Computational Investigation of photocatalytic activity of Sn-doped Zinc Oxide nanoparticles

Abstract

Photocatalysts, such as zinc oxide (ZnO), are used in various industrial processes, including manufacturing photovoltaic solar cells, optical sensitisers, optoelectronic devices and degradation of organic pollutants. ZnO is particularly effective due to its strong oxidising power, hydrophobicity, notable antimicrobial properties, stability, and non-toxicity. However, its photocatalytic activity is confined mainly to the UV range of the solar spectrum because of its large band gap, resulting in lower efficiency and prompting research for improvement. Additionally, the recombination of photogenerated electron-hole pairs further reduces its photocatalytic efficiency by impeding charge separation. One of the methods used to enhance the photocatalytic performance of ZnO under visible light, is to reduce its band gap energy of 3.37 eV. Various techniques have been explored to achieve this, including doping with metals and non-metals, quantum dot sensitisation, alloying, surface modification, and nano-structuring. Doping with metals is an efficient approach to band gap reduction, because it creates new electronic states within the band gap, narrowing it and allowing light absorption to extend into the visible light spectrum. This versatile method enables the selection of specific dopants to achieve desired electronic and optical properties. Additionally, doping improves charge separation and reduces recombination rates, enhancing photocatalytic performance. This study focused on tin (Sn) as the dopant due to its superior performance in previous studies. A computational model was developed to explore the potential of Sn-doped ZnO as photocatalysts. Pure hexagonal wurtzite, the cubic rock salt, and the cubic zinc-blende (sphalerite) crystal structures were sourced from crystal structure databases and expanded to 2x2x2 bulk structures. These bulk structures were optimised, and their electronic properties, such as density of states (DOS), d-band centre, band structure, band gap, and X-ray diffraction, were calculated using Density Functional Theory (DFT) based calculations. The solid-state module Cambridge Serial Total Energy Package (CASTEP) were used to calculate these properties. Validation was achieved by comparing them with literature values. After optimisation, the hexagonal ZnO bulk structure was cut along several Miller planes: (100), (002), (101), (102), (210), (103), (200), (112), and (201). While the cubic and zinc-blende ZnO bulk structures were cut along planes of (111), (200), (220), (222), (311), and (400). A 15 Å vacuum gap was added between adjacent surfaces to prevent periodic cell interactions. These surfaces were optimised, and their surface energies and work functions were calculated to evaluate their stability and activity, with further validation against literature values. The predicted morphology was determined using the Wulff construction method with Wulffmaker software. This approach established the equilibrium shape of the ZnO crystal systems, where the size of each surface slab reflects its surface energy and, thus, the crystal's stability. Following this analysis, the pure hexagonal ZnO crystal structure was doped with Sn using the Supercell program, which systematically created all possible inequivalent site configurations for doping percentages of 6.25%, 12.50%, and 18.75%. This was done by substituting specific Zn atoms with Sn. These configurations were imported into Materials Studio 2020 and optimised. The most stable configurations for each doping percentage were identified based on the lowest final energies. The properties of Sn-doped ZnO bulk structures (DOS, d-band centre, band structure, band gap, X-ray diffraction) were determined. Validation of these structures was done by comparing their properties with available literature data.The Sn-doped ZnO bulk structures were then cut along the same Miller planes as the pure hexagonal ZnO model, namely (100), (002), (101), (102), (210), (103), (200), (112), and (201). This cut was done to create surface models of the Sn-doped material. Similarly, a 15 Å vacuum gap (to prevent periodic cell interactions) was employed. These doped surfaces were optimised, and their surface energies were calculated and validated against literature values. Again, the morphologies of the doped materials were determined, to determine the equilibrium shape of Sndoped ZnO crystal systems.

Finally, the properties of pure ZnO and Sn-doped ZnO were compared. The Sn-doped ZnO with the most reduced band gap and lowest work function was identified and recommended for improving the photocatalytic activity of ZnO. However, direct experimental data is needed to support these findings fully. This study concludes that the Sn-doped ZnO model predicts significant improvements to the catalytic potential of Sn-doped ZnO for application as a photocatalyst in the degradation of organic pollutants.