Characterizations of Antimony Tri‐Sulfide Chemically Deposited with Silicotungstic Acid

A new chemical deposition method has been developed to prepare photoconducting n-Sb2S3 thin polycrystalline films. The solution composition of the deposition bath was 0.025M potassium antimonyl tartarate, 0.4M triethanolamine, 0.025M thioacetamide, and 5 • 10-7M silicotungstic acid (STA), respectively. The as-deposited and the annealed films were characterized through x-ray diffraction, neutron activation analysis, and the optical absorption investigations. The photoelectrochemical performances of these films were examined. The best photoresponse was observed on the film prepared from a chemical bath containing 5 • 10-7M STA. Photoconducting antimony tri-sulfide thin films are generally prepared by vacuum evaporation (1) or by sintering technique (2) using powdered Sb2S3 compound as the starting material. This creates the difficulty of achieving the stoichiometric films due to wide differences of the vapor pressures of the constituents at the deposition temperature. Recently there has arisen a sustained effort to produce different chalcogenide thin films by simple and inexpensive chemical deposition methods. Other than Cd-chalogenides (3, 4), this method is applied to produce good quality, stoichiometric layered semiconductors such as MoS2 (4, 5) and MoSe2 (6). The method has also been proven to be the least expensive, lowtemperature method, non-polluting, and with an ease for making films of large area of any configuration. Among the different metal sulfides, antimony tri-sulfide finds special applications in vidicon devices (7,8), microwave devices (9), switching devices (10) and various optoelectron i c devices (11). The activation of electrode surfaces by heteropolyacids (HPAs), e.g., H4SiW12040 or H3PW1204o has been well established and largely used in the electrocatalysis of the hydrogen evolution reaction at modified semiconductor or metal electrode surfaces (12, 13). The significant improvement of the hydrogen evolution reaction kinetics has also been observed on electrodeposited nickel with various HPAs whereas the electrode fabricated without HPAs do not show any improvement (14, 15). These interesting results have prompted us to examine the effects of incorporating silicotungstic acid (SiW~204o; STA) in the chemical deposition bath of antimony trisulfide thin films. In this paper we report a new chemical method for the deposition of Sb2S3 thin films and their characterizations using x-ray diffraction, neutron activation analysis, optical absorption, resistivity and Hall effect studies, and the photoelectrochemical (PEC) measurements. The pronounced effect of STA with a nominal concentration of 5 • 10-7M in the chemical deposition bath have been studied along with the effect of annealing. Experimental Procedure Thin films of Sb2S3 were deposited onto SnO2-coated glass substrates from an aqueous solution of 0.025M potassium antimonyl tartarate (BDH, AR), 0.4M triethanolamine (BDH, AR), 0.5M ammonia, and 0.025M thioacetamide (BDH, AR) under a magnetic stirrer. Before use, the SnO2-coated glass slides were cleaned ultrasonically in isopropyl alcohol and dried in pure N= atmosphere. The cleaned substrates were then clamped vertically in the plating solution at 300 K. During the Sb2S3 film formation, the solution color changed progressively from light to deep yellow and then orange-red at which a thick deposition of Sb2S3 films (=1.0 i~m) were formed on the substrates. After 60 h, the slides were removed and washed thoroughly by water and dried in air. In case of STA-incorporated Sb2S3 films, the same procedure was adopted except 5 • 10-7M STA has been used in the chemical bath. The coated glass slides were then introduced into an oven maintained at 300~ for 1 h in N2 atmosphere. The composition and the crystalline status of the as-deposited and the annealed films with and without STA were examined from the powder x-ray diffractograms with a Philips x-ray * Electrochemical Society Active Member. diffractometer. The Sb2S3 powder was collected by scrapping off the SnO2-coated glass substrates and used for this study. The chemical composition of the different films was determined from the neutron activation analysis. Optical absorption spectra of the different Sb2S3 films deposited on SnO2 coated glass substrates were recorded from 950 to 540 nm by a Shimadzu UV/VIS spectrophotometer at 300 K. The optical bandgap of the as-deposited and annealed films with and without STA incorporated films were determined from this study. Resistivity and Hall effect measurements were carried out by four-point probe van der Pauw method. Conducting silver paint was used for electrical contact. The I-V characteristics have been found to be linear within the voltage range of study (20 V) showing the ohmic nature of the contact Sb2S3 films. The photoelectrochemical (PEC) measurements were conducted on a standard three-electrode cell geometry. A largearea graphite electrode was used as a counterelectrode. The working redox electrolyte was 0.01M 12 and 1M KI and degassed thoroughly before and during the experiments. The Sb~S3 film surfaces which are not contacted with the electrolyte were covered by microtop lacquer adhesives. The PEC cell was illuminated with a broad band tungsten halogen lamp (Fibre-Lite). The intensity of illumination was calibrated using Oriel solar simulator radio meter (Model 81020). Results and Discussion The structural features of the films deposited with and without STA are shown in Fig. 1. The as-deposited film in both the cases, with or without 5 • 10-7M STA showed a broad x-ray spectrum (Fig. la) suggesting that they have an amorphous structure. But the annealed samples (300~ for 1 h, N2 atmosphere) showed well-defined crystallographic planes (Fig. l b) and these were identified in the recordings (16). The structural features are consistent with an orthorhombic cell with lattice constants ao = 11.269, bo = 11.299, and Co = 3.824. Figure l c