Graphene-Based Nanoarchitectures.

Graphene-Based Nanoarchitectures.Anchoring Semiconductor and Metal Nanoparticles

on a Two-Dimensional Carbon Support

Prashant V.Kamat*

Radiation Laboratory,Departments of Chemistry&Biochemistry and Chemical&Biomolecular Engineering, University of Notre Dame,Notre Dame,Indiana46556

ABSTRACT Graphene based two-dimensional carbon nanostructures serve as a

support to disperse catalyst nanoparticles.Reduced graphene oxide is used as a

support to anchor semiconductor and metal nanoparticles.Such a design strategy

would enable the development of a multifunctional catalyst mat.This Perspective

focuses on the interaction between graphene oxide-semiconductor(TiO2,ZnO)

and graphene oxide-metal(Au,Pt)nanoparticles and discusses potential applica-

tions in catalysis,light energy conversion,and fuel cells.

G raphene has been the major focus of recent research

to exploit an sp2hybrid carbon network in applica-

tions such as nanoelectronics,polymer composites, H2production and storage,intercalation materials,drug delivery,sensing,catalysis,and photovoltaics.1-7The utiliza-tion of graphene as a two-dimensional catalyst mat with the potential to harness graphene's redox properties opens up new opportunities for designing next-generation catalysts. Exfoliated graphene sheets have theoretical surface areas of ～2600m2/g,8making graphene highly desirable for use as a 2-D catalyst support.However,two major hurdles remain in developing large-scale graphene-supported catalyst systems. The first one is to obtain solution-processable individual graphene sheets,and the second one is to use attached catalyst particles with good dispersity.

Chemical oxidation of graphite to graphene oxide results in exfoliated https://www.sodocs.net/doc/2111109558.html,pared to pure graphene,graphene oxide(GO)exhibits a significant loss of conductivity.These sheets need to be reduced to restore the sp2hybrid network and thus reintroduce the conductive property.Furthermore, care should be taken to avoid aggregation effects.9,10The strong van der Waals interactions among these reduced graphene sheets could result in aggregation.Both electrostatic stabilization11and chemical functionalization12,13have pro-ven to be useful in suppressing aggregation of exfoliated graphene oxide sheets.Density gradient ultracentrifugation has been found to be effective in isolating graphene sheets with different thickness.14Incorporation of individual gra-phene oxide sheets into silica sols has enabled researchers to obtain electrically conductive graphene-based composite films on glass.4Incorporation of catalyst particles onto an individual graphene or reduced graphene oxide(RGO)sheet with good distribution can provide greater versatility in carrying out selective catalytic or sensing processes.Our recent research efforts have made use of the two-dimensional morphology of graphene oxide sheets to anchor metal and semiconductor nanoparticles(Scheme1).The strategies presented in this Perspective provide a simple and attr-active approach toward designing next-generation catalyst systems.

Incorporation of catalyst particles

onto an individual graphene sheet

can provide greater versatility in

carrying out selective catalytic or

sensing processes.

Graphene-Semiconductor Nanoparticle Composites.

Metal oxides such as TiO2and ZnO are large-band-gap semiconductors,and they are photocatalytically active under

UV irradiation.These oxides are capable of interacting with

graphene oxide via carboxylic acid functional groups.16,17 Thus,mixing of two suspensions of GO and TiO2(or ZnO) results in the binding of oxide particles to graphene oxide

flakes.These particles remain suspended in suspension,

minimizing any aggregation effects.

Upon UV irradiation of deaerated suspensions of TiO2

colloids,one observes charge separation.In the presence of

ethanol,the holes are scavenged to produce ethoxy radicals, thus leaving the electrons to accumulate within TiO2particles (reaction1).The accumulated electrons serve to interact

with

Received Date:November13,2009

Accepted Date:December11,2009

Published on Web Date:December28,2009

the graphene oxide sheets in order to reduce certain func-tional groups (reaction 2).

TiO 2th νf TiO 2eh te Ts f C 2H 5OH

TiO 2ee Tt?C 2H 4OH

e1T

TiO 2ee Ttgraphene oxide eGO Tf TiO 2treduced graphene oxide eRGO T

e2T

The reduction can be followed by UV irradiation of TiO 2suspension first,followed by incremental addition of GO suspension.Spectrum a in Figure 1A represents trapped electrons in an UV-irradiated TiO 2suspension,with a broad absorption maximum at around 650nm.The absorbance decreases with incremental addition of deaerated graphene oxide suspension,suggesting transfer of trapped electrons.The titration of trapped electrons by GO indicted that approxi-mately 50%of the oxygen sites are able to accept electrons from TiO 2and undergo reduction.16This estimate gives a quantitative value of sites that are reducible through the TiO 2electrons.

TiO 2is a relatively mild reductant with a conduction band at around -0.5V versus NHE at neutral pH.Under UV irradiation,it is capable of transferring electrons to fullerenes and carbon nanotubes.In the present experiments,the oxygen sites that are susceptible to reduction at this redox potential can only undergo transformation,leaving reduction-resistant sites undisturbed.It is not clear at this stage whether the photocatalytic reduction can fully restore the conjugated sp 2network.It is likely that the reduction process will result in the creation of turbostatic graphene,13which is known to exhibit limited conductivity compared to bulk graphite.The advantage of photocatalytic reduction is that it can be

triggered on demand using UV irradiation.Following the photocatalysis procedure,the solvent can be dried,and the graphene -TiO 2can be recovered in the powder form and redispersed in polar solvents.

Scheme 1.Pictorial Illustration of Attaching Metal and Metal Oxide Nanoparticles to Reduced Graphene Oxide (RGO )Sheets

a

a

(A )A dilute solution of NaBH 4is added to graphene oxide solution followed by the controlled addition of HAuCl 4dropwise under stirring.The ratio of RGO/metal ion concentration controls the particle density on the RGO.The photograph shows the gold nanoparticles in the aggregated (left )and well-dispersed (purple )forms as obtained with low and high concentrations of RGO.(B )Attachment of preformed TiO 2nanoparticles to RGO is achieved via a photocatalytic reduction method.A mixture of GO and TiO 2in ethanol was irradiated with UV light.Under band gap excitation,the TiO 2particles undergo electron -hole separation.The holes are scavenged by ethanol,and the electrons are transferred to GO to form RGO.The conversion of brown-to black-colored suspension is shown in the photograph.Adapted from refs 15and 16

.

Figure 1.(A )Absorption spectra of an UV-irradiated 10mM TiO 2suspension in ethanol (spectrum a )and after the addition of graphene oxide suspension (deaerated );(a )0,(b )50,(c )150,and (d )300μg of GO.(a 0corresponds to the TiO 2suspension prior to UV irradiation.)(B )TiO 2-graphene oxide (TiO 2-GO )and TiO 2-reduced graphene (TiO 2-RGO )powders obtained after solvent evaporation.(Adapted from ref 16.)

Effort has also been made to probe the interaction be-tween GO and an emissive semiconductor such as ZnO.The green emission (λmax ≈530nm ),arising from oxygen vacancies,serves as a probe to monitor the interfacial electron-transfer processes.16-18Figure 2shows the emis-sion of a 1mM solution of ZnO nanoparticles with varying amounts of graphene oxide added to the solution.The decrease in fluorescence yield suggests that an additional pathway for the disappearance of the charge carriers domi-nates due to the interactions between the excited ZnO particles and the GO sheets.

The development of semiconduc-tor -graphene or metal nanoparti-cle -graphene composites provides an important milestone to develop energy harvesting and conversion

strategies.

Figure 2B shows the emission decay of a ZnO nanoparticle suspension at varying concentrations of GO.17In the absence of GO,the fast component of the emission decay has a lifetime of 3.0ns,while the slower component has a lifetime of 32.7ns.Both lifetimes decrease with increasing GO concentration.A decrease in the average lifetime of ZnO emission from 30to 14ns is observed with increasing concentration of GO.The short component shows nearly a 20-fold decrease in life-time (2.9-0.67ns )when the GO concentration is increased to 0.25mg/mL.The electron-transfer rate obtained from the emission lifetimes is 1.2?109s -1.This estimate of the electron-transfer rate constant sets the upper limit with which electron transfer can occur between an excited semiconductor particle and graphene oxide.

Figure 3shows the AFM image of the graphene -TiO 2composite.16It is evident that the individual graphene sheets are coupled to TiO 2nanoparticles.These TiO 2nanoparticles are likely to interact through charge-transfer interaction with carboxylic acid functional groups or simple physisorption on the graphene oxide sheets.The cross-sectional analysis indi-cates a height of ～2nm for the graphene sheet and a particle diameter for TiO 2of 2-7nm.It is widely accepted that the graphene sheet thickness increases as a result of the

bulky

Figure 2.(A )Emission spectra of 1mM ZnO suspension at different GO concentrations,(a )0,(b )0.035,(c )0.09,(d )0.14,(e )0.20,and (f )0.24mg/L.(B )Fluorescence decay traces of 1mM ZnO with varying GO concentrations.An incident laser pulse wavelength of 279nm was used for this experiment.Emission was monitored at 530nm.(Adapted from ref 17.

)

Figure 3.(A )AFM image of graphene oxide -TiO 2following UV irradiation for 15min.(B )Depth profile of the line of interest on the graphene sheet.The red marker corresponds to a sheet height of 2.2nm,whereas the green and white markers correspond to nanoparticle sizes ranging from 2.6to 4nm,respectively.Additional cross sections exhibited individual particle heights as great as 7nm.(Adapted from ref 16.)

carbonyl,epoxy ,and carboxyl groups.19“Dry ”values of graphene oxide sheet thicknesses have been calculated to be as low as 0.6nm by XRD;however ,single-sheet GO is typically found to be on the order of 0.9-1.3nm when analyzed by AFM.20,21On the basis of this estimate,we can quantify the thickness of graphene sheets obtained through photocatalytic reduction to mono-or bilayer graphene.

Recent efforts to study the capacitive 22and field emission properties of ZnO and RGO composites demonstrate its future application in nanoelectronics.23

Graphene Oxide -Metal Nanoparticle Composites.Re-duced graphene oxide serves as a support material to stabilize metal nanoparticles.For example,the synthesis of silver and gold nanoparticles stabilized by graphene oxide sheets has been reported by several research groups.7,15,24-29These composites have also been assembled as ultrathin films using a layer-by-layer self-assembly technique.30The procedure to make these composites consists of mixing of GO and a metal salt solution (AgNO 3,HAuCl 4,or H 2PtCl 6)and adding a dilute borohydride solution.7,15The chemical reduction process reduces both GO and metal ions.The RGO acts as a support to stabilize metal nanoparticles.

The formation of metal nanoparticles can be followed from the changes in the absorption as well as electron microscopy.Figure 4shows the role of graphene oxide in producing well-dispersed Au nanoparticles.15A similar meth-od has also been used to anchor Pt nanoparticles on GO sheets with good distribution.7The graphene -metal nanoparticle composites can be cast as films on Pt or carbon fiber electro-

des for electrocatalytic applications.Secondary treatments,such as exposure to hydrazine followed by annealing at higher temperature,have shown to increase the electrocatalytic activity.

Goncalves et al.27have shown that the oxygen functionali-ties at the graphene surface act as reactive sites for the nucleation and growth of gold nanoparticles.They observed that the nucleation and growth of these metal nanoparticles were dependent on the degree of oxygen functional groups at the graphene surface sheets.A similar observation was also noted recently by Jasuja et al.31By controlling the rate of diffusion and catalytic reduction of gold ions on graphene oxide (GO ),dendritic “snowflake-shaped ”gold nanostruc-tures (SFGN )have been constructed.Figure 5shows FESEM micrographs of SFGNs with five and six primary branches and several secondary branches.A Raman spectrum for a RGO sheet exhibits two peaks,corresponding to the D band (1340cm -1)and the G band (1590cm -1).The enhancement seen in the intensity of these bands for SFGN on RGO was attributed to surface enhancement of Raman signals.

Graphene-Based Composites for Energy Conversion.The examples discussed above show simple synthetic strate-gies for anchoring single-nanoparticle systems on reduced graphene oxide.By incorporating two or more catalyst parti-cles on a single graphene or reduced graphene oxide sheet,it should be possible to carry out selective catalytic processes at separate sites.In addition,graphene's ability to store and shuttle electrons will be an important parameter in dictating catalytic activity.Thus,proper design of a catalyst mat can provide greater versatility in carrying out selective catalytic or sensing processes.

Toward the Design of a Catalyst Mat with a T ailored Reactive Site.The illustration in Scheme 2shows one example of incorporating a semiconductor and Pt nanoparticle in a catalyst mat for the water splitting reaction.The semiconduc-tor nanoparticle (e.g.,TiO 2)absorbs the light and induces the oxidation reaction.The RGO,on the other hand,can capture electrons and shuttle them across the two-dimensional car-bon network to the Pt site to facilitate hydrogen reduction.While such strategies are yet to be implemented in water splitting reactions,preliminary studies in our laboratory have shown the ability of RGO to capture and shuttle electrons.

Design of “smart ”catalyst mats should also be useful in photocata-lysis,with the capability of simul-taneous sensing and destruction of

pollutants.

Improving the Performance of Solar Cells.The property of capturing electrons by the RGO sheets can be further exploited in improving the performance of nanostructure-based solar cells.In other words,Scheme 2can be further modified to load light harvesting assemblies such as TiO 2-dye or TiO 2-CdSe onto RGO sheets.These

composite

Figure 4.Photograph (top )and absorption spectra (bottom )of 1mM gold nanoparticles in THF containing different concentra-tions of RGO.(Adapted from ref 15.)

sheets when assembled on the electrode can deliver the photoinduced charges to the collecting surface.Such a con-cept has already been demonstrated using carbon nanotubes as conducting scaffolds in dye-sensitized and CdSe-sensitized solar cells.33,34Electrochemical reduction of GO films cast on an electrode surface has also been successful.35A recent study demonstrates the use of GO as an electron acceptor in organic photovoltaic cells (Figure 6).The quenching of the polythiophene (P3HT )emission by RGO established the charge-transfer interaction between the two.The device containing 10%of RGO exhibited the best

performance.

Figure 5.Formation mechanism of snowflake-shaped gold nanostructures on graphene oxide (GO ).(a )Interfacing the -COOH and -OH groups on GO sheets with a freshly prepared solution of gold nuclei,formed during hydroxylamine-assisted reduction of gold salt,results in nuclei attachment and seed-mediated formation of snowflake-shaped gold nanostructures (SFGNs )on the GO surface.(b )FESEM of SFGNs templated on GO lying on a silica surface.(Adapted from ref 31.)Scheme 2.Illustration of Selective Oxidation and Reduction Pro-cesses on a Single Graphene Sheet at Two Different Catalytic Sites

a

a

The example shows the utilization of a catalyst mat in the water splitting reaction using a semiconductor

photocatalyst.

Figure 6.(a )The schematic chemical structure of graphene oxide and P3HT .(b )Schematic structure of the device with the P3HT/SPFgraphene thin film as the active layer.(Reprinted from ref 32with Copyright permission from Wiley Interscience.)

Efforts are currently underway in our laboratory to probe the electron capture properties of RGO films and to use them as conducting scaffolds in photoelectrochemical solar cells.As an Electrocatalyst in the Operation of a Fuel Cell.Pt particles dispersed on carbon nanoparticles can be employed as fuel cell electrocatalysts.The carbon support plays an important role in improving the charge-transfer efficiency.Proton-exchange membrane assemblies were constructed using carbon (Toray )paper electrode with Pt -RGO nano-particles as the cathode and Pt dispersed on carbon black (E-Tek CB-Pt )as the anode.7The performances of these assemblies in a hydrogen fuel cell are compared in Figure 7.An unsupported Pt cathode was used as a reference.Both catalyst assemblies produced similar open-circuit voltage,but the voltage drop varied as the current was drawn from the cell.The partially reduced Pt -RGO-based fuel cell delivered a maximum power of 161mW/cm 2compared to 96mW/cm 2for an unsupported Pt-based fuel cell.This work is presented as an example to demonstrate the usefulness of graphene as a support to anchor electrocatalyst particles.The complexity of incorporating such composites in devices demands strategies to improve the conductive properties of the graphene-based supports.In addition,the ability of graphene to uptake gases

such as H 2may further enhance their capability to concen-trate reactants in a fuel cell reaction.36

Another potential application of graphene is in the energy storage area.Rechargeable lithium batteries that employ exfoliated graphene oxide as anode materials have been investigated.37,38However ,poor cycling performance has limited its use for long-term application.More careful efforts are needed to develop metal oxide -graphene composites for their use in storage batteries.For example,SnO 2/graphene-based anodes have shown a reversible capacity of 810mAh/g with improved cycling performance.39

The recent surge in research activities related to graphene-based systems has opened up several new pathways to developing catalysts and sensors with tailored function.In particular ,chemical functionalization 41,42is likely to play a major role in tuning the properties of graphene-based sys-tems.For example,the electronic structure and transport properties of EG from near-metallic to semiconducting has been achieved by covalent bonding of nitrophenyl groups to epitaxial graphene (Scheme 3).40,43Similarly,self-assembled monolayers of the molecular semiconductor perylene-3,4,9,10-tetracarboxylic acid derivatives formed on epitaxial graphene grown on the SiC (0001)surface provide ways to achieve molecular ordering that is unperturbed by defects in the epitaxial graphene.44,45Such functionalization can further direct the attachment of site-specific binding of metal and semiconductor nanoparticles.Strategies to design multifunc-tional graphene-based composites will play a major role in the development of catalyst mats.

AUTHOR INFORMATION Corresponding Author:

*E-mail:pkamat@https://www.sodocs.net/doc/2111109558.html,.

Biography

Prashant V.Kamat is a Professor of Chemistry and Biochemistry,Senior Scientist at the Radiation Laboratory,and Concurrent Pro-fessor of the Department of Chemical and Biomolecular Engineer-ing,University of Notre Dame.His major research interests are in the areas of developing hybrid assemblies with semiconductor nanocrystals,metal nanoparticles,and carbon nanostructures for next-generation solar cells and the elucidation of photoinduced charge-transfer processes in such light harvesting assemblies.See https://www.sodocs.net/doc/2111109558.html,/～pkamat for further details.

ACKNOWLEDGMENT The research described herein was sup-ported by the Office of Basic Energy Science of the Department

of

Figure 7.T op:Schematic diagram illustrating the electrocatalytic reduction of O 2on PT -RGO and transmission electron micro-graph of Pt -RGO.Bottom:Galvanostatic fuel cell polarization (I -V )curves (a,b,c )and power characteristics (a 0,b 0,c 0)of a fuel cell at 60°C and 1atm of back pressure.The cathode was composed of (a,a 0)Pt,(b,b 0)1:1Pt -RGO,and (c,c 0)1:1Pt -RGO (hydrazine,300°C treated ).The electrocatalyst concentration was maintained at 0.2mg/cm 2Pt.The anode for all experiments consisted of 0.5mg/cm 2Pt of E-T ek CB-Pt.The anode and cathode catalyst films were heat pressed on a Nafion 115membrane.(Adapted from ref 7.)

Scheme 3.Schematic Illustration of the Spontaneous Grafting of Aryl Groups to Epitaxial Graphene via Reduction of 4-Nitro-phenyldiazonium (NPD )T etrafluoroborate (adapted from ref 40

)

the Energy.I would like to acknowledge R.Muszynski,G.Williams, B.Farrow,and I.Lightcap for their contributions to our graphene-based research.This is contribution number NDRL-4836from the Notre Dame Radiation Laboratory.

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