DFT Calculations of Silver Atomic Quantum Cluster on TiO2 and CeO2 Catalysts for Green Hydrogen Production.

Alotaibi, Moteb and Lambert, Colin (2023) DFT Calculations of Silver Atomic Quantum Cluster on TiO2 and CeO2 Catalysts for Green Hydrogen Production. PhD thesis, Lancaster University.

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Abstract

An important issue with photocatalysts in producing renewable energy such as hydrogen is the low efficiency in harvesting solar energy. Fortunately, the deposition of small metal clusters such as silver (Ag5) could enhance the photocatalysts’ absorption range to match visible light. In this thesis, I simulated different photocatalysts formed from Ag5 atomic quantum clusters (AQCs) on anatase TiO2 (101), rutile TiO2 (110), and CeO2 (111). Using the Vienna ab initio simulation package (VASP), I employed the generalised gradient approximation (GGA) and a hybrid functional combined with Hartree Fock (HF) theory to systematically study their geometric and electronic structures. The Hubbard term (U) is considered for all the calculations to account for the strongly interacted and localised d and f electrons. The first focus of this thesis is to elucidate the photocatalytic activities of rutile and anatase TiO2 surfaces. An in-depth investigation of stoichiometric and reduced rutile TiO2 (110) and anatase TiO2 (101) decorated with bipyramidal and trapezoidal Ag5 atomic quantum clusters (AQCs) is carried out. It is found that the silver AQCs donate electrons to both stoichiometric and reduced TiO2 surfaces resulting in the formation of a single polaron at either a five-fold coordinated (Ti5c) atom or a six-fold coordinated (Ti6c) atom, indicating improved surface activity. Furthermore, depositing Ag5 AQCs on both TiO2 surfaces can produce mid-gap states within the band gap of the bulk, thereby improving the optical response of the composite in the visible and infrared. As expected, the number of mid-gap energy states increases further when a single oxygen vacancy is introduced into the studied surfaces, which reveals that Ag5 AQCs and oxygen vacancies can reinforce each other, leading to higher efficiency photocatalytic activity. We also find that upon adsorption of Ag5 AQCs on an anatase TiO2 (101) surface, the energy required to form an oxygen vacancy is lower than that of rutile TiO2 (110). Moreover, the adsorption of both bipyramidal and trapezoidal Ag5 AQCs on both TiO2 surfaces generally leads to significant distortion of the clusters, which accounts for the significant reduction in the total energy compared to the pristine TiO2 and gas phase AQCs. This detailed investigation provides insight into new mechanisms for enhancing the photocatalytic efficiency of both rutile TiO2 (110) and anatase TiO2 (101) surfaces. The second focus of this thesis is to examine the influence of trapezoidal and bipyramidal Ag5 AQCs on the photocatalytic activity of stoichiometric and defective CeO2 (111). In addition, the interaction of silicate (SiO32-) with AQCs is considered, which is introduced experimentally when purifying the AQCs, and its effect on the electronic structures of ceria is investigated. It is demonstrated that there exist small gap states in the mid-gap after adsorbing these AQCs, which are attributed to charge transfer from these AQCs to the ceria. Furthermore, more gap states are observed when an oxygen vacancy is created, leading to improved photocatalytic activity. Importantly, the energy needed to form this oxygen vacancy is significantly lowered by the presence of AQCs. Finally, we noted that the silicate does not only contribute to purifying the AQCs, but also could play a crucial role in further increasing the photocatalytic activity.