Imperfections that disrupt the sp(2) conjugation of graphene can alter its electrical, chemical, and mechanical properties. Here we report on the examination of monolayer chemical vapor deposited graphene imperfections using scanning electrochemical microscopy in the feedback mode. It was found that the sites with a large concentration of defects are approximately 1 order of magnitude more reactive, compared to more pristine graphene surfaces, toward electrochemical reactions. Furthermore, we successfully passivated the activity of graphene defects by carefully controlling the electropolymerization conditions of o-phenylenediamine. With further electropolymerization, a thin film of the polymer was formed, and it was found to be insulating in nature toward heterogeneous electron transfer processes. The use of spatially resolved scanning electrochemical microscopy for detecting the presence and the "healing" of defects on graphene provides a strategy for in situ characterization and control of this attractive surface, enabling optimization of its properties for application in electronics, sensing, and electrocatalysis.

We present a catalog of electron transfer mediators for investigating the heterogeneous electron transfer kinetics of large-area, single-layer graphene electrodes. Scanning electrochemical microscopy (SECM) was used to probe the apparent standard electron transfer rate constant of mediators in aqueous solutions and in acetonitrile and dimethylformamide, allowing for studies of graphene electroactivity at different potentials and in both aqueous and nonaqueous media. In aqueous solution, iron(III) ethylenediaminetetraacetic acid, hexacyanoruthenate(II), hexacyanoferrate(II), hexacyanoferrate(III), octacyanomalybdate(IV), cobalt(III) sepulchrate, and hydroxymethylferrocene exhibited quasi-reversible electron transfer behavior. The electron transfer kinetics of hexaammineruthenium(III), methyl viologen, and tris(2,2'-bipyridyl)ruthenium(II) were found to be reversible in these studies. The electron transfer rate constant of hydroxymethylferrocene and ferrocene, in organic media, was similar to that for hydroxymethylferrocene in water, which, although fast, shows clear kinetic complications that we believe are inherent to graphene. A series of viologens, known to be reversible at metal electrodes, exhibited quasi-reversible electron transfer. For [Co(dapa)(2)](2+), where dapa is 2,6-bis[1-(phenylimino)ethyl]pyridine, in dimethylformamide, the oxidation state of the redox pair investigated affected the observed kinetics. Under similar experimental conditions, the Co(I/II) couple exhibited nearly reversible behavior whereas Co(II/III) had finite kinetics. This behavior was ascribed to the large difference in self-exchange rates for these two processes. To demonstrate the utility of using these mediators for examining graphene electrodes, the kinetics of two mediators with quasi-reversible electron transfer behavior, iron ethylenediaminetetraacetic acid and hexacyanoruthenate, were measured in the presence of a redox-active species [Os(bpy)(2)(dipy)Cl]PF(6), where bpy is 2,2'-bipyridine and dipy is 4,4'-trimethylenedipyridine, adsorbed onto the graphene surface. The kinetics of both mediators were enhanced in the presence of one-hundredth of a monolayer of the osmium complex, showing that even small amounts of impurities on the graphene surface are capable of enhancing the observed kinetics.

The surface diffusion of a cobalt bis-terpyridine, Co(tpy)(2)-containing tripodal compound (1·2PF(6)), designed to noncovalently adsorb to graphene through three pyrene moieties, has been studied by scanning electrochemical microscopy (SECM) on single-layer graphene (SLG). An initial boundary approach was designed in which picoliter droplets (radii ~15-50 μm) of the tripodal compound were deposited on an SLG electrode, yielding microspots in which a monolayer of the tripodal molecules is initially confined. The time evolution of the electrochemical activity of these spots was detected at the aqueous phosphate buffer/SLG interface by SECM, in both generation/collection (G/C) and feedback modes. The tripodal compound microspots exhibit differential reactivity with respect to the underlying graphene substrate in two different electrochemical processes. For example, during the oxygen reduction reaction, adsorbed 1·2PF(6) tripodal molecules generate more H(2)O(2) than the bare graphene surface. This product was detected with spatial and temporal resolution using the SECM tip. The tripodal compound also mediates the oxidation of a Fe(II) species, generated at the SECM tip, under conditions in which SLG shows slow interfacial charge transfer. In each case, SECM images, obtained at increasing times, show a gradual decrease in the electrochemical response due to radial diffusion of the adsorbed molecules outward from the microspots onto the unfunctionalized areas of the SLG surface. This response was fit to a simple surface diffusion model, which yielded excellent agreement between the two experiments for the effective diffusion coefficients: D(eff) = 1.6 (±0.9) × 10(-9) cm(2)/s and D(eff) = 1.5 (±0.6) × 10(-9) cm(2)/s for G/C and feedback modes, respectively. Control experiments ruled out alternative explanations for the observed behavior, such as deactivation of the Co(II/III) species or of the SLG, and verified that the molecules do not diffuse when confined to obstructed areas. The noncovalent nature of the surface functionalization, together with the surface reactivity and mobility of these molecules, provides a means to couple the superior electronic properties of graphene to compounds with enhanced electrochemical performance, a key step toward developing dynamic electrode surfaces for sensing, electrocatalysis, and electronic applications.

A major goal of energy research is to use visible light to cleave water directly, without an applied voltage, into hydrogen and oxygen. Since the initial reports of the ultraviolet (UV) activity of TiO 2 and SrTiO 3 in the 1970's, researchers have pursued a fundamental understanding of the mechanistic and molecular-level phenomena involved in photo-catalysis. [1][2][3][4][5][6][7] Although it requires UV light, after four decades SrTiO 3 is still the gold standard for splitting water. It is chemically stable and catalyzes both the hydrogen and the oxygen reactions without applied bias. While ultrahigh vacuum (UHV) surface science techniques have provided useful insights, 8 we still know relatively little about the structure of electrodes in contact with electrolytes under operating conditions. Here, we report the surface structure evolution of a SrTiO 3 electrode during water splitting, before and after training with a positive bias. Operando high-energy X-ray reflectivity measurements demonstrate that training the electrode irreversibly reorders the surface. Scanning electrochemical microscopy (SECM) at open circuit correlates this training with a tripling of the activity toward photoinduced water splitting. A novel first-principles joint density-functional theory (JDFT) simulation constrained to the X-ray data via a generalized penalty function identifies an anatase-like structure for the more active, trained surface.Wide bandgap, n-doped, metal oxide semiconductors such as SrTiO 3 , TiO 2 and WO 3 absorb UV light to form photo-generated (electrons and holes) charge carriers, capable of driving redox reactions at the interface with an electrolyte. 9-11 SrTiO 3 is a prototypic perovskite structure metal oxide ( Figure 1a). The perovskites exhibit a vast range of attractive physico-chemical properties including promising energy conversion activity. 12 In particular, SrTiO 3 has very attractive photocatalytic properties. It is highly stable in base, displays high quantum efficiency for the electro-oxidation of water under UV illumination, and performs the light-driven water splitting reaction (i.e., photo-generation of both O 2 and H 2 from H 2 O) under an applied bias, 2,13 at open circuit aided by an auxiliary metal electrode, 2,13 and on free-standing crystals. 4,5,[13][14][15][16] The fundamental mechanisms underlying surface photochemical reactions, however, remain unclear. While the bulk d-band structure can correlate with activity, 12 surface defects and surface structure are critically important and it is typically difficult to decouple the bulk and surface contributions to observed changes in reactivity. [17][18][19] Here, we illustrate the critical role that surface structure plays by demonstrating under operando conditions that the electrochemical activation (training) of n-doped SrTiO 3 (001) in basic media induces an irreversible surface reordering that enhances (by 260%) its activity for photocatalytic water splitting.The operando structural characterization of SrTiO 3 before and after training wa...

Silicon photoanodes patterned with thin-film Ni catalyst islands exhibited stable sunlight-driven O2 evolution for over 240 h of continuous operation in 1.0 M KOH.