Accelerating Photocatalytic Hydrogen Production and Pollutant Degradation by Functionalizing g-C3N4 With SnO2

Energy crises and environmental pollution are two serious threats to modern society. To overcome these problems, graphitic carbon nitride (g-C3N4) nanosheets were fabricated and functionalized with SnO2 nanoparticles to produce H2 from water splitting and degrade 2-chlorophenol under visible light irradiation. The fabricated samples showed enhanced photocatalytic activities for both H2 evolution and pollutant degradation as compared to bare g-C3N4 and SnO2. These enhanced photoactivities are attributed to the fast charge separation as the excited electrons transfer from g-C3N4 to the conduction band of SnO2. This enhanced charge separation has been confirmed by the photoluminescence spectra, steady state surface photovoltage spectroscopic measurement, and formed hydroxyl radicals. It is believed that this work will provide a feasible route to synthesize photocatalysts for improved energy production and environmental purification.


INTRODUCTION
Exhaustion of hydrocarbon fuels and addition of toxic and hazardous materials from agricultural, medicinal, dyes, and cosmetic industries to the environment have resulted in increased pressure on the scientific community to address these problems adequately. A number of methods have been chalked out such as cracking of hydrocarbons and thermal splitting of water at elevated temperature to get H 2 (future fuel). However, these methods require highly costly and controlled operational environment and huge labor under normal conditions. On the other hand, different pollutants removal technologies such as adsorption, coagulation, and electrochemical methods have their own shortcomings and did not receive much popularity in the purification of the environment (Zhao et al., 2015;Gautam et al., 2016;Li et al., 2016;Ali et al., 2018b;Wang et al., 2018;Ali S. et al., 2019). Therefore, modern techniques are urgently required to address energy and environmental issues properly with the least operational cost and time.
Photocatalysis has opened a brilliant chapter to realize the energy crises and environmental issues. The photocatalysts have shown their remarkable influence in the production of H 2 from water, production of hydrocarbon fuels from CO 2 , and removal of pollutants from air and wastewater with minimum cost and least working labor Li et al., 2018a,b;Qu et al., 2018;Ullah et al., 2019;Xu et al., 2019a). Although different photocatalysts such as TiO 2 , ZnO, and ZrO 2 have been widely utilized, there are some typical problems such as activeness only under ultraviolet (UV) light and fast recombination of photogenerated charges Qi et al., 2018aQi et al., ,b, 2020a. Since visible light contributes a major portion to electromagnetic radiations, therefore, photocatalysts active under visible light are more effective and efficient. Graphitic carbon nitride, g-C 3 N 4 , is a promising polymeric photocatalyst with a band gap of 2.7 eV. Its conduction band (CB) and valence band (VB) have characteristic abilities to reduce water and oxidize organic pollutants, respectively (Qi et al., 2019a,b). Its metal-free nature is of particular importance as its release to the environment does not produce harmful threats to the aquatic animals and plants (Nie et al., 2018;Ran et al., 2018;Fu et al., 2019;Liu M. et al., 2019). However, low surface area and poor excited charge separation capability of this photocatalyst is still a marked question on its utilization for fuel production and organic oxidation (Dong et al., 2019;Liu Y. et al., 2019;Zhu et al., 2019c;Qi et al., 2020b). Therefore, these problems need to be tackled in future generation of semiconductor photocatalysis.
SnO 2 is an excellent UV responsive metal oxide photocatalyst with a band gap of 3.5 eV. Its excellent stability and tunable optical properties make it a suitable candidate for photocatalysis, solar cells, and gas sensors. More interestingly, its CB is situated at a suitable position below the CB of g-C 3 N 4 and thermodynamically acts as a sink to accept the excited electrons from g-C 3 N 4 during photocatalysis (Jana and Mondal, 2014;Xu et al., 2018a;Qi et al., 2019c,d). Therefore, its heterojunctional combination with g-C 3 N 4 will significantly improve excited charge separation for enhanced photocatalysis.
In this work, we coupled SnO 2 nanoparticles with g-C 3 N 4 to form different ratio composites and applied for the photocatalytic production of H 2 and 2-chlorophenol (2-CP) degradation under visible light, keeping in view to excite only g-C 3 N 4 and use SnO 2 as excited charge acceptor. The optimized composite (6SnO 2 /g-C 3 N 4 ) showed much improved photoactivities for H 2 production and pollutant degradation compared to bare SnO 2 and g-C 3 N 4 . These activities are solely attributed to the better charge separation in the composites.

EXPERIMENTAL PART
Preparation of g-C 3 N 4 Polymeric g-C 3 N 4 was prepared from dicyandiamide. A given amount of dicyandiamide was taken in crucible and heated at 550 • C in a muffle furnace for 4 h at the rate of 2 • C/min. After the completion of the heating duration, the cooled sample was grinded into fine powder and used for further study.

Preparation of SnO 2 Nanoparticles
Chloride salt of Sn(IV) was dissolved in water, and the solution was made basic with the help of NaOH solution under vigorous stirring. During addition of NaOH, the solution initially became milky and then became clear with the addition of more NaOH.
Finally, when the pH reached about 12, the solution was taken in an autoclave and heated at 220 • C for 10 h in oven. The obtained white powder was purified three times with distilled water, dried in the oven overnight, and then calcined at 450 • C for 1 h at the rate of 5 • C/min.

Preparation of SnO 2 /g-C 3 N 4 Composites
Composites containing different mass percent of SnO 2 and g-C 3 N 4 (SnO 2 /g-C 3 N 4 ) were prepared by taking appropriate amounts of SnO 2 and g-C 3 N 4 in water-methanol mixture containing 1 ml of concentrated HNO 3 . Each mixture was vigorously stirred under heating at 80 • C till the whole solvent was evaporated. After that, each mixture was dried in the oven overnight and then calcined at 450 • C for 1 h. The as prepared composites were represented by XSnO 2 /g-C 3 N 4 where "X" shows the percent amount (2, 4, 6, and 8%) of SnO 2 in the given composite.

Characterization Techniques
The prepared samples were passed through different characterization techniques to confirm the formation of heterojunctional composites. The X-ray diffraction (XRD) technique was used to determine the crystalline structure of the samples with the help of Bruker D8 Advance diffractometer using CuK α radiation. The oxidation states of the constituent elements of the composites were examined by means of X-ray photoelectron spectroscopy (XPS) employing X-ray from mono-Al source with the help of a Kratos-Axis Ultra DLD apparatus. The obtained binding energies were calibrated with the binding energy of adventitious C-atom which is 284.55 eV. The microscopic structure was further revealed with the help of transmission electron microscope (TEM) and high-resolution TEM (HRTEM) operating at 200 kV. The optical properties were confirmed with the help of UV diffuse reflectance spectra, by taking BaSO 4 as a reference, measured with a Shimadzu UV-2550 spectrophotometer. The photoluminescence (PL) spectrum of each sample was realized with the help of spectrofluorophotometer (Perkin-Elmer LS55) at a 390-nm excitation wavelength. The steady state surface photovoltage spectroscopic (SS-SPS) measurement of each sample was carried in a controlled atmosphere employing a homemade equipment possessing a lock-in amplifier (SR830) synchronized with a light chopper (SR540). Each sample was first thoroughly grinded and then kept between two indium tin oxide (ITO) glass electrodes in an atmosphere-controlled sealed container. Radiations from a 500-W Xe lamp (CHF XQ500W, Global Xe lamp power) were passed through a double-prism monochromator (SBP300) to get a monochromatic light.

Evaluation of Photoactivity for Water Splitting
Water splitting photocatalysis was carried out with the help of an online H 2 production unit (Perfectlight, Beijing, Lab Solar III). About 0.1 g photocatalyst was taken in a glass-made reaction cell, and 100-ml aqueous solution of methanol (20% V/V) was added. The apparatus was first deaerated with the help of a vacuum pump to remove any traces of O 2 and CO 2 dissolved FIGURE 1 | X-ray diffraction (XRD) pattern (A) and diffuse reflectance spectroscopy (DRS) (B) of g-C 3 N 4 , SnO 2 , and XSnO 2 /g-C 3 N 4 . Transmission electron microscope (TEM) image (C) and high-resolution TEM (HRTEM) images (D) of 6SnO 2 /g-C 3 N 4 .
in aqueous solution. The mixture was irradiated under visible light (wavelength > 420 nm) coming from a 300-W Xe lamp under vigorous stirring. The produced gases were analyzed after each hour with the help of a gas chromatograph (7,900, TCD, molecular sieve 5 Å, N 2 carrier, Tec comp.).

Evaluation of Photoactivity for Pollutant Degradation
The photoactivities were further studied by selecting 2-CP as a pollutant. About 0.2 g of powder sample was mixed with 50 ml of aqueous solution (25 mg/L) of 2-CP and exposed to a 150-W (GYZ220) Xe lamp under visible light (wavelength > 420 nm). Before being exposed to light, each sample was first kept in complete dark for 30 min to attain adsorption-desorption equilibrium. The concentration of the pollutant was checked after each hour with the help of a Shimadzu UV-2550 spectrometer.

Evaluation of Hydroxyl Radicals (·OH)
About 0.05 g powder sample was mixed with 50 ml of aqueous solution of coumarin (0.001 M) and exposed to a 150-W (GYZ220) Xe lamp under visible light (wavelength > 420 nm). Before exposure to light, each sample was first kept in complete dark for 30 min to attain adsorption-desorption equilibrium. After each hour, the amount of formed 7-hydroxycoumarin was checked at 390-/460-nm excitation/emission wavelengths with the help of a spectrofluorophotometer (Perkin-Elmer LS55).

Structural Characterization
The crystal structure study of the pure g-C 3 N 4 shows two characteristic diffraction peaks at 13.04 and 27.31 • as shown in Figure 1A. The former peak is due to the interplanner stacking of the aromatic rings in conjugation while the later peak is related to the interlayer structural units (Liu et al., 2017;Guan et al., 2018;Xu et al., 2018bXu et al., , 2019b (Mahjouri et al., 2020;Shokrzadeh et al., 2020). Interestingly, all the composite samples show the two characteristic peaks of g-C 3 N 4 at 13.04 and 27.31 • and SnO 2 peaks at 33.8, 37.3, and 51.2 • . However, the diffraction peak at 13.04 • has been decreased progressively as the amount of SnO 2 is increased, indicating that SnO 2 nanoparticles are well packed in the nanosheets of g-C 3 N 4 .
The UV-vis diffused reflectance spectra of the samples are shown in Figure 1B. As can be seen, g-C 3 N 4 and SnO 2 , respectively, show optical thresholds at 475 and 360 nm, corresponding to band gaps of 2.61 and 3.45 eV, respectively. Although the composite samples show the same optical thresholds at 475 and 360 nm, their light absorption has been slightly decreased as SnO 2 is a wide-band-gap semiconductor and its coupling with g-C 3 N 4 slightly decreases light absorption Zada et al., 2019a;Zhu et al., 2019a,b).
The TEM image of composite shows uniform distribution of small SnO 2 nanoparticles of about 10-nm size on the surface of g-C 3 N 4 as shown in Figure 1C. The HRTEM image shows the lattice fringes of 0.33-nm interplanar distance corresponding to the (110) plane of SnO 2 (Figure 1D). This shows that both g-C 3 N 4 and SnO 2 are in close contact with each other to intensify the charge separation for better photoactivities.
The oxidation states of different elements in the samples were determined using XPS measurements, and the results are shown in Figure 2. The obtained binding energies were calibrated with the binding energy of the adventitious carbon atom with a binding energy value of 284.55 eV. It is obvious that C1s in pure g-C 3 N 4 shows two XPS peaks at binding energies of 284.7 and 288.2 eV (Figure 2A). These peaks are attributed to the sp 2 hybridized C-atoms, respectively, bonded with N-atom of the aromatic ring and NH 2 group linking different aromatic rings. Similarly, the XPS binding energies of N1s in pure g-C 3 N 4 are composed of two parts at 398.4 and 400.6 eV and are, respectively, contributed by sec. and ter. N-atoms ( Figure 2B) Li Q. et al., 2019). The XPS peaks of Sn in pure SnO 2 are deconvoluted into two parts at 486.82 and 495.26 eV, which are, respectively, contributed by Sn3d 5/2 and Sn3d 3/2 and show that Sn is present in +4 oxidation state in the nanocomposite ( Figure 2C) (Li H. et al., 2019). When g-C 3 N 4 nanosheets are coupled with SnO 2 nanoparticles, the C1s and N1s peaks are slightly shifted toward the low-energy side while those of Sn are shifted toward the high-energy side. The binding energies of O1s in Figure 2D are contributed at 529.6 and 531.1 eV, which are, respectively, contributed by the lattice O-atom and adsorbed oxygen molecules. The redistribution of charge density in both components of the nanocomposite indicates that both g-C 3 N 4 and SnO 2 are present in close contact with each other for improved charge separation.

Photocatalytic Activity
The photoactivities of composites were first evaluated by splitting water under visible light (wavelength > 420 nm) in the presence of methyl alcohol. As shown in the Figure 3A, the H 2 production activity of pure SnO 2 is negligible under visible light irradiation. However, pure g-C 3 N 4 produces about 10 µmol of H 2 in 1 h under the stipulated conditions. Interestingly, these H 2 photoactivities are significantly enhanced when both g-C 3 N 4 and SnO 2 are coupled to form heterojunctional composites. Further, photoactivities are increased along with the increase in the amount of SnO 2 nanoparticles and the highest activities are FIGURE 3 | Photoactivities for H 2 evolution (A), 2-CP degradation (B) of pure g-C 3 N 4 , and XSnO 2 /g-C 3 N 4 and stability test of 6SnO 2 /g-C 3 N 4 (C). contributed by 6SnO 2 /g-C 3 N 4 sample, which are 132 µmol/h. However, further increase in the amount of SnO 2 decreases H 2 production as SnO 2 is a wide-band-gap semiconductor and it covers most surface of the g-C 3 N 4 to prevent absorption of visible-light photons. These enhanced H 2 activities are solely attributed to the improved charge separation in the composites by transferring excited electrons from g-C 3 N 4 to SnO 2 for the reduction of water. We further extended the photoactivities by measuring the degradation of 2-CP under visible-light (wavelength > 420 nm) irradiation. Again, the degradation performance of pure SnO 2 is very low due to its high-band-gap nature. The composite materials showed much improved photoactivities, and the optimized 6SnO 2 /g-C 3 N 4 sample showed 42% degradation activities under the given conditions as shown in Figure 3B. We also carried out the stability test of the optimized sample for five consecutive cycles, each cycle composed of a 5-h run. It is obvious from Figure 3C that there is no detectable decrease in the H 2 production activities, indicating that the optimized sample is very stable.

Charge Separation
The improved photoactivities of the composites compared to pure g-C 3 N 4 are attributed to the extended charge separation. In order to determine it, we carried out PL spectra, keeping excitation λ at 390 nm. It is clear from Figure 4A that g-C 3 N 4 gives intense peak at 470 nm, indicating poor charge separation. However, the intensities of the composites are progressively decreased as the amount of SnO 2 nanoparticles is increased, and the optimized sample shows relatively low PL peak, indicating excellent charge separation in it Ali et al., 2016;Lu et al., 2018;Ali N. et al., 2019;Ali S. et al., 2019). The relatively low intensities of PL peaks indicate enhanced charge separation and are responsible for improved photoactivities.
We further extended the charge separation experiments by measuring the atmosphere-controlled steady state surface photovoltage spectra (SS-SPS), and the results are shown in Figure 4B. As evident, g-C 3 N 4 shows very low SPS intensity. However, the SPS peak intensities are much improved when both g-C 3 N 4 and SnO 2 are coupled and the optimized 6SnO 2 /g-C 3 N 4 sample shows the highest peak intensity. Since high is the intensity of the SPS peak, high is the charge separation ; therefore, we can say that the composites impart enhanced charge separation and contributing to the improved photoactivities.
We also measured the hydroxyl radical (·OH) activities of the fabricated samples by doing coumarin fluorescent experiments under visible-light irradiation. Since ·OH is the major contributor to charge separation during photocatalysis and react with coumarin to form 7-hydroxycoumarin; therefore, the higher the intensity of coumarin fluorescent peak, the higher is the charge separation. As can be seen from Figure 4C, pure FIGURE 4 | Photoluminescence spectra (A), SS-SPS (B), and coumarin fluorescent spectra (C) of pure g-C 3 N 4 and XSnO 2 /g-C 3 N 4 .
FIGURE 5 | Schematic representation of charge separation, H 2 production, and pollutant degradation by 6SnO 2 /g-C 3 N 4 . g-C 3 N 4 gives very low peak, which shows its inherited low charge separation. However, the ·OH radical activities are considerably improved when both g-C 3 N 4 and SnO 2 are coupled, indicating improved charge separation and hence extended photoactivities (Ali et al., 2018a;Yasmeen et al., 2019a).

DISCUSSION
The improved charge separation in the prepared composite results in the enhanced H 2 production and 2-CP degradation. This enhanced charge separation has been schematically shown in Figure 5. The band gap of g-C 3 N 4 is about 2.7 eV and absorbs visible-light photons (Raziq et al., 2015. Its CB present at −1.3 eV is most suitable for H 2 production and superoxide generation which require reduction potential of 0.00 and −0.33 eV, respectively. Its VB is present at 1.4 eV (Yasmeen et al., 2019b). On the other side, the band gap of SnO 2 is 3.5 eV, and its CB is present below the CB of g-C 3 N 4 . Under visible-light irradiation, only g-C 3 N 4 is excited, and electrons jumped to its CB, leaving positive holes in the VB. Since the excited electrons have a very short lifetime; therefore, they jumped to the CB of SnO 2 to achieve some stability for a while.
Here these electrons reduce water into H 2 while the holes in VB of g-C 3 N 4 are solely left to carryout oxidation of alcohol. In case of 2-CP degradation, these positive holes either directly oxidize pollutants or undergo the formation of more reactive species such as hydroxyl-free radicals, which then finally degrade the target pollutant into simple CO 2 and water .

CONCLUSION
In order to overcome energy crises and environmental pollution, here, we synthesized g-C 3 N 4 nanosheets and coupled them with SnO 2 nanoparticles. The optimized composite of 6SnO 2 /g-C 3 N 4 produced about 132 µmol of H 2 from water in 1 h and degraded 42% 2-CP pollutant under visible-light irradiation as compared to the photoactivities of bare g-C 3 N 4 and SnO 2 . These enhanced photoactivities are attributed to the better charge separation as the excited electrons thermodynamically transfer from g-C 3 N 4 to SnO 2 as has been confirmed from photoluminescence spectra, steady state surface photovoltage spectroscopic measurement, and formed hydroxyl radicals. It is believed that this work would provide a feasible route to synthesize photocatalysts for improved energy production and environmental purification.

DATA AVAILABILITY STATEMENT
The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation, to any qualified researcher.

AUTHOR CONTRIBUTIONS
All authors listed have made a substantial, direct and intellectual contribution to the work, and approved it for publication.