LITERATURE REVIEW
Strontium titanate
Strontium titanate (STO) is a perovskite material characterized by a cubic unit cell with a lattice parameter of 3.905 Å, forming a robust network through corner-shared TiO6 octahedra With a high melting point of 2080°C, STO is suitable for high-temperature applications and maintains its cubic perovskite structure under ambient conditions Upon cooling to 105 K or lower, STO undergoes a phase transition from a cubic structure (space group: Pm3m) to a tetragonal structure (space group: P4mm), accompanied by changes in its electrical properties Although such transitions typically induce ferroelectricity, STO does not exhibit this behavior down to 0 K, classifying it as a quantum paraelectric or incipient ferroelectric material.
STO, or strontium titanate, is a versatile material known for its unique properties and is commonly used as a substrate for the epitaxial growth of high-temperature superconductors Additionally, STO finds applications in macroelectronics, ferroelectric devices, and optoelectronics With a high dielectric constant of around 300 at ambient temperature, its dielectric properties significantly increase at lower temperatures, enhancing its utility in various technological fields.
104) [36] STO undergoes a metal-insulator transition upon light doping STO is a typical nonpolar band insulator with a band gap of 3.2 eV (indirect)
Strontium titanate (STO) can exhibit a metallic phase with adjustable electrical conductivity based on oxygen concentration While bare STO acts as an electronic insulator at room temperature, introducing point defects into its lattice creates free charge carriers In semiconducting STO, these charge carriers are primarily electrons, which result from donor impurity doping or exposure to a reducing atmosphere This process generates a comparable density of oxygen vacancies, known for their high lattice mobility, especially at elevated temperatures Consequently, STO is classified as a mixed electronic-ionic conductor.
Strontium titanate (STO) is an effective photocatalyst, particularly in water-splitting applications, due to its conduction band edge being approximately 200 mV more negative than that of TiO2 Additionally, STO exhibits remarkable thermal and chemical stability Notably, the Sr²⁺ ions within its structure can accept electrons from the conduction band, resulting in the formation of Sr⁺ This newly formed Sr⁺ can then transport electrons to generate •O2⁻, facilitating the decomposition of organic compounds and thereby enhancing the efficiency of charge separation in the photocatalytic process.
Strontium titanate (STO) has garnered significant interest from researchers in recent years due to its remarkable photocatalytic properties However, it is important to note that the band gap of pure STO is 3.2 eV (indirect), which limits its photocatalytic activity to the ultraviolet region (λ < 387.5 nm).
Figure 1.1 The lattice structure of STO [39]
Over the last two decades, research on Strontium Titanate Oxide (STO) materials has significantly increased, with nearly 7,000 publications emerging between 2014 and 2018 alone, highlighting their diverse applications.
Figure 1 2 Number of publications using “strontium titanate” or “STO” as the topic keywords in the past 20 years [39]
This indicates the importance and attraction of this demanding research It was reported that the STO could be produced in many techniques such as
6 hydrothermal, solid-state reaction, and ultrasound-assisted, but the different synthesis pathways would lead to different morphology, crystallinity, uniformity, shape, and size of the STO nanoparticles
Recent research highlights the significant potential of Strontium Titanate (STO) in practical applications like fuel cells and gas sensors However, there is a lack of comprehensive discussion regarding the synthesis techniques and applications of STO This article critically summarizes the impact of various STO synthesis methods and explores its diverse applications.
Various starting materials can be utilized to synthesize strontium titanate (STO) under different conditions, with the catalyst's activity being influenced by factors such as crystallinity, surface area, particle size, and crystal phase The short diffusion distance of photoexcited electrons to the surface highlights the advantage of nano-particulate photocatalysts, where a larger surface area enhances performance Therefore, precise control over the shape and size of STO is crucial for developing high-performance photocatalysts and assessing shape-dependent photoreactivity Furthermore, the outer surface and morphology of the photocatalyst significantly impact its photocatalytic activity.
Eg values of STO could be slightly different when the sample preparation method and different processing details were used
The band gap of STO nanoparticles has been measured at approximately 3.10 eV through conventional solid-state reaction and hydrothermal synthesis Additionally, a study indicated that STO nanoparticles synthesized via ball milling-assisted solid-state reaction exhibited a band gap of around 2.00 eV, demonstrating photoactivity under visible light irradiation The presence of metal impurities within the STO lattice structure was noted, raising concerns about their potential impact on the energy gap value Furthermore, when coprecipitation synthesis was employed, the band gap of STO nanoparticles was estimated to be 3.35 eV, while other researchers reported values ranging from 3.40 to 3.60 eV.
Numerous scientists have identified various methods for synthesizing strontium titanate (STO), including hydrothermal, solvothermal, solid-state, sonochemical, and sol-gel techniques Each synthesis pathway influences the resulting morphology, crystallinity, uniformity, size, and shape of STO nanoparticles.
Table 1.1 Recent publications (2008-2018) of the preparation methods of STO nano- materials [39]
Preparation Method Structure Particle Size, nm
Sol-precipitation coupled with hydrothermal synthesis
Ball milling–assisted solid-state reaction
Sol-gel Nanopowders not available
Molten salt reaction Submicron crystallites and nanocrystals
Microwave-assisted hydrothermal Nanospheres not available
Solid-state reaction (flux based) Nanocubes 300
Ultrasound-assisted wet chemical Nanocrystals 7-17
Solid-state crystal growth Single crystals Not available
Ultrasound-assisted hydrothermal Mesoporous nanopowder
The sol-gel method is a versatile technique for producing various oxide compounds through the hydrolysis and condensation of metal alkoxides, followed by high-temperature treatment that leads to polymerization and the formation of a metal oxide network This synthesis technique allows for precise control over the morphology, chemical properties, and texture of the final products Additionally, it offers advantages over other synthesis methods, such as enabling the impregnation of coprecipitation to introduce dopants effectively.
The sol-gel method is a widely used technique for synthesizing nanomaterials, but it often involves costly metal alkoxide precursors and requires high-temperature calcination for optimal crystallinity Insufficient calcination temperatures can result in defects and low crystallinity, negatively impacting photocatalytic performance Additionally, even minor variations in experimental conditions can significantly alter the properties and yields of the catalysts produced Therefore, meticulous attention to detail is essential when employing the sol-gel process for synthesizing strontium titanate (STO).
In 2001, STO was successfully synthesized using the sol-gel technique, revealing that the crystallinity of the final product significantly influences its bandgap energy (Eg) Higher crystallinity results in a lower Eg, which is associated with enhanced photocatalytic activity, while products with poor crystallinity exhibit a larger Eg compared to single crystals.
It was suspected that this is due to the quantum size effect and the existence of amorphous phase in STO
The Pechini method enhances sol-gel chemistry by utilizing small molecule chelating ligands to create a homogeneous solution of metal/citrate complexes This technique advances the process by transforming the mixture into a covalent polymer network that encapsulates metal ions By delaying the decomposition of the organic matrix at elevated temperatures, the Pechini method offers improved control over the development of ceramic products.
In a typical process, a metal salt is dissolved in water with ethylene glycol and critic acid to form a homogeneous starting materials solution that contains metal-
The Pechini method involves heating a citrate chelate complex solution to initiate the polyesterification process between ethylene glycol and citrate, resulting in the formation of an extended covalent network A key advantage of this method is its ability to create a polymeric precursor that allows for the homogeneous dispersion of two or more metals throughout the network.
Application of STO for photocatalysis
When photons strike the surface of a semiconductor, they can excite electrons from the valence band to the conduction band if the energy of the incident light meets or exceeds the semiconductor's band gap energy.
Strontium titanate (SrTiO3) is a semiconductor recognized for its photocatalytic activity, featuring a band gap of 3.2 eV that enables the generation of photogenerated charge carriers when exposed to UV light (λ ≤ 387.5 nm) Due to these properties, SrTiO3 has been utilized in various photocatalysis applications, as detailed in Table 1.2.
The CB edge of strontium titanate (STO) makes it an ideal candidate for the water splitting process due to its favorable redox potential Photocatalytic hydrogen production is considered a sustainable and environmentally friendly method for generating alternative green energy, as it is relatively simple and produces zero emissions This process primarily relies on a semiconductor, light energy, and water However, the water-splitting reaction is non-spontaneous (ΔG = 237.178 kJ/mol) under standard conditions, necessitating external energy, such as electricity, to break water molecule bonds In this context, STO serves as a photocatalyst, generating electron-hole pairs that facilitate the redox reaction.
In 2008, Liu et al investigated the water splitting performance of pristine strontium titanate (STO) under strong UV light Subsequently, Iwashina and Kudo achieved water splitting under visible light by doping STO with rhodium (Rh), which extended its photocatalytic response to 540 nm This successful modification of STO attracted significant interest from other researchers in the field.
In 2016, Su, Huang, and Wey highlighted the effectiveness of STO/TiO2 nanotube composites in water splitting applications These nanotubes offer several benefits, including strong redox capabilities, excellent corrosion resistance, and efficient electronic transport The combination of STO and TiO2 enhances the photogenerated charge separation, making these composites promising for renewable energy solutions.
The transfer of 11 electrons from the conduction band (CB) of strontium titanate (STO) to that of titanium dioxide (TiO2) facilitates effective charge separation, while photogenerated holes move from the valence band (VB) of TiO2 to the VB of STO This process enhances charge transfer from the bulk to the surface, improving overall efficiency.
In 2016, research revealed that specific facets of strontium titanate (STO) exhibit distinct functions; for instance, the STO single crystal (001) facet enhances active sites for photo-reduction, while the (023) facet facilitates active sites for photo-oxidation Additionally, hydrogen production can occur through processes beyond just water splitting.
In 2018, Guo et al explored hydrogen generation through the decomposition of formic acid using STO composites, which can decompose via dehydrogenation and dehydration processes facilitated by photocatalysts However, these methods may produce CO2 and CO as by-products Additionally, STO was utilized in water oxidation research by Guan et al in 2014.
Rapid industrialization and the overuse of non-renewable energy sources have resulted in their depletion and an increase in environmental pollutants In addition to hydrogen production, STO is effective in various advanced oxidation processes (AOPs) that do not generate hazardous sludge This method generates highly reactive species, such as hydroxyl (•OH) and superoxide radicals (•O2-), which are utilized to oxidize organic pollutants effectively.
The presence of organic dyes in water poses a significant environmental threat due to their low biodegradability and toxicity to aquatic life These dyes can hinder sunlight penetration in water, negatively impacting aquatic plants and ultimately disrupting the entire aquatic ecosystem.
In 2018, Faisal et al explored the use of methylene blue (MB) as an industrial dye in conjunction with mesoporous strontium titanate (STO) and polythiophene (PTh) PTh, a conducting polymer known for its efficient sensitizing properties in photocatalysis, benefits from its π-conjugated electron system, high absorption coefficients, and excellent electron-hole mobility The study demonstrated that the PTh-STO photocatalyst exhibited significantly enhanced efficiency, attributed to the migration of excited electrons from the lowest unoccupied molecular orbitals (LUMOs) of PTh to the conduction band (CB) of STO.
In addition to methylene blue (MB) dye, researchers have also explored the treatment of various industrial dyes, including methyl orange (MO), crystal violet (CV), rhodamine B (RhB), and phenol oxidation A 2015 survey revealed a concerning presence of 36 detectable antibiotics, such as lincomycin, chloramphenicols, and macrolides, highlighting the need for effective treatment methods for these pollutants.
12 and tetracyclines [TCs]) existed in China, which was approximately 53 800 tons
Concerns have arisen regarding the negative impact of multiresistant bacterial strains on human health, which diminishes the effectiveness of antibiotics In response to this issue, Wu et al explored the degradation of tetracyclines (TCs) using manganese-doped strontium titanate (Mn-doped STO) Their findings indicated that TCs could be effectively degraded under visible light, with enhanced light harvesting capabilities.
In 2018, Kumar et al conducted an experiment demonstrating the effective degradation of levofloxacin (LFC) using Ag/Fe3O4 bridged STO/g-C3N4, achieving complete degradation in just 90 minutes LFC, a widely used antibiotic from the fluoroquinolone family, is essential for treating severe bacterial infections.
Strontium titanate (STO) is effective in reducing heavy metals, notably converting chromium from its hexavalent form (Cr VI) to the trivalent form (Cr III) It demonstrates superior reduction capabilities compared to titanium dioxide (TiO2) and can enhance the separation of photogenerated carriers by forming a heterostructure with TiO2, thanks to its more negative conduction band energy.
Photocatalytic oxidation of organic dye
Research indicates that dyes tend to be more toxic and heavily colored following the dyeing process Unlike many organic compounds, dyes derive their color from their ability to absorb light within the visible spectrum (400-700 nm), the presence of at least one chromophore (a color-bearing group), a conjugated system featuring alternating double and single bonds, and the resonance of electrons, which stabilizes the molecular structure.
Rhodamine B (RhB), a widely used xanthene dye in the textile industry, is known for its high stability across various pH levels and its fluorescent bluish-red color However, due to its synthetic origin and complex structure, RhB is challenging to decolorize and has been banned in food and cosmetics due to potential toxicity and carcinogenicity Consequently, the photocatalytic degradation of RhB is crucial for effectively purifying dye effluents and mitigating environmental impacts.
Rose bengal (RB), a xanthene dye alongside RhB, is widely utilized in printing, insecticides, and dyeing industries Recognized as an emerging pollutant in surface and potentially drinking water, rose bengal features a linear arrangement of three aromatic rings with an oxygen atom at the center Additionally, it serves therapeutic purposes in treating specific cancers and skin conditions such as eczema and psoriasis, as well as in medical and biological diagnostics as a contrast agent.
The challenge of water depollution, particularly in treating contaminated aqueous solutions with low concentrations of pollutants, necessitates advanced oxidation processes Since the late 1990s, there has been a significant increase in scientific and engineering interest in photocatalytic materials and processes, aimed at developing more efficient and applicable water treatment technologies.
Traditional technologies that utilize a blend of physical, biological, and chemical processes are non-destructive and effectively convert pollutants from one phase to another However, this transformation necessitates additional treatment to prevent secondary pollution.
Advanced Oxidation Processes (AOPs) offer an effective and eco-friendly solution for purifying polluted water sources, significantly reducing pollution levels while conserving energy.
[80], [83], [84] The AOPs involve two main stages of oxidation: (i) the formation of highly active species and (ii) the redox reactions of active species with organic contaminants [80]
Advanced Oxidation Processes (AOPs) utilize external energy sources like electric power, ultraviolet (UV) radiation, or solar light, along with strong oxidizing agents such as hydrogen peroxide (H2O2), ozone (O3), and various catalysts Among these, AOPs powered by solar light are increasingly favored for wastewater treatment due to their cost-effectiveness, efficiency, and the abundant availability of solar energy Unlike conventional methods, AOPs offer the significant advantage of completely degrading organic compounds into environmentally benign products like carbon dioxide (CO2), water (H2O), and harmless inorganic substances.
Heterogeneous photocatalysis is a promising advanced oxidation process (AOP) for treating drinking water and wastewater This method utilizes solid catalysts activated by UV or visible light to effectively degrade pollutants in liquid-phase environments Extensive research over the past few decades has demonstrated its non-selective photocatalytic performance, which occurs through a combination of parallel and series reduction/oxidation reactions.
Heterogeneous photocatalysis has demonstrated its effectiveness in completely decomposing various organic contaminants through the production of holes (h+) and radical species such as hydroxyl (•OH), superoxide (•O2-), and hydroperoxyl (•OOH) radicals generated by the reduction and oxidation of photogenerated electrons and holes Additionally, solid-form catalysts are advantageous as they can be easily separated from the solution, allowing for reuse and further cost reduction in the process.
A photocatalytic mechanism typically involves three key steps: first, photo-excitation produces electrons and holes; second, these charge carriers migrate to the catalyst's surface; and third, the electrons and holes interact with adsorbed electron acceptors and donors, such as oxidizing agents, to finalize the photocatalytic reaction.
The energy-level diagram in Figure 1.4 illustrates the conduction band (CB) and valence band (VB) edge positions of various semiconductors at pH=0, alongside selected redox potentials The energy scales are referenced to both the vacuum level and the normal hydrogen electrode (NHE) [99].
For effective photocatalytic degradation of organic compounds, the positions of the valence band (VB) and conduction band (CB) edges of semiconductors must align with the redox potential of oxidizing agents within the semiconductor's band gap This specific band structure significantly influences both the light absorption characteristics and the redox capabilities of the semiconductor Consequently, semiconductor materials are predominantly utilized as photocatalysts in various studies focused on photocatalytic applications, particularly in the treatment of water and wastewater.
The difference in the band structure between semiconductor, insulator, and metal is shown in Fig 1.5 and is explained as follows [100], [101]
In typical conductors like metals, the valence band (VB) and conduction band (CB) either overlap in energy or are partially filled with valence electrons The VB arises from the interaction of the highest occupied molecular orbital (HOMO), while the CB is formed from the lowest unoccupied molecular orbital (LUMO) The energy difference between the highest point of the VB and the lowest point of the CB is known as the forbidden band gap, or energy gap (Eg), in solid-state physics, and is referred to as the HOMO/LUMO gap in chemistry.
When valence electrons completely fill one or more energy bands while leaving others vacant, the crystal behaves as an insulator at absolute zero (0K) At elevated temperatures, its conductivity can vary, potentially classifying it as either an insulator or a semiconductor, depending on the size of the energy gap between the filled and unfilled bands A larger band gap typically indicates insulating properties.
20 than 3 eV will commonly be regarded as an insulator An external electric field will not cause current flow in an insulator
Semiconductors share a similar band structure with insulators but feature a smaller band gap (Eg < 3 eV) In their ground state, without photo-excitation, electrons remain localized in the valence band (VB) However, when exposed to light, these electrons gain enough energy to surpass the band gap barrier and transition to the conduction band (CB).
Table 1.3 Oxidation potential (Eox) of various oxidizing agents at pH = 0.106
Oxidizing agent Eox/V vs NHE
EXPERIMENTAL
Synthesis of catalysts
Table 2.1 outlines all the chemicals utilized for synthesizing the catalyst in this thesis The reagents employed were used as received, without any additional purification, and all aqueous solutions were prepared using doubly distilled water.
Table 2.1 List of chemicals used in this work
Order Name of chemical Origin
2.1.2 Synthesis procedure a Synthesis procedure of strontium titanate (STO)
STO was synthesized using the polymeric citrate precursor method, beginning with the dissolution of 0.568 g of titanium isopropoxide in 5 mL of ethylene glycol at room temperature for 30 minutes During continuous stirring, 4.201 g of citric acid was added and mixed until fully dissolved Subsequently, 0.422 g of strontium nitrate was incorporated into the solution in a slow manner until it became transparent The mixture was then maintained at 70 °C for 2 hours to ensure complete dissolution of all reagents before further heating.
The reaction mixture was heated at 120 °C for 5 hours to facilitate polymerization, resulting in a transparent brown resin after solvent evaporation The subsequent calcination of the resulting powders at 700 °C for 6 hours produced pure STO powder, which exhibited a white color.
Figure 2.1 Synthesis procedure of STO by polymeric citrate precursor method b Synthesis of Ag/STO nanocomposites
Metallic silver (Ag) and strontium titanate (STO) composites were created using a room-temperature liquid-phase deposition method Initially, 0.5 g of STO was ultrasonically dispersed in 30 mL of deionized water Following this, an appropriate quantity of silver nitrate (AgNO3) was incorporated into the mixture and stirred for 15 minutes Finally, 5 mL of glycol was added dropwise while maintaining continuous stirring of the solution.
The products were obtained through centrifugation after a 60-minute reaction, followed by washing with deionized water and ethanol three times They were then dried at 60°C for six hours For clarity, the products are labeled as STO 0.5, STO 1.0, STO 1.5, STO 2.0, STO 3.0, and STO 5.0, based on the nominal weight ratio of silver (Ag) to strontium titanate (STO).
Dissolved in 5 mL of ethylene glycol dissolved in 5 mL of ethylene glycol
Adding 4.201 g of citric acid into the mixture
Kept the mixture at 70 o C for 2h
Raising temperature to 120 o C for polymerization
Figure 2.2 Synthesis procedure of Ag/STO nanocomposites.
Catalyst characterization method
X-ray diffraction (XRD) uses X-rays to investigate and quantify the crystalline nature of materials by measuring the diffraction of X-rays from the planes of atoms within the material It is sensitive to both the type of and relative position of atoms in the material as well as the length scale over which the crystalline order persists It can, therefore, be used to measure the crystalline content of materials; identify the crystalline phases present (including the quantification of mixtures in favorable cases); determine the spacing between lattice planes and the length scales over which they persist, and to study preferential ordering and epitaxial growth of crystallites In essence, it probes length scales from approximately sub angstroms to a few nm and is sensitive to ordering over tens of nanometers
When X-rays collide with an atom, they interact with the electron cloud, resulting in scattering in multiple directions The diagram illustrates that the wave scattered by the lower plane has traveled a greater distance compared to the parallel wave.
Dispersing into 30 ml of distilled water
Adding the appropriate amount of AgNO3
Continuing to stir for another 1h
Centrifugated, washed with distilled water and ethanol three times and then dried at 60 o C overnight
Figure 2.2 Synthesis procedure of Ag/STO nanocomposites
25 scattered by the upper plane Depending on the angle and the difference in distance traveled, these waves will sum to give either constructive or destructive interference
Figure 2.3 Constructive and destructive interferences when X-rays interact with crystals
X-rays can constructively interfere at specific angles to a crystal plane, resulting in a diffracted beam that exhibits significantly higher intensity compared to other angles This phenomenon is governed by Bragg's Law.
In X-ray diffraction, where n represents an integer, λ denotes the wavelength of the X-rays (specifically 1.54Å for a copper tube source), d indicates the spacing between planes in the atomic lattice of the sample, and θ signifies the diffraction angle measured in degrees.
Figure 2.4 Illustration for lattice plane in crystal
Bragg's law, derived through basic geometry, identifies the angle at which X-rays are in phase to create constructive interference To analyze plane spacing effectively, a single wavelength is essential, which is why laboratory instruments are typically monochromated The X-ray diffraction (XRD) patterns were obtained using a STADI-P X-ray diffractometer (STOE) with monochromatic Cu-Kα radiation (λ).
The Scanning Electron Microscope (SEM) utilizes a high-energy electron beam produced by an electron gun, which is then manipulated by magnetic lenses to focus on the specimen's surface This beam is systematically scanned across the surface, allowing for detailed analysis Unlike light microscopes, SEMs do not create real images of the sample, providing a unique method for observing fine surface structures.
The SEM image is created by illuminating the sample in a rectangular scanning pattern, where each point's signal strength reflects the sample's topographical or compositional differences This process involves a synchronized scan of the viewing screen, ensuring a one-to-one correspondence between the specimen points and the image display By reducing the scanned area on the specimen, higher magnification is achieved, allowing for more detailed observation.
To create contrast in an image, it is essential to measure the signal intensity from the interaction between the beam and the specimen across its surface The electron detector collects signals generated by the specimen, which are then converted into photons through a scintillator These photons are amplified by a photomultiplier and transformed into electrical signals, ultimately modulating the image intensity displayed on the viewing screen.
The major components of an SEM are the electron gun, the vacuum system, the water chilling system, the column, the specimen chamber, the detectors, and the imaging system
The morphology of synthesized titania, including pristine STO and Ag/STO nanocomposites, was analyzed using scanning electron microscopy (SEM) with a Hitachi S4800 field emission scanning electron microscope (FESEM) at an accelerating voltage of 5 kV.
Figure 2.5 SEM layout and function
Historically, nitrogen gas (N2) at liquid nitrogen temperature (77 K/-196.15°C) was the preferred choice for gas adsorption studies for the structural characterization of materials
In the 1985 IUPAC recommendations, physisorption isotherms were grouped into six types
Figure 2.6 Classification of physisorption isotherms
In the last three decades, new types of isotherms have been identified, revealing a strong correlation with specific pore structures Consequently, it is necessary to refine the original IUPAC classifications of physisorption isotherms and their associated hysteresis loops The updated classification of physisorption isotherms is illustrated in Fig 2.6.
Reproducible and permanent hysteresis loops in multilayer physisorption isotherms are primarily linked to capillary condensation, which arises from adsorption metastability and network effects In cylindrical open-ended pores, delayed condensation occurs due to the metastability of the adsorbed multilayer, indicating that the adsorption branch of the hysteresis loop is not in thermodynamic equilibrium Unlike evaporation, which does not require nucleation, the desorption phase mirrors a reversible liquid-vapor transition Consequently, when the pores contain a liquid-like condensate, thermodynamic equilibrium is achieved during the desorption process.
Figure 2.7 Classification of hysteresis loops
The Brunauer–Emmett–Teller (BET) method remains the most popular technique for assessing the surface area of porous and finely-divided materials, despite its theoretical limitations Under specific controlled conditions, the BET area of nonporous, macroporous, or mesoporous solids—exhibiting well-defined Type II or Type IV(a) isotherms—can be considered the 'probe accessible area,' representing the effective area available for the adsorption of a specific adsorptive.
The BET method involves two key stages: first, transforming a physisorption isotherm into a BET plot to derive the BET monolayer capacity (nm); second, calculating the BET area (a(BET)) from nm by using an appropriate molecular cross-sectional area (σ).
It is customary to apply the BET equation in the linear form
Where n is the specific amount adsorbed at the relative pressure p/p° and nm is the specific monolayer capacity
Nitrogen adsorption measurements were conducted using an ASAP 2010 sorption system Prior to the measurements, samples underwent pretreatment involving heating and pumping under reduced pressure at approximately 150 °C The nitrogen adsorption process was executed at a temperature of 77 K.
X-ray Photoelectron Spectroscopy (XPS) or Electron Spectroscopy for Chemical Analysis (ESCA) is a technique that analyzes the elements constituting the sample surface, its composition, and chemical bonding state by irradiating x-rays on the sample surface and measuring the kinetic energy of the photoelectrons emitted from the sample surface XPS instrument using Al Kα rays can generally obtain information on elements within a few nm of the sample surface
Catalyst activity
Photocatalytic activities of as-prepared pristine STO and Ag/STO nanocomposites were evaluated by measuring the photodegradation of Rhodamine B (RhB)
The photocatalytic treatment of Rhodamine B (RhB) was conducted in a 250 mL batch reactor, where varying amounts of catalyst powder were suspended in the RhB aqueous solution The reactor utilized Philips solarium lamps (15W) to irradiate the solution from above, delivering a total power of 60W and an experimental radiation intensity of 3.2 mW/cm² The lamps primarily emitted visible light with wavelengths between 400 nm and 700 nm, along with some UVA light emissions.
Various quantities of catalyst powder were mixed with 100 ml of aqueous RhB solution, continuously stirred in the dark for 30 minutes to establish an adsorption/desorption equilibrium of RhB on the catalyst surface.
At specific time intervals, approximately 3 mL of the suspension was extracted under irradiation and centrifuged to eliminate the photocatalyst for UV-vis absorption spectrum analysis using a Shimadzu UV-2550 spectrophotometer The concentration of Rhodamine B (RhB) was assessed by examining its characteristic absorption Subsequently, the filtered photocatalyst was washed three times with distilled water and ethanol in preparation for X-ray diffraction (XRD) measurement.
All experiments were conducted three times, and the mean values were calculated following standard deviation.
Density functional calculations
Density Functional Theory (DFT) calculations were conducted using a plane-wave basis set with a kinetic energy cutoff of 520 eV, employing the projector-augmented wave (PAW) method in the Vienna ab-initio simulation program (VASP) for total energy and molecular dynamics simulations.
The study conducted at the Faculty of Physics, University of Vienna, utilized a Hubbard correction of U = 4.36 eV within the PBE+U exchange correlation functional to address self-interaction errors in Ti atoms Spin-polarized calculations incorporated Grimme's D3 method to account for long-range dispersion interactions The research focused on the oxygen-terminated (1×3) STO(110) surface, recognized as the most stable exposed surface of SrTiO3 (STO), consisting of eight layers—four of TiO2 and four of SrO A k-point grid density of 2×6×1 was employed to sample the Brillouin zone for all STO system calculations To investigate the formation of active oxidizing agents on the Ag/STO catalyst, various models were analyzed, including Ag overlayer films and nanowires on the STO(110) surface, as well as clean STO(110), pure Ag(100), and pure Ag(111) surfaces.
To assess the affinity of various oxidizing agents produced by the Ag/STO catalyst, including O*, O2, OH*, OOH*, and H2O, we present their binding energies as the formation energy of the reaction outlined in equation (2.1).
4) 𝑂 2 (𝑔) → 𝑂 𝑥 𝐻 𝑦 ∗ (2.1) wherein *, H2O(g), and O2(g) are on the clean surface; H2O and O2 in gas phases, respectively Furthermore, to understand the electronic properties of Ag/STO interfacial site and the chemical of those interfacial Ag sites at different proximity from the substrate, we computed the adhesion energies (Eadh) and charge density difference () for the Ag(100) overlayer films on STO (denoted as Agn/STO) with different Ag thicknesses (one, two, three and four overlayers) using the equations (2.2) and (2.3) below The Bader charges of all atoms in those structures were also computed using the approach by Henkelman et al
The total energy of silver (Ag) over-layers supported on strontium titanate (STO) is represented by the equation \( E_{Ag} - n \cdot E_{STO} \), where \( E_{STO} \) denotes the total energy of clean STO, and \( E_{Ag \, gas} \) indicates the total energy of a single Ag atom in the gas phase.
Where 𝜌 𝐴𝑔 𝑛 ⁄ 𝑆𝑇𝑂 , 𝜌 𝑆𝑇𝑂 and 𝜌 𝐴𝑔 𝑛 are the charge densities of the Ag overlayers supported on STO, clean STO, and Ag n cluster, respectively
This work is in collaboration with the research group of Dr Trinh Quang Thang, Institute of High-Performance Computing (IHPC), A*STAR (Agency for Science, Technology and Research, Singapore
CHAPTER 3: RESULTS AND DISCUSSION 3.1 Catalyst characterization
The Ag/STO composite was synthesized with varying silver (Ag) dosages of 0.5 wt%, 1.0 wt%, 1.5 wt%, 2.0 wt%, 3.0 wt%, and 5.0 wt% The X-ray diffraction (XRD) patterns of both the pristine strontium titanate (STO) and the Ag/STO nanocomposites at different Ag loading levels are illustrated in Fig 3.1.
Figure 3.1 XRD patterns of pristine STO and Ag/STO photocatalysts
The peaks located at 22.8°, 32.5°, 40.0°, 46.6°, 52.5°, 57.9°, 68.0°, and 77.4° can be well-indexed to the characteristic diffraction peaks of (100), (110), (111),
(200), (210), (211), (220) and (310) planes of cubic STO (JCPDS No 35- 0734)[102], suggesting that pure STO phase is formed and well crystallized after calcination of the STOprecursor
The diffraction peaks of pristine strontium titanate (STO) are consistently preserved in all silver (Ag)/STO nanocomposites, indicating that the phase structure of STO remains intact Notably, there is no shift in the positions of the dominant diffraction peaks following Ag loading, suggesting that Ag does not enter the STO crystal lattice as a dopant.
The lattice constant and unit cell volume values are calculated and presented in Table 3.1 The structure of STO is identified as a cubic perovskite, characterized by equal lattice parameters (a = b = c) and angles (α = β = γ = 90°) The interplanar spacing for the cubic unit cell is defined by a specific equation.
The interplanar spacing (d) in a cubic unit cell can be determined using Bragg's law, expressed as nλ = 2dsinθ, where λ represents the wavelength of the incident X-rays at 1.5406 Å, θ is the peak position in radians, and n denotes the order of diffraction, typically set to 1 The Miller indices h, k, and l are essential for identifying crystal planes, while the lattice constant (a) is a crucial parameter in defining the cubic structure.
The volume of the cubic unit cell is calculated as V = a 3 (Å 3 )
The lattice parameters of pristine strontium titanate (STO) and silver (Ag)/STO nanocomposites are nearly identical, as the silver is deposited on the surface of the STO rather than substituting the lattice atoms.
The lattice parameter and unit cell volume of the synthesized strontium titanate (STO) closely align with the standard crystal structure, exhibiting axial lengths of a = b = c = 3.905 Å in its cubic form.
Table 3.1 Lattice parameters of pristine STO and Ag/STO photocatalysts
To analyze the morphological and structural features of the synthesized STO and STO1.0 products, FESEM characterizations were conducted The SEM images reveal that the STO sample exhibits a dense structure composed of spongy agglomerates, likely resulting from significant gas release during the preparation process.
During the calcination step, the surface of STO exhibits a rough texture characterized by a well-organized arrangement of worm-shaped particles The FESEM images of STO 1.0 reveal a structure that closely resembles that of the STO sample.
Figure 3.2 SEM images of STO (a), STO 1.0 (b)
The compositional information from the EDX spectrum of pristine STO (Fig 3.3a) shows the presence of Sr, Ti, O, C, Cu elements where the peaks for C and
The EDX measurement reveals the presence of Cu in the STO sample, while the EDX spectrum of STO 1.0 shows the elements Sr, Ti, and O, along with the detection of silver, which is absent in the STO sample The molar ratios of Sr, Ti, and O in the STO sample are 1:1 and 35:3.28, respectively, whereas in the STO 1.0 sample, the ratios are 1:1.30:2.78 Both ratios are relatively close to the stoichiometric ratio of SrTiO3, which is 1:1:3.
Table 3.2 Elemental compositions of the sample STO and STO 1.0 (atomic percentages)
Figure 3.3 EDX spectrum of STO (a) b a
Figure 3.3 EDX spectrum of STO 1.0 (b)
Nitrogen sorption experiments were conducted to determine the BET surface areas of pristine STO and STO 1.0, revealing that all isotherms exhibit an H3 type hysteresis loop, indicative of capillary condensation within the mesopore network formed by packed nanoparticles The specific surface areas of the samples, as shown in Table 3.3, are relatively small, aligning with the dense structures observed in the SEM micrographs of these photocatalysts Notably, the surface area of the composite remains largely unchanged compared to that of pristine STO.
Figure 3.4 Nitrogen adsorption-desorption isotherms of pristine STO and STO 1.0 b
Table 3.3 Summary of the BET surface areas, pore volumes, and pore sizes of the photocatalysts
Sample Surface area (m 2 /g) Pore volume (cm 3 /g) Avarage pore size (nm)