High Efficient Noble Metal Free Zn(O,S)

international journal of hydrogen energy 42 (2017) 5638e5648

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High efﬁcient noble metal free Zn(O,S) nanoparticles for hydrogen evolution

* Hairus Abdullah, Dong-Hau Kuo , Xiaoyun Chen

Department of Materials Science and Engineering, National Taiwan University of Science and Technology, No. 43, Sec. 4, Keelung Road, Taipei 10607, Taiwan article info abstract

Article history: Noble metal-free zinc oxysulfide (Zn(O,S)) nanoparticles for H2 evolution had been facilely Received 23 September 2016 synthesized by a low-cost chemical reaction at low process temperature of 70e90 C with Received in revised form zinc acetate dihydrate as the zinc source and different amounts of thioacetamide as the 5 November 2016 sulfur precursor. The as-prepared Zn(O,S) had a nano crystallite size of 3e10 nm and Accepted 18 November 2016 formed aggregates. Low temperature process has led to the difficulty in forming well- Available online 10 December 2016 crystalline nanoparticles, therefore the ionic bonding is weaker as compared to the high temperature one. Due to the entropy-controlled reaction, Zn(O,S) with different composi- Keywords: tions, phases, and bandgaps was formed and proposed to have a three-dimensional mul- ZnOS tibandgap-quantum-well (3D MQW) band structure, as supported by the selected area Hydrogen evolution electron diffraction of high resolution transmission electron microscopy. Photo reduction þ Photocatalyst of Cr6 was initially performed for our materials selection. The photocatalytic hydrogen evolution reaction (HER) of Zn(O,S) in different kinds of solutions under low power UV lamps (the intensity is 0.088 mW/cm2 or approximately 1/40 times UV light intensity of sunlight) was executed. A high efficient hydrogen evolution rate of 213 mmol/g h watt was achieved by considering the input light power. The active surface oxygen anion-involved photocatalytic mechanism for HER is proposed, which was derived from the reversible change in Zn(O,S) color and the results from HER reactions in different solutions. © 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

The utilization of renewable energy from the sources of sun- Introduction light, wind, rain, tides, waves, and geothermal heat to lower the consumption of fossil fuels has been widely investigated. Nowadays, global energy demands heavily depend on non- As only harnessing 0.01% energy of sunlight in one second renewable fossil fuels. The combustion of fossil fuels has illumination is sufﬁcient for a year energy consumption of generated such many greenhouse gas emissions that the human living, sunlight harvesting for renewable energy has ' Earth s surface temperature will increase to cause the global attracted attention [2,3]. Although solar cell provides the green warning and climate change. To alleviate climate change, energy, its cost, conversion efﬁciency, and post-treatment Paris Agreement is declared in 2015 to limit global tempera- face many challenges. Hydrogen as energy carrier is believed ture increase below 1.5 C above the pre-industrial level with to be the best future energy source to replace fossil energy due net zero greenhouse gas emissions between 2030 and 2050 [1]. to its clean combustion product of water.

* Corresponding author. Fax: þ886 2 27303291. E-mail address: [email protected] (D.-H. Kuo). http://dx.doi.org/10.1016/j.ijhydene.2016.11.137 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. international journal of hydrogen energy 42 (2017) 5638e5648 5639

Hydrogen fuel has been produced by electrolysis and thioacetamide was set to 1:0.25, 1:0.5, 1:1, and 1:2 to obtain steam-methane reforming process. Electrolysis is a feasible four different kinds of Zn(O,S) and they were denoted as method, if the costly input electricity finds a cheap source. ZnOS-0.25, ZnOS-0.5, ZnOS-1, and ZnOS-2, respectively. In our , The petroleum and natural gas steam reforming process is the typical preparation for ZnOS-0.5, 4.4 g Zn(Ac)2 2H2O and 0.75 g major technology for producing hydrogen, but it still emits thioacetamide were added in 900 mL distilled water and stir- CO2 to our atmosphere and is not sustainable for long term red at 70 Cor90 C for 4 h to obtain Zn(O,S) precipitates. The application [4,5]. The other concern about hydrogen applica- white Zn(O,S) precipitate was collected by centrifugation and tion is its safety problems in storage and transportation. washed for three times with alcohol followed by drying in a Solar light-assisted photocatalytic reduction of water for rotary evaporator. To prepare ZnO for the comparative pur- producing hydrogen fuel is also viewed as a potential tech- pose, a similar procedure to that of Zn(O,S) was done without nology. Photocatalysis for hydrogen evolution reaction (HER) adding thioacetamide. The complete ingredients of Zn(O,S) has been widely pursued since the first work on ultraviolet NPAs were provided in Table 1. light (UV)-driven TiO2 photocatalyst by Fujishima and Honda in 1972 [6]. TiO2-based catalysts have been greatly improved Characterization for H2 production if the noble metal of Pt is added as a co- catalyst and high power UV lamp is used for illumination [7]. The Zn(O,S) NPAs were carefully examined by field-emission To harvest most of the solar light, visible light-driven catalysts scanning electron microscopy (FE-SEM, JSM 6500F, JEOL, for water splitting, based upon the concepts of p-n hetero- Tokyo, Japan) and high resolution transmission electron mi- e e junction [8 10], defect engineering [11 14], and sensitization croscopy (HRTEM, Tecnai F20 G2, Philips, Netherlands) with with low bandgap materials [15,16], have been widely the element mapping function. Specific surface area, average searched. However, the visible light-driven catalysts with pore diameter and total pore volume of Zn(O,S) NPAs were good performance in HER always contain the environmentally measured by Brunauer-Emmett-Teller (BET) method. Powder e harmful CdS [17 26]. X-ray diffraction (XRD) patterns of Zn(O,S) NPAs were recor- ZnO and ZnS as non-toxic semiconductors have been ones ded by Bruker D2-phaser diffractometer using Cu Ka radiation of the most famous UV-driven photocatalyst materials. ZnO at a wavelength of 1.5418 A. The UVeVis diffuse reflectance þ has been used in many applications such as water splitting spectra (DRS) and peak absorbance of Cr6 degradation at e [27], biomedical application [28 31], and antibacterial appli- 542 nm were recorded using a Jasco V-670 UVeVisible-Near IR cations [32,33]. Meanwhile, ZnS also has been used in many Spectrophotometer. During the photocatalytic sessions, fields such as water splitting [34,35], light emitting diode (LED) hydrogen evolution was monitored through a well-calibrated application [36,37], and sensor [38,39]. ZnO and ZnS can also gas chromatography (GC) system equipped with thermal form solid solution of Zn(O,S) to replace CdS for thin film solar conductivity detector (TCD) and 99.99% Ar as a gas carrier. cells [40,41]. Persson et al. reported the atomic layer-deposited ZnO S solid solution exhibited a very strong valence band þ 1 x x Photo reduction of hexavalent chromium (Cr6 ) experiment offset bowing as a function of S content in ZnO1xSx [42]. Chen et al. synthesized Zn(O,S) nanowire with a hexagonal struc- Due to the time-consuming experiment of hydrogen evolu- ture for HER by depositing ZnS quantum dots on ZnO, fol- tion, different Zn(O,S) NPAs were initially evaluated toward lowed with a 450 C annealing step [43]. þ Cr6 reduction under UV light illumination, then Zn(O,S) NPA In this work, solid solution Zn(O,S) nanoparticle aggregates with the best performance was used for HER experiment. To (NPAs) were synthesized by facile chemical reaction at low execute the photo reduction performance, 20 mg of Zn(O,S) processing temperature of 70 C and 90 C. The effects of the powder was well dispersed in 100 mL of 10 ppm potassium sulfur content during powder preparation on photocatalytic dichromate solution as a pollutant model. Prior to being illu- reduction and the ethanol content in aqueous solution on HER minated by UV light, the Zn(O,S)-dispersed potassium di- had been evaluated. Hydrogen formation mechanisms for chromate solution was stirred for 30 min to reach the HER at different aqueous solutions are elucidated. adsorption and desorption equilibrium between Zn(O,S) pho- þ tocatalyst and Cr6 ions. The time when catalyst is added into potassium dichromate solution is marked as t ¼30 min and Experimental section the time when light is turned on is marked as t ¼ 0 min. The þ UV light source used for photocatalytic reduction of Cr6 was Materials 6 W black light UV tube lamp. The experiments did not use cooling system due to the low power of UV lamps. During the In this work, all chemical were commercially available and experiments, the photocatalyst-dispersed potassium dichro- used without any purification treatment. mate solution was stirred continuously to accelerate the

Preparation of Zn(O,S) NPAs

Table 1 e Amounts of precursors used for synthesizing To prepare Zn(O,S) NPAs, zinc acetate dihydrate and thio- Zn(O,S) with different amounts of thioacetamide. acetamide were used as zinc and sulfur sources, respectively. Precursor ZnOS-0.25 ZnOS-0.5 ZnOS-1 ZnOS-2 Zn(O,S) was prepared with constant amount of Zn precursor $ and different contents of thioacetamide. To obtain optimum Zn(Ac)2 2H2O 4.4 g 4.4 g 4.4 g 4.4 g Thioacetamide 0.375 g 0.75 g 1.5 g 3 g performances, the molar ratio of Zinc acetate dihydrate and 5640 international journal of hydrogen energy 42 (2017) 5638e5648

6þ diffusion of Cr to photocatalyst surfaces and prevent sedi- (ZnOS-0.5 after used for photocatalytic H2 evolution reac- mentation of catalyst particles. At different time intervals, an tion) were examined with X-ray diffractometry. Fig. 1 shows þ aliquot was taken from the Cr6 solution and centrifuged to the diffraction patterns of as-prepared Zn(O,S) NPAs and get the supernatant solution. The obtained supernatant so- standard files of cubic ZnO (JCPDS #65-2880) and cubic ZnS lution was added by one drop of 5 g/L 1.5-diphenyl carbazide (JCPDS #05-0566). Zn(O,S) powders prepared at different solution and one drop of sulfuric acid solution. After the so- amounts of thioacetamide had the cubic zinc structure. All the lution was well mixed, the existence of Cr(VI) would give pink peaks of Zn(O,S) prepared with different amounts of thio- þ color to the solution. The concentration of remaining Cr6 acetamide from (111), (220), and (311) planes shifted 1.10, ions was colorimetrically detected by the UVevis absorbance 1.44, and 1.40, respectively, to higher angles, as compared to intensity at 542 nm and calculated based on the ratio between those of cubic ZnS. The amount of thioacetamide for prepar- peak heights at t min and t ¼30 min. ing Zn(O,S) had no effects on peak position. The peak shifting is attributed to the formation of Zn(O,S) solid solution, in Photocatalytic HER experiments which the O2- anions (effective radius of 140 pm) [44] occupied the S2- anion (effective radius of 184 pm) [44] in ZnS leading to þ Zn(O,S) NPA with the best performance in reducing Cr6 smaller lattice parameter. Based on the Scherrer equation for under UV light illumination was then used for HER experi- the (111) peak, all Zn(O,S) powders had a calculated crystalline ments. To evaluate the performance in evolving hydrogen size of 2.5 nm. As pure ZnO prepared at the sulfur-free con- under 6 W or (4 6 W) UV tube lamp illumination, 225 mg dition had a hexagonal Wurtzite structure, the cubic ZnO Zn(O,S) NPA was separately dispersed in 450 mL pure water, pattern in Fig. 1 was based upon the JCPDS file. This experi- pure ethanol, 50% ethanol solution, and 10% ethanol solu- ment indicates that the small amount of thioacetamide in tion. The experiments were conducted in a 500 mL quartz aqueous solution for Zn(O,S) formation can stabilize the cubic reactor without water cooling. The UV lamp is safe to be structure. The similar peak positions of all Zn(O,S) with operated without any special treatment. The length of UV different amounts of thioacetamide indicated Zn(O,S) was lamp is longer than the available depth of reactor, therefore stoichiometrically formed at low temperature. The stoichio- only 2/3 of the UV lamp can be inserted into reactor. To have metric formation of Zn(O,S) was also confirmed by EDS anal- a precise calculation, the 6 W or (4 6 W) power of UV lamp ysis as shown in Table S1 in supplementary data. was used as 4 W or 16 W for calculating HER efficiency in our There are no different morphologies among ZnOS-0.25, work. To determine the intensity of 4 6 W black light UV ZnOS-0.5, ZnOS-1, and ZnOS-2 as shown in scanning electron tube lamp, a photometer was used to measure the UV illu- microscopy (SEM) images which are provided in Fig. S1 (sup- mination. A photometer detector was placed about 2 cm from plementary data). To further study the structure and the lamps and the maximum value of 600 lux or 0.088 mW/ morphology of Zn(O,S) NPA, ZnOS-0.5 was carefully examined cm2 was obtained. During the photocatalytic HER, the reactor by transmission electron microscopy (TEM) for its high resolu- was firstly connected to a gas collection bulb of 100 ml for tion image, element mapping, and electron diffraction pattern. safety consideration then gas chromatography (GC) system to detect gas evolved from reaction solution. To obtain the right H data, the GC system together with the mass flow system 2 25 30 35 40 45 50 55 60 65 70 and two gas cylinders of 7% H2 and 100% Ar was used to (111) ZnO Cubic (JCPDS#65-2880) establish a calibration line for hydrogen content. With the (200) (220) (311) dilution of 100% Ar flow, the hydrogen peak area related to the H2 contents at 1.8, 3.6, 5.4, and 7% were obtained. Our As-prepared ZnOS-A (111) reactor was located at position lower than GC machine so Zn (220) (311) that the low molecular weight of evolved hydrogen gas would As-prepared ZnOS-2 easily flow into GC system. Prior to being illuminated by UV (111) lamp, the reactor was purged by 99.99% Ar gas with the (220) (3 1 1 ) flowing rate of 100 mL/min for one hour to ensure all gas in As-prepared ZnOS-1 reactor was removed. After purging process, the gas in (111) (220) reactor was carried into GC system for analysis. If GC system (311) showed the signals from hydrogen, oxygen, and nitrogen As-pre pared ZnOS-0.5 gases, the reactor will be continuously purged or checked for (111) (220) (311) the leaking problem. After the purging procedure, the reactor Intensity (a.u) started to be illuminated by UV light. At each sampling in- As-prepared ZnOS-0.25 (111) terval, the Ar carrier gas flowed into GC system for several (220) (311) minutes and was analyzed by TCD. The HER experiment was conducted for 5 h for each test in this work. (111) ZnS Cubic (JCPDS #05-0566) (220) (311)

25 30 35 40 45 50 55 60 65 70 Results and discussion 2θ

To identify the crystal structure of Zn(O,S) powder, the as- Fig. 1 e XRD pattern of as-prepared Zn(O,S) powders and prepared ZnOS-0.25, ZnOS-0.5, ZnOS-1, ZnOS-2, and ZnOS-A JCPDS ﬁles of ZnO and ZnS. international journal of hydrogen energy 42 (2017) 5638e5648 5641

Fig. 2a shows the low magniﬁcation images of ZnOS-0.5 aggregate and the element mapping at the red rectangle- marked location. ZnOS-0.5 had an aggregate size less than 120 nm. Fig. 2b shows the selected area electron diffraction (SAED) pattern of ZnOS-0.5. SAED ring patterns from the (111), (220), and (311) planes explains its polycrystalline nature. The (111) ring pattern actually displayed a broad circular band instead of a sharp ring. With the help of the standard d spacing ¼ values of the (111) plane for cubic ZnO at d1 2.67 A and for ¼ cubic ZnS at d2 3.12 A, the broad circular band of the (111)

plane is bounded by circles of reciprocal radius R1 related to d1

and R2 to d2 for ZnO and ZnS, respectively. Each spot located in the (111) circular band corresponds to one reciprocal radius, one d-spacing value, one O/S composition ratio, one phase, and one bandgap value for a semiconductor. Fig. 2c shows HRTEM lattice fringes of ZnOS-0.5 from the black rectangle-marked zone in Fig. 2a. It revealed that ZnOS-0.5 aggregates contained many nanoparticles with the size of 3e10 nm. Therefore, our ZnOS- 0.5 powder is composed of nanoparticle aggregates (NPAs) with the specific values of surface area, total pore volume and average pore diameter of 18.77 m2/g, 0.074065 cm3/g, and 15.784 nm, respectively. The related analysis was provided in Fig. S4 (supplementary data). Some lattice fringes in Fig. 2c show the d-spacing values of 2.68, 2.89, and 3.10 A for the (111) plane. As each nanocrystallite in NPA is surrounded by many other nanoparticles with different phases and bandgap values, naturally grown ZnOS-0.5 has a multiple phase structure and a three-dimensional multibandgap-quantum-well (3D MQW) band structure. MQW has been popularly applied to the artifi- cially fabricated multiple-quantum-well InGaN/GaN structure for light emitting diodes. In InGaN/GaN MQW band structure, it is two dimensional and constituted by the alternative stacking of two layers with two different bandgap values. The stability of our 3D MQW Zn(O,S) is attributed to its Gibbs formation energy with the entropy term dominating over the enthalpy term. Its occurrence is related to low processing temperature that lower the enthalpy energy and to increase the configuration entropy from the mixing of O and S to form Zn(O,S). As the high temperature calcination has been applied for converting the chemically derived powder into a well crystallized phase [43], its utilization has prohibited the formation of 3D MQW Zn(O,S). DRS spectra of Zn(O,S) NPA was examined to show its optical properties. The results as shown in Fig. 3a revealed after Zn(O,S) solid solution was formed, the intensity of UV light absorption was significantly increased, as compared to those from commercial ZnS and as-fabricated ZnO. The highest UV light absorption was obtained by ZnOS-1 following with the order of ZnOS-0.25, ZnOS-0.5, and ZnOS-2. UV light absorption of ZnOS-1 was nearly increased two times higher than that of hexagonal ZnO. The increase in UV light absorption indicates Zn(O,S) is highly sensitive to UV light with the wavelength between 344 and 354 nm. Based on DRS data, the black UV tube lamp with the wavelength of 352 nm was chosen as light Fig. 2 e (a) HRTEM image with the Zn, O, and S element source to meet the optical property of Zn(O,S) material. The mapping indicated by red rectangle. (b) Broad diffraction ring patterns from the selected area electron diffraction of the black rectangle area marked in (a) with the two schematic circles to show the theoretical ring positions for marked in (a). (For interpretation of the references to colour pure ZnO and ZnS. (c) Lattice fringes of ZnOS-0.5 in this figure legend, the reader is referred to the web nanoparticle aggregates for the black rectangle area version of this article.) 5642 international journal of hydrogen energy 42 (2017) 5638e5648

(a) (b) 2.00E-037 ZnO (E = 3.2 eV) ZnO g

2 ZnOS-0.25 (E = 3.6 eV) ZnOS-0.25 ) g -1 ZnOS-0.5 1.50E-037 ZnOS-0.5 (Eg= 3.6 eV) ZnOS-1 ZnOS-1 (E = 3.5 eV) ZnOS-2 g

eV.cm ZnOS-2 (E = 3.5 eV) ZnS ( 1.00E-037 g 2 Absorbance ) ZnS (E = 4.1 eV)

ν g h α ( 5.00E-038

0.00E+000 300 400 500 600 700 800 2.0 2.5 3.0 3.5 4.0 4.5 Wavelength (nm) hν (eV)

Fig. 3 e (a) Diffuse reﬂectance spectra (DRS) of ZnOS-0.5 NPAs and (b) their bandgap energy value determination by absorbance spectrum ﬁtting.

average bandgap values for Zn(O,S) prepared at different thi- closed to those from EDS analysis as shown in Table S2 in oacetamide concentrations were deﬁned by ﬁtting each supplementary data. absorbance spectrum, as shown in Fig. 3b. The bandgap values X-ray photoelectron spectroscopy (XPS) analysis is an of Zn(O,S) were 3.5e3.6 eV between those of ZnO (3.2 eV) and elegant tool to probe the surface elemental composition and ZnS (4.1 eV) due to the formation of solid solution between chemical state of each element in Zn(O,S) NPAs. ZnOS-0.5 was them. Although MQW Zn(O,S) is supposed to show many taken as the sample for the XPS analysis. The elements of Zn, bandgap values, its nanocrystallite nature responses to light O, and S in ZnOS-0.5 are shown in XPS survey spectrum in illumination with an average optical bandgap value of Fig. 4a and the XPS high resolution scans of each element are 3.5e3.6 eV. Based on the bandgap values of ZnOS-0.5, the shown in Fig. 4bed. Fig. 4b shows the binding energy values of oxygen and sulfur amounts in lattice were roughly estimated Zn (2p3/2) and Zn (2p1/2) in ZnOS-0.5 were noticed at 1022.5 and to be 44% and 56%, respectively. The estimation values were 1045.9 eV, respectively which were consistent to the literature

Fig. 4 e (a) XPS full scan spectrum of ZnOS-0.5 and XPS spectra of (b) Zn 2p, (c) O 1s, and (d) S 2p peaks for ZnOS-0.5. international journal of hydrogen energy 42 (2017) 5638e5648 5643

data [45]. The high resolution scan of oxygen (1s) in ZnOS-0.5 was set at t ¼ 0 min. The ZnOS-0.25, ZnOS-0.5, ZnOS-1, ZnOS- had binding energy values of 530.4 eV, 531.3 eV, and 532.3 eV 2, ZnS, and ZnO photocatalysts adsorbed 24.4, 64.5, 56.2, 49.4, 6þ which were related to oxygen in lattice (OL), oxygen vacancy 7.6, and 1.2% Cr , respectively, after stirred in dark for 30 min.

(Vo), and adsorbed oxygen (Oads) as hydroxide bonded on Under illumination of 6 W UV tube lamp, ZnOS-0.25, ZnOS-0.5, surfaces, respectively [45,46]. The amounts of 60.35% OL, ZnOS-1, ZnOS-2, ZnS, and ZnO photocatalyst reduced 99.3% 6þ 18.87% Vo, and 20.78% Oads were calculated based on the peak Cr in 35 min, 99.5% in 15 min, 99.5% in 20 min, 89.6% in areas. These data can provide the information of oxygen and 35 min, 65.9% in 35 min, and 55.7% in 35 min, respectively. The þ sulfur ratio in ZnOS-0.5. The oxygen to sulfur ratio was photo image in Fig. 5 was related to the Cr6 reduction of calculated to be 49.93 to 50.07. The binding energy values at ZnOS-0.5 from t ¼ 0 mine35 min under 6 W UV light. To show

161.8 and 163.1 eV conﬁrmed the presence of S (2p3/2) and S the photo response, 20 mg of ZnOS-2 was dispersed in 100 mL

(2p1/2) peaks, respectively as indicated in Fig. 4d and were also potassium dichromate solution of 10 ppm by stirring without þ consistent with literature data [45]. lighting for 90 min. There was no Cr6 reduction for ZnOS-2 in þ Photocatalytic reduction of Cr6 is to ﬁnd materials with dark, as shown in Fig. 5. Based on the light absorbance and the good reduction capability before HER. In this experiment, reduction capability from Figs. 3a and 5, respectively, the HER 20 mg each of ZnOS-0.25, ZnOS-0.5, ZnOS-1, and ZnOS-2 experiments were carried out under UV light illumination photocatalysts that prepared at 70 C were tested for with the wavelength of 352 nm. reducing 100 mL potassium dichromate solution of 10 ppm To test the photocatalytic capability of ZnOS-0.5 NPA in under UV light (Fig. 5) illumination. The light-on condition evolving hydrogen, 225 mg of 70 C-prepared ZnOS-0.5 was dispersed in 450 mL of pure water, neat alcohol, 50% ethanol solution, and 10% ethanol solution. Prior to starting HER ex-

0’ 5’ 10’ 15’ 20’ 25’ 30’ 35’ periments, the reactor with the ZnOS-0.5 dispersed solution 1.0 was connected with GC system and was purged under Ar gas ZnOS-0.5 for 1 h with vigorous solution stirring. After purging, the gas

) 0.8 from reactor was ﬂowed into GC system for analysis to ensure 6+ there were no hydrogen, oxygen, and nitrogen gases remain- 0.6 ing in reactor. The reactor system was then started to be ZnOS-0.25 illuminated by 6 W UV tube lamp. The total evolved hydrogen 0.4 ZnOS-0.5 amounts after 5 h light illumination were 3159 mmol/g, (remaining Cr o ZnOS-1 m m m ZnOS-2 2945 mol/g, 2277 mol/g, and 595 mol/g in 10% ethanol so- C/C 0.2 ZnO lution, 50% ethanol solution, pure ethanol, and pure water, ZnS respectively, as shown in Fig. 6a. However, only 7 mmol/g and ZnOS-2 (dark) 0.0 21 mmol/g hydrogen could be evolved from the 10% ethanol solution for as-prepared ZnO and commercial ZnS, respec- -30 -20 -10 0 10 20 30 40 50 60 70 80 90 tively. The highest hydrogen amount achieved was 3159 mmol/ Time (min) g in 5 h or 158 mmol/g h watt. The unit of mmol/g h,watt is used þ Fig. 5 e Photocatalytic reduction of Cr6 in aqueous instead of mmole/g,h to consider the energy conversion solution with the presence of Zn(O,S) photocatalyst under efﬁciency. UV light illumination and the related inset shows the To improve the performances of photocatalytic activities, photo image of solution color change with the processing ZnOS-0.5 was also prepared at 90 C and was tested for time. hydrogen evolution experiment under 4 6 W UV tube lamp

(a) 3500 3500 ZnO in 10% ethanol (b) No UV ZnS in 10% ethanol 3000 ZnOS-0.5 in 10% ethanol 3000 ZnOS-0.5 in 50% ethanol UV 2500 ZnOS-0.5 in pure ethanol

mole/g) 2500

μ ZnOS-0.5 in pure water 2000 2000

1500 1500 1st run 1000 1000 2nd run rd 500 500 3 run Hydrogen evolution ( 0 0

012345 012345 Time (h)

Fig. 6 e (a) Photocatalytic hydrogen evolution for ZnOS-0.5 in different ethanol concentrations in aqueous solution and pure water and (b) the reusability of ZnOS-0.5 in 10% ethanol solution under 6 W UV tube lamp illumination for three runs in three days without evacuating the solution. Self-synthesized ZnO and commercial ZnS were also tested and their data were included in (a) for comparative purpose. 5644 international journal of hydrogen energy 42 (2017) 5638e5648

illumination. Fig. 7a shows the improved photocatalytic per- Fig. 6b shows the HER performances of 70 C-prepared ZnOS- formance of ZnOS-0.5 that prepared at 90 C to evolve 0.5 in three runs of reusability experiments. The amounts of hydrogen in different solutions under 4 6 W UV tube lamp evolved hydrogen were 3159 mmol/g, 3293 mmol/g, and m m illumination. The highest H2 amount of 17,023 mol/g for 5 h 2931 mol/g for the first, second, and third run, respectively. (3405 mmol/g$h or 213 mmol/g h W) was achieved in ethanol The highly efficient HER at a rate of 147e165 mmol/g h watt for m solution, while the H2 amounts of 1553 and 1322 mol/g were each run is encouraging. The amount of evolved hydrogen achieved in pure ethanol and pure water, respectively. To changed a little at the third run. The change in evolution rate evaluate our system with a standard photocatalyst material, can be related to the loss of ethanol due to vaporization or photocatalytic HER for the commercial available P-25 was also reaction. During the reusability experiments, the color of conducted under the same experiment condition. However, ZnOS-0.5 photocatalyst gradually changed from white to gray m only 299 mol/g H2 can be evolved in 5 h. The low performance under UV light illumination, as shown in the inset of Fig. 6b. of P-25 in our system is due to the low power of UV light However, the color gradually changed back to white color illumination (only 0.088 mW/cm2) and the absence of Pt as co- when the UV light was turned off. The reversible color change catalyst. in a sulfur-free solution is related to the formation of oxygen All the previous works [17e26,47e60] used high power vacancies on ZnOS-0.5 due to the HER reactions leading to Zn- lamps to activate their catalysts in HER experiments. The rich composition on the catalyst surfaces, as confirmed by hydrogen evolution rate in terms of input light power was XRD of ZnOS-A in Fig. 1 for ZnOS-0.5 after reusability tests. calculated to evaluate HER efficiency. High power lamp needs However, the color change of catalyst only slightly affects the cooling system to dissipate the heat, otherwise highly flam- photocatalytic activities during HER experiments. Fig. 7b mable hydrogen gas at high temperature environment can shows the HER reusability of 90 C-prepared ZnOS-0.5 with H2 initiate an explosive fire in a real application system. amounts of 17,023, 16,593, 15,382, and 13,353 mmol/g for four Furthermore, most of the works with a high rate of hydrogen runs in four days without evacuating the reaction solution. A evolution always involve the hazardous substance of CdS or slight decrease in H2 production was also observed, maybe the costly noble metal of Pt or Au as a co-catalyst. Therefore, due to the ethanol evaporation. To confirm the phase of black the safety, cost, and efficiency are the major issues for a ZnOS-A nanoparticles changing back to white one again, the photocatalytic HER system to be used as a renewable energy ZnOS-A powder was kept in ethanol solution without UV light source. With the 6 W or (4 6 W) UV lamp, it is obvious that illumination until the color changed back to white color. The the hydrogen evolution in this work is very special with a powder was collected by centrifugation and dried in a rotary simple and low-cost system and without burning heat, evaporator, then examined by X-ray diffractometer. The precious metal, and toxic Cd compounds. result showed the Zn phase was disappeared and all the peaks The reusability experiments of ZnOS-0.5 for HER under UV of ZnOS-0.5 S (denoted for ZnOS-A after its color changed back light illumination were conducted by using 70 C-prepared to white) were matched to other Zn(O,S) peaks with different ZnOS-0.5 NPA for three runs in three days and 90 C-prepared thioacetamide as shown in Fig. S3 in supplementary data. ZnOS-0.5 for four runs in four days without evacuating reac- The mechanism of photocatalytic HER on Zn(O,S) is eluci- tion solution or with the reactor solution untouched. To carry dated. The conventional HER mechanisms with the assistance out the reusability experiments, 225 mg of ZnOS-0.5 powder of photocatalyst involve water reduction by electron and was dispersed in 10% ethanol solution and HER experiments oxidation by hole. The important issue is to find photo- were conducted under 6 W and 4 6 W UV tube lamp illu- catalysts that can produce the long lifetime of the photo- mination for 70 C-prepared ZnOS-0.5 and 90 C-prepared induced electron and hole pairs. Our proposed 3D MQW ZnOS-0.5, respectively. The duration for each run was 5 h. structure for Zn(O,S) can have the function to transfer the

(a) 18000 18000 (b) st ZnOS-0.5 + 10% ethanol 1 run ) 16000 16000 ZnOS-0.5 + pure ethanol 2ndrun ZnOS-0.5 + pure water 14000 14000 3rd run

mole/g ZnO + 10% ethanol

μ th

( 12000 ZnS + 10% ethanol 12000 4 run P25 + 10% ethanol 10000 10000

8000 8000

6000 6000

4000 4000 2000

Hydrogen evolution Hydrogen evolution 2000 0 0 012345 012345 Time (h)

Fig. 7 e (a) Photocatalytic HER of ZnOS-0.5 in different solutions and (b) its reusability in 10% ethanol solution for four runs in four days without evacuating the solution under 4 £ 6 W UV tube lamp illumination. international journal of hydrogen energy 42 (2017) 5638e5648 5645

photo-induced electrons and holes into the nearby contact was observed in the sulfur-free solution, the catalyst should grains with energy valleys. As the crystallite size of Zn(O,S) is be involved in HER but can be recovered after the illumination about 3e10 nm, the travel distance for the charge carriers is lamp is turned off. We propose the surface oxygen anion from about 1.5e5nm.Fig. 8 shows the illustration of 3D MQW the interaction of Schottky defect with surface reaction par- Zn(O,S) nanoparticle aggregates with different d-spacing ticipates in HER. Due to the surface behavior and the charge- values and the transfer of photogenerated electrons and holes balance requirement, the intrinsic Schottky defects have the to energy valleys. The separated photo-induced charge car- different migration rates for cation and anion. If the oxygen riers of electrons and holes need to involve the chemical re- anion moves faster than cation, surface charge will be nega- actions to evolve hydrogen gas. As the reversible color change tive and be balanced by a space positive charge [61]. The

Fig. 8 e (a) Schematic drawing of multibandgap-quantum-well Zn(O,S) aggregates with nanoparticles at different d-spacing or bandgap values to form the energy valleys for the charge separation of photogenerated electron and hole pairs and (b) Illustration of 3D conduction and valence bands with multibandgap quantum wells for Zn(O,S) nanoparticle aggregates to evolve hydrogen gas during photocatalytic HER in ethanol solution. 5646 international journal of hydrogen energy 42 (2017) 5638e5648

Schottky defects were involved in our mechanisms since conventional one, which is the surface anion-involved reac- during the photocatalytic reaction, the reaction solution color tion. To have the surface anions involve HER, the anion changed to black color. The color changed was due to the bonding strength is important. If the anion bonding is strong, increased amount of surface oxygen anion used for photo- no oxygen vacancy can be formed. If the anion bonding is catalytic reaction to induce the non-equilibrium of catalyst weak, catalyst surface will be covered by Zn to prohibit the surface charges. Therefore, the condition will induce more further reactions. Conventional calcination procedure at high Schottky defects during the photocatalytic reaction and ﬁnally temperature to crystallize the catalyst precursor can lead to changed the color of Zn(O,S). strongly bonded compounds. Our low temperature synthesis The chemical reactions are shown with the following in pure water makes Zn(O,S) NPA has a chance to form the equations, based upon the KrogereVink notation [61], for HER weakly bonded oxygen anion to execute the oxidation re- in pure water: actions in Eqs. (1) and (5). As the prepared Zn(O,S) has a crystallite site of 3e10 nm, its small diameter leads to a big þ 2 þ þ/ þ 2þ H2O Osurf 2h 2OHaq: VO;surf (1) curvature and the weakly bonded surface anions, which can detach from or re-attach to Zn(O,S) surface for the reversible þ 2þ þ / þ 0þ H2O VO;surf 2e H2 OO;surf (2) HER. Therefore, surface anion bonding strength is strongly related to the interaction between catalyst and aqueous so- þ 2þ / þ þ 0þ lution. The idea of anion bonding for HER was not emphasized H2O VO;surf 2H OO;surf (3) by other reports, as photocatalyst was thought to promote but

þ þ not to participate in hydrogen evolution. With the involve- H O þ O2 þ V2 /H þ 2O0 (4) 2 surf O;surf 2 O;surf ment of surface anion, our reaction formula also can explain HER is initiated by the surface oxygen anion-involved no liberation of oxygen gas in our experiment. No liberation of oxidation of water in Eq. (1) to generate the available oxygen oxygen is an advantage of our Zn(O,S) system for the concerns vacancies for hydrogen production by the oxygen vacancy- of safety and gas separation of hydrogen from oxygen. þ involved reduction of water in Eq. (2). With H from Eq. (3) to HER research has shifted to visible light-driven catalysts balance OH from Eq. (1), the net reaction in Eq. (4) is obtained. due to the abundance of visible light from sunlight, but the 4% 2 2 At the light-off condition, Eq. (4) proceeds to form H2 with the (4 mW/cm if the sunlight intensity is 100 mW/cm ) UV light recombination of surface oxygen and oxygen vacancy and the from sunlight can be managed to validate our 6 W or (4 6) W catalyst color reversibly changes back to white. UV light-driven catalyst for feasible hydrogen evolution. The For HER in pure ethanol, similar chemical reactions as in number of sunlight intensity for UV light is much higher than pure water are shown with the following equations: that of UV lamp (0.088 mW/cm2) used in this work. The UV light-driven Zn(O,S) catalyst has one advantage of producing þ 2 þ þ/ þ þ 2þ C2H5OH Osurf 4h CH3CHO H2O VO;surf (5) photo carriers after illumination. If the catalyst semiconductor has low energy bandgap, it will have existing The highest rate occurs for Zn(O,S) in 10% ethanol solution. charge carriers and low electrical resistivity. Under the light With more water molecules surround the catalyst, water re- illumination, one type of its photo carriers will be neutralized actions for HER are still critical and cannot be ignored. As the by the existing carrier and the other type of carriers becomes ethanol can be easily oxidized to proceed chemical reactions, dominant. Therefore, either oxidation or reduction reaction we expect the hydrogen evolution with high efficiency comes was enhanced but not both to maximize the hydrogen evo- from the synergic effect of the water and ethanol contact to lution rate. For the above reason, semiconductor for HER is Zn(O,S) catalyst. As ethanol can be easily oxidized, the addi- better to be an intrinsic one with suitably high electrical tion of pure ethanol in HER is expected to enhance equation resistance in dark condition. Furthermore, composite catalyst (5). However, the HER reaction in pure ethanol remains slow, design is important for charge separation. Some challenges of because it needs to wait for the generated water that pro- composite catalyst for HER are the uniformity in distribution ceeded through equation (5). Therefore, the synergic effect of nanoparticles and the coverage ratio of nanoparticles on actually involves a series reaction, i.e. it needs to initiate the the catalyst surface, in addition to the bandgap engineering. water oxidation reaction first, then following the water Those two challenges, from the microstructural viewpoint, reduction reaction. It becomes interesting to note that water are related to the domain size effect of the oxidation and reduction reaction of forming H2 from solution is actually reduction zones controlled by the separation capability of the controlled by the oxidation reaction. To reach the highest photogenerated charge carriers. Overall, hydrogen evolution hydrogen production rate, both oxidation and reduction re- by photocatalyst has two important issues: (1) the domain size actions need to be enhanced together. To find the catalyst of oxidation and reduction zones and (2) the exchange capa- with the best reduction capability cannot be the best strategy bility of the surface oxygen anion. for HER. The efficient and environment friendly Zn(O,S) photocatalysts with hydrogen production rate of 158 and 213 mmol/ Conclusions gh,watt have been developed. Synthesized composite catalyst with co-catalyst to meet the criteria for oxidation by one Zn(O,S) nanoparticle aggregates had been easily synthesized component and reduction by the other component also has at low temperature with different thioacetamide contents and been widely applied for HER due to the charge separation. characterized as well as tested toward photo reduction of There is one more difference for our model from the hexavalent chromium and photocatalytic hydrogen evolution international journal of hydrogen energy 42 (2017) 5638e5648 5647

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