S-Coupled ZnO@ZnS CoreâShell Nanorods Decorated - American

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Efficient H2 production using Ag2S-coupled ZnO@ZnS core-shell nanorods decorated metal wire mesh as an immobilized hierarchical photocatalyst Mu-Hsiang Hsu, Chi-Jung Chang, and Hau-Ting Weng ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.5b01387 • Publication Date (Web): 14 Jan 2016 Downloaded from http://pubs.acs.org on January 17, 2016

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ACS Sustainable Chemistry & Engineering

Efficient H2 production using Ag2S-coupled ZnO@ZnS core-shell nanorods decorated metal wire mesh as an immobilized hierarchical photocatalyst Mu-Hsiang Hsu, Chi-Jung Chang*, Hau-Ting Weng Department of Chemical Engineering, Feng Chia University, 100, Wenhwa Road, Seatwen, Taichung 40724, Taiwan, ROC

*Corresponding Author: Chi-Jung Chang E-mail: [email protected] Tel: 886-4-24517250 ext 3678 Fax: 886-4-24510890 Abstract Ag2S-coupled ZnO@ZnS core-shell nanorods heterostructures were grown on stainless-steel wire mesh substrates along the c-axis by sulfidation of aligned ZnO nanorod arrays as immobilized hierarchical photocatalysts (Ag2S-coupled ZnO@ZnS/metal wire mesh) for photocatalytic H2 production. The effects of the sulfidation time and AgNO3 precursor concentration on the morphology, crystalline properties, optical property, photocurrent response, and photocatalytic H2 production activity of the photocatalysts under UV or visible light irradiation, together with the stability of photocatalytic activity for recycled photocatalysts were investigated. In comparison with ZnO/metal wire mesh under the same conditions, the ZnO@ZnS/metal wire mesh photocatalysts showed excellent photocatalytic H2 production activity which can be attributed to the formation of Ag2S-coupled ZnO–ZnS heterojunctions on conductive metal wire mesh substrate which favors the absorption of light, separation of photogenerated electron–hole pairs, and contact with the reactant solution. The H2 production rates for Ag2S-coupled ZnO@ZnS/metal wire mesh photocatalysts reached 5870 µmolg−1h−1 and 168 µmolg−1h−1 under UV and visible light irradiation.

KEYWORDS: Hierarchical, ZnO@ZnS core-shell, stainless-steel wire mesh, photocatalytic hydrogen production, visible-light.

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Introduction Formation of hydrogen and oxygen by water splitting using heterogeneous photocatalysts under UV or visible light irradiation is a promising process to achieve recyclable and clean hydrogen production1. Zinc oxide nanomaterials, with a wide band gap (3.37 eV), is applicable for a lot of applications such as photocatalyst 2-6, photodetector 7, gas sensor 8,9, oil capture 10, and ultraviolet laser 11. ZnS photocatalyst with a band gap (3.6 eV) wider than ZnO can facilitate the fast formation of photogenerated electron-hole pairs. ZnS is used for the photocatalytic degradation of organic molecules and photocatalytic H2 production 12-14. Decorating nanomaterials on the photocatalyst surface is an efficient route to form heterostructured photocatalysts with improved activity 15-18. a lot of materials combinations have been studied for the enhanced electron-hole separation of coupled semiconductor photocatalysts by the formation of interface contact potentials.19,20 Based on the theoretical calculations and experimental results, it is reported that the combination of CdS and ZnS can produce a solid solution photocatalyst which has the photoexcitation threshold energy lower than that of individual CdS and ZnS 21. The recombination of photoexcited electron-hole pairs of ZnO based photocatalysts can be suppressed by the formation of semiconductor/ZnO heterojunctions. 22-24 Some ZnO-based coupled heterostructures, including ZnO@ZnS core-shell nanorods 25,26, ZnO microrods/Cu2O nanocrystals 27, and Ag nanoparticle/ZnO nanorods 28, have been studied for photocatalytic hydrogen production. For photocatalyst nanoparticles, separation and recovery of the nanoparticles from the reaction medium are necessary for repeated photocatalytic operations. The separation process can be omitted for immobilized photocatalysts which are prepared by forming nanostructured photocatalysts on supporting substrates. The immobilized photocatalysts can be reused again after washing. Indium tin oxide (ITO) 29, copper plates 30, and polyethylene fibers 31 have been used as supports for the preparation of ZnO based immobilized photocatalysts. In addition, electrical conductive supports, such as indium tin oxide (ITO), copper plates, and stainless-steel wire mesh substrates 32 , can also help the separation of photogenerated charge in the photocatalysis process and enhance the photocatalytic activity. Wire mesh supported materials have been used as catalysts for the preferential oxidation 33, decomposition of CH3OH 34 and N2O 35. ZnO nanoparticles can be synthesized on stainless steel wire mesh 36. Wire mesh supports can be easily wetted by the reactants solution. Therefore, wire mesh based photocatalyst favor the effective contact between reactants and the photocatalysts. In this study, three-dimensional ZnO@ZnS–Ag2S core-shell nanorods heterostructures with various Ag2S/ZnS ratios were grown on stainless-steel wire 2


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mesh substrates and acted as immobilized hierarchical photocatalysts for photocatalytic H2 production. The motivation of this study is to investigate the possibility of combining the above-mentioned methods. Using a conductive stainless steel wire-mesh support for the photocatalysts maintains good wetting by the reactant solution and high surface area, enhancing the transport and adsorption of reactants. The conductive supports also favor the separation of photogenerated charges. The Ag2S coupled ZnO@ZnS core-shell nanorods heterostructures can enhance the separation of photo-induced electron–hole pairs, leading to high photocatalytic efficiency. The surface morphology, surface wetting, optical property, photocurrent, and photocatalytic H2 production activity of these hierarchical heterostructured photocatalysts under UV or visible light irradiation were studied.

Experimental Materials Zinc acetate dihydrate (Zn(CH3COO)2 ·2H2O) (J.T.Baker), zinc nitrate hexahydrate (Zn(NO3)2 · 6H2O) (J.T.Baker), hexamethylenetetramine (C6H12N4) (Riedel-de Haen) were used as received. Anhydrous sodium sulfide (Na2S) was supplied by Sigma-Aldrich. Sodium sulfite (Wako) was used as received. Sodium chloride (NaCl) was supplied by Uniregion Bio-tech. Silver nitrate (AgNO3) was provided by Showa. ZnO@ZnS core-shell nanorods decorated stainless steel wire mesh photocatalyst At first, 0.1 mol zinc acetate was dissolved in 100 ml of ethanol to prepare the seed solution. The seed solution was coated on the stainless steel wire mesh substrate by dip coating. The substrate was dried at room temperature and then annealed at 400 °C for 2 h to create a ZnO seed layer on the stainless-steel wire surface. ZnO nanorod arrays were grown on the stainless steel wire mesh substrate by a hydrothermal method. The modified substrates were immersed in the aqueous solution containing 40 ml of 0.1M zinc nitrate and 40 ml of 0.1M of hexamethylenetetramine at 95 oC for 6h. The substrate was washed with distilled water and then dried at room temperature. Conversion of the aligned ZnO nanorod films to ZnO-ZnS core-shell nanorods was carried out by immersing the ZnO nanorods decorated stainless steel wire mesh substrate in 30 mM Na2S solution at 60 oC water bath with stirring for different sulfidation time (2, 4 and 6 h) to create a thin ZnS shell around the ZnO nanorods with different shell thickness. The substrate was washed with distilled water and then dried at room temperature. ZnO@ZnS core-shell nanorods decorated stainless steel wire mesh was fabricated as a hierarchical photocatalyst. 3



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Ag2S coupled ZnO@ZnS core-shell nanorods decorated stainless steel wire mesh photocatalyst For the fabrication of Ag2S coupled ZnO@ZnS core-shell nanorods samples, 0.58g sodium sulfide nonahydrate was dissolved in 80 mL water. The AgNO3 solution then added into the sodium sulfide solution drop by drop. The AgNO3 precursor concentration =1, or 2, or 3M. For the growth of Ag2S coupled ZnO@ZnS core-shell nanorods, ZnO nanorods decorated stainless steel wire mesh sample was immersed in the solution and reacted at 60 oC for 4 h. The final products were washed by deionized water, and then dried at room temperature. The samples prepared by different AgNO3 precursor concentration were observed by FESEM and HRTEM to check whether Ag2S formed outside the ZnS shell after the shell formation process. Photocatalytic activity test The photocatalytic hydrogen production were conducted on the Ag2S-coupled ZnO@ZnS core-shell nanorods decorated metal wire mesh catalysts (2 cm x 4 cm) in a gas-closed Pyrex cell system illuminated by UV light or visible light. The temperature for the photocatalytic hydrogen production reactions remained at 30 ± 3 o C. The photocatalyst was immersed in a 100 mL aqueous solution consisting of 0.1 M Na2S, 0.04 M Na2SO3 and 3 M NaCl. The pH value of the aqueous solution is about 13. The amount of produced H2 gas was measured by gas chromatography which was equipped with a thermal conductivity detector (molecular sieve 5-A column and Ar carrier). Nomenclature The photocatalysts are denoted as MwZSTxNy. Mw means the distance between edges of adjacent wires is w µm. Z indicates the growth of ZnO nanorods is 6 h. STx represents that the sulfidation time for the growth of ZnO@ZnS nanorods is x h (x= 2, or 4, or 6). Ny indicates the AgNO3 precursor concentration is y mM (y=1, or 2, or 3). When the stainless steel wire mesh substrates were replaced by glass substrate, Mw was replaced by G. Characterization The surface morphologies and microstructures were analyzed by a field-emission scanning electron microscope (FE-SEM, HITACHI S-4800) and transmission electron microscopy (TEM, JEOLJEM-2010). The structures of the samples were investigated via an X-ray diffractometer (XRD, MACSCIENCE MXP3). The PEC measurements were carried out in a glass cell which can facilitate the transmittance of light onto the 4


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surface of photoelectrode when irradiated by a mercury lamp. The surface area of the working electrode is 2.25 cm2. Pt electrode and Ag/AgCl electrode acted as the counter and reference electrodes, respectively. The electrolyte solution was 0.2 M NaOH. For the FESEM, XRD, and reflection spectra of ZnO@ZnS core-shell nanorods decorated stainless steel wire mesh samples, measurements were directly carried out on the samples without removing the stainless steel wire mesh substrates. For the TEM measurement, ZnO@ZnS core-shell nanorods were removed and were dispersed into ethanol via ultrasound. Then, the samples were collected with carbon copper grids. For the DRS spectra and XPS, the stainless steel wire mesh substrates were replaced by glass substrate. XPS was performed with a VGESCA scientific theta probe spectrometer in constant analyzer energy mode.

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Results and discussion Morphology and chemical compositions

Figure 1. FESEM images of (a) ZnO nanorods decorated stainless steel wire mesh, and ZnO@ZnS core-shell nanorods decorated stainless steel wire mesh with various sulfidation times (b) M60ZST2 (c) M60ZST4 (d) M60ZST6. The FESEM images and EDX spectra of ZnO nanorods and ZnO@ZnS core-shell nanorods decorated stainless steel wire mesh with various sulfidation time were shown in Fig.1 and Fig.2 to investigate the morphology and chemical compositions of the photocatalysts. Figure 1(a) shows the FESEM image of ZnO nanorods decorated stainless steel wire mesh. The stainless steel wire mesh is covered with ZnO nanorods. The nanorods have smooth surfaces. The diameter of the nanorod for M60Z photocatalyst is about 100-125 nm. Figure 1(b), 1(c), and 1(d) present the FESEM images of ZnO@ZnS core-shell nanorods decorated stainless steel wire mesh photocatalysts with various sulfidation times. After being converted from ZnO nanorods to ZnO-ZnS core-shell nanorods via surface sulfidation, the diameters of the nanorods for M60ZST2, M60ZST4, and M60ZST6 photocatalysts are about 110-150, 150-200, and 150-200 nm, respectively. As the sulfidation time increased from 2h to 6h, the surface of the ZnO@ZnS core-shell nanorods became rougher and the diameters of the nanorods turned larger. Ghrib et al. 37 proposed a growth mechanism 6


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of ZnS shell on ZnO nanowires. At first, Na2S salt decomposes in aqueous solution with Na+ and S2− ions (equation 1). Then, a thin layer of ZnO nanorod reacts with hydroxide ions (OH−) in the aqueous solution (equation 2). The ZnS forms by the reaction between the unstable ZnO22− ions react with S2− ions (equation 3). The ZnS nanoparticles were formed on the ZnO nanorods to fabricate ZnO@ZnS core-shell nanorods. Na2S → 2Na+ + S2− ZnO + 2OH 2−

−

2−

(1) 2−

→ ZnO2 + H2O

ZnO2 + S + 2H2O → ZnS + 4OH

(2) −

(3)

Figure 2. EDX spectra of ZnO@ZnS core-shell nanorods of (a) M60ZST2 (b) M60ZST4 (c) M60ZST6 with various sulfidation times. 7



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Figure 2 shows the EDX spectra of ZnO@ZnS core-shell nanorods decorated stainless steel wire mesh photocatalysts, M60T6IM30T2, M60T6IM30T4, and M60T6IM30T6. The sulfidation time was 2, 4 and 6 h, respectively. The Na2S precursor concentration for the samples was 30 mM. The EDX spectra showed that the S content of ZnO@ZnS core-shell nanorods based photocatalysts increased with increasing sulfidation time. Atomic S contents of M60T6IM30T2, M60T6IM30T4, and M60T6IM30T6 photocatalysts are 5.5 %, 7.1 %, and 8.5 %, respectively. It reveals that ZnS shell layers were covered on the ZnO nanorods. X-ray diffraction patterns The XRD spectra of ZnO nanorods and ZnO@ZnS core-shell nanorods prepared by different sulfidation times were shown in Figure 3. The diffraction peaks of ZnO nanorods (Fig.3a) match well with hexagonal ZnO (JCPDS36-1451). No impurity peak is observed. Figure 3b, 3c, and 3d present the XRD patterns of ZnO@ZnS core-shell nanorods decorated stainless steel wire mesh photocatalysts M60ZST2, M60ZST4, and M60ZST6 with sulfidation times increasing from 2h to 6h. The patterns of the ZnO-ZnS core-shell nanorods consist of diffraction peaks of ZnS and ZnO. The peak at 2θ = 28.62o can be assigned to the face-centered-cubic ZnS (JCPDS65-0309). During the sulfidation process, sulfur diffused into the ZnO lattices to occupy the oxygen vacancies. The ratio of the ZnS/ZnO peak intensity increased gradually when the sulfidation time was extended to 6 h, indicating that more ZnO was transformed into ZnS with increasing sulfidation time. Since the peaks of (220) and (311) crystal plane of ZnS are close to the peaks of (102) and (101) crystal plane of ZnO38, these peaks may overlap. All diffraction patterns of photocatalysts exhibit sharp peaks, indicating that all samples consisting of ZnO nanorods decorated wire mesh, or ZnO@ZnS core-shell nanorods decorated wire mesh were highly crystalline.

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Figure 3. XRD spectra of (a) ZnO nanorods decorated wire mesh, and ZnO@ZnS core-shell nanorods decorated wire mesh with different sulfidation times (b) M60ZST2 (c) M60ZST4 (d) M60ZST6. (*: peaks related to stainless steel) TEM The HRTEM images of ZnO nanorods and ZnO@ZnS core-shell nanorods decorated stainless steel wire mesh with various sulfidation times were shown in Fig 4 to investigate the crystal structures of the photocatalysts. Figure 4a presents the TEM image of a ZnO nanorod of the M60Z sample with a diameter of 109 nm. The nanorod has a smooth surface. Figure 4e shows the HRTEM image of M60Z. There are crystal lattice fringes. The average distance between the adjacent lattice planes is 0.260 nm, matching well with the (002) plane lattice distance of hexagonal ZnO. Fig. 4b, 4c, and 4d present TEM images of ZnO@ZnS nanorods on M60ZST2, M60ZST4, and M60ZST6 photocatalysts. There is a clear interface between the outer ZnS shell and the inner ZnO core. The ZnO rod is covered by a layer of ZnS shell. In comparison with pristine ZnO nanorods, the surface of the ZnO@ZnS nanorods became rougher. In addition, the thickness of the ZnS shell became thicker when the sulfidation time increased from 2h to 6h. Fig. 4f, 4g, and 4h present HRTEM images of ZnO@ZnS nanorods on M60ZST2, M60ZST4, and M60ZST6 photocatalysts. The shell thicknesses of M60ZST2, M60ZST4, and M60ZST6 photocatalysts are 8, 10, and 15 nm, respectively. The HRTEM images revealed that the ZnS shell consisted of many ZnS nanoparticles. Shen et al. 39 － proposed a formation mechanism of ZnO@ZnS core-shell nanorods. At first, S2 released from the decomposition of thioacetamide (TAA) reacted with the Zn2

＋

which was slowly dissolved from the surface of ZnO nanorods to generate ZnS nanoparticles around the ZnO nanorods. A nonuniform ZnS nanoparticles shell was formed around the ZnO nanorods. As the sulfidation time increased, ZnO@ZnS core-shell nanorods formed as more and more ZnS nanoparticles were produced and piled up to form a ZnS shell. For heterostructured photocatalysts, intimate contacting between different semiconductor materials plays an important role in the hydrogen production activity of photocatalytic water splitting process. Such an effective contact can promote the transfer of charge carrier between different semiconductor materials in the heterostructured photocatalysts. TEM results revealed that the sulfidation process starts from the surface of the ZnO nanorods, and then increase inward gradually, retaining the morphology and following the crystal orientation of the ZnO core. It is consistent with the results illustrated in XRD spectra. There is close contact between ZnO core and ZnS shell.

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Figure 4. The TEM images of (a) ZnO nanorods decorated stainless steel wire mesh (M60Z), and ZnO@ZnS core-shell nanorods decorated stainless steel wire mesh with various sulfidation times (b) M60ZST2 (c) M60ZST4 (d) M60ZST6, and the HRTEM images of (e) M60Z (f) M60ZST2 (g) M60ZST4 (h) M60ZST6.

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Morphology and compositions of Ag2S-coupled photocatalyst

Figure 5. FESEM images (a) M60ZST4N1 (b) M60ZST4N2 (c) M60ZST4N3 and EDX spectra (d) M60ZST4N1 (e) M60ZST4N2 (f) M60ZST4N3 of Ag2S-coupled ZnO@ZnS nanorods grown on mesh photocatalysts prepared with different concentrations of silver nitrate precursor, and HRTEM images of (g) Ag2S-coupled ZnO@ZnS nanorods (h) Ag2S on ZnS shell of M60ZST4N2 photocatalyst. 11



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Ag2S-coupled ZnO@ZnS nanorods on wire mesh hierarchical photo-catalysts M60ZST4N1, M60ZST4N2, and M60ZST4N3 were prepared with 1, 2, and 3 mM AgNO3 precursor, respectively. Fig. 5a and 5b show the hexagonal ZnO@ZnS nanorods on the M60ZST4N1 and M60ZST4N2 photocatalysts are uniformly distributed. The average rod diameters ranged between 310 nm and 360 nm. When the AgNO3 precursor concentration increased to 3 mM, a lot of aggregate particles formed around the nanorods of M60ZST4N3 (Fig. 5c), which are confirmed (by XPS analysis, Fig.8) to be the aggregation of Ag2S on the shell. EDX spectra of M60ZST4N1, M60ZST4N2, and M60ZST4N3 photocatalysts were shown in Fig. 5d, 5e, and 5f, respectively. M60ZST4N2 has slightly higher Ag content than M60ZST4N1. There are some Ag2S particles located on the nanorods of M60ZST4N3 photocatalyst. Most of the Ag2S particles locate on the top of ZnO@ZnS nanorod arrays. As the AgNO3 precursor concentration increased from 2 mM to 3 mM, the Ag content increased sharply from 1.07% to 17.73%. A lot of Ag2S aggregate particles formed around the nanorods of M60ZST4N3 photocatalyst when the AgNO3 precursor concentration increased to 3 mM. Figure 5(g) shows the HRTEM image of Ag2S-coupled ZnO@ZnS nanorods of the M60ZST4N2 photocatalyst to demonstrate the distribution of small Ag2S particles on the shell of the ZnO@ZnS nanorod. Since most Ag2S particles locate on the top of ZnO@ZnS nanorod arrays, there are only some small particles around the ZnO@ZnS nanorod (Fig. 5g). Figure 5(h) shows HRTEM image of Ag2S particles on ZnS shell of the M60ZST4N2 photocatalyst, illustrating the lattice planes in these nanocrystals. The interplanar spacing of the Ag2S particles is approximately 0.25 nm, which corresponded to the planes of monoclinic Ag2S40. There is also an interplanar distance of 0.3 nm, which can be attributed to the (111) plane of cubic ZnS shell41. It confirms the loading of Ag2S particles on ZnS shell of the M60ZST4N2 photocatalyst. Ag2S nanoparticles of diameters ranging from 10 to 25 nm are loaded on the surface the ZnS shell of M60ZST4N2 photocatalysts.

Wettability of photocatalysts Figure 6(a) exhibits the image of a water droplet placed on the pristine stainless steel wire mesh film (M60). The water contact angle is about 109.4o ± 2.2. The pristine stainless steel mesh surface is hydrophobic. It prevents the aqueous reactant solution (containing Na2S, Na2SO3, and NaCl) from passing through the stainless steel wire mesh. When the ZnO nanorods were grown on the stainless steel wire mesh surface, the surface of mesh film M60Z changed from hydrophobic to 12


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superhydrophilic (Figure 6b). In addition, after the formation of ZnO@ZnS core-shell nanorods on the stainless steel wires mesh, the mesh film M60ZST4 remains superhydrophilic (Figure 6c). After being coupled with Ag2S, the mesh film M60ZST4 is still superhydrophilic (Figure 6d). In this study, the photocatalyst was placed in an aqueous salt solution containing Na2S and Na2SO3 as the hole scavengers. The superhydrophilic property of the M60ZST4N2 photocatalyst surface can help the flow of aqueous salt solution through the wire mesh photocatalysts. It can increase the colliding frequency between the photocatalyst and the active compositions (Na2S, Na2SO3, NaCl), which may help to enhance the photocatalytic H2 production activity of the photocatalyst.

Figure 6. The contact angle of one water drop on (a) pristine M60 stainless steel wire mesh (b) M60Z mesh film with ZnO nanorods, and (c) M60ZST4 mesh film with ZnO@ZnS nanorods (d) M60ZST4N2 mesh film with Ag2S-coupled ZnO@ZnS nanorods. Optical property DRS spectra Figure 7(a) shows diffuse reflection spectra of samples which are glass substrates decorated with ZnO nanorods (GZ), ZnO@ZnS core-shell nanorods (GZST4), and Ag2S-coupled ZnO@ZnS core-shell nanorods (GZST4N2). For the GZ sample, the spectrum locates only in the UV range. When ZnS shells were grown on the surface of ZnO nanorods as shell layers (GZST4), visible light absorption band tail are observed in addition to the band gap absorption band of the ZnO nanorod core. In comparison with GZST4, there is a red shift of the band edge for the DRS spectrum of GZST4N2 sample which the ZnS shell layer was coupled by Ag2S. The GZST4N2 sample showed enhanced UV and visible light absorption properties than that of GZST4. The increased absorption of Ag2S-coupled ZnO@ZnS photocatalyst 13



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GZST4N2 in the visible region is due to the Ag2S coupling effect which is similar to the absorption spectra ofAg2S reported by Motte et al. 42 Reflectance spectra Reflectance spectra of pristine stainless steel wire mesh and M60ZST4N2 photocatalyst were measured to study the effect of surface morphology on the light-trapping properties (Fig. 7b). It was reported that light absorption in solar cells or photocatalyst can be enhanced drastically by light trapping effect of the device with － properly patterned surface textures through of 43 44. In this study, compared with the pristine stainless steel wire mesh M60, the ZnO@ZnS core-shell nanorods decorated stainless steel wire mesh photocatalysts demonstrated lower reflection in the wavelength range from 300 to 800 nm because of light scattering. It confirmed that an enhanced light trapping effects was achieved by the diffraction of the light among the ZnO@ZnS core-shell nanorods on wire mesh surface of the M60ZST4N2 photocatalysts. The decrease in reflectance results from the multiple scattering within the interstices among the core-shell nanorods. Such an interface scattering extends the optical path length. The multiple interface scattering can lead to improved light absorption efficiency and enhanced photocatalytic hydrogen production activity. Both the light trapping phenomena and surface hydrophilicity of the wire mesh photocatalyst may contribute to the high photocatalytic activity.

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Figure 7. (a) Diffuse reflectance spectra (DRS) of GZ, GZST4, GZST4N2 samples grown on glass substrates, (b) reflectance spectra of pristine stainless steel wire mesh M60 and M60ZST4N2 photocatalyst. XPS spectra The Ag3d, S2p and O1s binding energy of the photocatalyst was investigated by X-ray photoelectron spectroscopy (XPS) to investigate the surface composition and chemical states of the ZnO nanorods, ZnO@ZnS nanorods, and Ag2S-coupled ZnO@ZnS nanorods, as shown in Figure 8. Figure 8a showed the XPS O1s peak of ZnO nanorods. The presence of lattice oxygen located at 530.1 eV indicated the formation of ZnO. Another peak located at 531.7 eV can be assigned to the － chemisorbed oxygen of surface hydroxyl groups 45 47. XPS S2p and O1s peaks of ZnO@ZnS core-shell nanorods are plotted in Fig. 8b and 8c, respectively. The peak at 160.8 eV is assigned to S 2p3/2 (Fig. 8b), indicating that the shell layer consists of ZnS. 48

In comparison with pure ZnO nanorods, O1s peak of ZnO@ZnS core-shell

nanorods (Fig. 8c) shifted to 531.1 eV. Meanwhile, a sharp decrease of the lattice oxygen peak at 530.1 eV was observed after the growth of ZnS shell layer. XPS S2p, O1s, and Ag3d peaks of Ag2S-coupled ZnO@ZnS core-shell nanorods are demonstrated in Fig.8d, 8e, and 8f, respectively. O1s peak appeared at 531.1 eV. The 15



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S 2p3/2 peak is found at 161.1 eV. The Ag 3d5/2 and Ag 3d3/2 peaks are observed at 366.8 and 372.8 eV, respectively, which are similar to the reported results for Ag2S nanoparticles

49

. The XPS analysis reveals the composition and chemical states of

Ag2S-coupled ZnO@ZnS core-shell nanorods. The XPS result reveals that the aggregates formed on the nanorods shown in FESEM images (Fig.5) were Ag2S nanoparticles.

Figure 8. XPS (a) O1s peak of ZnO nanorods, (b) S2p (c) O1s peaks of ZnO@ZnS core-shell nanorods grown on glass substrates, GZST4, and (d) S2p (e) O1s (f) Ag3d peaks of Ag2S-coupled ZnO@ZnS core-shell nanorods grown on glass substrates, GZST4N2.

16


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Photocatalytic H2 production Activity sulfidation time Core-shell nanorod arrays of ZnO@ZnS have been synthesized with various shell thickness by changing the sulfidation time. Their shell thickness dependent photocatalytic properties under UV light irradiation have been investigated. Figure 9 presents the photocatalytic H2 production of ZnO nanorods decorated stainless steel wire mesh M60Z, and ZnO@ZnS core-shell nanorods decorated stainless steel wire mesh with various sulfidation times. The photocatalytic H2 production of M60Z is 1500

µmolg−1h−1.

The

hydrogen

production

of

ZnO–ZnS

heterostructure

photocatalyst can be improved by coupling ZnO nanorod with ZnS shell. The photocatalytic H2 production of M60ZST2 becomes 3380 µmolg−1h−1. When the sulfidation time increased to 4h, the H2 production rate of M60ZST4 became 5310 µmolg−1h−1. Further increase in sulfidation time to 6h resulted in a decreased H2 production rate of 3620 µmolg−1h−1. The M60ZST4 photocatalyst exhibited the highest photocatalytic H2 production performance. Figure 9 shows that the photocatalytic H2 production activity of the ZnO@ZnS core-shell nanorods based photocatalysts increases to reach a maximum and then decreases with an increase in the sulfidation time. Fig. 4f, 4g, and 4h show the porous structures of ZnS shells which consist of some ZnS nanoparticles. Meanwhile, the thickness of the ZnS shell became thicker when the sulfidation time increased from 2h to 6h. Wang et al.

38

found that ZnO/ZnS-0.6 core/shell nanorods have higher surface areas and photo absorption than the other photocatalysts with different ZnS/ZnO molar ratios (ZnS thickness). They reported that ZnO/ZnS-0.6 nanorods exhibit a maximum H2 production of 2608.7 µmol h−1 g−1 under UV light irradiation. The ZnO/ZnS-0.6 nanorod sample has a proper deposition amount and ZnS shell thickness, which favor the separation and transport of photogenerated charge carriers. In this study, the M60ZST4 photocatalyst prepared with 4h sulfidation time has a maximum H2 production because that it has optimized ZnS shell thickness and surface area for the separation/transport of photogenerated charge and reduction of H+.

17



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Figure 9. The photocatalytic H2 production of (a) ZnO nanorods decorated stainless steel wire mesh M60Z, and ZnO@ZnS core-shell nanorods decorated stainless steel wire mesh with various sulfidation times (b) M60ZST2 (c) M60ZST4 (d) M60ZST6. AgNO3 precursor concentrations The influence of AgNO3 precursor concentrations on the photocatalytic hydrogen production was studied in the aqueous NaCl-Na2S-Na2SO3 solution. Figure 10 shows the hydrogen production of Ag2S-coupled ZnO@ZnS nanorods prepared with different silver nitrate concentration under UV light irradiation. The H2 production rate of M60ZST4 is 5310 µmolg−1h−1. The H2 production rate of Ag2S-coupled M60ZST4N1 photocatalyst became 5870 µmolg−1h−1 when 1 mM AgNO3 precursor was used. The ZnO@ZnS core-shell nanorods decorated stainless steel wire mesh photocatalysts exhibited a higher photocatalytic activity for H2 evolution from the Na2S, Na2SO3 and NaCl /water solutions than the ZnO nanorods decorated stainless steel wire mesh photocatalysts under the same conditions. When the AgNO3 precursor concentration increased to 2 mM, the H2 production rate became 6406 µmolg−1h−1. Further increase in AgNO3 concentration leads to a decreased H2 production rate of 4370 µmolg−1h−1. The M60ZST4N2 photocatalyst exhibited the highest photocatalytic H2 production. To demonstrate the importance of stainless-steel wire mesh substrates, Ag2S-coupled

ZnO@ZnS

core-shell

nanorods

were

grown

on

conductive

stainless-steel wire mesh substrates and nonconductive glass substrates to prepare the M60ZST4Ny and GZST4Ny photocatalysts, respectively. The photocatalytic H2 production rates of M60ZST4Ny and GZST4Ny photocatalysts are listed in Table 1. Ny indicates the AgNO3 precursor concentration is y mM (y=1, or 2, or 3). Comparing M60ZST4Ny and GZST4Ny photocatalysts with the same y value, M60ZST4Ny based photocatalysts were all higher than the GZST4Ny photocatalysts. The enhanced photocatalytic H2 production rates were due to the electrical conductive 18


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supports which can help the separation of photogenerated charge in the photocatalysis process.

Figure 10. The photocatalytic H2 production of (a) ZnO nanorods decorated stainless steel wire mesh M60Z, (b) M60ZST4, and Ag2S-coupled ZnO@ZnS core-shell nanorods decorated stainless steel wire mesh prepared with different AgNO3 precursor concentrations (c) M60ZST4N1 (d) M60ZST4N2 (e) M60ZST4N3. Table 1. Photocatalytic H2 production activity of Ag2S-coupled ZnO@ZnS core-shell nanorods grown on stainless steel wire mesh (M60ZST4Ny) or on glass plate (GZST4Ny) photocatalyst

M60ZST4N1

M60ZST4N2

M60ZST4N3

GZST4N1

GZST4N2

GZST4N3

5870

6406

4370

1322

3110

2374

Rate of H2 production −1 −1

(µmolg h )

Visible light driven photocatalytic activity Figure 11 shows the hydrogen production of Ag2S-coupled ZnO@ZnS nanorods prepared with different silver nitrate concentration under visible irradiation. When the AgNO3 precursor concentration increased, the visible light driven H2 production rate increased from 120 µmolg−1h−1 to 168 µmolg−1h−1. Further increase in AgNO3 concentration leads to decreased H2 production rate 99 µmolg−1h−1. The M60ZST4N2 photocatalyst exhibited the highest photocatalytic H2 production. In order to achieve an efficient Ag2S-coupled ZnO@ZnS nanorods decorated stainless-steel wire mesh based hierarchical photocatalysts for visible-light driven photocatalytic H2 production applications, controlling the AgNO3 concentration is critical. Liu

49

reported that

photogenerated electrons in ZnO and ZnS of Ag2S/ZnO/ZnS photocatalyst migrated to the conduction band of Ag2S and helped the separation of photogenerated carriers. For 19



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the M60ZST4N2 photocatalyst, the formation of some Ag2S nanoparticles on the ZnS shell (Fig. 5b) enhanced the H2 production because of the electron transfer at the Ag2S/ZnS interface. However, as shown in Fig.12, electrons transferred to Ag2S cannot produce H2 because the CB of Ag2S is lower than that of the reduction potential of H+/H2. H2 production occurred at the ZnS surface. There are large amounts of Ag2S aggregates formed on the top of ZnO@ZnS nanorods array for the M60ZST4N3 photocatalyst (Fig. 5c). These aggregates did not increase the Ag2S/ZnS interface. In addition, these aggregates also hindered the contact between ZnS and Na2S/Na2SO3. Therefore, the H2 production rate of the M60ZST4N3 photocatalyst prepared by more AgNO3 precursor is lower than that of the M60ZST4N2 photocatalyst. In this study, using a conductive stainless steel wire-mesh support for the photocatalysts provides good wetting by the reactant solution and high surface area, enhancing the transport and adsorption of reactants. The conductive supports also favor the separation of photogenerated charges. The Ag2S coupled ZnO@ZnS core-shell nanorods heterostructures on conductive supports can enhance the separation of photo-induced electron–hole pairs, leading to high photocatalytic H2 production efficiency. As shown in Fig.7a, in comparison with GZST4, a red shift of the band edge was observed for the DRS spectrum of Ag2S-coupled GZST4N2 sample. The enhanced light absorption properties of Ag2S-coupled samples were the reason for their photocatalytic activity under visible light irradiation.

Figure 11. The photocatalytic H2 production of (a) M60ZST4N1 (b) M60ZST4N2 (c) M60ZST4N3 under visible light irradiation. A proposed mechanism of the photocatalysis process of the Ag2S-coupled ZnO@ZnS/metal wire mesh photocatalyst was shown in Figure 12. As reported in the － literatures,50 51 the valence band (VB) edge potential and conduction band (CB) edge potential of ZnO are 2.9 and -0.32 eV (vs. the standard hydrogen electrode (SHE)), 20


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respectively. The VB edge potential and CB edge potential of ZnS are 2.48 and -0.95 eV, respectively. The VB edge potential and CB edge potential of Ag2S are 1.09 and － 0.17 eV, respectively. 52 54 The band gaps for ZnO and ZnS obtained from an average of Eg reported in cited literatures are 3.22 and 3.43 eV, respectively. 55 Based on the reported EVB and ECB edge potentials of ZnO, ZnS and Ag2S, the possible charge transfer process over the Ag2S-coupled ZnO@ZnS/metal wire mesh photocatalyst under light irradiation is proposed in Figure 12. The enhanced photoresponse of Ag2S-coupled ZnO@ZnS/metal wire mesh photocatalysts shown in the transient photocurrent

response

tests

(Figure

13)

revealed

effective

separation

of

photogenerated electrons and holes. The enhanced photocatalytic hydrogen production activity of Ag2S coupled ZnO/ZnS core/shell nanorods/metal wire mesh photocatalysts can be attributed to the heterostructure, which favors the absorption of light and the separation of photo-induced electron–hole pairs. Liu et al.

49

mentioned

that photogenerated electrons in ZnO and ZnS migrated to the conduction band (CB) of Ag2S and prevent the recombination of photogenerated electron-hole pairs for the Ag2S-coupled ZnO/ZnS core/shell nanorods. Yang et al. 54 found that the ZnS:Ag2S nanosheets which had a large surface area and loaded Ag2S showed better photocatalytic H2 generation rates than that of the pure ZnS. Because the conduction band (CB) and valence band (VB) energy levels of Ag2S are bracketed by those of ZnS, both electrons and holes can be transferred to Ag2S. However, since the CB of Ag2S is lower than that of the reduction potential of H+/H2, electrons transferred to Ag2S cannot produce H2. Therefore, Ag2S can neither produce H2 nor transfer electrons from its VB to H+. Photogenerated electrons from ZnS can react with protons in the water and produce H2. Besides, holes can transfer to Ag2S on the ZnS shell, which will promote carrier separation. Such a mechanism also happens to the Ag2S-coupled ZnO@ZnS/metal wire mesh photocatalysts. The better H2 production activity of the Ag2S-coupled ZnO/ZnS nanorods results from enhanced separation and transfer of photogenerated carriers. Besides, electrical conductive supports, such as graphene 14, indium tin oxide (ITO) 29, copper plates 30, and stainless-steel wire mesh substrates

32

can also help the separation of photogenerated charge in the

photocatalysis process and enhance the photocatalytic activity. In this study, with the formation of Ag2S on the shell of ZnO@ZnS core-shell nanorods decorated metal wire mesh photocatalysts, photogenerated electrons in ZnS can be separated effectively by transferring to the conduction band of Ag2S, or migrating toward conductive stainless steel wire mesh substrate. After the separation of electron–hole pairs (equation 4), the electrons can be scavenged by H+ to generate H2 (equation 5). Since there are Na2S and Na2SO3 which can act as the hole scavenger in the solution, － scavenging the hole by SO32 and S2− can proceed (equation 6 and 7). Then, 21



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Page 22 of 31

hydrogen can be produced stably by such a novel photocatalyst. ZnO@ZnS-Ag2S/metal wire mesh +hν h+ + e +

H +e － SO32 2−

－

－

H2

(5) +

2−

+ H2O+ 2h SO4 +2H +

2 S + 2h S2

(4)

+

(6)

2−

(7)

Figure 12. A proposed mechanism of the photocatalysis process of the Ag2S-coupled ZnO@ZnS/metal wire mesh photocatalyst. The photocurrent response Effects of Na2S sulfidation time The chopped current-time transient photocurrent response under UV light irradiation of the samples was measured by a photoelectrochemical test device. Figure 13(a) shows the chopped current-time transient responses of ZnO nanorods coated stainless-steel wire mesh M60Z and ZnO@ZnS core-shell nanorods coated stainless-steel wire mesh based photocatalysts with different Na2S sulfidation time as electrodes. The photocurrent of the ZnO@ZnS core-shell nanorods based M60ZST2 photocatalyst is about 2 times that of ZnO nanorods based M60Z photocatalyst under UV light irradiation. The highest photocurrent is obtained when the sulfidation time became 4 hr. The photocurrent of the M60ZST4 photocatalyst is about 3 times that of M60Z photocatalyst. Further increase of sulfidation time resulted in a decrease of the photocurrent. ZnS and ZnO are good photocatalysts. However, both materials encounter the same limitation of their wide band gap. Batzill et al.

56

showed that

although ZnO and ZnS had high photo energy thresholds, the band alignment at the 22


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ZnO@ZnS interface favored the reducing of the photoexcitation threshold energy. Hence, the combination of ZnO and ZnS photocatalysts with wide band gap can yield a composite photocatalysts with lower photoexcitation threshold. In this study, the increased photoresponse reveals that the photoinduced electrons and holes can be separated effectively. The recombination of the electron-hole pairs is hindered. The current increased rapidly under light irradiation and recovered quickly when the light was turned off. Reversible and stable photocurrent response of the photocatalysts was observed. These photocurrent results are consistent with the photocatalytic H2 production activity results. Effects of AgNO3 precursor concentration Figure 13(b) shows the chopped current-time transient responses of Ag2S-coupled ZnO@ZnS nanorods coated stainless-steel wire mesh based photocatalysts with different AgNO3 precursor concentrations as electrodes. The photocurrent of the Ag2S-coupled ZnO@ZnS core-shell nanorods based M60ZST4N1 photocatalyst is higher that of ZnO@ZnS core-shell nanorods based M60ZST4 photocatalyst under UV light irradiation. The photocurrent reached the highest value when the concentration of AgNO3 precursor was increased to 2 mM. The photocurrent decreased with further increase of AgNO3 concentration. The photocurrent of the Ag2S-coupled ZnO@ZnS nanorods coated stainless-steel wire mesh based photocatalysts is about 1.3 times that of ZnO@ZnS nanorods based photocatalyst under UV light irradiation. These photocurrent response trends were consistent with the observed photocatalytic activity results of Ag2S-coupled ZnO@ZnS nanorods coated wire mesh based photocatalysts. The enhanced photoresponse indicated effective separation of photogenerated electrons and holes.

23



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Figure 13. The chopped current-time transient photocurrent response of (a) ZnO nanorods coated stainless-steel wire mesh based photocatalyst (i) M60Z and Ag2S-coupled ZnO@ZnS nanorods coated stainless-steel wire mesh based photocatalysts with different Na2S sulfidation time (ii) M60ZST2 (iii) M60ZST4 (iv) M60ZST6. (b) uncoupled (i) M60ZST4 and Ag2S-coupled ZnO@ZnS nanorods coated stainless-steel wire mesh based photocatalysts with different AgNO3 concentrations (ii) M60ZST4N1 (iii) M60ZST4N2 (iv) M60ZST4N3. H2 production stability of recycled photocatalysts The stability of photocatalysts upon light irradiation is very important. It was reported that semiconductor photocatalysts usually suffer from instability problem caused by photocorrosion in longterm photocatalytic reactions.57 However, the stability and activity in hydrogen evolution from water splitting of semiconductor photocatalysts can be improved by using different preparation methods.58 Fig. 14 shows the H2 generation performances of recycled M60ZST4N2 photocatalysts during five repeated tests. After being washed with water, Ag2S-coupled ZnO@ZnS nanorods/wire mesh based immobilized photocatalyst can be recycled. The H2 generation stability of the recycled M60ZST4N2 photocatalyst from water containing 24


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sulfide and sulfite as hole scavengers is important to repeated operations. During the five repeated H2 generation experiments for recycled M60ZST4N2 photocatalysts, the limited activity loss was observed for Ag2S-coupled ZnO@ZnS nanorods/wire mesh based photocatalyst M60ZST4N2. The hydrogen production rate for the fifth test was 88.6 % of the first test. These ZnO@ZnS core-shell nanorods decorated stainless-steel wire mesh based photocatalysts exhibited high photocatalytic activity and good stability in H2 production. The stable photocurrent property of the photocatalysts (Fig. 13) may explain why the photocatalyst showed stable photocatalytic hydrogen production activity.

Figure 14. Photocatalytic H2 production stability of recycled M60ZST4N2 photocatalyst under UV irradiation for 5 runs.

Conclusions Formation of Ag2S coupled ZnO/ZnS core/shell nanorods on conductive metal wire-mesh favors the absorption of light and the separation of photogenerated electron–hole pairs, and contact between the reactant solution (containing Na2S, Na2SO3, and NaCl) and the photocatalysts, leading to efficient photocatalytic H2 production activity and anti-photocorrosion property. Photogenerated electrons in ZnO and ZnS can be separated effectively by transferring to the conduction band of Ag2S, or migrating toward conductive stainless steel wire mesh substrate. Ag2S coupled ZnO/ZnS core/shell nanorods coated wire meshes structure not only increased the surface area of photocatalysts but also changed their surface from hydrophobic (CA=109o) to superhydrophilic (CA=0o). The aqueous reactant solution can wet and pass through the wire mesh structure. The thickness of the ZnS shell and amounts of coupled Ag2S have great influence on the photocatalytic performance of the ZnO@ZnS core-shell nanorods based immobilized hierarchical photocatalyst. The maximum H2 production rates for Ag2S-coupled ZnO@ZnS nanorods decorated 25



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Page 26 of 31

stainless-steel wire mesh based hierarchical photocatalyst M60ZST4 reached 5870 µmolg−1h−1 and 168 µmolg−1h−1 under UV and solar-simulated light irradiation. The amount of hydrogen production for the fifth test is 88.6 % of the first test. The optimized Ag2S-coupled ZnO@ZnS/metal wire mesh photocatalysts exhibited high photocatalytic activity and good stability in H2 production. Acknowledgements The authors would like to thank the financial support from the Ministry of Science and Technology under the contract of NSC 102-2632-E-035-001-MY3. The authors appreciate the Precision Instrument Support Center of Feng Chia University in providing the measurement facilities.

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Table of Contents Efficient H2 production using Ag2S-coupled ZnO@ZnS core-shell nanorods decorated metal wire mesh as an immobilized hierarchical photocatalyst Mu-Hsiang Hsu, Chi-Jung Chang*, Hau-Ting Weng Department of Chemical Engineering, Feng Chia University, 100, Wenhwa Road, Seatwen, Taichung 40724, Taiwan, ROC Synopsis

Efficient H2 production was achieved by Ag2S-coupled ZnO–ZnS heterojunctions on conductive wire-mesh photocatalysts which favors contact with reactant solution, light absorption, and separation of photogenerated electron–hole pairs.

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S-Coupled ZnO@ZnS CoreâShell Nanorods Decorated - American

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