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Article

3-D/3-D Z-Scheme Heterojunction Composite Formed by Marimo-like Bi2WO6 and Mammillaria-like ZnO for Expeditious Sunlight Photodegradation of Dimethyl Phthalate

by
Ying-Hui Chin
1,
Jin-Chung Sin
1,2,3,4,*,
Sze-Mun Lam
2,3,4,5,
Honghu Zeng
2,3,4,*,
Hua Lin
2,3,4,
Haixiang Li
2,3,4,
Liangliang Huang
2,3,4 and
Abdul Rahman Mohamed
6
1
Department of Petrochemical Engineering, Faculty of Engineering and Green Technology, Universiti Tunku Abdul Rahman, Jalan Universiti, Bandar Barat, Kampar 31900, Perak, Malaysia
2
College of Environmental Science and Engineering, Guilin University of Technology, Guilin 541004, China
3
Guangxi Key Laboratory of Theory and Technology for Environmental Pollution Control, Guilin University of Technology, Guilin 541004, China
4
Collaborative Innovation Center for Water Pollution Control and Water Safety in Karst Area, Guilin University of Technology, Guilin 541004, China
5
Department of Environmental Engineering, Faculty of Engineering and Green Technology, Universiti Tunku Abdul Rahman, Jalan Universiti, Bandar Barat, Kampar 31900, Perak, Malaysia
6
School of Chemical Engineering, Universiti Sains Malaysia, Engineering Campus, Nibong Tebal 14300, Pulau Pinang, Malaysia
*
Authors to whom correspondence should be addressed.
Catalysts 2022, 12(11), 1427; https://doi.org/10.3390/catal12111427
Submission received: 3 October 2022 / Revised: 4 November 2022 / Accepted: 9 November 2022 / Published: 13 November 2022
(This article belongs to the Section Photocatalysis)
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Abstract

:
In the present work, we assessed the photocatalytic performance of the new 3-D/3-D Z-scheme heterojunction composite for the degradation of dimethyl phthalate (DMP). The composite was composed by marimo-like Bi2WO6 and mammillaria-like ZnO which was named BWZ. The composite was successfully fabricated using a hydrothermal-precipitation method and analyzed via different characterization techniques. Under natural sunlight irradiation, the optimal composite with 20 wt% of Bi2WO6/ZnO (20-BWZ) exhibited a photodegradation rate constant of 0.0259 min−1, which reached 2.3 and 5.9-folds greater than those of pure ZnO (0.0112 min−1) and Bi2WO6 (0.0044 min−1), respectively. That was predominantly attributed to the formation of a Z-scheme photocatalytic system in the as-synthesized composite reduced the charge carrier recombination and accelerated the photoactivity. Transient photocurrent response and electrochemical impedance spectroscopy analyses were performed to confirm this conclusion. The reusability test indicated that the 20-BWZ had no significant deactivation after four runs, which inferred good stability of the as-prepared composite. Furthermore, the quenching test demonstrated that the photogenerated hole, superoxide anion radical and hydroxyl radical were all involved in the photodegradation of DMP, among which •OH was the principal reactive species. This work revealed that the as-prepared BWZ composites have great potential applications for the degradation of refractory pollutants in the environmental remediation field.

Graphical Abstract

1. Introduction

Phthalate acid esters (PAEs) have been commonly applied in cosmetic, pharmaceutical and packaging products. They served as the plasticizer in polyvinyl products, moisturizer in beauty products and anti-cracking agent in nail polishes and are often found in children’s toys and food container products. PAEs have been classified as endocrine disruptors that can pollute the environment and pose threat to human health even when appearing at very low concentrations. As a short-chain ester, dimethyl phthalate (DMP) was identified as one of the most detected PAEs in the environment [1]. In natural water, DMP was detected at the concentration of 0.0057–1150 μg/L [2]. Due to its high chemical stability in the environment, DMP has a long hydrolysis half-life of 3 years [3]. According to the United States Environmental Protection Agency (USEPA), it was priority listed as a persistent organic pollutant. Although the Malaysian government has imposed restrictions on the discharge of municipal wastewater treatment plants via several parameters such as chemical oxygen demand (COD), biological oxygen demand (BOD) and pH, there is no discharge standard on DMP. Therefore, even though the wastewater complied with the discharge standards, the wastewater that contained DMP still affected the surrounding environment. DMP has been revealed to disrupt and damage the cell structure of Staphylocuccus Aures by inducing oxidative stress and inhibiting energy metabolism [2]. There was evidence that DMP can act as a teratogen in animals and reduced fertility in humans [4]. Therefore, treatment on wastewater-contained DMP is necessary before being discharged into the waterbody.
Among the treatment technologies, advanced oxidation processes, especially photocatalysis, are some of the promising techniques due to their high efficacy, sustainability, environmentally friendly and energy-saving properties [5,6]. ZnO is regarded as the most reliable photocatalyst owing to its low cost, chemical stability, magnificent structure and eco-friendly nature [7,8]. In spite of all these excellent features, some obstacles limited the practical applications of ZnO such as its large band gap (~3.2 eV) and quick electron-hole pair recombination [9]. Thus, numerous studies have been explored to improve photocatalytic performance. Recently, Z-scheme heterojunction has attracted huge attention as it can inhibit electron-hole recombination and maintain the strong redox ability simultaneously, which can boost photoactivity [10]. According to the previous studies, different Z-scheme composites have been reported such as ZnO/TiO2, ZnO/SnNb2O6, Ag2WO4/Bi2WO6 and MIL-101(Fe)/g-C3N4 [10,11,12,13]. However, it is still a challenge to exploit the Z-scheme catalyst with extraordinary performance.
At present, bismuth-based semiconductors have revealed outstanding photocatalytic potential due to their essential polarizabilities induced by Bi 6s2 single electrons, which are useful for electron-hole pair separation [14]. Especially, Bi2WO6 is a potential semiconductor in the field of photocatalysis due to its dual characteristics of bismuth photocatalyst and metal tungstate with a moderate bandgap (~2.7 eV) [15,16]. Moreover, according to report [17], the energy level of Bi2WO6 was found to be well-fitted with that of ZnO. Therefore, the combination of ZnO and Bi2WO6 to form a new Z-scheme composite can be a pristine route to create highly potential photocatalytic material due to their suitable staggered energy band structure.
On the other side, changing the surface morphology is another route to tailor the performance of photocatalysts. Various dimensional structures such as nanoparticles (0-D), nanowires/nanorods/nanotubes (1-D), nanoflakes/nanosheets (2-D) and nanospheres/nanoflowers (3-D) have been recognized as promising photocatalysts due to the increased number of active sites [18,19]. It was reported that the 3-D layered structure catalyst benefited light absorption by expanding the quantity of light traveling routes, thus improving the number of photogenerated charge carriers available to partake in photodegradation [18]. Therefore, the construction of 3-D/3-D Z-scheme ZnO/Bi2WO6 heterojunction is beneficial for establishing a superior photocatalytic performance.
On the basis of the above consideration, herein a series of 3-D/3-D Z-scheme heterojunction composites composed of marimo-like Bi2WO6 and mammillaria-like ZnO (BWZ) were prepared by a facile hydrothermal–precipitation route to enhance the DMP degradation under natural sunlight irradiation. A variety of characterization techniques were employed to systematically examine the phase structure, morphology, chemical composition, optical and photoelectrochemical properties of as-prepared materials. Meanwhile, the Z-scheme charge separation and migration mechanism in BWZ composite were also investigated. To the best of our knowledge, this is the first report on the 3-D/3-D Z-scheme BWZ composites with efficient sunlight photoactivity for the degradation of DMP.

2. Results and Discussion

2.1. Characterization of Prepared Photocatalysts

XRD analysis was used to analyze the crystalline structure of as-prepared products. As illustrated in Figure 1a, the peaks at 2θ angles of 31.8°, 34.4°, 36.3°, 47.6°, 56.6°, 62.9°, 66.4°, 67.9° and 69.1° were indexed to (100), (002), (101), (102), (110), (103), (200), (112) and (201) planes of hexagonal wurtzite structure of ZnO (JCPDS file No. 36-1451), respectively [20]. The diffraction patterns that occurred at 28.3°, 32.9°, 47.2°, 55.9° and 58.7° were matched with (131), (200), (202), (133) and (262) planes, which can be indexed to the orthorhombic phase of Bi2WO6 (JCPDS File No. 01-079-0208) [21]. In addition, the peaks related to hexagonal ZnO and orthorhombic Bi2WO6 were found in the diffraction patterns of as-prepared composites, unveiling the successful formation of BWZ composites. Moreover, the crystalline size (D) of the photocatalysts can be calculated by Scherrer’s formula [22,23]: D = Kλ/βcosθ, where K is the particle shape-constant (normally taken as 0.9), λ is the wavelength of X-ray, β is the full width at half maximum of diffraction peak and θ is the diffraction peak. The average crystallite size of Bi2WO6 and ZnO are 34.7 nm and 18.8 nm, respectively. In the 20-BWZ composite, the size of Bi2WO6 and ZnO were calculated to be 30.4 nm and 18.3 nm, respectively. The results indicated that the crystallite size of Bi2WO6 and ZnO did not change significantly in the composite.
To investigate further, the FTIR spectra were applied to characterize the functional groups of as-prepared samples (Figure 1b). The absorption bands at 3435 and 1646 cm−1 were designated for O-H stretching and bending vibration of free water [24,25]. The bands at 752 and 567 cm−1 corresponded to the W-O-W and Bi-O stretching vibrations in Bi2WO6, respectively [26,27], while the band at 439 cm−1 was attributed to the stretching vibration of Zn-O in ZnO [28]. In addition to the XRD results, it was observed from the FTIR findings that the prepared products were pure ZnO, Bi2WO6 and BWZ composites.
XPS analysis was then performed to assess the chemical state of 20-BWZ. The sample contained elements of Zn 2p, O 1s, Bi 4f and W 4f. Figure 1c shows that the binding energy peaks at 1023.2 and 1046.3 eV of Zn 2p, corresponding to the Zn 2p3/2 and Zn 2p1/2, respectively, and indicated the chemical states of zinc element was divalent [29]. In the case of O 1s spectrum (Figure 1d), two characteristic peaks were detected. The peak that appeared at 530.1 eV was ascribed to the lattice oxygen species while the peak detected at 531.6 eV was assigned to the hydroxyl groups [30,31]. The XPS spectrum of Bi 4f (Figure 1e) showed two asymmetric peaks with binding energies of 157.9 and 163.2 eV. These were ascribed to the doublets of Bi 4f7/2 and Bi 4f5/2, respectively, and the splitting energy of 5.3 eV was characteristic of Bi3+ [17,32]. In Figure 1f, the peaks at 31.4 eV and 34.2 eV corresponded to W4f7/2 and W4f5/2, respectively, indicating the existence of metallic tungstate in the composite [33].
The morphology of as-prepared products was initially evaluated via FESEM analysis. Figure 2a depicts that the pure ZnO had a mammillaria-like hierarchical morphology with sizes of approximately 1–4 μm, which piled up by the lamellar structure. In Figure 2b, the pure Bi2WO6 with sizes of 2–8 μm assembled by numerous nanosheets that whirl to form a marimo-like structure. For 20-BWZ (Figure 2c), mammillaria-like ZnO was found to grow and grip tightly on the aperture of the Bi2WO6 structure, this is beneficial to accelerate the charge migration of as-prepared composite in photocatalysis due to the close interface contact between both photocatalysts. Further increasing the Bi2WO6 to 25 wt%, the 25-BWZ did not show any significant changes in the overall morphology (Figure 2d). Furthermore, TEM analysis was used to study the detailed morphology of the 20-BWZ composite (Figure 2e,f). Apparently, the TEM observation was in good agreement with the FESEM finding. From the high-resolution TEM image in Figure 2f, the interplanar spacing of 0.260 nm and 0.317 nm belonged to (002) plane of ZnO and (131) plane of Bi2WO6, respectively [34,35]. The above results verified that ZnO integrated tightly on Bi2WO6, inferring the successful development of BWZ heterojunction. Moreover, the coexistence of Zn, O, Bi and W elements in the 20-BWZ composite in the EDX elemental mapping further indicated successful composite photocatalyst synthesis (Figure 2g–k).

2.2. Photocatalytic Performance

The photocatalytic performances of as-prepared products were assessed by DMP degradation under sunlight irradiation. Figure 3a shows the time-dependent UV-vis absorption spectra of DMP degradation over 20-BWZ composite. It was obviously demonstrated that the gradual decrease of the absorption peak (λ = 229 nm) of DMP stipulated the degradation of DMP during photocatalysis. To validate this speculation, a variation of DMP concentration (C/Co) with reaction times under different conditions was monitored. As depicted in Figure 3b, the concentration of DMP under photolysis and dark adsorption over 20-BWZ varied negligibly with time, indicating the necessity of both catalyst and light irradiation. Upon illumination for 90 min, the degradation efficiency of DMP over 20-BWZ composite was the greatest, reaching 86.6%, whereas the pure ZnO and Bi2WO6 only degraded 59.5% and 27.1% DMP, respectively. Concurrently, the DMP degradation efficiencies of 15-BWZ and 25-BWZ were 66.4% and 47.8%, respectively, implying an appropriate introduction of Bi2WO6 enhanced photoactivity. This can be attributed to the fact that decorating low Bi2WO6 can lead to less heterojunctions, while excessive Bi2WO6 can become a recombination center of charge carriers [36,37,38]. Therefore, the synergistic effect among the components of the 20-BWZ composite considerably enhanced the photoactivity.
In order to study the reaction kinetics of DMP degradation, a first-order equation was adopted: ln(Co/C) = kt, where k represents the observed rate constant, Co and C are the DMP concentration at the initial time and after the reaction time, t, respectively. In Figure 3c, the curve of ln(Co/C) versus t demonstrated good acceptable linearity, and the k value of all prepared products is depicted as a histogram. Remarkably, 20-BWZ obtained the highest k value (0.0259 min−1) among the catalyst, and its value was found to be about 2.3 and 5.9-fold greater than those of pure ZnO (0.0112 min−1) and Bi2WO6 (0.0044 min−1), respectively. In addition, the efficacy of the 20-BWZ composite was evaluated through synergy index using the equation as follows: synergy index = k20-BWZ/(kZnO + kBi2WO6), in which k20-BWZ, kZnO and kBi2WO6 are the k value for 20-BWZ, ZnO and Bi2WO6, respectively. The synergy index measured by substituting the data was 1.7 which was larger than 1, indicating a positive synergetic effect between ZnO and Bi2WO6 [39]. The above findings demonstrated that the decoration of Bi2WO6 with ZnO can considerably enhance the photoactivity of the composite in degrading the DMP.

2.3. Catalyst Recycle and Phytotoxicity Assessment

Considering the practical use of the catalyst, its recyclability is a crucial parameter index. As shown in Figure 4a, four-time cyclic DMP photodegradation tests were carried out over the optimized 20-BWZ. The 20-BWZ composite upon recycling use still maintained good photoactivity (86.6% to 78.1%, 72.6% and 72.0% by fresh, 2nd cycle, 3rd cycle and 4th cycle, respectively) with only approximately 14% loss in photoactivity after the fourth run. The decline in the DMP degradation efficiency upon recycling use can be attributed to the unavoidable loss of catalyst during the reusage processes. Furthermore, the phytotoxicity assessment is another important parameter in practical applications to understand the possible environmental risks of photocatalytically treated DMP solution. Impressively, the Vigna radiata seeds exposed to the photocatalytically treated samples showed a much higher germination rate compared to the untreated DMP solution, which validated the detoxification of parent DMP pollutants via the photocatalysis treatment. As shown in Figure 4b, higher radicle growth from seed germination in photodegraded samples as well as in deionized water as the control indicated a decrease in phytotoxicity. The untreated DMP solution displayed an explicitly high level of phytotoxicity (82.8%) as compared with the control (0%) and the sample after photocatalytic treatment (12.7%). Hence, the present findings revealed that the prepared 20-BWZ composite not only held good stability in degrading DMP but also efficiently detoxified the treated DMP solution.

2.4. Photocatalytic Mechanism

2.4.1. Optical Absorption and Energy Band Structure Analyses

The optical absorption property of as-prepared products was investigated by UV-vis DRS analysis as shown in Figure 5a. Compared to pure ZnO, the 20-BWZ composite displayed remarkably intensive visible-light harvesting capacity due to the presence of Bi2WO6. This indicated that the response region of BWZ composites under visible light was broadened and enhanced. Furthermore, according to the Kubelka–Munk analysis in Figure 5b,c, the band gap energies (Eg) of pure ZnO and Bi2WO6 were estimated to be 3.20 and 2.94 eV, respectively. More significantly, the band edge position of these semiconductors was also investigated by Mott–Schottky analysis. From Figure 5d,e, the conduction band potentials (ECB) of ZnO and Bi2WO6 were −0.750 and −0.003 eV, whereas their corresponding valence band potentials (EVB) were found to be 2.45 and 2.94 eV based on the formula EVB = ECB + Eg, respectively. Consequently, the energy band structure diagram of the BWZ composite is schematically displayed in Figure 5f, where the ZnO in the BWZ composite revealed higher CB and VB positions compared to the Bi2WO6. No doubt, the EVB of the ZnO was ideally able to receive the excited electron from ECB of Bi2WO6, forming a Z-scheme mechanism in BWZ composite. In addition, the intimate interface between ZnO and Bi2WO6 was favorable to the development of the Z-scheme mechanism.

2.4.2. Charge Carrier Dynamics

The transient photocurrent response has been regarded as one of the reliable techniques to evaluate the ability for separation and migration of photogenerated charge carriers. Generally, the larger the photocurrent intensity, the better the separation of the charge carrier [40,41]. In Figure 6a, 20-BWZ exhibited the highest photocurrent intensity among the products, implying a great charge separation over 20-BWZ. Concurrently, the charge transfer efficiency of as-prepared samples was also further examined using electrochemical impedance spectra (EIS) measurement as shown in Figure 6b. A smaller arc radius represented a greater charge carrier separation and migration [42,43]. As expected, the 20-BWZ exhibited the smallest arc radius in the Nyquist plot, inferring lower charge transfer resistance and greater charge migration efficiency. Therefore, the results insinuated that the constructed composite can indeed escalate the charge carrier migration efficiency and hence enhanced the photocatalytic performance.

2.4.3. Identification of Predominant Active Species

To further support the photodegradation reaction and to explore the influences of active species, radical quencher tests were carried out in the presence of different scavengers. As displayed in Figure 7a, the degradation of DMP was not considerably impacted by introducing Na2SO4; nevertheless, it cannot be denied that there was a slight contribution of photogenerated hole (h+) and superoxide anion radical (•O2−) in degrading DMP over the 20-BWZ. It was worth mentioning that the photodegradation efficacy was apparently repressed to 26.2% due to the introduction of IPA, indicating that the hydroxyl radical (•OH) played a predominant role in the photodegradation.
Furthermore, the PL spectra using TA as a probe were used to examine the existence of •OH in the 20-BWZ photocatalysis. As shown in Figure 7b, a noteworthy PL signal at about 425 nm indicated the formation of •OH [44,45]. The PL intensity was also increased gradually as the time of irradiation increased, which verified that the fluorescence was produced by chemical reactions of TA with •OH generated during photocatalysis. In light of these results, the indisputable participation of •OH in DMP degradation over 20-BWZ can be greatly revealed.

2.4.4. Mechanism for Improved Photoactivity of BWZ Composite

Based on the experimental findings discussed above, the plausible photocatalytic mechanism of BWZ composite is proposed in Scheme 1. Firstly, it was presumed that the BWZ composite fitted to the traditional heterojunction mechanism. Under sunlight irradiation, both ZnO and Bi2WO6 were photoactivated, causing the accumulation of photogenerated h+ on VB of ZnO and that the photogenerated e− in the CB of ZnO transferred to the CB of Bi2WO6, which efficiently separated the charge carriers. In this scenario, the CB position of Bi2WO6 was −0.003 eV, which was lower than the E°(O2/•O2¯) = −0.33 eV vs. NHE [46]. Hence, the e− in CB of Bi2WO6 was unable to photo-reduce O2 to generate •O2¯, which was inconsistent with the radical quencher findings. Consequently, a Z-scheme mechanism was put forward in the degradation of DMP over prepared composite, in which the photogenerated e− in the CB of Bi2WO6 transferred to the VB of ZnO and recombined with the h+ owing to intimate contact interface between two semiconductors [47]. At the same time, the e−, which remained in the CB of ZnO, can photo-reduce O2 to generate •O2¯, while the h+ left in the VB of Bi2WO6 can photo-oxidize H2O and OH− to form •OH. Finally, the produced radicals can further directly oxidize the DMP and obtained wastewater purification.
There are a few researchers who have reported the intermediate and final products of photocatalytic degradation of DMP. For example, Chen et al. [48] proposed the photocatalytic degradation pathway for DMP using Pt-TiO2/mPMMA under UV irradiation. They found that the DMP initially degraded into o-acetoxybenzoic acid and further degraded into other intermediates. The detected intermediates in their study included 5-acetoxy-2,4-heptadienedioic acid, 2-heptenedioic acid, 2-hexenedioic acid, 2-pentenedioic acid, 3-hydroxy-heptanedioic acid, 3-hydroxy-pentanedioic acid, 5-hydrox-2-heptenendioic acid, 3,5-dihydroxy-heptanedioic acid, phthalic acid, 2-hydroxy-2,4,6-octatrienedioic acid, 2,4-heptadienedioic acid, 2,4-hexadienedioic acid, 4-hydroxy-2-hexenedioic acid and malonic acid. These intermediates finally oxidized to CO2 and H2O as their end products.
Lee et al. [49] also studied the intermediates formed during the decomposition of DMP by TiO2. In their study, intermediates such as dimethyl 4-hydroxyphthalate, methyl salicylate and benzoic acid were found during the degradation process. In another study, Tan et al. [50] reported the photodegradation of DMP by MWCNTs/TiO2 photocatalysts under UV irradiation. Intermediates such as dicarbonic acid, dimethyl hydroxyphthalate, hydroxyl-methyl benzoates, monomethyl phthalate and benzoic acid were detected during irradiation (<180 min).
In a nutshell, dimethyl hydroxyphthalate and benzoic acid were the intermediates commonly detected during the degradation of DMP. These aromatic intermediates could further cleave into smaller aliphatic compounds which would eventually oxidize to CO2 and H2O. A comparison table of different photocatalysts with their performance in degrading phthalate pollutants is summarized in Table 1.

3. Materials and Methods

3.1. Chemicals and Materials

All the reagents used in this research study were of analytical grade and used without further purification. Bismuth nitrate pentahydrate (Bi(NO3)3·5H2O, purity ≥ 99%) nitric acid (HNO3, purity ≥ 65%), isopropanol (C3H8O, purity ≥ 99%), chloroform (CHCl3, purity ≥ 99%) were procured from R&M Chemicals, Selangor, Malaysia. Sodium tungstate (Na2WO4, purity ≥ 99%) and sodium hydroxide (NaOH, purity ≥ 98%) were obtained from Acros Organics, Geel, Belgium. Zinc nitrate hexahydrate (Zn(NO3)2·6H2O, purity ≥ 99%) was procured from Systerm, Selangor, Malaysia. Dimethyl phthalate (C10H10O4, purity ≥ 99%) was purchased from Alfa Aesar, Haverhill, United State. Sodium sulfate (Na2SO4, purity ≥ 99%) was obtained from Friendemann Schmidt Chemical, Kuala Lumpur, Malaysia. Ammonium oxalate (C2H8N2O4, purity ≥ 99%) was purchased from Sigma-Aldrich, Missouri, United State. Terephthalic acid (C8H6O4, purity ≥ 98%) was procured from Merck, Selangor, Malaysia.

3.2. Photocatalyst Preparation

Marimo-like Bi2WO6 was synthesized using a hydrothermal method. Initially, 6 mmol Bi(NO3)3·5H2O was dissolved in 0.4 M HNO3 under ultrasonication for 10 min. Subsequently, a 3 mmol Na2WO4 solution was added drop-wise into the Bi(NO3)3 solution under stirring. The mixture was stirred for 60 min before being placed into a Teflon-lined autoclave and treated for 16 h at 175 °C. The resulting product was washed using ethanol and deionized water and then dried at 65 °C for 24 h.
The typical synthesis of BWZ composites was as follows: 4 mmol Zn(NO3)2·6H2O and 0.0462 g Marimo-like Bi2WO6 were ultrasonicated in 80 mL deionized water for 30 min. After that, 0.024 mol NaOH was added slowly into the mixture and stirred for 17 h. The collected products were washed several times with ethanol and deionized water, and then dried at 65 °C in an oven. The dried products were placed into a muffle furnace under ambient pressure in air and calcined at 450 °C for 2 h to obtain the 15-BWZ. Moreover, a series of products with different amounts of Bi2WO6 (15, 20 and 25 wt%) were prepared under similar experimental conditions and the obtained composites were denoted as 15-BWZ, 20-BWZ and 25-BWZ, respectively. The pure mammillaria-like ZnO was also synthesized without adding Marimo-like Bi2WO6. The entire preparation process for synthesizing the BWZ composites is schematically depicted in Scheme 2.

3.3. Characterization

The X-ray diffraction (XRD) patterns were obtained by a Philip (Tokyo, Japan) PW1820 X-ray diffractometer with Cu Kα at 2° min−1 scanning rate. Fourier transform infrared spectroscopy (FTIR) was conducted via a Perkin Elmer (Waltham, MA, United State) Spectrum Two FTIR spectrometer with a scanning spectral ranging from 400–4000 cm−1. The X-ray photoelectron spectroscopy (XPS) spectra were recorded using a PHI (Chanhassen, MN, United State) Quantera II photoelectron spectrometer with Al-Kα radiation (1486.6 eV). Morphological studies were carried out by a JEOL (Peabody, MA, United State) JSM-6701F field-emission scanning electron microscopy (FESEM) installed along with an energy dispersive spectroscopy (EDX). The transmission electron microscopy (TEM) analysis was examined with a FEI (Hillsboro, OR, United State) Tecnai 20 microscope. The optical properties of the products were examined by a Perkin Elmer (Waltham, MA, United State) Lambda 35 UV-vis diffuse reflectance spectroscopy (UV-vis DRS) with BaSO4 as a standard reference.

3.4. Photoactivity Test

The photoactivity of the as-prepared products was assessed by the degradation of DMP. Typically, an aliquot of 75 mg photocatalyst was dispersed in 75 mL of 5.2 μM DMP solution in a beaker. Prior to the irradiation, the suspension was magnetically agitated for 30 min in the dark. Subsequently, the reaction solution was irradiated under natural sunlight. In the course of photocatalytic tests, solution samples were consistently taken out at certain time intervals and analyzed at λ = 229 nm with a JASCO (Tokyo, Japan) V-730 UV-vis spectrophotometer. All photocatalytic tests were conducted on sunny days between 12:00 and 14:00 during the months of November and December. The average irradiation strength was about 67,000 lux, as determined by a digital luxmeter.
Moreover, the stability of the best composite was evaluated by photocatalytic reactions each run for four consecutive cycles. After each cycle, the collected photocatalysts were rinsed thoroughly with deionized water and then placed into a new DMP solution. The phytotoxicity of treated solution on plant growth was also carried out via seed germination of Vigna radiata. The phytotoxicity evaluation was performed in the same manner as our earlier study [60]. Of note, all the photocatalytic tests were conducted in duplicates at least under similar conditions to make sure the reproducibility of the experimental results.

3.5. Photoelectrochemical Study

Photoelectrochemical experiments were performed using the Gamry (Warminster, England) Interface 1000 electrochemical workstation with a standard three-electrode system. The working electrode was prepared by coating the as-prepared products on a transparent fluorine-doped tin oxide glass, whereas the platinum wire and Ag/AgCl were employed as the counter and reference electrodes, respectively. Furthermore, 0.5 M Na2SO4 solution was utilized as the electrolyte in the photoelectrochemical studies. The transient photocurrent response was executed under a step voltage of 0.4 V and the maximum current was set as 0.3 mA. Moreover, the electrochemical impedance spectroscopy (EIS) measurement was operated under the frequency range from 0.01 to 103 Hz with 5 mV amplitude. The Mott–Schottky curve of as-prepared products was obtained with the initial voltage of 1.2 V, the final voltage of −1.2 V and a frequency of 1000 Hz. The conduction band potential of the products can be obtained from the tangential intercept of the Mott-Schottky curve. The valence band was determined through EVB = ECB + Eg, where EVB is the valence band potential, ECB is the conduction band potential and Eg is the band gap energy [61].

3.6. Detection of Active Species

The significance of photogenerated positive hole (h+), electron (e−), hydroxyl radical (•OH) and superoxide anion radical (•O2−) in the photoactivity were assessed by introducing 1 mM of ammonium oxalate (AO), Na2SO4, isopropanol (IPA) and chloroform, respectively [62,63,64]. The scavenger tests were performed using a similar method to that of the photoactivity test. In another test, the importance of •OH was also identified through the terephthalic acid-photoluminescence (TA-PL) probing method [65]. In the TA-PL experiment, the DMP solution was substituted by 5 × 10−4 M terephthalic acid in 2 × 10−3 M NaOH solution. The PL spectra were analyzed at 315 nm excitation wavelength using a Perkin Elmer (Waltham, United State)Lambda S55 spectrofluorometer.

4. Conclusions

In this work, the Z-scheme heterojunction composite formed by marimo-like Bi2WO6 and mammillaria-like ZnO has been successfully prepared using a hydrothermal-precipitation route. The as-prepared products were characterized by different techniques to acquire their crystallinity, surface morphology, physicochemical and optical properties. By varying the Bi2WO6 amount, the optimized 20-BWZ composite was validated, which demonstrated exceptional photoactivity towards the degradation of DMP. The rate of DMP degradation over 20-BWZ (0.0259 min−1) was 2.3 and 5.9-folds greater than those of pure ZnO (0.0112 min−1) and Bi2WO6 (0.0044 min−1), respectively. The improved photoactivity of 20-BWZ was predominantly attributed to the effective charge carrier separation by Z-scheme heterojunction. In addition, the 20-BWZ revealed good stability in the recycling test after four consecutive cycles. The results obtained from the phytotoxicity assessment suggested that the 20-BWZ photocatalysis effectively detoxified the treated DMP solution. Radical quenching tests discovered that the reaction over 20-BWZ proceeded majorly via •OH with a minor contribution from photogenerated h+ and •O2−. In summary, it is reasonable to consider that constructing a Z-scheme BWZ composite provided an efficient route for optimizing the charge separation efficiency, redox ability and photoactivity for DMP degradation.

Author Contributions

Conceptualization, J.-C.S. and S.-M.L.; Methodology, Y.-H.C. and J.-C.S.; Validation, J.-C.S. and S.-M.L.; Investigation, Y.-H.C.; Formal analysis, Y.-H.C., J.-C.S. and S.-M.L.; Writing—original draft, Y.-H.C.; Writing—review and editing, J.-C.S. and S.-M.L.; Supervision, J.-C.S., S.-M.L., H.Z. and A.R.M.; Funding acquisition, J.-C.S. and S.-M.L.; Resources, H.Z., H.L. (Hua Lin), H.L. (Haixiang Li), L.H. and A.R.M. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by Ministry of Higher Education (MoHE) through Fundamental Research Grant Scheme (FRGS/1/2019/TK02/UTAR/02/4 and FRGS/1/2022/TK08/UTAR/02/5). We also want to thank to the Universiti Tunku Abdul Rahman (UTARRF/2021-C2/L03), Research funds of The Guangxi Key Laboratory of Theory and Technology for Environmental Pollution Control, China (1801K012 and 1801K013), ASEAN Young Talented Scientist Program of Guangxi and special funding for Guangxi “Bagui Scholar” construction project for sponsoring this work.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Conflicts of Interest

The authors declare no conflict of interest.

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Catalysts 12 01427 g001 550
Figure 1. (a) XRD; (b) FTIR spectra of ZnO, Bi2WO6 and BWZ composites; (c–f) XPS spectra of 20-BWZ composite.
Figure 1. (a) XRD; (b) FTIR spectra of ZnO, Bi2WO6 and BWZ composites; (c–f) XPS spectra of 20-BWZ composite.
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Catalysts 12 01427 g002 550
Figure 2. FESEM images of (a) ZnO, (b) Bi2WO6, (c) 20-BWZ composite and (d) 25-BWZ composite; (e) TEM; (f) HRTEM images of 20-BWZ composite; (g) EDX spectrum; (h–k) elemental mapping images of 20-BWZ composite.
Figure 2. FESEM images of (a) ZnO, (b) Bi2WO6, (c) 20-BWZ composite and (d) 25-BWZ composite; (e) TEM; (f) HRTEM images of 20-BWZ composite; (g) EDX spectrum; (h–k) elemental mapping images of 20-BWZ composite.
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Catalysts 12 01427 g003 550
Figure 3. (a) UV-vis absorption spectra of DMP degradation over 20-BWZ composite; (b) Photodegradation of DMP under different conditions; (c) the corresponding kinetics of DMP degradation over different photocatalysts.
Figure 3. (a) UV-vis absorption spectra of DMP degradation over 20-BWZ composite; (b) Photodegradation of DMP under different conditions; (c) the corresponding kinetics of DMP degradation over different photocatalysts.
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Catalysts 12 01427 g004 550
Figure 4. (a) Recycling test using optimized 20-BWZ composite; (b) Phytotoxicity of DMP before and after the photocatalytic treatment using 20-BWZ composite (red line indicates phytotoxicity (%)).
Figure 4. (a) Recycling test using optimized 20-BWZ composite; (b) Phytotoxicity of DMP before and after the photocatalytic treatment using 20-BWZ composite (red line indicates phytotoxicity (%)).
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Catalysts 12 01427 g005 550
Figure 5. (a) UV-vis DRS spectra of ZnO, Bi2WO6 and BWZ composites; Kubelka–Munk curves of (b) ZnO and (c) Bi2WO6; Mott–Schottky plots of (d) ZnO and (e) Bi2WO6; (f) the band structure diagram of BWZ composite.
Figure 5. (a) UV-vis DRS spectra of ZnO, Bi2WO6 and BWZ composites; Kubelka–Munk curves of (b) ZnO and (c) Bi2WO6; Mott–Schottky plots of (d) ZnO and (e) Bi2WO6; (f) the band structure diagram of BWZ composite.
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Catalysts 12 01427 g006 550
Figure 6. (a) Transient photocurrent response; (b) EIS plots of ZnO, Bi2WO6 and BWZ composites.
Figure 6. (a) Transient photocurrent response; (b) EIS plots of ZnO, Bi2WO6 and BWZ composites.
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Catalysts 12 01427 g007 550
Figure 7. (a) Radical quenching test; (b) •OH trapping PL spectra over 20-BWZ composite.
Figure 7. (a) Radical quenching test; (b) •OH trapping PL spectra over 20-BWZ composite.
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Catalysts 12 01427 sch001 550
Scheme 1. Schematic illustration of charge transfer mechanism of BWZ composite.
Scheme 1. Schematic illustration of charge transfer mechanism of BWZ composite.
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Scheme 2. Schematic diagram of synthetic route for fabricating BWZ composites.
Scheme 2. Schematic diagram of synthetic route for fabricating BWZ composites.
Catalysts 12 01427 sch002
Table
Table 1. Comparative data analysis of phthalate pollutants degradation over different photocatalysts reported previously.
Table 1. Comparative data analysis of phthalate pollutants degradation over different photocatalysts reported previously.
PhotocatalystsPollutantsOperating ConditionsEfficiencyRate Constant, kRef.
TiO2Dimethyl phthalate
(1 ppm)		Catalyst
loading: 0.5 g/L
96 W UV light		71%
(180 min)		0.0069[51]
TiO2Dimethyl phthalate
(5 ppm)		Catalyst
loading: 1 g/L
25 W UV-vis light		60%
(150 min)		0.0061[52]
CuPc/TiO2/SiO2/Fe3O4Dimethyl phthalate
(10 ppm)		Catalyst
loading: 1.2 g/L
500 W simulated solar light		40%
(600 min)		0.0008[53]
La/TiO2Dimethyl phthalate
(20 ppm)		Catalyst
loading: 1 g/L
UV-C light		74.4%
(600 min)		0.0023[54]
Iron oxide/g-C3N4
/BiOBr/polythiopin 		Dimethyl phthalate
(20 ppm)		Catalyst
loading: 2 g/L
500 W simulated solar light		24%
(360 min)		0.0032[55]
g-C3N4Dimethyl phthalate
(1.94 ppm)		Catalyst
loading: 0.5 g/L
UV-C light		70%
(120 min)		0.0120[56]
PANI/CNT/TiO2Diethyl phthalate
(1 ppm)		10 W visible light
67%
(120 min)		0.0092[57]
Ca/α-Fe2O3Diethyl phthalate
(0.2 ppm)		Catalyst loading: 0.025 g/L
50 W UV light		60%
(300 min)		0.0031[58]
Ce/Gd-WS2Dibutyl phthalate
(1 ppm)		Catalyst
loading: 0.2 g/L
250 W simulated solar light		64%
(60 min)		0.0110[59]
20-BWZ Dimethyl phthalate
(1 ppm)		Catalyst
loading: 1 g/L
Sunlight		86.6%
(90 min)		0.0259This study
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Chin, Y.-H.; Sin, J.-C.; Lam, S.-M.; Zeng, H.; Lin, H.; Li, H.; Huang, L.; Mohamed, A.R. 3-D/3-D Z-Scheme Heterojunction Composite Formed by Marimo-like Bi2WO6 and Mammillaria-like ZnO for Expeditious Sunlight Photodegradation of Dimethyl Phthalate. Catalysts 2022, 12, 1427. https://doi.org/10.3390/catal12111427

AMA Style

Chin Y-H, Sin J-C, Lam S-M, Zeng H, Lin H, Li H, Huang L, Mohamed AR. 3-D/3-D Z-Scheme Heterojunction Composite Formed by Marimo-like Bi2WO6 and Mammillaria-like ZnO for Expeditious Sunlight Photodegradation of Dimethyl Phthalate. Catalysts. 2022; 12(11):1427. https://doi.org/10.3390/catal12111427

Chicago/Turabian Style

Chin, Ying-Hui, Jin-Chung Sin, Sze-Mun Lam, Honghu Zeng, Hua Lin, Haixiang Li, Liangliang Huang, and Abdul Rahman Mohamed. 2022. "3-D/3-D Z-Scheme Heterojunction Composite Formed by Marimo-like Bi2WO6 and Mammillaria-like ZnO for Expeditious Sunlight Photodegradation of Dimethyl Phthalate" Catalysts 12, no. 11: 1427. https://doi.org/10.3390/catal12111427

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