A Photoelectrochemical Investigation on the Synergetic Effect between CdS and Reduced Graphene Oxide for Solar-Energy Conversion

Loading Co3N nanoparticles as efficient cocatalysts over Zn0.5Cd0.5S for enhanced H2 evolution under visible light

Co₃N nanoparticles were decorated on Cd_0.5Zn_0.5S as highly efficient co-catalysts via a facile method. The H₂ evolution activity of Co₃N(2 wt%)/Cd_0.5Zn_0.5S is 25 times higher than that of pure Cd_0.5Zn_0.5S.

Enhanced Photocatalytic Hydrogen Evolution by Loading Cd0.5Zn0.5S QDs onto Ni2P Porous Nanosheets

Ni₂P has been decorated on CdS nanowires or nanorods for efficient photocatalytic H₂ production, whereas the specific surface area remains limited because of the large size. Here, the composites of Cd_0.5Zn_0.5S quantum dots (QDs) on thin Ni₂P porous nanosheets with high specific surface area were constructed for noble metal-free photocatalytic H₂ generation. The porous Ni₂P nanosheets, which were formed by the interconnection of 15-30 nm-sized Ni₂P nanoparticles, allowed the uniform loading of 7 nm-sized Cd_0.5Zn_0.5S QDs and the loading density being controllable. By tuning the content of Ni₂P, H₂ generation rates of 43.3 μM h^- 1 (1 mg photocatalyst) and 700 μM h^- 1 (100 mg photocatalyst) and a solar to hydrogen efficiency of 1.5% were achieved for the Ni₂P-Cd_0.5Zn_0.5S composites. The effect of Ni₂P content on the light absorption, photoluminescence, and electrochemical property of the composite was systematically studied. Together with the band structure calculation based on density functional theory, the promotion of Ni₂P in charge transfer and HER activity together with the shading effect on light absorption were revealed. Such a strategy can be applied to other photocatalysts toward efficient solar hydrogen generation.

Facile Synthesis of Graphene Sponge from Graphene Oxide for Efficient Dye-Sensitized H2 Evolution

Graphene is an advanced carbon energy material due to its excellent properties. Reduction of graphene oxide (GO) is the most promising mass production route of graphene/reduced graphene oxide (rGO). To maintain graphene's properties and avoid restacking of rGO sheets in bulk, the preparation of 3-dimensional porous graphene sponge via 2-dimensional rGO sheets is considered as a good strategy. This article presents a facile route to synthesize graphene sponge by thermal treating GO powder at low temperature of 250 °C under N2 atmosphere. The sponge possesses macroporous structure (5-200 nm in size) with BET specific surface area of 404 m(2) g(-1) and high conductivity. The photocatalytic H2 production activity of the rGO sponge with a sensitizer Eosin Y (EY) and cocatalyst Pt was investigated. The rGO sponge shows highly efficient dye-sensitized photocatalytic H2 evolution compared to that obtained via a chemical reduction method. The maximum apparent quantum yield (AQY) reaches up to 75.0% at 420 nm. The possible mechanisms are discussed. The synthesis method can be expanded to prepare other graphene-based materials.

Intergrowth of Cocatalysts with Host Photocatalysts for Improved Solar-to-Hydrogen Conversion

In the field of photocatalytic hydrogen generation, cocatalysts play a vital role in enhanced properties. Delicate control of the physicochemical properties of cocatalysts and systematic optimization of the coupling between cocatalysts and host photocatalysts are essential. Herein, a simple one-step hydrothermal method was proposed to synthesize noble-metal-free NiSx/CdS photocatalysts for the first time. Time-dependent growth studies revealed that NiSx cocatalysts and CdS host photocatalysts were intergrown with each other in the one-step hydrothermal process. Compared with NiSx@CdS photocatalysts prepared by the common two-step method, the intergrowth effect induced close contact between NiSx and CdS, as well as smaller size and better dispersity of NiSx nanoparticles. These specific characters of NiSx/CdS finally resulted in efficient charge separation and rapid surface reaction, giving rise to significantly improved photocatalytic activity with the apparent quantum efficiency at 420 nm as high as 60.4%. To our knowledge, this value is the highest efficiency for NiSx modified CdS photocatalysts and is among the best efficiencies for visible-light-driven photocatalysts. It is believed that the present work can provide a general guidance to develop an efficient heterostructured cocatalyst/photocatalyst system for hydrogen generation.

Graphene quantum dots: recent progress in preparation and fluorescence sensing applications

This paper reviews recent activities in the preparation and fluorescence sensing applications of graphene quantum dots.

2D Hybrid Nanostructure of Reduced Graphene Oxide-CdS Nanosheet for Enhanced Photocatalysis

Graphene-based hybrid nanostructures have recently emerged as a new class of functional materials for light-energy conversion and storage. Here, we have synthesized reduced graphene oxide (RGO)-semiconductor composites to improve the efficiency of photocatalysis. Zero-dimensional CdS nanoparticles (0D), one-dimensional CdS nanorods (1D), and two-dimensional CdS nanosheets (2D) are grafted on the RGO sheet (2D) by a surface modification method using 4-aminothiophenol (4-ATP). Structural analysis confirms the attachment of CdS nanocrystals with RGO, and the strong electronic interaction is found in the case of a CdS nanosheet and RGO, which has an influence on photocatalytic properties. The degradation of dye under visible light varies with changing the dimension of nanocrystals, and the catalytic activity of the CdS NS/RGO composite is ∼4 times higher than that of CdS nanoparticle/RGO and 3.4 times higher than that of CdS nanorod/RGO composite samples. The catalytic activity of the CdS nanosheet/RGO composite is also found to be ∼2.5 times than that of pure CdS nanosheet samples. The unique 2D-2D nanoarchitecture would be effective to harvest photons from solar light and transport electrons to reaction sites with respect to other 0D-2D and 1D-2D hybrid systems. This observation can be extended to other graphene-based inorganic semiconductor composites, which can provide a valuable opportunity to explore novel hybrid materials with superior visible-light-induced catalytic activity.

New insight into the enhanced visible light photocatalytic activity over boron-doped reduced graphene oxide

Boron-doped reduced graphene oxide (B-RGO) synthesized by a facile one-step reflux route is able to exhibit significantly higher photocatalytic activity than non-doped RGO under visible light irradiation. New insights accounting for this photocatalytic activity improvement are discussed, which is distinctly different from the case of B-RGO nanoribbons under UV light irradiation.

High-efficiency plasmon-enhanced and graphene-supported semiconductor/metal core-satellite hetero-nanocrystal photocatalysts for visible-light dye photodegradation and H2 production from water

Solar-driven photocatalytic process based on electron-hole pair production in semiconductors is a long sought-after solution to a green and renewable energy and has attracted a renaissance of interest recently. The relatively low photocatalytic efficiency, however, is a main obstacle to their practical applications. A promising attempt to solve this problem is by combined use of metal nanoparticles, by taking advantage of strong and localized plasmonic near-field to enhance solar absorption and to increase the electron-hole pair generation rate at the surface of semiconductor. Here, we report a semiconductor/metal visible-light photocatalyst based on CdSe/CdS-Au (QD-Au) core-satellite heteronanocrystals, and assemble them on graphene nanosheets for better photocatalytic reaction. The as-synthesized photocatalyst exhibits excellent plasmon-enhanced photocatalytic activities toward both photodegradation of organic dye and visible-light H2 generation from water. The H2 evolution rate achieves a maximum of 3113 μmol h(-1) g(-1) for the heteronanocrystal-graphene composites, which is about 155% enhancement compared to nonplasmonic QD-G sample and 340% enhancement compared to control QD-Au-G sample, and the apparent quantum efficiency (QE) reaches to 25.4% at wavelength of 450 nm.

"Signal-on" photoelectrochemical sensing strategy based on target-dependent aptamer conformational conversion for selective detection of lead(II) ion

A "signal-on" photoelectrochemical sensing strategy for selective determination of Pb(2+) is designed on the basis of the combination of Pb(2+)-induced conformational conversion, the amplified effect of reduced graphene oxide (RGO) and resonance energy transfer between CdS quantum dots (QDs) and gold nanoparticles (AuNPs). The RGO/CdS/aptamer platform is constructed via a stepwise modification method, and characterized by electrochemical impedance spectroscopy. In the absence of Pb(2+), the AuNP-labeled DNA, as a signal quenching element, can be introduced by hybridization with aptamer on the surface of sensing platform, which quenches the photocurrent of QDs via an energy transfer process. Upon addition of Pb(2+), the aptamer is induced into a G-quadruplex structure, which can greatly hinder the hybridization between aptamer and AuNP-labeled DNA due to the competitive occupation of binding sites and steric effect, leading to the recovery of photocurrent. Under optimized conditions, this "signal-on" photoelectrochemical biosensor shows a linear relationship between photocurrent variation and the logarithm of Pb(2+) concentration in the range of 0.1-50 nM with a detection limit of 0.05 nM. Meanwhile, it also exhibits good selectivity for Pb(2+) over other interfering ions, and is successfully applied to the detection of Pb(2+) in environmental water samples. By substituting the aptamers with other sequences, this proposed strategy could be conveniently extended to detect different targets as versatile photoelectrochemical devices.

TiO2 nanotube array-graphene-CdS quantum dots composite film in Z-scheme with enhanced photoactivity and photostability

The most efficient solar energy utilization is achieved in natural photosynthesis through elaborate cell membrane with many types of molecules ingeniously transferring photogenerated electrons to reactants in a manner similar to the so-called Z-scheme mechanism. However, artificial photosynthetic systems based on semiconductor nanoparticles are inevitably accompanied by undesired non-Z-scheme electron transfer and back reactions, which adversely affect the photoactivity and photostability of the systems. Herein, we report on a novel Z-scheme system with an electrochemically converted graphene (GR) film as the electron mediator interlayer contacted with both TiO2 nanotube (TNT) array and CdS quantum dots (CdS QDs) on two sides. The obtained TiO2 nanotube array-graphene-CdS quantum dots (TNT-GR-CdS) composite film shows higher photoelectric response and photocatalytic activities than other bare TNT, TNT-CdS, TNT-GR, and TNT-CdS-GR. Moreover, compared to TNT-CdS, the activity stability is significantly improved, and the residual amount of Cd element in reaction solution is reduced ∼8 times over TNT-GR-CdS. Various measurements of photoelectrochemistry and radicals reveal that the enhanced photoactivity and photostabilities of TNT-GR-CdS are due to the efficient spatial separation of the photogenerated electron-hole pairs and the restricted photocorrosion of CdS via an efficient Z-scheme mechanism under simulated sunlight.