Synthesis of g-C₃N₄ from Various Precursors for Photocatalytic H₂ Evolution under the Visible Light

Abstract

Graphitic carbon nitride (g-C₃N₄) fabricated from different precursors exhibits unique microstructures and photocatalytic performance under visible light. Herein, we synthesized five different microstructures of g-C₃N₄ by the thermal poly condensation method using guanidine hydrochloride, melamine, urea, dicyandiamide and thiourea as the precursors. The results indicated that g-C₃N₄ prepared from urea precursor (UCN) has a nanostructure, porous layered structure, large specific surface area, and high separation efficiency of photo generated hole-electron pairs, which showed the best photocatalytic activity among all of the as-prepared samples. As for the lowest cost among the above five precursors, urea is an ideal candidate material for preparing g-C₃N₄ photocatalyst for a huge potential of wide industrial applications. In addition, Pt or Ni were used as the co-catalyst and loaded onto the g-C₃N₄ surface for photocatalytic hydrogen production. In comparison with noble metal Pt co-catalyst, Ni co-catalyst is inexpensive and has a significant effect o enhancing the photocatalytic activity under visible light. Therefore, Ni exhibits a considerable prospect to replace noble metal co-catalysts in the photocatalytic reactions.

1. Introduction

The rapid development of science and technology have benefited mankind and created a civilized society, and play an indispensable role in the process of promoting the progress of all human society [1,2,3]. However, with the continuous development and deepening of industrialization, the demand for all kinds of energy is also gradually increasing [4,5,6]. With the use of natural energy combustion, some non-renewable energies such as coal, oil, natural gas, and other fossil fuels release many harmful toxic gases and greenhouse gases, including H₂S, SO₂, NO, CO, CO₂, etc. [7], which has caused many growing environmental problems, such as a serious energy crisis, environmental pollution problems, and the greenhouse effect, which is not conducive to human survival or the development of the climate or the environment [8,9,10]. Facing these huge challenges, it is urgent to develop a clean, safe, efficient, and environmentally friendly method to achieve renewable energy sources, thus greatly alleviating the problems of the environment and energy demands.

Solar energy is believed to be the most ideal energy for the future because of its unique advantages and characteristics [11,12]. Specifically, using solar energy processes will not produce any pollution, and provides unlimited access at very low cost [13,14]. However, due to the shortcomings of solar energy, such as that it cannot be stored and is intermittent, research on the conversion of solar energy into stable chemical energy has gradually become the focus. Hydrogen was a good energy carrier, being clean, pollution-free, highly efficient, widely distributed, and cheap for storage and transportation [15,16]. The development and utilization of hydrogen energy has gradually achieved far-reaching significance to help alleviate the energy crisis and solve environmental pollution problems [17,18]. In recent years, photocatalysis has been considered the most promising strategy to produce hydrogen by water splitting by solar energy, that is to convert small density solar energy into dense chemical hydrogen energy.

As a new non-metallic polymer semiconductor photocatalyst material, g-C₃N₄ has visible light response and suitable band gap to absorb more visible light. In addition, g-C₃N₄ does not contain heavy metal elements, showing environmentally friendly and pollution-free merits [17]. The two-dimensional structure gives it a unique electronic structure. In this regard, these advantages are making g-C₃N₄ gradually become a focus of attention in the fields of environment, energy, and photocatalyst [19]. Until today, the main precursors for the preparation of g-C₃N₄ are urea, thiourea, and melamine by heat treatment and poly condensation. Due to the different molecular structures of precursors, and the different steps in the thermal condensation process of g-C₃N₄ formation, g-C₃N₄ with different textures can be formed [20]. For example, the precursors of melamine and dicydiamine tend to form a compact and solid block structure, while urea will form a porous fluffy structure, which is mainly caused by the release of a large amount of gas in the thermal condensation process of urea. However, many previous reports only involve g-C₃N₄ prepared from one or two precursors, and there are few studies comparing the photocatalytic properties of g-C₃N₄ prepared from more precursors. Herein, we used five different precursors (dicyanodiamine, melamine, thiourea, urea, guanidine hydrochloride) to prepare five different textures of graphitic carbon nitride (g-C₃N₄). UCN obtained from urea precursor achieves the best photocatalytic hydrogen evolution activity because of advantages in the microstructure, optical, and electronic structures. Meanwhile, the non-precious metal Ni as a co-catalyst was to explore the possibility of replacing non-precious metals with precious metals for photocatalytic hydrogen evolution from water splitting. This work provides a new insight into expanding the preparation strategies of g-C₃N₄ and reducing the cost of co-catalysts.

2. Experimental Section

2.1. Synthesis of Photocatalysts

Synthesis of g-C₃N₄: 10 g urea was weighed on an electronic analytical balance, then placed in the crucible, wrapped in the tin foil at the cauldron mouth, and covered with the crucible lid. It was then placed into a muffle furnace and heated to 550 °C with a warming speed of 5 °C/min. Two hours later, g-C₃N₄ (UCN) was obtained by natural cooling at room temperature. The same experimental steps were also performed for dicyanodiamine, guanidine hydrochloride, melamine, and thiourea precursors; finally, obtaining g-C₃N₄ photocatalysts marked as DCN, GCN, MCN, and SCN, respectively.

Synthesis of g-C₃N₄/Pt and g-C₃N₄/Ni composites: 72 mL of distilled water was taken in the reaction bottle, and 50 mg of g-C₃N₄ samples was added and dispersed by a sonography instrument. Then, 0.39 mL chloroplatin acid or nickel nitrate solution was added, then stirred with a magnetic mixer for 30 min. After that, the entire three-neck bottle was placed into nitrogen for 30 min to remove the air in the reaction system, and then was placed under light for 30 min. After that, the above suspension was separated with a centrifuge, and the obtained precipitate was thoroughly dried with a vacuum freeze dryer.

2.2. Characterization

X-ray diffraction (XRD) was recorded on a Rigaku D/Max-2000 diffractometer. A field-emission scanning electron microscopy (FESEM, JF-7500, JEOL) was used to observe morphology of the samples. A Shimadzu UV-2600 spectrophotometer ( SHIMADZU, Tokyo, Japan) equipped with an integrating sphere was applied to measure the diffuse reflection spectra (DRS), using BaSO₄ as a reference. Nicolet iS50 in situ diffuse reflectance Fourier transform infrared spectrometer (SHIMADZU, Tokyo, Japan) was used for FTIR test in the experiment. The surface area and pore structure were measured on the ASAP 2020 nitrogen adsorption device (SHIMADZU, Tokyo, Japan) from Micromeritics. A CHI660C electrochemical (CHENHUA, Beijing, China) analyzer from Shanghai Chenhuawas used to measure the separation rate and migration performance of photo generated electrons and holes of samples at the open circuit potential.

2.3. Photocatalytic H₂ Evolution Activity

72 mL of distilled water and 8 mL of triethanolamine were taken in the reaction three-neck bottle, and 50 mg samples was added and dispersed by sonography instrument. After that, the whole three-neck bottle was placed into nitrogen for 30 min to remove the air in the reaction system, and then was placed under visible light (Cell-300W, Xenon lamp with filter, λ > 400 nm) for a photocatalytic hydrogen production experiment for an hour. Finally, the gas productions of 0.4 mL were sealed by the gas sampler, and then the amount of H₂ was detected by the gas chromatograph (GC-14C, Shimadzu, Japan).

3. Results and Discussion

Figure 1 shows the FESEM images of the samples MCN, DCN, GCN, SCN, and UCN obtained from the five different precursors. Among them, the particles of MCN appear in a significant agglomeration phenomenon, forming a bulk-like structure with a large particle without any pores. A very small part of the DCN, GCN, and SCN samples are sheet-shaped or granular, and most of them are massive. However, the micro morphology of UCN is quite different from the other four samples, showing that the UCN morphology is laminar, and stack with each other to form a porous fluffy structure, indicating that UCN has a large specific surface area. This is mainly because the UCN formation process is accompanied by some gas during the calcination of urea, and it is emitted from the sample surface, leaving many pores. This nano layers sheet structure will greatly improve the specific surface area and the number of active sites of UCN, which is very conducive to the mass transfer in the photocatalytic process, and then promotes the photocatalytic performance.

Figure 1f shows the XRD patterns of the MCN, DCN, GCN, SCN, and UCN. All of the prepared samples contained two or more significant diffraction peaks, located at 12.1° and 27.3°. The former weak diffraction peak belongs to the (100) crystal surface, and is indexed as the periodic stacking of the triazine structural repeat units in the surface [17,21]. The latter corresponds to a crystal surface index of (002), which originates from periodic stacking between the layers [22,23], indicating that the samples prepared through five different precursors are all graphitic carbon nitride, and all of them have similar crystal structures. However, the intensity of these two characteristic peaks was decreased successively from melamine, dicyanodiamine, thiourea, guanidine hydrochloride to urea. This means that the MCN samples were the best crystalline, and the UCN were the worst crystalline, indicating the highest condensation of melamine during calcination. It is worth noting that the peak intensity of the UCN (100) facet is significantly weak compared with other samples, which may be caused by the release of the gas products during the calcination process, leading to the formation of the UCN with crystal defects, poor structure, and poor crystalline properties.

To further investigate the frame structure of the MCN, DCN, GCN, SCN, and UCN, FTIR spectra was recorded as shown in Figure 2a. The results show that vibration peaks in the FTIR spectra of all prepared samples are virtually identical, further indicating that these five precursors can prepare the same g-C₃N₄ containing the basic structural motif. Specifically, the FTIR absorption peaks of all samples are mainly around 800 cm⁻¹,1250 to 1700 cm⁻¹ and around 3200 cm⁻¹, which is in good line with the reports [24,25,26]. Vibrational peaks in the range of 1250 to 1700 cm⁻¹ belong to the characteristic peaks of the extended vibrations of C-N bonds on the heterocyclic ring in the g-C₃N₄ [21,27]. The absorption vibration peak at 800 cm⁻¹ is the “breathing” vibration model of the triazine structure of g-C₃N₄ [20,28]. The stronger absorption peak located at 3200 cm⁻¹ is the telescopic vibration of amino groups, indicating that all five precursors did not fully condense during calcination. These amino groups provide rich riveting sites for the metal co-catalysts that favor the stability of metal nanoparticles.

Figure 2b shows the UV-visible diffuse reflection spectra of UCN, MCN, DCN, GCN,a and SCN samples. The results show that the absorption edges of all five samples are in the visible light region, indicating a visible-light response. This suggests that all prepared g-C₃N₄ exhibit great possibility and potential for photocatalytic hydrogen evolution from water splitting. Notably, the absorption edge of the UCN is located at 448 nm, corresponding to a band gap of 2.76 eV, while the absorption edges of the other four samples are around 466 nm, corresponding to band gap of 2.66 eV obtained in Kubelka-Munk curves from their UV-visible spectra as shown in Figure 2c. Compared with other samples, the absorption edge of UCN has a blue shift and a large band gap, which is mainly caused by the quantum confinement effect of UCN nanostructure [29]. This result is consistent with the FESEM results.

To further probe the pore structure of the prepared samples, N₂ adsorption/ desorption isotherms, and corresponding pore size distribution curves of UCN, MCN, SCN, DCN and GCN samples were examined, as shown in Figure 2d. According to Brunauer Deming Deming Teller (BDDT) classification, the isotherms of the above five samples belong to type IV with a H3 hysteresis loop [30,31,32]. In addition, compared with other samples, the nitrogen adsorption capacity of UCN is much higher than that of the remaining samples, which is mainly due to its relatively loose nanostructure. Meanwhile,

Table 1. Relevant physical parameters from N₂ adsorption/desorption isotherms.

Samples	SBET (m2/g)	V (m3/g)	D (nm)
DCN	9.82	0.054	22.0
GCN	12.89	0.076	22.7
MCN	8.46	0.049	23.4
SCN	13.35	0.065	20.2
UCN	108.83	0.27	18.69

summarizes the specific surface area, pore volume, and pore diameter of all as-prepared samples. The specific surface areas S_BET of MCN, DCN, SCN, UCN, and GCN were 8.46, 9.82, 13.35, 108.83, and 12.89 m³/g, respectively. Among them, MCN and DCN have small S_BET values, which may be caused by the high degree of condensation, high yield, and good crystallinity of precursors during the calcination process. Comparatively, the specific surface area S_BET of the UCN reached 108.83 m²/g, which is much higher than that of other samples. This can be attributed to the releasing of a large amount of gas during the calcination process, which acts as a pore forming agent in the formation of g-C₃N₄, endowing UCN with a fluffy nanostructure and high specific surface area, increasing the photocatalytic active sites [33]. Furthermore, in comparison with other as-prepared samples, UCN shows more average pore volume and smaller average pore diameter, as shown in Table 1, which is very conducive to the mass transfer process in the photocatalytic process [34], and thus, significantly improving the photocatalytic hydrogen evolution form water splitting under visible light.

Photocatalytic hydrogen production of DCN, GCN, MCN, SCN,, and UCN coated with 3% Pt or Ni is recorded in Figure 3. In addition, the hydrogen production performance of DCN, GCN, MCN, and SCN without any co-catalyst is basically micro or trace, and they are not listed here. The photocatalytic hydrogen production of Pt co-catalyst on GCN, SCN, UCN, DCN, and MCN is 2536, 2894, 7421, 2498, and 2453 μmolh⁻¹g⁻¹, respectively. However, the photocatalytic hydrogen production of all samples prepared with Ni co-catalyst is 498, 596, 1484, 481, and 473μmolh⁻¹g⁻¹, respectively. This result shows that the introduction of noble metal Pt and non noble metal Ni co-catalysts can significantly improve the photocatalytic activity, accompanying by the best photocatalytic hydrogen performance of UCN. The experimental results show that photocatalytic activity over UCN with% 3 Ni co-catalyst and pure UCN is 1,484 and 9 μmolh⁻¹g⁻¹ under the same light conditions, respectively: that is, the former is 164 times the latter. This remarkable enhancement of photocatalytic performance indicates that non noble metal Ni can partially replace the noble metal as a co-catalyst for effective photocatalytic hydrogen evolution from water splitting. Additionally, non noble metal Ni not only has the advantages of easier capture of electrons, accelerating the separation of photo generated electron-hole pairs and reducing the probability of their recombination, but also has the characteristics of extensive sources, stable properties, and prices far lower than those of precious metals, which greatly saves the use cost of photocatalysts, and is very conducive to the wide application of photocatalyst technology.

The separation and transfer efficiency of carriers in photocatalysts is very crucial for the photocatalytic activity. Therefore, the transient photocurrent response was adopted to further investigate the separation behavior of photo generated electron-hole pairs in the photocatalytic process, as shown in Figure 4. All samples were irradiated by intermittent visible light source with 60 s cycle to obtain transient photocurrent response (i–t) curves. At the beginning of irradiation, the photocurrent curves of DCN, GCN, and MCN all observed obvious anodic photocurrent spikes, which may be caused by the bias voltage during the discharge process. It can be clearly observed that the instantaneous photocurrent intensity of the five g-C₃N₄ is UCN > SCN > DCN > GCN > MCN. For all samples, UCN shows the highest photocurrent intensity, mainly attributed to its high light utilization rate and unique two-dimensional nanostructure. This not only increases the concentration of photo generated carriers, but also accelerates the migration of photoelectrons to the surface, and the effective separation of electronic-hole pairs [34,35,36], thus significantly promoting the improvement of photocatalytic hydrogen performance.

4. Conclusions

In summary, graphitic carbon nitride (g-C₃N₄) was prepared by the thermal polymerization of five different precursors, namely guanidine hydrochloride, dicyandiamide, melamine, thiourea, and urea. Among them, g-C₃N₄ prepared with urea as precursor has a layered nanostructure, a high specific surface area of 108.83 m²/g, and excellent separation efficiency of photo generated electron hole pairs, exhibiting the best photocatalytic activity. When Pt or Ni were used as co-catalysts, the photocatalytic hydrogen performance of UCN under visible light was significantly enhanced. The photocatalytic hydrogen performance of UCN with 3% non-noble metal Ni as a co-catalyst is 164 times higher than that of pure UCN. Meanwhile Ni is cheap and comes from a wide range of sources. As a co-catalyst, it can significantly improve photocatalytic activity. Therefore, Ni has great potential to replace precious metals, such as Pt. It can reduce the cost of photocatalysis, which is conducive to the promotion of photocatalysis technology, and has a highly considerable research prospect.