Several Catalytic Processes On Graphene In The Presence Of Dopants And Electric Field: A First Principles Study

Since the experimental observation in2004by the English scientists, graphenehave become the focus in many areas. Graphene, a single layer of sp2hybridizedcarbon atoms possesses two dimentional honeycomb-like lattice with thickness ofonly0.35nm. This special configuration endues graphene many excellent properties,such as elcellent electronic transport performance, ultrahigh mechanical strength,large optical transparency, high thermal conductivity and good biocompatibility. Thecharge mobility of graphene exceeds2×105cm2.V-1.s-1at room temperature, ten timeshigher than that of the conmercial silicon wafers. Also graphene possesses the largeststrength of130GPa. These characteristics have further stimulated its applications inmany important fields, such as high-performance composites, molecular electronics,field-emission devices, sensors, hydrogen storage materials and biomedicalapplications.However, most of previous studies focused on the photoelectric performance ofgraphene, and not on its surface properties. The surface properties of graphene, whichstrongly depend on chemical composition and morphology, are highly significant inthe applications. Density Functional Theory calculations and experimentalobservations indicated that the graphene surface is highly hydrophobic. With thedevelopments of smart surface, reversible transition of graphene from hydrophobic tohydrophilic is important in the presence of external stimulation. In the applications ofelectrode materials of supercapacitors and biomaterials supports, it is desirable thatgraphene is hydrophilic and conductive since the former improves the wettingbetween graphene and polar electrolytes or biological molecules, while the latterenhances the transport of free carriers. Therefore, the development of stablehydrophilic and conductive graphene surface is essential for the above applications.Hydrogen has attracted great interest because it is a clean and renewable energysource. Hydrogen is present widely in nuclear reactors, coal mines and semiconductormanufacturing, etc. Because hydrogen is highly flammable and explosive withvolume concentration of more than4%, developing highly sensitive hydrogen sensorsis thus important. The oxidation of carbon monoxide (CO) has attracted great interests due to its importance in applications such as cleaning air and atmosphere purificationfor CO2lasers, as well as removing CO from hydrogen gas fuel to avoid electrodepoisoning in fuel cells. Therefore, developing low cost catalysts for CO oxidation atroom temperature is desirable. Moreover, the large surface to volume ratio alsobenefits for graphene as a support for heterogeneous catalysts. Therefore, we expectthat graphene may be highly active as a catalyst for the H2sensors and CO oxidation.Base on the first principles calculations, we have investigated several catalyticprocesses on graphene in the presence of electric field and dopants.The research contents mainly divided into four parts:(1) Catalytic effect of a perpendicular electric field on the reversible transition ofgraphene with water from hydrophobic to hydrophilic has been investigated byusing first principles calculations. It is found that a negative electric field F canreduce the energy barrier for H2O dissociative adsorption on graphene, under F=0.39V/, the energy barrier becomes negative and the dissociativeadsorption occurs smoothly without any potential barrier, which results inhydrophilic graphene. While a positive electric field has an opposite effect, thepositive electric field of F=0.36V/leads to a negative desorption energybarrier for the desorption of H and OH from graphene, making the graphene behydrophobic again. Therefore, the electric field can act as a switch to reversiblychange the graphene from hydrophobic to hydrophilic in the presence of watervapor.(2) The effect of Al dopant on the dissociative adsorption of a H2O molecule ongraphene is investigated by using first principles calculations. It is found thatdoping Al into graphene can facilitate the dissociative adsorption of H2Omolecules. The dissociative energy barrier is reduced to0.456eV on Al dopedgraphene and the reaction releases energy of0.413eV, which indicates asmooth dissociative adsorption on Al doped graphene at room temperature. Thedissociative adsorption of H2O molecules can convert the Al doped graphenefrom hydrophobic to hydrophilic while obtaining conductive graphene withdoping concentration higher than5.56%. The mechanism of the above is that Alcan facilitate the dissociative adsorption of H2O on graphene through mixinghybridization between its p orbit and1b1orbit of the H2O molecule. This hydrophilic and conductive graphene has potential applications insupercapacitor and biomaterial supports.(3) To facilitate the dissociative adsorption of H2molecules on pristine graphene,mono atom vacancy is considered to be added into graphene, which leads toreduction of the dissociative energy barrier of H2molecule on graphene to0.805eV for the first H2and0.869eV for the second one by using the first principlescalculations. The electronic structure of graphene and conductivity significantlychange before and after H2adsorption. In addition, the related dissociativeadsorption phase diagrams under different temperatures and partial pressuresshow that this dissociative adsorption at room temperature is very sensitive(1035mol/L). Therefore, this defected graphene is promising for ultrasensitiveroom temperature hydrogen sensors.(4) The oxidation of CO molecule on Al embedded graphene has been investigatedby using the first principles calculations. Two possible oxidation mechanismsEley Rideal (ER) and Langmuir Hinshelwood (LH) mechanisms are bothconsidered. In the LH mechanism, O2and CO molecules are firstly co adsorbedon Al embedded graphene, the energy barrier for the rate limiting step (CO+O2â†'OOCO) is only0.32eV, much lower than that of ER mechanism, whichindicates that LH mechanism is more favourable for CO oxidation onAl embedded graphene. Hirshfeld charge analysis shows that embedded Alatom would modify the charge distributions of co adsorbed O2and COmolecules. The charge transfer from O2to CO molecule through the embeddedAl atom plays an important role for the CO oxidation along the LH mechanism.Our result shows that the low cost Al embedded graphene is an efficientcatalyst for CO oxidation at room temperature.