TiO2 photocatalysis: Design and applications

Thermodynamic and kinetic analysis of heterogeneous photocatalysis for semiconductor systems

Since the report of the Honda-Fujishima effect, heterogeneous photocatalysis has attracted much attention around the world because of its potential energy and environmental applications. Although great progresses have been made in recent years, most were focused on preparing highly-active photocatalysts and investigating visible light utilization. In fact, we are still unclear on the thermodynamic and kinetic nature of photocatalysis to date, which sometimes leads to misunderstandings for experimental results. It is timely to give a review and discussion on the thermodynamics and kinetics of photocatalysis, so as to direct future researches. However, there is an absence of a detailed review on this topic until now. In this article, we tried to review and discuss the thermodynamics and kinetics of photocatalysis. We explained the thermodynamic driving force of photocatalysis, and distinguished the functions of light and heat in photocatalysis. The Langmuir-Hinshelwood kinetic model, the ˙OH oxidation mechanism, and the direct-indirect (D-I) kinetic model were reviewed and compared. Some applications of the D-I model to study photocatalytic kinetics were also discussed. The electron transport mode and its importance in photocatalysis were investigated. Finally, the intrinsic relation between the kinetics and the thermodynamics of photocatalytic reactions was discussed.

Application of immobilized TiO2 photocatalysis to improve the inactivation of Heterosigma akashiwo in ballast water by intense pulsed light

Ballast water exotic discharge has been identified as a leading vector for marine species invasion. Here immobilized TiO2 photocatalysis is introduced to improve the performance of intense pulsed light. For intense pulsed light/TiO2 photocatalysis, a typical inactivation of 99.89±0.46% can be achieved under treatment condition of 1.78 L min(-1) flow rate, 300 V pulse peak voltage, 15 Hz pulse frequency and 5 ms pulse width. Moreover, within tested 220-260 V peak voltage, 18.37-40.51% elevation in inactivation is observed in comparison with intense pulsed light treatment alone. The rough energy consumption of the tested intense pulsed light/TiO2 treatment system is about 1.51-2.51 times higher than that of the typical commercial UV ballast water treatment system. The stability of the photocatalytic reactivity and intactness of loaded TiO2 film is proved within 20-d's test, while local erosion on stainless steel support is observed after 30-d's test. The results indicate that intense pulsed light/TiO2 photocatalysis is likely to be a competitive ballast water treatment technique, while further measures is needed to reduce the energy consumption and ensure the performance of TiO2 film in a long run.

Photocatalytic degradation of recalcitrant organic pollutants in water using a novel cylindrical multi-column photoreactor packed with TiO2-coated silica gel beads

A novel cylindrical multi-column photocatalytic reactor (CMCPR) has been developed and successfully applied for the degradation of methyl orange (MO), amoxicillin (AMX) and 3-chlorophenol (3-CP) in water. Due to its higher adsorption capacity and simpler molecular structure, 3-CP compared with MO and AMX obtained the highest photodegradation (100%) and mineralization (78.1%) after 300-min photocatalytic reaction. Electrical energy consumption for photocatalytic degradation of MO, AMX and 3-CP using CMCPR was 5.79×10(4), 7.31×10(4) and 2.52×10(4) kW h m(-3) order(-1), respectively, which were less than one-thousand of those by reported photoreactors. The higher flow rate (15 mL min(-1)), lower initial concentration (5 mg L(-1)) and acidic condition (pH 3) were more favorable for the photocatalytic degradation of MO using CMCPR. Five repetitive operations of CMCPR achieved more than 97.0% photodegradation of MO in each cycle and gave a relative standard deviation of 0.72%. In comparison with reported slurry and thin-film photoreactors, CMCPR exhibited higher photocatalytic efficiency, lower energy consumption and better repetitive operation performance for the degradation of MO, AMX and 3-CP in water. The results demonstrated the feasibility of utilizing CMCPR for the degradation of recalcitrant organic pollutants in water.

Photocatalytic degradation and antimicrobial applications of F-doped MWCNTs/TiO₂ composites

Multi-walled carbon nanotubes-fluorine-co-doped TiO2 composite was synthesized by the solid state method. The prepared photocatalysts were characterized by using XRD, FTIR and FE-SEM. In addition, the samples were evaluated for antimicrobial activity and photocatalytic activity. The composites exhibited enhanced absorption properties in the UV light range compared to pure TiO2. The MWCNTS-F-co-doped TiO2 composites showed significant photocatalytic activity in the generation of oxygen.

Confined-space synthesis of single crystal TiO₂ nanowires in atmospheric vessel at low temperature: a generalized approach

Extensive efforts have been devoted to develop innovative synthesis strategies for nanomaterials in order to exploit the true potential of nanotechnology. However, most approaches require high temperature or high pressure to favor crystallization. Here we highlight an unconventional approach for the confined-space synthesis of the single crystal TiO₂ nanowires in the atmospheric vessel at low temperature by cleverly manipulating the unique physical properties of straight-chain saturated fatty acids. Our method also applys to icosane due to its straight-chain saturated hydrocarbon structure and similar physical properties to the saturated fatty acids. Interestingly, we also found that the unsaturated fatty acids can facilitate the crystal growth, but their bent chains lead to the formation of TiO₂ particle aggregates. In addition, we demonstrate the growth of TiO₂ nanowires on arbitrary substrates, which are of great importance for their wider applications. We thus anticipate our presented method to be a possible starting point for non-classical crystallization strategies and be easily adapted for the fabrication of all other inorganic materials.

Recent progress in magnetic iron oxide-semiconductor composite nanomaterials as promising photocatalysts

Photocatalytic degradation of toxic organic pollutants is a challenging tasks in ecological and environmental protection. Recent research shows that the magnetic iron oxide-semiconductor composite photocatalytic system can effectively break through the bottleneck of single-component semiconductor oxides with low activity under visible light and the challenging recycling of the photocatalyst from the final products. With high reactivity in visible light, magnetic iron oxide-semiconductors can be exploited as an important magnetic recovery photocatalyst (MRP) with a bright future. On this regard, various composite structures, the charge-transfer mechanism and outstanding properties of magnetic iron oxide-semiconductor composite nanomaterials are sketched. The latest synthesis methods and recent progress in the photocatalytic applications of magnetic iron oxide-semiconductor composite nanomaterials are reviewed. The problems and challenges still need to be resolved and development strategies are discussed.

Hierarchical TiO2 spheres decorated with Au nanoparticles for visible light hydrogen production

Hierarchical TiO₂ spheres composed of nanosheets are successfully synthesized via a simple solvothermal route.

Vibrational and electronic peculiarities of NiTiO3 nanostructures inferred from first principle calculations

Structural, electronic and vibrational properties of nanostructured (NiTiO₃)_n clusters were calculated by numerical models based on DFT and semi-empirical quantum chemistry codes.

Fe2O3–AgBr nonwoven cloth with hierarchical nanostructures as efficient and easily recyclable macroscale photocatalysts

The Fe₂O₃–AgBr nonwoven cloth with hierarchical nanostructures synthesized by an electrospinning–calcination method exhibits efficient photocatalytic activity and easy recycling features.

Nano and micro-TiO2 for the photodegradation of ethanol: experimental data and kinetic modelling

In this work we show the possibility of using pigmentary TiO₂-powders as photocatalysts and we develop a kinetic model which is able to follow the behavior of nano-TiO₂versus micro-TiO₂ in ethanol degradation.

Nanomaterial-Based Photocatalysis

In this chapter, an introduction about nanostructured materials (NsM) and their applications is presented. Semiconducting nanomaterials are attractive because their physical properties are different from those of the bulk due to the quantum-size effect. Also, they provide opportunities to study the effect of spatial confinement and problems related to surfaces or interfaces, which is important for chemistry. Recently, one-dimensional (1D) nanomaterials, such as nanowires, nanobelts, nanorods, and nanotubes, have become the focus of intensive research owing to their potential applications in electronic, optoelectronic, electrochemical, electromechanical, and other fields. Further, various outstanding properties of NsM, such as optical absorbance, improved magnetism, and specific surface area, for efficient device fabrication have been summarized. Finally, applications of photocatalysis in different fields reported in the literature have been presented.

Synthesis and characterization of UV upconversion material Y2SiO5:Pr3+, Li+/TiO2 with enhanced the photocatalytic properties under a xenon lamp

The upconversion luminescence agents Y₂SiO₅:Pr³⁺, Li⁺, that can be effectively excited by the blue light from a xenon lamp 150 W, are fabricated by a hydrothermal method with mesoporous molecular sieves of MCM-48 as the suitable silica source.

Photocatalytic degradation and toxicity evaluation of diclofenac by nanotubular titanium dioxide–PES membrane in a static and continuous setup

Diclofenac is a commonly used anti-inflammatory drug, which has been found in surface waters. TiO₂ nanotubes with a polymer microfiltration membrane showed high photocatalytic degradation of diclofenac combined with an excellent membrane performance and long term stability.

Antibacterial textiles functionalized by layer-by-layer assembly of polyelectrolytes and TiO2photocatalyst

Antibacterial photocatalytic textiles with high self-decontaminating activity under solar light againstE. colihave been prepared by sprayed layer-by-layer assembly of TiO₂and polyethylenimine or polyanionic poly(styrene sulfonate) polyelectrolyte.

Acid-assisted hydrothermal synthesis of nanocrystalline TiO2 from titanate nanotubes: influence of acids on the photodegradation of gaseous toluene

In order to efficiently remove volatile organic compounds (VOCs) from indoor air, one-dimensional titanate nanotubes (TiNTs) were hydrothermally treated to prepare TiO2 nanocrystals with different crystalline phases, shapes and sizes. The influences of various acids such as CH3COOH, HNO3, HCl, HF and H2SO4 used in the treatment were separately compared to optimize the performance of the TiO2 nanocrystals. Compared with the strong and corrosive inorganic acids, CH3COOH was not only safer and more environmentally friendly, but also more efficient in promoting the photocatalytic activity of the obtained TiO2. It was observed that the anatase TiO2 synthesized in 15 mol/L CH3COOH solution exhibited the highest photodegradation rate of gaseous toluene (94%), exceeding that of P25 (44%) by a factor of more than two. The improved photocatalytic activity was attributed to the small crystallite size and surface modification by CH3COOH. The influence of relative humidity (20%-80%) on the performance of TiO2 nanocrystals was also studied. The anatase TiO2 synthesized in 15 mol/L CH3COOH solution was more tolerant to moisture than the other TiO2 nanocrystals and P25.

A facile hydrothermal method for the controllable synthesis of TiO2 nanocrystals with tunable shapes

Using OA and OM as the capping surfactants, a facile and harmless hydrothermal method was invented to prepare anatase TiO₂ nanocrystals with exposed {101} facets.

Zinc hydroxide/oxide and zinc hydroxy stannate photocatalysts as potential scaffolds for environmental remediation

ZnSn(OH)₆ cubes as potential material for the efficient photodegradation of organic dyes with high chemical stability.

Controllable synthesis and morphology-dependent photocatalytic performance of anatase TiO2 nanoplates

The morphology of TiO₂ anatase nanoplates was controlled through tuning the ratio of HAc to DMF, reaction time and temperature. The excellent photocatalytic performance can be ascribed to the exposed {010} facet and high surface area.

Photodeposition synthesis of a ZnO nanoporous layer

A photodeposition methodology is investigated to synthesize novel ZnO nanoporous layer with high yield and uniformity on TiO₂ nanotubes.

Optimized photocatalytic degradation of caffeic acid by sol-gel TiO₂

TiO₂anatase powder was prepared by means of the sol-gel method with titanium(IV) butoxide as precursor. The formation of a tetragonal crystal structure of TiO₂anatase at 500 °C was confirmed by X-ray powder diffraction. The characterization of the samples synthesized was complemented by scanning electron microscopy, diffuse reflectance infrared Fourier transform spectroscopy, nitrogen adsorption-desorption isotherms (Brunauer-Emmett-Teller) and diffuse reflectance spectroscopy. The photocatalytic activity of the TiO₂anatase powder was evaluated in the degradation of caffeic acid in aqueous solution under ultraviolet radiation. A central composite circumscribed design was used to assess the weight of the experimental variables, pH and amount of catalyst in the percentage of caffeic acid degraded and the optimal conditions. The optimized conditions were found to be pH = 5.2 and a load of TiO₂of 1.1 g L⁻¹. Under these conditions more than 90% of caffeic acid degradation was achieved after 30 min of lamp irradiation. At this time the mineralization reached was almost 60%.

Biotechnologically obtained nanocomposites: A practical application for photodegradation of Safranin-T under UV-Vis and solar light

This research was undertaken to determine the potential of biologically obtained ZnS-TiO2 nanocomposites to be used as catalysts in the photodegradation of organic pollutants, namely, Safranin-T. The photocatalysts were prepared by modifying the surface of commercial TiO2 particles with naturally produced ZnS, using sulfide species produced by sulfate-reducing bacteria and metal contaminated wastewaters. Comparative studies using powder X-ray diffraction (XRD) and scanning electron microscopy (SEM), prior and after photodegradation, were carried out in order to monitor possible structural and morphological changes on the particles. Adsorption properties and specific areas were determined by the Brunauer-Emmet-Teller (BET) method. The final solutions were characterized by UV-Vis and chemical oxygen demand (COD) content in order to determine Safranin-T concentration and toxicity. The influence of the catalyst amount, initial pH and dye concentration was also evaluated. Finally, the efficiency of the precipitates as catalysts in sunlight-mediated photodegradation was investigated, performing two scale experiments by using different volumes of dye-contaminated water (150 mL and 10 L). All tested composites showed potential to be used as photocatalysts for the degradation of Safranin-T, although the ZnS-TiO2_0.06 composite (0.06 g of TiO2 per 50 mL of the zinc solution) was the most effective. This substantiates the applicability of these biologically obtained materials as efficient photocatalysts for the degradation of organic pollutants, in laboratorial conditions and under direct sunlight.

Hybrid ZnO/TiO2Loaded in Electrospun Polymeric Fibers as Photocatalyst

Hybrid ZnO/TiO2nanoparticles were applied as potential photocatalyst agents incorporated into electrospun fibers of enteric block copolymers. The resistance of fibers avoids aggregation and contributes to prolonged action of semiconductor nanoparticles under continuous light irradiation. Such behavior is attributed to the minimal aggregation degree and elevated efficiency of photodegradation of modified nanoparticles in comparison with pristine ones. Hybrid ZnO/TiO2in nanofibers contributes to the association of different mechanisms such as N-deethylation and cleavage of dye molecules applied in the overall process of photodegradation.

Electrochemical sensors cleaned by light: a proof of concept for on site applications towards integrated monitoring systems

The potential for on site applications of a SiO₂–Ag NPs–TiO₂ self-cleaning electrode was demonstrated.

AgBr nanoparticles decorated BiPO4 microrod: a novel p–n heterojunction with enhanced photocatalytic activities

AgBr nanoparticles were decorated on the surface of BiPO₄ micro-rod to fabricate a novel p–n heterojunction photocatalyst via a facile deposition–precipitation method.

Reductive photocatalysis and smart inks

Examples of reductive photocatalysis and smart ink.

Ga doped RGO–TiO2 composite on an ITO surface electrode for investigation of photoelectrocatalytic activity under visible light irradiation

Ga doping in RGO–TiO₂ nanocomposite photoelectrode shows enhanced photocurrent density (4.6 mA cm⁻²) in CO₂ photoelectrocatalysis.

Increasing Rutile Phase Amount in Chromium-Doped Titania by Simple Stirring Approach for Photodegradation of Methylene Blue under Visible Light

The amount of rutile phase in chromium-doped titania photocatalyst was controlled by varying stirring time (0.5–2.0 h) at room temperature during a sol–gel synthesis process. The percentage of rutile phase increased from 15.1 % to 28.6 % when stirring time was prolonged from 0.5 to 1.5 h. Further increases in the stirring time had negligible effect on the rutile phase amount. As evidenced by analyses using diffuse reflectance ultraviolet–visible spectroscopy and X-ray photoelectron spectroscopy, a sufficient stirring time was important for more substitution of Cr3+ for Ti4+ in the lattice, resulting in anatase-to-rutile phase transformation. The formation of more rutile phase in Cr-doped TiO2 not only reduced the band gap energy, but also induced surface defects that retarded electron–hole recombination. It has been demonstrated that the Cr-doped TiO2 prepared with a stirring time of 1.5 h possessed the lowest band gap energy of 1.89 eV, and hence it achieved the highest photodegradation of methylene blue under visible light irradiation.

Optimizing the packing density of TiO2 nanorod arrays for enhanced light harvesting by a light trapping effect and its photocatalytic decomposition of gaseous benzene

A single-crystalline TiO₂ nanorod array with a preferred [002] axial orientation was grown on transparent conductive fluorine-doped tin oxide substrates by a hydrothermal process.

Patterning of polymer brushes made easy using titanium dioxide: direct and remote photocatalytic lithography

Titanium dioxide photocatalytic lithography of initiator functionalized surfaces is proved for the realization of micropatterned polymer brushes.

Stimuli responsive upconversion luminescence nanomaterials and films for various applications

This review highlights recent advances in upconversion luminescence materials in response to various stimuli for a broad spectrum of applications.

Electrically conductive polyaniline sensitized defective-TiO2 for improved visible light photocatalytic and photoelectrochemical performance: a synergistic effect

A proposed scheme for the synthesis of s-Pani@m-TiO₂ nanocomposites.

Fabrication of titanium dioxide with durable superhydrophilicity by anodization

A durable superhydrophilic titanium plate with micro-nano composite architecture synthesized by anodization.

Enhanced Photocatalytic Efficiency of N-F-Co-Embedded Titania under Visible Light Exposure for Removal of Indoor-Level Pollutants

N-F-co-embedded titania (N-F-TiO₂) photocatalysts with varying N:F ratios were synthesized and tested for their ability to photocatalyze the degradation of pollutants present at indoor air levels using visible light. The synthesis was achieved using a solvothermal process with tetrabutyl titanate, urea and ammonium fluoride as sources of Ti, N and F, respectively. Three selected volatile organic compounds (toluene, ethyl benzene and o-xylene) were selected as the test pollutants. The prepared composites were characterized using X-ray diffraction, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy and Ultra-violet (UV)-visible spectroscopy. The photocatalytic degradation efficiencies of N-F-TiO₂ composites were higher than those obtained using pure TiO₂ and N-TiO₂. Moreover, these efficiencies increased as the N:F ratio decreased from sixteen to eight, then decreased as it dropped further to three, indicating the presence of an optimal N:F ratio. Meanwhile, as retention time decreased from 12.4 to 0.62 s, the average photocatalytic efficiencies decreased from 65.4% to 21.7%, 91.5% to 37.8% and 95.8% to 44.7% for toluene, ethyl benzene and o-xylene, respectively. In contrast, the photocatalytic reaction rates increased as retention time decreased. In consideration of all of these factors, under optimized operational conditions, the prepared N-F-TiO₂ composites could be utilized for the degradation of target pollutants at indoor air levels using visible light.

Photoinduced reversible shape conversion of silver nanoparticles assisted by TiO₂

Silver nanoprisms were transformed into nanodecahedra through photoinduction of ultraviolet (UV) light in the presence of titanium dioxide (TiO2) quantum dots (QDs). Subsequently, the silver nanodecahedra were reconverted to silver nanoprisms under sodium lamp if there was sufficient citrate in the reaction system. The localized surface plasmon resonance (LSPR) optical properties of silver nanoparticles were tuned during photoinduced shape conversion. The photocatalytic activity of TiO2 QDs assisted the conversion of prisms to decahedra upon UV light irradiation. Nevertheless, the presence of TiO2 did not inhibit the photoinduced reconversion from decahedra to prisms by sodium light. It was demonstrated that citrate was indispensable in the photoinduction process. In addition, oxygen in solution played a vital role in the reversible shape conversion of silver nanoparticles. Moreover, simulated sunlight can convert silver nanoprisms to nanodecahedra instead of UV light with assistance of TiO2 QDs, which would promote the photoinduced reaction of silver nanoparticles based on a natural light source.

Photocatalytic oxidation of aqueous ammonia using atomic single layer graphitic-C3N4

Direct utilization of solar energy for photocatalytic removal of ammonia from water is a topic of strong interest. However, most of the photocatalysts with effective performance are solely metal-based semiconductors. Here, we report for the first time that a new type of atomic single layer graphitic-C(3)N(4) (SL g-C(3)N(4)), a metal-free photocatalyst, has an excellent photocatalytic activity for total ammonia nitrogen (TAN) removal from water. The results demonstrated that over 80% of TAN (initial concentration 1.50 mg · L(-1)) could be removed in 6 h under Xe lamp irradiation (195 mW · cm(-2)). Furthermore, the SL g-C(3)N(4) exhibited a higher photocatalytic activity in alkaline solution than that in neutral or acidic solutions. The investigation suggested that both photogenerated holes and hydroxyl radicals were involved the TAN photocatalytic oxidation process and that the major oxidation product was NO3(-)-N. In addition, SL g-C(3)N(4) exhibited good photocatalytic stability in aqueous solution. This work highlights the appealing application of an inexpensive metal-free photocatalyst in aqueous ammonia treatment.

Architecture engineering toward highly active palladium integrated titanium dioxide yolk-double-shell nanoreactor for catalytic applications

Rational design of the hierarchical architecture of a material with well controlled functionality is crucially important for improving its properties. In this paper, we present the general strategies for rationally designing and constructing three types of hierarchical Pd integrated TiO2 double-shell architectures, i.e. yolk-double-shell TiO2 architecture (Pd@TiO2/Pd@TiO2) with yolk-type Pd nanoparticles residing inside the central cavity of the hollow TiO2 structure; ultrafine Pd nanoparticles homogenously dispersed on both the external and internal surfaces of the inner TiO2 shell; and double-shell TiO2 architecture (@TiO2/Pd@TiO2) with Pd nanoparticles solely loaded on the external surface of the inner TiO2 shell, and double-shell TiO2 architecture (@TiO2@Pd@TiO2) with Pd nanoparticles dispersed in the interlayer space of double TiO2 shells, via newly developed Pd(2+) ion-diffusion and Pd sol impregnation methodologies. These architectures are well controlled in structure, size, morphology, and configuration with Pd nanoparticles existing in various locations. Owing to the variable synergistic effects arising from the location discrepancies of Pd nanoparticle in the architectures, they exhibit remarkable variations in catalytic activity. In particular, different from previously reported yolk-shell structures, the obtained yolk-double-shell Pd@TiO2/Pd@TiO2 architecture, which is revealed for the first time, possesses a uniform hierarchical structure, narrow size distribution, and good monodispersibility, and it creates two Pd-TiO2 interfaces on the external and internal surfaces of the inner TiO2 shell, leading to the strongest synergistic effect of Pd nanoparticles with TiO2 shell. Furthermore, the interlayer chamber between the double TiO2 shells connecting with the central cavity of the hollow TiO2 structure through the mesoporous TiO2 wall forms a nanoreactor for enriching the reactants and preventing the deletion of Pd nanoparticles during the reaction, thus greatly accelerating the reaction speed. Owing to its structural features, yolk-double-shell Pd@TiO2/Pd@TiO2 architecture exhibits extremely high catalytic performance on the Suzuki-Miyaura coupling reaction. The synthetic methodologies are robust for fabricating double-shell architectures with various configurations for applications such as in catalysis, drug delivery, and medicine release. The obtained double-shell architectures may be used as novel catalyst systems with highly efficient catalytic performance for other catalytic reactions.

A facile and versatile method for preparation of colored TiO2 with enhanced solar-driven photocatalytic activity

Colored TiO2 has attracted enormous attention due to its visible light absorption and excellent photocatalytic activity. In this report, we develop a simple and facile solid-state chemical reduction approach for a large-scale production of colored TiO2 at mild temperature (300-350 °C). The obtained sample possesses a crystalline core/amorphous shell structure (TiO2@TiO2-x). The oxygen vacancy results in the formation of a disordered TiO2-x shell on the surface of TiO2 nanocrystals. XPS and theoretical calculation results indicate that valence band tail and vacancy band below the conduction band minimum appear for the TiO2-x, which implies that the TiO2@TiO2-x nanocrystal has a narrow band gap and therefore leads to a broad visible light absorption. Oxygen vacancy in a proper concentration promotes the charge separation of photogenerated carriers, which improves the photocatalytic activity of TiO2@TiO2-x nanocrystals. This facile and general method could be potentially used for large scale production of colored TiO2 with remarkable enhancement in the visible light absorption and solar-driven H2 production.