A review of synthesis and morphology of SrTiO3 for energy and other applications

With the increasing demand and depleting trend of commercial energies, it has forced the researchers all over the world to accelerate research and development in the area of renewable energy. Currently, unique and interesting features of binary compounds have gained more attention by researchers, and it became a favourite research topic among various groups of researchers around this world. It was noticed that strontium titanate (SrTiO₃) consists of several extraordinary properties that can apply for miscellaneous applications especially for energy storage, fuel cells, as well as to generate hydrogen fuel via photocatalysis process. Besides that, it was noticed that SrTiO₃ can be synthesised in different pathways. The method of preparation and amount of precursors can affect the surface properties of SrTiO₃. Thus, this article presents a critical review on how SrTiO₃ synthesis methods affect its surface morphology and the applications of SrTiO₃ in various fields.     

The effect of imidazolium ionic liquid on the morphology of Pt nanoparticles deposited on the surface of SrTiO3 and photoactivity of Pt–SrTiO3 composite in the H2 generation reaction

Photocatalytic water splitting has great potential in solar-hydrogen production as a low-cost and environmentally friendly method. Different unique techniques used to obtain photocatalysts with various modifications to improve H₂ generation have been introduced. In the present work, SrTiO₃ was successfully synthesized via the solvothermal method in the presence of ionic liquid (IL) - 1-butyl-3-methylimidazolium bromide ([BMIM][Br]) followed by surface decoration with Pt particles using the photodeposition method. The effect of the noble metal content and presence of IL on the morphology, optical and surface properties of SrTiO₃, thereby the effectiveness of hydrogen generation, has been thoroughly examined and presented. Unexpectedly, the presence of [BMIM][Br] at the SrTiO₃ surface affected the interaction between the semiconductor surface and platinum particles formed throughout photodeposition. Platinum particles at the surface of SrTiO₃_IL were found to be in the form of 2D clusters with a size of 1 nm. In comparison, Pt deposited on SrTiO₃ photocatalyst without application of IL created larger, three-dimensional structures with a diameter exceeding 5 nm. This is the reason why the total amount of platinum deposited on the SrTiO₃_IL sample is smaller than that on SrTiO₃ and justifies a higher efficiency of hydrogen generation of Pt modified SrTiO₃ photocatalyst in comparison to SrTiO₃ prepared in the presence of IL. The mechanism of H₂ generation in the water-splitting reaction in the presence of SrTiO₃_Pt photocatalyst was discussed.     

Hydrogen production performance of active Ce/N co-doped SrTiO3 for photocatalytic water splitting

An efficient visible light responsive photocatalyst Ce/N co-doped SrTiO₃ was prepared via a hydrothermal method for hydrogen production. The phase structure, morphology, contents and valence states of the dopant elements, specific surface area, optical properties, and photocatalytic activity of the samples were characterized. The transient photocurrent response and electrochemical impedance spectra under visible light illumination indicated that Ce/N co-doped SrTiO₃ possessed a more intense photo-current response and lower surface resistance than N–SrTiO₃ and Ce–SrTiO₃. The water splitting rate of Ce/N-co-doped SrTiO₃ is 4.28 mmol/g/h, which is 84.49 times higher than that of pure SrTiO₃. The enhanced photocatalytic performance is due to the narrowing of the band gap of SrTiO₃ by Ce ion and N ion impurities.     

Hybrid DFT study of MWCNT/Zr-doped SrTiO3 heterostructure: Hydrogen production,electronic properties and charge Carrier mediator role of Zr4+ ion

Over the past decade, remarkable efforts have been made to design low-cost, non-toxic, stable and efficient photocatalyst for water splitting. In the present work, an effective alternative approach to enhance hydrogen production of SrTiO₃ was by coupling with MWCNT to form heterojunction followed by doping with Zr⁴⁺ ion. The observed type-II band alignment and the polarised electric field could promote the separation of photoexcited charge carriers and photocatalytic activity of these hybrid heterostructures. The theoretical calculation revealed that Zr⁴⁺ ion could act as a charge carrier mediator to transfer electrons to the SrTiO₃ surface. The MWCNT (6,12,18)/Zr-doped SrTiO₃(100) heterostructure exhibited excellent activity due to the combined effect of MWCNT (6,12,18) and Zr-doped SrTiO₃(100) monolayers compared with pure SrTiO₃. This study offers a novel understanding of designing highly active and stable SrTiO₃-based photocatalyst as efficient hydrogen generation material.     

Electronic and optical properties of nitrogen and sulfur doped strontium titanate as efficient photocatalyst for water splitting: A DFT study

One of the most effective option of photocatalysts for water splitting is doped strontium titanate, SrTiO₃. It has a high rate of photo-generated charge transfer and limited photocatalytic activity for water splitting. The search of an appropriate photocatalyst having a high visible light absorption as well as fast charge transportation is extremely needed, however it is a difficult task. The structural, electronic and optical properties of sulfur-doped SrTiO₃ and nitrogen-doped SrTiO₃ are investigated using calculations based on density functional theory (DFT). According to the band structure calculations, the O-2p states represented the higher levels of the valence band of pure SrTiO₃. When S and N atoms are introduced into the SrTiO₃ structure on the O site, electronic structure findings indicate that doping the Sulfur (S) atoms reduced the band gap significantly, whereas doping of N atoms increased the bandgap of SrTiO₃. According to our results, the N-doped SrTiO₃ has a sufficient band gap of 2.03 eV, as well as suitable high visible light absorption and charge carrier transportation. The optical properties showed that N-doped SrTiO₃ has good photosensitivity for visible light. In addition, we have found a significant impurity state that differs from O 2p-states, which can increase photocatalytic efficiency. The results of studies of electronic band structure showed that electron-hole transportation was well consistent with the experimental data. Thus, the N-doped SrTiO₃ in this study is indeed an attractive candidate for hydrogen evolution throughout the visible light range, providing a logical base for the establishment of innovative photocatalysts.     

Review of surface photovoltage spectra of nano-sized semiconductor and its applications in heterogeneous photocatalysis

Heterogeneous photocatalysis is a promising technique valuable for environmental purification. Nano-sized semiconductors such as ZnO and TiO₂, which is one of the most basic functional materials, have emerged as effective photocatalyst materials. The surface photovoltage spectra (SPS) can be an effective method for quickly evaluating the photocatalytic activity of semiconductor materials since it can provide a rapid, non-destructive monitor of the semiconductor surface properties such as surface band bending, surface and bulk carrier recombination and surface states, mainly showing the carrier separation and transfer behavior with the aid of light, especially the electric-field-induced surface photovoltage spectra (EFISPS), in which SPS is combined with the electric-field-modified technique. In this review, the basic principles, measurement and applications of the SPS and EFISPS are mainly discussed together with some fundamental aspects like the electric properties of semiconductor surface and the principle of electric field effect. In particular, the applications of SPS to nano-sized semiconductors such as ZnO and TiO₂ in heterogeneous photocatalysis are emphasized, which involve mainly evaluating the photocatalytic activity by analyzing semiconductor surface properties such as the separation efficiency of photoinduced carriers under illumination by the SPS measurement, highlighting our own contributions. The results show that the weaker the surface photovoltage signal is, the higher the photocatalytic activity is in the case of nano-sized semiconductor photocatalysts.     

Efficient visible-light-driven photocatalytic H2 production over Cr/N-codoped SrTiO3

Visible-light-response Cr/N-codoped SrTiO₃ was prepared by a sol–gel hydrothermal method. The comparison studies indicate that Cr-doped and Cr/N-codoped SrTiO₃ can be synthesized by this means, but not the N-doped SrTiO₃. The theoretical calculations exhibit the defect formation energy of the Cr/N codoping into SrTiO₃ is much smaller than that of the N doping into SrTiO₃, illuminating that the incorporation of Cr can promote the N doping into the O sites in the SrTiO₃. Compared to the Cr-doped SrTiO₃, the Cr/N-codoped SrTiO₃ photocatalyst shows the high photocatalytic activities for hydrogen production with the quantum efficiency of 3.1% at 420 nm, due to the smaller band gap and much less vacancy defects.     

A review on selected heterogeneous photocatalysts for hydrogen production

In this study, visible light‐driven heterogeneous photocatalysts for hydrogen production are comparatively assessed based on technical, environmental, and cost criteria. The photocatalysis systems are compared with respect to their (i) rate of hydrogen generation per gram; (ii) rate of hydrogen generation per m² of the specific surface area; and (iii) the band gap energy. The photocatalysis systems are also compared and discussed in terms of flammability, reactivity, and their impact on living systems' health. Furthermore, the costs of the required components of the photocatalysis systems are ranked. In addition to individual photocatalyst comparison, seven photocatalyst groups are ranked and compared. The results show that TiO₂‐C‐362 and Ag_0.03Mn_0.40Cd_0.60S show the highest in terms of µmol/h‐g_cat and µmol/h‐m²_cat, respectively, and TiO₂‐C‐362 has the highest overall rankings. The Zn/In/S‐based photocatalyst groups show the highest hydrogen production rate in terms of µmol/h‐gcat and µmol/h‐m²_cat. Overall, Cd/S/Zn has the highest rankings when cost and health and environmental impact criteria are taken into account. Copyright © 2014 John Wiley & Sons, Ltd.     

Organic-inorganic composite of g-C3N4–SrTiO3:Rh photocatalyst for improved H2 evolution under visible light irradiation

The organic-inorganic composite g-C₃N₄–SrTiO₃:Rh was prepared for the first time as a photocatalyst for hydrogen production and the resulting hydrogen evolution rate under visible light irradiation from aqueous methanol solution was measured. A high hydrogen evolution rate of 223.3 μmol h⁻¹ was achieved by using 0.1 g of as-prepared photocatalyst powder comprised of 20 wt.% g-C₃N₄ 80 wt.% SrTiO₃:Rh (0.3 mol%). The hydrogen evolution rate was greater than that obtained by SrTiO₃:Rh (0.3 mol%) by a factor of 3.24. The quantum efficiency of as-prepared composite photocatalyst was 5.5% at 410 nm for hydrogen evolution. The high activity of the composite photocatalyst for hydrogen evolution stemmed from its electron–hole separation and transportation capabilities due to the hetero-junctions of the organic-inorganic composite materials. The proposed mechanism for the electron–hole separation and hydrogen evolution of the g-C₃N₄–SrTiO₃:Rh composite under visible light irradiation featured the reduced recombination of the photo-generated charge carriers. The doping of Rh ions into the SrTiO₃ has contributed to the high photocatalytic activity by forming a donor level from the valance band to the conduction band.     

Effect of tri-doping on H2 evolution under visible light irradiation on SrTiO3:Ni/Ta/La prepared by spray pyrolysis from polymeric precursor

Tri-doped photocatalyst, SrTiO₃:Ni/Ta/La, was prepared by spray pyrolysis from aqueous and polymeric precursor solutions. The third dopant, La³⁺, contributed to the BET surface area and porous morphology by preventing crystal growth, and increased the Ni²⁺/Ni³⁺ ratio by affecting the electron configuration in the lattice structure, which is closely related to the hydrogen evolution rate. The hydrogen evolution rate of the tri-doped photocatalyst, SrTiO₃:Ni(0.2 mol%)/Ta(0.4 mol%)/La(0.3 mol%), was increased by about 60%–895.2 μmol g⁻¹ h⁻¹ from the value of 561.2 μmol g⁻¹ h⁻¹ for the co-doped photocatalyst, SrTiO₃:Ni(0.2 mol%)/Ta(0.4 mol%), and was further enhanced to 2305.7 μmol g⁻¹ h⁻¹ when a polymeric precursor was used instead of an aqueous precursor in spray pyrolysis. The optimum additive content for polymeric precursor solution was 300 mol%.     

Recent development in band engineering of binary semiconductor materials for solar driven photocatalytic hydrogen production

Photocatalytic hydrogen production from water splitting is a promising approach to develop sustainable renewable energy resources and limits the global warming simultaneously. Despite the significant efforts have been dedicated for the synthesis of semiconductor materials, key challenge persists is lower quantum efficiency of a photocatalyst due to charge carrier recombination and inability of utilizing full spectrum of solar light irradiation. In this review, recent developments in binary semiconductor materials and their application for photocatalytic water splitting toward hydrogen production are systematically discoursed. In the main stream, fundamentals and thermodynamic for photocatalytic water splitting and selection of photo-catalysts has been presented. Developments in the binary photocatalysts and their efficiency enhancements though surface sensitization, surface plasmon resonance (SPR) effect, Schoktty barrier and electrons mediation toward enhanced hydrogen production has been deliberated. Different modification approaches including band engineering, coupling of semiconductor catalysts, construction of heterojunction, Z-scheme formation and step-type photocatalytic systems are also discussed. The binary semiconductor materials such as TiO₂, g-C₃N₄, ZnO, ZnS, Fe₂O₃, CdS, WO₃, rGO, V₂O₅ and AgX (Cl, Br and I) are systematically disclosed. In addition, role of sacrificial reagents for efficient photocatalysis through reforming and hole-scavenger are elaborated. Finally, future perspectives for photocatalytic water splitting towards renewable hydrogen production have been suggested.     

Hydrogen production over metal-loaded mesoporous-assembled SrTiO3 nanocrystal photocatalysts: Effects of metal type and loading

Mesoporous-assembled SrTiO₃ photocatalysts with different loaded metal co-catalysts (Au,Pt, Ag, Ni, Ce, and Fe) synthesized by the single-step sol–gel method with the aid of a structure-directing surfactant were tested for the photocatalytic activity of hydrogen production from a methanol aqueous solution under both UV and visible light irradiation. The Au, Pt, Ag, and Ni loadings had a positive effect on the photocatalytic activity enhancement, whereas the Ce and Fe loadings did not. The best loaded metal was found to be Au due to its electrochemical properties compatible with the SrTiO₃-based photocatalyst and its visible light harvesting enhancement. A 1 wt.% Au-loaded SrTiO₃ photocatalyst exhibited the highest photocatalytic hydrogen production activity with a hydrogen production rate of 337 and 200 μmol h⁻¹ g_cat⁻¹ under UV and visible light irradiation, respectively. The hydrogen diffusivity from the liquid phase to the gas phase also significantly affected the photocatalytic hydrogen production efficiency. An increase in the hydrogen diffusability led to an increase in the photocatalytic hydrogen production efficiency.     

Hierarchically SrTiO3@TiO2@Fe2O3 nanorod heterostructures for enhanced photoelectrochemical water splitting

In this particular work, the fabrication of SrTiO₃@TiO₂@ Fe₂O₃ nanorod heterostructure has been demonstrated via hydrothermal growth of SrTiO₃ cubic on the rutile TiO₂ nanorod as a template and later sensitized with Fe₂O₃ for photocatalytic solar hydrogen production in a tandem photoelectrochemical cell and dye-sensitized solar cell (DSSC) module. The photocatalytic solar hydrogen production of this heterostructure was optimized by controlling the amount of Sr and Fe on the surface of photocatalyst. The details of the influencing parameters on the physicochemical and photoelectrochemical properties are discussed. It was found that the morphology and quality of the fabricated materials were greatly manipulated by the concentration of Sr and Fe. The optimized 0.025 M SrTiO₃@TiO₂@ Fe₂O₃ heterostructure exhibited a higher photoconversion efficiency with a long electron lifetime, low charge transfer resistance and large donor density at the electrode and electrolyte interface. This composite has significantly improved the photocatalytic hydrogen production, yielding 716 μmol/cm² of maximum accumulative hydrogen. These results show that morphology rendering and manipulation of energy band alignment is crucial in creating efficient heterojunctions for excellent contributions in photocatalytic applications.     

A review on graphitic carbon nitride (g-C3N4) – metal organic framework (MOF) heterostructured photocatalyst materials for photo(electro)chemical hydrogen evolution

Graphitic carbon nitride (g-C₃N₄)-based heterostructured photocatalysts have recently attracted significant attention for solar water splitting and photocatalytic hydrogen (H₂) evolution, because of their alterable physicochemical, optical and electrical properties, such as tunable band structure, ultrahigh specific surface area and controllable pore size, defect formation and active sites. On the other hand, metal-organic frameworks (MOFs) possess a favorable surface area, permanent porosity and adjustable structures that allow them to be suitable candidates for diverse applications. In this review, we therefore comprehensively discuss the structural properties of heterogeneous g-C₃N₄/MOF-based photocatalysts with a special emphasis on their photocatalytic performance regarding the mechanism of heterogeneous photocatalysis, including advantages, challenges and design considerations.     

H2 evolution under visible light irradiation from aqueous methanol solution on SrTiO3:Cr/Ta prepared by spray pyrolysis from polymeric precursor

SrTiO₃:Cr/Ta powders were prepared by spray pyrolysis from polymeric precursors. Effects of the amount of co-dopant and additives on the photocatalytic activity for hydrogen evolution from aqueous methanol solution under visible light irradiation (λ > 415 nm) were investigated. For the photocatalyst prepared by spray pyrolysis from polymeric precursor, the hydrogen evolution rate was increased by a factor of ∼100 and induction period was decreased by a factor of 8 compared with a photocatalyst prepared by solid state reaction. These enhancements result from increased roughness of surface, and the compositional uniformity which are intrinsic characteristics of spray pyrolysis. In addition, photocatalyst prepared by spray pyrolysis from polymeric precursor have large BET surface area and small amount of Cr⁶⁺ ion which is responsible for long induction period. It should be noted that the reduction of Cr⁶⁺ ion was achieved without hydrogen reduction process.     

Application of mixture experimental design for photocatalytic ammonia degradation by sunlight‐driven WO3‐Ag3PO4‐ZnO ternary photocatalysts

Using statistical mixture design, the best composition of a heterojunction photocatalyst containing ZnO, Ag₃PO₄, and WO₃ was determined to maximize the sunlight driven ammonia removal from aqueous solution via both photocatalysis and adsorption processes. All samples were prepared by coprecipitation and immobilized over perlite granules as floatable support. X‐ray diffraction (XRD), Fourier‐transform infrared (FTIR), field emission scanning electron microscope (FESEM), Brunauer, Emmett, and Teller (BET), ultraviolet–visible (UV‐vis), and photoluminescence (PL) analyses were used to characterize the catalysts. Three responses of ammonia removal by photocatalysis, adsorption, and the total ammonia removal were modeled by special cubic models, and the ANOVA confirmed the significance of them. The maximum ammonia removal, approximately 88%, was obtained by photocatalyst composed of 32.93‐wt% WO₃, 41.82‐wt% Ag₃PO₄, and 25.26‐wt% ZnO. The contribution of photocatalysis and adsorption was estimated to be 72.74% and 14.44%, respectively, indicating the dominance of photocatalysis process. According to kinetic study, the optimum photocatalyst showed the highest apparent rate constant and lowest half‐life time of ammonia removal. The maximum quantum yield of 1.7% was calculated from the best photocatalyst composite at the maximum intensity of visible light received from sunlight. The reuse ability test revealed that the optimum ternary photocatalyst is suitable for wastewater treatments in practical applications.     

Degradation of methylene blue-ciprofloxacin and hydrogen production simultaneously using combination of electrocoagulation and photocatalytic process with Fe-TiNTAs

The study reported in this paper combines the electrocoagulation and photocatalysis for the simultaneous degradation of methylene blue dyes (MB)-antibiotic ciprofloxacin (CP) and production of hydrogen. The pollutant removal process was conducted by combining adsorption by electrocoagulation and degradation by photocatalysis. Meanwhile, H₂ was produced by reducing the H⁺ on the cathode and the photocatalyst surface in a reactor made of acrylic equipped with aluminum as the anode, stainless steel 316 plates as a cathode, Fe-doped titania nanotube arrays (TiNTAs) as a photocatalyst, and a 250-W mercury lamp as the light source. TiNTAs were synthesized via anodization and followed by the successive ionic layer adsorption and reaction (SILAR) method to incorporate Fe as the dopant. In particular, the effects of Fe loading in the composite photocatalyst are investigated. XRD results showed that TiO₂ nanotubes arrays comprise a 100% anatase phase. FESEM, EDX, TEM, and HRTEM analysis confirmed the formation of the nanotubular structure of TiO₂ and the presence of Fe deposited on the surface. The UV–Vis DRS indicated that the bandgap of Fe-TiNTAs reduced with Fe introduction, as compared to that of the undoped TiNTAs. The results showed that accumulation of the produced hydrogen from the combination of electrocoagulation-photocatalytic system is greater than that which is obtained using individual electrocoagulation or photocatalytic system. The combined process exhibited an enhanced degradation ability of methylene blue and ciprofloxacin, as well as in the H₂ production.     

Hydrogen production from water splitting over Eosin Y-sensitized mesoporous-assembled perovskite titanate nanocrystal photocatalysts under visible light irradiation

The photocatalytic water splitting is a promising process for producing H₂ from two abundant renewable sources of water and solar light, with the aid of a suitable photocatalyst. In this work, a combination of sensitizer addition and noble metal loading was employed to modify perovskite photocatalysts in order to achieve the enhancement of photocatalytic H₂ production under visible light irradiation. The dependence of the H₂ production on type of mesoporous-assembled perovskite titanate nanocrystal photocatalysts (MgTiO₃, CaTiO₃, and SrTiO₃), calcination temperature of photocatalyst, Pt loading, type and concentration of electron donor (diethanolamine, DEA; and triethanolamine, TEA), concentration of sensitizer (Eosin Y, E.Y.), photocatalyst dosage, and initial solution pH, was systematically studied. The experimental results showed that the 0.5 wt.% Pt-loaded mesoporous-assembled SrTiO₃ nanocrystal synthesized by a single-step sol-gel method and calcined at 650 °C exhibited the highest photocatalytic H₂ production activity from a 15 vol% DEA aqueous solution with dissolved 0.5 mM E.Y. Moreover, the optimum photocatalyst dosage and initial solution pH for the maximum photocatalytic H₂ production activity were found to be 6 g/l and 11.6, respectively.     

Fabrication of ZnO/SrTiO3 nanoarrays and its photoelectrochemical performances

The ZnO/SrTiO₃ nanomaterials were fabricated by a chemical conversion hydrothermal method in order to utilize the high electron transfer rate of one-dimensional ZnO nanorods and photocatalytic activity of SrTiO₃. The technological parameters, such as TiO₂ sol concentration, TiO₂ sol dipping cycle, Sr(NO₃)₂ concentration and reaction temperature, were investigated in the synthetic process and the reaction mechanism of the ZnO/SrTiO₃ nanomaterials was proposed. A photocurrent density of 7.53 mA/cm² was obtained for the as-prepared ZnO/SrTiO₃ photocatalyst, attributed to its improved absorption spectrum and appropriate nanostructure, which indicates a potential application in photoelectrochemical water splitting.     

Effect of polystyrene characteristic on photocatalytic hydrogen production by porous polystyrene photocatalyst film under simulated solar light irradiation

In this study, we developed a polystyrene-platinum/nitrogen-doped titanium dioxide/strontium titanate composite-polyvinylpyrrolidone (PS-PNS-PVP) photocatalyst film, which is applied in the process of photocatalytic hydrolysis under simulated sunlight to produce hydrogen, is developed. PS, which is cheap, non-toxic, with high UV resistance, and chemical inertness, is used as a carrier, and a highly effective hydrogen production of Pt/N–TiO₂/SrTiO₃ as a photocatalyst. The influence of the PS concentration on the stability, optical, and electrical properties of the photocatalyst film is discussed. In addition, the influence of the photocatalyst dispersion in the film on the activity under various photocatalyst concentrations was investigated. A polyvinylpyrrolidone pore-forming agent was then used to examine the effect on the photocatalyst film structure and optical properties, and the subsequent influence on photocatalytic hydrogen energy activity. Adjusting the PS concentration to 20 wt% produced good film-forming stability, and the photocatalyst dispersibility in the film under different photocatalyst concentrations. A photocatalyst concentration of 2.5 wt% resulted in good film dispersibility and the realization of added pore-forming agent. The modified photocatalyst film changed the film from a blind pore structure to a connecting void structure, increasing the film's porosity and hydrophilicity. This increased the number of photocatalytic sites, and the optimal hydrogen production of the photocatalyst film reached 21,333 μmol h^?1 g^?1.