CdS-based Photocatalysts for Energy and Environmental Applications

Abstract

Over the past few years, many approaches have been progressively developed to produce hydrogen from water or organic compounds, and to degrade environmentally hazardous products under solar light irradiation. These processes of fuel production and photodegradation are clean, cost-effective and environment friendly. Herein, we report photocatalytic H2 production from both formic acid (FA) and water. Selective release of hydrogen from FA is deemed feasible to solve issues associated with the release and storage of H2. In this work, we present a new efficient photocatalytic system consisting of CdS nanorods (NRs), nickel (Ni), and cobalt (Co) to liberate H2 from formic acid. The optimised noble metal free catalytic system employs Ni/Co as a redox mediator to relay electrons and holes from CdS-NRs to the Ni and Co respectively, which also deters the oxidation of CdS-NRs. As a result, a high H2 production activity of 32.6 mmolh-1 g -1 from the decomposition of FA was noted. Furthermore, the photocatalytic system exhibits sustained H2 production rate for 12 hours with sequential turnover numbers surpassing 4×103 , 3×10 3 and 2×103 for Co-Ni/CdS-NRs, Ni-CdS-NRs and CoCl2/CdS-NRs, respectively. In addition to the foregoing green production of H2 through water splitting has prompted the search for solar energy harvesting materials; however, this remains an ongoing challenge in the field. We report here a novel and cost-effective photocatalytic system, CoCl2.6H2O stimulated Ni-CdS-NRs@g-C3N4 nanohybrid, with the potential to significantly accelerate the visible light-assisted water-splitting reaction. In this system, the co-catalysts (Co and Ni) are selectively incorporated as redox mediators to impart electrons and holes away from CdS-NRs@g-C3N4. As a result, an excellent H2 production rate of 11376 µmolg-1 h -1 , turnover number (TON; 1945 after 24 hours), and external quantum yield (EQY; 4.4 %) were obtained at 420 nm and under mild reaction conditions. It is anticipated that this unique cocatalysts supported nanohybrid can create a concerted synergistic effect between the cocatalysts and the CdS-NRs@g-C3N4 heterojunction, providing more active sites to the catalytic system for the subsequent water splitting redox reaction. Moreover, the process of a redox shuttle proceeds without adding sacrificial reagents, and relies entirely on the employed cocatalysts to competently separate the oxidation and reduction steps. The experimental results reveal that the selective and optimum use of dual cocatalysts is a promising approach to trigger both the v production of H2 and improve the stability of the photocatalytic system for overall water splitting. Finally, visible light driven photocatalytic potential of CdS was exploited to either degrade carcinogenic organic pollutants (Congo Red (CR) dye) or convert them into useful products i.e reduction of 4-Nitrophenol (4-NP) to 4-Aminophenol (4-AP). To proceed with the CR degradation, first a facile synthesis of CdS-NRs and CdS nanospheres (CdS-NS) was carried out using cadmium(II) dithiocarbamate as a precursor in the presence of two different thermalizing solvents, ethylenediamine (en) and octylamine (OA). The as-synthesized CdS-NRs and CdS-NS were characterized by powder X-ray diffraction (PXRD), transmission electron microscopy (TEM), selected area electron diffraction technique (SAED), fourier transmission infrared spectroscopy (FT-IR), energy dispersive X-Ray spectroscopy (EDS), UV-visible spectroscopy, steady-state and time resolved photoluminesce (PL). TEM analysis has confirmed the formation of nanorods and nanospheres in the presence of en and OA, respectively. The in situ generated different capping ligands, as confirmed by FT-IR analysis, may be responsible for variation in morphology. The PXRD results revealed hexagonal (nanorods) and cubic mixed hexagonal phases (nanospheres). Owing to the band gap in the visible region, both these nanostructures were tested as a solar light drive photocatalyst for the degradation of CR. The results indicated that CdS-NRs exhibit better catalytic efficiency (Kapp= 0.366 min−1 ) toward CR degradation as compared to CdS-NS (Kapp= 0.299 min-1 ). The better photocatalytic activity of nanorods can be attributed to the anisotropically grown structure which infers longer electron hole recombination time, as revealed by time resolved photoluminescence. In the 4-NP reduction study, CdS-NRs of 46.6 nm average length and diameter ≥ 3.30 nm, derived from cadmium(II) dibenzylcarbamodithioate have been used as a visible light-driven photocatalyst for the transformation of environmentally detrimental 4-NP to 4-AP of pharmaceutical significance. As prepared CdS-NRs were characterized by PXRD, TEM, SAED, FT-IR, EDS, steady-state, time resolved PL and UV-visible spectroscopy. PXRD results revealed that CdS-NRs exhibit pure hexagonal character. The steady-state and time-resolved PL have confirmed the low electron-hole recombination rate in CdS-NRs than the bulk CdS. The transformation of 4-NP to 4-AP on CdS-NRs follows pseudo-first order kinetics with a rate constant 0.202 min-1 and turnover frequency (TOF) 6.06 h-1 .