Low charge separation efficiency (eta sep) of TiO2 impedes its applications of photoelectrochemical (PEC) water splitting. A positive potential pretreatment approach has been developed to construct a surface polarization with -OH groups at a potential of 1.48 VRHE. When the pretreatment time of 6 h is used, the eta sep and photocurrent density (Jph) reach up to 54% (vs. 24 %, TiO2) and 0.92 mA cm-2 (vs. 0.32 mA cm-2, TiO2) under AM 1.5 G illumination, respectively. The photovoltage can be increased from 0.17 V to 0.52 V. When the pretreatment time is extended to 22 h, a higher Jph of 1.57 mA cm- 2 and eta sep of 87 % can be obtained. Furthermore, the positive potential pretreated TiO2 possesses excellent PEC stability. The PEC improvements can be attributed to the surface polarization with the -OH groups and favorable surface states of Ti-OH, but not to the formation of Ti3+ or oxygen vacancies.

Metal-organic frameworks (MOFs) possess a large specific surface area, porosity, and structural diversity, making them ideal candidates for catalysing the urea oxidation reaction (UOR). Even though MOF-based catalysts have been broadly investigated in urea electrocatalysis, few MOF catalysts are used for urea photoelectrocatalysis. The latter requires the MOF-based catalysts to efficiently separate and transport the photogenerated holes at the interface of photoanode-electrolyte junctions. In this study, a sulphur-containing organic ligand of 1,4-benzenedithiol (BDT) is innovatively employed to synthesize two-dimensional Ni-BDT MOFs from Ni(OH)2 nanosheets precursor. After the in-situ electrochemical activation of TiO2@Ni-BDT, the resultant MOF derivative modified TiO2 photoanode named as TiO2@Ni-BDT-A exhibits 3.83 times photocurrent in comparison with the pristine TiO2. A high photovoltage of 0.73 V and an impressive 82% photocurrent retention after a long-term stability test can also be observed. The improvements are attributed to the enhanced surface active sites and the active species of Ni(2−σ)+−SOx, which can efficiently promote charge separation, increase conductivity, improve donor concentration, and lower the charge transfer resistance. This work develops a sulphur-containing Ni-MOF as a co-catalyst for efficient urea photoelectrocatalysis, and an effective strategy of in-situ electrochemical activation to boost photoanode activity.

The severe photocorrosion of cadmium sulfide (CdS) restricts its practical application for solar hydrogen production. Although remarkable progress has been achieved with an overlayer strategy for isolating the CdS surface, the lifetime of CdS-based photoanodes is still far from the actual requirements. Herein, a hybrid overlayer of defective Ni-MOF and NiO nanoparticles has been developed through the chemical bath deposition method with postannealing. This hybrid overlayer of Ni-MOF-d is coated on the surface of the TiO2/CdS type-II heterojunction. The composite photoanode exhibits a photocurrent density of 4.41 mA cm-2 at 1.23 VRHE, which is 3.47- and 1.32-fold that of CdS and TiO2/CdS, respectively. The Ni-MOF-d overlayer gives rise to a negative shift of the onset potential by 59.51 mV. After a long-term stability test of 11 h, a photocurrent retention of 70% is observed, which is among the most robust CdS-based photoanodes. The kinetics studies reveal that the performance improvements can be attributed to the multiple functions of the Ni-MOF-d hybrid overlayer, including isolating the CdS surface from the electrolyte, cocatalyzing the electrode oxidation processes, passivating the surface defect states of CdS, and facilitating the charge injection from the photoanode to the electrolyte.

Due to the excellent stability of titanium dioxide (TiO2), there is still value in improving its solar-to-hydrogen conversion efficiency through tremendous attempts. Metal sulfides with a narrow bandgap are good candidates to broaden the ultraviolet light absorption of TiO2 into the visible light region. However, sulfides suffer from the photocorrosion issue, leading to poor stability. Herein, a type-II heterojunction of TiO2/In2S3 is fabricated by a hydrothermal method, and a NiFe Prussian blue analog (NFP) overlayer is deposited on the surface of TiO2/In2S3 through a chemical bath deposition technique. Under AM1.5G illumination, a photocurrent density of 1.81 mA cm-2 can be obtained with NFP coated TiO2/In2S3 at 1.23 V vs. reversible hydrogen electrode, which is six folds of the photocurrent of TiO2. This photocurrent value can reach up to about 90% of its theoretical photocurrent. During a 12 h stability test, the TiO2/In2S3/NFP photoanode exhibits a high photocurrent retention of 95.17% after an initial transient decrease. The type-II heterojunction of TiO2/In2S3 can efficiently boost the charge separation because of the built-in electric field and enhance the visible-light absorption because of the narrow bandgap of In2S3. A NFP overlayer serves as the cocatalyst for water oxidation reaction due to its valence changes of nickel and iron elements. NFP cocatalyst can rapidly extract the photogenerated holes from In2S3 and then improve the charge separation/injection efficiencies. Thanks to chemical stability of NFP, its coating can also make In2S3 resistant to photocorrosion by physically separating the photoanode from the electrolyte. Therefore, there is a good synergistic effect between the TiO2/In2S3 heterojunction and NFP cocatalyst. This work provides some crucial insights for the interface engineering and material design in photoelectrochemical systems.

Solar urea wastewater splitting is capable of producing hydrogen and degrading the urea pollutant simultaneously. Nickel hydroxide (Ni(OH)2) has been recognized as an effective cocatalyst for the urea oxidation reaction (UOR). But the lack of an efficient preparation method and a suitable Ni(OH)2 based cocatalyst limits the performances of solar urea wastewater splitting. Herein, a potential-cycling method is developed with a high-purity nickel plate serving as the counter electrode and nickel source in a three-electrode configuration. Spherical Ni0-doped Ni(OH)2 nanoparticles are successfully synthesized on the surface of TiO2 nanorod arrays. The photocurrent density of TiO2/Ni0:Ni(OH)2 can reach 0.56 mA cm-2 at 1.23 VRHE in 1 M NaOH and 0.33 M CO(NH2)2 mixed electrolyte under AM1.5G illumination, which is 1.75 and 1.93 times those of TiO2/Ni(OH)2 deposited using a normal potentiostatic method with nickel salt solution and pristine TiO2, respectively. Ni0 doping can significantly decrease the charge transfer resistance and provide a more favorable distribution of density of states of Ni(OH)2 for the UOR. Furthermore, Ni0:Ni(OH)2 decorated TiO2 photoanodes exhibit good photocurrent retention during 12 h continuous testing. This work expands the preparation technique of urea catalysts and the strategy for developing highly efficient nickel-based catalysts.

Sunlight concentration has been demonstrated as one promising strategy for practically photoelectrochemical (PEC) water splitting with exceeding 10% solar-to-hydrogen efficiency. However, the operating temperature of PEC devices, including the electrolyte and photoelectrodes, can be elevated to 65 ℃ naturally due to the concentrated sunlight and the thermal effect of near-infrared light. In this work, high temperature photoelectrocatalysis is evaluated using titanium dioxide (TiO2) photoanode as a model system, which is believed to be one of the most stable semiconductors. During the studied temperature range of 25-65 ℃, a linear increment of photocurrent density with a positive coefficient of 5.02 μA cm-2 K-1 can be observed. The onset potential for water electrolysis shows a significant negative shift by 200 mV. An amorphous titanium hydroxide layer and a number of oxygen vacancies generate on the surface of TiO2 nanorods, promoting the water oxidation kinetics. During long-term stability testing, the NaOH electrolyte degradation and TiO2 photocorrosion at high temperatures could cause the decaying photocurrent. This work evaluates the high temperature photoelectrocatalysis of TiO2 photoanode and reveals the mechanism of temperature effects on TiO2 model photoanode.

Photoelectrochemical (PEC) urea splitting is of great significance for urea wastewater remediation and hydrogen production with low energy consumption simultaneously. Nickel hydroxides as electrocatalysts have been widely investigated for urea electrolysis. However, it is an open question how to synthesize highly catalytic Ni(OH)2 for the PEC urea splitting. Herein, we take advantage of the instability of metal–organic frameworks (MOFs) to perform an in situ synthesis of Ni(OH)2 catalysts on the surface of TiO2 nanorod arrays. This transformed Ni(OH)2 (T-Ni(OH)2) possesses a superior PEC catalytic activity for water/urea splitting in comparison to the Ni(OH)2 prepared by the impregnation method. The in situ transition of a Ni-MOF is accomplished through an electrochemical treatment under AM1.5G illumination in a KOH-and-urea mixed electrolyte. The specific transition mechanism of Ni-MOFs is the substitution of ligands with OH− in a 1 M KOH electrolyte and the successive phase transition. The T-Ni(OH)2@TiO2 photoanode delivers a high photocurrent density of 1.22 mA cm−2 at 1.23 VRHE, which is 4.7 times that of Ni(OH)2@TiO2 prepared with the impregnation method. The onset potential of T-Ni(OH)2@TiO2 is negatively shifted by 118 mV in comparison to TiO2. Moreover, the decline of photocurrent during the continuous test can be recovered after the electrochemical and light treatments.

Metal-organic frameworks (MOFs) have been proved to be outstanding catalysts for oxygen evolution reaction (OER). When compared to monometallic MOFs, it's demonstrated that bimetallic MOFs have higher activity and stability. However, the coupling effect of the two metallic ions in bimetallic MOFs is unclear. To clarify the mechanism, bimetallic CoZn-MOFs with OER-inert Zn species are coated on BiVO(4 )photoanodes. Results show that the oxidation state of Co is improved through the addition of Zn. Combining with borate-irradiation treatment, the CoZnMOF/B-BVO achieves a photocurrent density of 4.27 mA/cm(2), which is 3.16 times that of the BVO electrode. Furthermore, the onset potential is negatively shifted by 253 mV, the surface charge injection efficiency is 91% (vs 43.5%, BVO), and the bulk charge separation efficiency reaches 88.2% (vs 51.3%, BVO). The photocurrent of CoZnMOF/B-BVO retains 90% after 10 h test. Oxidation state modulation of MOFs with the second metal is an effective strategy to improve MOFs' activity.