One-Electron Redox Processes during Polyoxometalate-Mediated Photocatalytic Reactions of TiO2 Studied by Two-Color Two-Laser Flash Photolysis

Catalytic activity of porous carbon nitride regulated by polyoxometalates under visible light

A series of porous carbon nitrides modified by different polyoxometalates (POMs) were prepared by the ultrasonic method. POMs were assembled on the surface of mpg-C₃N₄via electrostatic attraction. The catalyst has visible light degradation activity for phenol (λ > 420 nm). mpg-C₃N₄ modified by H₄SiW₁₂O₄₀ with a mass ratio of 1 : 5 showed the highest catalytic activity, which was 3.5 times higher than that of mpg-C₃N₄. As an electron acceptor, polyoxometalate can capture the photoelectron of C₃N₄, which can promote the separation of photocharge and improve the photocatalytic activity. ESR also confirmed that the superoxide radicals play a major role in degradation. The results show that the charge separation efficiency and catalytic activity can be enhanced by polyacids.

A porous composite catalyst is prepared by modifying mpg-C₃N₄ with polyacids through ultrasound. When the mass ratio of SiW₁₂ to mpg-C₃N₄ was 1 : 5, the degradation of phenol showed the best catalytic activity, which was 3.5 times that of mpg-C₃N₄.

Silicotungstate, a Potential Electron Transporting Layer for Low-Temperature Perovskite Solar Cells

Thin films of a heteropolytungstate, lithium silicotungstate (Li₄SiW₁₂O₄₀, termed Li-ST), prepared by a solution process at low temperature, were successfully applied as electron transporting layer (ETL) of planar-type perovskite solar cells (PSCs). Dense and uniform Li-ST films were prepared on FTO glass by depositing a thin Li-ST buffer layer, followed by coating of a main Li-ST layer. The film thickness was controlled by varying the number of coating cycles, consisting of spin-coating and thermal treatment at 150 °C. In particular, by employing 60 nm-thick Li-ST layer obtained by two cycles of coating, the fabricated CH₃NH₃PbI₃ PSC device demonstrates the photovoltaic conversion efficiency (PCE) of 14.26% with J_SC of 22.16 mA cm^-2, V_OC of 0.993 mV and FF of 64.81%. The obtained PCE is significantly higher than that of the PSC employing a TiO₂ layer processed at the same temperature (PCE = 12.27%). Spectroscopic analyses by time-resolved photoluminescence and pulsed light-induced transient measurement of photocurrent indicate that the Li-ST layer collects electrons from CH₃NH₃PbI₃ more efficiently and also exhibits longer electron lifetime than the TiO₂ layer thermally treated at 150 °C. Thus, Li-ST is considered to be a promising ETL material that can be applied for the fabrication of flexible PSC devices.

SiW12 -TiO2 Mesoporous Layer for Enhanced Electron-Extraction Efficiency and Conductivity in Perovskite Solar Cells

High quality electron-transport layer (ETL) with superior optical and electrical properties is an essential part in high efficient perovskite solar cells (PSCs). In this work, SiW₁₂ -TiO₂ mesoporous film is prepared by a facile one-step spin-coating deposition method and successfully applied as ETL in PSCs. Compared with pristine TiO₂ -based PSC, the SiW₁₂ -TiO₂ -based one shows a remarkable enhanced power conversion efficiency (PCE) from 12.00 to 14.66 %, which is owed to the higher conductivity, electron-extraction efficiency, and well-matched energy level alignment of SiW₁₂ -TiO₂ film. Moreover, the SiW₁₂ -TiO₂ -based device also shows a good long-time stability in under ambient conditions. This work demonstrates that using polyoxometalates (POMs) to modify the metal-oxide semiconductors is an effective approach for further enhancing the performance of PSCs.

Titania hollow spheres modified with tungstophosphoric acid with enhanced visible light absorption for the photodegradation of 4-chlorophenol

Titania hollow spheres were synthesized using silica nanospheres as the template. The core was removed using NaOH solution. They were subsequently impregnated with tungstophosphoric acid (TPA) solutions and annealed at two different temperatures (100 and 500 °C). These materials were characterized by several physicochemical techniques (XRD, BET, SEM, DRS, FT-IR, FT-Raman and ³¹P MAS-NMR). The ³¹P MAS-NMR and FT-IR characterization showed that the main species present in the samples was the [PW₁₂O₄₀]^3- anion, which was partially transformed into the [P₂W₂₁O₇₁]^6- anion during the synthesis and drying step. ³¹P MAS-NMR, and FT-Raman characterization revealed the evidence of a strong interaction between the Keggin anion of TPA and TiO₂ surfaces, possibly due to the formation of surface heteropolyacid-TiO₂ complexes. The DRS results showed that the absorption threshold onset continuously shifted to the visible region with increased TPA concentration and calcination at 500 °C. The enhanced visible light absorption could be related to the formation of a surface complex TPA Keggin anion-TiO₂. The catalytic activity of the materials in the photodegradation of 4-chlorophenol under UV and visible light irradiation increased when the TPA content and the calcination temperature of the samples were raised.

Structure effects of amphiphilic and non-amphiphilic quaternary ammonium salts on photodegradation of Alizarin Red-S catalyzed by titanium dioxide

The role of surfactants such as single- and double-tailed tetraalkylammonium bromide and various non-amphiphilic tetraalkylammonium salts was investigated on the TiO₂photocatalyzed degradation of Alizarin Red-S under UV light irradiation.

Tuning of the photocatalytic performance of g-C3N4 by polyoxometalates under visible light

Polyoxometalate/g-C₃N₄ composites were prepared with Keggin POMs and g-C₃N₄. POMs used as electron traps can efficiently promote the spatial separation of photogenerated electrons and holes of C₃N₄, which resulted in a high photocatalytic activity towards MO degradation.

Enhanced photocatalytic degradation of phenol over anatase TiO2 in neutral water on addition of iron(iii) substituted polyoxotungstate

PW₁₂O₄₀-type polyoxometalates have been widely used as electron mediators of TiO₂ photocatalysis for organic degradation in water, but they are stable only at pH 1-2, which greatly limits their application for water treatment. Herein we report an iron(iii)-substituted PW₁₁O₃₉ (PW₁₁Fe) capable of mediating the photocatalytic degradation of phenol and 2,4-dichlorophenol in an aerated aqueous suspension of anatase TiO₂ at pH 2.0-7.2. As the initial concentration of PW₁₁Fe or the initial pH of the suspension increased, the rate of phenol degradation increased, and then decreased. A maximum reaction rate was achieved at 2.0 mM PW₁₁Fe and pH 5.5, which was 3.9 times higher than that measured without PW₁₁Fe. In all cases, the rates of phenol degradation were of the first order in phenol, implying the recycling behavior of PW₁₁Fe. Through electrochemical measurement, a possible mechanism is proposed, which involves the interfacial electron transfer from the irradiated TiO₂ to PW₁₁Fe, and the reduction of O₂ by the reduced PW₁₁Fe. This would improve the efficiency of the charge separation of TiO₂, and consequently increase the rate of phenol degradation at interfaces.

Two polymolybdate-based complexes and their graphene composites with visible-light photo-responses

Two polymolybdate-based complexes with different band gaps show different visible-light photo-responses, and their graphene composite materials possess improved electrocatalytic activities for HER.

Efficient dye-sensitized solar cell with a pure thin film of a hybrid polyoxometalate covalently attached organic dye as a working electrode in a cobalt redox mediator system

A Keggin-type hybrid polyoxometalate is used as a pure semiconductor thin film in the working electrode of a dye-sensitized solar cell fabricated with a D35 organic dye and a cobalt redox mediator system, giving a maximum IPCE of about 91% .

Formation of a hybrid compound composed of a saddle-distorted Tin(IV)-porphyrin and a Keggin-type heteropolyoxometalate to undergo intramolecular photoinduced electron transfer

Nonplanar Sn(IV)-porphyrin complexes, [Sn(TMPP(Ph)(8))-Cl(2)] (1) and [Sn(TMPP(Ph)(8))(OMe)(2)] (2) (TMPP(Ph)(8): 5,10,15,20-tetrakis(4-methoxyphenyl)-2,3,7,8,12,13,17,18-octaphenylporphyrinato), were prepared and characterized by spectroscopic and electrochemical methods together with X-ray crystallography. Variable-temperature (1)H NMR study revealed that the coordination of the methoxo ligand of 2 is weak enough in solution to enhance the axial ligand exchange with a Keggin-type phosphotungstate (α-[PW(12)O(40)](3-)) due to the steric stress between the axial methoxo ligand and the peripheral phenyl groups of the porphyrin ligand. The formation of a novel 1:1 donor-acceptor complex, [Sn(TMPP(Ph)(8))(OMe)(α-[PW(12)O(40)])](2-) (4) was confirmed by (1)H NMR and UV-vis spectral titrations, and also by MALDI-TOF-MS measurements. Electrochemical measurements for the donor-acceptor complex in PhCN revealed that the Sn(IV)-TMPP(Ph)(8) moiety acts as an electron donor and the α-[PW(12)O(40)](3-) moiety acts as an electron acceptor and that the energy level of the electron-transfer (ET) state of the 1:1 complex (1.17 eV) is lower than that of the triplet excited states of the SnTMPP(Ph)(8) complex (1.31 eV). Femtosecond and nanosecond laser flash photolysis measurements indicate that intersystem crossing from the singlet excited sate to the triplet excited state occurs followed by intramolecular photoinduced electron transfer from the triplet excited state of the Sn(IV)-TMPP(Ph)(8) moiety to the α-[PW(12)O(40)](3-) moiety in the 1:1 complex in benzonitrile.

Structural-dependent photoactivities of TiO(2) nanoribbon for visible-light-induced H(2) evolution: the roles of nanocavities and alternate structures

Bicrystalline dehydrated nanoribbon (DNR) with alternate structure of TiO(2)(B) and anatase has been prepared by a short-time annealing method. It is photoactive for photocatalytic H(2) evolution from water in the visible region sensitized by a novel heteropoly blue sensitizer. The effects of annealing temperature and time on the DNR phase transformation process have been investigated. These results reveal that the nanocavity structure of TiO(2)(B) exhibits a narrow band gap and improves its absorbance coefficient in the visible region. The alternate structures of TiO(2)(B) and anatase improve interfacial electron separation and transfer. Compared with normal phase junction, the smoothing alternate joints in the band structure of DNR-600-30 provide an effective route for the movement of holes and electrons. This unique alternate bicrystalline structure has a significant advantage on its applications in photocatalysis and nanodevices.

Coupled sensitizer-catalyst dyads: electron-transfer reactions in a perylene-polyoxometalate conjugate

Ultrafast discharge of a single-electron capacitor: A variety of intramolecular electron-transfer reactions are apparent for polyoxometalates functionalized with covalently attached perylene monoimide chromophores, but these are restricted to single-electron events. (et=electron transfer, cr=charge recombination, csr=charge-shift reaction, PER=perylene, POM=polyoxometalate).A new strategy is introduced that permits covalent attachment of an organic chromophore to a polyoxometalate (POM) cluster. Two examples are reported that differ according to the nature of the anchoring group and the flexibility of the linker. Both POMs are functionalized with perylene monoimide units, which function as photon collectors and form a relatively long-lived charge-transfer state under illumination. They are reduced to a stable pi-radical anion by electrolysis or to a protonated dianion under photolysis in the presence of aqueous triethanolamine. The presence of the POM opens up an intramolecular electron-transfer route by which the charge-transfer state reduces the POM. The rate of this process depends on the molecular conformation and appears to involve through-space interactions. Prior reduction of the POM leads to efficient fluorescence quenching, again due to intramolecular electron transfer. In most cases, it is difficult to resolve the electron-transfer products because of relatively fast reverse charge shift that occurs within a closed conformer. Although the POM can store multiple electrons, it has not proved possible to use these systems as molecular-scale capacitors because of efficient electron transfer from the one-electron-reduced POM to the excited singlet state of the perylene monoimide.