Artificial Photosynthesis Research Articles

Impact of Formulation of Photocurable Precursor Mixtures on the Performance and Dimensional Stability of Hierarchical Cation Exchange Membranes.

This work presents a systematic approach to formulating UV curable ionomer coatings that can be used as ion-exchange membranes when they are applied on porous substrates. Ion-exchange membranes fabricated in this way can be a cost-effective alternative to perfluorosulfonic acid membranes, such as Nafion and similar thin ionomer film membranes. Hierarchically structured coated membranes find applications for energy storage and conversion (organic redox flow batteries and artificial photosynthesis cells) and separation processes (electrodialysis). Designing the ionomer precursor for membrane formulation requires the introduction of compounds with drastically different properties into a liquid mixture. Hansen solubility theory was used to find the solvents to compatibilize main formulation components: acrylic sulfone salt (3-sulfopropyl methacrylate potassium salt) and hexafunctional polyester acrylate cross-linker (Ebecryl 830), otherwise nonmiscible or mutely soluble. Among the identified suitable solvents, acrylic acid and acetic acid allowed for optimal mixing of the components and reaching the highest levels of sulfonic group content, providing the desired ion-exchange capacity. Interestingly, they represented a case of a reactive and nonreactive solvent since acrylic acid was built into the ionomer during the UV curing step. Properties of the two membrane variants were compared. Samples fabricated with acetic acid exhibit improved handleability compared with the case of acrylic acid. Acetic acid yielded a lower area-specific resistance (6.4 ± 0.17 Ohm·cm2) compared to acrylic acid (12.1 ± 0.16 Ohm·cm2 in 0.5 M NaCl). This was achieved without severely suppressing the selectivity of the membrane, which was standing at 93.4 and 96.4% for preparation with acetic and acrylic acid, respectively.

Photobiology: introduction, overview and challenges

Photobiology is a broad field of research that studies the biological effects of light and has made huge advances in understanding fundamental questions such as how Photosynthesis works and in the area of medicine, which has a direct impact on society. Various specialties of Photobiology have been proposed, and to simplify the overview, six major fields are considered here: Photophysics and Photochemistry, Photosynthesis, Photoreception, Photomedicine, Optogenetics and Optical Bioimaging, and Ecological Photobiology. In this review, an introduction, description and examples are given for each of these major research areas. Current challenges in Photobiology include the understanding of non-visual Photoreception, complex photoreceptive mechanisms and the development of novel therapeutic strategies that may depend on the results of advanced optogenetic approaches. Furthermore, the integration of Photobiology with other fields such as materials science and engineering may lead to artificial photoreceptors and bio-inspired light harvesting and artificial Photosynthesis systems. In an era of global change, understanding the effects of artificial light on organisms and ecosystems is essential to mitigate the ecological disruptions caused by artificial lighting. This non-exhaustive selection of research directions and challenges illustrates the complexity and breadth of Photobiology research.

Dipole field in nitrogen-enriched carbon nitride with external forces to boost the artificial photosynthesis of hydrogen peroxide

Artificial photosynthesis is a promising strategy for efficient hydrogen peroxide production, but the poor directional charge transfer from bulk to active sites restricts the overall photocatalytic efficiency. To address this, a new process of dipole field-driven spontaneous polarization in nitrogen-rich triazole-based carbon nitride (C3N5) to harness photogenerated charge kinetics for hydrogen peroxide production is constructed. Here, C3N5 achieves a hydrogen peroxide photosynthesis rate of 3809.5 µmol g−1 h−1 and a 2e− transfer selectivity of 92% under simulated sunlight and ultrasonic forces. This high performance is attributed to the introduction of rich nitrogen active sites of the triazole ring in C3N5, which brings a dipole field. This dipole field induces a spontaneous polarization field to accelerate a rapid directional electron transfer process to nitrogen active sites and therefore induces Pauling-type adsorption of oxygen through an indirect 2e− transfer pathway to form hydrogen peroxide. This innovative concept using a dipole field to harness the migration and transport of photogenerated carriers provides a new route to improve photosynthesis efficiency via structural engineering.

Hollow Plasmonic P-Metal-N S-Scheme Heterojunction Photoreactor with Spatially Separated Dual Cocatalysts toward Artificial Photosynthesis.

Semiconductor-based step-scheme (S-scheme) heterojunctions possess many merits toward mimicking natural photosynthesis. However, their applications for solar-to-chemical energy conversion are hindered by inefficient charge utilization and unsatisfactory surface reactivity. Herein, two synergistic protocols are demonstrated to overcome these limitations based on the construction of a hollow plasmonic p-metal-n S-scheme heterojunction photoreactor with spatially separated dual noble-metal-free cocatalysts. On one side, plasmonic Au, inserted into the heterointerfaces of CuS@ZnIn2 S4 core-shell nanoboxes, not only accelerates the transfer and recombination of useless charges, enabling a more thorough separation of useful ones for CO2 reduction and H2 O oxidation but also generates hot electrons and holes, respectively injects them into ZnIn2 S4 and CuS, further increasing the number of active carriers participating in redox reactions. On the other side, Fe(OH)x and Ti3 C2 cocatalysts, separately located on the CuS and ZnIn2 S4 surface, enrich the redox sites, adjust the reduction potential and pathway for selective CO2 -to-CH4 transformation, and balance the transfer and consumption of photocarriers. As expected, significantly enhanced activity and selectivity in CH4 production are achieved by the smart design along with nearly stoichiometric ratios of reduction and oxidation products. This study paves the way for optimizing artificial photosynthetic systems via rational interfacial channel introduction and surface cocatalyst modification.

Oxygen defect modulating the charge behavior in titanium dioxide for boosting photocatalytic nitrogen fixation performance

Extremely high-temperature and high-pressure requirement of Haber-Bosch process motivates the search for a sustainable ammonia synthesis approach under mild conditions. Photocatalytic technology is a potential solution to convert N2 to ammonia. However, the poor light absorption and low charge carrier separation efficiency in conventional semiconductors are bottlenecks for the application of this technology. Herein, a facile synthesis of anatase TiO2 nanosheets with an abundance of surface oxygen vacancies (TiO2-OV) via the calcination treatment was reported. Photocatalytic experiments of the prepared anatase TiO2 samples showed that TiO2-OV nanosheets exhibited remarkably increased ammonia yield for solar-driven N2 fixation in pure water, without adding any sacrificial agents. EPR, XPS, XRD, UV-Vis DRS, TEM, Raman, and PL techniques were employed to systematically explore the possible enhanced mechanism. Studies revealed that the introduced surface oxygen vacancies significantly extended the light absorption capability in the visible region, decreased the adsorption and activation barriers of inert N2, and improved the separation and transfer efficiency of the photogenerated electron-hole pairs. Thus, a high rate of ammonia evolution in TiO2-OV was realized. This work offers a promising and sustainable approach for the efficient artificial photosynthesis of ammonia.

Platinum single-atom catalysts anchored on a heterostructure cupric oxide/copper foam for accelerating photoelectrochemical hydrogen evolution reaction

Artificial photosynthesis, the process of producing green hydrogen via photoelectrochemical water splitting, requires the development of highly efficient and stable photoelectrodes. However, photoelectrode development is associated with critical challenges. Herein, we develop a reconfiguration strategy for platinum single-atom catalysts anchored on a cupric oxide/copper heterostructure foam to yield a highly active hydrogen evolution reaction. A cupric oxide/copper heterostructure foam fabricated by a simple chemical oxidation process has high carrier accessibility and abundant electrolyte diffusion pathways. Moreover, its catalytic activity is well maintained, which favors hydrogen production. Additionally, inserting platinum single-atom catalysts reduces the number of precious metals required while ensuring catalytic activity. The proposed photoelectrode enables an impressive hydrogen production of 59.2 μmolh−1cm−2 under 1 sun illumination. This study provides a facile strategy to impart an outstanding surface geometry for platinum single-atom catalysts coupled with a cupric oxide/copper heterostructure foam, which can serve as a photocatalyst for accelerating hydrogen evolution reactions.

Moral impact of technologies from a pluralist perspective: Artificial photosynthesis as a case in point

We propose and illustrate a model for evaluating the moral impact of technologies from a pluralist perspective. We conceptualize technological artefacts as having moral profiles that consist of the values served and disserved along five levels of decision-making: (1) problem, (2) strategy, (3) resources, (4) product and (5) design. The notion of complex equality, directly stemming from the pluralist philosophy of Michael Walzer, can function as a heuristic principle to guide the identification and analysis of imbalances along these five levels. We provide an illustrative case study of the moral profile of artificial photosynthesis (AP), an emerging technology for renewable fuel production that promises to resolve our current dependence on fossil fuels. We conclude by providing future directions for the implementation of pluralist ideas in R&D policy and in societal discourse on emerging and incumbent technologies.

Development of photocatalysts based on zeolite A with copper oxide (CuO) for application in the artificial photosynthesis process

This study explores the CO2 photoreduction process and its importance as a potential solution to mitigate carbon emissions. The co-precipitation method was used to produce composites formed between zeolite A and varying mass percentages of CuO, which demonstrated good activity and selectivity in CO2 photoreduction. The selectivity of the composites was linked to the presence of Lewis acid and base sites, with the predominance of Lewis base sites favoring the conversion of CO2 to CH4. Increased CuO content in the composite structure led to enhanced selectivity, reaching 100 % with ZE/CuO10 %. The presence of both Lewis acid and base sites facilitated the formation of two-carbon compounds such as ethylene, making the process of dimerization more accessible. Additionally, the study suggests that methanol and acetic acid can also be produced alongside other gaseous products such as CO, C2H4, and CH4. These findings highlight the potential of zeolite-based composites containing different active sites important during CO2 photoreduction for the production of valuable chemicals.

Boosting Photocatalytic Hydrogen Evolution Through Local Charge Polarization in Chemically Bonded Single‐Molecule Junctions Between Ketone Molecules and Covalent Organic Frameworks

AbstractEfficient charge carrier separation is widely recognized as crucial for effective photocatalytic solar energy conversion. Although certain covalent organic frameworks (COFs) exhibit visible light absorption, rapid carrier recombination often severely restricts their photocatalytic efficiency. This study presents a novel molecular‐engineering approach to enhance carrier separation in 2D substoichiometric COFs post‐treatment with polar ketone molecules. Remarkably, through molecular engineering, incorporating polar ketone molecules induces local charge polarization within the COFs, substantially improving photocatalytic hydrogen evolution performance. Specifically, the modified COFs single‐molecule junctions demonstrate a 5.6‐fold increase in efficiency compared to the unmodified COFs. Experimental and theoretical investigations provide comprehensive evidence that introducing ketone‐based single‐molecule junctions induces local charge polarization and delocalization, thereby facilitating efficient carrier separation, rapid charge transfer, and enhanced dispersion in water. These findings validate the potential of fine‐tuning the carrier separation properties of COFs at the molecular level, offering new prospects for artificial photosynthesis.

Oxygen-Doped Red Carbon Nitride: Enhanced Charge Separation and Light Absorption for Robust CO2 Photoreduction.

Utilizing artificial photosynthesis for the conversion of CO2 into value-added fuels has been recognized as a promising strategy for the ever-increasing energy crisis and the greenhouse effect. Herein, the element doping engineering of red spherical g-C3N4 having oxygen bonded with compositional carbon (C-O-C) for CO2 photoreduction has been explored to address this challenge. The C-O bond was formed by hydrothermal treatment with dicyandiamide and 1,3,5-trichlorotriazine. The experimental and DFT results displayed the optimum oxygen substitution sites and demonstrated that the oxygen doping greatly improved the light utilization efficiency, CO2 affinity, and charge carrier transfer, which enhanced photoreduction efficiency of CO2. The evolution rates of CO (47.2 μmol g-1) and CH4 (9.1 μmol g-1) using O-CN were much higher than that of bulk-CN without a cocatalyst. The main reason was the contribution of the O 2p orbital to the conduction band (CB) and valence band of O-CN, which effectively reduced the electron mass, facilitating electron/hole separation and enhancing its fluidity. Furthermore, the Fermi level also shifted to the bottom of the CB, leading to higher electron density, which further improved the CO2 reduction ability. Our study marks an important step for developing high-performance photocatalysts for reduction of CO2.

Revelations on photosystem II, thermoluminescence, and artificial photosynthesis: a retrospective of Govindjee from fundamentals to applications.

Photosynthesis, as one of the most important chemical reactions, has powered our planet for over four billion years on a massive scale. This review summarizes and highlights the major contributions of Govindjee from fundamentals to applications in photosynthesis. His research included primary photochemistry measurements, in the picosecond time scale, in both Photosystem I and II and electron transport leading to NADP reduction, using two light reactions. He was the first to suggest the existence of P680, the reaction center of PSII, and to prove that it was not an artefact of Chlorophyll a fluorescence. For most photobiologists, Govindjee is best known for successfully exploiting Chlorophyll a fluorescence to understand the various steps in photosynthesis as well as to predict plant productivity. His contribution in resolving the controversy on minimum number of quanta in favor of 8-12 vs 3-4, needed for the evolution of one molecule of oxygen, is a milestone in the area of photosynthesis research. Furthermore, together with Don DeVault, he is the first to provide the correct theory of thermoluminescence in photosynthetic systems. His research productivity is very high: ~ 600 published articles and total citations above 27,000 with an h-index of 82. He is a recipient of numerous awards and honors including a 2022: Lifetime Achievement Award of the International Society of Photosynthesis Research. We hope that the retrospective of Govindjee described in this work will inspire and stimulate the readers to continue probing the photosynthetic apparatuses with new discoveries and breakthroughs.

ALTERNATIVE ENERGY AND ARTIFICIAL PHOTOSYNTHESIS

Limited reserves of fossil fuels and the negative impact of their combustion products on the environment are two pressing problems of our time. The development of alternative energy sources, among which solar energy is the most accessible, is considered as a possible solution. Acquisition of skills of its effective and environmentally friendly use by creating artificial photosynthetic systems imitating the processes of natural photosynthesis, as well as the use of artificial photosynthesis for the production of biofuels can contribute to a way out of the current situation.

Bidirectional host-guest interactions promote selective photocatalytic CO2 reduction coupled with alcohol oxidation in aqueous solution

Existing artificial photosynthesis systems often underperform due to challenges in water oxidation and extensive photogenerated carrier transfer. Herein, we demonstrate that β-cyclodextrin-decorated CdS nanocrystals (CdS-CD) can simultaneously anchor cobalt tetraphenyl porphyrin (CoTPP) catalysts and alcohol reductants onto CdS surfaces through bidirectional host-guest interactions between β-CD and CoTPP/alcohol. This configuration ensures swift electron transfer from CdS to β-CD bonded CoTPP, facilitating CO2 reduction to HCOOH. Results showcase a remarkable yield of 1610 μmol g−1 h−1 and a 96.5% selectivity. Meanwhile, photogenerated holes in CdS are efficiently neutralized by β-CD bonded furfuryl alcohol, achieving a furfural yield of 1567 μmol g−1 h−1 and > 99% selectivity. In contrast, the discrete CoTPP-CdS system achieved a HCOOH yield of only 306 μmol g−1 h−1 and a poor HCOOH selectivity of 46.4%. It was also observed that the CoTPP@CdS-CD system maintained high CO2-to-HCOOH conversion rates when using other alcohols as reductants.

Tunable Band Engineering Management on Perovskite MAPbBr3 /COFs Nano-Heterostructures for Efficient S-S Coupling Reactions.

Efficient artificial photosynthesis of disulfide bonds holds promises to facilitate reverse decoding of genetic codes and deciphering the secrets of protein multilevel folding, as well as the development of life science and advanced functional materials. However, the incumbent synthesis strategies encounter separation challenges arising from leaving groups in the ─S─S─ coupling reaction. In this study, according to the reaction mechanism of free-radical-triggered ─S─S─ coupling, light-driven heterojunction functional photocatalysts are tailored and constructed, enabling them to efficiently generate free radicals and trigger the coupling reaction. Specifically, perovskites and covalent organic frameworks (COFs) are screened out as target materials due to their superior light-harvesting and photoelectronic properties, as well as flexible and tunable band structure. The in situ assembled Z-scheme heterojunction MAPB-M-COF (MAPbBr3 = MAPB, MA+ = CH3 NH2 + ) demonstrates a perfect trade-off between quantum efficiency and redox chemical potential via band engineering management. The MAPB-M-COF achieves a 100% ─S─S─ coupling yield with a record photoquantum efficiency of 11.50% and outstanding cycling stability, rivaling all the incumbent similar reaction systems. It highlights the effectiveness and superiority of application-oriented band engineering management in designing efficient multifunctional photocatalysts. This study demonstrates a concept-to-proof research methodology for the development of various integrated heterojunction semiconductors for light-driven chemical reaction and energy conversion.

Impact of Spatial Inhomogeneity on Excitation Energy Transport in the Fenna-Matthews-Olson Complex.

The dynamics of the excitation energy transfer (EET) in photosynthetic complexes is an interesting question both from the perspective of fundamental understanding and the research in artificial photosynthesis. Over the past decade, very accurate spectral densities have been developed to capture spatial inhomogeneities in the Fenna-Matthews-Olson (FMO) complex. However, challenges persist in numerically simulating these systems, both in terms of parameterizing them and following their dynamics over long periods of time because of long non-Markovian memories. We investigate the dynamics of FMO with the exact treatment of various theoretical spectral densities using the new tensor network path integral-based methods, which are uniquely capable of addressing the difficulty of long memory length and incoherent Förster theory. It is also important to be able to analyze the pathway of EET flow, which can be difficult to identify given the non-trivial structure of connections between bacteriochlorophyll molecules in FMO. We use the recently introduced ideas of relating coherence to population derivatives to analyze the transport process and reveal some new routes of transport. The combination of exact and approximate methods sheds light on the role of coherences in affecting the fine details of the transport and promises to be a powerful toolbox for future exploration of other open systems with quantum transport.

Synergistic Trimetallic Nanocomposites as Visible-NIR-Sunlight-Driven Photocatalysts for Efficient Artificial Photosynthesis.

Here, we report monodispersed tricomponent MnNSs-SnO2@Pt and MnNFs-SnO2@Pt nanocomposites prepared by simultaneous SnO2 and Pt nanodot coating on Mn nanospheres (MnNSs) and Mn nanoflowers (MnNFs) for highly efficient CO2 photoreduction in visible-NIR-sunlight irradiation. MnNFs-SnO2@Pt showed higher efficiency with a quantum yield of 3.21% and a chemical yield of 5.45% for CO2 conversion under visible light irradiation for HCOOH formation with 94% selectivity. Similarly, MnNFs-SnO2@Pt displayed significant photocatalytic efficiency in NIR and sunlight irradiation for HCOOH yield. MnNFs-SnO2@Pt nanocomposites also showed robust morphology and sustained structural stability with shelf-life for at least 1 year and were utilized for at least 10 reaction cycles without losing significant photocatalytic efficiency. The high surface area (94.98 m2/g), efficient electron-hole separation, and Pt-nanodot support in MnNFs--SnO2@Pt contributed to a higher photocatalytic efficacy toward CO2 reduction.

Solar-Driven Reforming of Methane and Nitrogen to Methanol and Ammonium on Iron-Modified Zeolite under Ambient Conditions in Water.

Artificial photosynthesis from selective methane oxidation or nitrogen reduction to value-added chemicals provides a promising pathway for the sustainable chemical industry, while still remaining a great challenge due to the extreme difficulty in C-H and N≡N bond cleavage under ambient conditions. Catalysts that can cocatalyze these two reactions simultaneously are rarely reported. Here, Fe-ZSM-5 with highly dispersed extra-framework Fe-oxo species enables efficient and selective photocatalytic conversion of methane and nitrogen to coproduce methanol and ammonia using H2O as the redox reagent under ambient conditions. The optimized Fe-ZSM-5 photocatalyst achieves up to 0.88 mol/molFe·h of methanol products with 97% selectivity. Meanwhile, the productivity of ammonia is 0.61 mol/molFe·h. In situ EPR and DRIFT studies disclose that water serves as a redox reagent to provide hydroxyl radicals for methane oxidation and protons for nitrogen hydrogenation. Quantum chemical calculations revealed that Fe-oxo species play a significant role in the coactivation of methane and nitrogen molecules, which lowers the energy barriers of rate-determining steps for methanol and ammonia generation.

Photocatalytic Conversion of CO2 By H2o As an Electron Donor over Mg-Doped SrTiO3

Atmospheric CO2 is considered as the main cause of climate change, which is known as one of the serious problems for the earth, and several approaches are employed to convert CO2 into value-added products, such as CO, HCOOH, CH3OH and CH4. Among those approaches, photocatalysis has been highly attracting the world-wide attention, because photocatalytic conversion of CO2, which is called as “artificial photosynthesis”, can be a solution for climate change in terms of its renewability, harmlessness and low cost. Ga-based mixed oxide photocatalysts such as ZnGa2O4 were reported as superior photocatalysts for the photocatalytic conversion of CO2 by using H2O3. Recently, our group reported that Ag and Co dual cocatalysts-loaded Al-doped SrTiO3 photocatalyst showed relatively high formation rate of CO for the photocatalytic conversion of CO2 by H2O under photoirradiation with a monochromatic light (365 nm)1, although Ga-based photocatalysts exhibited the activity under the photoirradiation with less than 300 nm. In this study, the effect of metal dopant in SrTiO3 on the photocatalytic conversion of CO2 was investigated. It was found that Mg-doped SrTiO3 showed higher formation rate of CO for the photocatalytic conversion of CO2 into CO under photoirradiation at 365 nm than Al-doped SrTiO3.Mg-doped SrTiO3 was synthesized by a flux method. After grinding mixture of SrTiO3, MgO, and SrCl2 flux, the obtained powder was calcined at 1418 K for 15 h. Ag and Co dual cocatalysts were loaded by chemical reduction method. An aqueous solution of NaH2PO2 reductant was added to the suspension containing Mg-doped SrTiO3, AgNO3, and Co(NO3)2, and it was kept at 353 K for 1.5 h with stirring. The photocatalytic conversion of CO2 by H2O was carried out using an external-irradiation-type reaction vessel. Ag and Co-loaded Mg-doped SrTiO3 was dispersed in 0.1 M NaHCO3 aqueous solution, and high purity CO2 gas was bubbled into the suspension at a flow rate of 30 mL min−1. Monochromatic UV LED lamp (365 nm) was used as a light source. The gaseous products were analyzed by TCD–GC (H2, O2) and FID–GC equipped with methanizer (CO).XRD patterns of pristine and Mg-doped SrTiO3 showed that diffraction peaks are shifted to the lower angle by Mg2+ doping, which might be corresponded to Mg2+ doping into Ti4+ site in the bulk of SrTiO3. SEM images displayed that a shape of Mg-doped SrTiO3 is an edge-shaved cube, while that of pristine SrTiO3 is irregular. In other words, {110} facets were observed in Mg-doped SrTiO3 in addition to {100} facets. According to the previous report2, the photoexcited carriers, electrons and holes, were transported to different crystal facets of the perovskite oxide, especially {100} and {110} facets, respectively. This anisotropic charge distribution would decrease the charge recombination rate in Mg-doped SrTiO3, resulting in the high photocatalytic activity for CO2 conversion.The attached figure shows formation rates of H2 (gray), O2 (white) and CO (black) and selectivity toward CO evolution (black diamond) for the photocatalytic conversion of CO2 by H2O over Ag and Co-loaded pristine SrTiO3, Al-doped SrTiO3, and Mg-doped SrTiO3. Mg-doped SrTiO3 exhibited much higher photocatalytic activity than pristine SrTiO3 and Al-doped SrTiO3. The formation rate of CO over Mg-doped SrTiO3 was 20 μmol h−1, which was 2.3 times larger than that over Al-doped SrTiO3. Surprisingly, the selectivity toward CO evolution was almost 100 % over both Al-doped and Mg-doped SrTiO3. Moreover, the stoichiometric amount of O2 evolution was observed during this reaction, indicating that H2O worked as an electron donor for the photocatalytic conversion of CO2.The control experiments revealed that the reduction of CO2 did not proceed in the absence of photocatalyst, photoirradiation, or additive (0.1 M NaHCO3). These results clearly indicated that the significant photocatalytic performance was observed by using the Ag and Co-loaded Mg-doped SrTiO3 photocatalyst under photoirradiation with a NaHCO3 additive in the suspension. To confirm that the generated CO is not derived from any organic contamination, the isotope-labeling experiments using 13CO2 gas was carried out. GC-MS analysis of the outlet gas in the photocatalytic conversion of 13CO2 over Mg-doped SrTiO3 clarified that 13C-labeled CO (m/z = 29) was evolved in priority to 12CO (m/z = 28). Accordingly, we concluded that CO gas evolved in our system is originated from CO2 gas bubbled into the suspension. Wang, et al., Chem. Sci., 2021, 12, 4940–4948Takata, et al., Nature, 2020, 581, 411–414Wang, et al., J. Mater. Chem. A 2015, 3, 11313–11319 Figure 1

(Invited) Enhanced Photocatalysis on TiO2-Passivated III-V Compound Semiconductors and 2D Material Heterostructures for Water Splitting and CO2 Reduction

We report enhanced photocatalysis for H2 evolution and CO2 reduction using TiO2-passivated InP and GaAs photocathodes.1-4 The TiO2 layer makes the InP semiconductor photochemically stable. This represents a major step forward in photocatalysis, which has typically been limited to metal oxide materials. In addition to making these surfaces stable, the TiO2 film, deposited by atomic layer deposition (ALD), also provides a substantial enhancement in the efficiency of H2 evolution. We find that passivating GaAs with just a few nm of TiO2 produces a shift in the onset potential of H2 evolution by +0.35 V at 1 mA/cm2 and enhances the photocurrent by 32-fold over bare GaAs (at 0 V vs. RHE). Here, thinner TiO2 films produce a larger enhancement than thicker films, which correlates with the higher density of O-vacancies (i.e., Ti3+ surface states) observed in these thinner films using X-ray photoemission spectroscopy (XPS). While TiO2 films 1-5nm thick produce large enhancements, no enhancement is observed for TiO2 thicknesses above 10 nm, which are crystalline and, therefore, considerably more insulating than thinner amorphous TiO2 films.Using transient reflectance spectroscopy (TRS), we measure the photoexcited carrier dynamics in a GaP/TiO2 photoelectrode, as well as the electrostatic field dynamics at this semiconductor-liquid interfaces in situ under various electrochemical potentials.5 Here, the electrostatic fields at the surface of the semiconductor are measured via Franz−Keldysh oscillations (FKO). These spectra reveal that the nanoscale TiO2 protection layer enhances the built-in field and charge separation performance of GaP photoelectrodes.We also demonstrated plasmon-resonant enhancement of photocatalysis on monolayer WSe2.6 In addition, we measured the stacking dependence and resonant interlayer excitation of monolayer WSe2/MoSe2 heterostructures for photocatalytic energy conversion.7 Qiu, J., G. Zeng, M. Ge, S. Arab, M. Mecklenburg, B. Hou, C. Shen, A.V. Benderskii and S.B. Cronin, Correlation of Ti3+ states with photocatalytic enhancement in TiO2-passivated p-GaAs. Journal of Catalysis, 337, 133 (2016).Qiu, J., G.T. Zeng, M.A. Ha, M.Y. Ge, Y.J. Lin, M. Hettick, B.Y. Hou, A.N. Alexandrova, A. Javey and S.B. Cronin, Artificial Photosynthesis on TiO2-Passivated InP Nanopillars. Nano Letters, 15, 6177-6181 (2015).Qiu, J., G.T. Zeng, M.A. Ha, B.Y. Hou, M. Mecklenburg, H.T. Shi, A.N. Alexandrova and S.B. Cronin, Microscopic Study of Atomic Layer Deposition of TiO2 on GaAs and Its Photocatalytic Application. Chemistry of Materials, 27, 7977-7981 (2015).Wang, Y. and S.B. Cronin, Performance Enhancement of TiO2-encapsulated Photoelectrodes Based on III–V Compound Semiconductors, in Ultrathin Oxide Layers for Solar and Electrocatalytic Systems. 2022, Royal Society of Chemistry. p. 103-134.Xu, Z.H., B.Y. Hou, F.Y. Zhao, Z. Cai, H.T. Shi, Y.W. Liu, C.L. Hill, D.G. Musaev, M. Mecklenburg, S.B. Cronin and T.Q. Lian, Nanoscale TiO2 Protection Layer Enhances the Built-In Field and Charge Separation Performance of GaP Photoelectrodes. Nano Letters, 21, 8017-8024 (2021).Chen, J., C.S. Bailey, Y. Hong, L. Wang, Z. Cai, L. Shen, B. Hou, Y. Wang, H. Shi, J. Sambur, W. Ren, E. Pop, and S.B. Cronin., Plasmon-Resonant Enhancement of Photocatalysis on Monolayer WSe2. ACS Photonics, 6, 787 (2019).Chen, J., C.S. Bailey, D. Cui, Y. Wang, B. Wang, H. Shi, Z. Cai, E. Pop, C. Zhou and S.B. Cronin, Stacking Independence and Resonant Interlayer Excitation of Monolayer WSe2/MoSe2 Heterostructures for Photocatalytic Energy Conversion. ACS Applied Nano Materials, DOI:10.1021/acsanm.9b01898 (2020).

(Invited) Theoretical Study on the Environmental Effects on Photoexcited Properties of Donor-Acceptor-Linked Molecules

Photoinduced charge separation process plays important roles in various fields such as natural and artificial photosynthesis, solar cell, and light-emitting diode. Toward an understanding of the molecular mechanism of photoinduced charge separation, we have been studying the excited and charge-separated states of donor-acceptor-linked molecules with theoretical methods such as QM/MM method and molecular dynamics (MD) simulation. In particular, we have focused on the environmental effects on photoexcited properties. In this presentation, we will present recent progress of our research. For example, we will talk about theoretical analysis of acceptor-donor-acceptor type non-fullerene acceptor called TACIC. The excited state of TACIC is known to have a longer lifetime in the film state than in solution. This is in contrast to a similar acceptor-donor-acceptor type non-fullerene acceptor molecule, ITIC, which has a shorter lifetime in the film state than in solution. However, the molecular mechanism is unknown. We analyzed the structures and excited states of the TACIC and ITIC by using quantum chemical calculations and MD simulation. First, it is found that the excited-state potential energy surface of TACIC monomer is similar to that of ITIC monomer, which is consistent with the experimental result that both have similar lifetimes in solution. Next, the packing structure of TACIC in the film state is much different from that of ITIC. Analysis of the excited states of dimer obtained from the film state structure demonstrated that the transition dipole of TACIC dimer is much smaller than that of ITIC dimer. These results could be related to the difference in lifetime of the excited state between TACIC and ITIC in the film state. Further analysis is currently in progress. We will also talk about the photoexcited properties of porphyrin-fullerene linked molecules.