ZnIn2S4 composite photocatalysts for the effective degradation of nitenpyram under visible light irradiation

Journal Pre-proof All-solid-state Z-scheme WO3 nanorod/ZnIn2 S4 composite photocatalysts for the effective degradation of nitenpyram under visible light irradiation Mengling Tang (Methodology) (Software) (Writing - original draft), Yanhui Ao (Conceptualization) (Writing - review and editing) (Visualization), Peifang Wang (Writing - review and editing) (Project administration) (Funding acquisition), Chao Wang (Supervision) (Project administration)

PII:

S0304-3894(19)31667-X

DOI:

https://doi.org/10.1016/j.jhazmat.2019.121713

Reference:

HAZMAT 121713

To appear in:

Journal of Hazardous Materials

Received Date:

30 September 2019

Revised Date:

13 November 2019

Accepted Date:

17 November 2019

Please cite this article as: Tang M, Ao Y, Wang P, Wang C, All-solid-state Z-scheme WO3 nanorod/ZnIn2 S4 composite photocatalysts for the effective degradation of nitenpyram under visible light irradiation, Journal of Hazardous Materials (2019), doi: https://doi.org/10.1016/j.jhazmat.2019.121713

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All-solid-state Z-scheme WO3 nanorod/ZnIn2S4 composite photocatalysts for the effective degradation of nitenpyram under visible light irradiation

Mengling Tang, Yanhui Ao*, Peifang Wang, Chao Wang

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Key Laboratory of Integrated Regulation and Resource Development on Shallow Lakes of

Ministry of Education, College of Environment, Hohai University, No.1, Xikang road, Nanjing,

author. Tel./ fax: +86 25 83787330,

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*Corresponding

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210098, China

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Graphical Abstract

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E-mail address: [email protected] (Yanhui Ao)

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Highlight 

All-solid-state Z-scheme WO3 nanorod/ZnIn2S4 composite photocatalysts were synthesized.



The photocatalyst exhibited high performance for the degradation of nitenpyram.

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The rate constant improved by a factor of 3.8 and 2.5 compared to WO3 and ZnIn2S4,.

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A deductive degradation pathway of nitenpyram was proposed.

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Abstract: A Z-scheme WO3/ZnIn2S4 photocatalyst was synthesized via a simple solvothermal method. Compared with pure WO3 and ZnIn2S4, photocatalytic

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experiments showed that these Z-scheme photocatalysts exhibited enhanced activity

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for the degradation of nitenpyram (NTP). The apparent rate constant (k) of NTP degradation on 50WZ (WO3/ 50 wt% Znln2S4) was 0.042 min-1 (~3.8 times higher

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than WO3 and ~2.5 times higher than ZnIn2S4). Photoluminescence (PL), photocurrent (PC), and electrochemical impedance spectroscopy (EIS) showed that

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the separation and transfer efficiency of photogenerated carriers in 50WZ was markedly enhanced, which was favorable for improving its photocatalytic activity. Active

species

capture

experiments

and

electron

spin

resonance

(ESR)

measurements showed that superoxide radicals and holes were the main active species for NTP degradation, and they confirmed the formation of the Z-scheme 2

structure. Furthermore, a possible NTP degradation pathway was deduced based on the results of high-performance liquid chromatography mass spectrometry (HPLC-MS).

Key words: photocatalysis; Z-scheme; nitenpyram; neonicotinoids

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1. Introduction

The degradation of nitenpyram (NTP), which is an effective and widely used

neonicotinoids pesticide in agriculture[1, 2], in the environment is of great concern

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because of its persistence and toxicity in water[3], resulting in a marked impact on

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the ecology and human health[4]. There are numerous problems with current NTP degradation methods: the microbiological method has restricted reaction conditions

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and a low efficiency in general[5]; both the electrocatalytic and low-temperature

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plasma methods have high energy consumption[6, 7]; in contrast, the photocatalytic method has attracted increasing attention because of its environmental friendliness 9]

. Photocatalysts that only respond to ultraviolet light have

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and efficiency[8,

difficulties degrading complex organic compounds, hence, visible light responsive

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photocatalysts have become a significant area of research[10]. Semiconductor materials, such as oxides, nitrides and sulfides, have been

extensively studied[11-13]. Graphite phase C3N4 is a visible light responsive metal-free semiconductor, however, it has poor absorption and rapid photogenerated carrier recombination, which greatly limits its application as a photocatalyst[14-17], and the 3

heterojunctions related to this material have been widely researched. Sulfide semiconductors have an excellent photoelectric property and an appropriate bandgap size, hence, they have a wider optical response range and higher photocatalytic activity than g-C3N4[18-21]. This makes them suitable for the photocatalytic degradation of complex organic compounds. ZnIn2S4 is a ternary sulfide, a typical semiconductor member of the AB2X4 group, and a good option for

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use as a visible light responsive photocatalyst because of its suitable bandgap energy (2.2–2.8 eV)[22, 23]. ZnIn2S4 has two common polymorphs based on the hexagonal and cubic phases, both of which have good charge storage capacity and

26]

, the cubic ZnIn2S4 displays stable

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photoluminescence and photoconductivity[25,

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electrochemical performance[24]. It has been reported that hexagonal ZnIn2S4 shows

thermoelectric properties[27], while it is a high-pressure phase[28]. Chen et al,

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synthesized these two ZnIn2S4 phases with different precursors and investigated

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their degradation properties: MO was almost completely degraded by the cubic phase, while the hexagonal phase had a degradation efficiency of only 52%, but it

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had a higher stability than the cubic phase[28]. Studies have considered the selection of suitable bandgap semiconductors to form heterojunctions to address

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the issues of sulfide photocorrosion and the high recombination rate of photogenerated electron-hole pairs in a single photocatalyst[29, 30]. Photocatalysts forming traditional heterojunction with ZnIn2S4 have been reported, such as g-C3N4/ZnIn2S4[31], ZnIn2S4/CdIn2S4[32], TiO2/ZnIn2S4[33]. However, the reduction and oxidation ability of photo-induced carriers in conventional heterojunction are usually 4

lower than one of its components[34], therefore, the Z-scheme structure is a better option[35-37]. However, few studies have investigated Z-type photocatalyst containing ZnIn2S4. This work investigated semiconductors with a suitable bandgap for the formation of a Z-type heterojunction with ZnIn2S4. WO3 is considered as a promising material because of its electrochromic and photocatalytic properties[38, 39]. Moreover, because of its narrow band gap (2.4–2.8 eV)

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and good stability, WO3 has been reported to have good photocatalytic activity for dye

degradation[40-42]. The ECB and EVB of ZnIn2S4 are at approximately -0.80 eV and

1.52 eV[43], while the ECB and EVB of WO3 are at approximately 0.74 eV and 3.44

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eV[44], respectively. Thus, photogenerated electrons can theoretically transfer from the

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CB of WO3 to the VB of ZnIn2S4 to form a Z-scheme structure. A composite ZnIn2S4 nanosheets/ WO3 nanorods core-shell structure composite has been reported via a

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solvothermal method[45], and a 2D/2D WO3/ZnIn2S4 photocatalyst has been prepared

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via a electrostatic self-assembly method[46]. However, these catalysts were only applied for photocatalytic hydrogen production. Furthermore, research on the

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photocatalytic degradation of neonicotinoids is minimal to date. In this paper, a composite photocatalyst was prepared via a simple one-step hydrothermal

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method and applied to the photocatalytic degradation of neonicotinoid pesticides.

In this work, a new Z-scheme WO3/ZnIn2S4 photocatalyst was prepared by a simple solvothermal method. The crystal structure, element composition and optical properties of the hybrids were characterized using various characterization methods. 5

NTP was chosen as a target contaminant to assess the photocatalytic activities of the samples. In addition, a possible pathway for NTP degradation by 50WZ and a rational Z-scheme mechanism are proposed.

2. Results and discussion 2.1 Characterizations of WO3/Znln2S4 The morphologies of WO3, Znln2S4 and 50WZ samples were investigated via

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TEM. As can be seen from Fig. 1a, the pure WO3 exhibited a nanorod morphology with a diameter of ~35 nm and a length of ~210 nm. HRTEM image (Fig. 1b) showed

that the lattice spacing of the WO3 crystal was 0.391 nm, which corresponds to the

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(001) plane of WO3. From Fig. 1c, we can see that pure ZnIn2S4 shows a flower-like

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morphology with a diameter of ~3.5 μm. From the high-magnification local micrograph, we can see a folded and flaky structure on the surface of the nanoflower.

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The HRTEM (Fig. 1d) showed that the lattice fringe of this nanoflower was ~0.32 nm,

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corresponding to the (102) plane of hexagonal ZnIn2S4. On observing the 50WZ hybrid (Fig. 1e), it could be determined that ZnIn2S4 did not present a flower-like

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structure in the mixture, but formed a sheet structure on the surface of the nanorods. Through HRTEM analysis (Fig. 1f), it was found that the lattice fringes of these two

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substances were still present in 50WZ, which can be ascribed to the important role of WO3 groups for crystal growth and the ZnIn2S4 self-assembly[43]. WO3, ZnIn2S4 and 50WZ were investigated via XPS to further demonstrate the presence of each element in the mixture and the interaction between components. From the survey figure (Fig. 2a), the 50WZ hybrid showed all the peaks of WO3 and 6

ZnIn2S4, and all the expected elements, W, O, Zn, In and S, were detected. For the high-resolution W 4f spectrum (Fig. 2b), the two peaks at binding energies of 35.7 eV and 37.8 eV could be attributed to W4f7/2 and W4f5/2[47]. For the O 1s spectrum (Fig. 2c), the peak at a binding energy of 530.4 eV belongs to the oxygen atom that forms the strong W=O bond in WO3[48]. In Fig. 2d, two peaks at 1021.3 eV and 1044.4 eV can be attributed to Zn 2p3/2 and Zn 2p1/2, respectively[49]. Two binding

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energy in the high-resolution In 3d spectrum (Fig. 2e) at 444.9 eV and 452.4 eV may

belong to In 3d5/2 and In 3d3/2[31]. In a high-resolution S 2p spectrum (Fig. 2f), the

two peaks at 161.5 eV and 162.7 eV can be assigned to S 2p3/2 and S 2p1/2,

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respectively[31]. The results indicate that the valence states of Zn, In and S are +2, +3

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and -2, respectively[31, 43].

The XRD spectra of the prepared powders are shown in Fig. 3a. The peaks of

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the bare ZnIn2S4 at 2θ = 21.6°, 27.7°, 30.4°, 47.2°, 52.4° and 55.6° were in

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agreement with those of the hexagonal phase ZnIn2S4 (JCPDS No. 65-2023). The typical diffraction peaks of bare WO3 at 2θ = 13.9°, 22.7°, 28.2° and 36.6° were

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indexed as the (100), (001), (200) and (201) planes of WO3 (JCPDS No. 33-1387). There were no impurity peaks in the pure sample patterns, and the main diffraction

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peaks were consistent with the standard materials, which indicates that the prepared samples had a good crystallinity and purity. The WO3/ZnIn2S4 hybrids exhibited almost all diffraction peaks associated with the two pure samples. Because the diffraction peak at 2θ = 28.2° of WO3 was very close to the ZnIn2S4 peak at 2θ = 27.7°, a combined higher intensity peak can be observed in the hybrids. 7

In general, the PC and EIS mainly investigated the separation efficiency of the photogenerated electron-hole pairs[50,

29]

. It can be seen from Fig. 3b that WO3,

ZnIn2S4 and 50WZ all had an immediate response at the moment of turning on and off the light, but the photocurrent generated by 50WZ was much higher than WO3 and ZnIn2S4, which demonstrated that the photogenerated carriers in 50WZ had the longest lifetime. The EIS diagram (Fig. 3c) displayed the impedance changes for the

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three samples. The arc of the Nyquist plot of 50WZ was the smallest, indicating that

the charge transfer resistance in 50WZ was relatively smaller than others. Therefore,

the separation efficiency of the photogenerated carriers in the hybrid 50WZ sample

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were much higher than the other two pure samples.

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In addition, from the PL spectrum (Fig. 3d), we can see obvious emission peaks of all the tested samples appear at ~460 nm, and the peak intensity for 50WZ was

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significantly lower than that for the other two pure samples. This demonstrates that

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the radiative recombination of photogenerated carriers in 50WZ was lower than that for the pure samples[51], which is in agreement with PC and EIS measurement

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results.

The UV-vis DRS spectrum shows the optical properties of WO3, ZnIn2S4 and

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50WZ. From Fig. 4a, the absorption edge is at ~450 nm for bare WO3 and ~600 nm for bare ZnIn2S4, showing a relatively wide visible light absorption range. The composite 50WZ sample shows broadened light absorption compared with the bare WO3, for the interaction between WO3 and ZnIn2S4 in the composite. The band gap energy (Eg) of a semiconductor can be computed according to the following formula: 8

αhv = A(hv-Eg)n/2

(1)

In which, α, h, v and A are the absorption coefficient, Planck's constant, light frequency and a constant, respectively. n is related to the type of semiconductor (n = 1 or 4, for a direct or indirect transition, respectively). Both n values of WO3 and ZnIn2S4 are 1[52, 43], and the Eg for WO3 and ZnIn2S4 can be computed to be 2.75 eV and 2.4 eV (Fig. 4b), respectively[44,

28]

. The valence band potential (EVB) and

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conduction band potential (ECB) can be calculated by the following formulas[53, 31]: EVB = χ - Ee + 0.5 Eg

(2)

ECB = EVB - Eg

(3)

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Wherein, χ is the absolute electronegativity of a compound, which is the geometric

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mean of the electronegativity of the constituent atoms. For WO3 and ZnIn2S4, the values of χ are 6.49 eV and 4.86 eV, respectively[44, 31]. Ee is the energy of free

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electrons on the hydrogen scale (4.5 eV)[53, 52, 43]. Thus, the EVB values of WO3 and

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ZnIn2S4 were computed as 3.36 eV and 1.56 eV, and the corresponding ECB are 0.61 eV and -0.84 eV, respectively, which are basically consistent with the values reported

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in other literatures[44, 43].

The N2 adsorption-desorption isotherms of the prepared samples are shown

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in Fig. 5. These samples exhibit type IV isotherms with H3 hysteresis loops, indicating that these samples have mesoporous structures. The BET surface areas of WO3, ZnIn2S4 and 50WZ are 30.04, 17.31 and 37.69 m2 g-1, respectively. As we know, a higher specific surface area provides abundant active sites for contact and interaction with pollutant molecules, improving the photocatalytic 9

effect. Therefore, the enhanced specific surface area of 50WZ can improve its photocatalytic activity. 2.2 Photocatalytic activities of WO3/Znln2S4 Fig. 6a shows the photocatalytic activities for NTP degradation on the as-prepared samples under a same condition. Under dark conditions for 1 h, the NTP adsorption rates by WO3, ZnIn2S4, 25WZ, 50WZ, 75WZ, 100WZ and

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PM-50WZ were 5.7%, 7.0%, 10.0%, 13.8%, 13.2%, 9.2% and 4.8%, respectively. The blank test demonstrated that there was minimal NTP degradation under

visible light irradiation without a photocatalyst, implying that NTP is relatively

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stable and difficult to hydrolyze. The photocatalytic activities of all the WZ mixtures

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were higher than those of pure WO3 and ZnIn2S4 and 50WZ had the best photocatalytic effect. After 40 min of irradiation, 3 ppm NTP was degraded about

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74.1% in the presence of 50WZ, while the degradation efficiency was only 31.8%

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with WO3 and 47.2% with ZnIn2S4. The reasons may be as follows, when the ZnIn2S4 content was relatively lower, a large amount of photogenerated electrons

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were present in WO3, which recombined with the photogenerated holes[31]. However, when the ZnIn2S4 content was relatively higher, the excess ZnIn2S4 flakes on the

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surface of WO3 nanorods agglomerated, resulting in a lower contact area with the NTP molecules and reduced photocatalytic activity[54], and the 50WZ composition was the optimal one for photocatalytic degradation. In addition, a physical mixture with the same mass ratio as 50WZ had a lower effect. The apparent rate constant (k) for 50WZ was 0.042 min-1, which is 3.8 times higher than that of WO3 and 2.5 10

times higher than that of ZnIn2S4 (Fig. 6b). Therefore, the results show that 50WZ has an enhanced photocatalytic activity. To find out the main active substances on degradation NTP by 50WZ, an active species capture experiment were carried out. From the degradation map after adding capture agents for different active species (Fig. 6c), the addition of TBA had little effect on NTP degradation by 50WZ, while the addition of BZQ has a great effect,

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with the degradation rate decreasing from 74.1% to 10.1%. After adding Na2-EDTA,

the degradation rates decreased from 74.1% to 40.1%. Thus, the effects of these three

investigated active species on NTP degradation by 50WZ are •O2− > h+ > •OH.

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Considering the CB of ZnIn2S4 is sufficient to produce •O2− to participate in the

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reaction for the CB potential of ZnIn2S4 (-0.84 eV) is higher than the standard reduction potential of •O2−/O2 (-0.33 eV), the result is rational.

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Total organic carbon (TOC) analysis is an effective method to evaluate the

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mineralization rate of pollutant. As can be seen from Fig. 6d, the TOC removal rate reached 66.5% after extending the photocatalytic time to 120 min.

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Accordingly, it can reasonably be hypothesized that as the photocatalytic degradation time continues to be increased, NTP can increasingly be mineralized

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by the prepared catalysts. 2.3 The stability of 50WZ photocatalyst In order to explore the reusability of our samples, we conducted 4 times repeated experiments by recycle and reuse 50WZ. After each photocatalytic test, the sample was centrifuged and rinsed with ultrapure water several times, and then dried 11

for the next run. From Fig. 7a, the degradation efficiency of NTP for the photocatalyst on the first cycle was 74.1%, and the degradation rate was 68.2% after four cycles. This indicated that the reusability and stability of the material was relatively good. A structural comparison for the fresh and used 50WZ samples is shown in Fig. 7b. The positions of the characteristic peaks for the used 50WZ are basically

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unchanged, indicating that the structure of the photocatalyst did not change after use.

In addition, XPS analysis of the 50WZ surface elements after four cycles

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was compared with the unused 50WZ. From Fig. ESI 2, it can be seen that the

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positions of the W 4f, O 1s, Zn 2p, In 3d and S 2p diffraction peaks of the fresh and used 50WZ were nearly the same. However, the S 2p diffraction peak

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intensity at 162.7 eV decreased, which may be due to the depletion of the S

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element in the 50WZ sample after 4 cycles. This change is relatively small compared with the severe photocorrosion of pure sulfide. Therefore, the results

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show that the prepared 50WZ photocatalyst had a good stability. 2.4 Possible photocatalytic mechanism in the WO3/Znln2S4 system

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To further reveal the electron transport modes in the 50WZ hybrid, ESR was

used to detect •O2− and •OH in this sample (Fig. 8a-b). Under dark conditions, no •O2− or •OH radicals formed in 50WZ, while the signals for both •O2− and •OH appeared and increased with extending irradiation time, which indicates that the 50WZ could produce •O2− and •OH under light irradiation. 12

Based on this, a possible photocatalytic mechanism is proposed (Fig. 9). If the composite photocatalyst was a traditional heterojunction, the photogenerated electrons would transfer from the CB of ZnIn2S4 to the CB of WO3, and •O2− would not be generated, because the CB position of WO3 (0.61 eV) was lower than the potential of •O2−/O2 (-0.33 eV). Photogenerated holes would transfer from the VB of WO3 to the VB of ZnIn2S4, and •OH would not be generated, for the VB position of

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ZnIn2S4 (1.56 eV) is higher than the potential of •OH/OH- (1.99 eV) and •OH/H2O (2.68 eV). However, this is not in agreement with our ESR results. In a Z-scheme

structure, photogenerated electrons can transition from the VB to the CB of the WO3

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semiconductor, then, they would transfer to the VB of ZnIn2S4 through a Z-scheme

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electron transport channel, and finally transition to its conduction band. In this structure, the VB position of WO3 would be lower than the potential required to

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produce •OH, and the CB position of ZnIn2S4 would be higher than the potential

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needed to generate •O2−, so that the 50WZ composite would produce both •O2− and •OH. This is consistent with the observed results of the above capture and ESR

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experiments. Thus, we believe that the 50WZ composite is more in line with a Z-scheme photocatalytic mechanism.

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2.5 Degradation pathway of NTP by 50WZ Based on the record of high-performance liquid chromatography mass

spectrometry (HPLC-MS), we analyzed the main intermediate structures and proposed a possible degradation pathway for NTP (m/z = 270.72 g mol-1) using 50WZ (Table 1 and Fig. 10). The results showed a major peak with m/z = 270.8, 13

corresponding to the NTP molecule. With extending reaction time, NTP was oxidized and degraded into many other small molecules with m/z values of 257.5, 227.9, 171.9, 224.9, 191.8, 163.8 and 122.7. Usually, the oxidation of organic compounds is mainly carried out at the oxidation site of organic molecules, and the oxidation sites are mainly located on the functional groups. In Fig. 10, the methyl group on the side chain of the NTP molecule may be removed under •O2−

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and h+ attacking to form P1 (m/z = 256.69 g mol-1). Then under the action of •O2− and •OH, the furthest nitro group from the pyridine ring would denitrify to form a

hydroxyl group, forming P2 (m/z = 227.69 g mol-1)[55]. Studies have indicated out

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that the intermediate P2 is unstable and readily converts to P3 (m/z = 227.69 g

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mol-1) in solution. With continuous attack of •O2− and h+, the ethyl group that connects the amino group and a ether bond in P3 may break off, forming P4 (m/z =

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170.64 g mol-1)[56]. We also detected the presence of 2-chloropyridine P12 (m/z =

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131.09 g mol-1) on the HPLC-MS spectrogram, which may be formed by the oxidation of the pyridine ring side chain groups in P4. In addition, the

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carbon-carbon double bond is susceptible to oxidation reaction, and NTP may also shed its methyl-linking nitro groups on side chain to form P5 (m/z = 225.72 g

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mol-1) under the attack of •O2−, •OH and h+[56]. Then, chlorine atoms in the 2-chloropyridine ring at position 2 may be break by active species to form P6 (m/z = 191.27 g mol-1)[57]. Under sustaining attack of •O2− and h+, methyl and nitro groups are continuously oxidized to form product P7 (m/z = 163.22 g mol-1). We all know that the amino group of organic compounds is vulnerable to attack by 14

oxidizing molecules, hence, P7 is easily oxidized to P8 (m/z = 122.17 g mol-1). However, although the oxidation reaction will first take place at oxidation sites, different initial oxidation sites will also lead to different oxidation paths. P11 (m/z = 286.67 g mol-1), a substance with a larger molecular weight than NTP, was also detected, which may be formed via oxidation of methyl groups on the side chain of NTP molecules into carboxyl groups. In the degrading solution, the and P10

(m/z = 131.09 g

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ring-opening product P9 (m/z = 155.09 g mol-1)

mol-1)[55] were also detected, which were the main intermediates and corresponded

with the strong signals observed in the HPLC-MS spectrum. Combined with the

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TOC degradation results, when the irradiation time was extended to 120 min, the

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TOC degradation efficiency reached 30%. Therefore, it can reasonably be inferred that NTP might eventually be completely oxidized to carbon dioxide and

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3. Conclusions

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water by extending the degradation time.

A series of WO3/ZnIn2S4 heterojunctions were synthesized via a facile solvent

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thermal method. These hybrids exhibit enhanced photocatalytic activity for the photocatalytic degradation of NTP, and the 50WZ sample exhibited the best

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photocatalytic performance compared with the pure WO3 and ZnIn2S4 samples and their other compounds of different proportions. This enhanced performance could mainly be ascribed to the Z-scheme heterostructure of the 50WZ, which helped extend the life of the photogenerated carriers. We believe this work provides a strong base for further study of the degradation of neonicotinoids in the environment 15

using photocatalysts.

Author Contributions Section Mengling Tang: Methodology, Software, Writing - Original Draft Yanhui Ao: Conceptualization, Writing - Review & Editing, Visualization Peifang Wang: Writing - Review & Editing, Project administration, Funding acquisition Chao Wang: Supervision, Project administration

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Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Acknowledgements

We are grateful for grants from Major Science and Technology Program for Water

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Pollution Control and Treatment (2017ZX07203002), National Science Funds for

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Creative Research Groups of China (No.51421006), Natural Science Foundation of China (51679063), The World-Class Universities (Disciplines) and Characteristic

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Development Guidance Funds for the Central Universities, the Key Program of National Natural Science Foundation of China (No. 91647206), the National Key Plan

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for Research and Development of China (2016YFC0502203), and PAPD.

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Figure Captions: Fig. 1 TEM and HRTEM images of WO3 (a, b), ZnIn2S4 (c, d) and 50WZ (e, f). Fig. 2

XPS survery spectra of WO3, ZnIn2S4 and 50WZ (a), high-resolution spectra of W 4f

(b), O 1s (c), Zn 2p (d), In 3d (e), and S 2p (f). Fig. 3 XRD patterns of as-prepared photocatalysts (a), transient photocurrent response (b), EIS changes (c) and PL spectra (d) of WO3, ZnIn2S4 and 50WZ.

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Fig. 4 UV-vis DRS spectra of WO3, ZnIn2S4 and 50WZ (a), and the plots of (ahv)1/2 versus hv for the band gap energies of WO3 and ZnIn2S4.

Fig. 5 N2 adsorption-desorption isotherms of WO3, ZnIn2S4 and 50WZ.

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Fig. 6 Photocatalytic activities of as-prepared samples for NTP degradation (a), the apparent

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rate constants (k) for NTP degradation (b), capture experiment diagram of 50WZ for NTP

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degradation (c) and TOC removal by 50WZ (d).

Fig.7 Photocatalytic stability of 50WZ for NTP degradation after 4 cycles (a), XRD patterns

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of freshand used 50WZ catalyst (b).

Fig. 8 ESR spectra of 50WZ photocatalyst under dark and visible light irradiation for

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DMPO-•O2- (a) and DMPO-•OH (b).

Fig. 9 Proposed possible photocatalytic mechanism of 50WZ composite.

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Fig. 10 Proposed possible degradation pathway of NTP in the presence of 50WZ. Table 1 HPLC-MS data for the intermediate products during NTP degradation.

26

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Fig. 1 TEM and HRTEM images of WO3 (a, b), ZnIn2S4 (c, d) and 50WZ (e, f).

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Fig. 2

XPS survery spectra of WO3, ZnIn2S4 and 50WZ (a), high-resolution spectra of W 4f

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(b), O 1s (c), Zn 2p (d), In 3d (e), and S 2p (f).

28

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Fig. 3 XRD patterns of as-prepared photocatalysts (a), transient photocurrent response (b),

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EIS changes (c) and PL spectra (d) of WO3, ZnIn2S4 and 50WZ.

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Fig. 4 UV-vis DRS spectra of WO3, ZnIn2S4 and 50WZ (a), and the plots of (ahv)1/2 versus hv

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for the band gap energies of WO3 and ZnIn2S4.

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Fig. 5 N2 adsorption-desorption isotherms of WO3, ZnIn2S4 and 50WZ.

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Fig. 6 Photocatalytic activities of as-prepared samples for NTP degradation (a), the apparent

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rate constants (k) for NTP degradation (b), capture experiment diagram of 50WZ for NTP

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degradation (c) and TOC removal by 50WZ (d).

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Fig.7 Photocatalytic stability of 50WZ for NTP degradation after 4 cycles (a), XRD patterns

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of freshand used 50WZ catalyst (b).

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Fig. 8 ESR spectra of 50WZ photocatalyst under dark and visible light irradiation for

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DMPO-•O2- (a) and DMPO-•OH (b).

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Fig. 9 Proposed possible photocatalytic mechanism of 50WZ composite.

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Fig. 10 Proposed possible degradation pathway of NTP in the presence of 50WZ.

36

Table 1 HPLC-MS data for the intermediate products during NTP degradation. Empirical formula

NTP

C11H15ClN4O2

Molecular structure

NH

P1

NH2

C10H13ClN4O2+H

Cl

NH2

C10H14ClN3O

Cl

N

C10H14ClN3O

256.69

227.9

227.69

227.9

NH2 O N N

-p

Cl

C8H11ClN2+H

170.64

C11H16ClN3

224.9

225.72

NH

191.8

191.27

NH

163.8

163.22

122.7

122.17

154.9

155.09

131.2

131.09

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N

227.69

171.9

NH Cl

P5

257.5

N OH

P4

270.72

NO2

N

P3

270.8

N

N

P2

Exact m/z value (g mol-1)

NO2

N Cl

Observed m/z value (g mol-1)

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Name

NH

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N

Cl

C11H17N3

N

na

P6

N

N

P9

N H N

C7H10N2

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P8

C9H13N3

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P7

C8H13NO2

N H N OHC

N O

P10

C6H13NO2

N OH

O

37

P11

C10H11ClN4O4+H

NH2 N Cl

P12

286.67

113.9

113.54

N COOH

C5H4ClN N

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na

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Cl

287.9 NO2

38