Generation of the Reactive Oxygen Species on the surface of nanosized titanium(IV) oxides particles under UV-irradiation and their connection with photocatalytic properties
Graphical abstract
Photocatalytic activity of TiO2 (difenoconazole, thiamethoxam, methyl orange, UV-irradiation) is determined by the content of ROS (OH, O2―, H2O2).
Introduction
In recent years (2000–2018), titanium dioxide with its broad spectrum of properties has been the object of researcher’s attention, and the transition to the nanosized form has extended the possibilities of TiO2 using allowing production of inorganic sorbents, fabrication of new catalysts and their carriers, ceramics with special thermal, optical, and piezoelectric properties, complex nanohybrid constructions for the addressed delivery of drugs, etc. Due to the high chemical inertness, the absence of toxicity, and low cost, nanodimensional TiO2 has found the increasingly larger application as efficient and environmentally pure photocatalyst (for the photodegradation of various contaminants in aqueous and air media) because of its ability to oxidize almost all toxic organic substances [[1], [2], [3], [4], [5]] and efficiency as a bactericidal agent [5,6] and resultant component for annihilation of tumor cells in the cancer treatment [[4], [5], [6]].
The essence of the photocatalytic oxidation of organic compounds is the formation of the electron-hole pair under the influence of the illumination energy (hν) in semiconductor TiO2 particles (the band gap width of titanium dioxide modifications is from 3.0 to 3.3 eV; therefore, all TiO2 modifications absorb the radiation only in the ultraviolet spectral region up to 380 nm). The use of nanodimensional TiO2 particles leads to a considerable increase in its photocatalytic activity (PCA), and the doubtless advantage of nanoparticles when compared with microparticles is a large probability of the charge escape on the catalyst surface. Herewith, the holes, when escaping on the particle surface in the solution, enter the reaction with electron donors or OH ions with the formation of strong oxidizers (hydroxyl OH). In their turn, conduction electrons on the surface of TiO2 nanoparticles interact with oxygen [[7], [8], [9]], which promotes the formation of superoxide anion radicals (O2− OO-), and with organic substances, which can serve as electron acceptors [10]:TiO2 + hv → TiO2 (e¯VB + h+CB) (CB – conduction band, VB – valence band)h+ + H2O → H++ OHh+ + OH¯ → OHe¯ + O2→ O2¯O2¯ + H+→ HO2¯O2¯ + 2H+ → H2O2OH + OH → H2O2O2¯ + OH → 1O2 + OH¯O2¯ + h+→ 1O2O2¯ + e¯ → 1O2organic substances + OH (or h+, or e―) → oxidation products [9,11,12].
All these reactions result in the totality of very active radicals (OO−, HOOH, OH, 1O2), which are called Reactive Oxygen Species, or ROS [13]. Due to the formation of ROS the titanium(IV) oxides nanoparticle surface is the strong oxidizer and, as result, complete decomposition of harmful organic pollutants to CO2 and H2O occurs [9,11,12]. As the penetration depth of the UV radiation into TiO2 particles is limited (∼100 nm), only their outer surface is active [14].
To directly develop the material based on titanium(IV) oxides (NTO), it is necessary to know their functional possibilities in the certain application area and reasons causing some operational properties implementation, which are determined by the NTO characteristics, in particular, by the form and content of ROS - one of the main factors of the photocatalytic activity of nanoobjects.
The direct method of ROS analysis is the electron paramagnetic resonance (EPR) method that makes it possible not only to detect but also to identify many radicals by the analysis of the hyperfine structure of EPR signals. However, it turns out to be insufficiently sensitive in some cases because of the extremely low time-independent concentration of radicals in the objects under study. The chemiluminescence (CL) method possesses a series of advantages [[15], [16], [17]]: as a rule, it is not associated with a change in the course of processes in different media where luminescence is recorded and is very sensitive in the detection of highly reactive radicals. Currently the CL method has been used in theoretical (physiology, biophysics, molecular biology, pharmacology, and biochemistry) and applied (laboratory-diagnostic service, toxicology, surgery, stomatology, oncology, pulmonology, and physiotherapy) disciplines, but not in the materials science of functional nanoobjects.
To detect O2¯ and OH radicals on the surface of titanium dioxide nanoparticles (anatase and rutile), the chemiluminescence method with the use of luminol and terephthalic acid has been applied [18]. The formation of H2O2 (in a presence of lucigenin), anion radicals OH (with coumarin) and O2¯ (with luciferin) on the surface of TiO2 nanoparticles with rutile or anatase structure were found using EPR method, IR spectra and chemiluminescence method under UV-irradiation [19]. To detect anion radicals OH and O2¯ on the surface of commercial Degussa P25 (a mixture of anatase and rutile) nanoparticles in the presence of luminol the chemiluminescence method has been used [20].
The goal of our work is the application of the chemiluminescence method to determine the content of Reactive Oxygen Species (ROS) in aqueous systems with nanodimensional titanium(IV) oxides of various modifications and to establish the connection between the photocatalytic properties and ROS content correlated with the maximal amount of emitted chemiluminescence photons.
Objects of research are well-known commercial samples (Hombifine N, Hombikat UV100, Degussa (Evonic) P25) and prepared in present research samples with nanosized anatase, little-known nanosized titanium(IV) oxide, i.e. η-phase (superstructure of anatase structure [21]) and peroxide phase, which is derived from η-phase [22]. All these objects exhibit high photocatalytic activity in the reaction of photodecomposition of model dyes methyl blue and methyl orange [22,23].
Section snippets
Experimental and methods
The objects of the investigation were three groups of the samples with nanodimensional titanium(IV) oxides – NTO.
Group I - commercial samples 1–5 with nanodimensional anatase: sample 1 – Hombifine N («Sachtleben Chemie GmbH», Germany), sample 2 – Hombikat UV100 («Sachtleben Chemie GmbH», Germany), sample 3 – “Anatase I” (Sigma-Aldrich), sample 4 – “Anatase II” (Russia), and sample 5 – Degussa P25 (Aeroxide), containing the mixture of nanosized anatase and rutile.
Group II - samples with
Results and discussion
Fig. 1, Fig. 2, Fig. 3 show X-ray diffraction patterns of studied samples.
Samples 1–4, 6 and 7 contain nanosized anatase (JCPDS № 89-4921) with different crystallite size (Fig. 1, lines 1–4 and Fig. 2, lines 6 and 7; Table 1). Commercial Degussa P25 (sample 5) is a mixture of anatase (∼85 %) and rutile (Fig. 1, line 5). Samples 9 and 10 (Fig. 3, lines 9, 10) contain nanoscale η-phase with composition TiO2–x×mH2O (m = 2.5–3.0) [21], the structure of which is a superstructure to anatase
Conclusions
The application of the chemiluminescence method for determining the content of reactive oxygen species (ROS) in nanoobjects of a broad spectrum (commercial samples with nanosized anatase, the samples with nanodimensional anatase prepared by hydrolysis of titanium-containing precursors of various compositions, the samples with the poorly known η-phase with the composition TiO2-x×mH2O and peroxide phase - TiOx(O2)2-x×mH2O - the substitutional solid solution of oxygen by the peroxide group in
CRediT authorship contribution statement
Sergey P. Mulakov: Formal analysis, Investigation. Pavel M. Gotovtsev: Investigation, Resources. Asiya A. Gainanova: Resources, Investigation, Writing - original draft, Validation. Galina V. Kravchenko: Resources, Investigation. Galina M. Kuz’micheva: Writing - review & editing, Validation, Funding acquisition. Vadim V. Podbel’skii: Software.
Declaration of Competing Interest
The author declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
We thank the Ministry of Education and Science of the Russian Federation (No.4.1069.2017/PCh.; 2017–2019) for support of this work.
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