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. 2020 Jul 22;5(30):18919-18934.
doi: 10.1021/acsomega.0c02142. eCollection 2020 Aug 4.

Hierarchical TiO2 Nanoflower Photocatalysts with Remarkable Activity for Aqueous Methylene Blue Photo-Oxidation

Jonathan Harris  1 , Ryan Silk  1 , Mark Smith  1 , Yusong Dong  1   2   3 , Wan-Ting Chen  1   2   3 , Geoffrey I N Waterhouse  1   2   3
Affiliations

Hierarchical TiO2 Nanoflower Photocatalysts with Remarkable Activity for Aqueous Methylene Blue Photo-Oxidation

Jonathan Harris et al. ACS Omega. .

Abstract

This study systematically evaluates the performance of a series of TiO2 nanoflower (TNF) photocatalysts for aqueous methylene blue photo-oxidation under UV irradiation. TNF nanoflowers were synthesized from Ti(IV) butoxide by a hydrothermal method and then calcined at different temperatures (T = 400-800 °C) for specific periods of time (t = 1-5 h). By varying the calcination conditions, TNF-T-t photocatalysts with diverse physicochemical properties and anatase/rutile ratios were obtained. Many of the TNF-T-1 photocatalysts demonstrated remarkable activity for aqueous methylene blue photo-oxidation at pH 6 under UV excitation (365 nm), with activities following the order TNF-700-1 > TNF-600-1 > TNF-500-1 > TNF-400-1 ∼ P25 TiO2 ≫ TNF-800-1. The activity of the TNF-700-1 photocatalyst (99% anatase, 1% rutile) was 2.3 times that of P25 TiO2 at pH 6 and 14.4 times that of P25 TiO2 at pH 4. Prolonged calcination of the TNFs at 700 °C proved detrimental to dye degradation performance due to excessive rutile formation, which reduced the photocatalyst surface area and suppressed OH• generation. The outstanding activities of TNF-700-1 and TNF-600-1 are attributed to their hierarchical nanoflower morphology which benefitted UV absorption, a near-ideal anatase crystallite size for efficient charge separation, and their unusually low isoelectric point (IEP = 4.3-4.5).

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Conflict of interest statement

The authors declare no competing financial interest.

Figures

Figure 1
Figure 1
SEM images for (a) TNF-AP and TNF-T-1 photocatalysts obtained by calcination of TNF-AP at T = 400–800 °C for 1 h. (b) TNF-400-1; (c) TNF-500-1; (d) TNF-600-1; (e) TNF-700-1; and (f) TNF-800-1. All images were collected at a magnification of 50 000×; scale bar is 1 μm.
Figure 2
Figure 2
SEM images for (a) TNF-AP and TNF-T-1 photocatalysts obtained by calcination of TNF-AP at T = 300–800 °C for 1 h. (b) TNF-400-1; (c) TNF-500-1; (d) TNF-600-1; (e) TNF-700-1; and (f) TNF-800-1. All images were collected at a magnification of 100 000×; scale bar is 500 nm.
Figure 3
Figure 3
(a) Average TNF-T-1 diameter and average TNF-T-1 petal thickness as a function of calcination temperature (T). (b) BET specific surface areas for the TNF-T-1 photocatalysts as a function of the calcination temperature (T).
Figure 4
Figure 4
(a) Powder XRD patterns; and (b) FT-IR absorbance spectra of TNF-AP and the TNF-T-1 photocatalysts obtained by calcination of TNF-AP at T = 400–800 °C for 1 h.
Figure 5
Figure 5
(a) Ti 2p XPS spectra, (b) O 1s XPS spectra, (c) Ti L-edge NEXAFS data, and (d) O K-edge NEXAFS data for TNF-AP and the TNF-T-1 photocatalysts obtained by calcination of TNF-AP at T = 400–800 °C for 1 h.
Figure 6
Figure 6
(a) UV–vis absorbance spectra and Tauc plots (inset); and (b) photoluminescence spectra collected in air under UV excitation for TNF-AP and the different TNF-T-1 photocatalysts. The inset in (b) shows the normalized photoluminescence intensity for the TNF-T-1 photocatalysts (normalized against the PL signal of TNF-AP).
Figure 7
Figure 7
(a) Plots of ln(A/A0) versus time for the photo-oxidation of aqueous methylene blue (C0 = 4 × 10–5 mol L–1) using TNF-AP and the different TNF-T-1 photocatalysts; and (b) pseudo-first-order rate constants for methylene blue photo-oxidation as a function of calcination temperature (T). Photocatalytic tests were conducted at pH 6 in phosphate buffer (I = 0.1264 mol L–1).
Figure 8
Figure 8
Plots of (a) normalized [OH•] and surface area normalized [OH•] for the different TNF-T-1 photocatalysts under UV excitation in pH 6 phosphate buffer (the [OH•] data was obtained from photoluminescence experiments in Figure S2) and then normalized against [OH•] data collected for TNF-500-1; (b) rate constants for methylene blue photo-oxidation at pH 6 weighted against the normalized [OH•] for the various TNF-T-1 photocatalysts.
Figure 9
Figure 9
Pseudo-first-order rate constants for methylene blue photo-oxidation at different pH values for TNF-600-1, TNF-700-1, and P25 TiO2.
Figure 10
Figure 10
Plots of (a) pseudo-first-order rate constants for methylene blue photo-oxidation as a function of TNF-T-t calcination time at 700 °C. Photocatalytic tests were conducted in pH 6 phosphate buffer (I = 0.1264 mol L–1); (b) surface area normalized rate constants for methylene blue photo-oxidation at pH 6 versus the percentage of rutile in various TNF-T-t photocatalysts.
Figure 11
Figure 11
(Top) Schematic diagram showing OH• generation by UV irradiation of an anatase TiO2 photocatalyst in water at pH 6. The hydroxyl radicals produced during excitation of photocatalysts are powerful oxidants, capable of readily oxidizing aqueous methylene blue. (Bottom) Schematic showing the working state of the TNF-700-1 and P25 TiO2 photocatalysts at different pH. The gray arrows indicate electrostatic attraction or repulsion of aqueous methylene blue, and the red broken arrows indicate OH• generation and migration into solution. Note: hydroxyl radical generation for TNF-700-1 at pH 4 is expected to be much greater than hydroxyl radical generation for P25 TiO2 at pH 6.
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