Solution-Phase Syntheses and Photochemical Properties of Silver Bismuth Sulfide Nanoparticles

Abstract

Introduction When semiconductor nanoparticles are reduced to a size of several nanometers or less, the physico-chemical properties start to change by the quantum confinement. The energy gap of particles is enlarged and their absorption spectrum is blue-shifted with a decrease in the particle size. These nanoparticles have been intensively investigated for developing efficient light energy conversion systems, such as photocatalysts [1] and photovoltaics [2]. Especially, high-efficiency quantum dot solar cells fabricated with PbS and PbSe nanoparticles, which showed the responsivity in near-IR region, have attracted much attention [3]. However, the contents of highly toxic element, Pb and Se, limit their use in a wide range of applications [4].Recently, a multinary AgBiS2 semiconductor can absorb visible and near-IR lights and seems to be a promising material for efficient light energy conversion [5], but there are few reports to prepare size-quantized nanoparticles and control their particle size. In this study, we report the preparation of AgBiS2 nanoparticles using a liquid-phase chemical synthesis method. Their size was varied depending on the preparation conditions. Furthermore, we investigated their photoelectrochemical properties as functions of particle size and composition. Experimental Precursors of bismuth acetate (Bi(OAc)3), silver acetate (Ag(OAc)), and sulfur powder, were dispersed in a mixture solvent of 1-dodecanethiol (DDT) and oleylamine (OLAm), followed by the heat treatment at 373, 393, 423 or 473 K for 30 min. Thus-obtained AgBiS2 nanoparticles were isolated by the addition of methanol as a non-solvent. Results and Discussion The particle size and composition of AgBiS2 nanoparticles were controlled by changing the heating temperature. With an increase of the heating temperature from 373 K to 473 K, the particle size was enlarged from 3.2 nm to 8.1 nm, accompanied by the increase of the Ag fraction in nanoparticles to the stoichiometric value. The onset wavelength in the absorption spectra of AgBiS2 nanoparticles was shifted from 1000 nm to 1200 nm with an increase in the particle size, resulting in the decrease of bandgap from 1.45 eV to 1.05 eV. Since the estimated bandgap values were larger than that of the bulk AgBiS2 (0.80 eV) [6], it was found that the obtained AgBiS2 nanoparticles showed a quantum size effect. The XRD analysis revealed that the obtained nanoparticles exhibited the cubic AgBiS2 crystal structure, regardless of the reaction temperature.The Ag/Bi ratio in the preparation also changed the particle size of obtained AgBiS2 nanoparticles as well as their chemical composition. By reacting precursors with Ag/(Ag+Bi) = 0.33 at 423 K, almost stoichiometric AgBiS2 nanoparticles were formed, that is, Ag: Bi: S = 1: 1: 2. In contrast, when Ag/(Ag+Bi) ratio in preparation was decreased from 0.5 to 0.25, the Ag fraction in nanoparticles decreased from the stoichiometric value of Ag/Bi=1, that is, non-stoichiometric AgBiS2 nanoparticles were formed. The particle size tended to decrease with a decrease in the Ag/(Ag+Bi) ratio in preparation.We carried out the photoelectrochemical measurement of AgBiS2 nanoparticles immobilized on ITO electrodes. Anodic or cathodic photocurrents were detected, depending on the Ag fraction in nanoparticles. The energy levels of AgBiS2 nanoparticles were determined by measuring the photoelectron yield spectroscopy in air. The onset potentials of photocurrent generation were significantly different from the levels of conduction band minimum or valence band maximum, suggesting that the thus-obtained AgBiS2 nanoparticles contained the large amount of defect sites which acted as a carrier recombination site. Acknowledgments This work is supported by the New Energy and Industrial Technology Development Organization (NEDO). References (1) S. Chen, D. Huang, P. Xu, W. Xue, L. Lei, M. Cheng, R. Wang, X. Liu, and R. Deng., J. Mater. Chem. A, 8, 2286 (2020).(2) G. H. Carey, A. L. Abdelhady, Z. Ning, S. M. Thon, O. M. Bakr, and E. H. Sargent, Chem. Rev., 115, 12732 (2015).(3) H. Lee, H. J. Song, M. Shim, and C. Lee, Energy Environ. Sci., 13, 404 (2020).(4) F. M. Winnik and D. Maysinger, Acc. Chem. Res., 46, 672 (2013).(5) M. Bernechea, N. Miller, G. Xercavins, D. So, A. Stavrinadis, and G. Konstantatos, Nat. Photonics, 10, 521 (2016).(6) S. N. Guin and K. Biswas, Chem. Mater., 25, 3225 (2013).