Near-IR Photoelectrochemical Response of Colloidally Synthesized (Cu,Ag)2ZnSnS4 solid Solution nanocrystals
Authors: Kameyama Tatsuya , Kakudo Ryosuke , Kuwabata Susumu , Torimoto Tsukasa 
Published: February 2020

Introduction

Colloidally synthesized nanocrystals (NCs) of multinary semiconductors have attracted much attention due to their light-harvesting properties and tunable electronic energy structure depending on their size and composition. One of important application of colloidal semiconductor NCs is a precursor to prepare semiconductor films with homogeneous composition across whole films via spin-coating. So far, a lot of study have been reported for the fabrication of photovoltaic devices using these colloidal nanocrystals as a precursor ink. Cu2ZnSnS4 (CZTS), a multinary chalcogenide with p-type semiconductor property, has been intensely studied as a new type of photovoltaic materials owing to their optimal properties for solar energy conversion, such as high absorption coefficient, being composed of earth-abundant and non-toxic elements, and band gap energy of 1.4-1.5 eV. In our previous papers, we successfully synthesized CZTS nanocrystals via thermal decomposition of precursors in hot organic solution and investigated their photoelectrochemical properties depending on the particle size and composition.[1-3] 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) in bulk system.[4] 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 increased ability for photocatalytic reduction and oxidation reaction with increased energy gap by making solid solution. Therefore the doping of Ag+ into CZTS nanocrystal to form solid solution is a promising strategy to enhance their photo-properties without changing nanocrystal size, but such attempt has not been carried out. In this study, we prepare the solid solution NCs of CZTS-AZTS 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

The X-ray diffraction patterns of thus-obtained NCs were assignable to kesterite crystal structure similar to that of CZTS and individual peaks were shifted to lower angle with increase in the Ag content in NCs, except for NCs with x = 1.0 in which sub-peaks assignable to orthorhombic AZTS were additionally observed. These indicated that the obtained NCs were not a mixture of pure CZTS and AZTS NCs but a solid solution between CZTS and AZTS. TEM measurement revealed that thus-obtained particles had polygonal shape with average diameter of ca. 10 nm, regardless of x value. The optical bandgap of CAZTS NCs in chloroform, estimated from absorption onset energy, was blue-shifted with an increase in Ag content from 1.1 eV (x = 0) to 2.0 eV (x = 1.0). We determined the composition-dependent electronic energy structure of CAZTS NCs with photoelectron yield spectroscopy. The energy level of valence band edge of CAZTS NCs became slightly higher from +0.8 to +0.2 V vs. Ag/AgCl with increasing Cu content in the products, while that of conduction band edge was nearly constant at ca. -1.0 ~ -1.1 V, irrespective of x value.
CAZTS thin films were prepared onto ITO electrodes by spin-coating chloroform solutions containing corresponding NCs at 1000 rpm for 10 second, followed by heating the electrodes at 300°C. The irradiation (λ > 350 nm) to thus-obtained films under appropriate potential application produced cathodic photocurrents except for NCs of x = 1.0 (AZTS), 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.[5] 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 the irradiation of visible and near-IR lights.

Reference

[1] T. Kameyama, et al., J.Mater. Chem., 2010, 20, 5319.
[2] H. Nishi, et al., Phys. Chem. Chem. Phys., 2013, 16, 672.
[3] H. Nishi, et al.,
J. Phys. Chem. C, 2013, 117, 21055.
[4] A. Kudo, et al.,
Chem. Mater.,
2010,
22, 1402.
[5] T. Sasamura, et al.,
Chem. Lett., 2012, 41, 1009.