Fabrication and Photocatalytic Applications of Perovskite Materials

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Fabrication and Photocatalytic Applications of Perovskite Materials with Special Emphasis on Alkali Metal based Niobates and Tantalates Tokeer Ahmad, Umar Farooq, and Ruby Phul Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b04641 • Publication Date (Web): 06 Dec 2017 Downloaded from http://pubs.acs.org on December 8, 2017

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Fabrication and Photocatalytic Applications of Perovskite Materials with Special Emphasis on Alkali Metal based Niobates and Tantalates Tokeer Ahmad*, Umar Farooq and Ruby Phul Nanochemistry laboratory, Department of Chemistry, Jamia Millia Islamia, New Delhi110025, INDIA

*Corresponding Author: Dr. Tokeer Ahmad Email: [email protected] Phone: 91-11-26981717, Extn: 3261


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Abstract: Population explosion have caused serious environmental and energy related problems that have increased the interest of researchers to develop new non-toxic, cheap, stable and efficient materials to address these environment and energy related issues. Among different efforts the development of photocatalysts is considered as an important way to utilize the sustainable solar energy for environment remediation and energy purposes. Presently, several hundred photocatalyts have been synthesized. Among them, alkali niobates and tantalates are important class of photocatalysts due to their nontoxicity, structural flexibility and simplicity. This review summarizes recent developments in synthetic strategies of alkali niobate and tantalates, their important application as photocatalysts for environment remediation and energy applications, and efforts being made to modify their physicochemical properties and extend their efficiencies by tuning different reaction conditions. The purpose of article is to discuss methods to regulate the efficiencies of these materials and future challenges faced for practical applications. Keywords: Alkali niobate/tantalates, Synthesis, Photocatalytic activity, Band gap, Optical absorption, Environment remediation, Photocatalyst, Charge carriers.



For Table of Contents Only 1. Introduction 2. Scope of the Review 3. Structure and Factors Affecting Structure of Alkali Niobates and Tantalates 4. Synthesis of Alkali Niobates and Tantalates 4.1 Solid State Method 4.2 Polymeric Citrate Precursor Method 4.3 Microemulsion or Reverse Micellar Method 4.4 Hydrothermal Method 5. Photocatalytic Applications of Alkali Niobates and Tantalates 5.1 Alkali Niobates 5.1.1 Alkali Niobates as Promising Photocatalysts for Removal of Organic Pollutants 5.1.2 Photocatalytic Water Splitting 5.1.3 Reactions of Water Splitting 5.1.4 Alkali Niobates as Photocatalysts for Water Splitting 5.2 Alkali Tantalates 5.2.1 Alkali Niobates as Promising Photocatalysts for Removal of Organic Pollutants 5.2.2 Photocatalytic Water Splitting by Alkali Tantalates 6. Conclusions & Perspective 6.1 Conclusions 6.2 Perspective


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1. Introduction: With increase in population of the world, the demand for energy is increasing exponentially and poses great risk to the environment. The supply of sustainable, clean energy and environment protection are the two key factors to develop and sustain life on this planet. Presently, for energy purposes we are mainly dependent on low efficient, nonrenewable toxic fossil fuels. Burning of fossil fuels produces less energy and more toxic gases like CO2, NOx and SOx which have highly abrogating effects on our environment. Apart from these toxic gases emitted from fossil fuel combustion lot of factors such as effluents from industries, households and toxic organic molecules from textile industries are responsible for degrading the quality of our environment. To address these issues of energy and environment different alternative renewable, clean and sustainable sources of energy are being explored. Among these alternative sources solar energy is being considered as an ultimate source of energy which could address both energy and environmental issues due to its availability, accessibility and cleaneness1,2. Each year earth receives approximately 3850000 exajoules (EJ) of solar power and consumption of only 1% of this solar energy would fulfill the energy demand of human beings at the present consumption rate1. Apart from energy applications utilization of solar energy in environmental remediation is being explored substantially3,4. To harness solar energy large number of solar conversion technologies such as solar thermal electricity, photovoltaics, solar heating and photocatalysis have attracted immense and increasing attention5-9. However, till date, human beings could consume only 0.014% of solar energy reaching the earth annually1. Among these variety of conversion techniques, semiconductor photocatalysis of water and CO2 reduction (artificial photosynthesis) are being considered as an important routes of solar energy conversion. Photocatalysis has been recognized as an important route for conversion of harmful organic wastes in water/air into harmless molecules, reduction of heavy metal ions and removal of



bacteria10-17 etc. Photocatalytic effect is strived by reactions caused due to generation of electrons and holes on the surface of heterogenous photocatalysts. During photocatalytic processes, several reactive species such as superoxide anion radical (•O2−), hydrogen peroxide (H2O2), singlet oxygen (1O2), and hydroxyl radical (•OH) are generated due to reaction with electrons and holes, which are believed to take part in redox reactions in photocatalysis. The acclamation of global interest by these semiconductor photocatalyts mainly followed by the landmark discovery of semiconductor photocatalyst (TiO2) by Fujishima and Honda18. With this discovery, new doors of research opened in the field of photocatalysis and efforts are being made to outperform TiO2. In this quest, a huge number of photocatalyts such as YFeO3, ZnO, SnO2, NaNbO3, ZrO2, CuCrO2, Cd4As2Br3 and Cd4Sb2I3 and so forth have been discovered19-25. During last decade researchers have devoted enormous efforts to develop efficient, cost effective and environment friendly photocatalyts with special focus on those catalysts which can be utilized under visible range of solar spectrum such as AgPO 4, hydrogenated TiO2, In1-xNixTaO4, and metal free visible active photocatalysts such as boron carbide, elemental α-Sulphur, anion doped graphene26-31 etc. The efficiency of the photocatalyst for production of hydrogen and degradation of organic molecules from water is dependent on the catalyst itself. Practically, splitting of water in suspension systems is not considered feasible due to recombination of generated H2 and O232-35, to overcome this drawback Z scheme systems are developed or loading of co-catalysts for segregation of H2 and O2 is done on different sites of the single particulate catalysts36-37. Besides being environment friendly semiconductor photocatalysts have fallowing important interesting features (1) no need of electrolytes (2) possess large surface areas, and small flux per unit area of photogenerated charge carriers compared to the bulk electrodes (3) simple to use for large scale applications1.


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Among vast variety of photocatalysts, perovskite oxides and their derivatives are considered as an important functional photocatalyst materials due to their huge applications, structural flexibility and simplicity and excellent photocatalytic performance. Perovskite family includes vast library of functional materials such as PbTiO3, SrTiO3, NaNbO3, NaTaO3, LiNbO338-42 etc. having huge applications. Different perovskite materials are represented in figure 1. Alkali niobate and tantalates are important class of perovskite oxide family having applications in lead free piezoelectric materials, bioimaging, photocatalysis etc. Alkali niobates and tantalates have attracted huge attention as photocatalysts due to their structural feasibility and environment friendly nature. Figure 2 shows statistical data of the research carried out on photocatalytic degradation of organic molecules from 2006-2016.

Figure 1. Different types of perovskite materials.



Figure 2. Statistical graph for research done on photocatalytic materials (Source: Web of Science). Wiegle have demonstrated that tantalates having perovskite structure facilitate the migration of photogenerated electron-hole pairs due to corner shared TaO6 octahedra in its crystal structure. Sodium niobate is an important candidate among alkali niobates which plays an important role in organic pollutant degradation, reduction of CO2 and photocatalytic hydrogen generation43-44. Many review articles focusing on photocatalysis have been published. However, few of these reviews focus on inorganic perovskites1,2,45,46 and with more little attention on alkali niobates and tantalates. In this review we summaries the recent developments in the synthetic strategies of alkali niobates and tantalates, effect of reaction conditions on structure, morphology, and photocatalytic activity of these perovskite oxides. The scope of this review is discussed in section 2. 2. Scope of the review: Despite sporadic reviews on the perovskite materials, there is no systematic and comprehensive effort covering the established photocatalytic applications of alkali niobates and tantalates. This review discusses the use of alkali niobates and tantalates as photocatlytic materials. Synthetic strategies of alkali niobates and tantalates based functional materials with ACS Paragon Plus Environment

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brief description is discussed in section 4. We believe that detailed discussion on synthetic strategies of alkali niobates and their applications in bulk and nano form would be beneficial for chemical engineering, nanotechnology, inorganic and material chemistry. In section 5 photocatalytic application of alkali niobates and tantalates are discussed and strategies to enhance the range of their application are also discussed. 3. Structure and factors affecting structure of alkali niobates and tantalates: Alkali niobates and tantalates are potentials alternate lead free piezoelectrics materials having wide range of piezoelectric, photocatalytic, ferroelectric properties due to which they found application in different fields. These materials have perovskite structure. Perovskites can be described by having general formula ABO3, “A” and “B” being two cations and “O” being an oxygen anion. In perovskites cation “A” can be alkaline, alkaline earth or lanthanum cation while as “B” is a metal element from the transition metals having 3d, 4d or 5d configuration. Sodium niobate is one of important member of alkaline niobate family having perovskite structure composed of NbO6 octahedron. Sodium niobate has a property of showing polymorphism based on perovskite structure47. Megaw (1974) summarized the state of art “the seven phases of sodium niobate”47. However, all the seven phases refer to those only which arises due to rise in temperature48,49. Peel et. al.50 has carried out the comprehensive analysis of phase behavior of sodium niobate by using combination of various techniques supported by DFT calculations. Sodium niobate possesses a complex phase diagram consisting of various complicated poorly understood temperature dependent phases51-55. Based on the published crystallographic data, most commonly reported phase of sodium niobate at room temperature is having an orthorhombic unit cell with space group, Pbcm, having a=5.506Å, b= 5.556Å and c= 15.220Å and characterization of this phase was first done by S. Cowley et.al.54. This phase exhibits an exceptional “octahedral tilting” having three tilts which are independent of each other. In this structure sodium atom has two



crystallographically distinct sites. However, based on peak broadening studies, Darlington and knight suggested that actual unit cell of this phase was having monoclinic geometry, having Y=89.9452. Despite of having such structural confusions, it has been established that sodium niobate is antiferroelectric at room temperature, and according to results given by Shuvaeva et. al.56, it undergoes antiferroelectric to ferroelectric induced phase transition under applied electric field with a space group P21ma, having unit cell parameters a=5.569, b=7.790 and c=5.518. This transition from antiferroelectric phase to ferroelectric phase occurs due to polarization of single crystal of sodium niobate under the influence of external electric field. Waser and co-workers proposed that, sodium niobate undergo various structural phase transitions as a function of particle size57, 58. Therefore, from different crystallographic studies it was observed that the property of sodium niobate to show polymorphism depends on temperature and particle size59. The polymorphic phase transition in sodium niobate is shown in figure 3.

Figure 3. Schematic representation of polymorphic phase transition in sodium niobate. [Reprinted with permission from ref 59 Wang, Y.; Xu, N.; Zhang, Q.; Yang, H., RSC Adv. 2015, 5, 61989. Copyright (2015) The Royal Society of Chemistry].


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Jhonston et.al.60 does the comparative studies of different methods of preparation like solid state, molten state preparation and sol-gel preparation and from analysis of the products obtained by these methods it was clear that polymorphism of sodium niobate also depends on the synthetic method. Two phases Pbcm and P21ma generally coexist as prominent room temperature polymorphs of sodium niobate. The thermodynamic stabilities of these two phases are almost similar, and slight change in reaction condition results in the preferential formation of one phase to another. J. Wu and D. Xue during the synthesis of sodium niobate via ion exchange method demonstrated that the concentration of NaOH and duration of the reaction can be used to tune the process of crystallization and thus controls the desired morphologies as well as crystal phase of the resulting sodium niobate microcrystallite61. Based on this important property to show polymorphism sodium niobate has found application in many fields. Sodium niobate is of considerable interest to many researchers and equipment designer’s due to its unique physical properties and its ability to become the foundation of many eco-friendly materials. Sodium niobate is considered as n-type semiconductor and has attracted lot of attention in the photocatalytic field. Since, structurally sodium niobate consists of NbO6 octahedron, which has the ability of separating the electronhole pair effectively and this property prompts its photocatalytic activity. Katsumata et. al.62 studied the photocatalytic activity of sodium niobate and summarized that the film of sodium niobate synthesized by sol-gel method exhibits photo-induced hydrophilicity under UV-light irradiation. Despite of fact that a very little photo-oxidation took place, the results obtained were still of great interest because these were the first results showing that sodium niobate has potential to undergo photo-induced hydrophilicity using UV irradiation. K. Saito and A. Kudo63 studied the photocatalytic splitting of water using sodium niobate nanowires and revealed that sodium niobate nanowires exhibits enhanced photocatalytic activities as compared to the bulk sodium niobate using platinum as a co-catalyst. This superior behavior



in photocatalytic activity arises from small particle size, structural anisotropy and large surface area. Li et. al.64 demonstrated that sodium niobate can show anisotropy (i.e. the band gap estimated form absorption studies was different along different planes) in its photocatalytic oxidization activity and OH- ion generation. They revealed that anisotropy in photocatalytic activity could be caused by some intrinsic property like anisotropy in ferroelectric properties. However, sodium niobate has wide band gap which makes it less photo-responsive in UV range of spectrum. Hence, range of strategies like doping, sensitizing the photocatalyst with narrow band gap semiconductor has been developed to improve the efficiency of sodium niobate in the visible region of solar spectrum. Fan et. al.65 synthesized composite CuO/NaNbO3 with enhanced photodegradation of methyl orange under visible light irradiation. Their findings suggested that CuO nanoparticles may act as sensitizer for the improvement of visible light absorption and results in the formation of p-n junction which could reduce the electron hole pair recombination rate. Lv et al.66 developed In2O3-NaNbO3 composite photocatalyst their enhanced photocatalytic H2 evolution under visible light. Shi et al.67 fabricated g-C3N4-NaNbO3 composite photocatalyst with enhanced photocatalytic activities for photo-reduction of CO2. Kumar et. al.68 synthesized CdS-NaNbO3 core-shell heterostructures which exhibit enhanced photo-reduction of methylene blue under visible light irradiation. Another important member of alkaline perovskites is metal tantalate. Recently metal tantalates such as ATaO3, ATaO4, ATa2O6, ATa2O7 and ATa2O9 where A=alkali metal, alkaline earth metal and rare earth metals or a mixture of these have been reported as representative photocatalysts69-76. Among these compounds, tantalates ATaO3 (A= Li, Na and K) were discovered by Kato and Kudo in 1998 and were demonstrated to be the effective photocatalyst for splitting water under UV light irradiation69. Crystal structure of these compounds exhibit perovskite type structure, and the band gap of these compounds strongly


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depend on nature of cations, like 4.7eV for Li, 4.0 for Na, and 3.9 for K as calculated from UV-visible light diffuse reflectance spectroscopy69-71. Although tantalates have high band gap they still show the unique catalytic property. They are the active materials in photo-splitting of water even in absence of co-catalyst. After loading with NiO co-catalyst perovskite type sodium tantalate shows highly effective water splitting under UV irradiation due to which it has gained much attention from researchers27,75,77. Sodium tantalate is reported as superior semiconductor

photocatalyst

in

photocatalytic

oxidation

because

of

its

positive

electrochemical potential over other semiconductors, such as TiO2, this positive electrochemical potential makes sodium tantalate based photocatalysts to be more efficient in photo-splitting of water to produce hydrogen and photo-degradation of organic dyes78-82. Sodium tantalate crystallizes into orthorhombic crystal system having space groups Pc21n and Pcmn having lattice parameter a = 0.5513, b = 0.7750 and c = 0.5494 nm, and a density of 7.129 gcm-3

83, 84

. Kennedy et.al85 used neutron diffraction to determine the structural

phase transition in sodium tantalate from room temperature to 933K and reported that perovskite type sodium niobate undergoes various structural phase transition changing its room temperature space group and crystal system from Pbnm orthorhombic (a=5.4768Å, b=5.5212Å and c=7.7890 Å) to orthorhombic Cmcm at around 700K (a=7.8337Å, b=7.8485Å and c=7.8552 Å), tetragonal P4/mbm (a=b=5.555Å and c=3.933Å) at 835K and above 890K changes to cubic Pm3m with (a = b = c = 3.9313Å). Su et. al.86 demonstrated the effect of dopant on structure and photocatalytic performance of sodium tantalate by using dual substitution of single dopant Cr3+ in perovskite sodium tantalate, and found that at lower concentration of dopant < 2.47 mol% Cr3+, Cr3+primarily substituted for Ta5+ and results in the creation of oxygen vacancies and exhibiting no effect on the lattice dimensions but increases the photo-catalytic activities. With increase in concentration of dopant Cr3+>2.47 mol% Cr3+, there occurs dual substitution for Na+ and Ta5+, resulting in creation of oxygen



vacancy concentration, reduction in lattice dimensions, increase in surface area but causes reduction in photocatalytic activities. This dual substitution results in the mid-gap energy level between conduction band and valance band and is highly useful for visible-light absorption.

Potassium niobate is an important functional perovskite material. Potassium niobate has a typical ferroelectric character with diverse emerging technological applications because of its nonlinear optical, piezoelectric, ferroelectric and photocatalytic properties87. For Example, potassium niobate is considered as a potential material for optical application such as holographic storage, frequency doubling and optical wave guiding88. Suyal et. al.89 demonstrated that nanowires of potassium niobate can be used as important element in nanometric electromechanical devices by employing their piezoelectric properties and polarization switching. Nakayama et. al.90 showed that potassium niobate nanowires has a potential application in nonlinear optical probes of scanning microscopy for physical and biological sciences. Potassium niobate found application in piezoelectric materials because single crystal of sodium niobate exhibits large piezoelectricity and high Curie temperature 435 °C91, 92. At room temperature, potassium niobate exhibits orthorhombic Amm2 symmetry where the ferroelectric character arises due to the displacement of Nb5+ within the NbO6 octahedra93. Above room temperature and at ambient pressure potassium niobate exists in various crystal phases like orthorhombic, tetragonal and cubic, with decrease in temperature potassium niobate undergoes two structural phase transitions: one from cubic phase to tetragonal at 691 K and another from tetragonal phase to orthorhombic phase at 498 K94. In addition to these two phase transitions another phase transition occurs at -10 °C, which involves the change in structure from orthorhombic to rhombohedral and this phase transition involves sharp change in dielectric constant94. The cubic phase shows paraelectric behavior whereas other two phases tetragonal and orthorhombic shows ferroelectric property. Chen et.


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al.95 have synthesized nanoscale potassium niobate powder with orthorhombic phase structure using sol-gel method and it was determined that heat changes have evident effect on potassium niobate morphology and grain size. It was demonstrated that orthorhombic ferroelectric phase was preserved at room temperature even when the particle size was reduced to less than 50 nm. Using temperature dependent XRD, it was reported that the actual transition temperature remained almost same and phase transition was not dependent on particle size.

4. Synthesis of alkali metal niobates and tantalates: This section of the review comprises the simple overview of synthetic strategies and the influence of reaction conditions on the final product. Synthesis of ternary complex oxide nanoparticles such as perovskite type alkaline niobates and tantalates with desired shape, size and crystal structure is very important as per their application in various fields, therefore, many approaches have been developed by various researchers. Large number of researchers have carried out on alkali niobates and tantalates and demonstrated that the shape, size, morphology, crystal structure, and properties greatly vary depending on the method of synthesis, the physical and chemical conditions employed during the reaction process. Based on literature present large number of chemical methods like solid state techniques, microemulsion mediated approach, polymeric method, hydrothermal method and other methods like electro chemical, photo-deposition, ion exchange, solid state techniques have been employed to synthesis ABO3 (A = Na, K and Li and B = Nb, Ta) type perovskite materials figure 4.



Others (Microemulsion, electrochemical solvothermal)

Synthesis of alkali niobates/tantalates

Figure 4. Major synthetic approaches for the synthesis of alkali niobates and tantalates. 4.1 Solid state method: Sodium niobate being an important member of ABO3 type pervoskites have been traditionally synthesized by solid state approach by employing alkali metal carbonate and Nb2O596,97. However, this method needs high calcination temperature (t >750) for a longer period which results in poor compositional homogeneity due to possible volatilization of the alkali metal, and low suitable purity97,98. Chaiyo et. al.99 synthesized a crystalline powders of sodium niobate having excellent homogeneity using modified solid-state method involving the reaction of Na2C2O4 and Nb2O5. The calcination temperature employed was low by about 275 °C than that used in the conventional solid-state approach. Koruza et. al.100 used top down processing of sodium niobate nanopowder by using combination of two methods such as solid-state approach and followed by milling in an agitator bead mill. Nanopowder of sodium niobate synthesized by this method was having average size of about 25 nm which was considerably close to the particle size obtained by solution based chemical routes. Liu et. al.101 have proposed temperature dependent solid phase oriented rearrangement route for the synthesis of sodium niobate by utilizing sandia octahedral molecular sieves (SOMS) Na2Nb2O6.H2O as precursors. Single crystalline sodium niobate was synthesized by thermal decomposition of Na2Nb2O6.H2O without any deformation in morphology. Different alkali ACS Paragon Plus Environment

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niobates/tantalates synthesized via solid state technique along with their precursors and particle size are given in Table S1. In contrast to solid state method low temperature solution techniques have been used for the synthesis of alkali niobates and tantalates. These methods allow the better mixing of reactants and hence results in excellent reactivity of the mixture and to produce the more homogeneous, pure and reproducible phases whose microstructure and morphology can be easily controlled124. These methods include sol-gel, polymeric precursor, microemulsion, solvothermal and hydrothermal methods. 4.2 Polymeric citrate precursor method (PCP): PCP is one of the low temperature synthesis method used for synthesis of alkali niobates and tantalate nanoparticles. This method is also called as amorphous complex method125,126. This technique is based on Pechani method, which involves the use of multifunctional organic acid such as citric acid, malic acid etc. which can chelate the metal ion and forming a stable complex and a diol like ethylene glycol. In this method, soluble complex is formed and changes into gel by elimination of solvent. Gel formation results in the random distribution of cations of the starting solution. Upon heating at high temperature, the organic component of gel is removed which results in very fine, homogeneous and highly crystalline oxide powders at temperature less than those employed in solid state techniques. However, there are only few reports present in the literature on synthesis of alkali niobate/tantalate nanoparticles by PCP method. Camargo et. al.124 synthesized sodium niobate powders at temperature as low as 450 °C by a wet chemical method employing water soluble complex of malic acid. Single phase with excellent purity was obtained at 550 °C. Use of malic acid reduces the carbon content in the final product which was confirmed by Raman spectroscopy. Al-Hartomy et. al.127 have synthesized nanosized lead free barium strontium zirconate based perovskite material by polymeric method using citric acid and ethylene



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glycol as precursor forming agents. The synthesized barium strontium zirconate was highly monophasic and highly crystalline. T. Ahmad et, al.128 have also demonstrated polymeric citrate precursor method to synthesis perovskite YCrO3 with grain size of 22 nm having high surface area, high dielectric constant and multiferroic properties. T. Ahmad et al.129 has also synthesized YMnO3 nanoparticles having narrow size distribution with high surface area. The crystal structure of synthesized YMnO3 was elucidated by using XRD and was found to possess hexagonal monophasic structure. The detailed pictorial representation of alkali niobate/tantalate nanoparticle synthesis by PCP method is given in figure 5.

3O

O O OH

OH

OH

2

O

Y3+

O

O

OH

Y

O

O

O

OH

O

O O

6H

HO

O

O

O

Y3+

2C6H807 (aq)

3(aq)

Y(C6H807)2

6H+

(a)

O O

n

Y

O

O

O

O

O

O

OH

O O

O O

HO

+ n HO

OH

Y

O

O

O

O O

O

O

OH

O

O O

O

O

OH

O

n

Polymeric structure

+ nH20 n Y(C6H807)2

3-

3(aq)

+ nC2H602 (l)

n Y(C6H807)(C6H807)

nH20 (s) +

Figure 5. Schematic representation of synthesis of metal oxide nanoparticles by polymeric citrate precursor method (Y = any metal ion).


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4.3 Microemulsion or Reverse micellar method: Another important method used for synthesis of sodium niobate/tantalate was microemulsion method which involves the formation of oil in water O/W microemulsion by adding surfactant in a vessel containing two immiscible polar and non-polar solvent130. Figure 6 shows the typical structure of microemulsion, which consists of hydrophilic polar head group which are directed inwards encapsulating small quantity of aqueous phase and a long hydrophobic hydrocarbon chain that projects into the oil phase and is directed outwards131. In reverse micelles/microemulsion water is soluble in polar core resulting in the formation of water pool content. This water pool content is characterized by W o ratio i.e., ratio of concentration of water to the concentration of surfactant.

Oil phase

H2 O

Oil

microemulsion or Micelle

W/O microemulsion or Reverse Micelle

Figure 6. Schematic representation of W/O micro-emulsions. When Wo< 15 reverse micelles are formed and if Wo> 15 then microemulsion is formed. The typical size of waterpool is 5-10 nm therefore, such reverse micelles can serve as uniformly sized nanoreactors with precise control over shape and size of synthesized



nanomaterials. Another important advantage of using this method is to get microemulsion having different shapes depending on position of microemulsion in phase diagram therefore, it is easy to control the morphology of the nanomaterials by choosing the microemulsion at different positions in phase diagram. T. Ahmad et. al.132 synthesized nanocrystalline ABO3 (A= Ba, Pb and B= Zr) type compounds using modified microemulsion route avoiding alkoxides and the results obtained shows that monophasic nanocrystalline Ba1-xPbxZrO3 with grain size in the range of 20-60nm was synthesized. All the oxides were monodispersed, uniform and spherical at 800°C. T. Ahmad et. al.39 also synthesized the SrTiO3 having grain size 40 nm by employing reverse micellar method. The dielectric constant of the synthesized SrTiO3 was extremely stable with respect to temperature. Shankar et. al.133 first time synthesized sodium niobate and sodium tantalate via reverse micellar method. The results obtained shows that monophasic and highly crystalline nanoparticles of sodium niobate and sodium tantalate with cuboidal morphology were successfully synthesized by modified reverse micellar method at 500-600 °C. The particle size of the obtained sodium niobate and sodium tantalates was in nanorange and was equal to 18 nm and 40 nm respectively. The space group of as-synthesized sodium niobate and sodium tantalate was Pmc21 and Pbnm respectively. It was determined that the antiferroelectric transition in nanocrystalline sodium niobate shits to lower temperature as compared to that of bulk sodium niobate. The mechanism of synthesis of nanorange ABO3 type perovskite via microemulsion/reverse micellar route is quite simple and logical. Mechanism of microemulsion synthesis of ABO3 nanoparticles by microemulsion method; The mechanistic steps for synthesis of ABO3 (A = Na, K and B = Nb, Ta) nanoparticles are schematically given in figure 7. Two microemulsion one containing component A and another containing component B are being prepared and then mixed together. During the mixing process, these two microemulsions forms the encounter pair (EP)


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and colloid with each resulting in the formation of fused dimer (FD) in which exchange of reactant takes place between two microemulsion but, according to liquid drop model the surface energy of the fused dimer is very high therefore, it breaks into smaller units containing both the reactants. The chemical reaction takes place in these smaller microemulsions resulting in the final product which is in the range of nanoscale. T. Ahmad et. al.134,135 have synthesized various materials e.g. nanorods of manganese oxalates MnO, Mn2O3, Mn3O4 using reverse micellar method and barium orthotitanate by modified reverse micellar method and found that the grain size of these materials can be easily controlled in the nanoscale range and morphology is also controlled depending upon position of reverse micelle in phase diagram.

Figure 7. Mechanistic steps involved in reverse micellar method. 4.4 Hydrothermal method: Another alternate and rapid growing approach to synthesis these alkali niobates and tantalates is the so called “Hydrothermal Method”. During last 10 years hydrothermal method has been used most extensively to synthesize alkali niobates/tantalates and their derivatives



due to various advantages like ease to reaction conditions like temperature, pressure, pH etc. and different morphologies of nanoparticles are obtained by varying the reaction conditions. Guo et. al.136 have developed convenient two step hydrothermal method for synthesis of water dispersible nanoparticles of sodium tantalate. In this approach precursor was prepared in first step which results in strong bonding between water soluble complex agent such as citric acid, bicine, triacetin, EDTA etc. and Ta5+ ion which was of great help in preventing growth of sodium tantalate nanoparticles by restricting the combination of Ta5+ with OH- and Na+. It was made clear that both the time of reaction and complex reagent could influence the particle size which ranged between 5-30 nm. Ji et al.137 have used hydrothermal method for synthesis of sodium niobate cubes and nanowires by employing thin foils of Nb and low concentration of NaOH solution in presence of H2O2. Na2Nb2O6.H2O nanowires were successfully synthesized hydrothermally at 200 °C only for 4 h. However, prolonged hydrothermal treatment and calcination of hydrothermal precursor results in the loss of water from Na2Nb2O6.H2O and transforms to sodium niobate microcubes. Sodium niobate nanowires were obtained by annealing Na2Nb2O6.H2O at 500 °C. Zhu et. al.138 has described a synthetic method for synthesis of sodium niobate at high temperature by mixing the reactant under hydrothermal conditions. A hexagonal lattice phase of sodium niobate was successfully synthesized by this method. Products with different morphologies in the form of irregular Na8Nb6O19.nH2O bars with monoclinic lattice, then crystallizes and grows into niobate fibers in OH- solution. With the progress of reaction, the fibers of niobate changes in spiky ball dandelion like structure and finally grows and crystallizes into octahedra sodium niobate with hexagonal lattices. The results obtained shows complex phase transitions with respect to reaction temperature and reaction time. Grewe et. al.139 have demonstrated a facile synthesis protocol for synthesis of sodium tantalate hydrothermally by using a mixture of tantalum and sodium ethoxide precursor dispersed in ethanol and using ammonium hydroxide solution as


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mineralize. It was determined that by adjusting the amount of mineralizer, a large variety of sodium tantalate with different morphologies, texture parameter, crystal phases, surface area band gaps and degrees of crystallinity were synthesized. Figure 8 represents morphologies of as-synthesized sodium tantalate at different mineralizer concentrations. Modeshia et. al.140 demonstrated fine control of products offered by hydrothermal method by changing the hydrothermal conditions such as pH lowering and utilizing the narrow range stoichiometric amounts of metal precursors. The in-situ studies show that careful control of hydrothermal conditions could be helpful in isolation of different phases.

Figure 8. TEM images of (a) 0-NaTa, (b) 0.25-NaTa, (c) 1.6-NaTa, (d) 3.2-NaTa, (e) 4.0NaTa, (f) 4.8-NaTa, and (g) 9.6-NaTa. All scale bars are 100 nm. [Reprinted with permission



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from ref 139 Grewe, T.; Tüysüz, H., ACS Appl. Mater. Interfaces, 2015, 7, 23153. Copyright (2008) American Chemical Society]. The obtained morphologies were like titanates ranging from agglomerated particles in acidic medium over rod like morphology in neutral pH and to cubic tantalate in basic medium. Cubical tantalate particles having pyrochlore structure were obtained at high concentration of base and this structure can show ion exchange property through the tunnels present in the structure by replacing alkali metal with H+ without any reasonable change in the morphology. Gao et al.141 have synthesized sodium tantalate cubes having well-defined and uniform single crystals by employing solvothermal method in presence of monodisperse meso-microporous Ta3N5 hollow spheres as precursors. Jiang et. al.142 synthesized potassium niobate nanostructures, which includes nanotowers, nanocubes, nanowires and nanorods via hydrothermal method using KOH and Nb2O5 as a starting material. It was established that by varying the reaction condition like concentration of alkali hydroxide, reaction time and reaction temperature during the synthesis the morphology of final product can be easily controlled. Other methods have been recently employed for the preparation of alkali niobates and tantalates which include microwave assisted method and ion exchange method. Wu et al.143 have synthesized sodium niobate micro-crystals by using flexible ion exchange method in solution phase. In this method potassium niobate hollow spheres were prepared hydrothermally which acts as precursor for synthesis of sodium niobate. Potassium niobate hollow spheres [KNHS] being metastable is chemically highly reactive in NaOH solution and resulted in the crystallization of sodium niobate via ion exchange and recrystallization process. Xu et. al.144 have synthesized 1D nanostructured niobates using ion exchange method based on molten-salt reactions. Sodium niobate was successfully synthesized via simple ion-exchange reaction from potassium niobate (K2Nb8O21) and this method is a selfsacrificing template process. Shi et. al.145 have developed a hybrid microwave assisted


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hydrothermal method for synthesis of perovskite sodium tantalate nanocrystals by using Ta2O5 and NaOH as a starting material. Sodium tantalate preparation via this method required less time and mild reaction conditions and the synthesis of sodium niobate occurs through the formation of intermediate pyrochlore phase. Figure 9 represents the low energy barrier reaction pathway for hydrothermal synthesis of pure NaTaO3 by utilizing the ball milled Ta2O5 powder as a raw material.

Figure 9. Schematic illustration of change in free energy during the course of reaction taking place directly from Ta2O5 to NaTaO3 (dashed line) and indirectly via pyrochlore Na2Ta2O6 phase (solid line). [Reprinted with permission from ref 145 Shi, J.; Liu, G.; Wang, N.; Li, C., J. Mater. Chem. 2012, 22, 18808. Copyright (2012) The Royal Society of Chemistry]. Wang et. al.146 hydrothermally synthesized KNbO3 nanorods in presence of sodium do-decyle surfactant at 180 °C for 48 h by employing Nb2O5 and KOH as a source of Nb and K respectively. It was demonstrated that addition of surfactant changes the morphology of the product from agglomerates to nanorods. The possible mechanism for the synthesis of KNbO3



was also demonstrated. This method provides the possible approach to synthesis complex ternary oxide nanostructures like NaNbO3, KTaO3 etc. Goh et. al.147 has also hydrothermally synthesized orthorhombic KNbO3 and NaNbO3 in presence of 6.7-15 M concentration KOH and NaOH with temperature ranging from 150 to 200 °C. During the synthesis process formation of an intermediate hexaniobate species occurs before changing to perovskite structure. From the studies, it was reported that with decrease in concentration of KOH in reaction mixture the stability of the intermediate was increased. From the results obtained it was demonstrated that why there is great variation in the obtained mass of the synthesized powder with change in concentration of KOH. Kanie et. al.148 have synthesized fine particles of sodium and potassium niobate having cubic shape and orthorhombic crystal structure by two step hydrothermal process by employing high concentration of sodium hydroxide, potassium hydroxide and niobium pentachloride solution. Table S2 includes the various lead free pervoskites/ piezoelectric materials synthesized via hydrothermal method. Suzuki et. al.84 have examined the epitaxial growth of orthorhombic sodium tantalate crystals on the (100) plane of SrTiO3 surface as shown in figure 10. The crystals of sodium tantalum were grown by employing a thin film of TaOx and NaNO3 by employing flux coating method. The obtained result shows that instead of forming a continuous film grown sodium tantalum forms an island-shaped crystal are developed. Attributing to the flux method, the synthesized sodium tantalate crystals were having cube-like structures which were highly developed with relatively smooth faces. In addition, few non-oriented crystals were also developed since some part of TaOx film remained between the sodium tantalate crystals and SrTiO3 substrate, which resulted in the poor reaction between NaNO3 and TaOx. From this result, it was evident that well oriented crystals were developed under the influence of the (100) crystal face of SrTiO3.


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Figure 10. (a) Low- and (b) high-magnification surface SEM images of crystals grown on a SrTiO3 substrate. [Reprinted with permission from ref. 84 Suzuki, S.; Wagata, H.; Yubuta, K.; Oishia, S.; Teshima, K. Cryst.Eng.Comm. 2015, 17, 9016 Copyright (2015) The Royal Society of Chemistry]. Wan et. al.156 have used high temperature Raman spectroscopy and density function theory to elucidate the structure of K2O rich KNbO3 melt and to study the mechanism of KNbO3 crystal growth. It was observed that the triangular pyramidal NbO3 group are anionic motif in the bulk melt, which are related to each other by isomerization reaction to produce chains of NbO2Ø2 near the crystal-melt interface. Around the potassium niobate crystal metal interface a boundary layer having the chain structure was observed, with about 5 micrometer



thickness. The melt structures in the bulk melt and the boundary layer were analyzed and verified by DFT calculations and the calculated results agreed with Raman peaks present in the melt spectra. The crystal growth mechanism of potassium niobate (KN) is given in figure 11.

Figure 11. KN crystal growth mechanism. [Reprinted with permission from ref 156 Wan, S.; Zhang, B.; Sun, Y.; Tanga, X.; You, J., Cryst. Eng. Comm., 2015, 17, 2636. Copyright (2015 The Royal Society of Chemistry]. Richter et. al.157 have demonstrated the fabrication and optical characterization of coreshell nanowires. The fabricated core-shell complex was composed of potassium niobate core and gold shell. The hybrid and increased optical signals in near infrared spectral range were observed due to the combination of nonlinear optical properties possessed by the core with the plasmonic resonance of the shell. Two functionalization schemes of the core were compared before the shell growth process: Salinization and polyelectrolyte. It was demonstrated that polyelectrolyte functionalization scheme results in the smoother and complete core-shell nanostructure and is an easy to use synthesis process. Spectroscopic


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studies showed that the core-shell nanowires show a strong surface Plasmon resonance close to 900 nm, which matches with the second harmonic generation resonance obtained for the same nanowires from nonlinear optical experiments. 5. Photo-catalytic applications of Alkali niobates and tantalates: Due to absence of lead alkali niobates/tantalates are being considered as environment friendly perovskite materials. In addition to that these materials have attracted lots of attention due to their ability to act as photocatalysts for the removal of many organic and inorganic pollutants from water. This section describes the use of bulk and nano sized alkali niobates and tantalates for photocatalytic applications. 5.1. Alkali niobates: With the aim of gaining sustainable energy and environment development alkali niobate/tantalates and their derivatives are considered as potentials materials for heterogeneous photocatalysis in hydrogen generation, organic pollutant degradation, and removal of toxic elements from water158-160 due to the rapid consumption of fossil fuels energy and environment concerns are growing. Due to rapid industrial growth and population explosion the demand for energy by 2050 is doubled to its present energy supply161,162. To replace fossil fuels as source of energy new alternatives are being explored in which solar energy is considered as the best possible alternative renewable, emission free clean source of energy. To utilize the solar energy for diverse potential energy and environmental applications semiconductor photocatalysts has gained considerable attention. Considering solar energy as a driving force, photocatalysis needs appropriate functional materials like semiconductors to carry out the catalytic reaction like generation of H2 from splitting of water, conversion of CO2 to hydrocarbons163-169. The pioneer work of photocatlytic water splitting using TiO2 as electrode under UV light irradiation carried out by Fujishima and Honda in1972 has revolutionized the field of semiconductor photocatalysis18. Since, then



enormous research has been carried out to develop semiconductor photocatalysts with high efficiencies. The efficiency of these photocatalysts has been the bottleneck in the utilization of whole solar spectrum as energy source. Therefore, search for highly efficient and robust photocatalysts is going on and alkali niobates/tantalates or their derivatives have introduced ripples of excitement among researchers due their environment friendly nature and stability. 5.1.1 Alkali Niobates as promising photocatalysts for removal of Organic pollutants: Among alkali niobates sodium niobate acts as a promising photocatalyst. Sodium niobate plays an important role in degradation of organic dyes, photolysis of H2O to produce H2 as source of energy152, 44. As discussed previously sodium niobate shows polymorphism hence, the properties of sodium niobate depends on the phase and the size of the sodium niobate particles that has been synthesized. Improved photocatlytic H2 production activity was reported in sodium niobate nanowires photocatalyst170. Sodium niobate nanowires photocatalyst was simultaneously having cubic crystal structure and one-dimension morphology. Sodium niobate nanowires exhibit enhanced photocatlytic activity as compared to the sodium niobate nanoparticle. This enhancement in photocatlytic activity was attributed to the large surface area, high chemical purity. To exploit sodium niobate as photocatalyst different morphologies with different geometrical shapes and different sizes have been developed by employing multiple synthesis techniques and different synthesis conditions. To enhance the photocatlytic activity of sodium niobates modified heterojunction photocatalysts with improved activity are being developed. Kumar et al. have developed sodium niobate/CdS core-shell heterostructures with enhanced visible-light driven photocatalysis68. Important characteristic feature of this system is that it shows the low charge recombination rate and rapid charge separation which proves beneficial for the photocatalysis. Degradation of organic dye pollutants from waste water by sodium niobate/CdS core-shell heterostructures was investigated. In this study dye degradation has been carried out by bare


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sodium niobate, pristine CdS as well as NaNbO3/CdS core shell heterostructures. From the investigation, it was confirmed that photocatlytic degradation of the core shell heterostructure has exceeded the individual counterparts. Efficiency of individual catalysts and core shell heterostructure is disclosed in figure 12.

Figure 12. (a) Photocatalytic performance and (b) degradation efficiency of NaNbO3 nanorods, CdS nanoparticles, Degussa P25, and NaNbO3/CdS core/shell heterostructures for the degradation of MB solution under visible light irradiation. [Reprinted with permission from ref 68 Kumar, S.; Khanchandani, S.; Thirumal, M. Ganguli, A. K., ACS Appl. Mater. Interfaces. 2014, 6, 13221. Copyright (2014) American Chemical Society]. The generation and separation of the charge by the photocatalysts are schematically represented in figure 13.



Figure 13. Schematic diagram showing separation of photogenerated charge carriers in NaNbO3/CdS core/shell heterostructures. [Reprinted with permission from ref 68 Kumar, S.; Khanchandani, S.; Thirumal, M. Ganguli, A. K., ACS Appl. Mater. Interfaces. 2014, 6, 13221. Copyright (2014) American Chemical Society]. The mechanism of the photocatlytic dye degradation by sodium niobate/CdS core shell heterostructure has been investigates by employing series of quenchers such as Benzoquinone (BQ), ammonium oxalate, AgNO3, t-BuOH were used for O2•-, h+ e−, OH• respectively in methylene blue solution before the addition of sodium niobate/CdS photocatalysts. Figure 14 represents the effect of scavengers on the photocatlytic efficiency of sodium niobate/CdS photocatalyst.


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Figure 14. Effects of a series of scavengers on the photodegradation efficiency of MB using NaNbO3/CdS core/shell heterostructures. [Reprinted with permission from ref 68 Kumar, S.; Khanchandani, S.; Thirumal, M. Ganguli, A. K., ACS Appl. Mater. Interfaces. 2014, 6, 13221. Copyright (2014) American Chemical Society]. S. Kumar et. al.68 proposed mechanism for the photocatlytic degradation of MB dye as follow: CdS + hν →CdS(e-CB h+VB) CdS(e-CB) +NaNbO3→ CdS +NaNbO3(e-CB) NaNbO3 (e-CB) + O2 → NaNbO3+ O2•O2•-+ H2O→ HO2•+ OH− HO2•+ H2O → OH•+ H2O2 H2O2→ 2OH• OH• +MB →CO2+ H2O CdS(h+VB) + MB→ Degraded products



e-CB and h+VB stands for electron and holes in the conduction band and valance band respectively. The main aim to develop such heterostructures is to not to allow the recombination of charges generated by the photocatalytic reaction. In such heterostructures electrons are generated from the valance band and transferred to the conduction band of the material having high potential energy, thereby resulting in the formation of photoactive species. Simultaneously the electrons can easily move to the conduction band of the material having low potential energy from where they can be easily trapped by molecular oxygen present in the system and thus resulting in the generation of activated molecular oxygen thus results in the inhibition of recombination of photo generated electron-hole pairs which results in the enhanced photocatlytic activity of the modified/heterostructured photocatalysts like sodium niobate/CdS. Li et al.64 suggested important characterizations of sodium niobate as photocatalyst. From this study, it was confirmed that photocatalytic behavior of sodium niobate depends on the physical properties like crystal structure, band gap of the photocatalyst. Photocatalytic property of sodium niobate show anisotropic behavior i.e. different crystal plans of sodium niobate developed by pulse laser deposition on LaAlO3 shows different catalytic activity. The order of photo degradation of Rhodamine B in Ar atmosphere with irradiation by Xe lamp is (100) < (110) < (111), which is consistent with the OH generation and is closely related to the ferroelectric property of the sodium niobate. Figure 15 shows the change in concentration with respect to time during photocatalytic dye degradation by different crystal planes of sodium niobate.


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Figure 15. (a) Photocatalytic degradation of RhB over NN/LAO (100) NN/LAO (110), and NN/LAO (111) during 1 h of full arc irradiation of Xe lamp under Ar atmosphere. (b) Fluorescence signal intensity of TAOH at 429 nm over NN/LAO (100), NN/LAO (110), and NN/LAO (111) after 1 h of full arc irradiation of Xe lamp. [Reprinted with permission from ref 64 Li, G.; Yi, Z.; Bai, Y.; Zhang, W.; Zhang, H., Dalton Trans. 2012, 41, 10194. Copyright (2012) The Royal Society of Chemistry]. Another important strategy that has been developed to enhance the range of photo absorption by alkali niobates is band gap modulation by doping metal or non-metal ions. From the literature, it is clear that large number doped alkali niobates have been designed with enhanced photocatlytic activities. Recently Kanhere et. al.171 has developed Bi doped ACS Paragon Plus Environment


sodium niobate photocatalyst and its visible light photocatalytic activities were investigated. It was established that the Bi occupancy depends on the molar ratio of Na/Ta of the starting material which in turn affects the visible light activity of the doped photocatalyst. From the studies, it was found that at low concentration of Na in the starting material Bi occupies Na sites, and this system doesn’t show visible light absorption. While as under sufficient Na condition Bi predominantly occupies positions at Ta sites. This system of high Na concentration and Bi occupied Ta sites showed visible light absorption up to 450 nm. Another condition was employed in which mild Na concentration was used, in this condition Bi occupies both Na and Ta sites and shows the visible light absorption at 550 nm which corresponds to the highest photocatalytic degradation of methylene blue by the Bi doped sodium niobate. The first principle Theoretical calculation were also carried out which agreed with the experimental data. From the results obtained it was clear that the photo catalytic activities of the sodium niobate/ tantalates can be tuned by appropriate doping of any dopant at particular lattice sites which could lead to fabrication of new photocatalysts with visible light sensitivity. To develop robust photocatlytic activities of sodium niobates in visible range another important polymeric semiconductor material, namely graphitic carbon nitride (g-C3N4), has introduced excitement among researchers. The use of (g-C3N4), to develop heterostructures with alkali niobates/tantalates starts only few years back. (g-C3N4) has found huge applications due to its stable and environment, friendly nature. (g-C3N4), possess surface property which can be easily modulated by surface functionalization and surface engineering for photocatalytic applications. Chengjie et al.172 has developed (g-C3N4) sensitized sodium niobate (NN) heterostructure by solid state reaction using urea and sodium niobate. Rhodamine B (RhB), methyl orange (MO) and tetracycline (TC) were employed as model organic pollutants in water to determine the photocatlytic dye degradation activity of (g-


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C3N4)/NN heterostructure. Figure 16 shows the photocatalytic activity and kinetics of dye degradation reaction by (g-C3N4)/NN heterostructure and pristine (g-C3N4) and sodium niobate.

Figure 16. (a)The photocatalytic activities of the as-prepared photocatalysts for RhB degradation under visible-light irradiation and (b) the degradation kinetics of RhB after 2.0 h irradiation with visible-light. [RhB] = 10 mg L−1, [photocatalyst] = 0.05 g L−1. [Reprinted with permission from ref 172 Chengjie, S.; Mingshan, F.; Bo, H.; Tianjun, C.; Liping, W.; Weidong, S., Cryst. Eng. Comm. 2015, 17, 4575. Copyright (2015) The Royal Society of Chemistry]. Another important aspect which influences the photo catalytic activity of the photocatalyst is the particle size. With the decrease in the particle size the surface area to volume ratio increases which results in the exposure of more surface-active sites for the catalysis. Li et al.22 have studied the effect of particle size of sodium niobate on photo-degradation of 2propanol. Depending on the amount of precursor loaded different sized nanoparticles of sodium niobate ranging from 3-50 nm were obtained. The photocatlytic activity was studied by employing 2-propanol degradation as a model reaction. Gaseous phase iso-propanol, (IPA) was used to elucidate the effect of particle size on photocatalytic activity of sodium



niobate. Photodegradation reaction of IPA proceeded via acetone route. Photocatalytic dehydrogenation of IPA occurs which results in the formation of acetone and then finally CO2 is produced by photo-oxidation. It was observed that the photocatlytic activity of nano sized NaNbO3 (NN) samples is different. The optimum size of NN which show the highest activity was 30 nm. The smallest particle size of 3 nm was also obtained but its activity was lowest due to large band gap. Therefore, from this study it was clear that not only the size of particles determines the activity of the photocatalyst but there are other factors like defect density, change in electronic structure and surface area which shows competing effect on activity of catalyst. Li et. al.22 have sensitized N-doped sodium niobate samples with carbon nitride by heating sodium niobate powders and urea mixture with different weight ratios and compared the photo-catalytic activity by analyzing the photodegradation of dyes by pristine sodium niobate and the sensitized samples of N-doped sodium niobate, it was demonstrated that the modified sodium niobate shows improved photo-catalytic activity. The highest activity was found in the sample having weight ratio of sodium niobate to urea of 1:60 which was attributed to efficient utilization of light and efficiently transfer of electron in the sample. From the results, it was concluded that metal free carbon nitride could function as a photosensitizer. Table S3 shows various alkali niobate/tantalate photocatalysts and their derivatives with enhanced photocatalytic activity. Shi et. al.67 have developed visible light responsive heterojunction photocatalyst of g-C3N4/NaNbO3 nanowires by encroaching g-C3N4 on NaNbO3. The intimate interface formed in g-C3N4/NaNbO3 heterojunction was characterized and revealed by HRTEM. The results obtained from the study of photocatalytic reduction of CO2 showed that photo-activity of g-C3N4/NaNbO3 heterojunction was 8 times enhanced as compared to that of individual C3N4 and NaNbO3 under visible light irradiation. This remarkable increase in the photo-catalytic activity was associated to efficient uncoupling and transfer of electron-hole pair generated at the interface of g-C3N4/NaNbO3 heterojunction


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using visible light. The enhancement in photo-catalytic activity was attributed to the wellaligned overlapping band structures of C3N4 and NaNbO3. The enhanced photocatalytic activity is diagrammatically represented as in figure 17.

Figure 17. Schematic diagram of the separation and transfer of photogenerated charges in gC3N4/NaNbO3 composite under visible light irradiation. [Reprinted with permission from ref 72 Yamakata, A.; Ishibashi, T.; Kato, H.; Kudo, A.; Onishi, H., J. Phys. Chem. B. 2003, 107, 14383. Copyright (2003) American Chemical Society]. Presence of radioactive elements in the portable water also possesses great threat to environment as well as to the human health. Radioactive ions like uranium, iodine is released into the atmosphere from nuclear industries and from medical techniques using radioactive materials. In developing application of radioactive isotope of iodine for medical techniques, prevention of its leakage is a matter of concern. An important study of sodium niobate was carried out for its application as adsorbent of radioactive ions present in water. Mu et. al.153 have demonstrated a new method for removal of radioactive ions from solutions by employing Ag2O implanted nanofibers of sodium niobate. For the safe disposal of radioactive iodine, uptake of iodine was studied in Ag2O implanted sodium niobate nanofibers, these were synthesized by developing an efficiently matched phase coherent interface in between the two components. The synthesized composite of Ag2O sodium niobate was having ACS Paragon Plus Environment


distinctive structural properties, like better resistance to basic conditions and highly selective which has made it better and irreversible adsorbent for elimination of radioactive iodine from water. The size and structure of the synthesized adsorbent in the nano-range provided large surface area which not only enhanced the uptake of radioactive iodine ions and increases the kinetics but also ensures the safe disposal by efficient removal of the adsorbent from the final solutions. 5.1.2. Photocatalytic water splitting: Another important application of alkali niobate is photo catalytic water splitting to generate H2 as an alternate sustainable and clean source of energy for future. Many photocatalysts were developed time to time to enhance the activity of H 2 generation from water splitting. Alkali niobates have shown considerable photocatlytic activity towards hydrogen generation. Their photocatlytic activity can be engineered by changing various parameters like morphologies, heterostructures, crystal structure, surface area, band gap and attachment of cocatalyts173. Table S3 shows UV-light active alkali niobates and tantalates used for photocatalytic splitting of water. 5.1.3. Reactions of water splitting: As discussed before H2 is considered as one of the most important clean and renewable alternative fuel for future. The main reactions involved in photocatalytic splitting of water is shown in figure 18. The first step of the process involves absorption of solar radiations by photocatalysts to generate electrons and holes. Most of the photocatalysts have semiconductor properties. Schematic representation of photocatalytic reactions on semiconductor materials is shown in figure 19. After irradiation with solar radiations having energy more than band gap of the semiconductor material, electron in the valance band are excited from valance band to conduction band, subsequently resulting in the formation of


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electron-hole pair. Photogenerated electrons and holes are responsible for the redox reaction like electrolysis. Electrons generated in the process reduces the water molecule to form H2 and the holes generated oxidize the water molecule to form O2 for complete water splitting178,179.

Figure 18. Processes involved in overall photocatalytic water splitting on aheterogeneous photocatalyst. [Reprinted with permission from ref 178 Maeda, K.; Domen, K., J. Phys. Chem. C. 2007, 111, 7851. Copyright (2007) American Chemical Society]. Important features of semiconductor photocatalysts are the band gap i.e. energy gap between conduction bands and the valance bands. For successful splitting of water, the value of lower level of conduction band needs to be more negative than the redox potential of water, while as the highest level of valance band must be more positive than oxidation reduction potential of O2/H2O (+1.23eV vs. NHE). Therefore, the minimum theoretical energy difference between conduction band and valance should be equal to 1.23eV that corresponds to light of approximately 1000 nm for complete of water splitting. However, in many metal oxides, experimentally the valance band is composed of O 2p orbitals, therefore, top of the valance band occupies its position above the redox potential of O2/H2O (+1.23eV vs. NHE).



Therefore, the reduction oxidation potentials of H+/H2 and O2/H2O likely takes its position in between the top of the valance band and bottom of the conduction band as shown in figure 19.

Figure 19. Basic principle of the overall water splitting on a heterogeneous photocatalyst. [Reprinted with permission from ref 178 Maeda, K.; Domen, K., J. Phys. Chem. C. 2007, 111, 7851. Copyright (2007) American Chemical Society]. Due to the activation barrier in the charge transfer process between photocatalysts and water practically much greater solar energy is required than the band gap of the photocatalysts. Also, due to large band gap these photocatalysts are mostly used in UV region of solar spectrum. However, the main component of the solar spectrum is composed of visible photons (50%), therefore the primary aim of developing photocatalysts for water splitting is to utilize the visible light of the spectrum. 5.1.4 Alkali Niobates as photocatalysts for water splitting: For overall splitting of water alkali niobates have been considered as an important and efficient UV-light photocatalysts. NaNbO3 is one of member of alkali niobates and has been recognized as important photocatalyst for generation on H2 and O2 from overall splitting of water66,150. Band gap of NaNbO3 is equal to 3.4 eV and it can be synthesized by different


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methods as discussed in section 4. As an example, Saito and Kudo63 reported synthesis of NaNbO3 nanowires by using niobium oxalate complex as a starting material. From their study it was confirmed that as compared to bulk NaNbO3, NaNbO3 nanowires show enhanced photocatalytic splitting of water into H2 and O2 under UV-light in presence sacrificial agent. In absence of the cocatalyst both NaNbO3-Bulk and NaNbO3-NW shows negligible activities. However, after loading of Pt as cocatalyst NaNbO3-NW showed enhanced activity compared to NaNbO3-bulk. The difference in their activities arises from surface area, large surface area contributes to high dispersion of cocatalyst particles that leads to enhanced photocatalytic activity. Similarly, Saito et al.180 synthesized nanowires of LiNbO3 by using lithium oxooxalate complex as lithium source and structure directing reagent. Overall photocatalytic splitting of water into H2 and O2 by LiNbO3 using RuO2 as a co catalyst was studied and it was observed that nano sized LiNbO3 show enhanced activities as compared to its bulk counterpart. At 0.5% of RuO2 the quantum yield for LiNbO3 nanowires was calculated to be 0.7%. Therefore, to increase the photocatalytic activities of alkali niobate surface area plays an important role. To increase the surface area, it is necessary to reduce the particle size and the size at nanoscale is more effective to increase the surface-active sites that plays crucial role in photocatalysis. So, to increase the surface area many synthesis approaches have been developed as discussed in section 4 of this review. In addition to engineering the size of alkali niobates much efforts have been devoted to enhancing their activities into the higher wavelength region by loading with different elements. Domen et al.181 reported the utilization of potassium niobate as novel photocatalyst for overall splitting of water into H2 and O2. For preparation of potassium niobate K2CO3 and Nb2O5 were calcined at 1200 °C in air and the prepared samples were characterized by XRD to confirm the crystal structure. XRD confirmed the formation of KNbO3, K2Nb6O1, and K4Nb6O17. Figure 20 (a) Shows the H2 evolution rate over the potassium niobate catalysts.



Among the three oxide K4Nb6O17 showed highest and stable activity during the redox reaction. Along with the holes CO2 was the product in this reaction. Unless treated with metal like Pt, TIO2 and SrTiO3 showed lower activities for H2 evolution in comparison to potassium niobate catalysts. Photocatalytic splitting of water was carried out over pure K4Nb6O17 and it was observed that only H2 was evolved with no detection of O2. It was also observed that activity of the catalyst decreased with time. However, after treating K4Nb6O17 with H2 at 500 °C and reoxidised at 200 °C considerable amount of H2 and O2 was evolved as shown in figure 20 but the amount of H2 and O2 evolved was not stoichiometric. In this study Domen and co-workers prepared NiO (O.1W%) - K4Nb6O17 catalysts by treating K4Nb6O17 with Ni(N03)2 aqueous solution followed by calcination in air and the catalysts was further reduced and reoxidised by H2 at 500 °C and O2 at 200 °C respectively. It was observed that catalyst treated with NiO showed an order of magnitude higher activity compared to treated K4Nb6O17 alone and stoichiometric evolution of H2 and O2 was also observed figure 20 (b).

Figure 20 (a). H2 evolution over potassium niobates from aqueous methanol solution (20 vol %) . : K4Nb6O17,

: K2Nb6O16;

: KNbO3. The photocatalytic hydrogen evolution from


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aqueous methanol solution (250 ml) was carried out over the catalysts (0.2 g) with a Xe lamp (500 W). (b) The photodecomposition of water over treated K4Nb6Ol7 and NiO (O.1 wt%)K4Nb6Ol7. (a)

: H2; : O2 over K4Nb6O17; (b) :H2;

:O2 over NiO(0.1 wt%)-K4Nb6Ol7.

The catalysts, K4Nb6Ol7 and NiO (0.l wt%)-K4Nb6Ol7 were reduced by H2 at 500 "C and then reoxidized by O2 at 200 "C. The reactions were carried out in an inner irradiation type reaction cell with 1 g of catalyst. A high-pressure mercury lamp was used. [Reprinted with permission from ref 181 Domen, K. Kudo, A.; Shinozaki, A.; Tanaka, A.; Maruya, K.; Onishia, T., J. Chem. Soc. Chem. Commun. 1986, 356. Copyright (1986) American Chemical Society]. Similarly, Sayama et al. studied the intercalation of Pt over K4Nb6O17. As K4Nb6O17 is a layered compound and is composed of two different alternating interlayer spaces (interlayer I and II) and the K+ ions present at interlayer spaces are exchanged by other cations. It was observed that Pt loaded KNb6O17 prepared by treating with H2PtC16 doesn’t show any activity towards photocatalytic splitting of water. This reason for the inactivity of Pt- K4Nb6O17 was inferred that Pt present on the external surface of K4Nb6O17 carried out the reverse reaction and produces water from H2 and O2. To stop the reverse reaction Pt intercalated K4Nb6O17 was prepared using [Pt(NH3)4]2+ cations instead of PtCI62- anions. After reduction of H2 Pt from the external surface of K4Nb6O17 was removed by treatment of catalysts with aqua regia. Photocatalytic decomposition of water was studied over both Pt-K4Nb6O17 catalysts treated with and without aqua regia. For pure Pt- K4Nb6017 small amount of H2 evolution was observed with negligible O2 evolution. However, in case of catalyst treated with aqua regia from the beginning evolution of H2 and O2 was observed and after several hours the ratio of H2 and O2 evolution became 2:1. From their study it was also observed that the activities of the catalyst also depend on the amount of Pt loaded. Figure 21 shows the effect of amount of



Pt loaded on the catalytic activities of K4Nb6O17 and activity increases with increase in amount of Pt and optimum activity was observed at 0.01% of Pt loading182-185.

Figure 21. Time course of H2 and O2 evolution from water over Pt (0.01wt %)/K4Nb6O17 catalyst: (a) nontreatment (H2, ; O2, ); (b) after aqua regia treatment (H2, ; O2, ). [Reprinted with permission from ref 182 Sayama, K.; Tanaka, A.; Domen, K.; Maruya, K.; Onishit T., J. Phys. Chem. 1991, 95, 1345. Copyright (2013) American Chemical Society]. Another important approach to enhance the overall water splitting to H2 and O2 is to sensitize the catalysts with certain organic molecules like dyes as shown in figure 22. These organic molecules act as photosensitizers. Like an example Abe et al. used coumarin and carbazole dyes as photosensitizers on layered niobates for H2 evolution. In this study tungsten (VI) oxide WO3 was used as photocatalyst for O2 evolution and triiodide and iodide redox


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couple was employed as shuttle electron mediator between them. When adsorbed on Pt loaded TiO2 Coumarin dye (C-343) shows the significant activity for H2 evolution from an aqueous solution containing I- electron donor. However, prolonged irradiation time results in decrease of the activity due to degradation of the dye molecules. Therefore, to enhance the stability of the dye molecule oligiothiophene moiety was inserted between the donor and acceptor portions of the coumarin dye and it was observer that stability of the oxidized state of dye in aqueous solution increases drastically. The stable oxidized state enables the use of dye molecules as an effective sensitizer for stoichiometric production of H2 and O2 from aqueous solution having I- as electron donor, in combination with suitable n-type semiconductor, layered niobates H4Nb6O17. Simultaneously the effect of dye structure on both stability and the efficiency of H2 evolution was examined by using series of coumarin and carbazole dyes. Also, to increase selectivity of the catalysts towards H 2 and O2 evolution nanoparticulate Pt and IrO2 coloaded with Pt inside the interlayer of layered niobate and on surface of WO3 respectively were applied. Electrochemical studies were carried out to check the stability of the dye molecules, their redox behaviour in aqueous and dehydrated acetonitrile solutions were examined by cyclovoltammetry (CV). Nanoporous TiO2 electrode adsorbed dye molecules were used for CV measurements due to insolubility of dyes in aqueous solution having pH less than 7. The results obtained from electrochemical studies are given in figure S1. From the results obtained it was observed that, the dye molecule having two or more thiophene rings shows the reversible behaviour even in water which indicated that even in water the life time of one oxidized state of dye is enough to get reduced back to its original state by accepting electron during the reverse cathodic scan. Similarly, for carbazole dyes having four thiophene showed the reversible behaviour even in water. Repeated CV scans clearly indicates the stability of both unmodified and modified dye molecules as given in figure 23 the stabilizing ability of these oligothiophene moieties helps



in achieving the visible active water splitting by using dye sensitized H2 evolving photocatalysts. The use of dyes for Z scheme production of efficient photocatalyts diversified the use of photocatalysts for overall water splitting by reducing the difficulties faced in tailoring the band gap of inorganic semiconductors186-189.

Figure 22. Use of organic molecules as sensitizers for inorganic catalysts to enhance their catalytic performance. [Reprinted with permission from ref 186 Abe, R.; Shinmei, K.; Koumura, N.; Hara, K.; Ohtani, B., J. Am. Chem. Soc., 2013, 135, 16872. Copyright (2013) American Chemical Society.]


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Figure 23. CV profiles of dyes adsorbed on a porous TiO2 electrode in water containing 0.1 M LiClO4 as a supporting electrolyte. The scan rate was 100 mVs−1. Ag/AgCl in saturated aqueous NaCl solution was used as a reference electrode. [Reprinted with permission from ref 186 Abe, R.; Shinmei, K.; Koumura, N.; Hara, K.; Ohtani, B., J. Am. Chem. Soc., 2013, 135, 16872. Copyright (2013) American Chemical Society.] 5.2 Alkali tantalates: 5.2.1 Alkali tantalates as photocatalysts for degradation of organic pollutants: Sodium tantalate has band gap of 4.0 eV and it has been recognized as an important photocatalyst for degradation of organic molecules and water splitting in UV range of solar spectrum. Synthesis of sodium/alkali tantalates by various methods is discussed in section 4 of this review. As an example, Li and Zang have synthesized nanosized sodium tantalate via single step hydrothermal route at 140 °C for 12 h. The photocatalytic activity of the sodium niobate catalyst was evaluated by employing safranine (SF) dye and gaseous formaldehyde as an activity descriptor. The optimum reaction conditions and morphologies of different samples were investigated. Figure 24 represents the degradation of SF in presence of sodium tantalate catalysts prepared at different temperatures190. From the results obtained it was



observed that catalysts prepared at low temperature 100 °C has the low reaction constant of 0.01201 min-1 and with increase in the synthesis temperature of the catalysts their activity increases with reaction constant of 0.026 min-1 and 0.039 min-1 for the samples prepared at 120 °C and 140 °C respectively. The increase in photocatalytic activity of different synthesized catalysts at high temperatures was attributed to better crystalline order of the catalysts. However, there was decrease in the catalytic activities of the samples prepared at 160 °C. The catalytic activity of these sodium tantalate nanocubes synthesized at 140 °C via hydrothermal method were 10 times enhanced compared to the sodium niobate catalyst synthesized by solid state reaction. Figure 25 shows the degradation of formaldehyde over catalysts prepared at different temperatures. It is observed that all samples prepared at different temperatures shows similar catalytic performance and whole formaldehyde sample is degraded in 60 min apart from increasing the surface area of the catalysts morphology also has significant effect on overall activity of the catalyst material.

Figure 24. The photocatalytic performance of samples synthesized at various temperatures for 12 h: (a) the degradation of ST, and (b) degradation curve for ST dye in the presence of


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NaTaO3 nanostructure under UV irradiation. [Reprinted with permission from ref 190 Li, X.; Zang, J., J. Phys. Chem. C. 2009, 113, 19411. Copyright (2009) American Chemical Society].

Figure 25. The photocatalytic performance of samples synthesized at various temperatures for degradation of gaseous formaldehyde (C/C0: concentration/initial concentration). [Reprinted with permission from ref 190 Li, X.; Zang, J., J. Phys. Chem. C. 2009, 113, 19411. Copyright (2009) American Chemical Society.] Gӧmpel et al.191 have hydrothermally synthesized different alkali tantalates MTaO3 (M= Na, K, Rb) having different morphologies depending pH of the reaction mixture. Morphology of different tantalates obtained ranges from agglomerates in acidic medium, rods at neutral pH and cubes at basic pH. Photocatalytic activities of these samples were evaluated by using rhodamine blue as a model pollutant. The results obtained are given in figure 26 and it was observed that complete degradation of dye occurs in 4, 8, and 12 minutes over cube shaped HTaO3, tantalum oxide rods, and cubic KTaO3 respectively. The higher activity of HTaO3 compared to KTaO3 was attributed to the acidic groups over the surface of HTaO3. Therefore, controlling the surface area, crystallinity and morphology has been effective



strategies in enhancing the catalytic activities of alkali tantalates. In addition of increasing the activity in the UV region several efforts were made to enhance the activity of alkali tantalate in the visible range of solar spectrum. As an example, Bajorowicz et al. have successfully synthesized the hybrid material of perovskite type KTaO3 and reduced graphene using solvothermal method192. Effect of graphene loading on surface area, morphology, structure and absorption properties of the catalyst were elucidated and were correlated with the photocatalytic activities of the hybrid composite. The photocatalytic activities of these hybrid composites were evaluated by using phenol as a model pollutant. The results obtained clearly indicated the enhanced photocatalytic performance of the all KTaO3-reduced graphene hybrid samples. As compared to pure KTaO3 the nanocomposites of KTaO3 shows enhanced catalytic performances under visible light irradiation. This enhanced activity could be attributed to the role of reduced graphene as photosensitiser in the composites and formation of p-n heterojunction with p-type graphene and n-type KTaO3 nanocubes. Further, the results obtained indicates that addition of graphene to KTaO3 nanocubes enhanced visible light absorption and separation of photogenerated charge carriers. The sample with a 30 wt.% content of graphene showed best performance with activity of 43% in the visible range of spectrum. This best activity could be attributed to the small crystallite size, enhanced absorption activity and efficient contact between KTaO3 and reduced graphene oxide sheets. This report may prove an important guide to enhance the activities of inorganic perovskites in the visible range by developing different organic-inorganic hybrid catalyst materials.


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Figure 26. (a) UV-VIS spectra of the samples after UV-light irradiation for 18 minutes and centrifugation of the nanoparticles. (b) Digital photograph of the corresponding solutions from left to right: control RhB solution, KTaO3, HTaO3, (cube shape), Ta2O5 (rods shape), Ta2O5 (cube shape) (750 °C), and Ta2O5 (rods shape) (850 °C). [Reprinted with permission from ref 191 Gӧmpel, D.; Tahir, M. N.; Panthӧfer, M.; Mugnaioli, E.; Brandscheid, R.; Kolbb, U.; Tremel, W., J. Mater. Chem. A. 2014, 2, 8033. Copyright (2014) The Royal Society of Chemistry]. In addition of controlling the surface area, morphology and crystal structure of alkali tantalates, much efforts have been made to achieve the higher catalytic activity of alkali niobate by doping with other elements70,73,193. Torres-Martínez et al. found the photocatalytic activity of NaTaO3 sol-gel doped with Sm and La. Solid state and sol gel reactions were used for the synthesis of Sm and La doped NaTaO3. The results obtained for catalytic degradation of methylene blue showed that samples doped with Sm and La possesses enhanced



photocatalytic performances. Sample calcined at 600 °C shows best activities towards degradation of methylene blue and it can be attributed to high specific surface area and high crystallinity of the samples at this temperature. Also, the compounds synthesized by sol-gel method show 4 times higher activities compared to samples synthesized via solid state route194. In addition of metallic doping Liu et al. investigated the effect of doping by nonmetallic N on the photocatalytic activity of NaTaO3 using methylene blue as model pollutant. The effect of calcination temperature on photocatalytic activity was evaluated from 700 °C to 1000 °C and it was observed that with increase in calcination temperature the catalytic activity of the samples decreased which can be attributed to the increase in grain size of the material. The effect of N doping on NaTaO3−xNx was studied and it was observed that with increase in amount of N the catalytic performances of NaTaO3−xNx increases when x ranges from 0 to 0.039, the reason is that with increase in concentration of N there occurs the formation of additional intra-bandgap states and these states lies close enough to the conduction band edge which induce electron coupling thus reduces the photogenerated charge recombination rate, so the photocatalytic activity is enhanced. With further increase in N content the catalytic activities decreased drastically. The reason is that the lattice defects acts as a recombination sites for the photogenerated electrons and holes. These studies demonstrate that doping with metals and non-metals is a useful approach in increasing the surface area and reducing the charge recombination, however the concentration of dopant need to be optimized for maximizing the photocatalytic performance195. Recently major efforts on NaTaO3 are trying to enhance the photocatalytic activity or absorption spectrum of NaTaO3 into the visible range of solar spectrum by doping with metals like Ta, Cr, Eu, Bi, Mn and Fe, and non-metals such as N, C, or co-doping of metal with non-metal or metal with metal and non-metal with non-metal for example, La/Cr, La/N and N/F196-203,86,107,111,178. The effect of doping on band gap narrowing is generally accepted


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that metallic d orbitals and s or p orbital of non-metals forms an intermittent band gap states of NaTaO3, which is generally formed by 5d orbital of Ta and 2p Orbital of O. As an example, Chen et al. synthesized 5 % Bi doped NaTaO3 having different ratio of Na/Ta in starting materials178. From the results it was observed that doping sites of Bi in lattices of NaTaO3 are strongly affected by Na/Ta molar ratio which in turn have influenced the optical and photocatalytic properties of the material. The Bi doped NaTaO3 prepared at mild concentration of Na-rich condition exhibited extended visible light absorption up to 550 nm because both Na and Ta sites were occupied by Bi dopants. As a result, the sample was more effective for degradation of methylene blue under visible light irradiation compared to those samples having Na deficiency or Na excess. Although doping with another element have successfully enhanced the activity of NaTaO3, it is worth noting that Cao and Hu et al. have synthesized self-doped (Ta4+) NaTaO3 nanoclusters by simple low temperature solvothermal method196. Two main important features of the method are that self-doping takes advantage of more homogenous features and low temperature synthesis reduces the particle size and avoids agglomeration occurring during the prolonged heating. Self-doping with Ta4+ reduces the band gap of NaTaO3 from 3.94 eV to 1.70 eV and thus can be used as photocatalyst under visible light. 5.2.2 Photocatalytic splitting of water using alkali tantalates: Alkali tantalates have been widely used for generation of H2 as a fuel from water splitting under UV range of solar spectrum. Being a member of perovskite family, their catalytic activities are mainly controlled by B site elements. Two main group of elements are used as active constituent of B site in a perovskite photocatalysts. The first group includes elements having empty d orbital such as Ti4+, Nb5+, Ta5+, W6+, Zr4+, and Ce4+ and second group includes elements having filled d orbitals such as Ga3+, In3+, Ge4+, Sn4+ and Sb5+ 178,179. In this section we will discuss the strategies used to enhance the photocatalytic activity of



different alkali tantalates under UV and visible range of solar energy. Different UV light active alkali tantalates for overall water splitting are listed in Table S3. As a first example, Kata and Kudo have synthesized series of alkali tantalates ATaO3 (A = Li, Na, and K) using conventional solid-state route. Their activity towards generation of H2 and O2 from water was evaluated. It was observed that the alkali tantalates prepared in excess of alkali, their photocatalytic activities were enhanced by order of magnitude 1-2. The order of activities of pristine alkali tantalates was KTaO3 < NaTaO3 < LiTaO3 and agreed with band gap as well as with transferring excited energy. NaTaO3 loaded with NiO cocatalyst showed highest activity among alkali tantalates loaded with NiO having apparent quantum yield of 28%204. To increase the range of activities of alkali tantalates in UV and visible range different methods were employed like decreasing the crystal size to nanodimensions to increase the surface area. Yokoi et al. synthesized colloidal array of NaTaO3 nanoparticles using confined space synthesis method. In this method three-dimensional mesoporous carbon was used, which was replicated by colloidal array of silica nanospheres having size of 20 nm. After heat treatment the mesoporous carbon was burnt out which leads to the formation of colloidal array of NaTaO3 nanoparticles with a size of 20 nm and a surface area of 34m2g-1. Photocatalytic properties of these arrays of NaTaO3 nanoparticles loaded with NiO cocatalyst were studied and it was found that compared to non-structured bulk NaTaO3 colloidal array of NaTaO3 nanoparticles show three times enhanced activity205. Later, Shi et al. used the microwave assisted hydrothermal route to synthesize the NaTaO3 perovskite nanocrystals. This facile route result in the formation of NaTaO3 nanocrystal in a short time and under mild condition due to indirect transformation route from Ta2O5 to Na2Ta2O6 and to NaTaO3. The results obtained for photocatalytic activities showed that NaTaO3 synthesized by microwave assisted hydrothermal method exhibited two times greater activity compared to that of NaTaO3 synthesized by conventional hydrothermal route148. Recently, Grewe et al.139


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developed a different variety of sodium tantalate with different crystallinity, morphologies, surface area and band gaps. All the systems show different photocatlytic activities. Out of these samples amorphous sample and composite sample containing both amorphous and crystalline phase have highest photocatalytic activity toward water splitting as compared to the sample containing crystalline phase only. Amorphous sample with low concentration of crystalline phase showed highest activity with a rate of 3.6 mmol h−1 from water/methanol in absence of cocatalyts. In comparison to the crystalline phase it shows the photocatalytic activity of 1200 μmol g−1L−1h−1W−1, which is four times greater than the crystalline phase. Table S4 shows the photocatlytic activity along with the band gaps of different samples prepared during the study carried out by Grewe139. Another important technique used enhance the activity of tantalates in UV and visible region of solar spectrum is doping with lanthanides or alkali earth cations. Kudo and Kato synthesized series of lanthanide doped NaTaO3. La, Pr, Nd, Sm, Gd, Tb and Dy were used as dopants and it was observed that NaTaO3 doped with La shows maximum efficiency for photocatalytic splitting of water. The apparent quantum yield was 50% at 270 nm which was 9 times higher as compared to non-doped NaTaO3. This enhanced activity of NiO/NaTaO3:La was explained that doping with La decreases the particle size of NaTaO 3 and improves the ordered surface of the material. The recombination of charge carriers generated during the process was reduced due to size reduction and high crystallinity. Also, the back recombination of H2 and O2 was inhibited by effective reaction sites for H2 and O2 evolution203. Recently Liu and Sohlberg made a theoretical calculation for La doped NaTaO3 and showed that La doping has strong relation with electronic structure of both cubic and orthorhombic NaTaO3. From the calculations it was observed that La doping increases that band gap of semiconductor as well as changes the direct band gap to indirect band gap which is significant for photocatalytic properties of these semiconductors. Effect of La doping in



NaTaO3 on effective mass of electrons and holes was also investigated. La doping has different effect on cubic and orthorhombic NaTaO3. The results obtained for effect on effective mass in orthorhombic La doped NaTaO3 agreed with that of experimentally obtained photocatalytic hydrogen generation rate versus doping concentration. These calculations showed that effective mass carrier mass has significant role in determining the efficiencies of semiconductor photocatalysts206. Kudo et al. explored the effect of alkaline earth metal doping on the overall splitting of water with NaTaO3. It was found that when the concentration of dopant such as (Ca, Sr and Ba) was greater than 0.5 mol. % nanostep structure are created on the surface of doped NaTaO3. These surface nanosteps are considered to dramatically enhance that water splitting efficiency185. Currently major research on alkali tantalate photocatalysts focuses on extending absorption spectrum of alkali tantalates into visible range of solar spectrum. As discussed earlier in this review doping with metals, nonmetal and co-doping are considered as an important technique to enhance the activities of alkali niobate into the visible light of solar spectrum. Recently Woo Kang et al. reported synthesis of La and Cr codoped NaTaO3 via spray pyrolysis of polymeric precursor and the synthesized Na1-xLaxTa1-xCrxO3 and NaTa1-xCrxO3 and their activities for H2 and O2 generation from overall splitting of water under visible light irradiation was evaluated. The H2 production rate of Na1-xLaxTa1-xCrxO3 was 6-fold higher compared to NaTa1-xCrxO3 photocatalyst prepared from aqueous precursor solution as well as the induction period was also reduced to 33%. Sample having optimum concentration of x = 0.003 exhibited the highest activity. The enhanced activity of the La and Cr doped NaTaO 3 was attributed to increase in surface area and to the increase of Cr3+ concentration compared to Cr6+ concentration on the photocatalyst surface. Doping with La cation was considered to increase the concentration of Cr3+ ions on the catalyst surface by adjusting the electronic configuration of the samples from A+B5+O3 to the form of (A+,A3+)2+(B5+,B3+)4+O3209. Modak and Ghosh


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theoretically investigated the synergetic effect of simultaneous doping with both cation and anion on the visible light photocatalytic activity of KTaO3207. N and Mo/W were used as anion and cations doping agents respectively. Although monodoping with these elements decreases the band gap of KTaO3 but it is not preferred as it introduces discrete impurity states which may acts as sites of recombination for electrons and holes generated during the photocatalytic splitting of water. In addition to significant reduction of band gap by (N, Mo) and (N, W) dopant pair no localized impurity state for charge recombination is being formed. It was also reported that localized defects may be intrinsic and their concentration depends on the synthesis method used. Therefore, it is of prime importance to choose suitable synthesis approach and doping ions to produce an efficient material. Another advantage of cation anion doping is that charge compensated systems are formed which reduces the chances of undesirable vacancy defect formation. From the theoretical calculations it was observed that band edge of (N, W) co-doped KTaO3 positioned suitably to satisfy the thermodynamic conditions for water splitting. However, for N and Mo co-doped system band gap is reduced considerably but only for other visible driven photocatalytic process and not for water splitting. Therefore, the obtained results provide the valuable information regarding the choice of strategies to improve the photocatalytic activities of KTaO3 for water splitting into the visible range. Alkali niobate and tantalate based semiconductor photocatalyts holds important position among different perovskite for promising UV-light active photocatalysis. Different techniques such as nanostructure engineering and doping have been used to dramatically enhance the performances of parent alkali niobates and tantalates as discussed above. In addition, to enhance activities of parent alkali niobates and tantalates in UV range doping with appropriate metals and non-metals enhances their activities into the visible range.



Other important application study was done by Dutto et. al.87. The study of nonlinear optical response in single alkaline niobate nanowires was carried out by synthesizing and characterizing three types of perovskite alkaline nanowires: ANbO3 (A= Na, K, Li). Efficiency of synthesized nanowires to generate second harmonic signal was compared by employing confocal imaging and it was demonstrated that LiNbO3 was showing strongest nonlinear response. Polarization responses of second harmonic generation (SGH) signal was also investigated in all the three systems. It was revealed from polarization responses that second harmonic (SH) signal depends on polarization and depends on extent of ordering in the nanowires structures. Based on their findings they have associated various future applications for all the three types of XNbO3 nanowires like based on highest nonlinear responses nanowires, they could be used as imaging markers, but the trapping stability of LiNbO3 was worst in comparison to KNbO3 or NaNbO3 hence, for use as opto-mechanical probes they have suggested to use KNbO3 nanowires based on their good trapping stability. All the three XNbO3 nanowires displayed similar wave-guiding efficiency, but based on aspect ratio they have recommended NaNbO3 nanowires as potential wave-guiding material. Ke et. al.149 have synthesized crystalline orthorhombic sodium niobate nanowires with approximate diameter of 100 nm and length of several hundred microns, as well as cubes of sodium niobate with edge length of about few hundred nanometers were synthesized by reaction between niobium oxide (Nb2O5) NaOH solutions having concentration of 10 and 12.5 M, respectively. The results obtained demonstrated that major initial products of the reaction were monoclinic Sandia octahedral molecular sieves (SOMS) nanowires, resulted from the intercalation of NaOH to Nb2O5, and heating up to 400 °C resulted in the formation of wiry orthorhombic sodium niobate by removal of H2O from these SOMS nanowires. Effective piezoelectric coefficient of individual sodium niobate nanowires along their vertical direction was measured by piezoelectric force microscopy (PFM) and was about few pm/V.


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This was the first report of synthesis of sodium niobate nanowires as well as determination of their piezoelectric coefficient. The study of piezoelectric property opens a new field of research which is now leading for hunt of lead free piezoelectrics. 6. Conclusions and Perspectives: 6.1 Conclusions: In this review, we have selectively highlighted the progress in synthetic strategies for the preparation of alkali niobate and tantalate perovskites and reviewed the progress in the field of enhancing the application of alkali niobates/tantalates as photocatalysts in UV and visible range of solar spectrum. Our aims were to develop an idea about the present research on the development of and strategies developed to utilize alkali niobates and tantalates for efficient solar energy conversion and environment remediation material to provide some insight and guidelines for more research. Although significant progress has been made over recent years still more research is needed to develop the alkali metal based niobates and tantalates for practical application in photocatalytic solar energy conversion and environmental remediation to attain sustainable and safe future. Important aspect that is broadly discussed in the review is the synthetic strategies of alkali niobates and tantalates and the influence of different reaction conditions on the structure, morphology and other physical properties of alkali niobates and tantalates. Reverse micellar and hydrothermal synthetic strategies seem to be the effective alternative methods to conventional high temperature synthesis approaches and some excellent results have been reported. These low temperature methods offer better control on reaction conditions to achieve optimal conditions for synthesis of alkali niobates and tantalates having efficient photocatalytic activities. Other methods are limited to synthesis specific alkali niobates and tantalates and are not considered as general methods for the synthesis of alkali niobates and tantalates.



With increase in fundamental research photocatalysis shows an alternate way which can solve the energy and environment related problems of the world. However due to rapid growth in the energy demands and industrialization, there are still many challenges faced by alkali niobates and tantalates as photocatalysts due to their low photocatlytic activity. For an efficient photocatalyst, it should have strong light absorption property. Absorption of light radiation is the first step of the photocatalysis, the energy absorbed by any photocatalyst directly controls the number of electrons or holes generated, thus affecting the efficiency of the photo catalyst. As discussed in this review most alkali niobates and tantalates shows the absorption of UV part of solar spectrum which is only the 8% of the solar spectrum. This review has discussed the extension of use of alkali niobate and tantalates in the visible region which accounts for 50% of the solar spectrum. Different strategies like doping, development of heterojunctions, conceptual Z-scheme design and core shell nano-composites have been used to manipulate the band gap of these alkali niobates and tantalates to extend their application in visible region. Absorption of light alone doesn’t determine the efficiency of a photocatalyst, but another important factor which influences the efficiency is the separation of charge generated during photocatalysis. Therefore, in addition to the efforts to enhance the light absorption, it is also of equal importance to enhance the charge separation during photocatalysis. Therefore, to increase the charge separation during photocatalysis the recombination sites like holes, defects should be decreased or eliminated. Although the strategies discussed in this article like development of heterojunction e.g. g-C3N4/NaNbO3, core-shell nanostructures e.g. CdS/NaNbO3, metal semiconductor composites can enhance the charge separation by providing additional force developed by built in electric field, but the driving force/electric field is very low and is confined to the interfacial area. Therefore, construction of materials with strong internal developed electric field throughout the whole material would increase the charge separation efficiency of the material.


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Although huge efforts have been employed, there is lot of work that needs understanding in photocatalysis of alkali niobate and tantalates. With better understanding of mechanism of their photocatalytic activity with the aid of new developed synthetic strategies and development of sophisticated characterization techniques and theoretical calculation their low photocatalytic activity could be enhanced in coming future. Finally, the use of the growth technique to produce functional metal oxide crystals results in the reduction of environmental damage (e.g. by exhausting greenhouse gases) and cost-effective products. 6.2 Perspectives: To overcome the problems faced in photocatalytic degradation of organic molecules and photocatalytic water splitting reaction, in addition to development of synthesis methods and search for new photocatalysts the mechanism behind the reaction should be investigated. To understand the reaction mechanism, focus of investigation should be towards the surface reactions as surfaces play important role in photochemical transformations. The generation and detection of reactive oxygen species (ROS) during the reaction should be investigated carefully which would be of great help in designing and modifying efficient photocatalysts. In addition to development of alkali niobate and tantalate photocatalysts focus of researchers should be towards the mass production of these catalysts for practical applications, catalyst immobilization techniques to provide cost effective, environment friendly separation technique and improving the operating range of catalysts (catalysts should operate for wider range of temperature and pH). For photocatalytic water splitting other technological problems must be addressed like supply of water and separation of O2 from reactive H2 and O2 mixture. Finally, in addition to above constrains, researchers should work together to understand the reactions involved and to develop cost effective and environment being materials for bright future of the planet earth.



Associated Content: Supporting Information Tables showing reaction conditions used in solid state synthesis and hydrothermal synthesis of alkali niobates and tantalates. The alkali niobates and tantalates based photocatalysts showing hydrogen evolution and oxygen evolution reaction are also tabulated. Cyclic voltammogram of dye sensitized layered niobate photocatalyst. Notes: The authors declare no competing financial interest. Acknowledgement: UF and RP thanks to UGC, New Delhi for Research fellowship. TA thanks to DST and CSIR, New Delhi, Govt. of India for financial assistance to Nanochemistry Research Laboratory at Jamia Millia Islamia.


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