Low-dimensional nanostructured photocatalysts

Abstract

Low-dimensional nanostructures are a promising class of ideal high-performance candidates for energy storage and conversion owing to their unique structural, optical, and chemical properties. Low-dimensional nanostructured photocatalysts have attracted ever-growing research attention. In this review, we mainly emphasize on summarizing the 0-, 1-, and 2-dimensional nanostructured photocatalysts systematically, including their photocatalytic performance, synthesis methods, and theoretical analysis. From the viewpoint of dimension, we try to figure out the way to design more high-efficiency photocatalysts towards numerous applications in the field of solar energy conversion, hoping to promote efficient control and rational development of photocatalysts.

Journal of Advanced Ceramics2015, 4(3): 159182 ISSN 2226-4108 DOI: 10.1007/s40145-015-0159-8 CN 10-1154/TQ

Review

Low-dimensional nanostructured photocatalysts

Hao-Min XU, Huan-Chun WANG, Yang SHEN, Yuan-Hua LIN*, Ce-Wen NAN

State Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and Engineering,

Tsinghua University, Beijing 100084, China

Received: June 03, 2015; Revised: June 21, 2015; Accepted: June 24, 2015 The Author(s) 2015. This article is published with open access at Springerlink.com

Abstract: Low-dimensional nanostructures are a promising class of ideal high-performance candidates for energy storage and conversion owing to their unique structural, optical, and chemical properties. Low-dimensional nanostructured photocatalysts have attracted ever-growing research attention. In this review, we mainly emphasize on summarizing the 0-, 1-, and 2-dimensional nanostructured photocatalysts systematically, including their photocatalytic performance, synthesis methods, and theoretical analysis. From the viewpoint of dimension, we try to figure out the way to design more high-efficiency photocatalysts towards numerous applications in the field of solar energy conversion, hoping to promote efficient control and rational development of photocatalysts.

Keywords: low-dimension; photocatalysis; nanostructure

1 Introduction

The utilization of solar energy, including solar cell, solar water splitting, and organic pollution degrading, varies in peoples daily life. The energy crisis and environmental problem have triggered photocatalysis research widely. Pollution could be degraded and clean energy could be generated from water by being illuminated with the sun light. As 43% of the sun light is in the visible range and 50% is near infrared, it is considerably essential to extend the absorbable range to improve the utilization efficiency of the sun light for photocatalysts (PCs).

In general, the optical absorption performance of a semiconductor is related to its electronic structure, microstructure, defect, band gap, etc. [1]. The key issues influencing photocatalytic (PC) capability of a semiconductor include its energy band configuration as well as the nature of its surface/interface, e.g., the

surface energy, chemisorption properties, and surface area [2]. Therefore, the advanced low-dimensional PCs have attracted researchers attention due to their unique advantages of large surface area, high electronhole pair (eh) separating efficiency, and other irregular surface properties [3].

Low-dimensional structured materials refer to those 0-dimensional (0-D), 1-dimensional (1-D), and 2-dimensional (2-D) materials composed of ultra-fine particles under 100 nm in size. In low-dimensional structured materials, 0-D materials include quantum dots, nanoparticles, and nanoclusters; 1-D materials include nanotubes, nanorods, and nanowires; and 2-D materials refer to the extension of the two spatial coordinates like nanofilms and superlattices. Generally, low-dimensional PCs have better PC performance than bulk materials mostly due to the larger surface area and size effect [46]. Take 0-D nanostructures for example. Nanoparticles and quantum dots (QDs) have tunable electronic and optical properties as a function of size, called quantum-size effect. Materials in quantum dot form display special properties such as variation in

* Corresponding author. E-mail: [email protected]

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band gap. Cui et al. [7] prepared Bi2WO6 nanosheets with Bi2WO6 quantum dots dispersed randomly by a facile solvothermal route. Their products possess excellent visible light absorption and PC efficiency compared to Bi2WO6 nanoparticles or nanoplates. It was believed that there are different gaps between the quantum dots and the nanosheet substrates in the Bi2WO6 nanostructure, which promote the migration of photo generated electrons and holes from one side to another, thus favoring the separation of carriers. Although sometimes the band gap increases with respect to the decrease of size of particles in specific range induced by quantum-size effect which makes it harder for PCs to utilize more light, the band gap would decrease with the decrease of particle size in a certain range which may further help improve the PC efficiency. For example, in TiO2 nanocrystalline, the band gap varies from 3.173 to 3.239 eV as the particle size increases from 17 to 29 nm [8,9]. They believed that the delocalization of molecular orbitals induced by crystal defects in the conduction band (CB) edge creates traps in electronic energy, and consequently brings about the red shift of the absorption spectra.

The low-dimensional nanostructured PCs vary in types, mechanisms, and applications, making them a promising prospect. In this review, we briefly cover the recent progress in the morphology control, mechanism explanation, and advanced synthesis methods of low-dimensional nanostructured PCs to make contribution to the further exploration on novel PCs.

2 0-dimensional: quantum dots, nano-/micro-particles

We first come to the 0-D structured PCs. Generally the 0-D structure refers to those particles of which three-dimensional sizes are in nanoscale, like the dots without specific shape. A quantum dot is a semiconductor nanocrystal that is small enough to exhibit quantum mechanical properties with its excitons confined in all three spatial dimensions. As mentioned above, QDs show extremely obvious size effect and always act as the decoration when composited with other materials. Thus we will discuss QD PCs later in Section 5.

Since the nano-/micro-sized paritcles with various morphologies occupy important position in modern PCs, we classify them into 0-D as well. The particles of powdered catalysts with nanoscale dimensions

could increase total surface area and the number of available reaction sites. However, the small size of the particles brings several disadvantages. When the dimensions of the crystal are comparable to the width of the depletion layer, the photo generated eh will be spatially confined in close proximity, and the interfacial space charge and band bending which tends to separate electrons and holes will not be present [10]. Moreover, the back reaction of intermediate chemical species would become more likely in close proximity. All of these features lead to a loss of efficiency. Good PCs balance in all aspects of factors including crystallite size and morphology. It is found that the size, morphology, and facet of nanoparticles have effect on the PC ability.

The shapes of the nano-/micro-particle PCs include cube, sphere, box, spindle, flower, cage, etc. (Fig. 1). The catalysts in similar size but different morphology could display different PC effect. The reason could be the difference of the oriented facet surface energy or the chemical potential gradient [1113]. Different theories have been used to explain the effect of the shape, facet, and morphology of particular PCs on their PC properties. It is very inspiring that people could select the facets exposed and control the PC reaction process. Bi et al. [12] believed a higher surface energy of Ag3PO4 {110} facets than that of {100} planes contributes significantly to the enhanced PC activity of the {110} facets. A study on the two types of WO3

crystals (Figs. 2(a)2(c)) showed that the difference in electronic band edges caused by crystal facet electronic structure effect would optimize the PC activity for targeted reactions [14]. The normalized oxygen evolution rate of the quasi-cubic-like WO3 crystals by specific surface area is 8.4 times higher than that of the rectangular sheet-like crystals (Fig. 2(e)), and the much higher oxygen evolution rate of the quasi-cubic-like crystals can be attributed to the stronger oxidative power of photo excited holes generated from the lower valence band maximum (VBM) (Fig. 2(d)). Harn et al. [15] obtained a series of Ag2O with shapes evolving from edge and corner-truncated cube (ECTC) to rhombicuboctahedron (RO), and to hexapods (HP) (Figs. 3(a)3(i)) by simply maintaining a certain ratio of AgNO3 to NH4NO3 and adding NaOH of various concentrations, and found that the {100} facets exhibit the highest PC activity in methyl orange (MO) solution under full-spectrum light irradiation, followed by the {110} facets, then the {111} facets (Fig. 3(j)). They believed that different amounts of catalytically active

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Fig. 1 SEM images of BFO particle photocatalysts in shapes of (a) microsphere, (b) microcube, (c) submicrocube, (d) spindle-like, and (e) hollow sphere. Reproduced with permission from (ac) Ref. [17], 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, and (d, e) Ref. [18], 2010 Royal Society of Chemistry.

Fig. 2 (a) and (b) SEM images of quasi-cubic-like WO3 crystals and rectangular sheet-like WO3 crystals, respectively. (c) TEM image of rectangular sheet-like WO3 crystals. (d) Determined VB and CB edges versus NHE (pH = 7) of (i) quasi-cubic-like WO3 crystals and (ii) rectangular sheet-like WO3 crystals. (e) Oxygen evolution rate from water splitting in the presence of AgNO3 as an electron acceptor under visible light ( > 400 nm) and CH4 generation rate from photo reduction of CO2 in the presence of H2O vapor under UVVis light. Reproduced with permission from Ref. [14], 2012 Royal Society of Chemistry.

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C/C 0

Irradiation time (min)

Fig. 3 SEM images of the Ag2O crystals with morphologies: (a) and (d) ECTC, (b) and (e) RO, and (c) and (f) HP. (g)(i) Corresponding schematic drawings of the Ag2O crystals. (j) Photo degradation rate of MO dye by various PCs under full-spectrum light irradiation. Reproduced with permission from Ref. [15], 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

sites resulted by the atomic arrangements of facets contribute to this phenomenon. And for the ferroelectric PCs, the polarization state along crystal orientation of the particle also affects the PC efficiency. Giocondi and Rohrer [16] found that patterns of reduced silver on microcrystals of BaTiO3 (Fig. 4) and Sr2Nb2O7 are consistent with the orientations of the ferroelectric domains within the samples, indicatingthat the associated dipolar fields that separate photo generated charge carriers cause the spatial localizationof reaction products on the surfaces of ferroelectric microcrystals. The thought of fabricating crystal-facet

controlled PCs in order to further enhance or optimize the PC properties can also be applied in 1-D and 2-D structured semiconductors, which we will discuss in the following sections.

Numerous methods have been employed to fabricate 0-D PCs including coprecipitation or chemical precipitation, hydrothermal/solvothermal process [19,20], solgel, solid state reaction (SSR), spray pyrolysis [2123], combustion [24,25], reverse- microemulsion [26,27], and modified approaches based on them. The morphology and performance of photocatalyst crystal intensely depend on the synthesis

Fig. 4 SEM images of faceted BaTiO3 crystals after reaction in aqueous AgNO3. The speckled white contrast corresponds to silver containing deposits. Silver was deposited on only one of the exposed {100} facets, which happened to be the orientation of polarization. Reproduced with permission from Ref. [16], 2008 Springer Science+Business Media, LLC.

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approach, even the parameters employed during the synthesis process. Different approaches always resultin different crystal appearance, different surface physicochemical property, different specific surfacearea, and finally different PC activity. Yin et al. [27] synthesized cage-like Bi2MoO6 hollow spheres successfully by SSR with the assistant of colloidalcarbon spheres. In fabrication of hollow-structured photocatalyst particles with SSR, a preprocessing of absorption and precipitation is often necessary. Moreover, the hollow structure is always composed of smaller crystalline grain, which leads to the porosityof the hollow sphere. Solgel is a low-cost and convenient method for making pure phase or element doping particles. Nano Bi2WO6 catalyst withvisible light response was prepared by solgel [28]. Hydrothermal/solvothermal treatments are widely employed to synthesize photocatalyst particles, especially ternary and quaternary complex oxides, withthe advantage of facile morphology and structure control. Zhang et al. synthesized highly crystalline orthorhombic Bi2MoO6 particles with high visible light PC activity via a facile hydrothermal process without adding any surfactant. The morphologies of Bi2MoO6 can be selectively obtainedby adjusting the pH value of the reactant. The similar phenomenon was reported by many other groups [29].

Nut-like, potato-like, and broccoli-like monoclinic BiVO4 were fabricated in different solvents. The crystal phase could also be modulated by varying the pH value of reaction system. Monoclinic BiVO4

nanoparticles with small crystal size and large bandgap exhibit remarkable PC performance [30]. Besides, surfactants affect the products in hydrothermal method [3133]. In the process of hydrothermal/solvothermal treatment, the precursor ions undergo a period of nucleation and growth (dissolution and recrystallization). The main effect of pH value is to modulate the kinetics of nucleation and growth of the crystal by controlling the surface free energy. The preferential adsorption of ions in solution onto different crystal facets directs the growth of particlesinto various shapes by controlling the growth rates along different crystal axes. The crystal plane with a higher surface energy is expected to have a faster growth rate.

3 1-dimensional nanostructures

Since the discovery of carbon nanotubes, 1-D nanostructures (including nanobelts, nanotubes (NTs), nanorods (NRs), nanofibers, and other large aspect ratio nanostructures (Fig. 5)) have attracted

Fig. 5 SEM images of (a) TiO2 nanotubes, (b) BFO nanofibers, (c) TiO2 nanobelts, and (d) ZnO nanorods. Reproduced with permission from (a) Ref. [34], 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, (b) Ref. [35], 2013 Xuehui Zhang et al., (c) Ref. [36], 2010 American Chemical Society, and (d) Ref. [37], 2013 Royal Society of Chemistry.

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ever-increasing attention in many fields for energy harvesting and storage, and sensors and electronic devices.

Besides the change of energy band structure brought by the novel structure, recombination of photo induced electronhole pairs decreases in this nanostructure, resulting in the improved PC efficiency [38]. The outstanding properties of 1-D nanostructure are considered to be pertinent to the facts that: the quantum mechanical effects emerge and count a lot when the radial dimension of the 1-D nanostructure comes to or below certain characteristic lengths, leading to large surface-to-volume ratio and large aspect ratio which are beneficial to chemical activity and energy transfer, which are different from those corresponding bulk counterparts. Moreover, facilitating fast and long-distance electron transport, light absorption, and scattering based on the high length-to-diameter ratio is thought to be made-to-order as PCs. More recently, many 1-D nanostructures are fabricated and investigated as heterogeneous photocatalysts. The greater charge mobility in the 1-D along the longitudinal dimension of the crystals, plus the enhanced charge separation and fewer localized states due to the surface state of specific facet of nanocrystalline, makes high-efficiency PCs.

In this part, we will emphasize on three kinds of 1-D structured PCs: nanorods, nanowires (nanofibers, nanobelts), and nanotubes.

3. 1 Nanorods

NR PCs have been proved to possess great PC ability, which could be synthesized by various methods like in situ hydrothermal growth reaction method and electro deposition method. For a photocatalyst, NRs could act better than their particle or 2-D forms. Bai et al. [39] have compared the PC activity of g-C3N4 nanoplates and NRs by transforming nanoplates into NRs in reflux process. They observed about 1.5 times enhanced PC activity and 2.0 times the intensity of the photocurrent response of g-C3N4 NRs under visible light. They proposed that the increase of active lattice face and elimination of surface defects in high-crystallinity g-C3N4 NRs may accelerate the transfer of photo induced charges and inhibit the recombination of eh.

The facet effect mentioned in the 0-D structure is also applicable in the 1-D. In single crystalline NRs, due to the anisotropic growth of the crystallographic planes, two different preferentially oriented single

crystalline NRs could be fabricated (Fig. 6). They have different optical properties and band gaps, which may affect the PC performance [40].

3. 2 Nanowires, nanofibers, and nanobelts

These kinds of 1-D structure are longer in length than NRs, always with the longitudinal dimension on the micrometer scale and the lateral dimension on nanoscale, and they also exhibit excellent PC ability. Cha et al. [41] reported enhanced PC behavior of TiO2

rutile nanobelts. They prepared hierarchical TiO2

nano-architectures by one-pot hydrothermal synthesis in concentrated HCl. Moreover, the dislocations in nanostructured materials strongly influence not only the construction of nanoscale architectures, but also the band structure and the behavior of photo induced electronhole pairs. High-crystallinity CdS nanowires in ethylenediamine were observed to have better PC activity over nanoparticles [42]. The mechanism of the enhanced property arising from 1-D CdS nanostructure could be in line with that in single phase 1-D TiO2

nanostructure [4348].

Single crystalline 1-D nanostructure with almost ideal crystal structure and morphology usually requests rigorous conditions and has low yields. From the viewpoint of engineering and application, polycrystalline materials are much easier to fabricate.

Fig. 6 SEM and SAED pattens of single crystalline rutile TiO2 NRs with two different preferentially oriented facets. Reproduced with permission from Ref. [40], 2014 Royal Society of Chemistry.

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Several mature methods can be employed to synthesize polycrystalline nanowires/nanofibers, including hydrothermal/solvothermal process, electro spinning technology, wet chemical method with proper templates, and even physical method based on the growth mechanism of crystal nature. Gao et al. [49] synthesized perovskite-type polycrystalline BiFeO3

(BFO) nanowires with anodic aluminum oxide (AAO) as a template. BFO nanowires with 50 nm in diameter and 5 m in length were obtained when removing the template, revealing good photo induced oxidation ability. Many polycrystalline nanofiber metal oxide PCs were fabricated by electro spinning method, e.g., Bi2WO6 [50], BFO [51], TiO2 [52], and Bi4Ti3O12 [53]. In the process of electro spinning, metal salts are dissolved in appropriate solvent stoichiometrically to form precursor sol, and polymers (e.g., PVP, PAN, PVB) are added to adjust the viscosity to a practicable value. The fibers are obtained impelled by the field between the sol and the collector after annealing in air. By adjusting the proportion and concentration of the precursor sol, the diameter of the fibers can be tuned. Obviously, this kind of fibers composed of nanoparticles connected end-to-end is different from single crystalline 1-D nanostructure derived from wet chemical method.

The nanobelt photocatalyst is similar to the nanowire or nanofiber, but it is more likely to get preferential oritantion facet in nanobelts which involves in determining the PC result. Wu et al. [36] designed and prepared single crystalline anatase TiO2

(101) nanobelts by hydrothermal treatment (Figs. 7(a) and 7(b)). Their calculations showed that the exposed (101) facet of the nanobelts yields an enhanced reactivity and exhibits a lower electronhole

recombination rate than the nanospheres, which may explain the better PC degradation ability of TiO2

nanobelts (Fig. 7(c)).

3. 3 Nanotubes

Since the effective surface area is vital for photocatalysis, the construction of hollow 1-D nanostructure, also called tube in nano or micro scale, will much enhance the PC performance. What is more, the tube structure appears favoring electron transfer and has lower charge recombination than that in particles or bulk, which occurs at the activity sites on the photocatalyst surface. Nanotube-type graphitic C3N4 shown in Fig. 8(a) exhibits excellent visible light PC performance compared to the bulk graphitic C3N4

and P25 (Fig. 8(b)) [54].

Nanotube PCs can be fabricated by several methods, either with the assistant of template or not [5557]. AAO and carbon with proper processing are the most representative hard templates. The polymer fiber or carbon fiber can also be used as templates. Bi2MoO6 microtube was fabricated by using electrospun polyacrylonitrile microfibers as template following with the ethylene glycol solvothermal method [58]. By optimizing the synthesis parameters, hollow structure with high crystallinity and moderate grain size could be obtained.

Based on the nanotube shape, more evolved nanotube-like structure can be fabricated and applied in PCs. For example, Liu et al. [59] synthesized 1-D hierarchical Bi2WO6 hollow tubes with porous walls (Fig. 9(a)) by solvothermal treatment using Bi2O3 rods as both template and reactant. The hierarchical Bi2WO6 hollow tubes show the best PC degradation efficiency of RhB under the sun light compared to P25 and sphere-shaped samples (Fig. 9(c)). The nanotube PCs are usually made into oriented arrays, which are

Methyl orange concentration change, C/C 0

Nanospheres Nanobelts

50 nm

Irradiation time (min)

100

Fig. 7 (a) SEM image of the anatase TiO2 nanobelts. (b) Bright-field TEM image of a TiO2 nanobelt; the inset is an SAED pattern taken along the [100] direction of the nanobelt. (c) Degradation of MO in an aqueous solution by the TiO2 nanobelts and the nanospheres as a function of exposure time to UV irradiation. Reproduced with permission from Ref. [36], 2010 American Chemical Society.

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C/C 0

Without light Photolysis P25Bulk g-C3N4

Tube-like g-C3N4

Time (min)

Fig. 8 (a) SEM image of C3N4 tubes. (b) PC degradation of MB over samples under visible light irradiation. Reproduced with permission from Ref. [54], 2013 Royal Society of Chemistry.

Fig. 9 (a) and (b) SEM images of Bi2WO6 hollow tubes. (c) Photo degradation efficiencies of RhB with different PCs under the sun light. Reproduced with permission from Ref. [59], 2013 Royal Society of Chemistry.

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suitable as electrodes. An oriented nanoarray possesses an independent point contacting with current collector, producing a large junction area with electrolyte, thus it provides expedite charge transport pathways throughout the electrode that could maximize the charge separation ability [60].

In general, 1-D nanostructure is more and more concerned for its outstanding performance in visible light photocatalysis. Besides the advantage of specific surface area and the energy band modulated by morphology, the superior charge transfer properties and multiple adjustability are also attractive. One photocatalyst in shape of nanorod, nanowire, or nanotube may display different PC activities. Considering the similar dimension of the comparable 1-D nanostructures (nanobelts, nanoflowers, and nanowires), the status of dislocations in nanobelts, nanowires, and nanoflowers can be considered as a primary reason for different PC activities.

4 2-dimensional structures

4. 1 Nanosheets and nanoplates

Nanosheet materials usually refer to sheet-like crystallites of molecular thickness and extremely high two-dimensional anisotropy [61,62]. A variety of promising PCs belong to this category, such as SnNb2O6 [63], ZnGa2O [64], Bi2WO6 [65], BiVO4 [13], as well as various oxides and sulfide, like ZnO and TiO2.

This kind of nanoscale materials can exhibit intriguing charge-bearing properties due to large area nonstoichiometry on the surface, and photo induced carriers can survive longer on the nanosheets, which contribute to the high PC activity. Meanwhile, through controlling growth conditions, some nanosheets provide almost 100% exposed active sites [64].

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Hydrothermal method is one of the most used methods to prepare nanosheet samples. For example, Li et al. [66] obtained layered La2Ti2O7 nanosheets through the one-step hydrothermal method at 200 for 24 h when the NaOH concentration equaled 1 M. La2Ti2O7 nanosheets of 150 nm in width and 300 nm in length display superior PC properties through photo decolonization on MO and the high H2

evolution rate from waterethanol solution (Fig. 10). Two-dimensional nanosheets can also be produced by liquid exfoliation of layered materials, especially some transition metal dichalcogenides like MoS2, which is possibly extended to transition metal oxides [62].

Moreover, nanosheets can form various constructions like nanoflowers, nanosheet-scaffolded microspheres, and nanowalls. Liu et al. [64] obtained hierarchical microspheres scaffolded from ultra-thin

ZnGa2O4 (Figs. 11(a) and 11(b)) through a solvothermal route with ethylenediamine/H2O binary solvents. They possessed greater PC performance than those of the bulk ZnGa2O4 and mesoporous ZnGa2O4 due to the higher surface area, and that the ultra-thin geometry allows charge carriers move quickly from the interior to the surface to participate in the photo reduction reaction. The nanowalls are aligned nanosheets, namely, vertically aligned nanosheet array. This kind of structure combines advantages of oriented nanoarray and 2-D nanosheets.

For nanoplates, thickness can be a little larger (Fig. 12(a)). The single nanoplate is always single crystal, which can be determined by TEM results. As we mentioned above, people have found that facets with higher surface energy have higher PC activity [12,67], thus it is possible that we can make nanoplate

(a)

(b)

Degree of decolourization (%)

1

2

3

Rate of H 2evolution (mol/(g calh))

1

2

3

Time (min)

Time (min)

Fig. 10 (a) Degree of decolourization of MO solution and (b) rate of H2 evolution with the role of La2Ti2O7 samples (synthesized by (1) hydrothermal, (2) SSR) and (3) Degussa P25 under UV light. Reproduced with permission from Ref. [66], 2006 IOP Publishing, Ltd.

(a) (b) (c)

(3)

CH 4(ppm)

(2)

(1)

Time (h)

Fig. 11 (a) and (b) FESEM images of ultra-thin nanosheet-scaffolded microspheres. (c) CH4 generation over various ZnGa2O4 samples as a function of irradiation time: (1) bulk ZnGa2O4 (obtained by the solid state), (2) mesoporous ZnGa2O4, and (3) nanosheet-scaffolded microspheres ZnGa2O4. Reproduced with permission from Ref. [64], 2014 American Chemical Society.

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Dark Light on

Nanoplates (pH 6.15) Nanorods (pH 7.0)

C/C 0

Nanoplates (pH 6.15) Nanorods (pH 7.0)

Fig. 12 (a) SEM and (b) TEM images of m-BiVO4 nanoplates. (c) SAED pattern recorded from the white framed area indicated in (b). (d) Schematic illustration of the crystal orientation of the nanoplate. (e) PC degradation of RhB and (f) PC O2

evolution from an aqueous AgNO3 solution over nanoplates, nanorods, and various m-BiVO4 microcrystals prepared at different pH values under visible light irradiation. Reproduced with permission from Ref. [13], 2010 Royal Society of Chemistry.

photocatalyst with high surface energy facet exposed through choosing proper surfactant. Xi and Ye [13] reported the synthesis of well-defined m-BiVO4

nanoplates with certain exposed facets by a straightforward hydrothermal route in which no template or organic surfactant was used so that most of facet active sites can be maintained (Figs. 12(a)12(d)). These nanoplates show considerable PC activity for the degradation of RhB and the evolution of oxygen, better than other m-BiVO4 microcrystals (Figs. 12(e) and 12(f)).

In summary, the advantages of high surface area, much more reactive facets exposed, photo induced carriers being able to survive longer on the nanosheets contribute to the high PC activity, making nanosheet

PCs become more popular. To further investigate the surface of nanosheets, more effort should be applied to the construction of parent compounds with suitable layered structures.

4. 2 Films

Conventional PCs are usually utilized as suspended powders which are difficult to recollect and cause great loss, while thin film PCs are more recyclable. Also, thin film is easier made into electrode, which is necessary in PC water decomposition. Thus using thin film PCs is a tendency. Moreover, abnormal properties can be induced by the heterointerface in thin film. Previous studies have shown that the nature of the

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substrate can influence on crystal phase and optical or PC properties of thin film, like the band gap shift or phase change, which could be used to modify the PC properties of the films [6,6871].

Compared to the powder or bulk samples which can be dispersed in solutions to do experiments of photocatalysis, PC properties of films can be tested by gaseous PC experiment in a gas system. For instance, the TiO2 and WO3/TiO2 films were tested as PCs for the decomposition of 2-propanol and benzene in the gas phase [72]. The gas reactor system used for this reaction is shown in Fig. 13. 2-Propanol (or benzene) and water vapor were added in the vacuum reactor. The gas mixtures in the reactor magnetically convected during the irradiation. After irradiation at regular intervals, a bit of gas sample in the reactor was automatically picked up and sent to a gas chromatograph by using an auto sampling valve system. CO2, reactants, and other intermediates were detected and measured in gas chromatography (GC) system. WO3/TiO2 compositions can also be made into porous structure. Pan and Lee [73] prepared highly ordered cubic mesoporous WO3/TiO2 films by spin coating via an evaporation-induced self-assembly process. The mesoporous film structure TiO2 can be an efficient photocatalyst due to their ultra-high surface area and greatly organized pore structures.

Actually, even though thin films do show some PC capability and are sensitive to light irradiation in small devices, the PC ability is relatively weak because of the low concentration of PCs. Therefore, we need to find a way to improve the PC ability of thin films, like increasing surface areamaking porous thin films. Besides WO3/TiO2 films, porous TiO2, Bi2WO6, and BFO thin films on solid supports have been developed and researched [74,75]. Bi2WO6 is one of the new ternary metal oxide semiconductors showing great potential in the utilization of solar energy. Zhang et al. [76] synthesized the porous Bi2WO6 thin films as efficient visible light active PCs using carbon-sphere

template. Ordered porous Bi2WO6 films with the average size of the open pores varied from 290 to 320 nm exhibit a much higher PC activity and photocurrent conversion efficiency than nonporous Bi2WO6 films (Fig. 14). Their stability experiment indicated that the porous Bi2WO6 film is fairly stable under the studied conditions. The porous films can be synthesized by template method and chemical solution deposition. After the calcining process, we can get the well crystalline, continuously porous films. The process is shown in Fig. 13. The sphere templates can be Pluronic block copolymer P123 ((EO)20(PO)70(EO)20, EO is ethylene oxide and PO is propylene oxide), carbon-sphere, and poly styrene sphere. To increase the connectivity between the spheres and the substrate, the template should be keptat a proper temperature for a while. Besides the porous films mentioned above, discontinues nanoporous films constructed by particle-like units are also studied. Chioet al. obtained nanoporous morphology (specific surface area of 31.8 m2/g) of BiVO4 electrode (Fig. 15). They applied electro deposition method to make a template (BiOI), and then added V2O5 solution to converse BiOI to BiVO4 through heating process. They found that this kind of structure effectively suppresses bulk carrier recombination without additional doping, manifesting an electronhole separation yield of 0.90at 1.23 V versus the reversible hydrogen electrode (RHE), being beneficial to the PC performance [77,78].

In the future, people are supposed to keep looking for other novel 2-D PCs with high PC efficiency. Cooperating with computational strategy, people can predict or explore more novel highly efficient 2D materials for PC materials [79].

5 Composites

To prevent the recombination of electronhole pairs,

Fig. 13 Left: the gas reactor system used for this PC reaction. Right: an illustration of the process of fabricating porous thin films.

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Fig. 14 (a) SEM image of a porous Bi2WO6 film synthesized with 390 nm carbon spheres. (b) Photo degradation of MB in the presence of porous Bi2WO6 films with pore sizes of 290 and 320 nm under visible light irradiation; the nonporous film was studied for comparison. (c) Potentiodynamic scans under chopped illumination for porous and nonporous Bi2WO6 films. (d) Action spectra of porous and nonporous Bi2WO6 film electrodes. Reproduced with permission from Ref. [76], 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Fig. 15 Morphologies of nanoporous BiVO4 electrodes: (a) SEM image of BiOI, (b) top-view of BiVO4, (c) and (d) side-view of BiVO4. Reproduced with permission from Ref. [78], 2014 American Association for the Advancement of Science.

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the approach that has generally been applied is to load co-catalysts such as Pt, Pd, NiO, and RuO2 on the semiconductor surface. Lots of experiments have proved that, compared to the composites with components simply mixed, those with components intimately attaching each other are more efficient in photocataysis. The heterojunctions formed between the host semiconductor and the co-catalyst provide an internal electric field that facilitates separation of the electronhole pairs and induces faster carrier migration. Furthermore, these co-catalysts exhibit better conductivity, lower overpotential, and higher catalytic activity than the host. Enlightened by this, people make kinds of composites to construct internal electric field induced by the bend of electric energy band, or modify the surface acidity. The internal fieldis the origin force for the separation of photo induced eh, which is important in improving the efficiency of utilization of carriers involved in the reaction and enhancing the PC efficiency [10].

The most typical internal field is that in pn junction, which is the foundation for conventional photovoltaic solar energy conversion. When p-type and n-type semiconductors contact each other, charges transfer because of the concentration difference of holes and electrons so that the Fermi levels can equilibrate. The charge redistribution creates an electric field in the space charge region from n-type side to p-type side, which helps to separate photo generated charge carriers and improve the efficiency of PC reactions. At phase boundaries, the difference in the positions of band edges can create an internal field and thus separate charges. The photo induced electrons always transfer through the interface (junction) from higher conductive band to the lower one, while the holes from lower valence band to the higher one. Besides, the transfer rate of holes (14 ps) is much faster (40 times) than that of electrons (40160 ps). Theoretically, three kinds of junction constructions are able to improve the efficiency of PC activity (Fig. 16). This is the separation caused by the band bending and the open

circle potential (OCP) of which should be smaller than the band gap of the photocatalyst. Meanwhile, the OCP arised from ferroelectric phenomena could be much larger than the band gap of the material, and this is the reason why ferroelectric materials with small band gap are now widely researched [80].

Internal electric field in ferroelectric compositions (capacitors) can be classified as two independent components and written as

E E E

where bi

bi p

E represents the built-in field and p

E is the

depolarization field:

F

d

P

E d

p

2 /2 / /

0 F F e

where F

is the relative dielectric constant of the ferroelectric thin film; e

relative of the electrode layer; the screening length; P the polarization value; d the ferroelectric film thickness [81]. The depolarization field p

E is proportional to the strength of polarization; the built-in field bi

E originates from boundary contact. Especially in the film photocatalyst, the equation helps people design high-efficiency structure.

The composite PCs could be constituted by two different semiconductors or two different nanostructures of one semiconductor. In this review,we present some examples, classifying them into 00, 01, 02, and other composites.

5. 1 00 composites

00 composites indicate PCs composed with two or more 0-D structures which are very common, including oxideoxide nanoparticle composites, quantum dots/nanoparticle noble metal, or sulfideoxide composites.

For the oxideoxide, we take TiO2 for example. People have tried to composite it with other oxides like ZrO2, WO3, BFO, etc. TiO2ZrO2 nanocomposite synthesized by hydrothermal method was observed a

Fig. 16 Three types of band bending for the carrier transferring.

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shift in the absorption edge towards longer wavelength, which indicates the sub band gap absorption arises from surface states of the TiO2ZrO2 material [82]. BFO/TiO2 coreshell structured nanocomposites (Figs. 17(a) and 17(b)) have good visible light absorption properties (Fig. 17(c)), which should be induced by Fe or Bi/Ti inter diffusion in interfaces and the band bend in the boundary (Fig. 17(d)) [83]. Other oxide composites have also been extensively investigated. n-Type NaTaO3 loaded with p-type co-catalyst NiO, synthesized by heated mix of NaTaO3

powders and aqueous Ni(NO3)26H2O solution, shows enhancement water splitting activity under UV illumination [84].

Among quantum dot (or nanoparticle) noble metal (or nonoxide semiconductor)oxide types, the widely used CdS, CdSe, or other sensitizing dye molecules

can strongly increase solar conversion efficiency. Ag, Pt, and Au are popular noble metals in decorating PCs to enhance the PC activity remarkably [85,86]. BFO particles with surface decoration of Ag nanoparticles displayed enhanced PC activity (Fig. 18(a)). The TEM image shown in Fig. 18(b) reveals that Ag nanoparticles with size of 620 nm are uniformly deposited on the surface of BFO crystals. Ag has a Fermi level of 0.4 V versus normal hydrogen electrode (NHE), which is positive to the CB potential of BFO (0.1 V vs. NHE) (Fig. 18(c)). Compared to bare BFO particles, in the Ag decorated samples, the photo generated electrons are thermodynamically favorable to transfer from the CB of BFO to Ag nanoparticles, consequently benefiting the PC process [87].

To clearly explain the internal electric field that facilitates separation of the electronhole pairs and

Fig. 17 (a) TEM and (b) HRTEM images of BFO/TiO2 nanocomposites. (c) Normalization concentration of CR in the solution with different PCs versus the exposure time under visible light irradiation: (1) without catalysts, (2) pure TiO2, (3) pure BFO, BFO/TiO2 nanocomposites with mass ratios of (4) 1:2, (5) 2:1; and (6) 1:1. (d) Schematic illustration for the calculated energy level diagram of TiO2 and BFO. Reproduced with permission from Ref. [83], 2009 AIP Publishing LLC.

Fig. 18 (a) PC ability of bare BFO and Ag decorated BFO. (b) TEM image of Ag decorated BFO. (c) Schematic illustration of the PC mechanism of Ag@BFO photocatalyst toward the degradation of RhB. Reproduced with permission from Ref. [87], 2014 Springer Science+Business Media New York.

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induces faster carrier migration formed in noble metaloxide heterojunctions, we take the n-type TiO2Pt photo electrochemical cell for example, which is similar to nanoscale powder photocatalyst (Fig. 19(a)). If the CB

E is higher than the hydrogen evolution potential, photo generated electrons can flow to the counter electrode and reduce protons, resulting in hydrogen gas evolution without an applied potential, although there should be big enough value versus the standard hydrogen electrode. The remaining holes in the valance band (VB) are active on the TiO2, available for oxidation reactions [88].

In the noble metal loaded system, besides the low electronhole recombination, the effective light absorption by noble metal like Au and Ag which subsequently inject the generated hot electrons to the CB of semiconductors is believed another main reason for the enhanced photocatalysis, called plasma resonance enhancement of photocatalysis [89]. The practical performance of plasmon-exciton strongly depends on the mutual arrangement, particle size, shape of the metal and semiconductor, and the electronic structure of metal [90]. To load these noble metals on PCs, people choose several methods. For example, Au can be added with coprecipitation, electrochemical deposition, chemical vapor deposition, laser vaporization, modified impregnation, and photo deposition method [91]. Among these methods, photo deposition method is facile, common, and widely applicable [77,92,93].

Sometimes, people make one low-dimension structure with two or more different materials, called poly-component 1-D, 2-D, or hybrid nanostructures, like bicomponent nanosheets, but we still classify them as 00 type because the components attach each other

in 0-D form. With two or even more components assembling into an integrated heterostructure, the internal field built in heterojunction and the transfer of the photo induced electronsholes could be more favorable for the conversion of visible light than the single phase alone. Several approaches could be used to fabricate 1-D hybrid nanostructures [9499].

5. 2 01 composites

Besides the nanoparticles, 1-D or 2-D structure composites also occupy an important position. 01 PC composites come from decorating 0-D semiconductors or noble metals on 1-D PCs, e.g., CdSe QDs/ZnO nanotubes, CdSe QDs/ZnO nanowires, CdS QDs or Ag/TiO2 nanotubes, Pt/WO3 nanotubes [100104]. These 01 PCs show incredibly enhanced visible light PC performance. And like that in the 00 type, special methods like solvothermal technique and sequential chemical bath deposition could be used to obtain composites with intimately attached boundary rather than simple mixture.

The bonding 01 composites have the advantages of 0-D and 1-D structures, and junction forms around the boundary. The TiO2 nanoparticles (with uniform diameter 5 nm) with single-walled carbon nanotube attached exhibit higher PC activity than that of Degussa P25 when photo degrading RhB aqueous solution under UV light. The excellent PC activity could be attributed to larger surface area, and especially the ester bonds formed at the junction [103]. Cu2O/TiO2 nanotube array (NTA) photo electrodes prepared by the ultrasonication-assisted S-CBD, made of n-type TiO2 NTAs with Cu2O nanoparticles on them (Figs. 20(a) and 20(b)) exhibit more effective photo conversion capability than TiO2 nanotubes alone.

Fig. 19 (a) Schematic representation of powder photocatalyst. (b) Schematic representation of photo electrochemical water electrolysis using illuminated oxide semiconductor electrode.

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Cu2O/TiO2 composite photo electrodes possess superior photo electrocatalytic activity and stability in the degradation of RhB (Figs. 20(c) and 20(d)) [105]. The composites always display a better PC performance than the individual part, and modulating the properties of the components in the composites

may further improve the PC efficiency. For example, in the composites of Cu2O nanoparticles/ZnO NR arrays (Figs. 21(a)21(c)), the samples with Cu2O prepared in stronger alkaline solution have the highest PC efficiency due to higher carrier concentrations (Fig. 21(e)). In Cu2O (pH = 11.0)/ZnO NR arrays, the

Fig. 20 (a) SEM image and (b) low magnification TEM image of Cu2O/TiO2 NTAs (top view). The inset in (a) is the close-up. (c) Photo response of Cu2O/TiO2 NTAs prepared for (1) 4 min, (2) 9 min, (3) 1 min, (4) 19 min, and (5) TiO2 NTAs. (d) PC degradation rates of RhB under visible light irradiation for TiO2 NTAs and Cu2O/TiO2 NTAs prepared for different time. Reproduced with permission from Ref. [105], 2013 Royal Society of Chemistry.

Fig. 21 (a) SEM and (b) and (c) TEM images of Cu2O/ZnO NR arrays. (d) Light intensity dependent surface photo voltage (SPV) of Cu2O/ZnO NR arrays at 532 nm. (e) Concentrations of methylviologen reduzate formed over different Cu2O/ZnO NR arrays under visible light ( > 400 nm) irradiation. Reproduced with permission from Ref. [37], 2013 Royal Society of Chemistry.

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higher carrier concentration was believed to generate a more positive Fermi level, leading to photo induced charge carriers being separated quickly and efficiently under the largest interfacial electric field (Fig. 21(d)) [37].

Usually, the metal part in 01 composites works as

0-D shape, quantum dots or nanoparticles, but 0-D oxide compositing with 1-D metal has also been studied, and their PC enhancement mechanisms are consistent. Zhou et al. [106] coated uniform TiO2 on Au NRs and Au/Ag coreshell NRs, and evaluated their PC activities by the degradation of MB under visible light irradiation. They observed that the presence of plasmonic metal NRs enhances the PC activity of TiO2, especially the bimetallic Au/Ag NRs.

5. 3 02 composites

02 composites indicate that 0-D organic molecules or premade semiconductor nanocrystals are grafted or deposited onto the surface of beforehand 2-D PCs. Lots of teams have been studied, like WO3 or MoO3 nanocrystal/TiO2 films, CdS nanoparticle/ TiO2 nanosheet, Pt-loaded WO3-sodium titanate (NaxH2x)Ti2O4(OH)2 network film, CdS or CdSe /TiO2 mesoporous thin films (Fig. 22(b)) [106108].

02 type shares some similarities with 01 type, also combining the advantages of 0-D and 2-D structures,and junction forms around the boundary together. Herewe present two examples of nanoparticles/mesoporousfilm PCs. The PC activity of highly ordered cubic mesoporous film composites WO3/TiO2 was 2.2 times higher than a mesoporous TiO2 film and

6.1 times higher than a nonporous TiO2 film in photo decomposing 2-propanol in the gas phase [106]. Meanwhile, the presence of WO3 strengthens the stabilization of the mesoporous structure. In this case, it was also believed the enhanced PC activity of WO3/TiO2 is ascribed to the increase in surface acidity induced by the WO3 present on the surface of the mesopores [73]. Bartl et al. [107] synthesized CdS/TiO2 mesoporous thin films by thermally post-treated Cd2+-doped TiO2 films with sulfur vapor, and the structure shows an enhanced sensitivity to visible light. In Fig. 23, by switching from UVVis to Vis-only illumination (right part of curves), the pure anatase titania mesoporous films fail to produce a measurable ph

I , whereas the composite CdSanatase titania mesoporous film is still able to generate a clear on/off ph

I response, which is around 25% of the total UVVis ph

I . These discoveries imply that making 02 structure broadens photo response band.

Although the 0-D particles used to decorate 2-D PC are usually noble metals or semiconductors, loading 0-D PC on non-PC with 2-D structure like graphene (Fig. 22(a)) is also effective [109,110].

5. 4 Others (11, 12, 22)

There are other combination types apart from those mentioned above, such as 11, 12, 22, and even 012.

The nanotree-like CdS/ZnO nanocomposite is one typical 11 structured photocatalyst, with ZnO NRs on the surface of CdS nanowires [111]. This kind of spatially branched hierarchical structure was proved

Fig. 22 (a) SEM image of BFOgraphene nanohybrids. Reproduced with permission from Ref. [109], 2012 Royal Society of Chemistry. (b) TEM image of CdSe/cubic mesoporous titania. Reproduced with permission from Ref. [107], 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

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Fig. 23 Periodic on/off photocurrent ph

I response curves of (a) nanocrystalline pure anatase titania and (b) nanocrystalline composite CdS/anatase titania cubic mesoporous films illuminated with a periodically light/dark modulated Xe lamp. Arrows indicate the switching from UV and visible photons simultaneously (left side of the arrow) to illumination with visible photons only (right side of the arrow). Reproduced with permission from Ref. [107], 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

with high light harvesting efficiency, which also boostscharge separation and fast charge transport. An intriguing structure was fabricated by Yang et al. [112]. They synthesized CdS/ZnO core/shell nanofibers, one of the 11 types (Figs. 24(a) and 24(b))by one-pot single-spinneret electro spinning. The CdS/ZnO core/shell nanofibers display high H2

production efficiency (Fig. 24(c)). An et al. [113] studied the WO3 NRs/graphene composites (could be classified to 12 type) (Fig. 25(a)), and the composites display high-efficiency visible-light-driven PC ability (Fig. 25(b)). Sun et al. [114] composited ordered m-BiVO4 nanotubes and 2-D graphene (Fig. 25(c)), and found the composites exhibit unprecedented visible-light-driven PC activities, over 15 times faster than that of commercial P25 or bulk BiVO4

(Fig. 25(e)). The improvement may be due to the efficient electron transport along the axial direction of nanotube, decreasing aggregation, and easy aqueous dispersion. The synergistic effects between the

microscopic crystal structure and macroscopic morphological features of 1-D quantum tubes2-D sheets composites help provide increased PC reaction sites, enhanced charge transportation, and separation efficiency simultaneously.

Last but not the least, we can make PCs by combining three dimensions together according to the need of the materials, which is the 012 type. Xie and his co-workers [115] coated TiO2 ultra-thin layers made with ALD method in different thickness on the TiO2 NRAs/CdS QDs, and observed that 2 nm thick TiO2 thin layer covered by TiO2 NRAs/CdS QDs (Figs. 26(a) and 26(b)) enhances the degradation rate of MO under visible light compared to the samples without TiO2 thin layer cover (Fig. 26(c)). The remarkable PC ability is due to the enhancement of the separation of eh and the reduced photo-corrosion of the CdS QDs induced by the introduction of the ultrathin TiO2 top layer.

Fig. 24 (a) TEM image of the samples (CdS)1/(ZnO)1. (b) HRTEM image of the interface between core CdS and shell ZnO. (c) PC H2 production efficiency of (CdS)x/(ZnO)y. Reproduced with permission from Ref. [112], 2013 Royal Society of Chemistry.

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Fig. 25 (a) TEM image of WO3 NRs/graphene composites. (b) Visible-light-driven photo degradation of RhB by WO3/graphene composites and other WO3-based PCs. Reproduced with permission from Ref. [113], 2012 Royal Society of Chemistry. (c) High magnification TEM image for ordered m-BiVO4 quantum tubesgraphene nanocomposites. The inset is a schematic illustration of 12 type. (e) RhB concentration changes over several PCs under visible light irradiation. Reproduced with permission from Ref. [114], 2012 Royal Society of Chemistry.

Fig. 26 (a) SEM and (b) HRTEM images of TiO2 NRAs/CdS QDs coated with ALD TiO2 with 60 cycles. The 2 nm coating layer can be clearly seen. (c) Visible light ( > 420 nm) degradation behavior over MO of the TiO2 NRAs/CdS QDs coated with different cycles of ALD TiO2, and the samples with 60 cycles did the best. Reproduced with permission from Ref. [115], 2014 Elsevier Ltd.

6 Conclusions

In this review, we have selectively discussed and summarized the recent progress in the field of low-dimensional PCs. These PCs with special

structures possess excellent optical and PC properties due to high eh separation efficiency and large surface area, representing an encouraging prospect in the realization of a sustainable society. By applying the advanced low-dimensional PCs, people could convert

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light energy to other forms, which is useful in decomposing harmful substances, producing storable fuels, and detecting light signals. In the future, these PCs are supposed to be dedicated to more practical real-life applications, including being used to treat factory pollutant, purify the air, design indoor degerming or self-cleaning household items, make photoelectric sensors, and manufacture novel batteries and hydrogen generator.

The promising opportunities offered by the low-dimensional structures are outstanding. On one hand, we can make delicate structure via those advanced methods mentioned before to take the place of the traditional bulk materials. On the other hand, we can carry out modification on the base of the low-dimensional structures. In addition to the structure construction, people have learnt that energy band configuration of a photocatalyst affects greatly the utilization of light and determines the redox potentials, that is why energy band engineering plays a significant role in the field of looking for high-efficiency PCs. As mentioned above, the band gap of a photocatalyst may vary with the morphology, size, dimension, but doping elements is the main method to achieve band gap modification. For the single phase PCs, people prefer those with small band gaps. According to the orbit hybridization and band theory, doping proper both non-metal and metal elements in photocatalyst can narrow the band gap. Introducing non-mental anions (e.g., N, C, S, P, B, F) on the O sites could raise the VBM, extending the light absorption region toward visible light region. Doping metal elements (e.g., Bi, Co, Ce, Mn) can either lower the conduction band minimum (CBM) or raise the VBM, and thus improve the PC ability. The relationship between doping concentration and PC efficiency is not liner, but the optimal value exists. Comparing to non-metal doping, metal doping obsesses more choices. People have tried almost every possible metal element to dope the PCs, including 3d-transition elements, p-block cations with d10 configuration, d-block cations that have a d0 electronic configuration, etc. Designing proper band combination via band modification that benefits carrier separation the most is worth considering. Considering the band modification and structure construction together, we can combine the element doping with the structure configuration, such as manufacturing element-doped nanotube oxide PCs loaded with noble metal quantum dots. In this way, the eh can be highly efficiently separated; the band gap can be narrowed

simultaneously, which are two of the key issues in photocatalysis. In the case of Ag loaded N-doped TiO2

nanotubes, the sample has higher light adsorption and better degradation of acid orange-II than both N-doped TiO2 NTs and Ag loaded TiO2 nanotubes [102].

Although we have incredible opportunities, challenges in the areas of materials science and engineering still exist. For example, theoretical prediction to the activity or explanation for the performance of PCs with various structures need develop. More specifically, although a number of theories have been proposed to explain the PC selectivity along different crystal orientations, like theories based on facet surface energy, polarization state, and effective mass, no satisfactory and comprehensive understanding of mechanisms involved has yet been arrived at. Moreover, from the point of view of industrialization and commercialization, how to get sophisticated structures much more efficiently remains a question. In the future, with a more complete theory system established and technical hurdles addressed, high efficient and low-cost low-dimensional PCs are expected to be developed.

It hopes that this review will provide useful information in the utilization of low-dimensional nanostructures for versatile PC applications, and inspire growing efforts in the area of low-dimensional nanostructure based photocatalysis.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Nos. 51272121, 51221291, 51328203, and 51025205).

Open Access: This article is distributed under the terms of the Creative Commons Attribution License which permits any use, distribution, and reproduction in any medium, provided the original author(s) and the source are credited.

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