TiO₂/Graphene Composite Catalyst And Its Application In Photocatalysis

In recent years, researchers found that the loading of graphene insemiconductor/graphene composite catalyst can effectively improve thephotocatalytic performance of semiconductors: Unique two-dimensionalmorphology of graphene can promote the dispersion of semiconductor and improvethe specific surface area of the composite; super-conducting capabilities of graphenecan promote electronic transfer and enhance photo-generated electron-hole pairseparation efficiency on semiconductor. Although many studies have confirmed thepromoting of graphene in semiconductor photocatalytic performance, Graphene is anew type of carbon material and has just discovered in recent years, the researcheson semiconductor/graphene composites also in its infancy. Many problemson compound technology, composite process and reaction mechanism are still leftunsolved. This dissertation focuses on the development of efficient TiO2/graphenecomposite catalyst, exploring the impact of oxidation degree, loading radio andpreparation method of graphene, type and morphology of TiO2, and the process andmethod of combination, on photocatalysis performance of composite catalysis, anddiscussing the mechanism of photocatalysis reaction.In this research, TiO2/reduced graphene oxide (rGO) powder catalyst wassynthesized and its performance was evaluated with photocatalytic hydrogenevolution. It was found that the hydrogen evolution rate of TiO2/rGO increasedsignificantly after injecting small amount of air into the vacuum pumped and UVirradiated sealed reaction cell. The IR, XPS, Raman and ESR spectra analysis indicatedthat the O·-2, which generated from the reaction of photoinduced electrons andthe injected O2can moderately and controllably increase the oxygen groups ongraphene planar of TiO2/rGO at ambient condition and thierfore aeffects theperformance of T-rGO for photocatalytic hydrogen evolution.Since powder catalyst is difficult to recycle and easily be dispersed by gaseous pollutants, in order to expand the scope of application of catalyst, stablenanoTiO2/rGO colloid was synthesized at low temperature (95°C). The colloid couldbe stable for at least6months and adapted the spraying/printing/brushingfilm-forming technology in industry. Ultrathin and transparent nanoTiO2/rGO filmson ITO-PET were also fabricated using this stable colloid. Adding proper quantity ofrGO could effectively increase the photocurrent to4.8times than the photocurrentgenerated by pure nanoTiO2. The as-prepared transparent flexible film can beshaped into triangular calandrias and make full use of the incident UV light, thusincrease the removal efficiency of the pass through gas-phase acetaldehyde. Insummary, we found that the nanoTiO2and rGO will improve the shortcoming of eachother. Negative charged nanoTiO2particles will obviously enhance the dispersion ofrGO in solvent and rGO will effectively improve the poor connection betweenlow-temperature synthesized nanoTiO2particles and the substrate while it wasdeposited on substrate to be a photoelectric film.In order to further expand the contact area between titania and graphene,thereby improving the photocatalytic performance of composite film, MTQDs with2D morphology and~0.4nm thickness were synthesized by treating TNTs in SCW.The XRD, PL, UV-vis and Raman detection of MTQDs showed special characteristicscomparing with TNTs and anatase TiO2. Two differences between SCW and lowtemperature hydrothermal treatment were considered to be the main reasons forthe formation and preservation of MTQDs: the high temperature, high pressure andhigh H+/OH-concentration of SCW dissolved TNTs into monolayer MTQDs; theintercalation property of the “active” water clusters formed from the brokenhydrogen bonding network facilitated part of the MTQDs detaching from TNTs tubewalls and dispersing into the SCW. Owing to the low concentration in SCW, Thedetached MTQDs were prevented from further phase transition while the leftadhered ones then transferred into anatase TiO2particles. This study not onlyprovides some evidences for the study of phase transform in SCW, but also providesa one-step method to synthesize novel monolayer colloidal quantum dots.Coffee-ring effect was suppressed by using colloidal monolayer MTQDs, therefore aneven and continuous transparent ultrathin titania film was prepared simply by dropdrying or spray coating at room temperature. The reasons for the suppression ofcoffee-ring effect were discussed in detail: Special2D morphology of MTQDsenhances the interparticle attraction and the attraction between particles andinterface; the dangling bonds and ligand-free form may further increase the connection firmness between MTQDs. Therefore, a self-assembly film consisted bybounding MTQDs is generated on droplet interface during drying. Because thisbounding structure changes the surface tension of the droplet and resists theoutward flow pointing to the pinned contact line, and Marangoni flow is reduced byusing pure water as the only solvent, the coffee-ring effect is suppressed.In order to improve the quality of graphene in composite film, high qualitygraphene was prepared in supercritical DMF, and a composite photocatalytic filmwas prepared from MTQDs and high-quality graphene. Because MTQDs andgraphene both own two-dimentinal planar morphlogy, the contact manner betweenthese two material is face-to-face. Photocatalytic performance experiments of thecomposite film comfirmed that, due to the lack of internal crystal structure alongz-direction, the photocatalytic performance of MTQDs is less than anatase particles;graphene loading could significantly improve the photocatalytic effeciency of MTQDs,this is not only due to the super conductivity and the “electrons transport bridge”role of graphene, but also depending on the face-to-face contact mode betweenMTQDs and graphene. Using MTQDs/graphene colloid, an ulrtathinMTQDs/graphene composite film could be conveniently prepare at low temperature.This photocatalytic film is flat, transparent, smooth and owing good photocatalyticeffeciciency. This thin film could be used in wide range of applications such as solarcells, anti-fog film, self-cleaning film and anti-bacterial film etc..