In recent years, the development of efficient semiconductor photocatalyst working under visible light for various photo-dependent applications has become as a hot research topic. Photocatalytic semiconductors can solve two serious issues in 21st century, energy and environment .
Dyes and pigments in industrial wastewaters due to their non-biodegradable structure and toxic nature are harmful for the human health and cause lots of environmental hazards. Advanced oxidation process by photocatalysts can overcome the limitations of conventional methods employed for decomposition of organic dyes in polluted waters . Photocatalysis is a promising candidate for environmental purification because of its advantages including nontoxicity, cheapness, and mild condition reaction.
Despite the appreciable developments of photocatalytic technology, its commercial application is significantly restricted due to the fast recombination of electron-hole pair and wide bang-gap of photocatalyst leading to activation of photocatalyst only under UV light irradiation . Thus, numerous efforts have been devoted to fabricate the semiconductor system with a narrow bandgap and delayed recombination of electron-hole pair.
The graphite-like carbon nitride (g-C3N4), as a shining star in the photocatalytic field, possesses many unique properties, including high thermal, chemical, and photochemical stability, suitable band position, and low price [4, 5]. It is a metal-free two-dimensional polymer composed of a layered sheet of C and N covalently bonded in the form of a tris-triazine ring structure with a high degree in condensation . The pure g-C3N4 suffers from disadvantages, including deficient sunlight absorption and high recombination rate of electron-hole pairs that affect its photocatalytic activity under visible light irradiation, notwithstanding the benefits of g-C3N4 .
One of the effective ways to modify g-C3N4 to improve catalytic performance is using heterojunction structure. Different semiconductors were integrated with g-C3N4 such as Ag3VO4 , AgX , NixCo1−xS2 , WO3 , MoS2 , and In2S3 [13, 14].
In2S3 is a narrow bandgap n-type semiconductor having three different forms including α, β, and γ- In2S3. β- In2S3 with strong visible light absorption, suitable band potential, high photoconductivity, stable chemical and physical characteristics, and low toxicity attracts much attention in photocatalysis [13-17].
The high conductivity and mobility of charge carriers lead to the withdraw of photo-generated electrons from g-C3N4. The high conductivity of β- In2S3 nanoparticles integrated with g-C3N4 helps to efficient charge transport and improves the photocatalysis performance .
Research findings exhibit that enhanced visible light absorption and improved separation of electrons and holes in In2S3/ g-C3N4 nanocomposites lead to better photodegradation of RhB . Kokane et al. reported that the photocatalytic activity of In2S3/ g-C3N4 composite for degradation of both cationic and ionic dyes was the same. Also, their studies revealed that light absorption and lifetime of photogenerated charge carriers are enhanced in this system .
Despite the efficient photocatalytic performance of In2S3/ g-C3N4 nanocomposite in dye degradation, its separation from reaction media and recycling limits its application in practical situations. To overcome this problem, the immobilization of catalysts in the suitable substrate can provide promising conditions including easy removal of catalyst without cost and time-consuming process, convenient reuse of catalyst, and unpolluted reaction medium by nanomaterials. Hydrogel as a three-dimensional porous structure can have an appealing potential application to serve as a substrate. They can be used as a desirable candidate for supporting nanocomposite due to their unique intrinsic properties such as porous network which leads to rapid mass transport, their large accessible surface area for adsorption and photoreaction, and inhibition of nanomaterials aggregation and prevention of efficiency decreasing [19-22].
Herein, we selected resorcinol formaldehyde hydrogel (RFH) obtained from the polycondensation of resorcinol with formaldehyde under alkaline condition. The covalent crosslinking of these clusters creates a 3D framework with a functionalized surface. The RFH is red dark .
In this study, we synthesized g-C3N4 and In2S3 semiconductors and then immobilized them into a RFH as a porous substrate. The main advantage of In2S3/ g-C3N4 in hydrogel composite was using a combination of visible light absorption and increased lifetime of electrons and holes provided by the heterojunction structure of In2S3/ g-C3N4 with rapid mass transport and easy recycling of catalyst provided by hydrogel as a substrate to obtain high-performance photocatalyst for dye removal from water.
The reagents used in the experiment were of analytical grade and used without any further purification. Indium (III) chloride (InCl3), melamine (C3H6N6), sodium sulfide nonahydrate (Na2S3.9H2O), RhB (C.I.45170), resorcinol (C6H6O2), formaldehyde (CH2O), and sodium carbonate (Na2CO3) were analytically pure and from sigma Aldrich Co.
Synthesis of samples
In2S3 nanoparticles were synthesized by a simple precipitation method.
First, 0.01 M InCl3 ethanol-water solution (1:1 v/v) was prepared. 0.03 M Na2S solution in the ethanol-water mixture (1:1 v/v) was prepared and slowly added to the InCl3 solution under constant stirring and kept for 2 h. The mixture was centrifuged and precipitate was washed three times with distilled water and ethanol and then dried at 120 oC for 12 h. Ethanol was served as a solvent due to make dispersing medium and preventing agglomeration during the growth process [14, 24].
To obtain graphitic carbon nitride (g-C3N4), 10 g of melamine was placed into a semi-closed combustion boat, which heated at a rate of 5 C min−1 to reach 600 °C and then was kept at this temperature for 2 h under ambient condition [19, 25].
Resorcinol formaldehyde (RF) hydrogel was made by polycondensation of resorcinol and formaldehyde in the presence of Na2Co3 as the catalyst. Resorcinol and Na2CO3 (R/C=300) were mixed and then dissolved in distilled water. The solution was heated to 70 oC under magnetic stirring in a sealed flask. In another flask, formaldehyde (37 wt. % in Water, stabilized by 10-15 wt. % Methanol) was heated to 70 oC. The solutions of the two mentioned flasks were mixed. The solution divided equally into sample holders with diameter and height equal to 13 and 20 mm, respectively. Then, each sample holder was sealed with paraffin film and solutions were put in an oven at 70 oC for 120 min .
To synthesize In2S3/ g-C3N4 stabilized in hydrogel, 10 mg of In2S3 nanoparticles and 5 mg of g-C3N4 per 1 ml of RF were added and the mixture was stirred for 4 min, and after casting in sample holders they were put in an oven at 70 oC for 120 min.
The phase purity and crystal structure of the sample were recorded by x-ray diffraction (XRD) measured on a D8 Advance, BRUKER with a Cu anode in 2θ=۵-۸۰°. The surface morphology and nanostructure of the samples were observed with a field emission scanning electron microscope (FESEM, scientific England Agar Company). The specific surface area (BET), pore volume, and size were recorded at 77 K using apparatus BEIOSORP Mini from Microtrac Bel Crop. Diffuse reflectance spectra (DRS) were recorded using a V-670, JASCO spectrophotometer in the range of 200-900 nm and transformed to the absorption spectra according to the Tauc relationship. The infrared spectra were obtained on a FT-IR 6300 using KBr as the reference sample within a wavelength range of 400 – 4000 cm-1.
Photocatalytic removal of RhB
3 ppm of RhB solution was selected as polluted water to investigate the photocatalytic performance of the catalyst. The hydrogel was put in 25 ml of 3 ppm RhB solution in a petri dish while stirring the solution in a dark condition. Every 10 min, the catalyst was removed from the solution and the concentration of RhB was monitored by a UV-Vis spectrophotometer till 120 min. Also, this progress was done under visible light irradiation. The results in dark condition reveal the absorption capacity of In2S3/ g-C3N4 stabilized in hydrogel and the obtained results under light exposure demonstrate the photocatalytic performance of the abovementioned catalyst.
RESULTS and DISCUSSION
Fig.1 shows the FTIR spectra of the hydrogel, g-C3N4, In2S3 nanoparticle, and In2S3/ g-C3N4 in hydrogel. In FTIR spectrum of the hydrogel, the broad absorption band at 3428 cm-1 is attributed to hydroxyl groups bonded to the benzene rings. The stretching vibrations of CH2 were observed at 2938 cm-1 and 1469 cm-1. The band at 1608 cm-1 is related to aromatic ring stretching. The bands at 1087 cm-1 and 1290 cm-1 can be attributed to the formation of the C-O-C bond due to polycondensation reaction between resorcinol and formaldehyde .
In FTIR spectrum of g-C3N4, the peak at 1641 cm-1 is related to C=N stretching vibrations mode. The peaks at 1237 cm-1, 1317 cm-1, and 1466 cm-1 are assigned to aromatic C-N stretching vibration modes . The sharp absorption peak at 809 cm-1 is attributed to the breathing vibration of the tri-s-triazine cycle. The characteristic peak in FTIR spectrum of In2S3 nanoparticles is observed at 807 cm-1 due to bonding of In-S . The peak at 477 cm-1 is related to formation of Na2S during synthesis of In2S3. The intense peak at 3354 cm-1 is due to adsorbed water molecules on the surface of samples.The peak at 1605 cm-1is related to C=O stretching vibration of adsorbed CO2 molecules . The FTIR spectrum of In2S3/ g-C3N4 stabilized in hydrogel is very similar to that of hydrogel because of small amounts of In2S3 and g-C3N4.
The crystal structure of the composite was examined by XRD. Fig.2 shows the XRD pattern of In2S3/ g-C3N4 in hydrogel. As shown in Fig.2, the composite has an amorphous phase that can be attributed to the hydrogel. The sharp peak at 44° matches with the most intensive peak of carbon with JCPDS.no 01-075-0409 (diamond). This peak probably confirms the crystalization of carbon in the hydrogel. The characteristic peaks of pure g-C3N4 and In2S3 are not observed at the XRD spectrum because of the small amount of these materials in comparison with hydrogel.
The optical properties of g-C3N4, In2S3 nanoparticle, and In2S3/ g-C3N4 in hydrogel were investigated using UV–Vis diffuse reflectance spectroscopy. The optical absorption spectra of g-C3N4, In2S3 nanoparticle, and In2S3/ g-C3N4 in hydrogel are shown in Fig. 3. The increased absorption edges of g-C3N4 and In2S3 were observed at 460 and 500 nm, respectively. The absorption edge of In2S3/ g-C3N4 heterojunction stabilized in hydrogel shifted to longer wavelength (about 700 nm) in comparison with g-C3N4 and In2S3, implying that this composite works with visible light [18, 14]. The optical band gap of g-C3N4, In2S3, and In2S3/ g-C3N4 in hydrogel were calculated using Tauc relation αhν=A(hν-Eg)n where α is the absorption coefficient. A, ν, Eg, and n are constant, light frequency, bandgap, and an index, respectively. The n value is determined by different typical optical transitions of the semiconductor (n= 1/2 for a direct transition and n=2 for an indirect transition). In2S3 is a direct transition semiconductor and its bandgap (Eg) was obtained from a plot of (αhν)2 vs hν. The measured band gap was found to be 1.7 eV for In2S3. g-C3N4 is an indirect semiconductor and its Eg was obtained by an extrapolation of the linear range of plot (αhν)1/2 vs hν about 2.3 eV. Because of the more amounts of In2S3 rather than g-C3N4 in heterojunction, the In2S3/ g-C3N4 in hydrogel is accounted as a direct bandgap semiconductor and the measured bandgap was about 2.1. The measured band gap of g-C3N4 in this research differs from reported values by others. Also, the bandgap of In2S3/ g-C3N4 is lower than the obtained bandgap of this heterojunction reported by Kokane et al (2.66 eV). This smaller bandgap can be attributed to two reasons:
• The smaller bandgap of our g-C3N4 in comparison with the bandgap of g-C3N4 reported in the literature.
• Presence of hydrogel with a high oxygen content which has interaction with heterostructure.
The hydrogels are porous materials. The technique of N2 adsorption-desorption isotherms was used to evaluate the surface area and porosity type. Fig 5 shows the isotherms of hydrogel and In2S3/ g-C3N4 in the hydrogel. The isotherms are type IV that imply the mesoporous nature of materials. The shapes of the hysteresis loop confirm that pores have a bottleneck shape. The specific surface area, pore-volume, and pore size of samples were investigated by BET and BJH, respectively. The BET surface area of the hydrogel is 10.066 m2.g -1 while the BET surface area of In2S3/ g-C3N4 in the hydrogel is 219.69 m2.g -1. It can be seen clearly from BET results that the addition of In2S3 and g-C3N4 to hydrogel not only does not decrease the porosity but also increases surface area about twenty times. The pore volumes of hydrogel and In2S3/ g-C3N4 in the hydrogel were obtained 0.00408 and 0.144 cm3.g-1, respectively. The most probable pore diameters for the above-mentioned samples measured by BJH were between 2.52 and 3.92 nm. BJH measurements confirm that the presence of nanomaterials enhances the pore diameter and pore volumes. It can be concluded that RFH has an amazing structure to stabilize the catalysts without losing its porosity. The BET and BJH results are tabulated in Table 1
To characterize the morphology of the hydrogel and In2S3/ g-C3N4 in the hydrogel, FESEM was performed as shown in Fig .6. The samples have a uniform grainy and porous structure. The size distributions are relatively narrow. The particles in hydrogel have an average diameter of about 12.25 nm. The pores have a diameter between 16.00 nm and 19.18 nm. The particles in In2S3/ g-C3N4 in hydrogel have a diameter from 19.04 to 28.55 nm. The EDX spectrum of In2S3/ g-C3N4 in hydrogel confirms the presence of C, N, In and S elements (Fig.7).
The removal activity of hydrogel and In2S3/ g-C3N4 in hydrogel was evaluated by the removal of 3 ppm of RhB under dark condition (adsorption) and visible light irradiation (synergy of adsorption and photocatalysis). The results are shown in Fig. 8 based on the plot of Ct/C0 versus time (t), where Ct is the concentration of RhB at the time t, and C0 is the initial concentration of RhB solution. As Fig. 8 shows, pure hydrogel has a low capacity to adsorb RhB from solution but by addition g-C3N4 and In2S3 to hydrogel the adsorption capacity increases severely. This can be due to the presence of g-C3N4 and In2S3 nanomaterials with high active surfaces for interaction with dye, and also, the enhancement of hydrogel porosity which improves mass transfer in hydrogel and expands effective interaction surfaces. The removal of RhB under light irradiation which includes adsorption and photocatalysis processes is higher than the removal only by adsorption. Fig .9 shows the removal percentages of hydrogel and In2S3/ g-C3N4 in hydrogel. After 120 min in dark condition hydrogel only adsorbs 7% of RhB while In2S3/ g-C3N4 in hydrogel can adsorb about 72% of dye in solution. Under light exposure, In2S3/ g-C3N4 in hydrogel removes RhB about 88.6%. It means that the contribution of photocatalysis in dye removal is equal to 16.8%. The RhB removal performance of In2S3/ g-C3N4 in hydrogel is compared with reported amounts of In2S3/ g-C3N4 in publications in Table 2. It can be seen from Table 2 that the immobilization of heterostructure in hydrogel has no considerable effect on the removal performance of In2S3/ g-C3N4. Also, the stabilization of heterostructure in hydrogel facilitates the exit of catalyst from reaction media without any further treatment to remove the catalyst from the aqueous solution.
The adsorption and photodegradation of organic pollutants follow the firs order kinetic as is given below:
where C0 is initial RhB concentration (ppm), Ct is RhB concentration at time t (ppm), and K is the first-order kinetic constant (min-1). Fig.10 displays the linear relationship between ln (C0/ Ct) and time for RhB removal using In2S3/ g-C3N4 in hydrogel in dark condition and under light irradiation. The K value for RhB adsorption over hydrogel In2S3/ g-C3N4 in hydrogel is 0.0105 min-1 (R2=0.99) while under light irradiation it will reach 0.016 min-1 (R2= 0.97).
Fig. 11 exhibits the schematic of photocatalytic degradation of RhB in the presence of In2S3/ g-C3N4 in hydrogel. Probably under light irradiation, In2S3 and g-C3N4 are excited, and subsequently, electron-hole pairs are generated. The electrons in the conduction band of g-C3N4 are transited to the conduction band of In2S3. Then, the risk of electron-hole recombination in g-C3N4 decreases and the effective separation of electrons and holes is obtained. The holes in the valence band of g-C3N4 oxidize H2O molecules and generate .OH. The electrons in In2S3 conduction band are combined with O2 to give strong superoxide ions (.O2-). RhB in aqueous solution reacts with these reactive redox agents and is degraded.
Although In2S3/ g-C3N4 heterojunction has previously been used to degrade textile dyes in effluents, herein the RFH based porous substrate has been used to stabilize and immobilize the photocatalyst that facilitates separation of the catalyst from solution and also reduces the band gap of the composite. Due to the presence of hydroxyl groups in RFH, this porous structure has the ability to adsorb rhodamine B as a cationic dye through electrostatic interactions, thus increasing the removing effect by adsorption.
A novel visible light heterogenic In2S3/ g-C3N4 in hydrogel photocatalyst was synthesized. The FTIR, DRS, XRD, BET, BJH, SEM, and EDX techniques were used to characterize chemical functional groups, optical properties, crystalline structure, the specific surface area of the porous hydrogel, size and volumes of pores, and morphology and chemical composition of samples, respectively. The addition of In2S3 and g-C3N4 increased the porosity of hydrogel. Porosity enhancement led to the more active sites and enhanced the efficiency of adsorption and finally improved the photocatalytic performance. By adding the nanomaterials to the hydrogel, the RhB adsorption percentage improved up to 10 times. The adsorption and photocatalytic progress of RhB in aqueous solution over In2S3/ g-C3N4 in hydrogel followed the first-order kinetic model and the rate constants were found to be 0.105 min-1 and 0.16 min-1, respectively. Based on our results, In2S3/ g-C3N4 in hydrogel can be effectively and easily separated from the solution, and it could be a promising candidate for practical applications.
The authors wish to appreciate the Najafabad Branch, Islamic Azad University for partial support of this research.
CONFLICT OF INTERESTS
The authors declare that there is no conflict of interest regarding the publication of this manuscript.