Visible-light Responsive Photoanode Research Articles

MOFs derived CdS/CdO heterojunction photoanode for high-efficient water splitting

Enabling the photogenerated electron-hole separation of visible light responsive photoanode is essential to the conversion and utilization of solar energy in the photoelectrochemical (PEC) water splitting. In this study, porous CdS/CdO-M heterojunction photoanodes on fluorine-doped tin oxide glasses were prepared by calcining the Cd-metal–organic framework (Cd-MOF) and loading the CdS via successive ionic layer adsorption and reaction method. The heterojunction and pores in these prepared photoanodes promoted the separation and transfer of photogenerated electron-hole in the PEC water splitting. The optimized photoanode (CdS/CdO-M−15) exhibited superior photoelectrochemical water splitting performance with a photocurrent density of 4.8 mA cm−2, which is 20, 6 and 1.5 times higher than that of CdO-M, CdS and CdS/CdO-C-15 (prepared by the commercial CdO). The applied bias photon-to-current conversion efficiency of CdS/CdO-M−15 was as highly as 4.5 % under simulated solar light of 100 mW cm−2 in 0.25 M Na2S + 0.35 M Na2SO3 aqueous solution. Moreover, CdS/CdO-M−15 was stable under continuous illumination, and its photocurrent density for it was well maintained after 1 h illumination.

Hot electron injection and electron relay integrated into PbS/Au/ZnO sandwich nanotube arrays boost visible-light responsive photoelectric property

Abstract In this study, by integrating Schottky heterojunction and electron relay into a ternary heterostructure, we demonstrate a visible light responsive photoanode of PbS/Au/ZnO sandwich nanotube arrays (NTAs) for photoelectrochemical water splitting. The localized surface plasmon resonance (LSPR) of Au nanoparticles (NPs) and the superior visible light harvesting ability of PbS NPs enable the fabricated PbS/Au/ZnO sandwich NTAs exhibit substantially improved visible light absorption from 400 to 800 nm. Experimental measurements reveal that the PbS/Au/ZnO sandwich nanotube array photoanode presents significantly enhanced photoelectrochemical (PEC) capability for water splitting. The photocurrent density of the PbS/Au/ZnO sandwich NTAs increases by 6 and 2 times, respectively, in contrast with the Au/ZnO and PbS/ZnO nanotube array photoanodes. Furthermore, the photoconversion efficiency is 18 and 5 times that of the Au/ZnO and PbS/ZnO heterojunction nanotube array photoanodes, respectively. Comparative studies for the PEC water splitting apparently demonstrate that the Au NPs plays two crucial roles as a provider to inject hot electrons into the conduction band of ZnO and an electron relay to transfer the photoinduced electrons from PbS to ZnO within the PbS/Au/ZnO sandwich NTAs. The synergetic effects of the LSPR and electron relay of the Au nanoparticle enable the photoexcited electrons within the PbS/Au/ZnO sandwich NTAs to be effectively used to conduct water splitting reaction, leading to drastically enhanced PEC property.

Anion-Exchange Membrane Electrode Assembled Photoelectrochemical Cell with a Visible Light Responsive Photoanode for Simultaneously Treating Wastewater and Generating Electricity

In this work, an anion-exchange membrane electrode assembled photoelectrochemical cell is designed for simultaneously degrading organics and generating electricity. The proposed photoelectrochemica...

Enhanced photoelectrochemical charge transfer on Mn-doped CdS/TiO2 nanotube arrays: The roles of organic substrates

The application of photoelectrochemical (PEC) systems to harness solar energy while treating wastewater or biomass-laden streams is an emerging area of sustainability. On this basis, we investigate the Mn-doped CdS quantum dots supported on TiO2 nanotube arrays (TNT) as the visible light-responsive photoanode. The introduction of Mn-dopant (optimum Mn:Cd = 0.15) enhanced the photoelectron lifetime of the composite photoanode. This made available more photoholes for surface transfer, as reflected by the increase in photocurrent. The addition of organic compounds drastically enhanced the photocurrent, given their faster oxidation rates compared to water. Depending on the type of organic compounds, whether they are direct hole scavengers (formic acid, diethanolamine, triethanolamine) or hydroxyl radical scavengers (methanol, glucose, urea), tend to impart different extent of photocurrents enhancement. While the measured photocurrent was in most cases higher for Mn-doped CdS/TNT than the undoped composite, we found that direct hole scavengers were up to an order of magnitude more efficient than the hydroxyl radical scavengers in promoting surface holes transfer. Interestingly, the Mn-dopant was highly beneficial in improving the oxidation of hydroxyl radical scavengers, presumably due to the enhanced charge carrier lifetime that was necessary for more efficient production of hydroxyl radicals. For the first time, the current doubling-like effects were verified for compounds with alcohol functional groups through the measurements of incident photon-to-current conversion efficiencies (IPCE) above unity.

A visible-light responsive micro photocatalytic fuel cell with laterally arranged electrodes

A micro photocatalytic fuel cell with a visible-light responsive photoanode and the lateral arrangement of the electrodes at the same plane was developed to enable simultaneous organics degradation and electricity generation. The developed micro photocatalytic fuel cell was assessed by using methanol as a representative organic compound in the alkaline environment. It is shown that good visible-light response was achieved. The effects of the light intensity, methanol concentration, KOH concentration and liquid flow rate were also explored. Experimental results showed that when the light intensity was increased, the cell performance was improved due to more photo-excited electron-hole pairs. Upgrading the KOH concentration led to the increased performance due to more efficient capture of the holes and enhanced cathodic reaction and ion transport. The increase of the methanol concentration in the testing range led to the improved performance as a result of the enhanced mass transport. The discharging performance was firstly increased and then decreased with increasing the liquid flow rate due to the competition of enhanced mass transfer and decreased residence time and increased methanol crossover.

Photoanode-Driven Photoelectrochemical CO2 Reduction By Using the TiO2 Photoanodes and Different Metal Counter Electrodes(Pt, Ti, Cu, Sn, Zn)

Introduction Recently, global warming has become a serious problem all over the world. Being a greenhouse gas, the high levels of CO2 are a major contributor toward global warming. For address this problem, the photoelectrochemical CO2 reduction by using semiconductor photoelectrodes has attracted much attention as a potential means of converting solar energy into chemical energy in the form of usable organic fuels (CO, HCOOH, CH3OH, CH4, etc.). Titanium oxide (TiO2) has been a sustainable photocatalyst for environmental remediation because of its nontoxicity, availability, high oxidative potential and chemical stability. It is well-known that the TiO2 has three polymorphs: rutile, anatase, and brookite. In previous work, we have successfully synthesized the rutile and brookite TiO2 nanorods, which showed high levels of activity for degradation of 2-propanol and acetaldehyde under UV irradiation compared to the activity levels of anatase fine particles (ST-01). Therefore, in this study, we fabricated the TiO2 photoelectrodes by using the rutile and brookite nanorods, and investigated their photoelectrochemical properties under UV light irradiation. Moreover, we performed photoanode-driven photoelectrochemical CO2 reduction by using the TiO2 photoanodes and different metal counter electrodes (Pt, Ti, Cu, Sn, Zn) under UV light irradiation. Experimental Two polymorphs of TiO2 nanorods (rutile and brookite) were prepared by hydrothermal synthesis method, which described in our previously reports [1],[2]. Their photoelectrodes were fabricated by electrophoresis deposition method on the FTO conducting glasses. To improve photocurrent response, the necking treatment of the TiO2 photoelectrode was demonstrated: 0.01M titanium(IV) isopropoxide solution was dropped on the TiO2 electrode and then annealed in air at 400 ℃. The photoelectrochemical properties were investigated a three−electrode configuration using a silver−silver chloride (Ag/AgCl) reference electrode and different metal counter electrodes (Pt, Ti, Cu, Sn, Zn). The electrolyte was 0.1M KHCO3 aqueous solution. The electrolyte was stirred and purged with CO2 gas for 30 min before measurement. The photoelectrochemical CO2 reduction was carried out by using Xe lamp irradiation (100 mW/cm2, λ<800 nm). After photo-irradiation for 1h, the CO2 reduction products were evaluated by using gas-chromatography and ion chromatography. Results and discussion We investigated the photoelectrochemical CO2 reduction by using TiO2 photoanode and various metal counter electrodes (Pt, Ti, Cu, Sn, Zn). The CO2 reduction products were strongly dependent on the metal counter electrodes. In the case of Pt counter electrode, H2 was main product, and no CO2 reduction products such as CO, HCOOH, and CH3OH, were detected. This is because Pt has low overpotential for H2 generation compared to the other four metals (Ti, Cu, Sn, Zn). On the other hand, CO and/or HCOOH generation was generated when other four metals (Ti, Cu, Sn, Zn) was used as counter electrode. This is due to a relatively high overpotential for H2 generation. Among them (Ti, Cu, Sn, Zn), the Sn counter electrode showed the highest CO and HCOOH generation, which mechanism will be discussed in poster section. As described above, the photoelectrochemical CO2 reduction was demonstrated by using TiO2 photoanode combined with various metal counter electrodes under UV light irradiation. However, one fault of TiO2 is that it is inactive in the absence of ultraviolet (UV) light because of its large band gap (~3.0 eV for rutile and 3.2 eV for brookite). As nearly 50% of the incident solar energy on the earth’s surface falls within the visible light energy range, visible light-responsive photoanodes are strongly demanded. Therefore, we also introduced the photoelectrochemical properties of black TiO2 which could be synthesized by thermal annealing in reducing atmosphere. The detail information will be discussed in poster section.

Fe/W Co-Doped BiVO4 Photoanodes with a Metal-Organic Framework Cocatalyst for Improved Photoelectrochemical Stability and Activity.

BiVO4 has been identified as one of the excellent visible light responsive photoanodes for use in photoelectrochemical (PEC) water splitting. However, pristine BiVO4 usually exhibits relative low photocatalytic properties owing to insufficient charge separation and transport characteristics. Although the marginal n-type doping of higher valence ions can obviously raise the photocurrent value, it by no means improves the PEC stability. In this work, we successfully enhanced the PEC stability of BiVO4 by doping Fe ions in substitution of Bi. Density functional theory calculations have illustrated that Fe-doping would result in an impurity band in the forbidden gap, and thus narrow its energy gap. More importantly, Fe-doping can synergize with other means to further improve the PEC activities of BiVO4 . Therefore, we fabricated a nanoporous Fe/W co-doped BiVO4 photoelectrode, and then loaded the metal-organic framework (MOF) MIL-100(Fe) as cocatalyst to further promote the separation of charge carriers. To the best of our knowledge, MOFs have not yet been utilized as a cocatalyst to facilitate the charge separation, which could increase the photocurrent density of Fe/W co-doped BiVO4 .

Respective electrode potential characteristics of photocatalytic fuel cell with visible-light responsive photoanode and air-breathing cathode

Photocatalytic fuel cell is a promising technology for simultaneous wastewater treatment and electricity generation. To more directionally improve the cell performance, a deep understanding of respective photoanode and cathode performances is of vital importance. In this work, therefore, a high-performance photocatalytic fuel cell with a visible-light responsive photoanode and an air-breathing cathode was developed to gain insight into each electrode potential characteristics. The potentials of each electrode were measured under different operational parameters. Experimental results showed that too low fuel concentration led to a serious mass transfer limitation at the photoanode and too high fuel concentration led to a low ionic conductivity, both of which deteriorated the cell performance. Therefore, there existed an optimum fuel concentration to achieve the best cell performance. It was also shown that an increase in the electrolyte concentration markedly improved both the photoanode and cathode performances due to enhanced reaction rates and lowered internal resistance, thereby leading to the increased cell performance. In addition, it was demonstrated that the light intensity significantly affected the photoanode potential and increasing the light intensity boosted the cell performance. The results obtained in this work provide the guidance for improving the photocatalytic fuel cell performance in future applications.

Photoelectrocatalytic hydrogen production by water splitting using BiVO4 photoanodes

Nanocrystalline bismuth vanadate was deposited by simple wet chemistry procedure on FTO electrodes in order to construct visible light responsive photoanodes, which were employed for photoelectrochemical hydrogen production by water splitting. The specific surface area of the films was controlled by the presence of a surfactant template (Triton X-100). It was, however, found that when the quantity of surfactant was relatively high, the stoichiometry of the photocatalyst as well as its activity was lost. The obtained materials were characterized by SEM, XRD, AFM, BET and UV–Vis spectroscopy. Optimized photoanodes were employed in photoelectrochemical cells for water splitting and hydrogen production under electric and chemical bias. Maximum hydrogen production rate was 0.15mmol/h under electric bias of 1.4V vs Ag/AgCl plus 0.37V chemical bias.

Preparation of Layered Double Hydroxide and its Graphene Composite Films as Electrodes for Photoelectrochemical Cells

Visible-light responsive photoanodes for water splitting into hydrogen and oxygen were fabricated via NiTi-layered double hydroxide (NiTi-LDH, [Ni1-xTix(OH)2]2x+(An-)2x/n・mH2O]) and NiTi-LDH/graphene composite prepared by homogeneous precipitation methods. The NiTi-LDH and NiTi-LDH/graphene films were formed on transparent fluorine-doped tin oxide (FTO) glass substrates by electrophoretic deposition (EPD). The EPD films showed the pattern of hydrotalcite structure under x-ray diffraction, indicating that NiTi-LDH has been deposited on FTO by EPD without degradation. NiTi-LDH powder was obtained from formic acid intercalated LDH by homogeneous precipitation in formamide with hexamethylenetetramine, and NiTi-LDH/graphene composite was obtained from graphene and NiTi-LDH aqueous suspension with urea. The formed NiTi-LDH and NiTi-LDH/graphene composite films have different morphologies. Both NiTi-LDH and NiTi-LDH/graphene films prepared showed photovoltage of 15-55 mV under visible-light irradiation. Their photoelectrochemical properties were affected by the film morphology.

Modified TiO2 nanotube arrays (TNTAs): progressive strategies towards visible light responsive photoanode, a review

Since first introduced by Zwilling and co-workers in 1999, titania nanotube arrays (TNTAs) fabricated by simple electrochemical anodization method have attracted great interest due to their outstanding photoelectrochemical properties which render them the most promising candidate for many solar energy harvesting applications. In this contribution, the fabrication, properties, and applications of TiO2 nanotube arrays have been reviewed, with special focus on synthesis by anodization in fluoride-containing electrolytes. The effect of anodization process parameters such as electric potential, pH, anodization duration and electrolyte composition on the size, and morphology of TNTAs has been discussed in detail. Electronic property modification strategies of the wide band gap TNTAs to enhance the material responsiveness to visible light irradiation have also been reviewed. Modification strategies include nonmetal doping such as nitrogen, carbon, boron and sulfur; metal ion doping such as Fe, Zn, Zr and Cr; surface decoration with precious metal nanoparticles such as Pt, Ag, Au; and sensitization with CdS nanoparticles.