MISNISTRY OF EDUCATION AND TRAINING PHENIKAA UNIVERSITTY DINH THI KIM HUE SYNTHESIS AND PHOTOCATALYTIC PROPERTIES OF BLACK TiO2 NANOMATERIALS Major: MATERIALS SCIENCE Code: 8440122 MASTE
Purposes of research
The purposes of this research are threefold First, we aim to synthesize black TiO2 nanomaterials by combining a hydrothermal method with one or more treatment steps in different gases, i.e., NH3 and H2 Second, we target to achieve insights into the evolution of morphology, crystalline and electronic structures, composition, and optical properties of the materials upon the treatments in different gases at different temperatures Finally, we expect to understand the correlation between the material properties and their photocatalytic performance under visible-light irradiation.
Subjects and scopes of research
This research focuses on the fabrication of black TiO2 nanomaterials and the study of their photocatalytic properties under the irradiation of visible light More specifically, a 1D TiO2 nanostructure, i.e., TiO2 nanorods (NRs), is selected as the starting point due to its potential application in many other environmental and energy-related applications In addition, the black TiO2 materials in this research are limited to those obtained by NH3 and H2 treatment, which could result in N-doped TiO2 or O-vacant TiO2
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Methodology of research
This research is conducted by employing both experimental and theoretical approaches, which are presented in detail in Chapter 2 of this thesis Briefly, the experimental aspects of this research include:
▪ Synthesis of black TiO2 nanomaterials by employing a hydrothermal method in combination with high temperature treatments in various atmospheres
▪ Characterizations of the material morphology, crystalline structure, elemental composition and optical properties by using advanced characterization techniques such as scanning electron microscopy (SEM), X-ray diffraction (XRD), energy- dispersive X-ray analysis (EDX) and X-ray photoelectron spectroscopy (XPS), and UV-Vis absorption spectra
▪ Investigation of the photocatalytic performance of the synthesized materials via the degradation of organic dye, i.e., methylene blue (MB), under the irradiation of visible light generated by a white-LED lamp
The theoretical aspects of this research include employing density-functional theory (DFT) modeling to investigate the electronic structures, and the correlation between the materials properties and their performance achieved by experimental study.
Significance of research
The experimental results of this research have drawn a complete picture of the evolution of the morphology, crystalline structures, optical properties, and visible-light photocatalytic performance of TiO2 NRs treated in NH3 (and partially in H2 gas) under various experimental conditions in a broad range of temperatures (i.e., 400 – 1100 C)
Our research focuses on the controlled production of TiO2 nanorods and their modifications, such as N-doped TiO2, TiON, and TiN, for diverse applications This sets our work apart from previous studies Furthermore, we use theoretical modeling to gain a deeper understanding of their electronic structure, opening up new avenues for material design.
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4 of N-doped TiO2 NRs in correspondence with their optical and photocatalytic properties
These include the findings on how the band structure of TiO2 changes due to the N doping with different concentrations and the role of N dopant atoms in the adsorption properties of the N-doped TiO2 nanomaterials.
Structure of thesis
The thesis is composed of three chapters:
▪ Chapter 1 – Introduction: This chapter gives an overview of the black TiO2 nanomaterials with the focus on their properties, fabrication, and applications in photocatalysis
▪ Chapter 2 – Methodology: This chapter describes the experimental procedures employed for the material synthesis and characterizations Besides, the DFT parameters are also presented for investigating the electronic structure of N-doped TiO2 NRs and the adsorption of methylene blue – a target pollutant on the surface of the photocatalyst
Chapter 3, "Results and Discussion," presents the main findings obtained from comprehensive material synthesis and characterization experiments, as well as DFT calculations The chapter interprets these results based on both the research team's own findings and relevant literature, providing valuable insights into the behavior and properties of the materials under investigation.
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INTRODUCTION
Wastewater treatment
With the rapid industrial development and population growth over several past decades, water pollution has become one the most serious global challenges In fact, while hundreds of billion cubic meters of wastewater are discharged each year all over the world, only 52% of this volume is treated before discharging into the environment in a friendly way [9] Untreated wastewater, which contains various pollutants, such as dyes, pharmaceutical compounds, heavy metals, and inorganic compounds, gives rise to harmful effects on both the environment and human health Thus, it is crucial to develop effective and eco-friendly methods for decomposing the pollutant and recycling wastewater Nowadays, there are several approaches for wastewater treatment applications, such as photocatalytic degradation, physical adsorption, membrane filtration, biological treatment, and chemical processes [10] Among these technologies, photocatalytic degradation using photocatalysts has emerged as a powerful technique to solve this challenge due to their ability to use sunlight as a sustainable and cost-effective energy source
Photocatalytic degradation using advanced photocatalysts, as a green process, has become popular in recent years In this process, the photocatalysts are irradiated by a light source generating photons with suitable wavelengths, which are commonly in the UV and visible regions Upon the absorption of the incident lights, charge carriers, i.e., electrons and holes, are generated inside the bulk of photocatalysts (reaction 1) These carriers then diffuse to the surface of the catalysts and participate in the oxidation and reduction reactions with other molecules (e.g., O2, H2O) to generate active species [11,12], such as hydroxyl radical (OH • ) and superoxide (O 2 •− ), as described by reactions 2 and 3 Furthermore, the generated superoxide (O 2 •− ) can absorb a hydrogen ion in the valence band to produce another type active radicals, i.e., HO 2 • (reaction 4) These HO 2 •
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6 radicals can be decomposed to form hydrogen peroxide (H2O2, reaction 5), which can be further decomposed to form OH • + OH − by absorbing photogenerated electrons in the conduction band (reaction 6) The generated active species then take part in the degradation of the organic pollutant molecules, as described by reaction 7
Organic pollutant + OH • /O 2 •− /HO 2 • /H 2 O 2 → CO 2 + H 2 O (7)
Several organic molecules can be directly decomposed by the absorption of photogenerated electrons and holes, as described by reactions 8 and 9, which are commonly known as the direct oxidation/reduction processes [12]:
It can be seen that the photocatalytic reactions taking place during the photodegradation of an organic pollutant are rather complicated and may involve many more reactions [12] Moreover, besides participating in the oxidation and reduction reactions, electrons and holes can recombine at the surface or within the bulk of the catalyst [11,12], which reduces the photocatalytic efficiency of catalyst This is a major issue of photocatalysis Developing photocatalysts that can both absorb sunlight efficiently and hinder the unwanted recombination of photogenerated carriers is one of the main topics in photocatalysis
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Titanium dioxide (TiO 2 ) nanomaterials
Among photocatalysts, titanium oxide (TiO2) stands out for its photocatalytic efficiency, stability, and non-toxicity It exists in three crystal structures: anatase, rutile, and brookite Anatase and rutile, with identical tetragonal structures but distinct atomic coordination, are the most common phases, exhibiting differences in chemical properties and ionization potentials Anatase has higher photocatalytic activity despite its wider bandgap, owing to its indirect bandgap that minimizes recombination However, mixtures of anatase and rutile in specific ratios can enhance performance due to improved charge transfer and reduced recombination Degussa P25 TiO2 nanopowder exemplifies this concept.
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Figure 1.1 Three main polymorphs of TiO2: a) Anatase, b) Rutile, and c) Brookite Ti and O atoms are denoted as the cyan and red balls, respectively [21]
TiO2 nanomaterials can be synthesized by various techniques, such as chemical vapor deposition (CVD), physical vapor deposition (PVD), solvothermal method, microwave- assisted technique, green method, and hydrothermal method [22] In CVD, a precursor, typically a metal-organic compound, is vaporized and transported onto a substrate, where it decomposes or reacts with other reactants to generate TiO2 nanomaterials Important factors in this method include precursor concentrations, decomposition and reaction temperatures, and the initial geometry of the substrate PVD techniques, such as sputtering [23], evaporation [24], and pulse laser deposition [25], involve the vaporization of a solid source of TiO2 and their subsequent deposition onto a substrate
Unlike chemical vapor deposition (CVD), physical vapor deposition (PVD) involves a process without any chemical reactions Both CVD and PVD techniques are categorized as dry methods and commonly used for TiO2 thin film deposition on planar substrates In contrast, solvothermal synthesis is a wet-chemistry technique that entails dissolving titanium precursors in a solvent, followed by heating under high pressure to create TiO2 This approach offers advantages such as ease of implementation and the flexibility to engineer diverse TiO2 nanostructures by adjusting experimental parameters, including precursor and solvent types, reaction time, and temperature.
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9 Among the aforementioned methods, hydrothermal is an effective and simple approach to synthesize different structures of TiO2 by simply controlling the experimental parameters [22] In this technique, a precursor is first dissolved in a solvent (commonly water) and then transferred to an autoclave Here, additives or surface-active reactants can be added to control the growth of materials Similar to solvothermal method, hydrothermal approaches enable the fabrication of numerous TiO2 nanostructures Besides, hydrothermal methods can provide additional advantages such as low temperature (i.e., typically below 200 C) as well as the use of eco-friendly solvents (i.e., mostly H2O) Crucial factors in this process include reaction temperature, reaction time, precursor concentration, pH of solution, and type of additives By adjusting these factors, TiO2 in various geometries can be obtained For instance, Kim et al synthesized mesoporous TiO2 by using titanium(IV) isopropoxide (TTIP) as a precursor and a non-ionic surfactant in an acidic environment for a hydrothermal process at 55 °C for 2h [26] Using TTIP precursor, however, in a surfactant-free solution in hydrothermal process at 140 °C for 3 h, Wategaonkar et al demonstrated that rutile TiO2 microflowers were grown [27] By another hydrothermal approach, Kwon et al synthesized rutile-anatase TiO2 nanobranches by heating titanium butoxide in an acidic environment at 150 °C for 20 h [28] These examples demonstrate the versatility of hydrothermal methods Importantly, hydrothermal methods have been mostly the method of choice for the synthesis of 1D TiO2 nanostructures, such as nanorods, nanobelts, nanotubes, and nanowires, which are widely employed for catalytic applications because of their high specific surface area and excellent charge transfer [29]
The former is beneficial for the adsorption and the subsequent degradation (i.e., in photocatalysis) of the targeted molecules, whereas the latter can reduce the recombination rate of photogenerated electron-hole pairs before they reach the catalyst surface
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Despite exceptional photocatalytic activity, TiO2 nanomaterials suffer from a wide bandgap that restricts their usefulness This limits their absorption to UV radiation, comprising only a small fraction of sunlight To overcome this, enhancing TiO2's visible-light absorption is crucial for efficient solar energy utilization in photocatalytic reactions.
Among various strategies that have been developed, doping is one of the most common and effective approaches [31] Generally, introducing other elements in the host lattice of TiO2 (e.g., either metals or non-metals) can create new energy levels within the bandgap of TiO2 This facilitates the electronic transitions under the excitation of photons with energies lower than the bandgap energy Consequently, the light absorption of TiO2 extends significantly into the visible region, leading to a reduction in its bandgap
Doping TiO2 with impurties can induce a color change, leading to "black TiO2" materials with non-white hues These black TiO2 materials exhibit superior visible-light photocatalytic performance debido to enhanced absorption and reduced recombination of charge carriers.Introducing defects into TiO2 through disorders, oxygen vacancies, and Ti3+ defects can significantly alter its properties and contribute to the color change Reducing gases, such as H2 and H2S, can also be used to synthesize black TiO2 by creating oxygen vacancies and Ti3+ defects, which enhance visible-light absorption.
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11 relatively high temperature is the most common approach for producing black TiO2 in this manner (Fig 1.2) [1,34]
Figure 1.2 Synthesis of black TiO2 by H2 treatment at different temperatures [34]
Nitrogen-doped titanium dioxide (N-doped TiO2) photocatalysts achieve significant absorption in the visible-light range and mitigate the recombination rate of electron-hole pairs, resulting in excellent catalytic activities Nitrogen doping introduces localized energy levels within the bandgap, enhancing absorbance, charge carrier separation, and migration These effects reduce the recombination rate and improve the photocatalytic performance of N-doped TiO2 materials.
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12 of N could change the surface chemistry of TiO2 [41], which can strongly influence the adsorption behavior of the pollutants
N-doped TiO2 nanomaterials can be synthesized by various methods using various nitrogen sources, as shown in Table 1.1 These methods can be classified into two groups, i.e., wet- and dry-chemistry approaches The wet-chemistry approaches include hydrothermal, hydrolysis, and solvothermal techniques, in which titanium chloride, titanium alkoxides, and titanium sulfate are often used as the Ti precursors, whereas ammonia, urea, thiourea, nitric acid, and amines are used as the N sources These methods provide several advantages, such as the low temperature synthesis, the ability to produce TiO2 with various nanostructures, and the capability of controlling the concentration and the incorporation of N atoms in the host material Nevertheless, the wet chemical approaches also have several disadvantages, including time-consuming procedures, the necessity of post-treatment processes, and environmental concerns associated with the use of liquid precursors and solvents
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Table 1.1 Nitrogen sources for N-doped TiO2
N Nitrogen source Synthesis Structure Application Ref
1 CH3COONH4 Sol-Gel/Calcination Particle Phenol degradation [42]
2 IPAN Solvothermal/Calcination Microsphere RhB degradation [43]
3 NH4OH Hydrothermal Hollow Spheres Catalysis [44]
4 NH4OH Wet chemical exfoliation Nanospheres H2 production [45]
5 NH4F Anodization/Calcination Nanotubes Removal Of DP [46]
6 NH3/TBAOH Wet chemical exfoliation Nanosheets Water oxidation [47]
7 NH3 Anodization/Annealing Nanotubes Cathodic protection [48]
8 NH3 Hydrothermal/ Annealing Nanotubes Water oxidation [49]
11 PVP Calcination Nanofibers MB degradation [51]
12 TiN Electrochemical/Anodization Nanotubes MB degradation [52]
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14 Compared to the wet-chemistry approaches, the dry techniques, such as CVD, sputtering, and implantation, are only associated with the solid or gas precursors (e.g., NH3, N2O) and high temperatures [53–56] This approach allows for the precise control of N impurity levels by tuning the concentration of the precursor and the reaction temperature Additionally, dry techniques eliminate the necessity of liquid precursors and solvents, reducing environmental concerns and simplifying the synthesis process In dry methods, N-doped TiO2 catalysts are commonly synthesized by using TiN or TiO2 as starting materials For instance, N-doped TiO2 can be realized by oxidizing TiN in air in the range temperature of 300 – 900 °C [53] In this manner, the N doping concentration can be controlled by varying the oxidation time and the annealing temperature [53]
Otherwise, N-doped TiO2 can be achieved by NH3 treatment of TiO2 This process allows for not only tailoring the concentration of N dopant in broad range but also applying for various TiO2 nanostructures Hence, this method is widely used for synthesizing N- doped TiO2 nanomaterials [54,57]In general, the incorporation of N atoms into the TiO2 lattice starts from 400 to 600 °C For example, Irie et al demonstrated that the NH3 treatment of TiO2 powder at 550, 575 and 600 °C resulted in the substitution of N atoms at the O sites [6] Wang et al reported that the presence of N atoms are detected in TiO2 nanobelts via the NH3 treatment at different temperatures ranging from 525 to 600 °C [38] N-doped TiO2 could even be achieved by annealing the commercial P25 TiO2 in the NH3 treatment at 400 °C [54] It has been reported that the N doping concentration increases monotonically with increasing the treatment temperature or treatment time and can be achieved in a broad range, i.e., from less than 1% to several tens of percent [6,38]
Hydrogen doping has also been demonstrated as an effective method to enhance catalytic activity of TiO2 Besides causing the reduction effects as mentioned above (i.e., causing surface disorder, creating oxygen vacancies and Ti 3+ ions), the hydrogen
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15 treatment can also result in the doping effect, which generates Ti–H and Ti–OH bonding states that can affect the photocatalytic efficiency of the material [1] H-doped TiO2 can be achieved by different treatment methods The first approach is hydrogen treatment in which TiO2 is annealed in H2 atmosphere at high temperatures This method, similar to N-doped TiO2 obtained by NH3 treatment, allows for controlling the optical properties through the treatment temperature and time For example, Wei et al synthesized H- doped TiO2 nanotubes with a thin disordered layer on the surface by H2 treatment in the temperature range of 450 – 600 °C for 2 h [58] Zhang et al produced various H-doped TiO2 membranes by varying the treatment time (i.e., 5 h, 15 h, 30 h) while maintaining the treatment temperature at 600 °C [59] Another approach involves hydrogen treatment at a very high pressure For example, Lu et al produced black TiO2 from Degussa P25 at a high pressure of 35 bar at room temperature [60] The synthesized materials turned black after 20 days under these conditions The slow process was due to the low temperature process Furthermore, H2 plasma treatment can also be applied to form H- doped TiO2, as demonstrated for the synthesis of black TiO2 nanotubes by Teng et al
1.2.1.3 N and H co-doped TiO 2 nanomaterials
Co-doping of N and H into TiO2 has been investigated in several studies, which demonstrated a synergistic effect that can significantly improve the catalytic activity For example, Hoang et al prepared N- doped TiO2, H- doped TiO2, and H,N co-doped TiO2 for the water splitting process [49] The results showed that H,N co-doped TiO2 exhibited a superior efficiency over the others Similar results were reported by Li et al who synthesized N-doped TiO2, H- doped TiO2, and N,H co-doped TiO2 by annealing in NH3 and H2 atmospheres 600 °C [62] The enhancement in the activity of materials after the H2 treatment was explained by that the addition of H atoms could stabilize the localized energy levels created by N defects by forming N–H bonding states [63] This provides
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16 another feasible route for the fabrication of black TiO2 nanomaterials with a further enhancement of the photocatalytic activity by dry-chemistry approach.
Summary
The above short review indicates that NH3 and H2 treatment of TiO2 may bring interesting effects to the photocatalytic properties of TiO2 nanomaterials This explains why research on these nanomaterials remains an attractive topic Nevertheless, although the synthesis of N-doped TiO2 nanomaterials by NH3 treatment has been extensively studied, there are still debates regarding the electronic structure, physical properties, the mechanisms of the doping as well as the nitridation of TiO2 that may take place at high temperatures For the electronic structure of N-doped TiO2, several studies reported that the presence of N in the TiO2 lattice induced a considerable bandgap reduction of TiO2
[6,64], while in other studies, the N dopant only generated deep energy levels in the bandgap [38,65] For the nitridation of TiO2, several studies demonstrated that complete nitridation could be achieved between 800 and 900 °C [66,67], whereas others suggested that a considerably higher temperature was required [68] To this end, a systematic study is essential to achieve more insights into how the materials evolve during the treatment
In addition, most studies have focused on other structures of TiO2, whereas studies on TiO2 NRs remain a major gap These points formulate the motivations of our current research
The purposes of this research are threefold First, we aim to synthesize black TiO2 nanomaterials based on N-doped and H-doped TiO2 NRs by using a hydrothermal method in combination with the subsequent treatments in NH3 and H2 Second, we target to achieve insights into the evolution of morphology, crystalline and electronic structure, composition, and optical properties of the materials during the treatments at different temperatures Finally, by combining experiments with density-functional theory (DFT) calculations, we expect to obtain a deeper understanding of the correlation between the
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17 material properties and their photocatalytic performance, which will be tested by the photodegradation of methylene blue (MB, C16H18ClN3S), a model pollutant in wastewater, under visible-light irradiation
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METHODOLOGY
Materials and equipment
Chemicals Raw materials for the synthesis of TiO2 nanorods were purchased and used as is, which included commercial anatase TiO2 powder (99% purity, Merck), NaOH powder (Merck), and HCl solution (37%, Merck)
Equipment The treatments were performed in a home-built CVD system, which allowed high temperature annealing up to 1300 °C The treatments used diluted H2 (5 vol.% in Ar) and NH3 (98 vol.%) gases, whereas N2 (99.999 vol.%) was used as the purging gas Other equipment included autoclaves for hydrothermal synthesis, LED lamp (30 W) as the light source for photocatalytic test, solar simulator and thermal camera (testo 875-1) for photothermal conversion study.
Synthesis of materials
TiO2 NRs were synthesized by a hydrothermal method (Fig 2.1) Commercial anatase TiO2 nanopowder was used as the starting material For each experiment, 2 g of the material were added into 50 mL of 10 M NaOH aqueous solution The mixture was heated to 70 °C and continuously stirred for 30 min In the following step, the solution was transferred into a Teflon autoclave contained in a stainless-steel shield and maintained at 180 °C for 14 h After that, the autoclave was cooled down to room temperature The resulting solid was collected and washed in deionized water 3 times, and then in a HCl solution (0.01 M, pH = 5) for 2 h Later, the solid was washed again in deionized water until the pH of the solution reached 7 The final solid product was collected and annealed in air at 80 °C for 6 h Copies for internal use only in Phenikaa University
Figure 2.1: Process flow of the synthesis of TiO2 NRs by hydrothermal method
2.1.2 Modification of TiO 2 NRs by NH 3 and H 2 treatments at high temperatures
The NH3 and H2 treatments of the TiO2 NRs were conducted at different temperatures using a home-built reactor, which allowed for controlling the temperature, pressure, and flow rate of the gases Process flow of the synthesis of black TiO2 NRs by NH3 and H2 treatments at high temperatures are shown in Fig 2.2 First, for each experiment, 500 mg of the as-synthesized TiO2 NRs were loaded into a ceramic boat and placed horizontally in the reactor Before heating, the reactor was evacuated to a base pressure of 2 × 10 –2 mbar using a mechanical pump Then, a constant N2 flow of 500 sccm was introduced into the reactor, while keeping a constant pump speed In the next step, the reactor was heated with a heating rate of 20 °C/min When the treatment temperature was reached, the reactor was evacuated again to the pressure of 2 × 10 –2 mbar Thereafter, a flow of 200 sccm of the reducing gases (NH3 or H2) was introduced into the reactor
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20 with different treatment time intervals After this treatment step, the reactor was cooled down to room temperature under a constant N2 flow of 500 sccm
Figure 2.2: Process flow of the synthesis of black TiO2 NRs by NH3 and H2 treatments at high temperatures.
Materials characterization
The surface morphology of the synthesized materials was characterized by field- emission scanning electron microscopy (FE-SEM) using a using a HITACHI S-4800 (Institute of Materials Science, Vietnam Academy of Science and Technology)
The crystalline structure of synthesized materials was investigated by X-ray diffraction (XRD) technique, using a PANalytical X’pert Pro diffractometer (Hanoi
University of Science and Technology) equipped with a Cu-Kα X-ray source (λ = 1.5418 Å) The spectra were acquired in the 2θ range of 20 – 80°
The elemental compositions were measured by energy-dispersive X-ray analysis (EDX) using a HORIBA 7593-H detector (Institute of Materials Science, Vietnam Academy of Science and Technology) The measurements were performed with an electron beam energy of 20 keV, a scanning time in the range of 5 – 15 min, and a working distance of 15 mm X-ray photoelectron spectroscopy (XPS) was employed to
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21 investigate the elemental composition of the samples that have low N concentrations using a Thermo Scientific K-Alpha X-ray photoelectron spectrometer (KU Leuven)
UV-Vis absorption spectra were measured using a Shimadzu 2600 UV-visible spectrometer (Institute of Materials Science, Vietnam Academy of Science and Technology) The optical bandgap energy of samples was calculated from obtained UV- Vis absorption spectra by using the Tauc method, which can be expressed by the following equation [69]:
(αℎ𝜐) 𝑛 = 𝐴 ∗ (ℎ𝜐 − 𝐸 𝑔 ), where α is absorption coefficient (cm -1 ); ℎ is the Planck constant (J.s), 𝜐 is the photon frequency (Hz), E g is the band gap energy (eV), and A is a constant The n factor is either 2 or 1/2 for the direct or indirect bandgap, respectively
Density-functional theory (DFT) simulations were conducted to investigate the electronic structure of N-doped TiO2 NRs and to study the adsorption behavior of methylene blue molecules on the N-doped TiO2 surface The simulations were performed using the high-performance computing systems at Phenikaa University
2.2.2.1 Test for convergency of the input DFT calculation parameters
Two crucial input parameters for setting up the simulations are meticulously tested: the energy cutoff (Ecut) for the plane wave expansion and the supercell size The selection of these parameters ensures the convergence of the valence band maximum (VBM) energy for the pristine TiO2 models To determine the appropriate Ecut value, the primitive cell for both TiO2 anatase and rutile structures are utilized The Brillouin-zone integration is performed using a 6×6×6 k-point grid sampling Regarding the supercell size, different sizes for both TiO2 anatase and rutile structures are investigated, while employing a 6×6×6 k-point grid sampling for the Brillouin-zone integration The value
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22 of Ecut is chosen based on the convergence observed from the first test To optimize computational resources, the tests are conducted using the semi-local exchange- correlation functional
2.2.2.2 The defect formation energy of different defect states
To determine the relative stability among the four different N-doped systems, the defect formation energy, 𝐸 𝑓 , were calculated using the following definition:
2𝐸 𝑂 2 ] where, 𝐸 𝑝𝑟𝑖𝑠 and 𝐸 𝑠𝑦𝑠 represent the total energy of the pristine TiO2 and the N-doped
TiO2 system, respectively 𝐸 𝑁 2 and 𝐸 𝑂 2 denote the total energy of nitrogen and oxygen molecules, respectively 𝑛 𝑁 and 𝑛 𝑂 represent the number of doped N and substituted O atoms in the systems
2.2.2.3 DFT simulations for MB adsorption on the surface
The model of MB adsorption on the TiO2(101) surfaces with and without N-doped defect and on the TiN(200) surface is built by using the supercells composed of a 5-layer symmetric slab geometry together with a 15 Å vacuum layer The size of vacuum layer is found to be sufficient to ensure negligible coupling between periodic replicas of the slab The molecule is absorbed on one side of the slab, and the lower three layers are fixed upon geometrical optimization The MB molecule is considered in the form of the Methylthioninium molecule, i.e., without the presence of a chloride atom to ensure the convergence of the calculations The calculations for the total energy are done using the DFT method using the GGA-PBE exchange-correlation functional
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Photocatalytic tests
The visible-light photocatalytic activity of the materials was investigated by the degradation of MB For each test, 10 mg of the material was mixed with 80 mL of the MB solution (concentration of 5 ppm) in a glass beaker The suspension was sonicated in an ultrasonic bath for 30 min and then stirred continuously by a magnetic stirrer for 30 min (both in the dark) to reach an adsorption-desorption equilibrium Then, the suspension was irradiated by visible light produced by a white-LED lamp (30 W) After every 1 h of irradiation, 5 ml of MB were taken out The solid catalysts were filtered out by using a syringe filter The concentration of MB was determined from the UV-Vis absorption spectra measured by a JENWAY 6850 UV–vis spectrometer (Phenikaa University)
To determine the concentration of the MB solution, a standard curve for MB solution was first constructed For this purpose, the absorbance of a series of MB solutions with known concentrations in the range of 1 – 10 ppm was measured using the UV–vis spectrometer The measured absorbance was then plotted as a function of the MB concentration A linear regression of the plotted data points was applied, which served as the standard curve (Fig 2.3) After that, the concentration of any MB solution from 1 to 10 ppm was determined using this standard curve
The MB adsorption was calculated by the following equation:
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24 where A is MB adsorption efficiency (%), C 0 and C a (ppm) are the initial concentration and the concentration after the adsorption process, respectively
The MB photodegradation of synthesized materials is calculated through the following equation:
𝐶𝑎 × 100% where P is MB photodegradation efficiency (%), C a and C t (ppm) are concentrations after the adsorption process and at the time t during the irradiation process, respectively
Figure 2.3: Standard curve for MB concentration
2.3.2 Solar-to-steam vapor generation
To estimate the solar steam generation of the synthesized materials, an evaluation system was utilized, resembling the system in study of Chen et al [70] The system of solar simulator, model 94023A solar sim, at Phenikaa University was employed with a
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25 light intensity of 0.6 sun (1 sun is 1000 W/m 2 ), as shown in Fig 2.4a The temperatures of the evaporator were captured by a testo 875-1 infrared camera In this work, 20 mg of materials were dispersed uniformly on filtration sheet (Fig 2.4b) The sheet was then sandwiched between two styrofoam layers, which floated in water and helped reduce the heat dissipation While the bottom styrofoam also functioned as a water transport medium, the top one contained a go-through hole with a surface area of 1 cm 2 This enabled the exposure of the filtration sheet containing the investigated materials (Fig
2.4c) The sandwiched filtration sheet was put in a Petri dish containing water and kept in the dark for 10 min before the exposure to simulated sunlight for 20 min The mass changes of water were recorded in real-time by an analytical balance during the evaporation
Figure 2.4 a) Photograph of solar-to-team generation system at Phenikaa University, b) the filtration sheet containing 20 mg of material, (c) the Petri dish containing water covered with a styrofoam, (d) a part of filtration sheet containing material after covered by another styrofoam.
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RESULTS AND DISCUSSION
Synthesis of black TiO 2 NRs by NH 3 treatment
3.1.1 Morphology of NH 3 -treated TiO 2 NRs
The morphological characteristics of the TiO2 NRs before and after the NH3 treatment at different temperatures are shown in Fig 3.1 The samples, denoted as N400 to N1100, correspond to treatment temperatures ranging from 400 to 1100 °C The SEM image in Fig 3.1a indicates that the as-synthesized TiO2 has a rod shape with a smooth surface
The diameters of the NRs are in the range of 200 – 400 nm and lengths of a few micrometers As shown in Fig 3.1b–d, the samples treated in NH3 at temperatures in the range of 400 – 600 °C remain unaffected in both the dimensions and the surface morphology Nevertheless, the color of these samples, as indicated by the photographs on the right side of the SEM images, changes gradually from white to gray and dark gray This observed trend is indicative of the incorporation of N atoms into the TiO2 lattice [71] The NH3 treatments at 700 and 800 °C significantly affect both the morphology and the color of TiO2 NRs, as demonstrated in Fig 3.1e–f First, the solid TiO2 NRs are transformed into a porous hollow structure with a rough surface, as shown clearly in high-resolution SEM images (Fig S1) in the Appendix Section This finding is consistent with the previous study of Zhao et al [72], which reported pore formation and surface roughening in NH3-treated TiO2 NRs at 800 °C to 900 °C due to the liberation of H2O vapor and N2 gas originated from the nitridation reaction The nitridation of the TiO2 NRs occurs rapidly in this temperature range, as demonstrated in the following parts Hence the pore formation and the transformation from the solid NRs into the hollow structure are ascribed to the generation of the gaseous species and a structural reconstruction due to the incorporation of N atoms Second, the color of these samples turns black, which is the typical color of TiN-based materials At 900 °C, the shrinkage of NRs takes place considerably, which is attributed to the mass loss due to
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27 the conversion of TiO2 to TiN [73] The treatment at 1000 °C breaks the NRs into nanoparticles, which is in line with the observation indicated by Gong et al for NH3- treated TiO2 nanotubes [74] The NH3 treatment at 1100 °C leads to an increasing particle size, which accounts for the sintering effect of the nanoparticles at high temperatures [68] Moreover, the color of the NH3-treated TiO2 NRs at 1000 °C and 1100 °C gradually changes from black to brownish-black
Figure 3.1 presents SEM images of TiO2 nanorods (NRs) as-synthesized and after treatment in NH3 for 2 hours at varying temperatures The images depict morphological changes with temperature, along with the corresponding color changes of the powders observed on the right side of each image.
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3.1.2 Crystalline structure of NH 3 -treated TiO 2 NRs
Fig 3.2 presents the XRD patterns of the NH3-treated TiO2 NRs with increasing temperatures The as-prepared TiO2 NRs, because of its poor crystallinity, shows only two peaks at around 25° and 48.5°, which correspond to the (101) and (200) atomic planes of anatase TiO2 (JCPDS card no 21-1272) In addition, no diffraction peak of the rutile phase is detected However, a peak originating from the diffraction on the (210) planes of rutile phase (JCPDS card no 21-1276) is observed at around 44° in NH3-treated TiO2 at 400 °C (N400) and 500 °C (N500) samples Additionally, the (110) peak of the rutile phase emerges at around 29° after the NH3 treatment at 550 °C (N550) In this sample, an additional peak at around 33° appears, which does not correspond to the anatase and rutile phase of TiO2 This peak could be ascribed to the brookite phase of TiO2 (JCPDS card no 29-1360) [75]
The XRD pattern of NH3-treated at 600 °C (N600) presents two peaks at around 53° and 55°, which correspond to the (105) and (211) planes of anatase phase, respectively
In this sample, additional peaks (the closed circles), not referring to the anatase or the rutile phase, are observed that could arise from the brookite phase or a transient phase due to the doping of N into TiO2 lattice After the NH3 treatment at 700 °C (N700), the presence of the TiN phase is confirmed by the emergence of three peaks at around 37.5°, 43.5°, and 62.5° These peaks originate from the diffraction on the (111), (200), and (220) atomic planes, respectively, of the face-centered cubic (FCC) structure of TiN (JCPDS card no 87-0633) [67,76] At this temperature, several peaks of the anatase TiO2 are still observed, while all the peaks of the rutile TiO2 no longer exist This observation indicates that the rutile phase is less stable than the anatase phase under the NH3 treatment At 800 °C (N800), only the diffraction peaks of the FCC TiN are noticed whereas those of TiO2 disappear This demonstrates that the conversion from anatase TiO2 to FCC TiN take place rapidly from 700 to 800 °C, which is in an agreement with
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29 the findings reported by Chen et al [77] It is important to note that a slight shift toward the smaller 2θ side of diffraction peaks of TiN is observed when the treatment temperature rises from 700 to 1100 °C This is due to the residual O in the material, which rapidly drops with increasing temperature Moreover, the full width at half maximum (FWHM) value of the peaks decreases, whereas their intensities increase remarkably at higher temperatures This indicates the crystal growth of TiN by reason of the treatment at high temperatures
Figure 3.2 XRD patterns of the as-prepared TiO2 NRs and the NRs after the treatment in NH3 at different temperatures in the range of 400 – 1100 °C
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3.1.3 Elemental compositions of NH 3 -treated TiO 2 NRs
Fig 3.3 shows the atomic concentrations of Ti, O, and N elements of the TiO2 NRs treated in NH3 at different temperatures, which are identified by EDX mapping analysis
Upon increasing the temperature from 400 to 600 °C, TiO2 reduction via NH3 leads to oxygen vacancies and the absence of nitrogen incorporation Above 600 °C, nitrogen doping of TiO2 occurs alongside rapid nitridation from 700 to 900 °C While EDX mapping reveals incomplete nitridation in N800, N900, and N1000 samples due to the presence of oxygen, complete nitridation is achieved at 1100 °C.
In contrast to the results obtained by the EDX mapping analysis, the XPS data identified the appearance of N atoms in TiO2 NRs treated in NH3 at 500 and 600 °C, as shown in Fig 3.3b–d In Fig 3.3b, the binding energies are calibrated using the C−C peak position of C 1s to 284.8 eV to eliminate the influence of charging effect on peak shift [78] The data show that the NH3-treated TiO2 at 500 °C contains N, as evidenced by a broad peak at approximately 395.4 eV of the N 1s region The low intensity of the peak demonstrates a small amount of N element in the sample Moreover, the asymmetry of the peak suggests the various bonding states of N atoms For more clearly, N atoms
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31 in TiO2 lattice may exist in the form of Ti–N and N–O bonds, which have different binding energies of N 1s [7] The N 1s peak is more intense for N600, confirming a higher nitrogen content in the material It is important to note that the existence of two different defective states of N (i.e., NO and Ni) in TiO2 results in two distinct XPS peaks
Figure 3.3 (a) Atomic concentrations of Ti, O and N in the as-synthesized TiO2 NRs and the NRs after the NH3 treatment at different temperatures analyzed by EDX mapping; core-level XPS spectra of (b) C 1s, (c) N 1s and (d) Ti 2p of the as-synthesized TiO2 NRs and the NRs treated in NH3 at 500 (N500) and 600 °C (N600)
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32 of the N 1s spectrum at approximately 396 eV and 400 eV [77,79] The former stems from the substitutional NO states, while the latter originates from the interstitial Ni states
Hence, the substitutional NO states are predominant in our work by reason of absence of the Ni peak in the N 1s spectra in Fig 3.3c The N 1s XPS spectra of N500 and N600 confirm the doping of N element into TiO2 lattice within the treatment in NH3 at the temperatures of 500 and 600 °C This is consistent with the results reported in the previous studies, which demonstrated that the NH3 treatment within the temperature range of 400 – 600 °C resulted in the successful incorporation of N atoms into the TiO2 material [38,54] The absence of the N element in the EDX mapping data could be due to the limit of detection of the measurements (i.e., ~2% for N and 0.5 % for O)
Nevertheless, the data obtained by EDX mapping analysis could provide a qualitative description about the transformation of the elemental compositions in the NH3-treated TiO2 NRs at high temperatures
The presence of N in TiO2 NRs in the NH3 treatment at 500 and 600 °C is also indicated by the Ti 2p spectrum in Fig 3d Initially, in the Ti 2p spectrum of the as- synthesized TiO2, two peaks at 464.2 and 458.5 eV are clearly shown, representing the doublet states, i.e., Ti 2p1/2 and Ti 2p3/2, respectively, originating from the spin-orbit coupling effect of the Ti 2p electrons [38] In the Ti 2p peaks of N500 sample, a slight shift towards the lower binding energy side is observed, which is in line with the observation reported by Hoang et al [49] and Wang et al.[38] This shift arises from the difference in electronegativity between N and O More specifically, owing to the smaller electronegativity of N (i.e., 3.04) in comparison to O (i.e., 3.44), as N is doped in TiO2, a partial replacement of the O–Ti–O by the O–Ti–N bond results in the greater negative charge density on the shell levels of the Ti atom This diminishes the action of the Ti nucleus on the 2p core-level electrons, therefore reducing its binding energy in comparison to the original O–Ti–O bonds of TiO2 Moreover, this substitution of N also causes the broadening of the two Ti 2p peaks with the rise of the shoulders on the lower
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33 binding energy side [7], which is indicated in the N500 spectrum in Fig 3.3d Both the peak shift and the peak broadening are more noticeable for the Ti 2p spectrum of N600 because of the higher N content incorporated in the material, which is evidenced by the N 1s spectrum in Fig 3.3c Besides the N doping effect, the generation of oxygen vacancies and the formation of non-stoichiometric TiO2 due to the reduction of TiO2 in NH3 can also contribute to the shift and the broadening of Ti 2p peaks in Fig 3.3d [35,39]
3.1.4 UV-Vis optical absorption properties of NH 3 -treated TiO 2 NRs
Fig 3.4a presents the UV-Vis absorption spectra of NH3-treated TiO2 at various temperatures The as-synthesized TiO2 exhibits solely an absorption edge in the ultraviolet region at around 390 nm, with optical bandgap of 3.13 eV determined by the Tauc method [69], as shown in Fig 3.4b This calculated bandgap is slightly smaller in comparison to the regular bandgap values reported for anatase TiO2 [80], which could be attributed to the presence of defects in the as-prepared TiO2 NRs In fact, the defects in TiO2, including surface disorder or point defects (e.g., O vacancies and Ti interstitials), could give rise to a narrowing in the bandgap of TiO2 [81–83] In contrast to the only absorption in UV region of as-prepared TiO2, the NH3-treated TiO2 NRs in the temperature range of 400 – 550 °C (i.e., N400, N500, and N550) display a significant redshift of the absorption edge to wavelength range of 550 – 700 nm This shift could be explained by an absorption of photons with energies smaller than the bandgap that stem from the formation of localized energy levels above the valence band maximum of TiO2 when N is doped in the lattice [35,38] The presence of these energy levels is demonstrated by the DFT calculations discussed in the next part Moreover, the UV–Vis absorption spectra of N400, N500 and N550 also exhibit a pronounced Urbach tail that covers the entire visible and near-infrared regions The optical bandgap of N400, N500, and N500 obtained by Tauc method is 2.34, 2.41, and 2.22 eV, respectively, as shown in
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34 Fig 3.4b and Table 3.1 These results demonstrate the considerable bandgap narrowing of TiO2 by the N doping
Table 3.1 : Bandgap of the as-synthesized TiO2 NRs and the N-doped TiO2 NRs
Sample Treatment conditions Bandgap (eV)
Synthesis of black TiO 2 NRs by H 2 and NH 3 /H 2 treatment
In addition to NH3 treatment, H2 treatment enhances TiO2 photocatalytic activity by introducing H dopant and reducing TiO2 H doping alone can significantly alter the material's properties Furthermore, simultaneous N and H doping synergistically enhances the catalytic performance of TiO2 [62,63] This study investigates the effects of H2 treatment on both pristine and N-doped TiO2 nanorods to determine its impact on their catalytic activity.
3.2.1 Morphology, crystalline structure and UV-Vis absorption spectra of the H 2 - treated TiO 2 NRs
The as-synthesized TiO2 NRs was treated in diluted H2 (5 vol.% in Ar) at 700 C for 2 h For comparison, the treatment of the TiO2 NRs in N2 under identical experimental conditions was also carried out The SEM images, XRD patterns and UV-Vis absorption spectra of the TiO2 NRs treated in two different atmospheres (i.e., H2 and N2) are shown in Fig 3.8 The results show that no distinct difference in morphology and crystalline structure of the NRs treated in these gases is observed However, compared to the NRs treated in NH3 at the same temperature, the change in morphology is noticeable (Fig 3.8 a and b, and Fig 3.1e): no porous structure and smoother surface are found for the NRs treated in N2 and H2 In addition, the XRD patterns of the NRs treated in N2 and H2 show a higher degree of crystallinity with only the anatase phase of TiO2 Notably, in contrast to the dark color obtained for the NH3-treated NRs at the same temperature (N700 sample, Fig 3.1e), the color of the NRs treated in N2 and H2 remains virtually unchanged compared to the as-synthesized NRs, which is similar to the effect observed by Hoang et al while studying the H2 treatment of TiO2 nanowires [49] Importantly, the treatments in N2 and H2 do not cause any significant effect on the optical absorption behavior of the TiO2 NRs, as shown in Fig 3.8d Nevertheless, a small increase in the visible-light absorption is observed, indicated by a small shoulder in the wavelength range of 400 – 500 nm, as shown in the figure on the right side of Fig 3.8d Accordingly, a slightly higher absorption is achieved for the H2-treated TiO2 NRs This could arise from the formation of the Ti 3+ centers on the surface of the NRs, which is a consequence of the reduction of TiO2 by H2 [59,89] The minor effect observed in Fig 3.8d suggests that a longer treatment time or a higher concentration of H2 could be needed to achieve a higher enhancement of the visible light absorption However, despite the small effect achieved, the H2 treatment could slightly enhance the visible-light photocatalytic activity of the TiO2 NRs, which will be discussed in the Section 3.3.2
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Figure 3.8 SEM images of TiO2 NRs treated in (a) N2 and (b) diluted H2 at 700 C for 2 h (c) XRD patterns and (b) UV-Vis absorption spectra of the N2- and H2-treated TiO2
(d) displays a magnified view of the spectral zone outlined by the dotted rectangle in (d) For comparative purposes, (c) and (d) include the XRD patterns and UV-Vis absorption spectra of TiO2 in its as-synthesized and NH3-treated forms.
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3.2.2 Morphology, crystalline structure and UV-Vis absorption spectra of the NH 3 - treated TiO 2 NRs followed by H 2 treatment
Fig 3.9 shows the SEM images of the TiO2 NRs after the NH3 treatment at different temperatures in the range of 400 – 600 C, followed by the H2 treatment at 500 C for 2 h By comparing the SEM images in Fig 3.9 with those without the H2 treatment step, it is found that the H2 treatment does not influence the morphology of TiO2 NRs
Figure 3.9 SEM images of TiO2 NRs after the treatment in NH3 at different temperatures for 2 h: (a) 400 °C, (b) 500 °C, (c) 550 °C, and (d) 600 °C, followed by the H2 treatment at 500 °C for 2 h The color of the powders before and after the H2 treatment is shown by the photographs on the right side of the SEM images
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45 Nevertheless, the H2 treatment clearly alters the color of the samples, as demonstrated by the photographs shown on the right side of the SEM images, in which the arrows represent the change from before to after the H2 treatment Obviously, the color becomes lighter (or less dark) after the H2 treatment, and this change is more pronounced for the higher treatment temperatures This color change could be attributed to Ti 3+ centers or the presence of the H in the N-doped TiO2, which could generate the H interstitial defects [1,15,63] Nevertheless, this does not rule out the possibility of the partial removal of N due to the reduction of H2
Fig 3.10a presents the XRD patterns of the TiO2 NRs treated in NH3 at different temperatures (400 – 600 C) for 2h, and subsequently treated in H2 at 500 C for 2 h In comparison with the XRD patterns of the NH3-treated TiO2 NRs shown in Fig 3.2 at the same temperature, it can be concluded that the treatment in H2 does not cause any additional effect on the crystalline structure of the material Furthermore, when comparing the XRD patterns of the TiO2 NRs treated separately in three different environments (NH3, N2, and H2) at different temperatures (500 and 700 °C), it is observed that the XRD patterns of the materials treated in N2 and H2 show only the peaks of the anatase phase of TiO2, whereas the rutile phase of TiO2 is found in the XRD patterns of the NRs treated in NH3 at 500 °C This suggests that the treatment in NH3 could promote the growth of the rutile phase Nevertheless, further studies are needed to clarify this observation
H2 treatment has a notable impact on the optical absorption characteristics of NH3-treated NRs despite not affecting their crystalline structure As illustrated in Fig 3.10b, the H2-treated samples exhibit altered optical absorption compared to their as-synthesized counterparts This effect is evident when observing the UV-Vis absorption spectra of TiO2 and the N400, N500, N550, and N600 materials before H2 treatment.
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46 treatment of the N-doped TiO2 NRs causes the opposite effect to the treatment in NH3 and the visible-light absorption of the N-doped TiO2 NRs is considerably reduced (the arrows in the figure show the change of the absorption curve from before to after the H2 treatment) Consequently, the bandgap reduction of the N-doped TiO2 NRs is lessened after H2 treatment, as shown in Table 3.3 The plots in Fig 3.10b also reveal that the effect of H2 treatment is more pronounced for the NRs treated in NH3 at higher temperatures, which is equivalent to the higher N-doping concentrations Therefore, we speculate that the change in the optical absorption of the N-doped TiO2 NRs due to the H2 treatment could be related to the removal of N that was incorporated into the TiO2 during the NH3 treatment This could explain the color change of the powder after the
Figure 3.10 (a) XRD patterns and (b) UV-Vis absorption spectra of the TiO2 NRs treated in NH3 followed by the H2 treatment In (a), the XRD pattern of the as- synthesized TiO2 is added as the reference; in (b), the absorption spectra of the as- synthesized TiO2 and the TiO2 treated in NH3 without the H2 treatment step are also added for comparison The arrows in (b) show the change in the absorption behavior after the H2 treatment
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treated TiO 2 NRs followed by H 2 treatment
Photocatalytic properties of black TiO 2 NRs
3.3.1 Adsorption and degradation of MB by NH 3 -treated TiO 2 NRs at various temperatures
Fig 3.11 shows the MB adsorption and photodegradation efficiency of the TiO2 NRs before and after the NH3 treatment at different temperatures for 2 h In terms of adsorption, the as-synthesized TiO2 NRs exhibit a high adsorption efficiency, which can reach 49% of the MB in the solution after 1 h This is approximately two times higher than that of commercial Degussa P25 TiO2 (i.e., 26%) under identical adsorption conditions – 10 mg of material dispersed in an 80 ml of 5 ppm MB solution The NH3
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48 treatment initially induces a considerable drop of the adsorption, resulting in only 23 – 25 % of the MB adsorbed by the TiO2 NRs treated at 400 and 500 °C This could be ascribed to a modification in surface states caused by the N dopant and the surface reduction due to the NH3 treatment Thereafter, with increasing the treatment temperature, the MB adsorption rises rapidly and reaches the highest value of 67% for the NH3-treated TiO2 NRs at 800 °C Eventually, the NH3 treatments at higher temperatures (i.e., 1000 and 1100 °C) leads to a significant drop in the adsorption The excellent adsorption efficiency obtained for N800 may be attributed to its porous structure, as shown in Fig 3.1f, which offers a larger surface area for the adsorption of MB The high adsorption efficiency of this material implies a great potential for dye adsorption applications
Figure 3.11 Adsorption and photodegradation of MB under visible-light irradiation by
TiO2 NRs before and after the NH3 treatment at different temperatures
To investigate the origin of the increasing adsorption of MB, DFT simulations of MB are conducted on various surfaces, which correspond to the pristine anatase TiO2(101),
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49 the anatase with a single N doping defect TiO2(101), and the TiN(200) material The binding energy (Eb) of MB molecules to the surface is calculated as:
Eb = Etot – (Esurf + Emol) (3.1) where Etot, Esurf, and Emol represent the total energies of the molecule-surface, surface, and isolated MB molecule, respectively The results reveal that MB on the TiO2 (101) surface with an N defect requires the highest value of Eb (i.e., 1.89 eV), which is approximately two times higher than that on the pristine TiO2(101) Particularly, the smallest binding energy (i.e., 0.48 eV) matches from MB on the TiN (200) surface In addition, it is found that the most energetically favorable N dopant in TiO2 structure is associated with N substitutional defects at the sub-surface layer Further analysis of charge density differences indicates that the presence of the N atoms enhances the transfer of electrical charge between the MB molecule and the surface of N-doped TiO2 in comparison to the pristine TiO2(101), which give raise to the increasing adsorption efficiency of MB in the low temperature from 400 – 800 °C However, because of the lower electronegativity of N in comparison with O element, the N–H bond is weaker than the O–H bond, leading to the lower adsorption of the sample as the complete conversion to TiN takes place at temperatures over 800 °C
Regarding photocatalytic efficiency, Fig 3.11 shows the decomposed efficiency of MB caused by the catalysts under the irradiation of the LED light after subtracting the self-degradation of MB (i.e., degradation under irradiation without any catalyst) The as- synthesized TiO2 NRs show low photocatalytic activity under the visible-light irradiation, which causes the decomposition of only less than 6% of the MB This is because it only absorbs light in the UV region, as evidenced by the UV-Vis absorption spectrum in Fig 3.4 The treatment in NH3 enables the synthesized catalysts with a significantly higher activity, resulting in a higher degradation of MB Particularly, 33% of the MB is decomposed by the N400 after 5 h of illumination, whereas the N500 sample
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50 shows the greatest activity with the MB photodegradation of 41% This significantly higher photodegradation is obviously attributed to the visible-light absorption of these materials enabled by the N doping effect, as shown in Fig 3.4a However, the N550 photocatalyst exhibits a slightly lower photodegradation efficiency (i.e., 38%) of the MB, despite their stronger visible-light absorption Remarkably, only 2% of the MB is decomposed by the N600 photocatalyst Interestingly, similar observations were reported in several previous studies of photocatalytic activity in NH3-treated TiO2 nanomaterials For instance, Irie et al investigated the efficiency in the CO2 photocatalytic conversion of N-doped TiO2 nanopowders, which were achieved by NH3 treatment in the temperatures of 550, 575 and 600 C [6] It was found that the catalyst obtained at 550 C exhibited the highest performance Similarly, Wang et al found that among the N-doped TiO2 catalysts achieved by the NH3 treatment in the temperature range of 525 – 600 C, the catalyst treated at 550 C provided the highest activity [38]
It should be noted that the treatment of TiO2 at the higher temperatures results in not only the higher N dopant concentration but also the higher density of oxygen vacancies, which can act as the recombination centers for holes and electrons [6] This consequently leads to the lower photocatalytic performance of the N550 and N600 photocatalysts The reported observations together with our results emphasize that besides the dependence on the visible-light absorption capability, the photocatalytic activity of the materials is heavily governed by the N doping concentration as well as the oxygen vacancy density in the materials The virtually zero visible-light photoactivity of the materials obtained by the NH3 treatment at the temperatures in the range of 700 – 1100 °C is another evidence of these influences
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3.3.2 Adsorption and degradation of MB by TiO 2 NRs treated in H 2 and NH 3 /H 2 atmospheres
Fig 3.12a presents the adsorption and photodegradation of MB under visible-light irradiation by TiO2 NRs treated in N2 and H2 atmospheres at 700 °C for 2 h, denoted as N2-700 and H2-700, respectively For comparison, the activity of the as-synthesized TiO2
NRs is also added The results show that the MB adsorption efficiency reaches only 17% and 26% for H2-700 and N2-700, respectively, which are significantly lower than the one for the as-synthesized TiO2 NRs (49%) and the N700 (57%) We speculate that this significant difference in the MB adsorption capability of the materials arises from the difference in the surface morphology and especially the defect density, which is expected to be much higher for the as-synthesized TiO2 NRs and the N700 The smoother surface indicated by SEM images in Fig 3.8a–b and the better crystallinity indicated by the XRD patterns in Fig 3.8c are appreciable support for our speculation Nevertheless, this requires further studies to verify Despite its significantly lower adsorption capability, the H2-700 photocatalyst exhibits a noticeable visible-light photocatalytic activity, which is not the case for the N2-700 photocatalyst This could originate from a slightly higher absorption in the wavelength range of 400 – 500 nm of the H2-700 photocatalyst as shown in Fig 3.8d
Fig 3.12b shows the adsorption and photodegradation efficiency of MB under visible-light irradiation by TiO2 NRs treated in NH3 in the temperature range of 400 – 600 °C for 2 h, followed by the H2 treatment at 500 °C for 2 h Here, the adsorption and photodegradation efficiencies caused by the TiO2 treated in NH3 without the H2 treatment step are also added for comparison The dotted arrows in the figure show the change in these efficiencies of the materials from before to after the H2 treatment The results clearly show the effect of the H2 treatment on both the adsorption and the photocatalytic activity of the N-doped TiO2 NRs On the one hand, after the H2 treatment,
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52 the adsorption capability of the materials significantly decreases This decrease can be explained by the reducing role of N doping atoms in the adsorption of MB molecules
As shown by DFT calculations, the N atoms enhance the charge transfer between MB molecules and the surface of N-doped TiO2, which enhances the adsorption of MB The H2 treatment could result in the formation of N–H bonds, as reported by Li et al [62] and Parmar et al [63], which subsequently can suppress the influence of N on the charge transfer phenomenon
Figure 3.12 Adsorption and photodegradation of MB under visible-light irradiation by
TiO2 NRs treated in a) N2 and diluted H2 at 700 °C for 2 h, b) NH3 in the temperature range of 400 – 600 °C for 2 h, followed by the H2 treatment at 500 °C for 2 h In a), the adsorption and photodegradation of MB by the as-synthesized TiO2 is added as the reference In b), the adsorption and photodegradation of MB by the TiO2 treated in NH3 without the H2 treatment step are also added for comparison The dotted arrows in (b) show the change in MB adsorption and photodegradation efficiency of the materials after the H2 treatment
On the other hand, the H2 treatment does not cause any considerable effect on the photocatalytic activity of the N-doped TiO2 obtained by NH3 treatment in the temperature range of 400 – 550 °C However, the effect is particularly pronounced for
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53 the N,H600, which shows a significant increase in the decomposition of MB (i.e., ~19%) in comparison with the photocatalyst treated only in NH3 (N600) The enhanced activity of N,H co-doped TiO2 NRs could stem from the presence of H in the host lattice
Co-doping with nitrogen (N) and hydrogen (H) enhances photocatalytic performance by stabilizing the N-doped structure through N–H bonding states This reduces charge recombination and increases the adsorption of small molecules, leading to the formation of more hydroxyl radicals and active oxygen, which are essential for decomposing organic pollutants Consequently, N and H co-doping has a beneficial effect on photocatalytic applications due to its ability to promote charge separation and enhance the generation of reactive species.
Photothermal conversion performance of TiN-based nanostructures achieved by
The NH3 treatment of TiO2 NRs at temperatures in the range of 700 – 1100 °C results in TiN-based nanostructures, which do not show any photocatalytic activity toward the photodegradation of MB under visible-light irradiation, as previously demonstrated
Despite strong light absorption across the visible spectrum, TiN-based materials exhibit promising photothermal properties This discovery opens avenues for exploring alternative applications beyond solar steam generation, given TiN's established efficacy in that domain.
Thus, in this work, the solar-to-steam generation for the as-synthesized TiO2 NRs and the NH3-treated TiO2 NRs in the temperature range of 700 – 1000 °C were estimated by employing the interfacial evaporation method as reported in study of Tran et al [92] In this technique, the solar light absorber (e.g., the NH3-treated TiO2) on a supporting substrate is placed on the surface of water (i.e., floating) Under simulated solar irradiation, the solar energy is converted into thermal energy, which vaporizes the water at the water-air interface to generate steam The supporting substrate is a self-floating
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To minimize heat loss to the external environment, an insulator layer is implemented The evaporation rate can be determined using the formula m* = mloss*t*A, where m* is the evaporation rate, mloss is the water mass loss over irradiation time t, and A is the projected area Additionally, an evaporation rate-light intensity ratio (H) is calculated using a specific equation.
𝐼 (3.3) where unit of H is [kg.h -1 (kW) -1 ], m * is the evaporation rate (kgm –2 h –1 ), I is the projected light intensity (kW.m -2 ) The higher the H factor, the better the evaporation rate generation of sample under the same light intensity Thus, the material providing the highest H factor is the optimal solar absorber
Fig 3.13 shows the surface temperature distribution of the as-synthesized TiO2 NRs, N1000 and N1100 before and after a continuous irradiation of simulated solar light at the power density of 0.6 sun for 20 min captured by an IR camera For as-synthesized TiO2 NRs, the surface temperature only increases by 8.6 °C (i.e., from 22.2 to 30.8 °C)
At the same time, N1000 shows a temperature increase of approximately two times higher (i.e., 16.2 °C), whereas N1100 exhibits an increase of 14.8 °C These results indicate the significantly higher photothermal conversion efficiency of the NH3-treated TiO2 NRs at high temperature The enhanced photothermal conversion is attributed to increased absorption in the UV-Vis region and the plasmonic effect observed in TiN- based materials, as demonstrated by their UV-Vis spectra (Fig 3.4) in the Section 3.1.4
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Figure 3.13 The surface temperature distribution measured for the as-synthesized TiO2
NRs, N1000, and N1100 before (a, c, and e, respectively) and after (b, d, and f, respectively) the irradiation by the simulated solar light of 0.6 sun for 20 min
Figure 3.14 presents the evaporation rate of the materials after irradiation at a power density of 0.6 suns for 20 minutes Notably, the as-synthesized TiO2 NRs exhibit an exceptional evaporation rate of 0.983 kgm-2h-1 This rate significantly surpasses those obtained for other materials evaluated under similar conditions.
NH3-treated TiO2 materials are significantly higher, which increases gradually with treated temperature and reaches the highest value of 1.401 kgm –2 h –1 for the N1000 sample This indicates the higher photothermal conversion efficiency of the TiN-based materials, as presented in the previous part Compared to N1000, the slightly lower rate achieved for the N1100 sample, i.e., 1.359 kgm –2 h –1 , despite its more pronounced plasmonic effect, as demonstrated by the UV-Vis spectra (Fig 3.4) in the Section 3.1.4
This could arise from the difference in particle size and composition On the one hand, the N1100 sample exhibits a slightly larger particle size, as shown in their SEM images (Fig 3.1i–h), which could potentially decrease the photothermal conversion efficiency, similar to the effect reported by Sakamoto et al.[93] On the other hand, as indicated by the EDX data shown in Fig 3.3a, the N1000 sample contains a significant amount of O, which could also contribute to the photothermal properties of the TiN Nevertheless, as presented in Table 3.4, the achieved evaporation rates and the H factor caused by our
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TiN-based materials possess competitive or superior catalytic performance compared to TiN and other reported materials Notably, NH3-treated TiO2 nanorods exhibit exceptional catalytic rates, indicating their promising potential for solar-to-steam energy conversion applications These findings highlight the significant advantages of NH3 treatment in enhancing the photocatalytic activity of TiO2 nanorods and open up new avenues for efficient solar energy utilization.
Figure 3.14 Evaporation rate of as-synthesized TiO2 NRs and NH3-treated TiO2 NRs at high temperatures
Table 3.4 Solar water evaporation performance of various photothermal materials reported in the literature
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The hydrothermal method was combined with high-temperature treatments under NH3 and H2 reducing atmospheres to synthesize black TiO2 nanorods The combination of experimental techniques and DFT modeling revealed the evolution of morphology, structures, composition, and properties of TiO2 nanorods upon treatment at temperatures ranging from 400 to 1100 °C.
1) TiO2 NRs with diameters in the range of 200 – 400 nm and lengths of a few micrometers are successfully synthesized by the hydrothermal method using TiO2 nanoparticles as the starting materials The as-synthesized TiO2 NRs are mostly amorphous and contain a high defect density The bandgap of the TiO2 NRs is found to be 3.13 eV, which only allows for optical absorption in the UV range As a consequence, the TiO2 NRs do not exhibit any significant photocatalytic activity under visible light irradiation However, due to the high defect density, this material exhibits excellent adsorption of MB molecules
2) N-doped TiO2 NRs with low N dopant concentrations are achieved via NH3 treatment in the temperature range of 400 – 600 °C This treatment enhances material crystallization with both the formation of rutile and anatase phases of TiO2 materials
The experimental data and the theoretical computations indicate that the formation of N substitutional defects (NO) is more energetically favorable than other defects, leading to a significant bandgap reduction These N-doped TiO2 materials extend their absorption into the visible-infrared spectrum with an optical bandgap in the range of 2.22 – 2.41 eV, thereby enabling visible-light photocatalytic activity Their photocatalytic activity depends not only on visible-light absorption but also on N doping concentration and O vacancy density
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58 3) NH3 treatment in the temperature range of 600 – 1100 °C causes nitridation of TiO2