Physical and chemical properties of carbon nanotubes and silica nano-composites on the photo-catalytic activity of titania nanoparticles for selected organic pollutants.

Where the work of others has been used, this is acknowledged accordingly in the text. This thesis has been prepared according to the format set out in the guidelines of the College of Agriculture, Engineering and Science of UKZN, (FHDR Approved 13 March 2007).

Plagiarism

The photocatalytic properties of the materials were tested using a 10 mg L-1 methylene blue (MB) dye solution. The effect of loading with TiO2 (5 and 20 wt.% TiO2 on the SBA-CNT composite) was studied, and the physicochemical properties of the catalyst were additionally evaluated and tested in MB discoloration experiments.

Background
Problem Statement
Aims and Objectives
Research Approach
Research Scope
References

Evaluate the activity of photocatalysts in the degradation of organic pollutants of interest (methylene blue and MCPA). Photo-catalytic experiments were used to understand the influence of physical and chemical properties of materials during heterogeneous oxidation catalysis.

Figure 1. 1 Schematic diagram of TiO 2 nanoparticle photo-catalyst.

General Introduction to Titanium Dioxide

Crystalline Structure of Titanium Dioxide

In the pharmaceutical sectors, TiO2 is used as a block for UV radiation in sunscreens, and it is also used as a pigment for various pills and syrups [9]. TiO2 is also used as a pigmenting agent for paper and printing ink due to its bleaching properties and opacity ability [0].

Morphology and Synthetic Approaches to Nanostructured Titanium Dioxide

Templates for Mesoporous TiO 2 Fabrication

The sol–gel methods enable the synthesis of mesoporous TiO2 nanoparticles, fibers and tubes. The synthesis of mesoporous TiO2 with different mesostructures, and the resulting different physical and chemical properties, have been investigated using various structure directing agents, precursors and synthetic chemistry (Table 2.1).

Principle of TiO 2 in Heterogeneous Photo-catalysis

Among the various applications, the most active area of the TiO2 photocatalyst is in the decomposition of organic compounds in water. Electron migration results in empty holes in the valence band (vb), the holes are highly oxidizing when they reach the surface of the particle.

Operational Factors that Influence Photo-catalytic Efficiency of TiO 2

Catalyst makeup
Light intensity and wavelength
Catalyst loading
pH
Nature and concentration of organic pollutants

17 Often, the difference in photocatalytic activity is related to the variation of the specific surface area, the degree of structural defects in the crystal framework, impurities or the density of hydroxyl groups on the surface of the catalyst [56]. The pH of the solution plays a key role in the photocatalytic degradation of organic pollutants, as organic substances in water exhibit different properties.

Challenges of TiO 2 Nanoparticles in Heterogeneous Photo-catalytic Oxidation

73] reported that doping TiO2 with the metal ions Zr4+, La3+ and Ce3+ enabled the incorporation of metals into the lattice and some metal bonding to the surface of TiO2. The morphology of a semiconductor photocatalyst appears to play a crucial role in improving the photocatalytic activity of TiO2 for various applications of interest.

Modification/Support of TiO 2 with Mesoporous Nano-materials

Mesoporous Material Classes and Synthetic Approaches
TiO 2 /SBA-15 Nano-composites
Multi-walled Carbon Nanotubes (MWCNT)
TiO 2 /MWCNTs Nano-composite
TiO 2 /SBA-15-MWCNTs Nano-composite

In particular (for the study of Yen et al.) the samples prepared by the sol-gel method were more efficient in the degradation of phenol and NOx than the samples prepared by hydrothermal treatment. The photo-catalytic properties of the compound will be investigated in the degradation of selected organic pollutants found in water.

Figure 2. 4 Schematic presentation of mesoporous material formation from use of surfactants and precursors in the sol-gel chemistry [88]

Photo-catalytic Degradation of Organic Pollutants

The enhancement of photo-catalytic activity can also be attributed to the high dispersion of semiconductor nanoparticles on the CNT surface [143]. To our knowledge, no study has reported the photocatalytic transformation of MCPA with composite carbon nanotube catalysts.

Summary

Bell, N., on the design and synthesis of titanium dioxide-graphene nanocomposites for enhanced photovoltaic and photocatalytic performance. and Dasgupta, S., Visible light-induced photocatalytic degradation of organic pollutants. Angewandte Chemie International Edition in English, 1995. and García, A., Study on the synthesis of high surface area mesoporous TiO2 in the presence of nonionic surfactants. Journal of Photochemistry and Photobiology A: Chemistry, 2010. and Hequet, E., Functionalization of a cotton fabric surface with titania nanosols: applications for self-cleaning and UV-protective properties.

Materials and Chemicals
Characterization Instruments

Nitrogen Sorption
Powder X-Ray Diffraction (XRD)
Electron Microscopy
Thermo-Gravimetric Analysis
Raman Spectroscope
Fluorescence Spectroscope
Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES)
UV-Vis Diffuse Reflectance Spectrometer (DRS)
High Performance Liquid Chromatography (HPLC)

Synthesis Procedures

Acid Treatment of Multi-walled Carbon Nanotubes (MWCNTs)
Synthesis of SBA-15
Synthesis of SBA-15 Coated MWCNTs (SBA-CNT Composites)
Synthesis of Mesoporous Titanium Dioxide
Synthesis of Mesoporous TiO 2 Nanoparticles Catalyst
Synthesis of TiO 2 Nanoparticles Supported on Different Materials

Photo-catalytic Activity Experiments
Data Analysis
References

The solutions were stirred for an additional 10 minutes before an addition of the deionized water (pH 1.06) was made. 63 The same methodology was used under the same conditions for the synthesis of 5 and 20 wt. The variation in the synthesis method was in the weight of the TIP solution added to the mixture.

Figure 3. 1 Micrometrics Tri-Star II 3030 N 2 sorption instrument employed for textural analysis

Raman Spectroscopy Data Analysis

An increased ID/IG ratio of aCNTs compared to rCNTs would have suggested an increase in the disordered carbon atoms due to attached functional groups and tube end openings [3, 6]. The ID/IG ratio of the CNTs increased with the coating of SBA-15, compared to aCNTs (Table 4.1). The increased ratio implies that the graphitic vibrations of the CNTs are minimized due to the silica coating [11].

Figure 4. 1 Raman spectrum of raw and acid treated CNTs, rCNTs and aCNTs respectively.

Electron Microscopy Investigations

Micrographic Analysis of Raw and Acid Treated Carbon Nanotubes
Electron Microscopy Analysis on SBA-15
Micrographic Analysis of SBA-15 Carbon Nanotube Composites

When the FWHM for the D-band was examined, the value decreased significantly after incorporating SBA-15 onto the CNTs. During the growth of SBA-15 on the CNTs, most of the vacant nucleation sites on the CNT walls may have been occupied or made less accessible by the surfactant used. Micrographs of composites, Figure 4.7 shows thin film coatings with uniform and thicker layers of SBA-15 at 20 wt.

Figure 4. 4 Micrographs of commercial CNTs and acid treated CNTs, denoted rCNTs and aCNTs respectively

Thermo-gravimetric Analysis (TGA) Studies

Thermo-gravimetric analysis on Raw and Acid Treated Carbon Nanotubes
Thermo-gravimetric analysis on SBA-15 coated Carbon Nanotubes

Thermograms presented in Figure 4.9 show that the decomposition onset temperature of the acid-treated CNTs is lower than that of raw CNTs. The thermal stability of the materials was inferred from the oxidation temperatures, which are the highest peak position in the derived weight versus temperature graph. The derived weight patterns (Figure 4.10b) confirm that the stability of the composites increases as the SiO2 content increases.

Figure 4. 9 Thermogram pattern of raw and acid treated CNTs, derivative weight pattern (insert)

X-Ray Diffraction Pattern (XRD) Analysis

XRD Pattern Analysis on Raw and Acid treated Carbon Nanotubes
XRD Pattern Analysis on SBA-15
XRD Pattern Analysis on SBA-15 Carbon Nanotubes Composites

The decrease in thermal stability of the aCNTs also confirmed the presence of functional groups on the tubes. After introducing SBA-15 onto the tube surface, the sharpness of the CNT peaks in the XRD gradually decreased as the silica content increased (Figure 4.13). Regardless of the overlap, it remains clear that the reflection of the tubes in the 30 wt.

Figure 4. 11 XRD pattern of raw and acid treated CNTs.

Nitrogen (N 2 ) Sorption Studies

Nitrogen Sorption Studies on Raw and Acid Treated CNTs
Nitrogen Sorption Studies on SBA-15
Nitrogen Sorption Studies on SBA-15 Carbon Nanotubes Composites

Analysis of nitrogen sorption on SBA-15 (Figure 4.15) shows that the SBA-15 material has a type IV isotherm associated with capillary condensation in the mesoporous material. The surface area of SBA-CNT nanocomposites increased after coating with higher mass percentages of SBA-15 (Table 4.4). This is attributed to SBA-15, as a peak is found in the pore size distribution pattern of SBA-15 (Figure 4.15) at similar pore diameter sizes and this narrow pore size in the 30 wt.

Electron Microscopy Investigations of Catalysts

HRTEM and SEM Analysis on Unsupported TiO 2
HRTEM and TEM Analysis on TiO 2 Nano-Composites

The crystal size of TiO2 as estimated by the Scherrer equation was 5.71 nm, and it was reduced to 4.57 nm when similar calculations were performed on SBA-15 titanium oxide composites. The incorporation of TiO2 on the surface rather than in the pores of SBA-15 could be a result of the small pore diameter of 5.03 nm of the SBA-15 support. The anatase particles are more concentrated on the silica-coated nanotubes than on the uncoated tubes as depicted.

Figure 4. 19 SEM micrographs of mesoporous TiO 2 nano-particles.

XRD Analysis of Catalysts

XRD Analysis on Unsupported TiO 2
XRD Analysis of 10 wt. % TiO 2 Nano-Particles on SBA-15
XRD Analysis of 10 wt. % TiO 2 Nano Composite

However, incorporation of TiO2 caused a shift in the 2θ˚ value of SBA-15, indicating a change in the SBA-15 structure (Table 4.5). Shifts in the low-angle XRD peaks suggest that the pores are smaller in TiO2/SBA-15 compared to SBA-15 itself. TiO2/SBA-15 composite showed small low intensity diffractions of the anatase peaks, this suggests that the TiO2 particles were well dispersed on SBA-15, this was consistent with the low angle XRD analysis of the composite and the electron microscopy analysis.

Table 4. 5 Properties of 10 wt. % TiO 2 catalyst on supports according to XRD pattern analysis using the (101) reflection peak of anatase TiO 2

Raman Spectroscopy of Unsupported TiO 2 and 10 wt. % TiO 2 Catalyst

98 Analysis of the XRD patterns using the overlapping (2θ = 26.00˚) peak showed a slight broadening of the anatase phase TiO2 particles in 10 wt. The general effect of the support frameworks on the titanium oxide particles was to limit the growth of TiO2 nanoparticles, as evidenced by the small size of the TiO2 crystals in the composites. None of the characteristic Raman spectroscopic bands of titanium oxide were identified in the various composites (Figure 4.25).

Figure 4. 24 Raman spectra of TiO 2 nano-particles.

Nitrogen Sorption Studies of Unsupported TiO 2 and 10 wt. % TiO 2 Catalysts

The specific surface area of TiO2 increased when supported on different materials compared to unsupported TiO2 nanoparticles (Table 4.7). There were slight variations of titanium dioxide content in the composition according to ICP-OES analysis. The support of the titania catalyst on different supports did not affect the meso-porosity of the materials.

Table 4. 7 Textural parameters of TiO 2 and 10 wt. % TiO 2 nano-composites according to N 2 sorption studies

Optical Studies and Photoluminescence Studies on TiO 2 and TiO 2 Nano-Composites 102

The red shift indicates a decrease in band gap energy and this was due to the unique way in which CNTs can absorb the visible and other parts of the electromagnetic spectrum. The recombination rate was slowed by the presence of CNTs in the catalyst due to the 1D structure of the tubes, which makes them prone to conduct electrons without resistance [40, 41] and consequently allow the generated electrons in TiO2 to be easily transferred to the CNTs [6]. The TiO2/SBA-CNT composite shows some agglomerates of TiO2 particles, which can be observed at such low mass percentages, possibly due to the silica surface groups.

Figure 4. 28 UV-Vis diffuse reflectance measurements (a) and Kubelka Munk plots (b).

XRD Pattern Analysis of Different [TiO 2 ] on SBA-CNT

The presence of the peak indicates that the crystallinity and probably the properties of the SBA-CNT composite were preserved during the chemical and thermal treatments. Interpretation of the FWHM and 2θ degree positions of the peaks was analyzed from the anatase TiO2 (101) reflection peak and (211) reflection peak (Table 4.8). Both reflection peaks indicated that the support of TiO2 on the SBA-CNT composite reduces the growth of TiO2.

Figure 4. 31 XRD patterns of TiO 2 and 5, 10 and 20 wt. % TiO 2 /SBA-CNT composites (rectangular shape highlights the emerging CNTs peaks within the composites)

Raman Spectroscopy and Thermo-gravimetric Analysis of 5, 10 and 20 wt. % TiO 2 on

109 Figure 4.32 shows that unsupported TiO2 is very thermally stable as can be seen from the percent weight loss of 5.68. TiO2 in the SBA-CNT composite decreased the decomposition temperature of the composite, and then the decomposition temperature gradually changed as the TiO2 content increased in the composite. An increase in TiO2 concentration in the SBA-CNT support caused a decrease in the weight loss percentage of the materials [32].

Figure 4. 32 Thermogram patterns of TiO 2 , SBA-CNT support and xTiO 2 /SBA-CNT composites (x denotes 5, 10 and 20 wt

Nitrogen Sorption, Optical and PL Studies of Different [TiO 2 ] on SBA-CNT

111 Figure 4.33 shows the PL spectra of materials and from the suppression of peaks in the different catalyst compositions it can be determined that the lower the titanium content, the lower the electron-hole recombination rate of self-trapped excitation in titanium. 23] also observed that the introduction of CNTs decreased the e- - h+ recombination rate according to PL studies, although their ratio of CNTs to TiO2 was.

Figure 4. 33 Photoluminescence spectra of TiO 2 and SBA-CNT supported TiO 2 .

Conclusions

The photocatalytic performance was attributed to the morphological structure of TiO2/SBA-15 composite, which showed a long order of the curved pore walls, which are reported to facilitate the movements of pollutants and products during catalytic reactions. The photoactivity of the nanotubes was attributed to the presence of functional groups, which could have acted as centers for radical species generation. 122 These results showed that the photocatalytic activity of materials, especially the efficiency of the catalyst composites, depends not only on the adsorption properties of the material surfaces, but rather on the physicochemical properties of the catalyst composites, which are brought about by the synergy between TiO2 and support.

Figure 5.1b represents the photo-activity of the catalysts after irradiation with a 32 W visible day light halogen lamp

Photo-catalytic Activity of 5, 10 and 20 wt. % TiO 2 supported on SBA-CNT

The activity of the catalyst is attributed to the small titanium crystals (4.28 nm) and low band gap energy (2.50 eV). The photo-catalytic activities of the catalyst were directly proportional to the band gap energy of the catalysts. The decolorization rate constant before normalization is attributed to the high surface area and porosity of the materials.

Figure 5. 2 Catalytic activity of materials before and after irradiation with visible light irradiation

Photo-catalytic Activity of 5, 10 and 20 wt. % TiO 2 on SBA-CNT on the Photo-

Effects of solution pH on catalyst efficiency
Effect of catalyst concentration
Effect of pollutant concentration

When the pH of the solution was 2.98, no significant adsorption of MCPA on the catalyst surface was observed. In a heterogeneous regime, the mass of the catalyst is directly proportional to the rate of the photocatalytic reaction. As the catalyst concentration in the solvent medium increases, the light scattering possibilities also increase with scanning effects [14].

Figure 5. 4 Photo-degradation of 8 mg L -1 MCPA with 0.1 g L -1 5 wt. % TiO 2 /SBA-CNT composite at different pH conditions.

Photo-catalytic transformation of MCPA under optimum conditions

132 The differences in the efficiency of our catalyst with two substrates of different structures show that the photocatalytic activity was strongly influenced by the molecular structure of the pollutant. The reduction of the titanium band gap and the increase in surface area do not always support photocatalytic degradation, depending on the target substrate; however, it offers the advantages of activity under visible light irradiation, adsorption, and slow electron–hole recombination rates. 16] illustrated that the efficiency of photocatalytic degradation, especially of herbicides, was strongly influenced by the molecular structure and not by the physicochemical properties of the catalyst.

We attributed the catalytic activity to the crystallinity and electron–hole recombination rate of the catalyst systems. The variation in the activity of the catalyst was attributed to the difference in the substrate structures that affected the oxidative mechanisms during the photocatalytic reaction. Further research on the TiO2/SBA-CNT catalyst could focus on studying the effect of SBA-15 concentrations on the photocatalytic activity of the catalyst.