Controlled Microwave-Assisted Synthesis of the 2D-BiOCl/2D-g-C3N4 Heterostructure for the Degradation of Amine-Based Pharmaceuticals under Solar Light Illumination.

Designing efficient 2D-bismuth oxychloride (BiOCl)/2D-g-C3N4 heterojunction photocatalysts by the microwave-assisted method was studied in this work using different amounts of BiOCl plates coupled with g-C3N4 nanosheets. The effects of coupling the 2D structure of g-C3N4 with the 2D structure of BiOCl were systematically examined by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy, X-ray diffraction, photoluminescence (PL), lifetime decay measurement, surface charges of the samples at various pH conditions, and UV-vis diffuse reflectance spectroscopy (UV-vis DRS). The prepared photocatalysts were used for the degradation of amine-based pharmaceuticals, and nizatidine was used as a model pollutant to evaluate the photocatalytic activity. The UV-vis DRS and other optical properties indicated the major effect of coupling of BiOCl with g-C3N4 into a 2D/2D structure. The results showed a narrowing in the band gap energy of the composite form, whereas the PL and lifetime analysis showed greater inhibition of the electron-hole recombination process and slightly longer charge carrier lifetime. Accordingly, the BiOCl/g-C3N4 composite samples exhibited an enhancement in the photocatalytic performance, specifically for the 10% BiOCl/g-C3N4 sample. Moreover, the zeta potential of this sample at different pH values was evaluated to determine the isoelectric point of the synthesized composite material. Consequently, the pH was adjusted to match the isoelectric point of the complex materials, which further enhanced the activity. Further degradation of pharmaceuticals was studied under solar light irradiation, and 96% degradation was achieved within 30 min.

Introduction

The research work nowadays is oriented toward treatment of wastewater and drinking water from various organic contamination, more specifically from pharmaceuticals and personal care products.[1−3] Amine-based pharmaceuticals such as nizatidine, ranitidine, doxylamine, and carbinoxamine have attracted most researchers’ attention because of the ability of these compounds to produce toxic nitrogenous disinfection by-products (NDMPs), which are formed during the disinfection step.[4,5] Unfortunately, in conventional wastewater treatment some NDMPs have been detected after applying commonly used oxidants such as chlorine, chlorine dioxide, and potassium permanganate.[6−8] However, most of the recent studies focused on the occurrence of NDMP compounds and the potential risk on the environment resulting from such compounds.[9−11] On the other hand, only few studies have been undertaken photocatalytic degradation of amine-based pharmaceuticals as an alternative way of wastewater treatment.[12,13]

The field of semiconductor photocatalysis has emerged as an innovative technology for application in many important fields such as water purification, environmental remediation, hydrogen evolution, optical sensing devices, energy harvesting, and storing devices.[14−17] The idea of utilizing the abundantly available solar radiation for driving important chemical reactions has potential for large-scale industrial applications. Different types of semiconductor photocatalysts such as oxynitrides, sulphides, oxides, and metal-free semiconductors have been investigated.[18−21]

The main issue with the excising photocatalysis materials can be summarized in two points; (1) possible use of the full solar spectrum, including the UV region along with the visible region and (2) the high recombination rate of photogenerated electrons and holes, which limit the reaction efficiency.[22−27] One way to utilize the full solar spectrum is a fabrication of heterojunction materials by coupling two semiconductors with compatible band energy potentials, which allows the overlapping of conduction bands and valence bands of both materials, resulting in an enhancement in the light absorption and photocatalytic activity.[28,29] The introduction of 2D/2D layered semiconductors is known as an effective way for separation of the charge carriers because of a larger contact surface.[30−32] It is relatively hard for TiO2 and ZnO to control the morphology of these photocatalyst nanoparticles. Therefore, bismuth oxychloride (BiOCl) is a very attractive catalyst as it can be synthesized in different morphologies and sizes. BiOCl has a band gap energy of 3.4 eV and responds to UV irradiation.[33−35] Recently, different structures of BiOCl have been reported such as spheres, plates, and flower-like structures.[36−39] Wang et al. reported the use of g-C3N4 for water splitting under visible light irradiation.[40] g-C3N4 has a band gap of 2.7 eV, indicating that its visible light active.[41,42] g-C3N4 exhibits the needed excellent properties.[43−45] However, it suffers from a rapid recombination rate, which reduces the overall efficiency.[46,47]

Therefore, the 2D/2D stacked BiOCl/g-C3N4 composite material is a very attractive catalyst from both experimental and theoretical aspects.[48] To date, only a few studies have demonstrated the synthesis of the BiOCl/g-C3N4 composite material via different methods such as hydrothermal, solvothermal, ionic-liquid, and hydrolysis route where the prepared samples have been used mainly for the degradation of dyes such as rhodamine B, methyl orange, and methylene blue. Compared to the mentioned preparation methods, the microwave-assisted method dramatically reduces the experimental time and enhances product purity.[49,50] To the best of our knowledge, the synthesis of the 2D/2D BiOCl/g-C3N4 composite material via microwave-assisted technique for the degradation of an amine-based pharmaceutical such as nizatidine has never been reported.

In this study, we demonstrate a microwave-assisted method for the synthesis of 2D/2D BiOCl/g-C3N4 nanocomposite material. The amount of BiOCl attached to the g-C3N4 sheets was investigated by varying the percentage of BiOCl from 0% up to 50% (pure g-C3N4, 10% BiOCl/g-C3N4, 30% BiOCl/g-C3N4, 50% BiOCl/g-C3N4, and pure BiOCl). The resulting samples were examined by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), UV–vis diffuse reflectance spectroscopy (UV–vis DRS), photoluminescence (PL), and time-resolved PL. The photocatalytic performance of the photocatalysts was evaluated by degradation of an amine-based pharmaceutical, nizatidine. Surface charges of the samples were explored through pH adjustment for better photocatalytic performance.

Results and Discussion

XRD Analysis

The crystalline properties of the prepared BiOCl, g-C3N4, and various BiOCl/g-C3N4 composite samples were investigated by XRD. The XRD pattern of the obtained samples is shown in Figure . The g-C3N4 sample had two unique diffraction peaks at 13.1° and 27.4° corresponding to the (100) and (002) planes of g-C3N4, respectively. The BiOCl sample peaks were indexed to the tetragonal crystal phase with diffraction peaks obtained at 2θ values of 11.98°, 24.10°, 25.86°, 32.50°, 33.45°, 40.90°, 46.64°, 49.70°, 54.10°, 55.12°, and 58.60° which correspond to the respective miller indices of (001), (002), (101), (110), (102), (112), (200), (113), (211), (104), and (212) planes, respectively. All results perfectly matched with previously reported studies[51] (JCPDS card no. 006-0249). For the prepared BiOCl/g-C3N4 composite structure, the diffraction peaks of BiOCl are clearly observable, as well as the typical peak for g-C3N4 (002), indicating the coupling between BiOCl and g-C3N4 where the intensity of the (002) diffraction peak decreased along with the increasing number of BiOCl microplates on the surface of g-C3N4. Moreover, the grain sizes of the synthesized structures were measured using the Maud Software with Cif card number 1011175.[52] For BiOCl sample, the space group and lattice parameters of the samples were P4/nmm, a = b = 3.89 Å, c = 7.37 Å, and α = β = γ = 90° and the grain sizes of 94.6, 90.1, 60.1, and 51.8 nm were found for pure BiOCl, 50% BiOCl/g-C3N4, 30% BiOCl/g-C3N4, and 10% BiOCl/g-C3N4 samples, respectively. The decrease in grain size values could be attributed to the presence of a limited concentration of a BiOCl precursor preventing further growth of the microplate. It was also observed that no other peaks are identified in XRD patterns, indicating the high purity of the BiOCl/g-C3N4 heterostructure.

Figure 1

XRD patterns of BiOCl/g-C3N4 composite samples.

UV–Vis DRS Spectra and Band Gap Analysis

The UV–vis diffuse reflectance spectra of the synthesized samples are shown in Figure a. From the figure, the absorption edge of BiOCl sample was approximately 364 nm, which is in the UV range, whereas the fundamental absorption edge of the g-C3N4 material was about 450 nm, which is considered to be in the visible light range. Moreover, the coupling of the BiOCl sample with g-C3N4 showed a red shift of the band edge absorption up to 476 nm, which is expected to enhance the visible light photocatalytic performance of the heterostructure. The optical band gap was calculated according to the following Tauc equation[53]where α, ν, A, and Eg are the absorption coefficient, light frequency, proportionality constant, and band gap, respectively, where the n value is equivalent to 4 because of the essential absorption of both g-C3N4 and BiOCl in an indirect transition between bands.[54] Therefore, the band gap energy is estimated from the slope drawn near to the band edge of (αhν)1/2 versus hν plots as shown in Figure b. The band gap values for BiOCl, g-C3N4, 50% BiOCl/g-C3N4, 30% BiOCl/g-C3N4, and 10% BiOCl/g-C3N4 samples were 3.40, 2.76, 2.77, 2.70, and 2.60 eV, respectively. All composite forms of BiOCl/g-C3N4 showed lower band gap values when compared to pure BiOCl. Moreover, the 10% BiOCl/g-C3N4 sample exhibited a smaller band gap value compared to all samples including g-C3N4. This observation could point to the sufficient overlapping of the lowest unoccupied molecular orbital (LUMO) of g-C3N4 and the conduction band of BiOCl shorting the initial band gap values of each g-C3N4 and BiOCl separately.[30,55]

Figure 2

(a) UV–vis diffuse reflectance spectra of the obtained samples; (b) the corresponding Tauc plot of the samples.

Scanning Electron Microscopy and EDS Analysis

The morphologies of g-C3N4, BiOCl, and BiOCl/g-C3N4 samples prepared by the microwave-assisted method were determined by scanning electron microscopy (SEM). Figure a,b shows the SEM images of pure g-C3N4 and pure BiOCl samples. The g-C3N4 image shows sheet-like microstructures, whereas the BiOCl product consists of a microplate structure. Moreover, the fabrication of BiOCl in the presence of g-C3N4 does not change the microplate structure of BiOCl obtained in the absence of g-C3N4 (Figure c–e). The number of BiOCl microplates on the surface of the g-C3N4 sheet increased because of increase in the percentage of the BiOCl precursor from 10% up to 50%. However, the 2D/2D type of composite is expected to enhance the photocatalytic performance of the material because of increased area of interaction.[48] In order to study the elemental distribution of BiOCl particles on the surface of g-C3N4 sheets, the elemental mapping analysis was performed as shown in Figure a. Quantitative analysis of the scanned area revealed a concentrated presence of Bi, O, and Cl, whereas the BiOCl particles were located as shown in Figure b–d and the presence of C and N was spread all over the scanned area as shown in Figure e,f. Moreover, the elemental composition of the prepared heterostructure sample was measured by energy-dispersive X-ray spectroscopy (EDX) analysis (Figure a). It is observed that the sample was composed mainly of five main elements: carbon, nitrogen, bismuth, oxygen, and chlorine. The atomic ratios of Bi/O/Cl/C/N were 9.5:3.5:1.2 wt %:45:40.8. These results further confirm the high purity of the produced BiOCl/g-C3N4 structure. The particle size distribution of BiOCl is shown in Figure b, and the average particle size was about 0.25 μm.

Figure 3

SEM images of (a) g-C3N4, (b) BiOCl, (c) 10% BiOCl/g-C3N4 sample, (d) 30% BiOCl/g-C3N4 sample, and (e) 50% BiOCl/g-C3N4 sample.

Figure 4

Elemental composition of the (a) 10% BiOCl/g-C3N4 sample; (b–f) color contrast of the individual elements Bi, Cl, O, N, and C.

Figure 5

(a) EDX spectrum of the BiOCl/g-C3N4 sample; (b) histogram of the average particle size distribution.

Optical Properties

The PL emission peak is mainly considered to result from the recombination process of the photogenerated electrons and holes pairs. In general, increase in PL emission peak intensity indicates a higher recombination rate for photogenerated electrons and holes.[56,57]Figure a shows the PL emission spectra of g-C3N4, BiOCl, and BiOCl/g-C3N4 composite samples. All samples were exposed to an excitation process at a wavelength of a 370 nm at room temperature and the main emission peak observed at about 450 nm. For the BiOCl sample, no emission peak was detected because of insufficient excitation energy for such a high band gap energy sample (3.4 eV). For the other samples, the PL intensities of g-C3N4 reduced dramatically after coupling it with BiOCl plates, indicating the inhabitation of the recombination process of free charge carriers in the heterostructure samples. Moreover, the 10% BiOCl/g-C3N4 and 30% BiOCl/g-C3N4 showed the lowest PL peak intensity when compared to other samples. Time-resolved PL measurement was performed in order to understand the photogenerated recombination process. The fitting of the normalized decay curves is shown in Figure b. The average fluorescence lifetime results of g-C3N4, 10% BiOCl/g-C3N4, 30% BiOCl/g-C3N4, and 50% BiOCl/g-C3N4 samples were 3.13, 3.95, 3.59, and 3.14 ns respectively. The 10% BiOCl/g-C3N4 sample showed a longer charge carrier lifetime compared to other samples. This enhancement in lifetime can be attributed to the formation of BiOCl/g-C3N4 heterojunction, which prevent fast photogenerated electrons and holes recombination.

Figure 6

(a) PL spectra of g-C3N4, BiOCl, and g-C3N4/BiOCl samples; (b) time-resolved fluorescence decay spectra of the obtained sample.

Isoelectric Point

Knowing the isoelectric point of photocatalysis is a very important factor to determine the electrostatic interaction behavior between the catalyst and the pollutant.[58] The isoelectric point could be determined by varying the pH value of photocatalyst suspension where the pH value resulting in the zero net charge of the photocatalyst is known as the isoelectric point. Thus, in order to check the surface charges of the obtained samples, their zeta potentials were analyzed. The zeta potentials of BiOCl, g-C3N4, and 10% BiOCl/g-C3N4 were −4.56, −24.08, and −21.34 mV, respectively. The zeta potential values of BiOCl, g-C3N4, and 10% BiOCl/g-C3N4 as a function of pH values are shown in Figure . As observed, the measured potential continuously declined with an increase in pH. The isoelectric points of BiOCl, g-C3N4, and 10% BiOCl/g-C3N4 were observed at pH values 3.84, 3.62, and 3.72, respectively. Moreover, Dumanović et al., reported that the protonation process of nizatidine occurs in neutral media at a pH value of 5–8.[59] The pH of the resulting solution after mixing nizatidine with water was 5.6. Therefore, all prepared samples were negatively charged at the time of the experiment.

Figure 7

Change of zeta potential of BiOCl, g-C3N4, and 10% BiOCl/g-C3N4 as a function of the pH.

Photocatalytic Activity Studies

The degradation of nizatidine using the BiOCl/g-C3N4 composite was investigated primarily under a light-emitting diode (LED) single wavelength (365 nm) light irradiation. The degradation profile of nizatidine was followed with the help of a UV–vis spectrophotometer as shown in Figure a. Figure b represents the concentration changes of nizatidine concentration with time starting from an initial concentration of 5 mg/L of the nizatidine aqueous solution at pH = 5.6. Moreover, a control experiment was performed to check the effect of the LED light on the pharmaceutical compound in the absence of the prepared catalyst. The initial concentration remained almost the same in the absence of the catalyst, indicating that nizatidine is very photostable. However, the degradation was dramatically enhanced after the addition of the prepared catalyst. Among the prepared samples, the pure BiOCl and pure g-C3N4 samples showed the lowest degradation performance. As expected, the BiOCl/g-C3N4 composite samples exhibited superior photocatalytic performance. The 10% BiOCl/g-C3N4 showed the best performance among all the prepared samples. The enhancement noticed for the 10% BiOCl/g-C3N4 sample could be attributed to the coupling of the two semiconductors, which results in the narrowing of the band gap energy along with an effective separation of the charge carriers as shown in UV–vis DRS and PL results. Moreover, the photocatalytic performance was further enhanced by adjusting the pH from 5.6 to 3.7 to reach an IEP of the 10% BiOCl/g-C3N4 sample as estimated from zeta potential results in Figure c. This further enhancement may be attributed to the improved interaction obtained between nizatidine and the 10% BiOCl/g-C3N4 catalyst. To confirm the above statement, another experiment was carried out at pH 10. At a higher pH, the photocatalytic performance of 10% BiOCl/g-C3N4 dropped down, which was expected because at a higher pH level the catalyst and the pollutant are negatively charged, resulting in a poor interaction.[59] A final test was performed under natural solar irradiation and the degradation dramatically increased to reach 96% within 30 min compared to the LED 365 nm of 120 min (Figure d). Finally, to investigate the stability of the 10% BiOCl/g-C3N4 sample, the photodegradation experiment was repeated five times under the same conditions. After each cycle, the sample was washed then collected and kept to dry overnight in an oven for the next run. Compared to the fresh sample, the recycled samples showed an acceptable stability in nizatidine degradation. Figure shows that the photodegradation efficiencies of 10% BiOCl/g-C3N4 remained almost the same after the second cycle. A schematic diagram of the proposed mechanism for the degradation of nizatidine on the surface of the BiOCl/g-C3N4 sample under solar light irradiation is shown in Figure . Photoexcited electrons from the conduction band (CB) of g-C3N4 are transferred to the CB of BiOCl upon irradiation. The electrons can reduce the surface-absorbed O2 to form a superoxide radical (•O2–) and hydrogen peroxide (H2O2) and these new species can interact to yield a highly oxidizing hydroxyl radical (•OH). On the other hand, holes remaining in g-C3N4 can react with surface-absorbed H2O to generate more •OH, enhancing the photoactivity. The radicals can oxidize the nizatidine to their intermediates and then eventually to CO2, water, and some other mineralized species.

Figure 8

(a) Degradation profile of nizatidine under LED 365. (b) Degradation rate of nizatidine at an initial concentration of 5 mg/L and pH = 5.6 with all the prepared samples. (c) Degradation rate of nizatidine with 10% BiOCl/g-C3N4 at different pH values under LED 365 nm. (d) Comparison of the degradation rate of nizatidine with 10% BiOCl/g-C3N4 under solar irradiation and LED 365 nm.

Figure 9

Cyclic runs for the photodegradation of nizatidine in the presence of 10% BiOCl/g-C3N4 at pH = 3.7.

Figure 10

Proposed reaction pathway for amine-based pharmaceuticals over the BiOCl/g-C3N4 composite structures.

Conclusions

In summary, the 2D-BiOCl/2D-C3N4 heterojunction composite photocatalytic materials were successfully synthesized by the microwave-assisted method. The results from SEM, EDX, UV–vis DRS, XRD, PL, lifetime decay, and surface charges measurement indicated that the selected method reduces experimental synthesis time down to 1 h and enhances the product purity compared to other techniques. The photocatalytic tests of the prepared samples showed high efficiency for the degradation of nizatidine as an amine-based pharmaceutical model under solar light irradiation. The sample with 10% BiOCl to g-C3N4 ratio showed the highest photocatalytic activity. The degradation under solar irradiation reached 96% within 30 min. Moreover, the prepared composite showed reasonable stability after the second run of recycling. Therefore, applying the 10% BiOCl/g-C3N4 for the degradation of the amine-based pharmaceutical can be taken into consideration as an alternative method for removal of such pollutants during water treatment.

Experimental Section

Materials and Methods

The g-C3N4 samples were prepared by using melamine powder (M2659 ALDRICH). Analytical grade bismuth nitrate pentahydrate Bi(NO3)3·5H2O, 98%, nitric acid HNO3, 99%, and potassium chloride KCl, 99%, were purchased from Sigma-Aldrich and were used without further purification for the preparation of BiOCl. The g-C3N4 synthesis was done in a muffle furnace by directly heating melamine powder in a semi-closed system. One gram of melamine powder was placed into an alumina crucible and covered with a lid and then direct heating was applied by increasing the temperature to 550 °C at a heating rate of 20 °C/min for 3 h. The BiOCl/g-C3N4 composite samples were prepared through the microwave-assisted technique. BiOCl/g-C3N4 samples with three wt % ratios were prepared, which were 10, 30 and 50% (wt %/wt). In a typical procedure for preparing the BiOCl/g-C3N4 composite, 0.2 g of g-C3N4 powder was dispersed in 40 mL of DI water and sonicated for 30 min and then depending upon the percentage, different amounts of Bi(NO3)3·5H2O and KCl were mixed and stirred for 2 h. The obtained solution was transferred into a microwave vessel and the microwave was maintained at 110 °C with a heating rate of 3 °C/min for 1 h. The product was then washed with water and ethanol. The pure BiOCl sample was obtained by the same method without g-C3N4.

Characterization

The crystal phases of all the samples were examined by XRD analysis using a MiniFlex600 X-ray diffractometer with graphite monochromatized Cu Kα radiation (λ = 1.540 Å) source. The sample morphologies were checked using a field emission scanning electron microscope (JSM-7800F JOEL, Japan) with a maximum working voltage of 30 kV, maximum resolution of 0.8 nm, and a working distance of 10 mm being used during measurements. UV–vis DRS measurements were conducted using the PerkinElmer Lambda 650S spectrometer. PL behavior was evaluated using a PerkinElmer LS 55 Luminescence Spectrometer. Lifetime decay measurement was done with help of EasyLife X equipped with 380 nm nanosecond pulsed LED’s. The surface charges of the samples at various pH conditions were measured by a Photal Otsuka Electronics.

Nizatidine is an amine-based pharmaceutical. It was chosen as a model pollutant to test the photocatalytic activity of the prepared samples. All photodegradation reaction experiments were carried out in a photocatalytic reactor batch system consisting of a cylindrical borosilicate glass reactor vessel with an effective volume of 500 mL. The experiment was carried out in the open atmosphere with an air diffuser fixed at the reactor to uniformly disperse oxygen into the solution. The reaction suspensions were prepared by adding 0.1 g of the prepared sample in 250 mL of aqueous nizatidine solution (0.4 mg/mL), with an initial concentration of 5 mg/L. The aqueous suspension was then irradiated with Mic-LED-365 (from Prizmatix 420 mW with an average intensity 13.37 W/m2). Prior to illumination, the reaction suspensions were magnetically stirred for 30 min in the dark to ensure adsorption–desorption equilibrium between the photocatalyst and nizatidine. Upon illumination, 6 mL aliquots of suspension were taken from the reactor at scheduled time intervals. The sample was centrifuged at 8000 revolutions per minute (rpm) for 5 min and then filtered to remove the catalyst. The filtrate was analyzed on the UV–vis spectrophotometer to evaluate the degradation of nizatidine.