Macroporous GaN/ZnO solid solution photocatalyst is synthesized through a novel sol-gel method under mild conditions. The performance of as-synthesized solid solution photocatalyst is evaluated for decomposition of gaseous 2-propanol (IPA). It is found that due to enhancement in both the adsorption to gaseous IPA and the absorbance to visible light, the porous GaN/ZnO solid solution exhibits a good photocatalytic performance for IPA decomposition. Moreover, the mechanism for photocatalytic degradation IPA over porous GaN/ZnO solid solution is also investigated in comparison with those for the two end materials ZnO and GaN. The trapping effects with different scavengers prove that both the photoexcited electrons and holes affect the IPA photodegradation process, simultaneously.

Photocatalysis using semiconductor and solar light is widely regarded as an ideal technology for solving both the environmental pollution and energy shortage issues. In last several decades, many semiconductor materials such as SrTiO3, GaN, and ZnO have been developed as photocatalysts.1 Most of them, however, can harvest only the ultraviolet light that occupies ∼4% of the solar spectrum, due to their wide energy bandgap. In order to effectively utilize the solar light, development of new visible-light-driven photocatalysts has been attracting more and more attention in recent years.2 Among the various methods, making solid-solution semiconductor is of particular interest for the flexibility in tuning the energy band structure and the derived advantageous performance that are unattainable by the individual end material.3–5 In recent years, some metal oxynitrides have been reported to exhibit excellent photocatalytic performance for overall water splitting under visible light as their bandgaps are less than 3.0 eV.6–9 Particularly, since the development of GaN/ZnO solid solution by Domen et al., a lot of works have been reported to promote its performance by means of doping noble metals, depositing cocatalysts, regulating the morphology, and so on.10–13 For instance, Rh2−yCryO3decorated GaN:ZnO is so far the most active catalyst for overall water splitting under visible light irradiation.13 

Previously, two ways have been developed for synthesizing GaN/ZnO solid solution. One method was developed by Maeda and Domen et al. who used a simple sintering process to get the particulate GaN/ZnO solid solution, the surface area of which was very low.12,13 On the other hand, it has been shown that the surface area of a material (e.g., TiO2) could be largely increased by making it porous, through which the photocatalytic activity is improved.14–17 In this context, Yan and Zou et al. used a two-step reactive template route to acquire mesoporous GaN/ZnO solid solution that had a high surface area, but the process was fairly complex.8 It should be pointed out that both of the aforementioned processes are needed to conduct under relatively high temperature, high NH3 flow (more than 250 ml min−1), and long soaking time (over 6 h), resulting in resource waste and also limiting the content of Zn in the solid solution.

In this paper, a novel synthetic method that needs low NH3 flow and short soaking time has been developed to synthesize porous GaN/ZnO solid solution. Gaseous 2-propanol (IPA) was used as a model organic pollutant for photodegradation so as to evaluate the behavior of the photocatalyst. In addition, the possible photocatalytic mechanism for IPA degradation over GaN/ZnO solid solution was discussed based on radical trapping experiments.18 

Powder samples of GaN (Aldrich), ZnO (Aldrich), Ga(NO3)3 ⋅ xH2O (Aladdin), and Zn(CH3COO)2 (Aladdin) were purchased for the synthesis of GaN/ZnO solid solution. The procedure for synthesis of macroporous GaN/ZnO solid solution is illustrated in Figure 1. Briefly, Ga(NO3)3 ⋅ xH2O (1.28 g) and Zn(CH3COO)2 (0.56 g) were first mixed in ethanolamine solution (0.6 ml) by heating at 65 °C for 1 h and subsequently aging at 0 °C for 1 week. The resulting gel-like material was sintered at 400 °C for 1 h, producing particulate ZnGa2O4 as the precursor. At last, the precursor was sintered at 900 °C for 1 h in ammonia atmosphere (80 ml min−1), obtaining the macroporous GaN/ZnO solid solution (denoted as Smacropore). In addition, particulate and mesoporous GaN/ZnO solid solution samples (denoted as Sparticle and Smesopore, respectively) were also synthesized for comparison (see Sec. S1 in the supplementary material19 for details).

FIG. 1.

Illustration of the formation of the macroporous GaN/ZnO solid solution. (a) Ga(NO3)3xH2O, Zn(CH3COO)2 powders and ethanolamine solution. (b) Formation of the gel-like material. (c) Formation of porous precursor by calcinations. (d) Formation of the macroporous GaN/ZnO solid solution by nitridation.

FIG. 1.

Illustration of the formation of the macroporous GaN/ZnO solid solution. (a) Ga(NO3)3xH2O, Zn(CH3COO)2 powders and ethanolamine solution. (b) Formation of the gel-like material. (c) Formation of porous precursor by calcinations. (d) Formation of the macroporous GaN/ZnO solid solution by nitridation.

Close modal

Phase constitution and crystal structure of the synthesized materials were determined by means of powder X-ray diffraction (XRD, D/MAX 2500, Rigaku Co., Japan) with Cu 1 radiation (λ = 0.154 nm). Morphology was observed on a scanning electron microscope (SEM, S-4800, Hitachi Co., Japan). UV-vis diffuse reflectance spectrum was measured on a spectrophotometer (UV-2700, Shimadzu, Japan; reference: BaSO4). The specific surface area was evaluated by nitrogen adsorption using the Brunauer–Emmett–Teller (BET) method with a surface area analyzer (Quantachrome Co., USA).

Gaseous 2-propanol (IPA, Sinopharm Chemical Reagent) was used as the model pollutant for photocatalytic degradation. The photocatalyst powder was uniformly dispersed on the circular bottom (10.2 cm2) of a small glass dish, which was put inside a reaction vessel (Pyrex glass body; net volume: 500 ml) with a quartz cover as the window for the incident light irradiated from a 300 W xenon lamp. Photocatalytic reactions were carried out under full-arc xenon lamp irradiation without any filter or under visible light irradiation by using a cutoff filter (HOYA, L42; λ > 420 nm). The reaction setup was first purged by artificial air (N2 : O2 = 4 : 1) for 10 min, and then the gaseous IPA was injected into the reaction vessel. Before light irradiation, the reaction vessel was put in the dark until an adsorption–desorption equilibrium of IPA in the photocatalyst powder was reached. During reaction under irradiation, about 0.5 ml of gas was taken from the reaction setup at an interval of 10 min for acetone concentration analysis by a gas chromatograph (GC-2014, Shimadzu Co., Japan). For mechanistic study, different scavengers including Benzoquinone (BQ), ammonium oxalate (AO), and t-butyl alcohol (TBA) (Sinopharm Chemical Reagent) were used. The scavengers (AO and BQ) (1 × 10−2 g ml−1 and 0.6 ml) and H2O (10 ml) were added into the photocatalyst powder (0.12 g), respectively, and then dried. The scavenger (TBA) was blown by artificial air into the reaction vessel containing IPA.

GaN, ZnO, and their solid solution have previously been reported to crystallize into the wurtzite structure at room temperature (∼25 °C).7 Figure 2 shows the XRD patterns of macroporous GaN/ZnO solid solution, together with the two end materials GaN and ZnO for comparison. By means of the least square method, their lattice constants were calculated to be as follows: macroporous GaN/ZnO solid solution (a = 3.202 Å, c = 5.205 Å), GaN (a = 3.190 Å, c = 5.189 Å), and ZnO (a = 3.250 Å, c = 5.207 Å). We can see that the lattice constants of GaN/ZnO solid solution were between those of two end materials GaN (JCPDS No. 76−0703) and ZnO (JCPDS No. 36-1451), confirming the formation of GaN/ZnO solid solution. It should be mentioned that owing to the relatively high nitriding temperature and short nitriding time, the crystallinity of macroporous GaN/ZnO solid solution appeared to be higher than that of mesoporous one but lower than that of the particulate sample (see XRD patterns in Figure S1 of the supplementary material19).

FIG. 2.

XRD patterns of GaN, ZnO, and porous GaN/ZnO solid solution powders.

FIG. 2.

XRD patterns of GaN, ZnO, and porous GaN/ZnO solid solution powders.

Close modal

The macroporous structure of as-synthesized GaN/ZnO solid solution sample can be seen from the SEM images under different magnifications as shown in Figure 3. In addition to the regular cores (Figure 3(b)) formed upon ammonia nitridation treatment, we can also observe some irregular larger pores (Figure 3(a)), which are believed to be resulted from the volatilization of organic solvent during sintering. BET surface area of the macroporous GaN/ZnO solid solution was measured to be ∼11.0 m2 g−1, which was smaller than that of the mesoporous sample (∼61 m2 g−1) but larger than that of particulate sample (∼7 m2 g−1). The average pore diameter of macroporous GaN/ZnO solid solution, calculated from the nitrogen adsorption isotherm using the Barrett–Joyner–Halenda (BJH) method, was around 60 nm, whereas the mean pore diameter of mesoporous GaN/ZnO solid solution was only 11 nm (see Figures S2a and S2c in the supplementary material19). These results are consistent with the observed TEM images (see Figures S2b and S2d in the supplementary material19). As a reference, the surface area of particulate GaN/ZnO solid solution was measured to be only ∼7 m2 g−1, suggesting decreased activity for IPA photodegradation due to decreased adsorption to gaseous IPA as will be discussed below.

FIG. 3.

SEM images of macroporous GaN/ZnO solid solution.

FIG. 3.

SEM images of macroporous GaN/ZnO solid solution.

Close modal

UV-vis diffuse reflectance spectra of the samples are presented in Figure 4. The bandgap of GaN/ZnO solid solution was roughly estimated based on its absorption edge to be ∼2.6 eV, which was similar to that of the mesoporous GaN/ZnO solid solution but narrower than that of the particulate sample (see Figure S3 in the supplementary material19). The conduction band and valence band of GaN/ZnO solid solution are mainly constructed of the Ga 4s4p states and (N 2p + Zn 3d) states, respectively. The p–d repulsion, which is related to the ratio of Zn/N (or Zn/Ga), dominates the level of valence band.20,21 In the synthesized macroporous solid solution sample, the Zn/Ga ratio was measured to be as high as 0.39, resulting in a higher valence band top and thus a narrow bandgap. The reason accounting for the large content of Zn in the macroporous GaN/ZnO solid solution could be attributed to the lower ammonia usage and shorter nitriding time, both of which decreased the volatilization of Zn. In the case of mesoporous GaN/ZnO solid solution, which was synthesized at low nitriding temperature, a large content of Zn seemed to be reserved as it had a similar absorption edge to that of the macroporous sample (see Figure S3 in the supplementary material19). In contrast, due to the much higher temperature and longer time for nitridization (see Sec. S1 in the supplementary material19 for details), the Zn/Ga ratio was only 0.12 in the particulate solid solution. Thus, its bandgap was much larger than that of the macroporous samples, being consistent with its UV-vis absorbance spectrum (Figure S3 in the supplementary material19).

FIG. 4.

UV–vis diffuse reflectance spectra of GaN, ZnO, and macroporous GaN/ZnO solid solution powders.

FIG. 4.

UV–vis diffuse reflectance spectra of GaN, ZnO, and macroporous GaN/ZnO solid solution powders.

Close modal

It is known that the mechanism for photocatalytic degradation of organic pollutant is closely related to the photocatalyst used,22–24 in which the potential levels of band edges play a crucial role. For example, Katsumata et al. used bisphenol A as the target organic pollutant and proved that the photocatalytic degradation mechanism over Ag3PO4 catalyst was direct hole-oxidation.22 Tian et al. revealed that the degradation of methyl orange over C3N4/Bi2WO6 heterojunctions was primarily dependent on the oxidation of O2.23 Bui et al. demonstrated that OH was the main oxygen species in the photocatalytic degradation of benzene by anatase TiO2.24 So far, however, few works have been reported in the photocatalytic degradation of organic pollutant, especially for the mechanistic investigation, over GaN/ZnO solid solution.

Considering the facts as mentioned above, it is necessary to examine the effect of different scavengers on the degradation of IPA so as to clarify the exact mechanism in photocatalytic degradation of IPA over GaN/ZnO solid solution. In this study, photocatalytic reaction was carried out under visible light irradiation (λ > 420 nm), and TBA, BQ, and AO were used as scavengers of the species OH, O2, and hole, respectively.

Illustrated in Figure 5(a) are the IPA photodegradation processes over porous GaN/ZnO solid solution without and with different scavengers. We can see that while the addition of TBA does not affect the degradation rate in comparison with that without using any scavenger, the presence of AO and BQ significantly decreases the photodegradation rate of IPA. These results indicate that in the IPA photodegradation process over GaN/ZnO solid solution, O2 and h+ are the main reactive oxygen species, and OH does not play an important role. It should be noted that the acetone evolution rate on macroporous GaN/ZnO solid solution (638 ppm h−1) was similar to that on mesoporous GaN/ZnO solid solution (659 ppm h−1) but was about 8 times higher than that on the particulate sample (74 ppm h−1) because of the relatively wide bandgap of the latter (see Figure S4 in the supplementary material19).

FIG. 5.

IPA photodegradation over (a) macroporous GaN/ZnO solid solution (under visible light irradiation), (b) GaN (under full-arc xenon lamp irradiation), and (c) ZnO (under full-arc xenon lamp irradiation) without and with different scavengers.

FIG. 5.

IPA photodegradation over (a) macroporous GaN/ZnO solid solution (under visible light irradiation), (b) GaN (under full-arc xenon lamp irradiation), and (c) ZnO (under full-arc xenon lamp irradiation) without and with different scavengers.

Close modal

For comparison, we also investigated the IPA photodegradation mechanisms over GaN and ZnO, respectively. Taking the relatively wide bandgap of either GaN or ZnO into account, the photocatalytic reactions were conducted under full-arc xenon lamp irradiation because IPA photodegradation could hardly be observed over these two materials under visible light. In Figure 5(b), it is clear that the hole scavenger could promote the photocatalytic degradation of IPA, probably because of the following reasons: the hole scavenger decreases the number of holes and reduces the recombination of photoexcited electrons and holes, making more electrons transport to the surface of GaN and react with O2 and generated O2. It implies that O2 is the main reactive oxygen species in the photocatalytic process over GaN.

From Figure 5(c), we can see clearly that the addition of BQ significantly inhibits the degradation rate of IPA over ZnO, indicating that O2 is also the main reactive oxygen species in the photocatalytic process over ZnO. As the potential of valence band top in ZnO (+2.89 V vs. NHE)25 is higher than the energy potential for oxidization of H2O (OH/H2O = + 2.68 V vs. NHE),26 h+ could oxidize the adsorbed H2O to OH. So, the addition of TBA also affects the degradation rate of IPA slightly.

CH 3 CH ( OH ) CH 3 + e + O 2 + H + CH 3 COCH 3 + OH + H 2 O ,
(1)
CH 3 CH ( OH ) CH 3 + h + CH 3 COCH 3 + 2 H + + e ,
(2)
h + + H 2 O OH + H + ,
(3)
e + O 2 O 2 ,
(4)
O 2 + H + HO 2 ,
(5)
CH 3 CH ( OH ) CH 3 + OH CH 3 C ( OH ) CH 3 + H 2 O ,
(6)
CH 3 C ( OH ) CH 3 + HO 2 CH 3 COOH ( OH ) CH 3 ,
(7)
CH 3 COOH ( OH ) CH 3 CH 3 COCH 3 + OH + OH .
(8)

Overall, the main mechanism for IPA degradation over both GaN and ZnO can be described by process (1), i.e., the O2 oxidizes 2-propanol to acetone. For ZnO, process (2) also plays a role, which is less important than process (1). For GaN/ZnO solid solution, the IPA photodegradation process is much more complex as described in processes (3)–(8): both e and h+ are very active simultaneously.27–29 Figure 6 schematically illustrates the whole reaction mechanisms of IPA photodegradation over ZnO, GaN, and GaN/ZnO solid solution, respectively.

FIG. 6.

Schematic illustration of mechanisms for photocatalytic degradation of IPA over ZnO, GaN, and GaN/ZnO solid solution, respectively.

FIG. 6.

Schematic illustration of mechanisms for photocatalytic degradation of IPA over ZnO, GaN, and GaN/ZnO solid solution, respectively.

Close modal

In summary, we have succeeded in developing a novel sol-gel method to synthesize porous GaN/ZnO solid solution under low NH3 flow and short soaking time. The relatively large surface area of the synthesized porous samples increases the adsorption to gaseous IPA and provides more reaction sites for IPA photodegradation. In addition, the developed synthetic method also secures the increase in Zn content and thus the significant decrease in the energy bandgap of GaN/ZnO solid solution. As a result, the performance of GaN/ZnO solid solution for IPA photodegradation is enhanced remarkably. Different from ZnO and GaN, over which photocatalytic IPA degradation mainly involves O2, photocatalytic IPA degradation over GaN/ZnO solid solution is a more complex process that involves both e and h+, simultaneously.

This work was partially supported by the National Basic Research Program of China (973 Program, Contract No. 2014CB239301), the Natural Science Foundation of Tianjin (Contract No. 13JCYBJC16600), and the Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), China.

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Supplementary Material