| In recent years,the treatment of Cr(Ⅵ)wastewater has become the top priority in controlling water pollution.Photocatalytic technology has great application potential in disposal of Cr(Ⅵ)polluted water.The stable and effective visible-light(about 46%of solar energy)photocatalysts are the keys to the industrial application of photocatalytic technology.SnS2 is a CdI2-type of layered semiconductor which has been widely studied in sewage treatment,water splitting,and reduction of CO2,etc.for its cost efficientiveness,nontoxicity,visible-light absorption ability and suitable band gap(2.2 eV).Although SnS2 shows bright development prospect in the photocatalysis field,it also has some weak points,such as the inefficient charge transport and the easy recombination of photogenerated electron-hole pairs.Various methods have been proposed by the researchers to overcome these limitations,such as structure regulation,elemental doping and heterojunction.But,as far as we know,no study has yet been reported on the modification of SnS2 photocatalyst by coupling with conjugated polymers.Conjugated polymers belong to narrow gap semiconductor materials with good visible-light absorption ability,which can result in increased quantity of photogenerated electrons and holes.In addition,conjugated polymers also have good hole transport performance,which can improve charge separating efficiency of the composite photocatalysts,thereby promoting their photocatalytic reactivity.Therefore,in this thesis,conjugated polyvinyl chloride(CPVC),conjugated polyvinyl alcohol(CPVA)and polyaniline(PANI)were employed to modify SnS2 or Zr-SnS2 for obtaining higher activity binary composite photocatalysts.Besides,And PANI and nitrogen-doped reduced graphene(NRG)were employed to co-modify SnS2 for synthesis of even higher efficiency ternary composite photocatalysts(PANI/SnS2/NRG).Taking the reduction of Cr(Ⅵ)as a probe reaction,the photocatalytic properties of the obtained composites were detected.The influences of the different synthesis conditions of the as-prepared composites on their photocatalytic properties were investigated.The kinetics,influencing factors and stability of photocatalytic Cr(Ⅵ)reduction by the composite catalysts were investigated.Moreover,the mechanisms underlying the improved photocatalytic efficiency of prepared composites are elucidated,based on comparison of the optical,photoelectric,and electrochemical properties of the prepared composites and SnS2,as well as the matched energy band structures of SnS2 and conjugated polymers.Besides,the mechanisms of photocatalytic reduction of Cr(Ⅵ)by the prepared composites and SnS2 were investigated.The specific contents are as follows:1.First,SnS2 nanosheets were prepared by hydrothermal method.Then a series of SnS2/CPVC nanocomposites were prepared by liquid-solid phase mixing,solvent removal and pyrolysis,etc.The effects of the dosage,pyrolysis temperature and time of PVC on the photocatalytic activity of SnS2/CPVC nanocomposites were investigated.In addition,based on the comparison of the optical,photoelectric and electrochemical impedance properties of SnS2 and SnS2/CPVC nanocomposites,the mechanism underlying the enhanced photocatalytic activity of SnS2/CPVC nanocomposites was elucidated.The results suggested that:(1)the combination with CPVC turned out to be effective in improvement of the photocatalytic performance of SnS2,and the optimized conditions for synthesizing the most efficient SnS2/CPVC nanocomposite(SnS2/CPVC-2)were that the mass ratio of PVC powder to SnS2 nanoflakes was 1%,and the heating temperature and time used for the dehydrochlorination of PVC in the SnS2/PVC nanocomposite were 150℃ and 2 h,respectively.Moreover,SnS2/CPVC-2 also demonstrated good photocatalytic stability and reusability.(2)SnS2/CPVC nanocomposites demonstrated increased absorption of light in the visible spectrum.(3)SnS2 and CPVC have matched electronic band structures,and they can form a type Ⅱ heterojunction,so the separation of e-and h+in the SnS2/CPVC nanocomposite can be effectively enhanced,and more e-and h+can accelerate the photocatalytic reactions.2.The preparation process of SnS2/CPVC needs the organic solvent(tetrahydrofuran),and the pyrolysis of PVC will also produces toxic HCl gas.Therefore,in this chapter,we replaced PVC with an environmental-friendly and safe material(water-soluble polyvinyl alcohol(PVA))to modify SnS2.First,SnS2 was prepared by hydrothermal method.Then a series of SnS2/CPVA nanocomposites were prepared by liquid-solid phase mixing,solvent removal and pyrolysis,etc.The effects of the dosage,pyrolysis temperature and time of PVA on the photocatalytic activity of SnS2/CPVA nanocomposites were investigated.In addition,the optical,photoelectric and electrochemical impedance properties of SnS2 and SnS2/CPVA nanocomposites were compared,for elucidating the mechanism underlying the enhanced photocatalytic activity of SnS2/CPVA nanocomposites.Besides,the effects of photocatalytic experiment parameters(such as the photocatalyst dosage,initial concentration and pH of Cr(Ⅵ)aqueous solution)on the efficiency of SnS2/CPVA-2 in treatment of Cr(Ⅵ)aqueous solution were also studied.The results suggested that:(1)all the SnS2/CPVA nanocomposites exhibit obviously larger dark adsorption and faster photocatalytic reduction of aqueous Cr(Ⅵ)than SnS2,and the optimized conditions for synthesizing the most efficient SnS2/CPVA nanocomposite(SnS2/CPVA-2)were that the mass ratio of PVA powder to SnS2 nanoflakes was 1%,and the heating temperature and time used for the dehydration of PVA in the SnS2/PVA nanocomposite were 180℃ and 2 h,respectively.(2)The preparation process of SnS2/CPVA composite photocatalyst is more cost-effective and environment friendly,and the SnS2/CPVA nanocomposite exhibits obviously larger dark adsorption and faster photocatalytic reduction of Cr(Ⅵ)than SnS2/CPVC nanocomposite.(3)The enhanced photocatalytic activity of SnS2/CPVA nanocomposite was attributed to its increased absorption of visible-light,more adsorption of Cr(Ⅵ),and higher efficiency in separation and transfer of photoinduced electrons and holes.(4)The photocatalytic experiment parameters(including dosage of photocatalyst,and initial pH and concentration of Cr(Ⅵ)aqueous solution)significantly affected the efficiency of SnS2/CPVA-2 in the treatment of aqueous Cr(Ⅵ).3.The preparation processes of SnS2/CPVC and SnS2/CPVA composite photocatalysts are complex,and the conjugated derivatives(CPVC and CPVA)produced by the pyrolysis of PVC and PVA have low conjugate degree and poor crystallinity which limit to the separation and transfer of photoinduced electrons and holes.Therefore,in this chapter,a simpler one-step liquid-solid phase mixing method was used to prepare SnS2/PANI binary nanocomposites.The effects of mass ratios of PANI and SnS2 on the photocatalytic reduction of Cr(Ⅵ)by SnS2/PANI nanocomposites were investigated.The optical,photoelectric and electrochemical impedance properties of SnS2 and SnS2/PANI nanocomposites were compared,for elucidating the mechanism underlying the enhanced photocatalytic activity of SnS2/PANI nanocomposites.Besides,the effects of photocatalytic experiment parameters on the efficiency of SnS2/PANI-7%in treatment of Cr(Ⅵ)aqueous solution were also studied.Moreover,the mechanisms of photocatalytic reduction of Cr(Ⅵ)by SnS2/PANI composite and SnS2 were investigated.The results suggested that:(1)the combination with PANI turned out to be effective in improvement of the photocatalytic performance of SnS2,and the optimized mass ratio of PANI to SnS2 was 7%.(2)The enhanced photocatalytic activity of SnS2/PANI nanocomposite was attributed to its increased absorption of visible-light,more adsorption of Cr(Ⅵ),and higher efficiency in separation and transfer of photoinduced electrons and holes.(3)SnS2 and PANI have matched electronic band structures,and they can form a type Ⅱ heterojunction,so the separation of e-and h+ in the SnS2/PANI nanocomposite can be effectively enhanced,and more e-and h+can be involved in the photocatalytic reactions.(4)The photocatalytic reduction of Cr(Ⅵ)over SnS2/PANI-7%was dominantly through the reduction by photo generated electrons.(5)The photocatalytic experiment parameters(including dosage of photocatalyst,and initial pH and concentration of Cr(Ⅵ)aqueous solution)significantly affected the efficiency of SnS2/PANI-7%in the treatment of aqueous Cr(Ⅵ).4.For SnS2/conjugated polymer binary composites,only the photogenerated electrons of SnS2 can transfer to conjugated polymers(the photogenerated holes of SnS2 still remain in its valence band),the separation and transfer of photogenerated electrons and holes are not inadequate,and the photocatalytic performance need further enhancement.In this chapter,N-doped reduced graphene oxide(NRG)and PANI were both employed to modify SnS2 for synthesis of a new more effient visible-light-activated PANI/SnS2/NRG ternary composite photocatalyst.The optimum preparation conditions of PANI/SnS2/NRG composite photocatalyst were explored for obtaining the most efficient product.The composition,structure,optical and electrochemical impedance properties of the as-prepared PANI/SnS2/NRG composites were characterized.Moreover,the mechanisms of photocatalytic Cr(Ⅵ)reduction by PANI/SnS2/NRG composite and SnS2 were investigated.Besides,the effect of various experimental conditions on the efficiency of PANI/SnS2/NRG-2%in treatment of aqueous Cr(Ⅵ)were also examined.The results suggested that:(1)The PANI/SnS2/NRG composite exhibited obviously larger dark adsorption and faster photocatalytic reduction of aqueous Cr(Ⅵ)than SnS2,SnS2/PANI and SnS2/NRG binary composites.(2)PANI/SnS2/NRG-2%exhibited the highest photocatalytic efficiency and good photocatalytic stability and reusability.This suggested that the optimal mass ratio of graphene oxide to SnS2/PANI-7%for preparation of PANI/SnS2/NRG ternary composite was 2%.(3)The lower initial pH and concentration of Cr(Ⅵ)solution,and larger dosage of photocatalyst can accelerate the photocatalytic treatment of Cr(Ⅵ)by PANI/SnS2/NRG-2%.(4)Both PANI and NRG have matched electronic band structures with SnS2,so the separation of e-and h+in the PANI/SnS2/NRG nanocomposite photocatalyst can be effectively enhanced.(5)The photocatalytic reduction of Cr(Ⅵ)over PANI/SnS2/NRG-2%was dominantly through the reduction by photogenerated electrons,whereas both photogenerated electron and superoxide anion radical reduction played important roles in SnS2-mediated photocatalytic reduction of Cr(Ⅵ).5.The preparation processes of PANI/SnS2/NRG composite photocatalyst are complex and high cost,and the reduction process of GO requires alkaline and toxic hydrazine hydrate as a reducing agent.In this chapter,elemental doping has been used to adjust the band structure and promote the visible-light absorption of SnS2.First,the Zr4+doped SnS2(Zr-SnS2)nanoflakes were synthesized by a simple one-step hydrothermal method.Then,a series of Zr-SnS2/PANI nanocomposites were prepared via simple liquid-solid phase mixing method.The effects of the Zr doping amount on the photocatalytic activity of Zr-SnS2 nanoflakes,and the effects of the mass ratios of PANI to Zr-SnS2 on the photocatalytic reduction of Cr(Ⅵ)by Zr-SnS2/PANI nanocomposites were investigated.The visible-light absorption,band structure,separation and transfer efficiency of photo-generated charge carriers of the as-prepared Zr-SnS2/PANI nanocomposites were characterized.Moreover,the mechanisms of photocatalytic reduction of Cr(Ⅵ)by Zr-SnS2/PANI composite and SnS2 were investigated.Besides,the influences of photocatalytic testing conditions on the efficiency of Zr-SnS2/PANI-6%in treatment of aqueous Cr(Ⅵ)were also examined.The results suggested that:(1)the Zr-doping in SnS2 significantly enhanced photocatalytic activity for the reduction of Cr(Ⅵ).The 8%Zr-SnS2 sample exhibited the maximum reduction efficiency under visible-light irradiation.(2)Zr-SnS2/PANI-6%demonstrated remarkably enhanced photocatalytic activity in the reduction of aqueous Cr(Ⅵ)under visible-light irradiation.Moreover,Zr-SnS2/PANI-6%also demonstrated certain photocatalytic stability and reusability.(3)The introduction of Zr4+adjusted the band structure and promoted the visible-light absorption of SnS2.(4)Zr-SnS2 and PANI have matched electronic band structures,and they can form a type Ⅱ heterojunction,so the separation of e-and h+ in the Zr-SnS2/PANI nanocomposite can be effectively enhanced.(5)The photocatalytic Cr(Ⅵ)reduction over SnS2 and Zr-SnS2/PANI-6%was mainly by photogenerated electrons and superoxide anion radical,whereas photogenerated electron reduction played a greater role in Zr-SnS2/PANI-6%photocatalytic Cr(Ⅵ)reduction.(6)The photocatalytic experiment parameters(including dosage of photocatalyst,and initial pH and concentration of Cr(Ⅵ)aqueous solution)significantly affected the efficiency of Zr-SnS2/PANI-6%in the treatment of aqueous Cr(Ⅵ). |
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