Preparation Of Functionalized Graphene Nanocomposite Materials And Their Photocatalytic Hydrogen Generation Properties

Nowadays, energy shortage and environmental contamination are serious issues andthus seeking for renewable and clean energy is an urgent task. The production of chemicalfuels by solar energy conversion has been considered as one of the major strategies forsolving the global energy problem. Photocatalytic water splitting for hydrogen productionhas been considered as an ultimate solution for clean, economical, and environmentallyfriendly production of hydrogen by using solar energy. For an efficient photocatalyst,longlived charge carriers, fewer charge trapping centers, proper energy level offsets, andstability against light are highly desirable for improving the photocatalytic reactivity.During the past decade, a variety of strategies, including textural design, doping, noblemetal loading, and surface sensitization of semiconductor photocatalysts, have beenemployed to improve the photocatalytic performance of photocatalysts. The discovery ofgraphene has opened up a new way to improve photocatalytic performance owing to aunique sp2hybrid carbon network. In this dissertation, we prepared various functionalizedgraphene-based nanomaterials for photocatalytic H2generation from water. In thisphotocatalytic systems, graphene can act as an electron acceptor. In addition, grapheneoxide (GO) or reduced graphene oxide (RGO) can also act as a photocatalyst for hydrogenproduction. The main points could be summarized as follows:(1) RGO/GO is a semiconductor with finite bandgap which can be tuned based on thedifferent oxidation level. RGO/GO itself has ability of extraction of hydrogen from waterin the presence of light. However, RGO/GO is not effective for absorbing visible light andhas a low photocatalytic activity for H2generation under visible light irradiation. Based ona similar consideration for dye sensitization of semiconductor, RGO is functionalized by eosin Y (EY) in aqueous media to form the stable aqueous EY functionalized graphene(EY-RGO) suspension through noncovalent modification. EY molecules are built on thesurface of the RGO via hydrogen bonding and-stacking interactions without causingdamage to the electronic properties of the RGO. UV-vis, Raman, fluorescence spectra, andphotoelectrochemical measurements reveal that EY molecules attached on the surface ofthe RGO play a role of an antenna: harvesting irradiation light to give more efficientphotoinduced electron transfers from EY to RGO. The EY-RGO is photocatalytic activefor water reduction to produce hydrogen. The average production rate of H2for thephotocatalyst (wEY/wRGO=1) in a10vol%triethanolamine aqueous solution can reach3.35mmol·g-1·h-1and0.40mmol·g-1·h-1under30h UV-vis and10h visible light irradiation,respectively. The photocatalytic activity of EY-RGO is superior to that of RGO, GO, andEY-GO. Modification EY-RGO with Pt nanoparticles can further improve photocatalyticactivity.(2) A novel composite composed of TiSi2, graphene and RuO2nanoparticles was fabricatedby one-pot deposition method. Graphene can act as an excellent supporting matrix forsemiconductors and as the electron acceptor due to its high specific surface area andsuperior electron mobility. RuO2serves as a cocatalyst in the system. The resultingRuO2/TiSi2/RGO composite was characterized by scanning electron microscopy, X-raydiffraction, Fourier transform infrared spectra, X-ray photoelectron spectroscopy, UV-visdiffuse reflectance spectra, photoelectrical response and electrochemical impedance spectra.The results indicated three components in the composite were effectively contacted, thusfacilitating the photogenerated charges transfer and separation through multiple routes. Thecomposite enhanced the absorption ability in the visible range. The composite shows ahigher rate of H2evolution (97.5mol·h-1·g-1) than that of the composite RuO2/TiSi2(71.9mol·h-1·g-1) and pure TiSi2(56.3mol·h-1·g-1) under visible-light irradiation (420nm)when the contents of graphene and RuO2were both1wt%. Incorporation of graphene andRuO2into TiSi2diversifies the electron transfer process, increases H2evolution active siteand reduces the probability of electron–hole recombination. O2can be evolved in high temperatures for the oxidation of water by holes in the valence band of TiSi2，leading tooverall water splitting.(3) Nitrogen doped graphene was synthesized from graphite oxide and urea by thermalsolid-state reaction. The nitrogen content in the graphene lattice may be tuned by simplychanging the ratio of reagents GO and urea. The sample prepared at a low ratio of GO andurea (wGO/wurea=0.3) has a relative high nitrogen content (~10at.%); while the sampleprepared at a high ratio of GO and urea (wGO/wurea=1or0.5) usually has a low nitrogencontent (~3.2at.%for wGO/wurea=1;~6.5at.%for wGO/wurea=0.5). Moreover, GOannealed with urea shows an evident reduction effect. The oxygen content in our samplesis lower than those of GO annealed in H2or in Ar at the same temperature, indicating GOcan be reduced more easily in the presence of urea. Oxygen-containing functional groupsin GO and amino-groups of urea are suggested to be essential for forming C-N bonds in thegraphene lattice. XPS investigations demonstrate a forming mechanism of N-dopedgraphene prepared by thermal solid-state reaction of GO and urea: a gradual thermaltransformation of nitrogen bonding configurations from amide form nitrogen to pyrrolic,then to pyridinic, and finally to “graphitic” nitrogen in graphene sheets with increasingannealing temperature. The electrical conductivity of the sample can reach ca.40S·cm-1,which is5orders of magnitude higher than that of graphene oxide. The increase inconductivity of the sample annealed at higher temperature may be attributed to bettergraphitization of C=C π-conjugation of the graphene basal plane and the decreased defectsformed within the plane associated with incorporation of nitrogen. This research providedthe foundation for the photocatalytic application of nitrogen doped graphene.(4) N-doped graphene (NGR)/TiO2nanocomposites was prepared using nitrogen dopedgraphene as a supporting matrix for TiO2nanoparticles. TEM image shows TiO2nanoparticles with an average diameter of ca.8nm were fairly well attached to the NGRsheets, while the average diameter of TiO2nanoparticles is ca.20nm for RGO/TiO2composite, indicating stronger coupling between TiO2and N-doped sites on the NGR thanRGO. Nitrogen-containing groups in graphene may serve as favourable nucleation and anchor sites for TiO2nanocrystals. The XPS spectra and Raman spectra of NGR/TiO2nanocomposites indicated the strong electronic interaction exist between TiO2and NGR.Photocurrent responses, electrochemical impedance spectra and cyclic voltammogramsresults show NGR has higher conductivity and higher carrier transfer rate than RGO. TheNGR/TiO2composite shows higher photocatalytic activity for hydrogen generationcompared with pure TiO2and RGO/TiO2. Furthermore, platinum nanoparticles modifiedNGR/TiO2composites (Pt/NGR/TiO2) enhanced the photocatalytic activity and stabilitycompared with Pt/TiO2and Pt/RGO/TiO2.