International Journal of Energy Research, Vol.45, No.4, 5922-5938, 2021
Band gap engineering of ZnO by amino acid capping for optoelectronic and energy applications
ZnO nanorods were capped with aliphatic, aromatic, and heterocyclic amino acids and the effect of capping on morphology, structural, optical, electrochemical, and electrical properties were studied in detail. The formation of coordination complexes between amino acids and ZnO were confirmed by XRD, XPS, and FTIR. The binding energy difference; increment E = 23 eV explicitly indicates the nanometer range of the complexes formed. The high I-uv/I-vis seen in PL, for all ZnO-AA complexes except ZnO-L-Phe and ZnO-D-Phe indicate the crystallinity and minimal defect in the complexes. The band gap of ZnO-D-Ala and ZnO-DL-Ala reduced by 0.15 eV compared to ZnO NRs (3.0 eV). This reduction in band gap is attributed to the (a) electronic percolation between C=O group of amino acid and Zn in ZnO and (b) increase in crystallite size due to amino acid capping. The free energy of activation for electronic conductivity, in case of ZnO-D-Ala and ZnO-DL-Ala remained identical to ZnO NRs (127 kJ) as supported by EIS analysis and Arrhenius kinetics. Out of all the aliphatic, aromatic, and heterocyclic amino acids capping on ZnO NRs, the results indicate that ZnO-D-Ala and ZnO-DL-Ala complexes possess lower band gap and can be used as a viable n-type semiconductor material in optoelectronic applications such as energy harvesting devices like solar cells.