Nanoparticles for the Degradation of Water Pollutants
3.3 Results and Discussion
3.3.1 Materials Characterization
mm; 5 μm particles), a Diode Array Detector (L-2450) and a solvent delivery pump (L-2130). An isocratic method set at a flow rate of 1 mL min–1 was used with the eluent consisting of an A:B (70:30) mixture of 20 mM NaH2PO4 acidified with H3PO4 at pH = 2.80 (A) and acetonitrile (B).
Absorbance was found to be linear over the whole range considered. The maximum relative standard deviation of HPLC measurements was never larger than 2%.
The photocatalytic oxidation of the tested pollutants can be described by a pseudo-first-order kinetic model, according to the following equation:
(3.1)
where C corresponds to the pollutant concentration, k is the pseudo-first- order kinetic constant, t is the reaction time and C0 is the pollutant concentration at t = 0. The values of k were obtained by non-linear regression using the Marquardt–Levenberg algorithm.
Table 3.1 Surface areas (SBET) and pHPZC determined for Au/TiO2 and for the Au/ carbon–TiO2 composites.
Catalyst SBET (m2/g) pHPZC
Au/TiO2 121 3.9
Au/C60–TiO2 78 5.6
Au/CNT–TiO2 61 5.2
Au/GO–TiO2 112 3.7
Au/ND–TiO2 85 3.6
The results show that the SBET area was comparable for Au/TiO2 (121 m2 g–1) and Au/GO–TiO2 (112 m2 g–1) and higher than for the other Au/carbon–TiO2 composites (85, 61 and 78 m2 g–1, for Au/ND–TiO2, Au/CNT–TiO2 and Au/C60–TiO2, respectively). The lower values obtained for the composites prepared with ND, CNT and C60 may be due to the agglomeration of the TiO2 particles induced by the presence of these carbon
materials, leading to the formation of larger TiO2 particles, compared to the Au/TiO2 catalyst. However, a significant development of the porosity was observed at high relative pressure in the case of GO–TiO2 composites, together with a clear hysteresis loop (Figure 3.2).
The materials loaded with gold were also analysed by SEM. It can be seen that images with topographic contrast (secondary electrons diffraction) provide a better view of the supports morphology, while images with composition/atomic number contrast (backscattered electrons detection) allow a better visualization of the gold nanoparticles.
The bare TiO2 (Figures 3.3a and b) shows an agglomerated material, consisting of anatase crystallites with an estimated size of 4–5 nm as previously reported [45, 54]. TiO2–C60 composites show much larger particles (Figures 3.3c and d). The TiO2–CNT composites show TiO2 particles with sizes in the order of hundreds of nanometres, homogeneously embedding carbon nanotubes (Figures 3.3e and f), as also shown in a previous publication [69]. Figures 3.3g and h show TiO2 particles aggregated on the top of GO layers, forming a kind of junction of GO platelets, as previously observed [54]. A well-balanced TiO2 distribution on both sides of the GO nanosheets seems to occur, indicating a good self-assembly of the TiO2 nanoparticles on GO during preparation. The TiO2 composites prepared with NDs show homogeneous particles, a little larger than those found for bare TiO2, as already reported [45].
Figure 3.3 SEM images of bare TiO2 (a and b), C60–TiO2 (c and d), CNT–
TiO2 (e and f), GO–TiO2 (g and h) and ND–TiO2 (i and j) with gold. On the left, SEM images with topographic contrast (secondary electrons diffraction) are shown (a, c, e, g and i). On the right, SEM images with
composition/atomic number contrast (backscattered electrons detection) can be found (b, d, f, h and j). Shown scale bars are 2 μm. Gold nanoparticles are seen as bright spots.
The size distribution histograms of gold nanoparticles on the TiO2-based supports were obtained from several SEM images and are shown in Figure 3.4 and Table 3.2. It can be seen that the lowest average size is for gold on CNT–TiO2 that showed 13.8 nm. C60–TiO2, ND–TiO2 and bare TiO2 have a similar size of ~17 nm. Interestingly, the size range of Au/ND–TiO2 is narrower (1–35 nm) than that of the other materials (2–40 nm). The largest sizes were obtained for Au/GO–TiO2 (23.7 nm in average). This material also shows the largest and broadest size range (6–50 nm).
Figure 3.4 Size distribution histograms of gold nanoparticles on bare TiO2 (a), C60-TiO2 (b), CNT-TiO2 (c), GO-TiO2 (d) and ND-TiO2 (e).
Table 3.2 Size ranges and average gold nanoparticle sizes for the Au/TiO2- based samples, obtained from measurements made on ~200–600 particles.
Catalyst Au size range (nm) Au nanoparticle size (nm)
Au/TiO2 2–40 17.1
Au/C60–TiO2 2–40 16.9
Au/CNT–TiO2 2–40 13.8
Au/GO–TiO2 6–50 23.7
Au/ND–TiO2 1–35 17.0
ATR-FTIR spectra of Au/TiO2 and the respective Au/carbon-based TiO2
composites are depicted in Figure 3.5. The ATR spectra recorded for Au/TiO2 and all the Au/carbon-based TiO2 composites show mainly a broad band between 2800 and 3600 cm–1, associated with stretching vibrations of water molecules and hydroxyl groups. This is confirmed by the presence of some weak bands around 1600 cm–1 caused by bending vibration of coordinated water, although the presence of Ti–OH bonds could also have certain contribution to this peak [70, 71]. The band corresponding to the vibration of Ti–O–Ti bonds was situated between 800 and 950 cm–1 [72].
Figure 3.5 ATR spectra of Au/TiO2 and Au/carbon–TiO2 composites.
The diffuse reflectance UV–Vis spectra (DRUV–Vis) of selected Au materials, expressed in terms of Kulbelka–Munk absorption units, are depicted in Figure 3.6. The DRUV–Vis spectra for the Au/samples show the characteristic absorption sharp edge rising at 400 nm due to the bandgap transition of the TiO2 semiconductor. The addition of any carbon material into the TiO2 increases the baseline absorbance in the visible region, leading
to a bandgap narrowing [47, 48]. Moreover, the addition of gold nanoparticles into the TiO2 reveals the presence of one band in the visible region with a wavelength maximum at around 550 nm, typical of the gold surface plasmon [73, 74]. The increase in absorption in the visible region depended on the type of nanocarbon used, a higher absorption in the visible region being obtained for nanocomposites prepared with GO, then with CNT, and finally with ND.
Figure 3.6 DRUV–Vis spectra of Au/TiO2 and Au/carbon–TiO2 composites.