Photoluminescence and Photoelectrochemical Properties of ZnTe-AgInTe2 Solid Solution Nanocrystals

Abstract

Introduction Much attention has been paid to near-infrared light (NIR)-absorbing and -emitting semiconductor nanocrystals (NCs) due to their unique physicochemical properties, such as tunable energy structure depending on their size and multiple exciton generation. The most widely studied NIR responsive NCs are based on II-VI and IV-VI semiconductors such as CdTe, PbS, and PbSe. However, their high toxicity is an obstacle to practical application of these NCs, because of potential risks for the human health and the environment. Recently, increasing attention has been paid to I-III-VI2 semiconductor NCs such as CuInS2, AgInSe2, and AgInTe2 [1 -2 ] for the application to in vivo imaging or photoenergy conversion system, because they contain no highly toxic heavy metals and have intense photoluminescence (PL) and absorption in near-infrared region. Furthermore their absorption and emission wavelength can be varied by making solid solutions between I-III-VI2 and II-VI semiconductors. We have successfully prepared nanocrystals of ZnS-AgInS2 [2] and ZnSe-AgInSe2 [3] solid solutions and controlled the optical properties by changing their chemical composition. In this study, we report a solution phase synthesis of ZnTe-AgInTe2 solid solution ((AgIn)xZn2(1 - x)Te2, ZAITe) NCs having a tunable optical properties in near-infrared light wavelength region and their photoelectrochemical properties. Experimental ZAITe NCs were synthesized by a thermal reaction of corresponding metal acetates and a Te precursor in 1-dodecanethiol at 180°C for 180 min. Photoelectrochemical properties of ZAITe NCs were measured in an aqueous solution containing 0.2 mol dm-3 Eu(NO3)3as an electron scavenger. The Ag/AgCl electrode and Pt wire were used as reference and counter electrodes respectively. Results and discussions TEM observations revealed that rod-shaped NCs with width of ca. 4 nm and length of ca. 11 nm were formed regardless of chemical composition of particles, that is, the x value. Thus-obtained NCs exhibited broad peaks in their XRD patterns, each peak position being gradually shifted to higher diffraction angle from the corresponding peak of hexagonal AgInTe2 with a decrease in the x value. Figure 1 shows the absorption and PL spectra of ZAITe NCs. The absorption onset was blue-shifted from 1100 to 810 nm with a decrease in x from 1.0 to 0.25. These results indicated that the obtained NCs were composed of ZAITe solid solution, the composition of which could be controlled by varying the fraction of Zn precursor in preparation, and that their energy gap could be varied depending on their chemical composition. The NCs exhibited near-band-edge PL peak located near the corresponding absorption onset except for x = 0.25, the peak wavelength being blue-shifted with a decrease in x value. The main PL peak observed for NCs with x = 0.25 was largely red-shifted from their absorption onset, suggesting that electronic states in band gap, such as donor and acceptor sites or surface defect sites, were formed at the xvalue smaller than 0.5. Layer-by-layer deposition enabled the accumulation of ZAITe NCs on an ITO electrode. The thus-obtained particle films exhibited a photoresponse similar to p-type semiconductor electrode regardless of x value. Action spectra of the photocurrent were in good agreement with absorption spectra of ZAITe NCs immobilized. The potentials of the valence band and conduction band edges were successfully determined from the onset potentials of cathodic photocurrents and the energy gaps estimated from absorption spectra. ZAITe NCs had a tunable electronic energy structure depending on xvalue. These demonstrate the potential use of ZAITe NCs as photovoltaic materials in near-infrared region. Reference [1] M. Langevin et al., Nanoscale Res. Lett., 2015, 10, 255. [2] T. Kameyama, et al., Nanoscale, 2016, 8, 5435. [3] T. Torimoto, et al., J. Am. Chem. Soc., 2007, 129, 12388. [4] T. Kameyama, T. Torimoto, et al., J. Phys. Chem. C, 2015, 119, 24740. Figure 1