Carrier separation and charge transport characteristics of reduced graphene oxide supported visible-light active photocatalysts.

Extending the absorption to the visible region by tuning the optical band-gap of semiconductors and preventing charge carrier recombination are important parameters to achieve a higher efficiency in the field of photocatalysis. The inclusion of reduced graphene oxide (rGO) support in photocatalysts is one of the key strategies to address the above-mentioned issues. In this study, rGO supported AgI-mesoTiO2 photocatalysts were synthesized using a sonochemical approach. The physical effects of ultrasound not only improved the crystallinity of AgI-mesoTiO2 but also increased the surface area and loading of the AgI-mesoTiO2 nanocomposite on rGO sheets. The low intense oxygen functionalities (C-O-C and COOH groups) peak observed in the high resolution C1s spectrum of a hybrid AgI-mesoTiO2-rGO photocatalyst clearly confirmed the successful reduction of graphene oxide (GO) to rGO. The interfacial charge transfer between the rGO and the p-n junction of heterostructured photocatalysts has decreased the band-gap of the photocatalyst from 2.80 to 2.65 eV. Importantly, the integration of rGO into AgI-mesoTiO2 composites serves as a carrier separation centre and provides further insight into the electron transfer pathways of heterostructured nanocomposites. The individual effects of photo-generated electrons and holes over rGO on the photocatalytic degradation efficiency of rhodamine (RhB) and methyl orange (MO) using AgI-mesoTiO2-rGO photocatalysts were also studied. Our experimental results revealed that photo-generated superoxide (O2(-)˙) radicals are the main reactive species for the degradation of MO, whereas photo-generated holes (h(+)) are responsible for the degradation of RhB. As a result, 60% enhancement in MO degradation was observed in the presence of rGO in comparison to that of the pure AgI-mesoTiO2 photocatalyst. This is due to the good electron acceptor and the ultrafast electron transfer properties of rGO that can effectively reduce the molecular oxygen to produce a large amount of reactive O2(-)˙ radicals. However, in the case of RhB degradation, h(+) is the main reactive species which showed a slightly increased photocatalytic activity (12%) in the presence of rGO support where the role of rGO is almost negligible. This study suggests the effective roles of rGO for the degradation of organics, i.e., the rate of photocatalytic degradation also depends on the nature of compound rather than rGO support.

Synergistically Enhanced Electrocatalytic Performance of an N-Doped Graphene Quantum Dot-Decorated 3D MoS2-Graphene Nanohybrid for Oxygen Reduction Reaction.

Nitrogen-doped graphene quantum dots (N-GQDs) were decorated on a three-dimensional (3D) MoS2-reduced graphene oxide (rGO) framework via a facile hydrothermal method. The distribution of N-GQDs on the 3D MoS2-rGO framework was confirmed using X-ray photoelectron spectroscopy, energy dispersive X-ray elemental mapping, and high-resolution transmission electron microscopy techniques. The resultant 3D nanohybrid was successfully demonstrated as an efficient electrocatalyst toward the oxygen reduction reaction (ORR) under alkaline conditions. The chemical interaction between the electroactive N-GQDs and MoS2-rGO and the increased surface area and pore size of the N-GQDs/MoS2-rGO nanohybrid synergistically improved the ORR onset potential to +0.81 V vs reversible hydrogen electrode (RHE). Moreover, the N-GQDs/MoS2-rGO nanohybrid showed better ORR stability for up to 3000 cycles with negligible deviation in the half-wave potential (E 1/2). Most importantly, the N-GQDs/MoS2-rGO nanohybrid exhibited a superior methanol tolerance ability even under a high concentration of methanol (3.0 M) in alkaline medium. Hence, the development of a low-cost metal-free graphene quantum dot-based 3D nanohybrid with high methanol tolerance may open up a novel strategy to design selective cathode electrocatalysts for direct methanol fuel cell applications.

Influence of electron storing, transferring and shuttling assets of reduced graphene oxide at the interfacial copper doped TiO2 p-n heterojunction for increased hydrogen production.

Herein we report simple, low-cost and scalable preparation of reduced graphene oxide (rGO) supported surfactant-free Cu2O-TiO2 nanocomposite photocatalysts by an ultrasound assisted wet impregnation method. Unlike the conventional preparation techniques, simultaneous reduction of Cu(2+) (in the precursor) to Cu(+) (Cu2O), and graphene oxide (GO) to rGO is achieved by an ultrasonic method without the addition of any external reducing agent; this is ascertained by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analyses. UV-visible diffused reflectance spectroscopy (DRS) studies (Tauc plots) provide evidence for the loading of Cu2O tailoring the optical band gap of the photocatalyst from 3.21 eV to 2.87 eV. The photoreactivity of the as-prepared Cu2O-TiO2/rGO samples is determined via H2 evolution from water in the presence of glycerol as a hole (h(+)) scavenger under visible light irradiation. Very interestingly, the addition of rGO augments the carrier mobility at the Cu2O-TiO2 p-n heterojunction, which is evidenced by the significantly reduced luminescence intensity of the Cu2O-TiO2/rGO photocatalyst. Hence rGO astonishingly enhances the photocatalytic activity compared with pristine TiO2 nanoparticles (NPs) and Cu2O-TiO2, by factors of ∼14 and ∼7, respectively. A maximum H2 production rate of 110 968 µmol h(-1) gcat(-1) is obtained with a 1.0% Cu and 3.0% GO photocatalyst composition; this is significantly higher than previously reported graphene based photocatalysts. Additionally, the present H2 production rate is much higher than those of precious/noble metal (especially Pt) assisted (as co-catalysts) graphene based photocatalysts. Moreover, to the best of our knowledge, this is the highest H2 production rate (110 968 µmol h(-1) gcat(-1)) achieved by a graphene based photocatalyst through the splitting of water under visible light irradiation.