Graphite-like C₃N₄ and Graphene Oxide Co-Enhanced the Photocatalytic Activity of ZnO Under Natural Sunlight

Abstract

To enhance the photocatalytic performance of ZnO, the ZnO/g-C₃N₄ (ZCN) composite was prepared by ZnO and g-C₃N₄ under ball milling, and then the ternary graphene oxide (GO)/ZnO/g-C₃N₄ (GZCN) composite was achieved by using sonicating, stirring, and liquid phase evaporating. The photocatalytic performance was tested under UV light and natural solar light, respectively. The experimental results displayed that the GZCN composite revealed excellent photocatalytic performance under UV light and natural sunlight. When the ratio of ZnO to g-C₃N₄ is 1:0.2 and the mass fraction of graphene oxide is 0.25% in GZCN composite, the modified ZnO possesses optimal photocatalytic activity under UV light or natural solar light. RhB dye is degraded by 94% within 20 min under UV light, which is 3.41 times that of pure ZnO. Moreover, GZCN can degrade 88% of RhB in 60 min under natural sunlight. The enhancement for photocatalytic activity is attributed to the excellent conductivity of GO and heterojunction interaction between ZnO and g-C₃N₄, where the special electronic structure of g-C₃N₄ expands the spectral response range of ZnO and accelerates the transmission of photogenerated electrons and holes.

1. Introduction

ZnO photocatalyst has been extensively researched due to its wide band gap (3.37 eV), high exciton binding energy (60 meV), numerous morphologies, and due to the fact that it is cheap and non-toxic [1,2,3,4]. However, pure ZnO can only absorb near-ultraviolet light, has a very high combination rate of photogenerated electron–hole pairs, and displays a low photocatalytic activity. Accordingly, there have been a number of reports about enhancing the photocatalytic activity of ZnO, such as element doping, constructing heterojunctions [5,6,7], searching for suitable co-catalysts [8], and catalyzer carriers [9]. Among them, semiconductors coupling and element doping can change the semiconductor band structure by introducing impurity into ZnO to increase the solar energy utilization rate [10]. Furthermore, catalyst carrier is an effective method to improve the photocatalytic performance of ZnO, which has a large amount of impurities and defects compared with pure ZnO materials owing to reducing the recombination of photogenerated e⁻-h⁺ hole pairs [11].

In recent years, graphite-like carbon nitride (g-C₃N₄) as a metal-free polymer n-type semiconductor has become a research hotspot because of its narrow band gap (2.74 eV), high chemistry stability, and unique structure and electric properties [12]. It has been studied in a variety of fields such as photocatalytic water splitting [13,14,15], carbon dioxide photoreduction [16], disinfection [17], the degradation of pollutants [18,19], and the battery field [20]. Currently, g-C₃N₄ is being utilized as a reinforcement material to improve photocatalytic performance, which signifies a great achievement. For instance, He and his co-workers have successfully prepared ZnO/g-C₃N₄ composite with high photocatalytic activity compared to ZnO or g-C₃N₄ [21]. Tang et al. have prepared a wide-spectrum response high potential composite through constructing the ternary Ag/g-C₃N₄/NaTaO₃ Z-scheme heterojunction [22].

Unfortunately, g-C₃N₄ has some serious drawbacks, such as poor conductivity, low surface area, small active sites, slow surface reaction kinetics, and moderate oxidation ability [23,24,25]. At this point, it is necessary to design a new material that has the ability to overcome those problems. Graphene has been widely used to improve the photocatalytic property of semiconductor oxide due to its unique electric properties and structure, large specific surface area, high thermal stability, and chemical stability [26,27,28,29,30]. Our previous report also illustrated that graphene can enhance the photocatalytic activity of ZnO nanomaterials [31]. Hence, using the excellent properties of graphene can neutralize the disadvantages of g-C₃N₄, which can further improve the photocatalytic performance of ZnO/g-C₃N₄ (ZCN) composite.

ZnO is most likely to be applied in practice because of its low cost, non-toxicity, and superior properties. However, few researchers have reported on the photocatalytic properties of ZnO under natural light. In this manuscript, graphene oxide (GO) and g-C₃N₄ are used as co-reinforcing materials for improving the photocatalytic of ZnO, and the ternary GO/ZnO/g-C₃N₄ (GZCN) composite was successfully obtained by the mechanical ball milling method, and the photocatalytic activity was studied under natural solar light and UV light. This work will promote the application of ZnO in sewage treatment and environmental protection.

2. Experimental Section

2.1. Preparation of GZCN Material

Zinc oxalate (ZnC₂O₄) was purchased from Zhengzhou Runbang Chemical Products Co., Ltd., Zhengzhou city, China. Urea was purchased from Shanghai Civic Chemical Co., Ltd., Shanghai city China, with a purity of 99%. Graphene oxide (GO) solution (0.5 g/L) was prepared by Hummers’ classic method.

GZCN composites were synthesized typically as follows: First, ZnO and g-C₃N₄ powder were attained by annealing zinc oxalate and urea at 550 °C, respectively, and the heating rate was 5 °C/min. Next, the ZnO/g-C₃N₄ (ZCN) powder was obtained by ball milling 5 g ZnO and 1 g g-C₃N₄ for 4 h at a speed of 150 r/min and then dried in an oven at 60 °C. To find the optimum ratio of ZnO to g-C₃N₄, ZCNx was prepared by the same method, x = 0.1, 0.2, 0.3. Next, 2 g ZCN was dispersed in 60 mL ethanol. Subsequently, 10 mL GO solution (0.5 g/L) was added into the suspension after sonicating for 30 min. The GZCN composites were successfully prepared via stirring in a water bath at 70 °C until the liquid phase evaporated. The xGZCN was synthesized through the same process to explore the best mass ratio of GO, x = 0.0125, 0.25, 0.0375, and 0.5.

2.2. Characterization

The X-ray diffractometer (XRD) measurement was performed by Philips PW1710 diffractometer (Made in Philips Co., Ltd., Amsterdam city, Netherlands) with Cu Ka₁ radiation to analyze the crystal structures. The morphological analysis of different samples was observed by the NovaNano SEM 200 (FEI Co., Ltd., Hillsboro city, Oregon state, USA) and JEM-3010 Transmission Electron Microscope (TEM, JEOL Co., Ltd., Tokyo city, Japan). UV–vis diffuse reflection spectra were recorded on the TU-2550 spectrophotometer (SHIMADZU Co., Ltd., Tokyo, Japan).

The photocatalytic activity and cycle performance were evaluated by the degradation of RhB molecular under UV light and natural sunlight, and these specific steps were referred to in our previous works [32]. Briefly, 0.1 g sample was added into 200 mL RhB solution (1 × 10⁻⁵ mol·L⁻¹), and the mixed solution was placed under a 100 W high-pressure mercury lamp (the light wavelength is 365 nm, and the distance is about 20 cm between lamp and liquid). The solution depth is approximately 10 cm. Moreover, 0.1 g sample was added into 300 mL RhB solution (1 × 10⁻⁵ mol·L⁻¹), and then the mixed solution was irradiated under outdoor natural sunlight, for which the light strength is approximately 700 lux and solution depth is approximately 15 cm. The cycle stability of GZCN was tested at outdoor sunlight from 10 am to 14 pm.

Electrochemical impedance spectroscopy (EIS) was performed with the CHI 660 electrochemical workstation (Shanghai Chenhua Instrument Co., Ltd., Shanghai city, China). Ag/AgCl was used as the reference electrode, and Pt wire was used as the counter electrode. ITO conducting glass coated with sample (1 cm × 1 cm) was exploited as the working electrode, and the electrolyte solution was 0.5 mol/L Na₂SO₄ solution (100 mL).

3. Results and Discussion

The crystal structures of ZnO, g-C₃N₄, ZCN, and GZCN were characterized by X-ray diffraction. As shown in Figure 1, pure g-C₃N₄ exhibits two distinct peaks at 13.7° and 27.4°, corresponding to (100) and (002) plane, which can be indexed to the characteristic in-planar structural packing and inter-planar stacking peaks of the aromatic systems in g-C₃N₄ [33]. The diffraction peaks of ZnO are well matched with the hexagonal crystalline structure, and the high intensity demonstrated its crystallinity and purity. Except the same peaks with ZnO, it was found that a characteristic peak at 27.4° of CN also existed in the XRD patterns of ZCN and GZCN, demonstrating that g-C₃N₄ has been successfully introduced into industrial ZnO. However, the peak of GO could not be found in the pattern of GZCN, which may mean that the content of graphene oxide was too small or that the intensity of the peak was too low. Furthermore, the intensity of the diffraction peak for ZnO did not changed, meant that the crystallinity of ZnO has not changed after adding GO and g-C₃N₄.

Figure 2a,b show the morphology of the ZnO and GZCN samples, respectively. It is clear that ZnO exhibits irregular particle morphology. The morphology of GZCN displays that ZnO particles are dispersed in the fluffy g-C₃N₄ sheet surface, and there are a few ZnO agglomerations, as shown in Figure 2b. Figure 3a is the TEM image of GZCN; it is obvious that ZnO particles were embedded in the g-C₃N₄ nanosheet, but GO could not be found, which is attributed to the low content of GO. The selected area electron diffraction pattern (Figure 3b) verified that ZnO has good crystal characteristics containing multiple single diffraction spots. In order to confirm the distribution of elements in the composite, element mapping is characterized, and the results are shown in Figure 4. According to the element mapping, the black particles are ZnO, and the flake structure is g-C₃N₄, demonstrating that ZnO particles are loaded on the g-C₃N₄ sheet.

The photocatalytic activity was evaluated by degrading organic dye (RhB) under ultraviolet light and natural solar light. Figure 5a shows the performance of ZCNx under UV light. t equals 0, indicating the signal for turning on/off the light. When t is less than 0, this indicates dark adsorption. C₀ reveals the initial concentration of organic dyes, and C denotes the concentration of organic dyes at a certain moment. When the content of g-C₃N₄ increases, the photocatalytic activity first increases and then decreases. When the mass ratio of ZnO to g-C₃N₄ is 1:0.2, ZCN possess optimum photocatalytic activity, of which the degradation rate reached 99.7% within 40 min. Figure 5b depicts the effect of GO on photocatalytic performance, and GZCN showed excellent photocatalytic performance when the content of GO was 0.25 wt%. For a comparison of the different samples, the photocatalytic degradation under UV light and reaction rate constants k are depicted in Figure 6. As shown in Figure 6a, it is obvious that the blank test only shows a small amount of degradation for RhB. ZCN shows a better photocatalytic performance than ZnO, which also demonstrates that introducing g-C₃N₄ can well improve the photocatalytic activity of ZnO. The GZCN composite exhibits the highest photocatalytic activity when comparing ZnO and ZCN, and RhB dye is degraded by 94% within 20 min. Furthermore, the degradation rate was calculated as shown in Figure 6b. The k value of GZCN is 0.14318 min⁻¹, in which it is 3.41 times faster than that of ZnO (0.04192 min⁻¹). The result proved that introducing GO can further improve the photocatalytic performance of ZCN.

The different samples’ visible activity was tested by decomposing RhB dye under natural solar light, as shown in Figure 7a. ZnO has a low photocatalytic activity, while the ZCN and GZCN composites exhibited a higher photocatalytic activity, demonstrating that the introduction of g-C₃N₄ and GO can enhance the natural light activity of industrial ZnO. Additionally, in order to evaluate stability, cycle tests were completed and are shown in Figure 7b. Figure 7b shows that the photocatalytic still has high activity after five cycles, meaning that GZCN has high photocatalytic stability.

The above experiments demonstrated that ZnO has very high photocatalytic activity after being modified with GO and g-C₃N₄. Figure 8a displays the UV-vis diffuse reflectance spectra of different samples. All samples have a typical absorption in the UV light region with an edge of 400 nm. After compounding with g-C₃N₄, the absorption edge starts moving to the visible light region, for which there is a small enhancement for visible light absorption. Furthermore, it is not hard to find that the GZCN composite has a stronger absorption ability than the ZCN composite in the visible light region, implying that GO can enhance the light absorption ability of ZnO for visible light. According to the Kubelka–Munk formula [αhv = A(Eg−hv)^n/2] [34], the band gap energies of ZnO, ZCN, and GZCN are 3.08 eV, 2.52 eV, and 2.43 eV, respectively (shown in Figure 8b). Notably, ZnO has a narrower band gap compared to the ordinary ZnO (3.20 eV), which can be caused by its impurities. Figure 9 shows the electrochemical impedance spectroscopy (EIS) of different materials. It is clear that ZCN has a larger slope compared to GZCN, illustrating that the conductivity of GZCN is higher than that of ZCN.

A possible enhancement mechanism of photocatalytic property was proposed, as shown in Figure 10. The pure ZnO is supported on g-C₃N₄ sheet, and a small amount of graphene oxide is dispersed on the g-C₃N₄ nanosheet or pure ZnO. The introduction of g-C₃N₄ not only enhances its absorption in the visible region but can also form a heterojunction with pure ZnO, reducing the composite probability of photogenerated electron–hole pairs. The photogenerated electrons transfer from the conduction band of g-C₃N₄ to the conduction band of ZnO, while the holes migrate from the valence band of ZnO to the valence band of g-C₃N₄, inhibiting the recombination of photogenerated electron–hole pairs and improving the photocatalytic activity of ZnO [35]. In addition, owing to its excellent conductivity ability, graphene oxide (GO) can reduce the electrical resistance of the composite, promoting the mobility rate of photogenerated electron–hole pairs, leading to a further increase in the photocatalytic activity of ZCN.

4. Conclusions

A GZCN composite with high activity, visible light response, and high stability was prepared using a low-cost method. Utilizing g-C₃N₄ and grapheme as co-modifiers can significantly enhance the photocatalytic performance of pure ZnO. The photocatalytic activity of GZCN reaches optimization when the ratio of ZnO to g-C₃N₄ is 1:0.2 and the mass ratio of graphene oxide is 0.25%. Compared to industrial ZnO, the photocatalytic degrading process of GZCN was accelerated by 2.41 times under UV light irradiation. Moreover, the GZCN composite exhibited an excellent photocatalytic performance in sunlight irradiation, which can degrade 88% of RhB in 60 min. The main reason for this improvement is that the GZCN composite displayed good ability in the absorption of visible light and high stability. More importantly, this material is inexpensive and can be prepared on a large scale, which can hopefully be applied in practice.