Efficient Solar Water Splitting Research Articles

CoGa-Layered double hydroxides modified tin-doped hematite photoanode for efficient solar water splitting

CoGa-Layered double hydroxides modified tin-doped hematite photoanode for efficient solar water splitting

Complementary photoelectrochemical performances between dual heterojunctions and homojunction to achieve efficient solar water splitting of ZnIn2S4-based photoanode

The broken verge of carrier separation efficiency is invaluable factor to enhance photoelectrochemical performances of ZnIn2S4-based dual heterojunction. Hence, additional internal electric field is introduced when hexagonal/cubic ZnIn2S4 homojunction interface is used as medium to tandem Sb2S3/hexagonal-ZnIn2S4 and cubic-ZnIn2S4/Cu2S heterojunctions. The enhanced photoelectrochemical properties of Sb2S3/hexagonal/cubic ZnIn2S4/Cu2S are initiated through complementary properties of heterojunctions and homojunction components: (i) carrier separation efficiency verge of heterojunctions is broken to 56.29 % due to cooperation of homojunction, (ii) photoinduced carrier excitation energy homojunction is reduced to 2.06 eV with the modification of heterojunctions, (iii) built-in electric field is reinforced and electron delocalization of homojunction is enhanced by heterojunction, (iv) photoelectrochemical reaction sites of homojunction are increased through optimizing surface states effect of heterojunctions. As a result, the upgraded to 31.80 times higher water oxidization photocurrent density of 4.77 mA/cm2 is achieved by Sb2S3/hexagonal/cubic ZnIn2S4/Cu2S with decreased reaction overpotentials to 0.60 V. This work raises new thoughts on modified PEC performances of ZnIn2S4 via composite methods.

Nesting BiVO4 nanoislands in ZnO nanodendrites by two-step electrodeposition for efficient solar water splitting

Photoanodes with a large electrochemically active surface area, rapid charge transfer, and broadband light harvesting capacity are required to maximize the photoelectrochemical (PEC) water splitting performance. To address these features, we demonstrate that 3D hierarchal ZnO nanodendrites (NDs) can be sensitized with BiVO4 nanoislands by chemical and thermal treatments of electrodeposited Bi metal films. The flat band measurements and optical characterization suggested that the resulting heterojunction had type-II band alignment with a viable charge transfer from BiVO4 to ZnO NDs. In parallel, PL analysis revealed inhibition of the charge recombination rate by the electron transfer between BiVO4 and ZnO NDs. Upon AM 1.5 G illumination, BiVO4/ZnO NDs heterojunction yielded the highest photocurrent efficiency (0.15 mA·cm−2 at 1.2 V vs. NHE), which was attributed to its enhanced surface area (due to the presence of small dendrite branches), extended broadband light absorption extending from UV to visible light regions, and the most efficient interfacial charge transfer as proven by electrochemical impedance spectroscopy (EIS) studies. Besides, the incident photon-to-current conversion efficiency and applied bias photon-to-current efficiency tests confirmed an improved spectral photoresponse of the heterojunction based photoanode, particularly towards the visible light spectrum. The results outline a promising synthesis route for building heterojunctions between visible light active and wide band gap semiconductors for the use as a highly efficient photoanodes in a PEC cell.

Tailoring multi-layered BiVO4/WO3 photoanodes for an efficient photoelectrochemical gas-phase solar water splitting

Photoelectrochemical gas-phase solar water splitting represents a promising avenue for sustainable hydrogen production, fulfilling the energy demands of the future. In this work, we present the fabrication and optimization of the functional performance of visible-light active BiVO4/WO3 photoanodes with a tailored multi-layered structure. The photoanodes are manufactured in a reproducible way by an automated spray pyrolysis technique, including BiVO4 (top layer) and WO3 (bottom layer) in a multi-layered structure with different ratios (80:20; 50:50; 20:80). The photoactive surfaces are fully characterized and then evaluated in a divided filter-press reactor operated in continuous mode under visible light irradiation (100 mW cm−2), where a platinized titanium plate is used as dark cathode. The results demonstrate the pivotal role of mass ratio in optimizing charge transfer kinetics and recombination rates in the photoanode. We demonstrate the synergistic effects between BiVO4 and WO3, elucidating their contributions to enhance light absorption, and reduce electron-hole recombination, determining an optimal ratio of 80:20 for both hydrogen production (155.7 μmol m−2 s−1) and on-off current density gap (0.66 mA cm−2). The developed multi-layered BiVO4/WO3 surfaces thus offer a compelling pathway for advancing gas-phase photoelectrochemical water splitting technology, underscoring the significance of electrode design and paving the way for sustainable solar hydrogen production.

A Br-triggered BiVO4 photoanode surface for efficient solar water splitting

Adding dopants to the structure and coupling cocatalysts on the surface are traditional strategies to inhibit carrier recombination and high catalytic barriers for enhancing the performance of water splitting of the BiVO4 photoanode.

Photo-driven electrochemical etching to construct porous structures with oxygen vacancies on ZnO photoanodes for efficient solar water splitting

Fast charge separation and oxidation kinetics are the key issues for photoelectrochemical (PEC) water splitting. Herein, photo-driven electrochemical etching (PEE) was applied to modify the morphology and surface properties of ZnO photoanodes to improve PEC performance. By regulating the applied potential and treatment time, the PEE generated a lotus root like porous structure with a pore diameter of 20–30 nm and depth of about 300 nm, and produced a large amount (30.1 %) of oxygen vacancies (Ov) on ZnO photoanode. PEE treated-ZnO array demonstrated a charge separation efficiency of 87 %, and achieved 1.8 times larger photocurrent (1.26 mAcm−2 at 1.23 VRHE) with a cathodic shift of onset potential of 0.12 VRHE than pristine ZnO. The introduced Ov and the shortened charge transport distance of porous microstructures by PEE treatment improved the electrode/electrolyte interface charge transfer and water oxidation kinetics. DFT calculations indicate that the construction of oxygen defects on the catalyst surface can reduce the overpotential of oxygen evolution reaction (OER) and further promote the catalytic activity of OER. Moreover, electrode surface pore structure can be tailored by adjusting illumination parameters and electrolytes. This work illustrates a novel and effective approach to tunable photo-corrodible electrode materials for solar water splitting.

Constructing iron-group doped metal–organic framework films on hematite photoanodes for efficient solar water splitting

Constructing iron-group doped metal–organic framework films on hematite photoanodes for efficient solar water splitting

Uniform deposition of metal-organic coordination networks on BiVO4 through chemical bonding for efficient solar water splitting

Uniform deposition of metal-organic coordination networks on BiVO4 through chemical bonding for efficient solar water splitting

Plasmonic coupling-boosted photothermal nanoreactor for efficient solar light-driven photocatalytic water splitting

Photothermal nanoreactor with rapid charge transfer and improved spectral utilization is a key point in photocatalysis research. Herein, silver sulfide quantum dots (Ag2S QDs) were coating on the surface of porous graphitic carbon nitride nano vesicles (PCNNVs) to form Ag2S/PCNNVs nanoreactors by a simple calcination method for obtaining efficient photothermal-assisted photocatalytic hydrogen (H2) evolution under simulated/real sunlight irradiation. In particularly, the as-prepared optimal 3% Ag2S/PCNNVs sample exhibited the H2 production rate of 34.8 mmol h−1 g−1, which was 3.5 times higher than that of bare PCNNVs. The enhancement of photothermal-assisted activity over the Ag2S/PCNNVs composite system is mainly attributed to the coupling of the photothermal conversion performance of Ag2S QDs and the thermal insulation performance of PCNNVs based on the plasmonic coupling-boosted photothermal nanoreactor. This study presents a promising strategy for the development of high-efficient photothermal-assisted photocatalysts.

Citrate modulation of CoAl(OH)x Catalyst/Sb–TiO2 nanorods interface boosting photocarrier separation and injection for enhanced water oxidation

The design of an oxygen evolution catalyst (OEC) coupled photoanode ensuring effective charge separation, transfer, and injection is always desirable for enhanced water oxidation kinetics. Here, we have designed Sb-doped TiO2 nanorods (NRs) photoanode coupled with the ultrathin amorphous CoAl(OH)x OEC via citrate anion bridging. The CoAl(OH)x performs as a hole scavenger OEC, a hole storage layer, surface defect passivation layer, and also generates a band banding leading to the built-in potential at the semiconductor/catalyst interface favoring rapid hole separation. The citrate linking improves the CoAl(OH)x/Sb–TiO2 interfacial bonding and reduces the interfacial impedance to boost the photocarrier separation. Citrate bridging also serves as an efficient hole extraction layer and transportation pathway and promotes oxygen vacancy formation, which favors adsorbing OH− during the OER. As a result, the CoAl(OH)x/cit/Sb–TiO2 NRs photoanode exhibits a superior photocurrent density of 1.73 mA cm−2 at 1.23 V vs. reversible hydrogen electrode (VRHE), close to the theoretical maxima of titanium dioxide, with a very low onset potential of 0.12 VRHE and an extremely high hole injection efficiency of 93.1%. This work demonstrates an effective design strategy to boost the performance of OER catalyst coupled photo harvesting semiconductor photoanodes to achieve efficient solar water splitting.

Ru-P pair sites boost charge transport in hematite photoanodes for exceeding 1% efficient solar water splitting

Fast transport of charge carriers in semiconductor photoelectrodes are a major determinant of the solar-to-hydrogen efficiency for photoelectrochemical (PEC) water slitting. While doping metal ions as single atoms/clusters in photoelectrodes has been popularly used to regulate their charge transport, PEC performances are often low due to the limited charge mobility and severe charge recombination. Here, we disperse Ru and P diatomic sites onto hematite (DASs Ru-P:Fe2O3) to construct an efficient photoelectrode inspired by the concept of correlated single-atom engineering. The resultant photoanode shows superior photocurrent densities of 4.55 and 6.5 mA cm-2 at 1.23 and 1.50 VRHE, a low-onset potential of 0.58 VRHE, and a high applied bias photon-to-current conversion efficiency of 1.00% under one sun illumination, which are much better than the pristine Fe2O3. A detailed dynamic analysis reveals that a remarkable synergetic ineraction of the reduced recombination by a low Ru doping concentration with substitution of Fe site as well as the construction of Ru-P bonds in the material increases the carrier separation and fast charge transportation dynamics. A systematic simulation study further proves the superiority of the Ru-P bonds compared to the Ru-O bonds, which allows more long-lived carriers to participate in the water oxidation reaction. This work offers an effective strategy for enhancing charge carrier transportation dynamics by constructing pair sites into semiconductors, which may be extended to other photoelectrodes for solar water splitting.

PbTiO3 Based Single-Domain Ferroelectric Photocatalysts for Water Splitting

ConspectusSolar-driven photocatalytic water splitting paves a way to produce green hydrogen for building a sustainable clean energy system, particularly within the framework of the carbon-neutral initiative. However, to date, the solar-to-hydrogen (STH) conversion efficiency of particulate photocatalysts falls far short of the demand of over 10% for industrial applications. Single-domain ferroelectric semiconductor materials with amazing unidirectional spontaneous polarization penetrating the bulk are promising candidates as photocatalysts for water splitting. Their existing inherent internal field set up by polarization can cause both two oppositely charged surfaces (namely, polar surfaces) and spatial separation of photogenerated charge carriers between them upon light excitation. These unique properties provide sufficient new room for flexibly engineering of bulk and surface/interface structures to enhance photocatalytic water splitting.In this Account, we offer a systematic overview of the exploration of PbTiO3 based single-domain ferroelectric photocatalysts for efficient solar water splitting. In detail, the controlled growth, surface modification, heterostructures, and Z-scheme system of single-domain PbTiO3 ferroelectrics are discussed by considering polar surfaces, spatial separation of the photoexcited charges, selective distribution of defects, and selective adsorption of reactive species. We start with the controlled synthesis of single-domain ferroelectric PbTiO3 nanoplates to realize the opposite directional transport of photoexcited charges driven by the polarized internal electric field, which can be verified by the fact that photochemical reduction/oxidation deposits occur on opposite polar surfaces. Then, we move to the selective modification of the specific polar surface to promote the transfer of photoexcited charges by applying the spatial separation effect of photoexcited charges and defects related internal screening mechanism. Furthermore, asymmetric heterostructures and Z-scheme systems are rationally designed by selective adsorption of reactive species on charged surfaces to promote charge separation and suppress redox mediator side reactions, respectively. Although significant advances have been achieved, many efforts remain needed toward the industrial production target. Based on single-domain ferroelectric photocatalysts, we present the following perspectives on the utilization of a wide solar spectrum for improving solar energy conversion efficiency. First, proper dopants can be incorporated homogeneously in the bulk to reduce the bandgap without obviously weakening ferroelectricity. Second, the epitaxial growth of narrow bandgap semiconductors on the ferroelectrics is feasible to induce additional polarization in the region close to their interface. These two aspects are anticipated to enable strong visible light absorption and charge separation efficiency. Third but not least, the assembly of Z-scheme systems with two narrow bandgap ferroelectrics is desirable to mimic natural photosynthetic systems with quantum yield approaching one unit.

CuAlO2/CuBi2O4 heterojunction photocathodes with improved charge collection for efficient solar water splitting

P-type heterojunction photoelectrode made of CuAlO2 and CuBi2O4 has been developed for the first time by using solution-based deposition method. Its advantageous bandgap size and favourable band positions make it a highly prospective option for photoelectrochemical (PEC) water splitting. The photoelectrode was characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), UV–vis absorption spectra, time-resolved photoluminescence (TRPL) and X-ray photoelectron spectroscopy (XPS). Compared to CuBi2O4 alone, the heterojunction photoelectrode showed improved photoelectrochemical activity with the highest photocurrent density of − 0.62 mA/cm2 at 0.6 V vs. RHE. The improvement is attributed to decreased recombination of photo-generated electrons and holes and improved charge separation and transport in the heterostructure. This research offers an efficient method to enhance the visible-light-driven activity of CuBi2O4 and a new strategy for fabricating heterogeneous nanostructure photoelectrodes for solar-driven water splitting.

All-Perovskite Tandem Photoelectrodes for Unassisted Solar Hydrogen Production

Unassisted solar water splitting by multijunction tandem photoelectrodes is a sustainable approach to generating green hydrogen fuels. Here, we report on the fabrication of monolithically integrated all-perovskite tandem photocathodes for efficient unassisted solar water splitting. All-perovskite tandem photocathodes wired to an iridium oxide anode deliver a high photovoltage of more than 2 V with an operating photocurrent density of 12.5 mA cm–2 at zero applied bias under simulated AM1.5G 1 sun illumination, yielding a solar-to-hydrogen (STH) conversion efficiency of 15%. The tandem photoelectrode demonstrates continuous operation in water for more than 120 h under simulated 1 sun illumination with less than 5% efficiency loss. A technoeconomic analysis shows the projected production cost of all-perovskite tandem photoelectrodes is $30 m–2, promising a levelized cost of hydrogen of less than $1/kg if sufficient device longevity is realized. This work provides a path toward achieving cost-effective unassisted solar hydrogen fuel production.

Boosting charge separation and transfer at the boron-triggered BiVO4 interface for efficient and stable solar water splitting

Photoelectrochemical (PEC) water splitting is a promising strategy for solar energy conversion and storage. However, poor charge transfer and sluggish water oxidation reaction restrain the PEC performance. Herein, we propose a simple and effective interface etching strategy on the semiconductor surface to modulate the photoanode/cocatalyst interface, following by the decoration of NiFeBi oxygen evolution catalyst (OEC) for significantly improved oxygen evolution activity and stability of the BiVO4 photoanode. The resultant photoanode achieves a record photocurrent density of 6.33 mA cm−2 at 1.23 VRHE under one sun illumination, over approximately 3.63 times higher than that of the pristine BiVO4. The applied bias photon-to-current efficiency of 2.3% can be attained with showing a good durability of 40 h. Systematic studies have demonstrated that the introduction of electron-deficient state of B accelerates the surface trapping hole and the charge transfer, which enhances the interfacial coupling between BiVO4 and NiFeBi OEC, and strongly drives the hole from the BiVO4 to the NiFeBi surface for capable water oxidation. Density functional theory calculations prove that the surface B treatment increases the effective number of reaction paths of OER and the density of electronic states near the Fermi energy level, which improves the electrical conductivity of the BiVO4/OEC interface. This work provides a feasible strategy for tailoring the interface engineering to significantly enhance the PEC activity and stability of the photoanodes.

In-situ cation-exchange strategy for engineering single-atomic Co on TiO2 photoanode toward efficient and durable solar water splitting

Single-atomic catalysts (SACs) have been emerging as one of potential candidates in catalysts, owing to their unique merits with extremely high specific surface area as well as remarkably exposed active sites. Herein, we develop an in-situ gas-phase cation exchange strategy for engineering single-atomic Co on the surface of TiO2 photoanode toward solar water splitting. It is verified that the atomically-dispersed Co with Co-O coordination could optimize the surface electronic structures, enhance the light absorption, promote the photoinduced charge transfer, lower the reaction barrier and accelerate the reaction kinetics, which consequently enable the overall improved photoelectrochemical (PEC) behaviors for photoanodes. As a proof of concept, the as-constructed TiO2-based photoanodes deliver robust stability up to 100 h and a photocurrent density up to 1.47 mA cm−2 at 1.23 V vs. RHE, which are superior to those of pristine TiO2, representing their significance for potential applications.

Assembly of a novel Fe2TiO5-impregnated donor-π-acceptor conjugated carbon nitride for highly efficient solar water splitting

Carbon nitride (CN) exhibits low photocatalytic performance for the energy conversion during photocatalytic water splitting (PWS) under visible light spectrum. Designing novel high-performance CN-based photocatalysts via providing advanced intrinsic characteristics to pristine CN material may further endorse the PWS process. Integrating organic molecules within the skeleton of CN is an interesting strategy to lessen the extent of charge recombination by promoting effective charge separation and transportation, thereby boosting light absorption and photocatalytic performances. Yet, limited progresses have been made along this line. We have carefully chosen and fused previously unexplored organic donor-π-acceptor-type 2,6-diaminopurine (DP) molecules within the skeleton of CN via a superficial organic molecular engineering. Moreover, the subsequent introduction of Fe2TiO5 with copolymerized CN-DP10.0 and pristine CN provide considerably more surface-active sites to form heterojunction material, synergistically extending both the optical absorption range and the lifetime of photoinduced electron. More importantly, with a typical type-II heterojunction formation, the CN and Fe2TiO5 energy levels are overlapped and compatible, thereby significantly accelerating both electron and hole transfer processes. These beneficial characteristics in turn lead to an outstanding apparent quantum yield (AQY) of 61.7% at a wavelength of 420 nm, together with highly efficient and greatly improved photocatalytic hydrogen (H2) (165.7 μmol/h) along with oxygen (O2) (6.2 μmol/h) evolution reactions in visible spectrum at 420 nm. These values are 15.7 and 11.4 times more than those of the pristine CN (9.9 μmol/h and 0.5 μmol/h). Consistent with these experimental findings, our computational simulations show that integration of Fe2TiO5 and DP molecules within the CN framework decreases the band gap from 3.25 to 2.55 eV, suggesting remarkable photocatalytic activities of Fe2TiO5/CN-DP10.0. As a result, it is anticipated that this work will offer a significant avenue for scientific investigation into the next generation of renewable energy employing improved heterostructure photocatalyst predicated on organic ligands.

Defect-rich spinel ferrites with improved charge collection properties for efficient solar water splitting

Spinel zinc ferrite (ZnFe₂O₄, ZFO) is a potential photoanode material for photoelectrochemical (PEC) water splitting because of its ideal bandgap (1.9–2.1 eV) and superior chemical stability in aqueous solutions. However, the low charge collection efficiency significantly hinders the improvement in PEC activity. Herein, we report an ultrafast and effective flame activation route to enhance the charge collection properties of ZFO. First, high-temperature flame (&gt; 1300 ℃) facilitated surface and grain boundary diffusions, increasing the grain size and connectivity of the ZFO nanoparticles. Second, the reducing atmosphere of the flame enabled the formation of surface defects (oxygen vacancy and Fe²⁺), thereby increasing the charge carrier density and surface adsorption sites. Significantly, these two factors promoted charge transport and transfer kinetics, resulting in a 10-fold increase in the photocurrent density over the unactivated ZFO. Furthermore, we deposited a thin Al₂O₃ overlayer to passivate the ZFO surface and then the NiFeO_<i>x</i> oxygen evolution catalyst (OEC) to expedite hole injection into the electrolyte. This surface passivation and OEC deposition led to a remarkable photocurrent density of ~1 mA/cm² at 1.23 V versus the reversible hydrogen electrode, which is the highest value among all reported ZFO photoanodes. Notably, the NiFeO_<i>x</i>/Al₂O₃/F-ZFO photoanode achieved excellent photocurrent stability over 55 h (96% retention) and superior faradaic efficiency (FE &gt; 94%). Our flame activation method is also effective in improving the photocurrent densities of other spinel ferrites: CuFe₂O₄ (93 times), MgFe₂O₄ (16 times), and NiFe₂O₄ (12 times).

Pd/PdO and hydrous RuO2 difunction-modified SiO2@TaON@Ta3N5 nano-photocatalyst for efficient solar overall water splitting

A novel Pd/PdO and hydrous RuO2 difunction-modified SiO2@TaON@Ta3N5 core-shell structured nano-photocatalyst was synthesized successfully, which displayed excellent photocatalytic activity for overall water splitting into H2 (473.52 μmol−1·g−1·h−1), about 2.86 times higher than unmodified SiO2@TaON@Ta3N5 (165.74 μmol−1·g−1·h−1), under the visible-light irradiation with the wavelength ≥420 nm, without any sacrificial agent, as well as excellent stability against photocorrosion. The apparent quantum yield (AQY) reaches to 0.253% under irradiation intensity of 12 mW cm−2 at 420 nm. The spatially separated Pd, PdO and RuO2 clusters were decorated on the Ta3N5 surface to construct local multi-heterojunctions, which were confirmed to enhance the light absorption capability, drive efficient separation of charge carriers and directional transfer, and promote surface redox reaction kinetics of HER and OER. The trace modification of metallic Pd clusters and TaN could mainly contribute to the significant decrease in the HER overpotential, while PdO exhibited a stronger contribution than RuO2 for OER catalytic activity. The synergetic mechanism of enhanced photocatalytic overall water splitting for hydrogen production was discussed in detail. Thus the combination of core-shell heterojunction construction and surface difunction modification provides a promising strategy for develop efficient all-in-one photocatalysts for solar overall water splitting.

Janus β-Te2X (X = S, Se) monolayers for efficient excitonic solar cells and photocatalytic water splitting

In this work, we have investigated the novel 2D allotropes i.e., β-Te2X (X = S, Se) using first-principles calculations and study their potential applications in light harvesting devices.