Introduction Solar energy is the only renewable and carbon neutral energy source of sufficient scale to replace fossil fuels. Direct conversion of this energy into clean fuels is one of the most important scientific challenges of the 21st century. In photocatalysis, both hydrogen production from solar water splitting and carbon dioxide conversion to organics are obviously attractive, being potentially convenient and clean methods to store solar energy in energy rich molecules which can be employed as virtually inexhaustible fuel sources.
Introduction According to the Intergovernmental Panel on Climate Change(IPCC 2001),1 the Earth’s surface temperature has risen by approximately 0.6 K in the past century, with particularly significant warming trends over the past two decades. The primary contributor to this phenomenon is carbon dioxide (CO 2 ) emissions from fossil fuel combustion.
Introduction Development of photocatalytic CO2 reduction to useful chemicals using solarlight systems, should be one of the best solutions for serious problems, i.e.,shortage of energy, shortage of carbon resources, and the global worming problem.
Photoexcitation in solid followed by deexcitation hv D+D+ D A A-A- Volume Recombination Surface Recombination A C B D VB CB hv Introduction
In 1979, Inoue et al. first reported the photocatalytic reduction of CO 2 in aqueous solution to produce formaldehyde(HCHO), formic acid (HCOOH), methyl alcohol (CH 3 OH), and trace amounts of methane (CH4) using various semiconductors, such as tungsten trioxide (WO 3 ), titanium dioxide (TiO 2 ), zinc oxide (ZnO), cadmium sulfide (CdS), gallium phosphide (GaP),and silicon carbide (SiC). As well known, titanium oxide is most widely used photocatalyst for wastewater treatment and destruction of volatile organic compounds and photoreduction of CO 2.
Photocatalysis employing various metal oxide semiconductors such as Ga2O3,GaP,InTaO4,MgO,ZrO2, BiVO4,and ATaO3 (A =Li, Na, K) have been reported. Metal sulfides such as ZnS, CdS, and CdxZn1+xS can work as CO2 photoreduction catalysts in the presence of a sacrificial electron donor.
Although the efficiency of CO2 convertion in this way,photocatalysis is a potentially economical and environmental CO2 removal process.Three general methods are listed follow for enhancing the efficency. The first is choosing semiconductors with appropriate band-gap energies. The second improvement method involves reductant development. The third and last method is to optimize operating conditions including temperature,pressure,light intensity,and operating wavelength.
It was reported that Cu,Pb,Ni or Bi-doped ZnS photocatalysts show high activities for H2 evolution from aqueous solutions in the presence of a sacrificial reagent such as sulfite ions under visible-light irradiation (λ > 420 nm) even without a platinum cocatalyst. H. Yoneyama / Catalysis Today 39 (1997) 169-175
Ternary sulfides ZnIn2S4, as the only member of the AB2X4 family semiconductor with a layered structure, has attracted farranging interests because of its potential applications in different fields such as charge storage,thermoelectricity,photoconduction and so on. Lei et al. synthesized ZnIn2S4 by a simple hydrothermal method and firstly treated ZnIn2S4 as an efficient visible-light-driven photocatalyst for hydrogen production.Thus, ZnIn2S4 turned to be a good candidate for photocatalytic hydrogen production from water under visible light irradiation.
The band edge potentials of the semiconductors were estimated using the equation related to Mulliken electronegativity. Herein, the electronegativity of an atom is the arithmetic mean of the atomic electron affinity and the first ionization energy. The conduction band (CB) potential at the point of zero charge can be calculated according to an empirical equation: E CB = X- E e +0.5Eg Eg is 2.2~2.3eV reported in some literatures, E e is the energy of free electrons on the hydrogen(4.5eV),E VB is determined by E CB = E VB –Eg.So the conduction band and valence band have been calculated to be E CB =-0.75,E VB =1.45
In2S3, existing in three different structure forms: i.e., α- In2S3(defect cubic structure), β-In2S3 (defect spinel structure) and γ-In2S3 (layered hexagonal structure), is an extensively studied narrow band gap(2.0~2.2) semiconductor because of its defect structure. The photocatalytic activity is closely related to its crystal structure, the distorted electric field in the crystal can enhance the separation of photoinduced electron–hole pairs.As a result, the defect structure of In2S3 may promise a good photocatalytic activity. In fact, β -In2S3 has already been investigated as a visible- light driven photocatalyst recently. The the conduction band and valence band of In2S3 is E CB =-0.8,E VB =1.2
Experiment 1 wt%Cu-doped ZnS, 0.5%Bi-doped ZnS,0.3%Ni-doped ZnS and In 2 S 3,1 wt%Cu-doped In 2 S 3,ZnIn 2 S 4 have been synthesized using hydrothermal method. For In 2 S 3 : In(NO3)3.5H2O+TAA+Deionized water Stirring 15 min Dried at 70~80 ℃ In2S3 Cu/In 2 S 3,ZnIn 2 S 4 and Cu/ZnS,Bi/ZnS,Ni/ZnS was obtained in similar ways. 160 ℃ for 24hr
catalysts µmol/g/h 20 ml of methanol solution and 20 mg of catalyst powders, 6h
Plan Continue to prepare Ni-doped ZnS(0~2%) using hydrothermal method and find the optimal concentration.In addition,their photocatalytic activity and morphology will be compared with those prepared with surfactant such as CTAB.Then observe their stability after 5 runs.(2012.11~2014.1) Prepare other metal doped ZnS(Cu,Bi,Mn) and study dopant effects on the photocatalytic activity of ZnS.The possible photocatalytic mechanism will be presented.(2014.1~2014.3)
Plan Consider using ZnIn2S4 as photocatalyst and control its morphology,then enhance its photocatalytic activity by doping metal.(2014.4~2014.7) Investigate photocatalytic reduction of CO2 on In2S3 and its morphology, improve its photocatalytic activity by doping metals or combining with other catalyst.(2014.8~2014.11) Develop efficient photocatalyst materials with visible light response.(2014.12~ )