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., 2009; Wang et al., 2012; Ong et al., 2016). In the last several decades, graphitic carbon nitride (g-C₃N₄) the organic semiconductor showed a specific graphite-like sp²-bonded C–N structure that exhibits significant potential in the field of CO₂ 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-C₃N₄ 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-C₃N₄.

A large number of strategies acting to resolve these problems and reinforce the photocatalytic performances of g-C₃N₄, involving non-metal element doping [S (Cao et al., 2018), O (Jiang et al., 2019), V (Ding et al., 2013), B (Yan et al., 2010), etc.], noble metals decoration [Pd (Wang et al., 2011) Pt (Maeda et al., 2009) and Au (Li et al., 2012)], 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-C₃N₄ to collaborate with other materials, layers structure configuration is deemed as a prospective method to compound g-C₃N₄ as they can accelerate the diffusion of reactant, strengthen the light harvesting, expand the exposed surface areas, and promote the charge delivery.

Generally speaking, microstructural g-C₃N₄ photocatalysts with controllable morphology and structure can be divided into six categories: mesoporous (Zhao et al., 2018; Chen et al., 2019), nanosheets (Yang et al., 2017; Gao et al., 2018), nanorods (Cui et al., 2012; Bai et al., 2013), nanotubes (Gao et al., 2012), and nanospheres (Zheng et al., 2015). Among them, one-dimensional (1D) nanostructured photocatalyst, especially fabricating 1D photocatalyst with large surface area, is especially interested as the charges can vectorially transfer along with the 1D structure (Tahir et al., 2014).

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-C₃N₄ 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.

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-C₃N₄, 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-C₃N₄ 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-C₃N₄ 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 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-C₃N₄ material will possess a vast specific surface area, which is more conducive to enhance the photocatalytic performance. The transmission electron micrographs of chain g-C₃N₄ are shown in Figures 1C,D. We can see the chain g-C₃N₄ presents a similar sponge hole that exists on the surface of the chain g-C₃N₄, 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-C₃N₄.

X-ray diffraction (XRD) patterns for chain g-C₃N₄ and bulky g-C₃N₄ 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° is 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 (Li et al., 2020a; 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-C₃N₄ were compared to the bulky g-C₃N₄ 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 to the stretching modes of terminal NH₂ or NH groups at the defect sites of the aromatic ring (Li et al., 2020b; Li et al., 2020c).

The exposed surface area and pores distribution of chain g-C₃N₄ and bulky g-C₃N₄ were further researched. The adsorption-desorption isotherm curve of chain g-C₃N₄ and bulky g-C₃N₄ 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-C₃N₄ was counted to be 47.85 m²g⁻¹ through the Brunauer-Emmett-Teller (BET), which is approximately three times larger than pure bulky g-C₃N₄ (16.15 m²g⁻¹). The inset in Figure 2 exhibits the pore-size distribution of the chain g-C₃N₄ and bulky g-C₃N₄. The pore distribution for chain g-C₃N₄ is mainly located at 2.46 nm, and the pore volume is 0.233 cm³g⁻¹. But for the bulky g-C₃N₄, the pore volume is just 0.097 cm³g⁻¹. The mesoporous structure and large surface area of the chain g-C₃N₄ 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-C₃N₄ 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 (Hou et al., 2020; Li et al., 2020d; Wang et al., 2020a; Zhang et al., 2020a; Zhang Y. et al., 2020), the optical bandgap of chain g-C₃N₄ and bulk g-C₃N₄ 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; Zhang S. 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-C₃N₄.

According to mentioned above, the prepared chain g-C₃N₄ has a one-dimensional superstructure with a wide surface area, making it a more suitable candidate material for photocatalytic H₂ production. To evaluate the photocatalytic performance of chain g-C₃N₄ photocatalyst with the visible light irradiation, the H₂ production performance was tested and compared with that of bulky g-C₃N₄ photocatalytic performance was compared.

As the result shown in Figure 4A, the H₂ evolution rate of the bare bulky g-C₃N₄ sample was measured to be 9.6 μmolh⁻¹, separately. However, the photocatalytic properties of chain g-C₃N₄ were markedly improved, and the H₂ generation rate increased as high as 28.7 μmolh⁻¹. The chain g-C₃N₄ sample shows rather superior photocatalytic activity for H₂ evolution, which could be ascribed to its vaster surface area (Figure 2). Figure 4B shows the stability of the photocatalytic H₂ production system using chain g-C₃N₄ as the photocatalyst under visible light irradiation. Therefore, we studied the chemical stability of chain g-C₃N₄, as described in Figure 4B. After four consecutive cycle experiments, we found that the evolution rate of the chain g-C₃N₄ 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 (Huang et al., 2020a; 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-C₃N₄ emerge severe charge recombination, while the PL spectrum of chain g-C₃N₄ is intense quenched (Figure 5A). The reorganizing of the photo-generated e-h pairs of chain g-C₃N₄ can be restrained. The restraining of the e-h pair recombination is powerfully verified by the increased photocurrents for chain g-C₃N₄, as shown in Figure 5B. Apparently, the chain g-C₃N₄ 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-C₃N₄ and chain g-C₃N₄ were obtained through these six cycles. It can be seen that the transient photocurrent spectrum of the chain g-C₃N₄ electrode was much better than the bulky g-C₃N₄. Test results indicate that chain g-C₃N₄ greatly improves the separation of interface carriers. The photocurrent research consequences coincide with the impedance and photoluminescence research consequences, demonstrating that the chain g-C₃N₄ material will have better photocatalytic performance.

In summary, we proposed an annealing method to achieve bulky g-C₃N₄ and chain g-C₃N₄ utilizing melamine as reactant materials. The chain g-C₃N₄ nanostructures illustrated improvement on the photocatalytic H₂ 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-C₃N₄ nanostructures demonstrate a promising candidate for exploring more actual applications of carbon nitride.