Template-Free Synthesis of One-Dimensional g-C3N4 Chain Nanostructures for Efficient Photocatalytic Hydrogen Evolution

The development of graphite-carbon nitride (g-C3N4) photocatalyst is of great significance for various visible utilization applications. Control the nanostructures of g-C3N4 can tailor its photocatalytic performance. In this paper, one-dimensional chain-like g-C3N4 was successfully synthesized by heat-induced polymerization of melamine which was saturated in ethylene glycol. The photocatalytic hydrogen production rate (HER) of the prepared g-C3N4 chain enhanced about 3 times than that of bulk g-C3N4, increasing from 9.6 μmolh−1 to 28.7 μmolh−1. The improved photocatalytic activity of the g-C3N4 chain was attributed to the advantages of porosity and nanostructure. The extraordinary nanopores result in an enlarged specific surface area for adsorption and the production of abundantly available channels for charge transfer. The one-dimensional chain-like structure can facilitate the exposure of internal/external active sites as many as possible, and induce the directional migration of charge carriers.


INTRODUCTION
With the rapid pace of industrialization and urbanization growing in the past few decades, global crises related to environmental degradation and energy shortage have become the most critical topics to the world and threaten the survival environment of humankind (Yu et al., 2017;Yu et al., 2019). Photocatalytic water splitting for hydrogen evolution has been considered as a sustainable strategy to convert and store plentiful solar energy for future energy requirements Wang et al., 2012;Ong et al., 2016). In the last several decades, graphitic carbon nitride (g-C 3 N 4 ) the organic semiconductor showed a specific graphite-like sp 2 -bonded C-N structure that exhibits significant potential in the field of CO 2 conversion, water splitting, and environmental remediation on account of its inexpensive preparation, brilliant visible light response, thermal stability, and well-developed electronic band structure (Ding et al., 2018;He et al., 2019;Rong et al., 2020;Shu et al., 2020;Zhang D. et al., 2020). However, g-C 3 N 4 accompanied by the defaults such as poor quantum efficiency, low specific surface area, and rapid charge recombination have adversely affected the photocatalytic application of g-C 3 N 4 .
A large number of strategies acting to resolve these problems and reinforce the photocatalytic performances of g-C 3 N 4 , involving non-metal element doping [S (Cao et al., 2018), O , V (Ding et al., 2013), B (Yan et al., 2010), etc.], noble metals decoration [Pd (Wang et al., 2011) Pt ) and Au ], heterojunction designing (Liang et al., 2019;Shi et al., 2020a;Shi et al., 2020b) and coupling with graphene (Li et al., 2013;Han et al., 2017). Expect for the capacity of g-C 3 N 4 to collaborate with other materials, layers structure configuration is deemed as a prospective method to compound g-C 3 N 4 as they can accelerate the diffusion of reactant, strengthen the light harvesting, expand the exposed surface areas, and promote the charge delivery.
After discovering the carbon nanotubes, 1D nanostructures (lines, rods, tubes, strips, fibers, etc) have attracted extensive attention from researchers. 1D nanostructures have greatly satisfied the increasing demand for microelectronics and optoelectronic devices such as optical waveguides, field-effect transistors, and photodetectors in recent years. 1D nanostructures showed brilliant phonon, gas sensitivity, field emission, photoconductivity, and electron transport performance due to their higher surface volume ratio and more active position. Furthermore, the growth of 1D nanostructures has an immense effect on improving the mechanical energy, thermal and electrical capabilities of materials. Because nanoparticles in 1D nanostructures are interconnected in three dimensions, an extremely fast interparticle, vectorially transport of photogenerated charge carriers (electrons and holes) is likely to emerge through the grain boundaries. This represents that the redox reaction sites are far away related to the photoexcitation sites, which seems to be responsible for the high activities of photocurrent generation and hydrogen production.
In the present work, 1D g-C 3 N 4 chain nanostructures were obtained by regulating the saturation of melamine and ethylene glycol solution for photocatalytic hydrogen evolution. The enhanced photocatalytic properties are known to be caused by an extension of the life of the photoinduced charge carrier. In addition, increasing specific surface area is also a vital factor in promising photocatalytic performance. Owing to the composite method is convenient, environmentally friendly, and low-cost, it is suitable for an expanded range of practical applications.

Synthesis of the g-C 3 N 4 Chain Nanostructures
The one-dimensional g-C 3 N 4 chain nanostructures were prepared by a controllable approach. Firstly, excessive amounts of melamine powders (3.0 g) were dissolved in 60 ml of ethylene glycol to form a saturated solution at room temperature. Subsequently, add 1 ml of concentrated nitric acid solution to 59 ml of water, drop by drop add the solution to 20 ml of supernatant and stir continuously until a white flocculent precipitate is obtained. The white flocculent precipitate was bleached with ethanol five times to remove nitric acid and ethylene glycol. Lastly, the collected sample was transferred into the muffle furnace for heating 2 h at 550°C with a heating rate of 20°C·min −1 . Meanwhile, as the contrast sample, the bulk g-C 3 N 4 was obtained by the heating process at 550°C for 2 h with a heating rate of 20°C·min −1 of the melamine powders.

Characterization
The crystal structure was characterized by X-ray diffraction (XRD, D/max2600, Rigaku, Japan) using the Cu Ka radiation (k 1.5418 Å). The morphologies of one-dimensional g-C 3 N 4 chain nanostructures were characterized by scan electron microscopy (SEM, SU70, Hitachi, Japan). And specific surface areas of the one-dimensional g-C 3 N 4 chain nanostructures were measured by a Micromeritics ASAP 2010 instrument and analyzed by the Brunauer-Emmett-Teller (BET) method. Photoluminescence (PL) spectra of photocatalysts were performed on a Jobin Yvon HR800 micro-Raman spectrometer including a 325 nm line from a He-Cd laser. UV-Vis diffuse reflectance spectra (DRS) of the samples are obtained by using a UV-Vis-IR spectrometer (Perkin-Elmer, Lambda 850). Photocurrent measurements were characterized by CHI 660 E electrochemical workstation (Chenhua, Shanghai) by applying a three-electrode cell accompanied by a visible light source. A platinum filament, Ag/AgCl electrode, and 0.2 M of Na 2 SO 4 were acted as the counter electrode, reference electrode, and an electrolyte solution, separately. The as-fabricated sample mixed a certain amount of Nafion solution was painted on FTO glass as a working electrode (the effective area was 1 cm × 1 cm).

Photocatalytic Test
Photocatalytic H 2 evolution was injected into a 250 ml of quartz reactor with a visible-light source irradiation. As a typical synthesis experiment, 0.1 g of photocatalyst with a certain Pt cocatalyst (1 wt%) was dispersed in a mixed solution of aqueous solution (90 ml) and methanol (10 ml). The amount of H 2 evolution was measured in a gas chromatograph (GC-2014C Shimadzu Corp., N 2 as carrier gas). The time interval of sampling was performed at 40 min during the water splitting process.

RESULTS AND DISCUSSION
The morphology of the as-fabricated products was scrutinized by scanning electron microscope (SEM). Figure 1A depicts the SEM image of the as-prepared bulk g-C 3 N 4 , in which the sample possesses a hierarchical component with a particle size of greater than 20 μm. However, in Figure 1B, we found that the morphology of g-C 3 N 4 changed greatly compared with the bulk structure, and a new one-dimensional chain structure appeared. It can be illustrated from Figure 1B that the lengths of these irregular-oriented chains g-C 3 N 4 one-dimensional structure could reach dozens of micrometers, and the diameters of those fibers range from 1 to 2 µm. Each one-dimensional structure was Frontiers in Chemistry | www.frontiersin.org March 2021 | Volume 9 | Article 652762 separated from each other, and further secondary structures could be found. Through the observation of the morphology characteristics of the two structures, we can infer that the chain g-C 3 N 4 material will possess a vast specific surface area, which is more conducive to enhance the photocatalytic performance. The transmission electron micrographs of chain g-C 3 N 4 are shown in Figures 1C,D. We can see the chain g-C 3 N 4 presents a similar sponge hole that exists on the surface of the chain g-C 3 N 4 , the morphology can effectively improve the capacity in the course of the photocatalytic reaction area. Meanwhile, the product can shorten the time and distance of the charge transfer, and promote the charge separation order to improve the photocatalytic activity of the material. As we all known, in the pyrolysis period of supramolecular precursor, the by-products were formed with varieties of gases gradually released, giving rise to the nitrogen defects obtained in the framework of g-C 3 N 4 . X-ray diffraction (XRD) patterns for chain g-C 3 N 4 and bulky g-C 3 N 4 are expressed in Figure 1E. Obviously, both patterns contain two diffraction peaks, which are located at 13.2°and 27.6°, respectively. The former peak at 13.2°could be indexed as (100) lattice plane, which is associated with interlayer stacking. The corresponding interlayer spacing value has been calculated to be 0.676 nm. And the later peak at 27.6°i s a feature interlayer stacking peak of aromatic systems, which could be indexed as (002) lattice plane. The calculated interplanar distance of aromatic units is 0.326 nm Zhang et al., 2020b). Figure 1F shows the Fourier transform infrared (FT-IR) spectra of the as-prepared samples. The FT-IR spectra of the synthesized chain g-C 3 N 4 were compared to the bulky g-C 3 N 4 exhibit with similar characteristics. The FT-IR the bands located at 1,240, 1,321, 1,415 cm −1 , and 1,570 cm −1 are mainly from the typical stretching modes of C-N heterocycles. And the band at 810 cm −1 is attributed to out-of-plane bending modes of C-N heterocycles. The C-N stretching mode has IR band at 1,635 cm −1 . And the broadband near 3,200 cm −1 corresponds FIGURE 1 | SEM images of (A) bulky g-C 3 N 4 and (B) chain g-C 3 N 4 at low and high magnification, and (C, D) TEM image of chain g-C 3 N 4 at low and high magnification; (E) XRD and (F) FT-IR patterns of bulky g-C 3 N 4 and pearl-chain g-C 3 N 4 .
Frontiers in Chemistry | www.frontiersin.org March 2021 | Volume 9 | Article 652762 to the stretching modes of terminal NH 2 or NH groups at the defect sites of the aromatic ring Li et al., 2020c). The exposed surface area and pores distribution of chain g-C 3 N 4 and bulky g-C 3 N 4 were further researched. The adsorption-desorption isotherm curve of chain g-C 3 N 4 and bulky g-C 3 N 4 demonstrate the type IV curve, as shown in Figure 2, indicating the presence of uniform mesoporous with high specific surface area and large total pore volume. The specific surface area of chain g-C 3 N 4 was counted to be 47.85 m 2 g −1 through the Brunauer-Emmett-Teller (BET), which is approximately three times larger than pure bulky g-C 3 N 4 (16.15 m 2 g −1 ). The inset in Figure 2 exhibits the pore-size distribution of the chain g-C 3 N 4 and bulky g-C 3 N 4 . The pore distribution for chain g-C 3 N 4 is mainly located at 2.46 nm, and the pore volume is 0.233 cm 3 g −1 . But for the bulky g-C 3 N 4 , the pore volume is just 0.097 cm 3 g −1 . The mesoporous structure and large surface area of the chain g-C 3 N 4 are conducive to the absorption of more active substances and reactants on the surface, enhancing the photocatalytic reaction. Therefore, we can assume that the chain g-C 3 N 4 material can have superior photocatalytic activity.
The UV-vis diffuse reflectance spectroscopy illustrated by Figure 3. from the (αhv)1/2 vs. photon energy (hv) plot Li et al., 2020d;Wang et al., 2020a;Zhang et al., 2020a;, the optical bandgap of chain g-C 3 N 4 and bulk g-C 3 N 4 was calculated to be approximate 2.88 and 2.93 eV, respectively. When the particle size descends to a certain value, the electron energy level near the Fermi energy level of the metal changes from quasi-continuous to discrete energy level (Miao et al., 2019;Huang et al., 2020b;Wu et al., 2020;, and the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital energy level (LUMO) of the nano-semiconductor particles have discontinuous energy gap, which leads to the blue shift of the chain g-C 3 N 4 . According to mentioned above, the prepared chain g-C 3 N 4 has a one-dimensional superstructure with a wide surface area, making it a more suitable candidate material for photocatalytic H 2 production. To evaluate the photocatalytic performance of chain g-C 3 N 4 photocatalyst with the visible light irradiation, the H 2 production performance was tested and compared with that of bulky g-C 3 N 4 photocatalytic performance was compared.
As the result shown in Figure 4A, the H 2 evolution rate of the bare bulky g-C 3 N 4 sample was measured to be 9.6 μmolh −1 , separately. However, the photocatalytic properties of chain g-C 3 N 4 were markedly improved, and the H 2 generation rate increased as high as 28.7 μmolh −1 . The chain g-C 3 N 4 sample shows rather superior photocatalytic activity for H 2 evolution, which could be ascribed to its vaster surface area ( Figure 2). Figure 4B shows the stability of the photocatalytic H 2 production system using chain g-C 3 N 4 as the photocatalyst under visible light irradiation. Therefore, we studied the chemical stability of chain g-C 3 N 4 , as described in Figure 4B. After four consecutive cycle experiments, we found that the evolution rate of the chain g-C 3 N 4 did not decline significantly, indicating that the material has good chemical stability.
The key factor of photocatalytic reaction is the effective generation and rapid separation of photoexcited carriers Li X. et al., 2020;Wang L. et al., 2020;Wang et al., 2020b;Zhang et al., 2020c). The optical performances of the samples were measured by photoluminescence (PL). The intensity of PL spectra can state the extent of the recombination of photo-generated charges. In Figure 5A, it illustrates bulky g-C 3 N 4 emerge severe charge recombination, while the PL spectrum of chain g-C 3 N 4 is intense quenched ( Figure 5A). The reorganizing of the photo-generated e-h pairs of chain g-C 3 N 4 can be restrained. The restraining of the e-h pair FIGURE 2 | BET nitrogen adsorption/desorption isotherms of bulky g-C 3 N 4 and chain g-C 3 N 4 .
FIGURE 3 | UV-vis absorption spectra of bulky g-C 3 N 4 and chain g-C 3 N 4 .
Frontiers in Chemistry | www.frontiersin.org March 2021 | Volume 9 | Article 652762 recombination is powerfully verified by the increased photocurrents for chain g-C 3 N 4 , as shown in Figure 5B. Apparently, the chain g-C 3 N 4 was provided with a better separation efficiency of e-h pairs. To further confirm the ability of several samples to separate and transfer charges and to respond to light (Cheng et al., 2019;Li et al., 2021), we performed photocurrent tests on them, as shown in Figure 5B six cycles were tested under λ > 420 nm xenon lamp. As can be seen from the diagram, two working electrode transient photocurrent response of bulky g-C 3 N 4 and chain g-C 3 N 4 were obtained through these six cycles. It can be seen that the transient photocurrent spectrum of the chain g-C 3 N 4 electrode was much better than the bulky g-C 3 N 4 . Test results indicate that chain g-C 3 N 4 greatly improves the separation of interface carriers. The photocurrent research consequences coincide with the impedance and photoluminescence research consequences, demonstrating that the chain g-C 3 N 4 material will have better photocatalytic performance.

CONCLUSION
In summary, we proposed an annealing method to achieve bulky g-C 3 N 4 and chain g-C 3 N 4 utilizing melamine as reactant materials. The chain g-C 3 N 4 nanostructures illustrated improvement on the photocatalytic H 2 production under visible light irradiation owing to unique inimitable one-dimensional structure, high specific surface area, excellent light-harvesting properties, and low recombination rate of electron-hole pairs. This simple preparation and facile composition of one-dimensional g-C 3 N 4 nanostructures demonstrate a promising candidate for exploring more actual applications of carbon nitride.

DATA AVAILABILITY STATEMENT
The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding author.  Frontiers in Chemistry | www.frontiersin.org March 2021 | Volume 9 | Article 652762 5 Zhang et al.
One-Dimensional g-C 3 N 4 Chain Nanostructures