**1. Introduction**

Imidacloprid (IMI), which is the most widely used pesticide in the group of neonicotinoids, is a pesticide that is used in agriculture such as in crop protection against aphids, leafhoppers, psyllids beetles, etc. [1], and parasite management [2]. The use of neonicotinoids has been registered in approximately 120 countries worldwide [3], and IMI is one of the top ten global agrochemicals used as a pesticide worldwide [4]. It acts as a nicotinic acetylcholine receptor (nAChR) agonist that interferes with the transmission in the central nervous system of insects and results in paralysis and death [5]. With their widespread use, persistent nature, and high solubility (610 mg/L in 20 ◦C H2O; log Kow = 0.57), IMI can cause damage to the environment via transportation in water, soil, and air [6]. Furthermore, the use of IMI can affect human health which includes neurological effects [7,8], in addition to gastrointestinal symptoms, lethargy [9], emaciation thyroid lesions, and cardiorespiratory failure [10]. Thus, the removal of these pollutants from water is essential due to their harmful influence on human health and aquatic ecosystems. Various methods can be applied for the degradation of IMI from aqueous solutions such as microfiltration membrane [11], biological degradation [12], adsorption [13,14], and advanced oxidation processes (AOPs) [15,16]. Among the AOP methods, photocatalytic activity has been used effectively in wastewater treatment for the removal of organic pollutants due to its simplicity, high activity, low cost, and ability to reduce CO2 [17,18].

**Citation:** Kobkeatthawin, T.; Trakulmututa, J.; Amornsakchai, T.; Kajitvichyanukul, P.; Smith, S.M. Identification of Active Species in Photodegradation of Aqueous Imidacloprid over g-C3N4/TiO2 Nanocomposites. *Catalysts* **2022**, *12*, 120. https://doi.org/10.3390/ catal12020120

Academic Editors: Ioan Balint and Monica Pavel

Received: 23 December 2021 Accepted: 16 January 2022 Published: 19 January 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Graphitic carbon nitride (g-C3N4) has attracted significant attention as a visible photocatalyst for water purification due to its stability, high surface area, eco-friendliness, and facile synthesis [19,20]. However, the disadvantage of pure g-C3N4 is the fast recombination of photogenerated electron–hole pairs which lead to low photocatalytic efficiency [21]. Many strategies have been tried to improve the photocatalytic performance such as nanostructure design [22,23], metal and non-metal doping [24], and composite photocatalysts [25–28]. Among the various strategies, photocatalysis by coupling with other semiconductor materials is a beneficial method to improve the electron recombination process and extend the visible light absorption, which can enhance the photocatalytic performance. TiO2 is an n-type semiconductor that has been widely used owing to the high efficiency, low cost, non-toxicity, and long-term stability of this compound. However, because of the large band gap energy of 3.2 eV of TiO2, this results in the ineffective utilization of visible light, low quantum efficiency, and fast recombination [29]. It is expected that coupling TiO2 with g-C3N4 can improve electron–hole pair recombination, broaden the photo-response range, and promote oxidation and reduction processes.

Herein, g-C3N4/TiO2 photocatalysts were synthesized by a simple hydrothermal method. The phase structure, chemical composition, morphology, and scavenger trapping were investigated in detail. The g-C3N4/TiO2 photocatalysts were used to degrade imidacloprid pesticide in wastewater under UV-Vis light irradiation. The recyclability of the composite was studied. In addition, the possible photodegradation mechanism was also proposed in this study.

### **2. Results and Discussion**

#### *2.1. Characterization*

The XRD patterns of bulk-CN, CNS, TiO2, and 0.5TiO2/g-C3N4 are shown in Figure 1a. The g-C3N4 has two main diffraction peaks at 13.1◦ and 27.5◦, which corresponds to the (001) plane caused by the arrangement of the tri-s-triazine units and the (002) plane caused by the interlayer stacking of the conjugated aromatic ring (JCPDS 87-1526) [29]. After the exfoliation of the bulk-CN, the decrease of CNS intensity (002) peak indicated that the interlayer structure was partially destroyed [30,31], and the slight shift of the (002) peak is attributed to the decreased distance of the basic sheets in the nanosheets [32]. The peaks of pure TiO2 at 25.3, 37.8, 48.0, 53.9, 62.7, 68.8, 70.3, 75.0, and 82.6◦ correspond to the (101), (004), (200), (211), (204), (116), (220), (215), and (224) crystal planes of anatase TiO2 (JCPDS 21-1272) [33]. Hydrothermally synthesized g-C3N4/TiO2 photocatalysts showed the patterns related to both pure g-C3N4 and TiO2. In addition, there is no obvious change in the peaks of TiO2 in the composites, indicating that coupling with g-C3N4 did not influence the phase structure of TiO2.

The Raman spectra of g-C3N4, TiO2, and g-C3N4/TiO2 composites are shown in Figure 1b. The characteristic peaks of g-C3N4 appeared at 707 cm−<sup>1</sup> and 1230 cm−<sup>1</sup> which were assigned to the breathing modes of the tri-s-triazine ring and C-N heterocycles, respectively [34]. Moreover, all the Raman bands observed for bulk-CN can be found in the CNS. The Raman spectrum of pure TiO2 exhibited peaks at 148, 395, 510, and 640 cm−<sup>1</sup> corresponding to anatase-phase TiO2 [35]. 15CNS/TiO2 showed a combination peak of g-C3N4 and TiO2 which confirms the formation of composites. No peak shifts were observed, which means no structural changes occurred during the preparation of the composites with pure TiO2 and g-C3N4.

**Figure 1.** (**a**) Powder XRD patterns of bulk-CN, CNS, TiO2, and 0.5CNS/TiO2. (**b**) Raman spectra of bulk-CN, CNS, TiO2, and 15CNS/TiO2. Figure S1 shows the PXRD of the rest of nanocomposites.

The chemical binding states of g-C3N4, TiO2, and composites were studied through XPS analysis. Figure 2a displays the survey scan of bulk-CN, CNS, TiO2, and g-C3N4/TiO2 in various weight ratios which confirmed the presence of C, N, Ti, and O atoms in the composites. Figure 2b shows three high-resolution C 1s spectrums at binding energies of 285.0, 288.3, and 289.2 eV, assigned as C-C, N–C=N, and sp<sup>2</sup> hybridized carbon in the tri-s-triazine ring (N2-C=N) for g-C3N4.

Four binding energies in N 1s spectra (Figure 2c) can be observed, which can be classified into to sp2 hybridized nitrogen C-N=C (398.8 eV), tertiary nitrogen N-(C)3 (399.2 eV), amino functional groups N–H (400.3 eV), and π-excitation (401.2 eV), respectively [36–38]. The C 1s and N 1s spectra are slightly shifted from primitive g-C3N4 which suggests that there is a chemical bond connection between g-C3N4 and TiO2 [39]. The C/N ratio of g-C3N4 is 0.90, indicating the presence of nitrogen vacancies that probably occurred during the thermal reduction process [40]. EPR spectra can provide evidence for probing the surface vacancies in photocatalysts. As shown in Figure 2f, the EPR intensity signal of CNS is significantly enhanced, revealing the increase of nitrogen vacancies generated in gC3N4 [41]. Figure 2d shows the high-resolution Ti 2p spectrum. The binding energy peaks of Ti 2p3/2 and Ti 2p1/2 appeared at 459.3 and 465.0 eV, which represent Ti4+ species in the form of TiO2 clusters [42]. In addition, there might be another Ti species in the material due to the poor XPS peak fitting for the Ti4+ alone. A better XPS profile fitting was later obtained by including a peak at 460.2 eV, being assigned as the Ti3+ defects on the composite surface [43,44]. The O 1s spectrum in Figure 2e can be devised into three peaks in TiO2 with the binding energy of 530.5, 531.9, and 533.2 eV which can be assigned to (Ti-O), oxygen vacancy (Vo), and water molecules adsorbed on the surface of TiO2, respectively [45]. Figure 2f shows the result of the solid ESR measurement which was used to confirm the presence of Ti3+. A strong EPR signal of TiO2 and the composites was observed with g of 1.997, which corresponds with Ti3+ defect (3d1, S = 1/2) and oxygen vacancy (Vo) [46]. It is possible that Ti4+ was reduced to Ti3+ by the loss of oxygen from the surface of TiO2 because of the hydrothermal treatment at a high temperature [47].

**Figure 2.** (**a**) The survey scan of all samples. (**b**) The C1s spectra. (**c**) The N1s spectra. (**d**) The Ti 2p spectra. (**e**) The O1s spectra. (**f**) Solid EPR spectra of g-C3N4, TiO2, and g-C3N4/TiO2.
