Zn(O,S) nanodiodes on

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Oriented p-n heterojunction Ag2O/Zn(O,S) nanodiodes on mesoporous SiO2 for photocatalytic hydrogen production Noto Susanto Gultom, Hairus Abdullah, Dong-Hau Kuo, and Wen-Cheng Ke ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.9b00079 • Publication Date (Web): 17 Apr 2019 Downloaded from http://pubs.acs.org on April 18, 2019

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ACS Applied Energy Materials

Oriented p-n Heterojunction Ag2O/Zn(O,S) Nanodiodes on Mesoporous SiO2 for Photocatalytic Hydrogen Production

Noto Susanto Gultom, Hairus Abdullah, Dong-Hau Kuo*, Wen-Cheng Ke

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

ACS Paragon Plus Environment

ACS Applied Energy Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT Hydrogen is a great candidate fuel to replace fossil fuels in the future since it has high energy density, zero-emission, and renewability. Here, we report our design with p-type Ag2O and n-type Zn(O,S) loaded on mesoporous silica to form SiO2/Ag2O/Zn(O,S) with the nano p-n heterojunction to improve the efficiency of photocatalytic hydrogen evolution reaction (HER). The photocatalysts were systematically characterized to identify their properties. Through the optimization of the Zn(O,S)-loaded amount and position of p-Ag2O, the highest hydrogen production rate of 9,200 µmol. g-1cat.h-1 was achieved by SiO2/Ag2O/Zn(O,S)-0.6 catalyst with corresponding apparent quantum yield of 10.3%, which was about 2.7 times higher than pure Zn(O,S). By placing nZn(O,S) of diodes outwards was proposed for the electron-rich part to enhance the reduction reaction, while placing p-Ag2O inwards for the hole-rich part to modulate the electron concentration and establish the built-in electrical nano field. The relative mechanism for improving the efficiency of HER has been investigated in this paper. Keywords: p-n heterojunction, built-in electric field, nanodiodes, hydrogen production, oxygen vacancy, electron-hole separation


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1. INTRODUCTION Energy and environment have the most concerns in each country worldwide due to high energy demands as results of rapid population and industry growth. According to data in 2017, about 81% of global energy needs are derived from fossil fuels such as oil, natural gas, and coal 1. Those energy resources have emitted lots of carbon dioxide (CO2) to the environment, which causes air pollution, global warming, climate change, and other environmental issues. Moreover, the fossil fuels have depletion issue, i.e. it will be exhausted due to its limitation in the earth. Therefore, developing new alternative energy with renewable, sustainable, zero emission, high efficiency, and cost-effective requirements is urgently needed. Inspired by the natural photosynthesis on the plant that utilizes the solar energy to convert the carbon dioxide to glucose and oxygen as the final products, artificial photosynthesis, so-called photocatalysis, is a potential approach to solve both energy and environmental issues. Environmentally benign hydrogen with high energy density can be used as fuel for hydrogen car and fuel cell vehicle in the future. Hydrogen also can be converted to generate electricity for home and industry needs. By using this hydrogen fuel, the efficiency can reach to about 65 %, which is much higher than the efficiency of fossil fuels of ca. 20% 2. However, hydrogen does not exist as gas alone but bonding with other elements such as hydrocarbon (CnHm) and water (H2O). At present, about 95% of the total hydrogen production in the world is obtained by steam reforming and gasification technique 3. Unfortunately, those techniques have several disadvantages such as the requirements of high pressure, high temperature, and unrenewable feedstock. Both techniques still produce the CO2 emission as the side product. Therefore, to produce hydrogen with green and renewable techniques is also an important concern. Besides those mentioned techniques above, hydrogen can be produced from water via photocatalytic water splitting. This technique is much better than steam reforming and gasification from an environmental viewpoint, because it utilizes the renewable feedstock, green process, and ambient operation condition without applying pressure and heat. After the invention of Honda and Fujishima for photo-electrochemical water splitting in 1972 4, numerous semiconductor materials have been reported in previous works 5-9. However, most of them have low efficiency because the electron-hole pairs rapidly recombine after photoexcitation process. To overcome this critical issue, surface modification by loading co-catalyst, heterojunction formation, and the introduction of sacrificial reagent have been widely and extensively conducted 10-14.



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Silver oxide (Ag2O), as one of the important p-type semiconductors, has been extensively studied for several applications such as antibacterial, photovoltaics, catalyst, and gas sensor 15-18. Recently, our research has focused on developing n-type semiconductor Zn(O,S) solid solution for hydrogen evolution reaction (HER) and chemical conversion through hydrogenation reaction 19-23. Our Zn(O,S) has the unique property of three-dimensional multibandgap-quantum-well (3D MQW) band structure that has an important role in facilitating the electron-hole separation

23.

Some

previous works by forming heterojunction through oriented two semiconductors with different bandgap value had shown the enhancement of activity due to efficient the electron-hole separation 24-26.

Moreover, the p-n heterojunction also has been one of the most effective approaches to

promote the electron-hole pairs separation

27-28,

we intend to enhance the HER performance of

Zn(O,S) by forming p-n heterojunction. The formation of the built-in electric field can effectively suppress the electron-hole recombination through the band bending at the interfacial region for the electron and hole to be drifted in the opposite direction instead of being recombined each other. Furthermore, the formation of p-n heterojunction enhanced the optical absorption at UV-visible region and photocatalytic activity 29-31. Based on above considerations, we design the nanodiode with p-Ag2O and n-Zn(O,S) loaded on mesoporous silica (SiO2) support in order to have the uniform distribution of Ag2O and Zn(O,S) nanoparticle as well as to control their location through the incorporation of SiO2 sphere and particle size through the mesoporous property of SiO2. Herein, we investigate the effects of the Zn(O,S)-loaded amount and the position of p-Ag2O either inward or outward for hydrogen production. We find that amount of the loaded Zn(O,S) should be optimized and the position of Ag2O significantly affects the HER rate. The built-in electric field after formation of p-n heterojunction in Ag2O/Zn(O,S) is proposed to promote the electron-hole separation as well as to improve the HER rate of Zn(O,S).


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2. EXPERIMENTAL SECTION 2.1 Chemicals. Tetraethyl Orthosilicate

(TEOS)

was

obtained

from

Seedchem.

Cetyltrimethylammonium bromide (CTAB), zinc acetate dehydrates (C4H6O4Zn·2H2O), and thioacetamide (C2H5NS) were commercially purchased from Alfa Aesar. Silver nitrate (AgNO3) and sodium hydroxide (NaOH) were obtained from Sigma Aldrich and Showa, respectively. 2.2 Synthesis SiO2. SiO2 spherical with diameter about 300 nm was synthesized as follows. First, 1.2 g CTAB was dissolved into a mixture solution of 240 mL DI water and 160 mL ethanol under ultrasonication treatment for several minutes. Then, 3.2 mL NH4OH was added into the abovementioned solution with vigorous stirring. After stirring for 30 min, 4 mL TEOS was slowly dropped and stirred for another 2h. Lastly, the precipitation was washed with ethanol and calcined in air at 550 oC for 3h. 2.3 Deposition of Ag2O on SiO2. Deposition of Ag2O on the surface SiO2 was conducted by dispersing 0.5 g SiO2 powder into 100 mL DI water in an ultrasonication bath for 30 min. Then, 2.5 mL of AgNO3 (0.018 M) was added to the above solution with continuously stirring for 20 min. Subsequently, another solution of 3 mL NaOH (0.2 M) was added for the precipitation of Ag2O on SiO2. The as-synthesized SiO2/Ag2O powder was denoted as SA. Based on the EDS analysis, the weight of Ag2O was about 2 wt%. 2.4 Preparation of SiO2/Ag2O/Zn(O,S). To prepare SiO2/Ag2O/Zn(O,S), 0.3 g SA was dispersed in 100 mL DI water. Then, certain amounts of zinc acetate dehydrates and thioacetamide (listed in Table S1) were added into the above solution under vigorous stirring and heating at 90 oC for 4 h. After cooling down to room temperature, the solid powders were collected and washed with ethanol for 3 times. Finally, the powders were dried in a vacuum oven at 80 oC for overnight. The SiO2/Ag2O/Zn(O,S) composite powder was abbreviated as SAZ. Figure 1 schematically illustrates the syntheses of SiO2, SiO2/Ag2O, and SiO2/Ag2O/Zn(O,S) by the facile sol-gel and chemical precipitation method.



2.5 Sample characterizations. The morphology and microstructure of the as-synthesized photocatalysts were analyzed by field emission scanning electron microscopy (FE-SEM) JEOL 6500 and transmission electron microscopy (TEM) Tecnai F20 G2, Philips. The elemental mapping was collected by high-resolution transmission electron microscope Tecnai F20 G2 Philips under a working voltage of 200 kV. The optical absorption and photoluminescence spectra were evaluated by Jasco V-670 UV-Vis NIR and Jasco FB-8500 spectrophotometer, respectively. Xray diffraction pattern was recorded by Bruker D2 phaser with monochromatic CuKα radiation. Xray photoelectron spectroscopy (XPS) measurements were performed by VG ESCA Scientific Theta Probe spectrometer system with Al Kα (1486.6 eV) radiation. The electrochemical impedance spectra (EIS) and Mott-Schottky were conducted by BioLogic SP-300 in a threeelectrode cell with Ag/AgCl, glassy carbon, and Pt as reference, working, and counter electrodes, respectively. The life time was measured by WCT-120 Photoconductance Lifetime Tester made by Sinton Consulting Inc. 2.6 Photocatalytic hydrogen production test. Photocatalytic hydrogen evolution experiment was conducted in the gas-tight system with argon the gas carrier, as schematically illustrated in Figure 2. For each experiment, 0.225 g as-synthesized catalyst was dispersed in a 450 mL aqueous solution with 10 vol.% ethanol. Then, argon flowed into the reactor at a flow rate of 100 standard cubic centimeter per minute (sccm) for 30 min to remove all atmospheric gas. After argon purging, 4 UV lamps with 6 W per each were turned on. The amount of generated hydrogen was measured by gas chromatography with a thermal conductivity detector (TCD). The HER measurement was done every 30 min for a span of 5 h. 4 lamps are suggested, as the HER rate is proportional to the number of lamps23. The apparent quantum yield (AQY) was calculated by below formula and the details of calculation was provided in supplementary information.

A.Q.Y 

number of envolved hydrogen molecules x 2 x 100% number of incident photon


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3. RESULTS AND DISCUSSION Morphology and microstructure analyses. Figure 3 depicts the FE-SEM images of SiO2, SiO2/Ag2O, and SiO2/Ag2O/Zn(O,S) with different amounts of Zn(O,S) loaded on the surface of SiO2/Ag2O. The as-synthesized SiO2 had a uniform spherical shape with a diameter of about 300 nm and its surface was smooth, as shown in Figure 3a. After depositing Ag2O, the SiO2 surface became rougher due to the nanometer-sized Ag2O particles successfully loaded on SiO2, as depicted in Figure 3b. Figures 3c-3f show the FE-SEM images of SiO2/Ag2O/Zn(O,S) with different Zn(O,S) amounts. It can be clearly observed that Zn(O,S) surface became rougher at a higher loading content. These FE-SEM images proved that Zn(O,S) was successfully deposited on SiO2/Ag2O. The concentration of each element in catalysts SiO2/Ag2O/Zn(O,S) with different amounts of Zn(O,S) is provided in Table S2. Figure 4 exhibits the low and high magnification TEM images of SiO2/Ag2O/Zn(O,S)-0.6 nanocomposite. It is obviously observed that Zn(O,S) nanoparticle was well decorated on the surface of spherical SiO2/Ag2O with the tiny particles of several ten nanometers. To study the composition distribution of Zn(O,S) nanoparticle, elemental mapping was performed by TEM in a dark-field imaging mode. For the scan area in Fig. 5a, the element mapping images of silicon and oxygen are shown in Figures 5b and 5c, respectively. Those images depict the high concentrations of silicon and oxygen as the main elements of the catalyst. The Ag of Ag2O was uniformly distributed and has much lower concentration than other elements, as displayed in Figure 5d. Figures 5e and 5f represent the distributions of zinc and sulfur, respectively. Both of them also had the uniform distributions over the SiO2 with their concentrations much higher than Ag but lower than Si and O. Overall, the elemental mapping reveals the homogenous distribution for each element in SiO2/Ag2O/Zn(O,S)-0.6 catalyst.



XPS analyses. The surface chemical and oxidation state of each element in SiO2/Ag2O/Zn(O,S)0.6 catalyst were further studied by XPS and calibrated with C1S (Figure S2). This C1S was contributed by the adventitious carbon contamination since the sample was exposed to the atmosphere before being tested. Its binding energy values are 284.5, 285.9, 288.5 eV which correspond to the bonding of C-C, O-C-O, and O-C=O, respectively 32. Figure 6a depicts the XPS survey of SiO2/Ag2O/ Zn(O,S)-0.6 catalyst and confirms the presence of all element constituents including Si, Zn, Ag, O, and S. The high resolution XPS of Si is shown in Figure 6b with a binding energy of 100 eV. This binding energy is contributed by tetravalent Si4+ for SiO2 phase 33. Figure 6c shows the asymmetric peak of oxygen (O 1s) to be fitted into three symmetric peaks. The peak with binding energies of 533.2 eV corresponds to the O-Si contribution 34. The other two peaks of 530.4 and 531.3 eV are contributed from oxygen in the Zn(O,S) phase as oxygen in the lattice and oxygen vacancy, respectively works

36-38.

32, 35.

These binding energies have good agreement to early

The percentage of oxygen in the lattice and oxygen vacancy are about 70 and 30%,

respectively for Zn(O,S) phase. Based on the peak area, the oxygen ratio between oxygen in SiO2 and in Zn(O,S) is 6:1. According to previous report 39, the binding energy value of oxygen with silver for Ag2O was 530.4 eV and it was exactly the same as the binding energy of oxygen with zinc in our Zn(O,S) phase. Figure 6d shows the high resolution XPS of silver. The low intensity and high noise were attributed to its much lower concentration. With the aid of fitting technique, the spin-orbit splitting energies of Ag 3d5/2 and Ag 3d3/2 were found at 374.7 and 368.8 eV, respectively. Those binding energies confirmed the monovalent Ag+ in Ag2O phase and are consistent with the literature data 17. The binding energies of Zn were located at 1023.2 eV for Zn 2p3/2 and 1046.3 eV for Zn 2p1/2. With a peak separation of 23.1 eV, our zinc belongs to bivalent Zn2+ 32. Figure 6f depicts the high resolution XPS of S with binding energies of 164.4 and 163.2 eV for the spin-orbit splitting of S 2p3/2 and S 2p1/2, respectively. These binding energy values confirm the S2- state, as consistent with literature reports 19, 21, 23


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XRD analysis. Figure 7 shows the XRD patterns of SiO2, Ag2O, and SiO2/Ag2O/Zn(O,S) with different Zn(O,S) contents. The broad peak of SiO2 confirmed its amorphous nature. The XRD peaks for Ag2O did not appear in SiO2/Ag2O/ Zn(O,S) composite catalyst due to its little amount of ~2 wt%. The X-ray patterns of SAZ with different Zn(O,S) contents showed similar peaks to pure Zn(O,S) but at a lower intensity. The X-ray pattern of Zn(O,S) obviously located in between the ZnS and ZnO phases according to the JCPDS No. 05-0566 (ZnS Cubic) and No. 65-2880 (ZnO Cubic), respectively. Its pattern is similar to ZnS phase with peak shift to the higher angle due to oxygen with smaller ionic radius partly occupy the sides of sulfur with larger ionic radius. The more details about Zn(O,S) phase had been well explained in our early work 23. We observed that by increasing the Zn(O,S) content from 0.25 mmol (SAZ-0.25) to 1.5 mmol (SAZ-1.5), the XRD peaks of Zn(O,S) clearly appeared with higher intensity. Optical properties analysis. Optical properties of as-synthesized catalyst were evaluated using the diffuse reflectance spectra (DRS) and photoluminescence (PL) techniques. Figure 8a shows the DRS spectra of SiO2/Ag2O, SiO2/Zn(O,S), and SiO2/Ag2O/Zn(O,S)-0.6. It is obviously seen that SiO2/Zn(O,S) and SiO2/Ag2O only have absorption at UV and visible light regions, respectively. However, after the formation of SiO2/Ag2O/Zn(O,S), the absorption peak appears not only at UV region but also at the visible light region. Moreover, the addition of Ag2O to SiO2/Zn(O,S) to form the SiO2/Ag2O/Zn(O,S) could significantly improve the absorption spectra for both at UV and visible light regions. To evaluate the electron-hole recombination of catalyst, PL spectra were performed with a Laser excitation wavelength of 250 nm and are presented in Figure 8b. The PL emission intensity decreased after introducing the Ag2O into SiO2/Zn(O,S) to form SiO2/Ag2O/Zn(O,S) nanocomposite. The formation of p-n heterojunction helps in promoting the electron-hole separation with the lower PL emission intensity, which is expected to enhance photocatalysis. To confirm the prolonged lifetime, the photoconductance lifetime measurement method developed by the Sinton Instrument was used for measurement and shown in Figure 8c. The charges carrier lifetimes of SZ and SAZ-0.6 are 7.04 and 9.12 µs, respectively. The lifetime is enhanced by 30% after formation of SiO2/Ag2O/Zn(O,S) p-n heterojunction.



Electrochemical properties analyses. The electrochemical properties of catalyst were investigated by Mott-Schottky, EIS and photocurrent response measurements. To provide an evidence of forming p-n heterojunction of the synthesized sample, we conducted the MottSchottky experiment for catalysts of SiO2/Ag2O, SiO2/Zn(O,S), and SiO2/Ag2O/Zn(O,S)-0.6, as shown in Figure 9. It is well known that positive and negative slopes in Mott-Schottky plot represent the characteristics of n-type and p-type semiconductors, respectively 40. Figure 9a shows the Mott-Schottky plot of SiO2/Zn(O,S) with a positive slope, indicating the n-type semiconductor with electron as majority carrier. The Mott-Schottky plot of SiO2/Ag2O in Figure 9b shows a negative slope, suggesting the characteristic of p-Ag2O with electronic hole as majority carrier. As expected, the Mott-Schottky analysis of SiO2/Ag2O/Zn(O,S)-0.6 catalyst in Fig. 9c exists both of positive and negative slopes with a V-like shape, which reveals the formation of p-n heterojunction between p-Ag2O and n-Zn(O,S). The similar shape of Mott-Schottky plots for CeO2/Ag2O and CdS/PbS for proving the formation of p-n heterojunction was also reported elsewhere 41-42. The electrochemical properties of catalysts were further studied by electrochemical impedance spectra (EIS), as shown in Figure 10a. The EIS measurement was done at frequency range of 100 mHz to 200 kHZ with a scan rate of 10 mV/s at amplitude of 10 mA. The EIS test has been considered as a powerful characterization technique to evaluate the charge transfer on the interface between the catalyst coated on the working electrode and the electrolyte solution. The diameter of arc in Nyquist plot correlates to the charge transfer resistance (Rct), and the small radius of arc refers to the low Rct. The charge transfer resistance also represents the easiness of electrons and holes to transport on the catalyst surface. After fitting with Randle circuit, the Rct values were obtained as 4,760, 5,590, and 4,050 Ω for catalysts of SA, SZ, and SAZ-0.6, respectively. It is found that SAZ-0.6 catalyst has the lowest charge transfer resistance to reveal its easiest surface transport for the photo carriers of electron and hole. To evaluate the response of catalyst against light, the photocurrent response was performed by transient photocurrent-time technique. The UV LED lamp was used as a light source and it was turned ON/OFF for every 20 seconds. Figure 10b displays the photocurrent responses of catalysts of SA, SZ, and SAZ-0.6. The photocurrent quickly raised to the highest level with the light ON and kept at constant for several seconds before light OFF. With the light ON, a number of charge carriers (electrons and holes) were generated and did not highly recombine then to contribute the photocurrent

43.

The photocurrent was proportional to the number of photogenerated and un-


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trapped carriers. Till the light OFF, the photocurrent quickly dropped to the baseline before the light turned on again. The photocurrent of SAZ-0.6 is quite high and is about 12 and 6 times higher than those of SA and SZ, respectively. This huge increase in photocurrent reflects its efficient charge transfer and separation with the assistance of the formed p-n heterojunction. The highest photocurrent from SAZ-0.6 is expected to lead the highest photocatalytic activity. Besides, the repetition of transient photocurrent with ON-OFF several times did not cause any significant photocurrent degradation, revealing its outstanding stability and repeatability property. Photocatalytic hydrogen activity. Figure 11a shows the photoactivity of the as-synthesized catalysts for hydrogen production. Firstly, the SiO2/Zn(O,S) had a HER rate of 4,100 µmol.g1cat.h-1,

20% higher than pure Zn(O,S) in our previous report 23. This enhancement is due to the

well distribution of Zn(O,S) on the SiO2 to prevent the aggregation and grain growth and to keep the photocatalyst in nano size with high activity. The hydrogen production rates of SiO2/Ag2O/Zn(O,S) with different Zn(O,S) loading amounts were 7,300, 9,200, 7,600, and 4,800 µmol.g-1cat.h-1 for catalyst SAZ-0.3, SAZ-0. 6, SAZ-0.9, and SAZ-1.2, respectively. We find that SAZ-0.6 is the best catalyst to give the highest HER rate with corresponding apparent quantum yield of 10.3%. There is a 2.24-fold increase with the incorporation of p-Ag2O between SiO2 and n-Zn(O,S), as compared to the 4,100 µmol.g-1cat.h-1 for Ag2O-free SZ-0.6. The higher amounts of Zn(O,S) such as SAZ-0.9 and SAZ-1.2 led to aggregation and the bigger grain size, as shown in the FE-SEM image of Figure 3. While the lower amount of Zn(O,S) for SAZ-0.3 had shown the lower activity even though there was no aggregation. There is an optimal amount of Zn(O,S) for our rector. The optimization involves the maximization of the solar light harvesting by using porous SiO2 to disperse photocatalyst, the p-n nanodiode concept, and the Zn(O,S) at an optimal amount. Overall, the formation of p-n heterojunction significantly enhances the hydrogen production rate. Introducing a little amount of 2% Ag2O between SiO2 and Zn(O,S) to form the p-n heterojunction between Ag2O and Zn(O,S) is expected to efficiently improve the photocatalytic activity by having n-Zn(O,S) outwards for forming the electron-rich part to further enhance the reduction reaction, while having p-Ag2O inwards to modulate the electron concentration with the hole concentration and establish the built-in electrical nano field. To prove the concept of the dominant n-type semiconductor for enhancing the reduction reaction, the position of p-Ag2O has been purposely changed to outwards to form the configuration of SiO2/Zn(O,S)/Ag2O (SZA). Its



HER rate was 4,600 µmol.g-1cat.h-1m, as shown in Figure 11b, which was 50% lower than SiO2/Ag2O/Zn(O,S). The degraded performance tells us that the outward p-Ag2O has covered the active and useful surface sites not only to reduce the area for the reduction reaction to proceed but also to introduce the unfavored holes. With the help of the porous SiO2 sphere, the Ag2O core@ Zn(O,S) shell-type p-n nano diodes has been successfully used to enhance the photocatalytic hydrogen evolution. To further study the effect of other p-type semiconductor, Ag2S was prepared in the form of SiO2/Ag2S/Zn(O,S), as shown in Figure 11b. This design is also used to understand the possibility of converting Ag2O in SAZ into Ag2S during sample preparation. The HER rate of SiO2/Ag2S/Zn(O,S) was 7,000 µmol.g-1cat.h-1, which was 24% lower than SiO2/Ag2O/Zn(O,S). pAg2S remains useful because its incorporation still improves the HER rate of SiO2/Zn(O,S). Overall, p-Ag2O as an inter-phase for SiO2/Zn(O,S) showed the better performance than p-Ag2S. With the right p-type semiconductor in SiO2/Zn(O,S), we believe the HER rate can be further improved. Although the concept of p-n heterojunction in the core@shell form for photocatalysis has been used 28, 30, their reaction mainly involves the dye degradation and chemical conversion. This work provides an example in enhancing photocatalytic HER rate with the concept of core@shell-type p-n nano diodes of Ag2O/Zn(O,S). One of the most important things in photocatalysis is the ability of the catalyst to be able for re-use for several times, i.e. the so-called reusability. Figure 11c depicts the photocatalytic performance of SiO2/Ag2O/Zn(O,S)-0.6 for four-run re-used tests in four days without any evacuation and re-installation. Therefore, no new ethanol was added and the catalyst was not washed for de-contamination. The HER rates were close one another, even after being re-used for the fourth run. The stable reusability test proves the outstanding photocatalytic stability of our SiO2/Ag2O/Zn(O,S)-0.6 catalyst. The similar stable behavior also had been confirmed by the photocurrent response tests in Figure 10b. As most of the HER tests conducted their reusability tests with catalyst washing and solution refilling with new sacrificial agents, our system is relatively easy for operation.


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Photocatalytic mechanism. The band structure of SiO2/Ag2O/Zn(O,S) was determined by combining the bandgap and Mott-Shocttky data. The bandgap values of Ag2O and Zn(O,S) are 1.36 and 3.53 eV, respectively (see Figure S1). It has been known that the gradient of MottSchottky plot represents the position of conduction and valence bands of n-type and p-type semiconductor, respectively30, 44. Hence, from the Mott-Schottky plot in Figure 9, the conduction band of Zn(O,S) is -0.24 V and valence band to be 3.29 V. The valence band of Ag2O is 0.8 V and its conduction band to be -0.56 V. Based on these values, the band structure is schematically drawn in Figure 12. Active lattice oxygen and its induced oxygen vacancy have been considered as one of the important factors to produce hydrogen from the HER reaction for Zn(O,S)-based photocatalyst in our earlier works

19-21, 23.

Figure 12 also shows a typical heterojunction II and

illustrates the photocatalytic mechanism of SiO2/Ag2O/Zn(O,S) catalyst in generating hydrogen from the mixture solutions of ethanol and water. Each process is explained in the following: Firstly, the SiO2/Ag2O/Zn(O,S) catalyst absorbs the photon energy (h) larger than their band gap energies to generate electrons and holes in the conduction band and valence band, as described in Eq. 1 for Zn(O,S) and Eq. 2 for Ag2O. These carriers have sufficient lifetime to transport due to the 3D MQW band structure. The 3D MQW of Zn(O,S) is combination structure of valleys and hills in 3-dimension due to the difference in d-spacing, as had been illustrated in previous work23. This structure can trap the electrons in the valley while keeping the holes in the peak to inhibit electron-hole recombination. Zn(O,S)+ h   e - (Zn(O,S)) + h + (Zn(O,S))

(1)

 e- (Ag 2 O) + h + (Ag 2 O) Ag 2 O + h 

(2)

Secondly, the photogenerated electrons from the Ag2O migrate to the conduction band of Zn(O,S) while the holes from the valence band of Zn(O,S) transfer to the valence band of Ag2O through the interface region due to the formation of p-n heterojunction with the built-in electric field to drift carriers. Those electrons and holes will be used to execute the chemical reaction. Photogenerated holes start up the oxidation reactions with the aid of surface active oxygen anion on Zn(O,S) under the influences of ethanol (Eq. 3) and water (Eq. 4). Oxygen vacancy is an important product of those reactions. The evidence for this formation oxygen vacancy is based on the color change of Zn(O,S) powder from the white to grey during the photoreaction, while from



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the grey back to while rest overnight. This reversible color change behavior not only provide the evidence of oxygen vacancy but also indicate the oxygen out for water oxidation and oxygen back in for water reduction. 22+ C2 H 5OH + Osurf + 4h +   CH 3CHO + H 2 O + VO,surf

(3)

22+ H 2 O + Osurf + 2h +   2OH -aq. + VO,surf

(4)

Thirdly, the oxygen vacancy traps the water molecule. After water being trapped, its O-H bond weakens and finally breaks to release the adsorbed protons (H+) on surface, as shown in Eq. 5 2+ H 2 O + VO,surf   2H + + O0O,surf

(5)

Lastly, the electrons reduce the adsorbed protons (H+) and produce the hydrogen gas by a reduction reaction in Eq. 6.

2H + + 2e-   H2

(6)

After analyzing all the characterization results, the mechanism for enhancing HER rate of SiO2/Ag2O/Zn(O,S) nanocomposite is proposed. The Mott-Schottky plot provides an evidence of forming the p-n heterojunction in Ag2O/Zn(O,S) nanocomposite. The addition of p-Ag2O has the function to modify the electron concentration of n-Zn(O,S). It is also well known that the formation of p-n heterojunction can generate the special built-in electric field at interface region. This property is powerful to promote the charges separation after photo illumination for electrons and holes, as revealed by the photoluminescence data in Figure 8b. That means SiO2/Ag2O/Zn(O,S) catalyst has more survived electrons and holes than SiO2/Zn(O,S) catalyst without recombination instead to conduct the reduction and oxidation reaction. As results, SiO2/Ag2O/Zn(O,S) catalyst gives the highest HER rate. Based on the results of absorption spectra, the absorption of SiO2/Ag2O/Zn(O,S) significantly improve after formation of p-n heterojunction. This is a very important property in harvesting photon energy which the SiO2/Ag2O/Zn(O,S) catalyst absorbs more light and generates more charge carriers to be consumed in redox reaction. The EIS and lifetime results also support the data for SiO2/Ag2O/Zn(O,S) with lower charge transfer resistance, longer lifetime, and bigger chance to execute the redox reaction. Furthermore, the photocurrent data shows the obviously higher photocurrent compared to those of single materials of Zn(O,S)


Page 15 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


and Ag2O. As photocurrent value represents the number of survived photogenerated charge carriers, then SiO2/Ag2O/Zn(O,S) catalyst obviously provides more electrons to produce hydrogen by reduction reaction according to equation 4. At the end, the enhancement of hydrogen production rate is contributed by the synergetic effect of 3D MQW band structure, active surface oxygen, high photo-absorption, easy electron-hole separation, longer lifetime, low charge transfer resistance, and the p-n heterojunction in the SiO2/Ag2O/Zn(O,S) catalyst. 4. CONCLUSIONS Nanodiode of p-type Ag2O and n-type Zn(O,S) loaded on mesoporous silica support to form the nano p-n heterojunction SiO2/Ag2O/Zn(O,S) have been designed and successfully utilized for hydrogen production. The photocatalytic efficiency of hydrogen evolution reaction could be significantly improved by having n-Zn(O,S) outwards for the electron-rich part to further enhance the reduction reaction, while having p-Ag2O inwards for modulating the electron concentration and establishing the built-in electrical nano field. The amount of loading Zn(O,S) should be optimized in order to have the maximum light harvesting by using porous SiO2 to prevent the aggregation and grain growth and to keep the photocatalyst in nano size with high activity. SiO2/Ag2O/Zn(O,S) with 13.77 wt.% Zn(O,S) and 2 wt.% Ag2O (SAZ-0.6) produced the highest hydrogen rate of 9,200 µmol.g-1cat.h-1 with corresponding apparent quantum yield of 10.3 % which was about 2.7-fold higher than pure Zn(O,S). The photocatalytic stability and reusability showed outstanding performances even after being re-used for the fourth times in four days without any evacuation and re-installation. The significant HER improvement was contributed to the synergetic effects of 3D MQW band structure, active surface oxygen, enhanced photoabsorption, easy electron-hole separation, longer lifetime, low charge transfer resistance, and the p-n heterojunction in the SiO2/Ag2O/Zn(O,S) catalyst. This work provides a strategy to enhance the photocatalytic activity for HER by having the oriented p-n heterojunction concept and by utilizing mesoporous SiO2 as a design tool to control the catalyst in nano size and to vary the diodes in different orientations.



AUTHOR INFORMATION * Corresponding author E-mail: [email protected] ORCID: Noto Susanto Gultom : 0000-0001-5418-4296 Dong-Hau Kuo

: 0000-0001-9300-8551

Hairus Abdullah

: 0000-0002-1775-535X

Wen-Cheng Ke

: 0000-0002-8603-4070

CONFLICTS OF INTEREST There are no conflicts to declare ACKNOWLEDGMENTS This work was supported by Ministry of Science and Technology of Republic of China under grant number MOST: 107-2221-E-011-141-MY3 and HA was supported under 107-2811-E-011-008. REFERENCES (1) Company, E. Global Energy Statistical Yearbook 2018. https://yearbook.enerdata.net/totalenergy/world-consumption-statistics.html (accessed 22 October ). (2) Gupta, R. B. HYDROGEN FUEL Production, Transport and Storage, CRC press: United States, 2008. (3) Zou, X.; Zhang, Y. Noble metal-free hydrogen evolution catalysts for water splitting. Chem Soc Rev 2015, 44, 5148-80, DOI: 10.1039/c4cs00448e. (4) Fujishima, A.; Honda, K. Electrochemical Photolysis of Water at a Semiconductor Electrode. Nature 1972, 238, 37-38. (5) Cao, S.; Yu, J. g-C3N4-Based Photocatalysts for Hydrogen Generation. J. Phys. Chem. Lett. 2014, 5, 2101-2107, DOI: 10.1021/jz500546b.


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LIST OF FIGURES Ag2O

SiO2

Zn(O,S)

AgNO3

Zn+S (seeds)

NaOH

Heat 90C

SiO2/Ag2O

Figure 1. Schematic synthesis procedures of SiO2/Ag2O/Zn(O,S)


SiO2/Ag2O/Zn(O,S)


Page 22 of 31

Gas chromatography (GC) UV lamp Ar

Reactor

MFC

Argon tank

Magnetic stirrer

*MFC= mass flow controller

Figure 2. Schematic photocatalytic hydrogen production experiment (a)

(b)

(c)

(d)

(e)

(f)

Figure 3. FE-SEM images of (a) SiO2, (b) SiO2/Ag2O, and SiO2/Ag2O/Zn(O,S) with different Zn(O,S) contents, being labeled as (c) SAZ-0.3, (d) SAZ-0.6, (e) SAZ-0.9, and (f) SAZ-1.


Page 23 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(a)

(b)

100 nm

50 nm

Figure 4. (a) Low and (b) high magnification TEM images of SiO2/Ag2O/Zn(O,S)-0.6

(a)

(b)

(c)

(d)

(e)

(f) (f)

Figure 5. (a) Dark field image of SiO2/Ag2O/Zn(O,S)-0.6 and elemental mapping images of (b) Si, (c) O, (d) Ag, (e) Zn, and (f) S.



(a)

(b)

Zn

125

Si 2p

Intensity (a.u)

Intensity (a.u)

O

Ag

Si S

250

375

500

625

750

Binding energy (eV)

(c)

O 1s

875

110

1000

108

106

104

102

Binding energy (eV)

(d)

533.2 eV (O-Si)

100

98

Ag 3d5/2

531.3 eV (OV) 530.4 eV (OL)

540

538

536

534

532

Binding energy (eV)

(e)

530

528

Intensity (a.u)

Intensity (a.u)

Ag 3d3/2

526

378

376

374

372

370

368

Binding energy (eV)

366

364

(f)

Zn 2p3/2

S 2p3/2

Intensity (a.u)

S 2p1/2

Intensity (a.u)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 31

Zn 2p1/2

1048

1040

1032

1024

Binding energy (eV)

1016

168

166

164

162

Binding energy (eV)

160

Figure 6. (a) XPS survey spectrum and high resolution (b) Si, (c) O, (c) Ag, (e) Zn, and (f) S of SiO2/Ag2O/Zn(O,S)-0.6 catalyst.


Page 25 of 31

(111)

8000

*ZnS (JCPDS#05-0566) ZnO (JCPDS#65-2880)

(311)

(220)

7000

Intensity (a.u)

6000

Zn(O,S) SAZ-1.2

5000

SAZ-0.9 SAZ-0.6 SAZ-0.3

4000 3000

Ag2O

2000

20

30

40

(311)

(220)

*

(311)



(220)

0

*

SiO2

(111)

1000

(111)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60






*

50

60

70

2 theta (degree) Figure 7. X-ray diffraction patterns of SiO2, Ag2O, Zn(O,S), and SiO2/Ag2O/Zn(O,S) with different Zn(O,S) amounts.



(a)

(b)

SA SAZ-0.6 SZ

SZ SAZ-0.6

Intensity (a.u)

Absorbance (a.u) 350

400

450

500

550

600

350

Wavelength (nm) 0.020

(c)

400

450

Wavelenght (nm)

500

SZ SAZ-0.6

0.015

Intensity (a.u)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 31

0.010

0.005

0.000 0.000

0.002

0.004

0.006

0.008

0.010

0.012

Time (s) Figure 8. (a) Diffuse reflectance spectra, (b) photoluminescence spectra and (c) emission lifetime decay of SiO2/Zn(O,S) and SiO2/Ag2O/Zn(O,S)-0.6


550

Page 27 of 31

10

5x10

(b)

(a) 10

6.0x10

10

4x10

4.0x10

SiO2/Ag2O

2

1/C (F)

10

2x10

10

2.0x10 10

1x10

0 -0.5

0.0

-0.4

-0.3

-0.2

-0.1

0.0

0.4

0.1

0.5

8.0x10

0.6

0.7

0.8

0.9

Potential ( V vs. Ag/AgCl)

Potential ( V vs. Ag/AgCl) 10 (c)

10

6.0x10

p-type Ag2O

2

1/C (F)

2

10

SiO2/Zn(O,S)

10

3x10

1/C (F)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


10

4.0x10

SiO2/Ag2O/Zn(O,S) n-type Zn(O,S) 10

2.0x10

0.0 -0.6

0.80 V

-0.20 V -0.4

-0.2

0.0

0.2

0.4

0.6

0.8

Potential ( V vs. Ag/AgCl)

Figure 9. Mott-schottky plots of (a) SiO2/Zn(O,S), (b) SiO2/Ag2O, and (c) SiO2/Ag2O/Zn(O,S)0.6.


1.0


2500 2000 1500 1000 500

35

(b) On

SAZ-0.6

SZ

SA

30 -2

SA SAZ-0.6 SZ

Current density (mA/cm )

3000 (a)

25 20 15 10 5 Off

0

0 0

1000

2000

3000

Z' (ohm)

4000

5000

60

6000

80

100

120

140

Time (s)

160

180

200

Figure 10. (a) EIS analysis and photocurrent response of SiO2/Ag2O (SA), SiO2/Zn(O,S) (SZ), and (b) SiO2/Ag2O/Zn(O,S)-0.6 (SAZ-0.6)

10000 9000

(a)

9,200

8000

7,600

7,300

7000 6000

0

560

SA

1000

SAZ-1.2

2000

SAZ-0.3

3000

SAZ-0.9

4000

4,800 4,100 SAZ-0.6

5000

SZ

-1

-1

H2 rate (mol g cat h )

-Z'' (ohm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 31

Samples


Page 29 of 31

10000 9000

(b)

9,200

7,000

7000 6000

4000

2000

SZA

3000

SAg2SZ-0.6

4,600

5000

SAZ-0.6

-1 -1

H2 rate (mol g h )

8000

1000 0 10000 9000

Samples

(c) 9,200

9,200

9,100

9,000

8000 -1 -1

H2 rate (mol g h )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


7000 6000 5000 4000 3000 2000 1000 0

1

st

2

nd

Cycles

3

rd

4

th

Figure 11. Photocatalytic hydrogen production of (a) SiO2/Ag2O/Zn(O,S) with different Zn(O,S) contents, (b) SiO2/Ag2O/Zn(O,S)-0.6, SiO2 /Zn(O,S)-0.6/Ag2O, SiO2/Ag2S/Zn(O,S)-0.6 and (c) reusability of SiO2/Ag2O/Zn(O,S)-0.6.



Depletion Region H2+OO ee- ee p-type Ag2O - E h+ e h+ + VB (0.8) h

CB (-0.56) e e

h

+

-

e

h

+

O+O2-

H2 h+

e-

e - h+

SiO2 Zn(O,S) Ag2O Hole Electron Nano-diode

H2O+Vo2+

h+ h+

2OH-+Vo2+

-1

CB (-0.24) 0

n-type Zn(O,S)

h+

Depletion Region

-2

VB (3.29)

C2H5OH+O2-

1 2 3

Potential (V vs Ag/AgCl

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 31

4

CH3CHO+H2O+Vo2+

Figure 12. Schematic mechanism for generating hydrogen through oxygen vacancy from water and ethanol by catalyst SiO2/Ag2O/Zn(O,S).


Page 31 of 31

Table of content

Depletion Region H2+OO H2O+Vo2+

ee- ee p-type Ag2O - E h+ e h+ + VB (0.8) h

CB (-0.56) e e

h+

-

e

h

n-type Zn(O,S)

+

O+O2-

H2 h+

e-

e - h+

SiO2 Zn(O,S) Ag2O Hole Electron Nano-diode

h

VB (3.29) +

h

2OH-+Vo2+


-1

CB (-0.24) 0

h+

Depletion Region

-2

+

C2H5OH+O2-

1 2 3

Potential (V vs Ag/AgCl

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4

CH3CHO+H2O+Vo2+

Zn(O,S) nanodiodes on

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