Surface Of P25 Research Articles

Application of Carbon Nanosphere Templates for Preparation of Inverse Opal Photonic Crystals in Dye‐Sensitized Solar Cells

Carbon nanospheres are successfully synthesized with a particle size of about 140 nm using a two‐step hydrothermal method and used the carbon nanospheres as the deposition template material on the surface of P25(TiO2) by means of vertical deposition method. The antiopaline TiO2 photonic crystal (PC) structure is further prepared via immersed carbon nanospheres in TiO2 precursor solution, and the antiopal PC is used as the scattering layer of the photoanode of dye‐sensitized solar cell (DSSCs). As a comparison, the antiopal PC reflective layer is also prepared on the photoanode of DSSCs using polystyrene nanospheres as the template material. The photoanodes with those PC scattering layers prepared with different template materials are fabricated into complete DSSCs. The final photoelectric conversion efficiency of the DSSCs prepared with carbon nanospheres as the template material reaches 7.02%, which is comparable to the cells prepared with polystyrene nanospheres as the template material. The efficiency of the cell with the scattering layer is increased by about 16% compared with that of the normal DSSC based on P25.

Reduction and Diffusion of Cr-Oxide Layers into P25, BaLa4Ti4O15, and Al:SrTiO3 Particles upon High-Temperature Annealing.

Chromium oxide (Cr2O3) is a beneficial metal oxide used to prevent the backward reaction in photocatalytic water splitting. The present work investigates the stability, oxidation state, and the bulk and surface electronic structure of Cr-oxide photodeposited onto P25, BaLa4Ti4O15, and Al:SrTiO3 particles as a function of the annealing process. The oxidation state of the Cr-oxide layer as deposited is found to be Cr2O3 on the surface of P25 and Al:SrTiO3 particles and Cr(OH)3 on BaLa4Ti4O15. After annealing at 600 °C, for P25 (a mixture of rutile and anatase TiO2), the Cr2O3 layer diffuses into the anatase phase but remains at the surface of the rutile phase. For BaLa4Ti4O15, Cr(OH)3 converts to Cr2O3 upon annealing and diffuses slightly into the particles. However, for Al:SrTiO3, the Cr2O3 remains stable at the surface of the particles. The diffusion here is due to the strong metal-support interaction effect. In addition, some of the Cr2O3 on the P25, BaLa4Ti4O15, and Al:SrTiO3 particles is reduced to metallic Cr after annealing. The effect of Cr2O3 formation and diffusion into the bulk on the surface and bulk band gaps is investigated with electronic spectroscopy, electron diffraction, DRS, and high-resolution imaging. The implications of the stability and diffusion of Cr2O3 for photocatalytic water splitting are discussed.

Evonik P25 photoactivation in the visible range by surface grafting of modified porphyrins for p-nitrophenol elimination in water

<abstract> <p>An Evonik P25 TiO<sub>2</sub> material is modified using a porphyrin containing Si-(OR)<sub>3</sub> extremities to extend its absorption spectrum in the visible range. Two different loadings of porphyrin are grafted at the surface of P25. The results show that the crystallinity and the texture of the P25 are not modified with the porphyrin grafting and the presence of the latter is confirmed by Fourier-transform infrared spectroscopy (FTIR) measurements. All three samples are composed of anatase/rutile titania nanoparticles around 20 nm in size with a spherical shape. The absorption spectra of the porphyrin modified samples show visible absorption alongside the characteristic Soret and Q bands of porphyrin, despite slightly shifted peak values. The <sup>29</sup>Si solid state nuclear magnetic resonance (NMR) spectra show that the porphyrin is linked with Ti–O–C and Ti–O–Si bonds with the Evonik P25, allowing for a direct electron transfer between the two materials. Finally, the photoactivity of the materials is assessed on the degradation of a model pollutant—<italic>p</italic>-nitrophenol (PNP)—in water. The degradation is substantially enhanced when the porphyrin is grafted at its surface, whereas a very low activity is evidenced for P25. Indeed, with the best sample, the activity increases from 9% to 38% under visible light illumination. This improvement is due to the activation of the porphyrin under visible light that produces electrons, which are then transferred to the TiO<sub>2</sub> to generate radicals able to degrade organic pollutants. The observed degradation is confirmed to be a mineralization of the PNP. Recycling experiments show a constant PNP degradation after 5 cycles of photocatalysis of 24 h each.</p> </abstract>

Easy stabilization of Evonik Aeroxide P25 colloidal suspension by 4-hydroxybenzoic acid functionalization

A new functionalization method was developed with the aim of producing stable TiO2-based colloids starting from commercial Evonik Aeroxide P25 powder. The stability of P25 colloids was efficiently increased by grafting 4-hydroxybenzoic acid (4-HBA) on the P25 surface. Results highlighted that 4-HBA interacted with the surface of P25 through its carboxylic group.The key parameter for an optimized stability is the 4-HBA/P25 ratio. If the amount of 4-HBA is too high compared to the TiO2 available surface, 4-HBA molecules preferentially interact with each other, therefore reducing their adsorption on P25.Eight different amounts of 4-HBA were grafted on P25, producing stable colloids in both ethanol and water medium. In both media, the optimal properties and highest stability were obtained for a 4-HBA/TiO2 mass ratio equal to 0.333.This optimal sample was further tested and compared to pure P25 (used as reference material) for p-nitrophenol (PNP) degradation under UV-visible light without any stirring to highlight the beneficial effect of the powder stability. The photocatalytic results showed that the grafted sample reached a degradation up to 8 times higher than pure P25. This functionalization method is easy to implement and stabilizes colloids starting from any pre-synthesized powder. This versatility widens the field of applications, and opens the door for further developments in areas such as coatings.

The importance of intermediates ring-opening in preventing photocatalyst deactivation during toluene decomposition

The deactivation of photocatalysts is the main barrier that diminishes the commercial value of photocatalytic air-purification technology. It is essential to explore the intrinsic origins for the deactivation and develop effective strategy to address this key issue. This work investigates the intrinsic reasons for VOCs-decomposition induced intermediates accumulation on the surface of TiO2 (P25) based on the combined experiments and DFT calculations. The main culprit of deactivation is the accumulation of benzaldehyde intermediate on the surface of P25 because of its high reaction energy of aromatic ring opening. This will lead to the facilitated conversion of benzaldehyde to benzoic acid that will thus occupy the active sites on the surface of P25. However, the wide gap β-Ga2O3 achieved the excellent deactivation resistance during prolonged VOCs degradation, which is derived from that the fact that the adsorbed intermediates will be activated on the surface of β-Ga2O3 and strengthen the aromatic ring opening via •OH radicals, thus realizing efficient mineralization of the ring-containing intermediates into inorganic small molecules (CO2 and H2O). This work points out that the deactivation-resistant property of photocatalyst is closely related to the adsorption activation and the ring-opening characteristics of the intermediates, which could provide a new perspective in development of efficient and deactivation-resistant photocatalysts for practical applications.

Performance of surface fluorinated P25-TiO2 on the photocatalytic degradation of volatile organic compounds in indoor environment

Volatile organic compounds (VOCs) are considered as a major group of indoor air contaminants with several adverse health effects. Photocatalytic oxidation process (PCO) has been applied for the removal of indoor VOCs. In the PCO process, the adsorption extent of water molecules on the surface of photocatalysts is a decisive factor for PCO efficiency. Water molecules contribute in PCO process either as; a) the main hydroxyl radical’s source and enhance the removal efficiency, or b) compete with VOCs to adsorb on the super-hydrophilic surface of P25 and reduce the photodegradation efficiency. In this study, the effects of surface fluorination on the photoactivity and surface hydrophilicity of commercial P25 were investigated. The fluorinated P25 (F-P25) nanoparticles were synthesized to remove methyl ethyl ketone (MEK) from the indoor environment. Surface fluorination formed the surface ≡ Ti – F by an exchange reaction between the surface hydroxyl groups (OH) and fluoride ions (F−) and decreased the water vapor adsorption on the photocatalyst’s surface. Herein, the photocatalytic efficiency of F-P25 was compared to P25 under a continuous-flow condition with a small residence time (0.026 sec) at four different relative humidity levels (0, 20, 40, and 60%). Furthermore, the performances of F-P25 and P25 on the generation of by-products, including formaldehyde and acetaldehyde were also evaluated. The results revealed that the F-P25 enhanced the photodegradation of MEK compared to bare-P25 in all tested relative humidity levels; however, the generation of the by-products increased as relative humidity increased.

Application of Fenton-like Photocatalysts Based on Defect Reconstruction in Degradation of Dye Wastewater

In recent years, improving the performance of catalysts through defect modulation has attracted extensive attention. However, the impact of defect rearrangement on Fenton-like photocatalytic reactions has not been studied. In this study, Fe-containing polyoxometalate molecules were grafted onto the surface of defective TiO2.The influence of oxygen vacancy formation and the defect arrangement process on the catalytic activity was investigated. The results indicated that the calcination of cyanamide modified defective TiO2 is beneficial for the spatial reconstruction of oxygen vacancies generated by H2 atmosphere treatment. With rearranged structural defects, photo-generated electrons were prone to transfer from the surface of P25 to Fe-POM nanoparticles. Due to the enhanced charge separation in TiO2 and the increased reactive sites on the Fenton-like reagent, Fenton-like photocatalysts with rearranged defects showed a 13-fold increase in catalytic activity during the degradation of dye molecules.

Hydrothermal synthesis of carbon quantum dots using different precursors and their combination with TiO2 for enhanced photocatalytic activity

In this study, carbon quantum dots (CQDs) were successfully synthesized by a facile and green hydrothermal method, and further coupled with P25 (commercial TiO2) to form CQDs/P25 heterojunctions for enhanced photocatalytic activity. Two typical precursors, glucose and citric acid were selected as precursors for the synthesis of CQDs. The effects of precursors on the properties of CQDs and the photocatalytic activity of resultant CQDs/P25 was indeed investigated. The TEM, Raman and XPS analysis revealed the good combination of CQDs on the surface of P25. The photodegradation experiments indicated that CQDs/P25 exhibited enhanced photocatalytic performance than that of pure P25. In addition, with the better crystallinity of CQDs, the higher photocatalytic activity of CQDs/P25 was achieved. More importantly, based on the comprehensive and detailed discussion, the hydrothermal carbonization process of various precursors and the possible mechanism for the enhanced photocatalytic activity of different CQDs/P25 composites were significantly put forward.

Cu and Zr surface sites in the photocatalytic activity of TiO2 nanoparticles.

The rate of methylene blue and terephthalic acid degradation assisted with double metal-modified catalyst (0.1mol% Cu and 1.0mol% Zr) was enhanced as compared with single metal-modified catalysts (0.1, 0.5mol% Cu and 1.0mol% Zr). The wet impregnation method was used for copper and zirconium modification of the surface of Aeroxide P25 TiO2 particles. Simultaneous loading of both metals on the surface of P25 leads to an increased specific surface area of the obtained material despite negative Cu influence. The tendency of stabilization and agglomerate size rising with the time for Cu and Zr-modified catalysts were traced by dynamic light scattering (DLS) measurements. The observed optical characteristics suggest that Cu compensated the broadening of band gap caused by Zr loading. Crystal structure of obtained photocatalysts was explored by XRD; morphological data and particle size were obtained by SEM. EDX was used for Zr and Cu content determination. Cu K-edge extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) analytical techniques were used to investigate the local Cu neighbourhood in the samples and to identify copper coordination and valence state of copper species in the synthesized nanocomposites.

Titania-Supported Palladium/Strontium Nanoparticles (Pd/Sr-NPs@P25) for Photocatalytic H2 Production from Water Splitting

Here we demonstrate a new strategy of boosting the photocatalytic activity of titania (P25) for photocatalytic H2 production from the water splitting reaction by depositing palladium/strontium nanoparticles, forming Pd/Sr-NPs@P25. The Pd/Sr-NPs are in situ prepared on the surface of P25. The effects of Pd and Sr in the photocatalytic reactions are further revealed. Strontium in the form of strontium oxide promotes electron transfer from the semiconductor surface to palladium nanoparticles by increasing the Fermi level of the P25 support. The structural and morphological characterizations of the Pd/Sr-NPs nanocomposite are carried out using UV–vis DRS, XRD, TEM, and XPS techniques, based upon which the mechanistic insights are discussed.

A facile preparation of Ag2O/P25 photocatalyst for selective reduction of nitrate

Ag2O/P25 and Ag/P25 catalysts were synthesized and applied in the photocatalytic reduction of nitrate with formic acid as a hole scavenger. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and transmission electron microscope (TEM) analysis demonstrate that Ag2O or Ag nanoparticles were well distributed on the surface of P25. Under UV irradiation, 5% Ag2O/P25 showed a fast rate (0.95h−1, 6.3 times of P25), a high conversion (97.2%, 3.63 times of P25) and a high N2 selectivity (83.1%, 1.15 times of P25) for nitrate reduction. The improved separation of electron–hole pairs on Ag2O/P25 results in the high conversion of nitrate. Moreover, compared with the Ag/P25 catalyst, Ag2O/P25 exhibited a good reusability in nitrate reduction. The formed Ag–Ag2O structure [Ag(0) present on the surface of Ag2O] over P25 by partial photoreduction of deposited Ag2O contributed to the better stability of Ag2O/P25 in nitrate reduction. The present study provides a new sight into the use of Ag2O/P25 catalyst in the reduction of nitrate.

On the redox mechanism operating along C2H2 self-assembly at the surface of TiO2.

The interaction of acetylene with the TiO2 surface at room temperature entails a complex set of self-assembly reactions with the formation of products having relatively high molecular weight. In a previous paper by some of us (Jain, S. M.; et al. J. Mater. Chem. A 2014, 2, 12247-12254), the C2H2-TiO2 reaction has been monitored, essentially by Fourier transform infrared spectroscopy, at the surface of P25 (a mixture of anatase and rutile, typical benchmark material in the field of photocatalysis) in order to elucidate the nature of the products of this surface reaction. In the present paper, the same process was followed, for the first time, using electron paramagnetic resonance (EPR) and monitoring by the thermogravimetric analysis the weight loss of the material upon heating in order to further investigate the complex mechanism of the surface reaction. This was done using pure anatase and comparing the EPR results with those concerning both rutile and P25. The self-assembly mechanism occurring at the interface is accompanied by the formation of EPR visible Ti(3+) centers due to electrons injection in the TiO2 substrate. This finding clarifies that at least one of the reaction channels of this complex process (namely, the formation of polycyclic aromatic hydrocarbons) is based on the heterolytic dissociative chemisorption of acetylene, followed by a redox interaction between the adsorbate and the solid, which allows the creation of the building blocks necessary to assemble polyaromatic molecules.

Visible-Light-Driven Photocatalytic Performance of Ag@TiO<sub>2</sub> Nanocrystals: The Effects of Phases and Morphologies

Silver decorated TiO2 nanoparticles (Ag@syn-TiO2, Ag@P25) were synthesized by photoreducing Ag+ species in the presence of the synthetized TiO2 (syn-TiO2) nanocrystals or P25. The samples obtained were characterized by XRD, XPS, UV-vis and TEM. The XRD results showed the syn-TiO2 nanoparticles were pure anatase, and the P25 nanoparticles were the mixture of antase and rutile. The TEM observations and XPS spectra indicated that Ag species anchored on the surfaces of syn-TiO2 nanoparticles as Ag0 and AgO nanoclusters (NCs). The Ag NCs have a size range of 13 nm and the AgO NCs have a size range of 1015 nm. The Ag NCs on the surface of P25 have a size range of 57 nm, and no AgO species were found. The photocatalytic performance of the Ag@syn-TiO2 and Ag@P25 samples was evaluated by degradating Rhodamine B (RhB) under visible-light irradiation (λ ≥ 420 nm). The results indicated that the phases and morphologies of TiO2 nanocrystals have effects on their photocatalytic properties, and that the syn-TiO2 nanocrystals wiht the pure anatase are more active than P25 with the mixture of antase and rutile in RhB degradation. The possible mechanisms were discussed.

Carbon coated nitrogen doped P25 for the photocatalytic removal of organic pollutants under solar and low energy visible light irradiations

The commercially available TiO2 Degussa P25 was modified using a simple technique to produce a visible light active carbon (C) coated and nitrogen (N) doped P25. The modified photocatalyst was prepared by mechanical mixing of P25 powder with various amount of urea and followed by thermal heating under semi-closed reactor at different temperatures. The results showed that a layer of carbon graphite was deposited on the surface of P25 and N atoms were incorporated into the lattice of the nano-scaled photocatalyst. It was observed that the UV–Vis spectra of the modified photocatalyst exhibited red shift to the visible region as compared to that of the pristine P25, implying the decreased of band gap energy for the modified photocatalyst. Moreover, the modified photocatalyst was found to yield efficient photocatalytic degradations of the RR4, MB and phenol pollutants under visible light and solar light irradiations.

Electron scavenger of thin layer Carbon coated and Nitrogen doped P25 with enhanced photocatalytic activity under visible light fluorescent lamp

The commercially available and relatively cheap Degussa P25, TiO2 (80% anatase and 20% rutile) was modified to produce Carbon coated and Nitrogen doped P25. The modified photocatalyst was produced by using peat as the carbon precursor. The results showed that a thin layer of carbon in graphite form was deposited on the surface of P25 by which N atoms were also incorporated into the lattice of TiO2 by substituting the sites of oxygen atoms. From PL analysis, it was shown that the Carbon coated and Nitrogen doped P25 improved photocatalytic activity by acting as electron scavengers as well as reduced the electron–hole pair recombination. The photocatalytic activity of Carbon coated and Nitrogen doped P25 was examined by degradation of anionic Cibacron Brilliant Red or otherwise known as reactive red 4 (RR4) dye under visible light fluorescent lamp irradiations. The results indicated that the Carbon coated and Nitrogen doped P25 had enhanced photocatalytic activity and excellent photocatalytic stability even up to the 5th cycle of reuse. Its photocatalytic activity was almost 2 times faster than pristine P25. Carbon coated and Nitrogen doped P25 was not visibly active under low intensity visible light source (45-W) but showed clear photoactivity under a higher intensity visible light source (125-W).

Facile preparation and size-dependent photocatalytic activity of Cu2O nanocrystals modified titania for hydrogen evolution

Degussa TiO2 powder (P25) with Cu2O nanocrystals of different sizes deposited on the surface were prepared by a facile ethanol-induced deposition followed with a calcination process at 350 °C and characterized by XRD, XPS, HRTEM and UV–Vis diffuse reflectance spectra, respectively. In our protocol, Cu2O nanocrystals is formed by reducing Cu(II) with the ethanol adsorbed on the surface of P25. The absorption edge as well as photocatalytic activity for H2 evolution are closely related to the size of the Cu2O nanocrystals depending on the Cu content in the as-prepared composites. The 0.9 mol% Cu2O/TiO2 composite with ca. 4 nm Cu2O nanocrystals exhibits the highest photocatalytic H2 evolution rate of 318 μmol h−1 under the illumination of simulant solar light and an apparent quantum efficiency of 28.6% at 365 nm, 397 times higher than that of P25. This enhancement is mainly attributed to the quantum confinement effect of quantum-sized Cu2O nanocrystals (q-Cu2O), which can upgrade the conduction band bottom of q-Cu2O, and thus facilitates the effective separation of photogenerated charge carriers to enhance the photocatalytic activity. The results provided here not only offers a green and simple way for depositing q-Cu2O on the surfaces of P25 to get the photocatalyst with an excellent photocatalytic activity for H2 evolution, but also sheds a new insight for the fabrication of efficient p–n heterojunctions for the separation of electron-hole pairs by quantum confinement effect.

The nature of the surface of pyrogenic titanium dioxide according to the optical spectroscopy data

The coordination state of titanium atoms on the surface of P25 two-phase pyrogenic titanium dioxide (Degussa) was studied by electron diffuse reflectance spectroscopy. Various methods for processing the complex electronic diffuse reflectance spectrum were considered, and two variants that could be used to obtain both qualitative and quantitative characteristics of titanium-oxygen polyhedra were suggested. Coordination structures characteristic of anatase and rutile were simultaneously present on the surface of P25. The ratio between them (54: 46) substantially differed from that determined by X-ray diffraction.

Evidence for O2- Radical Stabilization at Surface Oxygen Vacancies on Polycrystalline TiO2

The characterization and stability of superoxide radicals (O2-) over polycrystalline TiO2 (Degussa P25) was investigated using electron paramagnetic resonance (EPR) spectroscopy. The adsorbed oxygen molecules act as efficient electron scavengers and were therefore used to indirectly probe the sites of electron transfer at the surface of the anatase component of the mixed phase P25 material. A distribution of various stabilization sites on the surface was identified by analysis of the g values and further confirmed by identification of several well-resolved 17O hyperfine patterns. For the first time, on a polycrystalline TiO2 surface evidence for stabilization of superoxide radicals specifically at anion vacancy sites is presented by EPR. These radicals, labeled [Vac...O2-], are characterized by the spin Hamiltonian parameters of gxx = 2.005, gyy = 2.011, gzz = 2.019, and 17OAxx = 7.64 mT (Ayy = Azz > 1 mT). The [Vac...O2-] radicals exhibit pronounced reactivity under the influence of thermal, photochemical, and chemical treatment compared to the remaining surface O2- anions bound at nonvacancy sites. The extent of site occupancy was found to be sensitive to the oxygen adsorption temperature and the extent of O2- radical migration on the surface. Thus, the stability and lifetime of the surface O2- anions are directly correlated to the structure of the adsorption site itself at the anatase surface of P25.