Near-IR Sensitization of ZnO Rod Electrode By Non-Toxic Ag8SnS6 Nanoparticles

Introduction

Near-Infrared (NIR) light-responsive semiconductor nanoparticles have attracted much attention as an efficient light absorbing material in quantum dot solar cells due to their suitable energy gap for solar light spectrum. Furthermore, semiconductor nanoparticles with narrow energy gap have a potential to exceed the theoretical conversion efficiency of conventional solar cells by utilizing their unique physicochemical properties such as the increase in the probability of multiple exciton generation and the hot electron injection. So far binary chalcogenide semiconductor nanoparticles (e.g. CdTe, PbS and PbSe) have intensely investigated for their use in quantum dot solar cells, but the application of these semiconductor nanoparticles appears to be more and more limited due to the high toxicity of Cd or Pb in view of the recent environment regulations. A multinaly chalcogenide of Ag₈SnS₆(ATS) with energy gap of NIR region contains only non-toxic elements seems to be one of the most potential alternatives for conventional toxic nanoparticles. However there was no report about the preparation of colloidal ATS nanoparticles.

Here we report the preparation of ATS nanoparticles in hot organic solvent and investigated their photosensitization abilities of ZnO rod electrode.

Experimental

ATS nanoparticles were prepared by thermal decomposition of organic metal complexes in oleylamine at various reaction temperatures under N₂ atmosphere. ZnO nanorods were deposited on fluorine-doped SnO₂ (FTO) in an aqueous solution containing hexamethylenetetramine and zinc nitrate.^[1] ATS nanoparticles are immobilized on ZnO rod/FTO electrode by dip-coating technique. The ZnO rod electrode was dipped for 10 second into ATS nanoparticles solution and pulled at a rate of 1 cm min^-1. Un-adsorbed particles were removed by immersing into ethanol. These dip-coating and washing cycles were repeated for 30 sets to densely immobilized ATS on ZnO rod electrodes (ATS/ZnO rod). Thus-obtained ATS/ZnO electrode was irradiated in acetonitrile solution containing triethanolamine as a hole scavenger, using 300 W Xe lamp (λ>350 nm). The potential was determined against an Ag/AgCl (sat. KCl) reference electrode and a Pt wire was used as a counter electrode.

Results and discussions

XRD patterns of obtained particles exhibited broad peaks, which were assignable to diffraction of orthorhombic ATS. The absorption spectrum (Fig. 1a) of ATS nanoparticles prepared at 200°C had an absorption onset around 920 nm, being corresponding to the reported energy gap of bulk ATS (1.35 eV). TEM observation revealed that spherical particles were formed at 200°C (Fig. 1b). The particle size could be controlled from 7 to 13 nm only by changing the reaction temperature from 150°C to 350°

To investigate the ability of ATS nanoparticles as a photosensitizer, we immobilized them onto a ZnO rod electrode by dip-coating technique. The immersion of ZnO rod electrode in ATS solution changed their color from white to black. Diffuse reflectance spectra of the thus-obtained films agreed well with absorption spectra of corresponding ATS nanoparticles in solution. Figure 2 shows SEM images of thus-obtained ZnO rod electrodes before and after deposition of ATS nanoparticles. It was found that ATS nanoparticles fully covered the surface of ZnO rod.

The irradiation to ATS/ZnO rod electrodes immersed in the acetonitrile solution containing triethanolamine produced an anodic photocurrent, the intensity being much larger than that of the ZnO rod electrode. These results indicated that ATS/ZnO exhibited the similar behavior of n-type semiconductor photoelectrodes. Action spectra of photocurrent exhibited the onset wavelength at ca. 1000 nm, being in good agreement with the absorption onset wavelength observed in diffuse reflectance spectra of the corresponding electrodes. These results indicated that ATS nanoparticles effectively worked as a photosensitizer driven by irradiation of visible and NIR light (λ<1000 nm).

Reference

[1] Y. J. Lee et al., J. Cryst. Growth., 2007, 304, 80.