Theoretical insights into low dimensional materials for water splitting

Abstract

The semiconductor based water splitting method is a promising strategy to produce hydrogen carrier to fulfill the global energy demand. The water splitting method involves two half-cell reactions named as oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). However, the overall conversion efficiency (solar to chemical energy) is low due to the rapid electron-hole recombination. Thus, an efficient catalyst is highly required for the water splitting reaction at the macroscopic level. Nowadays, low dimensional materials of few nanometer sizes have shown their potential towards OER/HER in comparison to their bulk metal surfaces due to the presence of high surface unsaturation, and high surface to volume ratio which increases the charge carrier separation and suppresses the carrier recombination. These nanomaterials have been synthesized in different morphologies such as nanosheet, nanotube, nanorod, and nanospheres and have been explored for water splitting reaction. The electronic properties of these nanomaterials can be easily tuned as compared to bulk counterparts because of more flexibility. However, the dimensionality of the materials is very important factor for significant change in the activity because the valence band of the material gets stabilized by reducing the dimensionality which may lead to reduce the possibility of carrier recombination. However, the overall efficiency of water splitting is still very low. Therefore, more efforts need to be done to improve the scope of low dimensional materials in the field of water splitting. In this context, computational screening along with experimental methods is necessitated in the near future to develop highly efficient materials for water splitting. The contentsof each chapter included in the thesis are discussed briefly as follows:3.1. Introduction In this chapter, a brief overview of the water splitting reaction mechanism and their working principles have discussed, putting much emphasis on semiconductor based catalysts for water splitting reaction. Besides, the particular half-cell reaction pathways of water splitting (OER/HER) have discussed here. The potential applicability of low dimensional semiconductors in water splitting has explored. The recent advances on nanosheet based catalysts have been reviewed. Furthermore, the applicability of hollow materials like nanotube based catalysts and their possible application in water splitting has been discussed. Besides, the methods of tuning the electronic properties of low dimensional materials for enhancing their activity have also been surveyed. The density functional theory (DFT) calculations have been used to investigate the catalytic properties of the reported catalysts. Hence, this chapter includes a brief discussion based on DFT and its uses in the catalytic system. Furthermore, we have discussed the basic theory behind phonon dispersions, molecular dynamics, and charge related calculations.3.2. Hexagonal Planar CdS Monolayer Sheet for Visible Light Photocatalysis In this chapter, we have proposed two-dimensional (2D) stable CdS monolayer (ML) sheets using the state-of-the-art theoretical calculations. We found three different conformers (planar, distorted, and buckled) of CdS MLs which are separated by low energy barriers. These monolayer sheets are not only thermodynamically, mechanically, and dynamically stable but also can withstand temperature as high as 1000 K. The density of states analysis indicates that all the three MLs are direct band gap semiconductor with band gap higher than bulk due to its low dimensionality. Furthermore, the semiconducting properties of CdS ML persist under a high percentage of strain. The band edge alignment of these monolayer sheets and bulk CdS is done with respect to the wateroxidation and reduction potential to evaluate their photocatalytic activities. We found that these MLs have not only perfect band edge potentials for water splitting but have more stabilized valence band compared to CdS bulk, which favours hole transfer process and reduces the risk of electron-hole recombination. On the other hand, such materials can be very promising for Z-scheme photocatalysis due to their low dimensionality and flexibility.Our results suggest that the planar CdS ML has a smaller band gap (2.77 eV) than others, and thus it can be a better material for visible light photocatalysis.3.3. Role of Dimensionality of CdS for Photocatalytic Water Splitting: Bulk versus Monolayer versus Nanotube In this chapter, we have modelled a hexagonal facetted CdS nanotube (NT) catalyst for photocatalytic water splitting reaction because these nanotubes are experimentally realized. The CdS NT is dynamically, thermally and energetically more stable structure than their 2D ML of nanosheet based structures. This could be the reason that the nanotube based CdS structures are most commonly synthesized over ML structure. Moreover, our density of states analysis shows that CdS NT has strong p-d/s-p mixing at the valence/conduction band as compared to ML, which indicates that Cd-S boding is stronger in the NT compared to that in the ML. The overall photocatalytic activity of the CdS photocatalyst has been predicted based on the electronic structures, band edge alignment, and overpotential calculations. For comparisons, we have also investigated the water splitting process over bulk CdS. The band edge alignment along with the oxygen evolution reaction/hydrogen evolution reaction (OER/HER) mechanism studies help us to find out the effective overpotential for the overall water splitting on these surfaces. Our study shows that the CdS NT has a highly stabilized valence band edgecompared to that of bulk CdS and ML structure owing to strong p–d mixing. The highly stabilized valence band edge is essential for the hole-transfer process and reduces the risk of electron-hole recombination. Hence, CdS NT could be a promising material for suppressing the carrier recombination over bulk CdS and ML structure too. Moreover, CdS NT requires less overpotential for water oxidation reaction than the bulk CdS. Our findings suggest that the efficiency of the water oxidation/reduction process further improves in CdS as we reduce its dimensionality that is going from bulk CdS to the two-dimensional nanosheet to the one-dimensional nanotube. Furthermore, the stabilized valence band edge of CdS nanotube also improves the photostability of CdS, which is a problem for bulk CdS.3.4. Computational Screening of Electrocatalytic Activity of Transition Metal Anchored CdS Nanotubes for Water Splitting In this chapter, we have studied the catalytic activity of late transition metal (TM = Fe, Co, Ni, Cu, Ru, Rh, Pd, Ag, Os, Ir, Pt, and Au) doped CdS nanotubes (TM@CdS NT) for both oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). As CdSderived nanostructures have identified as potential catalysts for water splitting for decades. In spite of the significant enhancement in photocatalytic activity exhibited by these CdS nanotubes, the electrocatalytic activity of CdS nanostructures has not been improved considerably for OER/HER because of the high overpotential compared to the state-of-the-art catalysts. In particular, transition metal doping is an efficient way in the desirable tuning of the properties of nanostructures, and it also affects the extent of interaction with OER/HER intermediates. In this context, systematic screening of stability as well as an activity among the doped NT structures is carried out and compared the results with pristine CdS NT and bulk CdS. The doping of transition metals is found to be accompanied by an enhancement of impurity d states near the Fermi level, suggesting an efficient electrocatalytic activity. Majority of transition metal doped structures are associated with significant stability and are observed to improve both theOER as well as HER activity. We found that Pd@CdS and Ru@CdS as optimal catalysts for OER and HER, respectively with the lowest overpotential, outperforming pristine CdS NT as well as bulk CdS. The origin of the activity trend is attributed to the differences in the interaction to the reaction intermediates across the series of doped nanotube structures. Complete scrutiny of the adsorption energetics of elementary reactions for all the TM@CdS NT structures is provided and on that basis, an activity plot is constructedtocorrelate the overpotential and adsorption energetics.3.5. Stanene and Doped Stanene for Photocatalysis In this chapter, we have discussed the group-IV based two-dimensional material stanene which is a single layer of tin. Stanene is a quantum spin Hall insulator and a promising material for optoelectronic and electronic devices. Recently, graphene and doped graphene based materials have been reported for improving the photocatalytic activity because they can be used as an efficient electron acceptor for enhancing the photoinduced charge carrier separation. Stanene has similar electronic properties like graphene and could show dissipationless conduction at room temperature. Therefore, here we have studied the effect of elemental doping (In, Sb, and In-Sb) in stanene towards photocatalytic applications. Here we have shown that such In/Sb mono-doped stanene behaves as a degenerate semiconductor, whereas In-Sb co-doping opens the band gap (0.10 eV) in stanene. Moreover, such doped stanene based systems show excellent energetic, thermal, dynamical, and mechanical stability, which further suggests that they can be synthesized and used for various applications. Our band edge alignment study indicates that Sb@stanene systems have more negative conduction band than stanene and graphene with respect to water reduction potential, which further indicates that Sb@stanene systems can bepromising for photocatalysis process.