Functional one?dimensional metallic oxides have attracted much attention because of their potential use in electronic, optical and spintronic devices ?. Among them, zinc oxide (ZnO), titanium dioxide (TiO2), and vanadium pentoxide (V2O5) nanostructures including nanowires, nanobelts and nanorods have been extensively studied due to their special electronic, chemical and optical properties ?. Incorporating these metallic oxide materials into an integrated structure with nanoscale dimension is of great interest because the resulting hierarchical nanostructures often possess much large surface areas and improved physical and chemical properties, providing potential applications for sensing, light emission and photocatalysis ?.
It is known that ZnO material can be made with the most diverse and abundant configurations of nanostructures such as quantum dots, rods, wires, belts, springs, bows, helices and prisms ?. Accordingly, ZnO based composite nanostructures are easily built up combined with other metallic oxides ?. The outside metallic oxides layers can be deposited onto the surfaces of the ZnO nanostructure substrates by different ways including sputtering methods or vapor deposition, and then the so?called “hierarchical nanostructures” are formed. Some interesting configurations or morphologies can be obtained for these combined nanomaterials. Special and improved optical or electronic properties are often reported , ?. Currently, the typical composite nanomaterials such as core?shell nanostructure formed by the inside nanorods with the outside covered layers. Examples including TiO2/ZnO, ZnO/MgO and ZnO/Er2O3 core?shell structures have been fabricated ?. Increase in light emission or efficiency enhancement for dye?sensitized solar cells based on those core?shell structures are observed, which certainly proves that these types of composite nanostructures have enhanced optical and electronic properties that may have practical applications.
In the current review, we summarize our recent work on the ZnO nanorod based composite nanostructures including the fabrication processes, microstructure characterization and the related photocatalytic applications. We would also like to discuss the current research status on these composite nanostructures and demonstrate the promising photocatalytic applications through the way of incorporating these metallic oxide materials into an integrated structure.
2. FABRICATIONS AND MICROSTRUCTURE
The microstructure and properties of metallic oxide are very sensitive to the processing techniques and parameters. In our study, we mainly use ZnO nanorods as the nanotemplate; some other oxides such as V2O5 and TiO2 were then deposited onto ZnO nanorods by magnetron sputtering to form controllable composite structure , . Combined with annealing treatment under controlled atmosphere at suitable temperature, novel hierarchical nanostructures can be obtained, with much improved optical or electronic properties than simple ZnO, V2O5, or TiO2 nanostructures.
2.1 Simple nanorods array template
For the composite nanostructure preparation, ZnO nanorods and V2O5 nanorods are selected as the basic templates since these two metallic oxides have stable characters; and the nanorods structure can be prepared in a well controllable and effective way ?.
2.1.1 ZnO nanorods array
Aligned ZnO nanorods have been successfully synthesized by catalyst?assisted vapor phase synthesis and catalyst?free metal?organic chemical vapor deposition ?. We use a relatively simple method, template?assisted hydrothermal growth based on wet chemical reaction , which provide a low?cost, low?temperature, and environmental friendly way for highly ordered ZnO nanorods array preparation.
The template?assisted hydrothermal growth method includes two steps: first a thin ZnO seeds layer (∼100 nm) was sputtered onto glass or silicon substrates, and then the coated substrates were suspended vertically in a sealable glass bottle filled with aqueous solutions of zinc nitrate and hexamethylenetetramine for ZnO growth. Diluted nitride acid or ammonia hydroxide solution was used to adjust the pH values. The sealed bottle was put into an oven at 95℃ for several hours for the ZnO nanorods to grow. ZnO nanorods with different surface distribution densities can be successfully obtained by controlling the initial pH value of the growth solution, as shown in Fig. 1. The ordered ZnO nanorods have a preferential growth direction of , and the rod formed is a single crystal. Furthermore, the morphologies of the nanorods array including the length, radius and orientation can also be controlled to certain extent by adjusting the solution concentration, thickness of the seed layer, growth temperature and growth time. Obviously, this facile preparation method provides a feasible way for the basic ZnO nanorods
[Fig. 1.] Zinc oxide nanorod arrays with different surface distribution densities prepared by low?temperature template assisted growth in solutions: (a) sparse and (b) dense rods grown in solutions with low and high pH values, respectively.
array preparation, thus supplying good templates for the following oxide composite structure fabrication.
2.1.2 β-V2O5 nanorods
Vanadium oxides have several stoichiometries including VO, V2O3, VO2, and V2O5, among which vanadium pentoxide, V2O5, is the most stable oxide and show semiconductor property with an energy gap of ∼2.2 eV at room temperature ?. Due to its layered structure and special characteristics including chemical sensing, multi?colored electrochromism and photochromism, V2O5 has attracted much interest, and has been considered to be a promising material for applications in Li?ion battery electrode, optical?electrical switches and color memory devices ?.
Several approaches have been reported to synthesize nanostructured vanadium oxides including hydrothermal method, sol?gel method and vapor phase epitaxy growth method , ?. Recently an alternative approach to produce single crystalline β?V2O5 nanorods has been developed in our labs , . The sputtered amorphous V2O5 films on glass substrate showed a temperature?sensitive crystallization behavior and transferred to β?V2O5 nanorods or β phase nanoslices with highly oriented structure after an annealing in O2 atmosphere at 500?550℃ as shown in Fig. 2. The related X?ray diffraction (XRD) spectra indicate that the β?V2O5 nanorods obtained after an annealing at 500℃ possess a highly ordered crystal structure.
[Fig. 2.] (A) The temperature sensitive crystallization process of vanadium pentoxide (V2O5): (a) the as?prepared amorphous V2O5 film; (b) annealed at 450℃; (c) β?V2O5 nanorods formed after annealing at 500℃and (d) flat?lying nanoslices formed at 550℃. The inserts show the related cross?section morphology for each sample. (B) X?ray difffraction patterns for the as?prepared and annealed samples.
Based on this temperature sensitive crystallization, V2O5 can be made to complex composite nanostructures. Interesting morphologies and hierarchical structures can be achieved after suitable annealing treatment, which will be shown below.
By using the ZnO nanorods array as the basic template, different composite nanostructures can be effectively fabricated with large?scale production. In the following part, we introduce one of these fabrication progresses: producing ZnO based composite nanostructures combined with magnetron sputter method.
2.2.1 ZnO/TiO2 core-brush array
ZnO and TiO2 nanowires have been extensively studied due to their potential use in solar cells and as photocatalysts ?. The principles of these applications are quite similar: based on their light absorption, light?electron conversion and electron transportation properties. To enhance the light conversion efficiency and improve the performance of the solar cells or photocatalysts, it is necessary to achieve high external quantum yields across a broad visible light spectrum, to
[Fig. 3.] (a) Zinc oxide (ZnO) nanorods array before titanium dioxide(TiO2) deposition; (b) ZnO/TiO2 bottlebrush nanostructures after 90min deposition; (c) transmission electron microscopy images ofZnO/TiO2 bottlebrush nanostructures with deposition time of 60 minand 90 min; (d) the insert shows the enlarged image; and (e) schematicsfor the formation of ZnO/TiO2 nanobrush.
increase the diffusion length of electrons within the nanocrystalline materials, and to increase the speed of the electron transport process . As TiO2 and ZnO materials have different band gaps, their composites should cover a broader light absorption spectrum than the individuals can cover. Furthermore, it is known that the electron transport rate in ZnO nanowires is much faster than in TiO2 due to its higher electrical conductivity .
Based on the above factors, ZnO/TiO2 composites, or coreshell nanostructures, have attracted much attention for solar cell and photocatalyst applications , , ?. Currently we use two?step process to prepare ZnO/TiO2 heterostructured bottle brush. Firstly, ZnO nanorods arrays are synthesised on glass substrates by the seed?assisted low temperature (95℃) hydrothermal method as described previously . TiO2 layers are then deposited on the top and sidesurfaces of ZnO nanorods by using magnetron sputtering with low deposition rate at room temperature. Pure TiO2 is used as the target and Ar as the working gas. The deposition time is controlled to be 30 min and 60 min.
The obtained ZnO/TiO2 nano?bottlebrush structures are shown in Fig. 3. It is clear that a TiO2 shell has been coated on the surfaces of ZnO nanorods, and the initial ZnO nanorods became the torch?shaped microstructure from the scanning electron microscopy (SEM) images. To further investigate the bottlebrush nanostructure, transmission electron microscopy (TEM) was used for more detailed microstructure of the obtained bottlebrushes. It can be seen that fine TiO2 nanowires are just like “hairs,” and orderly grown on the top and side surfaces of ZnO nanorods. With longer deposition time, the TiO2 hairs grow longer on the ZnO nanorods surfaces. Finally, the initially bare ZnO nanorods became bottlebrush?like nanostructures. A schematic drawing of formation mechanism is shown in Fig. 3(e).
The crystal structures of the ZnO/TiO2 bottlebrush nanostructures were investigated by X?ray diffraction, which shows
[Fig. 4.] (a) Photoluminescence (PL) spectra at room temperature from zinc oxide (ZnO) nanorods sample and ZnO/titanium dioxide (TiO2) bottlebrush samples with deposition time of 60 and 90 min; (b) the curve fitting of the PL spectra for the sample of 60 min deposition; (c) schematic illustration of the enhanced PL process from the ZnO/TiO2 bottlebrush nanostructures with resonant effect. Two excitation channels are shown in the figure.
the basic ZnO nanorods array has good crystalline structure with the preferred oriented growth direction of , while the outside?covered TiO2 layer shows several peaks with the rutile (110) dominant peak. From the current XRD, however, it is difficult to determine the individual TiO2 nanowires deposited on the ZnO nanorods are polycrystalline or single crystalline, though they look much like single crystals. Further highresolution TEM (HR?TEM) is needed to confirm the microstructure and crystallinity of the covered TiO2 nanowires.
The growth mechanism for the TiO2 nanowire coating is suggested to be in two stages. The first stage is that the initial nucleation of TiO2 nanocrystals on the ZnO nanorods surfaces. For this stage, it should be attributed to the heterogeneous nucleation, in which the interfacial energetics plays a key role. After the nucleation stage, the TiO2 nanowires will growth along the preferential  direction of rutile phase, which is the second stage of TiO2 nanowire formation. For the second stage, surface energetics should dominate the crystal growth mechanism to minimize the surface energy .
It should be noted that the relatively low depositing rate under a low sputter power in the magnetron sputter is another key point that favors the TiO2 nanowires growth. If a high sputtering power and high bias voltage were applied, the Ti4+ and O2? ions will have much higher kinetic energy, which tends to form an amorphous TiO2 film at room temperature.
The composite ZnO/TiO2 nano?bottlebrush structures exhibited a much enhanced optical properties as shown in Fig. 4(a). The photoluminescence (PL) emission for the initial ZnO nanorods show a strong UV light emission peak at ∼379 nm (curve a), and no other visible light emission peaks can be seen, evidence of the good crystalline quality of the ZnO nanorods prepared by hydrothermal method. After TiO2 deposition for 60 minutes, the PL peak from ZnO at the UV range reduced dramatically, and the TiO2 related PL peaks in the visible light range appeared (curve b). After 90 minutes deposition, the ZnO related PL peak at 379 nm disappeared entirely, and the TiO2?related PL peaks became weaker (curve c) compared to the sample with 60 minutes deposition. The detailed PL analysis of the 60 minutes deposited ZnO/TiO2 bottlebrush sample is shown in Fig. 4(b). The peak at 379 nm is the UV peak from the ZnO nanorods and the other peaks at 415 nm, 437 nm, 469 nm and 497 nm are from the TiO2 nanowires. The peak at 415 nm is likely to come from the selftrapped excitons localized at TiO6 octahedral sites, and the other longer wave peaks could be attributed to the oxygen vacancies on the surface area of TiO2 nanowires ?.
From the different PL spectra, it is interesting to discuss the origin of the enhancement of light emission from the integrated composite nanostructures. Obviously, the disappearance of the ZnO related ultraviolet (UV) peak for the bottlebrush of 90 minutes should be due to the relatively thick and dense TiO2 nanowire layer, thus the laser source has low penetration intensity to excite the inner ZnO nanorods. For the 60 minutes deposited ZnO/TiO2 bottlebrush sample, PL came from both the core part of ZnO and the shell part of TiO2 hairs by the laser excitation. Thus both the UV peak from ZnO and peaks from TiO2 appeared in the PL spectra. At the same time, a coupling mechanism may have taken place between the ZnO and TiO2 nanostructures: Some UV photons from ZnO may have been absorbed by the outside TiO2 nanowires, and the outside TiO2 hairs were excited by both the laser light and UV emission from the inner ZnO nanorods, as illustrated by the schematic in Fig. 4(c). These two excitation mechanisms may have formed a so?called resonant effect for the PL process, and may have been the reason that the intensity of the TiO2?related PL peak has been enhanced significantly .
2.2.2 ZnO/V2O5 bilayer
Our experimental results indicated that V2O5 films prepared by magnetron sputtering on glass substrates have temperature sensitive crystallization behavior within a narrow annealing temperature range from 450℃ to 550℃ , . Especially, when annealed at 500℃, highly ordered β?V2O5 nanorods will form.
According to the previous research, V2O5 material always shows visible light PL, and the emission intensity strongly depends on their microstructure ?. However, V2O5 has hardly been considered as a candidate for light emitting due to the unsatisfying emission intensity. Recent papers reported that GaN based light emission diodes capped by ZnO layer or nanostructures showed enhanced light emission, which may provide an effective way to increase the light emission efficiency of luminescence materials , . Thus it is interesting to investigate the light emission of V2O5/ZnO bi?layer composites in an effort to obtain enhanced light emission from ZnO incorporated V2O5 nanostructures.
V2O5 films capped by a thin ZnO layer were prepared by magnetron sputtering method at room temperature. The microstructure of the post?annealed samples was characterized by SEM and XRD as shown in Fig. 5. It can be observed
[Fig. 5.] (a) Surface morphology of as?deposited vanadium pentoxide/ zinc oxide (V2O5/ZnO) samples, the insert shows the cross?section image; (b) sample annealed at 400℃; (c) sample annealed at 450℃; (d) sample annealed at 500℃; (e) X?ray diffraction spectra for V2O5/ZnO bi?layer composite samples before and after annealing with different temperatures; and (f) a schematic drawing to show the changes of the microstructure.
that the as?deposited V2O5/ZnO sample has a quite smooth surface comprising compact particles with small grain size. The cross?section (the insert image) shows a clear interface between the V2O5 amorphous film (∼350 nm) and the capped ZnO layer (∼120 nm).
After an annealing in O2 ambience at 400℃, the ZnO small grains grew up and became larger particles at the top surface as shown in Fig. 5(b). When the annealing temperature increases to 450℃, the surface grains grew further larger, Fig. 5(c). After an annealing at 500℃, the samples show very different surface microstructures as in Fig. 5(d). Clusters and nanorods with porous structures clearly appear on the sample surface. Combined with the XRD spectra in Fig. 5(e), it can be seen that those nanorods (slices) grew from the interstices of the porous particles are β?V2O5 crystals. The annealed composite samples here show similar β?V2O5 nanorods comparing with those pure V2O5 films after annealed at 500℃, since V2O5 compound always show strong temperature sensitive crystallization. At the same time, the top?layer continuous ZnO thin film transferred into porous particles. This situation is understandable considering the interfacial interaction such as the effects of strain and stress during the annealing process. The capped ZnO layer on V2O5 films tends to depress the crystallization process of V2O5 to form standing β?V2O5 nanorods. However, due to the strong temperature sensitive crystallization behavior, the crystallization of V2O5 may not be suppressed by the covered ZnO layer. As a competition reaction, the annealed V2O5 became ordered β?V2O5 nanorods and the continuous ZnO layer was tore into
[Fig. 6.] (a) and (c) Photoluminescence (PL) spectra for vanadium pentoxide/zinc oxide (V2O5/ZnO) samples, (b) the integrated intensities for the different samples, and (d) PL spectrum and corresponding second?derivative spectrum for a V2O5/ZnO sample annealed at 500℃.
porous particles because of the interfacial strain. The porous ZnO particles were therefore dispersed to cover the β?V2O5 nanorods surfaces with an amorphous?like state. This process is schematically illustrated in Fig. 5(f). The XRD spectra showing the absence of ZnO related diffraction peaks give direct evidence for the above mechanism. Furthermore, the change of surface morphology for the samples annealed at 500℃ is unlikely caused by the solid?state reaction between V2O5 and ZnO according to the binary phase diagram of V2O5 and ZnO .
The optoelectronic properties of V2O5/ZnO samples are characterized by PL at room temperature with a 325 nm He?Cd laser source. From Fig. 6(a), it is clear that the broad green light emission from the V2O5/ZnO sample and pure V2O5 sample after an annealing at 500℃ is much stronger than other samples. A great enhancement for the light emission has been observed, especially for the V2O5/ZnO sample after annealing. The main peak is centered at 560 nm, which is quite consistent with the reported band gap of crystal V2O5 of 2.2 eV, confirming that the enhanced light emission should be mainly generated from the near band edge emission of crystal V2O5 nanostructures.
Figure 6(b) shows that annealing temperature has a very strong effect on the integrated intensity of light emission. It can be seen that the intensity enhancement is not significant when the annealing temperature is below 500℃. After an annealing at 500℃, however, the visible light emission increased dramatically. The enhancement for the pure V2O5 sample is ~8 times, while for V2O5/ZnO composite sample, the enhancement reaches ∼25 times compared with the as prepared samples. This tremendous enhancement was attributed to the coupling between V2O5 nanorods and ZnO nanoparticles as well as the improved V2O5 crystallinity. From the SEM images of Fig. 5(d) and the schematics of Fig. 5(f), the originally continuous ZnO layer becomes individual (amorphous) particles that surrounded the V2O5 nanorods, forming a porous surface. As a result, the V2O5 nanorods well embed into those amorphous ZnO nanoparticles. Thus, the absorption for the excitation laser beam is much enhanced for the annealed composite sample via the coupling with the surrounding ZnO particles. Accordingly, the light emission from V2O5 will be greatly improved, as the UV emission from ZnO particles (∼3.4 eV band gap) acts as another exciting source, added to the laser source to excite V2O5 nanorods, resulting in a much higher light emission efficiency for the composite V2O5/ZnO sample compared to the pure V2O5 or ZnO.
More detailed PL information is shown in Fig. 6(c). It can be seen that the ZnO related peaks at ∼392 nm is quite clear for the as?prepared V2O5/ZnO sample and the annealed samples below 500℃. The enhanced ZnO peak for the sample annealed at 450℃ should be due to the improved ZnO crystallinity. The V2O5/ZnO samples annealed at 500℃ show decreased ZnO UV?peak and prominent blue shift to 380 nm. The obvious near?band?edge blue shift of ZnO is considered to come from the phase and microstructure variation of ZnO layer after annealing, since the blue shift of ZnO UV emission was reported to be associated with small particle size and amorphous state , . Obviously, the observed blue shift of ZnO UV peak also provides a strong support for the above explanation that the capped ZnO layer becomes porous particles due to the interfacial strain after an annealing at 500℃.
Figure 6(d) shows the PL spectra for the V2O5/ZnO sample annealed at 500℃ with the corresponding second?derivative PL spectrum. Vibrational PL is clearly observed, characterizing a fine structure. The related peaks positions are labeled in the figure. According to previous literatures , , the vibrational fine structures are usually caused by the V = O vanadyl group involved in a tetrahedral coordination. Previous studies reported that the fine features of PL spectra for V2O5 were prominent at low temperature experiment. For the current samples, vibrational fine structures can be observed clearly at room temperature, which should be due to the high ordered structure of V2O5 nanorods.
V2O5/ZnO composite nanostructures show greatly enhanced visible light emission after an annealing at suitable temperature due to the coupling between V2O5 nanorods and ZnO nanoparticles. The improved V2O5 crystallinity after suitable annealing may have been another reason. This composite structure with significantly enhanced light emission suggests its potential applications as new luminescence or phosphorescence materials. Further work on other similar oxide pairs may discover more promising light emission systems.
2.2.3 ZnO/V2O5 nano-“lollipop” array
ZnO/V2O5 bi?layers showed strong photoemission due to the temperature sensitive crystallization of V2O5 material and interaction of V2O5 with ZnO. What will happen if the ZnO layer is replaced by ZnO rods? This session reports our work on the ZnO nanorods based ZnO/V2O5 composite nanostructures and their properties. ZnO nanorods arrays were synthesised on glass substrates (25 mm×10 mm) by using the seedassisted low temperature (95℃) hydrothermal method described above . V2O5 shell layer was then deposited onto the surfaces of ZnO nanorods by using magnetron sputter at room temperature. High purity V2O5 (∼99.9%) was used as the
[Fig. 7.] (a) Scanning electron microscopy cross?section and topsurface images for as?prepared zinc oxide (ZnO) nanorods array;(b) ZnO nanorods array covered by vanadium pentoxide (V2O5)layer with deposition time of 60 minutes; (c) cross?section sampleof the annealed ZnO/V2O5 composite nanorods arrays; (d) transmissionelectron microscopy (TEM) image for the annealedZnO/V2O5 composite nanorods; (e) The high?resolution TEM (HRTEM)images for the ZnO nanorods as the stem; (f) HR?TEM imagesfor the V2O5 crystal ball as the head; (G) a schematic to showthe growth process of the “lollipop” structure during annealing.
target and Ar as the working gas. The base vacuum of the sputtering chamber was lower than 4×10?6 torr. The working pressure and the flux were controlled to be 10 mtorr and 10 sccm, respectively. After above preparation, the samples were annealed under oxygen ambience at different temperatures.
The morphologies of ZnO/V2O5 composite nanorods arrays are shown in Fig. 7.It can be observed that a V2O5 shell has been coated on the top and side surfaces of ZnO nanorods. After deposition, the initial ZnO nanorods became a torchshaped core?shell structure according to the SEM images. After an annealing at 500℃ in oxygen ambience, the ZnO/V2O5 core?shells changed their morphologies to the interesting heterogeneous nano?lollipops array, as shown in Fig. 7(c). Comparing with the microstructures of as?deposited ZnO/V2O5 core?shell samples, the annealed samples show heterogeneous nano?lollipops shape with the top heads of V2O5 crystal balls and the stems of ZnO nanorods.
Figures 7(d)?(e) shows the TEM images for the ZnO/V2O5 nanolollipop structure. The lattice image for the ZnO nanorods as the stem can be clearly observed in HR?TEM image,which indicates the growth direction of the ZnO nanorods prepared by hydrothermal method. The lattice structure of V2O5 crystal is revealed in Fig. 7(f). The lattice distance is about 0.72 nm, which is close to the (200) inter?plane distance of V2O5 single crystal.
Figure 7(g) illustrates the microstructure change from the as?deposited samples to the annealed composite samples. It is believed that this phenomenon is related to the temperature?sensitive crystallization behavior of V2O5 material which was discussed in details in one of our previous reports . The surface diffusion of V2O5 at 500℃ may also play an important role in the formation of nano?lollipops array based on ZnO/V2O5 core?shell structure. The initially covered V2O5 particles with amorphous state have diffused along the surface of ZnO nanorods, and aggregated together during the heat treatment process, forming ball?shape V2O5 nanocrystals on the top of the ZnO nanorods.
3. PHOTOCATALYTIC APPLICATIONS
It can be observed that the metallic oxides composite nanostructures including TiO2/ZnO and ZnO/V2O5 nanostructures show much enhanced PL emission as well as broadened PL spectra. Accordingly, these composite nanostructures may have potentials for photocatalysis or dye sensitized solar cell applications due to the broadened spectra, which covers the spectra band from ultraviolet to visible light band.
3.1 ZnO/TiO2 composite nano-brush for decomposition of Bromo-Pyrogallol red dye
ZnO/TiO2 composite nano?brush show enhanced photo emission for the TiO2 covered “brush?hair,” which should have good photocatalytic ability. Here the photocatalytic performance of as formed ZnO/TiO2 composite nano?brush was studied by decomposition of Bromo?Pyrogallol red (Br? PGR) dye under UV?radiation.
The decomposition tests of Br?PGR dye aqueous solution were conducted as follows: 1×1 cm of the ZnO/TiO2 composite sample was immersed into 2 ml Br?PGR solution with a concentration of 0.02 mg/L in a photoreaction chamber. A mercury tube lamp (25 W) was used as a UV irradiation source with centre wavelength at 254 nm. The lamp was located 10 cm away from the reaction chamber. Before exposure the sample to the UV light, the dye solution with catalyst slide was stirred thoroughly in the dark for 30 minutes to reach the adsorption equilibrium of the dye on the catalyst. The change of Br?PGR concentration in accordance with the irradiation time was measured using a UV?Vis spectrophotometer (Agilent 8453 UV/Visible [Vis] Spectrometer). The Br?PGR decomposition was recorded at the absorption band maximum (546 nm) in the UV?Vis spectrum. Pure TiO2 thin film and TiO2/ZnO bi?layer film were used as reference catalysts in order to study the effect of microstructure on the catalytic behaviour. They were prepared on clear glass substrates with predeposited ZnO seed layer by using the identical sputtering parameters as those of the ZnO/TiO2 composite nano?brush. The tests of photocatalytic decomposition of Br?PGR dye
solution using these reference catalysts were conducted under the same experimental conditions as did with ZnO/TiO2 composite nano?brush samples.
Figure 8(a) shows the temporal evaluation of UV?Vis spectrum changes accompanying the photo?degradation of Br?PGR using the ZnO/TiO2 composite nano?brush sample sputter deposited for 90 minutes. The Br?PGR dye initially showed a major absorption band at 546 nm, gradually decreased in absorption and slightly shifted the band to the short wavelength with increasing irradiation time under the UV light. Fig. 8(b) shows the temporal degradation rate of Br?PGR over ZnO/TiO2 composite nano?brush (90 minutes) compared with TiO2 thin film and TiO2/ZnO bi?layer film. C0 and C are the equilibrium concentration of Br?PGR before and after UV irradiation, respectively.
As can be seen in Fig. 8(b), for the ZnO/TiO2 composite nano?brush sample, the concentration of Br?PGR is reduce to 19% after UV irradiation of 210 minutes, whereas the degradation effectiveness of the TiO2 film and TiO2/ZnO bi?layer film are 68% and 64%, respectively. It is obvious that ZnO/TiO2 composite nano?brush structure have a better photocatalytic property than TiO2 thin films coated either on glass substrate or on ZnO seeds layer. In addition, the blank Br?PGR sample without any catalyst shows almost no decomposition under the same UV irradiation condition.
An important factor contributing to the enhanced photocatalytic activity of ZnO/TiO2 composite nano?brush nanostructure is that the special structure has obviously a much larger specific surface area compared to the simple layer deposited films. The large surface area comes from ZnO nanorods array and the covered nanosized TiO2 “hairs,” which can supply a very high density of active site for surface reactions as well as a high interfacial charge carrier transfer. These factors are believed greatly beneficial for the enhancement of photocatalytic performance. In addition, the coupling of TiO2 and ZnO nanostructure could induce an extension of the light wavelength range, which increases the light absorption and decrease the photocatalytic reaction energy, therefore improving the photocatalytic activity , .
Another advantage of using the ZnO/TiO2 composite nanobrush structure as catalyst is from the unique hierarchically core?brush structure, which can immobilize the TiO2 nanostructures on the surface of ZnO nanorods. Accordingly, there will be less separation and recovering issues when utilize ZnO/TiO2 composite nano?brush in water or air purification process, which are currently a major obstacle that limited a wider application of nanostructured photocatalysts. So the ZnO/TiO2 composite nano?brush structure may be used as an effective and recyclable photocatalyst for practical applications in the near future.
3.2 ZnO/V2O5 composite nano-lollipop array for 2,6-dichlorophenol degradation
Session 2.2.3 reported that ZnO/V2O5 composite core?shell nanostructures convert to the ordered ZnO/V2O5 composite nano?“lollipop” array after annealing treatment. Tests also conducted to investigate the photocatalytic properties of this heterogeneous nanostructure.
The UV?Vis absorption property of the V2O5/ZnO composite nano?lollipops array was investigated by UV?Vis spectroscopy as shown in Fig. 9(a). The absorption of pure ZnO nanowire array shows a sharp edge at ∼380 nm, which obviously correspond to its band edge absorption since the band gap of ZnO material is about 3.3 eV. For the as?prepared V2O5/ZnO composite core?shell structure, the absorption edge has a clear shift towards the long wavelength direction. While for the annealed sample with the nano?lollipops shape, the absorption edge becomes ambiguous. The average absorption for UV light weakens a little while the absorption in the range of visible light strengthens greatly, indicating the potential for the photocatalytic applications with visible light or natural sunlight.
Accordingly, we tested the photocatalytic activity of these V2O5/ZnO composite nanostructures by examining the decay reaction of 2,6?dichlorophenol (2,6?DCP) under the radiation of 420 nm light source. The initial concentration of 2,6?DCP solution was fixed at 10 mg/L with an initial pH at 7.0. The results are shown in Fig. 9(b). It can be observed that the photocatalytic activity of these V2O5/ZnO composites is quite good in terms of the degradation of 2,6?DCP. After 6 hours, more than 60% removal of 2,6?DCP was achieved while without the catalysts, less than 5% decomposition of 2,6?DCP could be observed.
It is interesting to note it appears that the annealing temperature has little effect on the photocatalytic performance of the V2O5/ZnO composite. The UV?Vis spectra in Fig. 9(a)
[Fig. 9.] (a) Ultraviolet visible spectra for pure zinc oxide (ZnO) nanowire array, as?deposited vanadium pentoxide (V2O5)/ZnO core?shell sample and the V2O5/ZnO nano?lollipops array after annealed at 500℃; (b) The concentration of 2,6?dichlorophenol solution as the function of irradiation time by using V2O5/ZnO composite nanostructures as catalyst.
show the annealed sample has a relatively high absorption over the visible light range from 500 to 800 nm. The photocatalytic light spectra centered at 420 nm, however, contain mainly photos with wavelength ranging from 400 to 500 nm (data are not shown). Thus the annealing treatment could not demonstrate much influence on the photocatalytic activity of these V2O5/ZnO composites. We believe that the annealed samples may have better performance under the visible light at the wavelength longer than 500 nm.
Furthermore, we found that the annealed composite samples are much more stable and strong than those asdeposited samples. The as?deposited V2O5 layered samples with amorphous state are easily to flake off in solution from the substrates or from the surface of ZnO nanorods. Thus the as?deposited samples are not suitable for long?term use or reuse, while the annealed V2O5/ZnO lollipop?array sample are quite strong and stable after several times tests. The next step we will test the phtocatalysis performance for the annealed heterogeneous nano?lollipops V2O5/ZnO array under visible light with longer wavelength or even with natural sunlight.
We have reviewed our recent work on ZnO, TiO2 and V2O5 related composite nanostructural materials including the preparation, characterization, microstructure and the possible photocatalytic applications for decomposition of organic chemicals. Combined with hydrothermal growth, magnetron sputtering and thermal treatment, different and complex integrated hierarchical structures at nanoscale and low dimension have been prepared. These processing techniques are relatively easy and cheap, providing a convenient route for various composite nanostructures fabrication in the future. It is interesting to see that these composite samples show much improved optoelectronic properties and superior photocatalytic performances compared to the single oxide or simple layered oxide films.
Transition metal oxides with semiconductor nature attract great attention in recent years due to their special electronic and chemical properties. These oxides can often be made as low dimensional nanostructured composites such as nanowires, nanorods and core?shell structures. These types of nanostructures frequently show much improved and/or special electronic and optical properties because of the coupling effect between the different compounds as well as the enlarged surface areas. The mechanisms of these coupling effects are far from well understood. Research in this area is of both scientific and applied significance.