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This article appeared in a journal published by Elsevier. The attachedcopy is furnished to the author for internal non-commercial researchand education use, including for instruction at the authors institution
and sharing with colleagues.
Other uses, including reproduction and distribution, or selling orlicensing copies, or posting to personal, institutional or third party
websites are prohibited.
In most cases authors are permitted to post their version of thearticle (e.g. in Word or Tex form) to their personal website orinstitutional repository. Authors requiring further information
regarding Elsevier’s archiving and manuscript policies areencouraged to visit:
Multifunctional composite multilayer coatings on glass with self-cleaning,hydrophilicity and heat-insulating properties
Wen Zhu a,b,⁎, Dali Tong a, Jinbang Xu c,⁎⁎, Yong Liu a, Jian Ma a
a State Key Laboratory of Material Processing and Die and Mould Technology, Huazhong University of Science and Technology, Wuhan 430074, People's Republic of Chinab Shenzhen Key Lab of Information Function Ceramic, Shenzhen 518057, People's Republic of Chinac School of Control Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, People's Republic of China
a b s t r a c ta r t i c l e i n f o
Article history:Received 27 February 2012Received in revised form 4 November 2012Accepted 6 November 2012Available online 21 November 2012
Keywords:FilmsOptical propertiesGlassTiO2
Functional applications
Multifunctional coated glass with self-cleaning, hydrophilicity and heat-insulating properties is prepared by re-activemagnetron sputtering. A SiO2(top)/TiO2(under) double-layer structure is deposited on the outer surface ofthe glass to integrate self-cleaning and hydrophilicity functions. The inside of the glass is a similar six-layer coat-ing that has heat-insulating function. Detailed optimization process for constructing the composite coatings onthe glass is reported, and its efficacy is evaluated. TiO2 nanotubes sensitizedwith narrow-band-gap semiconduc-tor drastically improve the photocatalytic activity. SiO2/sensitized TiO2 nanotube double-layer also exhibits ex-cellent photo-induced super-hydrophilicity. The optical properties of each individual layer have been studiedin order to achieve a balance between the heat-insulation performance and the visible-light transmittance ofthe heat-insulating multilayers. The spectra of the two-side composite coatings on the glass show that thecolor appearance can be tailored, which present potential applications in the architecturalwindows and automo-tive glazing.
The surface coated glass is becoming more and more attractive forits versatile features, such as anti-reflection [1], low-emission [2], trans-parent conduction [3], thermo- or electro-chromogenesis [4–7], and soon. In some countries, surface coated glass is employed in double- andmultiple-pane windows almost without exception. Among these fea-tures, the self-cleaning, hydrophilicity and heat-insulating propertiesare very useful in the application as architectural window and automo-tive glazing. Self-cleaning commonly uses photocatalytic behavior todegrade organic molecules as well as microorganisms adsorbed on theglass surfaces under the solar irradiation [8–10]. Thus, hazardous or un-pleasant pollutants can be decomposed into tasteless, odorless, or atleast less toxic compounds. On the other hand, small water dropletsthat remained on the glass surface can reduce the visibility because ofthe lens effect. Hydrophilic treatments are suitable for inhibiting thelens effect, and can offer a clear view in rainy day [11]. However, thegeneral hydrophilic materials are easily contaminated by the organic
compounds, especially by that of having polar function groups. One so-lution is to give the dual function to the surface coated glass, which canintegrate the hydrophilicity with photocatalytic activity in the samepiece of coated glass. Furthermore, self-cleaning processing will bemore efficient by merging the two functions together. Combining thephotocatalytic surfaces with photo-induced super-hydrophilicity meansthat water droplets spread more or less evenly over the surface so thatlight scattering tends to be insignificant. Thus the water is invisible, andmoreover, drying-related contamination residues do not appear, whichcan be a considerable asset for self-cleaning application. It is knownthat the TiO2 films have excellent photocatalytic activity and photo-induced hydrophilicity, and have beenwidely used in environmental ap-plications such as air purification [12,13], sterilization [14], antifogging[15,16] and self-cleaning [17–20]. TiO2 coatings on the exposed outerside of windows can almost eliminate water condensation which mayotherwise occur under certain climatic conditions. TiO2 coatings couldalso be used on the inside of windows for buildings and vehicles, wherethey can serve as reflectors for thermal radiation and avoidwhat is some-times referred to as “cold radiation”, i.e., the coatings can reflect thermalradiation from a human body back to the same body, which can play amajor role in architectural and automotive energy conservation. Inthis study, we report the formation of multifunctional glazing withTiO2-based composite coatings on the glass substrate using a reactivemagnetron sputtering. We demonstrate a simple but effective proce-dure to perform the multifunction with self-cleaning, hydrophilicityand heat-insulating properties.
Thin Solid Films 526 (2012) 201–211
⁎ Correspondence to: W. Zhu, State Key Laboratory of Material Processing and Die andMould Technology, Huazhong University of Science and Technology, Wuhan 430074,People's Republic of China. Tel./fax: +86 27 87558476.⁎⁎ Corresponding author. Tel./fax: +86 27 87558476.
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2. Experimental details
2.1. Materials
Commercially available 99.999% pure disks of titanium, silicon,silver, nickel, and chromium with 100 mm in diameter and 4–5 mmin thickness were used as sputtering target materials. Ammoniumfluoride, ethylene glycol, methylene blue (MB, C16H18-ClN3S·xH2O),Bi(NO3)3·5H2O (99 wt.%), CdSO4·8/3H2O (99 wt.%), and Na2S·9H2O(99.9 wt.%) were purchased from Tianjing Chemicals Co. Ltd. (China);EDTA2− (C10H14O8N2Na2·2H2O) was from Sinopharm Chemical Re-agent Co., Ltd.; SeO2 (99.99 wt.%) and TeO2 (99.99 wt.%) were obtainedfrom Johnson Matthey Co. Ltd.; all other chemicals were analyticalgrade, and solutions were prepared with twice-distilled water.
2.2. Preparation of self-cleaning and hydrophilicity coating
Double-layer coating was produced using a reactive magnetronsputtering systemwith a base pressure of less than 7×10−4 Pa. Quartzglass plates (20 mm×25 mm) were used as substrates. Prior to beingloaded into the chamber, the substrates were immersed for 24 h in amixed acid solution,which consists of amixture of H2SO4+H2O2+H2Owith a volume ratio of 1:1:5. Subsequently, an ultrasonic agitation of20 min was performed in the mixed acid solution. After this, the sub-strates were ultrasonically cleaned orderly with acetone, ethanol anddeionized water, and then were dried in nitrogen. The TiO2 underlayerand the SiO2 top layer were sequentially deposited on the substratesto form the SiO2/TiO2 double-layer structure by dc sputtering a Ti targetand rf sputtering a silicon target, respectively, in a mixture of Ar and O2
gases. Substrate temperature was kept at 300 °C during the deposition.The total working pressure and the oxygen partial pressurewere inves-tigated to optimize the fabrication technology.
2.3. Preparation of TiO2 nanotube arrays
Vertically oriented TiO2 nanotubes (NTs) were prepared on a Ti/glasssubstrate by the electrochemical anodization method. First, a metal Tithin film was coated on the glass surface by dc magnetron sputtering ti-tanium target at room temperature using a pure Ar pressure of 0.16 Pa.The sputtering time was 6 min, resulting in a film thickness of about620 nm. Then, the metal Ti thin film was anodized to form the TiO2
NTs. Anodization was performed for 13 min under constant potential(35 V) at room temperature, approximately 28 °C, in an electrolytemix-ture, which contains 0.28 g NH4F and 1.0 vol.% deionized H2O dissolvedin 100 mL ethylene glycol. A two-electrode configuration was used withthe Ti/glass substrate as the working electrode and platinum foil as thecounter electrode. The anodized samples were annealed at 300 °C in ox-ygen ambient for 2 h with heating and cooling rate of 2 °C/min to con-vert the amorphous phase to crystalline phase. Then the as-preparedcrystalline phase samples were sensitized by various narrow-band-gapsemiconductors, such as Bi2S3, CdS, CdSe, and CdTe. Bi2S3 sensitizingwas carried out from 0.005 to 0.01 M Na2S 9H2O, 0.005 M Bi(NO3)35H2O, and 0.1 M EDTA2− solution at pH 10.5 adjusted with nitric acidand ammonia. The Bi solutionwith EDTAwasprepared byfirst dissolving5 mM Bi(NO3)3 5H2O and 0.05 M EDTA2− into nitric acid, and thenslowly dripping ammonia to adjust the pH value to 10.5. The S solutionwith EDTA was prepared by dissolving 5 mM Na2S 9H2O and 0.05 MEDTA2− directly into twice-distilled water, and also using ammonia toadjust the pH value to 10.5. Both as-prepared Bi and S solutions thenwere mixed to form the Bi2S3 sensitizing solution. The crystalline nano-tube arrays were immersed into the sensitizing solution for 24 h, andthenwere dried at 110 °C for 2 h. Finally, the sensitized TiO2 NT sampleswere annealed again at 300 °C in muffle furnace for 2 h, and then wereprocessed for characterization and SiO2 deposition. Theexperimental de-tails of other narrow-band-gap semiconductor sensitizing were analo-gous to Bi2S3, only these precursor solutions were 10 mM CdSO4
8/3H2O+5 mM Na2S 9H2O for CdS, 10 mM CdSO4 8/3H2O+2 mMSeO2 for CdSe, and 60 mM CdSO4 8/3H2O+10 mM TeO2 for CdTe,respectively.
2.4. Preparation of heat-insulating coating
Amultilayer coatingwith TiO2/NiCrOx/Ti/Ag/NiCr/Si3N4 stacked struc-turewas designed to perform the heat-insulating function. The TiO2 layerin the stackwas done by dc sputtering the Ti target in amixture of Ar andO2 gases at 20%oxygenpartial pressure and1.0 Pa totalworking pressure,which produced a sputtering rate of 7.74 nm/min. NiCrOx dielectric layerwas formed by dc reactive sputtering the nickel and chromium targets al-ternately, in the mixed gas of Ar and O2 with a volume ratio of 9:1,resulting in a working pressure at ~0.2 Pa and a sputtering rate of0.18 nm/s. This process was carried out by controlling the swing of thesubstrate to the top of both targets for the alternating deposition of nickeland chromium atoms. The swing frequency and the anchoring time uponeachmetal target were handled carefully to determine the film thickness.Substrate temperature was kept at 300 °C during the alternating deposi-tion. Ti, Ag, and NiCr metal layers were done by dc magnetron sputteringtitanium, silver, nickel and chromium targets at room temperature usinga pure Ar pressure of 0.16 Pa with a sputtering rate of 1.75 nm/s,3.28 nm/s and 1 nm/s, respectively. The outermost Si3N4 layer was de-posited by rf reactive sputtering the silicon target in the Ar and N2 gasmixtures. The total working pressure and the nitrogen partial pressurewere set to 0.35 and 0.2 Pa, respectively, resulting in a sputtering rate of13.3 nm/min.
2.5. Characterization
The crystallizing structure was characterized by X-ray diffraction(XRD) in the 2θ range from 20 to 80° on a PANalytical X'pert PROX-ray diffractometer with Cu Kα radiation (λ=1.54050 Å) using astep size of 0.02° and step time of 0.3 s. A field emission scanning elec-tron microscopy (FESEM, Sirion 200) was used to analyze the morphol-ogy of both TiO2 particle films and TiO2 NTs on glass substrate. Beforethe FESEM experiments, nano-platinum was sprayed onto the samplesurfaces to increase its conductivity. The spectral transmittances be-tween 250 and 2800 nm in wavelength were measured using a spectro-photometer (Jasco, V570). The photocatalytic activity of each samplewasevaluated by measuring the photodegradation of MB under the 365 nmwavelength light irradiation (intensity ~1.5 mW/cm2), that was pro-duced by using the full-spectrum sunlight simulator (Trusttech, CHF-XM500, ~100 mW/cm2) with an optical filter. The sample was im-mersed in unstirred MB solution of 20 mL, with an initial concentrationof about 10 mg/L, for an hour each time under irradiation. The MB con-centration after irradiation was measured using a photoelectric colorim-eter (Shanghai Huaguang, 581-S). Surface stylus profiler (KLA-TencorP16+) was used to characterize the film thickness. The hydrophilicitywas evaluated by measuring the water contact angle using a JC2000Acontact angle instrument (Shanhai Zhongchen). This instrument allowsthe deposition of a 2 μL water droplet onto the sample under test via asyringe coupled to a 27 gauge needle with a 90° bevel tip held abovethe test sample. The droplet profile was recorded after 30 s of its deliv-ery. The contact angle was determined using an instrument software toanalyze the resulting recorded profile image.
3. Results and discussion
3.1. Multifunctional integration
Fig. 1 shows the schematic drawing of the multi-functional com-posite coated glass. The exposed outside of glass is SiO2/sensitizedTiO2 NT double-layer coating, which gives the outer surface of glasswith both self-cleaning and hydrophilicity dual function. The insideof glass is a multilayer coating to fulfill its heat-insulating function.
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3.2. Fabrication technology of TiO2 nanoparticle film
Typically, TiO2 nanoparticle film is formed using argon gas as thesputtering gas to bombard the titanium metal target, resulting inthe formation of titanium atoms vapor, which then reacts with theoxygen reactive gas, to produce the condensing particles upon thesubstrate. The mechanical characteristics of the as-prepared TiO2
film, in particular adhesion and compactness, depend upon the ener-gy of the condensing particles, which necessitates a suitable workingpressure to create mechanically robust, stoichiometric TiO2 films.Fig. 2 shows the XRD patterns of three film samples prepared usingdifferent working pressures with a common oxygen partial pressureof 20%. Peaks of TiO and Ti2O3 were observed at a lower working pres-sure of 0.41 Pa. There was no stable stoichiometric ratio for the for-mat of TiO2 compound. Increasing the working pressure from 0.41to 0.77 Pa, the diffractive peaks of single-phase TiO2 anatase structurewithout the superfluous peaks of other formats of titanium-oxygencompounds were observed. Further increasing the working pressureto 1.0 Pa, it could be seen that only the diffractive peak of (101) planereflection with preferential orientation was presented. Zhou et al. [21]proposed that the crystalline structures found at different workingpressures are due to the variation of the energy of the sputtered parti-cles according to a collision mechanism. In this collision mechanism,the sputtered particles (including atomic Ti, molecular TiO and TiO2)from the target have to pass through a plasma region before reachingthe substrates, where they will undergo collisions with the plasma spe-cies, which can randomize their directions and then reduce the depos-iting flux to the substrate. The collision intensity depends on theplasma density in front of the substrate that is directly related to theworking pressure. At a low working pressure, the plasma in front ofthe substrate is very dilute. The formation of the mixed TiO and Ti2O3
structure and the absence of TiO2 phase at 0.41 Pa mean that even ifthe sputtered species could arrive at the substrate without suffering se-vere collisions at low total pressure, the energy transferred to the grow-ing film on the substrate by these species is insufficient to the formationof single-phase TiO2 films with stable stoichiometric ratio. At a highertotal working pressure (0.77 Pa), the plasma density is moderate. Al-though the sputtered species underwent a certain degree of collisionsat the working pressure, they still had a higher impinging energy andthus had a higher surface mobility when they were deposited onthe substrates. As a result, the single-phase TiO2 compound couldbe obtained. It is interesting to find that only a narrow anatase(101) diffraction peak could be seen for sample prepared at 1.0 Pa.This means that the film was mainly composed of anatase crystalswith preferential orientation. This result is similar to that of Miaoand Xirouchaki [22,23] who explained the phenomena with thezone model and atomic mobility. According to their explanation,some clusters could be formed in a dense plasma due to severe
collisions, such as a three-body collision between two sputtered particlesand an argon atom. Andwhen these clusters arrived at the substrate theywould exhibit as good nucleation sites for crystal formation. These clus-ters, being small, still do not have a crystalline orientation, but when de-posited they tend to orient themselves with the substrate, forming amaterial with regular crystalline structure. A combination of gas phaseclustering and the surface conditions on the substrate has been used tocontrol crystallinity in such environments [24,25]. Continually increasingthe working pressure to be greater than 1.0 Pa will increase the collisionfrequency, which would result in larger clusters. Such a strong collisionwill reduce the incident energy of the sputtered particles and the mobil-ity of the surface atoms provides sufficient time for rough growth of thethin film, and ultimately forms a coarser surface morphology. Such aphenomenon has been reported in [25,26]. It suggests that the workingpressure of 1.0 Pa is a suitable technical parameter to obtain TiO2 nano-particle films with high-quality.
The oxygen partial pressure is another important technical param-eter which affects the quality of nanoparticle films. Figs. 3–5 show theXRD pattern, FESEMmicrographs, and kinetic plot of MB degradation,respectively, of as-prepared TiO2 nanoparticle films formed usingthree different oxygen partial pressures with a common total workingpressure of 1.0 Pa. A polycrystalline single-phase TiO2 deposit with-out preferential orientation was observed for oxygen partial pressureof 10% (Fig. 3). In contrast, the oxygen partial pressure of 20% couldlead to the regular crystalline arrangement with preferential orienta-tion, only the single diffraction peak of (101) plane reflection wasdetected. However, as shown in Fig. 3, the characteristic (101) peakbecomes weaker at an oxygen partial pressure of 30%, indicatingthat the coverage of the TiO2 deposit coating obtained at 30% oxygenpartial pressure is low. Furthermore, the FESEM micrographs show
Fig. 1. The schematic drawing of the multi-functional two-side composite coated glass.
Fig. 2. XRD diffractograms of TiO2 nanoparticle films fabricated using different totalworking pressures. The oxygen partial pressure was held on a constant level of 20%.
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that the TiO2 films prepared using various oxygen partial pressureshave significantly different morphological characteristics. As seen inFig. 4a, the film formed at oxygen partial pressure of 10% has roughmorphology with greater average columnar diameter about 75 nm.The TiO2 nanoparticle films fabricated using 20% oxygen partial pres-sure reveal flat and smooth surface morphological characteristicswith small and uniform average columnar diameter about 15–20 nm(Fig. 4b). As is evident from the FESEMmicrographs in Fig. 4c, the coat-ings have desquamated partially from the nanoparticle films formed at30% oxygen partial pressure. The impact of oxygen partial pressure onthe crystalline structures and themorphologies should be directly relat-ed to the changes of deposition kinetics. The thickness of the TiO2 filmsdecreases with enhancing oxygen partial pressure, that is connected tothe typical decrease of the deposition rate, has been previously reportedby Toku and Serio [24,27]. The decrease is related with the transitionfrom themetallic to reactive sputteringmode. If the total working pres-sure was kept at a constant value, a lower oxygen partial pressure of10% means faster deposition kinetics, which allows the rapid growthof condensing particles. As a result, the random crystal structure with-out preferential orientation could be formed. Furthermore, it is alsowell known that the oxygen as the reactive gas content in plasma hasan important influence on the charging efficiency of the system[26,28]. The lower oxygen partial pressure reduces the charging effi-ciency of the system. A lower charging efficiency allows more clustersto grow to a larger size via agglomeration prior to charging, easy to ob-tain the films with larger crystallite size. In contrast, the oxygen partialpressure of 20% can get a relatively high charging efficiency. The clustershave not yet had time to agglomeration growth, and have already beensubjected to charging on account of the higher charging efficiency. Onceit is charged, it cannot grow by agglomeration due to coulombic repul-sion, which limits the cluster size. According to Miao's theory of zonemodel and atomicmobility [22], a meta-stable anatase phase with pref-erential orientation can be formed when these small clusters arrivedwith lower atomic mobility at the substrate surface. On the otherhand, the moderate deposition kinetics and the smaller clusters at 20%oxygen partial pressure provide the increasing nuclei on the substrate,thus the crystal growth was suppressed, resulting in the formation ofsurfacemorphologywith small crystallite size as shown in Fig. 4b. How-ever, when the oxygen partial pressure was higher than a critical value,the target surface would be poisoned and be covered by an oxide layer[29]. Consequently, the deposition rate would decrease remarkably[30,31]. As a result the adhesion of the condensing particles to the un-derlying substrate would be impaired, and the deposit coatings wereprone to desquamate, resulting in lower coverage of deposits. In fact,the X-ray diffraction and the FESEM micrograph indicate for the filmsgrownwith 30% oxygen partial pressure that the value of oxygen partialpressure in this study is too high so that a poisoning effect has beenestablished. The crystalline phase and morphology of the as-prepared
films directly determine the photocatalytic degradation performance.As shown in Fig. 5, the MB photocatalytic degradation rate ofas-fabricated films using the oxygen partial pressure of 20% is higherthan that of 10% and 30% in each illumination period, and has aphotodegradation efficiency of 78±0.9% after 4 h of illumination. Twofactors contributed to the optimal condition of photodegradation effi-ciency for the films deposited at the oxygen partial pressure of 20%.The first is that the anatase crystals grown in the films were small, asshown in Fig. 4b. It has been reported that the filmwith larger crystallitesize could cause stronger reflection and interference of light, which
Fig. 3. XRD diffractograms of TiO2 nanoparticle films fabricated using different oxygenpartial pressures. The total working pressure was held on a constant value of 1.0 Pa.
Fig. 4. FESEM images of TiO2 nanoparticle film samples fabricated at different oxygenpartial pressures using a constant working pressure of 1.0 Pa: (a) 10% oxygen partialpressure; (b) 20% oxygen partial pressure; (c) 30% oxygen partial pressure.
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resulted in lower transmittance, as well as lower absorption [24,32]. Thefilm with small crystallite size can absorb more light irradiation, moreelectron–hole pairs can be generated in the TiO2 film, and consequentlygives rise to the enhancement of the photocatalytic activity. The secondis that the surface area is always a key factor in heterogeneous catalysisprocess. As the crystallite size was small and the amount of crystallinecontent was high for the film deposited at 20% oxygen partial pressure,there were more nano-sized holes among the titanium oxide particles.It can be concluded that the surface area of the TiO2 film deposited at20% oxygen partial pressure must be larger than that of deposited atlow or high oxygen partial pressure. This may be another reason for theimproved photocatalytic activity of fine TiO2 film because larger surfacearea means more active sites available for photodegradation.
3.3. Fabrication technology of SiO2/TiO2 double-layer film
SiO2 film has excellent hydrophilicity properties, while it is also su-perior to TiO2 in hardness and chemical stability, so a double-layerstructure of the SiO2(top)/TiO2(under) filmwill reflect the perfect com-bination between hydrophilicity and photocatalytic activity. The TiO2
nanoparticle underlayer with the thickness of about 200 nm was pre-pared using the aforementioned optimization process. The SiO2 toplayer was deposited by rf sputtering the silicon target 3.5 min in theAr and O2 gas mixtures. Similarly, the oxygen partial pressure and thetotalworkingpressure are investigated to optimize the fabrication tech-nology of SiO2 film. At a constant total working pressure, it was foundthat increasing the oxygen partial pressure in the range of less than20% can significantly improve the photocatalytic performance, but haslittle efficacy in the range of more than 20%. As shown in Fig. 6, after4 h of illumination the SiO2 top layer formed at 20% oxygen partial pres-sure can obtain a photodegradation efficiency of 64±0.7%, which is sig-nificantly higher than the value of 44±0.7% produced at 8% oxygenpartial pressure. However, increasing the oxygen partial pressure to60% just causes a slight enhancement of the photodegradation efficien-cy to 66±0.6%. Using the vacuum evaporation method, Nakamura pre-pared the SiO2/TiO2 double-layer film, and his results very clearlyshowed that a sparse surface structure of the SiO2 top layer is conduciveto improving the hydrophilicity of as-prepared composite double-layerfilm, and also allows the TiO2 underlayer to fully perform its photocata-lytic activity [11]. When the oxygen partial pressure increases, the filmstructure changes from dense into porous, in this case, the film hasmore voids, and part of the light can pass through the filmwithout scat-tering, resulting in an increase of the transmittance [33]. Further, the
increase in porosity upon increasing oxygen partial pressure has beencalculated using effective medium theories [34]. In comparison withusing oxygen partial pressure of 8%, using the oxygen pressure of 20%will produce relatively sparse surface structure of the SiO2 top layer,resulting in the significant improvement of the photocatalytic perfor-mance. However, continually increasing the oxygen partial pressure tobe greater than 20%, it seems that the efficacy of the increase in porosityupon increasing oxygen partial pressure has reached a certain threshold,therefore, the oxygen partial pressure of 60% caused only a slight en-hancement of the photodegradation efficiency as shown in Fig. 6. For fur-ther confirming the assumption of the porosity efficacy, a contrastiveexperiment of using two different work pressures to prepare the SiO2
top layer was performed. Figs. 7 and 8 show the hydrophilicity and pho-tocatalytic comparative experiments, respectively, of as-preparedSiO2/TiO2 double-layer film with the SiO2 top layer formed usingtwo different total working pressures at constant oxygen partialpressure of 20%. As expected, the results reveal that using the totalworking pressure of 0.5 Pa is better than that of 1.0 Pa to fabricatethe SiO2 top layer of the double-layer film in terms of both the hydro-philicity and the MB photocatalytic degradation rate. As mentionedabove, the higher total working pressure could form a coarser sur-face morphology with larger crystallite size and compact structure,which can cause stronger reflection and interference of light,resulting in lower transmittance. Consequently, the hydrophilicityand the photodegradation efficiency of the resulting double-layerfilm would decrease correspondingly. Because the photocatalytic ac-tivity of TiO2 is a surface reaction, it will be meaningful to assesswhat amount of TiO2 surface exposed in the SiO2/TiO2 double-layer.However, the exposure area of the TiO2 surface is difficult to measurequantitatively. Moreover, Nakamura has demonstrated that the pho-tocatalytic efficacy in the SiO2/TiO2 double-layer structure systemdoes not just come from the exposed surface of TiO2 underlayer[11]. Using the Fourier transform infrared spectrometry and thetime-of-flight secondary-ionmass-spectrometry, it is indicated thatthe SiO2 top layer is chemically bonded with the TiO2 underlayer.The stable chemical bond structure can assist the decompositionof the organic contaminants on the double-layer film surface(where the SiO2 top layer is very thin, about 20 nm). Though thedetailed reaction path is under investigation, we consider that theporous SiO2 structure would play an important role in decomposingorganic contaminants by the UV light irradiation on the SiO2/TiO2
double-layer films.
Fig. 5. Kinetic plots of MB photocatalytic degradation of TiO2 nanoparticle films formedat different oxygen partial pressure using a constant working pressure of 1.0 Pa. Atleast five degradation experiments were carried out on each evaluated point by five in-dependent samples and the reported degradation rate was the mean value. Thephotodegradation efficiency of 78±0.9% after 4 h of illumination was observed forfilms formed at oxygen partial pressure of 20%.
Fig. 6. Kinetic plots of MB photocatalytic degradation of SiO2(top)/TiO2(under)double-layer films with the SiO2 top layer formed at different oxygen partial pressureusing a constant working pressure of 0.5 Pa. The TiO2 underlayer was prepared at 20%oxygen partial pressure using the total working pressure of 1.0 Pa. At least five degra-dation experiments were carried out on each evaluated point by five independent sam-ples and the reported degradation rate was the mean value. The photodegradationefficiencies of the double-layer films with the SiO2 top layer formed at oxygen partialpressure of 8%, 20% and 60% after 4 h of illumination were 44±0.7%, 64±0.7% and66±0.6%, respectively.
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3.4. Fabrication technology and photocatalytic modification of TiO2 NTs
Although the SiO2 top layer has excellent surface hydrophilicity, andcan significantly enhance the hardness and the chemical stability of thedouble-layer structure, however, its coverage can also lead to an ob-served attenuation of photocatalytic activity (from 78±0.9% down to64±0.7%, compared Fig. 5 with Fig. 6). As the highly ordered TiO2 NTscan offer large specific surface areas and great capability of fully captur-ing solar energy, they show good optical catalytic activity than TiO2
nanoparticle films [35,36]. Therefore, substituting TiO2 NTs for TiO2
nanoparticle underlayer of the double-layer structure will promotethe photocatalytic performance. Fig. 9 shows the top surface views ofas-prepared TiO2 NTs formed by anodizing the Ti thin film on a glasssubstrate. Since the sputtering titanium layer is very thin (about620 nm), the electrochemical anodization of such a thin layer is re-stricted within a very short period of time, that is difficult to precise-ly control, often resulting in a sponge-like over-oxidation tissue (Fig. 9a).To avoid the formation of “sponge” structure, the pretreatment anodiza-tion method was developed to obtain highly defined and ordered trans-parent TiO2 NTs. Our previous work has shown that the chemicaldissolution of titania in the acidic electrolyte plays a key role in the NT
Fig. 7. Closed circuit television (CCTV) profiles of a water droplet on the surface of SiO2/TiO2 double-layer films with the SiO2 top layer formed using two different total work-ing pressures at constant oxygen partial pressure of 20%. The TiO2 underlayer was pre-pared at 20% oxygen partial pressure using the total working pressure of 1.0 Pa. Theaverage contact angles were 0.7° and 3.29° for that using the total working pressureof 0.5 Pa and 1.0 Pa, respectively.
Fig. 8. Kinetic plots of MB photocatalytic degradation of SiO2/TiO2 double-layer filmswith the SiO2 top layer formed at 20% oxygen partial pressure using the total workingpressure of 0.5 Pa and 1.0 Pa, respectively. The TiO2 underlayer was prepared at 20%oxygen partial pressure using the total working pressure of 1.0 Pa. At least five degra-dation experiments were carried out on each evaluated point by five independent sam-ples and the reported degradation rate was the mean value. The photodegradationefficiencies of the double-layer films with the SiO2 top layer formed at the total work-ing pressure of 0.5 Pa and 1.0 Pa after 4 h of illumination were 64±0.7% and 52±0.9%,respectively.
Fig. 9. Top-surface FESEM micrographs of TiO2 NTs formed by anodizing the Ti/glasssubstrate in an electrolyte comprising 1.0 vol.% H2O and 0.28 g NH4F in 100 mL ethyl-ene glycol at 35 V. The Ti/glass substrate was done by dc magnetron sputtering titani-um target at room temperature using a pure Ar pressure of 0.16 Pa. The sputteringtime was 6 min, resulting in a film thickness of about 620 nm. (a) The sponge-likeover-oxidation tissue via the route of direct anodization, where the Ti/glass substratedirectly underwent the anodization for 13 min. (b) The regular and highly orderednanotube arrays via the route of the pretreatment anodization, where the Ti/glass sub-strate was annealed in a muffle furnace at 500 °C for 40 min prior to anodization, andafter this treatment, the substrate was put back into the pool to undergo the anodiza-tion for 13 min.
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formation [36]. In order to generate regular NT arrays on such a thintitanium surface, the chemical dissolution rate must be sloweddown. Schmuki [37] has reported that a compact thermal rutilelayer formed on the Ti surface can protect the tube tops againstchemical etching in the fluoride-containing electrolyte during anodi-zation. Therefore, in this work, the Ti/glass electrodes were annealedin a muffle furnace at 500 °C for 40 min prior to anodization, and avery thin rutile oxide layer was formed on the surface of titanium foil.The compact rutile layer formed on the titanium foil surface couldserve as a protective layer for the tube tops to prevent the excess chem-ical dissolution during the follow-up anodization. After this treatment,the electrodes were put back into the pool to undergo the anodizationfor 13 min, and the regular and highly ordered NT arrays surfacecould be obtained (Fig. 9b). The anodized samples were then annealedat 300 °C in oxygen ambient for 2 h to convert the amorphous phase tocrystalline phase.
Because TiO2 is a large-band-gap semiconductor (3.0 and 3.2 eV forthe rutile and anatase phases, respectively), its activation is limited onlyin the UV region, which accounts for only 4–5% of the spectrum of solarenergy [38]. In order to modify the spectral absorption range of TiO2
NTs, a narrow-band-gap semiconductor/TiO2 NT composite structuremay be an advantage to employ not only UV light but also the less ener-getic butmore abundant visible light for photocatalysis.Wehave recentlyreported that the photoactivity of TiO2 NTs can be enhanced by sensitiz-ing them with the narrow-band-gap semiconductor [39]. Shown inFig. 10a is a top-surface FESEM image of Bi2S3-sensitized TiO2 NT film,which reveals that well ordered pore structures were maintained. This
suggests that the sensitization process does not damage the orderedTiO2 NT array structure. Only the diffractive peaks of single-phase Bi2S3and the TiO2 substrate without the superfluous peaks of elemental Bior S are observed in the XRD pattern of the film sample (Fig. 10b).Fig. 11a shows the photodegradation efficiencies of the SiO2/sensitizedTiO2NTdouble-layerfilms using Bi2S3, CdS, CdSe, andCdTe as sensitizers,respectively, after 4 h of illumination. This is not a significant differencein using different types of narrow-band-gap semiconductor as sensitizeramong the samples investigated. However, as shown in Fig. 11a, thephotodegradation efficiency of as-fabricated films using Bi2S3 sensitizerseems to be slightly higher than that of using other sensitizers such asCdS, CdSe, and CdTe. This phenomenon can be explained by the factthat Bi2S3 can absorb more visible light (Eg=1.28 eV), and the conduc-tion band of Bi2S3 is closer to the corresponding band of TiO2, resulting inmore favorable injection of electron [40]. Fig. 11b features kinetic plots ofphotocatalytic degradation of as-prepared SiO2/TiO2 double-layer filmswith the TiO2 underlayer using nanoparticles, pure NTs and sensitizedNTs, respectively. The sensitized TiO2 NTs by the narrow-band-gap semi-conductor of Bi2S3 exhibit the bestMB degradation rate in each illumina-tion period among the investigated samples, indicating that the opticalcatalytic activity was facilitated by the narrow-band-gap semiconductorsensitizer. Furthermore, the enhanced photocatalytic performance ofSiO2/sensitized TiO2 NT double-layer can also promote photo-inducedsuper-hydrophilicity that causes insignificant light scattering, and there-fore invisiblewater droplets. Fig. 12 shows the hydrophilic dynamic test-ing of the sensitized double-layer film by continuously recording theCCTV profiles in the interval of every 3 s. As is evident from the dynamicdroplet profiles, thewater droplet quickly spreads along the surface onceit fell onto the film, and disappeared in a very short period of time.
Fig. 10. Topography and crystallographic structure of TiO2 NTs sensitized with Bi2S3.TiO2 NT substrate was formed by anodizing the Ti/glass substrate via the route of thepretreatment anodization. The as-prepared NT substrate was immersed into the Bi2S3sensitizing solution for 24 h at room temperature, and then was dried at 110 °C for 2 hto produce Bi2S3-sensitized TiO2 NT film. Bi2S3 sensitizing solution was consisted of0.005–0.01 M Na2S·9H2O, 0.005 M Bi(NO3)3·5H2O, and 0.1 M EDTA2− at pH 10.5 adjustedwith nitric acid and ammonia. (a) FESEM images of Bi2S3-sensitized TiO2 NT film. (b) XRDpattern recorded for Bi2S3-sensitized TiO2 NT film.
Fig. 11. Photocatalytic comparative experiments of SiO2/TiO2 double-layer films. (a) Thephotodegradation efficiencies of the SiO2/sensitized TiO2 NT double-layer films usingBi2S3, CdS, CdSe, and CdTe as the sensitizers, respectively, after 4 h of illumination. (b) Kineticplots of MB photocatalytic degradation of SiO2/TiO2 double-layer films with the TiO2
underlayer usingnanoparticles, pure nanotubes andBi2S3-sensitizednanotubes, respectively.The SiO2/Bi2S3-sensitized TiO2 NT double-layer films exhibit the best MB degradation rate ineach illumination period among the investigated samples.
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3.5. Fabrication technology and performance of multilayer heat-insulatingcoating
On the other hand, some heat-insulatingwindows, which obstruct asmuch as possible of the invisible solar radiation in the wavelength inter-vals of 800–2500 nm, can play a major role in architectural energy con-servation [41]. Here, a multilayer coating with TiO2/NiCrOx/Ti/Ag/NiCr/Si3N4 stacked structure was designed to perform the heat-insulatingfunction. In order to determine the optical properties of each individuallayer films in the stacked structure, we formed the individual thin filmson the glass substrates, independently, and carried out measurementsand analyses. Fig. 13 features the spectral transmittances of TiO2 filmsformed in various sputtering times on the multilayer side of glass.Using the surface stylus profiler, a sputtering rate of 7.74 nm/mincan be determined, so the film thickness was accurately identifiedas 46.44 nm, 54.18 nm, and 61.92 nm in the sputtering time of 6, 7,
and 8 min, respectively. As shown in Fig. 13, the deposited TiO2 film in6 min displays some unwanted transmittance at shorter wavelengths,but this feature can be alleviated by prolonging the sputtering time to7 min. The film sputtered in 7 min shows extraordinary transmittancein a band centered at a wavelength of 515 nm, presenting a soft greenluster that is comfortable to human eyes. However, continued increaseof the sputtering time to 8 min leads to a higher transmittance in theband centered at a wavelength of 800 nm, which is unfavorable for theheat-insulating effect. A similar beneficial effect exists in Ag thin film.Fig. 14 shows the spectral transmittances of a five-layer stack comprisingTiO2(7 min)/NiCrOx(13 s)/Ti(2 s)/Ag/NiCr(2 s), where the Ag layers areformed in various sputtering times. The corresponding thickness of9.84 nm, 13.12 nm, and 16.40 nm was calculated in the sputteringtime of 3, 4, and 5 s, respectively, using a sputtering rate of 3.28 nm/s.The Ag layers sputtered in 4 s observably enhanced the transmittancein the visible region and also improved the heat insulating performance
Fig. 12. Continuous CCTV profiles of a water droplet on the surface of SiO2/sensitized TiO2 NT double-layer film in the interval of every 3 s. (a) The droplet profile in the momentbefore the dripping, showing the droplet was suspended on the needle tip, and yet did not fall onto the film. (b) The droplet profile recorded after 3 s, showing the droplet had fallenonto the film, and quickly spread along the surface. (c) The droplet profile recorded after 6 s, showing the droplet was disappearing rapidly. (d) After 9 s the droplet had completelydisappeared, not visible water droplet on the film surface.
Fig. 13. Spectral transmittances through TiO2 films formed in various sputtering timeson the multilayer side of glass. The TiO2 individual layer films were done by dcsputtering the Ti target in a mixture of Ar and O2 gases at 20% oxygen partial pressureand 1.0 Pa total working pressure.
Fig. 14. Spectral transmittances of a five-layer stack comprising TiO2(7 min)/NiCrOx(13 s)/Ti(2 s)/Ag/NiCr(2 s), where the Ag layers are formed in various sputtering times. The Agmetal layer was done by dc magnetron sputtering silver target at room temperature usinga pure Ar pressure of 0.16 Pa.
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in the long-wave near infrared region. It has been reported that the Agfilm formed within a shorter sputtering time is composed of discontinu-ous small island-like particles, which can enhance the sheet resistance ofthe Ag film, resulting in the increase of emissivity [42,43]. However, alonger sputtering time can increase the surface roughness of Ag thinfilm, leading to an increase of light absorbance [44]. It suggested thatthe Ag layers sputtered in 4 s have a combinative advantage to meetthe optimal condition of the optical and electrical properties because ofan absorption threshold or continuous film limitation at the thicknessof 13.12 nm. Such a thickness value is very close to Szczyrbowski etal.'s report of 13.50 nm [2].
It is now possible to appreciate experimental data on optical proper-ties of Ti metal films as a thin interfacial layer. Fig. 15 shows the mea-sured transmittances of a five-layer structure comprising TiO2(7 min)/NiCrOx(13s)/Ti(2s)/Ag(4s)/NiCr(2s) and a similar four-layer structureonly without the Ti interlayer. The titanium layer thickness was calcu-lated as 3.50 nm in the sputtering time of 2 s using the sputtering rateof 1.75 nm/s. As seen in Fig. 15, the very thin Ti interlayer betweentheNiCrOx and the upper Ag layer noticeably improved the heat insulat-ing performance and also enhanced the transmittance in the visible re-gion. This efficacy should be attributed to the dual role played by thethin interface layer. First of all, its good conductivity can reduce thesheet resistance of the upper Ag film, resulting in a lower emissivity.Lastly, the interlayer can also promote the formation of continuous Aglayer even in the case of superthin Ag coating, which contributes tothe double effect of reducing the infrared radiation and enhancing thevisible-light transmittance.
The barrier layers applied to both sides of the Ag film can protectthe Ag film from the aggressive environment, and provide better ad-hesion to the upper and lower dielectric layers. As a blocker the NiCrsuboxide or NiCr pure metal is usually used [41,45]. In our system, theupper barrier layer is the NiCr pure metal (without oxygen), whichprevents oxidation of the underlayer silver film during the sputteringprocess of upper dielectric layer. The under barrier layer is investigat-ed by using the NiCr suboxide and NiCr pure metal as the blockers, re-spectively. Fig. 16 features the spectral transmittances of six-layerstack showing the impact of two different under barrier layers.There is no significant difference in using the NiCr suboxide andNiCr pure metal as the under barrier layers. However, in comparisonwith the NiCr pure metal, the NiCr suboxide as under barrier layerseems to be slightly more effective in reducing the near-infraredtransmittance. This efficacy should be attributed to the slower depo-sition rate of 0.18 nm/s for NiCr suboxide than that of 1 nm/s forNiCr pure metal. In other words, if the same 2 nm thickness of theunder barrier layer is deposited, the sputtering time of NiCr suboxideneeds 12–13 s, while that of NiCr pure metal only needs 2 s. The
slower deposition rate makes the under barrier layer of NiCr suboxidesmoother, resulting in the improvement of the Ag conductivity.
Fig. 17 shows data from a six-layer stack with the Si3N4 film formedin various sputtering times as the top layer. The Si3N4 top layer can ren-dermechanical durability to this type of coatings owing to its high hard-ness and scratch resistance [2]. However, this effect must be based onthe premise that the top layer cannot destroy the optical properties ofthe complete stack coatings. This can be reconciledwith the occurrenceof an optimal geometric structure. It appears that the sputtering film in4 minwith a thickness of 53.20 nmcan generate higher heat-insulatingeffect in the wavelength intervals of 800–2500 nm (Fig. 17). Usingmulti-angle ellipsometry, the optical constants (refractive indexand extinction coefficient) of Si3N4 thin film sputtered in 4 min aredetermined to be 2.05 and 0.001, respectively. The high opticalstability and heat-insulating effect are due to the proper choice ofthe optical constants for the dielectric layer adjacent to the silverfilm (TiO2 and Si3N4). Fig. 18 shows the comparative data from asix-layer stack with the Si3N4 top layer versus without the toplayer. Although the Si3N4 top layer slightly impairs the optical trans-mission, there is a very low emissivity in the near infrared regioncompared with that of the absence of top layer.
According to the above analysis, the six-layer stackwith TiO2(7 min)/NiCrOx(13 s)/Ti(2 s)/Ag(4 s)/NiCr(2 s)/Si3N4(4 min) configuration is
Fig. 15. Spectral transmittances of a five-layer structure comprising TiO2(7 min)/NiCrOx(13 s)/Ti(2 s)/Ag(4 s)/NiCr(2 s) and a similar four-layer structure only withoutthe Ti interlayer. The Ti interlayerwas done by dcmagnetron sputtering titanium target atroom temperature using a pure Ar pressure of 0.16 Pa. Its thickness was calculated as3.50 nm in the sputtering time of 2 s using the sputtering rate of 1.75 nm/s.
Fig. 16. Spectral transmittances of a six-layer stack with two different configurationsshowing the impact of under barrier layers. The under barrier layer was investigatedby using the NiCr suboxide and NiCr pure metal as the blockers, respectively. NiCrsuboxide barrier layer was formed by dc reactive sputtering the nickel and chromiumtargets alternately, in the mixed gas of Ar and O2 with a volume ratio of 9:1, resulting ina working pressure at ~0.2 Pa. NiCr metal barrier layer was done by dc magnetronsputtering the nickel and chromium targets alternately at room temperature using apure Ar pressure of 0.16 Pa.
Fig. 17. Spectral transmittances of a six-layer stack with the Si3N4 outermost layerformed in various sputtering times. The Si3N4 top layers were deposited by rf reactivesputtering the silicon target in the Ar and N2 gas mixtures. The total working pressureand the nitrogen partial pressure were set to 0.35 and 0.2 Pa, respectively.
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designed to perform the heat-insulating function. The correspondingindividual layer thickness of the TiO2, NiCrOx, Ti, Ag, NiCr, and Si3N4
was calculated as 54.18 nm, 2.34 nm, 3.5 nm, 13.12 nm, 2 nm, and53.2 nm, respectively, resulting in a total thickness of 128.34 nm.This thickness value is very close to the FESEM measured value of137.25 nm (Fig. 19a), which is slightly greater than the calculatedvalue. The deviation may be derived from the FESEM measurementerrors. Fig. 19b shows the XRD pattern of the multilayer stackedfilm sample. Only the diffractive peaks of TiO2 and Ag without thecharacteristic peaks of NiCrOx, Ti or NiCr were observed. The obscu-ration of NiCrOx, Ti and NiCr peaksmay be explained to the followingtwo reasons: on one hand, the amount of these substances in the
multilayer stacking is very small, only a few nanometers in thickness,may not be detected; on the other hand, the characteristic peaks ofNiCrOx, Ti, or NiCr are very close with Ag and TiO2 peak positions,resulting in peak overlapping. It is possible, as shown in Fig. 19b,that these peaks are relatively broad andmay be composed of severalpeaks overlapped.
The heat insulating performance of the multilayer coating is testedusing a homemade apparatus (Fig. 20a). A heat insulated containerkeeps an opening covered by the coating glass samples, which were ir-radiatedwith a 275 W infrared bulb. The internal temperature of the in-sulated container is displayed by a digital thermometer. As shown inFig. 20b, using the blank glass as the cover achieved an internal temper-ature rise of 30 °C under continuous irradiation for 8 min. The temper-ature rise decreased to 8 °C while substituting the coated glass for theblank glass, exhibiting the excellent heat-insulating performance.
3.6. Spectra of the two-side composite coatings
Fig. 21 illustrates the optical property of the multi-functionaltwo-side composite coated glass. In addition to the further improve-ment of the heat insulation in the near-infrared region, the two-sidecomposite coatings can create the multiple transmission bands inthe visible region. The maxima of transmission spectra can be regulat-ed by the TiO2 NT length on the outer surface of glass so that thetransmittance can be tailored to different color appearance, which isextremely important for industrial applications. It must also mentionthat the transmittance in the visible region from the two-side compos-ite coated glass is lower (maximum transmittance of 60.75% from
Fig. 18. Spectral transmittances of a six-layer structure comprising TiO2(7 min)/NiCrOx(13 s)/Ti(2 s)/Ag(4 s)/NiCr(2 s)/Si3N4(4 min) and a similar five-layer structureonly without the Si3N4 top layer.
Fig. 19. Cross-sectional FESEM micrograph (a) and XRD pattern (b) of the multilayercoating with a six-layer structure comprising TiO2(7 min)/NiCrOx(13 s)/Ti(2 s)/Ag(4 s)/NiCr(2 s)/Si3N4(4 min).
Fig. 20. (a) Homemade experimental setup for testing the heat insulating performance.(b) The internal temperature rise of the container using the multilayer coated glass andthe blank glass as the cover is 8 °C and 36 °C, respectively, under continuous irradiationfor 8 min, describing that the multilayer coated glass has an excellent heat-insulatingperformance.
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Fig. 21). However, the multi-functional composite coated glass haspotential versatile application in some fields, such as architecturalglazing and automotive side windows, which have lower demandto the visibility.
4. Conclusion
We have reported a suitable architecture of two-side compositecoated glass with multifunctional integration of self-cleaning, hydro-philicity and heat-insulating properties. The fabrication technology ofeach individual layer films is optimized to perform the multifunctionalintegration. A SiO2(top)/TiO2(under) double-layer structure is designedto reflect the perfect combination between hydrophilicity and photo-catalytic activity. The substituting TiO2 NTs for TiO2 nanoparticleunderlayer of the double-layer structure shows significant improve-ment in photocatalytic performance. To overcome the formation obsta-cles of the superthin TiO2 NTs, a pretreatment anodization method hasbeen developed to obtain highly defined and ordered transparent TiO2
NTs. Furthermore, TiO2 NTs sensitized with the narrow-band-gap semi-conductor are prepared to extend the photoresponse of NTs to visiblelight. The resulting SiO2/sensitized TiO2 NT double-layer has been veri-fied with excellent photocatalytic performance and photo-inducedsuper-hydrophilicity. To fulfill the energy conservation, the multilayercoating is deposited on the backside of glass.We found that the geomet-ric structure of the individual layer films, related to the optical constantshave a crucial impact on the optical properties of themultilayer coating.In addition, the very thin Ti interlayer between the NiCrOx and theupper Ag layer can noticeably improve the heat insulating performanceand also enhance the transmittance in the visible region. The Si3N4 toplayer exhibits a dual function to secure the excellent mechanical dura-bility of themultilayer coating and to serve also as heat-insulating treat-ment. Finally, the multi-functional two-side composite coated glassshows a tailored ability in color appearance. We expect that the multi-functional complex has a wide application in architectural and automo-tive glazing.
Acknowledgment
Thisworkwas cofinanced by theNational Natural Science Foundationof China (21173090, 30970717). Technical assistance from the Analyticaland Testing Center of HUST is gratefully acknowledged.
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Fig. 21. Spectral transmittance of the multi-functional two-side composite coatedglass.
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