Preparation And Hydrogen Production Performance Of Two-Dimentional Nano MoS₂and Graphene Based Photocatalysts

Photocatalytic hydrogen production technology is the use of the photogenerated charges of semiconductor to split water into hydrogen. This technology is expected to convert solar energy to hydrogen energy to replace the daily exhausting and environmental unfriendly fossil energy. The kernel of this technology is development and application of highly efficient photocatalyst. However, traditional photocatalysts suffer from photogenerated charge recombination and low surface reactive rates, which makes traditional photocatalysts far from the application requirements. Therefore, development of highly efficient photocatalysts with high photogenerated carrier seperation efficiency and surface reaction rate is a focus of research in the field of photocatalysis. Recently, two-dimensional nano MoS2and graphene is emerging as the prior choice to sovle the problems facing traditional photocatalyst becouse their single atomic layer structure and high carrier mobility rate could facilitate the photogenerated carrier seperation and transfer as well as their huge surface and active sites on the edges could speed the surface reaction rate. Based on the above analysis, three kind of photocatalyst based on graphene and (or) MoS2were synthesised. The photogenerated carrier seperation efficiency, surface reaction rate and hydrogen production were studied by the methods of transient absorption spectroscopy (TA), surface potovoltage spectroscopy and electron paramagnetic resonance spectrum as well as the comparison analysis of the as-prepared photocatalyst and typical photocatalyst. In this dissertation, several works have been done as follows:Three gaphene based photocatalyst CdS/Gr, Au/Gr/TiO2and Au/GO have been synthesized by solvent-thermal, hydrothermal and photoreduction methods. The results of TEM and XRD revealed that graphene inhibited the aggragation and promoted the crystallization of CdS. The results of transient absorption spectroscopy and surface potovoltage spectroscopy (SPV) showed that, the photogenerated carrier lifetime of CdS/Gr is87.86μs, which is1.5times as long as that of CdS (57.86μs), indicating the enhancement of photogenerated charge seperation. When the weight ratio of graphene was8wt%, the highest rate44μmol·h-1was obtained which was2.9times as high as that of CdS. Moreover, CdS/Gr showed good stability in the continuous experiment. After deposition of Pt on this photocatalyst, the hydrogen production rate of CdS/Gr/Pt is100%higher than that of CdS/Pt, indicating that graphene is a potential substitute of Pt; Electron paramagnetic resonance spectrum was employed to study the photogenerated electron transfer under incident light irradiation with different wavelenghs. The results showed that there was a vector transfer from Au to TiO2and then to graphene. The results of SPV revealed that, compared with that of Au/TiO2, the photogenerated electrons of Au/Gr/TiO2could be separated more effectively. The results of hydrogen production experiments showed that, the hydrogen production rate of Au/Gr/TiO2and was44mol·h-1in water where no hydrogen was detected over TiO2and TiO2/Gr; In methanol, the hydrogen production rate was712mol·h-1,2times as high as that of Au/TiO2; Graphene oxide (GO) was directly used as a photocatalyst in photocatalytic hydrogen production. The results showed that GO could produce hydrogen from pure water and its photocatalytic hydrogen production rate was0.04μmol·h-1. Au nanoparticles with surface plasmon resonance effect could absorb visible light, and thus improve the photocatalytic hydrogen production rate of GO, and this rate was about3umol·h-1. Moreover, Au/GO showed good stability in the continuous experiment.MOS2based photocatalyst MoS2/CdS has been fabricated through a green hydrothermal method. The research results showed that the highest hydrogen production rate was192μmol·h-1which was17times as high as that of CdS when the synthetic temperature and time as well as the ratio of MoS2were200℃,24h and6.9wt%. The results of transient absorption spectroscopy showed that the photogenerated electron lifetime of asprepared MoS2/CdS was167.03μs, about3times as long as that of CdS, indicating the enhanced photogenerated charge seperation. The hydrogen production rate of MoS2/CdS was higher than that of Pt/CdS, which showed the advantage of MoS2with supporting active sites for hydrogen production and presented the potential of MoS2as a cheap cocatalyst. The MoS2/CdS photocatalyst remained good stability in Na2S and Na2SO3solution in the continuous experiments. Moreover, this photocatalyst showed high activity for hydrogen production in the solution of formic acid, one of the main sources of acid rain, indicating it can realize the pollutant degradation and hydrogen production simutanously. In addition, this preparation method avioded high temperature calcination step (>400℃) and toxic sulfur source.Graphene and MoS2based photocatalyst MoS2/Gr/CdS has been synthesized for H2production. The results showed that MoS2/Gr with95wt%MoS2and5wt%graphene could maximize the photocatalytic hydrogen production rate of CdS. The photocatalytic hydrogen production rate and photogenerated carrier lifetime of MoS2/Gr/CdS were390.7μmol·h-1and183.12μs. Compared with MoS2/CdS and CdS/Gr, the higher photocatalytic hydrogen production rate and longer photogenerated carrier lifetime of MoS2/Gr/CdS indicated that both photogenerated carrier separation and surface reaction rate of CdS could be improved by MoS2/Gr. Moreover, MoS2/Gr/CdS showed good stability in the continuous experiment.In summary, two-dimensional MoS2and graphene could significantly improve the photogenerated charge separation efficiency and surface reactive rates of photocatalyst. The application of them in photocatalytic hydrogen production paves the way to enhance the ultilization efficency of solar light and promotes the development of photocatalytic hydrogen production technology.