Photocatalytic Degradation of Rhodamine B by C and N Codoped TiO 2 Nanoparticles under Visible-Light Irradiation

CandNcodopedTiO 2 nanoparticlesweresynthesizedviaasolvothermalmethod.ThedegradationofRhodamineBbythephotocatalyst C, N-TiO 2 wasinvestigated under visible-light irradiation generatedby using a 36W compact ﬂuorescent lamp which is characterized by wavelengths from 400 to 650nm. The structure and properties of the obtained photocatalyst have been systematically investigated using X-ray diﬀraction, TEM, UV-Vis, FT-IR, and BET techniques. The experimental results revealed that C, N codoped TiO 2 nanoparticles were successfully synthesized, with an average diameter of 9.1nm. C, N-TiO 2 nanoparticles exhibited an energy band gap of 2.90eV, which were lower than pristine TiO 2 (3.34eV), C-TiO 2 (3.2eV), and N-TiO 2 (3.03eV). The degradation of Rhodamine B by C, N-TiO 2 indicated that, under visible-light irradiation, the optimal dose of the photocatalyst was 1.8g/L, and the removal of Rhodamine B was almost complete after 3 hours of reaction. The photocatalytic degradation of Rhodamine B in the range of 5–100mg/L showed that the process followed the ﬁrst-order kinetics according to the Langmuir–Hinshelwood model. The highest apparent rate constant (0.0427min − 1 ) was obtained when theinitial concentrationof Rhodamine B was 5mg/L, whereas the former decreased with the increase in the initial concentration of Rhodamine B. Moreover, C and N codoped TiO 2 nanoparticles presented a high potential for recycling, which was characterized by a removal eﬃciency of more than 86% after three cycles.


Introduction
TiO 2 is one of the well-known photocatalysts because of its nontoxicity, chemical stability, ease of processing, low cost, and multiple reusability. However, TiO 2 is only active under the ultraviolet light (Eg∼3.2 eV in the anatase phase), which prevents it from various applications. e doping of nonmetals such as N, C, S, P, and halogens in TiO 2 would lead to an increase in catalytic activities in the visible-light region [1][2][3][4][5]. Particularly, the modification of TiO 2 by carbon increases the photosensitivity of the obtained materials [1,2]. It has been reported that N-doping TiO 2 could result in a significant decrease in the energy band gap and an increase in catalytic activity in the visible-light region [3,4].
Rhodamine B (RhB) has been widely used in histologic specimens; textile, paper, and cosmetics industries; RhB is also a well-known water tracer fluorescent [6]. RhB is harmful to humans and animals. Expose to RhB was reported to cause acute symptoms such as burning of the eyes, excessive tearing, nasal burning and itching, chest pain/ tightness and burning, rhinorrhea, cough, burning of the throat, headache, and nausea [7]. e carcinogenicity and reproductive and developmental toxicity of RhB to humans and animals have been experimentally proven [8,9]. erefore, a number of efforts to remove RhB from the aqueous medium have been made. Several techniques have been deployed to deal with RhB residues in waters such as flotation [10,11], activated sludge process [12], chemical oxidation [13,14], adsorption [15][16][17], and membrane filtration [18,19]. Recently, advanced oxidation processes (AOPs) in a particular photocatalytic approach have been widely applied for the degradation of RhB in waters. For instance, Phuruangrat et al. [20,21] studied parameters influencing RhB degradation in a solution by Bi 2 WO 6 -based catalysts and found that at optimal conditions, RhB degradation efficiency could reach more than 98%. Similarly, Lee et al. [22] reported that a photocatalyst based on a polydimethylsiloxane (PDMS)-TiO 2 -gold (Au) composite was able to degrade about 85% of RhB after 90 min of visiblelight irradiation. at photocatalytic system also presented a high potential of recyclability, with a degradation efficiency of more than 80% after four cycles of RhB treatment. Le et al. [23] revealed synergic effects in the Fe-C-TiO 2 /AC photocatalytic system and reported a very good catalytic activity of the system for the degradation of RhB in solutions under visible-light irradiation. After five cycles, there were still more than 80% of RhB in solutions degraded by the Fe-C-TiO 2 /AC photocatalytic system. Similar results were reported by previous studies [24,25]. Until now, no work reports on the degradation of RhB by the C, N-TiO 2 photocatalyst.
In this work, C and N codoped TiO 2 was prepared by combination of the sol-gel and hydrothermal methods. e main objective is to shift light absorption from the UV region to the visible-light region apart from the increase in photocatalytic activity of as-synthesized photocatalysts. Different parameters influencing the photodegradation of RhB by C and N codoped TiO 2 photocatalysts were studied, while the photodegradation of C and N codoped TiO 2 was described by using the Langmuir-Hinshelwood equation.

Catalyst Preparation.
Chemicals: TIOT (tetraisopropyl orthotitanate 98%), nitric acid (HNO 3 68%), ethyl alcohol (C 2 H 5 OH 99.7%) and ammonium chloride (NH 4 Cl) pure, and Rhodamine B (C 28 H 31 ClN 2 O 3 ) were used. Synthesis of C, N-TiO 2 : firstly, two solutions A and B were prepared separately. Solution A was obtained by well mixing 6 mL TIOT with 50 mL C 2 H 5 OH, while solution B contained a mixture of 28 mL C 2 H 5 OH, 0.4 mL HNO 3 (68%), 1.6 mL of distilled water, and 850 mg NH 4 Cl. Solution A was dropped into the solution B under vigorous agitation at room temperature for 2 hours to form a uniform and transparent sol solution. Sol was then aged for 2 days at room temperature to form gel which was then placed into a Teflon-lined stainless steel autoclave and heated at 180°C for 10 hours. After the hydrothermal treatment, the obtained solid was washed using distilled water, prior to being dried at 100°C for 24 hours. e C, N-TiO 2 catalyst was collected after being pulverized in an agate mortar [26][27][28].

Photocatalytic Degradation Experiment.
To investigate the optimal catalyst dosage, 100 mL RhB (20 mg/L) was firstly poured into a beaker (250 mL), which was then added with x (g/L) of the catalyst (x � 1.4, 1.8, 2.6, and 3.0 g/L). en, the obtained solution was stirred with a constant speed for 30 min in the black light to reach the absorption equilibrium. Finally, the solution was irradiated by a 36 W compact fluorescent lamp, and the decomposition time was counted since then. e RhB concentration during the reaction was measured by the photometry method. e removal efficiency of the RhB bay photocatalyst (H%) is calculated based on the initial concentration of RhB (C 0 ) and concentration of RhB (C t ) at time (t), according to the following equation: e effect of irradiation conditions on the RhB decomposition was also investigated.
e experiments were carried out in the dark, natural light at noon, and lights from a 36 W compact fluorescent lamp. e light characteristics of the deployed compact lamp were examined by using a PMS-50 spectrophotocolorimeter which showed that the deployed compact lamp emitted light in the region of 400−650 nm ( Figure S1).

Characterization Techniques.
Crystallite phases of the obtained materials were identified by X-ray diffraction (XRD) measurements (D8 Advance 5005). A scanning electron microscope (SEM, Hitachi S4800) and a transmission electron microscope (TEM, JEOL JEM-1010 electron microscope) have been used to investigate the particle size and morphology of the samples. Wavelength absorption was conducted by UV-Vis (Jasco-V670 photospectrometer). Elemental composition of the catalyst was determined by energy-dispersive X-ray spectroscopy, EDX (JEOL-JSM 6490). Functional groups were identified by IR spectroscopy (IR prestige 21). Nitrogen isothermal adsorption (Brunauer-Emmett-Teller (BET)) was done by TriStar 3000 V6.07 A. RhB concentrations were determined by UV-Vis at 553 nm (the absorption maximum wavelength of RhB).

Characterization of C and N Codoped TiO 2 .
As can be seen in Figure 1, unmodified TiO 2 exhibits the characteristic peaks of both anatase (A) and rutile (R) crystal phases, while modified TiO 2 only shows the characteristics peaks of the anatase crystal phase at 2θ of 25.26°; 37.78°; 38.56°; 48.00°; 53.90°; and 63.92°. is indicates that the C and N codoping in TiO 2 has an effect on the phase formation of the obtained material, which may be attributed to an increase in its catalytic activity [27]. e average crystallite size of C, N-TiO 2 calculated by using Scherrer's equation is 9.1 nm.
is is in good agreement with the results of TEM (Figure 2) measurement. It is noted that the average crystallite size of C and N codoped TiO 2 is smaller than that of unmodified TiO 2 .
e UV-Vis spectra of the prepared photocatalysts are presented in Figures 3 and 4.
e results show that the absorption band of C and N codoping TiO 2 (C, N-TiO 2 ) was significantly expanded to the visible region compared to the 2 Journal of Chemistry absorption bands of C-TiO 2 , N-TiO 2 , and pristine TiO 2 ( Figure 3). e band gap energies estimated by using the Tauc method are 3.34, 3.20, 3.03, and 2.90 eV for pristine TiO 2 , C-TiO 2 , N-TiO 2 , and C, N-TiO 2 , respectively ( Figure 4). e decrease in the energy band gap can be explained as follows: (i) when adding the carbon element, a part of carbon atoms will occupy the interstitial site in the crystal structure which is attributed to the decrease in the energy band gap [26]; (ii) N substitutes O, leading to a significant decrease in the energy band gap [3,27]. Moreover, a part of carbon atoms cover on the surface of TiO 2 in the form of graphite or carbonate groups which would increase the photosentivity [3,29]. e absorption edge shifts to the visible regions would be promising for the application of photocatalysts under sunlight irradiation. e IR spectra showing a broad peak appearing at around 3450 cm −1 could be assigned to vibrations of hydroxyl groups on the surface of TiO 2 in the form of Ti-OH ( Figure 5). In addition, the peak at 1647 cm −1 is likely due to vibrations of hydroxyl groups in the form of Ti-O-OH. e relatively weak peaks at 1540 cm −1 and around 1385 cm −1 could be attributed to the vibration of C�O and C-O in the carbonate group, respectively ( Figure 5). e presence of those functional groups on the TiO 2 surface would lead to an increase in photosensitivity of the as-synthesized catalysts [26]. e presence of the N element in the catalyst obtained is approved by the peak at 1400 cm −1 , which is likely due to vibrations of N-H bonds. In addition, multiple bands at the region of 536-484 cm −1 are assigned to vibrations of Ti-O and Ti-O-C bonds. is is in agreement with the results of EDX analysis (Figure 6), which reveal the presence of Ti, O, C, and N elements in the samples. It is noted that the formation of bonds between C and N elements and TiO 2 could be examined by using the X-ray photoelectron spectroscopy (XPS) method [24,25].    BET analysis was carried out under nitrogen atmosphere to figure out the porous features of the prepared synthesized materials, which is presented in Figure 7. e adsorption/ desorption plots of C, N-TiO 2 show a common type IV isotherm with a H3 hysteresis loop. Furthermore, the BJH analysis shows a wide distribution of hierarchical pores with diameters in the range of meso-macro pores ( Figure S2); the pore volume for C, N-TiO 2 is 0.040 cm 3 /g.
It is noted that the BET specific surface area of the prepared C, N-TiO 2 is 20.3 m 2 /g. is figure is slightly higher when compared to the values previously reported for pristine TiO 2 . For instance, Hussain et al. [30] reported that TiO 2 commercial (anatase/technical) could present a BET specific surface area in the range of 10-15 m 2 /g, while the increase in rutile percentage could lead to an augmentation of the surface area of TiO 2 . Similarly, Shi et al. [31] revealed that anatase TiO 2 exhibited a BET surface area lower than 5 m 2 /g, and the latter would decrease when the treatment temperature increases. Mahlambi et al. [32] synthesized TiO 2 nanoparticles and found that at 550°C their specific surface areas were 22.72 m 2 /g, and this value decreased to 7.78 m 2 /g when the treatment temperature increased to 600°C.

Photocatalytic Performance of C, N-TiO 2 towards RhB
Degradation.
e results in Figure 8 indicate that the RhB degradation efficiency under visible-light condition of C, N-TiO 2 is higher than that of pure TiO 2 . As for C, N-TiO 2 , the degradation efficiency reached up to 94% after 90 min of irradiation. is is consistent with the characteristics of the obtained catalyst.
Studying the effect of the C, N-TiO 2 content on the RhB degradation efficiency (Figure 9) suggests that the optimal amount of the catalyst is 1.8 g/L. Using a large amount of catalyst caused the photoresist, resulting in a decrease in catalytic activity.
According to the results obtained from liquid UV-Vis measurements of the RhB solution by C, N-TiO 2 as a function of treatment time (Figure 10), RhB changed over time. As for the initial RhB solution of 920 mg/L, the characteristic peak at 533 nm moved to the peaks of the intermediate organic compounds. e peak intensity of these compounds decreases with increasing treatment time and is extremely low (near zero) after 3 hours of treatment.
is suggests that the role of C, N-TiO 2 in the RhB degradation is different from the normal absorption processes [33,34].
It should be noted that the RhB degradation efficiency is extremely low in the dark (Figure 11), indicating that the catalyst obtained is a photocatalyst. e RhB degradation efficiency under compact lamp and natural light irradiation is almost the same because of the 36 W compact lamp containing wavelength at 450 nm near absorption maximum of the catalyst. Hence, the obtained catalyst works well in visible light and natural light, and it is in agreement with previous reported works [35][36][37].     Journal of Chemistry

Kinetics of RhB Degradation by C, N-TiO 2 under Visible-Light Irradiation.
Kinetics of the RhB degradation reaction was studied by using different initial concentrations of RhB (0-100 mg/L) while keeping the catalyst load constant at 1.8 g/L. RhB decomposition reaction can be described by the Langmuir-Hinshelwood kinetics equation that is expressed as follows [27]: where k is the reaction rate constant, K is the adsorption coefficient, t is the time, and C is the reactant concentration (the target organic compound). In the case of low initial concentrations, KC << 1, equation (2) can be rewritten as the following: where k' is the apparent rate constant which is also known as the reduced first-order reaction. en, the following equation is derived: where C o and C t are the reactant concentrations at t � 0 and t (min), respectively. Figure 12 shows the dependence of lnC o / C t on time during the degradation of RhB by the C, N-TiO 2 photocatalyst. As can be seen in Figures 12 and 13, when the initial concentrations of RhB were in the range of 0-100 mg/L, the photodegradation was divided into 3 distinct groups, which were characterized by the corresponding distinct apparent rate constants. e decomposition of RhB was observed unstable when the initial concentration was relatively low (0-10 mg/L).
e corresponding apparent rate constant k' is 0.0427 min −1 . In contrast, when the initial concentration was relatively high (>10 mg/L), the RhB decomposition occurred very slowly, resulting in smaller apparent rate constants (k ' � 0.0041 min −1 for 100 mg/L). is is because the bold color of RhB causes the photoresist [23,27]. e suitable initial concentration for stable decomposition is about 20-40 mg/L. e optimal initial concentration of RhB is 20 mg/L at which the apparent rate constant k ' is 0.0205 min −1 . e results obtained in this work is in agreement with previous studies [27,32]. Hence, with the RhB initial concentration of 20 mg/L, equation (4) can be expressed by the following: Similarly, the relation between C o /C t and t for other initial concentrations of RhB could be plotted as in Figure 13. e photocatalytic degradation of RhB by the C, N-TiO 2 system can be explained by the following mechanism ( Figure 14): (1) and (3) • OH and • O 2 − radicals are mainly responsible for the degradation of RhB in solutions [38]. e mechanism for the degradation of RhB in solutions by the C, N-TiO 2 system is proposed in Figure 14.

Recoverability and Reusability of C, N-TiO 2 Photocatalyst.
e catalytic stability in RhB decomposition under visible-light irradiation was studied. After each decomposition cycle, the catalyst was centrifuged, washed with distilled water, and then used for further treatment of RhB in solutions. As can be seen in Figure 15, C and N codoped TiO 2 exhibited good catalytic     activity with the degradation efficiency of above 86% after three cycles, and the obtained results were in agreement with previous studies [39]. It is noted that the absence of RhB and other substances on the surface of the catalyst and no release of catalysts to the water medium need to be proved prior to each subsequent cycle. is could be revealed by using TEM, FT-IR, BET, and extinction spectrum change of the water solution [22]. In this work, the photocatalytic performance is still very high after three cycles (86%), suggesting that the presence of substances on the catalyst's surfaces is insignificant.

Conclusions
In this work, C and N codoped TiO 2 nanoparticles are successfully prepared by solvothermal synthesis and then used for the study on their catalytic activities regarding RhB degradation in solutions under visible-light irradiation. e obtained results showed that the as-synthesized nanoparticles mainly contained anatase crystallites, with an average particle diameter of 9.1 nm. C, N-TiO 2 presented a high catalytic activity in the RhB degradation under visible-light irradiation. e optimal catalyst dosage was 1.8 g/L, while optimal initial concentration of RhB was 5 mg/L. e photocatalytic degradation kinetics was found to follow the first-order rate law of the Langmuir-Hinshelwood model. e apparent rate constant depended on the initial concentration of RhB, which was higher at more diluted solutions of RhB.

Data Availability
All the data used to support the findings of this study are provided within the manuscript.

Conflicts of Interest
e authors declare that there are no conflicts of interest regarding the publication of this paper.