Photocurrent Measurements Research Articles

Photocatalytic dye degradation of V2O5 Nanoparticles—An experimental and DFT analysis

V2O5 nanoparticles were synthesized by using a facile method towards its visible-light photocatalytic dye decomposition of two industrial standard dyes, methylene blue (MB) and methyl violet (MV). From the XRD data, the crystalline phase was identified as orthorhombic, and the particle size was estimated as 52 nm. FTIR and XPS studies were performed to confirm the presence of functional groups, ‘V’ and ‘O’ elements. The photocatalytic dye degradation efficiency of the prepared nanoparticles against MB and MV was found to be 92% and 85% , respectively. Photocurrent measurement confirms the charge carrier generation during exposure to visible light. A density functional theory (DFT) calculation was performed to uphold the experimental results and to understand the mechanism of photocatalytic degradation of the prepared V2O5 nanoparticles. The obtained results through DFT are compared with experimental results and are discussed.

Novel flake-like Z-Scheme Bi2WO6-ZnBi2O4 heterostructure prepared by sonochemical assisted hydrothermal procedures with enhanced visible-light photocatalytic activity

In this work, novel Bi2WO6-ZnBi2O4 p-n heterojunction was successfully synthesized through combination of sonochemical and hydrothermal procedures. The results revealed that the sonochemically prepared nanocomposites had spherical shape with amorphous structure which became crystalline particles with flak-like morphology after hydrothermal treatment. Bi2WO6-ZnBi2O4 nanocomposite with 20 wt% ZnBi2O4 content demonstrated the highest photocatalytic performance compared to the pure Bi2WO6 and ZnBi2O4 samples toward the degradation of Malachite Green (MG). This finding is associated with the efficient separation of photo-generated electron/hole pairs due to the formation of interfacial charge transfer between Bi2WO6 and ZnBi2O4. However, further ZnBi2O4 loading had no significant effect on the enhancement of photocatalytic activity of nanocomposite samples owing to the shielding effect of ZnBi2O4 against incident light. The obtained results were further verified by optical and photocurrent density measurements. Based on the obtained results from active species trapping experiments, electron spin resonance and Mott-Schottky calculations, the Z-scheme charge transfer pathway was proposed as the predominant mechanism for the photocatalytic reactions. Also, the photocatalytic degradation of 2,4-dichlorophenol, as a colorless model pollutant, was conducted on optimal composite sample to study the possible contribution of MG dye sensitization mechanism in the enhancement of photocatalytic efficiency.

Investigation on Al3+ and Al2O3 Coexist in BiVO4 for Efficient Methylene Blue Degradation: Insight into Surface States and Charge Separation.

A series of Al-doped BiVO4 composites have been synthesized via the hydrothermal method for methylene blue (MB) degradation application. The reasons for the improvement of photocatalytic performance was explained from the perspective of optics. Transient photovoltage (TPV) measurements suggested that the surface states have the priority to capture photogenerated carriers, and the Al2O3 surface passivation layer can prolong the lifetime of charge carrier. The results of surface photovoltage (SPV), transient photovoltage (TPV), and surface photocurrent (SPC) measurements suggested that the coexistence of Al3+ and Al2O3 caused by the appropriate doping would improve the transfer property and prolong the lifetime of photogenerated carriers. Finally, the possible photocatalytic mechanism is expounded to illustrate the photogenerated charge behavior under visible light irradiation. This work provides a better understanding of the synergistic effect of Al-doping and Al2O3 passivation layer on enhancing the photocatalytic performance.

Oxygen-vacancy engineering approach to bismuth basic nitrate/g-C3N4 heterostructure for efficiently photocatalytic hydrogen evolution

Formation of oxygen vacancy is emerging as one type of promising defect engineering in the preparation of inorganic functional composites. In this work, a novel composite upon the calcination of the nanosheets Bi6O6(OH)3(NO3)3·1.5H2O (BBN) and g-C3N4 was prepared. A large amount of oxygen vacancies was revealed by electron spin resonance (ESR) and Raman spectroscopy. These intriguing structural characters render the as-prepared composite with excellent interfacial charge transfer and photo-stimulated response as shown by electrochemical impedance spectroscopy (EIS) and photocurrent measurement. Upon visible-light activation, such composite enables photocatalyse H2 evolution with a rate of 181 μmol h−1 g−1 around the 4.2 times that of pure g-C3N4 as a reference. Such composite catalyst exhibits excellent recycling and reusable behaviors. The formation of BBN/CN heterojunction and the existence of oxygen vacancies are the main reasons for the distinctly improvement of its photocatalytic activity, which has been verified by theoretical calculations.

Mesoporous BiVO4/2D-g-C3N4 heterostructures for superior visible light-driven photocatalytic reduction of Hg(II) ions

In this contribution, a Z-scheme mesoporous BiVO4/g-C3N4 nanocomposite heterojunction with a considerable surface area and high crystallinity was synthesized by a simple soft and hard template-assisted approach. This material demonstrates superior visible light-driven photocatalysis for the photoreduction of Hg(II) ions. TEM and XRD results show that the mesoporous BiVO4 NPs, with a monoclinic phase and an ellipsoid-like shape, are highly dispersed onto the porous 2D surfaces of g-C3N4 nanosheets with a particle size of 5–10 nm. The obtained BiVO4/g-C3N4 nanocomposites with a p-n heterojunction show significantly enhanced Hg(II) photoreduction efficiency compared to the mesoporous BiVO4 NPs and pristine g-C3N4. Among all synthesized photocatalysts, the 1.2% BiVO4/g-C3N4 nanocomposite indicated the highest photoreduction of Hg(II) performance, reaching ~ 100% within 60 min; this result is 3.9 and 4.5 –fold larger than that of the BiVO4 NPs and pristine g-C3N4. The Hg(II) photoreduction rates highly increase to 208.90, 314.95, 411.23 and 418.68 μmol g−1min−1 for the mesoporous 0.4, 0.8, 1.2 and 1.6% BiVO4/g-C3N4 nanocomposites, respectively. The reduction rate of the mesoporous 1.2% BiVO4/g-C3N4 nanocomposite demonstrated a 5.2 and 3.8 times larger increase than that of the pristine g-C3N4 nanosheets and pure BiVO4 NPs. The superior Hg(II) photoreduction efficiency was ascribed to decreased carrier recombination and the improved utilization of visible light by constructing BiVO4/g-C3N4 nanocomposites with a p-n junction. Transient photocurrent measurement and photoluminescence spectra were employed to confirm the possible Hg(II) photoreduction mechanism over these BiVO4/g-C3N4 photocatalysts. This research provides an accessible route for the nanoengineered design of mesoporous BiVO4/g-C3N4 heterostructures that demonstrated unique photocatalytic performance.

Exploring the Coordination Modes of a Keggin-Type [ZnW12O40]6- Anionic Cluster: Bonding Patterns, Crystal Structure, and Semiconducting Properties.

Polyoxometalate-based organic-inorganic hybrid compounds (POIHCs) have been greatly developed due to their wide application prospects, but the pursuit of their directed synthesis via molecular design still remains a challenge. Herein, we demonstrate that the coordination modes of the Keggin-type [ZnW12O40]6- anion can be tuned, which leads to different semiconductor characteristics. Using the same building block, ligand, and metal ion (ZnW12, phen, Cu2+), we synthesized three new POIHCs with different bonding patterns by means of different coordination modes of ZnW12. The three POIHCs (H2phen){ZnW12O40[Cu(phen)2]2}·3H2O (1), {ZnW12O40[Cu(phen)(H2O)2]2[Cu(phen)(H2O)]}n·3H2O (2), and (Me4N)2{ZnW12O40[Cu(phen)(H2O)]2}n·5H2O (3) (phen = 1,10-phenanthroline) have been structurally characterized by single-crystal X-ray diffraction. Compound 1 appears as a zero-dimensional coordination complex cluster, while compounds 2 and 3 are both 1D chain structures with different Cu2+ bridge linkages. Although these three POIHCs possess the same chemical components, their semiconductor properties are different, which is demonstrated by measurements of transient photocurrent and band gap (Eg) values. Furthermore, we carried out comparative experiments on the photoconductivity performance of compounds 1-3 and their photocatalytic reduction from O2 to H2O2, indicating the significant influence of the energy level matching on the photocatalytic activity.

Near-infrared photodetection using random networks of single-walled carbon nanotubes with asymmetrical work-function electrodes

We investigate the use of random networks of single-walled carbon nanotubes for near-infrared photodetection. By increasing the number of nanotubes between asymmetrical work-function electrodes using dielectrophoretic assembly, the effect of Fermi-level pinning of nanotube-Schottky contacts was revealed in the linear current-voltage characteristic. The extracted device resistance showed an abrupt drop when the numerous intertube junctions formed densely packed networks in the electrode channel. Under the excitation of a near-infrared laser, we performed the photocurrent measurement at ambient temperature at different light powers. Our devices with densely packed nanotube networks showed enhanced photoconductive detection of responsivity, detectivity, and detection response. This is attributed to the increase in the photoabsorption area, the decrease of the channel resistance, and the formation of continuous conducting paths for high-efficient charge percolation. The photoconductive responsivity of up to 8.0 μA W−1 was found with a detectivity of about 4.9 × 105 cm Hz1/2 W−1, which is 4 orders of magnitude greater than that achieved in the channel with individual nanotubes deposited and comparable to that of suspended nanotube bolometers. The densely packed nanotube devices had a detection response of ∼ 4 ms under a finite bias that can be explained by the short-diffusion length of the photoexcited electrons and holes. However, the decrease in the photocurrent with time observed in our devices that exhibited photovoltaic characteristics indicates that electron-hole pair recombination in the nanotube networks occurs with differing characteristic time scales of the injected electrons and holes.

Spatially resolved Fourier transform impedance spectroscopy: A technique to rapidly characterize interfaces, applied to a QD/SiC heterojunction

We demonstrate a technique to quickly build and spatially map the frequency response of optoelectronic devices. The transfer function of a linear system is the Fourier transform of its impulse response. Such an impulse response is obtained from transient photocurrent measurements of devices such as photodetectors and solar cells. We introduce and apply Fourier transform impedance spectroscopy (FTIS) to a PbS colloidal quantum dot SiC heterojunction photodiode and validate the results using intensity-modulated photocurrent spectroscopy. Cutoff frequencies in the devices were as high as ∼10 kHz, showing their utility in advanced thin film and flexible electronics. The practical frequencies for FTIS lie in the mHz–kHz range, ideal for composite materials such as quantum dot films that are dominated by interfacial trap states. These can lead to characteristic lengths for charge collection ∼20–500 μm dominated by transmission line effects, rather than intrinsic diffusion and drift length scales, enabling extraction of interfacial capacitances and series/parallel resistances.

Multiplexed Bioanalysis Based on Light-Sensitive Electrodes

IntroductionQuantum dots (QDs) have become attractive tools in bioanalysis because of their advantageous optical and electrochemical properties. This relates to applications exploiting fluorescence, electrochemiluminescence or photocurrent generation [1]. For the latter case QDs are immobilized on conductive electrodes. By light excitation charge carriers are formed inside the nanoparticles which can be used for photocurrent generation. Signal chains can be constructed when appropriate donor or acceptor components are present in solution. The special feature of such electrodes is that the response is triggered by illumination of the sensing electrode [1]. In this study, a light-addressable photoelectrochemical sensor was built by coupling QDs with oxidases to detect different enzyme substrates.Results and ConclusionsFirst CdSe/ZnS QDs have been immobilised via a dithiol compound on gold electrodes. Two different enzymes, glucose oxidase (GOD) and sarcosine oxidase (SOD) have been deposited on this QDs-electrode at spatially well resolved places. Here small spots have been achieved by a dropping method. A laser beam is moved over the spots to detect photocurrents. Since oxygen can act as electron acceptor at the illuminated QDs the photocurrent is correlated to the oxygen concentration in solution. By action of the enzymes in the presence of the corresponding substrate a competitive situation for oxygen is created in front of the QD layer and thus, the photocurrent decreases [2, 3]. Since this happens in a concentration dependent way, analysis of the enzyme substrate is feasible.Furthermore, due to the spatially separated immobilisation of the two enzymes, an independent read-out of the two enzyme reactions is possible. The results indicate good lateral resolution and selectivity of the multi-detection system without any crosstalk between the two sensing regions and also without any response at regions, which have not been modified with an enzyme [4].After that, the possibility of an imaging of the enzyme spot by photocurrent measurement has been studied. When the laser is moved over the spot of GOD with a constant speed, the size of the spot can be determined from the photocurrent - time curve (with a defined moving speed). The photocurrent correlates to different local enzyme activities. Consequently, an imaging of the enzyme activity distribution on the surface has also been achieved by this method.

(Invited) Single Nanosheet Photoelectrochemistry: Probing Charge Recombination and Transport Pathways in Monolayer Transition Metal Dichalcogenide Photoelectrodes

Monolayer heterojunctions such as MoS2/WS2 are attractive for solar energy conversion applications because the interfacial electric field spatially separates charge carriers in less than 100 femtoseconds. Photoelectrochemical cells represent an intriguing platform to collect the spatially separated carriers. However, the recombination, transport, and interfacial charge transfer processes that take place following the ultrafast charge separation step have not been investigated. Here we demonstrate novel charge recombination and transport pathways in monolayer MoS2/WS2 photoelectrochemical cells by spatially resolving the net collection of carriers (i.e., the photocurrent) at the single nanosheet-level. We discovered an excitation wavelength-dependent recombination pathway that depends on the heterojunction stacking configuration and the carrier generation profile in the heterostructure. Photocurrent mapping measurements revealed that charge transport occurs parallel to the layers over micrometre-scale distances even though the indium tin oxide electrode and liquid electrolyte provide efficient charge extraction pathways via intimate electron- and hole-selective contacts. Our results reveal how composition heterogeneity influences the performance of bulk heterojunction electrodes made from randomly oriented nanosheets and provide critical insight into the design of efficient heterojunction photoelectrodes for solar energy conversion applications. Figure 1

Effect of film morphology on water oxidation enhancement in NiFeCo modified hematite photoanodes

Hematite (α-Fe2O3) is a promising candidate for water oxidation applications due to its abundance in the earth crust and its suitable bandgap. However, hematite performance is severely limited by electron-hole recombination at its interface with the electrolyte; something that can be addressed using electrocatalysts. In this report, we evaluate the influence of a ternary NiFeCo co-catalyst to enhance the water oxidation performance of hematite photoanodes. Thus, NiFeCo co-catalyst is optimized for hematite thin films with i) dense and ii) porous (nano-rod) morphologies. Both hematite films are prepared using electron beam evaporation method followed by an annealing step and NiFeCo co-catalyst is processed using electrodeposition. The results demonstrate optimal thickness of NiFeCo co-catalyst is thinner for porous hematite layers relative to the dense ones indicating that the optimal condition of the co-catalyst is affected by hematite morphology. The optimized dense NiCoFe/hematite photoanode shows nearly than 2.8-fold enhancement relative to the pristine hematite photoanode, whereas the optimized porous NiCoFe/hematite photoanode shows 2.2-fold enhancements. Transient photocurrent measurements confirm that NiFeCo facilitates hole injection from hematite into the electrolyte. These findings have important implications for optimizing electrocatalysts for photoelectrochemical water splitting photoanodes and taking into account the effect of surface morphology.

Preparation of g-C3N4/BMO heterojunction for visible photocatalytic degradation of O-Nitrophenol and actual pharmaceutical wastewater

A novel Bi2MoO6 (BMO) modified g-C3N4 heterostructure composite photocatalyst (g-C3N4/BMO) was prepared by hydrothermal method. The orthorhombic crystal phase of the material was studied by the X-ray diffraction assay (XRD). The microscopic morphology of the material was investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), indicating a heterojunction structure of the composite. The electrons and holes separation was evaluated by photo-luminescence (PL) and photocurrent measurements. The removal rate of O-Nitrophenol was as high as 97.3% over the catalyst with the optimal ratio of 1: 0.09 (BMO:g-C3N4) in 180 min. COD removal percentage of actual pharmaceutical wastewater was as high as 71.40%. The active species in the photocatalytic system were studied, indicating superoxide radicals (·O2-) and holes (h+) as the main active species.

Identification of Two Regimes of Carrier Thermalization in PbS Nanocrystal Assemblies

We bring fresh insight into the ensemble properties of PbS colloidal quantum dots with a critical review of the literature on semiconductors followed by systematic comparisons between steady-state photocurrent and photoluminescence measurements. Our experiments, performed with sufficiently low powers to neglect nonlinear effects, indicate that the photoluminescence spectra have no other noticeable contribution beside the radiative recombination of thermalized photocarriers (i.e., photocarriers in thermodynamic quasi-equilibrium). A phenomenological model based on the local Kirchhoff law is proposed that makes it possible to identify the nature of the thermalized photocarriers and to extract their temperatures from the measurements. Two regimes are observed: For highly compact assemblies of PbS quantum dots stripped from organic ligands, the thermalization concerns photocarriers distributed over a wide energy range. With PbS quantum dots cross-linked with 1,2-ethanedithiol or longer organic ligand chains, the thermalization concerns solely the fundamental exciton and can quantitatively explain all the observations, including the precise Stokes shift between the absorbance and luminescence maxima.

Controlling and Optimizing Photoinduced Charge Transfer across Ultrathin Silica Separation Membrane with Embedded Molecular Wires for Artificial Photosynthesis.

Ultrathin amorphous silica membranes with embedded organic molecular wires (oligo(p-phenylenevinylene), three aryl units) provide chemical separation of incompatible catalytic environments of CO2 reduction and H2O oxidation while maintaining electronic and protonic coupling between them. For an efficient nanoscale artificial photosystem, important performance criteria are high rate and directionality of charge flow. Here, the visible-light-induced charge flow from an anchored Ru bipyridyl light absorber across the silica nanomembrane to Co3O4 water oxidation catalyst is quantitatively evaluated by photocurrent measurements. Charge transfer rates increase linearly with wire density, with 5 nm-2 identified as an optimal target. Accurate measurement of wire and light absorber densities is accomplished by the polarized FT-IRRAS method. Guided by density functional theory (DFT) calculations, four wire derivatives featuring electron-donating (methoxy) and -withdrawing groups (sulfonate, perfluorophenyl) with highest occupied molecular orbital (HOMO) potentials ranging from 1.48 to 0.64 V vs NHE were synthesized and photocurrents evaluated. Charge transfer rates increase sharply with increasing driving force for hole transfer from the excited light absorber to the embedded wire, followed by a decrease as the HOMO potential of the wire moves beyond the Co3O4 valence band level toward more negative values, pointing to an optimal wire HOMO potential around 1.3 V vs NHE. Comparison with photocurrents of samples without nanomembrane indicates that silica layers with optimized wires are able to approach undiminished electron flux at typical solar intensities. Combined with the established high proton conductivity and small-molecule blocking property, the charge transfer measurements demonstrate that oxidation and reduction catalysis can be efficiently integrated on the nanoscale under separation by an ultrathin silica membrane.

Carrier Tunneling from Charge Transfer States in Organic Photovoltaic Cells

AbstractCharge transfer (CT) states play a key role in the functioning of organic solar cells; however, understanding the mechanism by which CT states dissociate efficiently into free charges remain a conceptual challenge. Here, the electric field dependent dynamics of charge generation in planar cyanine/fullerene photovoltaic cells is probed over a wide temperature range using time‐resolved Stark effect experiments, transient absorption, and photocurrent measurements. Results indicate that dissociation of thermalized CT states is the rate‐limiting step for all temperatures. The dissociation rate strongly depends on the field, but is temperature independent. The results also suggest that the yield of generated charges is temperature independent. Model electrostatic calculations illustrate that specific orientations of the cyanine crystal relative to C60 create a repulsive potential for an electron near the interface that is largely due to the quadrupole moment of the unit cell. In combination with the electron‐hole coulomb attraction and the electric field‐induced barrier lowering, a high‐energy potential barrier forms with a narrow width of a few nanometers. It is proposed that charge separation occurs via a field‐dependent electron tunneling mechanism through that barrier, which is temperature independent. The results support a thus far overlooked pathway for CT state dissociation via carrier tunneling.

ZnO Nano-Rod Arrays Synthesized with Exposed {0001} Facets and the Investigation of Photocatalytic Activity

Zinc oxide (ZnO) possesses superior chemical and physical properties so that it can occupy an essential position in the application of nanostructures. In this paper, ZnO nano-rod arrays were synthesized by a simple one-step hydrothermal approach with the assistance of cetyl trimethyl ammonium bromide (CTAB). Exposure of the {0001} facets could be controlled by adjusting the amount of CTAB and the maximum exposure of the {0001} facets of ZnO nanorods is obtained at 1.2 g of CTAB. The photocurrent, EIS, and PL measurements support the facile charge transfer with minimum recombination of the photogenerated excitons of the ZnO nano-rod arrays obtained at 1.2 g of CTAB. Consequently, the obtained ZnO nano-rod arrays at the optimal CTAB of 1.2 g exhibit an excellent photocatalytic degradation rate of 99.7% for rhodamine B (RhB), while the degradation rate of RhB by the ZnO obtained without CTAB is only 35%.

A 3D electropolymerized thin film based on an isoporphyrin and on a pyridine end-decorated molybdenum(II) halide cluster: Photoelectrochemical and impedance properties

Herein, the first example of formation of a 3D hybrid organic-inorganic copolymer based on an isoporphyrin and on a pyridine end-decorated molybdenum(II) halide cluster. The isoporphyrin copolymer thin film was obtained by electropolymerization of zinc-5,15-bis(p-tolyl)porphyrin in the presence of polyiodometalate with 6 pendant pyridyl ligands (((Bu4N)2[{Mo6I8}(OOC-C5H4N)6]). Electrogenerated porphyrin radical cation is a powerful electrophile which can rapidly react with the pyridine end-decorated Mo(II) halide cluster as nucleophile to form 3D copolymer containing stable isoporphyrin.The electropolymerization was monitored by electrochemical quartz crystal microbalance (EQCM). The copolymer was characterized by UV-Vis-NIR spectroscopy, X-ray photoelectron spectroscopy (XPS), electrochemistry, and atomic force microscopy (AFM). The existence of the radical of the isoporphyrin copolymer was proved by electron spin resonance spectroscopy (EPR).Interestingly, the UV-Vis-NIR spectra of the copolymer exhibited broadening and splitting of the Soret band as well as one additional band in the NIR between 750 nm and 1000 nm.The photoelectrochemical properties have been studied by electrochemical impedance spectroscopy and by photocurrent transient measurements under visible-NIR light irradiation.The 3D copolymer containing stable isoporphyrin radical exhibits interesting photocurrent response even under only NIR illumination.In the solid state, the 3D copolymer could be stored for over several months and even one year without any degradation under ambient conditions in air.

Si-based photodiode and material characterization of TiO2 thin film

This study proposes a different technique known as the thermionic vacuum arc to produce a TiO2/Si heterojunction photodiode with better electrical properties than literature like the ideality factor indicating that the method is very suitable to form an outstanding quality heterojunction interface. The heterojunction is highly sensitive to different light intensities and has stable photocurrent characteristics as a photodiode. Structural and morphological properties of the produced TiO2/Si heterostructure surfaces were investigated via XRD and AFM, respectively. According to XRD analysis, it was observed that the TiO2 thin film was in a polycrystalline structure with the Anatase and Brookite phases. Also, the film surface is homogenous, and a low roughness value was measured as 3 nm. The thin film thickness and the bandgap values (Eg) were determined based on optical methods. The electrical parameters of TiO2 thin film, such as conductivity type, charge carrier density, and mobility, were also determined by Hall Effect Measurement. The Ag/TiO2/Si/Ag heterojunction device characteristics were determined by conducting the current-voltage (I-V) measurement. The ideality factor (n) and the barrier height (Φb) values were determined as 1.7 and 0.65 eV, respectively. The photo-response performance was measured via transient photocurrent (I-T) measurements for different light intensities.

Gold Nanorod@Ruthenium Oxide Core–Shell Heterostructures: Synthesis, Single‐Particle Characterizations, and Enhanced Hot Electron Generation

AbstractA synthetic approach has been developed towards encapsulating gold nanorods (AuNRs) with conductive ruthenium oxide (RuO2) by the hydrolysis of ruthenium (III) acetylacetonate in the presence of cetyltrimethylammonium bromide (CTAB) in an autoclave. The reaction conditions are investigated systematically. The resultant AuNR@RuO2 heterostructures are encapsulated with uniform RuO2 shells and are monodispersed in aqueous solutions. Correlated dark‐field microscopic (DFM) and scanning electron microscopic (SEM) characterizations on AuNR@RuO2 nanostructures are performed at single‐particle level, providing the plasmonic properties in correspondence with the size and structure details of the core–shell nanostructures. The complex refractive indexes of RuO2 shell on single nanostructures are further determined by fitting the calculated spectra to the measured ones. Hot electron (HE) photocurrent measurement employing a photoelectrochemical cell shows the encapsulation of the AuNR with 8.2‐nm RuO2 shell results in a 2.9‐fold enhancement at 638 nm, in good agreement with the theoretical calculation. The calculation indicates enhancement ratio exceeding 50 at 800 nm. The mechanism behind is the substantial enhancement of the electric field inside the RuO2 shell as well as less loss in comparison to plasmonic metals, while the conductive nature of RuO2 allows for the transfer of HEs generated inside the shell to the environment.

New insights on photocatalytic hydrogen evolution of ZnFe2−xGaxO4 (0 ≤ x ≤ 2) solid solutions: Role of oxygen vacancy and ZnO segregated phase

The exponential increase in global energy and clean energy demands lead to the necessity for development of efficient photocatalyst for hydrogen evolution. Herein, photocatalytic hydrogen evolution activity with nano-structured ZnFe2-xGaxO4 (0 ≤x ≤ 2) solid solution samples synthesized by citrate-gel method at 550 °C is reported. The prepared materials have been characterized for their structure and optical properties. The formation of solid solution with spinel type structure in the complete range of composition and a systematically decreasing trend of unit cell parameters with increasing Ga3+ concentration is observed from powder XRD studies. The band gap of these solid solutions could be tuned from 1.9 to 3.1 eV by increasing the Ga3+ concentration. Without any co-catalyst, ZnFe2O4 shows poor catalytic activity (378μmolh−1g−1) for photocatalytic hydrogen evolution from water. Whereas its activity enhances with loading of co-catalyst and touches a maximum activity of 3089μmolh−1g−1 with loading 1 wt% of Pt. The catalytic activity increases systematically with increasing Ga concentration in the solid solutions and reaches to 3989μmolh−1g−1 for ZnGa2O4 nanomaterials. Electrochemical impedance spectroscopic and transient photocurrent measurements also support the observed increasing trend. The lower catalytic activity of ZnFe2O4 has been attributed to the faster e-h recombination due to lower band gap and inherent oxygen vacancies. A smaller amount of oxygen vacancies and segregated ZnO phase observed in Ga substituted ZnFe2O4 samples favor the e-h pair separation and that facilitate the hydrogen generation from water.