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Synthesis, characterization and electronic structure modulation of perovskite oxides for photocatalytic applications

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Mohammad Qureshi, in the Department of Chemistry, Indian Institute of Technology Guwahati, Assam, India, for the award of the degree of Doctor of Philosophy. It verifies that the work described in this thesis entitled "Synthesis, Characterization and Modulation of the Electronic Structure of Perovskite Oxides for Photocatalytic Applications".

A brief overview of the synthesis and banding technique of photocatalysts/perovskite oxides and their application in photocatalytic activities is given. The structure-property correlation of the compounds is presented based on Rietveld refinement combined with the experimental data.

Rietveld refinements reveal that the parent compound LMO crystallizes in the rhombohedral phase, while with an increase in the doping concentration of ruthenium, the phase of the compounds changed from rhombohedral to cubic. The high alkaline condition under the hydrothermal condition leads to the formation of oxygen vacancies and lattice disorder in the compounds, confirmed by UV-visible diffuse reflectance spectroscopy (UV-Vis DRS), X-ray photoelectron spectroscopy (XPS) and electron spin resonance (ESR) analysis.

The synergistic effect of decreasing the band gap due to Ce doping and the presence of oxygen vacancies are the reasons for the increased photocatalytic efficiency of BaZr1−xCexO3 (x under UV-visible light irradiation. The synergistic effect of increasing light absorption together with higher Photogenerated charge carrier transfer efficiency in the presence of CDs and the presence of oxygen vacancies are the reasons behind the improved photocatalytic efficiency of x wt.

Introduction

  • AN OVERVIEW OF ENERGY DEMAND AND HYDROGEN PRODUCTION
  • GENERAL MECHANISM FOR SEMICONDUCTOR PHOTOCATALYSIS AND WATER SPLITTING
    • Photocatalytic H 2 or O 2 Evolution from Water Reduction or Water Oxidation
  • STRATEGIES TO DEVELOP EFFICIENT PHOTOCATALYSTS
    • Band Gap Engineering of a Material by Metal or Non-metal Ion Doping
    • Band Gap Engineering by Designing Semiconductor Alloy
    • Enhancing H 2 or O 2 Evolution by Introducing Cocatalysts
  • PEROVSKITE OXIDES

Therefore, the minimum required band gap of the photocatalyst should be ≥ 1.23 eV (λ ≤ 1000 nm) to generate H2 and O2 from overall water splitting. Therefore, to exploit the visible light of the solar spectrum, the band gap of the material must be less than 3 eV.

Oxygen

This lattice distortion has a direct impact on the physical and electronic structure of the material and can also influence the formation, transport and catalytic activity of the charge carrier. Therefore, the nature of the B-site element essentially determines the electronic properties of a perovskite oxide and thus determines the catalytic activity of the perovskite materials.

B” cation

Apart from the ideal cubic structure, perovskite oxides can exhibit certain degrees of distortion due to the different ionic radii and valence of the constituent elements and ion vacancies within their lattice, which can form different crystal phases. 62 – 65 Typically, in ABO3 the valence band is mainly derived from the 2p orbitals of the oxygen atom and the conduction band is derived from the d orbitals of B atom and the degree of hybridization of B‒O depends on the electronegativity of the B atom.

A” cation

  • PHOTOCATALYTIC DYE DEGRADATION
  • REFERENCES
  • INTRODUCTION
  • CHEMICAL DETAILS
  • SYNTHESIS DETAILS
    • Synthesis of La 1−x Sr x MnO 3 (x = 0.0 – 0.5)
    • Synthesis of Graphene Oxide (GO)
    • Synthesis of La 1−x Sr x MnO 3 (x = 0.0 – 0.5)/GO Composite
    • Synthesis of Carbon Dots (CDs)
    • Synthesis of Carbon Dots (CDs)_BaZrO 3 (BZO) Hybrid Nanomaterials
  • PHOTOCATALYTIC ACTIVITY EVALUATION
    • Photocatalytic Water Oxidation and Reduction
    • Photocatalytic Methyl Orange (MO) Dye Degradation
  • CHARACTERIZATION
  • APPARENT QUANTUM YIELD (AQY) CALCULATION
  • REFERENCES

Therefore, further improvement of the electronic structure of the perovskite oxides is in great demand. The lamp's emission profile gives the incident light intensity in the nm range.

Modulating the Electronic Structure of Lanthanum Manganite by Ruthenium

Oxidation

INTRODUCTION

In this process, the oxygen release reaction is one of the main bottlenecks that limits the overall catalytic efficiency of the catalyst.1,. 2 To alleviate this problem, remarkable progress has been reported in the recent past in the search for efficient photochemical catalysts for oxygen generation. 3 – 7 Among them, perovskite oxides have gained much interest in the fields of oxygen release reaction and oxygen reduction reaction since the report of Meadowcroft in 1970. 8 Perovskite oxides oxides have been widely used as efficient catalysts for hydrogen production, organic pollutant removal, automotive emission control, hydrocarbon oxidation, removal of CO, NOx, volatile organic compounds, etc.9 – 15 Among perovskite oxides, LaMnO3 (LMO) is one of the extensively investigated materials in the field of catalysis. 16 – 21 The catalytic activity of LaMnO3 is attributed to vacancies in the network or the presence of Mn(III)/Mn(IV) average valence of Mn in it. 22, 23 Oxygen stoichiometry also plays an important role in maintaining the occurrence of mixed valence states of Mn (Mn (III)/Mn (IV)) in LMO.24.

EXPERIMENTAL SECTION

RESULTS AND DISCUSSION

  • Powder X-ray Diffraction (PXRD) Patterns and Rietveld Refinement

The double peak of the highest intensity at around 32.5°, corresponding to (hkl) values ​​of (110) and (104), gradually merged into one peak with gradual Ru doping in LMO. This phenomenon clearly indicates the presence of modified crystallographic symmetry in Ru-doped LMO leading to the formation of a cubic phase, space group Pm3m (#221), as revealed by Rietveld refinement of the PXRD patterns (Figure 3.2).

2 (Degree)

X-ray Photoelectron Spectroscopy (XPS) Spectra

Whereas Figure 3.4 represents (A) deconvoluted Mn 2p3/2 peak, inset to (A) shows the Mn 2p core level spectra, (B) deconvoluted Mn 3s core level spectra, (C) deconvoluted O 1s core level spectra and (D) ) deconvoluted Ru 3p3/2- spectra of LaMn0.7Ru0.3O3. In Figure 3.4(D), the data show that the Ru 3p3/2 XPS peak could be deconvoluted into two peaks with B.E.

Fourier Transform Infrared (FTIR) Spectra

Such binding energy values ​​indicate that Ru exists in the Ru(IV) and Ru(V) oxidation states.41 From the above analysis of the XPS data it can be concluded that both Mn and Ru enter mixed valence states, Mn(III) /Mn( IV) and Ru (IV) /Ru (V). Ruthenium is more electronegative than that of manganese, so the doping of ruthenium can shift the Mn-O band to a higher wave number by forming a Ru-O bond.

Ultraviolet–visible Diffuse Reflectance Spectra and Band Gap Calculation

Wavenumber (cm -1 )

The investigated calculation is a semi-empirical method, which involves some uncertainty, and the band position obtained may be slightly inaccurate. Nevertheless, the calculated values ​​of the band gap are almost in agreement with the previously reported values ​​by T.Arima et.al.47.

Wavelength (nm)

Calculation of Band Position

The valence band position of an inorganic semiconductor can be calculated using the following formulas reported previously by Xu and Schoonen,52. Where EVB are valence band maxima (VBM), ECB are conduction band minima (CBM), e.g. is the estimated band gap of the semiconductor evaluated from the Tauc plot, Ee is the energy of free electrons on the hydrogen scale (- 4.5 eV), χ𝑀and χ𝑋 are the absolute electronegativities of M and X atoms respectively, χ is the electronegativity of ​​the individual atoms in the multiatomic semiconductor, calculated using equation (ii), 53 IP is the ionization potential and EA are the electron affinity values ​​for each atom.

Material Morphology and Elemental Analysis

To check for the homogeneous elemental distribution in the compounds, energy dispersive X-ray (EDX) spectroscopic mapping was performed.

Photocatalytic Water Oxidation and Dye Degradation

O RuMn

Before illumination, the catalyst in the dye solution was allowed to stir in the dark for 15 min to achieve adsorption–desorption equilibrium. The decrease in concentration of the dye solution due to adsorption before irradiation is plotted as −15 to 0 minutes in the time scale in Figure 3.11.

Time (Minute)

CONCLUSIONS

In summary, this work reports for the first time the synthesis of LaMn1−xRuxO3 (x) and its use as a photocatalyst for water oxidation and dye degradation. Changes in the band gap and various redox reactions that take place due to doping were studied.

PXRD Rietveld processing studies of the compounds confirm the reduction of the lattice distortion as well as the increase of the Mn‒O‒Mn bond angle to 180° with a progressive increase of the ruthenium doping, which facilitates the charge carrier mobility of the catalysts.

Doped Lanthanum Manganite /Graphene Oxide Composite for Efficient and Robust

INTRODUCTION

Graphene and its derivative, such as graphene oxide (GO) are better charge transport materials.11 Graphene oxide (GO) is a hydrophilic layered carbon-based material with different functional groups containing oxygen (hydroxyl , epoxy) in its basal plane as well as at the edge (carboxylic acid or carbonyl groups) of its lattice.12 – 17 Fully oxidized graphene oxide is an insulator, while incompletely oxidized graphene oxide is a semiconductor in nature and previous reports state that the band gap of graphene oxide is actually controlled by the aro (sp2) and aliphatic (sp3) domains in its structure.18. 20 Due to their high surface area, charge carrier separation and transport ability, GO and related graphene materials are widely used in photocatalytic systems.21 – 25.

EXPERIMENTAL SECTION

  • Synthesis of La 1−x Sr x MnO 3 (x = 0.0 – 0.5)
  • Synthesis of Graphene Oxide (GO)
  • Synthesis of La 1−x Sr x MnO 3 (x = 0.0 – 0.5)/GO Composite Photocatalyst

RESULTS AND DISCUSSION

  • Powder X-ray Diffraction (PXRD) Patterns

This could be explained by the formation of Mn (IV) ions from Mn (III) ions with Sr doping in LMO, as reported previously. 31, 32 The doublet of the highest intensity peak at around 32.5° gradually merged into one peak with progressive Sr doping in LMO. This change in peak symmetry with Sr doping clearly indicates the presence of a modified crystallographic symmetry in Sr-doped LMO, which may lead to the formation of cubic phase, space group Pm3m (no. 221).

2  (Degree)

  • X-ray Photoelectron Spectroscopy (XPS) Spectra
  • Ultraviolet–visible Diffuse Reflectance Spectra
  • Field Emission Transmission Electron Microscopic (FETEM) Image and Elemental Analysis
  • Room Temperature Resistivity Measurement
  • Photocatalytic Water Oxidation

However, it can be observed in Figure 4.1 that with gradual Sr doping, the PXRD diffraction patterns move towards higher diffraction angles. From the resistivity plot shown in Figure 4.8, it should be noted that the resistivity of LaMnO3 initially increases from 0.166  cm to 80  cm in La0.9Sr0.1MnO3 and then decreases with increasing Sr (A) doping.

Strontium (x)

Migration of holes to the surface reaction site of the photocatalyst depends on the transport phenomenon of the system. The resistance values ​​of the four probes of the lanthanum manganites show a minimum resistance of 0.118 Ω cm for x = 0.3 strontium doping.

O 2 Produced (mol/h/g)

  • CONCLUSIONS
  • REFERENCES

Progressive doping of strontium in lanthanum manganite causes the bond angle to change from 150° (x = 0.0) to 180° (x = 0.3) due to internal chemical pressure; which allows maximum conduction of electrons in the system. Strontium doping at the lanthanum site facilitates the formation of an equivalent amount of holes (Mn(IV)) by promoting the conduction pathway, which helps the formed charges to migrate to the surface of the reaction site to oxidize water.

Synergistic Effect of Cerium Doping and Oxygen Vacancies in Photocatalytic

Hydrogen Production Efficiency of BaZrO 3−δ

INTRODUCTION

Among several studied perovskite oxide photocatalysts, barium zirconate, BaZrO3 (BZO), a typical broad-bandgap cubic perovskite oxide, is a promising material with a wide range of technological applications in modern industries.1 ‒ 3 BZO has been studied widely due to its high conductivity and ability to accommodate a wide range of doping substances in it. To increase the photocatalytic activity of BZO material, a number of efforts have been made in recent years, such as Zou et al.

EXPERIMENTAL SECTION

RESULTS AND DISCUSSION

  • Powder X-ray Diffraction (PXRD) Patterns
  • Material Morphology and Elemental Analysis
  • Ultraviolet-visible Diffuse Reflectance Spectra and Band Gap Calculation
  • X-ray Photoelectron Spectroscopy (XPS) Spectra
  • Electron Spin Resonance (ESR) Spectra

By increasing the Ce doping concentration from x, the absorption onset of the doped compounds is slightly red-shifted. Anindya Sundar Patra Chapter 5 to a very low concentration of Ce in BaZr0.97Ce0.03O3, the shape of the spectra is not as pronounced as for pure CeO2.

Magnetic field (mT)

Therefore, it is important to understand the nature of the oxygen vacancies present in doped BZO. In order to verify the presence of oxygen vacancies in the fired photocatalysts, we performed ESR analysis for all compounds and observed a sharp peak at around the g-tensor value ~2.005, which corresponds to singly ionized paramagnetic oxygen vacancies (Figure 5.11).

Binding energy (eV)

Calculation of Band Position

Here EVB indicates the valence band maxima (VBM), ECB indicates the conduction band minima (CBM). E.g. is the band gap of the semiconductor calculated from the Tauc plot, Ee represents energy of free electrons on the hydrogen scale (– 4.5 eV), χM and χX are the absolute electronegativities of M and X atoms, respectively. The valence band maxima and conduction band minima calculated using equations (i) and (iii) are shown in Table 5.2.

Photocatalytic Hydrogen Production

From the O 1s XPS spectra and ESR analysis, we found that even after calcination, the catalysts possess certain amount of oxygen vacancies in their lattice. Therefore, the increase in photocatalytic efficiency in these catalysts after calcination may be due to the reduction of crystal defects and increase in crystallinity.

Hydrogen production (µmol/h/g)

CONCLUSIONS

The synthesized compounds produced hydrogen upon illumination without any cocatalysts, but in the presence of sacrificial donor molecules. Among all the synthesized compounds, BaZr0.97Ce0.03O3, BaZr1−xCexO3 (x) shows the highest efficiency in the production of hydrogen gas upon oxidation of the sacrificial donor.

In summary, barium zirconate and cerium doped barium zirconate hollow spheres have been successfully synthesized using a template-free low-temperature hydrothermal method. As shown by UV-Vis DRS,

Synergistic Effect of Upconversion Luminescent Carbon Dots and Oxygen

Efficiency of Stable BaZrO 3−δ Hollow Nanospheres

INTRODUCTION

It is known that carbon-based nanomaterials can pave an efficient way to direct the flow of photogenerated carriers due to their better electron acceptance and transport properties.11 So, the photocatalytic efficiency of the semiconductor could be increased by designing a carbon-based nanomaterial composite. The best performing hybrid photocatalyst was evaluated by the photocatalytic rate of H2 generation and the degradation efficiency of methylene blue (MB) dye.

EXPERIMENTAL SECTION

  • Preparation of Carbon Dots (CDs)
  • Preparation of BaZrO 3-δ (BZO)
  • Preparation of CDs_BZO Hybrid Nanomaterials

RESULTS AND DISCUSSION

  • Fourier Transform Infrared (FTIR) Spectrum
  • Raman Spectrum

In the hybrid compounds, we cannot find any peak of carbon dots, which may be due to a very low content of carbon dots in the hybrid compounds and very high crystallinity of BZO compared to CDs.19 We did not notice any PXRD peak shift of bare and hybrid compounds, confirming phase and the structural retention of BZO in the hybrid compounds. 101 The presence of hydroxyl, amines and carbonyl groups ensures high solubility of CDs in water.20 The presence of various groups such as sp2 carbon, secondary amines and carbonyl groups proves the formation of polyaromatic structures during the synthesis of CDs. 21.

Raman shift (cm -1 )

From the Raman spectra in Figure 6.3, we notice two distinct peaks at 1325 cm-1 and 1548 cm-1, corresponding to the D band and the G band, respectively. The D band arises due to out-of-plane stretching vibration of sp3 carbon atoms in the disordered states, whereas the G band arises from in-plane stretching vibration of sp2 carbon atoms inside the ordered aromatic region.22 Therefore, the D band corresponds to the amount of disordered states or defect sites and G band corresponds to the amount of ordered states inside CDs, a ratio of peak intensities between these two bands, i.e.

D band

ID/OG can give an idea to assess the extent of defects in it.23 After fitting these two peaks by Gaussian distribution, we have found the ratio of ID/IG.

G band

Material Morphology and Elemental Analysis

During the course of the reaction, the dense spheres undergo recrystallization to form hollow crystal spheres by the Ostwald annealing process. In this process, the smaller crystallites from the core of the sphere tend to dissolve more over time and move onto the larger particles on the surface of the spherical particles.

Ultraviolet-visible Diffuse Reflectance Spectra

It is found that the carbon dots are spherical in shape and 2–7 nm in size and uniformly dispersed without major agglomeration. The SAED pattern of CDs shown in Figure 6.6(H) indicates the low crystallinity of the synthesized CDs.

X-ray Photoelectron Spectroscopy (XPS) Spectra

This (Oads.) region is also known as the sign of oxygen vacancies in a sample.34 The number of oxygen vacancies in a compound can be evaluated by calculating the relative peak area ratio of Oads. CDs increase the number of oxygen vacancies in the hybrid material we also analyzed the O1s core level XPS spectra of CDs.

Binding energy (eV)(A)

Electron Spin Resonance (ESR) Spectra

It is known from the literature that three different types of oxygen vacancies can be present in a compound, such as neutral, singly ionized and doubly ionized. In Figure 6.13(A), we can see that BZO and 3C_BZO show a broad peak at around the g − tensor value of 2.005, which is the result of singly ionized paramagnetic oxygen vacancies (VO•).39 Figure 6.13(B) shows the ESR spectrum of synthesized CDs at room temperature and shows an intense ESR signal at a g tensor value of 2.005.

Among these vacancies, the number of trapped electrons present is two, one and zero in neutral, ionized and double oxygen vacancies respectively. these compounds. Therefore, from the UV-visible DRS study, XPS analysis and ESR study, we can say that all the studied compounds have a certain amount of oxygen vacancies and disordered states in their structures, which can be useful for their photocatalytic activities.

Time (ns)

Photocatalytic Hydrogen Production

Photocatalytic H2 evolution from water by xC_BZO (x = 0 - 4) hybrid nanomaterials under UV-visible light were analyzed in the presence of a mixture of 0.25 M Na2SO3/0.35 M Na2S as a sacrificial hole trap and shown in Figure 6.15. When the semiconductor is excited by light with an energy greater than or equal to the band gap energy, electrons are excited to the conduction band, leaving an equivalent number of holes in the valence band, as shown in Equation 1.

Photocatalytic Dye Degradation

Hydrogen Production (µmol/h/g)

CONCLUSIONS

We observed that 3 wt% CDs loaded on BZO showed the highest efficiency in both photocatalytic production of H2 and degradation of MB dye.

Handbook of X-ray Photoelectron Spectroscopy: A Reference Book of Standard Data for Use in X-ray Photoelectron Spectroscopy, Physical Electronics Division, Perkin-Elmer Corp., 1979.

THESIS OVERVIEW AND

FUTURE SCOPE

Overall Summary and Salient Features of the Thesis

From our aforementioned findings, we found that manganite-based perovskite oxides are better photocatalyst than zirconium-based perovskite oxides in terms of water oxidation due to their small band gap and suitable band positions. On the other hand, zirconium-based perovskite oxides are capable of both water oxidation and reduction and act as overall water-splitting photocatalyst.

Future prospects

LIST OF PUBLICATIONS AND

CONFERENCES ATTENDED

4th International Conference on Nanomaterials and Advanced Nanotechnology (ICANN December, 2015, Indian Institute of Technology Guwahati, Guwahati, India (Participant). Poster Presented).

References

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