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Enhanced photocatalytic hydrogen production of noble-metal free Ni-doped Zn(O,S) in ethanol solution
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 2 ( 2 0 1 7 ) 2 5 8 9 1 e2 5 9 0 2
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Enhanced photocatalytic hydrogen production of noble-metal free Ni-doped Zn(O,S) in ethanol solution Noto Susanto Gultom, Hairus Abdullah, Dong-Hau Kuo* 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:
High efficient hydrogen evolved Ni-doped Zn(O,S) photocatalyst with different Ni amounts
Received 28 June 2017
had been successfully synthesized with a simple method at low temperature. Our Ni-doped
Received in revised form
Zn(O,S) catalyst reached the highest hydrogen generation rate of 14,800 mmol g1 h1 or
6 August 2017
0.92 mmol g1 h1 W1 corresponding to apparent quantum yield 31.5%, which was 2.3
Accepted 29 August 2017
times higher compared to the TiO2/Pt used as a control in this work. It was found that a
Available online 20 September 2017
small amount of Ni doped into Zn(O,S) nanoparticles could increase the optical absorbance, lower the charge transfer resistance, accordingly decrease the electron-hole recombination
Keywords:
rate, and significantly enhance the photocatalytic hydrogen evolution reaction (HER). The
Nickel doping
as-prepared catalyst has the characteristics of low cost, low power consumption for acti-
Hydrogen production
vating the catalytic HER, abundant and environmental friendly constituents, and low
Zinc oxysulfide
surface oxygen bonding for forming oxygen vacancies. The photocatalytic performance of
Oxygen vacancy
Ni-doped Zn(O,S) was demonstrated with a proposed kinetic mechanism in this paper. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Nowadays, the major energy source is derived from fossil fuels. It is well known that the utilization of non-renewable, limited, and CO2-generated fossil fuels leads to global warming and climate change. One of the undoubted solutions is the utilization of renewable, abundant and environmental friendly resources to overcome those issues. Among the renewable energy sources, solar energy is the most promising one due to its abundance, only 0.01% energy from the sunlight could satisfy the human energy necessity [1]. The challenge is to convert and store the energy in an appropriate compound as energy carrier, therefore it could be practically used in
future. Photocatalytic hydrogen production has been considered as an ideal method to produce hydrogen as energy carrier. As the final product of hydrogen combustion is only water, hydrogen is believed as an alternative clean energy carrier in the future. Since the invention of Fujishima and Honda in 1972 [2], photocatalytic water splitting has been widely investigated. More than hundreds of photocatalysts (metal oxide, metal sulfide, oxynitride, oxysulfide, etc.) [3e7] have been reported for overall or half-cell reactions of photocatalytic water splitting. High efficiencies of photocatalytic reactions have been achieved with the utilization of toxic CdS-related photocatalysts [8e10]. Furthermore, most of the catalysts for water splitting used the precious metal like Pt as co-catalyst [11,12]. In addition, water
* Corresponding author. E-mail address: [email protected] (D.-H. Kuo). http://dx.doi.org/10.1016/j.ijhydene.2017.08.198 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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splitting also used high power and harmful sacrificial reagent Na2S/Na2SO3 to improve its performance [13]. Therefore, to design an efficient photocatalyst with earth-abundant and environmental friendly material is needed to promote wide scale applications. To replace the precious metal, low cost and green materials such as carbon, manganese, cobalt and nickel have the potential to be further explored [14e17]. The abilities of photocatalyst to absorb light (UV or visible) and suppress the photo-generated electron-hole recombination are the keys to improve photocatalytic activity. The concepts such as p-n heterojunction, solid solution formation, sacrificial reagent, dye sensitizer, co-catalyst, and doping have been proved in previous works [11,18e22]. Doping concept has been commonly known as a strategy to increase the surface area [23], to enhance the absorbance against light and to suppress the electron-hole recombination of semiconductors [24e26]. Nickel as a transition metal has been considered as an effective dopant in improving activity of several photocatalyst systems. Chang et al. [27] reported photocatalytic hydrogen production with ZnS/GO, which could be improved from 5700 to 8683 mmol/g h after the Ni substitution at the Ni/Zn ratio of 0.00167. Wang et al. showed the increased hydrogen production rate from 402 to 746 mmol/h for Zn0.4Cd0.6S solid solution after adding 4% nickel [28]. Ganesh et al. successfully enhanced the performance of TiO2 after doped with Ni for photocatalytic methylene blue (MB) degradation [29]. Cai et al. demonstrated Ni-doped ZnO could improve rhodamine B (RhB) degradation [30]. Those works suggested a relatively low amount of Ni doping in a photocatalyst material could significantly promote the photocatalytic reaction. In our previous work [7], we had successfully designed Zn(O,S) solid solution by combining ZnO and ZnS to form solid solution for hydrogen production. The highest rate of hydrogen production of Zn(O,S) was 3405 mmol g1 h1. The present work is to enhance the photocatalytic hydrogen evolution of Zn(O,S). In this present work, we have successfully enhanced the photocatalytic hydrogen production of Zn(O,S), as compared with previous work. The Ni-doped Zn(O,S) has been synthesized with simple method at low temperature processing. Earth-abundant materials, non-precious metal, low power UV lamp, and non-harmful ethanol solution as sacrificial reagent were used in this work. The photocatalytic hydrogen production with different nickel and ethanol contents was investigated. The hydrogen evolution mechanism is also proposed and elucidated in this paper.
chloride hexahydrate were dissolved to 800 mL distilled water under vigorous stirring. This solution was kept heating at 90 C for 3 h, then 0.5 mL hydrazine was added. After cooling down, the obtained precipitates were collected and washed for several times with ethanol. Finally, the precipitates were dried by rotary evaporator. The nickel chloride hexahydrate amounts of 0, 3, 10 and 20% based on the amount of zinc acetate dihydrate were used in this work and the produced powders were denoted as ZnNiOS-0, ZnNiOS-3, ZnNiOS-10, and ZnNiOS-20, respectively.
Characterizations The X-ray diffraction (XRD) pattern of photocatalyst was collected by X-ray diffractometer (D2 phaser) with CuKa ¼ 1.5418 A as an anode. A field emission scanning electron microscope (FE-SEM) JSM 6500F was performed to observe the morphology of photocatalyst. The diffuse reflectance spectra (DRS) and photoluminescence (PL) of the photocatalyst were recorded by UV-VIS Jasco V-670 and Jasco FP8500 spectrophotometer, respectively. The X-ray photoelectron spectroscopy (XPS) was carried out using PHI 5700 ESCA to determine the surface chemical elements of photocatalyst. The GC 1000 (China Chromatography Co.) equipped with thermal conductivity detector (TCD) was used to confirm the calibrated flow rate of the evolved hydrogen that generated from photocatalytic reaction.
Electrochemical measurement The electrochemical measurement was carried out with impedance spectra (EIS) and cyclic-voltammetry (CV) using Princeton Applied Research Versa STAT by utilizing Ag/AgCl, Pt, and glassy carbon electrodes as reference, counter, and working electrodes, respectively. To coat the samples on working electrode, 5 mg catalyst powder was dispersed in 5 mL Nafion solution. The mix solution was dropped on glassy carbon and dried in oven at 100 C for 10 min. The EIS was measured at voltage amplitude of 10 mV, frequency range of 10e20 kHz, whereas the CV was measured at potential range from 1.0 to 1.0 V with a scan rate of 50 mV. All measurements were conducted in the solution contained of 1 M KCl, 5 mM K4Fe(CN)6, and 5 mM K3Fe(CN)6 as electrolyte solution.
Photocatalytic hydrogen production experiments
Experimental section Materials Zinc acetate dihydrate (98%), thioacetamide (99%), and nickel chloride hexahydrate (99.95%) were obtained from Alfa Aesar. All chemicals were directly used without any further purification treatment.
Sample preparation Ni-doped Zn(O,S) was prepared with the similar procedure as previous work [7]. 20 mmol of zinc acetate dihydrate, 10 mmol of thioacetamide, and an appropriate amount of nickel
The photocatalytic hydrogen production experiments were carried out in a tightly closed 500 mL quartz reactor. Practically, 225 mg of photocatalyst was well dispersed in 450 mL the mixture of ethanol and water solution. The ethanol percentage was fixed 0, 10, 50 and 100% based on the total volume. The argon gas was flowed for 1 h before starting photocatalytic reaction to remove all atmospheric gases in the reactor. Four black light UV lamp tubes with 6 W for each were used as a light source. However, only 2/3 of the lamps could be inserted into reactor, therefore the total power of the light source was calculated as 4 4 W. The percentage of hydrogen that generated from reaction was determined by GC-TCD, which was set at 50 C with argon as a gas carrier at a flow
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rate of 100 mL/min. The details of GC calibration and purging procedures had been elucidated in our previous work [7]. Apparent quantum yield (A.Q.Y) was calculated by using following equation. A$Q$Y ¼
Table 1 e Crystallite size and the nickel content of Nidoped Zn(O,S).
number of envolved hydrogen molecules 2 100% number of incident photon
Results and discussion
Sample
Crystalline size (nm)a
[Ni]/([Ni]þ[Zn]) 100%
ZNOS-0 ZNOS-3 ZNOS-10 ZNOS-20
2.428 2.435 2.465 2.481
0 0.16 4.13 7.24
a
Calculation was done by Scherrer equation at (111) plane.
X-ray diffraction pattern analysis pattern of ZNOS-10(A) did not differ from that of ZNOS-10, indicating that Ni-doped Zn(O,S) was stable.
Fig. 1a shows the XRD patterns of Ni-doped Zn(O,S) with different nickel precursor contents and the standard files for cubic ZnS (JCPDF#05-0566) and ZnO (JCPDF#65-2880). The diffraction patterns of all samples were related to the cubic ZnS structure with diffraction peaks slightly shifted to the higher angles. To clearly observe the peak shift, the peaks with the highest intensity at (111) plane were verified with a slower scan rate as shown in Fig. 1b. The XRD pattern of ZNOS-3 did not shift due to the low concentration of nickel, as confirmed by energy dispersive X-ray spectroscopy (EDS) analysis shown in Table 1. The actual nickel amounts in ZNOS-0, ZNOS-3, ZNOS-10, and ZNOS-20 were 0, 0.16, 4.13, and 7.24%, respectively. Until the higher Ni precursor contents up to 10% and 20%, the ZNOS catalyst had the higher Ni content and the observable peak shift. The crystallite size of Nidoped Zn(O,S) did not significantly change (Table 1), even though the Ni precursor concentration reached 20%. The peaks shift is due to Ni with smaller ionic radius (Ni2þ ¼ 69 pm) [31] to occupy the sites of Zn with larger ionic radius (Zn2þ ¼ 74 pm) [31] in Zn(O,S), which leads to the shrinkage of lattice parameter. There were no other observable peaks in the XRD pattern, indicating it is free of secondary phases. After being used for hydrogen production experiment the powder of ZNOS-10 was collected and denoted as ZNOS-10(A). The XRD
ZnOS-20
ZnOS-20
ZnOS-10
ZnOS-10
ZnOS-3
ZnOS-3
ZnOS-0
ZnOS-0
(111)
(311)
(220)
(111)
40
50
(111)
(311)
(220)
ZnOS-10 (A)
ZnS cubic (#05-0566)
30
ZnO cubic (#65-2880)
(b)
ZnOS-10 (A)
Intensity (a.u)
Zn
20
Fig. 2 shows the FE-SEM images of Ni-doped Zn(O,S) with different nickel precursor contents. The morphology of Zn(O,S) did not change before and after doping with nickel. The particle size had no difference among the Zn(O,S) particles with different Ni contents. Based on our previous work, each of the particles actually contained the aggregated tiny nanoparticles. The morphology of Ni-doped Zn(O,S) was not different from that of Zn(O,S) in our previous work [7]. As confirmed by Scherrer calculation in Table 1, the tiny nanoparticles (about 2.4e2.5 nm) of Ni-doped Zn(O,S) were naturally aggregated to form larger particles of 10e60 nm, as shown in Fig. 2. Fig. 3 shows the FE-SEM image and elemental mapping of ZNOS-10. It revealed that the ZNOS-10 catalyst was composed of Zn, Ni, O, and S, and had it elements uniformly distributed. From the element mapping, it can be clearly seen that nickel concentration is obviously lower compared to zinc concentration. The content of oxygen was also lower than that of sulfur in Zn(O,S), indicating it has the ZnS-based structure.
ZnO cubic (#65-2880)
(111)
(a)
Morphology and microstructure
60
70
26
28
ZnS cubic (#05-0566)
30
32
34
2 theta (de gree) Fig. 1 e X-ray diffraction patterns of Ni-doped Zn(O,S) with different nickel precursor contents.
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Fig. 2 e FE-SEM images of Ni-doped Zn(O,S) with different nickel precursor contents at (a) 0%, (b) 3%, (c) 10%, and (d) 20%.
Fig. 3 e FE-SEM element mapping of Zn, Ni, O, and S elements for ZNOS-10 catalyst powder.
Diffuse reflectance spectra and photoluminescence spectra The optical properties of Ni-doped Zn(O,S) were evaluated by using DRS and PL measurements, as shown in Fig. 4. It was observed that the increased nickel content in Zn(O,S) significantly enhanced the light absorbance at UV region (Fig. 4a). However, the edges of optical absorbance spectra did not obviously shift even with the increasing of Ni precursor content to 20%. The highest absorbance was obtained for 10% Ni-doped Zn(O,S) (denoted as ZNOS-10), which was almost twice higher than that for pure Zn(O,S). To further study the optical property of Ni-doped Zn(O,S) catalyst, photoluminescence (PL) experiment was carried out. It is a powerful characterization to evaluate photo carrier recombination by comparing the emission peak height of different catalysts with the same baseline. Higher intensity of PL
emission is related to high recombination rate of photo carriers after excitation. Fig. 4b shows the PL spectra of Ni-doped Zn(O,S) catalysts when photon with the excitation wavelength of 250 nm was given. The emission peaks for all samples were located about 393 nm. The PL spectra shows that pure Zn(O,S) powder had the highest emission intensity. However, after doping with nickel the emission intensity became lower, indicating nickel as dopant could successfully suppress the photo carrier recombination. PL spectrum of ZNOS-10 had the lowest emission intensity due to the lower recombination rate between hole and electron after photo excitation. This result indicates that ZNOS-10 can provide longer life time for photo carriers to execute redox reaction, therefore it had the highest photocatalytic activity as further confirmed by its photocatalytic hydrogen evolution reaction in Fig. 8.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 2 ( 2 0 1 7 ) 2 5 8 9 1 e2 5 9 0 2
Absorbance (a.u)
0.7
(a)
2500
0.6 ZNOS-0 ZNOS-3 ZNOS-10 ZNOS-20
0.5 0.4 0.3
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(b) ZNOS-0 ZNOS-3 ZNOS-10 ZNOS-20
2000 1500 1000
0.2
500
0.1 0
300
350
400
450
350
400
450
500
Waveleng th (nm) Fig. 4 e (a) Diffuse reflectance spectra (DRS) and (b) photoluminescence (PL) spectra of Ni-doped Zn(O,S).
Fig. 5 shows Tauc plots of Ni-doped Zn(O,S) catalysts to determine their band gap energy values. The calculated band gap values were 3.50, 3.51, 3.52, and 3.51 eV for ZNOS-0, ZNOS-3, ZNOS-10, and ZNOS-20, respectively. These band gap values were slightly different and were not varied too much with the varying amounts of Ni in Zn(O,S) system. It was observed the variation amounts of Ni in catalyst only influenced the optical absorbance of Zn(O,S) catalysts. The band gap values were matched to our black light UV tube lamp emission which had a wavelength of 352 nm. Therefore, low intensity of the UV lamp was sufficient to activate Ni-doped Zn(O,S) during the photocatalytic HER.
X-ray photoelectron spectroscopy analysis To study the chemical composition of elements in ZNOS-10, XPS analysis was performed and the results are shown in
Fig. 6. The XPS survey in Fig. 6a shows the presence of Zn, Ni, O, and S elements in catalyst. The binding energy values of Zn were located at 1023.05 eV and 1046.25 eV, corresponding to Zn2p3/2 and Zn2p1/2, respectively as shown in Fig. 6b. These values shifted 1.2 eV to higher binding energy values compared to the reference due to the surface band bending [32]. However, the difference between the two peaks of Zn2p3/2 and Zn2p1/2 (i.e. 23.2 eV) are well consistent with the literature data [33]. Although the high resolution XPS of Ni in Fig. 6c shows the low intensity and relatively high noise, the peaks of Ni2p3/2 and Ni2p1/2 still can be observed at 852.7 eV and 869.9 eV, respectively [33]. The low intensity is attributed to the low concentration of Ni in ZNOS-10, as confirmed by EDS analysis. The state of oxygen was confirmed with the binding energy values at 530.6 eV, 531.54 eV, and 532.38 eV which were related to oxygen in lattice, adsorbed oxygen, and oxygen vacancy, respectively, as shown in Fig. 6d. These binding
Fig. 5 e Tauc plots for the band gap determinations of (a) ZNOS-0, (b) ZNOS-3, (c) ZNOS-10, and (d) ZNOS-20 catalysts.
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Zn 2p
(a) Intensity (a.u)
10000 8000
Ni 2p O 1S
6000 4000
S2p
2000 0 200
(b)
300
400
500
600
700
4700
Zn 2p3/2=1023.05 eV
800
900 1000 1100 1200
(c)
2p1/2=869.9 eV
4500
Zn 2p1/2=1046.25 eV
4000
2p3/2=852.7 eV
4500
3500 3000
Intensity (a.u)
4600
4400
2500
4300 2000 1500
4200 845
1015 1020 1025 1030 1035 1040 1045 1050 1055
(d)
3500
850
855
860
865
870
875
(e)
3000
S 2p1/2=163.80 eV O1s=530.6 eV (lattices)
2800
3000
S 2p3/2=162.60 eV
O1s=531.54 eV (adsorb)
2600
2500
O1s=532.38 eV (Vacancy)
2400
2000
2200
1500 2000
524
526
528
530
532
534
536
538
540
1000 154
156
158
160
162
164
166
168
Bindingenergy (eV) Fig. 6 e (a) XPS survey of ZNOS-10 confirmed the presence of all elements in catalyst powder and high resolution XPS spectra of (b) Zn, (c) Ni, (d) O, and (e) S elements. energy values were well agreed with the literature data [33,34]. Based on the surface area of oxygen spectra, the percentages of oxygen in lattice, adsorbed oxygen, and oxygen vacancy were calculated as 59.82%, 21.19%, and 18.98%, respectively. The binding energy values of sulfur are observed at 162.60 eV and 163.80 eV for 2p3/2 and 2p1/2, respectively, which were also consistent with the literature data [33]. The chemical composition analysis of Ni-doped Zn(O,S) was performed and the amounts of Zn, Ni, O, and S were respectively 25.54%, 1.37%, 34.28%, and 38.95%. Lower oxygen content was also confirmed by the oxygen mapping in Fig. 3 and the XRD structure analysis in Fig. 1.
Electrochemical impedance spectroscopy analysis The purpose of this characterization is to measure the electrical resistance of as-prepared catalysts for understanding the effect of the Ni doping content on Zn(O,S). EIS is one of powerful characterization tools to study the resistance between catalyst-coated electrode and electrolyte which forms the capacitive double layer with charge transfer resistance at the interface contact. Randle equivalent circuit was conducted
to fit the experimental data. Fig. 7 shows the Nyquist plot of EIS measurement for Ni-doped Zn(O,S). The inset in Fig. 7 indicated Rs as the solution resistance, Rp the charge transfer resistance, and CPE the constant phase element to represent the capacitance of double layer [35]. ZNOS-10 had the smallest diameter of semicircle spectra, indicating the most efficient charge transfer. This result well agrees with the PL data in Fig. 4b which shows the lowest emission spectrum due to the efficient charge transfer to hinder the electron-hole recombination rate. The fitting plot using Z scheme software determined the values of charge transfer resistances and it was found the resistivity values were 4203, 3292, 862 and 1604 U for ZNOS-0, ZNOS-3, ZNOS-10, and ZNOS-20, respectively. The properties of Zn(O,S) can be optimized with an appropriate amount of the nickel substitution. Therefore, the systematic work of the Ni doping on Zn(O,S) in this study is important.
Photocatalytic hydrogen production Fig. 8a shows the photocatalytic activities of Ni-doped Zn(O,S) for hydrogen production in 10% ethanol solution under 24 W
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3000 CPE
2500
-Z''(ohm)
Rs
2000
Rp
1500 1000
ZNOS-0 ZNOS-3 ZNOS-10 ZNOS-20
500 0 0
500
1000
1500
Z'(ohm) Fig. 7 e Electrochemical impedance spectra of Ni-doped Zn(O,S) with different Ni precursor amounts.
Hydrogen production (µmol/g)
UV lamp illumination. The hydrogen production rates of ZNOS-0, ZNOS-3, ZNOS-10, and ZNOS-20 were 3405, 3440, 4907, and 1875 mmol g1 h1, respectively. ZNOS-10 was found as a catalyst with the highest photocatalytic activity which was 1.44-fold higher than that of pure Zn(O,S). However, a higher amount of Ni content at 20% with the degraded photocatalytic activity can be attributed to its low optical absorbance, higher emission intensity, and the higher recombination rate of photo carriers, which had been observed in other previous works [28,36,37]. To study the effect of ethanol as sacrificial reagent, the ZNOS-10 was tested in the ethanol solution at different ethanol concentrations. Fig. 8b shows the effect of ethanol concentration on the activity of hydrogen production by the Ni-doped ZNOS-10 photocatalyst. Surprisingly, it was found that the photocatalytic activities dramatically improved at the higher ethanol concentrations of 50% and 100%, as compared to 10%. The highest hydrogen production rate of 14,800 mmol g1 h1 was found to happen in a 50% ethanol solution corresponding to apparent quantum yield 31.5%. For the comparison purpose, P25 (TiO2) with the precious metal of platinum (Pt) as a co-catalyst, the most famous material for hydrogen production, was prepared
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(a)
and tested at the same system and condition. The hydrogen evolution rate of P25/Pt was obtained to be 6414 mmol g1 h1. The rate of our ZNOS-10 in a 50% ethanol solution is 2.3 times higher than TiO2/Pt, 3.0 times higher than ZNOS-10 in a 10% ethanol solution, and 4.3 times higher than Ni-free ZNOS-0. The present work has several advantages compared to the reported works such as (1) the utilization of earth-abundant materials without precious metal, (2) low power consumption (only 0.088 mW/cm2 which is 1/40 times lower than UV light intensity of sunlight [7]), (3) employing economic and environmental friendly sacrificial reagent, and (4) high hydrogen production rate. Table 2 lists some important works on HER by different photocatalyst systems under UV and visible light illumination. Most of the previous works used high power lamps to activate their catalysts in HER experiments. The hydrogen evolution rate in terms of light power input was calculated to evaluate HER efficiency. The unit of mmol g1 h1 W1 was used to consider the input power as an important parameter in hydrogen evolution work. High power lamp needs cooling system to dissipate the heat, otherwise highly flammable hydrogen gas in high temperature environment can initiate an explosive fire in a real application system. Furthermore, most of the works with a high rate of hydrogen evolution always involve the hazardous substance such as CdS or the costly noble metal of Pt or Au as a co-catalyst. Therefore, the safety, cost, and efficiency are the major concerns for a photocatalytic HER system to be used as a renewable energy source. The utilization of 4 6 W black light UV lamps obviously shows that our HER is very special with a simple and low-cost system without burning heat, precious metal, and toxic Cd compounds.
Cyclic voltammetry (CV) analysis To show the reusability or stability of ZNOS-10 catalyst powder, a brief cyclic voltammetry experiment was conducted in 1 M KCl with 5 mM K3Fe(CN)6 and K4Fe(CN)6 as active redox couple. The CV measurement was conducted with the voltage range between 1.0 and 1.0 V and a scan rate of 50 mV/s at 25 C. A well-known Nernstian system with the equation as shown in Eq. (1) can be employed to the analysis of cyclic voltammogram.
75000
(b) 0% 10% 50% 100% P25/Pt(50% ethanol)
60000
20000
ZNOS-0 ZNOS-3 ZNOS-10 ZNOS-20
15000
45000
10000
30000
5000
15000
0
0
0
1
2
3
4
0
5
1
2
3
4
5
Time (h) Fig. 8 e Photocatalytic hydrogen production of (a) Ni-doped Zn(O,S) with different nickel precursor contents in 10% ethanol solution and (b) ZNOS-10 in ethanol solution with different ethanol concentrations, together with P25/Pt as a control.
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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
Materials (Zn, Ni)(O,S) CdS nano rod/CdSe ZnO/Cd0.8Zn0.2S PtePdS/CdS Graphene/ZnIn2S4/CdS quantum dots TiO2 nano wire Titanate nano disks/CdS g-C3N4/Zinc phthalocyanine Carbon nano tubes/CdS ZSM-5 type metal silicates/CdS nanoparticles Ga2O3/CdS quantum dots Nano sized MoS2/graphene Hybrid/CdS TiO2/hexagonal CdS g-C3N4/Au Carbon nano tubes/ZnxCd1xS In2O3/Gd2Ti2O7 CdS/MWCNTs AgInZn7S9 ZnO/In2O3 (AgIn)xZn2(1-x)S2 WO3/Au
Co- catalyst Sacrificial reagent
Light source
Hydrogen evolution rate (mmol g1 h1 W1)
Ref.
e Pt Pt e Pt Pt Ni Pt NiS e Pt MoS2
Ethanol 2-propanol Benzyl alcohol Na2SeNa2SO3 Na2SeNa2SO3 Methanol Ethanol Ascorbic acid Na2SeNa2SO3 Na2SeNa2SO3 Lactic acid Lactic acid
24 W UV tube lamp (Actual power: 16 W) 300 W Xe lamp 450 W Xe lamp 300 W Xe (l > 420 nm) 300 W Xe lamp (l > 420 nm) 450 W Hg lamp 300 W Xe lamp (l > 420 nm) 350 W Xe lamp (l > 420 nm) 350 W Xe lamp (l > 420 nm) 500 W Osram lamp (l > 420 nm) 300 W Xe lamp (l > 420 nm) 300 W Xe lamp (l > 420 nm)
923.0 133.0 81.1 96.7 90 47.8 51.1 35.7 34.7 22 30.2 30
Present work [38] [39] [9] [40] [41] [42] [43] [44] [45] [46] [47]
e e e e e Pt e Pt Pt
Na2SeNa2SO3 Triethanol amine Na2SeNa2SO3 Methanol Na2SeNa2SO3 Na2SeNa2SO3 Methanol Na2SeK2SO3 Glycerol
500 300 500 300 300 300 300 300 300
18 29.6 12.1 19.3 16.6 7.4 6 1.1 0.4
[48] [49] [50] [51] [52] [53] [54] [55] [56]
W W W W W W W W W
Osram lamp Xe lamp (l > Xe lamp Xe lamp (l > Xe lamp (l > Xe lamp Xe lamp (l > Xe lamp Xe lamp (l >
420 nm) 420 nm) 420 nm) 420 nm) 420 nm)
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Table 2 e Various kinds of photocatalyst systems used for hydrogen evolution reactions under UV and visible light illumination. No.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 2 ( 2 0 1 7 ) 2 5 8 9 1 e2 5 9 0 2
i.h i h FeðCNÞ3 E ¼ E 0:0592 log FeðCNÞ4 6 6 0
(1)
where E is the applied potential and E0 is the formal electrode potential. As the applied potential was negatively increased, will decrease at the ZNOS-10 the concentration of Fe(CN)3 6 was reduced to coated electrode surface as the Fe(CN)3 6 Fe(CN)4 6 . Otherwise, if the applied potential was positively increased, the concentration of Fe(CN)3 6 will increase as the Fe(CN)4 6 was oxidized. The reversible redox reaction occurred on the electrode surface, indicating ZNOS-10 catalyst could provide oxidation and reduction reactions. The cathodic and anodic peaks were turned on at 0.33 and 0.11 V, respectively which were related to oxidation and reduction potential 3 values of Fe(CN)4 6 and Fe(CN)6 as shown in Fig. 9. After 100 runs of CV measurement, ZNOS-10 still showed its capabilities in oxidizing and reducing the redox species in solution. Based on the CV quantitative simulation of Nicholson and Shain [57,58], the diffusion coefficient of redox species could be calculated using Eq. (2) as follows: Ip ¼ 2:69 105 n3=2 AD1=2 v1=2 C
(2)
where Ip is the peak current, n is the number of electrons transferred, A is the electrode area, D is the diffusion coefficient of redox species, v is the scan rate, and C is the bulk concentration of species. Based on the equation Eq. (2), the 3 diffusion coefficients of Fe(CN)4 6 and Fe(CN)6 species were 12 12 2 and 1.93 10 m /s, respectively. The CV analysis 3.2 10 has confirmed the stability of ZNOS-10 catalyst to continuously oxidize and reduce the K4Fe(CN)6 and K3Fe(CN)6 redox species, respectively, in a KCl solution.
Photocatalytic reaction mechanism Based on the color change of our catalyst as shown in Fig. 10 during the photocatalytic reaction, oxygen vacancies are
anodic peak
Current (A)
50.00µ
25899
expected to form and involved in the photocatalytic mechanism. Fig. 10 shows the schematic drawing of surface photocatalytic reaction of Ni-doped Zn(O,S) catalyst to generate hydrogen. When the photon energy larger than band gap of Ni-doped Zn(O,S), the electrons and holes will be generated in the valence and conduction bands, respectively. Water will be oxidized by the holes and surface oxygen to form the oxygen vacancies and hydroxyl ions, and water will be reduced by the electrons and oxygen vacancies to produce hydrogen. Hydrogen evolution reaction of ZNOS-10 is initiated by oxidation reaction of ethanol (Eq. (3)) and water (Eq. (4)) to provide oxygen vacancy sites, which will trap oxygen from water and make the OeH bonding of water molecule weak and finally form Hþ cation and H2 gas as shown in Eqs. (5) and (6), respectively. As the highest rate occurs in a 50% ethanol solution, the oxygen vacancies are faster produced in ethanol compared to that in pure water. The role of oxygen vacancies to enhance photocatalytic activity had been proposed in the other work by Moya et al. and Gan et al. [7,59,60]. However, the HER rate is lower for pure ethanol solution, indicating water oxidation reaction is still critical and cannot be ignored. During the oxidation of ethanol, the produced water and the ethanol reagent in solution would synergically enhance the hydrogen evolution rate. þ
2þ C2 H5 OH þ O2 surf þ 4h /CH3 CHO þ H2 O þ VO;surf
þ
(3)
2þ H2 O þ O2 surf þ 2h /2OHaq: þ VO;surf
(4)
0 þ H2 O þ V2þ O;surf /2H þ OO;surf
(5)
0 H2 O þ V2þ O;surf þ 2e /H2 þ OO;surf
(6)
There is no oxygen liberated during the reaction because the surface oxygen anions, the intrinsic Schottky defect, involved in the water redox reactions. The surface oxygen anion, originated from the interaction of point defects close to interface, is crucial for our HER mechanism. To produce the water reduction for a large amount of hydrogen, the Ni-doped Zn(O,S) has to form many oxygen vacancies, therefore the catalyst changes its color from white to dark grey. However,
25.00µ
0.00
cathodic peak -25.00µ 0.33 V
0.11 V
hν
-50.00µ -1.2
-0.8
-0.4
0.0
0.4
0.8
1.2
Potential (V) Fig. 9 e Cyclic voltammogram of ZNOS-10 powder coated glassy carbon electrode in a mixed solution of 1 M KCl, 5 mM K3Fe(CN)6, and 5 mM K4Fe(CN)6 with Ag/AgCl and platinum as reference and counter electrodes, respectively, for 100 cycles.
VB
Fig. 10 e Schematic drawing of surface photocatalytic reaction of Ni-doped Zn(O,S) catalyst to generate hydrogen.
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 2 ( 2 0 1 7 ) 2 5 8 9 1 e2 5 9 0 2
the catalyst color can reversibly change back from dark grey to white after the experiment is terminated and the reactor stands still for overnight. The dark grey Ni-doped Zn(O,S) with many oxygen vacancies did show the Zn peak after the XRD structure analysis. As for the photocatalytic hydrogen evolution reactions, the charge separation of photo generated electrons and holes are the light absorbance of catalyst are always the key issues. The incorporation of nickel as dopant in Zn(O,S) lattice could efficiently lower the recombination rate of photo carriers and lower the charge transfer resistance. It is well known that dopant could serve as trapping center and recombination center of charge carrier [61]. Dopant as trapping center is very useful since it can decrease the charge recombination and enhance the properties or photocatalytic activity of a material. In the other side dopant as recombination center is useless because it will be a favorable site for charge carriers to recombine. However too much nickel in the lattice of Zn(O,S) might become the center for electron-hole recombination [28]. In the case of Ni-doped Zn(O,S), when nickel content is relatively low (10%) it serves as trapping center for charge carriers leading to efficient separation. In the other hand, when the nickel content is high (increased to 20%), it is assumed it serves as recombination center. Furthermore, some similar results from other works also showed the dopant concentration had the optimum point to enhance the photocatalytic activity [27,30,61]. The optimum point in our work is ZNOS-10. However, the 4.3-fold difference between the Ni-free Zn(O,S) and 4.13%Ni-doped ZNOS-10 in HER should involve other factors. We propose that the substitution of Ni in Zn(O,S) with 4.13%Ni leads to the weakest surface oxygen bonding to easily proceed the reactions in Eqs. (3) and (4) for producing the oxygen vacancies. With this easy formation of oxygen vacancies, HER can be accelerated. However, the oxygen formation cannot guarantee the HER enhancement. TiO2, as an example, becomes n-type semiconductor with many oxygen vacancies after it is reduced. It has no holes to proceed the water oxidation reactions of Eqs. (3) and (4), so its HER cannot sustain. The challenge of HER is to have not only the low recombination rate in the photo induced electrons and holes but also the reversible exchange of surface oxygen anions. Furthermore, the lattice distortion occurred due to the Ni dopant in Zn(O,S) lattice was also one of our consideration that induced the charge transfer resistances were lower in (Zn,Ni)(O,S) system [62e64]. The more amount of Ni dopant, the more distortion in lattice was occurred. Accordingly, the charge transfer resistance would increase and lower the photocatalytic reaction. Therefore, at high Ni content for ZNOS-20, the photocatalytic performance is lower than that of ZNOS-10.
Conclusions Ni-doped Zn(O,S) had been successfully synthesized with different nickel precursor contents by simple chemical method at low temperature. It was found that a small amount of 4.13% Ni in ZNOS-10 had significantly enhanced photocatalytic activity of Zn(O,S) for hydrogen production. The highest activity was obtained from ZNOS-10 in a 50% ethanol
solution with the hydrogen production rate of 14,800 mmol g1$h1 corresponding to apparent quantum yield 31.5%, which was 4.3 times higher than that of pure Zn(O,S). The great activity of Ni-doped Zn(O,S) in HER is partially contributed by the synergetic effect of high light absorbance, high separation rate of photo induced carriers, and relatively low charge transfer resistance. The generated oxygen vacancies also play the important role in photocatalytic HER, as evidenced by the catalyst color change from white to dark grey under UV irradiation. The Ni substitution to the Zn site in Zn(O,S) is demonstrated to weaken the surface oxygen bonding for facilitating the reversible oxygen exchange in forming oxygen vacancies.
Acknowledgement This work was supported by the Ministry of Science and Technology of Republic of China under Grant no. MOST 1063111-Y-042A-093.
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Enhanced photocatalytic hydrogen production of noble-metal free Ni-doped Zn(O,S) in ethanol solution Noto Susanto Gultom, Hairus Abdullah, Dong-Hau Kuo* 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:
High efficient hydrogen evolved Ni-doped Zn(O,S) photocatalyst with different Ni amounts
Received 28 June 2017
had been successfully synthesized with a simple method at low temperature. Our Ni-doped
Received in revised form
Zn(O,S) catalyst reached the highest hydrogen generation rate of 14,800 mmol g1 h1 or
6 August 2017
0.92 mmol g1 h1 W1 corresponding to apparent quantum yield 31.5%, which was 2.3
Accepted 29 August 2017
times higher compared to the TiO2/Pt used as a control in this work. It was found that a
Available online 20 September 2017
small amount of Ni doped into Zn(O,S) nanoparticles could increase the optical absorbance, lower the charge transfer resistance, accordingly decrease the electron-hole recombination
Keywords:
rate, and significantly enhance the photocatalytic hydrogen evolution reaction (HER). The
Nickel doping
as-prepared catalyst has the characteristics of low cost, low power consumption for acti-
Hydrogen production
vating the catalytic HER, abundant and environmental friendly constituents, and low
Zinc oxysulfide
surface oxygen bonding for forming oxygen vacancies. The photocatalytic performance of
Oxygen vacancy
Ni-doped Zn(O,S) was demonstrated with a proposed kinetic mechanism in this paper. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Nowadays, the major energy source is derived from fossil fuels. It is well known that the utilization of non-renewable, limited, and CO2-generated fossil fuels leads to global warming and climate change. One of the undoubted solutions is the utilization of renewable, abundant and environmental friendly resources to overcome those issues. Among the renewable energy sources, solar energy is the most promising one due to its abundance, only 0.01% energy from the sunlight could satisfy the human energy necessity [1]. The challenge is to convert and store the energy in an appropriate compound as energy carrier, therefore it could be practically used in
future. Photocatalytic hydrogen production has been considered as an ideal method to produce hydrogen as energy carrier. As the final product of hydrogen combustion is only water, hydrogen is believed as an alternative clean energy carrier in the future. Since the invention of Fujishima and Honda in 1972 [2], photocatalytic water splitting has been widely investigated. More than hundreds of photocatalysts (metal oxide, metal sulfide, oxynitride, oxysulfide, etc.) [3e7] have been reported for overall or half-cell reactions of photocatalytic water splitting. High efficiencies of photocatalytic reactions have been achieved with the utilization of toxic CdS-related photocatalysts [8e10]. Furthermore, most of the catalysts for water splitting used the precious metal like Pt as co-catalyst [11,12]. In addition, water
* Corresponding author. E-mail address: [email protected] (D.-H. Kuo). http://dx.doi.org/10.1016/j.ijhydene.2017.08.198 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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splitting also used high power and harmful sacrificial reagent Na2S/Na2SO3 to improve its performance [13]. Therefore, to design an efficient photocatalyst with earth-abundant and environmental friendly material is needed to promote wide scale applications. To replace the precious metal, low cost and green materials such as carbon, manganese, cobalt and nickel have the potential to be further explored [14e17]. The abilities of photocatalyst to absorb light (UV or visible) and suppress the photo-generated electron-hole recombination are the keys to improve photocatalytic activity. The concepts such as p-n heterojunction, solid solution formation, sacrificial reagent, dye sensitizer, co-catalyst, and doping have been proved in previous works [11,18e22]. Doping concept has been commonly known as a strategy to increase the surface area [23], to enhance the absorbance against light and to suppress the electron-hole recombination of semiconductors [24e26]. Nickel as a transition metal has been considered as an effective dopant in improving activity of several photocatalyst systems. Chang et al. [27] reported photocatalytic hydrogen production with ZnS/GO, which could be improved from 5700 to 8683 mmol/g h after the Ni substitution at the Ni/Zn ratio of 0.00167. Wang et al. showed the increased hydrogen production rate from 402 to 746 mmol/h for Zn0.4Cd0.6S solid solution after adding 4% nickel [28]. Ganesh et al. successfully enhanced the performance of TiO2 after doped with Ni for photocatalytic methylene blue (MB) degradation [29]. Cai et al. demonstrated Ni-doped ZnO could improve rhodamine B (RhB) degradation [30]. Those works suggested a relatively low amount of Ni doping in a photocatalyst material could significantly promote the photocatalytic reaction. In our previous work [7], we had successfully designed Zn(O,S) solid solution by combining ZnO and ZnS to form solid solution for hydrogen production. The highest rate of hydrogen production of Zn(O,S) was 3405 mmol g1 h1. The present work is to enhance the photocatalytic hydrogen evolution of Zn(O,S). In this present work, we have successfully enhanced the photocatalytic hydrogen production of Zn(O,S), as compared with previous work. The Ni-doped Zn(O,S) has been synthesized with simple method at low temperature processing. Earth-abundant materials, non-precious metal, low power UV lamp, and non-harmful ethanol solution as sacrificial reagent were used in this work. The photocatalytic hydrogen production with different nickel and ethanol contents was investigated. The hydrogen evolution mechanism is also proposed and elucidated in this paper.
chloride hexahydrate were dissolved to 800 mL distilled water under vigorous stirring. This solution was kept heating at 90 C for 3 h, then 0.5 mL hydrazine was added. After cooling down, the obtained precipitates were collected and washed for several times with ethanol. Finally, the precipitates were dried by rotary evaporator. The nickel chloride hexahydrate amounts of 0, 3, 10 and 20% based on the amount of zinc acetate dihydrate were used in this work and the produced powders were denoted as ZnNiOS-0, ZnNiOS-3, ZnNiOS-10, and ZnNiOS-20, respectively.
Characterizations The X-ray diffraction (XRD) pattern of photocatalyst was collected by X-ray diffractometer (D2 phaser) with CuKa ¼ 1.5418 A as an anode. A field emission scanning electron microscope (FE-SEM) JSM 6500F was performed to observe the morphology of photocatalyst. The diffuse reflectance spectra (DRS) and photoluminescence (PL) of the photocatalyst were recorded by UV-VIS Jasco V-670 and Jasco FP8500 spectrophotometer, respectively. The X-ray photoelectron spectroscopy (XPS) was carried out using PHI 5700 ESCA to determine the surface chemical elements of photocatalyst. The GC 1000 (China Chromatography Co.) equipped with thermal conductivity detector (TCD) was used to confirm the calibrated flow rate of the evolved hydrogen that generated from photocatalytic reaction.
Electrochemical measurement The electrochemical measurement was carried out with impedance spectra (EIS) and cyclic-voltammetry (CV) using Princeton Applied Research Versa STAT by utilizing Ag/AgCl, Pt, and glassy carbon electrodes as reference, counter, and working electrodes, respectively. To coat the samples on working electrode, 5 mg catalyst powder was dispersed in 5 mL Nafion solution. The mix solution was dropped on glassy carbon and dried in oven at 100 C for 10 min. The EIS was measured at voltage amplitude of 10 mV, frequency range of 10e20 kHz, whereas the CV was measured at potential range from 1.0 to 1.0 V with a scan rate of 50 mV. All measurements were conducted in the solution contained of 1 M KCl, 5 mM K4Fe(CN)6, and 5 mM K3Fe(CN)6 as electrolyte solution.
Photocatalytic hydrogen production experiments
Experimental section Materials Zinc acetate dihydrate (98%), thioacetamide (99%), and nickel chloride hexahydrate (99.95%) were obtained from Alfa Aesar. All chemicals were directly used without any further purification treatment.
Sample preparation Ni-doped Zn(O,S) was prepared with the similar procedure as previous work [7]. 20 mmol of zinc acetate dihydrate, 10 mmol of thioacetamide, and an appropriate amount of nickel
The photocatalytic hydrogen production experiments were carried out in a tightly closed 500 mL quartz reactor. Practically, 225 mg of photocatalyst was well dispersed in 450 mL the mixture of ethanol and water solution. The ethanol percentage was fixed 0, 10, 50 and 100% based on the total volume. The argon gas was flowed for 1 h before starting photocatalytic reaction to remove all atmospheric gases in the reactor. Four black light UV lamp tubes with 6 W for each were used as a light source. However, only 2/3 of the lamps could be inserted into reactor, therefore the total power of the light source was calculated as 4 4 W. The percentage of hydrogen that generated from reaction was determined by GC-TCD, which was set at 50 C with argon as a gas carrier at a flow
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rate of 100 mL/min. The details of GC calibration and purging procedures had been elucidated in our previous work [7]. Apparent quantum yield (A.Q.Y) was calculated by using following equation. A$Q$Y ¼
Table 1 e Crystallite size and the nickel content of Nidoped Zn(O,S).
number of envolved hydrogen molecules 2 100% number of incident photon
Results and discussion
Sample
Crystalline size (nm)a
[Ni]/([Ni]þ[Zn]) 100%
ZNOS-0 ZNOS-3 ZNOS-10 ZNOS-20
2.428 2.435 2.465 2.481
0 0.16 4.13 7.24
a
Calculation was done by Scherrer equation at (111) plane.
X-ray diffraction pattern analysis pattern of ZNOS-10(A) did not differ from that of ZNOS-10, indicating that Ni-doped Zn(O,S) was stable.
Fig. 1a shows the XRD patterns of Ni-doped Zn(O,S) with different nickel precursor contents and the standard files for cubic ZnS (JCPDF#05-0566) and ZnO (JCPDF#65-2880). The diffraction patterns of all samples were related to the cubic ZnS structure with diffraction peaks slightly shifted to the higher angles. To clearly observe the peak shift, the peaks with the highest intensity at (111) plane were verified with a slower scan rate as shown in Fig. 1b. The XRD pattern of ZNOS-3 did not shift due to the low concentration of nickel, as confirmed by energy dispersive X-ray spectroscopy (EDS) analysis shown in Table 1. The actual nickel amounts in ZNOS-0, ZNOS-3, ZNOS-10, and ZNOS-20 were 0, 0.16, 4.13, and 7.24%, respectively. Until the higher Ni precursor contents up to 10% and 20%, the ZNOS catalyst had the higher Ni content and the observable peak shift. The crystallite size of Nidoped Zn(O,S) did not significantly change (Table 1), even though the Ni precursor concentration reached 20%. The peaks shift is due to Ni with smaller ionic radius (Ni2þ ¼ 69 pm) [31] to occupy the sites of Zn with larger ionic radius (Zn2þ ¼ 74 pm) [31] in Zn(O,S), which leads to the shrinkage of lattice parameter. There were no other observable peaks in the XRD pattern, indicating it is free of secondary phases. After being used for hydrogen production experiment the powder of ZNOS-10 was collected and denoted as ZNOS-10(A). The XRD
ZnOS-20
ZnOS-20
ZnOS-10
ZnOS-10
ZnOS-3
ZnOS-3
ZnOS-0
ZnOS-0
(111)
(311)
(220)
(111)
40
50
(111)
(311)
(220)
ZnOS-10 (A)
ZnS cubic (#05-0566)
30
ZnO cubic (#65-2880)
(b)
ZnOS-10 (A)
Intensity (a.u)
Zn
20
Fig. 2 shows the FE-SEM images of Ni-doped Zn(O,S) with different nickel precursor contents. The morphology of Zn(O,S) did not change before and after doping with nickel. The particle size had no difference among the Zn(O,S) particles with different Ni contents. Based on our previous work, each of the particles actually contained the aggregated tiny nanoparticles. The morphology of Ni-doped Zn(O,S) was not different from that of Zn(O,S) in our previous work [7]. As confirmed by Scherrer calculation in Table 1, the tiny nanoparticles (about 2.4e2.5 nm) of Ni-doped Zn(O,S) were naturally aggregated to form larger particles of 10e60 nm, as shown in Fig. 2. Fig. 3 shows the FE-SEM image and elemental mapping of ZNOS-10. It revealed that the ZNOS-10 catalyst was composed of Zn, Ni, O, and S, and had it elements uniformly distributed. From the element mapping, it can be clearly seen that nickel concentration is obviously lower compared to zinc concentration. The content of oxygen was also lower than that of sulfur in Zn(O,S), indicating it has the ZnS-based structure.
ZnO cubic (#65-2880)
(111)
(a)
Morphology and microstructure
60
70
26
28
ZnS cubic (#05-0566)
30
32
34
2 theta (de gree) Fig. 1 e X-ray diffraction patterns of Ni-doped Zn(O,S) with different nickel precursor contents.
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Fig. 2 e FE-SEM images of Ni-doped Zn(O,S) with different nickel precursor contents at (a) 0%, (b) 3%, (c) 10%, and (d) 20%.
Fig. 3 e FE-SEM element mapping of Zn, Ni, O, and S elements for ZNOS-10 catalyst powder.
Diffuse reflectance spectra and photoluminescence spectra The optical properties of Ni-doped Zn(O,S) were evaluated by using DRS and PL measurements, as shown in Fig. 4. It was observed that the increased nickel content in Zn(O,S) significantly enhanced the light absorbance at UV region (Fig. 4a). However, the edges of optical absorbance spectra did not obviously shift even with the increasing of Ni precursor content to 20%. The highest absorbance was obtained for 10% Ni-doped Zn(O,S) (denoted as ZNOS-10), which was almost twice higher than that for pure Zn(O,S). To further study the optical property of Ni-doped Zn(O,S) catalyst, photoluminescence (PL) experiment was carried out. It is a powerful characterization to evaluate photo carrier recombination by comparing the emission peak height of different catalysts with the same baseline. Higher intensity of PL
emission is related to high recombination rate of photo carriers after excitation. Fig. 4b shows the PL spectra of Ni-doped Zn(O,S) catalysts when photon with the excitation wavelength of 250 nm was given. The emission peaks for all samples were located about 393 nm. The PL spectra shows that pure Zn(O,S) powder had the highest emission intensity. However, after doping with nickel the emission intensity became lower, indicating nickel as dopant could successfully suppress the photo carrier recombination. PL spectrum of ZNOS-10 had the lowest emission intensity due to the lower recombination rate between hole and electron after photo excitation. This result indicates that ZNOS-10 can provide longer life time for photo carriers to execute redox reaction, therefore it had the highest photocatalytic activity as further confirmed by its photocatalytic hydrogen evolution reaction in Fig. 8.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 2 ( 2 0 1 7 ) 2 5 8 9 1 e2 5 9 0 2
Absorbance (a.u)
0.7
(a)
2500
0.6 ZNOS-0 ZNOS-3 ZNOS-10 ZNOS-20
0.5 0.4 0.3
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(b) ZNOS-0 ZNOS-3 ZNOS-10 ZNOS-20
2000 1500 1000
0.2
500
0.1 0
300
350
400
450
350
400
450
500
Waveleng th (nm) Fig. 4 e (a) Diffuse reflectance spectra (DRS) and (b) photoluminescence (PL) spectra of Ni-doped Zn(O,S).
Fig. 5 shows Tauc plots of Ni-doped Zn(O,S) catalysts to determine their band gap energy values. The calculated band gap values were 3.50, 3.51, 3.52, and 3.51 eV for ZNOS-0, ZNOS-3, ZNOS-10, and ZNOS-20, respectively. These band gap values were slightly different and were not varied too much with the varying amounts of Ni in Zn(O,S) system. It was observed the variation amounts of Ni in catalyst only influenced the optical absorbance of Zn(O,S) catalysts. The band gap values were matched to our black light UV tube lamp emission which had a wavelength of 352 nm. Therefore, low intensity of the UV lamp was sufficient to activate Ni-doped Zn(O,S) during the photocatalytic HER.
X-ray photoelectron spectroscopy analysis To study the chemical composition of elements in ZNOS-10, XPS analysis was performed and the results are shown in
Fig. 6. The XPS survey in Fig. 6a shows the presence of Zn, Ni, O, and S elements in catalyst. The binding energy values of Zn were located at 1023.05 eV and 1046.25 eV, corresponding to Zn2p3/2 and Zn2p1/2, respectively as shown in Fig. 6b. These values shifted 1.2 eV to higher binding energy values compared to the reference due to the surface band bending [32]. However, the difference between the two peaks of Zn2p3/2 and Zn2p1/2 (i.e. 23.2 eV) are well consistent with the literature data [33]. Although the high resolution XPS of Ni in Fig. 6c shows the low intensity and relatively high noise, the peaks of Ni2p3/2 and Ni2p1/2 still can be observed at 852.7 eV and 869.9 eV, respectively [33]. The low intensity is attributed to the low concentration of Ni in ZNOS-10, as confirmed by EDS analysis. The state of oxygen was confirmed with the binding energy values at 530.6 eV, 531.54 eV, and 532.38 eV which were related to oxygen in lattice, adsorbed oxygen, and oxygen vacancy, respectively, as shown in Fig. 6d. These binding
Fig. 5 e Tauc plots for the band gap determinations of (a) ZNOS-0, (b) ZNOS-3, (c) ZNOS-10, and (d) ZNOS-20 catalysts.
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Zn 2p
(a) Intensity (a.u)
10000 8000
Ni 2p O 1S
6000 4000
S2p
2000 0 200
(b)
300
400
500
600
700
4700
Zn 2p3/2=1023.05 eV
800
900 1000 1100 1200
(c)
2p1/2=869.9 eV
4500
Zn 2p1/2=1046.25 eV
4000
2p3/2=852.7 eV
4500
3500 3000
Intensity (a.u)
4600
4400
2500
4300 2000 1500
4200 845
1015 1020 1025 1030 1035 1040 1045 1050 1055
(d)
3500
850
855
860
865
870
875
(e)
3000
S 2p1/2=163.80 eV O1s=530.6 eV (lattices)
2800
3000
S 2p3/2=162.60 eV
O1s=531.54 eV (adsorb)
2600
2500
O1s=532.38 eV (Vacancy)
2400
2000
2200
1500 2000
524
526
528
530
532
534
536
538
540
1000 154
156
158
160
162
164
166
168
Bindingenergy (eV) Fig. 6 e (a) XPS survey of ZNOS-10 confirmed the presence of all elements in catalyst powder and high resolution XPS spectra of (b) Zn, (c) Ni, (d) O, and (e) S elements. energy values were well agreed with the literature data [33,34]. Based on the surface area of oxygen spectra, the percentages of oxygen in lattice, adsorbed oxygen, and oxygen vacancy were calculated as 59.82%, 21.19%, and 18.98%, respectively. The binding energy values of sulfur are observed at 162.60 eV and 163.80 eV for 2p3/2 and 2p1/2, respectively, which were also consistent with the literature data [33]. The chemical composition analysis of Ni-doped Zn(O,S) was performed and the amounts of Zn, Ni, O, and S were respectively 25.54%, 1.37%, 34.28%, and 38.95%. Lower oxygen content was also confirmed by the oxygen mapping in Fig. 3 and the XRD structure analysis in Fig. 1.
Electrochemical impedance spectroscopy analysis The purpose of this characterization is to measure the electrical resistance of as-prepared catalysts for understanding the effect of the Ni doping content on Zn(O,S). EIS is one of powerful characterization tools to study the resistance between catalyst-coated electrode and electrolyte which forms the capacitive double layer with charge transfer resistance at the interface contact. Randle equivalent circuit was conducted
to fit the experimental data. Fig. 7 shows the Nyquist plot of EIS measurement for Ni-doped Zn(O,S). The inset in Fig. 7 indicated Rs as the solution resistance, Rp the charge transfer resistance, and CPE the constant phase element to represent the capacitance of double layer [35]. ZNOS-10 had the smallest diameter of semicircle spectra, indicating the most efficient charge transfer. This result well agrees with the PL data in Fig. 4b which shows the lowest emission spectrum due to the efficient charge transfer to hinder the electron-hole recombination rate. The fitting plot using Z scheme software determined the values of charge transfer resistances and it was found the resistivity values were 4203, 3292, 862 and 1604 U for ZNOS-0, ZNOS-3, ZNOS-10, and ZNOS-20, respectively. The properties of Zn(O,S) can be optimized with an appropriate amount of the nickel substitution. Therefore, the systematic work of the Ni doping on Zn(O,S) in this study is important.
Photocatalytic hydrogen production Fig. 8a shows the photocatalytic activities of Ni-doped Zn(O,S) for hydrogen production in 10% ethanol solution under 24 W
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3000 CPE
2500
-Z''(ohm)
Rs
2000
Rp
1500 1000
ZNOS-0 ZNOS-3 ZNOS-10 ZNOS-20
500 0 0
500
1000
1500
Z'(ohm) Fig. 7 e Electrochemical impedance spectra of Ni-doped Zn(O,S) with different Ni precursor amounts.
Hydrogen production (µmol/g)
UV lamp illumination. The hydrogen production rates of ZNOS-0, ZNOS-3, ZNOS-10, and ZNOS-20 were 3405, 3440, 4907, and 1875 mmol g1 h1, respectively. ZNOS-10 was found as a catalyst with the highest photocatalytic activity which was 1.44-fold higher than that of pure Zn(O,S). However, a higher amount of Ni content at 20% with the degraded photocatalytic activity can be attributed to its low optical absorbance, higher emission intensity, and the higher recombination rate of photo carriers, which had been observed in other previous works [28,36,37]. To study the effect of ethanol as sacrificial reagent, the ZNOS-10 was tested in the ethanol solution at different ethanol concentrations. Fig. 8b shows the effect of ethanol concentration on the activity of hydrogen production by the Ni-doped ZNOS-10 photocatalyst. Surprisingly, it was found that the photocatalytic activities dramatically improved at the higher ethanol concentrations of 50% and 100%, as compared to 10%. The highest hydrogen production rate of 14,800 mmol g1 h1 was found to happen in a 50% ethanol solution corresponding to apparent quantum yield 31.5%. For the comparison purpose, P25 (TiO2) with the precious metal of platinum (Pt) as a co-catalyst, the most famous material for hydrogen production, was prepared
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(a)
and tested at the same system and condition. The hydrogen evolution rate of P25/Pt was obtained to be 6414 mmol g1 h1. The rate of our ZNOS-10 in a 50% ethanol solution is 2.3 times higher than TiO2/Pt, 3.0 times higher than ZNOS-10 in a 10% ethanol solution, and 4.3 times higher than Ni-free ZNOS-0. The present work has several advantages compared to the reported works such as (1) the utilization of earth-abundant materials without precious metal, (2) low power consumption (only 0.088 mW/cm2 which is 1/40 times lower than UV light intensity of sunlight [7]), (3) employing economic and environmental friendly sacrificial reagent, and (4) high hydrogen production rate. Table 2 lists some important works on HER by different photocatalyst systems under UV and visible light illumination. Most of the previous works used high power lamps to activate their catalysts in HER experiments. The hydrogen evolution rate in terms of light power input was calculated to evaluate HER efficiency. The unit of mmol g1 h1 W1 was used to consider the input power as an important parameter in hydrogen evolution work. High power lamp needs cooling system to dissipate the heat, otherwise highly flammable hydrogen gas in high temperature environment can initiate an explosive fire in a real application system. Furthermore, most of the works with a high rate of hydrogen evolution always involve the hazardous substance such as CdS or the costly noble metal of Pt or Au as a co-catalyst. Therefore, the safety, cost, and efficiency are the major concerns for a photocatalytic HER system to be used as a renewable energy source. The utilization of 4 6 W black light UV lamps obviously shows that our HER is very special with a simple and low-cost system without burning heat, precious metal, and toxic Cd compounds.
Cyclic voltammetry (CV) analysis To show the reusability or stability of ZNOS-10 catalyst powder, a brief cyclic voltammetry experiment was conducted in 1 M KCl with 5 mM K3Fe(CN)6 and K4Fe(CN)6 as active redox couple. The CV measurement was conducted with the voltage range between 1.0 and 1.0 V and a scan rate of 50 mV/s at 25 C. A well-known Nernstian system with the equation as shown in Eq. (1) can be employed to the analysis of cyclic voltammogram.
75000
(b) 0% 10% 50% 100% P25/Pt(50% ethanol)
60000
20000
ZNOS-0 ZNOS-3 ZNOS-10 ZNOS-20
15000
45000
10000
30000
5000
15000
0
0
0
1
2
3
4
0
5
1
2
3
4
5
Time (h) Fig. 8 e Photocatalytic hydrogen production of (a) Ni-doped Zn(O,S) with different nickel precursor contents in 10% ethanol solution and (b) ZNOS-10 in ethanol solution with different ethanol concentrations, together with P25/Pt as a control.
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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
Materials (Zn, Ni)(O,S) CdS nano rod/CdSe ZnO/Cd0.8Zn0.2S PtePdS/CdS Graphene/ZnIn2S4/CdS quantum dots TiO2 nano wire Titanate nano disks/CdS g-C3N4/Zinc phthalocyanine Carbon nano tubes/CdS ZSM-5 type metal silicates/CdS nanoparticles Ga2O3/CdS quantum dots Nano sized MoS2/graphene Hybrid/CdS TiO2/hexagonal CdS g-C3N4/Au Carbon nano tubes/ZnxCd1xS In2O3/Gd2Ti2O7 CdS/MWCNTs AgInZn7S9 ZnO/In2O3 (AgIn)xZn2(1-x)S2 WO3/Au
Co- catalyst Sacrificial reagent
Light source
Hydrogen evolution rate (mmol g1 h1 W1)
Ref.
e Pt Pt e Pt Pt Ni Pt NiS e Pt MoS2
Ethanol 2-propanol Benzyl alcohol Na2SeNa2SO3 Na2SeNa2SO3 Methanol Ethanol Ascorbic acid Na2SeNa2SO3 Na2SeNa2SO3 Lactic acid Lactic acid
24 W UV tube lamp (Actual power: 16 W) 300 W Xe lamp 450 W Xe lamp 300 W Xe (l > 420 nm) 300 W Xe lamp (l > 420 nm) 450 W Hg lamp 300 W Xe lamp (l > 420 nm) 350 W Xe lamp (l > 420 nm) 350 W Xe lamp (l > 420 nm) 500 W Osram lamp (l > 420 nm) 300 W Xe lamp (l > 420 nm) 300 W Xe lamp (l > 420 nm)
923.0 133.0 81.1 96.7 90 47.8 51.1 35.7 34.7 22 30.2 30
Present work [38] [39] [9] [40] [41] [42] [43] [44] [45] [46] [47]
e e e e e Pt e Pt Pt
Na2SeNa2SO3 Triethanol amine Na2SeNa2SO3 Methanol Na2SeNa2SO3 Na2SeNa2SO3 Methanol Na2SeK2SO3 Glycerol
500 300 500 300 300 300 300 300 300
18 29.6 12.1 19.3 16.6 7.4 6 1.1 0.4
[48] [49] [50] [51] [52] [53] [54] [55] [56]
W W W W W W W W W
Osram lamp Xe lamp (l > Xe lamp Xe lamp (l > Xe lamp (l > Xe lamp Xe lamp (l > Xe lamp Xe lamp (l >
420 nm) 420 nm) 420 nm) 420 nm) 420 nm)
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 2 ( 2 0 1 7 ) 2 5 8 9 1 e2 5 9 0 2
Table 2 e Various kinds of photocatalyst systems used for hydrogen evolution reactions under UV and visible light illumination. No.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 2 ( 2 0 1 7 ) 2 5 8 9 1 e2 5 9 0 2
i.h i h FeðCNÞ3 E ¼ E 0:0592 log FeðCNÞ4 6 6 0
(1)
where E is the applied potential and E0 is the formal electrode potential. As the applied potential was negatively increased, will decrease at the ZNOS-10 the concentration of Fe(CN)3 6 was reduced to coated electrode surface as the Fe(CN)3 6 Fe(CN)4 6 . Otherwise, if the applied potential was positively increased, the concentration of Fe(CN)3 6 will increase as the Fe(CN)4 6 was oxidized. The reversible redox reaction occurred on the electrode surface, indicating ZNOS-10 catalyst could provide oxidation and reduction reactions. The cathodic and anodic peaks were turned on at 0.33 and 0.11 V, respectively which were related to oxidation and reduction potential 3 values of Fe(CN)4 6 and Fe(CN)6 as shown in Fig. 9. After 100 runs of CV measurement, ZNOS-10 still showed its capabilities in oxidizing and reducing the redox species in solution. Based on the CV quantitative simulation of Nicholson and Shain [57,58], the diffusion coefficient of redox species could be calculated using Eq. (2) as follows: Ip ¼ 2:69 105 n3=2 AD1=2 v1=2 C
(2)
where Ip is the peak current, n is the number of electrons transferred, A is the electrode area, D is the diffusion coefficient of redox species, v is the scan rate, and C is the bulk concentration of species. Based on the equation Eq. (2), the 3 diffusion coefficients of Fe(CN)4 6 and Fe(CN)6 species were 12 12 2 and 1.93 10 m /s, respectively. The CV analysis 3.2 10 has confirmed the stability of ZNOS-10 catalyst to continuously oxidize and reduce the K4Fe(CN)6 and K3Fe(CN)6 redox species, respectively, in a KCl solution.
Photocatalytic reaction mechanism Based on the color change of our catalyst as shown in Fig. 10 during the photocatalytic reaction, oxygen vacancies are
anodic peak
Current (A)
50.00µ
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expected to form and involved in the photocatalytic mechanism. Fig. 10 shows the schematic drawing of surface photocatalytic reaction of Ni-doped Zn(O,S) catalyst to generate hydrogen. When the photon energy larger than band gap of Ni-doped Zn(O,S), the electrons and holes will be generated in the valence and conduction bands, respectively. Water will be oxidized by the holes and surface oxygen to form the oxygen vacancies and hydroxyl ions, and water will be reduced by the electrons and oxygen vacancies to produce hydrogen. Hydrogen evolution reaction of ZNOS-10 is initiated by oxidation reaction of ethanol (Eq. (3)) and water (Eq. (4)) to provide oxygen vacancy sites, which will trap oxygen from water and make the OeH bonding of water molecule weak and finally form Hþ cation and H2 gas as shown in Eqs. (5) and (6), respectively. As the highest rate occurs in a 50% ethanol solution, the oxygen vacancies are faster produced in ethanol compared to that in pure water. The role of oxygen vacancies to enhance photocatalytic activity had been proposed in the other work by Moya et al. and Gan et al. [7,59,60]. However, the HER rate is lower for pure ethanol solution, indicating water oxidation reaction is still critical and cannot be ignored. During the oxidation of ethanol, the produced water and the ethanol reagent in solution would synergically enhance the hydrogen evolution rate. þ
2þ C2 H5 OH þ O2 surf þ 4h /CH3 CHO þ H2 O þ VO;surf
þ
(3)
2þ H2 O þ O2 surf þ 2h /2OHaq: þ VO;surf
(4)
0 þ H2 O þ V2þ O;surf /2H þ OO;surf
(5)
0 H2 O þ V2þ O;surf þ 2e /H2 þ OO;surf
(6)
There is no oxygen liberated during the reaction because the surface oxygen anions, the intrinsic Schottky defect, involved in the water redox reactions. The surface oxygen anion, originated from the interaction of point defects close to interface, is crucial for our HER mechanism. To produce the water reduction for a large amount of hydrogen, the Ni-doped Zn(O,S) has to form many oxygen vacancies, therefore the catalyst changes its color from white to dark grey. However,
25.00µ
0.00
cathodic peak -25.00µ 0.33 V
0.11 V
hν
-50.00µ -1.2
-0.8
-0.4
0.0
0.4
0.8
1.2
Potential (V) Fig. 9 e Cyclic voltammogram of ZNOS-10 powder coated glassy carbon electrode in a mixed solution of 1 M KCl, 5 mM K3Fe(CN)6, and 5 mM K4Fe(CN)6 with Ag/AgCl and platinum as reference and counter electrodes, respectively, for 100 cycles.
VB
Fig. 10 e Schematic drawing of surface photocatalytic reaction of Ni-doped Zn(O,S) catalyst to generate hydrogen.
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the catalyst color can reversibly change back from dark grey to white after the experiment is terminated and the reactor stands still for overnight. The dark grey Ni-doped Zn(O,S) with many oxygen vacancies did show the Zn peak after the XRD structure analysis. As for the photocatalytic hydrogen evolution reactions, the charge separation of photo generated electrons and holes are the light absorbance of catalyst are always the key issues. The incorporation of nickel as dopant in Zn(O,S) lattice could efficiently lower the recombination rate of photo carriers and lower the charge transfer resistance. It is well known that dopant could serve as trapping center and recombination center of charge carrier [61]. Dopant as trapping center is very useful since it can decrease the charge recombination and enhance the properties or photocatalytic activity of a material. In the other side dopant as recombination center is useless because it will be a favorable site for charge carriers to recombine. However too much nickel in the lattice of Zn(O,S) might become the center for electron-hole recombination [28]. In the case of Ni-doped Zn(O,S), when nickel content is relatively low (10%) it serves as trapping center for charge carriers leading to efficient separation. In the other hand, when the nickel content is high (increased to 20%), it is assumed it serves as recombination center. Furthermore, some similar results from other works also showed the dopant concentration had the optimum point to enhance the photocatalytic activity [27,30,61]. The optimum point in our work is ZNOS-10. However, the 4.3-fold difference between the Ni-free Zn(O,S) and 4.13%Ni-doped ZNOS-10 in HER should involve other factors. We propose that the substitution of Ni in Zn(O,S) with 4.13%Ni leads to the weakest surface oxygen bonding to easily proceed the reactions in Eqs. (3) and (4) for producing the oxygen vacancies. With this easy formation of oxygen vacancies, HER can be accelerated. However, the oxygen formation cannot guarantee the HER enhancement. TiO2, as an example, becomes n-type semiconductor with many oxygen vacancies after it is reduced. It has no holes to proceed the water oxidation reactions of Eqs. (3) and (4), so its HER cannot sustain. The challenge of HER is to have not only the low recombination rate in the photo induced electrons and holes but also the reversible exchange of surface oxygen anions. Furthermore, the lattice distortion occurred due to the Ni dopant in Zn(O,S) lattice was also one of our consideration that induced the charge transfer resistances were lower in (Zn,Ni)(O,S) system [62e64]. The more amount of Ni dopant, the more distortion in lattice was occurred. Accordingly, the charge transfer resistance would increase and lower the photocatalytic reaction. Therefore, at high Ni content for ZNOS-20, the photocatalytic performance is lower than that of ZNOS-10.
Conclusions Ni-doped Zn(O,S) had been successfully synthesized with different nickel precursor contents by simple chemical method at low temperature. It was found that a small amount of 4.13% Ni in ZNOS-10 had significantly enhanced photocatalytic activity of Zn(O,S) for hydrogen production. The highest activity was obtained from ZNOS-10 in a 50% ethanol
solution with the hydrogen production rate of 14,800 mmol g1$h1 corresponding to apparent quantum yield 31.5%, which was 4.3 times higher than that of pure Zn(O,S). The great activity of Ni-doped Zn(O,S) in HER is partially contributed by the synergetic effect of high light absorbance, high separation rate of photo induced carriers, and relatively low charge transfer resistance. The generated oxygen vacancies also play the important role in photocatalytic HER, as evidenced by the catalyst color change from white to dark grey under UV irradiation. The Ni substitution to the Zn site in Zn(O,S) is demonstrated to weaken the surface oxygen bonding for facilitating the reversible oxygen exchange in forming oxygen vacancies.
Acknowledgement This work was supported by the Ministry of Science and Technology of Republic of China under Grant no. MOST 1063111-Y-042A-093.
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