X-ray diffraction (XRD) analysis was performed to determine the crystalline phase and crystallite size of TiO2 samples (Fig. 1 and Table 1). The peaks appearing at 25º and 48º, correspond to the lattice planes of 101 and 200 (JCPDS 21-1272) of TiO2 anatase phase. The peaks at 27º, 36º and 54º with lattice planes of 110,101 and 211 (JCPDS 21-1276), respectively reveal the presence of rutile phase. As displayed in Fig. 1 the diffraction peaks of both anatase and rutile phases of TiO2 samples become intense as the calcination temperature increases, suggesting that TiO2 samples are composed of irregular polycrystalline materials, and the crystallinity for both phases is improved by rising the temperatures.
Representative SEM images were obtained to visualise the morphology of the different TiO2 samples (Fig. 2). Results reveal an irregular shape of TiO2 particles with the increase of the calcination temperature. For TiO2-500 and TiO2-600 (Fig. 2a and b), spherical-like TiO2 particles are observed with distinct sizes aggregate into clusters. At temperatures over 700 ºC, it seems that small particles coalesce to form larger particles (Fig. 2c), due to the transformation of anatase into rutile phase (Fig. 1). At 900 ºC the transformation of anatase-rutile is completed and agglomerations of more a uniform size of TiO2 particles are noticed (Fig. 1e).
The TEM and high-resolution transmission electron microscopy (HRTEM) were used to further investigate the microstructure and composition of representative TiO2 sample (TiO2-700, Fig. 2c). In this case, the particles show a typical size of ca. 100 nm, for its crystallized phase (Fig. 1c-i, and 1c-ii, respectively). In Fig. 2c-iii, the disordered layers on the surface of the TiO2-700 sample are visible and marked with orange dashed lines, suggesting the presence of defects that can provide higher carrier concentration and more active sites19.
HRTEM image revealed well-resolved crystal lattices in a representative TiO2-700 (Fig. 3c-iv), and the inter-planar spacing values of 0.32, 0.35 nm correspond to the (110) and (101) crystal planes of rutile and anatase TiO2, respectively, verifying the co-existence of rutile and anatase in a single nanoparticle, consistent with the XRD results.
Specific surface areas of TiO2 samples were determined by nitrogen adsorption at 77 K (Table 1). As expected, the results show a decrease of SBET with the enhancement of the calcination temperature, agreeing with the SEM observations.
The TiO2 samples were examined by PL spectroscopy at room temperature under an excitation energy of 4.43 eV (280 nm) to investigate the incidence of charge separation upon light excitation (Fig. 3). The intense PL emission in the UV and visible regions is commonly attributed to excitons recombination and the presence of defect sites, respectively20. The samples TiO2-500 (Fig. 3a) and TiO2-600 (Fig. 3b) show a broad peak at 3.0 eV ascribed to the crystallinity of TiO2. By increasing the annealing temperature (samples c, d and e), a noticeable red shift is observed for 2.93 eV, suggesting that the TiO2700, TiO2-800 and TiO2-900 samples exhibited higher crystallinity, as confirmed by HRTEM observations (Fig. 2). Additionally, these strong peaks (2.93 and 3.00 eV) have been ascribed to the self-trapped excitons in anatase and free excitons in rutile, respectively20. The peaks at 2.63–2.82 eV arise from the excitation electrons/holes recombination via oxygen vacancies and defects in both anatase and rutile phases of TiO2.
The diffuse reflectance spectra of the TiO2 samples were performed (Supplementary Fig. S1,) and the respective bandgap values were obtained by indirect Tauc plot analysis (Table 1). As displayed, increasing the calcination temperature the bandgap of TiO2 samples slightly diminish, which may indicate faster electron/hole recombination rates21.
Table 1
– Crystallization parameters, specific surface area (SBET), bandgap and oxygen defects of TiO2 samples.
Sample | Anatase (A)/ rutile (R) ratio (%) | Crystallite size (nm) (A) | Crystallite size (nm) (R) | SBET (m2 g− 1) | Bandgap (eV) | Oxygen defects (%) |
TiO2-500 | - | 19.4 | - | 36.8 | 3.21 | 16.8 |
TiO2-600 | 57.8 | 17.4 | 30.1 | 24.6 | 3.20 | 24.5 |
TiO2-700 | 88.8 | 43.7 | 49.2 | 17.1 | 3.13 | 31.7 |
TiO2-800 | 73.1 | 25.3 | 34.6 | 8.3 | 3.08 | 30.7 |
TiO2-900 | - | - | 43.7 | 6.2 | 3.05 | 26.1 |
We further have examined the surface chemical states of TiO2 using XPS spectroscopy. Given that XPS allows investigating the surface atomic constituents, it has been used to characterise the degree of oxygen defect according to the relative element contents, and the intensities and positions of peaks22,23. Among the defects identified in TiO2, the presence of oxygen vacancies (Ov) can act as active adsorption sites, and strongly influence the reactivity of the photocatalysts22. Additionally, the formation of Ov commonly leads to the creation of unpaired electrons that can generate donor levels in the electronic structure of the TiO224,25. As observed in Fig. 4 the O 1s XPS spectra of the TiO2 samples were deconvoluted into three main peaks at 528.8 eV, 529.3 eV and 529.9 eV energies. The peak located at 528.8 eV is assigned to Ti-O-Ti bonding and lattice oxygen (OL) within the TiO2 structure24. In contrast, the peaks at 529.3 eV and 529.9 eV energies are associated to surface chemisorbed hydroxyl groups (Oads) and Ov, respectively. As displayed in Table 1, the area proportional to oxygen defects increase with the calcination temperature of TiO2 samples until 700 ºC (TiO2-500, TiO2-600, and TiO2700; 16.8%, 24.5%, and 31.7% respectively). For highest calcination temperatures (TiO2-800, TiO2-900; 30.7% and 26.1%, respectively) the oxygen defects start to decrease, suggesting that surface Ov concentration gradually diminishes with temperatures above 700 ºC, being consistent with the literature24, 26.
Oxygen evolution reactions
Photocatalytic oxygen evolution was evaluated by TiO2 samples calcined at various temperatures under UV-LED radiation using an aqueous Fe(NO3)3·9H2O solution acting as sacrificial electron acceptor. Although the mechanism of O2 evolution using silver cations (Ag+) as electron acceptors is well known in the literature15,27 the photocatalytic generation of molecular oxygen is followed by the deposition of metallic silver nanoparticles on the surface of the photocatalyst. Irreversible reactions accompany this phenomenon due to the plasmonic adsorption band of the silver particles15. Due to this issue and the high documented studies using Ag+ ions as sacrificial agent in photocatalytic water oxidation preliminary experiments were performed under the same operational conditions using this electron acceptor for comparison purposes (Supplementary Fig. S2). The results revealed a slight increasing efficiency for O2 production in the first 30 min using Ag+ ions solution compared with the Fe3+. However, with the reaction time a progressive reduction of Ag+ into Ag0 and the oxygen evolution rates decreases.
Concerning the blank experiments (i.e., dark conditions, absence of catalyst and sacrificial agent), using a Fe3+ aqueous solution no formation of O2 was noticed. Contrary, photocatalytic water oxidation reactions revealed that all the tested samples were found to generate O2 upon UV-LED irradiation, although the efficiencies were dependent on the calcination temperature of TiO2 samples (Fig. 5). In general, the results show that when the calcination temperature is raised, the efficiency of the photocatalytic process is enhanced. Nevertheless, after a specific temperature (800 ºC), a decrease in O2 evolution was observed. Among the photocatalysts, the TiO2-700 sample showed the best performance for water oxidation with 8.95 µmol·min-1·gcat-1 of dissolved oxygen being detected.
The calcination treatment commonly affects the physicochemical properties of optical semiconductors, such as crystalline phase and crystallite size, specific surface area, and oxygen surface defects. Therefore, it is commonly accepted that a higher specific surface area (SBET) promotes a significant number of active adsorption sites and improves reactivity. Yet, from the results it is unlikely that the efficiency of the TiO2 samples is related to their specific surface area once was found a decrease in the SBET with the rise of the calcination temperature. This diminution on the SBET was more noticeable for the samples calcined at 800 and 900 ºC due to the enhancement of the TiO2 particles size as observed by SEM micrographs (Fig. 2). Additionally, the presence of higher percentage of rutile phase in these samples may indicate faster electron/hole recombination, which may explain the decrease in the efficiency of the water oxidation process24.
Although in terms of oxygen evolution, effect of the particle size and the crystallite phases of the photocatalysts is not clearly understood, some reports suggest that these factors may contribute to the efficiency of photocatalytic water oxidation. Hiroshi K. et al. 28, ascribed the performance of the water oxidation photocatalytic process to the crystallization from anatase to rutile phase. More recently, Maeda et al.29 have attributed the high efficiency of TiO2 catalysts for oxygen evolution to the enhancement of the particle size which seems to hinder the electron/hole recombination. Higher efficiencies for O2 production have been assigned to the TiO2 rutile phase unlike what happens in the degradation of organic compounds using TiO2 anatase30,31. To understand the efficiency of the developed TiO2 samples in terms of crystallization, experiments using commercial 100% TiO2 anatase and 100% rutile were tested under the same experimental conditions (Fig. 5; A and R, respectively). As shown, high efficiency was found with the commercial TiO2 rutile. Nevertheless, for the synthesised TiO2 calcined at 900 ºC 100% rutile (Table 1) the performance for O2 production was lower compared with the TiO2-700 and also with TiO2-800 samples, suggesting that the other factors account to the efficiency of the process using the TiO2 samples prepared by sol-gel method. As documented, the ratio between surface anatase and rutile particles increases the photocatalytic activity in water splitting reactions32,33. As displayed in Fig. 6, the performance of the photocatalytic water oxidation for O2 evolution was achieved when the ratio between anatase and rutile crystalline phases are superior (88%, Table 1), corroborating with the reported investigations33,34.
Moreover, defects at catalysts surfaces can play an essential role in the physical-chemical properties of TiO2. Among the defects in TiO2, oxygen vacancies are crucial to improving the reactivity of the photocatalytic processes and are thought to be the predominant defects in metals oxides19,24. Amano et al. 33 investigated the photocatalytic activity of a TiO2 commercial sample treated at 700 ºC under H2 flow. The authors ascribed the photocatalytic performance to an increase in the density of the oxygen vacancies at the photocatalyst surface, which enhanced the electrons availability in the conduction band.
Regarding the characterisation results, it can be inferred that the high reactivity of the TiO2‑700 sample for water oxidation is due to the higher percentage of oxygen vacancies at the catalyst surface as was found by XPS analysis. This higher density of oxygen vacancies may facilitate the charge carrier diffusion in TiO2, improving the photocatalytic activity for O2 production. Although, the percentage of oxygen vacancies in the TiO2-800 sample (30.7%) is negligible compared with the TiO2-700 sample (31.7%), a decrease in the rutile particle size was observed (Table 1). The same behaviour was detected for TiO2‑900, i.e., smaller rutile particle size than the TiO2-700, suggesting that excess thermal treatment is being used to decrease the oxygen vacancies, leading to the photocatalytic decreasing performance, being reliable with literature26. Additionally, tries to obtain electrochemical evidence of the oxidation potential of the photocatalysts were performed by supporting the TiO2 samples on conductive ITO electrode. The TiO2-700 and TiO2-800 samples (Supplementary Fig. S3) revealed an oxidation peak between 0.5 and 0.7 V vs SCE, suggesting that positive holes can be generated in the photocatalysis process, compatible with the observed results for photocatalytic oxygen production35.
As described above, a significant number of investigations have been devoted to understanding the photocatalytic water oxidation using Ag+ cations as sacrificial electron acceptors, yet, using Fe3+ aqueous solutions few reports have been documented. Concerning the results obtained with the TiO2 samples it seems reasonable to propose that using the Fe3+ ion solutions the overall process follows the following equations:
![](https://myfiles.space/user_files/58853_cdc0f79cc190fa60/58853_custom_files/img1628626696.png)
Additionally, as the O2 is being accumulated in the reaction system the oxidation of Fe2+ into Fe3+ may also occur (eq. 8). Although in practice the oxidation in acid media is hard, at a moderate pH the iron(III) oxide hydrate (Fe2O3) can precipitate, and a reduction reaction of Fe3+ to Fe2+ can take place quickly (eq. 9). As a result, hydrogen peroxide (HOOH) could be formed, occurring again the oxidation of Fe2+ into Fe3+ (eq. 10). Nevertheless, this last hypothesis seems to be challenging to occurs, as the maximum pH measured during the reactions was ca. 2.28.
![](https://myfiles.space/user_files/58853_cdc0f79cc190fa60/58853_custom_files/img1628626727.png)
Although the set of equations 3 –7 seems to describe the results obtained with the TiO2 samples, a maximum amount of oxygen was evaluated after 60 min in the reaction conditions; after that, the reaction system achieved the steady state (Supplementary Fig. S4). This can be ascribed to the reaction system configuration used, i.e., as the process performance was evaluated in terms of in situ O2 generation with the product remaining in reaction, which may indicate the O2 solubility in the aqueous media was reached.
Therefore, considering the above results, a potential technological application for oxygen production was assessed using the TiO2-700 immobilised in a glass slide and tested under continuous mode (Supplementary Scheme S1 b). Using this reactor configuration we can overcome the practical problems that arise from using a catalyst in powder form and ensure continuous feed of sacrificial electron acceptors. The apparent quantum efficiency (AQE) was examined for both reaction systems by the following equation:
![](https://myfiles.space/user_files/58853_cdc0f79cc190fa60/58853_custom_files/img1628626774.png)
where n is the amount of O2 molecules, NA the Avogadro constant, h the Plank constant, c the speed of light in vacuum (m/s), A the irradiated area (78.5 and 19.8 cm2, batch, and continuous systems, respectively), I the exposed irradiation intensity in each system, t the time of the reactions (s), and λ the wavelength of the monochromatic LED (384 nm).
As displayed in Fig. 7 a, after 60 min of reaction the concentration of O2 is lower operating in continuous mode (5.46 µmol·min-1·gcat-1) comparing with slurry mode (8.95 µmol·min‑1·gcat-1). Nevertheless, it is important to refer that a larger amount of catalyst per volume is used in the slurry system (0.120 g) in contrast with the continuous mode (0.013 g). Nevertheless, the results reveal that the immobilisation of TiO2-700 as film enables a remarkable efficiency for O2 production, exhibiting AQE exceeding 12% (Fig. 7 b) at 385 nm compared with the slurry mode. This configuration system may constitute a step forward for developing efficient and viable systems capable of producing O2 in continuous-flow reactors, reducing the operating costs and allowing large-scale production.