pubs.acs.org/NanoLett
Anchoring Semiconductor and Metal Nanoparticles on a Two-Dimensional Catalyst Mat. Storing and Shuttling Electrons with Reduced Graphene Oxide Ian V. Lightcap, Thomas H. Kosel, and Prashant V. Kamat* Radiation Laboratory, Departments of Chemistry & Biochemistry, Chemical & Biomolecular Engineering, and Electrical Engineering, University of Notre Dame, Notre Dame, Indiana 46556-0579 ABSTRACT Using reduced graphene oxide (RGO) as a two-dimensional support, we have succeeded in selective anchoring of semiconductor and metal nanoparticles at separate sites. Photogenerated electrons from UV-irradiated TiO2 are transported across RGO to reduce silver ions into silver nanoparticles at a location distinct from the TiO2 anchored site. The ability of RGO to store and shuttle electrons, as visualized via a stepwise electron transfer process, demonstrates its capability to serve as a catalyst nanomat and transfer electrons on demand to adsorbed species. These findings pave the way for the development of next generation catalyst systems and can spur advancements in graphene-based composites for chemical and biological sensors. KEYWORDS Graphene composite, catalysts, photocatalysis, metal nanoparticles, carbon support, electron storage
P
romising results have been achieved using graphenebased systems for nanoelectronics, polymer composites, H2 production and storage, intercalation materials, drug delivery, sensing, catalysis, and photovoltaics.1-14 The realization of a viable, two-dimensional material itself contradicted the previous thought that these materials would be too volatile to exist as stable solids.15,16 Discovery of Dirac-fermions and the quantum Hall effect in graphene added to a frenzy of interest in the material, which has also demonstrated excellent conductivity and mechanical strength.16-19 The utilization of graphene as a twodimensional catalyst mat with the potential to harness graphene’s redox properties remains a promising opportunity (Scheme 1). Exfoliated graphene sheets have theoretical surface areas of ∼2600 m2/g,2 making graphene highly desirable for use as a two-dimensional (2-D) catalyst support. Suspensionbased sheets of functionalized graphene, or graphene oxide (GO), provide a convenient route to keep sheets exfoliated and available for ion or nanoparticle intercalation.20,21 Single nanoparticle systems, i.e., Pd, Pt, and Au have been achieved using GO substrates.22-24 Compared to pure graphene, GO suffers from a significant loss of conductivity. This problem can be mitigated by a partial reduction of its functional groups. To date, few studies have examined using reduced graphene oxide (RGO) as a substrate for catalytic systems.14,25-27 Furthermore, no multicomponent catalyst systems on graphene-based supports have been reported.
The development of a template for semiconductor-graphene and metal nanoparticle-graphene composites is an important milestone toward the development of 2-D nanostructured catalyst and sensor systems. Incorporation of two or more catalyst particles onto an individual graphene or reduced graphene oxide (RGO) sheet at separate sites can provide greater versatility in carrying out selective catalytic or sensing processes. This work focuses on two important aspects of graphene-based systems: (1) the room temperature construction of a 2-D graphene-based nanomat with semiconductor and metal nanoparticles anchored at separate sites and (2) demonstration of the ability of RGO to store and shuttle electrons. The results that illustrate a stepwise transfer of electrons from photoexcited TiO2 nanoparticles into GO, and RGO to Ag+ ions, as well as the anchoring of catalyst particles on a single RGO sheet are discussed here. The GO, as obtained from the oxidation of graphite powder, is readily dispersed in polar solvents. Functional SCHEME 1. Reduced Graphene Oxide as a 2-D Conducting Support to Carry Out Selective Catalysis at Different Sites
* To whom correspondence should be addressed,
[email protected]. Received for review: 10/20/2009 Published on Web: 01/07/2010 © 2010 American Chemical Society
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DOI: 10.1021/nl9035109 | Nano Lett. 2010, 10, 577-583
FIGURE 1. Photographs showing the color changes observed during stepwise transfer of electrons. (1) Storing electrons in TiO2 by irradiating the deaerated ethanol suspension with UV light (λ > 300 nm) for 30 min. (2) Addition of deaerated ethanol suspension of GO until no blue color remains. Gray-colored solution results due to the formation of RGO. (3) Reduction of Ag+ to Ag nanoparticles (red color) by stored electrons in RGO following the addition of deaerated AgNO3 solution.
groups such as epoxides, hydroxides, and carboxylic groups adorn the surface of GO. These groups are responsible for forming single-layer sheets of GO as they disrupt the sp2bonded network and exfoliate the stacked layers of graphene (graphite). The ensuing loss of conductivity due to functionalization can be mitigated through the subsequent reduction of GO sheets.18,28,29 Reduction of GO to RGO has been accomplished using chemical, hydrothermal, and photocatalytic methods.29-32 Suspension-based RGO sheets are readily processable and can be used to anchor semiconductor and metal nanoparticles.25,32 In order to anchor TiO2 and Ag nanoparticles on the same RGO sheet, a solution-based, stepwise electron transfer process was conducted (Figure 1). The first step involved the excitation and storing of electrons in TiO2 nanoparticles using UV irradiation (λ > 300 nm) from an Eimac 300 W xenon arc lamp for 30 min. The colloidal titanium dioxide suspension (6.5 mM, degassed) was prepared by the hydrolysis of titanium(IV) isopropoxide in 200 proof ethanol. In the second step, a GO suspension in ethanol was added incrementally to the blue-colored TiO2 suspension to induce GO reduction and store excess electrons. The GO was obtained using a modified Hummers’ method and was sonicated and degassed prior to use.33 In the third step, stored electrons in RGO sheets reduce silver ions, causing deposition of silver nanoparticles at different sites on the RGO surface. Transfer of Electrons from Excited TiO2 into Graphene Oxide. Under UV illumination (λ < 380 nm), colloidal TiO2 nanoparticles suspended in ethanol undergo charge separation, forming electron-hole pairs. While the vast majority of these charge carriers recombine, a fraction of holes are scavenged by ethanol. The excess electrons become trapped at surface defect Ti4+ sites.34 The stored electrons remain stable for many days if the system is kept free from electronscavenging species such as O2. These trapped electrons absorb broadly in the red regime of the visible spectrum, and thus their concentration can be monitored using UV-vis © 2010 American Chemical Society
spectrophotometry. The number of trapped electrons can be quantified from the absorption spectrum using an extinction coefficient of 760 M-1cm-1 (at 650 nm).35 The maximum absorption peak appearing at ∼700 nm in the absorption spectrum (spectrum a in Figure 2A) represents the trapped electrons resulting from the photoexcitation of TiO2. A decrease in absorption is seen following the addition of GO suspension (spectrum b in Figure 2A). This decrease in absorption occurs as electrons are transferred from TiO2 to GO, where they are used in its reduction. A detailed account of the reduction of GO by photoexcited TiO2 has been previously reported.32 The GO suspension is sequentially added until the broad absorption peak ∼700 nm is fully bleached, indicating a complete transfer of stored TiO2 electrons to GO. The decrease in absorption at 650 nm was used to estimate the number of electrons transferred to GO. Figure 2B shows the moles of electrons (6.7 µmol total) consumed with increasing addition of GO (0.5 mg/mL). (The absorption spectra recorded following incremental addition of GO are shown in Figure S1 in the Supporting Information). At low GO concentrations (
