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Design, Fabrication, and Modification of Nanostructured Semiconductor Materials for Environmental and Energy Applications Xianluo Hu, Guisheng Li, and Jimmy C. Yu* Department of Chemistry, Environmental Science Programme and Centre of Novel Functional Molecules, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong, China Received June 15, 2009. Revised Manuscript Received August 12, 2009 Considerable effort has been made to design, fabricate, and manipulate nanostructured materials by innovative approaches. The precise control of nanoscale structures will pave the way not only for elucidating unique size/shapedependent physicochemical properties but also for realizing new applications in science and technology. Nanotechnology offers unprecedented opportunities for improving our daily lives and the environment in which we live. This review mainly describes our recent progress in the design, fabrication, and modification of nanostructured semiconductor materials for environmental applications. Their potential applications in the field of energy are briefly introduced. The scope of this article covers a variety of semiconductor materials, focusing particularly on TiO2based nanostructures (e.g., pure, doped, coupled, nanoporous, mesoporous, hierarchically porous, and ordered mesoporous TiO2). The preparation of nanoparticles, hierarchical nanoarchitectures, thin films, and single crystals by sol-gel, microemulsion, hydrothermal, sonochemical, microwave, photochemical, and nanocasting methods is discussed.

Introduction The environment and energy are the biggest challenges of the 21st century. Ironically, the solution to these large problems may lie in something very small. Nanomaterials, with attractive chemical and physical properties, are being explored for potential uses in energy and environmental applications. During the past decade, rapid advances in materials science have led to significant progress in environmental remediation and renewable energy technologies such as photocatalytic oxidation, adsorption/ separation processing, solar cells, fuel cells, and biofuels. The design and creation of new materials and substances chemically modified from the molecular and atomic levels to sizes on the nanoscale promise significantly enhanced functions for environmental and energy applications. Meanwhile, the development of advanced characterization techniques has facilitated a fundamental molecular-level understanding of structure-performance relationships, which are strongly related to grain size and size distribution, shape, chemical composition, presence of interfaces (grain boundaries and free surfaces), and interactions between the constituent domains. This knowledge, together with effective synthesis strategies, has inspired the design and fabrication of novel nanostructured materials for a wide variety of applications. Recently, a number of excellent reviews and reports on the preparation, modification, assembly, characterization, properties, engineering, and applications of nanostructured materials have been published.1 This feature article reviews our recent progress in the design, fabrication, and modification of semiconductor nanostructured materials. It also highlights their environmental and energy applications, including photocatalytic treatment, environmental monitoring, water splitting, and hydrogen storage. The synthesis of nanostructured materials is a very active research field.2 The *Corresponding author. E-mail: [email protected]. (1) Burda, C.; Chen, X. B.; Narayanan, R.; El-Sayed, M. A. Chem. Rev. 2005, 105, 1025. (2) Yin, Y.; Alivisatos, A. P. Nature 2005, 437, 664.

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ability to fabricate and process nanostructured materials lies at the heart of nanotechnology, paving the way for understanding novel properties and realizing their potential applications. To date, many technologies have been explored to synthesize nanostructured materials. These technical approaches can be essentially grouped in two paradigms: top-down and bottom-up.3 In particular, versatile bottom-up methods based on chemistry have attracted considerable attention because of their relatively low cost and high throughput.4 Bottom-up approaches refer to the buildup of a material from the bottom: atom-by-atom, moleculeby-molecule, or cluster-by-cluster. Growth species such as atoms, ions, and molecules, after impinging on the growth surface, assemble into crystal structures one after another. In recent years, a number of techniques, including coprecipitation, sol-gel processes, microemulsions, freeze drying, hydrothermal processes, laser pyrolysis, ultrasound and microwave irradiation, templates, and chemical vapor deposition, have been developed to control the size, morphology, and uniformity of nanostructures simultaneously.5,6 The successful implementation of the bottom-up strategy requires, in the end, the controlled growth of nanostructures. Among various media for crystal growth, the solutionbased method offers significant advantages, including (i) low reaction temperatures, (ii) size-selective growth, (iii) morphological control, and (iv) large-scale production. The liquid-phase approach to the synthesis of inorganic nanostructures has been recently reviewed.7,8 This approach is also the core idea that has guided the work presented in this review article. (3) Xu, Q. B.; Rioux, R. M.; Dickey, M. D.; Whitesides, G. M. Acc. Chem. Res. 2008, 41, 1566. (4) Xia, Y. N.; Rogers, J. A.; Paul, K. E.; Whitesides, G. M. Chem. Rev. 1999, 99, 1823. (5) Xia, Y. N.; Yang, P. D.; Sun, Y. G.; Wu, Y. Y.; Mayers, B.; Gates, B.; Yin, Y. D.; Kim, F.; Yan, Y. Q. Adv. Mater. 2003, 15, 353. (6) Fernandez-Garcia, M.; Martinez-Arias, A.; Hanson, J. C.; Rodriguez, J. A. Chem. Rev. 2004, 104, 4063. (7) Cushing, B. L.; Kolesnichenko, V. L.; O’Connor, C. J. Chem. Rev. 2004, 104, 3893. (8) Kwon, S. G.; Hyeon, T. Acc. Chem. Res. 2008, 41, 1696.

Published on Web 09/09/2009

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Environmental Applications In 1972, Fujishima and Honda achieved ultraviolet (UV) lightinduced water cleavage using a titanium dioxide photoanode in combination with a platinum counter electrode soaked in an electrolyte aqueous solution.9 Since then, semiconductor photocatalysis has attracted considerable attention because of its promising applications in environmental purification as well as solar energy conversion. Semiconductors (e.g., TiO2, ZnO, Fe2O3, WO3, and CdS) can act as photocatalysts for light-induced chemical transformations because of their unique electronic structure, which is characterized by a filled valence band and an empty conduction band. When a photon with an energy of hv matches or exceeds the band gap energy (Eg) of the semiconductor, an electron in the valence band (VB) is excited into the conduction band (CB), leaving a positive hole (hþ) in VB. The asphotogenerated CB electrons and VB holes in the excited states can recombine and dissipate the input energy as heat, become trapped in metastable surface states, or react with electron donors and electron acceptors adsorbed on the semiconductor surface. During the past decade, research on nanostructured semiconductors as the new building blocks to construct light-energy-harvesting assemblies has grown rapidly and has drawn from a number of scientific disciplines.10 Of the various semiconductors tested to date, TiO2 is the most promising photocatalyst because of its appropriate electronic band structure, photostability, chemical inertness, and commercial availability. A variety of morphologies of nanostructured TiO2 including nanoparticles, nanorods, nanowires, nanostructured films or coatings, nanotubes, and mesoporous/nanoporous structures have been reported, and many TiO2-based composites have also been prepared. Significant progress has been made in a variety of areas ranging from photovoltaics and photocatalysis to photo/electrochromics and sensors. The effective utilization of clean, safe, and abundant solar energy by the TiO2 photocatalyst will lead to promising solutions not only for the energy crisis but also for serious environmental challenges. Reports on the preparation, properties, and applications of nanostructured TiO2-based materials appear frequently in scientific journals.11 In most cases, nanostructured TiO2 materials can be prepared either by dry or wet processes. The reaction conditions (e.g., reactant concentration, reaction medium, temperature, and pH of solution) can be optimized easily in a wet process. For example, the sol-gel processing in combination with template synthesis and hydrothermal/solvothermal treatment is a popular method. Other preparation methods, such as micelles and inverse micelles, sonochemical, microwave, direction oxidation, electrodeposition, chemical vapor deposition (CVD), and physical vapor deposition, have been described in a recent review by Chen and Mao.11 TiO2 occurs in nature in three different polymorphs, namely, rutile, anatase, and brookite (in order of abundance). Additional synthetic phases are called TiO2(B), TiO2(H), and TiO2(R), and several high-pressure polymorphs have also been reported. As shown in Figure 1, TiO2 in the anatase crystal form has a band gap of 3.2 eV. Under light illumination, the photogenerated electrons and holes can initiate redox reactions with chemical species adsorbed on the surface or interface of the photocatalyst. A general mechanism for heterogeneous photocatalysis on TiO2 on the basis of electron paramagnetic resonance (ESR) and laser flash photolysis measurements has been proposed by Hoffmann (9) Fujishima, A.; Honda, K. Nature 1972, 238, 37. (10) Robel, I.; Bunker, B. A.; Kamat, P. V. Adv. Mater. 2005, 17, 2458. (11) Chen, X.; Mao, S. S. Chem. Rev. 2007, 107, 2891.

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Figure 1. Photoexcitation of a semiconductor (e.g., TiO2) and the subsequent generation of an electron and hole, which are trapped by an oxidant (Ox) and a reductant (Red), respectively. For TiO2 photocatalysis, the “Ox” is a surface-adsorbed oxygen molecule, and the “Red” is a surface-bound hydroxyl group.

and co-workers.12,13 This mechanism includes four primary processes, namely, charge-carrier generation, charge-carrier trapping, charge-carrier recombination, and interfacial charge transfer. The photocatalytic processes using TiO2 or other semiconductors have demonstrated that the limitations in achieving higher photoconversion efficiencies must be overcome. For photocatalysis to be sustainable, the recombination of electron-hole pairs must be inhabited. This can be accomplished either by trapping the photogenerated electrons or the holes or both. Considerable effort has also been expended to improve interfacial charge-transfer efficiency to enhance photocatalytic activity. Improved charge separation and inhibition of charge carrier recombination are essential in improving the overall quantum efficiency for interfacial charge transfer. Several approaches have been developed to achieve this goal, mostly involving the optimization of particle size/structure and surface area,14 surface modification with redox couples or noble metals,15 and coupling two semiconductors with different electronic energy levels.16,17 Dye sensitization18 and doping to enhance the photoresponse of TiO2 have been extensively studied.19 Non-TiO2 photocatalysts, such as In1 - xNixTaO4 (x = 0-0.2),20 Sm2Ti2S2O5,21 InNbO4,22 CaBi2O4,23 and Ag/AgCl,24 that can respond in the visible-light region have also been widely developed in recent years. Nanoparticles. As the size of a semiconductor particle is reduced, both the fraction of atoms located at the surface and the surface area to volume ratio are increased. This may enhance the available surface-active sites and interfacial charge-carrier transfer rates, thus leading to higher catalytic activities over the (12) Martin, S. T.; Herrmann, H.; Choi, W.; Hoffmann, M. R. J. Chem. Soc., Faraday Trans. 1994, 90, 3315. (13) Martin, S. T.; Herrmann, H.; Hoffmann, M. R. J. Chem. Soc., Faraday Trans. 1994, 90, 3323. (14) Chhabra, V.; Pillai, V.; Mishra, B. K.; Morrone, A.; Shah, D. O. Langmuir 1995, 11, 3307. (15) Subramanian, V.; Wolf, E. E.; Kamat, P. V. J. Am. Chem. Soc. 2004, 126, 4943. (16) Vinodgopal, K.; Bedja, I.; Kamat, P. V. Chem. Mater. 1996, 8, 2180. (17) Shiragami, T.; Fukami, S.; Wada, Y.; Yanaida, S. J. Phys. Chem. 1993, 97, 12882. (18) Hong, A. P.; Bahnmann, D. W.; Hoffmann, M. R. J. Phys. Chem. 1987, 91, 2109. (19) Asahi, R.; Morikawa, T.; Ohwki, T.; Aoki, K.; Taga, Y. Science 2001, 293, 269. (20) Zou, Z. G.; Ye, J. H.; Sayama, K.; Arakawa, H. Nature 2001, 414, 625. (21) Ishikawa, A.; Takata, T.; Kondo, J. N.; Hara, M.; Kobayashi, H.; Domen, K. J. Am. Chem. Soc. 2002, 124, 13547. (22) Zhang, L. Z.; Djerdj, I.; Cao, M. H.; Antonietti, M.; Niederberger, M. Adv. Mater. 2007, 19, 2083. (23) Tang, J. W.; Zou, Z. G.; Ye, J. H. Angew. Chem., Int. Ed. 2004, 43, 4463. (24) Wang, P.; Huang, B. B.; Qin, X. Y.; Zhang, X. Y.; Dai, Y.; Wei, J. Y.; Whangbo, M. H. Angew. Chem., Int. Ed. 2008, 47, 7931.

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corresponding bulk.14 However, research findings in some cases suggest a contrary trend.25 This can be explained by the increase in the number of surface defects and electron-hole recombination rates. For a specific photoinduced reaction system, it is desirable to optimize the particle size as well as select a suitable synthesis method. An ultrasound-assisted method was developed to prepare highly photoactive nanosized TiO2 photocatalysts consisting of anatase and brookite phases by the hydrolysis of titanium tetraisopropoxide in pure water or a 1:1 EtOH-H2O solution.26 The photocatalytic activity of TiO2 particles prepared by this method exceeded that of commercial Degussa P25. Transition-metal cation doping is a commonly used strategy to improve the efficiency of TiO2 photocatalysis. The doping of metal cations is believed to change the photochemical behavior of electron-hole pairs of TiO2 and thus to affect its photocatalytic activities. Karakitsou and Verykios27 reported that doping TiO2 with cations of valence higher than that of the parent cation (Ti4þ) enhanced the rate of H2 production. Also, the photocatalytic efficiency of higher-valence-doped TiO2 went through a maximum with increasing dopant concentration in the TiO2 matrix. Hoffmann et al.28 systematically investigated metal ion doping in quantum-sized (2-4 nm) TiO2 colloids by measuring their photoreactivities and the transient charge carrier recombination dynamics. It was found that the activity of doped TiO2 appeared to be a complex function of dopant concentration, energy level of dopants within the TiO2 lattice, d electronic configuration, distribution of dopants, electron donor concentration, and light intensity. Importantly, transition-metal doping and nonmetal (e.g., C, N, S, F, B, and I) doping may extend the response of TiO2 into the visible-light region.19,29-32 We discovered a simple method of preparing a highly photoactive nanocrystalline F-doped TiO2 photocatalyst with anatase and brookite phases by the hydrolysis of titanium tetraisopropoxide (TTIP) in a mixed NH4F-H2O solution.33 In a typical procedure, TTIP was added dropwise to an aqueous NH4F solution (RF = F/Ti, RF ranges from 0 to 0.2) under vigorous stirring at room temperature. The sol obtained by the hydrolysis process was aged in a closed beaker at room temperature for 24 h and then dried at 100 °C for about 8 h in air to vaporize water and alcohol in the gel. After the product was ground, the fine powder was calcined at 400, 500, 600, or 700 °C. We studied the effects of both RF and calcination temperature on the phase structure of F-doped TiO2 nanoparticles (Figure 2). We found that the crystallinity of anatase was improved upon F doping. With increasing RF, not only was the formation of the brookite phase suppressed but also the anatase to rutile phase transition was prevented. The crystal size and phase were changed by calcination at different temperatures. Importantly, we found that F-doped TiO2 calcined at 500 °C with RF in the range of 0.5-3 had a higher photocatalytic activity upon the oxidation of acetone to CO2 than did commercial Degussa P25. The coupling of different semiconductor systems may result in improved photocatalytic activity.16,17 Figure 3 shows the scheme for the charge-transfer processes involved in coupled (25) Lepore, G. P.; Langford, C. H.; Vichova, J.; Vlcek, A. J. Photochem. Photobiol., A 1993, 75, 67. (26) Yu, J. C.; Yu, J. G.; Ho, W. K.; Zhang, L. Z. Chem. Commun. 2001, 1942. (27) Karakitsou, K. E.; Verykios, X. E. J. Phys. Chem. 1993, 97, 1184. (28) Choi, W.; Termin, A.; Hoffmann, M. R. J. Phys. Chem. 1994, 98, 13669. (29) Irie, H.; Watanabe, Y.; Hashimoto, K. J. Phys. Chem. B 2003, 107, 5483. (30) Liu, G.; Zhao, Y. N.; Sun, C. H.; Li, F.; Lu, G. Q.; Cheng, H. M. Angew. Chem., Int. Ed. 2008, 47, 4516. (31) Mitoraj, D.; Kisch, H. Angew. Chem., Int. Ed. 2008, 47, 9975. (32) Livraghi, S.; Paganini, M. C.; Giamello, E.; Selloni, A.; Di Valentin, C.; Pacchioni, G. J. Am. Chem. Soc. 2006, 128, 15666. (33) Yu, J. C.; Yu, J. G.; Ho, W. K.; Jiang, Z. T.; Zhang, L. Z. Chem. Mater. 2002, 14, 3808.

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Figure 2. (A) XRD patterns of the TiO2 xerogel powders prepared from the H2O-NH4F mixed solution with RF = 0 (pure water) (a), 1 (b), 5 (c), 10 (d), and 20 (e) and dried at 100 °C for 8 h. (B) XRD patterns of TiO2 powders prepared from the H2O-NH4F mixed solution with RF = 1 and calcined at (a) 100, (b) 400, (c) 500, (d) 600, and (e) 700 °C for 1 h.33

semiconductor systems. The electrons photoinduced on the conduction band of a higher level can be injected into the lower conduction band of the second semiconductor. As a result, more efficient charge-carrier separation can be achieved. To this end, we have synthesized CdS, CdSe, MoS2, and WS2-sensitized TiO2 nanoparticles by ultrasound-driven, in situ photoreduction deposition and microemulsion-mediated solvothermal methods.34-36 Quantum-sized photosensitizers such as CdS not only extended the spectral response of TiO2 into the visible region but also allowed interparticle electron transfer. The Ti3þ signal observed by ESR confirmed the vectorial displacement of electrons from quantum-sized nanoclusters to TiO2. Such coupled semiconductor systems may well become important candidate materials for novel solar energy conversion devices. Much effort has been dedicated in this regard. For instance, Lee et al.37 prepared a composite CdS/TiO2 photocatalyst by coupling CdS (34) Ho, W. K.; Yu, J. C. J. Mol. Catal. A: Chem. 2006, 247, 268. (35) Ho, W. K.; Yu, J. C.; Lin, J.; Yu, J. G.; Li, P. S. Langmuir 2004, 20, 5865. (36) Yu, J. C.; Wu, L.; Lin, J.; Li, P. S.; Li, Q. Chem. Commun. 2003, 1552. (37) Jang, J. S.; Choi, S. H.; Park, H.; Choi, W.; Lee, J. S. J. Nanosci. Nanotechnol. 2006, 6, 3642.

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Figure 3. Schematic illustration of charge transfer in a coupled semiconductor system.

nanoparticles with TiO2 nanosheets for hydrogen production and methylene blue degradation under visible-light illumination (λ > 420 nm). Very recently, Tachibana and co-workers38 reported that photoelectrochemical solar cells based on very stable CdS quantum dot-sensitized TiO2 exhibited a high IPCE of ∼70%. Another beneficial effect is that the presence of foreign oxides can greatly improve the antisintering capability during the heat treatment. For example, the role of a potential promoter (ZrO2) was investigated in enhancing a visible-light photocatalyst (TiO2 - xNx) for the oxidation of gaseous organic compounds.39 We found that the introduction of ZrO2 into TiO2 - xNx considerably inhibited the undesirable crystal growth during calcination. Consequently, ZrO2-modified TiO2 - xNx displayed a higher porosity, higher specific surface area, and improved thermal stability compared to the same parameters for the corresponding unmodified TiO2 - xNx samples. However, the coupling of semiconductors does not always enhance the charge separation. This is because the design of a coupled photocatalyst relies on the band structures of its components. They are generally determined by many complex factors, including surface area, defect density, crystallinity, and quantum size effects. Transition-metal oxides (TMOs) are of particular interest in the construction of porous or multiscale porous materials because of their enhanced catalytic, magnetic, electronic, and optical properties arising from variable oxidation states. A good example is the use of porous TiO2 for environmental purification or as a photoelectrode material for solar cells. A number of interesting porous TMOs, especially mesoporous structures, have been reported (as pioneered by Stucky in synthesizing porous silica, niobia, and titania).40-42 Despite these advances, the development of new methods to fabricate stable porous TMO materials remains a challenge. In general, several factors, including the hydrolysis and condensation of TMO precursors, structural integrity collapsing during the redox reactions, possible phase (38) Tachibana, Y.; Akiyama, H. Y.; Ohtsuka, Y.; Torimoto, T.; Kuwabata, S. Chem. Lett. 2007, 36, 88. (39) Wang, X. C.; Yu, J. C.; Chen, Y. L.; Wu, L.; Fu, X. Z. Environ. Sci. Technol. 2006, 40, 2369. (40) Yang, P.; Deng, T.; Zhao, D.; Feng, P.; Pine, D.; Chmelka, B. F.; Whitesides, G. M.; Stucky, G. D. Science 1998, 282, 2244. (41) Yang, P. D.; Zhao, D. Y.; Margolese, D. I.; Chmelka, B. F.; Stucky, G. D. Nature 1998, 396, 152. (42) Antonelli, D. M.; Ying, J. Y. Chem. Mater. 1996, 8, 874.

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transitions, and crystallization processes, affect the quality and function of porous TOM materials. Our group aims to develop efficient synthesis methods capable of producing high-quality porous TOMs for photocatalytic applications. A rapid ultrasound-induced agglomeration approach was developed to synthesize highly active, 3D, thermally stable mesoporous TiO2 without using organic templates.43 By using a high-intensity ultrasound probe, we also synthesized mesoporous TiO2 with a bicrystalline framework of anatase and brookite.44 We found that the asprepared mesoporous TiO2, consisting of both anatase and brookite phases, showed high photocatalytic activity. This increased even more after calcination because of better crystallization. The combined effects of high brookite content, high surface area, and the existence of mesopores may contribute to the high photocatalytic activity of mesoporous TiO2 with a bicrystalline framework. To stabilize the mesoporous structure of TiO2, incorporating phosphorus into the framework of mesoporous TiO2 using H3PO4 as a phosphorus source was studied.45 The amorphous titanium phosphate with embedded crystalline anatase nanoparticles of several nanometers existed in the calcined phosphate mesoporous TiO2 (Figure 4). The more complete condensation between surface Ti-OH and the inhibition of crystalline grain growth of the embedded anatase TiO2 during the calcinations led to the stabilization of the phosphated mesoporous TiO2 framework. The calcined phosphated mesoporous TiO2 had a high photocatalytic activity related to its extended band gap energy, larger surface area, and the existence of Ti ions in tetrahedral coordination. An evaporation-induced self-assembly (EISA) method was developed to prepare visible-light-driven Cr-doped TiO2 with a 3D mesoporous framework.46 The visible-light photocatalytic activity of the resulting ordered cubic Im3m mesoporous Cr-TiO2 was superior to that of pure mesoporous TiO2. During the past decade, a quickly growing family of nontitania-based visible-light-driven semiconductor photocatalysts (e.g., YFeO3, RbPb2Nb3O10, InTaO4, BiVO4, BaCr2O4, La2TiO2N, TaON, and Ta3N5) has been identified.20,23,47-52 The electronic band structures of these photocatalysts are evidently different from those of doped TiO2. They possess steep absorption edges in the visible-light region with the result that their visible-light absorptions arise from the band-band transitions instead of the impurity. For instance, it is known that the valence bands of these oxide semiconductors consist of hybridizations of certain transition-metal orbitals (e.g., Bi 6s, Ag 4d, Ta 5d, V 3d, and Sn 5s) or nonmetal orbitals (e.g., N 2p) with O 2p. The hybridization results in an increase in the valence-band level in comparison to that of the O 2p orbital in TiO2 and a smaller band-gap energy sufficient to absorb visible light. We have recently proposed a nanocasting approach to prepare ordered mesoporous BiVO4 crystals using silica (KIT-6) as a template.53 After a mild thermal process, monoclinic scheelite BiVO4 crystals were formed inside (43) Yu, J. C.; Zhang, L. Z.; Yu, J. G. New J. Chem. 2002, 26, 416. (44) Yu, J. C.; Zhang, L. Z.; Yu, J. G. Chem. Mater. 2002, 14, 4647. (45) Yu, J. C.; Zhang, L. Z.; Zheng, Z.; Zhao, J. C. Chem. Mater. 2003, 15, 2280. (46) Yu, J. C.; Li, G. S.; Wang, X. C.; Hu, X. L.; Leung, C. W.; Zhang, Z. D. Chem. Commun. 2006, 2717. (47) Ye, J. H.; Zou, Z. G.; Oshikiri, M.; Matsushita, A.; Shimoda, M.; Imai, M.; Shishido, T. Chem. Phys. Lett. 2002, 356, 221. (48) Hitoki, G.; Takata, T.; Kondo, J. N.; Hara, M.; Kobayashi, H.; Domen, K. Chem. Commun. 2002, 1698. (49) Yoshimura, J.; Ebina, Y.; Kondo, J.; Domen, K.; Tanaka, A. J. Phys. Chem. 1993, 97, 1970. (50) Kudo, A.; Omori, K.; Kato, H. J. Am. Chem. Soc. 1999, 121, 11459. (51) Kudo, A.; Ueda, K.; Kato, H.; Mikami, I. Catal. Lett. 1998, 53, 229. (52) Hara, M.; Hitoki, G.; Takata, T.; Kondo, J. N.; Kobayashi, H.; Domen, K. Catal. Today 2003, 78, 555. (53) Li, G. S.; Zhang, D. Q.; Yu, J. C. Chem. Mater. 2008, 20, 3983.

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Figure 5. Schematic illustration of the formation of mesoporous

BiVO4.53

Figure 4. (A, B) TEM images of as-prepared and calcined pure mesoporous TiO2 (MT) and phosphated mesoporous TiO2 (PMT). a and b denote MT and PMT, respectively. AP following a and b denotes an as-prepared sample. Numbers following a and a denote the calcination temperatures. (C) Photocatalytic activities of MT and PMT calcined at different temperatures.44

the mesopores of silica, as shown in Figure 5. After the removal of the silica template by NaOH, the BiVO4 framework made of small nanocrystals with a large surface area and an ordered structure was obtained. The product exhibited enhanced photocatalytic performance in the photochemical degradation of methylene blue and the photocatalytic oxidation of NO gas in air under visiblelight irradiation. Hierarchically Porous Nanoarchitectures. There has been rapidly growing interest in recent years in the construction of functional materials with complex hierarchical structures. Materials with multimodal or multiscale pores are desirable in catalysis and separation processes, where optimization of the diffusion and confinement regimes is required.40 The host-guest interactions can be promoted because micropores and mesopores can provide Langmuir 2010, 26(5), 3031–3039

size and shape selectivity for the guest molecules. The presence of macrochannels should also make the guest molecules more accessible to the active sites and avoid pore blockage. Theoretical calculations and simulations also show that catalytic processes occur more efficiently in materials with hierarchical micropores or mesopores.54 Recently, thermally stable, hierarchically porous mesoscopic titania balls were prepared by Breu, Lu, and coworkers.83 Nanoscopic rutile was electrostatically assembled around colloidal spherical polyelectrolyte brushes (SPBs) consisting of a solid polystyrene (PS) core and a dense surface layer of polyelectrolytes. After subsequent calcination of the rutile/SPB hybrid, the noncapped surfaces of the nanocrystals were crosslinked, yielding thermally and mechanically stable mesoscopic rutile balls with interparticle mesoporosity. Dionysiou et al.56 reported a sol-gel dip-coating process to fabricate hierarchically mesoporous TiO2 nanocrystals with markedly improved photocatalytic performance. Our group has designed and synthesized several hierarchically porous materials including TiO2, F-doped TiO2, and ZnIn2S4 for photocatalysis.57-62 It is worth noting that microwaves can often be used to speed up chemical reactions. Microwave dielectric heating is becoming an increasingly popular method for nanomaterial synthesis.63 It can address the issue of conventional heating inhomogeneity and slow reaction kinetics. As a clean, cheap, and convenient method of heating, microwave irradiation often achieves higher yields and shorter reaction times.64 In particular, modern microwave systems possess the capabilities of time and temperature programming, allowing fast (54) Coppens, M. O.; Sun, J. H.; Maschmeyer, T. Catal. Today 2001, 69, 331. (55) Yelamanchili, R. S.; Lu, Y.; Lunkenbein, T.; Miyajima, N.; Yan, L. T.; Ballauff, M.; Breu, J. Small 2009, 5, 1326. (56) Choi, H.; Sofranko, A. C.; Dionysiou, D. D. Adv. Funct. Mater. 2006, 16, 1067. (57) Zhang, L. Z.; Yu, J. C. Chem. Commun. 2003, 2078. (58) Hu, X. L.; Yu, J. C.; Gong, J. M.; Li, Q. Cryst. Growth Des. 2007, 7, 2444. (59) Li, G. S.; Yu, J. C.; Zhu, J. A.; Cao, Y. Microporous Mesoporous Mater. 2007, 106, 278. (60) Ho, W.; Yu, J. C.; Lee, S. Chem. Commun. 2006, 1115. (61) Wang, X. C.; Yu, J. C.; Ho, C. M.; Hou, Y. D.; Fu, X. Z. Langmuir 2005, 21, 2552. (62) Yu, J. G.; Yu, J. C.; Zhang, L. Z.; Wang, X. C.; Wu, L. Chem. Commun. 2004, 2414. (63) Komarneni, S. Curr. Sci. 2003, 85, 1730. (64) Galema, S. A. Chem. Soc. Rev. 1997, 26, 233.

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Figure 6. (A) Annealed PS colloidal monolayer soaked in the sol. (B) Colloidal monolayer with the sol in the interstices. (C) Profile view of the colloidal monolayer and the sol film divided into two layers. (D) Lateral view of the colloidal monolayer and gel shells. (E) Lateral view of the ring arrays. (F) Top SEM view of the nanoporous ring arrays.67

and easy optimization of experimental factors.65,66 This is very beneficial for the creation of numerous synthetic recipes, the preparation of high-quality nanomaterials, and production scale up. Besides developing TiO2-based powders, we have also fabricated TiO2 hierarchical nanoporous ring arrays on a solid surface by an annealed template-induced sol-soaking strategy.67 The ring size was tuned by changing the sol concentration and the annealing time of the polystyrene sphere colloidal monolayer. As shown in Figure 6, the sol-gel process generates porous nanocrystalline networks in the ring walls, which can be further modified by incorporating functional particles into the pores. More importantly, the rings can overcome the restrictions of surface morphology and grow on any solid support. This method can also be extended to fabricate other hierarchical semiconductor materials including ZrO2, SnO2, Fe2O3, and ZnO. These unique rings with hierarchically porous architectures may find applications in energy-transfer photovoltaic devices, photocatalytic surfaces, and gas sensors. Films. It should be noted that conventional powder photocatalysts have limitations when they are used in a slurry system for environmental remediation (e.g., elimination of toxic and hazardous substances and metals in water). In this case, there is usually a need for post-treatment separation. To help overcome the poor recyclability and tedious treatment, considerable effort has been made to achieve immobilized semiconductor particles as thin films on a solid substrate, which can be traced back to the 1980s.68 The nanostructured film-based materials are likely to find new industrial applications, such as antibacterial and self-cleaning glass and ceramic tiles. Likewise, semiconductor-film materials are of great importance to the fields of photovoltaic devices and environmental sensing. In recent studies, our group has devised functional films of flake-array, porous, mesoporous, and zeolitelike mesoporous TiO2 films on glass and steel substrates.69-73 The (65) Hu, X. L.; Yu, J. C. Adv. Funct. Mater. 2008, 18, 880. (66) Hu, X. L.; Yu, J. C. Chem. Mater. 2008, 20, 6743. (67) Sun, F. Q.; Yu, J. C.; Wang, X. C. Chem. Mater. 2006, 18, 3774. (68) Matthews, R. W. J. Phys. Chem. 1994, 98, 6797. (69) Ho, W. K.; Yu, J. C.; Yu, J. G. Langmuir 2005, 21, 3486. (70) Yu, J. C.; Ho, W. K.; Lin, J.; Yip, K. Y.; Wong, P. K. Environ. Sci. Technol. 2003, 37, 2296. (71) Yu, J. C.; Ho, W. K.; Yu, J. G.; Hark, S. K.; Iu, K. Langmuir 2003, 19, 3889. (72) Wang, X. C.; Yu, J. C.; Hou, Y. D.; Fu, X. Z. Adv. Mater. 2005, 17, 99. (73) Yu, J. C.; Wang, X. C.; Fu, X. Z. Chem. Mater. 2004, 16, 1523.

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photodegradation efficiency, photoinduced hydrophilicity, and antibacterial effect were systematically investigated. For example, surface modification by trifluoroacetic and sulphuric acids (Figure 7A,B) has been demonstrated to enhance the photocatalytic activities of TiO2-based films for the photocatalytic oxidation of acetone or CH3Br in air.71,72 Metal-semiconductor composites have proved to be useful in promoting light-induced electron-transfer reactions. Exploring the catalytic activity of such composite structures could pave the way for the design of novel light-harvesting systems. We therefore further incorporated highly dispersed noble-metal nanoparticles into the mesoporous TiO2 films by a sonochemical and photochemical approach.74 These fine nanoparticles are effectively confined and stabilized in the nanopores of the TiO2 film, curbing their intrinsic tendency toward agglomeration (Figure 7C). We also exploited the possibility of sonochemistry to deposit poorly dissolved phthalocyanine dyes onto the surfaces of highly ordered zeolite-like mesoporous TiO2 films.75 The aggregation-free phthalocyanines are effectively encapsulated and stabilized in the nanopore arrays of the TiO2 films. This enables highly dispersed dyeTiO2 heterojunction arrays with a large contact area to be obtained. The resulting nanocomposite materials may have a variety of applications, for example, as electrode materials in dyesensitized solar cells and as photocatalysts for environmental remediation. Selective Exposed Facets of Anatase Crystals. The improvement of the photocatalytic efficiency of anatase TiO2 has long offered a challenge to researchers. Electron-hole pairs are transferred to surface-adsorbed reactants with the competition of mutual recombination. Because recombination tends to occur on grain boundaries and crystalline defects, the use of single crystals with highly reactive exposed facets and a low defect density is one possible strategy. It is well known that the surface structure and free energy of a crystal are closely related to its crystallographic orientation.76 Under equilibrium conditions, a crystal evolves and possesses a specific shape with distinctive exposed facets at the moment that the total surface free energy is minimized. Surface binding is a general approach to achieving a desired crystal shape. The absorbed species on different facets have different adsorption energies, which induce crystal growth in accordance with the equilibrium-state crystallographic orientation. Toward this end, Lu and co-workers77 achieved the synthesis of high-purity anatase TiO2 single crystals with a large percentage of reactive {001} facets by using hydrofluoric acid (HF) as a capping agent under hydrothermal conditions. They also reported a solvothermal method using 2-propanol as a synergistic capping agent and reaction medium together with HF to synthesize high-quality anatase TiO2 single-crystal nanosheets with 64% {001} facets.78 The as-formed nanosheets exhibited superior photoreactivity (more than 5 times) compared to that of P25 as a benchmark material. Ohtani et al. prepared 50-250 nm decahedral single-crystalline anatase particles and reactive {001} facets through a gasphase reaction using TiCl4 as a titanium source.79 Nanosized anatase TiO2 particles with a high {001} facet percentage have (74) Yu, J. C.; Wang, X. C.; Wu, L.; Ho, W. K.; Zhang, L. Z.; Zhou, G. T. Adv. Funct. Mater. 2004, 14, 1178. (75) Wang, X. C.; Yu, J. C. Macromol. Rapid Commun. 2004, 25, 1414. (76) Selloni, A. Nat. Mater. 2008, 7, 613. (77) Yang, H. G.; Sun, C. H.; Qiao, S. Z.; Zou, J.; Liu, G.; Smith, S. C.; Cheng, H. M.; Lu, G. Q. Nature 2008, 453, 638. (78) Yang, H. G.; Liu, G.; Qiao, S. Z.; Sun, C. H.; Jin, Y. G.; Smith, S. C.; Zou, J.; Cheng, H. M.; Lu, G. Q. J. Am. Chem. Soc. 2009, 131, 4078. (79) Amano, F.; Prieto-Mahaney, O. O.; Terada, Y.; Yasumoto, T.; Shibayama, T.; Ohtani, B. Chem. Mater. 2009, 21, 2601.

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Figure 7. (A) TEM image of ordered mesoporous sulfated TiO2. (B) Schematic representation of mesoporous nanocrystalline SO42-/TiO2

superacid molecular sieve film formation.72 (C) Sonochemical and photochemical approachs for the deposition of Au nanoclusters in mesoporous TiO2 nanocrystalline thin films.74

also been prepared by annealing-induced shape evolution from nanotubes80 and solution-based methods.81,82 Very recently, we have reported a microwave-assisted hydrothermal route for the preparation of microsized sheetlike anatase TiO2 single crystals with a remarkable 80% level of reactive {001} facets.83 The high reactivity of {001} facets makes these single crystals highly photocatalytically active. They are easily recyclable and thermally stable up to 800 °C.

Sensors As public awareness of environmental issues rises and governments make international commitments to reduce emissions, sensor arrays (electronic noses) can play a very important role in the protection of our environment.84 Sensors capable of detecting and even quantifying both simple and complex gas mixtures present a far more facile analytical method than capturing samples and analyzing them using conventional equipment. In particular, electronic noses are very attractive for on-site monitoring of priority pollutants as well as for addressing other environmental needs. Yet despite its great potential for environmental monitoring, broad applications of electronic nose technology for the control of air and water quality are still in their infancy. In particular, recent advances in novel nanostructured materials serving as sensing units will certainly expand the scope of the sensors toward a wide range of organic and inorganic contaminants. Gas-Sensing Mechanism of n-Type Oxides. It is highly desirable to design flexible, low cost, lightweight, disposable sensors for environmental applications. n-Type semiconductor metal oxides such as SnO2, Al2O3, ZnO, Fe2O3, and TiO2 are (80) Alivov, Y.; Fan, Z. Y. J. Phys. Chem. C 2009, 113, 12954. (81) Han, X. G.; Kuang, Q.; Jin, M. S.; Xie, Z. X.; Zheng, L. S. J. Am. Chem. Soc. 2009, 131, 3152. (82) Dai, Y. Q.; Cobley, C. M.; Zeng, J.; Sun, Y. M.; Xia, Y. N. Nano. Lett. 2009, 9, 2455. (83) Zhang, D. Q.; Li, G. S.; Yu, J. C. Chem. Commun. 2009, 4381. (84) Fryxell, G. E.; Cao, G., Environmental Applications of Nanomaterials: Sensors, Sorbents and Sensors; Imperial College Press: London, 2007.

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known for their outstanding gas-sensing properties in the detection of flammable and toxic gases at relatively low cost. Normally, electrically and chemically active oxygen vacancies exist on the surface of metal oxides. As n-type donors, they often significantly increase the conductivity of metal oxides. When the charge-accepting molecules (e.g., NO2 and O2) are adsorbed at the vacancy sites, electrons are effectively depleted from the conduction band, thus resulting in the decreased conductivity of the n-type oxide. Also, oxygen molecules are generally chemisorbed onto the surface of oxides as O2-, O-, or O2- species that exist in an equilibrium state (O2- S O- S O2- S Ο2).85 When the n-type oxide is exposed to a reductive gas such as H2, CO, or an alcohol, the surface-adsorbed oxygen species react with the gas. Hence, the surface concentration of oxygen species is reduced, and the conductivity is increased. By measuring the variation in resistance or conductance, gas sensors made of these n-type oxides can intelligently detect different kinds of gases. The performance of the sensors with respect to the sensitivity, selectivity, stability, and reproducibility is generally determined by several factors, including the nature of the reactions occurring on the oxide surface, temperature, catalytic properties of the surface, and electronic structure of the bulk and micro/nanosized oxide. Examples of Gas Sensing. Nanostructured TiO2 films have been widely explored as sensor units for various gases. Grimes et al. reported that TiO2 nanotubes prepared by anodization exhibited high sensitivity to hydrogen at temperatures as low as 180 °C.86 It was reported by Birkefeld and co-workers that the conductivity of anatase TiO2 varied in the presence of CO and H2 at temperatures above 500 °C but Al-doped TiO2 (10%) became selective to H2.87 For ZnO, a variety of nanostructures such as nanowires, nanorods, nanotetrapods, and nanoparticles have (85) Huang, X. J.; Meng, F. L.; Pi, Z. X.; Xu, W. H.; Liu, J. H. Sens. Actuators, B 2004, 99, 444. (86) Varghese, O. K.; Gong, D. W.; Paulose, M.; Ong, K. G.; Grimes, C. A. Sens. Actuators, B 2003, 93, 338. (87) Birkefeld, L. D.; Azad, A. M.; Akbar, S. A. J. Am. Ceram. Soc. 1992, 75, 2964.

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Indeed, environmental monitoring based on unique nanoarchitectured materials is increasingly attracting commercial interest. It seems very likely that nanodevices with reduced weight, cost, and power consumption; high selectivity, sensitivity, and reproducibility; and long-term stability will conquer important new markets in the field of environmental monitoring.

Energy Applications Figure 8. Schematic illustration of the sensing unit made of ZnO

colloidal nanocrystal clusters.92

Figure 9. (A) Schematic illustration of R-Fe2O3 nanoring formation. (B, C) TEM images of R-Fe2O3 nanorings. (D) Resistance changes for a sensor made of R-Fe2O3 nanorings when the surrounding gas was switched between alcohol vapor (∼6% in air) and air (ON for alcohol vapor and OFF for air).93

been investigated for gas sensing.88-91 Recently, we developed a rapid microwave-polyol process that offers an efficient pathway to monodisperse ZnO colloidal clusters in large quantities.92 The size of the clusters composed of small primary nanocrystals can be tuned continuously and precisely by simply varying the amount of Zn-complex precursors. The unique secondary and complex architecture of size-tunable ZnO nanocrystal clusters should render them flexible building blocks for advanced functional devices. Our preliminary results demonstrate that sensors made of ZnO-assembled clusters exhibit high sensitivity for humidity measurement at room temperature (Figure 8). Through a microwave-assisted hydrothermal process, we also synthesized R-Fe2O3 nanorings in aqueous solution (Figure 9).93 The asobtained R-Fe2O3 nanoring, a unique iron oxide nanostructure, exhibits high sensitivity for gas sensing of alcohol vapor at room temperature. (88) Wan, Q.; Li, Q. H.; Chen, Y. J.; Wang, T. H.; He, X. L.; Li, J. P.; Lin, C. L. Appl. Phys. Lett. 2004, 84, 3654. (89) Wang, J. X.; Sun, X. W.; Yang, Y.; Huang, H.; Lee, Y. C.; Tan, O. K.; Vayssieres, L. Nanotechnology 2006, 17, 4995. (90) Chu, X. F.; Jiang, D. L.; Djurisic, A. B.; Yu, H. L. Chem. Phys. Lett. 2005, 401, 426. (91) Tang, H. X.; Yan, M.; Ma, X. F.; Zhang, H.; Wang, M.; Yang, D. R. Sens. Actuators, B 2006, 113, 324. (92) Hu, X. L.; Gong, J. M.; Zhang, L. Z.; Yu, J. C. Adv. Mater. 2008, 20, 4845. (93) Hu, X. L.; Yu, J. C.; Gong, J. M.; Li, Q.; Li, G. S. Adv. Mater. 2007, 19, 2324.

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The dramatic increase in energy demand and the growing pressure to address environmental concerns have spurred the development of alternative energy and sustainable environmental technologies. Of the various renewable energy options, solar energy stands out as the most ultimately sustainable choice in terms of its availability and vast potential. During the past decade, nanostructured materials have emerged as unique building blocks to construct light-energy-harvesting assemblies. They have opened up new and promising avenues for the utilization of solar energy. A large number of reports, reviews, and books on possible methods of solar energy transfer, such as solar cells and hydrogen energy production by photocatalytic water splitting, have already been published.94-98 Hydrogen stands a high chance of becoming the most environmentally acceptable fuel of the future because it is an excellent source of clean energy. One promising method of producing hydrogen from water is photocatalytic water splitting. This involves chemical processes similar to photosynthesis by green plants, and such processes are known as artificial photosynthesis. For overall water splitting, water molecules are reduced by the electrons in the conduction band to form H2, and water molecules are oxidized by the holes in the valence band to form O2. Additional catalysts may catalyze the two half-reactions. Normally, to create an ideal nanostructured semiconductor photocatalyst for solar photocatalytic water splitting, the following properties should be considered: (1) the engineering of a suitable band gap to absorb the solar energy; (2) high crystallinity to facilitate the migration of electron/hole pairs to the surface; and (3) active sites and a large surface area for redox reactions that take place on the catalyst surface. A variety of photocatalysts including oxides consisting of metal cations with d0 and d10 configurations, metal (oxy)sulfides, and metal (oxy)nitrides have already been reported for photocatalytic water decomposition. 96,99-101 Although possible methods of fabricating photocatalysts with an optimal band gap have been extensively studied, much work remains to be done on surveying new high-efficiency photocatalytic nanomaterials and understanding the fundamental band gap engineering and mechanism. Deriving plentiful electricity from sunlight at a modest cost is also a formidable challenge.102-104 Semiconductor materials with nanoscale structural features have recently emerged as building (94) Law, M.; Greene, L. E.; Johnson, J. C.; Saykally, R.; Yang, P. D. Nat. Mater. 2005, 4, 455. (95) Kamat, P. V. J. Phys. Chem. C 2007, 111, 2834. (96) Kudo, A.; Miseki, Y. Chem. Soc. Rev. 2009, 38, 253. (97) Kamat, P. V. J. Phys. Chem. C 2008, 112, 18737. (98) Gratzel, M. Resources through Photochemistry and Catalysis; Academic Press: New York, 1983. (99) Jang, J. S.; Kim, H. G.; Reddy, V. R.; Bae, S. W.; Ji, S. M.; Lee, J. S. J. Catal. 2005, 231, 213. (100) Kato, H.; Asakura, K.; Kudo, A. J. Am. Chem. Soc. 2003, 125, 3082. (101) Maeda, K.; Teramura, K.; Lu, D. L.; Saito, N.; Inoue, Y.; Domen, K. Angew. Chem., Int. Ed. 2006, 45, 7806. (102) Oregan, B.; Gratzel, M. Nature 1991, 353, 737. (103) Gur, I.; Fromer, N. A.; Geier, M. L.; Alivisatos, A. P. Science 2005, 310, 462. (104) Wang, P.; Zakeeruddin, S. M.; Moser, J. E.; Nazeeruddin, M. K.; Sekiguchi, T.; Gratzel, M. Nat. Mater. 2003, 2, 402.

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blocks in solar cells with greatly improved light harvesting.105-108 For example, Lieber and co-workers reported miniature solar cells consisting of a p-type silicon nanowire core surrounded by an n-type silicon sheath.109 Electrons and holes are generated by light in the intervening layer of silicon and are swept into the n shell and p core, respectively, by a built-in electric field. These miniature solar cells could potentially be used to power microelectronic systems where downsizing to the nanoscale or microscale requires nanoelectronic components. Yang et al. proposed a dye-sensitized solar cell in which the traditional nanoparticle film was replaced by a dense array of oriented, crystalline ZnO nanowires.94 The direct electrical pathways provided by the nanowires ensure the rapid collection of carriers generated throughout the device. More recently, semiconductor quantum dot-based solar cells have attracted considerable attention because they promise to boost the energy conversion efficiency beyond the traditional Shockley and Queisser limit of 32% for Si-based solar cells. The future is bright for the development of commercial high-efficiency, lowcost solar cells based on nanostructured electrode architectures as well as the sensitization phenomenon. By incorporating thirdgeneration concepts based on quantum confinement, it should be possible to realize sensitized nanostructure systems with efficiencies beyond the theoretical (Shockley-Queisser) limit in the foreseeable future.

Conclusions and Perspectives In this feature article, we have described recent advances in the fabrication and modification of nanostructured semiconductor materials for environmental remediation and monitoring. These generally fall into four categories, namely, particles, hierarchical architectures, films, and single crystals. In particular, TiO2-based nanostructures (e.g., pure, doped, coupled, nanoporous, mesoporous, hierarchically porous, ordered mesoporous TiO2, and single-crystalline TiO2), mesoporous BiVO4, R-Fe2O3 nanorings, and ZnO nanocrystal clusters have been synthesized by sol-gel, microemulsion, hydrothermal, sonochemical, microwave, photochemical, and nanocasting strategies. We have also touched briefly on the possible applications of nanostructured semiconductor materials in the fields of photocatalytic water splitting and solar cells. Besides the applications mentioned in this review, other applications that may benefit from work in this field include hydrogen storage, fuel cells, thermoelectric conversion, (105) Gratzel, M. MRS Bull. 2005, 30, 23. (106) Zhang, Q. F.; Chou, T. R.; Russo, B.; Jenekhe, S. A.; Cao, G. Z. Angew. Chem., Int. Ed. 2008, 47, 2402. (107) Wang, K.; Chen, J. J.; Zhou, W. L.; Zhang, Y.; Yan, Y. F.; Pern, J.; Mascarenhas, A. Adv. Mater. 2008, 20, 3248. (108) Yuhas, B. D.; Yang, P. D. J. Am. Chem. Soc. 2009, 131, 3756. (109) Tian, B. Z.; Zheng, X. L.; Kempa, T. J.; Fang, Y.; Yu, N. F.; Yu, G. H.; Huang, J. L.; Lieber, C. M. Nature 2007, 449, 885.

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electronics, and clean processing.110-113 We hope that this survey provides some useful pointers for the future design, fabrication, and modification of semiconductor materials with nanoscale features for environmental and energy applications. It should be stressed that almost all of the environmental and energy-related research and advancements are closely linked to materials science and technology. The rapid development of nanofabrication techniques in the past few years has resulted in the creation of many different types of advanced nanostructural materials. Interesting properties have also been explored by controlling the size, shape, composition, and microstructure. We predict that the application of these materials in environmental and energy fields will benefit from this fundamental research in the design and fabrication of nanostructured semiconductor materials. As global environmental and energy issues increasingly demand our attention, numerous challenges in materials science and technology will arise. The utilization of the superior functions of nanostructured semiconductor materials is expected to contribute to environmental improvement and conservation as well as environmentally friendly technologies for renewable energy and energy savings. Therefore, the conceptual development for the production of nanostructured materials in a cost-efficient way is indispensable. In some cases, learning from nature and living things may help us to design and create novel nanoarchitectures with both refined simplicity and beauty and mind-boggling complexity and detail. As far as the field of semiconductor photocatalysis for environmental remediation is concerned, there is an urgent need to develop new photocatalytic materials that respond to sunlight by band structure control. This imperative will require researchers around the world to carry out systematic experimental studies and establish general design guidelines in band control engineering. Future work is also required to elucidate the mechanisms involved in the photocatalysis reactions. Our understanding of the interfacial properties and charge transport in solar cells and sensing arrays for environmental monitoring will also significantly benefit from advances in surface science and membrane-fabrication technologies. Acknowledgment. This work was supported by grants from the Research Grants Council of the Hong Kong Special Administrative Region (N_CUHK433/00, CUHK4027/02P, 402904), the Innovation and Technology Fund (ITS/118/01, ITP/021/ 07NI), and the Strategic Investments Scheme administrated by The Chinese University of Hong Kong. (110) (111) (112) (113)

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