Guiji Liu

A Tantalum Nitride Photoanode Modified with a Hole-Storage Layer for Highly Stable Solar Water Splitting**

Photoelectrochemical (PEC) water splitting is an ideal approach for renewable solar fuel production. One of the major problems is that narrow bandgap semiconductors, such as tantalum nitride, though possessing desirable band alignment for water splitting, suffer from poor photostability for water oxidation. For the first time it is shown that the presence of a ferrihydrite layer permits sustainable water oxidation at the tantalum nitride photoanode for at least 6 h with a benchmark photocurrent over 5 mA cm(-2) , whereas the bare photoanode rapidly degrades within minutes. The remarkably enhanced photostability stems from the ferrihydrite, which acts as a hole-storage layer. Furthermore, this work demonstrates that it can be a general strategy for protecting narrow bandgap semiconductors against photocorrosion in solar water splitting.

Interface Engineering of a CoOx/Ta3N5 Photocatalyst for Unprecedented Water Oxidation Performance under Visible‐Light‐Irradiation

Cocatalysts have been extensively used to promote water oxidation efficiency in solar-to-chemical energy conversion, but the influence of interface compatibility between semiconductor and cocatalyst has been rarely addressed. Here we demonstrate a feasible strategy of interface wettability modification to enhance water oxidation efficiency of the state-of-the-art CoO(x)/Ta3N5 system. When the hydrophobic feature of a Ta3N5 semiconductor was modulated to a hydrophilic one by in situ or ex situ surface coating with a magnesia nanolayer (2-5 nm), the interfacial contact between the hydrophilic CoO(x) cocatalyst and the modified hydrophilic Ta3N5 semiconductor was greatly improved. Consequently, the visible-light-driven photocatalytic oxygen evolution rate of the resulting CoO(x)/MgO(in)-Ta3N5 photocatalyst is ca. 23 times that of the pristine Ta3N5 sample, with a new record (11.3%) of apparent quantum efficiency (AQE) under 500-600 nm illumination.

Visible light driven overall water splitting using cocatalyst/BiVO4 photoanode with minimized bias†

BiVO4 and many other semiconductor materials are ideal visible light responsive semiconductors, but are insufficient for overall water splitting. Upon loading water oxidation cocatalyst, for example Co-borate (denoted as CoBi) used here, onto BiVO4 photoanode, it is found that not only the onset potential is negatively shifted but also the photocurrent and the stability are significantly improved. And more importantly, PEC overall water splitting to H2 and O2 is realized using CoBi/BiVO4 as photoanode with a rather low applied bias (less than 0.3 V vs. counter electrode) in a two-electrode scheme, while at least 0.6 V is needed for bare BiVO4. This work demonstrates the practical possibility of achieving overall water splitting using the PEC strategy under a bias as low as the theoretical minimum, which is the difference between the flat band and proton reduction potential for a photoanode thermodynamically insufficient for water reduction. As long as the water oxidation overpotential is overcome with an efficient cocatalyst, the applied bias of the whole system is only used for that thermodynamically required for the proton reduction.

Enabling an integrated tantalum nitride photoanode to approach the theoretical photocurrent limit for solar water splitting

The feasibility of photoelectrochemical (PEC) water-splitting cells relies on the development of high-performance photoanodes. Significant progress has been made in the discovery of narrow bandgap semiconductors as promising photoanodes. However, the rational design of photoanode architecture that brings the potentials of narrow bandgap semiconductors into fruition for efficient PEC water oxidation still remains a key challenge. Herein, we show a highly efficient photoanode system consisting of a tantalum nitride (Ta3N5) semiconductor for light harvesting, hole-storage layers (Ni(OH)x/ferrhydrite) that mediate interfacial charge transfer from Ta3N5 to coupled molecular catalysts (Co cubane and Ir complex) for water oxidation and a TiOx blocking layer that reduces the surface electron–hole recombination. The integrated Ta3N5 photoanode exhibits a record photocurrent of 12.1 mA cm−2 at 1.23 V vs. the reversible hydrogen electrode (RHE), which is nearly its theoretical photocurrent limit under sunlight (12.9 mA cm−2), suggesting that almost each pair of photogenerated charge carriers in Ta3N5 has been efficiently extracted and collected for solar water splitting.

Microwave-assisted hydrothermal synthesis of perovskite NaTaO3 nanocrystals and their photocatalytic properties

A fast and facile process for the preparation of perovskite NaTaO3 nanocrystals with Ta2O5 and NaOH as starting materials by a microwave-assisted hydrothermal (MHT) technique is reported. By pretreating the Ta2O5 powder by ball milling, pure-phase NaTaO3 can be synthesized under quite mild conditions through the formation of an intermediate pyrochlore Na2Ta2O6 phase, while much longer times are required in a conventional hydrothermal (CHT) process. After loading with NiO co-catalyst, the NaTaO3 nanocrystals prepared by the MHT method showed photocatalytic activity for overall water splitting more than two times greater than those prepared under CHT conditions.

Efficient Hole Extraction from a Hole-Storage-Layer-Stabilized Tantalum Nitride Photoanode for Solar Water Splitting

One of the major hurdles that impedes the practical application of photoelectrochemical (PEC) water splitting is the lack of stable photoanodes with low onset potentials. Here, we report that the Ni(OH)x/MoO3 bilayer, acting as a hole-storage layer (HSL), efficiently harvests and stores holes from Ta3N5, resulting in at least 24 h of sustained water oxidation at the otherwise unstable Ta3N5 electrode and inducing a large cathodic shift of ≈600 mV in the onset potential of the Ta3N5 electrode.

Photoelectrochemical Water Splitting Promoted with a Disordered Surface Layer Created by Electrochemical Reduction

The recent discovery of colored TiO2 indicated that the disordered surface layer over the TiO2 particles/photoelectrodes is beneficial for higher photocatalytic performance; however, the role of the disordered surface TiO2 layer is not well understood. Here, we report an electrochemical strategy for tuning the surface structure of TiO2 nanorod arrays (NRAs) and try to understand the role of the disordered surface TiO2 layer. Photoelectrodes of TiO2 NRAs with a disordered shell were prepared by an electrochemical reduction method. The photocurrent of the NRAs with a disordered shell can reach as high as ∼1.18 mA/cm(2) at 1.23 V, which is 2.2 times of that of the pristine TiO2 NRAs. Our results show that the surface disordered layer not only improves the bulk charge separation but also suppresses the charge recombination at the electrode/electrolyte interface, acting as an efficient water oxidation cocatalyst of photoelectrochemical cell for solar water splitting.

Abnormal Effects of Cations (Li+, Na+, and K+) on Photoelectrochemical and Electrocatalytic Water Splitting

The electrode-electrolyte interface chemistry is highly important for photoelectrochemical (PEC) and electrocatalytic water splitting where cations in the electrolyte are often crucial. However, the roles of cations in an electrolyte are much debated and not well-understood. This work reports that the PEC and electrocatalytic water oxidation (WO) activities in basic electrolytes with different cations follow an unexpected trend (Li(+) > K(+) > Na(+)) especially for long-term reaction. Such an abnormal order of activity is found to be the balance effect of two factors: the distinct extents of the weakening of O-H bond on electrode surface after interacting with cations in different electrolytes and the different rates of oxygen reduction reaction (ORR) which turns out to be dominant. Li(+) not only brings the most significant decrease of O-H bond strength but also is most effective for avoiding back reaction, while Na(+) shows the most detrimental effect on WO because of ORR. Our results provide important insight into the roles of cations in WO and demonstrate a new strategy of tailoring the electrode-electrolyte interface via judicious choice of cations in electrolyte for more efficient PEC and electrocatalytic water splitting.

Enhancing photoresponsivity of self-powered UV photodetectors based on electrochemically reduced TiO2 nanorods

Electrochemically reduced TiO2 nanorod arrays (R-NRAs) have been used for the first time to construct a self-powered, visible light blind ultraviolet (UV) photodetector. The fabricated R-NRAs device demonstrated superior photodetector performance with high photon-to-current efficiency of up to 22.5% at an applied bias of 0 V. The enhancement is attributed to a disordered surface layer which greatly improves the charge separation and transfer efficiency at the electrode/electrolyte interface.