Structure, Synthesis, and Applications of TiO 2 Nanobelts
Zhenhuan Zhao , Jian Tian , Yuanhua Sang ,* Andreu Cabot ,* and Hong Liu *
Dr. Z. Zhao, Dr. J. Tian, Dr. Y. Sang, Prof. H. Liu State Key Laboratory of Crystal Materials Shandong University Jinan 250100 , P. R. China E-mail: [email protected]; [email protected] Dr. Z. Zhao, Prof. H. Liu Beijing Institute of Nanoenergy and Nanosystems Chinese Academy of Science Beijing 100083 , P. R. China Prof. A. Cabot Catalonia Institute for Energy Research – IREC Sant Adria del Besos Barcelona 08930 , Spain E-mail: [email protected] Prof. A. Cabot Institució Catalana de Recerca i Estudis Avançats – ICREA Barcelona 08010 , Spain
DOI: 10.1002/adma.201405589
nanomaterial-based technologies having been commercialized. [ 1 ] In particular, since TiO 2 was found to be a chemically stable, low cost, environment-friendly, and effi cient hydrogen production photo-catalyst in 1972, [ 2 ] it has been widely used for several applications, such as photo-catalysis, [ 3 ] solar cells, [ 4 ] gas sensors, [ 5 ] and biosensors. [ 6 ] Most initial investigations focused on granulated TiO 2 and dem-onstrated the excellent performance of TiO 2 nanoparticles in removing organic pollutants and generating hydrogen by water splitting, as well as selective col-lection or charge transport in solar cells, gas sensors and biosensors. [ 7 ] However, granulated TiO 2 nanoparticles have also shown some unavoidable disadvantages. For photocatalysis or solar cell applica-tion, the large interface area associated with small particles strongly affects charge transport properties. Besides, the high concentration of surface defects associated with nanoparticles results in signifi cant charge recombination. [ 8 ] Another serious disadvantage of TiO 2 nanoparticles for practical photocatalytic applications is the high recycling cost when used in aqueous
media. These drawbacks limit further improvement of the per-formance of TiO 2 nanoparticles in several applications.
1D nanostructures have shown distinctive advantages with respect to 0D and 2D materials. [ 9 ] 1D nanostructures have the smallest dimensionality that allows two key properties for photocatalysis and nanodevices to be combined in the same material: fast electron transport and effective charge transfer. [ 10 ] Thus, 1D nanostructures are both an ideal model system for exploring dimensionality dependent electronic, optoelectronic and electromechanical properties, and an ideal building block for the production of high performance multifunctional het-erostructures. In particular, since the fi rst synthesis of belt-like ZnO nanostructures, [ 11 ] 1D nanobelts have attracted much attention due to their unique structural and functional proper-ties. Among the different materials obtained with a belt-like geometry, TiO 2 nanobelts — with a large concentration of exposed active facets and exceptional electronic, optical and mechanical properties — have raised particular interest and have been proposed for a remarkably wide range of applica-tions. [ 12 ] Nowadays, thanks to the large effort in developing synthesis methods to produce this material, single crystal TiO 2 nanobelts with exceptional crystallinity and a very low concen-tration of defects and dislocations are available. These nano-structures are characterized by exceptional charge transport
TiO 2 semiconductor nanobelts have unique structural and functional proper-ties, which lead to great potential in many fi elds, including photovoltaics, photo-catalysis, energy storage, gas sensors, biosensors, and even biomaterials. A review of synthetic methods, properties, surface modifi cation, and applications of TiO 2 nanobelts is presented here. The structural features and basic proper-ties of TiO 2 nanobelts are systematically discussed, with the many applications of TiO 2 nanobelts in the fi elds of photocatalysis, solar cells, gas sensors, biosen-sors, and lithium-ion batteries then introduced. Research efforts that aim to overcome the intrinsic drawbacks of TiO 2 nanobelts are also highlighted. These efforts are focused on the rational design and modifi cation of TiO 2 nanobelts by doping with heteroatoms and/or forming surface heterostructures, to improve their desirable properties. Subsequently, the various types of surface hetero-structures obtained by coupling TiO 2 nanobelts with metal and metal oxide nanoparticles, chalcogenides, and conducting polymers are described. Further, the charge separation and electron transfer at the interfaces of these hetero-structures are also discussed. These properties are related to improved sensi-tivity and selectivity for specifi c gases and biomolecules, as well as enhanced UV and visible light photocatalytic properties. The progress in developments of near-infrared-active photocatalysts based on TiO 2 nanobelts is also highlighted. Finally, an outline of important directions of future research into the synthesis, modifi cation, and applications of this unique material is given.
1. Introduction
The increasing research activities and great progresses in fundamental aspects and practical applications of nano-materials and nanofabrication technologies are changing the world. The amazing properties of nanomaterials have revolu-tionized many industries, with numerous nanomaterials and
properties, with fast charge transportation along the axial direc-tion, and an easier recyclability than granule-like nanoparticles. Furthermore, their electronic performance can be further con-trolled and enhanced through quantum confi nement effects. At the same time, chemically stable and ultralong TiO 2 nanobelts can be manipulated for tuning of functional properties and optimization for numerous potential applications, such as con-tinuous photocatalysis and sensors.
Nevertheless, although TiO 2 nanobelts have great poten-tial as photocatalysts and solar cell electrodes, they also pos-sess some intrinsic drawbacks. One main concern is the wide optical bandgap of TiO 2 , which limits its light harvesting capa-bility to the UV light range, leaving about 95% of the solar light energy wasted. [ 13 ] A second major limitation is the low photo-electronic coupling ability characterizing the poor photo-active facets of TiO 2 nanobelts. To overcome these drawbacks, and be able to develop high performance photo-electric con-version materials and devices based on TiO 2 nanobelts, is a present major challenge. In recent years, many works aiming to overcome the optoelectronic limitations of TiO 2 nanobelts have been reported. These works are based on several design principles, such as enlarging the photocatalytic active surface, enhancing/broadening the light harvesting range, forming Schottky or p–n junctions, or engineering their band structure to match particular energy levels. The very large surface area of TiO 2 nanobelts also provides a great platform to assemble a second nanomaterial to improve the relatively poor photo-elec-tric conversion ability of bare TiO 2 nanobelts.
The goal of this article is to provide a complete picture of the state of the art in TiO 2 nanobelts by reviewing the most impor-tant and most recent papers. The main areas of focus of this review article are as follows. 1) To provide a full introduction to TiO 2 nanobelts including their basic properties, commonly used
synthetic methods, and applications. The wide applications of TiO 2 nanobelt are highly related to the properties of TiO 2 nano-belt. In this Review, the basic properties of TiO 2 nanobelts are introduced before the synthetic methods, as we want readers fi rstly to fully understand the properties. 2) To clearly address the design principles of TiO 2 -nanobelt-based heterostructures and band-engineered nanobelts, in order to aid rational design and construction of specifi c nanostructures using proper mate-rials. 3) To highlight the very recent progress in the develop-ment of full solar light photocatalysis based on TiO 2 nanobelts.
2. Structure and Basic Properties of TiO 2 Nanobelts
TiO 2 has four crystalline polymorphs, including rutile, anatase, brookite, and TiO 2 (B). Rutile- and anatase-phase TiO 2 nano-belts have been most intensively investigated for photocatalysis applications because of their better catalytic activity and easier synthesis. Therefore, in this section, we focus mainly on the structural characteristics and properties of rutile and anatase TiO 2 nanobelts.
2.1. Structural Characteristics
Rutile and anatase TiO 2 nanobelts have an octahedral arrange-ment (very similar to that of TiO 6 ) with Ti and O atoms having the same coordination in their unit cell. [ 14 ] The layered arrangements of octahedrons in these two phases facilitates their growth in a belt-like geometry. [ 15 ] Both rutile and anatase nanobelts display a similar morphology, i.e., rectangular cross section with six exposed smooth facets ( Figure 1 ). Rutile TiO 2
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Figure 1. a) Typical scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM) images of rutile TiO 2 nano-belts obtained by acid-hydrothermal process. Reproduced with permission. [ 16 ] Copyright 2011, RSC. b) Typical transmission electron microscopy (TEM) and SEM images of anatase TiO 2 nanobelts. The inset energy dispersive X-ray (EDS) spectrum reveals the presence of only Ti and O in the nanobelt (Si comes from the substrate). The inset high-magnifi cation SEM image shows the width and fractured cross-section of a nanobelt. Reproduced with permission. [ 17 ]
nanobelts have very small thicknesses of about 20 nm, widths of 50–100 nm and can be several micrometers in length. [ 16 ] Rutile nanobelts, having a tetragonal phase with lattice parameters a = b = 0.459 nm, and c = 0.296 nm are obtained with very high crystalline quality. Although the growth direction for 1D nanostructures depends on the growth method used, for rutile TiO 2 nanobelts, the growth direction is generally the [001] direction. The geometric parameters of anatase TiO 2 nano-belts are very close to those of rutile TiO 2 nanobelts, having widths of about 50–200 nm and thicknesses of 20–50 nm. [ 17 ] The crystalline structure of anatase TiO 2 nanobelts can also be assigned to a tetragonal phase, but with different lattice parameters: a = b = 0.3766 nm and c = 0.9486 nm. The growth direction of anatase TiO 2 nanobelts is different from that of rutile TiO 2 nanobelts. After recording a series of selected area electron diffraction (SAED) patterns of single anatase TiO 2 nanobelts in different crystallographic orientations, keeping the [101] axis horizontal, [ 18 ] it was found that the growth direction of anatase TiO 2 nanobelts was the [010] direction, with a typical spacing of 0.38 nm.
2.2. Basic Properties of TiO 2 Nanobelts
While the differences between the crystalline structure of rutile and anatase TiO 2 nanobelts are tiny, they are suffi cient to result in dissimilar electronic structure and optical, mechan-ical and surface properties. These properties affect the perfor-mance of rutile and anatase TiO 2 nanobelts in their multiple applications.
2.2.1. Electronic Band Structure
Electronic properties are known to control light absorbance, redox potential, and charge carrier mobility, and consequently to strongly affect the functional properties of semiconduc-tors. [ 19 ] Full understanding of the electronic band structures of 1D TiO 2 nanostructures, especially those of TiO 2 nanobelts, is critical for their application in several fi elds, such as photo-catalysis, solar energy conversion or gas sensing. The wide bandgaps of bulk anatase TiO 2 (3.2 eV) and bulk rutile TiO 2 (3.0 eV) are related to the existence of the O 2p and Ti 3d states in the valence band (VB) and the conduction band (CB). [ 20 ] 1D TiO 2 nanostructures usually display thickness-dependent band-gaps. [ 21 ] When the thickness of the TiO 2 nanobelt is less than 2.5 nm, the bandgap is enlarged to a value above that of bulk anatase and bulk rutile TiO 2 , because of quantum confi nement effects arising with the decrease in thickness of the nanobelt ( Figure 2 a). [ 22 ] However, in general, thicker TiO 2 nanobelts are produced, and are therefore usually characterized by bandgaps narrower than the 3.4 eV theoretically predicted for the 2.5 nm TiO 2 nanobelts. The bandgaps of TiO 2 nanobelts are typically much lower than those of TiO 2 nanotubes, which are estimated to be about 3.87 eV for room-temperature aqueous colloids of TiO 2 nanotubes. The wider bandgaps of TiO 2 nanotubes are very close to the bandgaps of 2D titanate nanosheets, causing the 1D TiO 2 nanotube to display apparent 2D behavior at room temperature. [ 23 ]
The band structure of TiO 2 nanobelts is not only affected by their diameter, but also by their surface physical and chemical properties, and their growth orientation. [ 22,24 ] Different growth orientations result in different facet terminations, i.e, termi-nated with Ti or O atoms. However, it is found that both arm-chair and zigzag TiO 2 nanobelts with bandgaps larger than 2.93 eV prefer stable, oxygen-terminated facets.
TiO 2 nanobelts without oxygen vacancies are proven to be spin-unpolarized wide-bandgap semiconductors with variable bandgaps depending upon the growth direction and width, as shown in Figure 2 b. On the other hand, TiO 2 nanobelts containing oxygen vacancies (V o ) show narrower bandgaps, depending on the concentration of oxygen vacancies. Thus, the wide bandgap of TiO 2 nanobelts can be narrowed by intention-ally introducing oxygen vacancies.
It is well known that anatase TiO 2 is more photoactive than rutile TiO 2 . Because of this, details of the electronic structure of rutile TiO 2 nanobelts are hardly reported. Taking into account the important roles that electron mobility and lifetime play in photocatalysis, the main reason for this difference in photo-activity is probably the 40-times-larger electron mobility of anatase TiO 2 compared with rutile TiO 2 . [ 25 ] In this [010] direc-tion, anatase TiO 2 nanobelts are characterized by electrical conductivities of about 10 −7 Ω −1 cm −1 (Figure 2 c), [ 17 ] well above those of anatase TiO 2 nanoparticles, at 10 −9 Ω −1 cm −1 . [ 18 ]
Electron lifetimes depend on the kinetics of trapping and detrapping of electrons at states located in the bandgap of the oxide, and signifi cantly infl uence the electron transport and electron diffusion. [ 26 ] TiO 2 nanobelts have signifi cantly higher diffusion coeffi cients and much lower charge recombination than granulated TiO 2 (Figure 2 d). [ 27 ] Because of the longer electron lifetime, larger electron diffusion length, and lower charge carrier recombination, TiO 2 nanobelts have signifi cantly reduced charge loss during transportation, improving charge transfer and charge collection effi ciency with respect to granu-lated TiO 2 . Nanobelts also have almost two-fold higher effective donor concentrations than nanoparticles, with concentration of 3.85 × 10 19 cm −3 , compared with 1.90 × 10 19 cm −3 . [ 27 ]
2.2.2. Optical Properties
The light absorption ability of a semiconductor is mainly related to interband electron transitions and is thus controlled by the bandgap. Excitation of an electron to a point within the conduc-tion band occurs on absorbtion of a photon with energy equal to the sum of the bandgap energy and the energy within the conduction band, minus the energy of the hole left within the valence band. [ 28 ] These electron-hole pairs are those required for a photoreaction in a photocatalyst, or those extracted in a photovoltaic device. [ 29 ] For nanometer-sized semiconductors, the optical bandgap may be affected by the size and surface states of the nanoparticles. TiO 2 shows an excellent UV light photocatalytic activity; [ 30 ] however, its wide bandgap permits TiO 2 to only absorb light with wavelength below about 400 nm, which only accounts for 4% of the solar energy reaching the earth surface.
Absorbance signifi cantly depends on the TiO 2 crystal phase and therefore on the calcination temperature used during the
synthesis process. It has been reported by Zhou et al. that the light absorption edge of TiO 2 nanobelts changes with the calcina-tion temperature, while the crystalline phase evolves from as-syn-thesized H 2 Ti 3 O 7 , to TiO 2 (B), to anatase, and fi nally to rutile. [ 31 ] This dependence of the bandgap on the annealing temperature up to 800 ºC has been also demonstrated by Fu et al. [ 32 ] The bandgap of TiO 2 nanobelts obtained at 100, 400, 600, 800, 900 and 1000 °C are 2.85, 2.8, 2.9, 3.05, 3.1 and 2.9 eV, respectively, slightly smaller than that of bulk TiO 2 . It is suggested that the increase in the bandgap values is associated with the phase tran-sition from TiO 2 (B), with a bandgap value of 2.8 eV, to anatase, with a bandgap of 3.1 eV. However, notice that materials with a combination of phases are usually obtained. As an example, the sample calcined at 1000 °C contains both anatase and rutile phases. [ 31 ] Notice also that the rutile nanobelts obtained after the highest temperature process have higher degree of crystal-linity than the other TiO 2 phases. Direct measurements of the light absorption ability of rutile TiO 2 nanobelt have been hardly reported. Though rutile TiO 2 has lower photocatalytic activity
than anatase TiO 2 , more investigation is needed for better under-standing of the different optical and optoelectronic properties of rutile TiO 2 and anatase TiO 2 nanobelts. The light absorption of TiO 2 nanotubes is different from that of TiO 2 nanobelts. The wider bandgap of the TiO 2 nanotubes makes the position of the cut-off edge of light absorption of TiO 2 nanotubes blue-shifted with respect to that of the TiO 2 nanobelts. At the same time, the absorption characteristics of TiO 2 nanotubes show apparent 2D behavior, very similar to that of TiO 2 nanosheets.
In order to build a high performance photocatalyst, sup-pressing the recombination of charge carriers — occurring both at the surface and in the bulk of the semiconductor — is critical. Both anatase and rutile TiO 2 nanobelts have intensive light emission, [ 16,33 ] indicating relatively high radiative recom-bination effi ciencies of the photogenerated charge carriers. [ 34 ] The presence of dislocations and local defects acting as electron and hole trapping centers is responsible for both a deterioration of the material electrical properties and an increase in the non-radiative recombination of the charge carriers. [ 18 ]
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Figure 2. a) Energy gap evolution of ZTiO 2 NRs (zigzag TiO 2 nanoribbons, triangles) and ATiO 2 NRs (armchair TiO 2 nanoribbons, squares) as a function of nanobelt or ribbon width. The solid symbols represent the nanobelts with N = 2 m while the hollow symbols represent the nanobelts with N = 2 m + 1, where N = N z or N A and m is a positive integer. The bandgaps are fi tted to the formula E g = a + b / W , where the fi tting parameters a and b are 3.49 and 0.82 eV Å for ATiO 2 NRs with even N A ; 3.49 and –2.07 eV Å for ATiO 2 NRs with odd N A ; 3.62 and 1.23 eV Å for ZTiO 2 NRs with even N z ; and 3.62 and –1.68 eV Å for ZTiO 2 NRs with odd N z . Reproduced with permission. [ 22 ] Copyright 2010, ACS. b) Isosurfaces of spin density ρ = ρ ↑ – ρ ↓ (top) and band structures (bottom) of 6-ZTiO 2− x NRs with V o -defected one edge (left), two edges (middle), and hydrogenated V o defects at two edges (right). The isovalue is 0.01 | e |/Å 3 . The yellow and cyan surfaces represent spin-up and spin-down, respectively. The band lines of spin-up and spin-down branches are represented by the red and blue lines. The energy at the Fermi level ( E F ) is set to zero. Reproduced with permission. [ 22 ] Copyright 2010, ACS. c) Current–voltage ( I–V ) characteristics of a TiO 2 -nanobelt-based membrane showing a constant resistivity. Reproduced with permission. [ 17 ] d) Electron lifetimes in TiO 2 nanoparticles (TNP), TiO 2 nanobelts (TNR) and a mixture of TiO 2 nanoparticles and TiO 2 nanobelts (TPR10). Reproduced with permission. [ 27 ] Copyright 2011, ACS. e) Diffusion coeffi cient and diffusion length for electrons in TiO 2 nanoparticle (TNP), TiO 2 nanobelt (TNR), and TPR10. Reproduced with permission. [ 27 ] Copyright 2011, ACS.
One important application of TiO 2 nanobelts is their use as building blocks for fabrication of various micro/nano devices such as various single belt-based sensors or electronics. [ 35 ] The advantages of TiO 2 nanobelts in this context are ascribed not only to their unique geometry and electronic and optical properties, but also to their excellent mechanical properties, [ 36 ] which are totally different from the bulk material. [ 37 ] Compared with TiO 2 nanobelts, TiO 2 nanotubes are very fragile and are easily broken up by mechanical forces.
Present studies on the mechanical properties of TiO 2 mainly focus on rutile TiO 2 nanobelts, and few works have reported the mechanical behavior of anatase TiO 2 nanobelts. This may be related to the synthetic methods that will be discussed in the following sections. Wang and co-workers have investigated the mechanical properties of TiO 2 nanobelts with thicknesses of 60 nm, lengths of 8 µm and widths of 1.6 µm. [ 38 ] They found that a suspended nanobelt breaks under a critical frac-ture stress of about 4.3 µN ( Figure 3 ). The fracture strength of the TiO 2 nanobelt was reported to be about 250 GPa, and the Young’s modulus about 120 GPa. The Young’s modulus of TiO 2 nanobelts with an average cross-section dimension of about 30 nm was about 260 GPa. For thicker TiO 2 nano-belts, the bending modulus rapidly decreases with the increase of the thickness of the nanobelts. [ 39 ] As a result, the cor-responding shear modulus is suggested to be in the range of 0.07–0.4 GPa, ruled by the interlayer interactions and the defect density. [ 40 ]
The Young’s modulus of rutile TiO 2 nanobelts with an average thickness of 30 nm and a half-width of 80 nm was measured to be about 360 GPa by nanoindentation, after care-fully subtracting the substrate effect. [ 41 ] The average modulus of elasticity of anatase nanobelts with a thickness of 294 nm
and width of 210 nm was suggested to be 11.87 GPa, [ 42 ] which is considerably lower than that of bulk TiO 2 (280 GPa). The rea-sons behind the large difference between the rutile and anatase phases is attributed to the different levels of vacancies within the nano-belts obtained from different preparation methods. [ 43 ] In this [010] direction, the con-centration of the free surface atoms is sug-gested to determine the mechanical proper-ties of TiO 2 nanobelts.
2.2.4. Surface Properties
The photocatalytic activity and selectivity strongly depend on surface properties. As an example, the chemical dissociation of water molecules is energetically favored on the (001) plane; in contrast, water molecules are preferentially physically adsorbed on the (101) plane of TiO 2 . [ 44 ] The dissociated water molecule probably facilitates the transfer of photoinduced charge carriers and promotes the formation of reactive radicals. It has been
suggested that this is the reason for the much more effective photoreaction occurring at the (001) plane, compared with that at the (101) plane. [ 45 ]
The surface energy of the different facets is one main parameter determining its photoactivity. The surface energy of rutile TiO 2 facets is γ (001) > γ (100) > γ (101) (0.90 J m −2 > 0.53 J m −2 > 0.44 J m −2 ). Consistently with this order, Liu et al. have reported an enhanced photoactivity of TiO 2 with exposed (001) and (110) facets. [ 46 ] For the photodegradation of organic pollutants, it is also well accepted that the high-energy (001) facet is more effective than the (101) facet in the dissociative adsorption of reactant molecules. This is attributed to the reduction of organic pollutant activation energy on the (001) facet. The chemically dissociated molecules react more effi -ciently with photoproduced hydroxyl radicals on the surface of TiO 2 and in consequence can be rapidly converted to inac-tive surface hydroxyls. In other words, the photodegradation kinetics and the charge transfer process can be enhanced by the dissociated adsorption of pollutant molecules. However, TiO 2 nanostructures with exposed energetic (001) facets are hard to produce, and most synthetic routes reported involve the addition of highly toxic chemicals to stabilize the active (001) facet.
As expected, rutile and anatase TiO 2 nanobelts have different catalytic properties, in a large part due to the different exposed crystalline facets which rule the photocatalytic activity. [ 45 ] As an example, superoxide radicals act as an intermediate in the oxidization of organic molecules. [ 47 ] The superoxide radical is reported to originate from the adsorption of oxygen molecules on the surface of TiO 2 , which subsequently traps the photogen-erated electrons. As stated above, anatase TiO 2 nanobelts have two dominant (101) crystalline facets. Though the (101) facet on anatase TiO 2 is less energetically favored, it has been demon-strated to have the ability to promote the adsorption of oxygen
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Figure 3. a) Schematic diagram of the test of a suspended TiO 2 nanobelt-beam by an atomic force microscope (AFM) with a nanoindenter. b) F load varying with time. c) SEM images of suspended TiO 2 nanobelt-beams. d) Defl ection along the suspended nanobelts-beam at three fi xed load forces. a–d) Reproduced with permission. [ 38 ] Copyright 2012, RSC.
IEW molecules, which is benefi cial to the production of superoxide
radicals. The oxygen chemisorption onto the TiO 2 surface not only
depends on the specifi c facet, but is also strongly affected by the surface state of TiO 2 . It is suggested that the reactivity of the TiO 2 (110) facets is primarily governed by the surface species, such as oxygen vacancies and bridging hydroxyls, [ 48 ] changes in which may change the surface electronic structure of TiO 2 . [ 49 ] For annealed TiO 2 surfaces, one oxygen vacancy can adsorb two oxygen molecules. When the TiO 2 surface is exposed to water, one oxygen vacancy is converted to two bridging hydroxyls. In addition, different crystalline planes have different reduc-tion or oxidation potentials, therefore, upon photoexcitation, photogenerated electrons and holes may migrate to different planes separating from each other.
3. Synthetic Methods to Produce TiO 2 Nanobelts
Nucleation and growth are the two fundamental steps in the synthesis of 1D nanomaterials. Based on these two steps, a variety of methods for the synthesis of TiO 2 nanobelts have been developed. These methods show different levels of con-trol over the synthetic parameters; for example, introducing a liquid-solid interface to reduce the symmetry of the seed, employing various templates with 1D morphology, adding cap-ping reagents to control the growth rates of different facets, and so on. [ 10 ] There are several general approaches to the syn-thesis of TiO 2 nanobelts, such as hydrothermal processes, [ 50 ] electrochemical anodization, [ 51 ] and metallorganic chemical vapor deposition, [ 52 ] which are described in the following sections.
3.1. Hydrothermal Methods
As one of the most popular methods toward the synthesis of TiO 2 nanobelts, the hydrothermal method can be an effective and facile approach to the formation of highly crystallized TiO 2 nanobelts. The hydrothermal process is divided into two routes, i.e., the alkali-hydrothermal process and the acid-hydrothermal process. In the alkali-hydrothermal process, several materials can be used as the titanium precursors including titanium oxide powder, titanium butoxide (Ti(OBu) 4 ), titanium chloride (TiCl 4 ), TiF 4 , TiS 2 , and so on. [ 53 ] In a typical procedure, the tita-nium precursor is fi rst dispersed into a NaOH aqueous solution with a concentration of 10 M and then hydrothermally heated at 180–200 °C for at least 12 h. It is commonly accepted that sodium titanate nanobelts with similar crystalline structure are formed during the alkali-hydrothermal process. The chemical formula of the sodium titanate nanobelt is determined by the adopted titanium precursor. For example, TiO 2 powder tends to form sodium titanate nanobelts in the form of Na 2 Ti 3 O 7 or Na 2 Ti 4 O 9 . [ 17 , 53b ] The Ti(OBu) 4 and TiCl 4 precursors tend to form sodium titanate nanobelts in the form of Na 2 Ti 2 O 4 (OH) 2 , while TiF 4 is reported to form Na 2 Ti 5 O 11 nanobelts. These sodium titanate nanobelts have a similar layered structure. It is thought that the formation process of sodium titanate nanobelts involves a dissolution/recrystallization mechanism. [ 54 ] There are three
main steps in the formation of TiO 2 nanobelts according to the following three equations. First, the TiO 2 precursor is dissolved by breaking the Ti–O–Ti bonds, leading to the formation of sodium titanate nanobelts. Secondly, the sodium titanate nano-belt is converted to hydrogen titanate nanobelt by ion-exchange. Finally, the hydrogen titanate nanobelt is annealed to form TiO 2 nanobelts.
3TiO + 2NaOH Na Ti O + H O2 2 3 7 2→ (1)
Na Ti O + 2HCl H Ti O + 2NaCl2 3 7 2 3 7→ (2)
H Ti O 3TiO + H O2 3 7 2 2→ (3)
Proceeding with these three steps, the nanobelt morpholo-gies of Na 2 Ti 3 O 7 , H 2 Ti 3 O 7 and TiO 2 can be maintained if the reaction processes is carefully controlled, as shown in Figure 4 a. TiO 2 nanobelts obtained from alkali-hydrothermal process typi-cally have an anatase phase with the growth direction along with the [010] direction, as shown in Figure 4 b. [ 18 ] It should be pointed out that by carefully controlling the hydrothermal con-ditions, anatase TiO 2 nanobelts with a dominant (001) facet can also be prepared by the alkali-hydrothermal process. [ 55 ]
The key point for formation of the belt-like morphology is the formation of sodium titanate nanobelts: thus the mechanism of formation of such sodium titanate nanobelts has been widely investigated. [ 22,57 ] Figure 5 illustrates the growth mechanism of TiO 2 nanobelts. Initially during the hydrothermal process, six-coordinated monomers [Ti(OH) 6 ] 2− are formed and inclined to combine with each other form a nucleus. These nuclei con-sequently grow into nanosheets consisting of layers of octahe-dral [TiO 6 ] units. Large amounts of Na + cations are dispersed among the layers to balance the negative charges of the [TiO 6 ] octahedrals. These layered structures facilitate the replacement of the Na + by larger H 3 O + cations, weakening the static interac-tion due to the enlarged interlayer distance. As a result, the lay-ered sodium titanate is gradually exfoliated to form hydrogen titanate nanosheets that split to form nanobelts, in order to release the strong stress and lower the total energy. [ 28 ]
The size and morphology of the TiO 2 nanobelt can be tuned by adding a surfactant. Xie and Shang have reported a solvo-thermal method using titanium tetraisopropoxide (TTIP) as a precursor. Ethylenediamine (EDA) and ethylene glycol (EG) were used to control the morphology. [ 58 ] It is found that the morphology of the TiO 2 nanobelts was strongly dependent on the relative concentration of EDA. At very low concentrations, EDA reduced the diameter but kept the belt form of the TiO 2 . In this case, EDA plays a chelating-ligand role, infl uencing the solubility, reactivity, and diffusivity of the reactants. In this sce-nario, the polarity and coordination ability of solvents like EDA can infl uence the crystal morphology of the fi nal product. On further increasing the concentration of EDA, TiO 2 is formed with a nanorod morphology, and at very high EDA concentra-tions, far exceeding the amount of TTIP, TiO 2 assumes a fi ne fi bril form.
Another, completely different, route for producing TiO 2 nanobelts is the acid-hydrothermal process by the hydrolysis of a titanium precursor, e.g., tetrabutyl titanate. The nature of
this route is delicate control of the rate of hydrolysis of titanium precursor in acid conditions, usually resulting in the forma-tion of rutile TiO 2 nanobelts with the [001] growth direction, as shown in Figure 4 c. [ 56 ] Zhou et al. systematically investigated
the morphological evolution of TiO 2 in acid-hydrothermal con-ditions and found that rutile TiO 2 nanobelts can be formed at higher acid concentrations (9–10 M ). [ 16 ] In a typical proce-dure, 0.2 mL tetrabutyl titanate was homogenously mixed with
9–10 M HCl aqueous solution, and then hydrothermally treated at 180 °C for 2 h in a 20 mL Tefl on vessel. Compared with the alkali hydrothermal method, this route has the advantages of being a one-step synthesis, with much shorter hydrothermal treatment time.
The above mentioned dependence of the morphology of rutile TiO 2 nanobelt on acid concentration is further demonstrated by Cha et al. [ 56 ] In their experiments, a 12 M HCl aqueous solution was used to prepare rutile TiO 2 nanobelts. Interestingly, they found that the length of the nanobelt increased with increasing reaction time, while the nanobelts obtained after longer times (8 h) were much thinner that those formed at shorter times (4 h). They observed the phenomenon of a thin rutile nanobelt of 20 nm wide splitting into several thin nanobelts each about 5 nm wide, caused by a lattice misorientation originating from an array of dislocations.
A Ti foil has been also used as the tita-nium source for the hydrothermal synthesis of TiO 2 nanowire or nanobelts. [ 59 ] Before the hydrothermal reaction, the Ti foil was ultra-sonically cleaned using water, acetone, and
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Figure 4. a) Typical SEM images of Na 2 Ti 3 O 7 , H 2 Ti 3 O 7 and TiO 2 nanobelts. Reproduced with permission. [ 17 ] b) Bright-fi eld TEM image of an anatase nanobelt and the selected area electron diffraction pattern taken along the [100] direction as well as the HRTEM image. Reproduced with permission. [ 18 ] Copyright 2010, ACS. c) TEM image of a single rutile nanobelt and the selected area diffraction pattern (inset) of a nanobelt prepared using 12 M HCl and titanium(IV) butoxide by hydrothermal synthesis, as well as the HRTEM image. Reproduced with permission. [ 56 ] Copyright 2013, Royal Society of Chemistry.
Figure 5. Schematic illustration showing the growth mechanism of TiO 2 nanobelts. The [TiO 6 ] octahedral consists of one Ti atom and six O atoms, with the Ti atom in the center and six O atoms in the corners of the octahedral.
IEW ethanol, respectively. It was then placed into the Tefl on-lined
stainless steel autoclave, which contained 30 mL 1 M aqueous NaOH solution. The hydrothermal process was conducted at 220 °C for 24 h in an electric oven. Note that the concen-tration of the alkali solution required with this Ti precursor is much lower than that needed when using TiO 2 powder as the precursor. The hydrothermal reaction leads to the forma-tion of Na 2 Ti 2 O 4 (OH) 2 nanobelt arrays, which change into H 2 Ti 2 O 4 (OH) 2 nanobelts by immersion into the HCl solution. A post-calcination treatment at 500 °C can easily convert the H 2 Ti 2 O 4 (OH) 2 into TiO 2 . The width of the as-prepared TiO 2 nanobelt was about 72 nm. The growth direction was along the [100] direction of anatase crystal.
The hydrothermal method always involves three steps to pro-duce TiO 2 nanobelts — hydrothermal synthesis, acid treatment, and calcination — which makes this process time consuming. However, the advantages of this method are its mass-produc-tion potential, low cost, and easy control. Therefore, up to now, most TiO 2 nanobelts in literature have been synthesized by the hydrothermal method.
3.2. Electrochemical Anodization
The successful synthesis of TiO 2 nanobelts through elec-trochemical anodization without templates was reported by Chen and coworkers. [ 60 ] This method provides an effi cient way to directly prepare TiO 2 -nanobelt-based photoelectrodes for water splitting and biosensor applications. To conduct the electrochemical anodization, a standard two-electrode confi gu-ration was employed, using platinum foil and titanium foil as the cathode and anode, respectively. The electrolyte contained ammonium fl uoride, water, and ethylene glycol. By controlling the concentration of NH 4 F, the applied voltage, and the dis-tance between the two electrodes, anatase TiO 2 nanobelts can be synthesized on the surface of the titanium foil anode. The TiO 2 nanobelts kept growing continuously when the anodiza-tion time was increased.
The electrochemical growth of the TiO 2 nanobelts involves three simultaneous reaction: electric fi eld-assisted oxidation of Ti metal to form TiO 2 , electric fi eld-assisted etching of the oxide layer, and the chemical dissolution of TiO 2 by fl uoride. [ 61 ] It should be pointed out that another key role of the fl uoride ions is to control the growth of nanostructures with specifi c exposed crystalline facets. [ 44 ] Though this electrochemical method is simple, and the morphology of the formed TiO 2 nanobelts can be controlled by the experimental parameters such as applied voltage and concentrations, these TiO 2 nanobelts have very poor degrees of crystallinity.
3.3. Chemical Vapor Deposition
The chemical vapor deposition (CVD) method, employing cata-lytic substrates, has been developed as a convenient technique to obtain high-quality 1D nanostructures. [ 9,62 ] The CVD method is appropriate for the synthesis of 1D TiO 2 on silicon substrates coated with Ti, in the presence or absence of catalysts. [ 63 ] Typi-cally, for the growth of 1D TiO 2 , a TiO 2 seed layer must be
pre-coated onto the substrate, and the temperature is typically higher than 850 °C. Amin et al. successfully grew TiO 2 nano-belts at a decreased temperature (850–920 °C) by using nickel as the catalyst. [ 64 ] In this case, it was found that the formation of the TiO 2 nanostructure was substrate-independent. In this case, the nanobelt growth is suggested to proceed through a vapor–liquid–solid (VSL) growth mechanism. While several metals like Ni, Au, Ag, Pt, and Pd can assist the growth of TiO 2 nanostructures, Ni and Au are preferred as they result in higher yields.
The high deposition temperature required for the CVD growth of TiO 2 nanobelts should be attributed to the high melting point of Ti metal. Therefore, replacing metal Ti by other materials, like TiCl 4 , can signifi cantly decrease the dep-osition temperature. This was probed by Shi and Wang, who produced TiO 2 nanobelts at 650 °C using TiCl 4 and H 2 O as the raw materials. [ 65 ]
An alternative growth method is metallorganic chemical vapor deposition (MOCDVD), which has several advantages — in terms of compatibility with Si substrate and growth fl exibility of various materials — compared to wet-chemical approaches and traditional CVD methods. MOCVD typically employs organic titanium as the precursor, which allows reduction of the growth temperature. For example, polycrystalline TiO 2− δ nano-belts with mixed rutile and anatase phase can be grown on bare Si(100) substrate with [001] growth direction along at 510 °C using (C 11 H 19 O 2 ) 2 (C 3 H 7 O) 2 Ti as the metallorganic source. [ 52 ] The as-grown TiO 2− δ nanobelts were highly conductive.
The CVD method is generally used to synthesize rutile phase TiO 2 nanobelts and TiO 2 (B) nanobelts. Because of the diffi culty in acquisition of the raw materials required, few works report the synthesis of anatase TiO 2 nanobelts by CVD. In addition, though CVD has the advantage of yielding 1D nanostructures with excellent electronic and optical properties of a large variety of materials, it seems not to be the best method to produce TiO 2 nanobelts. This is because the morphologies of the TiO 2 nanostructures currently obtained from CVD or MOCVD are irregular, obviously different from belt-like.
3.4. Path-Directed Method
Although the synthesis of TiO 2 nanobelts is well developed, the production of highly ordered TiO 2 nanobelt arrays is still challenging. 1D nanostructures with highly ordered patterns are desirable for patterned microelectronics. For example, Xia and co-workers have fabricated TiO 2 nanostructure arrays using lithographically defi ned templates. [ 66 ] Kim et al. reported the synthesis of well-ordered TiO 2 nanostructures using surface relief gratings on polymer fi lms. [ 67 ] The polymer functioned as the template in the sol–gel process.
Highly ordered TiO 2 nanobelts with precisely controlled width, thickness, and length can also be fabricated using a simple, markless, path-directed method. [ 38 ] The path-directed method of synthesis of TiO 2 nanobelts involves three main steps. Firstly, a titanium fi lm with a thickness of 40 nm is deposited on a clean glass substrate by electron-beam evapora-tion. Secondly, a writing process is applied to the Ti fi lms using a frequency-doubled Nd:YAG 532 nm laser. The high-energy
laser induces the oxidation of the irradiated Ti metal. Upon Ti oxidation, the height of the oxidized regions increases due to the interfusion of oxygen. Finally, a wet etching process is conducted in a dilute HF solution to remove the residual, non-oxided Ti fi lm, and the oxidized regions (TiO 2 nanobelts) remain. This method results in polycrystalline TiO 2 nanobelts.
4. Applications of TiO 2 Nanobelts
TiO 2 is a multifunctional material, widely used in photo-catalysis for water splitting and organic pollutant removal, solar cells, gas sensors, biosensors, and so on. TiO 2 nanobelts, as one of the most important 1D TiO 2 nanostructures, have special electronic and optical properties that bring about signifi cant enhancement of their performance, and unique structural fea-tures that widen their potential applications. In this section, we summarize the basic applications of TiO 2 nanobelts in photo-catalysis, solar cells, gas sensors, biosensors, and electrochem-ical energy storage devices.
4.1. Photocatalysis
Since Fujishima and Honda reported the possibility of photo catalytic water splitting on a TiO 2 electrode in 1972, [ 2 ] researchers have made numerous efforts to widen the photocatalytic applications of TiO 2 in the photodegradation of various pollutants and the photogeneration of clean hydrogen fuel from water. [ 68 ] In one of the most signifi cant works, it was demonstrated that TiO 2 nanobelts have unique advantages over nanospheres in photocatalytic applications. [ 18 ] This work clearly addressed the physical mechanism of the improved photocatalytic activity of TiO 2 nanobelts over TiO 2 nanospheres. Very recently, our group has developed a hierarchical TiO 2 nanowire/graphite fi ber photoelectrocatalysis setup involving a wind-driven nanogenerator. [ 69 ]
In photocatalysis, the major and most important role of TiO 2 is to transfer the energy from the light to the charge carrier. Bulk TiO 2 has a bandgap of about 3.2 eV and strong absorp-tion in the UV, making it an ideal UV light photocatalyst. As a result, TiO 2 has the capability to effi ciently photodegradate rho-damine B, [ 70 ] methylene orange, [ 71 ] and organic pollutants such as 2-mercaptobenzothiazole, [ 72 ] benzotriazole, [ 73 ] and 4-chloro-phenol, [ 74 ] as well as to photocatalytically purify waste water from the papermaking industry. [ 75 ]
However, as stated above, several problems still exist with the use of granulated TiO 2 photocatalysts, the main issue being recovery and recycling. Larger TiO 2 nanobelts can well solve the recycling problems, while having photocatalytic activity and degradation effi ciency comparable to granulated TiO 2 photocat-alysts. [ 32 ] Its 1D nanostructure allows easy collection from sus-pension for later reutilization, or its manipulation into a paper-like morphology for continuous photodegradation of organic pollutants using a custom-made experimental set-up. [ 76 ] TiO 2 -nanobelt-based nanopaper has good mechanical properties and porosity. TiO 2 nanobelts also show excellent photocatalytic performance. They can almost completely degrade methylene blue within 55 min under UV light irradiation, comparable
with the well-known performance of granulated TiO 2 (P25). When the TiO 2 nanobelts were manipulated into a nanopaper morphology, the nanopaper still had excellent photocatalytic performance, with about 60% degradation within 40 min under UV light illumination. Note that the TiO 2 -nanobelt-based nanopaper facilitates the recycling of the photocatalyst during the photodegradation experiments, and that the slightly lower photocatalytic performance can be easily compensated by increasing the number of layers within the nanopaper, as the photocatalytic performance increases with the number of layers.
The photocatalytic reactions proceeding on the surface of the TiO 2 nanobelts involve both the hydroxyl radicals that come from the reaction between valence band holes and chemisorbed hydrogen groups or water molecules, [ 29 ] and also the superoxide radicals originating from reactions between the conduction band electrons and the adsorbed oxygen molecules. [ 77 ] How-ever, for TiO 2 nanobelts, the effi cient photocatalytic degradation of methyl orange is mainly caused by superoxide radicals, and the hydroxyl radicals take a lesser role in the oxidation of the methyl orange molecule. The better photocatalytic activity of TiO 2 nanobelts compared with TiO 2 nanospheres is due to the underlying physical mechanisms. Firstly, TiO 2 nanobelts have greater charge mobility along the longitudinal dimension of the crystals than that of TiO 2 nanospheres. Secondly, there are fewer localized states near the band edges and in the bandgap, due to the fewer unpassivated surface states in the TiO 2 nano-belts. Thirdly, the charge separation is enhanced due to trap-ping of photogenerated electrons by chemisorbed molecular oxygen on the (101) facet. [ 18 ] The following photoreactions typi-cally represent the key steps of the formation of radicals,
hvTiO + TiO (h + e )2 2+→ −
(4)
TiO (e ) + O O2 2 2→− −
(5)
TiO (h ) + H O OH2+
2 → ⋅
(6)
Another important photocatalytic application of TiO 2 nano-belts is water splitting, for the photogeneration of hydrogen fuel. [ 32 ] For TiO 2 nanobelts prepared from the electrochemical anodization method, a maximum photoconversion effi ciency of up to 4.51% at an applied voltage of about 0.1 V (vs. SCE (saturated calomel electrode)) can be reached, which is much higher than that of TiO 2 nanotubes obtained using the same synthetic method (2.43% at 0.39 V). [ 78 ] Photoelectrochemical (PEC) experiments have demonstrated that TiO 2 nanobelts have more effi cient separation of photoinduced charge carriers, com-pared with TiO 2 nanotubes. The excellent PEC performance should be associated with the long lifetime of the photoinduced charge carriers, which has its origin in the effi cient separa-tion, as well as a lower recombination rate. [ 60 ] For hydrogen evolution using TiO 2 nanobelts from alkali hydrothermal process, hydrogen production rates up to 2.41 m −2 h −1 have been measured, higher than that of commercial TiO 2 powder (P25, 1.91 m −2 h −1 ). [ 79 ]
TiO 2 is also used for the fabrication of solar cells, particularly dye-sensitized and organometallic halide perovskite solar cells. In both cases, TiO 2 plays the role of the electron transport layer. In dye-sensitized solar cells (DSSCs), a low cost alternative to mono- and polycrystalline silicon solar cells, TiO 2 nanomate-rials are photosensitized by dye molecules. [ 80 ] When TiO 2 nano-particles are used, the weak contact between the neighboring TiO 2 nanoparticles, as well as defects on the nanoparticle sur-faces signifi cantly decrease the electron diffusion length. [ 28,81 ] Highly crystalline 1D nanostructures usually have a lower trap density and offer more direct paths for charge transpor-tation, displaying lower electrical resistances for charge trans-port and much longer electron diffusion lengths of over tens of micrometers. [ 82 ] These properties make them ideal candidates for DSSCs. For example, solar cells using single crystalline anatase TiO 2 nanorods from a hydrothermal method display a high photon-to-current conversion effi ciency of 7.29%. [ 83 ] Feng et al. used vertically aligned single crystal rutile TiO 2 nanowires to assemble DSSCs with an open circuit voltage of 0.78 V, a fi ll factor of 0.68 and a short-circuit photocurrent den-sity of 6.95 mA cm −2 , obtaining a solar conversion effi ciency of 3.68%. [ 84 ]
Like nanorods and nanowires, TiO 2 nanobelts are also ideal candidates for DSSCs, and even exhibit better performance than the other 1D nanostructures. Kai Pan et al. have compared the performance of DSSCs using TiO 2 nanobelts and TiO 2 nanorods. [ 85 ] Under similar conditions and a using a xenon lamp with intensity 50 mW cm − 2 , the DSSC made from TiO 2 nanobelts shows a better photoelectrical energy conversion effi -ciency of 2.2% than those made from TiO 2 nanorods (1.8%). In addition, the TiO 2 nanobelt exhibited a bigger photovoltage than that of TiO 2 nanorods cells, indicating that the conduc-tion band level of TiO 2 nanobelts was more negative and more adjacent to the lowest unoccupied molecular orbital (LUMO) of the dye molecule. As another example, the IPCE (incident photon-to-electron conversion effi ciency) of DSSCs based on TiO 2 nanobelts produced from hydrothermal synthesis exhibits a maximum at 530 nm, with a conversion effi ciency of 33.2%, under illumination of AM 1.5 G (100 mW cm −2 ). [ 27 ] There is no doubt that TiO 2 nanobelts will attract more attention for DSSC applications as the understanding of their electrical proper-ties is improved. Although, to the best of our knowledge, TiO 2 nanobelts have not yet been used to replace TiO 2 nanoparticles in the fabrication of perovskite-based solar cells, they may also be able to provide advantages in this emerging type of photo-voltaic device.
4.3. Biosensors
As stated above, TiO 2 nanobelts are highly stable and have good compatibility with biomaterials. At the same time, they are able to enhance enzyme catalytic performance. [ 86 ] These merits posi-tion TiO 2 as an excellent candidate for various biosensors. In addition, good electronic properties and high surface to volume ratio ensure the fast transportation of charge carriers, which guarantees a high sensitivity. [ 87 ] TiO 2 has excellent aqueous
stability and a large index of refraction, and is considered an ideal candidate for optical biosensors. [ 6a , 88 ] For example, glucose detection is still challenging, and numerous detection methods have been developed to realize fast and precise detection of glu-cose using biosensors. [ 89 ] Our group has systematically investi-gated the performance of TiO 2 -nanobelt-based electrochemical biosensors. It was found that TiO 2 nanobelts exhibited excel-lent electrocatalytic activity in the oxidation of nucleobases ( Figure 6 b). [ 90 ] Two irreversible oxidation peaks appearing at 0.62 V and 0.89 V can be clearly observed for the TiO 2 -nano-belt-modifi ed electrode. An even higher electrocatalytic perfor-mance can be obtained by hydrothermal treatment of the TiO 2 nanobelt in diluted HCl aqueous solution. It is suggested that this is related to the increased specifi c surface area of the acid corroded TiO 2 nanobelt.
Taking advantage of the excellent electrocatalytic oxida-tion of nucleobases using TiO 2 nanobelts, a biosensing elec-trode employing acid corroded TiO 2 nanobelts was rationally designed to selectively detect the match and mismatch of single nucleobases. [ 91 ] As a result, the TiO 2 nanobelts displayed dis-tinguished selectivity and sensitivity. Figure 6 c shows the elec-trochemical responses of single base-pair interactions at the TiO 2 nanobelts modifi ed electrodes in PBS (phosphate buffer solution) at pH 7.4. Obviously, the surface-coarsened TiO 2 nanobelts (CTNs) can selectively detect the perfect match or mismatch of single nucleobases. It is shown that the oxidation peak potential has a negative shift of 30 mV for G (guanine)–U (uracil) mismatch and G–T (thymine) mismatch, compared to the perfect match of G–C (cytosine). Similar phenomena can be observed for the A (adenine)–C mismatch, A–U perfect match and A–T perfect match. Based on the excellent sensitivity and selectivity of TiO 2 -nanobelt-based electrochemical biosensor, TiO 2 nanobelts were also used to investigate cancer cells and anticancer drug effect (Figure 6 d). [ 92 ]
4.4. Gas Sensors
Metal oxide semiconductor gas sensors are based on the varia-tion of electrical resistance of a porous fi lm upon gas molecules adsorption at the surface. [ 93 ] The electrical resistance of the sem-iconductor sensor depends upon the type and concentration of the adsorbed gas, which is the basic mechanism providing selectivity. [ 94 ] For practical applications, the sensing material should fulfi ll particular requirements critical to the operation of real sensors, including sensitivity, selectivity, and speed of response requirements. [ 95 ] Among the numerous metal oxides, TiO 2 is a particularly interesting gas sensor material, which has proven excellent performance in detecting different species, such as O 2 , [ 96 ] H 2 S, [ 97 ] H 2 , [ 98 ] CO, [ 99 ] NO 2 , [ 100 ] NH 3 , [ 5,101 ] and so on. Besides, 1D nanostructures with high surface-to-volume ratio, are especially suitable in various gas sensor architectures, such as nanowire-based fi lms, nanowire arrays, and even single nanowire sensors. [ 102 ]
Our group has fabricated gas sensors using nanostructured sheets based on anatase TiO 2 nanobelts ( Figure 7 a). [ 17 ] It was found that TiO 2 nanobelts showed good response to O 2 . With increasing O 2 concentration, the resistance increased almost linearly from about 3.9 MΩ at 100 ppb to 5.4 MΩ at 1000 ppb.
Figure 6. a) SEM image of TiO 2 nanobelts. Reproduced with permission. [ 90 ] Copyright 2011, Royal Society of Chemistry. b) Cyclic voltammograms of various chemically-modifi ed electrodes in 0.1 M PBS (pH 7.4) without and with 0.1 m M guanine (G) and adenine (A). Potential sweep rate 100 mV s −1 . TNs/CA/GCE refers to the TiO 2 -nanobelt-modifi ed glassy carbon electrode. CTNs/CA/GCE refers to the glassy carbon electrode modifi ed by surface-coarsened TiO 2 nanobelts. Reproduced with permission. [ 90 ] Copyright 2011, Royal Society of Chemistry. c) SWV of single base-pair matches: TNs/CA/GCE and CTNs/CA/GCE in 0.1 M PBS (pH 7.4) containing 0.1 m M nucleobases. Reproduced with permission. [ 91 ] Copyright 2011, Royal Society of Chemistry. d) Cyclic voltammetry (CV) curves for various modifi ed electrodes in 0.1 M PBS (pH 7.4) with and without 0.1 m M O 6 -benzylguanine (O 6 BG), and the magnifi ed cyclic voltammogram of the partial area in, sweep rate, 100 mV s −1 . Reproduced with permission. [ 92 ] Copyright 2013, Royal Society of Chemistry.
At room temperature, detection levels down to 100 ppb, with still-high resistance variations of up to one order of magni-tude, was accomplished. The good O 2 sensing performance of TiO 2 nanobelts is due to the high density of surface sites available for gas adsorption and the high infl uence these sites have on the overall electrical properties of the material because of the high surface-to-volume ratio. The suggested
sensing mechanism is based on the variation of concentration of surface oxygen vacancies in the TiO 2 nanobelts with the oxygen partial pressure. This change in the concentration of oxygen vacancies translates into a modifi cation of the concen-tration of trapped electrons, and thus into a variation of the charge carrier density, measured as a change in the electrical resistivity.
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Figure 7. a) Low-magnifi cation SEM image of the TiO 2 nanobelt paper and high-magnifi cation image of a small area of the same paper. Reproduced with permission. [ 17 ] b) Response of nanostructured TiO 2 sheet to different oxygen concentrations: resistance–concentration curve and gas sensitivity curve of TiO 2 sheet at room temperature, as well as the sensor response to O 2 at different concentrations. Reproduced with permission. [ 17 ] c) (left) Response curves and (right) sensitivity profi les of ethanol vapor sensors based on TiO 2 nanobelts, Ag–TiO 2 nanobelts, surface-coarsened TiO 2 nano-belts, and surface-coarsened Ag–TiO 2 nanobelts upon exposure to different concentrations of ethanol vapor at 200 °C. Reproduced with permission. [ 103 ] Copyright 2010, American Chemical Society.
As expected, TiO 2 nanobelts also exhibit excellent perfor-mance in the sensing of other target gases like ethanol vapor, which leads to potential application in breath alcohol tests. TiO 2 nanobelts show a rapid response to ethanol gas, even at con-centrations as low as 20 ppm. [ 103 ] This sensing performance can be further improved by hydrothermally treating the TiO 2 nanobelts in a diluted acid solution, which creates surface coarsened TiO 2 nanobelts, or by assembling noble metal nano-particles on the surface of the TiO 2 nanobelts. Metal nanoparti-cles can lower the gas sensing temperature of TiO 2 nanobelts, and the surface coarsening can improve the sensitivity of TiO 2 nanobelts. Ag nanoparticle assembled on surface-coarsened TiO 2 nanobelts display a signifi cantly improved sensing perfor-mance, which is much better than many other sensors based on ZnO nanowires, [ 104 ] ZnO nanopillars, [ 105 ] SnO 2 nanorods [ 106 ] and Pt–TiO 2 nanowires arrays. [ 107 ]
4.5. Energy Storage Devices
TiO 2 nanostructures, and particularly TiO 2 nanobelts, have also been used in the fi eld of energy storage as supercapacitor electrodes, [ 108 ] and especially as anode materials in lithium ion batteries (LIBs). [ 109 ] The advantages of TiO 2 are its low cost, mechanical stability, environmental friendliness, safety, relatively high lithium storage capability, and good cyclability. As a supercapacitor electrode material, TiO 2 nanobelts shows improved energy storage performance, including signifi cant enhancement of specifi c capacitance, rate capability, energy density, and power density, when coupled with reduced gra-phene oxide (rGO), due to their unique shape, better charge transport properties, and larger area of contact with the rGO nanosheet compared with TiO 2 nanoparticles. [ 108a ] This work clearly demonstrates that it is necessary to consider the shape and coupling effects of metal oxide and carbon materials for supercapacitors. All main TiO 2 polymorphs have been proven to be excellent anode materials for LIBs. Among them, the anatase phase is found to have the best properties, with a theo-retical capacity of 335 mA h g –1 , a fl at operating potential and a low volume expansion, <4%, during Li + charge/discharge cycles. [ 109f , 110 ] However, excellent capacities and cycleabilities have been also obtained for TiO 2 (B) nanobelts, [ 109a , 111 ] and also for Magnéli-phase Ti n O 2 n− 1 nanobelts. [ 109c ] The main drawbacks of TiO 2 are its relatively poor Li + and electronic conductivities. The use of nanostructured fi lms allows for improvements in the electrode/electrolyte interface, and thus leads to reduction in the Li + transport path within the electrode. This has been experimentally demonstrated for rutile TiO 2 by measuring increasingly higher capacities, up to 378 mA h g −1 , when reducing the TiO 2 crystal domain size down to 15 nm.
However, the use of nanoparticles further reduces the elec-tronic conductivity of the electrode. On the other hand, the use of nanobelts allows improvements in the electron transport proper-ties of the material, while still providing a network of electrolyte channels for effi cient Li + transport. Moreover, the nanobelt geom-etry maintains the high interface area with the electrolyte and the excellent ability to accommodate lithiation-induced stresses. [ 110 ]
To further improve the performance of TiO 2 -nanobelt-based LIBs, the use of coaxial nanobelts heterostructures such as
TiO 2 @Co 3 O 4 and TiO 2 @MoS 2 has been proposed. [ 109e , 112 ] The goal of the shell, made of a high capacity material, is to facili-tate and increase Li uptake, while the TiO 2 nanobelt provides a suitable and mechanically stable framework that provides both a high electrode–electrolyte interface and effi cient charge transport.
To improve performance in LIBs, particularly with regard to electrical conductivity, TiO 2 nanobelts can be also wrapped with graphene or other forms of carbon, which act as a highly effi -cient porous current collector. [ 109a , b , e ] Moreover, very recently, a novel hybrid Li-ion capacitor combining a graphene-based electrochemical double layer capacitor-type cathode with a TiO 2 nanobelt based LIB-type anode has been demonstrated to have high energy and power densities. [ 113 ]
5. Modifi cation of TiO 2 Nanobelts
Pristine TiO 2 , the fi rst-generation material, has been employed in photocatalytic degradation and mineralization of a large number of organics, as well as the production of clean hydrogen fuel from water. Unfortunately, as mentioned above, TiO 2 , either in rutile or anatase phase, has a wide bandgap resulting in a photo-threshold that extends from the UV region to about 400 nm. This inherent limitation of TiO 2 curtails the effi ciency of conversion of the solar energy that reaches the earth’s surface. Like bulk TiO 2 , TiO 2 nanobelts also have a wide bandgap and can only absorb UV light, which imposes a major challenge for solar energy conversion. Numerous efforts have been made to overcome this challenge. These efforts can typically be divided into two dif-ferent approaches. The fi rst approach is the modifi cation of TiO 2 nanobelts with heteroatoms, including cations and anions. The second is the modifi cation of TiO 2 nanobelts by constructing a secondary phase on the TiO 2 nanobelt surface, to form a sur-face heterostructure. In this section, we review the modifi cation of TiO 2 nanobelts based on the above two routes, summarizing the achievements, and providing an overview on solutions to the challenges associated with the use of TiO 2 in photocatalysis.
5.1. Modifi cation with Heteroatoms
It is thought that introducing heteroatoms into the crystalline structure of the TiO 2 nanobelt can reduce the photothreshold energy by extending the light absorption edge to visible light. Numerous attempts toward achieving this goal have been made, employing various approaches including hydrothermal, [ 114 ] sol–gel, [ 115 ] electrochemical methods, [ 116 ] and so on. The het-ero-elements introduced into TiO 2 include not only anions (non-metal doping) but also the cations (metal doping). The following section aims to address the corresponding changes in electronic properties and optical properties originating from the introduction of anions and cations.
5.1.1. Modifi cation with Cations
Cation doping is usually realized by introducing transition metal ions, such as V, [ 117 ] Fe, [ 118 ] Co, [ 114a , 119 ] and Cr [ 120 ] and so
on, into the crystalline phase of TiO 2 . The most commonly utilized method for the modifi cation of TiO 2 nanobelts with cation heteroatoms is the hydrothermal method. [ 114c , d , 121 ] The cation modifi cation of TiO 2 has shown both positive and negative effects on the photocatalytic activity of TiO 2 . [ 122 ] The positive aspect is that cation modifi cation can decrease the photothreshold energy of TiO 2 , while the negative aspect is that the introduced metal ions act as new recombination center for electrons and holes. [ 48 ] For example, Fe, Mo, Ru, Os, Re, V, and Rh atoms signifi cantly increased the photoreactivity for both oxidation and reduction, while Co and Al decreased the photoreactivity, which is largely dependent on the concentra-tion, energy state, electron confi guration and distribution of the heteroatoms. [ 123 ]
TiO 2 nanobelts have a layer structure which is benefi cial for the introduction of heteroatoms. For example, lithium titanate nanobelts are an ideal anode material for high performance Li-ion batteries, as discussed above. [ 109c ] To date, modifi cation of TiO 2 nanobelts with metal ions is limited to the hydrothermal-process-assisted ion-exchange method. Most current literature is focused on the introduction of Bi, [ 114b ] Co, [ 119,124 ] Ni, [ 114a ] Fe, [ 118,123 ] and rare earth metal ions. [ 121,125 ] With the introduc-tion of different metal ions into the crystalline structure of TiO 2 nanobelts, different properties are usually expected. For example, Co or Ni doped TiO 2 nanobelts displayed room tem-perature ferromagnetism, while Bi doped TiO 2 nanobelts can extend the absorption edge to the visible light range to enhance the light harvesting ability. [ 114a , 124 ] Generally, introducing hetero
metal atoms into the TiO 2 nanobelt does not change the crystal structure and the morphology. However, even a tiny amount of heteroatoms can trigger signifi cant changes. For example, com-pared to the undoped TiO 2 nanobelts, the Bi-doped TiO 2 nano-belts show a red-shift of the absorption edge, and have strong responses to visible light irradiation, as shown in Figure 8 a. [ 114b ]
Another interesting group of dopants are the rare earth (RE) metals. Rare earth ions are known to have excellent upcon-version ability. [ 126 ] Upconversion refers to optical processes featuring the successive absorption of two or more photons through intermediate, long-lived energy states, followed by the emission of output radiation at a shorter wavelength than the incident wavelength. [ 127 ] This effect can be introduced into TiO 2 nanobelts by doping with rare earth ion such as Er 3+ and Yb 3+ . [ 121 ] According to results of experiments investigating upconversion fl uorescence emission, compared with undoped TiO 2 nanobelts, Er-doped TiO 2 nanobelts showed two dif-ferent emission peaks under excitation illumination of wave-length 980 nm at room temperature. The two emission peaks are a strong green emission and a relatively weak red emis-sion. Other results show that by modifying the TiO 2 nanobelts with rare earth ions, 980 nm light can be converted into two different wavelength ranges of 520–570 nm and 630–680 nm (Figure 8 b). [ 121 ] This upconversion effect may be used to realize the preparation of near-infrared photocatalysts which can largely enhance the utilization and conversion effi ciency of solar light energy. However, more research efforts are needed, because introducing different metal atoms usually brings about
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Figure 8. a) SEM and HRTEM image of Bi doped TiO 2 nanobelt, as well as the corresponding UV–vis spectra. Reproduced with permission. [ 114b ] Copyright 2009, Elsevier. b) SEM image, HRTEM image of the RE doped TiO 2 nanobelt, and upconversion (UC) fl uorescence emission spectra of the RE-doped TiO 2 nanobelts ( λ ex = 980 nm): (left) the Er–TiO 2 -01 and Er–TiO 2 -02; (right) the RE–TiO 2 -01 and RE–TiO 2 -02 (inset is the magnifi ed spec-trum around wavelength 660 nm). For comparison, the UC emission spectra of the Er–LTO-01 and RE–LTO-01 as the precursors of the Er–TiO 2 -01 and RE–TiO 2 -01 are also listed in the fi gure. LTO referes to layered titanate nanobelts. The number (01 and 02) refers to the concentration level of dopant. Reproduced with permission. [ 121 ] Copyright 2010, Elsevier.
different properties to the TiO 2 nanobelts. Due to this com-plexity, there are no general rules for metal ion modifi cation of TiO 2 nanobelts.
5.1.2. Modifi cation with Anions
An alternative approach for lowering the photothreshold of TiO 2 is to modify the TiO 2 nanobelts by doping with anions. The modifi cation strategies include the solvothermal method, [ 128 ] hydrothermal method [ 129 ] and high temperature annealing in specifi c gas atmosphere. [ 130 ] Since the fi rst report of the vis-ible light activity of N doped TiO 2 , [ 131 ] it has been found that N-doped TiO 2 shows extended light absorption behavior from the UV to the visible light regions, leading to the visible light photocatalytic activity of TiO 2 . However, the fundamental aspects of the origin of the visible light photocatalytic activity of the N-doped TiO 2 were still unclear until the work of Wu’s group. [ 132 ] Actually, since the early stages of research into anion-doped TiO 2 , it has been well accepted that introducing anions usually results in a bandgap narrowing or introduces interband levels, as shown in Figure 9 a. [ 133 ] These positive effects of anion modifi cation of TiO 2 have already been probed for TiO 2 nano-
belts. [ 128, 129 , 122a , 134 ] Determining the in-depth mechanism of vis-ible light photocatalytic activity of anion doped TiO 2 has proven controversial. Wu’s group has conducted systematic investiga-tions on the N-doped TiO 2 nanobelts. It is found that N-doping of TiO 2 nanobelt results in the presence of mid-gap states in the energy band of the TiO 2 nanobelt, rather than narrowing of the bandgap. N-doping of TiO 2 nanobelts can result in both positive and negative effects on the photocatalytic activity of TiO 2 nanobelts, i.e., limited visible light photocatalytic activity as well as the introduction of mid-gap energy states as charge recombination centers.
Usually, TiO 2 nanobelts modifi ed with anion heteroatoms are characterized by a change in color. For example, N-modifi ed anatase TiO 2 nanobelts display a pale yellow color compared to the white color of pristine TiO 2 nanobelts. This color change typ-ically originates from the red-shift of the light absorption edge of the TiO 2 nanobelt. This red-shift effect of the anion modi-fi ed TiO 2 nanobelts is very common, as shown in Figure 9 b. [ 135 ] The advantage of this effect is the extension of light absorption range from the UV into to the visible light range, increasing the utilization effi ciency of solar light. [ 130 ] Meanwhile, the introduced anion atoms usually have two possible sites in the crystalline structure of TiO 2 nanobelt, i.e., interstitial sites and
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Figure 9. a) Various schemes illustrating the possible changes that might occur to the bandgap electronic structure of anatase TiO 2 on doping with various non-metals: 1) bandgap of pristine TiO 2 ; 2) doped TiO 2 with localized dopant levels near the VB and the CB; 3) bandgap narrowing resulting from broadening of the VB; 4) localized dopant levels and electronic transitions to the CB; and 5) electronic transitions from localized levels near the VB to their corresponding excited states for Ti 3+ and F + centers. Ti 3+ and F + refer to the color centers which can absorb visible light radiation. Repro-duced with permission. [ 134 ] Copyright 2006, American Chemical Society. b) Diffusive refl ectance spectra of pristine and nitrogen doped anatase TiO 2 nanobelts, and electronic density of states (EDOS) for TiO 2 nanobelts. Reproduced with permission. [ 135 ] Copyright 2009, American Physical Society. c) UV–vis absorption spectra of N–F co-doped TiO 2 nanobelts. Reproduced with permission. [ 128 ] Copyright 2012, American Chemical Society. d) UV–vis absorption spectra of as-prepared N–S co-doped TiO 2 nanobelts. Reproduced with permission. [ 129 ] Copyright 2011, Elsevier.
IEW substitutional sites. As a result, oxygen vacancies are usually
created with anion modifi cation. Varying the concentration of the anion heteroatoms can change the related concentrations of the interstitial atoms and oxygen vacancies.
Aside from N, other elements like C, S, F, can also be intro-duced into TiO 2 nanobelts. However, though there exist few reports on the doping of TiO 2 nanobelts with just C, S, or F, the existing literature is mainly focused on the co-doping of TiO 2 nanobelts with N and F, or N and S, as shown in Figure 9 c and 9 d. [ 129 ] These co-doped TiO 2 nanobelts display obvious extension of their optical absorption edge from UV to visible light regions. However, theoretical analysis of the co-doping of TiO 2 nanobelts has thus far never been reported.
5.2. Modifi cation with Surface Heterostructures
Though TiO 2 nanobelts have several improved properties com-pared to TiO 2 nanoparticles, such as better electronic properties, mechanical properties and the ability to be easily recycled and reutilized as photocatalysts, they still suffer from the aforemen-tioned wide bandgap, which limits their applications to the UV light range, and from relatively fast charge-carrier recombina-tion, which reduces the effective number of electrons and holes for degradation reactions.
Ideally, for photocatalytic applications, the structure should make full use of the solar light spectrum, from UV to visible light, and even infrared light. The material should also have the ability to effectively separate the photoinduced charge carriers, to increase solar light conversion effi ciency. However, presently, no individual photocatalyst has been found that can meet the above criteria. One promising protocol to solve these problems is the construction of surface heterostructures on TiO 2 nano-belts. In this section, we systematically summarize the design principles for the construction of TiO 2 nanobelt surface hetero-structures. Based on these design principles, several kinds of TiO 2 nanobelt surface heterostructures that displayed enhanced photocatalytic activity have been successfully synthesized.
5.2.1. Design Principles of TiO 2 Nanobelt Surface Heterostructures
The construction of rationally designed, high performance TiO 2 nanobelt heterostructures should obey several basic design principles in order to control fundamental aspects such as charge separation, interfacial charge transfer, and electronic properties of materials.
Enlarge the Active Photocatalytic Surface Area : Photocatalytic reactions occur at the active sites on the surface of a photo-catalyst. Thus, the number of active sites directly infl uences the performance of the photocatalyst. The most common method for increasing the number of active sites is to increase the TiO 2 nanobelt surface area by depositing nanoparticles onto its sur-face. However, some drawbacks still exist, such diffi culties in controlling particle size, distribution uniformity, and density and a poor connectivity between the nanoparticle and the TiO 2 nanobelt substrate. One very effective way to increase the sur-face area of TiO 2 nanobelts is the application of hydrothermal acid corrosion, [ 136 ] which can create numerous nanoparticles
and pits on the TiO 2 nanobelt surface through an in situ dis-solution–precipitation process. In addition, the acid corrosion method results in intimate contact between the in situ created nanoparticles and the TiO 2 nanobelts, facilitating charge-carrier separation and transfer.
Broaden the Light Absorption Range : As previously discussed extensively, TiO 2 nanobelts have a wide bandgap and can only absorb UV light, leaving the majority of solar light energy wasted. To solve this issue, two different routes can be consid-ered. One route is optimization of the surface and bulk phys-ical and chemical properties by introducing oxygen vacancies, tuning crystal phases, and/or modifi cation with hetero-atoms. Another route is the construction of surface heterostructures by introducing narrow bandgap semiconductors, dye mol-ecules, and up-converting materials at the TiO 2 nanobelt sur-face. The former route is typically based on bandgap narrowing and the introduction of interband energy levels in the bandgap of TiO 2 . However, this route usually leads to some negative effects on the resulting photocatalyst, such as poor stability and even worse charge separation. The latter route is based on a synergistic effect between the TiO 2 and the secondary phase in the heterostructure. In fact, this route has shown tremen-dous advantages and can be split into two different strategies, depending on the range of light absorption of the secondary phase.
The modifi cation of TiO 2 nanobelts with visible-light photo-catalysts to form surface heterostructures can extend the light absorption into the visible. Semiconductors with a bandgap energy lower than 3.2 eV, i.e., a cut-off edge of light absorp-tion above 400 nm, can be used as potential visible light absorbers. [ 137 ] Transition metal oxides, transition metal chalco-genides, and their corresponding composite oxides and chalco-genides are potential materials for consideration. [ 138 ]
The modifi cation of TiO 2 nanobelts with near-infrared light absorbers can result in materials covering nearly the whole of the solar light spectrum. These kind of surface heterostructures should signifi cantly increase the light conversion effi ciency, because infrared light accounts for a signifi cant part of the solar light energy.
One alternative and effective method to use near-infrared light is to load upconversion materials onto TiO 2 nanobelts. The upconversion of infrared light to visible light usually hap-pens in materials containing d-block or f-block elements. [ 139 ] The most famous upconversion material is NaYF 4 doped with rare earth ions. [ 140 ] However, it is not possible to make full use of the infrared light by means of upconversion, because the excitation of the upconverting materials is limited to a single or few wavelength of light. Presently, there is a large requirement for the development of more effi cient near-infrared or infrared light photocatalysts.
Surface Plasmon Resonance Effects : TiO 2 nanobelts are UV-light photocatalysts with strong absorption ability of UV light. However, there is still room for further enhancement of the UV-light photocatalytic activity. One effective way is to use the plasmon resonance effect originating in noble metals, such as Au, [ 141 ] Ag, [ 142 ] and their alloys with Pt and Pd. [ 143 ] The solar energy conversion effi ciency of semiconductor–metal hetero-junctions can be improved by the transfer of plasmonic energy from the plasmonic metal to the semiconductor via three
underlying mechanisms: [ 144 ] i) photonic enhancement, [ 145 ] ii) direct hot electron transfer, [ 146 ] and iii) plasmon-induced res-onance energy transfer (PIRET). [ 147 ] So far, direct hot electron transfer processes have shown limited contribution to solar energy conversion effi ciency in semiconductor–metal hetero-junctions. In contrast, PIRET has been reported to enhance the solar energy conversion of semiconductors remarkably. [ 148 ]
Schottky Junctions : When a metal and a semiconductor come in to contact, an energy barrier is formed at the interface between them if the Fermi level of the metal is below that of the semiconductor. Such a structure is is termed a Schottky junction. [ 149 ] The main feature of the Schottky barrier is the Schottky barrier height which, to a fi rst approximation, is the energy difference between the work function of the metal and the electron affi nity of the semiconductor. By reasonable manipulation, the barrier can be engineered such that that photoinduced charge carriers in the semiconductor do not accumulate at the interface but fl ow into the metal, leaving the holes in the valence band of the semiconductor, and hence achieving the goal of charge carrier separation. [ 150 ]
p–n Junctions : When a p–n junction is formed at the interface between two types of semiconductor, a p-type and an n-type, a local electric fi eld appears across the space charge region. [ 151 ] Driven by the electric fi eld, the photogenerated electrons move towards the n-type semiconductor, and holes fl ow into the p-type semiconductor, accomplishing an effective separation of photoinduced charge carriers. TiO 2 nanobelts are n-type semi-conductors, and hence p-type semiconductors, such as NiO, Ag 2 O, Cu 2 O, can be coupled onto the surface of TiO 2 nanobelt to form the desired p–n junction.
Band Structure Matching : Photogenerated electrons and holes can be effectively separated through the use of p–n and Schottky junctions. However, these two types of heterostruc-tures have some disadvantages. Firstly, in most of the present studies the metals used for photocatalysis are precious and rare, which increases the potential cost. Secondly, most semi-conductors used in photocatalysis are n-type, and few of the possible p-type semiconductors are suitable for photocatalysis. In addition, the direct preparation of p-type semiconductors is not easy. Fortunately, it is still possible to construct TiO 2 -nano-belt-based heterostructures as long as the band structure of the semiconductor matches that of TiO 2 . The band structure of the secondary semiconductor should be such that it forms a type-II alignment that facilitates the transfer of electrons and/or holes between the two semiconductors. In this way, the range of available semiconductors suitable for the construction of useful heterostructures with TiO 2 nanobelts can be widened.
5.2.2. Classifi cation of TiO 2 Nanobelt Surface Heterostructures
According to the design principles stated in Section 5.2.1, and the different compositions of the second phase on TiO 2 nano-belts, TiO 2 nanobelt surface heterostructures can be classi-fi ed into several categories. In this section, the composition, microstructures, and properties of different categories of TiO 2 nanobelt surface heterostructures are summarized, including metal–TiO 2 nanobelt surface heterostructures, metal oxide–TiO 2 nanobelt surface heterostructures, chalcogenide–TiO 2
nanobelt surface heterostructures, and conducting polymer–TiO 2 nanobelt surface heterostructures.
Metal–TiO 2 Nanobelt Surface Heterostructures : Noble metal nanoparticles can be assembled onto the surface of TiO 2 nano-belts to form surface heterostructures, including Schottky junc-tions and plasmon-resonance-effect structures. Because of the excellent control over the electron transfer processes in storing and shuttling photogenerated electrons, noble metals such as Au, [ 152 ] Ag, [ 153 ] Pt, and Pd [ 154 ] have attracted much attention and have been coupled with various TiO 2 nanostructures. [ 155 ] In the heterostructure, the TiO 2 nanobelt acts not only as a photocata-lytic active platform, but also provides a semiconductor band to couple with the metal to form a Schottky barrier. [ 156 ] The noble metal nanoparticles can either be grown in situ or immobi-lized on the surface of the TiO 2 nanobelt. [ 132,133 ] In situ growth typically proceeds through photoreduction [ 103,155 ] or chemical reduction in the presence of reducing agents, [ 157 ] while immo-bilization usually involves the utilization of specifi c functional groups as linking agents. The key point for fabrication of such a heterostructure is control of the size and distribution of the noble metal nanoparticles.
The synthetic process involves the photoreduction of the noble metal salt in aqueous solution. The deposited noble metal nanoparticles can improve both the physical and chemical per-formance of the photocatalyst. Consequently, the surface energy structure of the TiO 2 nanobelt and the recombination dynamics of photogenerated electrons and holes can be adjusted to enhance the photoelectronic properties. These positive effects have been demonstrated in the excellent gas sensing perfor-mances and photocatalytic activities. With delicate control over the experimental parameters, the noble metal nanoparticles can be uniformly distributed on the surface of TiO 2 nanobelt — rather than forming a core–shell nanostructure — as shown in Figure 10 . [ 103,155,158 ] By taking advantage of photoreduction, both the Ag and Pd nanoparticles can be grown with intimate contact with the TiO 2 nanobelt, thus having excellent charge-carrier transfer properties and improved photocatalytic activity. At the same time, the Ag nanoparticle–TiO 2 nanobelt surface heterostructure and the Pd nanoparticle–TiO 2 nanobelt surface heterostructure display excellent gas sensing performance with very high sensitivity to ethanol gas. Pt nanoparticles can also be assembled onto the surface of TiO 2 nanobelts to form Pt nanoparticle–TiO 2 nanobelt surface heterostructures, as shown by the formation of Pt nanoparticles with a mean size of about 3.3 nm, homogeneously distributed on the surface of TiO 2 nanobelt. [ 159 ] These noble metals are found to form a bimetallic phase on the TiO 2 nanobelt to form the surface heterostructure. For example, Au and Pd can form bimetallic alloy nanoparticles on TiO 2 nanobelts for enhanced photocatalytic aerobic oxida-tion of benzyl alcohol. [ 160 ]
Metal Oxide–TiO 2 Nanobelt Surface Heterostructures : Metal oxide–TiO 2 nanobelt surface heterostructures can be divided into three different types according to the light absorption properties of the metal oxide nanoparticle: heterostructures formed by UV light-active metal oxide nanoparticles and TiO 2 nanobelts; heterostructures formed by visible light-active metal oxide nanoparticles and TiO 2 nanobelts; and heterostructures formed by near-infrared or infrared light-active metal oxide nanoparticles and TiO 2 nanobelts. From the point of view of
band structure and carrier separation, these metal oxide–TiO 2 nanobelt surface heterostructures are based on several of the design principles mentioned previously. These surface hetero-structures can be tuned to optimize charge transfer processes and extend the light absorption to the visible light and the near-infrared light regions.
To fabricate metal oxide nanoparticle–TiO 2 nanobelt surface heterostructures, many methods have been devel-oped, including sol–gel, [ 150,151 ] solution decomposition pro-cesses, [ 152,153 ] wet chemical deposition, [ 161 ] hydrothermal pro-cesses, [ 162 ] electrodeposition, [ 163 ] and so on. To achieve effi cient function of the surface heterostructure, two key points must be taken into consideration. The fi rst is control of the size of metal oxide nanoparticles and their distribution on the surface of the TiO 2 nanobelts. The second is the use of eco-friendly tech-niques. That is to say, the synthetic method should be easily scalable, and involve no toxic materials.
As an important basic work regarding the modifi cation of TiO 2 nanobelts to improve the photocatalytic activity, our group developed an effective approach to modify the TiO 2 nanobelts in situ, to fabricate TiO 2 nanoparticle/TiO 2 nanobelt hetero-structures based on an acid–hydrothermal process. [ 136 ] The morphology and photocatalytic performance in the degradation of organic molecules using TiO 2 –nanoparticle/TiO 2 –nanobelt surface heterostructures depend on the acid corrosion condi-tions. With an increase of acid corrosion time, the photocata-lytic activity of the heterostructures gradually increases. The
obvious enhancement of the photocatalytic performance of the heterostructure is ascribed to the enlarged specifi c surface area, increase in the number of active sites, and the heterostructure formed between the TiO 2 nanoparticle and TiO 2 nanobelt. It is notable that the TiO 2 nanoparticle and the TiO 2 nanobelt have intimate contact, which is benefi cial for the interfacial charge transfer.
By using acid–corroded TiO 2 nanobelts as a substrate, our group has developed several TiO 2 nanobelt-surface heterostruc-tures. The secondary materials of these surface heterostruc-tures include Ag 2 O nanoparticles, [ 33 ] NiO nanoparticles, [ 164 ] CeO 2 nanoparticles, [ 165 ] and Cu 2 O nanoparticles, [ 166 ] as well as metal oxide compound Ag 3 PO 4 nanoparticles. [ 167 ] These sur-face heterostructures display enhanced UV light photocatalytic activity, even better than that of P25, and excellent visible light photocatalytic performance. The common aspects of these sur-face heterostructures lie in the following: i) the secondary mate-rial has intimate contact with the TiO 2 nanobelt. For example, Ag 2 O nanoparticles can be assembled onto the surface through electrostatic reaction between the negatively charged TiO 2 nanobelts and positively charged Ag 2 O nanoparticles. ii) The secondary phase in the heterostructure has uniform distribu-tion and appropriate coverage which does not prevent the absorption of UV light by the TiO 2 nanobelt. iii) Enhanced charge separation due to the interface formed between the two parts of the heterostructure. For example, NiO is a p-type semi-conductor and TiO 2 is an n-type semiconductor. Through the
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Figure 10. a) SEM and TEM images of Ag nanoparticle–TiO 2 nanobelt surface heterostructure. SEM image: Reproduced with permission. [ 103 ] Copy-right 2009, American Scientifi c Publishers; TEM images: Reproduced with permission. [ 158 ] 2010, American Chemical Society. b) Pd nanoparticle–TiO 2 nanobelt surface heterostructure. Reproduced with permission. [ 155 ] Copyright 2012, Elsevier. c) TEM images of Pt nanoparticle–TiO 2 nanobelt surface heterostructures. Reproduced with permission. [ 159 ]
in situ decomposition by high temperature calcination, a p–n junction can be formed between the NiO nanoparticles and TiO 2 nanobelts. [ 164 ] The NiO–TiO 2 nanobelt p–n junction also shows excellent biosensing performance. [ 168 ] iv) The secondary part usually has high activity under visible light irradiation. It should be pointed out that the surface heterostructure formed of CeO 2 nanoparticles@TiO 2 nanobelts shows a different photocatalytic mechanism, the capture–photodegradation–release mechanism. [ 165 ] In this mechanism, the organic molecules were captured by CeO 2 nanoparticles, degraded by the photoinduced free radicals, and then released to the solu-tion. The CeO 2 /TiO 2 nanobelt surface heterostructure has high degradation effi ciency and broad active light wavelength, as well as good stability.
The good chemical stability and special geometry of the TiO 2 nanobelt makes it especially suitable for the preparation of a double heterostructure. As mentioned previously, the surface coarsened TiO 2 nanobelts easily form double heterostructures because the post-formed TiO 2 nanoparticles on the TiO 2 nano-belt act as new energetic sites for adsorption and reaction. For example, the above mentioned Ag nanoparticles can be depos-ited randomly onto the TiO 2 nanoparticles on the TiO 2 nanobelt surface to randomly form a double heterostructure. The double heterostructure can further enhance the photocatalytic activity of TiO 2 nanobelts in a dramatic fashion. This effect has been proven by the controllable synthesis of a new double surface heterostructure of Bi 2 O 3 /Bi 4 Ti 3 O 12 /TiO 2 nanobelt, fabricated through wet-chemical coprecipitation and post-solid reac-tion. [ 169 ] The Bi 4 Ti 3 O 12 interphase plays the role of a bridge, linking Bi 2 O 3 and TiO 2 . Under light irradiation, the special crystalline structure of the Bi 4 Ti 3 O 12 interphase can effectively promote the separation of photogenerated electrons and holes.
The second oxide phase in the TiO 2 nanobelt surface heter-ostructure can also be in the form of 1D nanostructure, such as ZnO nanorod@TiO 2 nanobelt surface heterostructure and branched TiO 2 nanobelt. [ 170 ] These 1D–1D surface heterostruc-tures further prove that TiO 2 nanobelts are the ideal platforms for the construction of various surface heterostructures.
Chalcogenide–TiO 2 Nanobelt Surface Heterostructures : Dif-ferent from metal oxide semiconductors, chalcogenides are typically narrow bandgap semiconductors which have wide and strong absorption from UV- to visible-light regions. [ 171 ] They are also famous for the controllability of their bandgap and band structure. [ 172 ] Chalcogenides have been widely used for many years as photocatalysts in water splitting and the degradation of organic pollutants. [ 173 ] By constructing chalcogenide nano-structures on the surface of TiO 2 nanobelts, the light harvesting behavior and photocatalytic performance of TiO 2 nanobelts can be obviously increased. [ 174 ] Under light irradiation, electrons in the valence band of chalcogenides are excited and easily trans-ferred into TiO 2 nanobelts. TiO 2 nanobelts with good charge mobility have the ability to guide electron transfer. For example, PbS quantum dot-decorated (QD-decorated) TiO 2 nanobelts exhibited extended light absorption up to a wavelength of 1400 nm ( Figure 11 a). [ 175 ] Meanwhile, the band structure of the surface heterostructure can be tuned by adjusting the size of the PbS quantum dots. This quantum dot surface heterostruc-ture displayed an increased fl uorescence lifetime and charge-transfer rate constant.
Quantum dots have the advantage of band structure manipu-lation, while layered 2D chalcogenides like MoS 2 are famous for their high electrolytic activity due to the numerous active sites on the edges. [ 178 ] Recently, a 2D MoS 2 layer was synthe-sized in situ on the surface of a TiO 2 nanobelt to form a novel 2D–1D surface heterostructure (Figure 11 b). [ 176 ] The 2D MoS 2 nanosheet–1D TiO 2 nanobelt surface heterostructure has increased contact area and intimate contact because of the special in situ grown nanostructure. Fast electron transport and interfacial transfer were expected in this novel surface heterostructure, ensuring high photocatalytic performance for water splitting for hydrogen evolution and photodegradation of organics. Similarly, the amount of MoS 2 loaded on the TiO 2 nanobelt surface was taken into account to get high photo-catalytic performance, since too low loading of MoS 2 leads to insuffi cient visible light absorption, while excessive loading of MoS 2 blocks the photo-electron transfer between the core part of the TiO 2 nanobelt and the MoS 2 shell. The unique advan-tages of 2D–1D surface heterostructure based on TiO 2 nano-belts was further demonstrated the by rGO@TiO 2 (Figure 11 c) and g-C 3 N 4 @TiO 2 nanobelt surface heterostructures assembled through the photoreduction method. [ 177 ] The GO was photo-catalytically reduced and wrapped around the TiO 2 nanobelt in situ to form the 2D–1D surface heterostructure. As a result, the surface area and electrical conductivity were signifi cantly enhanced, which ensures high photocatalytic performance. More interestingly, the graphene wrapped TiO 2 nanobelt can be a potential functional platform for the construction of ternary heterostructures. [ 179 ]
Conducting Polymer –TiO 2 Nanobelt Heterostructures : Con-ducting polymers are at the center of investigation for a wide range of applications. Their unique applications and extensive interest are mainly due to the elevated conductivity, catalytic activity, gas sensitivity and optoelectronic properties. These conducting polymers typically include polyaniline (PANi), polypyrrole (PPy),poly(3-hexylthiophene) (P3TH), and so on. [ 180 ] When the conducting polymers are coupled with semiconduc-tors, the hybrid heterostructures formed have some unique physical and electrical properties. The large conjugated systems in these polymer molecules are especially good for charge-carrier mobility, and this effect can be further enhanced when assembled with 1D nanostructures, demonstrated to be a very appealing strategy.
However, the synthesis of one-dimensional core–shell struc-tures with high surface area and effective interfacial charge transfer is still challenging. Hydrogen titanate nanobelts have a hydrophilic surface, and aniline, which is the precursor monomer of PANi, is water soluble. Therefore, the p-type PANi can form a uniform shell on the surface of hydrogen titanate nanobelts through in situ polymerization. [ 181 ] Individual hydrogen titanate–PANi core-shell nanowires are a prototypical system, for which electrical properties have been explored.
The highly thermal stable hydrogen titanate–PANi hetero-structures have a wide light absorption range of 200–800 nm, dramatically larger than pure hydrogen titanate nanobelts that have a light absorption cut-off edge at about 400 nm. Its resistance strongly depends on temperature. When the temper-ature is higher than 100 °C, the heterostructure shows ohmic contact properties, with high resistance. Up to a temperature
of 100 °C, the resistance of the heterostructure decreases dra-matically with increases in the temperature. The reason is that at temperatures lower than 100 °C, the PANi displays a more p-type nature and the hole concentration is increased by dona-tion from the PANi molecule toward the water molecules. Mean-while, the heating process-induced molecular conformation
is more favorable for electron delocalization. At temperatures higher than 100 °C, the water molecules are released and the chain alignment of the PANi polymer is disturbed.
Although still very few reports exist on polymer–TiO 2 nanobelt heterostructures, with the progress in synthesis of TiO 2 nanobelts, and great demand of organic–inorganic
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Figure 11. a) TEM images demonstrate the confi guration of the PbS quantum dots/TiO 2 nanobelt heterostructure. Reproduced with permission. [ 175 ] Copyright 2010, American Chemical Society. b) Low-magnifi cation TEM images of TiO 2 @MoS 2 heterostructures (50 wt% of MoS 2 ) and the corre-sponding SAED pattern. Reproduced with permission. [ 176 ] c) AFM scan showing the topography of the graphene oxide (GO) sheets deposited on mica, and TEM and HRTEM images showing the morphology of surface-coarsened TiO 2 nanobelt and rGO/TiO 2 nanobelts. (lower-right) Dashed lines in HRTEM image outline a portion of the interface between the TiO 2 nanobelt and TiO 2 nanoparticle, as well as that between TiO 2 nanoparticle and rGO sheet; (inset) schematic model of the rGO/TiO 2 nanobelt heterostructure. Reproduced with permission. [ 177a ]
one-dimensional composites, polymer–TiO 2 nanobelt hetero-structures will attract much attention.
6. Full Solar Light Spectrum Photocatalysts Based on TiO 2 Nanobelts
Solar energy — free, clean, and inexhaustible — is the most promising renewable energy source for human beings. [ 182 ] The enhancement of solar energy harvesting and conver-sion is a major goal in photo-electric conversion fi elds. This challenging mission is especially important in photocatalysis and photovoltaic devices because of the highly increased environmental problems and energy crisis. Thus, the search for photocatalysts that can harvest light across the full solar spectrum, from UV to near-infrared wavelengths, and effi -ciently convert the solar energy for further applications is critical.
6.1. Bi 2 WO 6 Nanosheet–TiO 2 Nanobelt Surface Heterostructures
The above mentioned challenge may be solved with the recently-reported broad spectrum photocatalyst consisting of hybrid Bi 2 WO 6 nanosheets–TiO 2 nanobelts surface heterostruc-tures. This hybrid heterostructure was demonstrated to pos-sess the ability to harvest UV, visible, and near-infrared light to decompose organic contaminants in aqueous solution. The hydrothermally synthesized Bi 2 WO 6 nanosheets showed good visible light absorption and even had a strong absorption in the near-infrared light range, indicating excellent visible and near-infrared light catalytic performance ( Figure 12 e). [ 183 ] The proportion of MO degraded using Bi 2 WO 6 nanosheets–TiO 2 nanobelt surface heterostructures under UV, visible, and near-infrared light irradiation for 120 min was up to 24%, 84% and 58%, respectively. The photocatalytic activity using visible light may come from the Bi 2 WO 6 itself, [ 85,184 ] while the near infrared
Figure 12. a) Representative SEM and TEM images of Bi 2 WO 6 /TiO 2 nanobelt heterostructures. Photocatalytic degradation of MO in the presence of P25, TiO 2 nanobelts, Bi 2 WO 6 nanosheets and Bi 2 WO 6 /TiO 2 nanobelt heterostructures under b) UV light, c) visible light, d) near-infrared light and e) simulated solar light. f) The experimental visible and near-infrared absorption spectrum of TiO 2 nanobelts, Bi 2 WO 6 nanosheets and Bi 2 WO 6 /TiO 2 nanobelt heterostructures. g) Visible and near-infrared absorption spectra of TiO 2 nanobelts, Bi 2 WO 6 nanosheets, and Bi 2 WO 6 /TiO 2 nanobelt hetero-structures. a–g) Reproduced with permission. [ 183 ]
photocatalytic ability is suggested to originate from its oxygen vacancies.
Achieving the use of solar light to the utmost extent is the main direction and object of photocatalysis. However, full spec-trum photocatalysts are diffi cult to realize because of the lack of infrared-light active photocatalysts. Therefore, the discovery of the near-infrared photocatalytic activity of Bi 2 WO 6 should provoke new investigations into to highly effi cient utilization of solar energy.
Carbon quantum dots (CQDs) with typical sizes below 10 nm are newly emerging carbon nanomaterials with promising applications because of their benign, abundant and inexpensive nature. The structure of CQDs, i.e., amorphous or crystalline, depends on the synthetic methods. [ 185 ] Notably, CQDs exhibit both up- and down-conversion PL (photoluminescence), and photogenerated electron transfer ability. The broad NIR absorp-tion spectrum, upconversion PL property, and laser excitation make CQDs an excellent candidate photocatalyst for harvesting NIR light.
The photocatalytic properties of CQDs under NIR light illumination were demonstrated recently by the highly selec-tive oxidation of benzyl alcohol using CQDs as a photocata-lysts under NIR light irradiation. [ 186 ] In this work, CQDs were obtained from alkali-assisted electrochemical methods and had sizes in the range 1–4 nm. The conversion effi ciency and selectivity of CQDs can reach values as high as 92% and 100%, respectively, well beyond those of carbon nanoparticles (with sizes in the range 100–150 nm and conversion effi ciency and selectivity values of 71% and 78%, respectively). The photoelec-trochemical results showed that, under NIR light irradiation, the CQD electrode exhibited an obvious photocurrent response, while the carbon nanoparticles displayed no obvious photocur-rent under the same conditions. CQDs displayed a broad lumi-nescence peak at about 400 nm, with excitation wavelength of 830 nm, and its PL can be effi ciently quenched by either elec-tron-acceptor or electron-donor molecules in solution. All these investigations confi rm that CQDs can be an ideal near-infrared light photocatalyst and can be used to modify TiO 2 nanobelts to form surface heterostructures. Our recent work regarding CQD–TiO 2 nanobelt heterostructures proved that they possess UV–vis–NIR light photocatalytic activity, which opens a new door for the synthesis of full solar spectrum light photocatalyst using carbon-based quantum dots. [ 187 ]
6.3. TiO 2 Nanobelt Surface Heterostructures based on Other Materials
Materials with NIR or infrared photocatalytic properties are not only limited to the above Bi 2 WO 6 and CQDs. Gold clus-ters also exhibit photocatalytic performance under NIR light irradiation, and gold cluster-modifi ed TiO 2 shows both vis-ible light and NIR light activity. [ 188 ] Au 25 cluster-modifi ed TiO 2 nanoparticles can effi ciently drive the oxidation of phenol
derivatives and ferrocyanide, as well as the reduction of Ag + , Cu + and oxygen under visible light and NIR light (<860 nm). One main and important feature of gold and silver is the local-ized surface plasmon-induced (LSP–induced) characteristic bands of optical attenuation at visible light and infrared wave-lengths. Nishijima and co-workers reported photoelectric con-version from light in the visible to NIR range using electrodes in which Au nanorods were elaborately arrayed on the surface of a TiO 2 single crystal. [ 141 ] This system was thought to be able to use light with wavelengths from 800 to 1200 nm. Further-more, the LSP signifi cantly increased the photocurrent genera-tion. Qin et al. have reported a novel NIR photocatalyst of Yb 3+ and Tm 3+ co-doped YF 3 nanocrystal core/TiO 2 shell core/shell nanoparticles. [ 189 ] The core/shell nanoparticles could effectively decompose methylene blue under the irradiation of 980 nm laser, which was ascribed to the upconversion mechanism. Actually, upconversion should be an effective way to use NIR light energy, because materials with upconversion ability can be easily realized by doping of materials like NaYF 4 . [ 124 , 125 , 138a ] However, such upconverting materials also have their intrinsic disadvantages, such as the narrow wavelength range for NIR light excitation.
7. Concluding Remarks
The synthetic methods to produce TiO 2 nanobelts are well developed, even at large-scales. This provides good support for the wide application of TiO 2 nanobelts. Because of their specifi c geometry, TiO 2 nanobelts have special physical and chemical properties which causes surprising performance in various fi elds such as environment protection, solar cell and water split-ting for hydrogen fuels, and numerous sensors. However, TiO 2 nanobelts by themselves have some drawbacks, which can be overcome using TiO 2 nanobelt surface heterostructures. Such heterostructures have been demonstrated to provide a very effective way to solve the problems faced by researchers when using bare TiO 2 nanobelts, e.g., lack of absorbance in the vis-ible and IR ranges of the spectrum. While TiO 2 nanobelt surface heterostructures have already become a well-accepted strategy to improve performance, studies on these surface heterostructures show further potential for development beyond the current state of the art. Directions of future research should focus on the fol-lowing aspects: 1) theoretical understanding of the mechanism of the interfacial bandgap tuning of TiO 2 nanobelt surface heter-ostructures, based on calculations and simulations, and reaching an effi cient charge-carrier separation based on a profound theo-retical understanding; 2) discovery, design and fabrication of vis-ible and infrared light photocatalysts with high performance, to realize the synthesis of high performance photo catalysts that are active in the whole solar light spectrum; 3) design and assembly of devices based on the 1D nanostructured TiO 2 nanobelt sur-face heterostructure for practical applications.
Acknowledgements Z.Z. and J.T. contributed equally to this work. The authors are thankful for funding from the National Natural Science Foundation of China
(Grant No. 51372142), the Innovation Research Group (IRG: 51321091) and the “100 Talents Program” of the Chinese Academy of Sciences. The authors also acknowledge the support from the “Thousands Talents Program” for pioneer researcher and his innovation team, China. A.C. acknowledges the Spanish MINECO for fi nancial support under contract ENE2013–46624-C4–3-R.
Received: December 6, 2014 Revised: February 9, 2015
Published online: March 19, 2015
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