Band alignment and depletion zone at ZnO/CdS and ZnO/CdSe hetero-structures for temperature independent ammonia vapor sensing

Abstract

An optical-fiber based evanescent ammonia vapor sensor was constructed with surface-passivated growth of zinc oxide (ZnO) nanostructures, which was achieved through a three-step wet chemical process. Initially, the ZnO nanostructures were synthesized using a wet-chemical method and subsequently surface-passivated with chalcogenide material compounds namely cadmium sulphide (CdS) and cadmium selenide (CdSe) nanoparticles individually using a citric acid assisted chemical synthesis technique. Finally, surface-passivated ZnO was deposited on the cladding modified optical-fiber using a dip coating process. X-ray diffraction (XRD), tunneling electron microscopy (TEM), energy dispersive spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS) analyses confirmed the growth of CdS and CdSe nanoparticles on the surface of ZnO nanoparticles. The atomic composition and the full width at half maximum (FWHM) of the oxygen O 1s oxidation state represented in the X-ray photoelectron spectra were lower and narrower for ZC2 nanostructures implying that the available surface oxygen had reacted well and promoted the uniform shell-like growth of CdSe nanoparticles on the ZnO. The significance of the surface-passivated ZnO was realized from UV-Vis diffuse reflectance spectroscopy (DRS) and a photo-luminescence (PL) study and was implemented in a room temperature optical-fiber based evanescent ammonia vapor sensor. The nano-sized CdS particles decorated on the surface of ZnO demonstrate a high vapor sensing behavior. The sensing enhancement was nearly 3 times larger than the core-shell like ZnO/CdSe (ZC2) nanostructures and was attributed to the effective interaction of the incident light and the sensing media, the change in the refractive index of the modified cladding regime, the rate of vapor adsorption and the effective charge-carrier transport between the so-formed hetero-junction interfaces. The ZC2 shows insignificant ammonia vapor adsorption and sensing due to decreased free carrier density produced within the ZnO host lattice and an increased potential barrier width between the ZnO/CdSe hetero-structures.

Photocapacitive CdS/WOx nanostructures for solar energy storage

“…In the case of the CdS sample, it is clear that both the Cd 3d and S 2p peaks are well-fitted by a single pair of doublet components with spin-orbit separation values of 6.74 eV 76,77 and 1.18 eV 78,79 , respectively, and the form of the Cd MNN Auger signal is characteristic of CdS 80,81 . The purity of the CdS surface is belied, however, by the relative areas of the Cd 3d and S 2p peaks: the measured Cd/S atomic ratio of 1.42 is significantly higher than the expected value of unity, indicating the presence of other Cd-containing compounds.…”

Preparation of CdS–TiO2-Based Palladium Heterogeneous Nanocatalyst by Solvothermal Route and Its Catalytic Activity for Reduction of Nitroaromatic Compounds

“…The binding energies of chemical states of elements Cd(3d 5/2 , 3d 3/2 ), S(2p 3/2 , 2p 1/2 ), Ti(2p 3/2 , 2p 1/2 ), O(1s), and Pd(3d 5/2 , 3d 3/2 ) match well with the literature values. 71−75 In ternary nanocatalyst, the presence of all of the elements, Cd, S, Ti, O, and Pd, can be seen before and after catalytic reduction of nitroaromatic compounds.…”

One-Pot Synthesis of Pd-Based Ternary Pd@CdS@TiO2 Nanoclusters via a Solvothermal Route and Their Catalytic Reduction Efficiency toward Toxic Hexavalent Chromium

“…These observations suggest that cadmium is in the Cd 2+ chemical state. 46−49 The binding energy of sulfur is 161 eV for S 2p 3/2 and 163 eV for S 2p 1/2 for TNC-I (Figure 6). For the material obtained from TNC-II , the binding energy for S 2p 3/2 is 163 eV as shown in Figure 7.…”