Investigation of metal and semiconductor nanoparticle/2D layer decorated TiO2 nanostructures for visible light photo(electro)-catalysis and optoelectronic applications

Giri, 'Enhanced Visible Light Photocatalytic Activity of Ag2O Decorated Nanoporous TiO2(B) Nanorods Heterostructure', International Conference on Nanoscience, Nanotechnology and Advanced Materials (NANOS-2015), Gitam University, Vizag, India, December 14-17, 2015 (best Poster Prize). Giri, “Role of Surface Plasmons and Hot Electrons on the Strong Visible Light Photocatalysis by Ag@TiO2 Nanostructures,” International Conference on Materials for Advanced Technologies (ICMAT-2017), Singapore, June 18-23, 2017.

Introduction

Crystal Structure of TiO 2

Band Structure of TiO 2

Fabrication of TiO 2 Nanostructures

Sol-gel and Template Assisted Method
Hydrothermal Method
Solvothermal Method
Electrochemical Anodization Method
Electrospinning Method
Vapor Deposition Technique

Fabrication of TiO 2 Based Heterostructures

Application of TiO 2 Nanostructure and Its Heterostructures

Photocatalysis: Hydrogen Production and Environmental Remediation 14
Photodetector .…
Sensor: Gas Sensor and Biosensor
Dye-Sensitized Solar Cells

Challenges in Fabrication and Applications of TiO 2 Based Heterostructures .…. 22

Organization of the Thesis

Introduction

In addition, recombination of the photogenerated electron-hole pairs in a homogeneous semiconductor is most likely by simply releasing heat (non-radiative transition) or light (radiative transition), which prevents their migration to the surface of the semiconductor, causing low photocatalytic effect . Despite having stability problems, the TiO2(B) phase offers a relatively open structure than that of the other polymorphs of TiO2.

Experimental Details

Sample Preparation …

Synthesis of TiO 2 (B) NRs
Growth of Ag 2 O/TiO 2 (B) NRs heterostructures …

Visible Light Photocatalytic Measurements
Morphology and Composition Studies
Structural Analysis

XRD and XPS Analyses
Raman Analysis

Brunauer–Emmett–Teller (BET) Surface Area Analysis
Optical Analysis

UV-Vis Absorption Study
Photoluminescence Study
Time-Resolved Photoluminescence Study

Visible Light Photodegradation Studies

Photodegradation and Reaction Kinetics of MO
Photodegradation and Reaction Kinetics for MB
Cyclic Stability
Mechanism of Enhanced Visible Light Photocatalytic

A comparison of the XPS core level O 1s spectra of pristine TiO2(B) NRs and TA2 HS is shown in Fig. Interestingly, the effective band gap of the TiO2(B) NRs was significantly reduced after loading with Ag2O NPs. The modification of the band gap energy for the HS samples may be due to the efficient coupling between the Ag2O NPs and TiO2(B) NRs.

A comparison of steady-state PL spectra at room temperature for pristine TiO2(B) NRs, Ag2O NPs, and TA2 HSs is shown in Fig. visible area (Fig.

Summary and Conclusions

Since the dye MO is anionic in nature, in aqueous solution, the anionic dye radicals feel columbian attraction to the cations (h+) on the HS surfaces. The anionic dye radicals are therefore very quickly adsorbed to the HS surface and react with the hydroxyl radicals (step 6). Thus, in aqueous solution there is coulombic repulsion between the cationic MB radicals and cationic Ag2O surface in the HS samples.

Thus, the MB dye radical adsorption rate on the HS surface is slower than the MO radical adsorption rate, resulting in a slower MB degradation rate, as shown in Table 2.2.25. Nevertheless, the degradation rate is quite high due to the high density of photoinduced carriers present in HS, resulting in significant degradation of MB by superoxide and hydroxyl radicals. Thus, Ag2O/TiO2(B) HS are very efficient for the photodegradation of dyes in visible light regardless of their ionic nature.

Role of Surface Plasmons and Hot Electrons on the Multi-Step Photocatalytic

Experimental Details

Synthesis of TiO 2 NRs
Growth of Ag@TiO 2 NRs heterostructure …

Photocatalytic Degradation under Visible Light
Characterization Techniques

To confirm the crystal structure and phase of the obtained product, XRD and micro-Raman analyzes were performed. The photocatalytic activity of pure TiO2 NRs and Ag@TiO2 HSs was evaluated considering the photodegradation of Rhodamine-B (RhB) as a model dye under visible light irradiation. The details of the visible light source, experimental arrangements and photocatalysis methods were discussed in Chapter 2, Section 2.2.2.

At selected irradiation intervals, 3 mL of solution was collected and centrifuged to remove the catalyst particles to estimate the remaining concentration of the dye solution. The details of the characterization techniques (XRD, XPS, Raman, FESEM, TEM, PL, etc.) used to study the systems were described in Chapter 2, Section 2.2.3.

Results and Discussion

XRD and Raman Analyses
XPS Analysis

Optical Analysis

Visible Light Photodegradation Studies

Photodegradation of RhB
Degradation Rate Kinetics of RhB
Modelling of Degradation Kinetics
Cyclic stability of Photocatalyst …
Mechanism of Enhanced Visible Light Photocatalytic

Interestingly, the distribution of Ag NPs is highly uniform in the TA32 sample compared to the other HS samples. The phases of anatase TiO2 and Ag coexist in the Ag@TiO2 HS crystals. Note that the XRD data show a marginal shift in the TiO2 peaks in the HS sample compared to the pristine TiO2 NRs.

Details of the time constants (𝜏) of pristine TiO2 NR and TA32 HS are tabulated in the inset of Figure 3.7(d). XPS analysis before photocatalysis showed that 48.17% of Ag was present in the form of Ag+ ions.

Summary and Conclusions

Trap State Mediated Plasmonic Hot Electron Injection and Efficient Charge

Introduction

Experimental Details

Synthesis of TiO 2 (B) NRs
Preparation of g-C 3 N 4 Nanosheets …
Preparation of Au and Ag NPs …
Preparation of Ag-TiO 2 , Au-TiO 2 , and Its Decoration over

Photocatalytic Measurements

Photodegradation of RhB and Phenol
Scavenging Test
Hydroxyl Radical Test
Superoxide Radical Test

Photoresponse Study

Results and Discussion

Morphology Studies

XPS Analysis

BET Surface Area Analysis
Optical Analysis

Photocatalytic Studies
Photoresponse Study
Mechanism for Enhanced Photocatalysis

Strongly Enhanced Visible Light Photoelectrocatalytic Hydrogen Evolution

Introduction

Hydrogen is believed to be one of the most promising alternatives to fossil fuels and a source of renewable green energy due to its high energy density and carbon-free combustion emission. Solar light-driven electrocatalysis using semiconductor heterostructures (HS) is one of the most promising sustainable technologies for hydrogen generation from water splitting. cost and insufficiency in nature prevent their use on an industrial scale.3 Two-dimensional (2D) transition metal dichalcogenide nanosheets, especially MoS2 have already been recognized as a co-efficient. However, the system was not well characterized and the performance of the composite system was comparable or inferior to that of the binary systems.

Herein, a solvothermally grown porous TiO2(B) NBs is considered as a new platform for the decoration of few-layer exposed MoS2 at the edge. Subsequently, marginal Pt NPs are preferentially decorated at the edge of MoS2 and porous sites of TiO2(B) to exploit its application in next-generation photoelectrocatalysis.

Experimental Details

Synthesis of TiO 2 (B) NBs
Growth of MoS 2 /TiO 2 NBs HSs …
Decoration of Pt NP on the TiO 2 NBs, MoS 2 Nanosheets

Photoelectrocatalysis Measurements under Visible Light …

Before loading the catalyst, the surface of the glassy carbon substrate is polished with 50 μm alumina powders on a polishing cloth and washed with DI water. Thus, the active area 'A' of the electrode is calculated from the cyclic voltammetry (CV) response of 5 mM [Fe(CN)6]3–/4– in 0.1 M KCl at different scan rates. Schematic representation of the experimental setup for the photoelectrochemical (PEC) measurements and hydrogen evolution reaction (HER).

The details of the FESEM, EDS, XRD, Raman, TEM and XPS were described in Chapter 2, Section 2.2.3. The details of the room temperature (RT) steady state PL measurement were discussed in Chapter 4, Section 4.2.2.

Results and Discussion

XRD Analysis
Raman Analysis
XPS Analysis

UV-Vis Absorption and Photoluminescence Study
Hydrogen Evolution Reaction (HER) Study
Mechanism of Enhanced HER Activity …

However, in the case of PTB, most of the Pt NPs are preferentially attached to the porous sites of TiO2 nanostructures. The additional Pt2+ in the HS sample can form Pt-O bond due to the chemisorption of oxygen and hydroxyl group on the surface of the Pt NP and MoS2 layer. After the decoration of MoS2, it is observed that the Ov concentration in the TiO2 lattice is more than doubled from 21.1% to 46.1%, which may be due to the replacement of O atoms in TiO2 with the S atoms during the in -situ growth of MoS2 over TiO2 platform.

Loading of Pt NPs onto TB or MSTB results in the decrease of Ov concentration from its initial value, perhaps due to the presence of Pt 2+ . After the growth of the MoS2 layer on the TiO2 NB, the PL intensity corresponding to the F+ center increases strongly, indicating an increase in the concentration of OV defects.

Summary and Conclusions…

It was found that a marginal amount of Pt NP (1.4 wt%) on the MSTB decorates the catalytically active edge sites of MoS2 and porous sites of TiO2 with Pt NPs. Furthermore, the presence of Pt NPs can also serve as beneficial metallic current collector nodes to facilitate lower resistance transport routes of photoexcited electrons from TiO2 domains to Pt NPs via MoS2 (see Figure 5.14), thus improving overall conductivity.14 significantly increased HER activity observed by PMSTB with a minimum charge transfer resistance of 14 Ω and a maximum exchange current density of 0.296 mA/cm2. On a separate note, the adsorbed hydroxyl group in TiO2 lattice increases sharply after the decoration of MoS2 layers (32.7%), which further increases to 47.3% after loading Pt NPs on MSTB, as confirmed by XPS- analysis.

Marginal Pt NPs (1.4 wt.%) are selectively decorated on the edge sites/defect sites of MoS2 and porous sites of TiO2 NBs. The Pt NPs activate the inert basal plane, MoS2 edge zones, and TiO2 porous sites and thus provide an integrated network that facilitates a facile injection of photogenerated electrons from TiO2 into Pt NPs through the MoS2 layer, which represents an approach of possible to increase the HER activity of the multicomponent catalyst under visible light.

Introduction

Tunable quantum yield and high photoluminescence from self-grown TiO2 quantum dots on doped fluorine. In this chapter, we demonstrate a tunable color and high photoluminescence quantum yield (PL QY) from spherical TiO2 quantum dots (QDs) self-grown on fluorine (NF)-doped mesoporous TiO2 (F-TiO2) nanotubes. , synthesized by a hydrothermal process followed by a rapid post-growth thermal annealing (RTA) under vacuum. QDs are associated with shallow and deep traps, and a record high PL QY of ~5.76% is achieved at room temperature even with its indirect gap.

To the best of our knowledge, there is no report on fluorine doping of mesoporous TiO2 nanostructure and the extremely high PL quantum yield (QY) of home-grown anatase fluorine-doped TiO2 (F-TiO2) nanocrystals (NCs) on mesoporous F-TiO2 nanoflowers (NFs). TiO2 NCs with high PL QY could be useful for sensing and bioimaging considering their good biocompatibility.

Experimental Details

Sample Preparation

Preparation of F-TiO 2 NFs
Growth of F-TiO 2 QDs on F-TiO 2 NFs

Details of the sample preparation conditions, post-synthesis treatment, crystal structure and surface morphology of the F-TiO2 NFs. XPS measurement was performed with a photoelectron spectrometer (AXIS Supra, Kratos Analytical, UK) using Al Kα X-ray beam (1486.6 eV). The PL QY is measured using an integrating sphere (Horiba FM sphere) incorporated into the Fluoromax-4 system.

Fluorescence confocal microscopy is performed using a confocal laser scanning microscope (LSM 880 microscope (Carl Zeiss)) with a laser excitation at 355 nm. XPS measurements are performed with a photoelectron spectrometer (AXIS Supra, Kratos Analytical, UK) using Al Kα X-ray beam (1486.6 eV).

Results and Discussion

FESEM and EDS Analyses
FETEM Analysis

XRD Analysis
Raman Analysis
XPS and ESR Analyses

Optical Analysis

Temperature Dependent PL Study

Mechanism of PL Enhancement in F-TiO 2 QDs/NFs

It is observed that the size of TiO2 NCs becomes larger with increasing HF molar concentration (from 60 mM to 80 mM) (see Fig. At higher HF concentration, the size of TiO2 NCs increases more due to of intense surface etching effect Therefore, after vacuum annealing, band gap narrowing results in visible light sensitization of F-TiO2 NF.

After RTA (oven) annealing in ambient air, the PL intensity increases by ∼12 (7) times without any major change in the nature of the spectrum. The details of the time constants (𝜏) of T20VR and T20VA are shown in the inset of Fig.

Summary and Conclusions

Note that despite the presence of TiO2 NPs/QDs, the band edge PL intensity was negligibly low in all the samples due to the indirect gap nature of the TiO2. However, the defect-mediated high yield and stable PL of the F-TiO2 NPs are extremely important for its bioimaging and drug delivery applications.

In-Situ Chemical Vapor Deposition Growth of Monolayer MoS 2 on TiO 2

Introduction

Experimental Details

Sample Preparation

Preparation of 3D TiO 2 NFs on Titanium Foil …
Direct Growth of Single Layer MoS 2 on Various Substrates

Device Fabrication for Photocurrent Measurements

Raman Analysis
Growth Mechanism of Monolayer MoS 2 over Various
XPS Analysis

Optical Analysis

Effect of Oxygen Plasma Treatment

Raman Study
XPS Study
Photoluminescence and Photoresponse Study

Performance Study of 1L-MoS 2 @TiO 2 Heterojunction Photodetector 196

Summary and Outlooks

Summary and Highlights of the Thesis Contribution

Scope of Future Work