Special Topic: Water Science doi: 10.1007/s11433-014-5430-4
Interaction between water molecules and 3C-SiC nanocrystal surface
ZHAO PuQin1,2, ZHANG QiZhen3 & WU XingLong1,4*
1 National Laboratory of Solid State Microstructures, Department of Physics, Nanjing University, Nanjing 210093, China; 2 Department of Applied Physics, Nanjing University of Technology, Nanjing 210009, China; 3 Department of Physics, Nanjing Normal University, Nanjing 210046, China; 4 Laboratory of Modern Acoustics, MOE, Department of Physics, Nanjing University, Nanjing 210093, China
Received August 12, 2013; accepted September 27, 2013; published online March 13, 2014
The influence of water permeates almost all areas including biochemistry, chemistry, physics and is particularly evident in phenom-ena occurring at the interfaces of solid surface such as SiC nanocrystals, which are promising nanomaterials and exhibit unique sur-face chemical properties. In this paper, the quantum confinement effect and stability of 3C-SiC nanocrystals in aqueous solution as well as photoluminescence properties in water suspensions with different pH values are reviewed based on design and analysis of surface structures. On this basis, the significant progress of 3C-SiC nanocrystals in efficiently splitting water into usable hydrogen is summarized and the relative mechanisms are described. In addition, the water-soluble 3C-SiC quantum dots as robust and nontoxic biological probes and labels also are introduced as well as future prospects given.
water, 3C-SiC nanocrystals, interactions
PACS number(s): 92.40.Qk, 61.46.+w, 79.20.Rf
Citation: Zhao P Q, Zhang Q Z, Wu X L. Interaction between water molecules and 3C-SiC nanocrystal surface. Sci China-Phys Mech Astron, 2014, 57: 819828, doi: 10.1007/s11433-014-5430-4
1 Introduction
As in so many interfacial systems, water with an abundance of molecular properties has become one of the most attrac-tive and significant absorbates. Depending on the system, adsorbed water may be present as a whole molecule or dis-sociated ions. It may become a cation by accepting a proton or an anion by donating a proton. As an adsorbed molecule, it may bind to certain surfaces through many different methods such as electrostatics, charge transfer or hydrogen bonding. It has been detected on surfaces with different structures such as monomers, dimers, larger multimers, two dimensional bilayers and three dimensional clusters. The molecular dipole easily responds to the surrounding envi-
ronment such that orientations from hydrogen-end “up” to hydrogen-end “down” have been observed through control of coverage, surface structure, coadsorbate or applied po-tential. Water molecule can exhibit the inertia on some sub-strates, while for others it readily causes oxidation of the surface and near-surface regions. In interacting with coad-sorbates, it is primarily used as a solvent by hydrating/ solvating proximal molecules. It also reacts with surround-ing molecules, or competes aggressively for adsorption sites by displacing proximal coadsorbates from the surface. The water molecule is also a predominant probe for studying material properties such as luminescent, biological proper-ties, catalytic reactivity, redox processes, site distributions and defect influence. The above characteristics of water molecules show that they are critical in many areas includ-ing catalysis, electrochemistry, material science, electronic
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devices, photocatalysis and photoconversion, corrosion, geochemistry, sensors, astrophysics, astrochemistry, and membrane science. More properties of water molecules have been described elsewhere [1,2].
Silicon carbide (SiC) is a promising material and has at-tracted considerable research focus because of many ad-vantages such as mechanical stability, biocompatibility as well as high compatibility with processing technologies evolved from silicon device fabrication. Since SiC nano-crystal consists of carbon or silicon outermost layers, they exhibit complicated surface chemistry in different surroun- dings, especially in liquid solution. Thus, the interaction between SiC nanocrystal surface and water molecules has made some research progress. Cicero et al. [3] investigated in detail the properties of water molecules on SiC(001) by means of ab initio molecular dynamics simulations, and revealed that Si-terminated surface is hydrophilic while C-terminated one is hydrophobic. Atomic control of hydro-philic and hydrophobic environments by epitaxially grown SiC substrate with adjacent islands forming on either Si- or C-terminated surface can fabricate high-density arrays of SiC patterned surface for biomolecular or other molecular adsorption. Yang et al. [4] studied the structures and proper-ties of water molecules confined inside both armchair and zigzag single-walled silicon carbide nanotubes (SiCNTs) using density functional theory calculations. They further showed that the weak interactions between the ordered sin-gle-file water or multi-file water networks and SiCNT wall, which mainly originate from van der Waals interactions and charge transfer from SiCNT to the adsorbed water mole-cules. Here, H-bond strength has an important role in such interactions. Because of hydrophilic characteristics of SiC surface, SiCNTs may also be used as artificial biological channel. Soares et al. [5] experimentally studied the interac-tion of SiO2 films thermally grown on Si and on 6H-SiC with water vapor. The experimental results showed that higher incorporation of hydrogen and isotopic exchange at higher temperature can cause electrical instabilities and higher defect concentration, which is adverse in all fabrica-tion steps, particularly in SiC-based device technology. Therefore, a strict control of water vapor contents is crucial during sample preparation.
In this review paper, we will primarily focus on discuss-ing the interaction between water molecules and 3C-SiC nanocrystal surface including several important research aspects: (i) observation of quantum confinement effect and relative stability of 3C-SiC nanocrystals in aqueous solution as well as photoluminescence (PL) properties of 3C-SiC nanocrystals in water suspensions with different pH values; (ii) efficient splitting of water into hydrogen based on dif-ferent 3C-SiC nanostructures. (iii) functionalization of wa-ter-soluble 3C-SiC nanocrystals and the applications as bi-ological labels and probes.
2 Influence of water on PL properties of 3C- SiC nanocrystals
2.1 Quantum confinement effect of 3C-SiC nanocrys-tals in aqueous solution
Since the discovery of porous silicon in 1990, researchers expect that quantum confinement effect can also occur in silicon carbide nanostructures in order to realize the blue light emission. However, for a long period, the quantum confinement effects have not been clearly demonstrated from some composite structures containing 3C-SiC nano-crystallites due to the complicated surface states. Therefore, a distinctive preparation method is urgently needed to ob-tain SiC nanostructures having luminescence properties according to the quantum confinement effect. Recently, an intense luminescence with readily apparent quantum con-finement was observed for the first time from the water suspensions of 3C-SiC nanocrystals [6].
The preparation method of 3C-SiC nanocrystals can be briefly described as follows: 3C-SiC polycrystalline target are firstly electrochemically etched in HF-ethanol (HF: C2H5OH = 2:1) under illumination by a halogen lamp, then the obtained porous 3C-SiC is treated in an ultrasonic bath, and finally the porous layers on the surface crumble into small crystallites dispersed in the aqueous solution during ultrasonic processing. Figure 1(a) shows a typical TEM image of the 3C-SiC nanocrystallites at an etching current density of 60 mA/cm2. The TEM image and the high resolu-tion TEM image show that the crystallites are almost spher-ical particles with an average size of 3.9 nm and have clear lattice fringes assigned to the {111} plane of 3C-SiC (Fig-ure 1(b)). The 3C-SiC crystallites in aqueous suspension exhibit intense emissions (Figure 2) with two distinct fea-tures: one is that the spectral lines are smooth and display a typical full width at half maximum of about 120 nm; the other is that the emission wavelength is continuously shifted from 450 to 540 nm as the excitation wavelength alters from 250 to 500 nm. The emission nearly disappears when the excitation wavelength is longer than 500 nm. This can be clearly explained by the quantum confinement effect. The 3C-SiC nanocrystallites with different sizes in suspension conforms to an approximate Gaussian distribution (Figure 1(c)). According to the theory of quantum confinement ef-fect [7,8], the smaller the particles size, the greater is the band gap. Therefore, as the excitation wavelength increases, carriers in many small nanocrystals cannot be excited lead-ing to the continuous redshift of wavelength. When the ex-citation wavelength approximates 500 nm, only the largest nanocrystals which have the smallest band gap can be ex-cited. Actually, the emission wavelength of 540 nm is con-sistent with the band gap of bulk 3C-SiC which is 556 nm (2.23 eV) [9]. When the excitation wavelength increases further (>540 nm), no emission peaks can be detected.
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Figure 1 (a) Typical TEM image of the 3C-SiC nanocrystals fabricated using an etching current density of 60 mA/cm2. (b) HRTEM image of one nanocrystal and the clear lattice fringes corresponding to the {111} plane of 3C-SiC. (c) Nanocrystal number histogram showing with an average size of 3.9 nm based on Gaussian fitting [6].
Figure 2 (Color online) PL spectra taken under different excitation wavelengths for the 3C-SiC nanocrystals fabricated using a current density of 40 mA/cm2 [6].
It is very important to understand such a problem: why can the colloidal 3C-SiC crystallites show the blue PL with quantum confinement? There are two possible reasons which can be used to explain this phenomenon: firstly, after the ultrasonic treatment, the as-etched porous 3C-SiC changes from interconnected particles to dispersive indi-vidual crystallites with sizes of several nanometers that are sufficiently small to exhibit quantum confinement; secondly, etching of SiC in hydrofluoric acid leads to the formation of a hydroxyl termination, which provides a good surface pas-
sivation for the crystallites. Therefore, the observed lumi-nescence can be attributed to the two main factors.
2.2 Stability of luminescent 3C-SiC nanocrystals in aqueous solution
Luminescent stability of nanocrystals is very important to future application of nanocrystals. Fan et al. [10] have stud-ied experimentally the long-term stability of luminescent colloidal 3C-SiC nanocrystals with diameters smaller than 7 nm. They studied two types of different samples. One was a newly prepared sample, the other an aged sample. By com-paring the PL curves of the two kinds of samples (Figures 3(a) and 3(b)), it was found that the emission peak of the newly prepared sample showed continuous redshift and the PL peak intensity increases first and then decreases along with increasing excitation wavelength. While the emission property of the aged sample seems to be consistent as that in the as-prepared sample, the emission peak disappeared un-der higher excitation energy of 440 nm, which indicates that some larger 3C-SiC nanocrystals have aggregated. The maximum emission intensity also appears with an appar-ently obvious blue shift in the aged sample because smaller nanocrystals have greater optical gaps [7,8]. These experi-mental phenomena show that most probable nanocrystals contributed to the emission are much smaller in the aged sample than those in the newly prepared sample. This result further indicates that smaller 3C-SiC nanocrystals could be more easily suspended in an aqueous solution owing to light weight and large surface area and they still retain lumines-cence properties after a long-time storage.
Smaller 3C-SiC crystallites could correspondingly keep steady in water as colloid-like nanocrystals and the sur-
Figure 3 PL spectra of the as-prepared (a) and stored for over 7 months (b) 3C-SiC nanocrystal suspensions under different excitations [10].
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rounding oxygen has less influence on the emission of the suspended 3C-SiC crystallites. This stability seems only suitable for the small particles. In order to further improve the stability of 3C-SiC nanoparticles with greater nanometer size scope in water, the conventional method is the addition of polymeric dispersants which leads to the improvement of the dispersion stability of SiC nanoparticles due to the gen-eration of steric repulsive forces [11,12]. It is quite difficult to determine the optimum additive condition to stabilize the nanoparticles under different conditions. Iijima et al. [13] reported a new method of chemical modification of SiC nanocrystal surface with azo radical initiators (such as AIBN, AMPA and AMCPA) in a reaction with unsaturated hydrocarbons of SiC surface. The modification technology also can easily tune the properties of SiC surface such as surface potential, wettability and Raman shift. The stability of modified SiC nanoparticles in water with various pH values has been estimated by observing the sedimentation behavior of SiC/water (Figure 4) and the corresponding agglomerate size in water (Figure 5). The experimental re-sults indicate that azo radical initiators have successfully reacted with unsaturated hydrocarbons, which leads to the density increase of the hydrophilic groups such as amines, imines and carboxyl groups on the surface of the SiC nano-particles. Therefore, the stability of SiC nanoparticles in water was improved significantly. These modified SiC na-noparticles need to have stable luminescent properties, which requires further experiments to verify this.
2.3 PL properties of 3C-SiC nanocrystals in water suspensions with different pH values.
PL is sensitive to the surface structure of SiC nanocrystals. The complicated surface modified by suitable species on SiC nanocrystals provides many practical applications such as luminescent probes in medical imaging [14]. Cicero et al. [15] have theoretically predicted that water molecules on Si-terminated surface of bulk 3C-SiC (001) can be dissoci-ated and cause surface reconstruction of SiC nanocrystals.
Figure 4 Stability of SiC nanoparticles in water with different pH values: (a) before surface modification; (b)–(d) after surface modification with AIBN, AMPA and AMCPA, respectively [13].
Figure 5 Averagely aggregated size of SiC nanoparticles in water with various pH values before and after surface modification with different azo radical initiators [13].
Hence, it is possible and significant to use PL to monitor such reconstructed surfaces and dissociative H2O mole-cules.
Recently, the PL characteristics of aqueous suspensions with different pH values containing 3C-SiC nanocrystals less than 6.5 nm in size have been investigated [16]. In ad-dition to a blue PL band usually stemming from the quan-tum confinement effect [6,17], an additional PL band at 510 nm (the green band) (Figure 6(c)) was observed when the excitation wavelength was longer than 350 nm. Wu et al. [16] showed that the green band could arise from the sur-face and water molecule states based on the following rea-sons: firstly, scanning tunneling microscopy have revealed that several Si-terminated surfaces on 3C-SiC crystals have different structures [18,19]; secondly, theoretical studies have predicted that water molecules dissociate into –OH and –H groups bonding onto the surface of 3C-SiC nano-crystals with Si-terminations and only Si-terminated surfac-es react with H2O molecules forming some OH and H+ bonding structures [20,21]. Infrared and x-ray photoelectron spectral results also showed similar conclusion. The above analyses indicate that the surface/water-molecule states caused by OH and H+ bonding process has a crucial role in the emergence of the green band [22], while similar green band cannot be observed in ethanol suspension of 3C-SiC nanocrystals, as shown in Figure 6(d).
The PL spectra of the 3C-SiC nanocrystals in water sus-pensions with various pH values adjusted by HCl and NaOH were examined and the corresponding results are presented in Figure 7. These results indicate that the green band appears mainly in the acidic suspensions and the in-tensity relative to the blue band decreases with decreasing pH value. It almost vanishes in the strong acidic suspension with a pH value of 1.0 and in all the alkaline suspensions. For the blue band, it shows an obvious red-shift with in-
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Figure 6 (Color online) (c,d) Normalized PL spectra of the water and ethanol suspensions excited in the wavelength range of 310–410 nm. (e) Dependence of the blue PL peak positions on excitation wavelengths for the water and ethanol suspensions [16].
Figure 7 PL spectra excited at 360 nm of the 3C-SiC nanocrystals in water suspensions with different pH values [16].
creasing excitation wavelength (>350 nm) in these acidic and alkaline suspensions. The above results imply that a specific concentration range of the OH and H+ bonding structures is necessary to produce the green emission. However, the introduction of excess OH can easily damage the surface luminescent centers. The green luminescent centers are complexes of the Si-terminated (001) surfaces and absorbed water molecules dissociated into OH and H+ group, as they are energetically favorable. Fourier transform infrared, X-ray photoelectron spectroscopy, and X-ray ab-sorption near-edge structure analyses have clearly revealed that the 3C-SiC nanocrystals in the water suspension have Si-H and Si-OH bonds on the surface, which further con-firms that water molecules only react with a Si-terminated surface [16]. Theoretical calculation of the electronic struc-ture of the layered 3C-SiC with surface reconstruction and dissociation of adsorbed water molecules based on the den-sity functional theory (DFT) agrees well with the published
experimental results [16].
3 Efficiently splitting water into hydrogen based on 3C-SiC nanocrystals
3.1 Significance of efficient semiconductor-based pho-toelectrochemical water splitting
Collecting and storing solar energy in chemical bonds through photosynthesis is a highly desirable approach with minimal environmental impact to solving the energy crisis. By utilizing the photoelectrolysis of water using semicon-ductors as both the light absorber and energy converter, the solar energy can be converted and stored in the simplest chemical bond, H2, which is a green energy carrier having a high energy capacity [23,24]. Efficiently splitting water into usable hydrogen may become a new industrial photosynthe-sis that would provide clean fuel and only use water as the waste product. To accomplish this new photosynthesis, it is imperative to develop an economically viable water split-ting cell directly at the semiconductor surface [25]. The advantage of water splitting cells with direct semiconduc-tor/liquid contacts is to avoid significant fabrication and complex systems costs involved with the use of separate electrolyzers wired to p-n junction solar cells and to gener-ate easily an electric field between the semiconductor/liquid junction [26].
3.2 Electrochemical hydrogen evolution based on sur-face autocatalytic effect of ultrathin 3C-SiC nanocrystals
Since hydrogen production from water splitting has signifi-cant potential in renewable energy production, much re-search in seeking materials with efficient electrocatalytic and photoelectrochemical water splitting capability is cur-rently being done [27–29]. However, it is crucial to thor-ough understanding of the hydrogen evolution reaction (HER) mechanism on the surface of semiconductor/liquid junction in order to discover and develop the appropriate catalysts. The interaction between water and solid surfaces has many potential applications such as heterogeneous ca-talysis, electrochemistry, and corrosion processes [1]. But contrary to general view, water molecules cannot be ad-sorbed on the surfaces of some solid materials, instead dis-sociating via a surface autocatalytic process which forms a complex consisting of –H and –OH species [15,30–32]. He et al. [33] have demonstrated experimentally 3C-SiC nano-crystals with surface autocatalytic effects in high-efficiency water splitting for production of hydrogen and further pro-posed a molecular level qualitative understanding of the HER mechanism.
The detailed procedure to prepare the water suspension containing ultrathin 3C-SiC nanocrystals smaller than 8 nm in size is similar to the method given elsewhere [6,34]. The
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details of experiments are briefly described as follows: the 3C-SiC nanocrystal film deposited on the glassy carbon (GC) substrate is used as working electrode (Figure 8), while Ag/AgCl (3 mol L1 KCI-filled) and a platinum wire are served as the reference and counter electrodes, respec-tively; 0.5 mol L1 Na2SO4 solution (PH=6.0) is the electro-lyte; 500 W Xe lamp is for the light source. The electro-chemical measurement was performed at room temperature in such a three-electrode cell connected to a CHI 660D workstation. Figures 9(a) and 9(b) show the linear sweep voltammetry curves at different times (curves A–F) for the freshly prepared 3C-SiC nanocrystal film electrode (mean nanocrystal size of 3.6 nm). The sample shows a small onset potential which increases and shifts to more negative poten-tials with time during scanning process, and finally reaches a stable value. The increasing onset potential in the negative direction could be caused by the desorption of protons from the nanocrystal surface formed during chemical etching [16]. Concurrently, several 3C-SiC nanocrystal film electrodes with different nanocrystal sizes were also tested for compar- ison and the corresponding current-potential curves showed in Figure 9(c). The lower opening voltage, the higher electr- ocatalysis activeness is achieved. For the commercial 3C- SiC nanocrystal film with similar catalyst loadings (mean nanocrystal size of 20 nm), in comparison, the currents are almost negligible. These results indicate that the electrocat-alytic activity of large 3C-SiC nanocrystals is rather low, in spite of formation of –H and –OH groups on the surface. Thus the nanocrystal size plays an important role in the
electrocatalytic HER activity. The mechanism why ultrathin 3C-SiC nanocrystals can
accelerate hydrogen production is illustrated in Figure 10. When water molecules diffuse to a favorable dissociation site that is Si-terminated surface on the 3C-SiC nanocrystals, they spontaneously split into two fragments (–H and –OH) bonded to two adjacent Si atoms (Si dimer). This autocata-lytic effect of ultrathin 3C-SiC nanocrystal surface can sig-nificantly reduce the activation barrier for dissociation. Hence the smaller nanocrystal size with larger specific sur-face area also has higher autocatalytic activity. Under fa-vorable conditions, hydrogen atoms on the nanocrystal sur-face are reduced to produce hydrogen molecules when elec-trons are transferred from the electrode to –H. The hydrogen generation by the route has a lower activation barrier than
Figure 8 Representative SEM image of the ultrathin 3C-SiC nanocrystal film on a GC substrate. The inset shows the local enlargement [33].
Figure 9 (Color online) Linear sweep voltammetry curves of the as-prepared nanocrystalline 3C-SiC electrode with nanocrystal sizes of 3.6 (a) and 3.0 nm (b) at different time (A−F) (the curve of a blank GC substrate for comparison). (c) Stable polarization curves of the two 3C-SiC samples with different nano-crystal sizes (the curves of a commercial 3C-SiC sample and a bare Pt foil as reference). (d) Current-time plot of the ultrathin 3C-SiC nanocrystal film elec-trode at certain applied potential. The inset shows two digital pictures of the 3C-SiC nanocrystal film electrode emitting hydrogen [33].
Zhao P Q, et al. Sci China-Phys Mech Astron May (2014) Vol. 57 No. 5 825
Figure 10 (Color online) Schematic illustration of the mechanism ex-plaining the electrocatalytic HER on the ultrathin 3C-SiC/GC electrode [33].
that directly through water electrolysis. Finally, the sus-tained adsorption and dissociation of H2O molecules on 3C-SiC nanocrystals surface in this dynamic process allows for efficient hydrogen production.
3.3 Atomic-scale mechanism of efficient hydrogen evolution at 3C-SiC nanocrystal electrodes
Efficient electrochemical hydrogen evolution at ultrathin 3C-SiC nanocrystal electrodes in acid solution has been reported in the previous literature [33]. Shen et al. [35] suggested that it has not been sufficiently determined how water molecules spontaneously dissociate and form Si-H and Si-OH and the atomic-scale mechanism of the HER needs further specific explanation. Wherefore they present-ed a theoretical study of the HER at the SiC nanocrystal electrode by quantum mechanical calculations in order to explore the possible reactions at a hydrogenated surface Si site, Si-H. Based on the results that H2 can be generated from a Si-H bond in a nonaqueous environment [36], they investigated other possible reactions for the hydrogen evo-lution at the SiC/aqueous interface:
Si-H+H3O+→Si++H2+H2O. (1)
However, reaction (1) does not occur by relaxing the structure of the cluster model with a hydronium ion H3O
+ placed near the center of the top Si surface, the possible reaction should be
Si-H+H3O+→Si-H…H3O
+. (2)
Thus the HER from Si-H site at the semiconductor- aqueous interface needs additional activation provided by free electrons from the conduction band of SiC electrodes, reaction (2) is accordingly changed as follows:
Si-H+H3O++e→Si*+H2+H2O. (3)
One extra electron is added to the SiC cluster by the ap-plied bias voltage. The Si-H bond is broken after relaxation, and the two H atoms originally forming the H…H bond can now combine and form an H2 molecule, leaving a Si dan-gling bond and an H2O molecule (Figure 11). The reaction belongs to the Heyrovsky-Volmer mechanism [23] and usu-ally occurs in some metal electrodes. It is a two-step process: Heyrovsky reaction is the first step as the Si-H bonds are already present due to the dissociation of water, then the second step is for an electron from the substrate to react with a hydronium adsorbed at a Si-H site, generating an H2 molecule and a Si dangling bond. The ordering of the reac-tions is supported by the fact that the hydrogen coverage on SiC electrodes does not depend on the applied voltage [37]. Quantum confinement effect accounts for the fact that only ultra small SiC nanocrystals are electrochemically active, which is consistent with He et al. [33] findings. Hence, the ability to bind a hydronium ion on hydrogenated surface Si site is a key factor for the HER to occur at high rate.
3.4 3C-SiC film on Si substrates and other types of SiC film for solar water splitting
Recently, Ma et al. [38] investigated the photoelectro- chemical (PEC) behavior of p-type 3C-SiC films on p-Si substrates for solar water splitting. The electrochemical (EC) properties of single-crystalline p-type 3C-SiC films on heavily doped p-type Si substrates was studied. The thick-ness of the SiC films is approximately 5.3 m and carrier concentration is 1015–1016 cm1. The SiC films were first etched by HF solution or aqua-regia-HF solution before PEC test. The photoresponse measurements were carried out by means of a potentiostat in a three-electrode setup with Ag/AgCl as the reference electrode and Pt as the counter electrode in H2SO4 aqueous solutions. The depend-ence of current density on the potential in dark and under light illumination indicates that dark current is very low without light illumination. The photocurrents of the un-treated and etched SiC are generated in different directions of potential regions, respectively. In other words, the p-type 3C-SiC film on a p-Si substrate can generate a cathodic photocurrent as the photocathode corresponding to hydro-gen production and generate an anodic photocurrent as photoanode corresponding to oxygen evolution. Experi-mental results also show that the SiC films etched by HF
Figure 11 (Color online) An extra electron is added to the SiC cluster, the Si-H bond adsorbing a hydronium is broken after relaxation, creating an H2 molecules and a Si dangling bond [35].
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solution with bigger shift of the current-potential cut-off edge have better PEC behavior for solar water splitting.
The photoelectrochemical properties of SiC as photoe-lectrodes are usually measured in a three-electrode setup driven by a potentiostat as an electrical power supply. Ya-suda [39] reported for the first time the self-driven wa-ter-splitting system based on SiC photoelectrodes in the two-electrode system. Band edge potentials were estimated from C-V measurements for 4H-, 6H-, and 3C-SiC, and all the polytypes seem to have suitable band edge potentials for water-splitting. The photocurrents were observed for all n-type samples under illumination by a solar simulator (Figure 12). Among the n-type samples, epitaxial 4H-SiC displayed the largest photocurrent, however, the current rapidly declined due to the formation of oxide layer. While the photocurrent of epitaxial p-type 4H-SiC is not only larger than those of bulk samples, but also the observed photocurrents remain stable, almost no decline with time was observed (Figure 13). The solar-to-hydrogen efficiency for epitaxial p-type 4H-SiC was estimated to be 0.10% based on the following formula [40]:
Figure 12 Time dependence of photocurrents for the epitaxial n-type SiC samples under light illumination [39].
Figure 13 Time dependence of photocurrents for the epitaxial p-type SiC samples under light illumination [39].
1.23
(%) 100,I
L
where I is the photocurrent, L is the light intensity, and 1.23 represents the redox potential width between H+/H2 and O2/H2O. Although the efficiency of p-type SiC reported by Akikusa and Khan was 0.17% [41], the photoelectrochemi-cal cell was not self-driven.
In conclusion, these results mentioned above suggest that various types of SiC as photoelectrodes are potential candi-dates for water-splitting system.
4 Functionalization of 3C-SiC nanocrystal sur-face for biotechnological applications
Quantum dots have more advantages than dye molecules in many aspects such as size-dependent fluorescence, re-sistance as well as photobleaching, and they have been uni-versally used in bioscience as fluorescent probes [42,43]. However, the cytotoxicity of some quantum dots restricts the applications in biological systems [44,45] and develop-ment of green-emitting nanocrystals with low or no cyto-toxicity has become an important issue [46–49]. The study indicates that 3C-SiC nanocrystals with water-soluble and high-efficiency luminescent properties can be regarded as feasible biocompatible materials [15], particularly for blood [50]. In this section, the water-soluble 3C-SiC nanocrystals as fluorescent biological labels will be introduced.
The complex surface structures including H-terminated hydrophilic surface and C-terminated hydrophobic surface in 3C-SiC nanocrystals allows for ease of functionalization by connecting various biological ligands such as proteins and antibodies for probing and imaging. For example, it has been reported that water-soluble 3C-SiC quantum dots were introduced into Human fetal osteoblast (hFOB) cells [51].
The 3C-SiC nanocrystal-labeled hFOB cells displayed bright green-yellow fluorescence (Figure 14), which im-plied that the 3C-SiC nanocrystals may penetrate the cell membrane by endocytosis. The photostability tests and cy-totoxicity evaluation on 3C-SiC nanocrystals-labeled and PI-labeled hFOB cells (Figure 15) indicated that the photo-stability of 3C-SiC nanocrystals in biological imaging can
Figure 14 (Color online) (a, b) Fluorescence microscope images of 3C-SiC nanocrystal-labeled hFOB cells [51].
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Figure 15 (a) Photostability tests of the 3C-SiC nanocrystal-labeled and PI-dye-labeled hFOB cells. (b) Cytotoxicity evaluation. HeLa cells were incubated with different concentrations of 3C-SiC nanocrystals as indicated for different times [51].
be compared with that of organic dye propidium iodide (PI), therefore 3C-SiC nanocrystals with low photo-bleaching and long-term stability can be used as robust and nontoxic biological probes. Another advantage of 3C-SiC nanocrys-tals as biological labels is the tunable visible light emissions. By improving the size monodispersity and surface struc-tures of the nanocrystals, the 3C-SiC nanocrystals are ex-pected to have far wider applications in multicolor imaging in various biological systems.
Recently, Schoell et al. [52] reported the organic func-tionalization of 3C-SiC surfaces modified with organosilane molecules such as octadecyl-trimethoxysilane (ODTMS) via wet chemical method. Figure 16(a) shows a micropat-terned ODTMS-modified 3C-SiC surface, the change of wettability can be seen when the surface is exposed to water vapor. The preferential condensation on the hydrophilic plasma-exposed regions indicates the local modification of surface properties. In addition, bovine serum albumin (BSA) proteins have been successfully immobilized on the pat-terned surface using glutaric dialdehyde linker (Figure 16(b)). The possibility of the locally modification and par-
Figure 16 (Color online) (a) Wettability comparison image of a micro-patterned ODTMS-modified 3C-SiC surface by optical microscopy. (b) Fluorescence optical micrograph of 3C-SiC surface following micropat-terned ODTMS-modified layer and immobilization of fluorescently labeled BSA protein [52].
tial passivation of surface with organic molecules make 3C-SiC a more promising material for future biotechnology applications.
5 Summary
The importance of the interactions of water molecules with 3C-SiC nanocrystal surface in many fields is evident from the quantity and diversity of work published on the subject. This work also reflects the success with which researchers have had in charactering the molecular-level properties of adsorbed water. One may suggest that the behavior of water on other solid surfaces such as Si(100), ZnO and Fe3O4 has similar surface properties which still need to be understood, while significant progress in this respect will be made in the near future. New breakthroughs for different nanostructures will undoubtedly be seen in the future.
This work was supported by the National Basic Research Programs of China (Grant Nos. 2011CB922102 and 2013CB932901) and the National Natural Science Foundation of China (Grant No. 11374141). Partial sup-port was also from the Natural Science Foundation of Higher Education of Jiangsu (Grant No. 12KJB140007).
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