A formation model of superoxide radicals photogenerated in nano-TiO₂ suspensions

Abstract

A formation model of O₂˙⁻ produced in TiO₂ photocatalysis was established, and then a custom built continuous flow chemiluminescence (CFCL) system was used to confirm the model's reliability by monitoring the O₂˙⁻ formation process. This model may give deeper insights into O₂˙⁻ formation for TiO₂ and other photocatalysts.

---

In photocatalytic reactions, such as TiO₂, it is generally accepted that O₂˙⁻ is produced from the reduction of adsorbed oxygen by photogenerated electrons on the TiO₂ surface.^1–5 O₂˙⁻ formation is a rate-limiting process in TiO₂ photocatalytic reactions,⁶ and thus determines the efficiency of TiO₂ photocatalytic reactions by promoting the separation of photogenerated electrons and holes. Moreover, O₂˙⁻ is an intriguing active species, attracting a great deal of attention in recent years due to its unique role. Previous studies have confirmed that O₂˙⁻ plays an essential role in the photodegradation of pollutants.^7,8 Therefore, probing the dynamic formation of O₂˙⁻ over the course of time under UV irradiation is conducive to better understanding TiO₂ photocatalytic reactions. We have successfully identified surface long-lived O₂˙⁻ photogenerated on TiO₂ surface,⁹ however, the dynamic details of O₂˙⁻ formation in TiO₂ photocatalytic reactions still remain to be solved.

In photocatalytic reactions, O₂˙⁻ is continuously undergoing the processes of formation and deactivation simultaneously. Many methods have been developed to determine the O₂˙⁻, including electron spin resonance (ESR),¹⁰ spectrophotometric assays,^11–13 and fluorescence assays,^14,15 but the O₂˙⁻ could only be detected at discrete times, and thus the total quantity could only be given within the irradiation duration. Given these limitations, O₂˙⁻ dynamic monitoring is a great challenge. Chemiluminescence (CL) is inherently sensitive and rapid due to the relative ease with which light emission is instantly generated through a chemical reaction when two or more reactants are mixed. These qualities of CL make it suitable for the dynamic study of O₂˙⁻, despite its characteristic instability. In a previous study, we were able to successfully develop a continuous flow chemiluminescence (CFCL) method for dynamic monitoring of the formation process of O₂˙⁻ in TiO₂ photocatalytic reactions.¹⁶

For the formation mechanism of O₂˙⁻ in TiO₂ photocatalytic reactions, it is generally accepted that O₂˙⁻ formation occurs on TiO₂ surfaces by the following scheme:

where O₂ denotes the dissolved oxygen (DO) in solution, S denotes the oxygen adsorption site on TiO₂ surface, k₁ and k₋₁ denote the adsorption/desorption rate constant of O₂ respectively, O₂,_s denotes the adsorbed oxygen on TiO₂ surface, k_f denotes the formation rate constant of O₂˙⁻, O_2,s˙⁻ denotes the O₂˙⁻ formed on TiO₂ surface, and k_d denotes the rate constant of O₂˙⁻ decomposition. It was reported that DO was first adsorbed on the site of TiO₂ surface following the Langmuir isotherm,⁶ and then the adsorbed O₂ was reduced to O₂˙⁻ by photogenerated electrons under UV irradiation. Meanwhile, the formed O₂˙⁻ was transformed into other species by side reactions, such as H₂O₂ or ¹O₂ which deactivated to O₂ quickly. Specifically, it is suggested by some research that O₂˙⁻ could be produced from the oxidation of H₂O₂ by valence-band hole (h⁺) or hydroxyl radical (·OH) in solution, where H₂O₂ is produced by the two-step oxidation of water or the two-electron reduction of O₂.¹⁷ It is insignificant in this study because the O₂˙⁻ detected by CFCL method is the long-lived superoxide adsorbed on TiO₂ surface which is produced from the reduction of O₂ by photogenerated electrons, not in solution according to our previous study.⁹ The process of O₂˙⁻ formation was illustrated concretely as follows: when TiO₂ semiconductor is irradiated, the photogenerated electrons transferring to the TiO₂ surface are captured by five-coordinated surface Ti⁴⁺ to form the Ti³⁺ (eqn (1)). Then the O₂ adsorbed on TiO₂ surface react with Ti³⁺ to form O₂˙⁻ (eqn (2)).^1,2,18

–Ti⁴⁺–OH + e → –Ti³⁺ + OH⁻ (1)

–Ti³⁺ + O_2,s → –Ti⁴⁺⋯OO˙⁻ (2)

O₂˙⁻ + h⁺/·OH → ¹O₂ (3)

O₂˙⁻ + e + H⁺ → H₂O₂ (4)

O₂˙⁻ + O₂˙⁻ + H⁺ → H₂O₂ + O₂ (5)

Meanwhile, O₂˙⁻ formed also undergoes a decaying process possibly via the following three pathways (eqn (3)–(5)): (1) oxidation by h⁺ or ·OH, (2) further reduction by e, or (3) self-disproportionation in solution. Therefore, the number of O₂˙⁻ determined in photocatalytic reactions at any irradiation time (t) is what the total quantity of formed O₂˙⁻ subtract those decomposed via the aforementioned side reactions from t₀ s to t s upon irradiation. The formula can be expressed as follows:

In this equation, the t₀ and t of the lower and upper limit of the definite integral represent the starting and ending time of UV irradiation respectively. Based on the formation process of O₂˙⁻ mentioned above, the net rate of O₂˙⁻ formation at any time in photocatalytic reactions can be obtained as shown in eqn (7.1). The first term of the right side of the eqn (7.1) represents the formation rate of O₂˙⁻, and it is a second-order reaction with respect to the concentration of –Ti³⁺([Ti³⁺]) and adsorbed oxygen (O_2,s). k_f is a second-order formation rate constant. The second term (eqn (7.1)) represents the decay rate of O₂˙⁻, among which k_d is a second-order decay rate constant, [X] represents the concentration of e, h⁺, ·OH or O₂˙⁻ in terms of eqn (3)–(5). It is generally believed to contain the aforementioned three pathways. We have previously confirmed that the O₂˙⁻ adsorbed on TiO₂ surface is thermodynamically favored.⁹ Therefore, the two former pathways (eqn (3) and (4)) dominated the decay process under UV irradiation. These processes were considered to be pseudo-first-order due to the constant of h⁺, ·OH and e under steady-state irradiation. So is the pseudo-first-order apparent rate constant (eqn (7.2)). [O₂]_s is equal to oxygen coverage on TiO₂ surface (θ) by the number of adsorption sites ([S]) according to Langmuir isotherm (eqn (7.2)), in which θ is related to the DO concentration in solution ([O₂]), adsorption constant (k₁), desorption constant (k₋₁), and formation rate constant of O₂˙⁻ (k_f) (eqn (7.3)).

It has been reported that the electron transfer from TiO₂ to O₂ is the rate-limiting step in TiO₂ photocatalytic reaction.^6,19–21 Upon this, k_f is far less than k₁[O₂] and k₋₁. Furthermore, the [O₂] in solution is low, and thus k₁[O₂] is far less than k₋₁. If such speculation is true, the eqn (7.3) can be eventually transformed into the following:

where K₁ (K₁ = k₁/k₋₁) is the adsorption equilibrium constant of [O₂]. Assuming that [O₂˙⁻] = mCL + n, which is a linear relationship between O₂˙⁻ concentration and CL intensity, the eqn (8) can then be converted into the following:

Finally, the eqn (10) representing the formation model of O₂˙⁻, with respect to time as the independent variable and CL intensity as the dependent variable, would be obtained by integrating these variables with the eqn (9) from t₀ to t:

In eqn (10), k_f, K₁, [S], [O₂], m, and n are constants under certain conditions. The coefficient represents the theoretical maximum CL intensity (CL₀) corresponding to the steady-state concentration of O₂˙⁻ when t is infinite in irradiated TiO₂ suspensions. Herein t₀ is the time when irradiation starts for 50 s, due to the disturbance of background signal within the first 50 s of irradiation. According to eqn (10), given that k_f, K₁, and k_d are determined by the intrinsic property of TiO₂ photocatalyst regardless of experimental conditions, CL₀ is dependent on [Ti³⁺], [S], [O₂] and [X], which are closely related to experimental conditions. In photocatalytic reactions, [Ti³⁺] relies on the number of photogenerated electrons highly dependent on I; [S] is the total surface area of TiO₂ in suspension, closely related to [TiO₂]; [O₂] is dependent on [DO] in TiO₂ suspensions; [X], the aforementioned concentration of h⁺, ·OH and e, is also dependent on I.

In order to verify the formation model of O₂˙⁻, we fit the different CL curves with eqn (10) by changing [TiO₂], I, and [DO]. As shown in Fig. 1(a–i), the CL curves from different [TiO₂], I and [DO] were well-fit by eqn (10) with high correlation coefficients (R² > 0.99), indicating that the model could simulate dynamic process of O₂˙⁻ formation. Furthermore, according to the O₂˙⁻ formation model (eqn (10)), CL intensity is linearly correlated to [S], [Ti³⁺], and [O₂], indicating that CL intensity could increase linearly with the increase of [TiO₂], I and [DO] within set limits. In order to verify this assumption, CL intensity at 300 s, 600 s, 900 s, 1200 s and +∞ under different [TiO₂], I and [DO] conditions in TiO₂ suspensions was calculated by eqn (10) (Table S1†). Then the CL intensity from different [TiO₂], I and [DO] at different time points were linearly fit (Fig. S1–3†), and the R square values were calculated and summarized in Table 1. For instance, the CL intensity with TiO₂ concentration of 0.02 mg mL⁻¹, 0.05 mg mL⁻¹ and 0.1 mg mL⁻¹ at 300 s, 600 s, 900 s, 1200 s and +∞, respectively, had good linear fit with high correlation coefficients (R² > 0.99), except at +∞ (R² = 0.903), confirming that CL intensity increased linearly with the increase of [S] as indicated by eqn (10). There was also a good linear relationship between CL intensity and I with high correlation coefficients (R² > 0.9), except at +∞ (R² = 0.821), indicating that CL intensity increased linearly with the increase of [Ti³⁺]. At different [DO], the corresponding R² values were also high (R² > 0.95), indicating that CL intensity was also linearly dependent on [O₂]. Overall, the formation model of O₂˙⁻ could adequately describe the dynamic process of O₂˙⁻ formation in photo-irradiated TiO₂ suspensions.

Fig. 1 CL curves of luminol (50 μM) with photo-irradiated TiO₂ suspensions (pH = 11) under different experimental conditions: TiO₂ concentration (a–c), irradiation intensity (d–f), DO concentration (g–i). The black lines represent experimentally measured values and the red lines are the fitted values.

Table 1 The calculated value of R square upon different experimental conditions at different irradiation time

Time (s)	TiO2 concentration (mg mL^−1)	Irradiation intensity (mW cm^−2)	DO concentration (mg mL^−1)
300	0.998	0.921	0.999
600	0.999	0.946	0.982
900	0.998	0.960	0.971
1200	0.994	0.972	0.968
+∞	0.903	0.821	0.977

---

In the present work, a formation model of O₂˙⁻ in TiO₂ photocatalytic reactions was established. According to the model, the O₂˙⁻ formation was closely related with [TiO₂], [DO] and I, under which the dynamic process of O₂˙⁻ formation was successfully simulated by the model with high correlation coefficients (R² > 0.9), thereby confirming the model validity. This model can explicitly provide details on O₂˙⁻ formation which determines the photocatalytic efficiency in TiO₂ photocatalytic reactions, and give deeper insights into designing high-efficiency TiO₂ photocatalysts. In accordance with this model, Feng et al. reported the self-doped Ti³⁺ enhanced TiO₂ photocatalyst for hydrogen production through the reduction of the TiO₂ surface using a one-step combustion method.²² Furthermore, this model may have significant implications for other photocatalysts with respect to O₂˙⁻ formation.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors gratefully acknowledge financial support from the Agricultural Science and Technology Innovation Program (ASTIP-TRIC06), National Key Research and Development Program of China (2016YFA0203102), National Natural Science Foundation of China (31601483, 31801646, 21876184, 21677152), Fundamental Research Funds for Central Non-profit Scientific Institution (1610232017011, 1610232016008, 1610232017010, 1610232018002).

Notes and references

1. Y. Nakaoka and Y. Nosaka, J. Photochem. Photobiol., A, 1997, 110, 299–305.
2. A. L. Attwood, D. M. Murphy, J. L. Edwards, T. A. Egerton and R. W. Harrion, Res. Chem. Intermed., 2003, 29, 449–465.
3. M. A. Henderson, W. S. Epling, C. H. F. Peden and C. L. Perkins, J. Phys. Chem. B, 2003, 107, 534–545.
4. E. Carter, A. F. Carley and D. M. Murphy, J. Phys. Chem. C, 2007, 111, 10630–10638.
5. J. Green, E. Carter and D. M. Murphy, Chem. Phys. Lett., 2009, 477, 340–344.
6. M. R. Hoffmann, S. T. Martin, W. Choi and D. W. Bahnemann, Chem. Rev., 1995, 95, 69–96.
7. C. Chen, P. Lei, H. Ji, W. Ma and J. Zhao, Environ. Sci. Technol., 2004, 38, 329–337.
8. J. Yang, C. C. Chen, H. W. Ji, W. H. Ma and J. C. Zhao, J. Phys. Chem. B, 2005, 109, 21900–21907.
9. D. Wang, L. Zhao, D. Wang, L. Yan, C. Jing, H. Zhang, L.-H. Guo and N. Tang, Phys. Chem. Chem. Phys., 2018, 20, 18978–18985.
10. J. Sun, H. Zhang, L.-H. Guo and L. Zhao, ACS Appl. Mater. Interfaces, 2013, 5, 13035–13041.
11. M. W. Sutherland and B. A. Learmonth, Free Radical Res., 1997, 27, 283–289.
12. B. H. J. Blelski, G. G. Shiue and S. Bajuk, J. Phys. Chem., 1980, 84, 830–833.
13. Y. Li, W. Zhang, J. Niu and Y. Chen, ACS Nano, 2012, 6, 5164–5173.
14. S. T. Wang, N. G. Zhegalova, T. P. Gustafson, A. Zhou, J. Sher, S. Achilefu, O. Y. Berezin and M. Y. Berezin, Analyst, 2013, 138, 4363–4369.
15. K.-I. Ishibashi, A. Fujishima, T. Watanabe, K. Hashimoto, K. Ishibashi, A. Fujishima and T. Watanabe, J. Photochem. Photobiol., A, 2000, 134, 139–142.
16. D. Wang, L. Zhao, L. H. Guo and H. Zhang, Anal. Chem., 2014, 86, 10535–10539.
17. Y. Nosaka and A. Y. Nosaka, Chem. Rev., 2017, 117, 11302–11336.
18. T. Daimon, T. Hirakawa, M. Kiazawa, J. Suetake and Y. Nosaka, Appl. Catal., A, 2008, 340, 169–175.
19. C. M. Wang, A. Heller and H. Gerischer, J. Am. Chem. Soc., 1992, 114, 5230–5234.
20. H. Gerischer and A. Heller, J. Electrochem. Soc., 1992, 139, 113–118.
21. A. M. Peiró, C. Colombo, G. Doyle, J. Nelson, A. Mills and J. R. Durrant, J. Phys. Chem. B, 2006, 110, 23255–23263.
22. F. Zuo, L. Wang, T. Wu, Z. Zhang, D. Borchardt and P. Feng, J. Am. Chem. Soc., 2010, 132, 11856–11857.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c9ra06323d

---