Controllable Fabrication of Three-Dimensional Radial ZnO Nanowire

Dec 13, 2010 - Lei Wei , Qi-Xuan Liu , Bao Zhu , Wen-Jun Liu , Shi-Jin Ding , Hong-Liang Lu , Anquan Jiang , David Wei Zhang. Nanoscale Research Lette...
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DOI: 10.1021/cg101062e

Controllable Fabrication of Three-Dimensional Radial ZnO Nanowire/Silicon Microrod Hybrid Architectures

2011, Vol. 11 147–153

H. S. Song,† W. J. Zhang,*,† C. Cheng,‡ Y. B. Tang,† L. B. Luo,† X. Chen,†,§ C. Y. Luan,† X. M. Meng,§ J. A. Zapien,† N. Wang,‡ C. S. Lee,† I. Bello,† and S. T. Lee† †

Center of Super-Diamond and Advanced Films (COSDAF) and Department of Physics and Materials Science, City University of Hong Kong, Hong Kong SAR, China, ‡Department of Physics and the Institute of Nano Science and Technology, The Hong Kong University of Science and Technology, Hong Kong SAR, China, and §Laboratory of Optoelectronic Functional Materials and Molecular Engineering, and Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100080, People’s Republic of China Received August 13, 2010; Revised Manuscript Received November 11, 2010

ABSTRACT: A facile method by combining bottom-up and top-down approaches was developed to construct uniform threedimensional (3D) ZnO nanowires (NWs)/silicon microrod (SiMR) hybrid architectures. Patterned SiMR arrays with controlled geometry and density were structured by photolithography and chemical etching on single-crystal silicon wafers, which subsequently served as 3D scaffolds for the ZnO NW growth. In contrast to the top-down approach to fabricate SiMR scaffolds, the radial ZnO NWs grown conformally on the SiMRs follow a bottom-up method by employing a modified carbon-assisted self-catalytic growth via chemical vapor deposition. The light absorption and the photocatalytic capability of methyl red of ZnO NW arrays were demonstrated to improve significantly by the 3D constructions. The method is expected to be applicable to the synthesis of 3D hybrid structures of other nanomaterials. The heterojunction and ultralarge surface area of the 3D architectures are promising for diverse applications in photovoltaics, catalysts, and sensing. Introduction One-dimensional (1D) nanostructures have distinct chemical and physical properties, and have found applications as elementary units in electronic and optoelectronic devices.1-3 Nevertheless, difficulties in manipulating the nanostructures in a precise and controlled way for massive device fabrications have seriously limited their practical applications. In this concern, direct growth of three-dimensional (3D) architectures by integrating the 1D or two-dimensional (2D) nanomaterials may be a promising approach to overcome the problem. The synthesis and integration of 3D nanostructures enable the formation of ordered superstructures or complex function architectures by combining properties of component 1D and 2D materials.4-7 In particular, 3D structures may benefit applications in devices that require higher specific surface and porosity. For example, 3D optical fiber/ZnO nanowire (NW) hybrid structures showed an efficiency about 120% higher than the ZnO arrays applied in dye-sensitized solar cells (DSSCs).8 3D porous silicon nanowalls were shown to facilitate faster carrier transport and better intercalation kinetics of lithium ions, thus resulting in a high specific capacity at high charge-discharge currents in lithium secondary batteries.9 In sensing applications, the 3D Pt-coated Au nanoparticle arrays have demonstrated a 3-fold or more surface-enhanced Raman scattering (SERS) signal enhancement under identical sampling conditions and surface coverages.10 3D tungsten oxide nanowire networks have also revealed ultra-sensitivity and high selectivity in gas sensing.11 We report here novel 3D ZnO NWs/Si microrods (SiMRs) hybrid architectures constructed by combining bottom-up and top-down approaches in a controllable way. Metal-assisted etching12 was utilized to construct patterned SiMR array scaffolds. In comparison with other growth methods such

as chemical vapor deposition (CVD) via vapor-liquid-solid (VLS) mechanism 13 and oxide-assisted growth (OAG),14 this approach has the advantages of mild preparation conditions, low synthetic temperature, simple equipment, and time savings. The radial ZnO NW branches were grown conformably on SiMR trunks by carrying out a modified carbon-assisted CVD method, which enables the growth of ZnO NWs on selected areas with high controllability and uniformity15 in contrast to the hydrothermal method,16 vapor-solid method,17,18 and conventional carbon-assisted growth method (ZnO and graphite powder mixture as reaction source) for ZnO NW synthesis.19 Silicon and ZnO are two important semiconductor materials with distinct properties and promising industrial applications. Silicon-based nanostructures have been demonstrated to be high potential materials in new nanoelectronics and energy conversion, and sensing devices, such as Si NW-based field effect transistors (FETs),20,21 complementary logic gates,22 nanoelectromechanical systems (NEMS),23,24 biochemical sensors,25,26 and p-n junction and dye-sensitized solar cells (DSSC).27-29 On the other hand, ZnO nanostructures also have exciting application potentials in optoelectronic devices,30 sensors,31 highly efficient photonic devices,32 near-UV lasers,33 nanogenerators,34 and electrochromic displays,35 etc. The 3D hybrid architectures consisting of ZnO NWs and SiMRs as reported in this work could take advantages of the inherent properties of both ZnO and silicon with enlarged surface areas and the formation of ZnO/Si heterojunctions, which may lead to novel applications in nanoelectronics, optoelectronics, photonics, catalysis, and sensing. Experimental Section

*Author to whom correspondence should be addressed. E-mail: apwjzh@ cityu.edu.hk.

Preparation of SiMR Arrays. n- and p-type Si (100) wafers (1.5  1.5 cm2 ) with resistivity in the range of 1-10 Ω 3 cm were used as the starting materials. The wafers were ultrasonically degreased in acetone and ethanol at room temperature, cleaned

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Scheme 1. Schematic Diagram of the Fabrication Processes of SiMR Arraysa

prototype experiment, samples (0.25 cm 2, the size maintained the same for all tests) were immersed into 25 mL of MR solution (10-4 mol/L) under the irradiation of a UV lamp (MAXIMA ML3500S/F, 365 nm, 4.0 mW/cm2). To ensure that the reduction of MR concentration was due to the photocatalytic degradation rather than the adsorption by ZnO NWs, all samples for photocatalytic testing were first immersed in solution in the dark for certain duration, and the concentration of MR in the solution was monitored until the adsorption of dye was saturated. Then the UV irradiation was induced to study the photocatalytic capabilities of the samples. A 5 mL solution was withdrawn from the cell and tested by UV-vis spectroscopy at a 20 min interval. The degradation efficiency of MR was evaluated by η = ([MR])/([MR0])  100%, where [MR0] and [MR] correspond to the equilibrium concentrations of MR before and after the photocatalytic reaction. Because the MR concentration was in a linear proportion to the absorbance (A), [MR]/[MR0] was directly derived from (A)/(A0), where (A0) and (A) correspond to the absorbance before and after the photocatalytic reaction.

a (a) Si substrates coated uniformly with a photoresist (PR) layer by spin coating. (b) The PR layer was patterned by the standard photolithography procedure. (c) A patterned metal layer was formed by sputtering and the lift-off procedure of PR. (d) Uniform Si rod arrays were fabricated via metal-assisted etching in H2O2 and HF solutions. (e) The residual metal film was completely dissolved.

Results and Discussion

in a boiled solution of H2 SO4 /H2 O 2 (5:2) and RCA solution of NH4 OH/H2 O2 /H 2O (1:1:5) for 1 h each sequentially, and then immersed in diluted hydrofluoric acid (5%) solution for 1 min to remove the thin surface oxide layer. The fabrication process of the SiMR arrays is shown schematically in Scheme 1. The Si wafers were uniformly coated with a thin layer of positive photoresist (PR, SPR 6112-B) by spinning coating at a spinning speed of 3000 rpm for 90 s and treated by hot-baking at 90 °C for 15 min (Scheme 1a). The hardened film substrate was exposed for 10 s with a contact aligner (Karl Suss MJB-3). The pattern can be obtained by development for 50 s with a develop solution of AZ300. Further cleaning was carried out to remove the residual solvent with excess DI water (Scheme 1b). Thin Ti/Ag or Ti/Au films were sputtered onto the patterned wafer pieces. The metal patterned wafer pieces were obtained after the standard lift-off procedure (Scheme 1c). The metal patterned silicon pieces were immediately immersed into HF-H2 O 2 solution in sealed vessels and treated for the designed time. The solution concentrations of HF and H 2O 2 were chosen to be 5 and 0.02 mol/L, respectively. The silicon under the metal pattern was etched away, and a large-area vertically aligned SiMR array resulted on silicon substrate (Scheme 1d). SiMR arrays were cleaned in boiled aqua regia (a 3:1 mixture of HCl and HNO 3) for at least 1 h to remove all metal from the MW arrays (Scheme 1e). Growth of ZnO NWs on SiMR Arrays. Wafer pieces of SiMR arrays were coated with a thin layer of PR (AZ5206E) by spin coating at a speed of 8000 rpm for 60 s and then treated by hard-baking at 100 °C for 10 min. The growth of ZnO NW arrays on SiMR arrays was carried out in a traditional three-temperature zone CVD system: An alumina boat containing 3 g of ZnO powder was placed in the center of a tube furnace. SiMR arrays with PR coating were placed downstream for the growth of ZnO NWs. Argon was used as carrier gas at a flow rate of 50 sccm with additional 0.5 sccm oxygen flow to facilitate the reaction. The furnace was heated to 1100-1300 °C and kept for half an hour under vacuum conditions (10-2 Torr). The substrate temperature was about 600-800 °C. The morphologies of the prepared samples were characterized with scanning electron microscopy (SEM, Philips XL 30 FEG). Single NW was further analyzed by high-resolution transmission electron microscopy (HRTEM, Philips CM200 FEG operated at 200 kV). The UV-vis absorption spectra were obtained by an Aglient 8453 UV-vis diode array spectrophotometer. Photocatalytic Characterizations. Photocatalytic activities of the ZnO NW arrays grown on planar Si substrate (denoted as ZnO NWs@flat-Si) and 3D ZnO NW/SiMR arrays were investigated by studying the photocatalytic degradation of methyl red (MR) in aqueous solution. The evaluation experiments were carried out in a quartz photoelectrochemical cell at room temperature. In a

Scheme 1 shows schematically the fabrication processes of the SiMR arrays, which is described in detail in the experimental section. Well-aligned SiMRs were constructed uniformly over large areas by carrying out the metal-assisted etching method. The geometry and density of SiMR arrays could be controlled individually by varying the pattern and etching parameters. Figure 1a,b shows the top-view SEM images of SiMRs of 3 and 2 μm in diameter, respectively, and insets are the enlarged images. The spacing between adjacent rods was kept at 2 μm in both of the samples. In addition, the rod height could be well controlled by varying the etching durations. For examples, Figure 1c-f shows the SiMR arrays etched for 5, 20, 40, and 90 min, respectively, while the other etching parameters were kept constant. The heights of the microrods were measured to be about 2.5, 7.5, 10, and 20 μm, respectively. Ordered SiMR arrays etched from Si wafers greatly magnify their surface area and enhance light trapping.11,36 The rods were employed as a scaffold for the sequential growth of NW overlayers to construct a tree structure. Figure 2a-d depicts the top- and tilted-view SEM images of the 3D ZnO NW/SiMR structures grown by the modified carbon-assisted CVD with the source and substrate temperature at 1100 and 600 °C, respectively (denoted as Sample S). The diameter of the SiMRs and the rod-rod spacing were both 2 μm. It is revealed that highly ordered urchin-like groups of NWs grew uniformly on SiMR surfaces as shown in Figure 2a. The enlarged image in Figure 2b indicates that the average length of the NWs is ∼600 nm. Tilted view in Figure 2c,d depicts that the forest like morphology is made up of ordered 3D ZnO NW/SiMR trees. The faceted cross sections of ZnO NWs indicate the single crystal nature of the NWs. It is noted that the ZnO NWs grew also on the bottom flat silicon substrate, like a grass field in the forest, as shown in Figure 2c. The growth of ZnO NWs on SiMRs could increase the surface area of ZnO NWs (surface area of ZnO NWs per projected unit area of Si substrate) with respect to that grown on flat substrate. While the source and substrate temperatures were increased to 1200 and 700 °C, respectively (Sample M), ZnO NW branches were also grown uniformly on SiMR surfaces. The length of the ZnO NWs increased and the diameter decreased (Figure 2e-h). It is interesting to note that the NW branches grow in a hierarchical structure, that is, the ZnO NWs grown on one side of SiMRs are obviously longer than those grown on the opposite side of rods (∼1.7 μm vs ∼500 nm in length, marked by green and white arrows, respectively), as shown in

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Figure 1. SiMR arrays with different rod geometries and densities. (a) Rods of 3 μm in diameter, and rod-rod spacing 2 μm. Inset is the enlarged image. (b) Rods of 2 μm in diameter, rod-rod spacing 2 μm. Inset is the enlarged image. (c-f ) The rod heights of 2.5, 7.5, 10, and 20 μm were obtained by etching for 5, 20, 40, and 90 min, respectively.

Figure 2e,f. The enlarged cross-sectional image in Figure 2h also verifies the length difference of ZnO NWs grown on the different sides of SiMRs. The direction-dependent growth of ZnO NWs was considered to be due to the shading effect of SiMRs in the CVD growth of ZnO NWs. The sides of rods facing the source direction may absorb more source vapor and grow faster. As the source and substrate temperatures were further increased to 1300 and 800 °C, respectively, long ZnO NWs of about 6-10 μm in length covered the whole sample surface, and the outlines of ZnO NW/SiMR trees are difficult to identify (Sample L), as shown in Figure 2i,j. The white dashed circles in Figure 2j imply the centers of the long ZnO NWs. Cross-sectional image in Figure 2k reveals that high-density ZnO NWs grow radially on the walls of SiMR trunks, and the NWs at the top of SiMRs are much longer than those grown on the bottom part of SiMRs. The total height of the ZnO NW/SiMR trees is about 20 μm. Close observation of the SEM morphologies of the sample also indicates that the diameter of NWs shrinks along the wire axis direction from ∼200 nm at root to ∼50 nm at the tip. The as-synthesized 3D ZnO NW/SiMR architectures are gray in color. Figure 3a shows the absorption spectra of the above-mentioned samples. ZnO NWs@flat-Si (Figure 5d) was also studied as a reference. Two absorption peaks around 380 and 1020 nm were observed for all samples, which are attributed to the band edge absorption of ZnO and silicon, respectively. It is evident that relative to the ZnO NW array on

flat silicon substrate, the ZnO NW/SiMR 3D hybrid structures have significantly increased absorption in the wide range from UV to infrared. In particular, a nearly flat absorption band of ∼40% was achieved for sample M (Figure 2e-h) in the range from 400 to 1000 nm. The enhanced light absorption in the visible-IR range is a great advantage of the 3D hybrid structures in photocatalytic, optoelectronic, and photovoltaic applications. The crystalline structure of ZnO NWs was also studied by HRTEM. The HRTEM image (Figure 3b) of the ZnO NW from sample L and the corresponding electron diffraction (ED) pattern (not shown here) confirm the wurtzite structure and single-crystal nature of the ZnO NWs. The ZnO NWs grow along the [0001] direction, and the interplanar spacing (0.52 nm) marked in the figure agrees well with the reported value for (0002) planes of wurtzite ZnO. The elemental composition of the ZnO NW tips by energy dispersive spectroscopy shows only Zn to O at an atomic ratio of ∼6:5. Brunauer-Emmett-Teller (BET) gas-sorption method was utilized to measure the surface area of the samples. It was indeed revealed that the surface area increased to 580, 990, and 2500 mm2 for samples S, M, and L, respectively, for a constant Si substrate size of 5  5 mm2. The increased surface area of ZnO NWs is expected to enhance their applications such as optoelectronic devices and photocatalysts. In this work, we investigated the photocatalytic capability of the 3D ZnO NW/SiMR structures by studying the photocatalytic degradation of a typical dye, MR. The control experiments

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Figure 2. Top and tilted views of 3D ZnO/SiMR hybrid structures: (a-d) SEM images of Sample S from different views and of different magnifications, (e-h) SEM images of Sample M from different views and of different magnifications, (i-l) SEM images of Sample L from different views and of different magnifications.

Figure 3. (a) Absorption spectra of ZnO NWs@flat-Si (Figure 5d), Sample S, Sample M, and Sample L. (b) HRTEM image of single ZnO NW from the Si rod base.

were also conducted with ZnO NWs@flat-Si of the same size. The concentration of MR aqueous solution was characterized by the maximum absorption peak at about 515 nm (Amax) by

the UV-vis spectroscopy. The photocatalytic process was demonstrated by the variation of Amax. Figure 4a shows the evolution of degradation processes with and without the assistance of ZnO NW catalysts. It was revealed that the degradation of MR was very slow under the UV irradiation without any catalyst; only 5% of MR was degraded after 2 h reaction. The introduction of ZnO NWs@flat-Si arrays could improve the degradation efficiency to about 40% for the same reaction time. However, the 3D ZnO NW/SiMR architectures (Sample S, M, L) demonstrated significantly improved degradation abilities with respect to that of ZnO NWs@flat-Si. Among them, sample M showed the best capability and decomposed about 90% of MR after 2 h. Figure 5b shows the variation of absorption peak of MR solution after catalytic degradation with the assistance of sample M for different times. The observations agree with the fact that the increase of the effective surface areas of ZnO NWs improves the photocatalytic capabilities.37 However, sample L showed a reduced catalytic efficiency as compared sample M, which may be due to the shading of the bottom NWs by the dense NWs at sample surface and thus the decrease of effective surface area. In addition to the geometry effects, the existence of oxygen defect-related vacancies also enhances the photocatalytic abilities of ZnO NWs. It has been demonstrated that the oxygen vacancies can serve as the electron capture centers to restrain the recombination of the excited electron-hole pairs, and therefore promote the photoxidation by hole-generated free hydroxyl radicals.38 The mechanism of the modified carbon-assisted selfcatalytic growth of ZnO NWs is illustrated schematically in Scheme 2. In this approach, the SiMR surfaces were uniformly

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coated with a PR layer of about a few hundred nanometers thick (Scheme 2a). In contrast to the conventional carbonassisted growth of ZnO NWs, the carbonaceous species originated from the decomposition of PR layer in the present method instead of the graphite mixed in source.39 Raman measurements (spectra not shown here) verified that the PR layer was converted completely to amorphous carbon after heating at 600 °C. As discussed below, an obvious advantage of the present method is that it enables conformal coating of PR and uniform nucleation of ZnO NWs over SiMR trunks.

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In the synthesis of ZnO NWs, ZnO vapor was carried by Ar to the deposition zone which was set at a temperature of 600800 °C, and the ZnO vapor species could easily react with carbon due to the immiscibility of the ZnO-carbon system.40 Zinc and sub zinc oxide (ZnOx, where x < 1) were extracted to form Zn and ZnOx film by the following reactions:39,41 2ZnOðgÞ þ CðsÞ f 2ZnðlÞ þ CO2 ðgÞ or ZnOðgÞ þ CðsÞ f ZnðlÞ þ COðgÞ

ð1Þ

ZnOðgÞ þ ð1 - xÞCðsÞ f ZnOxðlÞ þ ð1 - xÞCO or 2ZnOðgÞ þ ð1 - xÞCðsÞ f 2ZnOx ðlÞ þ ð1 - xÞCO2 ð2Þ The crucial role of carbonized PR is the adsorption of ZnO vapor leading to the production of zinc and ZnOx (Scheme 2c), which in turn serve as the nucleation catalysts for ZnO NW growth.42,43 In the present modified carbon-assisted growth of ZnO by utilizing PR coatings, ZnO vapor was reduced to Scheme 2. Schematic Diagram of the Fabrication Processes of the 3D Radial ZnO/SiMR Arraysa

Figure 4. (a) Photodegradation of MR with the assistance of ZnO NW@flat-Si, Sample S, Sample M, and Sample L, as compared to the photodegradation of MR without any catalyst. (b) UV-vis spectra of MR solution (normalized to the initial concentration) under the photocatalysis of Sample M.

a (a) Bare SiMR array, (b) SiMR rod arrays coated with PR layer, (c) carbonized PR reacts with ZnO vapor and transforms to Zn and subzinc oxide nucleation film, (d) the nucleation film develops to 3D radial ZnO NW/SiMR arrays.

Figure 5. Comparative experimental results on ZnO NW growth without the assistance of PR coating: (a, b) on bare Si rod arrays, (c, d) on ZnO precoated Si rod arrays. ZnO and graphite mixtures were used as evaporation source.

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Zn and ZnOx by carbon in the PR layer on SiMRs; Zn and ZnOx liquid droplets were oxidized to ZnO; and adsorption and diffusion of ZnO were enhanced at the liquidized tips. Eventually, supersaturation of ZnO in liquid tips would result in the segregation of ZnO and NW growth (Scheme 2d). Energy dispersive spectroscopy (EDS) measurements (spectra not shown here) show no Zn particle at the tip, indicating the consumption of Zn and ZnOx during ZnO NW growth. Varying the source and substrate temperatures can manipulate the ZnO vapor pressure, reaction rate, and supersaturation of ZnO, and therefore lead to the variation of nucleation density, growth rate, and dimension of ZnO NWs. Moreover, the thickness of PR layer was in the range from 100 to 200 nm in our experiments, and no obvious influence of the PR layer thickness on the growth of ZnO NWs was observed. Control experiments were carried out to verify the growth mechanism proposed for modified carbon-assisted growth of ZnO NWs, in which ZnO NWs were grown on bare SiMR (without PR coating) array substrates by using ZnO and graphite powder mixture as source (the same as the conventional carbon-assisted growth).17 Other conditions were maintained the same as those for the samples in Figure 2. It was revealed that ZnO NWs could grow only on the bottom substrate surfaces between SiMRs, as shown in Figure 5a,b. No ZnO NW was observed on SiMRs, in sharp contrast to the 3D ZnO NW/SiMR tree structures formed by employing the PR precoating. In this work, we also studied the growth of ZnO NWs on bare SiMR array substrates using only ZnO powder as the source. It was revealed that only sparse NWs were synthesized (SEM morphology not shown here), which further verified the role of PR in the growth of ZnO NWs. Moreover, in an alternative approach a thin ZnO film was deposited by sputtering on SiMR arrays in place of the PR film. It has been demonstrated that the predeposition of ZnO film could serve as the nucleation enhancement layer for the growth of ZnO NWs on flat substrates.39 Nevertheless, in our case only nanocrystalline ZnO film was obtained on the SiMR surfaces, as shown in Figure 5c. ZnO NWs could grow uniformly on the flat Si substrate at bottom (Figure 5d), in agreement with the previous results in the literature.39 With the assistance of PR coating, ZnO could grow on various substrate surfaces, for example, vertical trunk surfaces with a height over 20 μm, and the small-radius circle surfaces (diameter ∼2 μm). This capability can facilitate the growth of complex hybrid structures. Conclusion 3D ZnO NW/SiMR hybrid structures with ultrahigh surface areas have been synthesized by combining the top-down and bottom-up methods; that is, SiMR arrays with tailored geometry and density were obtained by patterned metal-assisted etching, and ZnO NWs were grown by modified carbonassisted CVD on PR-precoated SiMRs. The 3D ZnO NW/ SiMR structures show a forest-like morphology, in which SiMRs serve as the trunks for supporting the radial ZnO NW branches. The process enables the versatile control of the dimensions and morphologies of SiMR arrays as well as ZnO NWs. The photocatalytic experiments reveal that the 3D ZnO NW/SiMR architectures can enhance drastically the photocatalytic efficiency to degrade MR. Optimized 3D structure decomposes about 90% of MR for 2 h UV irradiation, which doubles that of ZnO NWs grown on planar substrate. The modified carbon-assisted self-catalytic growth of ZnO NWs,

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superior to the conventional carbon-assisted CVD method, enables the synthesis of high-density, uniform ZnO NW arrays on curved substrate surfaces like SiMRs, and is applicable to the synthesis of other metal oxide nanomaterials such as TiO2, CuO, SnO2, In2O3, etc. The controllable morphology, significantly enlarged surface area, and the enhanced light absorption of the 3D hybrid architectures make them promising in various applications such as p-n junction solar cells, dye-sensitized solar cells, optoelectronic devices, photodetectors, and bio- and chemical sensors. Acknowledgment. The work was supported by the Strategic Research Grant of the City University of Hong Kong (Project No. 7002489), Research Grants Council of Hong Kong SAR, China - CRF Grant (No. CityU5/CRF/08) and GRF Grant (No. CityU110209).

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