Preparation And Characterization Of Graphene/Titanate Nanotubes Photocatalysts

Nowadays, semiconductor-based photocatalysts have attracted intense interesting due to their excellent ability on degrading pollution materials. Graphene, a semi-metal material with zero band gap, has attracted intense attention since it was discovered firstly by Geim et al. in 2004. The novel two-dimensional material possesses excellent electrical and optical properties, large specific surface area and outstanding mechanical property. Recently, graphene modified TiO₂ photocatalysts, such as graphene/P25, graphene/TiO₂ nanoparticals, have attracted much attention due to their outstanding photocatalytic activities under visible-light illumination. However, the P25 and anatase TiO₂ nanopaticals possess small BET areas resulting from the morphologies of them. The small BET area limits the adsorptive capacity as well as photocatalytic performance of these composite photocatalysts. In this subject, the graphene modified titanate nanotubes (TNTs) have been prepared. The characterizations of these photocatalysts were performed. Moreover, the study on photoactivities of the as-prepared photocatalysts under both visible-light and UV-light and photocatalytic mechanisms of them were carried out, too.(1) In this study, a series of graphene/titanate nanotubes (TNTs) photocatalysts using graphene and nanoscale TiO₂ or P25 as original materials were fabricated by hydrothermal method. It is the first time to achieve the combination of graphene and TiO₂-based material with tubular morphology. These as-prepared photocatalysts possess large BET surface areas (almost 10 times larger than that of previously reported graphene/TiO₂ nanoparticles), which evidently enhance their adsorptive capacity and photocatalytic performances (the rate constants of degrading Rhodamine-B are at least 5 times higher than that of previously reported photocatalysts).(2) The influences on the photocatalytic performances from hydrothermal method temperature and amount of graphene were studied. Low temperature during the hydrothermal reaction is good for preparing composite photocatalysts with large BET area. Therefore, hydrothermal reaction with low temperature is suggested to prepare the graphene/TNTs photocatalysts. When the amount of graphene is proper (5 wt%), the photocatalytic performance of the photocatalyst is the best. The reason can be explained as follows: insufficient graphene (1 wt%) could not provide enough photogenerated electrons and electron transportation channels to TNTs, while excessive graphene would shield the light reaching the surface of the TNTs (increased absorbance and scattering of photons by phonons of graphene). Therefore, the results indicate the synergetic effect of graphene and TNTs. Under UV-light illumination, graphene acts as an electrons sink to promote the separation of the photogenerated electron-hole pairs in TNTs, leading the enhanced photocatalytic performances. The visible-light photoactivities of all prepared samples are due to the sensitization of graphene rather than impurity levels of TNTs. The photocatalytic mechanism of the as-prepared photocatalysts is suggested as follows: under visible-light irradiation,Ï€state electrons were excited in the graphene by absorbing visible-light. Owing to theÏ€-d conjugate, theÏ€state electrons were subsequently injected into the conduction band of TNTs. The high carrier mobility of graphene makes the photogenerated electrons transferring to TNTs quickly, which is in favor of high photoactivity and the utilizing efficiency of photogenerated electrons.(3) Based on the results of TEM, XRD, UV-visible diffusion reflection spectroscopy, XPS measurements, the morphologies and elements'chemical states of these as-prepared photocatalysts have been analyzed. The influence of photocatalytic performance from the amount of graphene can be explained by these results. The EPR and photocurrent measurements prove that the visible-light activities of these composite photocatalysts result from the sensitization of graphene.(4) An elementary research on the hydrogen production from water splitting by using the as-prepared graphene/TNTs photocatalysts has been done. The influence on the hydrogen production amount from the content of graphene and hydrothermal temperature of the photocatalysts has been discussed preliminarily.