Photocatalysts For Solar Water Splitting Research Articles

Stability and degradation of (oxy)nitride photocatalysts for solar water splitting

The investigation of degradation processes provides opportunities for the stability improvement of photocatalysts and photoelectrodes. This perspective provides an overview on recent progress with a focus on (oxy)nitrides.

TiN anchored mesoporous single-crystalline LaTiO2N with ohmic contact to expedite photocarrier separation for efficient photocatalytic water splitting

LaTiO2N is a promising photocatalyst for solar water splitting but is generally subject to a low activity due to inadequate separation of photocarriers (i.e. e- and h+). Loading noble metal cocatalysts such as Pt often has limited improvements because of their Schottky-type interfaces with LaTiO2N, resulting in a deficient electron collection at the surface catalytic sites. In this work, TiN nanoparticles have been firmly anchored at the surface of mesoporous single-crystalline LaTiO2N. As opposed to Pt, TiN has an ohmic contact with LaTiO2N due to a comparable work function and the structurally matched heterointerfaces. Such a tight and ohmic connection enables TiN to readily collect electrons from LaTiO2N, which in turn substantially improves the photocarrier separation and photocatalytic activity. These findings justify that TiN is an ideal relay for electron transfer between n-type semiconductors and noble metal cocatalysts which would otherwise be very difficult when the Schottky-type interfaces are formed.

Synthesis and Structure of the Double-Layered Sillén-Aurivillius Perovskite Oxychloride La2.1Bi2.9Ti2O11Cl as a Potential Photocatalyst for Stable Visible Light Solar Water Splitting.

Exploring photocatalysts for solar water splitting is a relevant step toward sustainable hydrogen production. Sillén-Aurivillius-type compounds have proven to be a promising material class for photocatalytic and photoelectrochemical water splitting with the advantage of visible light activity coupled to enhanced stability because of their unique electronic structure. Especially, double- and multilayered Sillén-Aurivillius compounds [An-1BnO3n+1][Bi2O2]2Xm, with A and B being cations and X a halogen anion, offer a great variety in material composition and properties. Yet, research in this field is limited to only a few compounds, all of them containing mainly Ta5+ or Nb5+ as cations. This work takes advantage of the outstanding properties of Ti4+ demonstrated in the context of photocatalytic water splitting. A fully titanium-based oxychloride, La2.1Bi2.9Ti2O11Cl, with a double-layered Sillén-Aurivillius intergrowth structure is fabricated via a facile one-step solid-state synthesis. A detailed crystal structure analysis is performed via powder X-ray diffraction and correlated to density functional theory calculations, providing a detailed understanding of the site occupancies in the unit cell. The chemical composition and the morphology are studied using scanning and transmission electron microscopy together with energy-dispersive X-ray analysis. The capability of the compound to absorb visible light is demonstrated by UV-vis spectroscopy and analyzed by electronic structure calculations. The activity toward the hydrogen and the oxygen evolution reaction is evaluated by measuring anodic and cathodic photocurrent densities, oxygen evolution rates, and incident-current-to-photon efficiencies. Thanks to the incorporation of Ti4+, this Sillén-Aurivillius-type compound enables best-in-class photoelectrochemical water splitting performance at the oxygen evolution side under visible light irradiation. Thus, this work highlights the potential of Ti-containing Sillén-Aurivillius-type compounds as stable photocatalysts for visible light-driven solar water splitting.

Stress and defects cooperatively regulate the photocatalytic performance of AlN bulk phase materials

Due to the wide bandgap width, the lack of oxidation capacity, high carrier recombination rate and limited solar absorption seriously inhibit the photocatalytic activity of pure AlN. Using first principles, we report an efficient method, nonmetal-nonmetal (C–F and B–O) passivation co-doping and H interstitial synergistic biaxial stress action, to solve all the above problems in AlN. We found that by applying appropriate tensile strain, Al36N34CN1FN3H and Al36N34BN1ON3H doped structures can reduce the band gap and make the UV absorption edge redshift. The electron and hole can be separated quickly, and Al36N34CN1FN3H has better photocatalytic activity. It can also raise the potential of CBM and VBM to generate hydrogen and oxygen spontaneously and is beneficial to broadening the absorption range of sunlight. Through the theoretical evaluation of HER and OER activities of Al36N34CN1FN3H, we found that Al36N34CN1FN3H is suitable to be a good catalyst for the photocatalytic reaction. Our results are helpful for the design of high-efficiency sunlight responsive AlN photocatalysts for solar water splitting.

Efficient band structure tuning of single-layer group IV-N semiconductors for visible-light-driven water splitting

Discovering more novel two-dimensional (2D) materials for solar water splitting has long been an essential topic in photocatalysis. The present study employs the hybrid functional (HSE06) theory to systematically investigate the electronic structures and band alignments with respect to the water redox potential of the mono-doping (cationic C and anionic P) and co-doping (B + O, B + S, Al + O, Al + S, and C + P) in monolayer IV-N (IVSi, Ge, and Sn) semiconductors. These donor-acceptor doping pairs are charge-compensated so that the doping systems can keep semiconductor character without recombination centers. Importantly, we discovered several co-doped IV-N structures to be highly promising solar water splitting photocatalysts, including (B + S) co-doped SiN, and (B + O), (B + S), and (C + P) co-doped GeN. They possess ideal band gaps (∼2.0 eV) for the maximum utilization of solar light, suitable band positions for the water redox reactions, and enhanced visible light response. Moreover, we propose that the band gap of the monolayer IV-N can be slightly reduced by the (Al + S) or (Al + O) co-doping, while heavily tailed by the (B + O) or (B + S) co-doping. Our research provides valuable insights for designing and synthesizing novel 2D IV-N photocatalysts for solar water splitting.

Two-dimensional semiconducting Ag2X (X = S, Se) with Janus-induced built-in electric fields and moderate band edges for overall water splitting

Introduction of Two-Dimensional (2D) photocatalysts for solar water splitting has attracted significant attention in recent years, due to their considerably large surface areas and short pathways of charge carrier migration compared to their bulk counterparts. Herein, monolayers of Ag2S, Ag2Se, SAg4Se, SAg4Te, SeAg4Te and their heterostructures are predicted as promising overall-water-splitting photocatalysts according to first-principles calculations. Compared with Ag2S and Ag2Se monolayers, the Janus ones not only possess direct band gaps and moderate band edges for overall water splitting but also achieve the Janus-induced built-in electric fields of 0.14, 0.34 and 0.21 eV respectively, which could improve the photocatalytic efficiency via restraining the electron-hole recombination. Further investigations on their heterostructures show that they are also suitable for overall water splitting with enhanced optical absorbances and larger built-in electric fields. Our findings extend the scope of overall-water-splitting photocatalysts to monolayers of Ag2S, Ag2Se, SAg4Se, SAg4Te, SeAg4Te and their heterostructures, which is expected to inspire the experimental fabrications and relative investigations.

C7N6/Sc2CCl2 Weak van der Waals Heterostructure: A Promising Visible-Light-Driven Z-Scheme Water Splitting Photocatalyst with Interface Ultrafast Carrier Recombination

Designing and characterizing high-efficiency direct Z-scheme photocatalysts both from theoretical and experimental aspects still remain a big challenge. Here, based on extensive first-principles calculations combined with excited state dynamics simulations, we report that a weak van der Waals (vdW) C7N6/Sc2CCl2 heterostructure is a mediator-free direct Z-scheme photocatalyst for solar water splitting. Theoretical results clearly reveal that this heterostructure displays a nice light-harvesting performance extending to the near-infrared region. The relatively strong interfacial non-adiabatic coupling accelerates the interlayer carrier recombination in the time scale of sub-picoseconds. The well separated electrons and holes with a strong redox capacity make the hydrogen evolution reaction spontaneously occur on the C7N6 surface and the oxygen evolution reaction on the Se-doped Sc2CCl2 surface, respectively, when the proposed heterostructure is radiated with solar light.

Survey of Tetragonal Transition Metal Chalcogenide Hetero‐Bilayers for Promising Photocatalysts

AbstractThe formation of van der Waals (vdW) hetero‐bilayers have been emerging as unique structures to improve the performance of 2D layered nanomaterials for next‐generation photocatalysts. In this work, first‐principles calculations are employed to study the electronic and optical properties of a variety of tetragonal transition metal chalcogenide hetero‐bilayers. It is found that there are three stable semiconductor/semiconductor vdW hetero‐bilayers, and twenty‐one stable semiconductor/conductor vdW hetero‐bilayers. It is revealed that ZnS/ZnSe and ZnSe/ZnTe hetero‐bilayers possess a special band characteristic that is favorable for reducing the carrier recombination. In semiconductor/conductor hetero‐bilayers, the band edges of semiconductor are tuned via Schottky barrier, including p‐type and n‐type Schottky barrier. This work suggests that all hetero‐bilayers have an enhanced optical absorption within the visible light region, leading to more efficient solar energy harvesting. Based upon the theoretical calculations, tetragonal transition metal chalcogenide hetero‐bilayers will be great candidates as high performance photocatalysts for solar water splitting, as well as many other optoelectronic applications.

Single-layer honeycomb sheets of zinc selenide and beyond with superior electronic and optical properties

The freestanding four-atom-thick double-layer honeycomb sheets of zinc chalcogenides which were synthesized recently are outstanding two-dimensional (2D) materials for next-generation ultrathin optoelectronic nanodevice. In this work, we report a theoretical study on two different single-layer honeycomb sheets of MX (M = Zn, Cd; X = Se, S). The two kinds of single-layer sheets of MX would be thought by cutting along the zinc blende (ZB) bulk as well as wurtzite (WZ) bulk. By using state-of-the-art calculations based upon quasiparticle GW and Bethe–Salpeter equation (BSE), the single-layer sheets of MX possess wide band gaps and are capable of being used as photocatalysts for solar water splitting. The optical absorbance demonstrates that the single-layer sheets of CdSe and CdS have substantial absorption within the visible light region, and the optical absorption of the single-layer sheets of ZnSe can be easily modulated via strains. Furthermore, the electronic transport properties of the single-layer sheets of MX are investigated through the nonequilibrium Green’s function (NEGF) method combined with density functional theory (DFT). It is anticipated that these novel freestanding single-layer honeycomb sheets of MX would be great candidates for abundant potential applications in energy storage and conversion.

Hydrous RuO2 and CoxNy difunction-modified Ta3N5@Ta2N multi-heterojunction nanoplates for efficient visible-light-driven photocatalytic hydrogen production

To overcome the intrinsic drawbacks of Ta3N5 such as charge carrier recombination and serious self-oxidation, constructing difunctional multi-heterojunction is very promising and challenging for local selective modifications. Hydrous RuO2 and CoxNy difuntionally modified Ta3N5@Ta2N multi-heterojunction nanoplates were fabricated by high-temperature nitridation reduction of Co3+-modified Ta2O5@Ta3N5, followed by photooxidation deposition of Ru3+. The structural and compositional features were modulated by optimizing the nitridation time and Ru-modifying amount. Interestingly, synergetic effects of CoxNy/Ta2N/Ta3N5 and RuO2/Ta2N/Ta3N5 on charge carrier separation and transfer, and on decrease in the HER and OER overpotentials about 73 mV and 238 mV, respectively, were observed. Improved visible light absorption was attributed to a localized surface plasmon resonance (LSPR) effect by subnitride Co5.47N, sub-bandgap behavior of Ta2N, and anion defect N-vacancies. Increased charge transfer resistance by CoxNy/Ta2N interface was significantly reduced by further constructing hydrous RuO2/Ta2N heterojunction, where new Ru–O–Ta linkages facilitated fast separation and transfer of charge carriers, and increased surface hydroxyl groups was beneficial to the adsorption of water molecules. Micropores also provided more photoactive sites. All above factors cooperatively enhanced the visible-light photocatalytic H2-evolution activity, which was 1395.35 μmol·g−1·h−1, about 6.53 times higher than that of Ta2O5@Ta3N5. Its apparent quantum efficiency reaches to 5.76% under irradiance intensity of 15 mW cm−2 at 420 nm. Constructing difunctional multi-heterojunction provides a promising avenue to design efficient and stable Ta3N5-based photocatalysts for solar water splitting.

Hybrid density functional study of band gap engineering of SrTiO3 photocatalyst via doping for water splitting

Perovskite SrTiO3 (STO) is an attractive photocatalyst for solar water splitting, but suffers from a limited photoresponse in the ultraviolet spectral range due to its wide band gap. By means of hybrid density functional theory calculations, we systematically study engineering its band gap via doping 4d and 5d transition metals M (M=Zr, Nb, Mo, Tc, Ru, Rh, Pd, Hf, Ta, W, Re, Os, Ir and Pt) and chalcogen elements Y (Y=S and Se). We find that transition metal dopant M either has no effect on STO band gap or introduces detrimental mid-gap states, except for Pd and Pt that are able to reduce the STO band gap. In contrast, doping S and Se significantly reduces STO's direct band gap, thus leading to appreciable optical absorption transitions in the visible spectral range. Our findings provide that Pd, S and Se doped STO are potential promising photocatalysts for water splitting under visible light irradiation, thereby providing insightful theoretical guides for experiments to improve the photocatalytic activity of STO.

Modular Construction of Prussian Blue Analog and TiO2 Dual-Compartment Janus Nanoreactor for Efficient Photocatalytic Water Splitting.

Janus structures that include different functional compartments have attracted significant attention due to their specific properties in a diverse range of applications. However, it remains challenge to develop an effective strategy for achieving strong interfacial interaction. Herein, a Janus nanoreactor consisting of TiO2 2D nanocrystals integrated with Prussian blue analog (PBA) single crystals is proposed and synthesized by mimicking the planting process. In situ etching of PBA particles induces nucleation and growth of TiO2 nanoflakes onto the concave surface of PBA particles, and thus enhances the interlayer interaction. The anisotropic PBA–TiO2 Janus nanoreactor demonstrates enhanced photocatalytic activities for both water reduction and oxidation reactions compared with TiO2 and PBA alone. As far as it is known, this is the first PBA‐based composite that serves as a bifunctional photocatalyst for solar water splitting. The interfacial structure between two materials is vital for charge separation and transfer based on the spectroscopic studies. These results shed light on the elaborate construction of Janus nanoreactor, highlighting the important role of interfacial design at the microscale level.

Vertical SrNbO2N Nanorod Arrays for Solar‐Driven Photoelectrochemical Water Splitting

Although SrNbO2N with a bandgap of ≈1.9 eV has been considered as a promising photocatalyst for solar water splitting, its photocatalytic activity is generally very poor under ordinary conditions, primarily due to bad photocarrier generation, separation, and transportation. Herein, vertical SrNbO2N nanorod arrays are first achieved using a facile hydrothermal method with a nitridation process for solar‐driven photoelectrochemical (PEC) water splitting. The obtained SrNbO2N nanorod arrays exhibit vertical aligned nanorod morphology and high crystallinity, which are beneficial for photocarrier generation and charge carrier transportation. After being uniformly covered with CoOx nanoparticles, the SrNbO2N nanorod arrays present a photocurrent of 1.3 mA cm−2 at 1.23 VRHE, which is ≈2.6 times higher than that of a SrNbO2N nanoparticle thin film electrode, under irradiation by AM1.5G simulated sunlight. This PEC water splitting activity is one of the best values among all reported SrNbO2N electrodes. Thus, this work offers a new strategy to develop bimetal oxynitride nanorod arrays for PEC applications.

Graphene quantum dot with covalently linked Rhodamine dye: a high efficiency photocatalyst for hydrogen evolution

Designing metal free photocatalyst for solar water splitting is an emerging field of research. Although graphene quantum dot (GQD) has been explored as a potential photocatalyst, their performance remains inherently limited by poor visible light absorption and p-type conductivity. To address this problem, we have covalently functionalized GQD with visible light active photosensitizer Rhodamine 123. It was found that this dye-sensitized GQD could make quite stabilized photocatalytic hydrogen evolution reaction (HER) under visible light irradiation, which is in striking contrast to the virtually inactive photocatalytic HER performance of control GQD without dye. Efficient photo-induced electron transfer from Rhodamine 123 to GQD and also the transformation of p type GQD to the n-p ambipolar semiconductor, both boosted by covalent bonding, are responsible for such high photocatalytic HER performance. Observation of negative photocurrent at zero bias voltage during transient photocurrent measurement, also suggests its promising potential in the metal free photocathode system to achieve photoelectrochemical H2 production.

Efficient visible-light induced H2 evolution from T-CdxZn1-xS/defective MoS2 nano-hybrid with both bulk twinning homojunctions and interfacial heterostructures

Developing high-performance noble-metal-free photocatalysts for solar water splitting is of great urgency to address the growing energy and environmental crises. However, the current photocatalytic efficiency is unsatisfactory due to the low conversion of solar energy. Herein, we prepared a unique hybrid photocatalyst consisting of twinned CdxZn1-xS solid solutions grown with several layers of defect-rich MoS2 nanosheets (T-CdxZn1-xS/MoS2) to achieve a stable and efficient H2 evolution reaction (HER) activity. Noticeably, the alternating zinc blende/wurtzite (ZB/WZ) homojunctions in T-CdxZn1-xS and intimate heterojunctions formed between T-CdxZn1-xS and MoS2 facilitated the efficient separation and transfer of both bulk and surface charge carriers. Moreover, the good visible-light responses of T-CdxZn1-xS and MoS2 as well as the existence of abundant S22− and Mo5+ active species in defective MoS2 nanosheets also contributed significantly to the activity improvement of T-CdxZn1-xS/MoS2. Benefiting from the above advantages, T-CdxZn1-xS/MoS2 exhibited superior HER rates of 37.22 mmol∙h-1 g-1 in lactic acid solution and 69.25 mmol∙h-1 g-1 in Na2S/Na2SO3 solution, corresponding to the apparent quantum yield (AQY) of 36.3 % and 55.2 % at 420 nm, respectively. The HER capability of T-CdxZn1-xS/MoS2 exceeded that of Pt/T-CdxZn1-xS and most CdS-based photocatalysts reported previously. Our findings will facilitate the exploitation of advanced nanomaterials for sustainable energy conversion and utilization.

Carbon-based nanomaterials: in the quest of alternative metal-free photocatalysts for solar water splitting.

One of the alarming problems of modern civilization is global warming due to the inevitable rise of CO2 in the environment, mainly because of the excessive use of traditional fossil fuels. The gradual depletion of fossil fuels is another challenge regarding the future energy demand; therefore, alternative renewable energy research is necessary. One of the alternative approaches is the solar fuel generation by means of photocatalytic water splitting and more specifically, hydrogen evolution from water through the reductive half-reaction. Hydrogen is the cleanest fuel and does not produce any greenhouse gas upon direct combustion, or even while acting as a chemical feedstock for other transportable fuel generation. Therefore, it is desirable to produce efficient photocatalysts for solar water splitting. After the discovery of the first photocatalytic water splitting reaction by Fujisima and Honda, several advancements have been made with metal-based inorganic semiconductor photo-catalysts. However, their practical applicability is still under debate considering the environmental sustainability, stability and economical expenses. As a result, it is essential to develop alternate photocatalysts that are environmentally sustainable, cost-effective, stable and highly efficient. The metal-free approach is one of the most promising approaches in this regard. Herein, we discuss the recent developments in carbon-based materials and their hybrids as alternative metal free photocatalysts for solar water splitting. The present discussion includes g-C3N4, two-dimensional graphene/graphene oxides, one-dimensional carbon nanotubes/carbon nanofibers and zero-dimensional graphene QDs/carbon dots. We have focused on the rectification of exciton generation, charge separation and interfacial photochemical processes for photocatalysis, followed by possible optimization pathways of these typical all carbon-based materials. Finally, we have highlighted several fundamental challenges and their possible solutions, as well as the future direction on this particular aspect.

Optical and dielectric properties of Bi2Ti2O7/Bi4Ti3O12 nanocomposite

Bismuth titanate pure pyrochlore phase Bi2Ti2O7 (BT2) and its nanocomposite Bi2Ti2O7/Bi4Ti3O12 (BT2/BT4) are synthesized using the coprecipitation method at optimum reaction parameters. XRD results reveal the formation of pyrochlore cubic single phase in BT2 while a mixed-phase consisting of pyrochlore cubic and perovskite Aurivillius orthorhombic phase (Bi4Ti3O12) in BT2/BT4. The nanocomposite sample BT2/BT4 shows the smaller crystallite size (∼30 nm) and larger microstrain than BT2 nanoparticles. The average particle size is calculated using FESEM micrographs are ∼40 nm and ∼60 nm in BT2/BT4 and BT2 samples respectively. The presence of only Bi, Ti and O elements in both samples are confirmed by the EDX spectrum. FTIR spectra of BT2/BT4 confirms the vibrations corresponding to perovskite octahedral in additions to pyrochlore metal oxide vibrations. An optical band gap of nanocomposite BT2/BT4 is much smaller (1.65 eV) than pure phase BT2 sample (2.33 eV). Dielectric analysis reveals high dielectric constant and large a.c. conductivity in BT2/BT4 than the BT2 sample. The tuned optical and dielectric properties of BT2/BT4 nanocomposite shows that this nanocomposite can be further used in photovoltaic applications and as a photocatalyst for solar water splitting or organic waste degradation reactions.

Three-in-One: Opened Charge-transfer channel, positively shifted oxidation potential, and enhanced visible light response of g-C3N4 photocatalyst through K and S Co-doping

The insufficient oxidation capacity, high carrier recombination rate and limited sunlight absorption seriously suppress the photocatalytic activity of pure g-C3N4. Using state-of-the-art hybrid density functional theory, we report an efficient method to tackle all aforementioned issues of g-C3N4 by metal-nonmetal (S and K) co-doping here. We find the adsorption of K atom on hollow site causes dynamic strain of g-C3N4. The S + K co-doping not only shifts the band edges downwards to achieve a much large overpotential of ca. 0.76 V, but also significantly extends the visible-light absorption threshold of g-C3N4. More importantly, the newly established channel between neighboring heptazine units in the doped structure is highly favorable for the separation of charge carriers. Our results help the design of high-performance visible-light-responsive g–C3N4–based photocatalyst for solar water splitting.

Theoretical design of a novel 2D tetragonal ZnS/SnO hetero-bilayer as a promising photocatalyst for solar water splitting

Van der Waals (vdW) hetero-bilayers are emerging as unique structures to enhance the performance of two-dimensional (2D) layered nanomaterials for next-generation electronic and optoelectronic devices. In this work, we employ first-principles calculations to study the novel tetragonal ZnS/SnO hetero-bilayer (BL). The state-of-the-art computations based upon quasiparticle GW and Bethe−Salpeter equation (BSE) are utilized to study the electronic and optical properties of this novel vdW hetero-bilayer. We reveal that ZnS/SnO BL is a polarized semiconductor with a clear built-in electric field, and possesses a special band characteristic favorable for reducing the carrier recombination. It is also demonstrated that strain and external electric field are among the effective methods to modulate the electronic and optical properties of ZnS/SnO BL. Our work suggests that ZnS/SnO BL has an excellent optical absorption in the solar spectrum, rendering the material a viable candidate for optoelectronic applications, in particular for solar water splitting.

C/g-C3N4 hybrid nanosheets obtained by gaseous stripping to boost photocatalytic hydrogen evolution performance

Graphitic carbon nitride (g-C3N4) is a new generation metal-free polymer semiconductor photocatalyst for solar water splitting. In this work, we report a gaseous treatment in wet nitrogen ambience, which can realize carbon deposition and nanosheets stripping in one step. The free carbon functions as electron trapping and the bridge of g-C3N4 and co-catalyst Pt, producing a high charge separation efficiency. The carbon content in the C/g-C3N4 hybrid nanosheets can be adjusted by changing ethanol content in the treating ambience. The optimized C/g-C3N4 (5%) nanosheets achieve the highest hydrogen evolution rate of 10482.62 μmol h−1 g−1, which is 2.3 times that of g-C3N4 (0%) nanosheets, even 13.9 times that of bulk g-C3N4. Optical properties, charges transfer and specific surface area of the C/g-C3N4 hybrid nanosheets are measured to discuss the mechanism of photocatalytic hydrogen evolution. The combination of high optical absorption in visible region and high charge separation efficiency contribute to the enhanced hydrogen evolution performance.