Photocathodic Current Research Articles

A self-powered cathodic molecular imprinting ultrasensitive photoelectrochemical tetracycline sensor via ZnO/C photoanode signal amplification

Quantitative determination of tetracycline (TC) in environment and foods is of great importance, as excessive residues might have negative effects on human health and environmental risks. Herein, a self-powered molecularly imprinted photoelectrochemical (PEC) sensor based on the ZnO/C photoanode and the Fe-doped CuBi2O4 (CBFO) photocathode is developed for the sensitive detection of TC. The photocathodic current can be amplified by the efficient electron transfer caused by the Fermi energy level gap between the photoanode and photocathode. Furthermore, molecularly imprinted polymers (MIPs) at photocathode can selectivity identify the TC templates and thus improve the specificity. Under the optimal conditions, the sensor has a linear range of 10‒2–1.0 × 105 nmol/L, and a limit of detection (LOD) of 0.007 nmol/L (S/N = 3). More crucially, the milk sample detection is carried out using the as-prepared sensor, and the outcome is satisfactory. The research gives us a novel sensing platform for quick and accurate antibiotic (like TC) in environment and food monitoring.

CdS quantum dots sensitized Ti-MOFs as high-performance photocathodic materials for immunoassay of alpha-fetoprotein based on Ag+ exchange strategy

Cathodic photoelectrochemical (PEC) analysis has drawn much attention because of its excellent anti-interference capability toward reductive substances in samples, but the development high-performance photocathodic materials is challenging. We report here CdS quantum dots (QDs) sensitized titanium-based metal–organic frameworks (Ti-MOFs) as high-performance photocathodic materials for ultrasensitive immunoassay of alpha-fetoprotein (AFP) based on Ag+ exchange strategy. Due to the excellent sensitization effect of CdS QDs, the photocathodic current of CdS QDs decorated MIL-125-NH2(Ti) is 25-fold that of MIL-125-NH2(Ti) alone. The photocathodic current of CdS/MIL-125-NH2(Ti) can be quenched after Ag+ exchange, because CdS is partially converted to Ag2S that serves as a charge-recombination center. Based on the PEC response of CdS/MIL-125-NH2(Ti) to Ag+, a cathodic PEC immunoassay platform is constructed for AFP detection by using CdS/MIL-125-NH2(Ti) as the photoactive material and Ag nanoparticles modified SiO2 nanospheres as labels. The linear range for AFP detection is from 1.0 × 10−4 to 1.0 × 102 ng mL−1, and the detection limit is 20 fg mL−1. The constructed PEC immunosensor is successfully used for detecting AFP in human serum samples. In this work, a novel and high-performance photoactive material is developed for cathodic PEC analysis, and a simple and effective PEC immunoassay platform for AFP detection is constructed based on Ag+ exchange strategy.

High performance BiFeO3 ferroelectric nanostructured photocathodes.

Ferroelectric materials may be used as effective photoelectrocatalysts for water splitting due to enhanced charge carrier separation driven by their spontaneous polarization induced internal electric field. Compared to other ferroelectric materials, BiFeO3 exhibits a high catalytic efficiency due to its comparatively smaller bandgap, which enables light absorption from a large part of the solar spectrum and its higher bulk ferroelectric polarization. Here, we compare the photoelectrochemical properties of three different BiFeO3 morphologies, namely, nanofibers, nanowebs, and thin films synthesized via electrospinning, directly on fluorine-doped tin oxide (FTO) coated glass substrates. A significant photocathodic current in the range from -86.2 to -56.5 μA cm-2 at -0.4 V bias (vs Ag/AgCl) has been recorded for all three morphologies in 0.1M Na2SO4 aqueous solution (pH = 6.8). Among these morphologies, BiFeO3 nanofibers exhibit higher efficiency because of their larger surface area and improved charge separation resulting from rapid diffusion of photoinduced charge carriers along the axis of the nanofiber. In the case of BiFeO3 nanofibers, we obtained the highest photocurrent density of -86.2 µA/cm2 at -0.4 V bias (vs Ag/AgCl electrode) and an onset potential of 0.22 V. We also observed that the onset potential of the photocathodic current can be increased by applying a positive polarization voltage, which leads to favorable bending of band edges at the electrode/electrolyte interface resulting in increased charge carrier separation.

Hierarchical SnS2/CuInS2 Nanosheet Heterostructure Films Decorated with C60 for Remarkable Photoelectrochemical Water Splitting.

Rational architectural design and catalyst components are beneficial to improve the photoelectrochemical (PEC) performance. Herein, hierarchical SnS2/CuInS2 nanosheet heterostructure porous films were fabricated and decorated with C60 to form photocathodes for PEC water reduction. Large-size CuInS2 nanosheet films were first grown on transparent conducting glass to form substrate films. Then, small-size SnS2 nanosheets were epitaxially grown on both sides of the CuInS2 nanosheets to form uniform hierarchical porous laminar films. The addition of C60 on the surface of the SnS2/CuInS2 porous nanosheets effectively increased visible light absorption of the composite photocathode. Photoluminescence spectroscopy and impedance spectroscopy analyses indicated that the formation of a SnS2/CuInS2 heterojunction and decoration of C60 significantly increased the photocurrent density by promoting the electron-hole separation and decreasing the resistance to the transport of charge carriers. The hierarchical SnS2/CuInS2 nanosheet heterostructure porous films containing multiscale nanosheets and pore configurations can enlarge the surface area and enhance visible light utilization. These beneficial factors make the optimized C60-decorated SnS2/CuInS2 photocathode exhibit much higher photocathodic current (4.51 mA cm-2 at applied potential -0.45 V vs reversible hydrogen electrode ) and stability than the individual CuInS2 (2.58 mA cm-2) and SnS2 (1.92 mA cm-2) nanosheet film photocathodes. This study not only reveals the promise of C60-decorated hierarchical SnS2/CuInS2 nanosheet heterostructure porous film photocathodes for efficient solar energy harvesting and conversion but also provides rational guidelines in designing high-efficiency photoelectrodes from earth-abundant and low-cost materials allowing widely practical applications.

Integrating Water-Soluble Polythiophene with Transition-Metal Dichalcogenides for Managing Photoinduced Processes.

Transition-metal dichalcogenides (TMDs) attract increased attention for the development of donor-acceptor materials enabling improved light harvesting and optoelectronic applications. The development of novel donor-acceptor nanoensembles consisting of poly(3-thiophene sodium acetate) and ammonium functionalized MoS2 and WS2 was accomplished, while photoelectrochemical cells were fabricated and examined. Attractive interactions between the negatively charged carboxylate anion on the polythiophene backbone and the positively charged ammonium moieties on the TMDs enabled in a controlled way and in aqueous dispersions the electrostatic association of two species, evidenced upon titration experiments. A progressive quenching of the characteristic fluorescence emission of the polythiophene derivative at 555 nm revealed photoinduced intraensemble energy and/or electron transfer from the polymer to the conduction band of the two TMDs. Photoelectrochemical assays further confirmed the establishment of photoinduced charge-transfer processes in thin films, with distinct responses for the MoS2- and WS2-based systems. The MoS2-based ensemble exhibited enhanced photoanodic currents offering additional channels for hole transfer to the solution, whereas the WS2-based one displayed increased photocathodic currents providing supplementary pathways of electron transfer to the solution. Moreover, scan direction depending on photoanodic and photocathodic currents suggested the existence of yet unexploited photoinduced memory effects. These findings highlight the value of electrostatic interactions for the creation of novel donor-acceptor TMD-based ensembles and their relevance for managing the performance of photoelectrochemical and optoelectronic processes.

Insights on the Origination of Ambipolar Photocurrent of Ferroelectric and the Improvement of Photoanodic Current

Modulating the photoelectrochemical (PEC) reactions on one photoelectrode is of significance to the photoelectrochemistry. Herein, we fabricate ferroelectric photoelectrode based on Pb(Zr, Ti)O3 (PZT) without polarization that shows tunable PEC reactions in the form of ambipolar photocurrent, and demonstrate that the interesting behavior originates from the random distribution of ferroelectric domains and a selective transport mechanism of charge carriers to the surface with electrolyte under different potentials. The photoanodic current could be realized as 4.55 μA/cm2 at 0.8 V vs Ag/AgCl and the photocathodic current could be obtained as 16.3 μA/cm2 at −0.3 V vs Ag/AgCl. In addition, by inserting a thin layer of Au between PZT and FTO, the photoanodic current could be enhanced to 114 μA/cm2 at 0.8 V vs Ag/AgCl. This obvious enhancement is on account of the multiple effects of the contact between Au and PZT, which include the rectification effect by the formed Ohmic contact, the role of surface plasmon resonance and the reduction of the resistance. Thus, a new PEC system for tuning photoelectrochemical reactions smartly and efficiently is proposed.

Optimal Metal Oxide Deposition Conditions and Properties for the Enhancement of Hydrogen Evolution over Particulate La5Ti2Cu1−xAgxS5O7 Photocathodes

AbstractThis work examined the effects of the thickness and annealing conditions of TiO2, Nb2O5 and Ta2O5 layers deposited by radio‐frequency reactive magnetron sputtering over La5Ti2Cu1−xAgxS5O7 (LTC1−xAx) photocathodes on photoelectrochemical (PEC) H2 evolution performance. Annealing of the oxide‐deposited LTC1−xAx photocathodes in ambient air enhances PEC H2 evolution, whereas annealing in an inert or reductive atmosphere does not. Annealing in ambient air converts the as‐deposited oxygen‐deficient conductive oxides into semiconducting oxides that function as hole‐blocking layers. The photocathodic current associated with PEC H2 evolution was enhanced to the greatest extent when oxide layers 2 nm in nominal thickness were annealed in air at 473 K. This study determined the key metal oxide properties essential to enhancing PEC H2 evolution over particulate LTC1−xAx photocathodes, and is expected to assist in the development of particulate photoelectrodes for solar H2 production.

Enhancement of Charge Separation and Hydrogen Evolution on Particulate La5Ti2CuS5O7 Photocathodes by Surface Modification.

Particulate La5Ti2CuS5O7 (LTC) photocathodes prepared by particle transfer show a positive onset potential of 0.9 V vs RHE for the photocathodic current in photoelectrochemical (PEC) H2 evolution. However, the low photocathodic current imposes a ceiling on the solar-to-hydrogen energy conversion efficiency of PEC cells based on LTC photocathodes. To improve the photocurrent, in this work, the surface of Mg-doped LTC photocathodes was modified with TiO2, Nb2O5, and Ta2O5 by radio frequency reactive magnetron sputtering. The photocurrent of the modified Mg-doped LTC photocathodes was doubled because these oxides formed type-II heterojunctions and extended the lifetimes of photogenerated charge carriers. The enhanced photocathodic current was attributed to hydrogen evolution at a positive potential of +0.7 V vs RHE. This work opens up possibilities for improving PEC hydrogen evolution on particulate photocathodes based on surface oxide modifications and also highlights the importance of the band gap alignment.

Catechol-TiO2 hybrids for photocatalytic H2 production and photocathode assembly.

Visible-light driven H2 evolution in water is achieved using catechol-photosensitised TiO2 nanoparticles with a molecular nickel catalyst. Layer-by-layer immobilisation of catechol-TiO2 onto tin-doped indium oxide electrodes generates photocathodic currents in the presence of an electron acceptor. This approach represents a new strategy for controlling photocurrent direction in dye-sensitised photoelectrochemical applications.

Synthesis and their enhanced photoelectrochemical performance of ZnO nanoparticle-loaded CuO dandelion heterostructures under solar light

Here we report an easy and large-scale synthesis of three-dimensional (3D) ZnO nanoparticle-loaded CuO dandelion (denoted as n-ZnO/p-CuO nanoparticle/dandelion) heterostructures and their photoelectrochemical (PEC) water splitting under simulated solar light illumination. CuO dandelions were fabricated by a facile and cost-effective chemical strategy, in which the ribbon-like CuO nanoplates were first formed and then assembled into dandelion-like architectures. ZnO nanoparticle-loaded CuO dandelion heterostructures were fabricated by calcining Zn(Ac)2-loaded CuO dandelions. High resolution transmission electron microscope (HRTEM) studies demonstrate that intimate p-n junction is built between p-CuO and n-ZnO interface. The n-ZnO/p-CuO nanoparticle/dandelion photoelectrodes exhibit significant improvement in PEC water splitting to CuO dandelion photoelectrodes. The correlation between photocurrents and different loading contents of ZnO nanoparticles (NPs) is studied in which the n-ZnO/p-CuO nanoparticle/dandelion heterostructures with loading 4.6wt% ZnO NPs show higher photocathodic current. The efficient separation of the photogenerated electrons and holes driven by the intimate p-n junction between p-type CuO and n-type ZnO interface is mainly contributed to the enhanced photoanode current. The achieved results in the present study offer a very useful strategy for designing p-n junction photoelectrodes for efficiency and low-cost PEC cells for clean solar hydrogen production.

Photoelectrochemical Hydrogen Production Using a CaFe2O4 Electrode Coated with TiO2 As a Surface Protective Layer

CaFe2O4 is a p-type semiconductor with a bandgap of 1.9 eV that can function as a photocathode for photoelectrochemical (PEC) hydrogen production from water. However, there is one issue in that the CaFe2O4 photocathode is likely to decompose during the PEC reaction, which results in a decrease in the photocathodic current. In this study, a TiO2 thin layer was coated on the CaFe2O4 surface by pulsed laser deposition to suppress decomposition.The onset potential was slightly shifted to the negative side by coating of the photocathode with TiO2. Although the photocathodic current was slightly decreased compared with the CaFe2O4 electrode without the TiO2 layer, the decrease in the photocurrent during the PEC hydrogen production reaction was suppressed. In the case of amorphous-like TiO2-coated CaFe2O4, the photocurrent was increased during the PEC hydrogen production reaction. The ratio of the photocurrent after 6 h reaction to the photocurrent at the initial stage of the reaction (5 min) was 120/100. After the PEC reaction, there were no cracks evident on the electrode surface, and the surface morphology was almost the same as the initial morphology. This result indicates that surface degradation of the CaFe2O4 electrode can be suppressed by coating with a thin TiO2 layer.

Indium tin oxide as a semiconductor material in efficient p-type dye-sensitized solar cells

Indium tin oxide (ITO) is a well-known n-type degenerate semiconductor. Herein, mesoporous ITO is utilized as a photocathode material for p-type dye-sensitized solar cells in place of the commonly applied p-type semiconductors, such as nickel oxide. In conjunction with [Fe(acac)3]0/− as redox mediator and a new sensitizer, an impressive energy conversion efficiency of 1.96±0.12% was achieved. Photoelectron spectroscopy in air revealed that ITO exhibits a significant local density of states arising below −4.8 eV, which enables electron transfer to occur from ITO to the excited dye, giving rise to the sustained photocathodic current.

Overall Water Splitting By Series-Connected La5Ti2(Cu,Ag)S5O7 Photocathode and BaTaO2 N Photoanode Utilizing Visible Light up to 650nm

Introduction. A p/n photoelectrochemical (PEC) system composed of series-connected photocathode and photoanode is highly desirable for solar-energy conversion to storable and conveyable hydrogen via overall water splitting. To achieve a solar-to-hydrogen conversion efficiency (STH) as high as or higher than 10%, it is needed to use photocatalytic materials permitting visible light absorption up to >650 nm. We have recently reported that a p/n PEC system composed of a La5Ti2(Cu,Ag)S5O7 (LTCA) photocathode[1] and a BaTaO2N (BTON) photoanode[2], both of which have photoactivity under irradiation up to 650 nm, is capable of spontaneous overall water splitting under visible light irradiation without external bias voltage[3]. The STH of the p/n PEC system of LTCA and BTON was approximately 0.1 % in the initial stage[3]. In the p/n PEC system of LTCA-BTON, the photocurrent and the working-potential (E work) are determined by the intersection of the current-potential curves of the respective LTCA photocathode and the BTON photoanode used. Therefore, an increment of the photocurrent and a high stability in aqueous electrolyte solutions at E work are important. Intriguingly, both the water reduction reaction on LTCA photocathodes and the water oxidation reaction on BTON photoanodes are strongly affected by ionic species existing in the electrolyte solution. We will demonstrate in this paper the effect of coexistent ions in aqueous solutions and surface modification of the LTCA and BTON photoelectrodes upon the performance of the p/n PEC systems in the overall water splitting reaction. Experimental. LTCA photocathodes and BTON photoanodes were prepared using powder materials through the particle transfer method[4]. For LTCA photocathodes, Pt catalyst promoting hydrogen evolution reaction was deposited by potentiostatic photodeposition under irradiation of simulated sunlight (AM1.5G) [3]. For BTON photoanodes, CoPi catalysts for oxygen evolution reaction was deposited on the surface by electrodeposition[2]. The PEC measurement was carried out using an Ag/AgCl/sat’d KCl reference electrode and a coiled Pt wire counter electrode in an Ar gas atmosphere at room temperature. To examine the effect of coexistent ions in aqueous solution, M2SO4, MClO4, and M2HPO4 (M = Li, Na, K) were used as supporting electrolytes. The pH of aqueous solutions was adjusted to 11–13 by an aqueous MOH (M = Li, Na, K) solution. A p/n PEC system was prepared by connecting a LTCA photocathode and a BTON photoanode fabricated individually. The p/n PEC system was irradiated with visible light from a 300 W Xe lamp through a cut-off filter (l > 420 nm). The amounts of evolved H2 and O2 were quantified with a gas chromatograph. Results and Discussion. The photocurrents of LTCA photocathodes at 0.7 VRHE, typical E work for p/n PEC systems of LTCA-BTON, increased in the order of Li+ > Na+ > K+ and ClO4 - > SO4 2- >> PO4 3- when coexistent cations and anions were changed, respectively. The photocurrents of BTON photoanodes at 0.7 VRHE increased in the order of K+ > Na+ > Li+ and PO4 3- > (ClO4 - ~ SO4 2-) for variation of cationic and anionic species, respectively. It is likely that the dependence of the photocurrent of the respective photoelectrodes on the coexistent ions is attributable in part to the specific adsorption of ions. For the p/n PEC systems of LTCA-BTON, LiClO4 was employed as a supporting electrolyte because the LTCA photocathode in the presence of Li+ and ClO4 - generated the high photocathodic current. A p/n PEC system composed of a LTCA photocathode and a BTON photoanode in 0.1 M LiClO4 + LiOH (pH = 12) was active in overall water splitting without external voltage. Hydrogen and oxygen were generated at the stoichiometric ratio of 2:1 over 20 h. In this presentation, the effect of surface modification for respective photoelectrodes on the performance of the p/n PEC system will be discussed in detail. References. [1] J. Liu et al., Energy Environ. Sci., 7, 2239-2242 (2014). [2] K. Ueda et al., J. Am. Chem. Soc., 137, 2227-2230 (2015). [3] T. Hisatomi et al., Energy Environ. Sci., 8, 3354-3362, (2015). [4] T. Minegishi et al., Chem. Sci., 4, 1120-1124 (2013).

Efficient p-zinc phthalocyanine/n-fullerene organic bilayer electrode for molecular hydrogen evolution induced by the full visible-light energy

A photoelectrode featuring an organic p/n bilayer was used to generate H2 from H+, where n-type fullerene (C60) was used in combination with p-type zinc phthalocyanine (ZnPc). When a Pt catalyst was loaded on the surface of C60 in the organo-photoelectrode, a photocathodic current was successfully generated via H2 evolution at the Pt-coated C60 surface. On the basis of the results of action spectral measurements for photocurrent, the H2 evolution appeared to be induced over the entire visible-light wavelength range of <750 nm. In particular, the visible-light absorption of ZnPc/C60 bilayer efficiently contributed to carrier generation, consequently leading to the H2 evolution at the C60 surface via reducing power generated through a series of photophysical events within the bilayer.

A Novel Photoelectrochemical Secondary Battery Featuring Phthalocyanine Photocathode Toward the Application to Ubiquitous Sensors

The conventional photovoltaic cell can be one of the most suitable energy sources for driving ubiquitous sensors when it also has the function as a secondary battery in the dark. However, such a system tends to require high cost and is also of difficulty in the viewpoint of miniaturization. Alternatively, a photoelectrochemical cell can offer a simple and promising battery originating in chemical reaction under solar irradiation. In this study, we proposed a novel photoelectrochemical battery (denoted as PEB) with a photoelectrode of metal-free phthalocyanine (denoted as H2Pc, a p-type semiconductor) deposited on ITO. H2Pc has a great potential in terms of an efficient absorption over the visible-light energy range, low cost, harmlessness, and ease of deposition, which can be expected to be an ideal photoelectrode for versatile PEB. When the photoelectrode was irradiated using the visible-light energy of λ＞500 nm, FeIII(CN)6 3– (electron acceptor) was reduced at the H2Pc surface, whereas FeII(CN)6 4– (electron donor) was concurrently oxidized at the counter electrode. This result indicates that H2Pc worked as the photocathode (i.e., H2Pc induced the generation of photocathodic currents for charging the PEB under the visible-light irradiation). The photocurrent density during charging was ca. 30 μA/cm2. Furthermore, when turning off the light, discharge occurred between the counter electrode and the discharge electrode. Such charge-discharge phenomena were confirmed to occur repeatedly. These results showed that the PEB featuring H2Pc photoelectrode can be an attractive electric power source toward ubiquitous sensors. In addition to the details of the aforementioned results, the origin of the present photocurrent will be discussed in terms of the FeIII(CN)6 3– reduction at the H2Pc photoelectrode. Figure 1

Photoelectrochemical hole injection revealed in polyoxotitanate nanocrystals functionalized with organic adsorbates.

We find that crystallographically resolved Ti17O24(OPr(i))20 nanoparticles, functionalized by covalent attachment of 4-nitrophenyl-acetylacetonate or coumarin 343 adsorbates, exhibit hole injection into surface states when photoexcited with visible light (λ = 400-680 nm). Our findings are supported by photoelectrochemical measurements, EPR spectroscopy, and quantum dynamics simulations of interfacial charge transfer. The underlying mechanism is consistent with measurements of photocathodic currents generated with visible light for thin layers of functionalized polyoxotitanate nanocrystals deposited on FTO working electrodes. The reported experimental and theoretical analysis demonstrates for the first time the feasibility of p-type sensitization of TiO2 solely based on covalent binding of organic adsorbates.

Langmuir-Blodgett films of self-assembled (alkylether-derivatized Zn phthalocyanine)-(C₆₀ imidazole adduct) dyad with controlled intermolecular distance for photoelectrochemical studies.

A multilayer Langmuir-Blodgett (LB) film of the self-assembled electron donor-acceptor dyad of Zn phthalocyanine, appended with four long-chain aliphatic ether peripheral substituents, and an imidazole adduct of C60 was prepared and applied as a photoactive material in a photoelectrochemical cell. Changes in the simultaneously recorded surface pressure and surface potential vs area per molecule compression isotherms for Langmuir films of the dyad and, separately, of its components helped to identify phase transitions and mutual interactions of molecules in films. The Brewster angle microscopy (BAM) imaging of the Langmuir films showed circular condensed phase domains of the dyad molecules. The determined area per molecule was lower than that estimated for the dyad and its components, separately. The multilayer LB films of the dyad were transferred onto hydrophobized fluorine-doped tin oxide-coated (FTO) glass slides under different conditions. The presence of both components in the dyad LB films was confirmed with the UV-vis spectroscopy measurements. For the LB films transferred at different surface pressures, the PM-IRRAS measurements revealed that the phthalocyanine macrocycle planes and ether moieties in films were tilted with respect to the FTO surface. The AFM imaging of the LB films indicated formation of relatively uniform dyad LB films. Then, the femtosecond transient absorption spectral studies evidenced photoinduced electron transfer in the LB film. The obtained transient signals corresponding to both Zn(TPPE)(•+) and C60im(•-) confirmed the occurrence of intramolecular electron transfer. The determined rate constants of charge separation, kcs = 2.6 × 10(11) s(-1), and charge recombination, kcr = 9.7 × 10(9) s(-1), indicated quite efficient electron transfer within the film. In the photoelectrochemical studies, either photoanodic or photocathodic current was generated depending on the applied bias potential when the dyad LB film-coated FTO was used as the working electrode and ascorbic acid or methylviologen, respectively, as the charge mediator in an aqueous solution.

ZnO/CuO Heterojunction Branched Nanowires for Photoelectrochemical Hydrogen Generation

We report a facile and large-scale fabrication of three-dimensional (3D) ZnO/CuO heterojunction branched nanowires (b-NWs) and their application as photocathodes for photoelectrochemical (PEC) solar hydrogen production in a neutral medium. Using simple, cost-effective thermal oxidation and hydrothermal growth methods, ZnO/CuO b-NWs are grown on copper film or mesh substrates with various ZnO and CuO NWs sizes and densities. The ZnO/CuO b-NWs are characterized in detail using high-resolution scanning and transmission electron microscopies exhibiting single-crystalline defect-free b-NWs with smooth and clean surfaces. The correlation between electrode currents and different NWs sizes and densities are studied in which b-NWs with longer and denser CuO NW cores show higher photocathodic current due to enhanced reaction surface area. The ZnO/CuO b-NW photoelectrodes exhibit broadband photoresponse from UV to near IR region, and higher photocathodic current than the ZnO-coated CuO (core/shell) NWs due to improved surface area and enhanced gas evolution. Significant improvement in the photocathodic current is observed when ZnO/CuO b-NWs are grown on copper mesh compared to copper film. The achieved results offer very useful guidelines in designing b-NWs mesh photoelectrodes for high-efficiency, low-cost, and flexible PEC cells using cheap, earth-abundant materials for clean solar hydrogen generation at large scales.

3D Branched Nanowire Photoelectrochemical Electrodes for Efficient Solar Water Splitting

We report the systematic study of 3D ZnO/Si branched nanowire (b-NW) photoelectrodes and their application in solar water splitting. We focus our study on the correlation between the electrode design and structures (including Si NW doping, dimension of the trunk Si and branch ZnO NWs, and b-NW pitch size) and their photoelectrochemical (PEC) performances (efficiency and stability) under neutral conditions. Specifically, we show that for b-NW electrodes with lightly doped p-Si NW core, larger ZnO NW branches and longer Si NW cores give a higher photocathodic current, while for b-NWs with heavily doped p-Si NW trunks smaller ZnO NWs and shorter Si NWs provide a higher photoanodic current. Interestingly, the photocurrent turn-on potential decreases with longer p-Si NW trunks and larger ZnO NW branches resulting in a significant photocathodic turn-on potential shift of ~600 mV for the optimized ZnO/p-Si b-NWs compared to that of the bare p-Si NWs. A photocathode energy conversion efficiency of greater than 2% at -1 V versus Pt counter electrode and in neutral solution is achieved for the optimized ZnO/p-Si b-NW electrodes. The PEC performances or incident photon-to-current efficiency are further improved using Si NW cores with smaller pitch size. The photoelectrode stability is dramatically improved by coating a thin TiO2 protection layer using atomic-layer deposition method. These results provide very useful guidelines in designing photoelectrodes for selective solar water oxidation/reduction and overall spontaneous solar fuel generation using low cost earth-abundant materials for practical clean solar fuel production.

Mg-Doped CuFeO2 Photocathodes for Photoelectrochemical Reduction of Carbon Dioxide

Mg-doped CuFeO2 delafossite is reported to be photoelectrochemically active for CO2 reduction. The material was prepared via conventional solid-state methods, and subsequently assembled into an electrode as a pressed pellet. Addition of a Mg2+ dopant is found to substantially improve the conductivity of the material, with 0.05% Mg-doped CuFeO2 electrodes displaying photocathodic currents under visible irradiation. Photocurrent is found to onset at irradiation wavelengths of ∼800 nm with the incident photon-to-current efficiency reaching a value of 14% at 340 nm using an applied electrode potential of −0.4 V vs SCE. Photoelectrodes were determined to have a −1.1 V vs SCE conduction band edge and were found capable of the reduction of CO2 to formate at 400 mV of underpotential. The conversion efficiency is maximized at −0.9 V vs SCE, with H2 production contributing as a considerable side reaction. These results highlight the potential to produce Mg-doped p-type metal oxide photocathodes with a band structur...