Synthesis of g-C3N4/HC composite and its visible light catalytic performance

Thermal polymerization prepared g-C3N4/HC composite with good visible light catalytic ability using corn stover and urea as raw materials. The structure of the g-C3N4/HC composite was characterized by XRD, and it was determined that the composite was composed of g-C3N4 and HC. The morphology of the g-C3N4/HC composite was characterized by SEM. g-C3N4 is dispersed in sheet form on HC tubes with a regular porous structure. The degradation efficiency of RhB by g-C3N4/HC reached 99.14% after 70 min irradiation by a 70 W metal halide lamp. The cyclic degradation experiments proved that g-C3N4/HC has good reusability. Compared with g-C3N4, the visible light absorption capacity of g-C3N4/HC is enhanced, and the electron-hole recombination rate is also reduced, which results in the higher photocatalytic capacity of g-C3N4/HC. The free radical trapping experiments confirmed that the hole free radical and the hydroxyl free radical are the primary active substances in the photodegradation reaction.


Introduction
The discharge of various wastes causes the pollution of water bodies.A lot of research has been done to find solutions to pollution.In recent years, photocatalytic oxidation technology has attracted wide attention due to its features, such as effective utilization of sunlight, complete degradation, and no secondary pollution.
As a non-metallic semiconductor material, g-C 3 N 4 has non-toxicity characteristics, good biocompatibility, good thermal stability, and cheap raw materials.g-C 3 N 4 still has good stability when exposed to light in strong acid or base solutions due to the strong covalent bond between carbon and nitrogen atoms.However, g-C 3 N 4 has some shortcomings, such as a small visible light response range and rapid recombination of photogenerated electrons with holes, which limit its application in the field of photocatalysis.Qi et al. [1] prepared g-C 3 N 4 /Bi 2 Fe 4 O 9 composite photocatalysts using a mechanical mixing method.After 240 min irradiation by 300 W xenon lamp, the degradation efficiency of RhB (10 mg/L) reached 87.59%.Xu et al. [2] prepared Ag/g-C 3 N 4 /TiO 2 ternary composite nanomaterials.After 110 min irradiation by 300 W xenon lamp, 10 mg/L RhB was almost completely degraded.Alsulmi et al. [3] constructed AgI/g-C 3 N 4 heterojunctions containing g-C 3 N 4 and silver iodide nanoparticles by acoustic chemistry, which showed 92% degradation of RhB after irradiation with a 300 W xenon lamp for 120 min.
Biochar takes agricultural waste as raw material.It has a low price, good pore structure, and rich functional groups.Liu et al. [4] studied the biochar modification by S, CuS, and MgO.CuS-Mg/S-BC was synthesized by the one-pot sulfurization method.After 120 min irradiation by a 150 W xenon lamp, the degradation efficiency of RhB reached 95.7%.However, the above research still has shortcomings, such as high energy consumption and long degradation time.In this paper, biochar was combined with g-C 3 N 4 to enhance the photocatalytic degradation ability of the material.

Preparation of biochar
The corn stalk was dried naturally, and the powder was obtained after grinding and sifting.0.5 g powder was placed in a beaker, mixed with 20 mL deionized water, and transferred to a 30 mL polytetrafluoroethylene lined high-pressure reactor, hydrothermal reaction at 180℃ for 12 h.We cool and centrifuge to collect black solids.The product is washed, dried and ground into powder, recorded as HC.

Preparation of g-C 3 N 4
We weigh 10 g urea in a 30 ml crucible, put it into a Muffle furnace, heat it to 550℃ at the speed of 5℃/min, hold it for 4 h, cool it to room temperature, and take it out.The product is recorded as g-C 3 N 4 .

Preparation of g-C 3 N 4 /HC
A certain amount of biochar and 0.2 g g-C 3 N 4 powder was evenly mixed in a crucible, calcined for 2 h at 550℃, cooled to room temperature, and collected to obtain HC/ g-C 3 N 4 .

Characterization
The crystal structure of g-C 3 N 4 /HC was determined by XRD with Cu-kα ray.The wavelength was 0.154 nm, the scanning range of 2θ was 10 ~ 80°, the test voltage was 40 kV, and the step size was 0.02°.The morphology of g-C 3 N 4 /HC was observed by Hitachi 4800 SEM with an accelerated voltage of 20 kV.The light absorption ability of g-C 3 N 4 /HC was tested by UV-2600 UV-visible spectrophotometer (DRS) with an integrating sphere, and the scanning range was 220 ~ 850 nm.An F-7000 fluorescence spectrophotometer measured the fluorescence spectra (PL) of g-C 3 N 4 /HC.

Photocatalytic ability test
The photocatalytic degradation ability of g-C 3 N 4 /HC to RhB solution with a concentration of 10 mg/L was tested using a metal halide lamp with a power of 70 W to simulate a visible light source.The procedure was as follows: In a clean beaker, we add 100 mL of RhB solution at 10 mg/L, then add 0.05 g g-C 3 N 4 /HC, and place the beaker on the magnetic stirrer.RhB reached adsorption-desorption equilibrium on the catalyst surface after magnetic stirring for 30 min under dark conditions.A 70 W metal halide lamp was turned on, and 4 mL liquid was taken into a centrifuge tube every 30 min during the illumination period and then placed into a high-speed centrifuge and centrifuged for 8 min.The absorbance of the supernatant at 554 nm was measured.The degradation efficiency of samples was calculated according to the formula [5].

Composition and morphology analysis of samples
The composition of g-C 3 N 4 /HC was analyzed by XRD.It can be seen from Figure 1 that the XRD curve of g-C 3 N 4 /HC has a characteristic wide peak at 22.38 o , which belongs to biochar [6].The diffraction peaks at 13.10 o and 27.40 o correspond to the (100) and (002) crystal faces of g-C 3 N 4 (JCPDS 87-1526).XRD results showed that HC combined with g-C 3 N 4 successfully.
The morphology of the sample was analyzed by SEM. Figure 2 (a) shows that biochar is a large tubular structure with many holes on its surface.Figure 2 (b) shows that g-C 3 N 4 exists as granular aggregates.The SEM image of g-C 3 N 4 /HC (Figure 2 (c)) showed that there are many neatly arranged pores on the surface of the tubular biochar, and g-C 3 N 4 is dispersed in sheets on the surface of the biochar, and no agglomeration phenomenon is observed.Fluorescence spectra can illustrate the photogenerated electron and hole pair recombination rate of the catalyst.As can be seen from Figure 3 (b), the fluorescence peak intensity of g-C 3 N 4 /HC is significantly reduced, which is due to the recombination between HC and g-C 3 N 4 so that the photogenerated electrons can be effectively transferred from g-C 3 N 4 to HC, and the electron-hole separation rate is improved.At the same time, biochar can absorb part of the photoluminescence of g-C 3 N 4 and generate the quenching effect.The increase in electron-hole separation rate further boosts the photocatalytic degradation ability of g-C 3 N 4 /HC.

Figure 1 .
Figure 1.XRD pattern of the samples.

Figure 3 (
a) shows the UV-VIS DRS spectra of the samples.The edge of the absorption band of pure phase g-C 3 N 4 is about 456 nm, while the edge of the absorption band of the composite sample combined with biochar extends to 487 nm.It can be seen from the figure that the absorption intensity of the catalyst on the visible region is enhanced.The enhancement of visible light absorption capacity boosts the photodegradation ability of g-C 3 N 4 /HC.
3 N 4 /HC photocatalyst was prepared by thermal polymerization.When the doping amount of biochar was 4%, the prepared g-C 3 N 4 /HC had good photocatalytic degradation performance for RhB.The simulated pollutants can be almost completely degraded after 70 minutes of exposure to 70 W metal halide lamps.Adding biochar enlarges the photoresponse range and increases the photogenerated carrier migration rate.Cyclic degradation experiments proved that the samples have good cyclic stability.The primary active substances in the photocatalytic reaction of the composite photocatalyst were h + and ꞏOH.