Photoelectrochemical Properties of Colloidally Synthesized (Cu,Ag)2ZnSnS4 solid Solution nanocrystals

Abstract

Introduction Multinary semiconductors of Cu2ZnSnS4 (CZTS) composed of earth-abundant and non-toxic elements have been intensively studied as a new type of potential photovoltaic materials for the application to thin film solar cells[1] and photocatalyst for hydrogen evolution[2] due to its promising optical properties for absorbing solar light, such as band gap energy of 1.4-1.5 eV and high absorption coefficient. Recently, Kudo and co-workers reported that the photocatalytic activities for hydrogen evolution can be greatly enhanced by formation of solid solution between CZTS and Ag2ZnSnS4 (AZTS).[2]The hydrogen evolution rate under visible-light irradiation of CZTS-AZTS photocatalyst particles was 150 times higher than that of pure CZTS particles due to an increase in conduction band edge energy by making solid solution. Nanometer-sized crystals of such photoactive semiconductors have also attracted much attention due to their light-harvesting properties and tunable electronic energy structure depending on their size. In addition, semiconductor nanocrystals (NCs) have opened up new ways to generate multiple charge carriers with a single photon. In our previous papers, we successfully synthesized CZTS NCs via thermal decomposition of precursors in hot organic solution and investigated their photoelectrochemical properties depending on the particle size and composition.[3-5] The incorporation of Ag+into CZTS nanocrystal solid solution is expected to enhance photo-properties of nanoparticles, but such attempt has not been carried out. In this study, we prepare CZTS-AZTS NCs uniformly dispersed in organic solution and report their photoelectrochemical properties. Experimental  CZTS-AZTS solid solution ((Cu1-x Ag x )2ZnSnS4; CAZTS) NCs were prepared via thermal decomposition of metal diethyl dithiocarbamate precursors in hot oleylamine under N2 atmosphere. After removal of largely aggregated particles, the resulting NCs were dissolved in chloroform. Photoelectrochemical properties of CAZTS NCs immobilized on ITO electrodes were measured in an aqueous solution containing 0.2 mol dm-3 Eu(NO3)3 as an electron scavenger. The Ag/AgCl electrode and Pt wire were used as reference and counter electrodes respectively. Results and discussions  TEM measurement revealed that thus-obtained particles were polygonal shape with average diameter of ca. 10 nm regardless of x value. Their chemical composition could be controlled between 0 < x < 1 by varying the Cu/Ag ratio in precursors. The X-ray diffraction patterns of NCs were assignable to kesterite-type crystal structure and each peak was shifted to lower angle with increase in the Ag content in NCs, indicating that the obtained NCs were not a mixture of CZTS and AZTS NCs but a solid solution between CZTS and AZTS. Figure 1 shows absorption spectra of CAZTS NCs in chloroform. The absorption onset of CAZTS NCs was blue-shifted with an increase in Ag content. The bandgap of the NCs was tunable from 1.1 eV (x = 0) to 2.0 eV (x = 1.0) only by changing value of x.  To investigate the photoelectrochemical properties of CAZTS NCs, the NCs were immobilized onto ITO electrodes with spin-coating CAZTS NCs chloroform solution at 1000 rpm for 10 second, followed by heating the electrode at 300°C. Irradiation (λ > 350 nm) to thus-obtained films was carried out in an aqueous solution containing Eu(NO3)3 as an electron scavenger. Cathodic photocurrents were observed for CAZTS NC electrodes except for NCs of x = 1.0 (AZTS) under applied electrochemical bias, the magnitude being increased with negative shift of the electrode potential. The observed behavior was similar to that of a p-type semiconductor photoelectrode. On the other hand, AZTS NCs-immobilized ITO electrodes exhibited anodic photocurrent like as an n-type semiconductor as reported in our previous paper.[6]Action spectra of the photocurrent were in good agreement with absorption spectra of CAZTS NPs used, indicating that CAZTS NPs effectively worked as photovoltaic materials driven by irradiation of visible and near-IR light. Reference [1] D. Aldakov, et al., J. Mater. Chem. C, 2013, 1, 3756. [2] A. Kudo, et al., Chem. Mater., 2010, 22, 1402. [3] T. Kameyama, et al., J.Mater. Chem., 2010, 20, 5319. [4] H. Nishi, et al., Phys. Chem. Chem. Phys., 2013, 16, 672. [5] H. Nishi, et al., J. Phys. Chem. C, 2013, 117, 21055. [6] T. Sasamura, et al., Chem. Lett., 2012, 41, 1009. Figure 1