Green Synthesis Of RGO/In(OH)_yS_z: Zn_x Nanocomposites For Photocatalytic Hydrogen Evolution Under Visible Light Irradiation

The key for photocatalytic hydrogen production using solar energy towards practical applications is to develop novel visible-light-responsive photocatalysts that are highly efficient,stable,inexpensive and environmentally benign.This work will focus on the design and synthesis of novel S-substituted hydroxide photocatalysts with proper band gap level,matching well with redox potential of H2O by cation-and anion-doping.In addition,the in-situ growth of RGO/In（OH）y Sz:Znx nanocomposites has been developed in the presence of L-cystine as the sulfur source,the complexing molecule and the reducing agent by a facile hydrothermal technique,in which graphene is considered to be a promising candidate for Pt as cocatalyst in photocatalysis.The photocatalytic efficiency and stability of RGO/In（OH）_yS_z:Zn_x nanocomposites are expected to be highly improved based on the more efficient separation and transportations of photogenerated e-/h+ pairs.（1）Well-crystallized In（OH）₃ nanosheets with cubic structure are fabricated from the hydrolysis of In³⁺ in aqueous alkaline solution via a hydrothermal route by adopting In（NO₃）3·4.5H2O as in source and NaOH as the pH modifier.It is found from the Uv-Vis diffuse-reflection absorption spectrum that In（OH）₃ exhibits strong absorption in the deep UV region with an absorption edge at about 240 nm,corresponding to a band-gap energy of 5.15 eV.（2）A series of sulfur-substituted In（OH）₃ solid solutions with cubic-phase are prepared in an aqueous solution of sodium hydroxide（pH = 10 N11）through a mild,cost-effective,environmentally-benign,low-temperature biomolecule-assisted hydrothermal technique in the presence of L-cystine as the sulfur source and complexing agent.Compared to that of In（OH）₃,the absorption band gap edge of In（OH）ySz samples displays a gradual red shift,with their band gaps decreasing from 2.61 eV to 2.18 eV with the increase of S/In molar ratio.The narrowing of the band-gap of In（OH）ySz is due to the partial replacement of O₂-by S2-in the crystalline matrix of In（OH）₃,where the energy level of the S3p orbital is higher than that of the O₂p orbital,and the predominant contribution of the valence band of In（OH）ySz by a hybrid orbital between S3p and O₂p.（3）In（OH）_yS_z:Zn_x catalysts are successfully fabricated in aqueous alkaline environment via a hydrothermal approach with the assistance of In（NO₃）3·4.5H2O as In source,Zn（CH₃COO）2·2H2O as the Zn source,L-cystine as the S source and NaOH as the pH modifier.In（OH）_yS_z:Zn_x samples display strong absorption in the visible region with the band gap energy ranging from 1.8 eV～2.3 eV.Under visible light illumination（λ>420 nm）,In（OH）_yS_z:Zn_x photoctalysts are active for photocatalytic water decomposition to H2 in the presence of Na2S-Na2SO₃ aqueous solution as the sacrificial electron donor reagents.（4）RGO/In（OH）_yS_z:Zn_x nanocomposites are successfully grown in situ in alkaline medium with the pH value of 10～11 by the method of biomolecule-assisted hydrothermal route with L-cystine as the sulfur source,the complexing molecule and the reducing agent.Under visible light illumination（λ>420 nm）,the as-synthesized RGO/In（OH）_yS_z:Zn_x photocatalysts display excellent and stable photocatalytic activity for water reduction to H2 in the presence of sacrificial electron donor（Na2S-Na2SO₃）,over ten orders of magnitude higher than those of In（OH）_yS_z:Zn_x catalysts,indicating Graphene is a promising candidate for Pt as cocatalyst in photocatalysis.