Enhanced efficiency of hematite photoanode for water splitting with the doping of Ge

Ge doped α-Fe₂O₃ nanowires are synthesized through a hydrothermal procedure with GeO₂ as a precursor and investigated as photoanodes for water splitting. The content of Ge in the photoanode rises with the increase of the amount of GeO₂ in the precursor solution. A proper amount of Ge facilities the preferred oriented growth of the (110) plane of α-Fe₂O₃, while excessive Ge hinders the growth of α-Fe₂O₃ crystals. The doping of Ge increases the absorption efficiency and decreases the recombining rate of the photogenerated electrons and holes. Ge also improves the density and transfer rate of the charge carriers in the photoanode. Ge doped α-Fe₂O₃ photoanode exhibits a highest photocurrent density of 0.92 mA cm^?2 at 1.23 V vs. reversible hydrogen electrode under AM 1.5 G simulated sunlight, which is nearly twice of that obtained by pure α-Fe₂O₃ under the same condition.     

A heterostructured catalyst composed of poly-2,6-diaminopyridine and TiO2 microspheres used as a photoanode for efficient water splitting

Photoelectrocatalytic (PEC) water splitting provides an alternative to direct solar-to-fuel production. In this study, a novel heterostructure formed between a conjugated polymer [poly-2,6-diaminopyridine (PDAP)] and three-dimensional TiO₂ microspheres was grown in situ on a Ti substrate (PDAP-3DTiO₂MSs/Ti) and used as photoanode for water oxidation in alkaline media under AM 1.5G illumination. The PDAP-3DTiO₂MSs/Ti can produce applied bias photon-to-current efficiency of 0.85% at 0.44 V vs. Pt and a photocurrent density of 1.56 mA cm⁻² at 1.23 V vs. RHE. Moreover, PDAP-3DTiO₂MSs/Ti displays impressive photoelectrochemical stability with 93% of its initial photocurrent being retained after 4 h of reaction. Based on physical-chemical characterization and photo-/electro-chemical measurements, the superior PEC water splitting performance of PDAP-3DTiO₂MSs/Ti should benefit from the coexistence of Ti³⁺ and Ti⁴⁺ in 3DTiO₂MSs, the light harvest capability of PDAP and the type II heterojunction formed between 3DTiO₂MSs and PDAP, which result in the enhanced generation and separation of photocarriers.     

TiO2/CeO2 core/shell heterojunction nanoarrays for highly efficient photoelectrochemical water splitting

In this paper, novel TiO₂/CeO₂ core/shell heterojunction nanorod (NR) arrays were synthesized as photoanode for photoelectrochemical (PEC) water splitting via a simple and facial two-step hydrothermal approach. This synthesis route can obtain different amount of CeO₂ nanoparticles by controlling the hydrothermal time and eventually achieve uniform TiO₂/CeO₂ core/shell nanostructures. The uniform TiO₂/CeO₂ core/shell heterojunction nanoarrays exhibit a markedly enhanced photocurrent density of 5.30 mA·cm^?2 compared to that of pristine TiO₂ NR 1.79 mA·cm^?2 at 1.23 V vs. RHE in 1 M KOH solution. The superior PEC performance of the TiO₂/CeO₂ core/shell heterojunction is primarily due to much enhanced visible light absorption and appropriate gradient energy gap structure. This work not only offers the synthesis route for the novel TiO₂/CeO₂ core/shell heterojunction, but also suggests that this new core/shell heterojunction has a great potential application for efficient PEC water splitting devices.     

Nanostructured Ni:BiVO4 photoanode in photoelectrochemical water splitting for hydrogen generation

Photoelectrochemical water splitting using bismuth vanadate (BiVO₄) is drawing attention but on account of presence of high charge recombination and poor water oxidation kinetics its performance is restricted. Present study attempts to understand the role of dopant Ni on BiVO₄ in a) reducing the charge recombination and b) to improve water oxidation kinetics. Ni doped BiVO₄ thin films are prepared via electrodeposition method and photoelectrochemical properties are investigated in 0.1 M phosphate buffer solution with and without sodium sulfite hole scavenger. Photocurrent density of 1.36 mA/cm² at 1.23 V vs. RHE has been obtained using 1.5% Ni doped BiVO₄. This sample also offered lower flat band potential, high open circuit potential and applied bias photon-to-current conversion efficiency. Addition of hole scavenger significantly increases the photoelectrochemical performance. Ni as a dopant therefore can play an important role in not only suppressing the electron-hole pair recombination but also in offering significantly enhanced photoelectrochemical response.     

Improved photon management in a photoelectrochemical cell with Nd-modified TiO2 thin film photoanode

Photon management involving particularly an up-conversion process is proposed as a relatively novel strategy for improving the efficiency of hydrogen generation in photoelectrochemical cells (PEC) with wide-band gap photoanodes. Optically active photoanode has been constructed by electrodeposition of titanium dioxide nanopowders containing Nd³⁺ ions, synthesized via a sol-gel method, onto ITO/TiO₂(thin film) substrates. Thin films of TiO₂ have been deposited by means of RF magnetron sputtering in an ultra-high-vacuum system. X-ray diffraction, scanning electron microscopy, UV-VIS-NIR spectrophotometry, and photoluminescence have been applied to assess the properties of photoanodes. In experiments involving photon-assisted water splitting, an external up-converter containing Yb³⁺/Er³⁺ rare-earth ions has been used. Photocurrent as a function of voltage (V_B) under illumination with white light is relatively high (280 μA at V_B = 0 V) for pure TiO₂ thin films and it is not affected by the electrodeposition of TiO₂:Nd³⁺ powders. NIR-driven up-conversion with laser excitation at λ = 980 nm has been found responsible for a 13-fold increase in photocurrent at V_B = 0 V in the modified PEC configuration.     

The co-decorated TiO2 nanorod array photoanodes by CdS/CdSe to promote photoelectrochemical water splitting

A cascade structure of TiO₂/CdS/CdSe semiconductor heterojunction is synthesized using a three-step technique of facile hydrothermal growth for the enhancement of the photoelectrochemical performances. The optical and photoelectrochemical properties controlled by the deposition processing parameters have been investigated. It is shown that the patterns of semiconductor heterojunction enlarge the absorption range of solar spectra, and improve the properties of the photogenerated charge carriers describing separation and transportation, and reduce the interface resistance between the photoelectrode and electrolyte comparing with the pure TiO₂ and CdS-decorated TiO₂ nanorod array photoanodes. The higher photocurrent density and photoconversion efficiency are up to 4.23 mA cm⁻² and 4.2%, which are the 4.1 and 25.3 times superior than that of the pure TiO₂ photoanode. The hydrothermal growth time increment of CdSe yields greater photoelectrochemical water splitting performances. The underlying physics mechanisms have been discussed based on forming a type-Ⅱ energy band alignment structure.     

Facile fabrication of visible light driven tin oxide photoanode and its photoelectrochemical water splitting properties

Photoelectrochemical water splitting is a promise way to transfer solar energy to hydrogen as chemical energy carrier. In this paper, visible light driven tin oxide based photoelectrodes were obtained through dipping SnCl₂·2H₂O EtOH solution on FTO or metal Ti substrate and with further heat treatment process. Photoelectrochemical measurements with three electrodes configuration revealed that this obtained photoelectrode showed n-type responsive properties and the photocurrent density reached mA/cm² level without any modification under visible light irradiation (λ > 420 nm). XRD, UV–Vis spectrum and control experimental results proposed that the visible light driven mechanism for the tin oxide based photoanode maybe ascribed to Sn⁴⁺/Sn²⁺ transformation and surface oxygen deficiency, and the tin oxide can be denoted as SnO_2−x.     

Photoelectrochemical water splitting by engineered multilayer TiO2/GQDs photoanode with cascade charge transfer structure

Herein, for the first time, an efficient photoanode engineered with the cascade structure of FTO|c-TiO₂|few graphene layers|TiO₂/GQDs|Ni(OH)₂ assembly (Ni(OH)₂ photoanode) is designed. This photoanode exhibited much lower electron–hole recombination, fast charge transport, higher visible light harvesting, and excellent performance with respect to FTO|c-TiO₂|TiO₂ assembly (TiO₂ photoanode) in the photoelectrocatalytic oxygen evolution process. The photocurrent density of Ni(OH)₂ photoanode is 7 times (0.35 mA cm⁻² at 1.23 V vs. RHE) greater than that of TiO₂ photoanode (0.045 mA cm⁻² at 1.23 V vs. RHE). The compact TiO₂ (c-TiO₂) layer in Ni(OH)₂ photoanode plays a role of an effective hole-blocking layer. Few-layer graphene layer could speed up the transport of the photogenerated electrons from the conduction band of the TiO₂/GQDs to FTO. Ni(OH)₂ layer could transfer rapidly holes into electrolyte solution.     

Junction energetics engineering using Ni/NiOx core-shell nanoparticle coating for efficient photoelectrochemical water splitting

We report a sparse Ni/NiOx core-shell nanoparticle coating on an n-GaN photoanode that yields a high photocurrent by engineering junction energetics. A conventional thin film coating of high work function NiOx induces a large band bending that helps generate high photovoltage. However, the high work function causes a Fermi level downshift and compromises photopotential. The discrete core-shell nanoparticle coating balances these two effects. The Ni core creates localized large band bending to produce a high photovoltage. Meanwhile, the reduced coating surface area decreases Fermi level downshift. The resulting higher photopotential together with the catalytic NiOx shell enables a photocurrent 50% higher than NiOx thin film coating and multiple times higher than Ni nanoparticle or film coating. The localized large band bending also forms a potential well to deplete holes from semiconductor, thereby providing full surface protection against corrosion. This core-shell nanoparticle coating demonstrates a new junction energetics engineering paradigm useful for photoelectrode optimization.     

Plasmon Au modified 3D-nanostructrue CoOx decorated hematite for highly efficient photoelectrochemical water splitting

Surface modification and interface engineering are efficient strategies to address the serious charge recombination and the sluggish water oxidation kinetics in photoelectrochemical water splitting. In this work, CoO_x decorated hematite nanosheets (Fe₂O₃/CoO_x) are deposited on Nickel foam by the in-situ hydrothermal process. Au nanoparticles are incorporated on Fe₂O₃/CoO_x semiconductors (Fe₂O₃/CoO_x/Au) by electrochemical deposition. In photoelectrochemical test, Fe₂O₃/CoO_x attains a photocurrent density of 1.87 mA cm^?2 at 1.23 V_RHE, which is 4.45 times that for α-Fe₂O₃. The onset potential of Fe₂O₃/CoO_x decreases by 266 mV compared with α-Fe₂O₃. The 3D-nanostructrue Fe₂O₃/CoO_x/Au attains a photocurrent density of 3.88 mA·cm^?2 at 1.23 V_RHE, which is 9.24 times that of ɑ-Fe₂O₃. The applied bias photon-to-current efficiency, charge separation and charge injection efficiency of Fe₂O₃/CoO_x/Au are improved. EIS studies show the co-modification of CoO_x and Au reduces charge transfer resistance. This strategy would provide a potential approach to promote light absorption and charge separation for photoelectrochemical catalyst.     

Effects of NiO-loading on n-type GaN photoanode for photoelectrochemical water splitting using different aqueous electrolytes

n-type GaN photoanodes used for water splitting have stability problems. One means of resolving this is loading NiO catalyst on the n-type GaN surface. Aqueous electrolytes H₂SO₄, HCl, KOH, and NaOH are usually used for photoelectrochemical water splitting. However, suitable electrolytes for the NiO-loading on n-type GaN photoelectrode have not yet been evaluated. Therefore, we investigated the effects of changing electrolytes used for NiO-loading in this study. The photocurrent of NiO-loading on n-type GaN increased when KOH and NaOH electrolytes were used. In addition, the surfaces showed no corrosion after reaction when these electrolytes were used. However, the photocurrent was not stable using KOH electrolyte. Interestingly, stable photocurrent was observed with when the NaOH electrolyte was used. In the case of H₂SO₄, the photocurrent of GaN did not change with and without NiO. The surface morphologies became rough because of GaN corrosion, and NiO dissolved in the H₂SO₄ electrolyte.     

Sensitization of vertically grown ZnO 2D thin sheets by MoSx for efficient charge separation process towards photoelectrochemical water splitting reaction

Development of advanced materials for photoelectrochemical (PEC) water splitting has become an essential issue for efficient, green, and economical hydrogen production. In this context, vertically grown thin sheets of ZnO is developed, which can function as an efficient photoanode in PEC water splitting reaction. Further, the PEC activity of ZnO is enriched by decorating a newly developed co-catalyst, which is amorphous MoS_x through efficient charge transportation. MoS_x nanostructure is decorated on the surface of ZnO nanosheet via electrodeposition technique. MoS_x adorned ZnO shows enhanced activity towards photoanodic PEC water splitting compared to bare ZnO. ZnO@MoS_x can generate photocurrent density nearly three times higher compared to bare ZnO at an applied potential of ‘0.5998’ V vs. RHE. Sensitization of MoS_x on ZnO surface results in an enhancement in carrier density; ZnO@MoS_x shows nearly 7.4-times higher carrier density compared to bare ZnO. Maximum photoconversion efficiency, 0.934% is achieved in the case of ZnO@MoS_x. The determined band alignment of ZnO and MoS_x indicate the formation of type-II heterostructure which allow facile charge carrier separation. Efficient charge separation is also confirmed with the help of PL spectroscopy. It further restricts the electron-hole recombination in ZnO, leading to enhanced PEC activity. ZnO@MoS_x thin sheets are very stable even up to 1000 s under chopped illumination condition.     

Plasmonic Bi nanoparticle decorated BiVO4/rGO as an efficient photoanode for photoelectrochemical water splitting

We report the application of plasmonic Bi nanoparticles supported rGO/BiVO₄ anode for photoelectrochemical (PEC) water splitting. Nearly, 2.5 times higher activity was observed for Bi-rGO/BiVO₄ composite than pristine BiVO₄. Typical results indicated that Bi-rGO/BiVO₄ exhibits the highest current density of 6.05 mA/cm² at 1.23 V, whereas Bi–BiVO₄ showed the current density of only 3.56 mA/cm². This enhancement in PEC activity on introduction of Bi-rGO is due to the surface plasmonic behavior of BiNPs, which improves the absorption of radiation thereby reduces the charge recombination. Further, the composite electrode showed good solar to hydrogen conversion efficiency, appreciable incident photon-to-current efficiency and low charge transfer resistance. Hence, Bi-rGO/BiVO₄ provides an opportunity to realize PEC water splitting.     

A ternary Ag/TiO2/CNT photoanode for efficient photoelectrochemical water splitting under visible light irradiation

A ternary Ag/TiO₂/CNT photoanode was prepared by grafting Ag nanoparticles on the surface of as-synthesized TiO₂/CNT nanocomposite for the photoelectrochemical (PEC) water splitting under visible light irradiance. The ternary composite photoanode was observed to generate four times higher photocurrent density compared to binary TiO₂/CNT nanocomposite under visible light irradiance. The Ag nanoparticles on the surface of nanocomposite act as a surface plasmon resonance (SPR) photosensitizer under visible light. The enhanced photocurrent density of Ag/TiO₂/CNT ternary photoanode is attributed to the increased light absorption in the visible region, decrease in band-bending and effective interfacial electron transfer due to the synergetic effect of Ag nanoparticles and CNTs. The enhanced charge transfer within the Ag/TiO₂/CNT was also confirmed by the electrochemical impedance spectroscopy. This work demonstrates a feasible route to improve the PEC performance of TiO₂ towards water splitting under sunlight irradiation.     

An ultra-thin NiOOH layer loading on BiVO4 photoanode for highly efficient photoelectrochemical water oxidation

Monoclinic bismuth vanadate has been widely used as a promising n-type semiconductor for photoelectrochemical (PEC) water decomposition due to its high reserves, good stability in neutral solutions, and relatively narrow band gap. Here, we developed a simple method to prepare a thin NiOOH layer on the surface of BiVO₄ nanorod arrays. The heterostructured photoanode shows great enhancement for the photocurrent density of 2.7 mA cm⁻² at 1.23 V vs. RHE, which is ~2.3 times higher than that of pristine BiVO₄ electrode, due to NiOOH as an efficient oxygen-releasing catalyst with abundant oxygen vacancies. The NiOOH/BiVO₄ photoanodes are systematically studied with X-ray diffraction, Raman, X-ray photoelectron spectra, scanning electron microscopy, transmission electron microscopy, and UV–vis diffuse-reflectance spectrum. The heterostructured photoanode shows excellent PEC activity, which can provide a promising and easy strategy to prepare such photoanode with high-efficient oxygen evolution co-catalysts.     

Photoresponse and stability improvement of ZnO nanorod array thin film as a single layer of photoelectrode for photoelectrochemical water splitting

ZnO nanorod array thin film with Al-doping and hydrogen treatment was developed as a photoelectrode combining the functions of transparent conducting oxide thin film and photoactive 1-dimensional nanostructured semiconductor into a single layer for photoelectrochemical water splitting. It was demonstrated that hydrogen treatment and Al-doping enhanced the dark currents, photocurrents, and hydrogen generation efficiencies largely and the enhancement by hydrogen treatment was more significant. The maximum photoinduced hydrogen generation efficiency was about 0.020%. Furthermore, hydrogen treatment also improved the photosensitivity and the stability under illumination significantly. The minimum decay time constant and rise time constant were 1.71 and 1.22 s, respectively. And after current-voltage scanning upon illumination for 50 cycles, the 1-dimensional morphology still remained unchanged but those without Al-doping and/or hydrogen treatment were altered seriously. The good photoresponse and stability made the Al-doped ZnO nanorod array thin film with hydrogen treatment have wide applications in the photoelectrochemical field.     

Morphology control of the hematite photoanodes for photoelectrochemical water splitting

The morphology of hematite photoanode is a significant relevant factor in its photoelectrochemical (PEC) performance. Hematite nanowires and nanocubes as well as nanorods with intentional Sn doping were prepared by hydrothermal processes containing disparate additives. The band-gap decreases in the sequence of nanowires, nanorods and nanocubes. Compared with nanorods, nanowires show higher carrier density but a lower light absorbance. With both inhibited bulk and surface charge recombination, nanowires achieve an enhanced photocurrent. Meanwhile, it is more complicated for the charge conversion in the hematite nanocubes. Light absorption is limited due to the compact arrangement of nanocubes. Besides, nanocubes show a highly oriented (104) plane which is unfavorable to the charge conductivity. Despite the negative factors hindering its PEC performance, the extremely high carrier density in the nanocubes benefits to the distinctly enhanced photocurrent collected from the hematite samples annealed at 550 and 650 °C respectively. However, the superiority of hematite nanocubes annealed under 800 °C is restricted by the high onset potential. Still, attributed to the high surface charge transfer efficiency, the hematite nanocubes achieve the highest photocurrent among the samples at biases above 1.3 V. Electrodes made of hematite nanorods, nanowires and nanocubes annealed at 800 °C achieve a photocurrent of 1.01, 1.30 and 1.40 mA cm⁻² at 1.6 V vs. RHE, respectively.     

Photoelectrochemical performance of bismuth vanadate photoanode for water splitting under concentrated light irradiation

Photoelectrochemical (PEC) water splitting is a promising way to convert solar energy into hydrogen energy. It is typically carried out at room temperature (RT) and 1 sun illumination. The PEC water splitting under concentrated light is expected to be an effective route to improve PEC performance, but there are few studies on it. Herein, CoPi/Mo:BiVO₄ photoanode was selected to investigate the effect of concentrated light and the reaction temperature on its PEC performance. It was revealed that CoPi/Mo:BiVO₄ showed enhanced PEC performance under concentrated light. The photocurrent density was enhanced with increased light intensity and increased reaction temperature. At a high temperature (60 °C), the normalized photocurrent density (3.31 mA cm⁻² at 1.23 V vs. RHE) was found to be optimal at 4 suns, which was attributed to the synergistic effect of concentrating light and heating. It is proved that concentrated light can effectively improve the PEC performance, which has important guiding significance to realize the low-cost and efficient PEC water splitting.     

Metal oxide photoelectrodes for hydrogen generation using solar radiation-driven water splitting

Metal oxide semiconductors are today the most promising materials for photoelectrochemical production of hydrogen by the method of the photoelectrolysis of water as the problems of the stability of photoelectrodes are basically solved only for such materials. The aim of this short review paper is the presentation of results in this field obtained both worldwide and by the authors of this paper. The factors determining the efficiency of photoelectrolysis and possible ways to increase the photocatalytic activity of semiconductor photoelectrodes are analyzed. It is shown that the development of new photoelectrodes made of solid solutions and more complicated multicomponent compositions are most promising. Possibilities to use porous and nanocrystalline oxide photoelectrodes in photoelectrochemistry are also discussed. Photoelectrolysis setups for the photoelectrochemical conversion of solar energy into hydrogen are briefly described.     

ZnO thin films,surface embedded with biologically derived Ag/Au nanoparticles,for efficient photoelectrochemical splitting of water

ZnO thin films, showing nano-ridges at the surface and the top layer embedded with metal (Ag/Au) nanoparticles (MNP), were obtained by sol-gel synthesis, using zinc acetate dihydrate [(CH₃.COO)₂Zn.2H₂O] as precursor. The method involved prior synthesis of Ag and Au nanoparticles via biological reduction of AgNO₃ and HAuCl₄, respectively, using algae Spirulina platensis. The XRD analysis indicated dominant evolution of wurtzite ZnO phase. Low-angle shift in peaks, seen with nanoparticles embedded films, indicated partial diffusion of metals into ZnO lattice. Band gap energy was least affected and lied in the expected range. AFM and SEM analysis revealed the surface topography and morphology, while EDX analysis confirmed the elemental stoichiometry and existence of Ag/Au nanoparticles in samples. Significant gain in photoelectrochemical current using MNP embedded films is largely accountable to the improvement in electrical conductance and the role played by metal nanoparticles in charge-carrier separation, collection and transport.