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A Composite Photocatalyst Based on Hydrothermally-Synthesized Cu2ZnSnS4 Powders Shih-Jen Lin 1 , Jyh-Ming Ting 1, *, Kuo-Chin Hsu 2 and Yaw-Shyan Fu 2, * 1 2

*

ID

Department of Materials Science and Engineering, National Cheng Kung University, Tainan 701, Taiwan; [email protected] Department of Greenergy, National University of Tainan, Tainan 701, Taiwan; [email protected] Correspondence: [email protected] (J.-M.T.); [email protected] (Y.-S.F.); Tel.: +886-6-260-5031 (Y.-S.F.); Fax: +886-6-260-2205 (Y.-S.F.)

Received: 31 October 2017; Accepted: 17 January 2018; Published: 19 January 2018

Abstract: A novel composite photocatalyst based on Cu2 ZnSnS4 (CZTS) powders was synthesized and investigated for use as a photocatalyst. CZTS powders were first made using a conventional hydrothermal method and were then used to grow silver nanoparticles hybridized onto the CZTS under various conditions through a microwave-assisted hydrothermal process. After the obtained samples were subsequently mixed with 1T-2H MoS2 , the three synthesized component samples were characterized using X-ray diffractometry (XRD), scanning electron microscopy, transmission electron microscopy (FE-SEM, FE-TEM), UV-visible spectroscopy (UV-Vis), Brunauer-Emmet-Teller (BET), photoluminescence spectroscopy (PL), and X-ray photoelectron spectroscopy (XPS). The resulting samples were also used as photocatalysts for the degradation of methylene blue (MB) under a 300 W halogen lamp simulating sunlight with ~5% UV light. The photodegradation ability was greatly enhanced by the addition of Ag and 1T-2H MoS2 . Excellent photodegradation of MB was obtained under visible light. The effects of material characteristics on the photodegradation were investigated and discussed. Keywords: Cu2 ZnSnS4 powders; silver nanoparticles; 1T-2H MoS2 ; photodegradation

1. Introduction A photocatalytic technique is considered a promising method for treating organic dyes in wastewater [1,2]. I2 -II-IV-VI4 quaternary chalcogenide semiconductors Cu2 ZnSnS4 (CZTS) are non-toxic, Earth-abundant, and inexpensive, and they have a high absorption coefficient >10−4 cm2 for potential application as solar-cell absorbers [3–5]. However, due to the fact that their optimum band gap is ~1.5 eV, they easily recombine into photogenerated holes and electrons. Thus, it is necessary to reduce the recombination rate. TiO2 is a well-known n-type metal oxide photocatalyst in many photocatalytic applications, including hydrogen generation and water splitting in environmental and energy-related fields [1,6–9]. However, its relatively large band gap of 3.2 eV is only a small fraction (4%) of the solar spectrum. Several studies have investigated ways to achieve better absorption and charge carrier mobility under visible light through the use of a dopant [10–13]. Koichi Awazu et al. labeled this new phenomenon “plasmonic photocatalysis”, where silver (Ag) nanoparticles (NPs) are coated with a passive material such as SiO2 to prevent oxidation and to separate TiO2 . Their work indicated effective surface plasmon resonance transfer of photogenerated holes on the surface enhanced photocatalytic reaction [14]. Recently, it has been shown that silver orthophosphate (Ag3 PO4 ) has extremely high photo-oxidative capabilities for O2 generation from water splitting under visible light irradiation [15]. GO–Ag3 PO4 nanocomposites have been synthesized as an excellent hybrid photocatalyst [16,17].

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They not only exhibit enhanced visible light photocatalytic activity, but also show better stability and durability than their counterparts. Among them, molybdenum disulfide (MoS2 ) exhibits a graphene-like layered structure in which S–Mo–S covalent bonds exist within the plane, and van der Waals forces hold the plane [2,18]. MoS2 provides a high surface area, unique electrical conductivity consisting of 1T-2H hybridized phases, and a sheet-like morphology [19]. It has received attention for application in energy storage devices due to its unique nanostructure and electronic properties. The photocatalysts of the I2 -II-IV-VI4 quaternary group exhibit better absorption ability under visible light than TiO2 . Therefore, Xuelian Yu et al. prepared CZTS-Pt and CZTS-Au heterostructured NPs using a synthesis route [20]. The results showed enhanced photocatalytic activity toward photodegradation and hydrogen generation by water splitting. Feng Jiang et al. reported a multilayer of Pt/In2 S3 /CdS/CZTS heterostrucures, where the obtained photocathode showed significant enhancement in terms of stability during photoirradiation for PEC water splitting [21]. Hybridized 1T-2H MoS2 was then added to provide higher electrical conductivity and the specific surface area necessary to achieve superior catalytic ability [22–24]. Importantly, Ag was added in the structure to extend the light absorption of the photocatalyst to the visible light region, which was due to the surface plasmon resonance of Ag NPs, which enabled the photocatalyst to absorb light in the visible spectrum. Owing to these outstanding characteristics, we report a novel design of three-component Ag/hybridized CZTS/1T-2H MoS2 heterojunction photocatalysts using a facile hydrothermal method. Since their appropriate band gap structures and stability enhance properties that include light harvesting, charge separation, and transfer photoactivity, it is essential to develop this method on a photocatalytic mechanism. 2. Results and Discussion 2.1. Crystal Structure The XRD patterns of the Kesterite (KS) CZTS, 1T-2H MoS2 , and 1 wt% Ag-CZTS/1T-2H MoS2 are shown in Figure 1. Figure 1a shows that the major CZTS diffraction peaks observed at 28.47◦ , 32.99◦ , 47.35◦ , 56.14◦ , 68.99◦ , and 76.42◦ can be indexed to the crystal planes of (112), (200), (220), (312), (008), and (332) of the KS structure (JCPDS no. 26-0575), respectively. The formation mechanisms of the CZTS powder are summarized as Cu2 S + SnS2 + ZnS → Cu2 ZnSnS4 . It is necessary for the excess thiourea concentration to be a homogeneous solution in order to effectively induce a self-assembly reaction through the conventional hydrothermal (CHT) method [4]. No secondary phases of Cu2 S, SnS2 or ZnS were detected [25]. Figure 1b shows that the CHT synthesized MoS2 microflower sample obtained the major diffraction peaks at 14.42◦ , 32.69◦ , 33.49◦ , 39.52◦ , 49.76◦ , and 58.3◦ , which correspond to the (002), (100), (101), (103), (105), and (110) crystalline planes of MoS2 (JCPDS no. 37-1492), respectively. In addition, the calculated interlayer spacing (d-002) using the Bragg equation was 0.66 nm, with the reported well-controlled expansion of the interlayer d-spacing in the hydrothermal/solvothermal approaches ranging from 0.62 to 1.1 nm [26]. In Figure 1c for the 1 wt% Ag/CZTS and 2:1 ratio of 1T-2H MoS2 composite, no peaks were observed for Ag probably due to the very small amount of Ag added onto the CZTS. The XRD pattern corresponding to both the CZTS and MoS2 peaks can be observed.

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Figure1.1. XRD XRD pattern pattern of of (a) (a)CZTS CZTSpowders powdersusing usingthe theconventional conventional hydrothermal hydrothermal process process (CHT) (CHT) at at Figure Figure 1. XRD pattern of (a) CZTS powders using the conventional hydrothermal process (CHT) at 2 powder using the CHT process at 200 °C for 24 h; (c) 2:1 ratio of 1 wt% 180 °C for 72 h; (b) MoS ◦ C for 72 h; (b) MoS powder using the CHT process at 200 ◦ C for 24 h; (c) 2:1 ratio of 1 wt% 180 °C 180 for 72 h; (b) MoS22powder using the CHT process at 200 °C for 24 h; (c) 2:1 ratio of 1 wt% Ag/CZTS and 1T-2H 1T-2H MoS MoS2.. Ag/CZTS and 2.2 Ag/CZTS and 1T-2H MoS

2.2. Morphology 2.2.Morphology Morphology 2.2. The morphology of Ag/CZTS/1T-2H MoS2 NPs based on the SEM results is shown in Figure 2. Themorphology morphologyof of Ag/CZTS/1T-2H Ag/CZTS/1T-2HMoS MoS basedononthe theSEM SEMresults resultsisisshown shownininFigure Figure2.2. 2 NPs The 2 NPs based Figure 2a shows the SEM images of 1 wt% Ag hybridized KS CZTS NPs. Shapes with a minor Figure 2a 2a shows of of 1 wt% Ag hybridized KS CZTS NPs. Shapes with a with minoraamount Figure shows the theSEM SEMimages images 1 wt% Ag hybridized KS CZTS NPs. Shapes minor amount of irregular particles were obtained, but the addition of Ag on the CZTS was not observed of irregular particles were obtained, but thebut addition of Ag on the on CZTS not observed on the amount of irregular particles were obtained, the addition of Ag the was CZTS was not observed on the SEM images probably because the Ag particles were too small to be detected. Figure 2b SEM images probably because the Ag particles were too small to be detected. Figure 2b shows on the SEM images probably because the Ag particles were too small to be detected. Figure the 2b shows the images of CHT-synthesized MoS2 microflowers obtained as the petals, which consist of imagesthe of CHT-synthesized MoS2 microflowers obtained as obtained the petals,aswhich consistwhich of several MoS shows images of CHT-synthesized MoS2 microflowers the petals, consist of2 several MoS2 nanosheets. On the other hand, Figure 2c shows that the morphologies of the nanosheets. On the other hand, Figure 2c shows that the morphologies of the as-prepared 2:1 ratio several MoS2 nanosheets. On the other hand, Figure 2c shows that the morphologies of the as-prepared 2:1 ratio of 1 wt% Ag/CZTS and 1T-2H MoS2 are well grown on the surface of the MoS2 of 1 wt% Ag/CZTS and 1T-2H MoS2 are well grown thewell surface of the MoS thereby 2 powder, as-prepared 2:1 ratio of 1 wt% Ag/CZTS and 1T-2H MoSon 2 are grown on the surface of the MoS2 powder, thereby successfully forming the Ag/CZTS/1T-2H MoS2 heterostructure. successfully forming the Ag/CZTS/1T-2H MoS2 heterostructure. powder, thereby successfully forming the Ag/CZTS/1T-2H MoS2 heterostructure.

Figure 2. Cont.

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Figure 2. Scanning electron microscopy (SEM) images of the prepared CZTS powders with the (a) 1

Figure wt% 2. Scanning microscopy (SEM) of the CZTS with the (a) microflower; andprepared (c) the 2:1 ratio of 1powders wt% Ag/CZTS Ag doped electron on CZTS powder; (b) 1T-2H MoS2images 1 wt% Ag powder; (b) 1T-2H MoS2 microflower; and (c) the 2:1 ratio of 1 wt% Ag/CZTS 2. anddoped 1T-2H on MoSCZTS and 1T-2H MoS2 .

Figure2. 3a showselectron the TEM images (SEM) of Ag/CZTS where Ag NPs a 1darker Figure Scanning microscopy images ofpowders, the prepared CZTSthe powders withwith the (a) contrast were hybridized on the CZTS powder. Figure 3d shows the high resolution TEM images wt% Ag doped on CZTS powder; (b) 1T-2H MoS2 microflower; and (c) the 2:1 ratio of 1 wt% Ag/CZTS Figure 3a shows the TEMlattice images of Ag/CZTS powders, where the Agplanes NPs with a darker contrast wherein the interplannar spacing of 0.31 nm corresponds to the (112) [5]. Meanwhile, and 1T-2H MoS2. were hybridized onAg thecorresponds CZTS powder. Figure 3d shows the high resolution TEM images the the additional to the face-centered cubic (FCC) Ag (111) [27], with a spacing of wherein 0.24 nm. Figure Figure 3b,espacing show the that the morphologies MoS 2planes consist ofAg a Meanwhile, considerable 3a shows images of Ag/CZTS where the[5]. NPs with a number darker interplannar lattice ofTEM 0.31microflower nm corresponds topowders, theof(112) the additional of singlewere and structures. The MoS 2 lattice was also calculated, for which the contrast hybridized on the CZTS powder. Figure 3dspacing shows the high resolution TEM images Ag corresponds to few the layer face-centered cubic (FCC) Agfringe (111) [27], with a spacing of 0.24 nm. Figure 3b,e corresponding d-spacinglattice was 0.67 nm [22]. Figure 3c,f show thetomore aggregated nanoparticles of wherein the interplannar spacing of 0.31 nm corresponds the (112) planes [5]. Meanwhile, show that the microflower morphologies of MoS2 consist of a considerable number of single and few theadditional 1 wt% Ag/CZTS and 2:1 ratio offace-centered 1T-2H MoS2 and SAED of thewith 1 wt% Ag/CZTS and the Ag corresponds to the cubicthe (FCC) Agimage (111) [27], a spacing of 0.24 layer structures. The MoS fringe spacing was also calculated, forconsiderable which thenumber corresponding 2 2,lattice 2:1 Figure ratio of3b,e 1T-2H MoS which is consistent of with tetragonal KS CZTS, it shows the major nm. show that the microflower morphologies of MoS2 consist of a as d-spacing was(112), 0.67few nmlayer [22]. Figure 3c,f thethemore aggregated nanoparticles thewhich 1 wt% Ag/CZTS phases (220), andstructures. (312), indicating that hybridizing of the Ag NPs can be of attributed tothe the of single and Theshow MoS 2 lattice fringe spacing was also calculated, for and 2:1corresponding ratio of 1T-2H MoS(111) the image of3c,f theshow 1 wt% Ag/CZTS and of MoS 1T-2H diffraction from the plane of The corresponding results for2:1 theratio 1T-2H d-spacing was 0.67 SAED nmFCC. [22]. Figure theSAED more aggregated nanoparticles of 2 MoS2 , 2 and microflower match the (100),ofand planes [23,24]. 1 wt% Ag/CZTS and(002), 2:1 ratio 1T-2H MoS 2 as andit the SAED image of the 1 wt%(112), Ag/CZTS and which the is consistent of with tetragonal KS(110) CZTS, shows the major phases (220), and (312), 2:1 ratio of 1T-2H MoS 2, which is consistent of with tetragonal KS CZTS, as it shows the major indicating that the hybridizing of the Ag NPs can be attributed to the diffraction from the (111) plane phases (112), (220), and (312), indicating that the hybridizing of the Ag NPs can be attributed to the of FCC. The corresponding SAED results for the 1T-2H MoS2 microflower match the (002), (100), and diffraction from the (111) plane of FCC. The corresponding SAED results for the 1T-2H MoS2 (110) planes [23,24]. microflower match the (002), (100), and (110) planes [23,24].

Figure 3. Cont.

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Figure 3. TEM images (a) 1 wt% Ag doped on CZTS powders; (b) 1T-2H MoS2 microflower; (c) 2:1 ratio of 1 wt% Ag/CZTS and 1T-2H MoS2; HRTEM images of (d) 1 wt% Ag/CZTS; (e) 1T-2H MoS2 Figure 3. TEM images (a) 1 wt% Ag doped on CZTS powders; (b) 1T-2H MoS 2 microflower; (c) 2:1 microflower; and (f) (a) SAED micrographs ratiopowders; of 1 wt% Ag/CZTS MoS2. Figure 3. TEM images 1 wt% Ag dopedofon2:1 CZTS (b) 1T-2Hand MoS1T-2H 2 microflower; (c) 2:1 ratio of 1 wt% Ag/CZTS and 1T-2H MoS2; HRTEM images of (d) 1 wt% Ag/CZTS; (e) 1T-2H MoS2 ratio of 1 wt% Ag/CZTS and 1T-2H MoS2 ; HRTEM images of (d) 1 wt% Ag/CZTS; (e) 1T-2H MoS2 microflower; and (f) SAED micrographs of 2:1 ratio of 1 wt% Ag/CZTS and 1T-2H MoS2. 2.3.microflower; Surface Areas and (f) SAED micrographs of 2:1 ratio of 1 wt% Ag/CZTS and 1T-2H MoS2 .

Figure 4 shows the Brunauer–Emmett–Teller (BET) analysis using N2 adsorption-desorption 2.3. Surface Areas 2.3.isotherms Surface Areas and the corresponding Barrett–Joyner–Halenda (BJH) pore size distributions curves Figure 4 surface shows the Brunauer–Emmett–Teller (BET) analysis using N2 adsorption-desorption (inset). The areas of the CZTS powders and 1T-2H MoS2using are measured as 3.38 and 21.39 m2 Figure 4 shows the Brunauer–Emmett–Teller (BET) analysis N2 adsorption-desorption isotherms and the corresponding Barrett–Joyner–Halenda (BJH) pore size distributions curves g−1, respectively. The insets areBarrett–Joyner–Halenda the BJH pore volume are 0.045 and 0.096 cm3 g−1, and the pore size isotherms and the corresponding (BJH) pore size distributions curves (inset). (inset). The surface areas of the CZTS powders and 1T-2H MoS 2 are measured as 3.38 and 21.39 m2 2 − 1, distribution plot, for CZTS whichpowders the adsorption average diametersasare 31.74 and 18.02 The surface areas of the and 1T-2H MoS2 pore are measured 3.38 and 21.39 m g nm, 3 g−1, and the pore size g−1respectively. , respectively. The insets are the BJH pore volume are 0.045 and 0.096 cm It can be are seenthe that there arevolume differences in theand morphology in different respectively. The insets BJH pore are 0.045 0.096 cm3results g−1 , and the porespecific size distribution plot, for which the adsorption average pore diameters are 31.74 and 18.02 surface areas which indicates that the MoS has a higher the nm, caserespectively. for the nm, CZTS distribution plot,(SSAs), for which the adsorption average pore2 diameters areSSA 31.74than and is 18.02 respectively. It can be seen that the there are differences the morphology results inarea. different specific powders due to there the fact microflower shapeinprovided surface However, It can be seen that arethat differences in the morphology resultsainhigher different specific surface areasthe surface areasof(SSAs), which indicates that than the MoS a higher than isThis the may case for the CZTS pore which size the 1T-2H MoS smaller that2 has of the CZTS SSA powders. be because the (SSAs), indicates that the2 is MoS 2 has a higher SSA than is the case for the CZTS powders due powders due to the fact that the microflower shape provided a higher surface area. However, the solution of diethylenediamine (DETA) acted as anarea. etchant and the increased theofexcess to alkaline the fact that the microflower shape provided a higher surface However, pore size the pore size ofwhich the 1T-2H MoS2 is smallerforce thanand thatpolarity of the CZTS powders. This may be because the thiourea, is an intermolecular molecule during the synthesis of the CZTS 1T-2H MoS2 is smaller than that of the CZTS powders. This may be because the alkaline solution alkaline solution of diethylenediamine (DETA) acted as an etchant and increased the excess resulting in the formation pitsand on increased the surfacethe [28,29]. larger SSA of powders, diethylenediamine (DETA) acted as of anmore etchant excessTherefore, thiourea, the which is an thiourea, which is an intermolecular force and polarity molecule during the synthesis of the CZTS and pore diameter in polarity the Ag/CZTS/MoS 2 contributed to active enhancement of photo-catalytic intermolecular force and molecule during the synthesis of the CZTS powders, resulting in powders, resulting in the formation of more pits on the surface [28,29]. Therefore, the larger SSA efficiency [24]. the formation of more pits on the surface [28,29]. Therefore, the larger SSA and pore diameter in the and pore diameter in the Ag/CZTS/MoS2 contributed to active enhancement of photo-catalytic Ag/CZTS/MoS2 contributed to active enhancement of photo-catalytic efficiency [24]. efficiency [24].

Figure 4. Cont.

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Figure 4. N N2Nabsorption/desorption isotherms of of (a)(a) CZTS powders, and (b)(b) 1T-2H MoS 2 microflower Figure 4. 2 absorption/desorption isotherms CZTS powders, and 1T-2H MoS 2 microflower Figure 4. 2 absorption/desorption isotherms of (a) CZTS powders, and (b) 1T-2H MoS2 microflower powders; insets are BJH pore size distribution plots, respectively. powders; insets BJH pore distribution plots, respectively. powders; insets areare BJH pore sizesize distribution plots, respectively.

2.4. Optical Absorption 2.4. Optical Absorption 2.4. Optical Absorption The UV-VIS absorption spectrum results forfor thethe tetragonal KSKS CZTS powders, wt% Ag/CZTS, The UV-VIS absorption spectrum results tetragonal CZTS powders, 1 wt% Ag/CZTS, The UV-VIS absorption spectrum results for the tetragonal KS CZTS powders, 11wt% Ag/CZTS, 1 wt% Ag/CZTS, and 10:1 and 2:1 ratios of 1T-2H MoS 2 are shown in Figure 5. It can be seen that 1 wt% Ag/CZTS, and 10:1 and 2:1 ratios of 1T-2H MoS 2 are shown in Figure 5. It can be seen that 1 wt% Ag/CZTS, and 10:1 and 2:1 ratios of 1T-2H MoS2 are shown in Figure 5. It can be seen that doping Ag enhanced the light absorption, and this fact can be ascribed to the charge transfer doping enhanced the light absorption, andfact this fact can be ascribed to the chargeprocess transfer doping AgAg enhanced the light absorption, and this can be ascribed to the charge transfer process from the valence band to the conduction band by board surface plasmon resonance (SPR) process from the valence band to the conduction band by board surface plasmon resonance (SPR) from the valence band to the conduction band by board surface plasmon resonance (SPR) absorption in absorption in the visible region [30]. However, further increases in the 10 wt% and 50 wt% 1T-2H absorption in the visible region [30]. However, further increases in the 10 wt% and 50 wt% 1T-2H the visible region [30]. However, further increases in the 10 wt% and 50 wt% 1T-2H MoS2 microflower MoS 2 microflower resulted indue broad due aapparent higher SSA. Ita It is apparent a a MoS 2 microflower powder resulted in broad absorption due a higher SSA. is apparent that powder resulted inpowder broad absorption to aabsorption higher SSA. Ittoisto that high surfacethat area high surface enhances thethe visible light likely due to to thethe strong monotonic high surface area enhances visible light absorption. is likely due strong monotonic enhances the area visible light absorption. This isabsorption. likely due This toThis theisstrong monotonic absorption in the absorption in the visible light region of the hybridized 1T-2H MoS 2. 2Among the samples, thethe 2:12:1 absorption in the visible light region of the hybridized 1T-2H MoS . Among the samples, visible light region of the hybridized 1T-2H MoS2 . Among the samples, the 2:1 ratio of 1 wt% Ag/CZTS ratio of of 1 wt% Ag/CZTS and MoS 2 exhibited thethe best enhancement. ratio 1MoS wt% Ag/CZTS and 1T-2H MoS 2 exhibited best enhancement. and 1T-2H exhibited the1T-2H best enhancement. 2

Figure 5. UV-VIS UV-VIS absorption spectra of (a) (a)(a) CZTS; (b)(b) wt% Ag/CZTS; (c)(c) 10:1 ratio of1of 1wt% wt% Ag/CZTS Figure 5. UV-VIS absorption spectra of CZTS; 1 wt% Ag/CZTS; 10:1 ratio 1 wt% Ag/CZTS Figure 5. absorption spectra of CZTS; (b) 11wt% Ag/CZTS; (c) 10:1 ratio of Ag/CZTS and 1T-2H MoS 2; and (d) 2:1 ratio of 1 wt% Ag/CZTS and 1T-2H MoS 2. 2. and 1T-2H MoS 2; and (d) 2:1 ratio of 1 wt% Ag/CZTS and 1T-2H MoS and 1T-2H MoS2 ; and (d) 2:1 ratio of 1 wt% Ag/CZTS and 1T-2H MoS2 .

2.5. PLPL Analysis 2.5. Analysis AA PLPL analysis was then performed to to determine thethe effect of of AgAg onon thethe recombination rate of of analysis was then performed determine effect recombination rate thethe photocatalyst. Figure 6 shows that thethe PLPL intensity was greatly increased when a 4a and 8 wt% of of photocatalyst. Figure 6 shows that intensity was greatly increased when 4 and 8 wt%

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2.5. PL Analysis A PL analysis was then performed to determine the effect of Ag on the recombination rate of the Materials 2018, 11, 158 7 of 13 photocatalyst. Figure 6 shows that the PL intensity was greatly increased when a 4 and 8 wt% of Ag was added towas the CZTS and a furtherand decrease was observed the amount was decreased Ag added photocatalyst, to the CZTS photocatalyst, a further decrease waswhen observed when the amount was1decreased 2 and 1 wt%. suggestedpromoted that the electrons promoted inband the conduction bandto the to 2 and wt%. Thistosuggested thatThis the electrons in the conduction of CZTS jump of plasmon CZTS jump to of theAg, surface plasmon state of Ag, thus reducing the structure. recombination rate ofexcessive the surface state thus reducing the recombination rate of the However, structure. However, excessive amounts of electrons were generated in the structure, which caused amounts of electrons were generated in the structure, which caused the Ag nanoparticles to serve as a the Ag nanoparticles to serve as a recombination site in the system [30]. On the other hand, 1 wt% recombination site in the system [30]. On the other hand, 1 wt% Ag/CZTS hybridized a higher SSA Ag/CZTS hybridized a higher SSA of MoS2 that absorbed the visible light and efficiently transferred of MoS2 that absorbed the visible light and efficiently transferred the photogenerated charge carriers. the photogenerated charge carriers. The CZTS nanoparticles generated excited electrons and holes The CZTS nanoparticles generated excited electrons and holesthe from its valence band in to its from its valence band to its conduction band after which electrons generated theconduction CZTS band conduction after whichband the electrons generated in the CZTS conduction band jumped to the surface plasmon jumped to the surface plasmon state of the Ag nanoparticles to be either directly state transferred of the Ag nanoparticles to be either to MoSthe shuttledofacross the Ag to MoS2 or shuttled across directly the Ag totransferred the MoS2. Finally, hybridized Agto the 2 orpresence 1T-2HtheMoS 2 microflower powders indicate a 2decrease in the recombination rate MoS2and . Finally, presence of hybridized Agmight and 1T-2H MoS microflower powders might indicate a necessary photocatalysis.rate necessary for photocatalysis. decrease in thefor recombination

Figure 6. PL spectra of (a) 2:1 ratio of 1 wt% Ag/CZTS and 1T-2H MoS2; (b) 10:1 ratio of 1 wt%

Figure 6. PL spectra of (a) 2:1 ratio of 1 wt% Ag/CZTS and 1T-2H MoS2 ; (b) 10:1 ratio of 1 wt% Ag/CZTS and 1T-2H MoS2; (c) 1 wt% Ag/CZTS; (d) 2 wt% Ag/CZTS; (e) CZTS; (f) 4 wt% Ag/CZTS; Ag/CZTS and 1T-2H MoS2 ; (c) 1 wt% Ag/CZTS; (d) 2 wt% Ag/CZTS; (e) CZTS; (f) 4 wt% Ag/CZTS; and (g) 8 wt% Ag/CZTS. and (g) 8 wt% Ag/CZTS.

2.6. XPS Study

2.6. XPS Study

A calibration was made for each sample with respect to 284.6 eV of carbon C 1s. The XPS

A calibration was forfour eachconstituent sample with respectcopper to 284.6 of carbon Thesulfur XPS analysis analysis carried outmade on the elements, 2p,eVzinc 2p, tin C 3d,1s.and 2p. (Figure 7a),the showed the binding energy of the Cu 2p2p, 3/2 and 2ptin 1/2 peaks at 931.4 and eV, 7a), carried out on four constituent elements, copper zincCu2p, 3d, and sulfur 2p.952.2 (Figure respectively, withenergy a peak splitting at 2p 19.8 eV, suggesting Cu (I). Figure 7b showed the Zn 2p3/2 and showed the binding of the Cu 3/2 and Cu 2p1/2 peaks at 931.4 and 952.2 eV, respectively, Zn 2p 1/2 peaks at 1021.8 and 1044.8 eV, respectively, with a peak splitting at 23 eV, corresponding to with a peak splitting at 19.8 eV, suggesting Cu (I). Figure 7b showed the Zn 2p3/2 and Zn 2p1/2 peaks Zn (II). Figure 7c shows the Sn 3d5/2 and Sn 3d3/2 peaks at 485.8 and 494.2 eV, respectively, with peak at 1021.8 and 1044.8 eV, respectively, with a peak splitting at 23 eV, corresponding to Zn (II). Figure 7c splitting at 8.4 eV, which is in a good agreement with Sn (IV). Figure 7d shows the S 2p3/2, S 2p1/2 shows the Sn 3d5/2 and Sn 3d3/2 peaks at 485.8 and 494.2 eV, respectively, with peak splitting at peaks at 161.1, 162.3 eV, respectively, from S (II) [5]. The Ag 3d XPS spectra of Ag/CZTS samples 8.4 eV, which is in adifferent good agreement with Sn (IV). Figure 7d shows the S 2p3/28, wt% S 2p1/2 obtained from weight percentages of AgNO 3 precursor: 1, 2, 4, and are peaks shownatin161.1, 162.3Figure eV, respectively, from S (II) [5]. The Ag 3d XPS spectra of Ag/CZTS samples obtained 7e. The high resolution spectra of the Ag 3d core level for the samples showed two peaks,from different weight percentages AgNO precursor: 2, 4, peaks and 8 proved wt% are shown Figure 7e. The high which were located aroundof368.2 eV3and 374.3 eV.1,These that AgNOin 3 was successfully reduced to metallic AgAg through the level microwave-assisted hydrothermal process in water as awere solvent resolution spectra of the 3d core for the samples showed two peaks, which located [31]. Further confirmation of the peaks existence of thethat hybridized 1T-2H phase of MoS 2 is provided in Ag around 368.2 eV and 374.3 eV. These proved AgNO3 was successfully reduced to metallic Figure 7f, where a typical MoS 2 microflower shows Mo 3d5/2 and Mo 3d3/2 peaks at 229.7 eV and through the microwave-assisted hydrothermal process in water as a solvent [31]. Further confirmation eV for the 2H hybridized phase, respectively, an Sof2sMoS peak at 226.6 eV in the high resolution Mo 3d of the232.7 existence of the 1T-2H and phase 2 is provided in Figure 7f, where a typical spectrum. Furthermore, a minor peak at 235.7 eV appears to the Mo6+ 3d5/2. However, the samples MoS2 microflower shows Mo 3d5/2 and Mo 3d3/2 peaks at 229.7 eV and 232.7 eV for the 2H phase, shift toward lower energy peaks at 228.7 and 231.9 eV for the 1T phase, which shifts toward a lower respectively, and an S 2s peak at 226.6 eV in the high resolution Mo 3d spectrum. Furthermore, a minor binding energy with a difference of ~0.8 eV with respect to the peaks in the 1T-2H phase [18,19].

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peak at 235.7 eV appears to the Mo6+ 3d5/2 . However, the samples shift toward lower energy peaks at 2018, eV 11, 158 8 of 13 228.7Materials and 231.9 for the 1T phase, which shifts toward a lower binding energy with a difference of ~0.8 eV with respect to the peaks in the 1T-2H phase [18,19]. Similarly, Figure 7g shows the S 2p XPS Similarly, Figure 7g shows the S 2p XPS spectra of the MoS2 samples indicating that two additional spectra of the MoS2 samples indicating that two additional signatures at 161.7 eV and 162.8 eV appear signatures at 161.7 eV and 162.8 eV appear beside the two S 2p3/2 and S 2p1/2 2H MoS2 peaks at 162.6 beside S 2p and S 2p1/2 2H MoS atshift 162.6toeV 163.7 eV, respectively. These 2 peaks eVthe andtwo 163.7 eV,3/2 respectively. These two peaks also theand lower binding energy side and are two peaksassociated also shiftwith to the binding thelower 1T phase [19]. energy side and are associated with the 1T phase [19].

Figure 7. High-resolution XPS characterization of as-synthesized (a) Cu 2p; (b) Zn 2p; (c) Sn 3d; (d) S 2p;

Figure 7. High-resolution XPS characterization of as-synthesized (a) Cu 2p; (b) Zn 2p; (c) Sn 3d; (d) S (e) Ag NPs; (f) Mo 3d; and (g) S 2p. 2p; (e) Ag NPs; (f) Mo 3d; and (g) S 2p.

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The photocatalytic performance of all of the Ag/CZTS and hybridized 1T-2H MoS2 photocatalysts 2.7. Photocatalytic Activity with the photocatalytic performance. The curves of C/C0 , where C0 were measured and compared is the initialThe concentration of MB, and C isofthe concentration of MB at time t are shown in photocatalytic performance allreaction of the Ag/CZTS and hybridized 1T-2H MoS 2 photocatalysts were measured and compared with the photocatalytic performance. The In curves of of the Figure 8a. It can be seen that the hybridized Ag decreased as the Ag content increased. the case where MoS C0 is the initial concentration of MB, and C is the reaction concentration of MB at time t additionC/C of0,1T-2H 2 , regardless of whether the ratio was lower (10 wt%) or was 1 wt%, Ag doped are shown in Figure 8a. It can be seen that the hybridized Ag decreased as the Ag content increased. on the CZTS greatly enhanced the photocatalytic performance. Figure 8b shows the first order linear In the case of the addition of 1T-2H MoS2, regardless of whether the ratio was lower (10 wt%) or transform, which was determined by plotting –ln (C/Co) versus irradiation time. The corresponding was 1 wt%, Ag doped on the CZTS greatly enhanced the photocatalytic performance. Figure 8b k valuesshows for the 1, 2,linear 4, 8 wt% Ag, the 1 wt% and the 10:1 2:1 versus ratios of the the CZTS, first order transform, which was Ag/CZTS, determined by plotting –ln and (C/Co) 1T-2H MoS samples were be4,0.0025, 0.0039, 0.0024, 0.0019, irradiation time. Thepowders corresponding k values forestimated the CZTS, to 1, 2, 8 wt% Ag, the 10.0033, wt% Ag/CZTS, 2 microflower 1 , respectively. and 0.0213 the 10:1min and−2:1 ratios of the 1T-2H 2 microflower powders estimated be 0.016, and TheMoS photodegradation rate samples constantwere of the three to component −1, respectively. The photodegradation 0.0025, 0.0039, 0.0033, 0.0024, 0.0019, 0.016, and 0.0213 min Ag/CZTS/1T-2H MoS2 sample was faster than that for the CZTS and Ag/CZTS samples. A schematic rate constant of the three component Ag/CZTS/1T-2H MoS2 sample was faster than that for the diagram of the proposed mechanism is provided in Figure 9. The results were attributed to the CZTS and Ag/CZTS samples. A schematic diagram of the proposed mechanism is provided in combined effects ofresults enhanced visible light absorption by Agofand the efficient charge separation due Figure 9. The were attributed to the combined effects enhanced visible light absorption by to the MoS in the photocatalyst system. Due resonance, electrons below the Ag 2and the efficient charge separation due to to the the surface MoS2 in plasmon the photocatalyst system. Due to the surface resonance, electrons below Fermi level of the Ag nanoparticles excited togenerated their Fermi level of plasmon the Ag nanoparticles excited to the their plasmonic state. Then, the electrons in plasmonic state.band Then,jumped the electrons generated the CZTSstate conduction band jumped to the surface the CZTS conduction to the surfaceinplasmon of the Ag nanoparticles, thus reducing plasmon staterate. of the nanoparticles, thus in reducing the recombination rate. The electrons reacted the recombination TheAg electrons generated and transferred to the Ag nanoparticles generated in and transferred to the Ag nanoparticles reacted to O2 gas to form superoxide radicals • − to O2 gas• to− form superoxide radicals ( O2 ). These active radicals reacted with the methylene blue ( O2 ). These active radicals reacted with the methylene blue solution to degrade it into H2O and solutionCO to2.degrade it into H2 O COgenerated hand,reacted the holes in H the system reacted 2 . On the On the other hand, theand holes inother the system withgenerated the adsorbed 2O to form •OH). with thehydroxyl adsorbed H2 O (to form hydroxyl radicalsalso (• OH). These radicalsblue alsosolution reactedtowith the radicals These active radicals reacted withactive the methylene degrade into H2O to anddegrade CO2. methylene blueitsolution it into H2 O and CO2 .

Figure 8. Photocatalytic activity under solar irradiation through the hybridized various weight percentages of AgNO3 precursor: 1, 2, 4, and 8 wt% Ag/CZTS and the ratios of 10:1 and 2:1 of 1 wt% Ag/CZTS and 1T-2H MoS2 ; (a) the degradation of MB; and (b) the photocatalytic degradation kinetics evolution.

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Figure 8. Photocatalytic activity under solar irradiation through the hybridized various weight percentages of AgNO3 precursor: 1, 2, 4, and 8 wt% Ag/CZTS and the ratios of 10:1 and 2:1 of 1 wt% Ag/CZTS and 1T-2H MoS2; (a) the degradation of MB; and (b) the photocatalytic degradation Materials 2018, 11, 158 10 of 13 kinetics evolution.

Figure 9. Schematic diagram for proposed photodegradation mechanism of methylene blue solution Figure 9. Schematic diagram for proposed photodegradation mechanism of methylene blue solution in in the presence of the Ag/CZTS hybridized 1T-2H MoS2 photocatalyst. the presence of the Ag/CZTS hybridized 1T-2H MoS2 photocatalyst.

3. Materials and Characterization 3. Materials and Characterization 3.1. Materials Materials 3.1. All chemicals chemicals were were used used as as received receivedwithout withoutfurther furtherpurification. purification. Analytical Analytical grade grade materials materials All included copper (II) chloride anhydrous (CuCl 2, Choneye pure chemicals Co., Ltd., Taipei City, included copper (II) chloride anhydrous (CuCl2 , Choneye pure chemicals Co., Ltd., Taipei City, Taiwan,98%), 98%),zinc zincchloride chloride(ZnCl (ZnCl2,, Merck, Merck, Kenilworth, Kenilworth, NJ, NJ, USA, USA, 98%), 98%), stannous stannous chloride chloride dehydrate dehydrate Taiwan, 2 (SnCl 2∙2H2O, Shimakyu, Osaka, Japan, 98%), thiourea (NH2CSNH2, Riedel-de Haën, Mexico City, (SnCl2 ·2H2 O, Shimakyu, Osaka, Japan, 98%), thiourea (NH2 CSNH2 , Riedel-de Haën, Mexico City, Mexico, 99%), 99%),diethylenediamine diethylenediamine(C (C4H H13N3, Panerac quimica sa, Barcelona, Spain, 98%), silver nitrate Mexico, 4 13 N3 , Panerac quimica sa, Barcelona, Spain, 98%), silver nitrate (AgNO3,, Aencore, Aencore, Surrey Surrey Hills, Hills, Australia, Australia, 99.8%), 99.8%), sodium sodium molybdate molybdate dihydrate dihydrate (NaMoO (NaMoO4··2H 2O, J.T. (AgNO 3 4 2H2 O, J.T. Baker, Center Valley, PA, USA, 99.5%), and oxalic acid (Riedel-de Haën, 99.5%), and the deionized Baker, Center Valley, PA, USA, 99.5%), and oxalic acid (Riedel-de Haën, 99.5%), and the deionized water (DI (DI water) water)used usedwas wasfrom fromEMD EMDMillipore MilliporeCorporation. Corporation. water 3.2. CZTS CZTS Convental Convental Hydrothermal Hydrothermal Process Process 3.2. Deionized water water was was used used as as the the solvent. solvent. In In aa typical typical aqueous aqueous synthesis, synthesis, 30 30 mmol mmol of of thiourea thiourea Deionized was dissolved in 20 mL of deionized water in a 50 mL Teflon-lined autoclave and stirred for 15 min. min. was dissolved in 20 mL of deionized water in a 50 mL Teflon-lined autoclave and stirred for 15 2.5 mmol mmol of of ZnCl ZnCl22 and and SnCl SnCl22·2H in 55 mL mL of of deionized deionized water, water, and and 55 mmol mmolof ofCuCl CuCl22 2.5 ·2H22O O were were dissolved dissolved in (anhydrous) was used in 10 mL of deionized water. All of the precursors were stirred for 10 min (anhydrous) was used in 10 mL of deionized water. All of the precursors were stirred for 10 min individually and were then poured into the 50 mL Teflon-lined stainless-steel autoclave, with the individually and were then poured into the 50 mL Teflon-lined stainless-steel autoclave, with the addition 0.543 0.543 mL mL diethylenediamine diethylenediamine (DETA) (DETA) as asthe thechelating chelatingagent. agent.The Theautoclave autoclavewas was sealed sealed and and addition ◦ maintained at 180 °C for 72 h and then allowed to cool down to room temperature naturally. After maintained at 180 C for 72 h and then allowed to cool down to room temperature naturally. After hydrothermal synthesis, synthesis, the the solution solution was was washed washed and and centrifuged centrifuged with with dimethylformamide dimethylformamide and and hydrothermal deionized water waterthree three times to remove the byproducts and DETA. Black particles then deionized times to remove the byproducts and DETA. Black particles were thenwere obtained ◦ obtained by drying in an oven at 80 °C for 8 h. by drying in an oven at 80 C for 8 h. 3.3. Ag/CZTS Ag/CZTS Microwave-Assisted Microwave-Assisted Hydrothermal Hydrothermal Process Process 3.3. The obtained obtained CZTS CZTS powers powers were were charged charged 0.75 0.75 gg then then various variousamounts amountsof ofAgNO AgNO33 solution were were The added (1 (1 wt%, wt%, 22 wt%, wt%, 44 wt%, wt%, and and88wt% wt%in in20 20mL mLdeionized deionizedwater). water). The The solution solution was was then then loaded loaded added into a Teflon-lined autoclave at 200 ◦ C for 1 h in a microwave-assisted system for the hydrothermal process. Finally, the products were washed three times with ethanol and DI water, centrifuged, and dried overnight in a vacuum oven at 60 ◦ C to obtain the 1, 2, 4, and 8 wt% Ag/CZTS powders.

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3.4. MoS2 Conventional Hydrothermal Process MoS2 synthesized from 5 mmol NaMoO4 ·2H2 O and 5 mmol oxalic acid were dissolved in 20 mL DI water and then stirred for 20 min. Then, 20 mmol thiourea was added and dissolved in 10 mL DI water separately. The solutions were then loaded into a 50 mL Teflon-lined autoclave for the hydrothermal process at 200 ◦ C for 24 h. The MoS2 powders were then collected and washed three times with ethanol and DI water. They were then dried at 60 ◦ C overnight in a vacuum oven. 3.5. Ag/CZTS/MoS2 Hybridized Process Finally, three components of the Ag/CZTS/MoS2 composite were mixed in 20 mL of ethanol with the varying weight ratios of 1 wt% Ag/CZTS and MoS2 powder 10:1 and 2:1 for 1 h. The products were washed three times with DI water and ethanol and then dried at 60 ◦ C overnight in a vacuum oven. 3.6. Characterization The crystalline structure of the powders was examined using X-ray diffraction (XRD Bruker D8 advance, Billerica, MA, USA). The morphology was determined using field emission scanning electron microscopy (FESEM, JEOL JSM-6701F, Tokyo, Japan). The microstructure was investigated using transmission electron microscopy (TEM, JEOL JEM-2100F, Tokyo, Japan). X-ray photoelectron spectroscopy (XPS, ULVAC-PHI, Inc. PHI 5000 VersaProbe, Chigasaki-shi, Japan) spectra were obtained using a 1486.6 eV Al anode X-ray source. XPS peak 4.1 software was used for the peak fitting, where the resulting peak components were pure Gaussian. The surface area was measured according to Brunauer-Emmet-Teller (BET, Micromeritics Instrument Corp. Tristar II, Norcross, GA, USA). The optical properties of particle suspensions were determined using a UV-VIS absorption spectrum (PerkinElmer Lambda 7500, Waltham, MA, USA). The PL spectra were obtained using a LS-55 fluorescence spectrometer (PerkinElmer, Waltham, MA, USA). 3.7. Photocatalytic Behavior Methylene blue (MB), which is widely utilized in organic dye textile industries, is typically used in photocatalytic dye degradation studies. All the samples were carried out using 20 mg of the photocatalysts in 60 mL of 20 ppm methylene blue solution. The solution was stirred in the dark for 30 min to achieve the adsorption-desorption equilibrium. Furthermore, stirring was continued under a 300 W halogen lamp intended to simulate sunlight with ~5% UV light. Then, ~4 mL of the charged solution was collected every 30 min and centrifuged for 15 min to remove the photocatalyst completely. The concentration of MB solution was measured with the absorbance at 664 nm with the UV–VIS spectrophotometer (PerkinElmer Lambda 7500, Waltham, MA, USA). 4. Conclusions In this paper, we discuss the use of a conventional hydrothermal method for the synthesis of Cu2 ZnSnS4 powders well indexed into a high-purity KS phase. Furthermore, hybridized Ag/CZTS was successfully synthesized through a microwave-assisted hydrothermal process using water as the solvent for such processes as ultrasound and microwave irradiation to facilitate chemical reactions. It was found that the photocatalytic performance of the CZTS powders was greatly improved by the addition 1 and 2 wt% of Ag NPs deposited on the surface of the CZTS acting as the electron traps of the matrix. This prevented the recombination of electron-hole pairs and improved the charge transfer processes by hybridizing the 1T-2H MoS2 in the structure, thus increasing the light harvesting in photocatalysis due to the broad light absorption range. This technique was attributed to the strong enhancement of visible light absorption and efficient charge separation, leading to reduced recombination. A maximum of 91.1% MB degradation occurred in under 2 h, where the photocatalyst containing a 2:1 ratio of 1 wt% Ag/CZTS and 1T-2H MoS2 exhibited the highest photodegradation.

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Acknowledgments: The authors thank the Ministry of Science and Technology of Taiwan for financial support under grant no. NSC 106-2221-E-006-170-MY3. Author Contributions: All of the authors contributed to this work, discussed the results and implications, and commented on the manuscript. Jyh-Ming Ting conceived the strategies and designed the experiments. Shih-Jen Lin performed the experiments and wrote the paper. Kuo-Chin Hsu assisted with the analytical tools. Yaw-Shyan Fu reviewed and contributed to the final manuscript. Conflicts of Interest: The authors declare no conflict of interest.

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