Photoelectrocatalytic production of solar fuels with semiconductor oxides: materials, activity and modeling.

Oxide photoelectrochemistry has been under continuous development over the last half century. These decades have witnessed the use of electrodes of different nature (from single crystals to nanoparticulate films), new electrode materials (including ternary and multinary transition metal oxides), and different strategies for improving their efficiency and stability (e.g. doping or protective layers). Although the very high initial expectations for using oxide electrodes in solar energy conversion were not fully met, substantial efforts have been devoted to reach an in-depth understanding of the processes limiting their functioning, providing firm bases for further developments. In this article, we review our main contributions in this field; in particular, we focus on the water photooxidation (i.e. oxygen evolution reaction), water photoreduction (i.e. hydrogen evolution reaction) and full water splitting processes (in a tandem cell) with binary and ternary oxides, including metal hydroxides as co-catalysts. We emphasize the importance of modeling and obtaining mechanistic insights and we conclude with a reflection on the main issues to be tackled in this field, which in our opinion should experience major advances in the coming years.

Ag(I) ions working as a hole-transfer mediator in photoelectrocatalytic water oxidation on WO3 film.

Ag(I) is commonly employed as an electron scavenger to promote water oxidation. In addition to its straightforward role as an electron acceptor, Ag(I) can also capture holes to generate the high-valent silver species. Herein, we demonstrate photoelectrocatalytic (PEC) water oxidation and concurrent dioxygen evolution by the silver redox cycle where Ag(I) acts as a hole-transfer mediator. Ag(I) enhances the PEC performance of WO3 electrodes at 1.23 V vs. RHE with increasing O2 evolution, while forming Ag(II) complexes (AgIINO3+). Upon turning off both light and potential bias, the photocurrent immediately drops to zero, whereas O2 evolution continues over ~10 h with gradual bleaching of the colored complexes. This phenomenon is observed neither in the Ag(I)-free PEC reactions nor in the photocatalytic (i.e., bias-free) reactions with Ag(I). This study finds that the role of Ag(I) is not limited as an electron scavenger and calls for more thorough studies on the effect of Ag(I).

Homogeneous photocatalytic Fe3+/Fe2+ redox cycle for simultaneous Cr(VI) reduction and organic pollutant oxidation: Roles of hydroxyl radical and degradation intermediates.

The sustained oxidation of aqueous organic pollutants using hydroxyl radicals (HO) generated in the UV-irradiated solution of ferric ions was investigated in the presence of Cr(VI). The synergistic effect of simultaneous 4-chlorophenol (4-CP) oxidation and Cr(VI) reduction is explained in terms of the various roles of OH radical, degradation intermediates, and Fe3+/Fe2+ redox cycle. The photolysis of FeIII(OH)2+ generates OH radical which degrades the organic substrate. The reduction of Cr(VI) was inhibited by the OH radical-induced re-oxidation of Cr(III) in the absence of 4-CP. The complete removal of Cr(VI) was achieved only in the presence of phenolic substrates which not only reacts with OH radical (hence inhibiting the reoxidation of Cr(III)) but also generates reducing intermediates which effectively reduce Cr(VI). Fe2+ also converted Cr(VI) to Cr(III) with regenerating Fe3+, which makes the overall process photocatalytic. The photocatalytic activity for the simultaneous removal of 4-CP and Cr(VI) was largely maintained up to five cycles. Such simultaneous and synergic photoactivity was also observed for other phenolic compounds (4-bromophenol, 4-nitrophenol, phenol). The simultaneous and synergic removal of phenolic compounds and Cr(VI) can be enabled through the redox couple of Fe3+/Fe2+ working as a homogeneous photocatalyst.

Temperature-boosted photocatalytic H2 production and charge transfer kinetics on TiO2 under UV and visible light.

This study investigates the effect of reaction temperature (298-353 K) on photocatalytic H2 production in bare and platinized TiO2 (Pt/TiO2) suspensions containing various organic hole scavengers (EDTA, methanol, and formic acid) under UV (λ > 320 nm) and visible light (λ > 420 nm for ligand-to-metal charge transfer). H2 production rates are enhanced ∼7.8- and ∼2.5-fold in TiO2 and Pt/TiO2 suspensions, respectively, with EDTA under UV by simply elevating the reaction temperature from 298 K to 323 K (ΔT = 25 °C). Such a temperature-boosted increase in H2 production is always observed, regardless of the TiO2 crystalline structure (anatase, rutile, and an anatase/rutile mixture), type of hole scavenger, and irradiation wavelength range. It is estimated that approximately 90% of incident photons are utilized in H2 production, for which the activation energy is 25.5 kJ mol-1. Detailed photoelectrochemical analyses show the positive relationship between reaction temperature and photocurrent generation, with charge carrier mobility and interfacial charge transfer improving at higher temperatures. Other possible factors, such as H2 solubility and mass transport, play a limited role.

Tailoring Multilayered BiVO4 Photoanodes by Pulsed Laser Deposition for Water Splitting.

Pulsed laser deposition (PLD) is proposed as promising technique for the fabrication of multilayered BiVO4-based photoanodes. For this purpose, bare BiVO4 films and two heterojunctions, BiVO4/SnO2 and BiVO4/WO3/SnO2, have been prepared using consecutive ablation of assorted targets in a single batch. The ease, high versatility and usefulness of this technique in engineering the internal configuration of the photoanode with stoichiometric target-to-substrate transfer are demonstrated. The obtained photocurrent densities are among the highest reported values for undoped BiVO4 without oxygen evolution catalysts (OEC). A detailed analysis of the influence of SnO2 and WO3 layers on the charge transport properties because of the changes at the internal FTO/semiconductor interface is performed through transient photocurrent measurements (TPC), showing that the BiVO4/WO3/SnO2 heterostructure attains a significant decrease in the internal losses and reaches high photocurrent values. This study is expected to open the door to the fabrication of other systems based on ternary (or even more complex) metal oxides as photoanodes for water splitting, which is a promising alternative for obtaining materials able to fulfill the different requierements in the development of more efficient systems for this process.

Photooxidation of arsenite under 254 nm irradiation with a quantum yield higher than unity.

Arsenite (As(III)) in water was demonstrated to be efficiently oxidized to arsenate (As(V)) under 254 nm UV irradiation without needing any chemical reagents. Although the molar absorption coefficient of As(III) at 254 nm is very low (2.49 ± 0.1 M(-1)cm(-1)), the photooxidation proceeded with a quantum yield over 1.0, which implies a chain of propagating oxidation cycles. The rate of As(III) photooxidation was highly enhanced in the presence of dissolved oxygen, which can be ascribed to its dual role as an electron acceptor of photoexcited As(III) and a precursor of oxidizing radicals. The in situ production of H2O2 was observed during the photooxidation of As(III) and its subsequent photolysis under UV irradiation produced OH radicals. The addition of tert-butyl alcohol as OH radical scavenger significantly reduced (but not completely inhibited) the oxidation rate, which indicates that OH radicals as well as superoxide serve as an oxidant of As(III). Superoxide, H2O2, and OH radicals were all in situ generated from the irradiated solution of As(III) in the presence of dissolved O2 and their subsequent reactions with As(III) induce the regeneration of some oxidants, which makes the overall quantum yield higher than 1. The homogeneous photolysis of arsenite under 254 nm irradiation can be also proposed as a new method of generating OH radicals.

Role of Interparticle Charge Transfers in Agglomerated Photocatalyst Nanoparticles: Demonstration in Aqueous Suspension of Dye-Sensitized TiO2.

The interparticle charge transfer within the agglomerates of TiO2 nanoparticles in slurries markedly enhanced the dye-sensitized production of H2 under visible light. By purposely decoupling the light absorbing part of Dye/TiO2 from the active catalytic center of Pt/TiO2, the role of bare TiO2 nanoparticles working as a mediator that connects the above two parts in the agglomerates was investigated systematically. The presence of mediator in the agglomerate facilitated the charge separation and the electron transfer from Dye/TiO2 to Pt/TiO2 through multiple grain boundaries and subsequently produced more hydrogen. The dye-sensitized reduction of Cr(VI) to Cr(III) was also enhanced when Dye/TiO2 nanoparticles were agglomerated with bare TiO2 nanoparticles. The charge recombination between the oxidized dye and the injected electron was retarded in the presence of bare TiO2 nanoparticles, and this retarded recombination on Dye/TiO2 was confirmed by using transient laser spectroscopy. This phenomenon can be rationalized in terms of an interparticle Fermi level gradient within the agglomerates, which drives the charge separation.

The electrochemistry of nanostructured titanium dioxide electrodes.

Several of the multiple applications of titanium dioxide nanomaterials are directly related to the introduction or generation of charge carriers in the oxide. Thus, electrochemistry plays a central role in the understanding of the factors that must be controlled for the optimization of the material for each application. Herein, the main conceptual tools needed to address the study of the electrochemical properties of TiO(2) nanostructured electrodes are reviewed, as well as the electrochemical methods to prepare and modify them. Particular attention is paid to the dark electrochemical response of these nanomaterials and its direct connection with the TiO(2) electronic structure, interfacial area and grain boundary density. The physical bases for the generation of currents under illumination are also presented. Emphasis is placed on the fact that the kinetics of charge-carrier transfer to solution determines the sign and value of the photocurrent. Furthermore, methods for extracting kinetic information from open-circuit potential and photocurrent measurements are briefly presented. Some aspects of the combination of electrochemical and spectroscopic measurements are also dealt with. Finally, some of the applications of TiO(2) nanostructured samples derived from their electrochemical properties are concisely reviewed. Particular attention is paid to photocatalytic processes and, to a lesser extent, to photosynthetic reactions as well as to applications related to energy from the aspects of both saving (electrochromic layers) and accumulation (batteries). The use of TiO(2) nanomaterials in solar cells is not covered, as a number of reviews have been published addressing this issue.

Concentration-dependent photoredox conversion of As(III)/As(V) on illuminated titanium dioxide electrodes.

The photoconversion of As(III) (arsenite) and As(V) (arsenate) over a mesoporous TiO(2) electrode was investigated in a photoelectrochemical (PEC) cell for a wide range of concentrations (µM-mM), under nonbiased (open-circuit potential measurements) and biased (short-circuit current measurements) conditions. Not only As(III) can be oxidized, but also As(V) can be reduced in the anoxic condition under UV irradiation. However, the reversible nature of As(III)/As(V) photoconversion was not observed in the normal air-equilibrated condition because the dissolved O(2) is far more efficient as an electron acceptor than As(V). Although As(III) should be oxidized by holes, its presence did not increase the photooxidation current in a monotonous way: the photocurrent was reduced by the presence of As(III) in the micromolar range but enhanced in the millimolar range. This abnormal concentration-dependent behavior is related with the fate of the intermediate As(IV) species which can be either oxidized or reduced depending on the experimental conditions, combined with surface deactivation for the water photooxidation process. The lowering of the photooxidation current in the presence of micromolar As(III) is ascribed to the role of As(IV) as a charge recombination center. Being an electron acceptor, the addition of As(V) consistently lowers the photocurrent in the entire concentration range. A global concentration-dependent mechanism is proposed accounting for all the PEC results and its relation with the photocatalytic oxidation mechanism is discussed.

Effect of surface fluorination on the electrochemical and photoelectrocatalytic properties of nanoporous titanium dioxide electrodes.

Titanium dioxide is a widely used photocatalyst whose properties can be modified by fluoride adsorption. This work is focused on the effect of surface fluorination on the electrochemical and photoelectrocatalytic properties of TiO(2) nanoporous thin films. Surface fluorination was achieved by simple addition of HF to the working solution (pH 3.5). Open circuit potential as well as ex situ XPS measurements verify that surface modification takes place. Fluorination triggers a significant capacitance increase in the accumulation potential region, as revealed by dark voltammetric measurements for all the TiO(2) samples studied. The photoelectrocatalytic properties (measured as photocurrents under white light illumination) depend on the substrate being oxidized and, in some cases, on the nature of the TiO(2) sample. In particular, the results obtained for electrodes prepared with a mixed phase (rutile + anatase) commercial nanopowder (PI-KEM) indicate that the processes mediated by surface trapped holes, such as the photooxidation of water or methanol, are accelerated while those occurring by direct hole capture from the adsorbed state (formic acid) are retarded. The photooxidation of catechol and phenol is also enhanced upon fluorination. In such a case, the effect can be rationalized on the basis of a diminished recombination and a surface displacement of both the oxidizable organic substrates and the poisoning species formed as a result of the organics oxidation. Photoelectrochemical and in situ infrared spectroscopic measurements support these ideas. In a more general vein, the results pave the way toward a better understanding of the photocatalysis phenomena, unravelling the importance of the reactant adsorption processes.

Photoelectrochemical behavior of nanostructured WO3 thin-film electrodes: The oxidation of formic acid.

Nanostructured tungsten trioxide thin-film electrodes are prepared on conducting glass substrates by either potentiostatic electrodeposition from aqueous solutions of peroxotungstic acid or direct deposition of WO3 slurries. Once treated thermally in air at 450 degrees C, the electrodes are found to be composed of monoclinic WO3 grains with a particle size around 30-40 nm. The photoelectrochemical behavior of these electrodes in 1 M HClO4 apparently reveals a low degree of electron-hole recombination. Upon addition of formic acid, the electrode showed the current multiplication phenomenon together with a shift of the photocurrent onset potential toward less positive values. Photoelectrochemical experiments devised on the basis of a kinetic model reported recently [I. Mora-Seró, T. Lana-Villarreal, J. Bisquert, A. Pitarch, R. Gómez, P. Salvador, J. Phys. Chem. B 2005, 109, 3371] showed that an interfacial mechanism of inelastic, direct hole transfer takes place in the photooxidation of formic acid. This behavior is attributed to the tendency of formic acid molecules to be specifically adsorbed on the WO3 nanoparticles, as evidenced by attenuated total reflection infrared spectroscopy.