DEVELOPMENT OF PEROVSKITE OXIDES FOR PHOTOCATALYTIC HYDROGEN GENERATION

Abstract

The current climate crisis demands an urgent shift away from traditional fossil fuels, towards carbon neutral or carbon zero, green H2 future fuels. This shift can be driven by green energy sources, such as hydro, solar, wind, and wave generated energy. Solar energy offers a virtually inexhaustible source of power and switching to solar energy could be a practical way to reduce emissions, protect the environment, and bolster our energy security. Solar energy can be converted into chemical energy by various processes. Among all, photocatalysis is a promising process. This has opened up a range of possibilities in the fields of hydrogen production, oxygen evolution, hydrogen peroxide generation, overall water splitting, nitrogen fixation, carbon dioxide reduction, and organic pollutant decomposition. Water splitting is an important technique for converting solar energy into chemical energy. Photocatalytic water splitting can take place either by one step photoexcitation or by two step photoexcitation (also known as Z scheme). Efficient water splitting relies on a semiconductor photocatalyst with a minimum 1.23 eV band gap at pH = 0, where incident light energy matches or exceeds the band gap energy. When light activates the photocatalyst, electron-hole pairs are generated, leading to surface reactions. These reactions involve the reduction of electron acceptor species (e.g., H+) and the oxidation of electron donor species (e.g., water or hydroxyl anions), crucial for the overall process. Following the initial PEC water splitting on TiO2 in 1972, there has been a notable surge in the exploration of various photocatalysts, particularly since 1980. Until 2000, numerous photocatalysts capable of photocatalytic activity under UV-light were investigated for overall water splitting (OWS). These systems contain transition metal ions with d0 (e.g. Nb5+, Ta5+, W6+, etc.) or d10 (e.g. Sb5+, Ge4+, Sn4+ Ga3+, In3+, etc.) configuration, in which conduction band is composed of d and sp hybrid orbitals, while valence band is made up of O 2p orbitals. Examples of some highly efficient UV-light active oxides are La doped NaTaO3, Zn doped Ga2O3 and Al doped SrTiO3 having band gap 4.1, 4.4, and 3.2 eV, respectively. Afterwards, enormous efforts were devoted to explore the visible-light active photocatalysts. One of the efficient approaches is doping with transition metal ions, which leads to the introduction of mid gap states in the compound that helps in the extension of absorption edge in the visible region. There are multiple studies reported for the doping of cations in NaTaO3, SrTiO3 etc. Enhanced absorption by NaTaO3 after co-doping with La and Mn was documented, while doping with trivalent lanthanide ions (Eu3+, La3+) have been also shown to enhance the photocatalytic activity of the compounds. Similarly, doping of SrTiO3 with Rh, Al, La, Fe, Mn, Co and Cr was reported to enhance the activity of the materials. However, there are only few reports available for doping in niobates towards their activity in photocatalytic hydrogen evolution. Chapter-1 gives a brief overview of design strategies, various perovskite-type compounds, and their photocatalytic activity. The present thesis work focuses on the creation of new perovskite oxide based photocatalysts, which are mostly composed of inexpensive and earth abundant elements, and their use in photocatalytic hydrogen evolution reaction (HER). In Chapter-2, syntheses methods utilized for material preparation are extensively discussed, along with a comprehensive summary of the characterization techniques employed to thoroughly analyze the materials. Powder X-ray diffraction (P-XRD) for the phase purity and identification, FULLPROF program suite for Rietveld refinement, electron paramagnetic resonance (EPR) measurement and superconducting quantum interference device (SQUID) magnetometer for the spin state determination, FE-SEM, TEM and EDS to understand the microstructure and elemental composition, and BET for specific surface area measurement were used. The optical properties of the samples were analyzed using diffuse reflectance data in the range of 200-800 nm. X-ray photoelectron spectroscopy was used to investigate the chemical state of the constituent elements and Ultraviolet photoelectron spectroscopy (UPS) for recording the valence band spectra of the compounds. The TGA and DTG were used to understand the thermal stability and decomposition pattern of the compounds. Fluorescence lifetime spectroscopy (FLS) was used for the fluorescence lifetime measurement of the compounds. Electrochemical workstation was used for the electrochemical impedance spectroscopy (EIS), Mott-Schottky and photocurrent density measurements of the materials. Photocatalytic hydrogen evolution reaction was carried out in a closed photochemical reactor system using 250 W medium-pressure Hg-vapor lamp as the light source. Gas chromatograph was used for the quantification of the evolved gases. A brief overview of the principles for each of the techniques along with the methods of measurements, analysis and details of equipment are described in this chapter. Many 3D titanate and niobate perovskites are useful photocatalysts for hydrogen evolution but are active only under UV-light. Various strategies including use of noble metalbased catalysts/co-catalysts and heterojunction formation are adopted to induce visible-light absorption and enhance photocatalytic activity. Here, in Chapter-3 simple coupled-substitution approach in 3D niobate perovskite oxides, Na0.5Ca0.5M0.25Nb0.75O3 (M = Cr, Mn, Fe and Co), is demonstrated to show enhanced hydrogen evolution without using heterojunction systems or noble metal catalysts/co-catalysts. The approach is innovative since the incorporation of a non Jahn-Teller transition metal ion induces local octahedral distortion in the structure while the Jahn-Teller active ions show negligible to no distortion in the presence of Nb5+, a d0 system, that is known to undergo second-order Jahn-Teller (SOJT) distortions. The effect of octahedral distortion on the structure, optical absorption and in cocatalyst free hydrogen evolution are discussed. The highest rate of hydrogen evolution of ~200 μmol g-1 h-1 for the Cr compound is believed to originate from enhanced charge transfer dynamics due to reduced charge transfer resistance arising from the enhanced covalency of Cr-O bonds. On the other hand, the moderate octahedral distortion also helps in the breakdown of electronic transition rules for enhanced UV-Vis light absorption. The study recognizes the effect of first-order Jahn-Teller (FOJT) and SOJT distortions in dictating enhanced photocatalytic hydrogen evolution activity. The study provides insights for the future design of efficient and sustainable energy conversion systems based on the interplay of structural and octahedral distortions in 3D perovskites. While chapter-3 dealt with the effect of B-site transition metals in the 3D perovskite niobates, Chapter-4 was devoted to the study the effect of A-site cation on the phase formation, stability and photocatalytic activity of the 3D niobates with same B-cation combinations. For this, perovskite semiconductors of composition Na0.5Sr0.5M0.25Nb0.75O3 (M = Cr, Mn, Fe, Co) are synthesized using a simple heterovalent coupled substitution strategy via a solid-state reaction route. In this case, the compounds were crystallized in the cubic system rather than the orthorhombic one for the Ca-analogues. As a result, there is no octahedral distortion observed in these compounds. The Cr-containing catalyst of the Sr-analogue exhibits a hydrogen evolution rate of 188 μmol g-1 h-1, significantly surpassing the performance of contemporary catalysts that rely on cocatalysts. However, the activity was less as compared to the Caanalogue. The chapter discusses the details of photophysical properties in the light of their structures and furnishes a new perspective for the design and development of non-precious noble-metal-free perovskites for efficient photocatalytic hydrogen evolution. While the practice of aliovalent doping in perovskites has been well-established over time, relatively less attention has been directed towards understanding the intricate arrangement of the doped cations within the host structure. The work described in this Chapter-5 addresses this gap by synthesizing A-site deficient partially ordered perovskites, Sr1.9Cr0.9M1.9O6 (where M = Nb, Ta), through the co-substitution of two distinct cations (Cr and Nb/Ta) at the B-site of the ABO3-type structure. The compounds are synthesized by solid-state reactions and identified to be crystallized in the face centered cubic lattice with Fm3̅ m space group symmetry. Rietveld structure refinements indicated that Cr and Nb/Ta are distributed across two distinct Wyckoff sites to different extents in the compounds. The degree of disorder is calculated by refining the occupancy factor. Refinement results indicated that 25% of Cr ions are replaced by the Nb ions in the case of Sr1.9Cr0.9Nb1.9O6, while 45% of the Cr ions are replaced by Ta ions in Sr1.9Cr0.9Ta1.9O6. Thus, Sr1.9Cr0.9Nb1.9O6 compound is found to have a higher degree (~ 50 %) of ordered arrangement of Cr and Nb, while the Ta-containing compound displays a nearly disordered arrangement. The study sheds light on the impact of such effects on the cocatalystfree photocatalytic hydrogen evolution reaction (PC-HER), where the Ta-containing compound demonstrates a superior HER activity (150 μmol g–1 h−1) as compared to the Nb-containing compound (117 μmol g–1 h−1). The enhanced activity of the Ta-containing compound is attributed to its disordered arrangement coupled with rapid charge carrier dynamics. The findings provide valuable insights for designing future catalysts with enhanced performance, bridging the gap between fundamental understanding and practical application in the field of perovskite-based photocatalysts. While topotactic ion-exchange is ubiquitous in the preparation of many metastable solids with 2D-layered structures, the scope of chimie-douce ion-exchange has been extended to quasi-two- and three-dimensional structures including nanocrystals in recent times. The lowtemperature solid-state exchange is yet another unique synthetic tool, which has given access to preconceived structures enabling the rational design of solids. Although rational synthesis using inorganic synthons are rare, few examples exist among the inorganic solids with layered structures. In Chapter-6 we report an interesting solid-state ion-exchange between the preformed 3D simple perovskite niobate NaNbO3 and CuSO4·5H2O in a 2:1 molar ratio. NaNbO3 is a simple ABO3 type perovskite, however, the A-site exchanged transition metal adopts a new coordination environment after ion-exchange and it orders in 1:3 fashion with respect to the cation vacancies resulting in a ordered A-site deficient quadruple perovskite structure (A A′3B4O12). The transformation eventually stabilized a high-pressure quadruple perovskite at ambient pressure. The transformation is achieved by solid-state metathesis reaction at a moderate temperature and ambient pressure, wherein the exchanged transition metal adopts a new coordination environment. Detailed structural analysis was carried out to understand the factors that drive the structural transformation on ion-exchange. Tauc plot analysis of UV-Vis DRS data gave a band gap of 1.94 eV, which is substantially narrower than that of the parent NaNbO3 (3.35 eV). When compared with the photoresponse of NaNbO3, CuNb2O6 showed enhanced visible-light absorption and photocurrent response, which are vital requirements for efficient photocatalysis. Hence, the compound is also evaluated for its activity in photocatalytic hydrogen evolution and the details are presented in this chapter. In conclusion, the present work opens up opportunities for prognostic design and synthesis of many low volume or high-pressure phases under ambient pressure and identification of new solid-state synthons for rational design of solids. Overall conclusion and the future prospects of the work is presented in Chapter-7. In this comprehensive study, we explored the potential of coupled substituted 3D niobate perovskite oxides for cocatalyst-free photocatalytic hydrogen evolution, addressing the limitations of UV-light activation observed in many existing photocatalysts. The innovative coupled-substitution approach in the perovskite niobates (Na0.5Ca0.5M0.25Nb0.75O3, where M = Cr, Mn, Fe, and Co) demonstrated remarkable hydrogen evolution without the need for heterojunction systems or noble metal catalysts. The incorporation of a non-Jahn-Teller transition metal ion induced local octahedral distortion, influencing the charge transfer dynamics and enhancing the covalency of metal-oxygen bonds. The study highlighted the importance of octahedral distortions in dictating the enhanced photocatalytic hydrogen evolution activity. The study also encouraged the investigation of the Sr-analogues of the 3D perovskite niobates to understand the effect of A-cation manipulation on PHE activity. Although the Sr-analogues also exhibited significant PHE performance, the PHE activity of Crcontaining niobate of the Ca-series was higher as compared to that of the Sr-series of compounds. Additionally, our investigation into A-site deficient perovskite oxides, Sr1.9Cr0.9M1.9O6 (M = Nb, Ta), revealed the impact of aliovalent doping and cationic arrangement on the cocatalyst-free photocatalytic hydrogen evolution. The study demonstrated that the degree of disorder in the cationic arrangement significantly influenced the photocatalytic activity, with the Ta-containing compound exhibiting superior performance as compared to the Nb-containing compound. These findings emphasize the importance of understanding the intricate arrangement of doped cations within the host structure for designing future catalysts with enhanced performance. Furthermore, the synthesis of CuNb2O6 through topotactic solid-state ion-exchange from NaNbO3 showcased the potential of rational design in creating novel structures with improved properties. The compound exhibited a narrower band gap and enhanced visible-light absorption, essential for efficient photocatalysis. The work not only contributes to the understanding of solid-state ion-exchange but also opens up opportunities for the design and synthesis of new low-volume phases under ambient pressure. Overall, the present study not only contributes to the fundamental understanding of perovskite-based photocatalysis but also offers practical strategies for the development of efficient and sustainable energy conversion systems.The insights gained from this work may pave the way for several future perspectives in the field of photocatalysis. Firstly, the understanding of octahedral distortions and their impact on photocatalytic activity can guide for tailored design of perovskite oxides for improved performance. The exploration of cationic arrangement and aliovalent doping may open up avenues for further studies to optimize these factors and enhance the efficiency of photocatalysts. Further exploration of A-site deficient perovskites as well A-cation induced distortions keeping the B-cation ratio and combinations intact may also constitute the subject of future study. Finally, the success in the rational design of CuNb2O6 through topotactic solidstate ion-exchange suggests a promising approach for the synthesis of new materials with different properties. Future research can explore similar techniques for synthesising perovskites and other structural families.