Effects of graphene oxide and sacrificial reagent for highly efficient hydrogen production with the costless Zn(O,S) photocatalyst

international journal of hydrogen energy xxx (xxxx) xxx

Available online at www.sciencedirect.com

ScienceDirect journal homepage: www.elsevier.com/locate/he

Effects of graphene oxide and sacrificial reagent for highly efficient hydrogen production with the costless Zn(O,S) photocatalyst 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, Republic of China

highlights

graphical abstract


Graphene oxide (GO) served as an inexpensive cocatalyst in Zn(O,S)/ GO nanocomposite.
GO layer effectively decreased the resistance to promote the charge carrier separation.
The best hydrogen production rate is

enhanced

by

two

times

compared to without GO.
The efficiency of sacrificial reagents: ethanol > methanol > isopropanol > ethylene glycol.

article info

abstract

Article history:

Searching for noble metal-free co-catalyst is still a strenuous part in photocatalytic hydrogen

Received 19 November 2018

evolution reaction (HER), as most of the great catalysts contain noble metals like the expensive

Received in revised form

platinum. The present work demonstrates a feasible synthesis method of Zn(O,S)/GO nano-

6 August 2019

composite with graphene oxide (GO) to serve as an inexpensive co-catalyst. Raman spectra and

Accepted 9 August 2019

transmission electron microscopy (TEM) images clearly verified that GO was successfully

Available online xxx

loaded on the surface of Zn(O,S). This GO layer could effectively decrease the charge transfer resistance and promote the charge carrier separation for enhancing hydrogen production rate.

Keywords:

By optimizing the GO content, the best hydrogen production rate of 2840 mg h1 was achieved

Graphene oxide

with Zn(O,S)/0.5 wt% GO catalyst under 16 W UV lamp with illumination light at a wavelength

Co-catalyst

of 352 nm, which showed about two times higher for GO-free Zn(O,S). The effect of sacrificial

Hydrogen production

reagent on the hydrogen production rate of Zn(O,S)/0.5 wt% GO catalyst was also

Charges separation

evaluated. The sacrificial reagent showed the efficiency with the following trend: ethanol > methanol > isopropanol > ethylene glycol. The mechanism for enhancing hydrogen production rate is elucidated in this paper. We consider the simple synthesis method and its low cost to make Zn(O,S)/GO a great potential for practical application. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

* Corresponding author. E-mail addresses: [email protected] (N.S. Gultom), [email protected] (D.-H. Kuo). https://doi.org/10.1016/j.ijhydene.2019.08.066 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Gultom NS et al., Effects of graphene oxide and sacrificial reagent for highly efficient hydrogen production with the costless Zn(O,S) photocatalyst, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.08.066

2

international journal of hydrogen energy xxx (xxxx) xxx

Introduction Hydrogen has been considered as an alternative energy carrier in the future for transportation, industry, and other applications because of its high energy density, low cost, and zero emission. Among the several techniques to produce hydrogen gas, photocatalytic water splitting is one of the most expected ones. After the pioneer work of Fujishima and Honda in 1972 [1] for the invention of photocatalytic water splitting, numerous semiconductor photocatalysts have been developed. However, most of the activities for hydrogen evolution reaction (HER) have been relatively low even with using sacrificial reagents such as ethanol, methanol, ethanolamine, sodium sulfite, sodium sulfide, glycerol etc [2,3]. Generally, the low activity is caused by the rapid electron-hole recombination after photo excitation. The charges carrier (electron and hole) must be separated and have enough lifetime to execute the redox reaction on the active site of catalyst in order to improve its activity. Surface modification by loading co-catalyst on the surface of a photocatalyst is an effective strategy to promote the separation of photogenerated electron-hole pairs [4]. Furthermore, the co-catalyst could also provide more active site to facilitate the reduction/oxidation reaction for enhancing photocatalytic activity. Loading with the proper co-catalysts had significantly enhanced the hydrogen production rate in some previous works [5e7]. Platinum (Pt), the highly-priced metal, is the most widely used co-catalyst for several systems such as TiO2, CdS, (AgIn)xZn1-xS, CdxZn1-xS, etc [8e12]. For practical application, the usage of noble metal is an unfavorable way due to its cost consideration. Searching for noble metal-free cocatalyst is still a strenuous part in photocatalytic water splitting, as most of the great co-catalysts are noble metals like the expensive platinum. Therefore, some researchers did the great efforts to invent the suitable non-noble metal and low-cost cocatalysts such as Ni2P, Ni, MoS2, and graphene [13e17]. After the invention of graphene and graphene oxide (GO) in the early year of 2000s, graphene has highly attracted the attention of people [18]. Graphene has the unique structure of 2D and the outstanding properties such as electrical, physical, chemical, optical, and mechanical properties, so it has been applied in several applications such as battery, catalyst, H2 storage, etc. [19e23]. In catalysis area, GO has a critical role for enhancing the separation of the electron-hole pairs by the interfacial contact between a semiconductor catalyst and GO or graphene in order to improve the absorption at visible light region [24e27]. Some previous works had shown the significant improvement of photocatalytic activity after photocatalyst was combined with GO. Recently, Sheu et al. reported that GO could enhance photocatalytic activity for hydrogen generation, dye degradation, and anti-bacterial action [28]. Rahman and co-worker also reported that hydrogen evolution rate of carbon nitride could be increased by 5-fold after being combined with GO [29]. Another study by Jahangir and Kowsar reported the high efficiency of CdO/GO nanocomposite for degradation of organic pollutants such as methylene blue (MB), methyl orange (MO) and rhodamine-B (RhB) dyes. The efficient degradation of dyes was attributed to the high surface area of the CdO/GO nanocomposite and its easy charge separation [30].

Based on the above-mentioned results, we consider that GO could improve the properties as well as activity of Zn(O,S) for hydrogen production. Herein, we conducted the nanocomposite concept by loading GO to serve as co-catalyst on the surface of Zn(O,S) in order to enhance the hydrogen production rate. Our design is to propose GO as a charge separator and an electron sink for redox reaction. The effects of GO content and the different kinds of sacrificial reagents are investigated in this paper. Based on the suppression of the electron-hole recombination and the increase in electrical conductivity, the possible mechanism for enhancing photocatalytic hydrogen production rate is elucidated.

Experimental section Materials All chemicals in this work were used without purification treatment. Zinc acetate hydrate dihydrate Zn(CH3CO2)2 of 98% purity [60] and thioacetamide (C2H5NS, 99% of purity) [61] were obtained from Alfa Aesar. The graphene oxide with surface resistance of 106 U and purity of 95% was obtained from Pin Shuo Photoelectric Corp [62], Taiwan.

Synthesis of Zn(O,S) The synthesis of Zn(O,S) and Zn(O,S)/GO nanocomposites were briefly shown in the schematic 1. The details of schematic 1 were explained as follows: 0.75 g of thioacetamide as a sulfur source was added into 800 mL DI water containing 4.4 g of zinc acetate under vigorous stirring for 15 min. Then, the solution was heated to 90  C and stirred for another 4 h. After cooling down to room temperature, the solid precipitate was collected and washed with ethanol for three times. Lastly, the precipitate was dried in vacuum oven at 80  C for 12 h to obtain the solid powder.

Deposition of graphene oxide on Zn(O,S) The as-prepared Zn(O,S) of 0.5 g was dispersed in 100 mL DI water under ultra-sonication for 30 min. Another suspension of graphene oxide was slowly dropped into the Zn(O,S) solution. After sonication for another 10 min, the mixture solution was further stirred at room temperature for 1 h. Then, the composite precipitate was recovered and washed with ethanol. Finally, it was dried in vacuum oven at 80  C for overnight. We observed that the powder color changed from white to gray. The color change indicated that GO was successfully loaded on the surface of Zn(O,S). To optimize the performance of catalysts, different GO contents were prepared at 0, 0.1, 0.25, 0.5, and 1 wt%, based upon the weight of Zn(O,S) and thereby denoted as ZG-0, ZG-0.1, ZG-0.25, ZG-0.5, and ZG-1, respectively.

Characterizations The diffraction pattern of as-prepared catalyst was recorded by X-ray diffractometer (D2 phaser) that is equipped with

Please cite this article as: Gultom NS et al., Effects of graphene oxide and sacrificial reagent for highly efficient hydrogen production with the costless Zn(O,S) photocatalyst, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.08.066

international journal of hydrogen energy xxx (xxxx) xxx

3

Schematic 1 e Synthesis procedures of (a) Zn(O,S), and (b) Zn(O,S)/GO nanocomposite.

copper anode, CuKa (l ¼ 1.54  A). To show the bonding of GO with Zn(O,S), the Raman spectroscopy was conducted at the 352 nm excitation wavelength with silica glass as reference. The Raman spectra was recorded at the range of 500e2500 cm1 with laser power at a sampling point of 18 mW, confocal aperture of 50 mm, accumulation number of 5, and exposure time of 5 s. The field emission scanning microscopy (FE-SEM) images were recorded by using JEOL JSM6500F. The high-resolution transmission electron microscopy was performed to further study the microstructure of as-prepared catalyst by using (HRTEM, Tecnai F20 G2, Philips, Netherlands) at accelerating voltage of 200 kV. To evaluate the light absorption ability of catalyst, Jasco V-670 UVeVisiblenear IR spectrophotometer was used with BaSO4 to improve the light transmission. The emission after photoexcitation was recorded by a fluorescence spectrophotometer (JASCD FB8500). X-ray photoelectric spectra (XPS) was performed by Thermo VG Scientific (ESCALAB 250, England). The

electrochemical impedance spectra (EIS) was conducted using a Bio-Logic Science instrument for a three-electrodes cell containing glassy carbon as working electrode, Ag/AgCl as reference, and Pt as the counter electrode. The measurements were done in a 100 mL solution of 0.1 M KCl as electrolyte and in the frequency range of 2e200 kHz.

Photocatalytic hydrogen experiment The photocatalytic hydrogen experiment was carried out in a gas-tight circulation system connecting with GC instrument, reactor, and argon tank, as shown in schematic 2. For each experiment, 0.225 g of the catalyst powder was dispersed in a mixed solution of 400 mL DI water and 50 mL different sacrificial reagents (i.e. ethanol, methanol, isopropanol, and ethylene glycol). Then, the solution was transferred into reactor and purged with argon for 30 min in order to release all the atmospheric gases. After all gases were completely

Schematic 2 e Photocatalytic hydrogen experiment of the gas-tight circulation system. Please cite this article as: Gultom NS et al., Effects of graphene oxide and sacrificial reagent for highly efficient hydrogen production with the costless Zn(O,S) photocatalyst, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.08.066

4

international journal of hydrogen energy xxx (xxxx) xxx

Fig. 1 e Representative FE-SEM images of (a) Zn(O,S) and (b) Zn(O,S)/0.5 wt% GO (ZG-0.5).

purged, the reactor was illuminated by low power 16 Watt UV lamp with illumination light at a wavelength of 352 nm. The amount of hydrogen from the reaction was determined by flowing the argon as gas carrier through reactor in order to carry the generated gases for gas chromatography analysis (GC-1000). The measurement was done for every 30 min at a 5 h span of photoreaction.

Results and discussion

found due to its much less amount to form the interference under e-beam irradiation. To study the distribution of GO over the Zn(O,S), the EDS elemental mapping was conducted, as shown in Fig. 3. Fig. 3a exhibit the FE-SEM image of ZG-0.5 with the yellow line indicating the area of elemental mapping. Fig. 3bee shows the elemental mapping of zinc (Zn), oxygen (O), sulfur (S), and carbon (C), respectively, as constituted in our catalyst. It can be clearly seen that not only Zn, O, and S show the uniform distribution but also C. This element mapping proves again the success in loading GO on the surface of Zn(O,S).

Morphology and microstructure X-ray diffraction and Raman analysis To study the morphology and microstructure of Zn(O,S)/GO photocatalyst, the field emission scanning electron microscopy was performed at 15 kV accelerating voltage. Fig. 1 shows the image of the as-prepared Zn(O,S) before and after loading 0.5 wt% GO on the surface. Overall the FE-SEM images of Zn(O,S) and Zn(O,S)/GO nanocomposite were not significantly different. The aggregates of small nanoparticles with the size varying from several tens to hundreds nanometer are observed in Fig. 1. The GO could not be observed by FE-SEM image due to its pretty thin nature. Therefore, to successfully observe GO loaded on the surface of Zn(O,S), the TEM analysis was performed. Fig. 2 displays the high magnification TEM images, SAED, and HRTEM of Zn(O,S)/GO nanocomposite. Fig. 2aeb clearly exhibit the presence of GO layers as indicated by red circles. GO layers were expected to be well loaded on the surface of Zn(O,S), even though it could not be distinguished due to the very thin layers. Fig. 2c depicts the selected electron diffraction (SAED) pattern of Zn(O,S)/GO. The R1 and R2 are the radii in reciprocal lattice for ZnO and ZnS, respectively. The white ring spots between the R1 and R2 proved the formation of polycrystalline Zn(O,S) solid solution. It also exhibited the broad ring pattern which indicates some differences in dspacing value. Fig. 2d shows the high-resolution transmission electron microscopy of ZG-0.5. Some d-spacing values of Zn(O,S) could be found with values of 3.10, 2.89, and 2.68  A for (111) plane. Therefore, Fig. 2ced together with different dspacing values demonstrate the unique properties of 3dimension multi-bandgap quantum well (3DMQW) of Zn(O,S) [31]. However, the d-spacing value of GO could not be

Fig. 4 shows the X-ray diffraction patterns of Zn(O,S) and Zn(O,S)/GO nanocomposites. The GO-free Zn(O,S) showed the similar diffraction pattern to cubic ZnS with a zinc blende structure, according to the standard file of JCPDS #05-0566 for cubic ZnS but with a peak shift to a higher angle. The peak shift is caused by the occupation of the smaller ion-sized oxygen on the sulfur site, which had been well explained in our previous work [31]. Even loading with 1 wt% GO on the surface of Zn(O,S), the X-ray diffraction pattern did not show any other peak or peak shift contributed from GO due to the little GO amount. It seems that the (111) peak from the Zn(O,S) or GO-Zn(O,S) located at 25-35 is composed of more than two peaks. To elucidate this point, the Fig. 2c and d are used for the explanation, as shown in Fig. S1. Due to the broad (111) peak of Zn(O,S) located between the (111) diffraction rings of ZnO and ZnS, we can fit the XRD pattern at 20-40 corresponding to three selected d-spacing values from the SAED result. Three selected peaks that are located at 28.1, 29.5, and 31.2 correspond to R3, R4, and R5 in SAED pattern with d-spacing values of 2.68, 2.89, and 3.10  A, respectively. The (111) peak simulation with more than three peaks can be also possible. This demonstration can be a good explanation for our Zn(O,S), which is a solid solution structure but with a fluctuated composition in O and S at the anionic site. Therefore, this Zn(O,S) in Zn(O,S)/GO has a disordered structure with a random distribution of O and S at the anion sites. The disordered structure for Zn(O,S) has not been mentioned if the processing is proceeded at higher temperature [32]. For the

Please cite this article as: Gultom NS et al., Effects of graphene oxide and sacrificial reagent for highly efficient hydrogen production with the costless Zn(O,S) photocatalyst, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.08.066

international journal of hydrogen energy xxx (xxxx) xxx

5

Fig. 2 e (a, b) High magnification TEM images, (c) SAED, and (d) HRTEM image of Zn(O,S)/GO.

definition of phase with a different structure or composition, our composition-fluctuated phases support the concept of 3D MQW structure in our early works [31,33,34]. With this support, the image in Fig. 2d can show the same {111} diffraction plane but with different diffraction radii and lattice fringes.

To further show the evidence for GO successfully loaded on the surface of Zn(O,S), the Raman spectra was measured under photo excitation with a laser at a wavelength of 532 nm. It is well known that Raman spectrometer is a powerful characterization tool to analyze carbon materials

Fig. 3 e (a) FE-SEM image and elemental mapping of (b) Zn, (c) S, (d) O, and (e) C.

Please cite this article as: Gultom NS et al., Effects of graphene oxide and sacrificial reagent for highly efficient hydrogen production with the costless Zn(O,S) photocatalyst, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.08.066

6

international journal of hydrogen energy xxx (xxxx) xxx

Fig. 4 e X-ray diffraction patterns of ZG-0 and ZG-1 together with the standard files for cubic ZnO and ZnS.

like GO. Fig. 5 shows the Raman spectra of Zn(O,S)/GO with different GO contents. Based on the literature, the characteristic peaks of carbon in GO are located at ~1337 and ~1600 cm1, as D-band and G-band, respectively, with D band related to the defect in the graphene and G band to the vibration mode of carbon atoms [35]. The Raman spectra of pure Zn(O,S) did not show the peaks at 1337 and 1600 cm1. However, after loading very small amount of 0.1 wt% GO, the broad peaks around 1337 and 1600 cm1 appeared with weak intensity. However, with further increasing the amount of GO to 1 wt%, those peaks became clearly observed with stronger peak intensity. Interestingly, after loading GO, the peaks located at 934, 1425, and 2450 cm1 for the characteristic peaks of Zn(O,S) disappeared. The disappearance can be caused by the well covered GO sheets on the top of Zn(O,S) to block the Raman signals from Zn(O,S) [36].

XPS analysis Fig. 6 depicts the XPS analysis of Zn(O,S)/GO nanocomposite with zinc, oxygen, sulfur, and carbon as the constituent elements of our catalyst. The high resolution scan of zinc is shown in Fig. 6a. Binding energies of 1022.5 and 1045.7 eV correspond to the spin orbital splitting of Zn 2p3/2 and Zn 2p1/2, respectively [37]. Fig. 6b shows the high resolution of oxygen (O 1s). The peak can be divided into three peaks for oxygen in the lattice, oxygen bonding with carbon (C¼O), and oxygen as OH, with binding energy values of 530.0, 531.0, and 532.7 eV, respectively. Those binding energy values are similar with some previous works [37e39]. Fig. 6c depicts the binding energies of sulfur (S 2p). The spin orbital splitting of S 2p3/2 and S 2p1/2 are located at 161.5 and 162.3 eV, respectively [37]. The high resolution scan of carbon is shown in Fig. 6d. The asymmetric shape of carbon could be fitted into three peaks

Please cite this article as: Gultom NS et al., Effects of graphene oxide and sacrificial reagent for highly efficient hydrogen production with the costless Zn(O,S) photocatalyst, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.08.066

international journal of hydrogen energy xxx (xxxx) xxx

Fig. 5 e Raman spectra of Zn(O,S)/GO at different GO contents.

with binding energies of 285.0, 287.5, and 289.9 eV, identified for CeC, CeO, and C¼O bonds, respectively [38].

Absorption analysis Fig. 7 shows the absorption spectra of ZnOS/GO nanocomposite at different GO contents after repeating the

7

Fig. 7 e Absorption spectra of Zn(O,S)/GO at different GO contents.

measurements for samples from different batches. The GOfree Zn(O,S) catalyst only had the absorption at UV region. However, after forming the Zn(O,S)/GO nanocomposites, they not only had absorption at UV region but also at visible light region, which is contributed from the black-color nature of GO. It is clearly seen that the increase in the GO content leads to the gradual increase in absorption at the visible light region.

Fig. 6 e High resolution XPS spectra of (a) Zn, (b), O, (c) S, and (d) C. Please cite this article as: Gultom NS et al., Effects of graphene oxide and sacrificial reagent for highly efficient hydrogen production with the costless Zn(O,S) photocatalyst, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.08.066

8

international journal of hydrogen energy xxx (xxxx) xxx

This phenomenon also corresponds to the color change of catalyst from white to dark gray, as shown in the inset of Fig. 7. This result provides another evidence of the successful loading of GO on the Zn(O,S) surface.

Photoluminescence and electrochemical impedance analyses Photoluminescence spectroscopy is a powerful technique to evaluate the electron-hole recombination after photoexcitation by comparing the emission intensity. The higher intensity corresponds to the faster electron-hole recombination or slow charge separation. Fig. 8a shows the photo emission of Zn(O,S)/GO at different GO contents after excitation with a laser at wavelength of 250 nm. The ZG-0.5 catalyst had the lowest emission intensity, so ZG-0.5 can be assigned as the most efficient catalyst for charge separation with the slowest electron-hole recombination rate, relative to others. Electrochemical impedance spectra were performed at room temperature to study the charge transfer resistance of Zn(O,S)/GO nanocomposites, as shown in Fig. 8b. It is well known that the large semicircle on the Nyquist plot represents the high charge transfer resistance. As shown in the Fig. 8b, the diameter of arc semicircle is significantly decreased by increasing the amount of GO, which indicates that electron could easily transfer from the surface of catalyst to the electrolyte solution. The similar results also can be found for ternary Ag3PO4/TiO2/GO system [28]. After fitting the measurement data with the Randle circuit, the values of charge transfer resistance of ZG-0, ZG-0.1, ZG-0.25, ZG-0.5, and ZG-1 are 4,100, 3,800, 1,500, 740, and 600 U, respectively. The low charge transfer resistance help in reducing the electron-hole recombination and thereby enhance the photocurrent [40].

Fig. 9 e Photocatalytic hydrogen production of (a) Zn(O,S)/ GO at different GO contents and (b) ZG-0.5 at different kinds of sacrificial reagents.

Photocatalytic hydrogen production Fig. 9a shows the production of the hydrogen amount from Zn(O,S)/GO nanocomposites at different GO contents in a 10% aqueous ethanol solution under 352 nm UV lamp irradiation for 5 h. The GO-free Zn(O,S) had the hydrogen production rate of 1471 ± 76 mg h1. After loading 0.1 wt% GO, its hydrogen production rate slightly improved to 1944 ± 99 mg h1. Further increasing the amount of GO, the hydrogen production rate

gradually increased to 2263 ± 126 and 2839 ± 171 mg h1 for 0.25 wt% and 0.5 wt% GO, respectively. However, when the GO content was increased up to 1 wt%, the hydrogen production rate of Zn(O,S)/GO quickly decreased to 1359 ± 63 mg h1. The reason for this degradation might be caused by the overloaded GO to cover much more surfaces and to inhibit absorption of photon energy by Zn(O,S) catalyst. It is also caused

Fig. 8 e (a) Photoluminescence spectra and (b) electrochemical impedance spectra (EIS) of Zn(O,S)/GO at different GO contents. Please cite this article as: Gultom NS et al., Effects of graphene oxide and sacrificial reagent for highly efficient hydrogen production with the costless Zn(O,S) photocatalyst, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.08.066

9

international journal of hydrogen energy xxx (xxxx) xxx

by the fast electron-hole recombination, as we confirmed with the PL data in Fig. 8a. Sacrificial reagents play the important role in enhancing photocatalytic activity of hydrogen production by inhibiting electron-hole recombination [41,42]. Some previous works had shown the significant enhancement of hydrogen production rate after introducing sacrificial reagent [43e45]. Therefore, to have a better understanding about the effect of sacrificial reagent against the photocatalytic hydrogen production in our system, we tested the activity of ZG-0.5 photocatalyst in the aqueous solution with different sacrificial reagents belonging to the alcoholic groups such as methanol, ethanol, isopropanol, and ethylene glycol, which are of different carbon chains. Fig. 9b shows the photo-catalytic hydrogen production of ZG-0.5 catalyst in different sacrificial reagents. The hydrogen production rates were 2839.5 ± 171, 1863 ± 104, 1179 ± 59, and 1071 ± 45 mg h1 in ethanol, methanol, isopropanol, and ethylene glycol, respectively. We find that ethanol is the most suitable sacrificial reagent for Zn(O,S)/ GO nanocomposite to give the best hydrogen production rate. Our higher hydrogen production rate in ethanol system compared to methanol is contradictory with the common results, which had reported the data to show higher rate in methanol [46e48]. However, Strataki reported the hydrogen production rate for Pt/TiO2 to be much higher in ethanol, and for low methanol and ethanol concentrations [49]. The possible explanations for this phenomenon are investigated as follows. The C1 and C3 of methanol and isopropanol, respectively, show the relatively low hydrogen production rate, as compared with the C2 of ethanol to give the highest hydrogen production rate. Moreover, ethylene glycol with the same C2 as ethanol gave the lowest hydrogen production rate. Hence, it is clear that there is no relationship between hydrogen production rate and the number of carbon atoms. Furthermore, it is commonly confirmed that the hydrogen production with different hole scavengers or electron donors is related to their permittivity [50]. Table 1 briefly provides the permittivity, oxidation potential, and hydrogen rate for HER in methanol, ethanol, isopropanol, and ethylene glycol. Generally, the higher permittivity has the better photo activity because more electrons and holes would be available for redox reaction [51]. However, in the present work, the hydrogen rate seems not to be affected by the permittivity. For example, ethylene glycol with the highest relative permittivity of 38.9 gives the lowest hydrogen production rate. In addition, isopropanol with the lowest permittivity of 18.3 has the similar hydrogen production with the ethylene glycol. On the other hand, ethanol with intermediate permittivity of 25.7 gives the best hydrogen production rate. Another factor that influences the hydrogen production rate is oxidation potential. In this

regard, the low oxidation potential means that the hole scavengers would easily be oxidized and then more holes are trapped [52]. In our case, the lowest oxidation potential of methanol does not give the highest hydrogen production rate. Therefore, it can be summarized that the hydrogen production is not related to the oxidation potential. Since the hydrogen production rate is not affected by the number of carbon atoms, permittivity, and oxidation potential, we do further investigation by conducting photocurrent response with transient photocurrent-time technique. The ZG-0.5 catalyst was coated on the glassy carbon electrode and UV LED lamp was used as a light source. To start the measurement, the lamp was switched ON/OFF for every 20 s and the result is shown in Fig. 10. From the photocurrent response, we obtained the value of photocurrent, which was high for high concentration of electron carrier. Fig. 10 depicts the photocurrent response of catalyst ZG-0.5 in the aqueous solution of different kinds of sacrificial reagents during the onoff UV irradiation cycles. It is obviously observed that the highest photocurrent of 85 mA was achieved with ethanol as the sacrificial reagent, indicating that ethanol can effectively trap the holes for oxidation reaction to leave more electrons for the reduction reaction to convert the hydrogen ion into the hydrogen gas. The values of photocurrent of ZG-0.5 in methanol, isopropanol, and ethylene glycol were 42, 30, and 28 mA, respectively. The sequence of photocurrent agrees well with their photocatalytic hydrogen production rate in the following trend: ethanol > methanol > isopropanol > ethylene glycol. Point to note, finding a suitable sacrificial reagent is one of the most important priorities in photocatalysis to obtain the optimum hydrogen production rate.

A proposed photocatalytic mechanism In our early studies, active oxygen for oxygen vacancy plays the critical role in generating hydrogen for Zn(O,S)-based photocatalysts [31,34,43,53e56]. Based on the color change from light grey to the black color under illumination and the O2 generation-free HER, a modified photocatalytic mechanism from traditional water splitting is proposed. The photocatalytic mechanism of Zn(O,S)/GO nanocomposite to produce the hydrogen from the water-ethanol mixture solution is shown in schematic 3. The chemical reactions over the Zn(O,S)/GO catalyst to generate hydrogen are described by Kroger-Vink notation below [57]: ZnðO; SÞ=GO þ hv / e þ h

þ

(1)

þ

2þ C2 H5 OH þ O2 surf þ 4h / CH3 CHO þ H2 O þ VO;surf

(2)

Table 1 e Properties of methanol, ethanol, isopropanol, and ethylene glycol related to hydrogen production rate [46]. Sacrificial reagents Methanol Ethanol Isopropanol Ethylene glycol

Chemical formula

Permittivity

Oxidation potential (eV)

H2 rate (mg.h1)

CH3OH CH3CH2OH CH3CHOHCH3 CH2OHCH2OH

31.2 25.7 18.3 38.9

1.05 1.1 e 1.54

1863 2839 1179 1071

Please cite this article as: Gultom NS et al., Effects of graphene oxide and sacrificial reagent for highly efficient hydrogen production with the costless Zn(O,S) photocatalyst, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.08.066

10

international journal of hydrogen energy xxx (xxxx) xxx

Fig. 10 e Photo response of ZG-0.5 photocatalyst under UV illumination with different sacrificial reagents of (a) methanol, (b) ethanol, (c) ethylene glycol, and (d) isopropanol.

þ

Schematic 3 e Proposed photocatalytic mechanism of Zn(O,S)/GO nanocomposite to generate hydrogen from a water-ethanol solution.

 2þ H2 O þ O2 surf þ 2h / 2OHaq: þ VO;surf

(3)

0 þ H2 O þ V2þ O;surf / 2H þ OO;surf

(4)

2Hþ þ 2e /H2

(5)

Each equation could be briefly explained as follows. First, as the photon energy is higher than its bandgap of Zn(O,S), the photo-excited electron and hole pairs are generated in the conduction and valence band, respectively (Eq. (1)) [58]. Next, the photogenerated holes (hþ) react with active surface oxygen (O2 surf Þ and ethanol/water to form the oxygen vacancies according to Eqs. (2) and (3) [31]. Then, the freshly generated oxygen vacancies can trap the water and weaken its OeH bonding to form the hydrogen ion (Hþ), as shown in Eq. (4). Finally, the hydrogen ions are reduced by the presence of electrons to generate hydrogen gas based on the Eq. (5) [59]. These equations needs to show the generation of H2 and to prove no O2 generation. Although the absorption spectra of

Please cite this article as: Gultom NS et al., Effects of graphene oxide and sacrificial reagent for highly efficient hydrogen production with the costless Zn(O,S) photocatalyst, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.08.066

international journal of hydrogen energy xxx (xxxx) xxx

Zn(O,S)/GO show the absorption at visible light range in Fig. 8a, but it could not generate hydrogen under visible light irradiation. The reason might be the insufficient photon energy to excite the electron from valence band of Zn(O,S) and generate electron-hole pairs. The photon energy of visible light might can excite electron from GO to have electron-hole pairs. However, the GO amount is quite little and electrically conductive in nature, and then the electron-hole pairs are insufficient to perform the redox reactions. From the EIS data in Fig. 8b, GO significantly decreases the charge transfer resistance of Zn(O,S)/GO nanocomposite with an improved conductivity. This is a very important factor for photo charge carrier to easily migrate on the surface of catalyst. As the conductivity becomes better the charges carrier can migrate faster and more effective to execute the redox reaction on the active site of catalyst. Furthermore, the heterojunction of Zn(O,S)/GO effectively suppresses the recombination of the photogenerated electrons and holes, as we confirm from PL result. It has been proved that the slower electron-hole recombination favors the higher photocatalytic activity. Our design is simply to utilize GO as an electron sink. After the photo-generated electrons and holes move to the conduction and valence bands of Zn(O,S), respectively, then the excited electrons migrate to GO layers, as shown in the schematic mechanism. This electron sink provides electrons a lot that can be used for reduction reaction. The enhanced photocatalytic hydrogen production of Zn(O,S)/GO catalyst is contributed due to the lower charge transfer resistance, the suppression of photo-generated electron-hole recombination, and the electron sink of GO.

Conclusions The feasible synthesis of Zn(O,S)/GO nanocomposite with graphene oxide (GO) to serve as an inexpensive co-catalyst have been developed for photocatalytic hydrogen production. By optimizing the GO content, the best hydrogen production rate of 2840 mg h1 was achieved by Zn(O,S)/0.5 wt% GO catalyst, which was about two times higher than GO-free Zn(O,S). The effect of sacrificial reagent on the hydrogen production rate of Zn(O,S)/0.5 wt% GO catalyst was also evaluated. The sacrificial reagent showed the efficiency with the following trend: ethanol > methanol > isopropanol > ethylene glycol. The kinetic mechanism for enhancing hydrogen production rate was attributed to the lower charge transfer resistance, and then the photo-generated electron-hole pairs individually migrating on the catalyst surface as well as slower charge recombination. The simple and cost-effective synthesis method for Zn(O,S)/GO is its advantage for practical applications.

Acknowledgments This work was supported by Ministry of Science and Technology of Republic of Taiwan, China under grant number MOST: 107-2221-E-011-141-MY3 and Noto Susanto Gultom was supported under MOST 108-2811-E-011-505.

11

Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.08.066.

references

[1] Fujishima A, Honda K. Electrochemical photolysis of water at a semiconductor electrode. Nature 1972;238:37e8. [2] Foo WJ, Zhang C, Ho GW. Non-noble metal Cu-loaded TiO2 for enhanced photocatalytic H2 production. Nanoscale 2013;5:759e64. [3] Kumar DP, Kumari VD, Karthik M, Sathish M, Shankar MV. Shape dependence structural, optical and photocatalytic properties of TiO2 nanocrystals for enhanced hydrogen production via glycerol reforming. Sol Energy Mater Sol Cells 2017;163:113e9. [4] Yang J, Wang D, Han H, Li C. Roles of cocatalysts in photocatalysis and photoelectrocatalysis. Acc Chem Res 2013;46:1900e9. [5] Mu H, Wan J, Wu Y, Xu J, Wang L, Cao X. Novel polymer supported graphene and molybdenum sulfide as highly efficient cocatalyst for photocatalytic hydrogen evolution. Int J Hydrogen Energy 2018;43:18105e14. [6] Ha E, Lee LYS, Wang J, Li F, Wong K-Y, Tsang SCE. Significant enhancement in photocatalytic reduction of water to hydrogen by Au/Cu2ZnSnS4 nanostructure. Adv Mater 2014;26:3496e500. [7] Majeed I, Nadeem MA, Hussain E, Badshah A, Gilani R, Nadeem MA. Effect of deposition method on metal loading and photocatalytic activity of Au/CdS for hydrogen production in water electrolyte mixture. Int J Hydrogen Energy 2017;42:3006e18.  lvez P. Enhanced [8] Serrano DP, Calleja G, Pizarro P, Ga photocatalytic hydrogen production by improving the Pt dispersion over mesostructured TiO2. Int J Hydrogen Energy 2014;39:4812e9. [9] Lingampalli SR, Gautam UK, Rao CNR. Highly efficient photocatalytic hydrogen generation by solution-processed ZnO/Pt/CdS, ZnO/Pt/Cd1-xZnxS and ZnO/Pt/CdS1-xSex hybrid nanostructures. Energy Environ Sci 2013;6:3589e94. [10] Kim YG, Jo W-K. Photodeposited-metal/CdS/ZnO heterostructures for solar photocatalytic hydrogen production under different conditions. Int J Hydrogen Energy 2017;42:11356e63. [11] Kameyama T, Takahashi T, Machida T, Kamiya Y, Yamamoto T, Kuwabata S, et al. Controlling the electronic energy structure of ZnSeAgInS2Solid solution nanocrystals for photoluminescence and photocatalytic hydrogen evolution. J Phys Chem C 2015;119:24740e9. [12] Liu M, Wang L, Lu G, Yao X, Guo L. Twins in Cd1xZnxS solid solution: highly efficient photocatalyst for hydrogen generation from water. Energy Environ Sci 2011;4:1372e8. [13] Sun Z, Zheng H, Li J, Du P. Extraordinarily efficient photocatalytic hydrogen evolution in water using semiconductor nanorods integrated with crystalline Ni2P cocatalysts. Energy Environ Sci 2015;8:2668e76. [14] Tran PD, Xi L, Batabyal SK, Wong LH, Barber J, Chye Loo JS. Enhancing the photocatalytic efficiency of TiO2 nanopowders for H2 production by using non-noble transition metal co-catalysts. Phys Chem Chem Phys 2012;14:11596e9. [15] Ba Q, Jia X, Huang L, Li X, Chen W, Mao L. Alloyed PdNi hollow nanoparticles as cocatalyst of CdS for improved

Please cite this article as: Gultom NS et al., Effects of graphene oxide and sacrificial reagent for highly efficient hydrogen production with the costless Zn(O,S) photocatalyst, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.08.066

12

[16]

[17]

[18]

[19]

[20]

[21]

[22]

[23]

[24]

[25]

[26]

[27]

[28]

[29]

[30]

[31]

[32]

[33]

international journal of hydrogen energy xxx (xxxx) xxx

photocatalytic activity toward hydrogen production. Int J Hydrogen Energy 2019;44:5872e80. Du F, Lu H, Lu S, Wang J, Xiao Y, Xue W, et al. Photodeposition of amorphous MoSx cocatalyst on TiO2 nanosheets with {001} facets exposed for highly efficient photocatalytic hydrogen evolution. Int J Hydrogen Energy 2018;43:3223e34. Lv H, Liu Y, Tang H, Zhang P, Wang J. Synergetic effect of MoS2 and graphene as cocatalysts for enhanced photocatalytic activity of BiPO4 nanoparticles. Appl Surf Sci 2017;425:100e6. Novoselov KS, Geim AK, Morozov SV, Jiang D, Zhang Y, Dubonos SV, et al. Electric field effect in atomically thin carbon films. Science 2004;306:666e9. Song M-K, Zhang Y, Cairns EJ. A long-life, high-rate lithium/ sulfur cell: a multifaceted approach to enhancing cell performance. Nano Lett 2013;13:5891e9. Hsu H-C, Shown I, Wei H-Y, Chang Y-C, Du H-Y, Lin Y-G, et al. Graphene oxide as a promising photocatalyst for CO2 to methanol conversion. Nanoscale 2013;5:262e8. Guo CX, Wang Y, Li CM. Hierarchical graphene-based material for over 4.0 Wt % physisorption hydrogen storage capacity. ACS Sustainable Chem Eng 2013;1:14e8. Jia L, Wang D-H, Huang Y-X, Xu A-W, Yu H-Q. Highly durable N-doped graphene/CdS nanocomposites with enhanced photocatalytic hydrogen evolution from water under visible light irradiation. J Phys Chem C 2011;115:11466e73. Min S, Lu G. Promoted photoinduced charge separation and directional electron transfer over dispersible xanthene dyes sensitized graphene sheets for efficient solar H2 evolution. Int J Hydrogen Energy 2013;38:2106e16. Li Q, Guo B, Yu J, Ran J, Zhang B, Yan H, et al. Highly efficient visible-light-driven photocatalytic hydrogen production of CdS-Cluster-Decorated graphene nanosheets. J Am Chem Soc 2011;133:10878e84. Li H, Cui X. A hydrothermal route for constructing reduced graphene oxide/TiO2 nanocomposites: enhanced photocatalytic activity for hydrogen evolution. Int J Hydrogen Energy 2014;39:19877e86. Huang Y, Yan C-F, Guo C-Q, Lu Z-X, Shi Y, Wang Z-D. Synthesis of GO-modified Cu2O nanosphere and the photocatalytic mechanism of water splitting for hydrogen production. Int J Hydrogen Energy 2017;42:4007e16.  mez A, Lo  pez R, Quiroz-Cardoso O, Oros-Ruiz S, Solı´s-Go  mez R. Enhanced photocatalytic hydrogen production by Go CdS nanofibers modified with graphene oxide and nickel nanoparticles under visible light. Fuel 2019;237:227e35. Sheu F-J, Cho C-P, Liao Y-T, Yu C-T. Ag3PO4-TiO2-Graphene oxide ternary composites with efficient photodegradation, hydrogen evolution, and antibacterial properties. Catalysts 2018;8:57. Rahman MZ, Zhang J, Tang Y, Davey K, Qiao S-Z. Graphene oxide coupled carbon nitride homo-heterojunction photocatalyst for enhanced hydrogen production. Mater Chem Front 2017;1:562e71. Ahmad J, Majid K. Enhanced visible light driven photocatalytic activity of CdO-graphene oxide heterostructures for the degradation of organic pollutants. New J Chem 2018;42:3246e59. Abdullah H, Kuo DH, Chen XY. High efficient noble metal free Zn(O,S) nanoparticles for hydrogen evolution. Int J Hydrogen Energy 2017;42:5638e48. Chen HM, Chen CK, Liu R-S, Wu C-C, Chang W-S, Chen K-H, et al. A new approach to solar hydrogen production: a ZnOZnS solid solution nanowire array photoanode. Adv Energy Mater 2011;1:742e7. Abdullah H, Susanto Gultom N, Kuo D-H. Synthesis and characterization of La-doped Zn(O,S) photocatalyst for green

[34]

[35]

[36]

[37]

[38]

[39]

[40]

[41]

[42] [43]

[44] [45]

[46]

[47]

[48]

[49]

[50]

[51]

[52]

chemical detoxification of 4-nitrophenol. J Hazard Mater 2019;363:109e18. Abdullah H, Gultom NS, Kuo D-H. A simple one-pot synthesis of a Zn(O,S)/Ga2O3 nanocomposite photocatalyst for hydrogen production and 4-nitrophenol reduction. New J Chem 2017;41:12397e406. Kudin KN, Ozbas B, Schniepp HC, Prud'homme RK, Aksay IA, Car R. Raman spectra of graphite oxide and functionalized graphene sheets. Nano Lett 2008;8:36e41. Zhao Y, Wei Y, Wu X, Zheng H, Zhao Z, Liu J, et al. Graphenewrapped Pt/TiO2 photocatalysts with enhanced photogenerated charges separation and reactant adsorption for high selective photoreduction of CO2 to CH4. Appl Catal B Environ 2018;226:360e72. Moulder WFS JF, Sobol PE, Bomben KD. Handbook of X-ray photoelectron spectroscopy. Eden Prairie: Physical Electronic Inc; 1995. Hu Z, Huang Y, Sun S, Guan W, Yao Y, Tang P, et al. Visible light driven photodynamic anticancer activity of graphene oxide/TiO2 hybrid. Carbon 2012;50:994e1004. Akhavan O. The effect of heat treatment on formation of graphene thin films from graphene oxide nanosheets. Carbon 2010;48:509e19. Alizadeh M, Tong GB, Mehmood MS, Qader KW, Rahman SA, Shokri B. Band engineered Al-rich InAlN thin films as a promising photoanode for hydrogen generation from solar water splitting. Sol Energy Mater Sol Cells 2018;185:445e55. Guzman F, Chuang SSC, Yang C. Role of methanol sacrificing reagent in the photocatalytic evolution of hydrogen. Ind Eng Chem Res 2013;52:61e5. Kudo A, Miseki Y. Heterogeneous photocatalyst materials for water splitting. Chem Soc Rev 2009;38:253e78. Gultom NS, Abdullah H, Kuo D-H. Enhanced photocatalytic hydrogen production of noble-metal free Ni-doped Zn(O,S) in ethanol solution. Int J Hydrogen Energy 2017;42:25891e902. Schneider J, Bahnemann DW. Undesired role of sacrificial reagents in photocatalysis. J Phys Chem Lett 2013;4:3479e83. Daskalaki VM, Kondarides DI. Efficient production of hydrogen by photo-induced reforming of glycerol at ambient conditions. Catal Today 2009;144:75e80. Wang M, Shen S, Li L, Tang Z, Yang J. Effects of sacrificial reagents on photocatalytic hydrogen evolution over different photocatalysts. J Mater Sci 2017;52:5155e64. Yang YZ, Chang CH, Idriss H. Photo-catalytic production of hydrogen form ethanol over M/TiO2 catalysts (M¼Pd, Pt or Rh). Appl Catal B Environ 2006;67:217e22. Lin W-C, Yang W-D, Huang IL, Wu T-S, Chung Z-J. Hydrogen production from methanol/water photocatalytic decomposition using Pt/TiO2xNx catalyst. Energy Fuels 2009;23:2192e6. Strataki N, Bekiari V, Kondarides DI, Lianos P. Hydrogen production by photocatalytic alcohol reforming employing highly efficient nanocrystalline titania films. Appl Catal B Environ 2007;77:184e9. Chen W-T, Chan A, Al-Azri ZHN, Dosado AG, Nadeem MA, Sun-Waterhouse D, et al. Effect of TiO2 polymorph and alcohol sacrificial agent on the activity of Au/TiO2 photocatalysts for H2 production in alcoholewater mixtures. J Catal 2015;329:499e513. Sadanandam G, Lalitha K, Kumari VD, Shankar MV, Subrahmanyam M. Cobalt doped TiO2: a stable and efficient photocatalyst for continuous hydrogen production from glycerol: water mixtures under solar light irradiation. Int J Hydrogen Energy 2013;38:9655e64. Reza Gholipour M, Dinh CT, Beland F, Do TO. Nanocomposite heterojunctions as sunlight-driven photocatalysts for hydrogen production from water splitting. Nanoscale 2015;7:8187e208.

Please cite this article as: Gultom NS et al., Effects of graphene oxide and sacrificial reagent for highly efficient hydrogen production with the costless Zn(O,S) photocatalyst, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.08.066

international journal of hydrogen energy xxx (xxxx) xxx

[53] Abdullah H, Gultom NS, Kuo DH, Saragih AD. Cobalt-doped Zn(O,S)/Ga2O3 nanoheterojunction composites for enhanced hydrogen production. New J Chem 2018;42:9626e34. [54] Gultom NS, Abdullah H, Kuo D-H, Ke W-C. Oriented pen heterojunction Ag2O/Zn(O,S) nanodiodes on mesoporous SiO2 for photocatalytic hydrogen production. ACS Appl Energy Mater 2019;2:3228e36. [55] Gultom NS, Abdullah H, Kuo D-H. Facile synthesis of cobaltdoped (Zn,Ni)(O,S) as an efficient photocatalyst for hydrogen production. J Energy Inst 2018;92(5):1428e39. [56] Gultom NS, Abdullah H, Kuo D-H. The concept of stagnant capillarity water in nano porous SiO2@(Zn,Ni)(O,S) nanocomposite photocatalyst as a strategy to improve hydrogen evolution. ACS Appl Mater Interfaces 2019;11(31):27760e9.

13

[57] Chiang Y-M, Birnie DP, Kingery WD. Physical ceramics : principles for ceramic science and engineering. New York: Wiley; 1997. [58] Ross RT, Nozik AJ. Efficiency of hot-carrier solar energy converters. J Appl Phys 1982;53:3813e8. [59] Grozovski V, Vesztergom S, Lang G, Broekmann P. Electrochemical hydrogen evolution: Hþ or H2O reduction? A rotating disk electrode study. J Electrochem Soc 2017;164:3171e8.Website references [60] Aesar Alfa. Chemical catalog. 2019.https://www.alfa.com/en/ catalog/011559/. [Accessed 14 June 2019]. [61] Aesar Alfa. Chemical catalog. 2019.https://www.alfa.com/en/ catalog/030638/. [Accessed 14 June 2019]. [62] Shuo Pin. Photoelectric. Products. 2019.http://www.pinspc. com/about.php?id¼5920. [Accessed 14 June 2019].

Please cite this article as: Gultom NS et al., Effects of graphene oxide and sacrificial reagent for highly efficient hydrogen production with the costless Zn(O,S) photocatalyst, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.08.066