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Stable and Flexible CuInS2/ZnS:Al-TiO2 Film for Solar Light-Driven Photodegradation of Soil Fumigant Lili Yan, Zhichun Li, Mingxing Sun, Guoqing Shen, and Liang Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b05587 • Publication Date (Web): 14 Jul 2016 Downloaded from http://pubs.acs.org on July 18, 2016
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Stable and Flexible CuInS2/ZnS:Al-TiO2 Film for
2
Solar
3
Fumigant
Light-Driven
Photodegradation
of
Soil
Lili Yan,† Zhichun Li, ‡ Mingxing Sun, § Guoqing Shen*,† and Liang Li*,‡
4 5
†
6
Shanghai 200240, China
7
‡
8
Dongchuan Road, Shanghai 200240, China
9
§
School of Agriculture and Biology, Shanghai Jiao Tong University, 800 Dongchuan Road,
School of Environmental Science and Engineering, Shanghai Jiao Tong University, 800
Shanghai Entry-Exit Inspection and Quarantine Bureau, 1208 Minsheng Road, Shanghai
10
200135, China
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KEYWORDS: CuInS2/ZnS:Al QDs; TiO2; PET; Photocatalytic film; 1,3-Dichloropropene
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ABSTRACT
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Semiconductor quantum dots (QDs) are suitable light absorbers for photocatalysis because of
18
their unique properties. However, QDs generally suffer from poor photochemical stability
19
against air, limiting their applications in photocatalysis. In this study, a stable solar light-driven
20
QDs-containing photocatalytic film was developed to facilitate photocatalytic degradation of the
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soil fumigant 1,3-dichloropropene (1,3-D). A highly stable CuInS2/ZnS:Al core/shell QDs
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(CIS/ZnS:Al QDs) was synthesized by doping Al into the ZnS shell and controlling ZnS:Al shell
23
thickness; the CIS/ZnS:Al QDs was subsequently combined with TiO2 to form a CIS/ZnS:Al-
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TiO2 photocatalyst. The optimized ZnS:Al shell thickness for 1,3-D photodegradation was
25
approximately 1.3 nm, which guaranteed and balanced the good photocatalytic activity and
26
stability of the CIS/ZnS:Al-TiO2 photocatalyst. The photodegradation efficiency of 1,3-D can be
27
maintained up to more than 80% after five cycles during recycling experiment. When
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CIS/ZnS:Al-TiO2 was deposited as photocatalytic film on a flexible polyethylene terephthalate
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substrate, over 99% of cis-1,3-D and 98% of trans-1,3-D were depleted as they passed through
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the film during 15 h of irradiation under natural solar light. This study demonstrated that the
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stable CIS/ZnS:Al-TiO2 photocatalyst both in powder and film form is a promising agent for
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photodegradation and emission reduction of soil fumigants.
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INTRODUCTION
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Semiconductor quantum dots (QDs) are suitable light absorbers for photocatalysis owing to their
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extraordinary properties, such as tunable band gap, high extinction coefficient of light
40
absorption, and relatively high photostability compared with traditional organic dyes.1–5 To date,
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a considerable amount of efforts has been devoted on researches to utilize QDs in sensitizing
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TiO2, forming solar light-driven photocatalysts.6–8 However, some common narrowband gap
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QDs, such as CdS, CdSe, and PbS, contain the toxic elements Cd and Pb. These core-only QDs
44
remain unstable, limiting their application in photocatalysis.9 Developing a stable, green, and
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effective solar light-driven photocatalyst is an interesting endeavor. CuInS2 (CIS) QDs with a
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band gap of 1.5 eV are emerging alternatives for these toxic conventional QDs. However, CIS
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core also suffers from poor chemical-/photo-stability against air and moisture, limiting their
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actual application.9 Therefore, the key issue in the application of CIS QDs in photocatalysis is
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the need to improve their stability during photocatalytic reactions.
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One common method that is proposed to be used to enhance the stability of CIS core involves
51
growing a protective shell to form core/shell QDs.10–13 This approach effectively eliminates
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surface defects by reducing the number of dangling bonds on a surface, thereby improving
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photostability.14 Furthermore, our previous work demonstrated an innovative approach to
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drastically improve the photostability of QDs; this approach involves Al doping into a ZnS (Eg =
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3.7 eV) shell; Al subsequently formed a self-passivation oxide layer (Al2O3) during light
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irradiation.15,16 The conduction band edge of CIS is higher than that of TiO2, allowing the
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electrons to energetically transfer to the lower conduction band of TiO2. This process leads to an
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efficient and relatively long-charge separation by minimizing electron/hole recombination.17
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However, Jiang et al. showed that shell thickness influences the efficiency of electron transfer.18
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Sun et al. found that the electron transfer rate from CIS/ZnS QDs into porous TiO2 films
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decreases exponentially with increasing shell thickness.19 The shell acts as a physical barrier that
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may hinder the transfer of photogenerated electrons and holes from the core to the surface. 4
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Moreover, the electrons and holes in this heterojunction structure are sequestered in the core and
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become unavailable for photocatalytic reactions. Nevertheless, high-performance photocatalysts
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require the electrons to effectively transfer to the outer surface.20 Thibert et al. observed that
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CdSe/CdS QDs shows a higher photocatalytic activity for H2O reduction than CdSe core, and
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this property is due to passivation of surface–deep trap states in core/shell QDs.20 In addition,
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Huang et al. observed that growth of ZnS on CdS core can improve stability during
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photocatalytic reaction and is advantageous for hydrogen generation.21 However, the effects of
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shell and its thickness on photocatalytic reaction when CIS-based core/shell QDs is applied in
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photocatalytic degradation of pollutants is rarely reported. Meanwhile, thickness significantly
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affects the stability of QDs.22 Therefore, balancing the photocatalytic activity and stability of
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photocatalysts by controlling shell thickness is a pivotal problem.
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Moreover, for practical use, photocatalysts should be immobilized on the surface of a suitable
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substrate given that immobilized photocatalysts can be separated and used for continuous
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processes.23 Polyethylene terephthalate (PET) is one of the most widely used polymers. The
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advantages of PET include its low cost, flexible properties, and high performance, such as high
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transparency, high efficiency in gas barrier, high stability in dimension and good mechanical
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property.24 PET has been used in photovoltaic fields, such as in dye- or QDs-sensitized solar
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cells.25,26 To date, the use of photocatalysts based on QDs and TiO2 for photocatalysis in gas
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phase is rarely reported. A heterojunction with a semiconductor can reduce TiO2 deactivation,
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particularly in the removal of volatile organic compounds (VOCs), owing to the formation of a
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stable byproduct on a photocatalyst surface.27,28
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In this study, we optimized the photocatalytic activity of CIS/ZnS:Al-TiO2 photocatalyst by
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controlling the thickness of ZnS:Al shell during CIS/ZnS:Al QDs synthesis. The performance of
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the soil fumigant 1,3-dichloropropene (1,3-D, which includes cis and trans isomers) in
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photocatalytic degradation was evaluated. This fumigant, being a potential VOC in the formation
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of photochemical smog (near-surface ozone), can lead to excessive atmospheric emission, which
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can cause air pollution and pose potential risk to humans.29–32 Recycling experiments involving
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these photocatalysts were performed to study the stability and reusability of these photocatalysts.
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Finally, we designed a novel solar light-driven CIS/ZnS:Al-TiO2 photocatalytic film. The as-
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prepared photocatalytic film was sandwiched in a glass reactor, which was used to investigate the
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emission reduction of 1,3-D.
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EXPERIMENTAL SECTION
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Materials. Dodecanethiol (DDT, 98%), 1-octadecene (ODE, >90%), n-butylamine (>99%),
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copper iodide (CuI, 99.95%), zinc acetate (Zn(Ac)2, 99%), oleylamine (90%), thioglycolic acid
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(TGA, ≥98%), ethanolamine (99%), TiO2 (anatase, 25 nm), tetraethyl orthosilicate (TEOS,
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99.99%), and poly (diallyl-dimethyl-ammonium chloride) (PDDA, low molecular weight 100–
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200 kDa) 20% solution in water were provided by Aladdin Chemical (Shanghai, China).
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Aluminum isopropoxide (Al(IPA)3, ≥98%) and oleic acid (OA, 90%) were purchased from
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Sigma-Aldrich (St. Louis, MO, USA). Indium acetate (In(Ac)3, 99.99%) was obtained from Alfa
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Aesar (Ward Hill, MA, USA). Acetone (HPLC grade) was obtained from Merck. 1,3-D (96%,
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including 50:50 cis- and trans-1,3-D isomers) and titanium (IV) isopropoxide (TTIP, 98%) were
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obtained from J&K Scientific Ltd (Beijing, China). All chemicals were used without further
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purification.
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Synthesis of CIS/ZnS:Al QDs. The preparation of CIS/ZnS:Al QDs includes synthesis of
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CIS QDs, preparation of Zn(OA)2 precursor, and coating of ZnS:Al. The details are described in
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the Supporting Information.
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Preparation of photocatalytic film. Cleaned PET was first submerged in a 1% (v/v) aqueous
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solution of PDDA for 20 min, rinsed in DI water, and dried at room temperature. The details
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SiO2 sol and TiO2 paste preparation are shown in the Supporting Information. The PET was
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coated with two layers of prepared SiO2 through spin-coating and then dried at 80 °C for 2 h.
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Subsequently, TiO2 layer was deposited on top of the SiO2 through spin-coating. The films were
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dried at room temperature and subsequently at 80 °C for 1 h. The deposition of QDs solution was
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then used in layer-by-layer deposition method.33 After coating each sub-layer, the PET film was
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dipped into a methanol solution with 5% TGA for 30 s and completely rinsed with clean
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methanol; the solvent was evaporated in the vacuum oven at 50 °C for 10 min. This process was
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repeated for 10 times.
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Characterization. Transmission electron microscopy (TEM) images of the synthesized QDs
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and photocatalysts were obtained with JEM-2100 operated at 200 kV. The sizes of the QDs were
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determined by using the software Nano Measurer 1.2.5. The sizes and shell thickness of the QDs
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were determined by measuring the geometric diameter (d) and effective diameter (𝑑eff = 3 𝑑).34
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The surface morphologies of the photocatalytic films were examined using a field emission
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scanning electron microscope (SEM, FEI Nova NanoSEM 450) operated at 5 kV with a working
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distance of 5 mm. X-ray diffraction (XRD) patterns were recorded on a Shimadzu XRD-6100
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diffractometer. The elemental analysis of the photocatalyst was carried out using an inductively
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coupled
2
plasma
optical
emission
spectroscopy
(ICP-PS3500DD,
Hitachi).
The
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photoluminescence (PL) intensities of QDs and photocatalysts were collected with a Gangdong
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F-380 fluorescent spectrophotometer and Ocean Optics LS-450, respectively. UV-vis absorption
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spectra of photocatalysts and films were collected on a Perkin-Elmer Lambda 750 UV/VIS/NIR
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spectrometer. X-ray photoelectron spectroscopy (XPS) analysis was performed on a Thermo
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Fisher Scientific ESCALAB 250Xi spectrometer with Al Kα radiation source (hv = 1486.8 eV).
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Photocatalytic degradation of 1,3-D. A certain amount of CIS/ZnS:Al-TiO2 photocatalyst
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was weighted into a 21 mL headspace vial. The vial was injected with 10 μL of 1,3-D (10.4
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mmol L−1 in acetone) and immediately capped with a Teflon-faced butyl-rubber septum and an
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aluminum cap (Agilent Technologies, Inc.). The bottom of the vial placed on a quartz glass was
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irradiated under simulated solar light (Xeon lamp, CEL-S500, Aulight, Beijing, China, AM 1.5,
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90 mw cm−2). The light density was monitored using CEL-NP2000 light power meter. No filter
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was used. At certain time intervals, the vials were transferred into a freezer (−20 °C) and stored
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for analysis. Acetone (5 mL) was added into each frozen vial, which was immediately capped
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with a Teflon-faced butyl rubber septum and an aluminum cap. All sample vials were shaken for
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1 h. The supernatants were filtered using a 0.22 μm nylon syringe filter, and an aliquot (1.0 mL)
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was transferred to a 2.1 mL vial for 1,3-D analysis on an Agilent 6890N gas chromatograph (GC)
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equipped with an Agilent 5973 mass selective detector. Separation was performed using a DB-5
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MS capillary column (30 m × 250 μm × 0.25 μm). The inlet temperature was 250 °C, and the MS
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source temperature was 230 °C. The initial oven temperature was set as 40 °C for 1 min and
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ramped to 100 °C at a rate of 15 °C min−1 and then to 125 °C at a rate of 50 °C min−1. Under
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these conditions, the retention times of cis- and trans-1,3-D were 3.11 and 3.40 min, respectively.
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The performance of photocatalytic film was studied in two cylindrical borosilicate glass
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reactors. Each reactor had an inside diameter and height of 8 and 4 cm, respectively, and
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composed of one source chamber and one receiving chamber, respectively. The two glass
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reactors were fitted together with a photocatalytic film sandwiched between them and the joint
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was sealed with epoxy resin and scotch tape. The photocatalyst coating of the photocatalytic film
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faced the source chamber. The middle of each glass reactor wall had a sampling port, which was
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capped with a PTFE/silicone septum and an aluminum cap. The sampling ports were sealed with
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adhesive aluminum tape.
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1,3-D was introduced into a 400 mL glass cylinder where the air was to spike 0.08 ml of 1,3-D.
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After equilibration for 30 min at room temperature, about 10 mL of 1,3-D gas was injected into
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the source chamber by using a gastight syringe. The initial concentrations of cis- and trans-1,3-D
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in the source chamber were 1.13 ± 0.01 and 0.63 ± 0.01 mg L−1, respectively. Subsequently, the
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experiment was conducted under natural solar light between 9:30 a.m. and 14:30 p.m. The
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average solar intensity was 42.3 mw cm−2 and the solar intensity fluctuations were minimal.
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Gases were sampled with a gastight syringe from the source and receiving chambers for
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subsequent analysis. 1,3-D concentration was determined on an Agilent 6890N GC-MS (Agilent
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5973) and an interfaced HP7694E Headspace sampler. Separation was carried out using a DB-5
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MS capillary column (30 m × 250 μm × 0.25 μm). The initial oven temperature of 40 °C was
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held for 1 min and then increased to 56 °C at a rate of 2.5 °C min−1. The headspace sampler was
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used under the following conditions: vial temperature, 80 °C; loop temperature, 90 °C; transfer
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line temperature, 100 °C; and headspace vial equilibration time, 5.0 min. The retention times of
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cis- and trans-1,3-D were 3.32 and 3.85 min−1, respectively.
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RESULTS AND DISCUSSION
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Characterization. Figure 1 shows the PL spectra and TEM images of the CIS core and
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CIS/ZnS:Al QDs. Overgrowth of ZnS:Al shell led to a systematic blue shift of the PL spectra of
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CIS core, that is, from 708, 645, and 629 nm to 622 nm (Figure 1a); simultaneously, the PL
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intensity considerably increased with increasing coating time. Compared with the size of CIS
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core (2.65±0.27 nm, Figure 1b), that of CIS/ZnS:Al QDs considerably increased, and the ZnS:Al
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shell thickness increased to 0.3, 1.3, and 1.6 nm at coating times of 30, 420, and 630 min,
178
respectively (Figures 1c–e). (a)
(b)
(c)
(d)
(e)
179 180
Figure 1. Evolution of PL spectra (a) and TEM images (b–e) of CIS core during growth of
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ZnS:Al shell with increasing coating time of ZnS:Al from 0, 30, and 420 min to 630 min.
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Figure 2a shows the diffractograms of TiO2, CIS/ZnS:Al-TiO2 photocatalyst and CIS/ZnS:Al
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QDs. The typical diffraction peaks were found at 25.28°, 37.80°, and 48.05°, corresponding to
184
the (101), (004), and (200) planes of TiO2 nanoparticles, indicating that the TiO2 nanoparticles
185
are anatase structure according to JCPDS 21-1272. Addition of CIS/ZnS:Al QDs generated new
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broad peaks located at 28.11°, 46.87°, and 55.29°, which can be attributed to the reflection
187
direction of CIS/ZnS:Al QDs.15 Details on the morphology of the obtained materials are shown
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by the TEM images (Figures 2b–d). Compared with TiO2 without any treatment (Figure 2b),
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many small nanoparticles were deposited on the surface of TiO2 nanoparticles (Figure 2d). The
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coexisting lattice fringes of TiO2 nanoparticles and CIS/ZnS:Al QDs were evident in the intimate
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contact observed in the HRTEM image (Figure 2c). Lattice fringe of 0.341 nm was
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corresponding to the d-spacing value of the (101) plane of anatase TiO2 (JCPDS 21-1272). The
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lattice fringe of the tetrahedral nanoparticle (red line circled) was 0.317 nm, corresponding to the
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d-spacing value of the (112) plane from roquesite CIS (JCPDS 27-0159) or ZnS alloyed CIS.
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Another lattice fringe of 0.232 nm in the Figure 2c inset could be related to the d-spacing value
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(220) from sphalerite ZnS (JCPDS 05-0566).35 To better reveal the chemical states and
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compositions of CIS/ZnS:Al-TiO2 photocatalyst, XPS analysis was further carried out and Figure
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S1 shows the results. Elements of Ti, O, Cu, In, Zn, S, Al, and adventitious C existed in the
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CIS/ZnS:Al-TiO2 photocatalyst (Figure S1a). In Figure S1b, two peaks for Ti 2p were observed
200
at 464.5 and 458.8 eV, assigned to Ti 2p1/2 and Ti 2p3/2, respectively, which was characteristic of
201
Ti4+ in anatase TiO2.36 Meanwhile, the peak for O 1s can be devolved into two peaks (Figure
202
S1c). The peak located at 529.9 eV was attributed to lattice oxygen, whereas the other peak at
203
531.9 eV was attributed to the surface adsorbed oxygen.37 The binding energy of Cu 2p1/2 and Cu
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2p3/2 (Figure S1d) located at 952.0 and 932.2 eV was corresponding to chemical element state of
205
Cu+.38 The peaks of binding energy at 445.0 and 452.5 eV (Figure S1e) matched well with 3d
206
electron of In3+.38 The two peaks (Figure S1f) located at 163.2 and 162.0 eV were assigned to S
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2p with a valence state of −2.39 The binding energy of Zn 2p1/2 and Zn 2p3/2 (Figure S1g) located
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at 1044.9 and 1021.9 eV was corresponding to chemical state of Zn2+.40 The Al 2p peak (Figure
209
S1h) located at 74.6 eV could be associated with the formation of Al–O (Al–OH or Al2O3).15 In
210
addition, Figure S2 shows the Fourier transform infrared spectroscopy (FTIR) spectra in the
211
supporting information. The above characterization results by XRD, TEM, XPS, and FTIR can
212
demonstrate the successful deposition of CIS/ZnS:Al onto TiO2 nanoparticles. Moreover, based
213
on the XPS analysis, the Cu/In atomic ratio was close to 1 and the Zn/S atomic ratio was 0.79.
214
The major elements (Ti, Cu, Zn, and Al) of the photocatalyst were also analyzed by ICP-OES.
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The results showed that the mass ratio of CIS/ZnS:Al QDs to CIS/ZnS:Al-TiO2 was 38.4% and
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the Al/Zn molar ratio was 0.106. Figure 2a (inset) shows the absorption spectra of CIS/ZnS:Al-
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TiO2 photocatalyst. TiO2 absorbs in the near-UV region with an optical absorption threshold of
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3.2 eV. Addition of CIS/ZnS:Al QDs can enhance adsorption in the visible region, which
219
underlies efficient harvesting of light energy. (a)
(c)
(b)
(d)
220 221
Figure 2. (a) XRD patterns and absorption spectra (inset) of TiO2, CIS/ZnS:Al-TiO2, and
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CIS/ZnS:Al. (*) TiO2 and (#) CIS/ZnS:Al. (b) and (d) TEM images of TiO2 and CIS/ZnS:Al-
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TiO2. (c) HRTEM image of CIS/ZnS:Al-TiO2. CIS/ZnS:Al QDs was synthesized with a ZnS:Al
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shell thickness of 1.3 nm.
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Figure 3 presents the SEM images of PET films. The morphology of the prepared PET films is
226
remarkably different. “Single-layer” indicated that the photocatalytic film consisted only of one
227
semi-transparent TiO2 layer (medium size) and QDs layer. “Double-layer” indicated that the
228
photocatalytic film consisted of two TiO2 layers and QDs layer. One of the two TiO2 layers was
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transparent TiO2 layer (small size) and the other was opaque TiO2 layer (large size). Figure 4a
230
shows the XRD patterns of transparent and opaque TiO2. The broad peak at approximately 25°
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(red line) of transparent TiO2 resulting from the TTIP hydrolysis can be indexed to the
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amorphous TiO2. The other small, weak peaks can be attributed to the reflection direction of
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crystalline TiO2.25 The opaque TiO2 (black line) contains peaks attributed to anatase (JCPDS 21-
234
1272) and rutile (JCPDS 78-1508). The average size of opaque TiO2 was approximately 25 nm,
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which was calculated using the Scherrer equation. These TiO2 layers were obtained through
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gradient centrifugation of TiO2 paste. The coating procedure resulted in a relatively even coating
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of PET, formed from dispersing transparent TiO2 on the PET (Figure 3a). Addition of opaque
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TiO2 and QDs layer resulted in a cracked film, and the border of the cracks was considerably
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jagged, possibly indicating TiO2 and QDs accumulation in these areas (Figure 3e). A more
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obvious cracked morphology was observed in the presence of double-layer film than in the
241
presence of single-layer film (Figure 3c); this phenomenon can be ascribed to the fact that the
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former was coated with an addition TiO2 layer compared with the latter. This observation is
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possibly caused by deposition of PDDA, producing a cracked film that covered nearly the entire
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PET surface.41 Cracking can be prevented by precisely controlling the working conditions, such
245
as those in vacuum evaporation42,43 and ball-milling technique.44 Addition of QDs significantly
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modified the morphology of TiO2, resulting in the presence of many bulges on the PET surface
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(Figures 3d,f) compared with the morphology of the transparent TiO2 as shown in the SEM
248
image (Figure 3b). Particle distribution on the double-layer film became denser and more
249
obvious than that on the single-layer film. This relatively homogeneous distribution of TiO2 on
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PET surface is possibly related to the presence of the positively charged CH3–N+(R)4 groups in
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the PDDA film, which prefers the anchoring of negatively charged SiO2 particles (basicity) and
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subsequently favors the attachment of TiO2 nanoparticles.41 QDs can be attached to TiO2
253
nanoparticles via the bifunctional linker molecule TGA.33 Furthermore, Figure 4b shows the UV-
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vis adsorption of single- and double-layer films, which can underlie efficient harvesting light
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energy in the visible region. Figure 4b (inset) further demonstrates the digital photograph of
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double-layer CIS/ZnS:Al-TiO2 photocatalytic film on the PET surface. Therefore, TiO2
257
nanoparticles and CIS/ZnS:Al QDs can be firstly successfully deposited on the PET surface. (a)
(b)
(c)
(d)
(e)
(f)
258 259
Figure 3. SEM images of selected areas of PET substrates coated with (a, b) transparent TiO2,
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(c, d) single-layer film (semi-transparent TiO2 layer and QDs layer), (e, f) double-layer film
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(transparent and opaque TiO2 layers as well as QDs layer).
262 263 264 265
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Figure 4. (a) XRD patterns of transparent and opaque TiO2 after centrifugation. “a” denotes
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anatase and “r” denotes rutile. (b) UV-vis absorption spectra of photocatalytic films acquired in
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transmission mode. Inset shows the digital photograph of double-layer photocatalytic film (PET
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substrate, diameter of 8 cm, and thickness of 0.1 mm) prepared through spin-coating. Single-
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layer denotes the photocatalytic film composed of semi-transparent TiO2 layer and QDs layer;
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double-layer denotes the photocatalytic film composed of transparent and opaque TiO2 layers as
273
well as QDs layer.
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Effect of ZnS:Al shell thickness during QDs synthesis on 1,3-D degradation. Figure 5a
275
shows the effect of ZnS:Al shell thickness on 1,3-D degradation during QD synthesis. The
276
degradation efficiencies of cis- and trans-1,3-D first increased with increasing ZnS:Al shell
277
thickness and peaked at 1.3 nm. The degradation efficiencies of cis- and trans-1,3-D increased
278
from 17.96% and 23.27% to 75.47% and 82.39%, respectively. The efficiencies decreased to
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63.19% and 65.95% as the ZnS:Al shell thickness increased to 1.6 nm, respectively. The
280
efficiency when CIS/ZnS:Al-TiO2 photocatalyst was used for 1,3-D photodegradation was
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higher by approximately 3.8-fold than that when using CIS-TiO2 photocatalyst. The
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aforementioned phenomenon was related to the surface defects and to the band structure of the
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resultant CIS/ZnS:Al core/shell QDs. Within a relatively short coating time, coating of ZnS and
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cation exchange occurred on the core/shell interface, significantly eliminating surface defects
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and dangling bonds on CIS core, dramatically suppressing the non-irradiation recombination and
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thereby drastically enhancing the PL quantum yield (QY).45 Moreover, diffusion of zinc into the
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CIS core increased the band gap and formed a gradient alloy core/shell structure.10,15,46 This
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gradient alloy core/shell structure was further verified by HCl etching of QDs (Figure S3 and
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Table S3). This gradient core/shell structure resulted in the “smoothing” of the confinement
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potential interfacial.45 The high-energy electrons in the core can easily tunnel through the ZnS
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shell into TiO2. High PL QY indicates that most of the photogenerated holes and electrons can
292
recombine via the irradiation pathway (PL). After the combination of QDs with TiO2 and the
293
formation of heterojunction, the most photogenerated carriers separated on the interface and
294
electrons transferred to n-type TiO2, resulting in the high photodegradation efficiency mentioned
295
above. This transfer was significantly dependent on ZnS:Al shell thickness. When ZnS:Al shell
296
became too thick, the ZnS layer acted as a tunneling barrier, localizing the electrons and holes in
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the CIS core and partially suppressing the transfer of electron into TiO2.47 Figure 5c demonstrates
298
the band alignment diagram of CIS/ZnS:Al-TiO2 photocatalyst. The conduction band edge of
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CIS, which is higher than that of TiO2 effectively allowed the electrons to accumulate at the
300
lower conduction band of TiO2, minimizing electron/hole recombination.48 When the ZnS:Al
301
shell thickness was 1.3 nm, the electron transfer was easier than the ZnS:Al shell thickness of 1.6
302
nm.
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(b)
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Figure 5. (a) Effect of ZnS:Al shell thickness during QDs synthesis on the degradation
305
efficiency of 1,3-D. Experimental conditions: photocatalyst mass, 20 mg; solar light intensity, 90
306
mw cm−2; and irradiation time, 3 h. (b) Schematic of the CIS/ZnS:Al QDs structure. (c) Band
307
alignment diagram of CIS/ZnS:Al-TiO2 photocatalyst.
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Moreover, the photocatalytic activity of the photocatalyst was compared with that of their
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constituent TiO2 and CIS/ZnS:Al QDs (Table S1). The total 1,3-D degradation efficiency when
310
using CIS/ZnS:Al-TiO2 was 79.19%±3.38%, which is significantly higher than that when using
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TiO2 (21.57%±3.92%). This result is mainly ascribed to the efficient harvesting of visible-light
312
energy resulting from addition of CIS/ZnS:Al QDs (Figure 2a, inset). Compared with
313
CIS/ZnS:Al QDs, CIS/ZnS:Al-TiO2 photocatalyst significantly improved the degradation
314
efficiency of cis-1,3-D from 58.31%±9.13% to 75.47%±1.90%. In addition, CIS/ZnS:Al QDs is
315
considerably more costly than TiO2. The CIS/ZnS:Al QDs/CIS/ZnS:Al-TiO2 mass ratio was
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38.4%, indicating that the amount of CIS/ZnS:Al in CIS/ZnS:Al-TiO2 photocatalyst decreased
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by 61.6% compared with that of pure CIS/ZnS:Al QDs. These results indicated that CIS/ZnS:Al-
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TiO2 photocatalyst resulted in high photocatalytic activity of 1,3-D.
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Stability and reusability of the photocatalyst. The stability and reusability of the
320
photocatalyst can influence the photocatalytic film. The good reusability of the photocatalytic
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film is advantageous in fumigation degradation, reducing the processing and waste disposal costs
322
of field fumigants. As mentioned above, doping Al into the ZnS shell can further improve the
323
photostability of QDs by forming a passivation oxide layer during light irradiation. We thus
324
compared the photocatalytic activity of CIS/ZnS:Al-TiO2 photocatalyst with that of CIS/ZnS-
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TiO2 photocatalyst under simulated solar light, which can evaluate the stability and reusability of
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the photocatalyst.
327 328
Figure 6. Recycling activities of CIS/ZnS:Al-TiO2 (black column) and CIS/ZnS-TiO2
329
photocatalysts (diagonal column) for 1,3-D photodegradation (left, cis-1,3-D; right, trans-1,3-D).
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Experimental conditions: photocatalyst mass, 50 mg; solar light intensity, 90 mw cm−2, and
331
irradiation time for each cycle, 5 h.
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Recycling experiments were conducted using an equal amount of CIS/ZnS:Al-TiO2 and
333
CIS/ZnS-TiO2 photocatalysts. The photocatalysts were collected, washed, precipitated by
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acetone, and dried in vacuum at 60 °C for 1 h after each cycle. The degradation efficiencies of
335
both catalysts deceased with increasing recycling cycles (Figure 6). The same trends were
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observed between the two isomers of 1,3-D. During recycling, the CIS/ZnS:Al-TiO2
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photocatalyst always resulted in high photocatalytic degradation efficiencies of cis- and trans-
338
1,3-D. The efficiencies can be maintained by more than 80% after five cycles. The degradation
339
efficiency using CIS/ZnS:Al-TiO2 photocatalyst decreased at a slower rate than that when using
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CIS/ZnS-TiO2 photocatalyst, specifically after the third cycle. An extremely significant
341
difference between CIS/ZnS:Al-TiO2 and CIS/ZnS-TiO2 photocatalysts (p < 0.01) was observed
342
in the fourth and fifth cycle, as revealed by using SPSS software (version 19.0, SPSS Inc.,
343
Chicago, IL, USA). This significant difference can be ascribed to the photostability difference of
344
the QDs. After each cycle, the remnant PL intensity of the CIS/ZnS:Al-TiO2 photocatalyst was
345
correspondingly stronger than that of CIS/ZnS-TiO2 photocatalyst (Figure 7). After five cycles,
346
the remnant PL intensity of CIS/ZnS:Al-TiO2 photocatalyst was maintained at approximately 90%
347
of the initial PL intensity. By contrast, the PL intensity of CIS/ZnS-TiO2 photocatalyst was
348
approximately 65% of the initial value. Huang et al.21 reported that the presence of ZnS shell can
349
significantly improve the photostability of CdS NCs by preventing photocorrosion of CdS NCs.
350
It also can improve the stability during photocatalytic reactions. Therefore, photostability of the
351
photocatalyst can imply to some extent the stability and reusability of the photocatalyst.
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Reduction in photostability indicates the photodegradation of the photocatalyst itself. Doping of
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Al into the ZnS shell improved the photostability of CIS/ZnS QDs.15 The ZnS:Al shell not only
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considerably reduced surface defects but also yielded a highly photostable CIS/ZnS:Al structure,
355
which further improved the stability of CIS/ZnS:Al-TiO2 photocatalyst. However, the initial
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degradation efficiency when using CIS/ZnS:Al-TiO2 photocatalyst was lower than that when
357
using CIS/ZnS-TiO2 photocatalyst. Kim et al. found that an Al overlayer can retard electron
358
transfer into the dye in a dye-sensitized TiO2 with an initial thin coating of Al2O3.49 Rao et al.
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reported that Al in the CIS/ZnS:Al QDs could be further oxidized into Al2O3, which formed a
360
passivation oxide layer during light irradiation.15 Thus, Al2O3 prevented electron transfer to the
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outer surface to some extent. PL lifetime of these two photocatalysts were recorded using a
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steady-state and time-resolved fluorescence spectrofluorometer (QM/TM/IM, PTI, USA) (Table
363
S2). The lifetime of CIS/ZnS:Al-TiO2 (307.5 ns) was longer than that of CIS/ZnS-TiO2 (287.2
364
ns), which further verified the slower electron transfer rate in the CIS/ZnS:Al-TiO2. Therefore,
365
this new approach involving doping of Al into ZnS shell not only can enhance the stability of the
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photocatalyst but also maintain photocatalytic activity.
367 368
Figure 7. Photostability of CIS/ZnS:Al-TiO2 and CIS/ZnS-TiO2 photocatalysts after each cycle.
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1,3-D degradation on photocatalytic film under natural solar light. Figure 8 shows the
370
degradation of 1,3-D on photocatalytic films under natural solar light. The control used was a
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PET substrate without any nanocomposite. In the source chamber, 1,3-D rapidly disappeared in
372
the presence of single- and double-layer films. By contrast, a slight reduction was observed in
373
the control treatment and was possibly caused by penetration into the receiving chamber through
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the PET substrate. In the receiving chamber, the concentrations of cis- and trans-1,3-D remained
375
lower in the single- and double-layer films than in the control. The photocatalytic films are
376
obviously effective for 1,3-D photocatalytic degradation and for controlling emission of 1,3-D
377
from the source chamber into the receiving chamber.
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Within 15 h of irradiation, approximately 75% of cis-1,3-D and 79% of trans-1,3-D
379
disappeared in the single-layer film in the source chamber (Figure 8a). Over 99% of cis-1,3-D
380
and 98% of trans-1,3-D were depleted using the double-layer film, and a nearly complete
381
disappearance was reached during the irradiation period (Figure 8a). As shown in Figure 3f, the
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double-layer film is rough and displays obvious presence of aggregates, which served as light-
383
scattering centers. This layer can improve the light-harvesting behavior of light-sensitive
384
materials, such as QDs.50 This phenomenon can also be verified using the UV-vis absorption
385
spectra of the two photocatalytic films acquired in transmission mode (Figure 4b). The double-
386
layer film showed higher absorbance than the single-layer film at wavelength lower than 550 nm.
387
Moreover, 1,3-D can be degraded using CIS/ZnS:Al-TiO2 photocatalyst. The photocatalytic
388
degradation of 1,3-D in the presence of photocatalytic films probably resulted in the
389
disappearance of 1,3-D in the source chamber. The concentration of 1,3-D as a function of time
390
in the source chamber was fitted using a pseudo-first-order model to investigate the
391
disappearance characteristic of 1,3-D on the photocatalytic films. The equations are as follows:51
392
ln
393
𝑡1/2 =
394
where C0 is the initial 1,3-D concentration, C is the concentration at any time (t), k is the pseudo-
395
first-order rate constant, and t1/2 is the half-life. This kinetic equation is the simplification of
396
Langmuir–Hinshelwood kinetic expression, which is commonly used to explain the kinetics of
397
heterogeneous catalysis.52 Table 1 shows the obtained first-order rate constants (k) and half-life
398
(t1/2). The fitting curves of cis-1,3-D and trans-1,3-D were included in the Supporting
399
Information (Figure S4). The correlation coefficients at different levels were higher than 0.95,
400
indicating that 1,3-D degradation kinetic could be considered a pseudo-first-order model. t1/2 of
𝐶0 𝐶
= 𝑘t
(1)
ln2
(2)
𝑘
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1,3-D in the presence of the double-layer film was shorter than that in the presence of the single-
402
layer film (Table 1). t1/2 of the cis- and trans-1,3-D decreased from 7.6 h to 3.3 h and from 6.5 h
403
to 2.7 h, respectively. These values were much lower than the values of 1.7–53 days in the
404
aerobic soil.53 Degradation of 1,3-D by the photocatalytic film was more rapid compared with the
405
long t1/2 of 52 and 12 days for cis- and trans-1,3-D under atmospheric condition.54 In addition,
406
the trans isomer dissipated more rapidly than the cis isomer. The higher dissipation rate is
407
possibly caused by direct reaction of trans-1,3-D with nucleophilic groups, such as –NH2, –SH,
408
and –COOH.55 These groups may react rapidly with trans but not with cis isomer because of
409
steric difference.55 In the present synthesis of CIS/ZnS:Al QDs, CIS/ZnS:Al QDs were capped
410
with ligands, such as DDT and OA, which contain –SH and –COOH groups, respectively.
411
Similarly, these groups can participate in degradation and preferentially react with trans isomer,
412
which accelerated the degradation rate relative to that of the cis isomer.
413
After 15 h of irradiation, the concentrations of cis- and trans-1,3-D using the control in the
414
receiving chamber were 0.23 and 0.16 mg L−1, respectively (Figure 8b). The concentrations of
415
cis- and trans-1,3-D in the presence of the single- and double-layer films were 0.16 and 0.12 mg
416
L−1, respectively (Figure 8b). These values remained lower than that in the presence of the
417
control, indicating that the photocatalytic film was relatively less permeable than the control film.
418
In a previous report, two outer layers and an inner layer of HDPE were constructed using
419
activated materials to control the emission.56 By contrast, our fabricated film consisted only of
420
one PET coated with CIS/ZnS:Al-TiO2 photocatalyst, which can effectively control the
421
penetration of 1,3-D into the receiving chamber.
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(b)
(a)
422 423
Figure 8. (a) Disappearance of cis- and trans-1,3-D concentrations over time in the source
424
chamber. (b) Concentrations of cis- and trans-1,3-D over time in the receiving chamber.
425
Experimental conditions: solar light intensity, 42.3 mw cm−2; and irradiation time, 15 h. Single-
426
layer (Single) denotes the photocatalytic film composed of semi-transparent TiO2 layer and QDs
427
layer; double-layer (Double) denotes the photocatalytic film composed of transparent and opaque
428
TiO2 layers as well as QDs layer.
429 430
Table 1 Equilibrium Time, Rate Constant (k), Half-life (t1/2) of 1,3-D in Permeability Reactors Fumigant cis-1,3-D trans-1,3-D
PET film single-layer double-layer single-layer double-layer
k (h-1) 0.092 0.211 0.106 0.256
t1/2 (h) 7.6 3.3 6.5 2.7
R2 0.9573 0.9776 0.9684 0.9898
431 432
CONCLUSION
433
This study provided a basis for efficient extended VOCs photodegradation by using the as-
434
prepared CIS/ZnS:Al-TiO2 photocatalyst both in its powder and film forms. The photocatalytic
435
activity and stability of CIS/ZnS:Al-TiO2 photocatalyst for 1,3-D photodegradation can be
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balanced by controlling the thickness of ZnS:Al shell. The optimized ZnS:Al shell thickness was
437
approximately 1.3 nm. The results of recycling experiment showed that the degradation
438
efficiency of 1,3-D remained higher than 80% after five cycles. Using the developed flexible
439
CIS/ZnS:Al-TiO2 photocatalytic film, over 99% of cis-1,3-D and 98% of trans-1,3-D can be
440
degraded in the source chamber. Therefore, the proposed photocatalytic film can be effectively
441
utilized to photodegrade and to control the emission of the soil fumigant. In addition, the film
442
can maintain high reusability because the photostability of QDs can be improved by doping Al
443
into the ZnS shell. Further researches should be conducted to understand the effect of the doping
444
level of Al on the photocatalytic activity of CIS/ZnS:Al-TiO2 photocatalyst. Moreover, the effect
445
of the quality and shapes of the CIS/ZnS:Al QDs on the photocatalytic activity should be
446
evaluated.
447
ASSOCIATED CONTENT
448
Supporting Information
449
The supporting information includes the following: details on synthesis of CIS/ZnS:Al QDs,
450
preparation of CIS/ZnS:Al-TiO2 photocatalyst, preparation of SiO2 sol and TiO2 paste, etching of
451
CIS/ZnS:Al QDs, Table S1-S3 as well as Figure S1-S4. This material is available free of charge
452
via the Internet at http://pubs.acs.org.
453
AUTHOR INFORMATION
454
Corresponding Author
455
*(G. Q. Shen) E-mail:
[email protected]. Tel.: +86 21 34206143.
456
*(L. Li) E-mail:
[email protected]. Tel.: +86 21 54747567.
457
Notes
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458
The authors declare no competing financial interest.
459
ACKNOWLEDGMENT
460
This work was supported by the National Natural Science Foundation of China (NSFC 21477075,
461
21271179), and the national key research and development plan (2016YFD0800207).
462
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