Quantum chemical investigation of the photocatalytic activity of TiO2 nanoclusters with different structures

The structural, electronic and photocatalytic activity of (TiO₂)n (n =5;10;15;20) nanoclusters were studied by quantum chemical methods. According to the calculation results, the band gap in the n=5 cluster was calculated to be 4 eV and its wavelength was calculated to be 310 nm. The band gap energies in the n=10; 15; 20 clusters were calculated to be 3.61; 3.20; 2.90, respectively, and their wavelengths were calculated to be 344; 388 and 427 nm, respectively. Also, the values of the standard Gibbs energy (ΔG) and enthalpy (ΔH) become negative with increasing number of units, which indicates that the formation of large clusters is thermodynamically favorable. For example, the Gibbs free energy for (TiO₂)₅ was found to decrease from -560.24 kcal/mol to -2250.95 kcal/mol for (TiO₂)₂₀. The entropy (S) and heat capacity at constant storage (𝐶𝑝) showed a systematic increase with cluster size, additional dispersity, and Gibbs tendency. The heat capacity increased from -567.49 to -2364.82 kcal/mol, and the entropy capacity increased from 80.56 to 265.76 kcal/mol. The rotational and configurational degrees of freedom were found to be improved.

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