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† Electronic supplementary information (ESI) available: Experimental details, characterization of rGO/TiO 2 composites (UV-visible absorbance, FTIR, Raman, and XPS), the photocatalytic generation and decomposition of H 2 O 2 depending on rGO contents and noble metals, the slurry-and electrode-type photocurrent production, and characterization of rGO/TiO 2 /CoPi and TiO 2 /CoPi (XPS, TEM, and EELS). See

Artificial photosynthesis (AP) promises to replace society's dependence on fossil energy resources via conversion of sunlight into sustainable, carbon-neutral fuels. However, large-scale AP implementation remains impeded by a dearth of cheap, efficient catalysts for the oxygen evolution reaction (OER). Cobalt oxide materials can catalyze the OER and are potentially scalable due to the abundance of cobalt in the Earth's crust; unfortunately, the activity of these materials is insufficient for practical AP implementation. Attempts to improve cobalt oxide's activity have been stymied by limited mechanistic understanding that stems from the inherent difficulty of characterizing structure and reactivity at surfaces of heterogeneous materials. While previous studies on cobalt oxide revealed the intermediacy of the unusual Co(IV) oxidation state, much remains unknown, including whether bridging or terminal oxo ligands form O2 and what the relevant oxidation states are. We have addressed these issues by employing a homogeneous model for cobalt oxide, the [Co(III)4] cubane (Co4O4(OAc)4py4, py = pyridine, OAc = acetate), that can be oxidized to the [Co(IV)Co(III)3] state. Upon addition of 1 equiv of sodium hydroxide, the [Co(III)4] cubane is regenerated with stoichiometric formation of O2. Oxygen isotopic labeling experiments demonstrate that the cubane core remains intact during this stoichiometric OER, implying that terminal oxo ligands are responsible for forming O2. The OER is also examined with stopped-flow UV-visible spectroscopy, and its kinetic behavior is modeled, to surprisingly reveal that O2 formation requires disproportionation of the [Co(IV)Co(III)3] state to generate an even higher oxidation state, formally [Co(V)Co(III)3] or [Co(IV)2Co(III)2]. The mechanistic understanding provided by these results should accelerate the development of OER catalysts leading to increasingly efficient AP systems.

In the electrochemical CO2 reduction reaction (CO2RR), Cu has been spotlighted as the only electro-catalyst that can produce multi-carbon molecules, but the mechanism of the selective C2+ production reaction remains...

As the development of oxygen evolution co-catalysts (OECs) is being actively undertaken, the tailored integration of those OECs with photoanodes is expected to be a plausible avenue for achieving highly efficient solar-assisted water splitting. Here, we demonstrate that a black phosphorene (BP) layer, inserted between the OEC and BiVO 4 can improve the photoelectrochemical performance of pre-optimized OEC/BiVO 4 (OEC: NiOOH, MnO x, and CoOOH) systems by 1.2∼1.6-fold, while the OEC overlayer, in turn, can suppress BP self-oxidation to achieve a high durability. A photocurrent density of 4.48 mA·cm −2 at 1.23 V vs reversible hydrogen electrode (RHE) is achieved by the NiOOH/BP/BiVO 4 photoanode. It is found that the intrinsic p -type BP can boost hole extraction from BiVO 4 and prolong holes trapping lifetime on BiVO 4 surface. This work sheds light on the design of BP-based devices for application in solar to fuel conversion, and also suggests a promising nexus between semiconductor and electrocatalyst.

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The generation of oxidants on illuminated photocatalysts and their participation in subsequent reactions are the main basis of the widely investigated photocatalytic processes for environmental remediation and selective oxidation. Here, the generation and the subsequent diffusion of (·)OH from the illuminated TiO2 surface to the solution bulk were directly observed using a single-molecule detection method and this molecular phenomenon could explain the different macroscopic behavior of anatase and rutile in photocatalysis. The mobile (·)OH is generated on anatase but not on rutile. Therefore, the photocatalytic oxidation on rutile is limited to adsorbed substrates whereas that on anatase is more facile and versatile owing to the presence of mobile (·)OH. The ability of anatase to generate mobile (·)OH is proposed as a previously unrecognized key factor that explains the common observations that anatase has higher activity than rutile for many photooxidative reactions.

Doped carbon-based systems have been extensively studied over the past decade as active electrocatalysts for both the two-electron (2e-) and four-electron (4e-) oxygen reduction reaction (ORR). However, the mechanisms for ORR are generally poorly understood. Here we report an extensive experimental and first-principles theoretical study of the ORR at nitrogen-doped reduced graphene oxides (NrGO). We synthesize three distinct NrGO catalysts and investigate their chemical and structural properties in detail via X-ray photoelectron spectroscopy, infrared and Raman spectroscopy, high-resolution transmission electron microscopy and thin-film electrical conductivity. ORR experiments include the pH dependences of 2e-versus 4e-ORR selectivity, ORR onset potentials, Tafel slopes and H/D kinetic isotope effects. These experiments show very different ORR behavior for the three catalysts, both in terms of selectivity and the underlying mechanism which proceeds either via coupled proton-electron transfers (CPETs) or non-CPETs. Reasonable structural models developed from DFT rationalize this behavior. The key determinant between CPET vs. non-CPET mechanisms is the electron density at the Fermi level under operating ORR conditions. Regardless of the reaction mechanism or electrolyte pH, however, we identify the ORR active sites as sp2 carbons that are located next to oxide regions. This assignment highlights the importance of oxygen functional groups, while details of (modest) N-doping may still affect the overall catalytic activity, and likely also the selectivity, by modifying the general chemical environment around the active site. File list (2) download file view on ChemRxiv NrGO_ACS_Catal_updated.pdf (3.76 MiB) download file view on ChemRxiv NrGO_SI_ACS_Catal_updated.pdf (6.66 MiB)