Accurate exploration of synergistic thermal effect: Low-grade-heat-promoted charge recombination coupled with spherical incidence for efficient sunlight-driven photocatalysis.

The major objective of the emerging photo-thermo-catalysis is using waste heat to boost the photocatalytic reaction, especially that powered by sunlight. Because of the complex composition of light-intensity-dependent apparent activation energies, the issue that principally hinders the synergistic thermal effect to photocatalysis has hardly been accurately explored. In this work, by virtue of mutual match of theoretical simulation and experimental behaviors, we demonstrate that photocatalytic reaction rates exhibit a sensitively positive correlation with temperature under weak illumination, in which charge recombination predominates the rate-determining step of semiconductor-cocatalyst interfacial electron transfer. Under high-intensity irradiation, however, the aggravation of charge leakage inherently accompanied by thermionic emission severely weakens the synergistic thermal effect or even slows down the reaction by raising the temperature. Inspired by these, we manage to maximize the photocatalytic solar utilization by spherical incidence of sunlight with the assistance of low-grade heat.

Revealing Energetics of Surface Oxygen Redox from Kinetic Fingerprint in Oxygen Electrocatalysis.

The key step for rational catalyst design in heterogeneous electrocatalysis is to reveal the distinctive energy profile of redox reactions of a catalyst that give rise to specific activity. However, it is challenging to experimentally obtain the energetics of oxygen redox in oxygen electrocatalysis because of the liquid reaction environment. Here we develop a kinetic model that constructs a quantitative relation between the energy profile of oxygen redox and electrochemical kinetic fingerprints. The detailed study here demonstrates that the kinetic fingerprints observed from experiments can be well described by different energetics of oxygen redox. On the basis of the model, a feasible methodology is demonstrated to derive binding energies of the oxygen intermediates from electrochemical data. The surface property of different catalysts derived from our model well rationalizes the experimental trends and predicts potential directions for catalyst design.

Nanostructuring Confinement for Controllable Interfacial Charge Transfer.

Carbon nanostructures supported semiconductors are common in photocatalytic and photoelectrochemical applications, as it is expected that the nanoconductors can improve the spatial separation and transport of photogenerated charge carriers. Transfer of charge carriers through the carbon-semiconductor interface is the key electronic process, which determines the role of charge separation channels, and is sensitively influenced by band structures of the semiconductor near the contacts. Usually, this electronic process suffers from excessive energy dissipation by thermionic emission, which will undesirably prevent the interfacial charge transfer and eventually aggravate the recombination of photogenerated charge carriers. Unfortunately, this critical issue has hardly been consciously considered. Here, ultrathin dopant-free tunneling interlayers coated on the surface of graphene and sandwiched between the carbon sheets and the semiconductor nanostructures are adopted as a model system to demonstrate energy saving for the interfacial charge transfer. The nanostructuring confinement of band bending within the ultrathin interlayers in contact with the graphene sheets effectively narrows the width of the potential barriers, which enables tunneling of a substantial number of photogenerated electrons to the co-catalysts without unduly consuming energy. Besides, the dopant-free tunneling interlayers simultaneously block the transferred electrons in the sandwiched graphene sheets from leakage.

Identification of Surface Reactivity Descriptor for Transition Metal Oxides in Oxygen Evolution Reaction.

A number of important reactions such as the oxygen evolution reaction (OER) are catalyzed by transition metal oxides (TMOs), the surface reactivity of which is rather elusive. Therefore, rationally tailoring adsorption energy of intermediates on TMOs to achieve desirable catalytic performance still remains a great challenge. Here we show the identification of a general and tunable surface structure, coordinatively unsaturated metal cation (MCUS), as a good surface reactivity descriptor for TMOs in OER. Surface reactivity of a given TMO increases monotonically with the density of MCUS, and thus the increase in MCUS improves the catalytic activity for weak-binding TMOs but impairs that for strong-binding ones. The electronic origin of the surface reactivity can be well explained by a new model proposed in this work, wherein the energy of the highest-occupied d-states relative to the Fermi level determines the intermediates' bonding strength by affecting the filling of the antibonding states. Our model for the first time well describes the reactivity trends among TMOs, and would initiate viable design principles for, but not limited to, OER catalysts.

Tunneling Interlayer for Efficient Transport of Charges in Metal Oxide Electrodes.

Due to the limited electronic conductivity, the application of many metal oxides that may have attractive (photo)-electrochemical properties has been limited. Regarding these issues, incorporating low-dimensional conducting scaffolds into the electrodes or supporting the metal oxides onto the conducting networks are common approaches. However, some key electronic processes like interfacial charge transfer are far from being consciously concerned. Here we use a carbon-TiO2 contact as a model system to demonstrate the electronic processes occurring at the metal-semiconductor interface. To minimize the energy dissipation for fast transfer of electrons from semiconductor to carbon scaffolds, facilitating electron tunneling while avoiding high energy-consuming thermionic emission is desired, according to our theoretical simulation of the voltammetric behaviors. To validate this, we manage to sandwich ultrathin TiO2 interlayers with heavy electronic doping between the carbon conductors and dopant-free TiO2. The radially graded distribution of the electronic doping along the cross-sectional direction of carbon conductor realized by immobilizing the dopant species on the carbon surface can minimize the energy consumption for contacts to both the carbon and the dopant-free TiO2. Our strategy provides an important requirement for metal oxide electrode design.

One-dimensional hybrid nanostructures for heterogeneous photocatalysis and photoelectrocatalysis.

Semiconductor-based photocatalysis and photoelectrocatalysis have received considerable attention as alternative approaches for solar energy harvesting and storage. The photocatalytic or photoelectrocatalytic performance of a semiconductor is closely related to the design of the semiconductor at the nanoscale. Among various nanostructures, one-dimensional (1D) nanostructured photocatalysts and photoelectrodes have attracted increasing interest owing to their unique optical, structural, and electronic advantages. In this article, a comprehensive review of the current research efforts towards the development of 1D semiconductor nanomaterials for heterogeneous photocatalysis and photoelectrocatalysis is provided and, in particular, a discussion of how to overcome the challenges for achieving full potential of 1D nanostructures is presented. It is anticipated that this review will afford enriched information on the rational exploration of the structural and electronic properties of 1D semiconductor nanostructures for achieving more efficient 1D nanostructure-based photocatalysts and photoelectrodes for high-efficiency solar energy conversion.

Thermodynamically driven one-dimensional evolution of anatase TiO2 nanorods: one-step hydrothermal synthesis for emerging intrinsic superiority of dimensionality.

In photoelectrochemical cells, there exists a competition between transport of electrons through the porous semiconductor electrode toward the conducting substrate and back-reaction of electrons to recombine with oxidized species on the semiconductor-electrolyte interface, which determines the charge collection efficiency and is strongly influenced by the density and distribution of electronic states in band gap and architectures of the semiconductor electrodes. One-dimensional (1D) anatase TiO2 nanostructures are promising to improve charge transport in photoelectrochemical devices. However, the conventional preparation of 1D anatase nanostructures usually steps via a titanic acid intermediate (e.g., H2Ti3O7), which unavoidably introduces electronic defects into the host lattice, resulting in undesired shielding of the intrinsic role of dimensionality. Here, we manage to promote the 1D growth of anatase TiO2 nanostructures by adjusting the growth kinetics, which allows us to grow single-crystalline anatase TiO2 nanorods through a one-step hydrothermal reaction. The synthesized anatase nanorods possess a lower density of trap states and thus can simultaneously facilitate the diffusion-driven charge transport and suppress the electron recombination. Moreover, the electronically boundary free nanostructures significantly enhance the trap-free charge diffusion coefficient of the anatase nanorods, which enables the emergence of the intrinsic superiority of dimensionality. By virtue of these merits, the anatase nanorods synthesized in this work take obvious advantages over the conventional anatase counterparts in photoelectrochemical systems (e.g., dye-sensitized solar cells) by showing more efficient charge transport and collection and higher energy conversion efficiency.

Time Constant Estimation and Alleviation of Interior Charge Recombination for Photocatalytic Reaction Guided by Correlation with Photoelectrochemical Behaviors.

Interior charge recombination that inherently competes with the separation of photogenerated charges is crucial to the photocatalytic utilization of incident photons. We here propose to simultaneously promote both the desired hole extraction and semiconductor-cocatalyst interfacial electron transfer and suppress the undesired interior charge recombination by shifting the equilibrium potential of the semiconductor in an actual photocatalytic reaction. By correlating with these interfacial electronic processes, we estimate the time constants for the occurrence of interior charge recombination, which range from several milliseconds to several deciseconds in the actual photocatalytic reaction. This time scale estimated from photoelectrochemical behaviors not only provides a substantial guide to photocatalytic reaction design but also avoids relying on a very dense photon beam that greatly deviates from actual working conditions to generate discernible optical signals in certain common methods.

Evaluating a Second Time Scale for Hole Extraction in an Actual Photocatalytic Reaction: The Method.

Photoinduced hole extraction, which is widely believed to occur on the picosecond-nanosecond time scale, exhibits sensitive correlation with semiconductor-cocatalyst interfacial electron transfer that occurs on a second time scale in a photocatalytic reaction. This inherent correlation means that the required high density of electrons for overcoming the potential barrier severely suppresses the survival and interfacial extraction of photogenerated holes. We here propose to evaluate the time constant for the occurring hole extraction by separately monitoring the rise and decay of the photoinduced potential. The evaluated second time scale indicates that hole extraction crucially influences the actual photocatalytic reaction. To improve photocatalytic photon utilization, we facilitate hole extraction by lowering the semiconductor-cocatalyst contact barrier.

Revealing the Efficient and Constant Photon Utilization of Photocatalytic Fixed Bed Reaction.

Because of the very high potential barrier, semiconductor-co-catalyst interfacial electron transfer occurring on a second time scale severely slows the photocatalytic reaction. Besides, the undesired deprivation of electrons from the co-catalyst by photogenerated oxidative intermediates in a photocatalytic slurry suspension further lowers the light-intensity-dependent photon utilization. We demonstrate here that photocatalyst immobilization can flatten the potential barrier and increase the selectivity of electrons for the target reaction. This is because the induced spatial separation of half reactions in the formed fixed bed reactors can suppress the loss of photogenerated charge carriers and increase the density of electrons in the semiconductor. The photocatalytic fixed bed reaction thus exhibits efficient and constant photon utilization.

Self-Promoted H2 Formation: The Feasibility of Photoinduced CO Removal for Lossless Hydrogen Purification.

A photoinduced radical reaction operated at low temperature can be used to remove trace CO from a H2 stream by minimizing the reverse water-gas shift. However, H2 consumption resulting from nonselective oxidation by hydroxyl radicals becomes an obstacle to practical hydrogen purification. Inspired by hydrogen exchange transfer, we demonstrate here that molecular hydrogen can promote H2 formation from hydrogen radicals, which are generated from the reaction of CO and H2 with hydroxyl radicals. The slight increment in H2 along with the radical reaction encouraged us to configure a photocatalytic hydrogen purification fixed-bed reactor, which can reduce CO to ≤1 ppm in the H2 stream.

Three-Channel Electron Transfer for Estimating Time Constants Correlated with Photocatalytic Photon Utilization.

Although the potential barrier for semiconductor-cocatalyst interfacial electron transfer can be reduced by intensifying the irradiation, the photocatalytic reaction still suffers from a low photon utilization. We here propose a trichannel electron transfer model to demonstrate that the photogenerated oxidative intermediates deprive electrons from the photocatalyst and compete with the target reaction. This model can evaluate the time constant for each electronic process involved in the target reaction and predict photocatalytic photon utilization, which is closely related to the evolution of the oxidative intermediates.

Revealing the Role of Electronic Doping for Developing Cocatalyst-Free Semiconducting Photocatalysts.

Developing cocatalyst-free photocatalysts is highly desired because it could avoid the very slow interfacial electron transfer that makes photocatalytic photon utilization a dilemma. However, even in the optimal case, photocatalysts without the use of cocatalysts deliver comparable performance only for conventional construction. We demonstrate here that electronic doping not only provides catalytically active sites in cocatalyst-free photocatalysts but also plays certain additional roles. These electronic states can efficiently channel the trapped electrons to the semiconductor surface without suffering from time-consuming detrapping and can facilitate the extraction of photogenerated holes. These features endow our demonstrated tungsten-doped CdS with evident superiority in photocatalytic performance over conventional counterparts loaded with platinum cocatalysts.

Photocatalyst: To Be Dispersed or To Be Immobilized? The Crucial Role of Electron Transport in Photocatalytic Fixed Bed Reaction.

Photocatalytic fixed bed reactors allow a straightforward separation from the process stream and simplify the installation and operation in practical application. However, it is widely believed that the restriction on mass transport and volume activation severely slows the reaction. Here, we demonstrate that photocatalytic fixed bed reactors can deliver a superior reaction rate to the slurry suspension by rationally modulating the electronic process and the most concerning issue of mass transport occurring on a decisecond time scale does not retard the reaction. Although the long-distance transport of photogenerated electrons in porous semiconductor films toward catalytic sites encounters boundary scattering, this electronic process can be far faster than semiconductor-cocatalyst interfacial electron transfer occurring on the decisecond-second time scale. Besides, the fixed bed reaction can be freely amplified without losing photon utilization. Under irradiation provided by a 320 W Hg lamp, we realize a reaction rate of 0.262 mol/h with 65.2% quantum yield for anaerobic dehydrogenation of ethanol.

Revealing and Facilitating the Rate-Determining Step for Efficient Sunlight-Driven Photocatalysis.

Because of the complex composition of the apparent activation energy, the rate-determining step in a photocatalytic reaction like hydrogen evolution is still being explored even after sluggish oxygen evolution is replaced with efficient hole extraction. This issue severely limits the implementation of certain strategies like the synergistic thermal effect. Here, by developing a combined monitor method based on open-circuit potential decay, we demonstrate that semiconductor-cocatalyst interfacial electron transfer occurring on a decisecond to second time scale dominates photocatalytic hydrogen evolution. This time scale is approximately 6-12 orders of magnitude larger than the widely reported values of picoseconds to microseconds and is comparable to that predicted by Durrant et al. To improve photocatalytic hydrogen evolution, we manage to create more intermediate sites by electronically doping the semiconductor surface. This measure promotes semiconductor-cocatalyst interfacial electron transfer by charge recombination and makes the synergistic thermal effect very evident.

Noble metal nanowire arrays as an ethanol oxidation electrocatalyst.

Vertically aligned noble metal nanowire arrays were grown on conductive electrodes based on a solution growth method. They show significant improvement of electrocatalytic activity in ethanol oxidation, from a re-deposited sample of the same detached nanowires. The unusual morphology provides open diffusion channels and direct charge transport pathways, in addition to the high electrochemically active surface from the ultrathin nanowires. Our best nanowire arrays exhibited much enhanced electrocatalytic activity, achieving a 38.0 fold increase in specific activity over that of commercial catalysts for ethanol electrooxidation. The structural design provides a new direction to enhance the electrocatalytic activity and reduce the size of electrodes for miniaturization of portable electrochemical devices.

Light-Intensity-Dependent Semiconductor-Cocatalyst Interfacial Electron Transfer: A Dilemma of Sunlight-Driven Photocatalysis.

In photocatalytic reactions, the interfacial transfer of electrons from semiconductor nanostructures to cocatalysts is the key step that determines the utilization of photogenerated charges and is sensitively influenced by the behaviors of this electronic process. Under weak illumination, photocatalytic reaction rates deviate from linearity to incident light intensity (r = kss·Pincα, with α → 0.5), because charge recombination predominates interfacial transfer. When the irradiation intensity is high, theoretically, thermionic emission would be the major electronic process (r = kte·Pincα, with α → 2). The ratio of photocatalytic reaction rate to incident light intensity that mainly reflects the energy utilization would encounter a minimum along the variation of irradiation intensity. This crucial relationship, however, has hardly been consciously considered. In this work, inspired by theoretical simulation, we demonstrate that sunlight-driven photocatalysis is generally on the bottom of the energy utilization curves for certain common semiconductors (CdS, TiO2, or C3N4).

Nanostructuring Bridges Semiconductor-Cocatalyst Interfacial Electron Transfer: Realizing Light-Intensity-Independent Energy Utilization and Efficient Sunlight-Driven Photocatalysis.

Despite thermodynamic feasibility, the high activation energy originating from potential barriers and trap states kinetically prevents the interfacial transfer of electrons from semiconductor nanostructures to reduction cocatalysts, resulting in a lowered utilization of photogenerated charge carriers in photocatalysis. Nanostructuring-induced narrowing of potential barriers offers a rational solution to kinetically facilitate interfacial electron transfer by tunneling. Here, inspired by theoretical simulation, we manage to promote the separation of photogenerated charge carriers by coating the semiconductor nanostructures with a homogeneous interlayer. The low activation energy for interfacial electron transfer endows photocatalysis with nearly constant quantum yields and a quasi-first-order reaction to the incident photons and grants evident superiority over the photocatalyst without interlayers, especially under sunlight. In our demonstrated sunlight-driven hydrogen evolution integrated with benzylamine oxidation, the production rates for both reduction and oxidation half-reactions reach as high as ∼0.77 mmol dm-2 h-1, which are ∼10 times higher than that without an interlayer.

Photoelectrocatalytic Reduction of CO2 to Paraffin Using p-n Heterojunctions.

Nowadays, photoelectrocatalytic (PEC) reduction of CO2 represents a very promising solution for storing solar energy in value-added chemicals, but so far it has been hampered by the lack of highly efficient catalyst of photocathode. Enlightened by the Calvin cycle of plants, here we show that a series of three-dimensional C/N-doped heterojunctions of Znx:Coy@Cu are successfully fabricated and applied as photocathodes in the PEC reduction of CO2 to generate paraffin product. These materials integrate semiconductors of p-type Co3O4 and n-type ZnO on Cu foam to construct fine heterojunctions with multiple active sites, which result in excellent C-C coupling control in reduction of CO2. The best catalyst of Zn0.2:Co1@Cu yields paraffin at a rate of 325 µg·h-1 under -0.4 V versus saturated calomel electrode without H2 release. The apparent quantum efficiency of PEC cell is up to 1.95%.

Metal-Organic Frameworks as Promising Photosensitizers for Photoelectrochemical Water Splitting.

Ti-based metal-organic frameworks (MOFs) are demonstrated as promising photosensitizers for photoelectrochemical (PEC) water splitting. Photocurrents of TiO2 nano wire photoelectrodes can be improved under visible light through sensitization with aminated Ti-based MOFs. As a host, other sensitizers or catalysts such as Au nanoparticles can be incorporated into the MOF layer thus further improving the PEC water splitting efficiency.