Small molecule π-π stacking promotes efficient photoelectrocatalytic splitting of aqueous hydrogen production from polyaniline.

The efficiency of photoelectrocatalysis is fundamentally dependent on the sufficient absorption of light and efficient utilisation of photogenerated carriers, but is largely limited by the reactivity from the inefficient charge transfer and surface sites of the catalyst. In this study, π-π stacking of polar small molecules on aromatic ring-rich polyaniline (PANI) was carried out to improve its photoelectrocatalytic splitting of water for hydrogen production. Detailed photoelectrochemical experiments and density-functional theory (DFT) calculations show that small molecules of p-aminobenzoic acid (PABA) and PANI have the best π-π stacking (compared to p-toluenesulfonic acid (PTA)), which promotes the separation of carriers on the PANI surface. In addition, the polar effect of the small molecules also improves the reactivity of the PANI surface and also reduces the potential barrier for H2 evolution. The current density of PANI-PABA reached -0.12 mA/cm2 (1.23 V vs. RHE) 2.53 times higher than that of pure PANI in linear voltammetric scanning tests under light. This strategy of introducing polar small molecules into organocatalysts via π-π stacking will provide new ideas for the preparation of efficient organic photoelectrocatalysis.

Construction of [NbO]6-x -xS Structure to Change Charge Density and Regulate Spontaneous Polarization to Achieve Efficient Pyro-Photo-Electric Water Splitting System of NaNbO3.

Pyroelectric materials in the field of photoelectrochemical (PEC) water splitting still face the problems of difficult low spontaneous polarization intensity and excessive carrier recombination. Based on the above problems, we altered the interaction between S-Nb-S in the [NbO]6-x -xS structure, and the constructed [NbO]6-x -xS structure achieved the regulation of charge density change and spontaneous polarization. The results show that under the stimulation of light and temperature fluctuations, the current density of the NS-4 photoanode is as high as 0.574 mA/cm2 at 1.23 VRHE , which is about 1.59 times higher than the pure NaNbO3 current density value, and the NS -4 photoanode achieves IPCE value of 16.08 %. The first-principles density-functional theory calculations (DFT) reveal the principle of the [NbO]6-x -xS structure for the suppression function of the carrier recombination and the improvement function of the pyroelectric effect. The analysis shows that the S-doping leads to the weakening of S-Nb-S interactions in the [NbO]6-x -xS structure, which improves the pyroelectric effect and suppresses the photo/pyro-generated carrier recombination, and effectively enhances the performance of the pyro-photo-electric synergistic water splitting system. This work promotes the development of pyroelectric materials in the field of photoelectrochemical water splitting.

Modulation of Lewis and Brønsted Acidic Sites to Enhance the Redox Ability of Nb2O5 Photoanodes for Efficient Photoelectrochemical Performance.

Accelerated surface redox reaction and regulated carrier separation are the crux to the development of highly reactive oxide semiconductors for efficient photoelectrochemical water splitting. Here, we have selected Nb2O5 materials that combine unique surface acidity and semiconductor properties, and first used surface phosphorylation to change its surface acidic sites (Lewis and Brønsted acidic sites) to achieve efficient photoelectrochemical water splitting. The resulting photoanode born from this strategy exhibits a high photocurrent density of 0.348 mA/cm2 at 1.23 VRHE, which is about 2-fold higher than that of the bare Nb2O5, and a cathodic shift of 60 mV. Detailed experimental results show that the large increase in the Lewis acidic site can effectively modulate the electronic structure of the active sites involved in catalysis in [NbO5] polyhedra and promote the activation of lattice oxygen. As a result, higher redox properties and the ability to inhibit carrier recombination are exhibited. In addition, the weakening of the Brønsted acidic site drives the reduction of protons in the oxygen evolution reaction (OER) and accelerates the reaction kinetics. This work advances the development of efficient photoelectrochemical water splitting on photoanodes driven by the effective use of surface acidity and provides a strategy for enhancing redox capacity to achieve highly active photoanodes.

A promising ternary sulfide bidirectional p-n heterojunction for unassisted tandem photoelectrochemical cells.

There has been great demand for high-efficiency, low-cost, and abundant photoelectrode materials for photoelectrochemical (PEC) systems. Here, we studied the PEC performance of ternary sulfide photoelectrodes (ZnIn2S4 and CuInS2) with the same nanostructure and applied bidirectional p-n heterojunctions with energy levels that were well matched to the unassisted tandem PEC cell device. Moreover, ZnIn2S4/CuInS2 and CuInS2/ZnIn2S4 were used as a photoanode and photocathode, respectively, in the unassisted tandem PEC cell device, which achieved a relatively high photocurrent density of 0.06 mA cm-2. This work provides new ideas for the design and manufacture of high-efficiency unassisted tandem PEC cell devices.

A uniformly decorated and photostable polydopamine-organic semiconductor to boost the photoelectrochemical water splitting performance of CdS photoanodes.

Photoelectrochemical (PEC) water splitting to produce renewable H2 fuel by storage of solar energy has attracted increasing attention as it could reduce carbon footprint and solve the global consumption growth. Herein, a photostable polymer polydopamine (PDA) was introduced to enhance the PEC performance by forming a uniform inorganic-organic hybrid heterostructure with CdS. The organic semiconductor PDA not only forms a strong coordinate bond to facilitate the transfer of electrons, but also acts as a passivation layer, contributing to improve the stability of the photoelectrode. A photocurrent density of 1.08 mA cm-2 was achieved for CdS/1PDA, which was about 2.4 times that of bare CdS at 0.28 V vs. RHE, and CdS/1PDA featured a reasonable photocurrent stability compared with bare CdS. The Co-Pi co-catalyst, as a hole acceptor, further prohibited charge recombination and promoted the water oxidation kinetics. The photocurrent density of CdS/1PDA/5Co-Pi was up to 2.68 mA cm-2 (0.28 V vs. RHE), which was 5.7 and 2.5 times higher than that of bare CdS and CdS/1PD, respectively. The strategy provides a beneficial insight to design an inorganic-organic uniform heterostructure for the enhancement in PEC performance.

A promising p-type Co-ZnFe2O4 nanorod film as a photocathode for photoelectrochemical water splitting.

A p-type Co-ZnFe2O4 film with a one-dimensional (1D) rod-like morphology is fabricated for the first time on fluorine-doped tin oxide (FTO) through a hydrothermal reaction and sintering treatment. The p-type Co-ZnFe2O4 is obtained by doping Co ions into n-type ZnFe2O4, in which Zn sites are substituted by Co. Compared with the n-type ZnFe2O4, the light absorption edge of Co-ZnFe2O4 is clearly shifted from 589 to 624 nm, and the positions of the valence/conduction band of Co-ZnFe2O4 meet the thermodynamic requirements for water splitting. The photocurrent density of p-type Co-ZnFe2O4 is -0.22 mA cm-2 at 0 V vs. the reversible hydrogen electrode (RHE), which is enhanced 7.33-times vs. that of n-type ZnFe2O4 (-0.03 mA cm-2 at 0 V vs. RHE). This work provides useful insights into tuning the p-n character of semiconductors to realize efficient photoelectrochemical (PEC) water splitting.

A high-efficiency and stable cupric oxide photocathode coupled with Al surface plasmon resonance and Al2O3 self-passivation.

The key to achieving high performance in photoelectrochemical (PEC) water splitting is the design of efficient and stable photoelectrode structures. Herein, we firstly synthesized a novel and high-photoactivity CuO/Al photocathode and then the Al2O3 passivation layer was further introduced through a spontaneous oxidation process in air to protect the photocathode against photocorrosion. On account of the localized surface plasmon resonance (LSPR) of Al nanoparticles (NPs) in conjunction with surface passivation of the Al2O3 layer, the obtained CuO/Al/Al2O3 photocathode exhibits a high photocurrent density of -0.95 mA cm-2 at -0.55 V vs. Ag/AgCl and photocorrosion stability of 89.5% after 1 h.

1D WO3 Nanorods/2D WO3-x Nanoflakes Homojunction Structure for Enhanced Charge Separation and Transfer towards Efficient Photoelectrochemical Performance.

Designing and fabricating photoelectrodes with low carrier recombination, high carrier transfer, and high light-capture capability is of great significance for achieving effective photoelectrochemical (PEC) water splitting. Herein, for the first time, 2D nonstoichiometric WO3-x nanoflakes (NFs) were vertically grown by hydrothermal synthesis on 1D WO3 nanorods (NRs) obtained by a hydrothermal method and high-temperature annealing (HTA). In this 1D HTA-WO3 /2D WO3-x photoanode, the 2D WO3-x NFs with active areas could maximize light harvesting, and the unique 1D/2D homojunction structure could improve the carrier-separation efficiency. At the same time, the 1D WO3 NRs with high aspect ratio were more beneficial to charge transfer after HTA. As expected, the 1D HTA-WO3 /2D WO3-x photoanode yielded an enhanced photocurrent density of 0.98 mA cm-2 at 1.23 V versus reversible hydrogen electrode, which is approximately 3.16 times that of pristine WO3 . The improvement could be attributed to the synergistic effect of HTA and the homojunction structure in the 1D HTA-WO3 /2D WO3-x photoanode, which could effectively improve carrier separation and transfer. Furthermore, this work may provide a promising strategy for the design and fabrication of semiconductor-based photoelectrodes.

2D elongated polyhedral-like YVO4 films: a novel photoanode for photoelectrochemical water splitting.

YVO4 films, prepared on FTO substrates using a long-term hydrothermal method, possessing two-dimensional (2D) elongated polyhedral microcrystals and serving as a novel photoanode in the photoelectrochemical (PEC) field, are reported for the first time. The research indicates that YVO4 is a potential photoanode for PEC water splitting.

Accelerating the charge separation of ZnFe2O4 nanorods by Cu-Sn ions gradient doping for efficient photoelectrochemical water splitting.

Spinel zinc ferrite (ZnFe2O4) with an appropriate band gap (2.1 eV) is a promising photoanode in the photoelectrocatalysis field, however, the photoelectrochemical (PEC) performance of ZnFe2O4 is confined due to poor charge separation. Hence, improving charge separation efficiency is essential to modify the PEC performance of ZnFe2O4. Herein, the novel Cu-Sn ions doped ZnFe2O4 nanorods are fabricated for the first time. Cu2+ ions are doped into ZnFe2O4 nanorods from surface to inside with a degressive concentration, it is helpful to enhance the charge separation efficiency of ZnFe2O4 nanorods along horizontal direction. Sn4+ ions originated from fluorine-doped tin oxide (FTO) layer are doped into Cu-ZnFe2O4 nanorods with a decreasing concentration from bottom to top, it is conducive to reducing surface trapping states and accelerating charge separation along vertical direction. Therefore, Cu-Sn dual ions gradient doping synergistically boosts the PEC activity of ZnFe2O4. The Cu.Sn-ZnFe2O4 nanorods have an excellent photocurrent density of 0.46 mA⋅cm-2 at 1.23 V vs RHE, which is 4.18 times than that of ZnFe2O4. The bulk charge separation efficiency (ηbulk) and surface charge separation efficiency (ηsurface) of Cu.Sn-ZnFe2O4 are 17.52% and 55.22%, which are 5.84 and 3.78 times than that of ZnFe2O4, respectively. This work provides inspirations for promoting charge separation efficiency of photoanodes, thus achieving efficient PEC water splitting.

3D Branched Ca-Fe2 O3 /Fe2 O3 Decorated with Pt and Co-Pi: Improved Charge-Separation Dynamics and Photoelectrochemical Performance.

The construction of junctions on hematite is an effective way to overcome the problems of slow charge separation and transfer kinetics, but constructing the junction is a significant challenge in photoelectrochemical (PEC) water splitting. Herein, a considerable improvement in PEC performance for α-Fe2 O3 was achieved following the introduction of a p-n homojunction between n-type α-Fe2 O3 and p-type Ca-doped α-Fe2 O3 through a facile hydrothermal method. The resultant 3D branched Ca-Fe2 O3 /Fe2 O3 enhanced the absorption intensity and reached a photocurrent density of 2.14 mA cm-2 at 1.23 V vs. reversible hydrogen electrode (RHE). The merit of the desired lattice matching of the buried p-n homojunction structure built an internal electric field, which led to appropriate band alignment. These results were supported by a series of photoelectrochemical measurements, in particular, surface photovoltage (SPV) measurements. For further improvement of the charge-separation efficiency, a combination of separated cocatalysts was established on the homojunction structure, in which Pt acted as the electron collector and was deposited on the bottom, and Co-Pi as the hole-extraction cocatalyst was inserted to accelerate hole transfer on the surface of the photoanode. The resulting Co-Pi/Ca-Fe2 O3 /Fe2 O3 /Pt branched nanorods showed a significant improvement in charge-separation efficiency and photocurrent density (2.94 mA cm-2 at 1.23 V vs. RHE). The present strategy, both the construction of the p-n homojunction and the coupling electron- and hole-transfer cocatalyst, could be expanded to many unstable or low-efficiency semiconductors for the design and fabrication of cost-effective photoanodes in PEC water splitting.

Improved Mechanical and Electrochemical Properties of XNBR Dielectric Elastomer Actuator by Poly(dopamine) Functionalized Graphene Nano-Sheets.

In this work, graphene nano-sheets (GNS) functionalized with poly(dopamine) (PDA) (denoted as GNS-PDA) were dispersed in a carboxylated nitrile butadiene rubber (XNBR) matrix to obtain excellent dielectric composites via latex mixing. Because hydrogen bonds were formed between ⁻COOH groups of XNBR and phenolic hydroxyl groups of PDA, the encapsulation of GNS-PDA around XNBR latex particles was achieved, and led to a segregated network structure of filler formed in the GNS-PDA/XNBR composite. Thus, the XNBR composite filled with GNS-PDA showed improved filler dispersion, enhanced dielectric constant and dielectric strength, and decreased conductivity compared with the XNBR composite filled with pristine GNS. Finally, the GNS-PDA/XNBR composite displayed an actuated strain of 2.4% at 18 kV/mm, and this actuated strain was much larger than that of pure XNBR (1.3%) at the same electric field. This simple, environmentally friendly, low-cost, and effective method provides a promising route for obtaining a high-performance dielectric elastomer with improved mechanical and electrochemical properties.