Construction Of Mn_0.5Cd_0.5S Solid Solution Composite Materials And Investigation On Photocatalytic H₂ Production Properties

H₂ is an important green energy,and how to efficiently obtain"green hydrogen"is a research hotspot in the current energy field.Photocatalytic water splitting for H₂ production can directly convert low-energy-density solar energy into high-energy-density H₂ energy,which is one of the ideal ways to obtain cheap"green hydrogen".However,due to low catalyst activity,the technology of photocatalytic splitting water for H₂ production is difficult to be applied on a large scale.At present,the development of high-efficiency novel catalysts is still the core of research in this field.In view of this,this thesis takes the ternary sulfide Mn_0.5Cd_0.5S solid solution as the main research object.Aiming at its inherent defects,loading NiCo₂S₄ cocatalysts,construction of heterojunctions with Zn_0.76Co_0.24S and Ni ion doping are conducted respectively to enhance the H₂ production rate of Mn_0.5Cd_0.5S.The structure-activity relationship and carrier transport mechanism of each system were detailly studied.The details are as follows:（1）The twinned Mn_0.5Cd_0.5S（Twinned,T-MCS）solid solution nanoparticles formed by alternating hexagonal wurtzite（WZ-MCS）and cubic sphalerite（ZB-MCS）were prepared by an alkaline hydrothermal method.Under the irradiation of visible light（λ>420 nm）,with0.35 M Na₂S/0.25 M Na₂SO₃ as sacrificial agents,the H₂ production rate of T-MCS is 61.4mmol·g^–1·h^–1,which are 8.2 and 1.9 times that of WZ-MCS(7.5 mmol·g^–1·h^–1)and ZB-MCS(32.6 mmol·g^–1·h^–1),respectively.The analysis shows the CB and VB of WZ-MCS are slightly lower than those of ZB-MCS.The bulk type Ⅱ homojunction formed between ZB-MCS and WZ-MCS can enhance the bulk phase separation of photogenerated charge of T-MCS.In addition,NiCo₂S₄ was supported on the surface of T-MCS as a H₂ production co-catalyst to form a Schottky junction.When the loading of NiCo₂S₄ is 2%,the H₂ evolution rate of the composite can reach 127.3 mmol·g^–1·h^–1,and the apparent quantum efficiency under（400±7.5）nm light irradiation is 23.4%.The results show the Schottky junction and the bulk type Ⅱ homojunction synergistically promote the separation of charge carriers and enhance the catalytic activity.（2）Zn_0.76Co_0.24S nanoparticles were loaded on T-MCS by in situ hydrothermal method.The experimental results suggest under visible light（λ>420 nm）irradiation,using 0.35 M Na₂S/0.25 M Na₂SO₃ as sacrificial agent,when the loading of Zn_0.76Co_0.24S is 3%,the H₂evolution rate of the composites can reach 132.9 mmol·g^–1·h^–1,which are 1.9 and 332.2 times that of T-MCS(68.1 mmol·g^–1·h^–1)and Zn_0.76Co_0.24S(0.4 mmol·g^–1·h^–1),respectively.The analysis results show Zn_0.76Co_0.24S and T-MCS possess matching energy band structures,and the carrier migration path in the heterojunction formed by the two follows the S-scheme mechanism.Under the combined action of energy band bending,coulomb gravity and built-in field,the electron of T-MCS will recombine with the holes of Zn_0.76Co_0.24S,and the electron with strong reducing ability in the CB of Zn_0.76Co_0.24S is retained.Under the synergistic effect of bulk type Ⅱ homojunction and surface S-scheme heterojunction,not only the efficient separation of photogenerated carriers is achieved,but also the charges with strong redox ability are retained,which promotes the surface redox reaction.（3）Ni-doped Mn_0.5Cd_0.5S nanoparticles(Ni-Mn_0.5Cd_0.5S,Ni-MCS)were prepared by hydrothermal method.Using 0.35 M Na₂S/0.25 M Na₂SO₃ as sacrificial agent,the Ni-Mn_0.5Cd_0.5S nanoparticles were obtained in visible light（λ>420 nm）irradiation,when the Ni doping content is 3%,the H₂ production rate of the sample is 108.3 mmol·g^–1·h^–1,which is about the pure Mn_0.5Cd_0.5S(7.5 mmol·g^–1·h^–1)14.4 times.The analysis found that Ni doping introduced a new impurity energy level in the forbidden band of Mn_0.5Cd_0.5S,which significantly improved its photoresponse ability.Compared with Mn_0.5Cd_0.5S（520 nm）,the light absorption edge of 3%Ni-MCS reached 605 nm,which is significantly red-shifted and has stronger absorption in the visible region.In addition,Ni doping also effectively suppressed the random recombination of photogenerated charges.The enhanced light absorption capacity and carrier separation efficiency together lead to a substantial increase in their catalytic activity.