Novel Zn_xCd_1-xS Photocatalysts With Efficient Visible-Light-Responsive Hydrogen Evolution Properties

Due to the high energy capacity,hydrogen is considered to be an ideal fuel for a new environment friendly energy supply system.However,how to conveniently and cheaply get hydrogen from a sustainable and renewable source is still an critical technical challenge.Because of its abundant,renewable advantages,solar energy will occupy a pivotal position of new energy development and application in the future.Thus,photocatalytic splitting of water into H2 is considered to be one of the most promising strategies to get hydrogen.Sulfide catalysts are a group of promising visible-light-responsive photocatalysts.Among them,CdS has attracted considerable attention due to its suitable bandgap(ca.2.4 eV)for efficient utilization of visible light and the sufficient redox potential(-0.9 V vs.NHE,pH = 7)for H2 evolution.However,there are several issues that still limit the H2 production rate on CdS nanoparticles.For example,the CdS nanoparticles tend to aggregate,forming larger particles,which results in a reduced surface area and a higher recombination rate of photoinduced electron-hole pairs.ZnS,is found to be an effective photocatalyst for H2 evolution even in the absence of noble metal co-catalysts since it has a high conduction band potential.Unfortunately,ZnS is a wide band gap semiconductor(3.4 eV)and can only be active for H2 evolution under UV light irradiation.Tuning its bandgap width and band-edge position by construction of ZnxCd1-xS solid solutions is an efficient method to address these problems because of the little radius difference between Cd2+(0.97 A)and Zn2+(0.74 A).The photocatalytic H2 production performance of ZnxCd1-xS solid solution is superior than that of CdS and ZnS,but but inherently unstable.Many kinds of methods have been used to further improve their photocatalytic activity and stability.In this thesis,we have explored several methods to promot the photocatalytic activity as well the stability.The specific content are as follows:In chapter 1,the concept and principle of photocatalytic hydrogen production from water was introduced firstly.The research background and the synthetic method of ZnxCd1-xS solid solution and the type of common H3 evolution photocatalysts were introduced subsequently.Then,the methods to improve ZnxCd1-xS solid solution photocatalytic activity were briefly discussed,including morphology controling,ion doping,cocatalyst loading,sacrificial agent matching,amine coordination,and defects introducing.Finally,some application examples of ZnxCd1-xS solid solution in gas sensors,fluorescopy,silicon solar cell were expressed.In chapter 2,An organic/inorganic hybrid photocatalyst of Zno.7Cdo.3S and perylenetetracarboxylic diimide(PDI)was prepared with a facile co-precipitation-hydrothermal method.The structure and morphology of Zno.7Cdo.3S/PDI photocatalyst was investigated systematically by X-ray diffraction experiments(XRD),atomic force microscopy(AFM),transmission electron microscopy(TEM),and scan electron microscopy(SEM),etc.The RH of Zn0.7Cd0.3S/PDI nanocomposite is 5.166 mmol g-1 h-1,which is 6.5 times of that Zno.7Cdo.3S solid solution without PDI modification.The corresponding apparent quantum efficiency(QE)of Zn0.7Cd0.3S/PDI nanocomposite is 22.5%under monochromatic light irradiation at 420 nm,which is also much higher than that of Zn0.7Cd0.3S solid solution(3.35%).The efficient electron transfer from Zno.7Cdo.3S to PDI is responsible for this improvement on the photocatalytic activity.Moreover,the modification of Zn0.7Cd0.3S with PDI has also stabilized the catalyst.The results of this research demonstrated that modification of ZnxCd1-xS by organic electron acceptor is a promising strategy to achieve a good catalyst for H2 production from water.In chapter 3,with L-cysteine as sulfur source,we have successively prepared a series of Zn0.5Cd0.5S solid solution.The morphology and microstructure of the resulted nanoparticles can be controlled by the solvents.In mixed solvents of ethanolamine and water,nanorods were formed due to the capping effect of the ethanolamine.If pure water was used as solvent,nanotetrahedrons were formed instead.Nanospheres of Zn0.5Cd0.5S solid solution could be prepared with water/ethylenediamine(4/6)as solvent.Moreover,the nanotetrahedrons and nanospheres materials were found to have trace amount of ZnS impurity.The different crystal structures were revealed by transmission electron microscopy(TEM),scan electron microscopy(SEM),electron dispersive spectrum(EDS),and X-ray diffraction(XRD)experiments for the nanorods,nanotetrahedrons and nanospheres.HRTEM examination revealed the presence of phase junctions between WZ and ZB phases in nanotetrahedrons and twin crystal structure in nanospheres.Due to the presence of high density of homojunctions,both nanotetrahedrons and nanospheres exhibited excellent photocatalytic activity and stability for hydrogen production from water.Especially,the H2 production rate of nanospheres under visible light illumination(λ≥ 420 nm)in 0.75 M Na2S and 1.05 M Na2SO3 aqueous solutions is measured to be 83.50 mmol h-1 g-1,corresponding to a QEs of 47.52%.The results of this research suggested that with L-cysteine as sulfur source,Zn0.5Cd0.5S solid solutions with phase junction and/or twin crystal structure can be easily prepared in different solvents.In chapter 4,we have successfully prepared a novel hybrid catalyst composed of twinned Zn0.5Cd0.5S solid solution and PdP～0.33S～1.67 for H2 evolution from water by a template etching method.The tetragonal phase PdP～0.33S～1.67 was vectorially deposited on to the surface of the twined Zn0.5Cd0.5S solid solution and thus heterojunctons formed between the PdP～0.33S～1.67 and the twinned Zn0.5Cd0.5S solid solution.The examination on the morphology of Zn0.5Cd0.5S/PdP～0.33S～1.67 hybrid catalyst by SEM,TEM and HRTEM techniques revealed the presence of heterojunctions between the deposited PdP～0.33S～1.67 and Zno sCdo.sS phases,as well as homojunctions between the ZB and WZ phases in this hybrid catalyst.The synergetic effects of both homojunctions and heterojunctions have suppressed the charge recombination dramatically and thus the activities of the catalyst are significantly improved.The H2 production rate of Zn0.5Cd0.5S/PdP～0.33S～1.67 under visible light illumination(λ≥ 420 nm)in 0.75 M ascorbic acid(pH = 2.39)aqueous solution is 372.12 μmol h-1 mg-1,which is 67 times larger than that of pristine twinned Zn0.5Cd0.5S solid solution at the same conditions,which is the highest value ever reported so far for sulfide photocatalysts in the presence of acidic sacrificial reagents.In 0.7 M Na2S and 0.5 M Na2SO3(pH = 13.86)solution,the H2 production rate of Zn0.5Cd0.5S/PdP～0.33S～1.67 is 246.04 μ mol h-1 mg-1,which is 5.8 times larger than that of pristine twinned Zn0.5Cd0.5S solid solution.Based on the fact that the twinned Zn0.5Cd0.5S solid solution is the best pristine sulfide catalyst for hydrogen evolution up to date,the further improved catalytic activity of Zn0.5Cd0.5S/PdP～0.33S～1.67 is significant.By combining homojunctions and heterojunctions with carefully designed band alignment in one catalyst,the charge separation can be improved and thus excellent catalytic activities can be achieved.This strategy might be useful in the design of new photocatalyst.In chapter 5,an inorganic/organic nanocomposite,namely Zn0.5Cd0.5S/ZnS(en)0.5,was fabricated by a facile one-pot synthesis.The structure and morphology of Zn0.5Cd0.5S/ZnS(en)0.5 nanocomposite was investigated systematically by X-ray diffraction experiments(XRD),transmission electron microscopy(TEM),scan electron microscopy(SEM),and high resolution transmission electron microscopy(HRTEM),etc.ZnS(en)0.5 nanosheets with a thickness of～8.85 nm have been synthesized through an Ar-assisted high-temperature calcination exfoliation of Zn0.5Cd0.5S/ZnS(en)0.5,and thus C-Zn0.5Cd0.5S/ZnS(en)0.5 was prepared.Higher density of 1D/2D heterojunction were found in C-Zn0.5Cd0.5S/ZnS(en)0.5 in comparison with that in Zn0.5Cd0.5S/ZnS(en)0.5.The C-Zn0.5Cd0.5S/ZnS(en)0.5 nanocomposite exhibited high efficiency for photocatalytic H2 production from 0.025 M N2aH2PO2 and 0.03 M Na2HPO4 sacrificial reagent.Moreover,the disproportionation of NaH2PO2 induced by photooxidation process was happened.The P-containing product of the disproportionation reaction lead to a in situ phosphorus dopping in C-Zn0.5Cd0.5S/ZnS(en)0.5 and formed a nanocomposite C-Zn0.5Cd0.5S/ZnS(en)0.5(P).Under visible light irradiation,Zn0.5Cd0.5S in C-Zn0.5Cd0.5S/ZnS(en)0.5(P)was excited and then free electrons were generated on CB and holes on VB.Driven by the potential difference of CB and VB between Zn0.5Cd0.5S and ZnS(en)0.5 the electrons transfer from the CB of ZnS(en)0.5 into the CB of Zn0.5Cd0.5S,and holes migrate from the VB of Zn0.5Cd0.5S into the defect level of ZnS(en)0.5 inversely.Thus the effective carriers spatial separation is achieved.Finally,the separated electrons in CB of Zn0.5Cd0.5S and holes on the defect level of ZnS(en)0.5 participate in reducing H+ into H2 and oxidizing the sodium hypophosphite,respectively.The H2 production rate of C-Zn0.5Cd0.5S/ZnS(en)0.5(P)under visible light illumination(λ>420 nm)in 0.025 M NaH2PO2 and 0.03 M Na2HPO4 sacrificial reagent is 6.23 p.mol h-1 mg-1,which is 4.1,986.3,and 15.7 times larger than that of Zn0.5Cd0.5S/ZnS(en)0.5,ZnS(en)0.5,and CdS at the same conditions,respectively.The synergetic effects of homojunctions and heterojunctions on the separation of the generated electron-hole pairs are responsible for the significantly promoted catalytic activities of the hybrid catalyst.More importantly,we demonstrated here for the first time an in situ P-doping method for Zn0.5Cd0.5S,which can carried at mild conditions.This method maybe useful in prepapration of other P-doped nanocomposite.