Antonio Otavio T. Patrocinio

Layer-by-Layer TiO2/WO3 Thin Films As Efficient Photocatalytic Self-Cleaning Surfaces

New TiO2/WO3 films were produced by the layer-by-layer (LbL) technique and successfully applied as self-cleaning photocatalytic surfaces. The films were deposited on fluorine doped tin oxide (FTO) glass substrates from the respective metal oxide nanoparticles obtained by the sol-gel method. Thirty alternative immersions in pH = 2 TiO2 and pH = 10 WO3 sols resulted in ca. 400 nm thick films that exhibited a W(VI)/Ti(IV) molar ratio of 0.5, as determined by X-ray photoelectron spectroscopy. Scanning electron microscopy, along with atomic force images, showed that the resulting layers are constituted by aggregates of very small nanoparticles (<20 nm) and exhibited nanoporous and homogeneous morphology. The electronic and optical properties of the films were investigated by UV-vis spectrophotometry and ultraviolet photoelectron spectroscopy. The films behave as nanoscale heterojunctions, and the presence of WO3 nanoparticles caused a decrease in the optical band gap of the bilayers compared to that of pure LbL TiO2 films. The TiO2/WO3 thin films exhibited high hydrophilicity, which is enhanced after exposition to UV light, and they can efficiently oxidize gaseous acetaldehyde under UV(A) irradiation. Photonic efficiencies of ξ = 1.5% were determined for films constituted by 30 TiO2/WO3 bilayers in the presence of 1 ppm of acetaldehyde, which are ∼2 times higher than those observed for pure LbL TiO2 films. Therefore, these films can act as efficient and cost-effective layers for self-cleaning, antifogging applications.

Charge carrier dynamics and photocatalytic behavior of TiO2 nanopowders submitted to hydrothermal or conventional heat treatment

The sol–gel technique followed by conventional (TiO2-1) and hydrothermal (TiO2-2) thermal treatment was employed to prepare TiO2-based photocatalysts with distinct particle sizes and crystalline structures. The as prepared metal oxides were evaluated as photocatalysts for gaseous HCHO degradation, methanol, and dye oxidation reactions. Additionally, metallic platinum was deposited on the TiO2 surfaces and H2 evolution measurements were performed. The photocatalytic activities were rationalized in terms of morphologic parameters along with the electron/hole dynamics obtained from transient absorption spectroscopy (TAS). TiO2-2 exhibits smaller particle size, poorer crystallinity, and higher surface area than TiO2-1. Moreover the hydrothermal treatment leads to formation of the metastable brookite phase, while TiO2-1 exhibits only the anatase phase. TAS measurements show that the electron/hole recombination of TiO2-2 is faster than that of the latter. Despite that, TiO2-2 exhibits higher photonic efficiencies for photocatalytic oxidation reactions, which is attributed to its larger surface area that compensates for the decrease of the surface charge carrier concentration. For H2 evolution, it was found that the surface area has only a minor effect and the photocatalyst performance is controlled by the efficiency of the electron transfer to the platinum islands. This process is facilitated by the higher crystallinity of TiO2-1, which exhibits higher photonic efficiency for H2 evolution than that observed for TiO2-2. The results found here provide new insights into the correlations between thermal treatment conditions and photocatalytic activity and will be useful for the design of high performance photocatalysts.

A hole inversion layer at the BiVO4/Bi4V2O11 interface produces a high tunable photovoltage for water splitting

The conversion of solar energy into hydrogen fuel by splitting water into photoelectrochemical cells (PEC) is an appealing strategy to store energy and minimize the extensive use of fossil fuels. The key requirement for efficient water splitting is producing a large band bending (photovoltage) at the semiconductor to improve the separation of the photogenerated charge carriers. Therefore, an attractive method consists in creating internal electrical fields inside the PEC to render more favorable band bending for water splitting. Coupling ferroelectric materials exhibiting spontaneous polarization with visible light photoactive semiconductors can be a likely approach to getting higher photovoltage outputs. The spontaneous electric polarization tends to promote the desirable separation of photogenerated electron- hole pairs and can produce photovoltages higher than that obtained from a conventional p-n heterojunction. Herein, we demonstrate that a hole inversion layer induced by a ferroelectric Bi4V2O11 perovskite at the n-type BiVO4 interface creates a virtual p-n junction with high photovoltage, which is suitable for water splitting. The photovoltage output can be boosted by changing the polarization by doping the ferroelectric material with tungsten in order to produce the relatively large photovoltage of 1.39u2009V, decreasing the surface recombination and enhancing the photocurrent as much as 180%.

Structural characterization of Ag-doped TiO2 with enhanced photocatalytic activity

Ag-doped TiO2 nanoparticles with different metallic content (0.5, 2.0, and 5.0% m/m) were prepared using a simple and cost-effective method based on the sol–gel technique, followed by thermal treatment. The addition of Ag+ ions during the hydrolysis/condensation of the Ti(IV) molecular precursor leads to homogeneous dispersion of the Ag+ cations on the titania matrix. As the amount of silver is increased, the resulting TiO2 nanoparticles exhibit smaller particle size (from 27 nm for bare TiO2 to 12 nm for TiO2–Ag 5.0%) and larger surface area. X-ray photoelectron spectroscopy (XPS) confirms that during the sintering step of the resultant powder at 400 °C for 5 hours, ca. 34% of the silver content is converted to Ag0 via thermal decomposition of Ag2O. The data also indicates the presence of highly oxidized silver species, such as Ag2+ and Ag3+. X-ray diffractograms and Raman spectroscopy confirm that the crystalline structure of the TiO2 matrix corresponds to the anatase polymorph; however, the presence of the dopant leads to an increase in the system disorder due to a higher concentration of oxygen vacancies, as also confirmed by XPS. TiO2–Ag 5.0% exhibited the highest photocatalytic activity towards mineralization of the E102 tartrazine azo-dye, being 78% faster than bare TiO2 at optimum pH conditions (pH = 6.9). Upon light excitation, the oxidized silver cations are reduced to Ag0, leading to an improvement in visible light absorption due to the surface plasmon resonance effect. The recycling of the photocatalyst showed that the enhanced photocatalytic activity is maintained, which can be associated with the reduction of charge recombination at the oxide surface and the enhanced visible light harvesting.

New layer-by-layer Nb2O5–TiO2 film as an effective underlayer in dye-sensitised solar cells

Highly efficient all-inorganic TiO2–Nb2O5 underlayers for dye-sensitised solar cell applications were produced by the layer-by-layer technique (LbL). TiO2 and Nb2O5 nanoparticles were prepared by the sol–gel method under acidic and alkaline conditions, respectively. The LbL films exhibited a very compact and homogeneous surface, as shown in FESEM and AFM images, which ensured a physical barrier between the electrolyte and the FTO surface, decreasing the dark current at this interface. Moreover, the rough film surface improved the physical interaction between the mesoporous TiO2 layer and the conductive substrate. The Ti(IV)/Nb(V) molar ratio in the films was 1.6, as determined by XPS, and it is controlled by the pH employed during the deposition process. The relative concentration of nanoparticles in the film plays a major role in its electronic properties: a higher TiO2 concentration allows an efficient transport of photoinjected electrons. Additionally, the presence of Nb2O5 nanoparticles imposes an electronic barrier for charge transfer from the FTO to the electrolyte, as shown by electrochemical impedance spectroscopy. Thus, all the DSC photoelectrochemical parameters increased, leading to an impressive improvement in the overall conversion efficiency.

Characterization of a highly efficient N-doped TiO2 photocatalyst prepared via factorial design

The preparation of titanium dioxide nanoparticles doped with nitrogen for application as a photocatalyst in the decomposition of azo dyes was optimized by factorial planning. Five variables were evaluated and the results showed that the stirring method of the reaction medium, the nitrogen source and the calcination temperature are the determining parameters that affect the photocatalytic activity. With this methodology, it was possible to obtain an optimized photocatalyst (K1) with high surface area and high mineralization efficiency (100%) of the dye Ponceau 4R under solar irradiation. K1, its non-doped version and the worst photocatalyst obtained by the factorial planning (K2) were characterized by several techniques to rationalize the different behaviors. The observed mineralization rate constants under artificial UV-A radiation were in the order of 10−2, 10−4 and 10−3 min−1, respectively, for K1, K2 and the non-doped oxide. As shown by N2 sorption isotherms, the powders exhibited large variations in porosity as well as in the specific surface area, with values ranging from 63.03 m2 g−1 for K1 to 12.82 m2 g−1 for K2. Infrared spectra showed that the calcination of the doped oxides between 300 and 500 °C leads to considerable loss of the nitrogen content, which is corroborated by XPS measurements that also indicate the presence of oxygen vacancies on their surfaces. Nanosecond transient absorption measurements show that the electron–hole half-lifetime in K1 is 870 ns, ca. two times longer than that observed for the other photocatalysts. Additionally, dye degradation studies under solar radiation reveal that K1 is ca. 28% faster than the non-doped TiO2 under similar conditions. This higher photoactivity for K1 is attributed to its extended visible light absorption and the optimized morphological and electronic properties.

Applications of Mesoporous Ordered Semiconductor Materials — Case Study of TiO2

TiO2 is a promising material for technological applications for its versatility [1-4], abundance, low toxicity, good chemical stability, photosensitivity and photostability [5-8]. In nature, it is found mainly in the mineral ilmenite [9], that can be processed industrially by two different routes [10]: The first, involves the reaction of the concentrate of ilmenite with hot sulphuric acid, resulting in the formation of sulphates of titanium, Fe(II) and Fe(III), being these last eliminat‐ ed by centrifugation, after cooling. The final solution is then purified and hydrolyzed to produce pure TiO2 [11]. The other usual way of obtaining consists in combining the ore with coke and gaseous chlorine under heating, resulting in CO2 and a spongy material rich in TiCl4. The reaction product is subjected to successive fractional distillation, with the formation of TiCl2 and TiCl3, due to stability of titanium in other degrees of oxidation. The different precursors of titanium are hydrolyzed, forming titanium dioxide [12].

Efficient Mineralization of Paracetamol Using the Nanocomposite TiO2/Zn(II) Phthalocyanine as Photocatalyst

The photocatalytic performance of a composite based on the association of TiO2 and 2.5 wt.% of zinc(II) phthalocyanine (TiO2/ZnPc) was evaluated towards the mineralization of paracetamol and compared to that observed for the bare oxide in different pH and H2O2 concentrations. The results show that the photocatalytic performances were influenced by the pH, with maximum efficiency around the isoelectric point. Mineralization efficiencies between 86-91% was obtained using TiO2/ZnPc in pH 5.5-6.8, with 33 mg L-1 of H2O2, ca. 15% higher than that observed with TiO2. The mineralization efficiencies using bare TiO2 and TiO2/ZnPc were respectively 112 and 18% lower in the absence of H2O2. The better performance of TiO2/ZnPc is related to its extended light absorption and non-uniform coating of the TiO2 surface by ZnPc aggregates. Above pH 6.8, the mineralization efficiencies decrease for both photocatalysts, although the consumption of H2O2 remains above 90%, due to its decomposition in alkaline pH.

Rapid Preparation of (BiO) 2 CO 3 Nanosheets by Microwave-Assisted Hydrothermal Method with Promising Photocatalytic Activity Under UV-Vis Light

Crystalline (BiO)2CO3 nanosheets were synthesized by a rapid one-step reaction via microwaveassisted hydrothermal method using urea as a morphology mediator and carbon source. The hydrothermal method combined with microwave heating allowed to obtain sheet-like (BiO)2CO3 particles at shorter reaction times when compared to the conventional heating hydrothermal method. The photocatalytic activity of the as prepared samples was evaluated towards degradation of Ponceau 4R (C.I. 16255) under artificial UV-Vis light irradiation. The results show that good photocatalytic efficiency can be obtained for powders prepared with reaction times as low as 2 minutes.

Photochemistry of fac-[Re(CO)3(dcbH2)(trans-stpy)]+: New Insights on the Isomerization Mechanism of Coordinated Stilbene-like Ligands

In this work, a novel complex fac-[Re(CO)3(dcbH2)( trans-stpy)]+, (dcbH2 = 4,4-dicarboxylic acid-2,2-bipyridine; trans-stpy = trans-4-styrylpyridine) was synthesized and characterized toward its spectroscopic, photochemical, and photophysical properties. The experimental data provide new insights on the mechanism of photochemical trans-to- cis isomerization of the stilbene-like ligand coordinated to Re(I) polypyridyl complexes. The new complex exhibits an unusual and strong dependence of the isomerization quantum yield (Φt →c) on the irradiation wavelength. Φt →c was 0.81 ± 0.08 for irradiation at 365 nm and continuously decreased as the irradiation wavelength is shifted to the visible. At 405 nm irradiation Φt →c is almost 2 orders of magnitude lower (0.010 ± 0.005) than that observed at 365 nm excitation. This behavior can be explained by the low-lying triplet metal-to-ligand charge-transfer excited state (3MLCT) that hinders the triplet photoreaction mechanism under visible light absorption. Under UV irradiation, direct population of styrylpyridine-centered excited state (1IL) leads to the occurrence of the photoisomerization via a singlet mechanism. Further experiments were performed with the complex immobilized on the surface of TiO2 and Al2O3 films. The nonoccurrence of isomerization at the oxide surfaces even under UV excitation evidences the role of energy gap between the 1IL/1MLCT states on the photochemical/photophysical processes. The results establish important relationships between the molecular structure and the photoelectrochemical behavior, which can further contribute to the development of solid-state molecular switches based on Re(I) polypyridyl complexes.