Photocatalyst Sheet Research Articles

Unlocking Copper-Free Interfacial Asymmetric C-C Coupling for Ethylene Photosynthesis from CO2 and H2O.

Solar-driven carbon dioxide (CO2) reduction into C2+ products such as ethylene represents an enticing route toward achieving carbon neutrality. However, due to sluggish electron transfer and intricate C-C coupling, it remains challenging to achieve highly efficient and selective ethylene production from CO2 and H2O beyond capitalizing on Cu-based catalysts. Herein, we report a judicious design to attain asymmetric C-C coupling through interfacial defect-rendered tandem catalytic centers within a sulfur-vacancy-rich MoSx/Fe2O3 photocatalyst sheet, enabling a robust CO2 photoreduction to ethylene without the need for copper, noble metals, and sacrificial agents. Specifically, interfacial S vacancies induce adjacent under-coordinated S atoms to form Fe-S bonds as a rapid electron-transfer pathway for yielding a Z-scheme band alignment. Moreover, these S vacancies further modulate the strong coupling interaction to generate a nitrogenase-analogous Mo-Fe heteronuclear unit and induce the upward shift of the d-band center. This bioinspired interface structure effectively suppresses electrostatic repulsion between neighboring *CO and *COH intermediates via d-p hybridization, ultimately facilitating an asymmetric C-C coupling to achieve a remarkable solar-to-chemical efficiency of 0.565% with a superior selectivity of 84.9% for ethylene production. Further strengthened by MoSx/WO3, our design unveils a promising platform for optimizing interfacial electron transfer and offers a new option for C2+ synthesis from CO2 and H2O using copper-free and noble metal-free catalysts.

An Artificial Leaf with Patterned Photocatalysts for Sunlight-Driven Water Splitting.

Plant leaves can turn entirely absorbed light into chemical energy due to their spatially separated photosystems I and II in the thylakoid membrane that enables unidirectional Z-scheme type charge transfer between them. In artificial systems that mimic leaves, a lack of spatial and interfacial control of active units (i.e., hydrogen evolution photocatalyst/HEP and oxygen evolution photocatalyst/OEP) introduces competitive charge transfer channels between them, resulting in deficient Z-scheme type charge transfer. Herein, we demonstrate that a patterned photocatalyst sheet, namely, an artificial leaf, comprising an ordered and separated distribution of the OEP and HEP strips on a conductive substrate, achieves unidirectional Z-scheme type charge transfer as the leaves do. It represents a next-generation photocatalytic system that mimics the leaves to bring breakthrough in photocatalytic over water splitting performance with the combination of highly active HEP and OEP photocatalysts, opening up a promising avenue toward solar energy conversion by artificial photosynthesis.

Activation of Nonoxide Photocatalyst Sheet Operable under Visible Light Irradiation up to 600 nm by Conductive Bilayer

Activation of Nonoxide Photocatalyst Sheet Operable under Visible Light Irradiation up to 600 nm by Conductive Bilayer

(Invited) Photocatalyst Sheets for Scalable Solar Fuels Production from Water and CO2

The development of sunlight-driven water splitting or CO2 reduction systems with high efficiency, scalability, and cost-competitiveness is a central issue for mass production of fuels and energy-rich chemicals.1 Z-scheme photocatalytic systems, employing the two-step photoexcitation of photocatalyst I (PC I) and photocatalyst II (PC II), have garnered significant attention. These systems can utilize narrow bandgap semiconductors capable of either water/CO2 reduction or oxidation potential, enabling the conversion of solar energy. The challenge in developing a Z-scheme system lies in ensuring efficient electron transfer between PC I and PC II.2 To address these challenges, we have developed all-solid-state devices—photocatalyst sheets—for photocatalytic water splitting.3,4 These sheets establish a direct connection between PC I and PC II particles via a conductive layer or conductive nanoparticles, effectively facilitating electron transfer between them. Consequently, these photocatalyst sheets achieve a high solar-to-H2 conversion efficiency exceeding 1%. They outperform analogous powder suspension and photoelectrochemical systems in their optimal reaction solutions due to the effective electron transfer enabled by the conductive layer or nanoparticles bridging PC I and PC II. Compared to traditional photoelectrochemical systems, the photocatalyst sheet design significantly reduces the effects of H+/OH− concentration overpotentials and the IR drop due to the proximity of PC I and PC II in the sheet. Hence, the design of these photocatalyst sheets proves highly suitable for efficient large-scale applications.Additionally, efforts to demonstrate fuel production from CO2 powered by sunlight are currently hampered by the requirements of sacrificial electron donors or external bias, and lack of efficiency, selectivity and scalability. To overcome those barriers, we integrate a selective molecular catalyst on semiconductor light absorbers to form a wireless and monolithic photocatalyst sheet.5 For instance, by incorporating a phosphonated cobalt(II) bis(terpyridine) catalyst (CotpyP) onto a modified SrTiO3:La,Rh|Au|BiVO4:Mo sheet, we have achieved a solar-to-formate conversion efficiency of 0.08%. This configuration harnesses the high selectivity of molecular catalysts for CO2 reduction and the strong water oxidation power of semiconductors, yielding a formate selectivity of 97±3% through the coupled process of CO2 reduction and water oxidation. We have also developed a bio-abiotic hybrid system by combining a photocatalyst sheet, capable of scalable photocatalytic water splitting, with nonphotosynthetic bacteria acting as biocatalysts.6 When pairing CO2-fixing acetogenic bacteria, Sporomusa ovata, with a SrTiO3:La,Rh|ITO|BiVO4:Mo sheet, a solar-to-acetate conversion efficiency of ~0.7% was achieved (as shown in the figure). This coupling exhibited an impressive selectivity for acetate production in CO2 reduction reactions, reaching ~100%.Our study offers novel and versatile strategies toward sustainable solar fuel production.References Q. Wang, C. Pornrungroj, S. Linley, E. Reisner. Nat. Energy 7 , 13–24 (2022)Y. Sasaki, H. Nemoto, K. Saito, A. Kudo. J. Phys. Chem. C 113, 17536–17542 (2009)Q. Wang, T. Hisatomi, Q. Jia, H. Tokudome, M. Zhong, C. Wang, Z. Pan, T. Takata, M. Nakabayashi, N. Shibata, Y. Li, I. D. Sharp, A. Kudo, T. Yamada, K. Domen. Nat. Mater. 15,611–615 (2016)Q. Wang, T. Hisatomi, Y. Suzuki, Z. Pan, J. Seo, M. Katayama, T. Minegishi, H. Nishiyama, T. Takata, K. Seki, A. Kudo, T. Yamada, K. Domen. J. Am. Chem. Soc. 139, 1675–1683 (2017)Q. Wang, J. Warnan, S. Rodríguez-Jiménez, J. J. Leung, S. Kalathil, V. Andrei, K. Domen, E, Reisner. Nat. Energy 5, 703–710 (2020)Q. Wang, S. Kalathil, C. Pornrungroj, D. C. Sahm, E. Reisner. Nat. Catal. 5, 633–641 (2022) Figure 1

An indirect Z-scheme heterostructure of nano-gold layer immobilized Cu-Al-doped SrTiO3 and CoOx-WO3 for photocatalytic CO2 reduction

Perovskite oxides have emerged as promising photocatalysts for CO2 reduction to valuable chemicals and fuels. However, conventional perovskite oxide photocatalysts often suffer from inefficient charge separation and bulk charge recombination, negatively impacting overall photoactivity. In this work, we present a novel immobilized perovskite oxide-based photocatalyst sheet featuring an indirect Z-scheme heterostructure composed of Cu-loaded Al-doped SrTiO3 (Cu-ASTO) and CoOx-loaded WO3 (CoOx-WO3) with an ultrathin (20 nm) gold interlayer serving as a conductive bridge. We tested this photocatalyst sheet in a sealed reactor containing a CO2-saturated aqueous solution of 0.1 M NaHCO3 and a side window to allow irradiation. Under irradiation with a 300 W Xe lamp, the Cu-ASTO/Au/CoOx-WO3 immobilized photocatalyst sheet produced methane (CH4) at a rate of 2.676 µmol cm-2 h−1 and methanol (CH3OH) at 0.517 µmol cm-2 h−1. We attribute the superior activity of this catalyst to the all-solid-state indirect Z-scheme heterostructure that greatly extends the lifespan of photoinduced charge carriers and enhances the CO2 photoconversion reaction. In experiments conducted outside, we demonstrated the immobilized photocatalyst's performance under natural sunlight, producing CH4 at 0.307 µmol cm-2 h−1 and CH3OH at 0.069 µmol cm-2 h−1. This work provides design ideas for developing robust immobilized photocatalyst systems for the scalable operation of the CO2 photoreduction process and broadening the scope of applications for perovskite heterostructure photocatalysts.

Nanoparticulate TiN Loading to Promote Z-Scheme Water Splitting Using a Narrow-Bandgap Nonoxide-Based Photocatalyst Sheet.

Some oxide-based particulate photocatalyst sheets exhibit excellent activity during the water-splitting reaction. The replacement of oxide photocatalysts with narrow-bandgap photocatalysts based on nonoxides could provide the higher solar-to-hydrogen energy conversion efficiencies that are required for practical implementation. Unfortunately, the activity of nonoxide-based photocatalyst sheets is low in many cases, indicating the need for strategies to improve the quality of nonoxide photocatalysts and the charge transfer process. In this work, single-crystalline particulate SrTaO2N is studied as an oxygen evolution photocatalyst for photocatalyst sheets applied to Z-scheme water splitting, in combination with La5Ti2Cu0.9Ag0.1O7S5 and Au as the hydrogen evolution photocatalyst and conductive layer, respectively. The loading of SrTaO2N with CoOx provided increases activity during photocatalytic water oxidation, giving an apparent quantum yield of 15.7% at 420nm. A photocatalyst sheet incorporating CoOx-loaded SrTaO2N is also found to promote Z-scheme water splitting under visible light. Notably, the additional loading of nanoparticulate TiN on the CoOx-loaded SrTaO2N improves the water splitting activity by six times because the TiN promotes electron transfer from the SrTaO2N particles to the Au layer. This work demonstrates key concepts related to the improvement of nonoxide-based photocatalyst sheets based on facilitating the charge transfer process through appropriate surface modifications.

Oxysulfide-Based Photocatalyst Sheet Modified with Silica Layer for Z-Scheme Overall Water Splitting at Atmospheric Pressure

Photocatalytic water splitting using particulate photocatalysts is considered as an ideal route to produce economically cheap renewable, and transportable H2 utilizing sunlight [1]. Photocatalyst sheets are directly scalable and have recently gained interest because a benchmark STH conversion efficiency of 1.0% was achieved at near ambient pressure using the one composed of Rh, La-codoped SrTiO3 and Mo-doped BiVO4 (BVO:Mo) as the H2- and O2- evolution photocatalysts (HEP and OEP), respectively. However, such systems absorb only a little portion of the sunlight due to the weak visible light absorption of the doped SrTiO3 [2]. To utilize longer wavelength visible light, our group has recently studied photocatalyst sheets composed of Ga-doped La5Ti2Cu0.9Ag0.1O7S5 (LTCA:Ga) (HEP) and BVO:Mo (OEP) with Au as conducting layer. The combination of LTCA:Ga/Au/BVO:Mo modified with Rh/Cr2O3 and CoO x as reduction and oxidation cocatalysts, respectively, achieved an STH efficiency of 0.4% under reduced pressure, but this was far lower than the oxide-based sheet system and the activity was decreased sharply against background pressure [3]. Considering intense visible light absorption of LTCA (λabs ~ 700 nm) and the associated maximum theoretical energy conversion efficiency for this combination, improvements in the net STH efficiency and activation under atmospheric pressure is highly indispensable.In this study, LTCA photocatalysts with different dopants were applied to the sheet system in combination with hydrothermally synthesized BVO:Mo and the resulting sheets were modified with CoO x /Cr2O3/Rh cocatalysts in an attempt to upgrade the water splitting activity. Moreover, surface modification onto LTCA-based photocatalyst sheets with a thin amorphous TiO2 and SiO2 layer were evaluated at elevated pressure and high temperature.Doped LTCA were synthesized by a conventional solid-state reaction in sealed evacuated quartz tubes similar to previous studies [3]. Precursors were blended inside a glove box. BVO:Mo (Mo 0.1 mol%) was synthesized by a hydrothermal method [4]. Photocatalyst sheet was fabricated by the particle transfer method [3]. SiO2 and TiO2 layers functioning as molecular sieves were deposited onto the cocatalyst-modified sheet from titanium tetraisopropoxide (TTIP) or tetraethyl orthosilicate (TEOS) by photo-decomposition and hydrolysis method, respectively, to prevent backward reactions at elevated pressure [5]. Photocatalytic water splitting reaction was carried out at different temperatures and pressures using a closed gas circulation system.Rh cocatalyst was first adsorbed and photo-deposited onto the doped LTCA/Au/BVO:Mo(h) sheet [3]. Gas evolution rates were significantly enhanced upon deposition of Cr2O3 layer as the oxygen reduction reaction (ORR), a backward reaction, was majorly suppressed promoted by uncovered Rh surface under the measured condition. Photocatalyst sheet composed of doped LTCA/Au/BVO:Mo(h) with optimal dopant amounts and cocatalysts loadings showed the H2 and O2 evolution rates of 110 and 54 µmol/h, respectively, under visible light irradiation and yielded an STH of 0.67% at 4 kPa and temperature 301 K [6]. The water splitting activity was improved by 1.6 times under reduced pressure compared to the previously studied LTCAGa/Au/BVO:Mo. However, this activity was highly sensitive against background pressure as nearly 90% reduced upon increasing pressure from 2 to 90 kPa owing to the predominating ORR occurring on LTCA and exposed Au surface. To suppress this backward reaction, TiO2 layer was coated to suppress ORR but the sheet failed to work at a temperature higher than 318 K due to the condensation of such layer. However, SiO2 layer coated sheet effectively suppressed ORR backward reaction and stably work under hot water and elevated pressure [6]. Consequently, under optimized condition SiO2 deposited sheet achieved an STH of 0.4% at 90 kPa at temperature 333 K.Doped LTCA materials were applied to the photocatalyst sheet systems as HEP along with BVO:Mo and Au layer as OEP and conductive layer, respectively, for Z-scheme pure water splitting under visible light. This combination achieved an STH of 0.67% at reduced pressure. This STH is considered as one of the highest values recorded for OWS systems involving non-oxide photocatalysts. The activity of the photocatalyst sheet was found negatively dependent onto background pressure due to the ORR on LTCA and Au layer. Surface modifications with SiO2 layer activated the sheet under atmospheric pressure. These findings will contribute to the improvement in the STH for other systems under realistic condition where OWS is likely to be conducted at an ambient pressure. References Nandy et al. Chem. Sci. 2021, 12, 9866-9884.Wang et al. Nat. Mater. 2016, 15, 611-615.Chen et al. ACS Catal. 2023, 13, 3285-3294.Li et al. Energy Environ. Sci., 2014, 7, 1369-1376.Pan et al. Angew. Chem. Int. Ed. 2015, 54, 2955-2959.Nandy et al. Joule, 2023, under revision.

PDIP/carbon paper photocatalyst sheet for enhanced photocatalytic water oxidation

Photocatalytic water splitting is a highly promising future energy technology. In contrast to the extensive work on particulate photocatalysts, there is still a lack of sufficient investigation for regularized photocatalysts. Here, from perylene diimide polymer (PDIP) and commercial carbon paper (CP), we develop a self-assembled PDIP/CP photocatalyst sheet that is easily scalable and can be used for photocatalytic water oxidation. Driven by strong interfacial π-π interactions, the PDIP spontaneously adsorbs onto the CP, which leads to a narrowed band gap, down-shifted VB edge, and enhanced separation of the photogenerated charge carriers of the PDIP. Under visible light, the PDIP/CP photocatalyst sheet shows significantly enhanced photocatalytic water oxidation activity (4 h cumulative oxygen evolution is 1.7 times higher than that of the powder-suspended system) and considerable cycling stability. The PDIP/CP photocatalyst sheet has the potential for large-scale device applications.

Oxide layer coating enabling oxysulfide-based photocatalyst sheet to drive Z-scheme water splitting at atmospheric pressure

•Photocatalyst sheets made of La5Ti2Cu0.9Ag0.1O7S5 (LTCA) and BiVO4 split water •Backward reactions on LTCA suppress the STH at elevated pressure •SiO2 coating allows the photocatalyst sheet to maintain an STH of 0.41% at 90 kPa •Pathways to activate photocatalyst sheets under realistic conditions are identified A photocatalyst sheet consisting of doped La5Ti2Cu0.9Ag0.1O7S5 as a hydrogen evolution photocatalyst (HEP), BiVO4 as an oxygen evolution photocatalyst (OEP), and Au as a conductor layer exhibits a solar-to-hydrogen energy conversion efficiency (STH) of 0.67% at 4 kPa in Z-scheme overall water splitting (ZOWS). However, the photocatalyst sheet loses its activity at elevated pressure due to backward reactions. Here, we show that this loss of activity can be suppressed by coating the sheet with amorphous SiO2 layer. In addition, the SiO2 coating maintained its performance at elevated temperatures, unlike the widely used amorphous TiO2 coating, and exhibited an STH of 0.41% at 90 kPa and 333 K. The results of the present study demonstrate the ability to functionalize photocatalyst sheets of narrow-band-gap materials prone to backward reactions, allowing their use in outdoor environments, and paves the way for practical solar hydrogen production. A photocatalyst sheet consisting of doped La5Ti2Cu0.9Ag0.1O7S5 as a hydrogen evolution photocatalyst (HEP), BiVO4 as an oxygen evolution photocatalyst (OEP), and Au as a conductor layer exhibits a solar-to-hydrogen energy conversion efficiency (STH) of 0.67% at 4 kPa in Z-scheme overall water splitting (ZOWS). However, the photocatalyst sheet loses its activity at elevated pressure due to backward reactions. Here, we show that this loss of activity can be suppressed by coating the sheet with amorphous SiO2 layer. In addition, the SiO2 coating maintained its performance at elevated temperatures, unlike the widely used amorphous TiO2 coating, and exhibited an STH of 0.41% at 90 kPa and 333 K. The results of the present study demonstrate the ability to functionalize photocatalyst sheets of narrow-band-gap materials prone to backward reactions, allowing their use in outdoor environments, and paves the way for practical solar hydrogen production.

In situ growth of a heterojunction CdS/TiO2 nanofiber monolithic photocatalyst sheet for enhanced hydrogen evolution

A heterojunction CdS/TiO2 nanofiber monolithic photocatalyst sheet was developed via facile in situ hydrothermal growth on Ti foil. The photocatalyst sheet can greatly enhance photocatalytic H2 production.

Recent advances in photocatalyst sheet development and challenges for cost-effective solar hydrogen production

This article presents progress and challenges in the development of photocatalyst sheets for scalable efficient production of renewable hydrogen via water splitting reaction.

One-step electrodeposition of amorphous MoSx - CdS on Ti mesh as a monolithic heterostructured photocatalyst for enhanced photocatalytic H2 evolution

Exploiting large-size monolithic semiconductor photocatalyst sheet/layer with superior photocatalytic hydrogen production performances through facile and scalable methods is an important research direction in promoting the practical application of traditional particulate semiconductor photocatalyst, yet challenging. Here we constructed an amorphous MoSx - CdS monolithic heterostructured photocatalyst sheet (a-MoSx - CdS/Ti) with maximal size of ∼100 cm2 through a one-step potentiostatic electrodeposition process of 5 min on titanium mesh used as photocatalyst support. Owing to synergic effects of favorable light absorption, abundant active sites for proton reduction, effective photogenerated charge separation and robust structural integrity, the a-MoSx - CdS/Ti not only exhibits an optimized H2 generation rate of 4476 μmol h−1g−1, but also demonstrates excellent operability during photocatalytic H2 generation and subsequent recycling processes, greatly improving its applicability. This work presents a straightforward and practical route to produce efficient, robust and easily operable photocatalytic H2 production system which expectantly finds promising practical applications in photocatalysis, photoelectrocatalysis and beyond.

Publisher Correction: Bacteria–photocatalyst sheet for sustainable carbon dioxide utilization

Publisher Correction: Bacteria–photocatalyst sheet for sustainable carbon dioxide utilization

(Invited) Solar Hydrogen Production with Particulate Photocatalysts

Sunlight-driven water splitting is studied actively for production of renewable solar hydrogen on a large scale. Both the efficiency and scalability of water splitting systems are essential factors for practical application of renewable solar hydrogen because of the low areal density of the solar energy. In this context, overall water splitting using particulate photocatalysts has been attracting growing interest, because such systems can be spread over wide areas by inexpensive processes potentially [1], although it is essential to radically improve the solar-to-hydrogen energy conversion efficiency (STH) of particulate photocatalysts and develop suitable reaction systems. In my talk, recent progress in photocatalytic materials and reaction systems will be presented.The author’s group has studied various semiconductor oxides, (oxy)nitrides, and (oxy)chalcogenides as photocatalysts for water splitting [2]. SrTiO3 is an oxide photocatalyst that has been known to be active in overall water splitting under ultraviolet irradiation since 1980 [3]. Recently, the apparent quantum yield (AQY) of this photocatalyst in overall water splitting has been improved drastically. The author's group has found that doping Al3+ into the titanium site of SrTiO3 boosts the water splitting activity by two orders of magnitude [4]. By refining the preparation conditions of the Al-doped SrTiO3 (SrTiO3:Al) photocatalyst and the loading conditions of cocatalysts working as hydrogen and oxygen evolution sites, the AQY has been improved to more than 90% at 365 nm [5], equivalent to an internal quantum efficiency of almost unity. This quantum efficiency is the highest yet reported and indicate that a particulate photocatalyst can drive the greatly endergonic overall water splitting reaction at a quantum efficiency comparable to values obtained from photon-to-chemical or photon-to-current conversion in photosynthesis or photovoltaic systems, respectively.The author's group has also been developing panel reactors for large-scale applications. Photocatalyst sheets based on SrTiO3:Al contained in a panel-type reactor split water into hydrogen and oxygen and release gas bubbles at a rate corresponding to a solar-to-hydrogen energy conversion efficiency of 10% under intense ultraviolet irradiation even when the water depth is merely 1 mm [6]. Moreover, the photocatalyst can maintain 80% of its initial activity during 1300 h of constant simulated sunlight irradiation at ambient pressure with appropriate surface modifications [7]. A prototype 1-m2-sized panel reactor containing SrTiO3:Al photocatalyst sheets splits water under natural sunlight irradiation without a significant loss of the intrinsic activity of the photocatalyst sheets. More recently, a solar hydrogen production system based on 100-m2 arrayed photocatalytic water splitting panels and an oxyhydrogen gas-separation module was built, and its performance and system characteristics including safety issues was reported [8].For practical solar energy harvesting, it is essential to develop photocatalysts that are active under visible light irradiation. Ta3N5 and Y2Ti2O5S2 photocatalysts are active in overall water splitting via one-step excitation under visible light irradiation [9,10]. Particulate photocatalyst sheets efficiently split water into hydrogen and oxygen via two-step excitation, referred to as Z-scheme, regardless of size. In particular, a photocatalyst sheet consisting of La- and Rh-codoped SrTiO3 and Mo-doped BiVO4 splits water into hydrogen and oxygen via the Z-scheme, showing a STH exceeding 1.0% [11,12]. Some other (oxy)chalcogenides and (oxy)nitrides with long absorption edge wavelengths can also be applied to Z-scheme photocatalyst sheets. Hisatomi et al., Nat. Catal. 2019, 2, 387.Chen et al., Nat. Rev. Mater. 2017, 1, 17050.Domen et al., J. Chem. Soc., Chem. Commun. 1980, 543Ham et al., J. Mater. Chem. A 2016, 4, 3027.Takata et al., Nature 2020, 581, 411.Goto et al., Joule 2018, 2, 509.Lyu et al., Chem. Sci. 2019, 10, 3196.Nishiyama et al., Nature 2021, 598, 304.Wang et al., Nat. Catal. 2018, 1, 756.Wang et al., Nat. Mater. 2019, 18, 827.Wang et al., Nat. Mater. 2016, 15, 611.Wang et al., J. Am. Chem. Soc. 2017, 139, 1675.

Development of Double Perovskite Oxide Photocatalysts for Efficient Visible-Light Driven Photocatalytic Water Splitting and CO2 Reduction

Energy and environment are the key global challenges in the 21st century. Solar energy is considered the most promising clean and sustainable energy resource due to its university, inexhaustible and environmental friendliness. One of the most viable means of solar energy conversion and utilization is artificially converting solar energy into chemical energy as natural photosynthesis does. H2 produced from water splitting and CO2 reduction, are the major research topics of artificial photosynthesis. However, the effective conversion of solar energy into chemical energy by cost-effective artificial means on a large scale remains elusive. Metal oxide-based photocatalysts are the most studied materials for photocatalytic water splitting and CO2 reduction. Particularly, perovskite oxides with the chemical formula of ABO3 have been intensively studied as semiconductor photocatalysts. However, overwhelmingly of the perovskite oxides are only active under UV-light-irradiation, which limited their potential in solar energy application. Therefore, breakthrough technology and step-change materials, particularly, visible-light-responsive photocatalysts are highly desirable for the development of photocatalytic systems. In recent years, copious progress has been made in designing materials that function under visible-light-irradiation. So far, the most successful strategy is anion doping of oxide semiconductors, such as nitrogen or sulfur dope to form oxynitrides and oxysulfides, respectively. For example, nitrogen-doped oxynitrides such as (Ga1-xZnx)(N1-xOx) and ANbO2N (A=Sr, Ba, and La)1, and sulfur-doped oxysulfides such as Sm2Ti2S2O5 2 showed efficient photocatalytic overall water splitting activities under visible-light-irradiation. Particularly, oxynitrides (Ga1-xZnx)(N1-xOx) based photocatalyst sheet, showed remarkable photocatalytic overall water splitting activity with AQE of more than 30% at l»420 nm3. However, these oxynitrides and oxysulfides suffer stability problems due to photocorrosion, hindering their potential practical application in photocatalytic applications. Recently, it has been theorized that double perovskite oxides (DPOs) with the chemical formula of A2BʹB"O6 can function as efficient and stable visible-light-responsive photocatalysts for photocatalytic water splitting and CO2 reduction. However, DPOs have been rarely studied for photocatalytic water splitting and CO2 reduction due to the difficulty of obtaining pure phase materials and the paucity of exposed active sites. Thus, developing efficient and stable visible-light-responsive DPOs photocatalysts for photocatalytic water splitting and CO2 reduction becomes important. In this regard, recently, we have demonstrated a series of efficient and stable visible-light-responsive DPOs photocatalysts for photocatalytic water splitting. For instance, Sr2CoWO6 and Sr2CoTaO6 can serve as an efficient and stable bifunctional photocatalyst for both photocatalytic oxygen evolution reaction (OER) and hydrogen evolution reaction(HER)4, 5, and Sr2NiWO6 can efficiently drive the photocatalytic OER6. Even though these DPOs have been demonstrated as visible-light-responsive photocatalysts with suitable CB and VB positions that straddle the theoretical potentials for overall water splitting, however, one-step overall water splitting has not been achieved so far, and their potential for photocatalytic CO2 reduction has not been studied yet. Overall water splitting under visible-light-irradiation based on particulate photocatalysts is a challenging reaction, which is regarded as one of the “Holy Grail” of sciences. And oxide semiconductors showing both photocatalytic OER and HER activities are rare. Nevertheless, bifunctional photocatalytic OER and HER were successfully demonstrated based on Sr2CoWO6 and Sr2CoTaO6. Further improvement of the material designs and developing appropriate cocatalysts may play a key role in achieving one-step overall water splitting under visible-light-irradiation based on DPOs photocatalysts. This work is mainly to explore the possibility of utilizing DPOs materials as visible-light-responsive photocatalysts for photocatalytic water splitting and CO2 reduction reaction, which is challenging work but has great potential value in advancing science and technology in photocatalysis. We anticipated that this work will provide a novel and rational strategy for improving the light absorption, charge separation and charge utilization in DPOs photocatalysts. References Maeda, K.; Teramura, K.; Masuda, H.; Takata, T.; Saito, N.; Inoue, Y.; Domen, K. The Journal of Physical Chemistry B 2006, 110 (26), 13107-13112.Ma, G.; Kuang, Y.; Murthy, D. H.; Hisatomi, T.; Seo, J.; Chen, S.; Matsuzaki, H.; Suzuki, Y.; Katayama, M.; Minegishi, T. The Journal of Physical Chemistry C 2018, 122 (25), 13492-13499.Kato, H.; Asakura, K.; Kudo, A. Journal of the American Chemical Society 2003, 125 (10), 3082-3089.Idris, A. M.; Liu, T.; Shah, J. H.; Zhang, X.; Ma, C.; Malik, A. S.; Jin, A. Solar RRL 2020, 4 (3), 1900456.Idris, A. M.; Liu, T.; Hussain Shah, J.; Han, H.; Li, C. ACS Sustainable Chemistry&Engineering 2020, 8 (37), 14190-14197.Idris, A. M.; Liu, T.; Hussain Shah, J.; Malik, A. S.; Zhao, D.; Han, H.; Li, C. ACS Applied Materials&Interfaces 2020, 12 (23), 25938-25948.

Bacteria–photocatalyst sheet for sustainable carbon dioxide utilization

The clean conversion of carbon dioxide and water to a single multicarbon product and O2 using sunlight via photocatalysis without the assistance of organic additives or electricity remains an unresolved challenge. Here we report a bio-abiotic hybrid system with the non-photosynthetic, CO2-fixing acetogenic bacterium Sporomusa ovata grown on a scalable and cost-effective photocatalyst sheet consisting of a pair of particulate semiconductors (La and Rh co-doped SrTiO3 (SrTiO3:La,Rh) and Mo-doped BiVO4 (BiVO4:Mo)). The biohybrid effectively produces acetate (CH3COO–) and oxygen (O2) using only sunlight, CO2 and H2O, achieving a solar-to-acetate conversion efficiency of 0.7% at ambient conditions (298 K, 1 atm). The photocatalyst sheet oxidizes water to O2 and provides electrons and hydrogen (H2) to S. ovata for the selective synthesis of CH3COO– from CO2. To demonstrate utility in a closed carbon cycle, the solar-generated acetate was used directly as feedstock in a bioelectrochemical system for electricity generation. These semi-biological approaches thus offer a promising strategy for sustainably and cleanly fixing CO2 and closing the carbon cycle. Conversion of CO2 to fuels or chemicals via artificial photosynthesis usually requires the assistance of organic additives or electricity. Now, a biohybrid system is reported consisting of a photocatalyst sheet and bacteria producing acetate and O2 from CO2 and H2O using sunlight as the sole energy input.

Charge carrier mapping for Z-scheme photocatalytic water-splitting sheet via categorization of microscopic time-resolved image sequences

Photocatalytic water splitting system using particulate semiconductor materials is a promising strategy for converting solar energy into hydrogen and oxygen. In particular, visible-light-driven ‘Z-scheme’ printable photocatalyst sheets are cost-effective and scalable. However, little is known about the fundamental photophysical processes, which are key to explaining and promoting the photoactivity. Here, we applied the pattern-illumination time-resolved phase microscopy for a photocatalyst sheet composed of Mo-doped BiVO4 and Rh-doped SrTiO3 with indium tin oxide as the electron mediator to investigate photo-generated charge carrier dynamics. Using this method, we successfully observed the position- and structure-dependent charge carrier behavior and visualized the active/inactive sites in the sheets under the light irradiation via the time sequence images and the clustering analysis. This combination methodology could provide the material/synthesis optimization methods for the maximum performance of the photocatalyst sheets.

Preparation of Nanoparticle Porous-Structured BiVO4 Photoanodes by a New Two-Step Electrochemical Deposition Method for Water Splitting

In the synthesis method of a BiVO4 photoanode via BiOI flakes, a BiOI film is formed by electrochemical deposition in Step 1, and a vanadium (V) source solution is placed by drop-casting on the BiOI film in Step 2. Following this, BiVO4 particles are converted from the BiOI–(V species) precursors by annealing. However, it is challenging to evenly distribute vanadium species among the BiOI flakes. As a result, the conversion reaction to form BiVO4 does not proceed simultaneously and uniformly. To address this limitation, in Step 2, we developed a new electrochemical deposition method that allowed the even distribution of V2O5 among Bi–O–I flakes to enhance the conversion reaction uniformly. Furthermore, when lactic acid was added to the electrodeposition bath solution, BiVO4 crystals with an increased (040) peak intensity of the X-ray diffractometer (XRD) pattern were obtained. The photocurrent of the BiVO4 photoanode was 2.2 mA/cm2 at 1.23 V vs. reversible hydrogen electrode (RHE) under solar simulated light of 100 mW/cm2 illumination. The Faradaic efficiency of oxygen evolution was close to 100%. In addition, overall water splitting was performed using a Ru/SrTiO3:Rh–BiVO4 photocatalyst sheet prepared by the BiVO4 synthesis method. The corresponding hydrogen and oxygen were produced in a 2:1 stoichiometric ratio under visible light irradiation.

Linking in situ charge accumulation to electronic structure in doped SrTiO3 reveals design principles for hydrogen-evolving photocatalysts.

Recently, high solar-to-hydrogen efficiencies were demonstrated using La and Rh co-doped SrTiO3 (La,Rh:SrTiO3) incorporated into a low-cost and scalable Z-scheme device, known as a photocatalyst sheet. However, the unique properties that enable La,Rh:SrTiO3 to support this impressive performance are not fully understood. Combining in situ spectroelectrochemical measurements with density functional theory and photoelectron spectroscopy produces a depletion model of Rh:SrTiO3 and La,Rh:SrTiO3 photocatalyst sheets. This reveals remarkable properties, such as deep flatband potentials (+2 V versus the reversible hydrogen electrode) and a Rh oxidation state dependent reorganization of the electronic structure, involving the loss of a vacant Rh 4d mid-gap state. This reorganization enables Rh:SrTiO3 to be reduced by co-doping without compromising the p-type character. In situ time-resolved spectroscopies show that the electronic structure reorganization induced by Rh reduction controls the electron lifetime in photocatalyst sheets. In Rh:SrTiO3, enhanced lifetimes can only be obtained at negative applied potentials, where the complete Z-scheme operates inefficiently. La co-doping fixes Rh in the 3+ state, which results in long-lived photogenerated electrons even at very positive potentials (+1 V versus the reversible hydrogen electrode), in which both components of the complete device operate effectively. This understanding of the role of co-dopants provides a new insight into the design principles for water-splitting devices based on bandgap-engineered metal oxides.

Effective solar-light-driven photocatalytic production of hypobromous acid on film-like photocatalyst sheets

Photocatalytic production of HBrO on a photocatalyst sheet under solar light irradiation is a promising way to store solar energy.