Tantalate-based Perovskite for Solar Energy Applications

To realize a sustainable society in the near future, the development of clean, renewable, cheap and sustainable resources and the remediation of environmental pollutions using solar energy as the driving force would be important. During the past few decades, plen‐ ty of efforts have been focused on this area to develop solar light active materials to meet the increased energy and environmental crisis. Owning to the unique perovskite-type structure, tantalate-based semiconductors with unable chemical composition show high activities toward the conversion of solar radiation into chemical energy. Moreover, vari‐ ous engineering strategies, including crystal structure engineering, electronic structure engineering, surface/interface engineering, co-catalyst engineering and so on, have been developed in order to modulate the charge separation and transfer efficiency, optical ab‐ sorption, band gap position and photochemical and photophysical stability, which would open a realm of new possibilities for exploring novel materials for solar energy applica‐ tions.


Introduction
In view of global energy crisis and environmental pollution, the search for renewable and clean energy resources and the development of eco-friendly systems for environmental remediation have received great attention. Solar energy is the prime renewable source of energy for every life on the earth. The amount of solar energy that strikes the Earth yearly in the form of sunlight is approximately ten thousand times the total energy that is consumed on this planet [1]. However, sunlight is diffuse and intermittent, which impedes its collecting and storage that play critical roles in the full exploitation of its potentials. As one of the most promising solutions for storing and converting solar energy, semiconductor photocatalysis has attracted much attention, since it provides an environmental benign strategy for splitting water into hydrogen and oxygen, reducing carbon dioxide into useful chemicals and fossil fuels, and completely eliminating all kinds of contaminants under the sunlight illumination under ambient conditions [2][3][4]. Generally, the fundamental principles of semiconductor photocatalysis have been extensively reported in previous works [5]. A photocatalytic reaction is initiated on the basis of the formation of photogenerated charges (such as electrons and holes) after capture of sunlight by a semiconductor. Consequently, electrons can transit from valence band (VB) to conduction band (CB) leaving behind holes in the VB. If the photogenerated electron-hole pairs' separation is maintained, the photogenerated carriers can move to the semiconductor surfaces to react with the adsorbed small molecules (dioxygen and water), generating the redox foundations of active species which lead to water splitting and/or destruction of organic compounds [6]. It is also noted that the photogenerated electrons in the CB can also recombine with the photogenerated holes in the VB to dissipate the input energy in the form of heat or radiated light (see Figure 1). From the perspective of efficient utilization of solar energy, the recombination between the photogenerated electrons and holes is not desired, which limits the efficiency of a semiconductor photocatalyst. For better photocatalytic performance, the photogenerated electrons and holes must be separated effectively, and charges have to be transferred rapidly at the mean time across the photocatalysis to astrict the recombination. To date, several semiconductors, including TiO 2 , ZnO, SnO 2 , BiVO 4 and so forth, have been extensively investigated [7][8][9][10]. Among them, tantalate-based semiconductors with perovskite-type structure have certainly verified to be one of the brilliant photocatalysts for producing hydrogen from water and the oxidative disintegration of much organic containment [11]. For instance, NaTaO 3 with a perovskite-type structure showed a quantum yield of 56% under ultraviolet light irradiation after lanthanum doping and NiO co-catalyst loading [12]. Nevertheless, because of their broad band gap, most tantalate-based semiconductors can only react under ultraviolet or near-ultraviolet radiation, which reduces the utilization of ~43% of the solar spectrum To efficiently utilize the sunlight in visible region, the design of visible-light response tantalate-based catalysts is current demanded. Up to now, numerous methodologies have been developed to prepare different visible-light-driven tantalate-based photocatalysts, including doping strategy, heterojunction, facet control and so on [13,14].
This chapter emphasizes certain topical works that concentrate on tantalate-based photocatalysts for solar energy application. The aim is to display that the rational design, fabrication and modifications of tantalate-based semiconductors have tremendous effects onto their final photocatalytic activity, simultaneously, providing some stimulating perspectives on the future applications.

Synthetic strategies of alkali tantalates
Alkali tantalate-based perovskite semiconductors (likewise LiTaO 3 , NaTaO 3 and KTaO 3 ) have a general formula of ABO 3 and have drawn a lot of attention due to the peculiar superconductivity, photocatalytic property, electrochemical reduction and electromagnetic features. There are two kinds of totally different cationic sites in a perovskite photocatalysis, in which A-site is coordinated by twelve O 2-, and is usually occupied by relative bigger cations (Li, Na and K). The B-site is taken up by smaller cations (Ta) with a coordination of six, as illustrated in Figure 2. The bond angles of Ta-O-Ta are 143 o for LiTaO 3 , 163 o for NaTaO 3 and 180 o for KTaO 3 , respectively [15]. Wiegel and coworkers have reported the relationship between crystal structures and energy delocalization for alkali tantalates. When the Ta-O-Ta bond angle is close to 180 o , the migration of excitation energy can be accelerated and the band gap decreases [16]. Thereby, the delocalization of excited energy of LiTaO 3 , NaTaO 3 and KTaO 3 increases in turn. This result suggests that KTaO 3 may be predicted to be with the best photocatalytic activity. Alkali tantalate with different sizes, morphologies and compositions can be prepared via traditional solid-state method, solvothermal, sol-gel, molten salt and other methods. Basically, the traditional solid-state method is quite often used to prepare alkali tantalates, which includes the high temperature processing of the combination of alkali salts and tantalum pentaoxide. Kudo and coworkers successfully prepared ATaO 3 (A = Li, Na and K) materials with high crystallinity via solid-state method. It is found that all alkali tantalate showed superior photocatalytic activity toward stoichiometric water splitting under ultraviolet condition [17]. The high photocatalytic activity is chiefly depending on the high CB level consisting of Ta 5d orbitals [15]. Among them, KTaO 3 is the most photocatalytic active, which may be ascribed to the fact that KTaO 3 can absorb the most of photons and possesses the least distorted perovskite structure, being consistent with the above-mentioned discussion. The evolution rate of H 2 and O 2 was determined to be 29 and 13 μmolh -1 , respectively. To improve more photocatalytic activity of ATaO 3 , a modified solid-state method was adopted by adding extra amount of alkali to compensate the loss [18]. When preparing the alkali tantalates with the existence of excess alkali, the photocatalytic activity of LiTaO 3 , NaTaO 3 and KTaO 3 materials were improved ten to hundred times. LiTaO 3 is the naked alkali tantalate photocatalyst which showed the highest activity. This is because LiTaO 3 possesses higher conduction band levels than that of NaTaO 3 and KTaO 3 , which may predict higher transfer rate of excited energy and the subsequent higher phtocatalytic activity. This type of phenomena was likewise observed for CaTa 2 O 6, SrTa 2 O 6 and BaTa 2 O 6 photocatalysts with similar crystal structures [19]. One should note that the synthetic strategy also has great influence on the structural features as well as photocatalytic activity. For instance, sol-gel method was also used to prepare NaTaO 3 nanoparticles. By using CH 3 COONa 3H 2 O and TaCl 5 as the raw materials and citric acid as the complexing agent, NaTaO 3 nanoparticles with monoclinic phase that shows an indirect band gap, high densities of states near the band edges and a Ta-O-Ta bond angle close to 180 o (Figure 3) are obtained. This result is quite different to NaTaO 3 that was synthesized via solid-state method, which formed the orthorhombic phase that has a direct band gap and a Ta-O-Ta bond angle of 163°. It is found that monoclinic NaTaO 3 has lots of effective states available for the photogenerated charge pairs. Meanwhile, the larger surface area and the advantageous features in the electronic and crystalline structures for the monoclinic NaTaO 3 have resulted in a remarkably higher photocatalytic activity for the sol-gel synthesized NaTaO 3 than that for the solid-state derived orthorhombic NaTaO 3 [20]. Besides sol-gel method, the molten-salt approach is also adopted to prepare alkali tantalate materials [21,22]. By a convenient molten-salt process, a series of NaTaO 3 and KTaO 3 efficient photocatalysts is successfully synthesized, which are highly crystallized single crystal nanocubes (about 100 nm large). Doping tetravalent Zr 4+ and Hf 4+ in NaTaO 3 and KTaO 3 efficiently increases the activity and stability of catalyst at the same time, although the energy levels have no change. Moreover, Zr 4+ and Hf 4+ doping can also led to particle size reduction and nearly monodispersed feature of NaTaO 3 and KTaO 3 nanoparticles. In the absence of co-catalyst, the photocatalytic activity can reach 4.65 and 2.31 mmolh -1 toward H 2 and O 2 production, respectively [22]. A novel kind of strontium-doped NaTaO 3 mesocrystals was also prepared by a common molten-salt way. The obtained three-dimensional architectures showed high crystallinity, preferred orientation growth and high surface area. The ability for hydrogen generation of photocatalyst achieves 27.5 and 4.89 mmolh -1 for methanol aqueous solution and pure water splitting under ultraviolet light irradiation [23]. However, either solid-state method or molten-salt approach often leads to ultra-low surface areas of alkali tantalates, which limits the photocatalytic activity. Hydrothermal synthesis is advantageous for regular nucleation of nanocrystals with welldefined particles, morphologies, crystallinity and surface areas [24]. For instance, nano-sized Ta 2 O 5 and NaTaO 3, KTaO 3 and RbTaO 3 cubes are prepared by a facile hydrothermal method [25]. It is observed that pH influences much in the process of tantalum compound nanoparticles preparation. The obtained morphologies ranging from agglomerated particles in acidic medium over sticks at neutral pH value to cubes in elementary media can be achieved, which are similar to titanates [26]. A microwave-assisted hydrothermal technique was reported using Ta 2 O 5 and NaOH as starting materials under quite mild conditions with short reaction time. The BET surface area of NaTaO 3 nanoparticles prepared by microwave-assisted hydrothermal method is about 1.5 times than that prepared by conventional hydrothermal method [27]. After loading NiO as co-catalyst, this photocatalyst showed photocatalytic activity for overall water splitting more than two times greater than those prepared by conventional hydrothermal process [28]. As an outstanding example, NaTaO 3 nanoparticles through hydrothermal treatment highly improved the photocatalytic activity by a factor of 8 toward water splitting in comparison with the photocatalysts obtained by traditional solid-state method, which is attributed to their smaller particle size, larger surface area and higher crystallinity [29,30].

Doping strategies
Introducing foreign elements, including metal ions or non-metal ions, into semiconductor host matrix is one of the most effective methods to modulate the electronic structure of the host semiconductor and produce enhanced photocatalytic performance. Owing to a big difference in radius of A-and B-site ions in alkali tantalates, the dopants can selectively permeate into the A or B sites, which determine chemical composition, surface features, electronic structure and their photocatalytic properties. To date, studies on the alkali tantalates derived by doping strategy are thoroughly investigated. La-doped NaTaO 3 is the most active photocatalyst in photocatalytically splitting water area [12]. In this case, the catalytic activity of NaTaO 3 is extremely modulated by doping with La 3+ . For example, the crystallinity growths and a surface stair structure with nanometer-scale features are constructed, which improve the separation efficiency of the photogenerated electron-hole pairs and the photocatalytically splitting water activity. The surface step structure is also formed in alkaline earth metal ion doped NaTaO 3 , which showed improvement of photocatalytic water splitting properties [31]. Bi 3+ doped NaTaO 3 nanoparticles are prepared under different initial stoichiometric ratio by traditional solid-state reaction, which showed visible light absorption and tunable photocatalytic activity. Controlling the original molar ratio of the reactants, the intrusion of bismuth at sodium site and tantalum site in NaTaO 3 can be well-modulated and the optimum performance can be easily changed. Occupancy of Bi atom at Na site of NaTaO 3 is not contributing to increase visible light absorption while occupancy of Bi at Ta site or at both Na and Ta site induces visible light absorption and the subsequent methyl blue degradation under visible light [32,33]. La, Cr codoping NaTaO 3 system is also developed by spray pyrolysis from aqueous and polymeric precursor solution. The hydrogen evolution rate of La, Cr codoped NaTaO 3 was enhanced 5.6 times to 1467.5 μmol g -1 h -1 , and the induction period was shortened to 33%, compared to the identical values achieved by the Cr-doped NaTaO 3 photocatalyst prepared from aqueous precursor solution [34]. Besides metal ion doping, several non-metal ions are also incorporated into the host matrix of alkali tantalates for improved visible light absorption and photocatalytic performance [35,36]. A plane-wave-based density functional theory calculation is conducted to predict the doping effects on the variations of the band structure of non-metal ions doped NaTaO 3 . There were studies about nitrogen, sulfur, carbon and phosphorus monodoping and nitrogen-nitrogen, carbon-sulfur, phosphorus-phosphorus and nitrogen-phosphorus codoping. Nitrogen and sulfur monodoping can improve the valence band edge to higher and keep the ability to split water into H 2 and O 2 remain unchanged, as is shown in Figure 4. Double hole-mediated codoping can decrease the band gap dramatically. Nitrogen-nitrogen, carbonsulfur and nitrogen-phosphorus codoping could narrow band gap to 2.19, 1.70 and 1.34 eV, respectively, which could absorb visible light.

Defect chemistry engineering
Defect chemistry plays an important role in modulating the electronic structure, charge carrier conductivity and photocatalytic performance [38]. Defect chemistry often shows different impacts on the photocatalytic efficiency for most of the semiconductors. Previous literature on NaTaO 3 indicated that the accretion of the extra quantity of Na in the synthesis of NaTaO 3 blocked construction of sodium ion defects in NaTaO 3 crystals, leading to the extreme enhancement of photocatalytic activity [18]. Basically, the native defects, such as oxygen vacancies and sodium ion defects, are often observed in NaTaO 3 . Oba and coworkers investigated the formation energies and electronic structure of lattice vacancies, antisite defects and lanthanum impurities in NaTaO 3 using first-principles calculations based on density-functional theory [39]. Under oxygen-poor environments, oxygen vacancy as a double donor is a main defect. In La-NaTaO 3 , the replacement of La at Ta site is similar to make up as a shallow acceptor under oxygen-rich environments whereas the replacement of La at Na site forms as a double donor under oxygen-poor environments. The location predilection of lanthanum leads to self-compensation in heavily doped cases, which have great impact on the change in carrier concentration and photocatalytic activity [12]. Defective center not only alters the carrier concentration but also induces visible light absorption. In Eu 3+ doped NaTaO 3 , a nonstoichiometric Na/Ta molar ratio led to site-selective occupation of Eu 3+ dopant ions, which resulted in a monotonous lattice expansion and local symmetry distortion [11]. The site-selective occupation of Eu 3+ gave rise to certain types of defective centers due to the charge difference between Eu 3+ ions and Na + and/or Ta 5+ ions, which is crucial to the modification of absorption in visible region and photocatalytic activity.

Heterojunction of nano-/microarchitectures
The constructions of heterojunction by combining a semiconductor with other semiconductors have attracted much research attention because of their perfect effectiveness in the separation of the photogenerated charge carriers and boosting the photocatalytic activity. In the past few years, a lot of significant findings have been described on the heterojunction of nano-/ microarchitectures. Nano-Cu 2 O/NaTaO 3 composite for the degradation of organic pollutants have also been successfully developed [13]. Nano-Cu 2 O/NaTaO 3 composite exhibits highly enhanced photocatalytic activity in comparison to their individual counterpart. Furthermore, C 3 N 4 /NaTaO 3 and C 3 N 4 /KTaO 3 composite photocatalysts were also developed [40,41]. Loading of C 3 N 4 is a good strategy to achieve the visible light photocatalytic activity ( Figure 5). Photogenerated electron jumped from the VB to CB of C 3 N 4 could unswervingly insert into the conduction band of NaTaO 3 or KTaO 3 , making C 3 N 4 /NaTaO 3 and C 3 N 4 /KTaO 3 as visible light-driven photocatalyst. Both of the composites showed superior photocatalytic activity toward Rhodamine B degradation under visible light irradiation, being close to commercial P25. Yin and coworkers reported the preparation of novel C-NaTaO 3 -Cl-TiO 2 composites via a facile solvothermal method. When C-NaTaO 3 is joined with Cl-TiO 2 to construct a core shell configuration, the visible light-induced degradation activity toward NO x of the catalysts under visible light irradiation could be highly improved because of the suppression of the recombination of photogenerated charge carriers [42]. Zaleska et al. prepared a series of novel binary and ternary composite photocatalysts based on the combination of KTaO 3 , CdS and MoS 2 semiconductors via hydro/solvothermal precursor route. They found that the highest photocatalytic activity toward phenol degradation under both UV-Vis and visible light irradiation and superior stability in toluene removal was observed for ternary hybrid obtained by calcination of KTaO 3 , CdS and MoS 2 powders at the 10: 5: 1 molar ratio [43].

Mesoporous structures construction
As one of the most important factors, surface area also imposes a big effect on the photocatalytic activity of the semiconductors. The majority of photocatalytic reactions occur at semiconductor surfaces, and therefore the photocatalytic activities of semiconductor oxides are usually greatly improved by the increase in surface area [44]. To further improve the surface area, nanocrystalline NaTaO 3 thin films with ordered three-dimensional mesoporous and nanostick-like constructions were successfully produced by PIB-b-PEO polymer-based sol-gel method. NaTaO 3 prepared at 650 ο C exhibits a BET surface area of about 270 m 2 cm -3 , which is much larger than the ever reported values [45]. These nanocrsytalline mesoporous NaTaO 3 samples show both enhanced ultraviolet light photocatalytic activity and can keep steady performance. A confined space synthesis process was also used for preparing colloidal array of NaTaO 3 by using three-dimensional mesoporous carbon as the hard template. This method brings about the creation of a colloidal collection of mesoporous NaTaO 3 particles (20 nm). After NiO loading, the mesoporous NaTaO 3 nanoparticles showed photocatalytic activity for overall water splitting more than three times as high as non-structured bulk NaTaO 3 particles [46]. A carbon modified NaTaO 3 mesocrystal nanoparticle was also successfully synthesized by a onepot solvothermal method by employing TaCl 5 , NaOH and glucose as the starting materials and distilled H 2 O/ethylene glycol mixed solution as a reaction solvent. The as-synthesized mesocrystal nanoparticles exhibited a high specific surface area of 90.8 m 2 g -1 with large amounts of well-dispersed mesopores in the particles. The carbon-modified NaTaO 3 mesocrystal demonstrated excellent efficiency for continuous NO gas destruction under visible light irradiation, which is considerably superior to those of the unmodified NaTaO 3 specimen and commercial Degussa P25, owning to large specific surface area, high crystallinity and visible light absorption [47].

Noble metal co-catalyst engineering
As well documented in previous literatures, co-catalyst introduces two positive factors into the photocatalyst, including promotion on the separation of photogenerated charge carriers and construction of active sites for reduction and/or oxidation reaction. Several noble metals have been commonly used as co-catalysts for photocatalytic applications. For example, water splitting activity of NaTaO 3 :La was improved when Au was loaded either by photodeposition method or by impregnation method. Moreover, Au/NaTaO 3 :La prepared by impregnation method exhibits much higher and more stable photocatalytic activity toward water splitting due to the fact that O 2 reduction on photodeposited Au co-catalyst was more efficient than that of impregnated Au co-catalyst [48]. Besides Au nanoparticles, Pt is also frequently used as cocatalyst for increasing the photocatalytic activity of alkali tantalates. With the deposition of Pt nanoparticles as co-catalyst, rare earth (including Y, La, Ce and Yb) doped NaTaO 3 exhibits a clear improvement of the hydrogen evolution, which is due to the fact that Pt nanoparticles act as electron scavengers reducing the photogenerated charge carrier recombination rate and facilitating the electron move to metal sites from the CB of NaTaO 3 , being as the catalytic center for hydrogen generation [49]. Moreover, Pd nanoparticles are also used as a co-catalyst for H 2 production from water containing electron donor species. Su et al. prepared novel Pd/NiO core/shell nanoparticles as co-catalyst, which are placed on the surface of La doped NaTaO 3 photocatalyst. It is noted that Pd nanoparticles are more effective for H 2 generation from water containing methanol, while Pd/NiO core/shell nanoparticles exhibit a higher H 2 generation by splitting pure water. The presence of NiO not only provides hydrogen evolution sites and suppresses the reverse reactions on Pd-based catalysts but also improves the stability of the Pd nanoparticles on the La doped NaTaO 3 surfaces [50]. In another case, when RuO 2 (1 wt.%) was introduced as co-catalyst, the ability for H 2 generation of NaTaO 3 prepared by an innovative solvo-combustion reaction was improved significantly, reaching around 50 mmol of H 2 after 5 h, which is the best of other reports in literature [51].

Earth abundant elements co-catalyst engineering
Due to too much scarcity and expense of noble metal co-catalyst to apply for wider scope solar energy applications, the development of high-efficiency and low-cost noble-metal-free cocatalysts is acutely necessary. Lately, co-catalysts composed of earth abundant elements have been explored extensively to replace noble metal co-catalysts for solar energy applications [52]. NiO is a p-type semiconductor with a band gap energy ranging within 3.5-4.0 eV, which is widely used as the co-catalyst of tantalates-based semiconductors for enhancing photocatalytic activity [53]. In the case of NiO/NaTaO 3 :La photocatalyst with high photocatalytic reactivity, NiO acts as co-catalyst loading as ultrafine NiO particles, which possesses characteristic absorption bands at 580 and 690 nm, The ultrafine NiO particles were highly active for hydrogen evolution as well as Pt of an excellent co-catalyst [12]. A detailed study on the structural features of NiO nanoparticles indicated that the interdiffusion of Na + and Ni 2+ cations created a solid-solution transition zone on the outer sphere of NaTaO 3 . The high photocatalytic activity resulting from a low NiO loading suggests that the interdiffusion of cations heavily doped the p-type NiO and n-type NaTaO 3 , reducing the depletion widths and facilitating charge transfers through the interface barrier [54]. Besides NiO, Ni metallic nanoclusters were also used as co-catalyst. For instance, a series of nickel-loaded La x Na 1-x TaO 3 photocatalysts was synthesized by a hydrogen peroxide-water based solvent method. Systematical investigation indicated that the activity of hydrogen generation from pure water is in sequence: Ni/NiO > NiO > Ni, whereas the activity sequence with respect to aqueous methanol is: Ni > Ni/NiO > NiO. Ni metallic nanoclusters exhibit the most active sites and facilitate the formation of hydrogen from aqueous methanol. In the case of Ni/NiO core/shell structure, Ni metallic nanoclusters induce the migration of photogenerated electrons from the bulk to catalyst surface, while NiO acts as H 2 evolution site and prevents water formation from H 2 and O 2 [55].

Molecular co-catalyst engineering
Molecular co-catalyst engineering have received much research attention in recent years. In a molecular/semiconductor hybrid system, the noble-metal-free molecular complex as cocatalyst can not only facilitate the charge separation but also help us to understand the mechanisms of hydrogen evolution and carbon dioxide reduction at molecular level [56]. Although the study on molecular sensitized alkali tantalates is limited, an excellent research has been done by Hong and coworkers. In this case, by using a molecular co-catalyst [Mo 3 S 4 ] 4+ , the photocatalytic activity of NaTaO 3 was significantly improved. The hydrogen production rate is about 28 times higher than pure NaTaO 3 because [Mo 3 S 4 ] 4+ clusters can provide a large number of effective active sites for hydrogen evolution and the matching of the conduction band of NaTaO 3 and the reduction potential of [Mo 3 S 4 ] 4+ also acts as one of the major determinants for the enhancement of the photocatalytic activity [57].

Synthetic methodologies of alkaline earth and transition metal tantalates
Solid-state reaction method and hydrothermal method are used routinely to synthesize alkaline earth and transition metal tantalates. Almost all the alkaline earth and transition metal tantalates can be obtained by high-temperature solid-state method using Ta [65] have been synthesized successfully by this method, which show prospects in many application including photocatalytic semiconductor, solar cells and electronic device. The high-temperature treatment of traditional solid-state reaction will increase the size of particles and thus decrease the surface area. Sr 2 Ta [67] were reported to be obtained by similar way with extra alkali, which can supply the loss at high temperature to suppress defects formation. This makes the crystal structure grow well and has better catalytic efficiency than others synthesized with a theoretical ratio in most cases. This improved solid-state reaction method would efficiently inhibit the recombination of photocarrier to enhance the photocatalytic activity. A new polymerizable complex technique is one of the preparation methods of alkaline earth tantalates, which has a relative moderate condition. This method includes the provision of Ta-base compound and then come into being the sticky sol-gel, after the treatment at 600-700 °C. Comparing with solid-state method, the tantalate-based photocatalysts synthesized by a polymerizable complex way often have greater crystallinity and better crystal size, which will lead to remarkably increase the photocatalytic efficiency [68]. Comparing with solid-state method, hydrothermal method has been widely used in synthesizing perovskite tantalates with very lower reaction temperature. Lots of alkaline earth tantalate could be prepared by the hydrothermal method exhibiting higher activity. In 2006, Zhu and coworkers synthesize monomolecular-layer Ba 5 Ta 4 O 15 nanosheets by hydrothermal method [65], which show enhanced activity ten times better than that of solid-state method-derived Ba 5 Ta 4 O 15 particles in photodegradation reactions of Rhodamine B solution. Perovskite Ca 2 Ta 2 O 7 has also been synthesized by hydrothermal process in aqueous KOH solution at 373 K for 120 h, which shows photocatalytic water splitting activity under UV-light irradiation [69]. Moreover, sol-gel route, as a common way to prepare the nanomaterials, also can be used for preparation of some perovskite tantalates. One typical case is that the ferroelectric SrBi 2 Ta 2 O 9 [70] and SrBi 2 Ta 2 O 9 nanowires [71] were synthesized using ethylene glycol as solvent, which showed greater dielectric and ferroelectric properties than the ceramics prepared by the solid-state reactions owning to a denser and more homogeneous microstructure with a better distribution of grain orientations. Sol-gel method is also used to prepare metastable phase like Sr 0.5 TaO TaO 3 reported by many groups [61,65,70,72,74,75]. This kind of perovskite composites are promising materials with multiple elements, perovskite framework and layer-like structures, which can be classified into three category structures by the different interlayer structure. On the other hand, it has been reported that some perovskite alkaline earth tantalates with double-perovskite structure also show photocatalytic activity under ultraviolet light irradiation [79]. And some transition metal tantalates have simple cubic perovskite-type structure like AgTaO 3 , Ba 3 ZnTa 2 O 9 , Sr 2 GaTaO 6 [80] and NaCaTiTaO 6 , NaCaTiNbO 6 , NaSrTiTaO 6 and NaSrTiNbO 6 [81].

Metal/non-metal doping strategies for band gap engineering
Introducing external ions into crystal structure has been generally approved as a positive way to improve the visible-light photocatalytic activity of semiconductors with larger band gaps. For nitrogen-doped layered oxide Sr 5 Ta [84]. It is found that, in most perovskite cases, the valence band levels were shifted upwards, in which the maximum contribution to valence band maximum comes from the p orbitals of the dopant anions, which shift. On the other hand, the dopant cations shift the CB level downwards because the CBM is chiefly governed by the d orbitals of foreign cations. This conclusion was applicable to perovskite structure tantalates system (Figure 7) directly by Liu and coworkers [85].

Multi-component heterojunction
Multi-component semiconductor combination tactic shows effectivity to improve photocatalytic activity by separation of the photo-generated charge carriers with a formation of a heterojunction structure. Heterojunction structure is the interface that is located at two areas of different crystalline semiconductors. This kind of material has to consider the following points, including near crystal structure, similar interatomic spacing and close coefficient of thermal expansion. Otherwise, they should have discrepant band gap values, which is exact contrary to a homojunction. It is benefited to regulate the electronic energy bands. To promote the redox ability and photocatalytic activity, composite photocatalysts involving two or more components were extensively studied. One type of such composites is usually constructed by coupling semiconductors with larger band gap for the purpose of the higher redox ability. A charming work is the Ba 5 Ta 4 O 15 /Ba 3 Ta 5 O 15 composite reported by Roland Marschall et al., which synthesized through the sol-gel method showed brilliant activities in OH radical generation and photocatalytic hydrogen production [86]. The outstanding activity is expected to come from enhanced charge carrier separation. In 2011, Wang and coworkers present Ptloaded graphene-Sr 2 Ta 2 O 7-x N x (Figure 8) with enlarged visible light absorption region and enhanced photocatalytic hydrogen generation [87].

Co-catalyst surface modification
Transition metals and their oxides are usually used as practical co-catalysts for photocatalysis. The role of the co-catalysts attached on the surface of the semiconductor material is particularly significant. It increases the overall photocatalytic activity by helping to separate charge pairs, which can work for both bulk and surface electron/hole pathway. The chemical reaction that took place at surface is promoted by the co-catalysts. Various metals and oxides loaded on the surface of semiconductor show different effects. In most photocatalytic water splitting systems, several metals like Au and Pt can accelerate the rate of reduction of hydrogen observably [88,89]. On the other hand, some oxides like NiO, NiO x and RuO 2 can promote the rates of both hydrogen and oxygen production [90][91][92]. Among them, NiO x exhibited highest activity in photocatalytic process [78]. As a hydrogen evolution site, the co-catalyst has to extract the photogenerated electrons from the CB of host materials. Thus, the conduction band level of co-catalyst should be below that of photocatalyst. In addition, photocatalytic water splitting is sensitive to the deposition methods of co-catalysts. Kudo et al. reported that photocatalysts show diversity in photocatalytic water splitting with different deposition methods [59]. Moreover, transition-metal sulfides like MS (M = Ni, Co, Cu) have also been developed as cocatalysts to improve the photocatalytic activity. These sulfides have the same effects with other co-catalysts in reaction process [93].

Summary and outlook
Tantalate-based perovskite semiconductors are well known for their wide spread applications in photocatalysis, ionic conductors, luminescence host materials and ferroelectric ceramics. Drawbacks of wide band gap and low charge separation efficiency inhibit the further development of tantalate-based perovskite semiconductors as superior photocatalysts. The combination of various strategies, such as doping, heterojunction and co-catalyst engineering, induces a thrilling beginning for exploring visible light active and highly efficient photocatalysts for solar energy applications. However, the studies on tantalate-based perovskite semiconductors are currently unsystematic. Meanwhile, the as-mentioned strategies and the derived photocatalytic systems with high efficiency and stability still need to be further developed.