Facets Of TiO2 Research Articles

Growth and in situ transformation of TiO2 and HTiOF3 crystals on chitosan-polyvinyl alcohol co-polymer substrates under vapor phase hydrothermal conditions

A chitosan-polyvinyl alcohol (CS/PVA) co-polymer substrate possessing a large number of amino and hydroxyl groups is used as a substrate to induce the direct growth and in situ sequential transformation of titanate crystals under HF vapor phase hydrothermal conditions. The process involves four distinct formation/transformation stages. HTiOF3 crystals with well-defined hexagonal shapes are formed during stage I, and are subsequently transformed into {001} faceted anatase TiO2 crystal nanosheets during stage II. Interestingly, the formed anatase TiO2 crystals are further transformed into cross-shaped and hollow squareshaped HTiOF3 crystals during stages III and IV, respectively. Although TiO2 crystal phases and facet transformations under hydrothermal conditions have been previously reported, in situ crystal transformations between different titanate compounds have not been widely reported. Such crystal formation/transformations are likely due to the presence of large numbers of amino groups in the CS/PVA substrate. When celluloses possessing only hydroxyl groups are used as a substrate, the direct formation of {001} faceted TiO2 nanocrystal sheets is observed (rather than any sequential crystal transformations). This substrate organic functional group-induced crystal formation/transformation approach could be applicable to other material systems.

Graphene quantum dots to enhance the photocatalytic hydrogen evolution efficiency of anatase TiO2 with exposed {001} facet.

Hydrogen evolution through photocatalysis is promising with respect to the environmental problems and challenges of energy shortage that we encounter today. In this paper, we have combined graphene quantum dots (GQDs) and {001} faceted anatase TiO2 (with an exposed percentage of 65-75%) together for effective photocatalytic hydrogen evolution. A series of characterizations including X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, Fourier-transform infrared spectroscopy and UV-visible absorption spectroscopy have been carried out to study the structure of the as-prepared GQDs/{001}TiO2 composite. It turns out that GQDs could be effectively decorated on {001}TiO2 sheet without changing its intrinsic structure. With an optimum loading amount of GQDs (0.5 wt% to {001}TiO2), GQDs/{001}TiO2 exhibits a hydrogen evolution efficiency 8 times higher than that of bare {001}TiO2, which is a significantly more obvious improvement than many other photocatalytic systems relevant to GQDs and TiO2 hybrids. In addition, GQDs/{001}TiO2 could stand long-term photocatalytic experiments. Photocurrent tests show that such an improvement of the photocatalytic efficiency over GQDs/{001}TiO2 may originate from a higher charge separation efficiency. The present study could offer reference for the construction of photocatalytic hydrogen evolution systems with low cost and long term stability.

A facile hydrothermal etching process to in situ synthesize highly efficient TiO2/Ag nanocube photocatalysts with high-energy facets exposed for enhanced photocatalytic performance

A facile hydrothermal etching process was developed to synthesize TiO2/Ag nanocubes for enhanced photocatalytic application. The synthesis was carried out by chemically etching hollow @TiO2 spheres in the presence of an AgF or AgF/NaF solution under hydrothermal conditions. Utilizing F− as a morphology-directing agent and Ag+ as an Ag source during the etching process, TiO2 nanocubes can be easily formed with their active high-energy facets exposed and Ag nanoparticles can be in situ deposited simultaneously on the surface of the TiO2 nanocubes to generate hybrid TiO2/Ag nanocubes. The resulting TiO2/Ag nanocubes were well controlled with Ag nanoparticles uniformly distributed on the surface of the TiO2 nanocubes. The morphologies of TiO2/Ag can be altered from ellipsoidal to cubic shapes depending on the amounts of AgF or AgF/NaF used for chemical etching. Due to the exposure of the active high-energy facets of TiO2 and the uniform deposition of Ag nanoparticles, the obtained TiO2/Ag nanocubes showed superior catalytic performance for the photocatalytic degradation of rhodamine B under visible-light radiation. It is proposed that the surface plasmon resonance effect arisen from Ag nanoparticles on TiO2 nanocubes can effectively enhance the visible-light driven photocatalytic properties of TiO2/Ag photocatalysts. Meanwhile, due to the etching of F ions during the synthetic process, the morphology variation of TiO2 from spherical to cubic shapes may be beneficial to the exposure of high energy {001} facets, which is liable to produce more electron–hole pairs in the photocatalytic process, and electron–hole pairs can combine with OH− to produce large amounts of hydroxyl radicals, eventually accelerating the decomposition of the organic dye.

Enhanced photoelectronic properties of single crystal TiO2 nanosheet array films by selective deposition of CdS nanoparticles on their {101} facets

The selective deposition of CdS nanoparticles on the {101} facets of TiO2 enhances the spatial separation of electrons and holes on different facets, thereby improving their photoelectronic properties.

Methanol Conversion into Dimethyl Ether on the Anatase TiO2(001) Surface

Exploring reactions of methanol on TiO2 surfaces is of great importance in both C1 chemistry and photocatalysis. Reported herein is a combined experimental and theoretical calculation study of methanol adsorption and reaction on a mineral anatase TiO2(001)-(1×4) surface. The methanol-to-dimethyl ether (DME) reaction was unambiguously identified to occur by the dehydration coupling of methoxy species at the fourfold-coordinated Ti(4+) sites (Ti(4c)), and for the first time confirms the predicted higher reactivity of this facet compared to other reported TiO2 facets. Surface chemistry of methanol on the anatase TiO2(001)-(1×4) surface is seldom affected by co-chemisorbed water. These results not only greatly deepen the fundamental understanding of elementary surface reactions of methanol on TiO2 surfaces but also show that TiO2 with a high density of Ti(4c) sites is a potentially active and selective catalyst for the important methanol-to-DME reaction.

Methanol Conversion into Dimethyl Ether on the Anatase TiO2(001) Surface

AbstractExploring reactions of methanol on TiO2 surfaces is of great importance in both C1 chemistry and photocatalysis. Reported herein is a combined experimental and theoretical calculation study of methanol adsorption and reaction on a mineral anatase TiO2(001)‐(1×4) surface. The methanol‐to‐dimethyl ether (DME) reaction was unambiguously identified to occur by the dehydration coupling of methoxy species at the fourfold‐coordinated Ti4+ sites (Ti4c), and for the first time confirms the predicted higher reactivity of this facet compared to other reported TiO2 facets. Surface chemistry of methanol on the anatase TiO2(001)‐(1×4) surface is seldom affected by co‐chemisorbed water. These results not only greatly deepen the fundamental understanding of elementary surface reactions of methanol on TiO2 surfaces but also show that TiO2 with a high density of Ti4c sites is a potentially active and selective catalyst for the important methanol‐to‐DME reaction.

Tailored Synthesis of Porous TiO₂ Nanocubes and Nanoparallelepipeds with Exposed {111} Facets and Mesoscopic Void Space: A Superior Candidate for Efficient Dye-Sensitized Solar Cells.

Anatase TiO2 nanocubes and nanoparallelepipeds, with highly reactive {111} facets exposed, were developed for the first time through a modified one pot hydrothermal method, through the hydrolysis of tetrabutyltitanate in the presence of oleylamine as the morphology-controlling capping-agent and using ammonia/hydrofluoric acid for stabilizing the {111} faceted surfaces. These nanocubes/nanoparallelepipeds were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM) and high angle annular dark-field scanning TEM (HAADF-STEM). Accordingly, a possible growth mechanism for the nanostructures is elucidated. The morphology, surface area and the pore size distribution of the TiO2 nanostructures can be tuned simply by altering the HF and ammonia dosage in the precursor solution. More importantly, optimization of the reaction system leads to the assembly of highly crystalline, high surface area, {111} faceted anatase TiO2 nanocubes/nanoparallelepipeds to form uniform mesoscopic void space. We report the development of a novel double layered photoanode for dye sensitized solar cells (DSSCs) made of highly crystalline, self-assembled faceted TiO2 nanocrystals as upper layer and commercial titania nanoparticles paste as under layer. The bilayered DSSC made from TiO2 nanostructures with exposed {111} facets as upper layer shows a much higher power conversion efficiency (9.60%), than DSSCs fabricated with commercial (P25) titania powder (4.67%) or with anatase TiO2 nanostructures having exposed {101} facets (7.59%) as the upper layer. The improved performance in bilayered DSSC made from TiO2 nanostructures with exposed {111} facets as the upper layer is attributed to high dye adsorption and fast electron transport dynamics owing to the unique structural features of the {111} facets in TiO2. Electrochemical impedance spectroscopy (EIS) measurements conducted on the cells supported these conclusions, which showed that the bilayered DSSC made from TiO2 nanostructures with exposed {111} facets as the upper layer possessed lower charge transfer resistance, higher electron recombination resistance, longer electron lifetime and higher collector efficiency characteristics, compared to DSSCs fabricated with commercial (P25) titania powder or with anatase TiO2 nanostructures having exposed {101} facets as the upper layer.

Pt deposited TiO2 films with exposed {0 0 1} facets for photocatalytic degradation of a pharmaceutical pollutant

TiO2 films with exposed {001} facets and uniform Pt nanoparticles on the crystal facets were fabricated on a Ti substrate using a simple low temperature electrostatic self-assembly method. Proper Pt colloidal pH value was selected by determining ξ potential of Pt NPs and TiO2 film with exposed {001} facets. Ultra high-resolution FESEM observation shows that Pt NPs with diameter of 2–4nm were physically separated and dispersed on TiO2 facets. The Pt NPs were found to be apt to deposit on the {101} facet with more positive surface potential of TiO2 film and the mean size and the number density of Pt NPs on different facet could be rationally tailored by controlling immersion time of TiO2 film with exposed {001} facets. The Pt-TiO2 film with exposed {001} facets prepared at immersion time of 5min shows considerably higher UV photocatalytic activity than TiO2 film with exposed {001} facets and single UV irradiation for all the experimented initial ethenzamide concentrations, and also no deactivation was observed during 3 recyling runs. The UV photocatalysis employing Pt-TiO2 film with exposed {001} facets as photocatalyst shows significant potential for application in degradation of pharmaceutical and personal care products (PPCPs) with amide bonds.

Solvent-free in situ synthesis of g-C3N4/{0 0 1}TiO2 composite with enhanced UV- and visible-light photocatalytic activity for NO oxidation

Many possible applications of anatase TiO2 nanosheets with exposed {001} facets ({001}TiO2) are limited by its low quantum efficiency and no-spectral response in the visible region. We report on a simple solvent-free in situ method for synthesizing g-C3N4/{001}TiO2 composite that over comes these drawbacks. The in situ growth of g-C3N4 and the removal of F− ions on the {001}TiO2 surface that remain from the synthesis process were simultaneously achieved during the calcination of a mixture of prepared {001}TiO2 and urea, resulting in a compact connection between the two components, which facilitated the interfacial charge transfer process. The growth of g-C3N4 during the calcination process preserved the {001} facets of TiO2. The composite samples exhibited higher photocatalytic activity than either the pure g-C3N4 or {001}TiO2 nanosheets separately. Specifically, as measured by the removal of NO, the optimal composite sample, with a g-C3N4 content of 10%, was 2.4 times more effective than pure {001}TiO2, 4.1 times more effective than g-C3N4 under UV light, and 5.8 times more effective than g-C3N4 under visible light irradiation. Also, the composite prepared by this method showed much higher activity than similar composites that were prepared by evaporation and mechanical methods. The new method is simple and holds the promise of expanded large-scale production and wider use of {001}TiO2 based catalysts in the future.

Coexistence of Square Pyramidal Structures of Oxo Vanadium (+5) and (+4) Species Over Low-Coverage VOX/TiO2 (101) and (001) Anatase Catalysts

We investigate the structural properties and coordination of VOX species supported on the facets of anatase TiO2 as the oxidation state changes during the catalytic cycle of selective catalytic reduction (SCR) of NOX. Density functional theory (DFT) calculations reveal how the molecular structures of hydrated, low-coverage V sites depend on the surface facet of the TiO2 (anatase) support. Thus, the support effect is not limited to only disperse the active phase but also dictates its shape and ultimately its reactivity. We find that hydrated VOX species, where V is present in oxidation state +5, are present in distorted tetrahedral configurations on the most stable TiO2(101) facet, whereas on the less stable TiO2(001) facet they are present in distorted octahedral configurations. Furthermore, we find the reduced states of the same species where V is present in oxidation state +4 also exhibit unchanged local coordination with the vanadyl unit V═O intact. We support the findings by electron paramagnetic reso...

Trifunctional TiO2 Nanoparticles with Exposed {001} Facets as Additives in Cobalt‐Based Porphyrin‐Sensitized Solar Cells

In this study, highly mesoporous TiO2 composite photoanodes composed of functional {001}‐faceted TiO2 nanoparticles (NPs) and commercially available 20 nm TiO2 NPs are employed in efficient porphyrin‐sensitized solar cells together with cobalt polypyridyl‐based mediators. Large TiO2 NPs (approximately 50 nm) with exposed {001} facets are prepared using a fast microwave‐assisted hydrothermal (FMAH) method. These unique composite photoanodes favorably mitigate the aggregation of porphyrin on the surface of TiO2 NPs and strongly facilitate the mass transport of cobalt‐polypyridyl‐based electrolytes in the mesoporous structure. Linear sweep voltammetry reveals that the transportation of Co(polypyridyl) redox is a diffusion‐controlled process, which is highly dependent on the porosity of TiO2 films. Electrochemical impedance spectroscopy confirms that the FMAH TiO2 NPs effectively suppress the interfacial charge recombination toward [Co(bpy)3]3+ because of their oxidative {001} facets. In an optimal condition of 40 wt% addition of FMAH TiO2 NPs in the final formula, the power conversion efficiency of the dye‐sensitized cells improves from 8.28% to 9.53% under AM1.5 (1 sun) conditions.

Visible-light-driven g-C3N4/Ti3+-TiO2 photocatalyst co-exposed {0 0 1} and {1 0 1} facets and its enhanced photocatalytic activities for organic pollutant degradation and Cr(VI) reduction

Novel g-C3N4/Ti3+-TiO2 photocatalyst co-exposed {001} and {101} facets of TiO2 was synthesized via a hydrothermal–sonication assisted strategy. The photocatalytic activities of the as-obtained photocatalyst were evaluated by the degradation of rhodamine B (RhB) and the reduction of Cr(VI) under visible-light irradiation. It was found that the g-C3N4/Ti3+-TiO2 composites with 6wt% g-C3N4 exhibited the highest visible-light photocatalytic efficiency, which is also higher than the pure g-C3N4 and Ti3+-TiO2. A possible photocatalytic mechanism was discussed on the basis of the theoretical analyses and scavenger experiments. Results show that holes (h+) and superoxide anions (O2−) reactive species participated in the degradation of RhB solution over the g-C3N4/Ti3+-TiO2 composites. The enhanced photocatalytic activities of g-C3N4/Ti3+-TiO2 composites can be attributed to the wide optical adsorption of g-C3N4 and Ti3+ as well as the effectively separation and transportation of photo-generated electrons and holes pairs, which was resulted from the surface heterojunction between the g-C3N4 and Ti3+-TiO2 nanosheets co-exposed {101} and {001} facets of anatase TiO2.

Enhanced photoelectrochemical performance of {001}TiO2/{001}SrTiO3 epitaxial heterostructures

The anatase TiO2 films with controllable {001} highly reactive facets were fabricated on {001}SrTiO3 substrate with various total work pressure (TWP) by direct current facing-target magnetron sputtering (DCFTMS). X-ray diffraction (XRD), scanning electron microscopy (SEM), surface roughness tester (SRT), and X-ray photoelectron spectroscopy (XPS) were employed to confirmed the influence of different TWPs on the interface of TiO2/SrTiO3 heterostructured with epitaxial preferred growth of the anatase {001}TiO2 facet. The mechanisms of enhanced photoelectrochemical properties were explored by transient photocurrent (TP), current–potential (I–V), cyclic voltammetry (CV), polarization (Tafel), electrochemical impedance spectroscopy (EIS), and Mott–Schottky (M–S) curves, which can be ascribed to the formation of high-quality heterojunction between {001}TiO2 thin film and {001}SrTiO3 substrate, which suppress the recombination of photogenerated electron–hole pairs effectively.

Effects of the interaction between TiO2 with different percentages of exposed {001} facets and Cu(2+) on biotoxicity in Daphnia magna.

Anatase TiO2 nanosheets (NSs) with exposed {001} facets have been widely used because of their high activity and particular surface atomic configuration. However, investigations on their biotoxicity are rare. In this study, bioaccumulation of five different TiO2 (with 10%, 61%, 71%, 74% and 78% exposed {001} facets), as well as copper and enzyme activities in Daphnia magna, are systematically investigated and rationalized. The results indicated that the addition of Cu2+ enhanced agglomeration–sedimentation of TiO2, resulting in the reduction of TiO2 bioaccumulation by 10% to 26%. TiO2 nanoparticles (NPs) increased copper bioaccumulation by 9.8%, whereas the other four TiO2 nanosheets (NSs) decreased it by 43% to 53%, which depended on TiO2 variant adsorption and free Cu2+ concentrations in the supernatant. The levels of superoxide dismutase (SOD) enzyme and Na+/K+-ATPase activities suggested that oxidative stress, instead of membrane damage, was the main toxicity in D. magna. Meanwhile, the SOD enzyme activities increased with decreasing Cu accumulation and increasing Ti accumulation because of the different functions of Cu and Ti in organisms. This research highlighted the important role of the percentage of exposed {001} facets in nanostructured TiO2 on bioaccumulation and biotoxicity of TiO2 and Cu2+ in Daphnia magna.

NOx Adsorption on ATiO3(001) Perovskite Surfaces

Density functional theory calculations have been used to explore NOx adsorption on perovskite oxides surfaces ATiO3(001) with A = Ca, Sr, Ba. NO adsorbs weakly on all facets with no apparent A-ion dependence, whereas NO2 adsorbs preferably over AO-terminated surfaces with adsorption energies that correlate with the ionization potentials of the alkaline earth atoms. Simultaneous adsorption of NO and NO2 is found to substantially enhance the stability of the adsorbates owing to an oxide mediated electron-pairing mechanism. Stabilization is also predicted for NO/O2 adsorption, and it is suggested that the presence of oxygen favors the formation of nitrite/nitrate pairs. It is found that the NOx adsorption properties can be modified by mixing alkaline earth cations in the perovskite framework. The results are put in context by comparison with NOx adsorption on the corresponding (001) facets of alkaline earth metal oxides and TiO2(110).

Enhanced Visible‐Light Photoelectrocatalytic Activity of {001}TiO2 Electrodes Assisted with Carbon Quantum Dots

AbstractAiming at further developing the application of highly reactive {001}TiO2 in photoelectrocatalytic oxidation, which is only limited by its response in UV light, carbon quantum dots (CQDs) are introduced to provide the excitation sources for {001}TiO2 through upconversion from visible to UV light. The CQDs/{001}TiO2 photoelectrode is prepared by loading the CQDs on truncated square pyramid‐shaped TiO2 spheres exposed to high‐activity {001} facets, using a simple impregnation–drying method. The results indicate that the CQDs/{001}TiO2 possess enhanced photoelectrocatalytic ability with an increased photocurrent of 12 mA cm−2 and photocurrent decay time of 1.75 s. Compared with pure {001}TiO2, the surface photochemical reaction resistance of CQDs/{001}TiO2 decreases, whereas the number of photogenerated carriers increase greatly. This visible‐light photoelectrocatalytic activity results from the synergistic effect of CQDs and the {001} facet of anatase TiO2.

Critical roles of co-catalysts for molecular hydrogen formation in photocatalysis

Surface atomic structure can play a critical role in the reactivity of materials, because many physicochemical processes occur at surfaces. As an important semiconductor material, titanium dioxide (TiO2) crystals with tailored facets have opened up intensive research in surface chemistry, such as photocatalytic hydrogen generation. However, the convincing measurement and explanation of the crystal facets effect for molecular hydrogen formation in photocatalysis has remained elusive. One longstanding controversy is between the exposed {001} and {101} facets. Here we demonstrate, through a combination of state-of-the-art electron microscopy and quantum chemical calculations, that the crystal facets of TiO2 do not play the critical role in photoreactivity; the bare TiO2 semiconductors without co-catalysts are catalytically inactive, whereas the location and species of the co-catalysts essentially determine the capacity for the hydrogen evolution reaction. Our results provide a unified explanation for the disparate results in the previous literature and highlight a feasible route to fabricating efficient photocatalysts.

The effect of hydrothermal temperature on the structure and photocatalytic activity of {001} faceted anatase TiO2

The effect of the hydrothermal temperature on the crystal structures and properties of {001} faceted anatase TiO2 are systematically investigated. Results indicate that the reaction temperature has little effect on the particle size, but results in fusing the adjacent {001} facet and eliminating the cross-linked structure. Along with the decreased {001} facet percentage, the decreased crystal defects, increased crystallinity, and raised conduction band (CB) minimum dramatically increase the methylene blue (MB) decomposing efficiency. The present study indicates that it’s important to take into consideration of various factors in designing efficient photocatalysts, but not only to expose the high-reactive facets.

Heterojunction of SrTiO3/TiO2 nanotubes with dominant (001) facets: Synthesis, formation mechanism and photoelectrochemical properties

Controllable growth of anatase TiO2 with dominant high reactive crystal facets has attracted intense interest in the past few years due to their unique structure-dependent properties. In this work, SrTiO3/TiO2 heterostructure nanotubes (SrT) arrays films with high reactive dominant (001) facets of anatase TiO2 were successfully prepared through the part conversion of TiO2 by a hydrothermal method. Various characterizations were used to investigate the morphologies, crystal phases and chemical compositions of the prepared samples. Structure analyses using X-ray diffraction and a high-resolution transmission electron microscope revealed that, the treatment sequence of hydrothermal reaction and annealing was a crucial influence factor on controlling the growth of preferred orientation of (001) facets of TiO2 in SrTiO3/TiO2 heterojunction, whereas the inverse treatment sequence yielded randomly oriented facets. The formation mechanism of dominant (001) facets of TiO2 in the heterojunction was related to crystal structures of TiO2 and SrTiO3. The heteroepitaxial growth on (001) plane of SrTiO3 led to dominant (001) facets of anatase TiO2 attributing to their excellent surface lattices match. The photoelectric performances of pure TiO2 nanotubes and SrT were investigated by photoluminescence, UV–visible diffuse reflectance, photocurrent and current–voltage response. The semiconductor characteristics of them were studied by electrochemical impedance spectroscopy and Mott–Schottky analysis in detail. The results demonstrated that the heterostructure of SrT not only increased light absorption, but also effectively shifted the Fermi level and promoted charge transfer. Among all the samples, the SrTiO3/TiO2 heterojunction prepared by 1h hydrothermal treatment (1SrT) exhibited the best photoelectrochemical performances due to the synergetic contributions of the high active (001) facets of anatase TiO2 and the charge transport properties of the heterostructure.

Synthesis of Crystal-Controlled TiO2 Nanorods by a Hydrothermal Method: Rutile and Brookite as Highly Active Photocatalysts

Rutile, anatase, and brookite TiO2 nanorods (TNs) with specific exposed crystal facets have been obtained by a hydrothermal method using peroxide titanic acid (PTA) solution of different pH values. Below pHPTA 10, the condensing complex is uncharged Ti2O5(OH)2(H2O)4 or cations Ti2O5(H2O)62+, and the corner-shared bonding easily occurs to form rutile phase. Above pHPTA 10, the condensing species is anions Ti2O5(OH)4(H2O)22–, resulting in the larger probability of edge-shared bonding which favor the formation of anatase TiO2. Brookite whose structure contains both corner-shared and edge-shared bonding can be formed when pHPTA equals to 10. Rutile TN synthesized from PTA solution at pH 4 showed the optimal photocatalytic oxidation activity for MB degradation; Brookite TN exhibited the highest photocatalytic activity for reduction of Cr (VI) despite that it does not exhibit the largest surface area. The presence of a spatial separation of redox sites in faceted rutile TiO2 nanorod determined the photocatalyti...