The Ga0.25Zn4.67S5.08 catalysts were prepared by ultrasonic spray pyrolysis with thiourea as a precursor. The crystal structure, optical properties, specific surface area, chemical composition, surface morphology, and internal structure of the prepared samples were characterized by XRD, UV-Vis DRS, BET, XPS, SEM, and TEM, respectively. The disinfection performance and the main active species of the synthesized catalysts under visible light irradiation were investigated with E. coli K12. The study results indicate that Ga0.25Zn4.67S5.08 prepared at 400°C exhibited the best antibacterial performance with complete kill of E. coli K12 within 4 h. The antibacterial performance of the prepared sample is also higher than that prepared by the hydrothermal method. The result of the radical trapping experiment indicates that the photogenerated holes play a dominant role in the disinfection process.

1. Introduction

Various pathogenic bacteria living in waters have always been the greatest threat to human health. The statistic data show that millions of people were disabled or die because of bacteria every year [1]. Traditional sterilization techniques include chemical sterilization and ultraviolet (UV) disinfection. Among them, the chemical disinfection method uses strong oxidants such as liquid chlorine, sodium hypochlorite, chloramine, chlorine dioxide, or ozone, which has an efficient sterilization effect and effectively protects people’s drinking water safety. However, liquid chlorine and sodium hypochlorite can increase the risk of disinfection by-product (DBP) formation [2, 3]. Chloramine disinfection may form carcinogenic nitrosamines and nitrite substances by reacting with nitrates and inorganic salts. Chlorine dioxide and chlorite may be produced during chlorine dioxide disinfection, causing blood erythrocyte damage and limiting oxygen transport [4]. Ozone disinfection has high energy consumption and may oxide some aqueous organic substances to form carboxylic acids, formaldehyde, ketones, and other carcinogenic and genetic toxic by-products [5].

The UV disinfection method can rapidly achieve the effect of sterilization and disinfection by damaging the molecular structure of microbial DNA or RNA without producing DBPs [6]. However, some bacteria, such as spores, fungal spores, Mycobacterium tuberculosis, and Bacillus subtilis, are highly resistant to ultraviolet light. In addition, some bacteria have the ability of self-repair or resurrection, so the sterilization is not durable. These problems limit the further popularization of UV disinfection [7]. Except for those mentioned above, the heavy metal ions such as Ag+ were also reported to have superior ability in cell killing; however, the high toxicity limits their application in disinfection [8]. Therefore, it is urgent to develop an economical, safe, and efficient disinfection technology for microbial inactivation in water [9].

In recent years, photocatalytic sterilization technology with photocatalyst as the core has attracted more and more attention due to its advantages of direct utilization and conversion of sunlight, high efficiency, stability, and no by-product formation [10]. Some traditional semiconductor materials such as TiO2 and ZnS have the ability of photocatalytic sterilization. However, these semiconductors are hampered by a large intrinsic band gap. Based on the above limitation, enormous efforts, such as doping and heterojunction construction, have been investigated to broaden the visible light response [1114], but the photocatalytic efficiency was still not obviously enhanced, which greatly limited their practical application. Therefore, it is imperative to develop a new type of high-efficiency photocatalyst to make up for the shortcomings of traditional materials. Recent series of studies have shown that ternary sulfides such as Cu3SbS3 [15], ZnIn2S4 [16], Cu2Mo6S8 [17], and [18] are highly attractive due to their low cost, excellent electrical properties, and nonlinear optical properties. Particularly, their physicochemical properties can be facile regulated by adjusting the composition of each element, making them stand out among many new photocatalysts [19]. The literature reports that Ga2S3 is an effective optoelectronic material; however, its wide band gap ( eV) limits the application of visible light [20]. Therefore, a new ternary sulfide photocatalyst with an adjustable and optimized band structure can be obtained by introducing new metal elements in the crystal phase structure of Ga2S3.

At present, the preparation methods of ternary sulfides mainly include the hydrothermal method [21], electrochemical method [22], and microwave-assisted method [23]. Most of these methods need high temperature, high pressure, or external template to control the morphology; thus, it is difficult to be widely used. In recent years, it has been reported that the ultrasonic spray pyrolysis (USP), which can be carried out under ambient pressure with no template, serves as a novel preparation method for photocatalyst. It mainly includes atomization of the solution by means of an atomizer and the physicochemical reaction of the droplets under heating conditions [24]. The catalyst particles with a high specific surface can be obtained by USP, and the photocatalytic efficiency can be significantly improved compared to conventional preparation methods.

In this study, a novel ternary sulfide photocatalyst Ga0.25Zn4.67S5.08 was prepared using USP. The effect of reaction conditions on the Ga0.25Zn4.67S5.08 was studied, and their photocatalytic activities were evaluated using bacterial disinfection under visible light irradiation.

2. Experimental Section

2.1. Catalyst Preparation

The Ga0.25Zn4.67S5.08 photocatalyst was prepared by a USP method. All chemicals used for synthesis were of analytical grade and used without further purification. In a typical process, 0.297 g Ga(NO3)3·XH2O, 0.511 g Zn(NO3)3·6H2O, and 0.761 g CH4N2S were added into 150 mL distilled water under vigorous stirring to form a transparent solution. Ultrapure water was used in all the experiments. The solution was nebulized at (YUYUE402AI, Shanghai) and then carried by air with different flow rates from 2 to 10 L·min-1 through a quartz tube surrounded by a furnace thermostated at 400~900°C. The quartz reaction tube with the diameter of 5.0 cm had a long length of 60 cm [25]. The products were collected in a percolator with ultrapure water and then separated by centrifugation and washed thoroughly with ethanol and ultrapure water. The products were finally dried under a vacuum at 60°C overnight. For comparison, the Ga0.25Zn4.67S5.08 were prepared by a modified hydrothermal method according to the previous study [11].

2.2. Characterization

The X-ray diffraction (XRD) pattern with a scan rate of 0.05° was recorded on a Shimadzu 6100 Advance X-ray diffractometer with Cu Kα radiation in the 2θ range from 20° to 60°. Scanning electron microscopy (SEM, SU8000) was used to characterize morphology of the obtained products. Transmission electron microscopy (TEM) study was carried out on a FEI Titan G2 60-300 electron microscopy instrument. The UV-Vis diffuse reflectance spectra (UV-Vis DRS) were performed on a UV-Vis spectrophotometer (Varian, Cary 500, Palo Alto, CA, USA) with BaSO4 as the reference sample in these measurements. The Brunauer-Emmett-Teller (BET) specific surface areas and porosities of the sample were carried out by using the ASAP 2020 equipment (Micromeritics, Atlanta, GA, USA). X-ray photoelectron spectroscopy (XPS) measurements were performed on a PHI Quantum 2000 XPS system (Physical Electronics, Portland, OR, USA); all the binding energies were calibrated to the C 1s peak at 284.8 eV of the surface adventitious carbon. Photocurrent was measured by an electrochemical workstation (Multi Autolab M20) in a three-electrode system with a working electrode, a Pt sheet counter electrode, and a standard Ag/AgCl reference electrode containing 0.2 mol·L-1 Na2SO4 solution as the electrolyte.

2.3. Photocatalytic Antibacterial Tests

The experiments of photocatalytic disinfection were carried out by using a 300 W Xe lamp with a UV cutoff filter () as the light source. All glassware and 0.9% saline were autoclaved at 121°C for 20 min before the experiments. Typically, 10 mg of nanocrystal powder was mixed with 50 mL saline in the quartz cell, followed by the addition to 1 mL of the suspension containing E. coli cells. The suspension was stirred in darkness for 1 h to achieve adsorption equilibrium of viruses on the photocatalyst. Then, at different time intervals, 0.1 mL of the reaction mixture was collected and immediately diluted with sterilized saline for a serial dilution. Following that, 0.1 mL of the diluted solution was spread uniformly on nutrient agar and incubated at 37°C for 12 h; the number of viable cells was counted. The survival rate was estimated by the following equation: , where and are the numbers of viable cells in darkness and after photocatalytic reaction with photocatalysts, respectively.

3. Result and Discussion

The X-ray powder diffraction analysis result is shown in Figure 1. The diffraction peaks at 28.6°, 47.6°, and 56.5° correspond to the (111), (220), and (311) planes of the cubic structure of Ga0.25Zn4.67S5.08 (JCPDS card 49-1626), respectively. Among all of the peaks, the diffraction peak at (111) gradually becomes more intense and narrow with pyrolysis temperature, suggesting an increasing degree of crystallinity [26, 27]. At the same time, these synthesized samples prepared at 400°C and 600°C have a wide diffraction peak, indicating that the samples have smaller crystals and possibly improved the catalytic efficiency. The average grain sizes calculated by the Scherrer equation of the samples prepared at 400, 600, and 800°C are 1.03, 1.32, and 3.18 nm, respectively [28]. Nevertheless, when the temperature rises to 900°C, there are some possible Ga2O3 and ZnS impurity diffraction peaks in the XRD pattern, which may be due to the oxidation and decomposition of Ga0.25Zn4.67S5.08 caused by high temperature.

Figures 2(a)2(c) show the SEM images of the samples fabricated at 400°C and 800°C, respectively. It can be seen from Figure 2(a) that the samples were composed of well-defined spherical microspheres, and their diameters are approximately distributed between 100 nm and 1200 nm. By comparing with Figures 2(b) and 2(c), it can be found that there is a rough surface of the sample prepared at 400°C, which will increase the specific surface area and the active sites in the photocatalytic process, eventually leading to the enhancement of photocatalytic activity. When the temperature was raised to 800°C, the surface of the particles becomes relatively smooth. This can be ascribed to the increase of grain size under high temperature, and the surface roughness gradually decreases.

The spherical and porous structures of the as-prepared Ga0.25Zn4.67S5.08 sample were further confirmed by TEM. The TEM images in Figures 2(d)2(f) illustrate that the Ga0.25Zn4.67S5.08 sample has porous structured microspheres and was built by the aggregation of plenty of nanoparticles. This structure will be beneficial to increase the specific surface area and the surface active sites of the catalyst. By enabling more contacts of the pollutant molecules with the catalysts, the performance in the photocatalytic process is thus improved. The composition of the product was confirmed by EDX analysis (Figure 2(g)), proving the presence of Ga, Zn, and S in the as-prepared sample.

On the basis of the above results, an illustration of the possible three-stage formation mechanism of Ga0.25Zn4.67S5.08 microspheres was suggested (Scheme 1). First, the solution was atomized into aerosol under ultrasonic conditions. The aerosols were then carried by air through the quartz tube surrounded by a furnace. Subsequently, the droplets were rapidly vaporized under high temperature, while the thiourea in the aerosol also decomposes quickly and produces H2S. The produced H2S rapidly reacted with the metal precursor to form ternary sulfide submicrospheres, while the mass and heat transfer were accelerated under gasification conditions. As the droplets were further evaporated and concentrated, the particle size gradually reduced to the obtained spherical particles.

The optical absorption properties of Ga0.25Zn4.67S5.08 calcined at different temperatures were characterized by diffuse reflectance UV-Vis spectra (DRS) in the range of 200~800 nm as shown in Figure 3(a). All the samples display remarkable absorption in the visible region. Besides, the blue shift of the absorption band edge can be observed when the preparation temperature decreases from 900°C to 400°C. The blue shift of the adsorption edge with the decrease in preparation temperature is mainly due to the quantum confinement effect [26]. The energy band gap () of the sample can be calculated by the formula [29]. The band gaps of the catalysts prepared at 400, 600, 800, and 900°C are calculated to be 2.21, 2.52, 2.45, and 2.29 eV, respectively, which indicates that the synthesized samples can respond well to visible light.

The chemical states of elements for the Ga0.25Zn4.67S5.08 prepared at 600°C were examined by XPS analysis. The wide scan XPS spectrum (Figure 4(a)) reveals that the predominant elements are C, O, S, Zn, and Ga. Among these elements, S, Zn, and Ga elements are from the prepared composites, while C and O elements are from the XPS instrument itself and the adsorbed hydroxyl groups, respectively. The high-resolution spectra of the Ga 2p with two characteristic peaks at 1117.8 eV and 1144.6 eV corresponded to the 2p3/2 and 2p1/2 levels (Figure 4(b)), suggesting the existence of Ga3+ species [30]. As shown in Figure 4(c), the two peaks at the binding energies of 161.4 eV and 162.6 eV can be attributed to the 2p3/2 and 2p1/2 levels of S2- [31]. The binding energies of two major peaks at 1021.0 eV and 1044.0 eV ascribed to Zn 2p3/2 and for 2p1/2, respectively (Figure 4(d)), in accordance with Zn2+ in Ga0.25Zn4.67S5.08 [32]. These results further indicate that Ga0.25Zn4.67S5.08 can be successfully synthesized via USP.

The nitrogen adsorption-desorption isotherms and pore size distribution curve of the as-prepared samples were depicted in Figure 5. As shown in Figure 5(a), the physical adsorption isotherms of all the samples show the typical type IV isotherms with H3 hysteresis loop. The pore size obtained from the desorption branch of the isotherms exhibits a narrow distribution in the range of from 2 to 10 nm (Figure 5(b)), indicating the mesoporous nature of the as-prepared sample [33]. The porous structures with high specific surface areas could adsorb more active species and reactants on its surface and provide efficient transport pathways for the reactants and products during the photocatalytic process, thus accelerating the photocatalytic reaction rate. The BET surface areas of the Ga0.25Zn4.67S5.08 prepared at 400, 600, and 800°C were 25.76, 13.91, and 14.00 m2·g-1, respectively (Table 1). The catalyst prepared at 400°C possesses the highest specific surface area and the largest average pore size of 13.85 nm, compared with that of samples prepared at 600°C and 800°C at 7.69 and 7.40 nm, respectively.

Efficient charge separation is also an important factor affecting photocatalytic performance [34]. The photocurrent response as a useful method to evaluate the charge separation was tested. As shown in Figure 6, all samples have great photoelectric performance, manifesting that they could respond to visible light. Obviously, photocurrent generation was shown to be affected by the calcined temperature. Notably, the photocurrent of the sample obtained at 400°C is higher than that of the samples obtained at 600°C and 800°C, which could be ascribed to the fact that the sample obtained at 400°C has a larger specific surface area and more surface active sites. As a result, much more photogenerated electrons and holes can be produced. The photocurrent density generated by the 800°C sample was higher than that of the 600°C sample, which can be ascribed to the fact that the sample prepared at higher temperature has a higher crystallinity. The higher crystallinity may decrease the electron-hole pair recombination rate and lead to higher photocurrent density. The falling edges also show that the lifetime of the photogenerated carriers of the samples prepared at 400°C is much longer than that of 600°C and 800°C, which indicates that the recombination of carriers of the samples prepared at 400°C is much lower than that of 600°C and 800°C. This is also benefit to photocatalytic performance.

The photocatalytic performances of the as-prepared Ga0.25Zn4.67S5.08 samples with different temperatures were evaluated by measuring the survival rate of E. coli under visible light irradiation. As shown in Figure 7, no evident loss of survival rate was observed in the dark, indicating negligible toxicity of the photocatalyst. For the light control experiment, the test was performed without photocatalyst under the irradiation of visible light. The survival rate is stable which indicates the E. coli cannot be killed by visible light under the present condition. For the photocatalytic test, the suspension was stirred in darkness for 1 h to achieve adsorption equilibrium of the cells on the photocatalyst. As observed, all samples prepared by USP have greater bactericidal performance, compared to the sample prepared by the hydrothermal method. Under the presence of Ga0.25Zn4.67S5.08 (400°C), nearly all bacteria cannot survive after 4 h of visible light irradiation, while the survival rate of other samples (600°C and 800°C) was only 27% and 12%, respectively. The bacterial suspension after the reaction was cultured for 96 h under dark reaction, and no change in the colony of E. coli was observed, indicating that the bacteria were indeed killed by the photocatalytic sterilization process, rather than simply suppressed. The higher photocatalytic performance of Ga0.25Zn4.67S5.08 (400°C) can be ascribed to its larger specific surface area and more active sites.

It is well known that active groups such as OH, e-, and h+ attack the bacterial cells during the reaction, eventually causing the decomposition of the cell wall [1]. In order to further explore which active substance played a decisive role in the photocatalytic sterilization process, different kinds of radical scavengers were added to a series of identical photocatalytic reaction systems. For example, sodium oxalate (0.5 mM as h+ quencher), Cr (VI) (0.05 mM as e- quencher), and isopropanol (0.5 mM as OH quencher) are added to the different batches of the Ga0.25Zn4.67S5.08 photocatalytic system, respectively, to capture h+, e-, and OH and to ensure that the scavengers under the concentration will not have any toxicity to E. coli [35]. As shown in Figure 8, the bacterial survival rate of photocatalytic systems added with isopropanol or Cr (VI) is almost the same with the system without a trapping agent, suggesting that OH and e- are not the main active substances in the photocatalytic sterilization, whereas when sodium oxalate was added as the hole trapping agent, the survival rate of bacteria reached 45% after irradiation for 4 h, suggesting that h+ oxidation was the dominant radical for the photocatalytic disinfection.

The stability of the samples was evaluated by successive cycles of the photocatalytic sterilization process. As shown in Figure 9, the results indicate that the photocatalytic activity of the sample did not decrease significantly even after three successive cycles. These results indicate that Ga0.25Zn4.67S5.08 microspheres possess excellent photocatalytic stability during the photocatalytic application.

Based on the above experimental and characterization results, a possible photocatalytic mechanism for disinfection by Ga0.25Zn4.67S5.08 microspheres was proposed and displayed in Figure 10. Under the visible light irradiation, the hole and electron pairs are generated and separated; then, the holes directly reacted with the cells adsorbed and produce inorganic matters.

4. Conclusion

In summary, Ga0.25Zn4.67S5.08 microspheres were synthesized by the USP process without adding template. The visible light photocatalytic antibacterial results indicate that the temperature of pyrolysis plays a dominant role in the disinfection process. On the basis of the DRS and BET, as well as the photocurrent, the superior photocatalytic activity of Ga0.25Zn4.67S5.08 prepared at 400°C was ascribed to the narrowed band gap, the high specific surface areas, and more photogenerated holes. We believe that this study shed light on engineering other novel porous structured photocatalysts for low-cost water disinfection.

Data Availability

The data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest

The authors declare that they have no conflicts of interest.


This work was supported by the Natural Science Foundation of Fujian Province (Grant No. 2016J05042), the Putian Science and Technology Bureau Project (2016S1001), and the Putian University Research Innovation Project (2018ZP03 and 2018ZP07) and the Putian University Research Project (2018048).