I-III-VI₂ semiconductor nanocrystals (NCs), including CuInS₂, Cu(In,Ga)Se₂, and AgInS₂, have received intense attention as light-absorbing materials in solar energy conversion systems owning to their unique physicochemical properties, such as multiple exciton generation and hot-carrier extraction, originated from quantum size effect. In addition, I-III-VI₂ semiconductors can easily form solid solutions with II-VI or other I-III-VI₂ semiconductors, resulting in wide tunability in their electronic energy structure with chemical composition. So far we have successfully prepared ZnS-AgInS₂^[1] and ZnSe-AgInSe₂^[2] solid solution NCs and controlled their optical properties in visible-light region by changing their chemical compositions. Recently, much attention has been paid to near-infrared light (NIR)-absorbing semiconductor NCs for the application to solar energy conversion systems. We also reported the synthesis of highly NIR-luminescent AgInTe₂ NCs of band gap energy at about 1.1 eV.^[3] In this study, we report solution phase synthesis and photoelectrochemical property of ZnTe-AgInTe₂ solid solution ((AgIn)_xZn_2(1-x)Te₂, ZAITe) NCs having a tunable optical properties in near-infrared light region.

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.

Prepared ZAITe 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 to hexagonal ZnTe with an increase in Zn content in NCs, that is, with a decrease in the x value. This indicated the formation of solid solution NCs between ZnTe and AgInTe₂. TEM measurement revealed that rod-shaped NCs were formed with width of ca. 4 nm and length of ca. 11 nm.

The optical property of ZAITe NCs was controllable in the near-IR region by changing the composition of solid solution. The absorption onset of ZAITe NCs was blue-shifted from 1040 to 770 with a decrease in x from 1.0 to 0.25, indicating that their energy gap was enlarged from 1.2 to 1.6 eV with an increase in the Zn content in ZAITe NCs. These NCs exhibited near-band-edge PL peak located near the corresponding absorption onset, except for x = 0.25. The main PL peak observed for NCs with x= 0.25 was largely red-shifted from their absorption onset and then could be assigned to the emission with donor-acceptor pair recombination or the emission from defect sites.

Photoelectron yield spectroscopy measurement enable to determine the composition-dependent electronic energy structure of ZAITe NCs. The energy level of conduction band edge (E_CB) of ZAITe NCs became slightly higher from -3.4 to -3.1 eV vs. vacuum level with increasing Zn content in the products, while that of valence band edge (E_VB) was nearly constant at ca. -4.7 eV, irrespective of xvalue.

The photoelectrochemical properties of ZAITe NCs immobilized on ITO electrodes were measured in an Eu(NO₃)₃ aqueous solution by the irradiation with Xe lamp light. Cathodic photocurrents were observed regardless of xvalue, indicating that ZAITe NCs exhibited a photoresponse similar to a p-type semiconductor. The photoresponse was observed with near-IR light irradiation and then the photocurrent action spectra were in good agreement with absorption spectra of ZAITe NCs used. These demonstrate the potential use of ZAITe NCs as photovoltaic materials in near-infrared region.

Reference

[1] T. Torimoto, et al., J. Phys. Chem. C 2015, 119, 24740.

[2] T. Kameyama, et al., J. Phys. Chem. C, 2014, 118, 29517.

[3] T. Kameyama, et al., Nanoscale, 2016, 8, 5435.