Photocatalytic Performance and Mechanistic Research of ZnO/g-C3N4 on Degradation of Methyl Orange.

Highly dispersed ZnO/g-C3N4 composites with different doping ratios of g-C3N4 were prepared by a hydrothermal method. The preparation method is simple and the energy consumption is low. The composite samples were used to degradate the methyl orange solution. They all showed excellent photocatalytic activity and cycling stability. The optimal loading content of g-C3N4 was investigated, and the mechanism of enhanced photocatalytic activity was studied in detail. This study provides a promising photocatalytic material for the removal of organic pollutants.

Introduction

In recent years, with the development of the economy and the popularization of industrialization, the rapid improvement of human living standards has exposed serious environmental pollution problems, exposing a serious threat to human health and hindering the sustainable development of society.[1−3] A lot of researchers have made great efforts to explore more catalysts or novel materials to degrade organic and toxic pollutants of wastewater.[4,5] Fortunately, energy shortage and water pollution problems can be partially solved by semiconductor photocatalysis technology, which has been the research focus recently.[6−9] Some semiconductors such as ZnO,[10] TiO2,[11,12] and BaTiO3[13] have been used in the photocatalysis field.

ZnO is a nontoxic, low-cost semiconductor that is abundant in the earth’s crust. It is widely used in photocatalytic degradation of organic dyes and photocatalytic decomposition of generating hydrogen from water.[14−16] However, there are still some problems with the ZnO semiconductor that it can respond to most ultraviolet light and little visible light. It recombined photogenerated electrons and holes easily and had low quantum efficiency. To improve the photocatalytic performance of ZnO and broaden the spectrum absorption in the visible range, researchers found three ways such as designing Z-scheme,[17] doping metal/nonmetal elements,[18] and coupling with other semiconductors.[8]

In 1993, Niu’s group published a paper on the g-C3N4 crystal with the hardness exceeding the diamond in Science.[19] Because of the dramatic result, this graphite-like nitrogen carbide semiconductor has attracted more attention.[20−22] Researchers found the g-C3N4 catalytic degradation of organic dyes under visible light and decomposition of hydrogen from water.[23] At present, g-C3N4 composite materials are widely used in photocatalysis. They also become one of the best photocatalytic materials due to their low raw material prices.

Inspired by the above analysis, it is a good strategy to construct a ZnO/g-C3N4 composite structure to improve the photocatalytic efficiency. Some methods for preparing ZnO/g-C3N4 composite photocatalysts have been reported before;[24−26] the composite photocatalyst exhibited higher degradation activity of methyl orange than pure ZnO and g-C3N4 but still has the following problems: the first one is that the preparation method is more complicated and the yield is lower; the second one is that the energy consumption is large; the third one is that the cycling stability is poor and it is difficult to reuse; and the last one is that the mechanistic research of ZnO/g-C3N4 is rarely reported. To solve the above problems, we synthesized the ZnO/g-C3N4 composite via a hydrothermal method. This preparation method is simple, low cost, and the recycle utilization has been improved. Besides, the effects of different g-C3N4 doping ratios on photocatalytic effects were systematically studied and the mechanism of photocatalytic reactions was analyzed in detail.

Results and Discussion

Characterization of the ZnO/g-C3N4 Composite Photocatalyst

X-ray diffraction (XRD) patterns of as-prepared samples are shown in Figure to investigate the phase structures of the samples. Figure a shows the typical diffraction patterns of pure ZnO, and all of the diffraction peaks of the patterns are well consistent with the wurtzite phase of ZnO (JCPDS: 36-1451). Figure b shows the typical diffraction patterns of pure g-C3N4, the well-defined peak at 2θ 27.4° was indexed for graphic materials as the peak of g-C3N4 (002).[27] Also, the diffraction patterns of the ZnO/g-C3N4 composite photocatalyst are shown in Figure c; all of the diffraction peaks of the patterns are well consistent with Figure a,b, and the diffraction pattern illustrates that the sample is a composite structure of ZnO and g-C3N4.

Figure 1

XRD patterns of pure ZnO (a), pure g-C3N4 (b), and ZnO/g-C3N4 (c).

Transmission electron microscopy (TEM) images of as-prepared samples are shown in Figure , which are used for an in-depth study of the morphology of composite materials. The TEM image of the prepared ZnO is shown in Figure a, and ZnO shows a random arrangement. Figure b shows the TEM image of the prepared g-C3N4. Figure c shows the TEM images of the ZnO/g-C3N4 composite material. It can be seen that the morphologies of ZnO and g-C3N4 are rodlike and sheet, respectively. The diameter of ZnO is about 300 nm, and two different phases combine each other closely. This combination of ZnO and g-C3N4 would extend the lifetime of photogenerated electrons and holes, which reduces the internal charge recombination. This is very beneficial to the improvement of photocatalytic activity, which will be confirmed in the following work.

Figure 2

TEM images of pure ZnO (a), pure g-C3N4 (b), and ZnO/g-C3N4 (c).

The UV–vis absorption spectra of pure ZnO, pure g-C3N4, and ZnO/g-C3N4 are shown in Figure to compare the light absorption capacity of different photocatalysts. As we know, it has a close relationship with photocatalytic activity. It can be seen that the ZnO sample displayed intrinsic absorption peaks in the ultraviolet area, while g-C3N4 has absorption peaks from the ultraviolet region to the visible range of about 460 nm, these are highly consistent with previous reports.[28] ZnO/g-C3N4 showed two distinct absorption peaks, which were located in the ultraviolet and visible regions, respectively. The absorption peak in the ultraviolet region is in accordance with the position of the intrinsic absorption peak of ZnO, and the absorption peak in the visible region also matches the absorption peak of g-C3N4. Compared with pure ZnO, the wider light absorption region of ZnO/g-C3N4 can make full use of visible light, resulting in higher photocatalytic activity.

Figure 3

UV–vis diffuse reflectance spectra (DRS) of pure ZnO, pure g-C3N4, and ZnO/g-C3N4.

Photocatalytic Activity

The photocatalytic activities of as-prepared samples were evaluated by the degradation experiment of methyl orange under visible light. First, the optimal g-C3N4 doping content of the ZnO/g-C3N4 composite photocatalyst was explored. Figure a shows the photocatalytic degradation activity of methyl orange by a pure ZnO catalyst, pure g-C3N4 catalyst, and composite photocatalyst with g-C3N4 contents of 10, 20, and 30% by weight, respectively. As can be seen from Figure a, ZnO/g-C3N4-20 wt % showed the highest photocatalytic activity.

Figure 4

Degradation curves of different samples of methyl orange under visible light irradiation (a) and cycle curve of the photocatalytic degradation of methyl orange by ZnO/g-C3N4-20 wt % (b).

At the same time, in the actual application process, the photocatalytic effect of photocatalyst recycling is an important indicator. To determine the recycling capacity of the ZnO/g-C3N4 composite photocatalyst by five cycles of degradation experiments on methyl orange. Figure b presents that after five cycles of degradation experiments, the degradation rate of methyl orange by the ZnO/g-C3N4 composite photocatalyst did not decrease significantly. The degradation ability could still reach more than 90%, indicating that the ZnO/g-C3N4 composite photocatalyst has good recyclability.

Photocatalytic Mechanism

The photoluminescence spectra of pure ZnO, pure g-C3N4, and ZnO/g-C3N4 are shown in Figure to investigate the migration, transfer, and recombination of electron–hole pairs. As we know, the separation efficiency of carriers can improve the photocatalytic performance of photocatalysts.[29−31] It can be seen from Figure that the pure ZnO sample displays an intense intrinsic fluorescence emission peak at around 380 nm, and at the same time, the defect fluorescence emission peak of ZnO can be observed between 600 and 650 nm.[32] The pure g-C3N4 sample revealed an intense fluorescence emission peak between 450 and 500 nm.[33] Compared with the above pure sample, ZnO/g-C3N4 shows intrinsic fluorescence emission peaks of ZnO and g-C3N4 simultaneously and the strength was reduced. The lower photoluminescence intensity meant that the high-efficiency electron transferred from the conduction band of g-C3N4 to the conduction band of ZnO, hindering the reorganization of photoinduced charge carriers. The occurrence of this quenching phenomenon provided effective evidence for the improvement of photocatalytic activity.[34]

Figure 5

Photoluminescence (PL) spectra of pure ZnO, pure g-C3N4, and ZnO/g-C3N4.

The photocurrent tests of pure ZnO and ZnO/g-C3N4 are indicated in Figure . As we know, the stronger the photocurrent means the more the photogenerated electrons, which implies that the electron–hole separation is better, resulting in higher photocatalytic activity. Figure shows the photocurrent–time curves of g-C3N4, ZnO, and ZnO/g-C3N4 in the dark and visible light in the switching cycle mode; obviously, the latter possesses higher corresponding photocurrent strength, which undoubtedly proves its higher electron–hole separation efficiency. This is consistent with the results obtained in the above PL experiment, which further provides evidence for the higher catalytic efficiency of the composite catalyst.

Figure 6

Photocurrent response curves of pure ZnO, pure g-C3N4, and ZnO/g-C3N4.

A free radical trapping experiment was carried out to investigate the mechanism of degradation of methyl orange by ZnO/g-C3N4 composite photocatalysts as shown in Figure . It is an accepted effective method for studying the photodegradation reaction pathway of organic molecules.[35−37] At present, there are three types of active substances in the photocatalytic degradation of organic pollutants, namely superoxide radicals (•O2–), hydroxyl radicals (•OH), and photogenerated holes (h+). A series of free radical trapping experiments were carried out using isopropanol (IPA), benzoquinone (BQ), and TEOA over the ZnO/g-C3N4 composite photocatalysts. This is the quencher corresponding to the above three substances.[38] The degradation results of methyl orange showed that when IPA was added, the photodegradation efficiency of methyl orange was only 19%; if BQ was used as the quencher, the photodegradation efficiency of methyl orange was 24%; when TEOA was used as the quencher, the degradation of methyl orange reached the highest efficiency, up to 37%. The above results indicate that superoxide radicals (•O2–), hydroxyl radicals (•OH), and photogenerated holes (h+) are all active substances of photocatalytic degradation of methyl orange and the order of influence of the activating substances in the photocatalytic degradation of methyl orange is •O2– > •OH > h+.

Figure 7

Photodegradation rates of methyl orange by ZnO/g-C3N4 of different quenchers.

Figure displays the mechanism of photocatalytic degradation of methyl orange by ZnO/g-C3N4 composites. It is well known that the degradation of methyl orange is mainly attributed to the production of three active substances, namely superoxide radicals (•O2–), hydroxyl radicals (•OH), and photogenerated holes (h+). ZnO itself cannot be excited by visible light; it can only work in ultraviolet light. The band gap of g-C3N4 is 2.7 eV, which can adsorb visible light. The photocatalytic degradation mechanism of g-C3N4 was mainly attributed to two processes: one is the reduction process initiated by photogenerated electrons, such as the degradation of MO; another has originated from oxidation by the photogenerated hole, such as the degradation of RhB.[39] The spectrum of the ZnO/g-C3N4 composite can be broadened to the visible region compared with the pure ZnO. Compared with the pure g-C3N4, the ZnO/g-C3N4 composite has a significantly enhanced photocatalytic effect for degrading methyl orange under visible light. This is attributed to the observation that the conduction band (CB) edge potential of g-C3N4 was more negative than that of ZnO, and the photoinduced electrons on g-C3N4 particle surfaces transferred to the CB of ZnO easily via combining closely. g-C3N4 can be charged to restore to the ground state by degrading methyl orange. At the same time, the electron–hole separations were also driven by the synergistic effect of ZnO and g-C3N4. This synergistic effect leads to large numbers of electrons on the ZnO surface and large numbers of holes on the g-C3N4 surface, respectively. These are all factors that promote the efficiency of photocatalytic degradation of methyl orange.

Figure 8

Photocatalytic mechanism of the ZnO/g-C3N4 under visible light irradiation.

Conclusions

In summary, the ZnO/g-C3N4 composite photocatalyst with high dispersibility was successfully prepared. The photocatalyst has a simple preparation method and high photocatalytic activity and recycling stability. The optimal loading content of g-C3N4 was ZnO/g-C3N4-20 wt %. The synergistic mechanism of ZnO and g-C3N4 was confirmed by a series of experiments. This study provides the possibility of exploring more photocatalysts.

Experimental Section

Materials

Melamine (99.9%), ammonia (25%), ZnO (>99.5%), poly(vinyl pyrrolidone) (PVP), benzoquinone (BQ), triethanolamine (TEOA), and isopropanol (IPA) were all supported by Shanghai Chemistry Co., Ltd. The above reagents are of analytical grade and have not been further purified. Deionized water is obtained from the analytical laboratory.

Synthesis of Samples

Synthesis of ZnO

To increase the surface area of ZnO, commercial zinc oxide is treated with ammonia water to obtain ZnO required for the experiment. The specific steps are as follows: 5 g of zinc oxide was dispersed in 100 mL of deionized water. To this, 10 mL of 25% ammonia was added dropwise and stirred at room temperature for 30 min and then centrifuged to obtain a sample, which was washed several times with deionized water and dried at 60 °C for 10 h.

Synthesis of g-C3N4

Pure g-C3N4 was synthesized by calcining melamine at a high temperature. Briefly, 10 g of melamine was placed in a crucible and nitrogen was passed through a tube furnace and heated from room temperature to 550 °C at a ramp rate of 5 °C/min for 3 h. After cooling to room temperature, the prepared g-C3N4 was ground into a powder and sonicated for 1 h.

Synthesis of ZnO/g-C3N4 Composites

ZnO/g-C3N4 composites were synthesized according to the following procedure: 1 g of zinc oxide was dissolved in 100 mL of deionized water. To this, a certain amount of PVP was added, stirred, and heated to 60 °C. Then, different amounts of g-C3N4 were added and the mixture was stirred at this temperature for 1 h. The mixture was transferred into a stainless steel autoclave and kept warm at 140 °C for 8 h under vacuum. Then, the product was naturally cooled to room temperature and centrifuged several times with deionized water and dried at 80 °C for 10 h.

Characterization

X-ray diffraction (XRD) patterns of the photocatalyst were recorded at room temperature. The patterns of the samples were acquired by a Shimadzu XRD-6000 diffraction system with high-intensity Cu Kα radiation (40 kV, 200 mA) at 20–70° with a scanning step of 10° min–1. Transmission electron microscopy (TEM) images were acquired on a JEM-2100 transmission electron microscope at an accelerating voltage of 200 kV. UV–vis diffuse reflectance spectra (DRS) were obtained using a Shimadzu UV-3600 spectrometer by using BaSO4 as a reference. The photoluminescence (PL) spectra of the photocatalyst were performed at room temperature with 325 nm as the excitation wavelength and a Xe lamp as the excitation source.

Photocatalytic Experiments

The photocatalytic activities were evaluated through decomposing methyl orange under visible light irradiation (a 500 W Xe lamp with a 400 nm cutoff filter). For this, 0.05 g of photocatalyst was dispersed in 20 mL of methyl orange solution with a concentration of 10–5 mol/L under magnetic stirring for 30 min in the dark to achieve effective adsorption between the photocatalyst and methyl orange. After starting the irradiation, 5 mL was taken to monitor the absorbance of methyl orange every 20 min. In addition, free radical capture experiments were performed by adding 1 mmol benzoquinone (BQ), triethanolamine (TEOA), and isopropanol (IPA), respectively.

Photoelectrochemical (PEC) Measurement

Before preparing the corresponding electrode, the following steps were operated. About 0.5 g of as-prepared sample was dispersed in 4 mL of ethanol. The dispersed catalyst was dip-coated onto a 1 × 1 cm2 fluorine-doped tin oxide glass electrode and then dried by a hairdryer. The photoelectrochemical measurements were carried out on normal three-electrodes with 0.5 M sodium sulfate solution as the electrolyte. The as-prepared photocatalyst thin films were used as the working electrodes, the counter electrode was platinum, and the reference electrode was silver/silver chloride. The illumination source is a 500 W Xe lamp, and the distance from the photoelectrode was fixed to 20 cm. The photoelectrochemical measurements were performed on a CHI-760D electrochemical analyzer from Shanghai ChenHua Instruments Co., Ltd.