Photoelectrochemical Water-splitting Performance Research Articles

Enhanced Photoelectrochemical Water-Splitting Property on TiO2 Nanotubes by Surface Chemical Modification and Wettability Control.

We report a simple method to control the photoelectrochemical (PEC) water-splitting performance of TiO2 nanotube arrays (NTs) by surface chemical modification. Four types of modifier molecules with different surface energy and functional groups, including amine (-NH2), n-alkane (-CnH2n+1), perfluoroalkyl (-F), and polymer molecule (-polymer), were self-assembled to the surface of TiO2 NTs, which could change the surface chemical composition and wettability from superhydrophilicity to hydrophobicity. Interestingly, different from expected results, photoelectrochemical measurement results show that the n-octadecyltrichlorosilane-modified TiO2 nanotube arrays with a contact angle of about 134° present the highest PEC property with doubled photocurrent density and more negative onset potential. And the total PEC performance order of the monolayers-modified TiO2 NTs is (-CnH2n+1) > (-F) > (-NH2) > (-OH, pristine TiO2) > (-polymer), which is due to the molecular monolayers modification being able to suppress the recombination of photogenerated electrons and holes and facilitate water oxidation by regulating the interface electric double layer, whereas a thick polymer layer on the photoelectrode surface would affect the light absorbance and decrease the PEC performance. Further investigation indicates that the surface energy and wettability of the TiO2 photoelectrode adjusted by surface modification also have an important influence on the interface reaction of water oxidation and the adsorption/desorption of newly formed oxygen, which also provides a new method for controlling the surface photocatalytic reactions.

Enhancement of the photoelectrochemical water splitting by perovskite BiFeO3via interfacial engineering

Ferroelectric semiconductors like BiFeO3 are increasingly being investigated for applications in solar energy conversion and storage due to their intrinsic ability to induce ferroelectric polarization-driven separation of the photogenerated charge carriers resulting in above-bandgap photovoltages. Nevertheless, the BiFeO3 has been commonly prepared using complex and expensive fabrication techniques, e.g., epitaxial growth, radio frequency sputtering and pulsed laser deposition, which are not economically viable for large-scale production. Herein, we report a facile and scalable method for the fabrication of porous perovskite BiFeO3 photoanodes, as well as sequential interfacial engineering methods to enhance their photoelectrochemical performance for water splitting. Upon atomic layer deposition of a TiO2 overlayer and photo-assisted electrodeposition of a cobalt oxide/oxyhydroxide co-catalyst, the photocurrent density of the engineered photoanode for oxygen evolution reaction (1 M NaOH) significantly increased from negligible photocurrent of the pristine BiFeO3 to 0.16 mA cm−2 at 1.23 V vs. reversible hydrogen electrode (RHE) under simulated 1 sun irradiation (100 mW cm−2, AM1.5G spectrum). Furthermore, such functionalization of the BiFeO3 photoanodes shifts the photoelectrochemical oxidation onset potential by 0.7 V down to 0.6 V vs. RHE. The significantly enhanced photoelectro-oxidation activity is facilitated by the improved charge transfer and electrochemical kinetics.

Hexagonal phase/cubic phase homogeneous ZnIn2S4 n-n junction photoanode for efficient photoelectrochemical water splitting

One of the most efficient ways to achieve a high performance of photoelectrochemical (PEC) water splitting is to accelerate the separation and transfer of photogenerated carriers. Homojunction has the perfect advantages due to the appropriate band gap and the close combination of interphase similar lattices between the two semiconductors. Herein, we report a rational and low-cost design that hexagonal phase/cubic phase homogeneous ZnIn2S4 n-n junction on FTO conductive substrate is fabricated through a simple two-step hydrothermal method to realize enganced PEC performance. In addition, the chemical reaction process and mechanism are discussed in detail. The hexagonal phase/cubic phase homogeneous ZnIn2S4 n-n junction have an excellent photocurrent density of 0.53 mA/cm2 at 1.23 V vs RHE, which is 1.36 and 1.89 times increased than that of the hexagonal phase ZnIn2S4 and cubic phase ZnIn2S4, respectively. Moreover, this work will be instructive for the design of efficient photoelectrode system which used in the photoelectrochemical water splitting.

Effect of Morphology on the Photoelectrochemical Activity of TiO2 Self-Organized Nanotube Arrays

In the present work, highly ordered titanium dioxide (TiO2) nanotube anodes were grown using a rapid anodization process. The photoelectrochemical performances of these electrodes strongly depend on the anodization conditions. Parameters such as electrolyte composition, anodization potential and anodization time are shown to affect the geometrical parameters of TiO2 nanotubes. The optimal anodization parameters are determined by photocurrent measurements, linear sweep voltammetry and electrochemical impedance spectroscopy. The thickness of the tube wall and its homogeneity is shown to strongly depend on the anodization potential, and the formation mechanism is discussed. This study permits the optimization of the photocurrent density and contributes to further improvement of the photoelectrochemical water-splitting performance of TiO2 nanotube photoelectrodes.

Hierarchical ZnO nanorod/ZnFe2O4 nanosheet core/shell nanoarray decorated with PbS quantum dots for efficient photoelectrochemical water splitting

The construction of a uniform heterojunction photoelectrode with rational band alignment and component control has been proved to improve the performance of photoelectrochemical water splitting. Here we prepared ZnO nanorod/ZnFe2O4 nanosheet core/shell nanoarray photoanodes, and PbS quantum dots were further decorated to promote charge transport and act as light absorber. The optimized ZnO/ZnFe2O4/PbS photoanode displays a significantly enhanced photocurrent density and excellent photoelectrochemical stability, and its photoconvertion efficiency is 1.34 and 3.54 times higher than that of ZnO/ZnFe2O4 and ZnO nanorod array photoanodes, respectively, and 87.4% of the initial photocurrent density is obtained after 2 h light irradiation. This work provides a facile strategy for the design of highly efficient and stable photoelectrodes in photoelectrochemical water splitting.

Bi-functional Fe2ZrO5 modified hematite photoanode for efficient solar water splitting

Surface modification on hematite is widely used to improve its photoelectrochemical (PEC) performance for water splitting. Here by using Zr-based metal-organic frameworks as precursor, a thin Fe2ZrO5 layer has been created on hematite surface, which can both significantly improve the photocurrent and reduce the onset potential with a prominent value of 180 mV. Moreover, by coupling with Ti-based modification and Co-Pi cocatalyst, the hematite photoanode decorated with Fe2ZrO5 layer can achieve an excellent photocurrent of 2.88 mA cm−2 at 1.23 V vs. RHE, which is more than 3 times higher than that of the original hematite. To the best of our knowledge, it is the first report for the decoration of Fe2ZrO5 on hematite to improve the water splitting efficiency. The Fe2ZrO5 layer acts as a bi-functional material to enhance the PEC performance, which can form a perfect passivation layer to suppress the charge recombination and provide Zr for Zr-doping in hematite to highly increase the carrier density.

Constructing ordered paths to improve the charge separation and light harvesting capacity towards efficient solar water oxidation performance

Photoelectrochemical (PEC) water-splitting performance can be expressed as the product of efficiencies of light absorption (ηabs), charge separation (ηsep) and charge transfer (ηtrans) processes. In BiVO4 photoanodes, the ηtrans has been greatly enhanced by integrating various low-price oxygen evolution electrocatalysts but improving ηabs×ηsep efficiency remains a great challenge. Considering this challenge, here, we fabricate inverse opal (IO)-SnO2@BiVO4 type-II heterojunction photoanodes and investigate the ηabs×ηsep efficiency by tailoring the amount of BiVO4 on the IO-SnO2 nanostructures. The optimized IO-SnO2@BiVO4 photoanode exhibits ηabs×ηsep of 62.91 % at 1.23 V vs. reversible hydrogen electrode (RHE), which is much higher than that of the bare BiVO4 (16.14 %). The significantly improved ηabs×ηsep is attributed to the introduction of IO-SnO2, which provides a high solar light-harvesting capability by diffuse scattering and coherent multiple internal scattering. Moreover, it greatly improves the intrinsic charge transport and reduces interface contact resistance due to ordered paths for electron migration. Even though ηabs×ηsep is high, the majority of the photogenerated holes are lost at the surface/electrolyte recombination, resulting in a very low ηtrans of 24.93 % at 1.23 V vs. RHE. To improve the ηtrans performance, an oxygen evolution catalyst is deposited on the surface of optimized IO-SnO2@BiVO4, and it greatly increases the ηtrans (96.29 %) and achieves JH2O of 3.57 mA∙cm−2 with excellent stability for 10 h. In addition, we achieve an applied bias photon‐to‐current efficiency of nearly 1.02 % at 0.72 V vs. RHE. Overall, the obtained results and fabrication process are considered a significant step toward achieving sustainability.

Nanoporous 6H-SiC Photoanodes with a Conformal Coating of Ni-FeOOH Nanorods for Zero-Onset-Potential Water Splitting.

A surface-nanostructured semiconductor photoelectrode is highly desirable for photoelectrochemical (PEC) solar-to-fuel production due to its large active surface area, efficient light absorption, and significantly reduced distance for charge transport. Here, we demonstrate a facile approach to fabricate a nanoporous 6H-silicon carbide (6H-SiC) photoanode with a conformal coating of Ni–FeOOH nanorods as a water oxidation cocatalyst. Such a nanoporous photoanode shows significantly enhanced photocurrent density (jph) with a zero-onset potential. A dendritic porous 6H-SiC with densely arranged holes with a size of ∼40 nm on the surface is fabricated by an anodization method, followed by the hydrothermal deposition of FeOOH nanorods and electrodeposition of NiOOH. Under an illumination of AM1.5G 100 mW/cm2, the Ni–FeOOH-coated nanoporous 6H-SiC photoanode exhibits an onset potential of 0 V versus the reversible hydrogen electrode (VRHE) and a high jph of 0.684 mA/cm2 at 1 VRHE, which is 342 times higher than that of the Ni–FeOOH-coated planar 6H-SiC photoanode. Moreover, the nanoporous photoanode shows a maximum applied-bias-photon-to-current efficiency (ABPE) of 0.58% at a very low bias of 0.36 VRHE, distinctly outperforming the planar counterpart. The impedance measurements demonstrate that the nanoporous photoanode possesses a significantly reduced charge-transfer resistance, which explains the dramatically enhanced PEC water-splitting performance. The reported approach here can be widely used to fabricate other nanoporous semiconductors for solar energy conversion.

Preparation of heterometallic CoNi-MOFs-modified BiVO4: a steady photoanode for improved performance in photoelectrochemical water splitting

In this work, an efficient nanoporous BiVO4 composite photoelectrode with high photocurrent and low onset potential for photoelectrochemical water splitting is achieved through modification of BiVO4 with a stable heterometallic CoNi-MOF layer. Compared with pristine BiVO4 anode, the formation of CoNi-MOFs/BiVO4 increases the photocurrent density by about 260 %. In addition, the CoNi-MOFs/BiVO4 photoanode is of greater steadiness for water oxidation, without insignificant activity attenuation within 3 reaction hours at 1.23 V vs. RHE. It is postulated that bimetal-organic framework CoNi-MOFs could supply more open sites which account for the impressively enhanced PEC performance of BiVO4 photoanodes toward water splitting under illumination. The results demonstrate a new method to fabricate dual metal nanoparallelepiped-like MOF for steady hydrolysis photoanode material.

3D-printed Cu2O photoelectrodes for photoelectrochemical water splitting.

Photoelectrochemical (PEC) water splitting is an alternative to fossil fuel combustion involving the generation of renewable hydrogen without environmental pollution or greenhouse gas emissions. Cuprous oxide (Cu2O) is a promising semiconducting material for the simple reduction of hydrogen from water, in which the conduction band edge is slightly negative compared to the water reduction potential. However, the solar-to-hydrogen conversion efficiency of Cu2O is lower than the theoretical value due to a short carrier-diffusion length under the effective light absorption depth. Thus, increasing light absorption in the electrode–electrolyte interfacial layer of a Cu2O photoelectrode can enhance PEC performance. In this study, a Cu2O 3D photoelectrode comprised of pyramid arrays was fabricated using a two-step method involving direct-ink-writing of graphene structures. This was followed by the electrodeposition of a Cu current-collecting layer and a p–n homojunction Cu2O photocatalyst layer onto the printed structures. The performance for PEC water splitting was enhanced by increasing the total light absorption area (Aa) of the photoelectrode via controlling the electrode topography. The 3D photoelectrode (Aa = 3.2 cm2) printed on the substrate area of 1.0 cm2 exhibited a photocurrent (Iph) of −3.01 mA at 0.02 V (vs. RHE), which is approximately three times higher than that of a planar photoelectrode with an Aa = 1.0 cm2 (Iph = −0.91 mA). Our 3D printing strategy provides a flexible approach for the design and the fabrication of highly efficient PEC photoelectrodes.

Enhanced photoelectrochemical water-splitting performance of SrNbO2N photoanodes using flux-assisted synthesis method and surface defect management

Particle-assembled SrNbO2N photoanodes were obtained using flux-assisted nitridation reaction. The optimal experimental parameters were studied for SrNbO2N particles with minimum surface defects and high crystallinity.

Tio2/Co3O4 Composite as Photoanode of Photoelectrochemical Water Splitting

Titanium dioxide is one of the most investigated materials for photoelectrochemical (PEC) water splitting. However, TiO2 has a wide band gap of approximately 3.2 eV, which limits its absorption energy to UV only, and the photoexcitation products (i.e., electron and hole) easily recombine. Developing a composite with other semiconductor materials is one of the efforts for improving the performance of TiO2 photoelectrode. In this study, a composite of TiO2/Co3O4 thin films was developed, and its performance in PEC water splitting was investigated. Co3O4 was synthesized using a hydrothermal method, and TiO2 P25 was used. TiO2 and Co3O4 were mixed by ball milling before coating on a clean FTO and annealing at 550 °C. Two weight variations of TiO2:Co3O4 were used (95:5 and 90%:10%). XRD and FESEM analysis were used to investigate crystallinity phase and surface morphology. The optical band gap of the thin films was determined by UV-Vis spectroscopy, where the film with 90:10 weight ratio of TiO2:Co3O4 obtained the smallest band gap of 2.9 eV. Co3O4 improved the current density and TiO2 performance by approximately three times.

Deposition of heterojunction of ZnO on hydrogenated TiO2 nanotube arrays by atomic layer deposition for enhanced photoelectrochemical water splitting

The heterojunction of ZnO was deposited on hydrogenated TiO2 nanotube arrays (H–TiO2) by atomic layer deposition (ALD) with various cycles. The ZnO was uniformly wrapped with the H–TiO2 samples and the thickness could be accurately controlled by the cycle numbers of ALD. The higher growth rate ~2.7 Å/cycle was obtained due to the surface amorphous layer, compared with the air-treated samples (A-TiO2), ~2.3 Å/cycle. When the cycle numbers increased to 200, nanowire arrays appeared. Interestingly, the absorption in the visible light region improved more significantly when ALD ZnO was employed for the H–TiO2 rather than the A-TiO2 samples. The H–TiO2 samples with 42 nm of ALD ZnO exhibited enhanced photoelectrochemical water splitting performances, compared with the A-TiO2 with 42 nm of ALD ZnO. This was related to the higher degree of the electronic band bending and improved photo-response in the UV and visible light region, resulting from the oxygen vacancies.

Surface Electronic Structure Reconfiguration of Hematite Nanorods for Efficient Photoanodic Water Oxidation

Hematite (α‐Fe2O3) is a promising candidate as a semiconducting photoanode for photoelectrochemical (PEC) water splitting. However, its PEC performance is much limited by the sluggish charge transfer kinetics at the α‐Fe2O3/electrolyte interface. Herein, an insulative metal oxide, hafnium dioxide (HfO2), is deposited on the surface of α‐Fe2O3 to engineer the photoelectrode/electrolyte interfacial electronic structure. With the conformal HfO2 overlayer coating, the surface defects of α‐Fe2O3 are effectively passivated, whereas the charge migration from α‐Fe2O3 to the electrolyte is blocked by the continuous HfO2 overlayer, leading to a moderate PEC enhancement. In contrast, with HfO2 nanoparticles deposited, the photogenerated holes are not only effectively extracted from the bulk of α‐Fe2O3 but are also promptly injected into the electrolyte for water oxidation, due to the reconfigurated surface electronic structure. Consequently, the HfO2 nanoparticles‐decorated α‐Fe2O3 photoanode achieves an onset potential cathodic shift by 180 mV and a 460% photocurrent density enhancement, reaching up to 1.20 mA cm−2 at 1.23 V versus reversible hydrogen electrode as compared with pristine α‐Fe2O3. An alternative approach to engineer the photoelectrode/electrolyte interfacial electronic structure to improve the PEC performance for water splitting is demonstrated herein.

Optical, water splitting and wettability of titanium nitride/titanium oxynitride bilayer films for hydrogen generation and solar cells applications

TiN/TiOxNy bilayer films with various TiN thicknesses were prepared using direct current (d.c.) reactive magnetron sputtering. The ratio of N/O in the bilayer films increased with increasing TiN top layer thickness. The observed crystalline peaks for the bilayer films, for different TiN layer thicknesses, were indexed to anatase TiO2 with tetragonal structure. Only 0.31° peak shift, to lower 2θ angles was detected. FESEM indicated that the morphology of TiOxNy single layer is consisted of elongated nanocrystals with diameters in the range of 27.9–46.8 nm and the diameters of the nanocrystallites changed slightly with increasing the thickness of TiN layer. The transmittance, absorbance, reflectance spectra and the optical constants of the bilayer films were presented. The photoelectrochemical performance for water splitting of TiN/TiOxNy bilayer films was studied by examination both the continuous and chopped photocurrent density under light illumination. The photocurrent density increases with increasing TiN top layer up to a thickness of 9 nm, after which a reduction in photocurrent is observed. The wettability of TiN/TiOxNy bilayer films could be transformed by UV illumination from hydrophobic to hydrophilic.

Decorating Cu2O photocathode with noble-metal-free Al and NiS cocatalysts for efficient photoelectrochemical water splitting by light harvesting management and charge separation design

Developing low-cost but effective cocatalyst is of crucial importance for strongly enhancing the photoelectrodes performance in photoelectrochemical (PEC) water splitting. Herein, we firstly report a rational and low-cost design of a ternary photocathode that Cu2O sandwiched by spatial noble-metal-free cocatalysts (NiS nanoparticles on the surface and Al nanoparticles on the bottom) to realize enhanced PEC performance. The integration of Al with surface plasmon resonance (SPR) effect and electron trapper NiS spatially loaded on the Cu2O nanocubes is indeed conducive to excite the hot electron-hole pairs, boost the solar light absorption and elevate ultrafast spatial transfer and separation of photogenerated electrons and holes, resulting in remarkable photocurrent density of −5.16 mA/cm2 at 0 V vs. RHE, an eight-fold enhancement than that of pristine Cu2O. The selection of noble-metal-free Al and NiS cocatalysts is superior substitute to noble metal Au and Pt from the comparison with our previous work. Therefore, this work could inspire ongoing interest in the design of highly efficient and cost-effective hybrid photoelectrodes which is able to replace noble metals in practical applications of PEC water splitting.

Something new under the sun for ultra low-cost single-junction PhotoAnodes for highly efficient photocatalytic water splitting

Novel, simple, and ultra low-cost single-junction photoanodes consisting of highly-oriented black nanopyramid-arrays of Bismuth(III) Sulfide (i.e. Bi2S3) grown at low temperature by all-solution chemistries onto common, inexpensive and industry-friendly 304 stainless steel show remarkable performance for photoelectrochemical solar water splitting without any co-catalysts. The applied bias photoconversion efficiency (ABPE) reaches 6% on stainless steel at 0.85V vs RHE and photocurrent densities of ~20 and ~45 mA/cm2 at 1.23V vs RHE at 1 and 2 Sun, respectively in aqueous Na2S/NaSO3 solutions due to highly efficient anisotropic charge separation at the apex of the nanopyramids which results in the generation of 0.8 mol h-1 of H2 per gram of Bi2S3 photocatalysts.

Efficient charge transfer and separation of TiO2@NiCo-LDH core-shell nanowire arrays for enhanced photoelectrochemical water-splitting

Efficient charge transfer and separation play a significant role in determining the photoelectrochemical (PEC) water-splitting performance of photocatalysts. Here, hierarchical nanowire arrays (NWAs) containing the TiO2 nanowire core and nickel-cobalt layered double hydroxide (NiCo-LDH) shell were synthesized by combining a hydrothermal method with a facile electrochemical-deposition process. The hierarchical structure not only forms heterojunctions between the TiO2 core and NiCo-LDH shell but also provides active sites for water-splitting reaction, achieving the charge transfer efficiency up to 87% and the charge separation efficiency up to 42% at 1.23 V vs. reversible hydrogen electrode (RHE) under one sun illumination. The excellent PEC water-splitting performance of TiO2@NiCo-LDH core-shell NWAs is attributed to the enhanced charge transfer and separation, resulting from the decoration of the NiCo-LDH cocatalyst. This facile and cost-effective strategy for integrating the light-harvesting TiO2 semiconductor and NiCo-LDH cocatalyst into a hierarchical core-shell nanostructure can be potentially applied in energy conversion and environmental applications.

3D cross-linked BiOI decorated ZnO/CdS nanorod arrays: A cost-effective hydrogen evolution photoanode with high photoelectrocatalytic activity

ZnO/CdS photoanode with an excellent photoelectrochemical (PEC) performance for water splitting to produce hydrogen, but its practical application is seriously hindered owing to the photocorrosion induced by photoholes on the surface of CdS. Herein, a 3D cross-linked heterostructure ZnO/CdS/BiOI nanorod arrays (NRAs) has been prepared by a simple solvothermal strategy. Under visible light illumination (λ > 420 nm), the ZnO/CdS/BiOI NRAs photoanode shows an excellent PEC activity and generates a photocurrent density of 9.12 mA cm−2 at 1.1 V vs. RHE, which is 1.8 times higher than that of the ZnO/CdS NRAs photoanode in the alkaline electrolyte. Furthermore, the photoanode achieves a high photo conversion efficiency of 3.49% and a long-time stability over 6000 s. It is proposed that the BiOI nanosheets not only serve as protecting layer to restrain the photocorrosion of CdS, but also facilitate the charge separation in CdS by the virtue of the p-n junction formed between CdS and BiOI.

Investigation of Some Characterizations of Black TiO\(_2\) Nanotubes Via Spectroscopic Methods

The black TiO\(_2\) with substantial Ti\(^3+\) and oxygen vacancies exhibit an excellent photoelectrochemical water-splitting performance due to the improved charge transport the extended visible light response. In this study, black TiO\(_2\) nanotube arrays synthesized by the anodization method, and then, they have been investigated some characterizations by spectroscopic methods such as UV-visible reflectance (UV-vis DRS), Fourier-transform infrared spectroscopy (FTIR), Raman spectroscopy, and photoluminescence spectrum. The results showed that some highlighted properties of the black TiO2 nanotube arrays and they could apply for water-splitting effect.