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Plasmonic Ag deposited TiO2 nano-sheet film for enhanced photocatalytic hydrogen production by water splitting
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Nanotechnology 25 165401 (http://iopscience.iop.org/0957-4484/25/16/165401) View the table of contents for this issue, or go to the journal homepage for more
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Nanotechnology Nanotechnology 25 (2014) 165401 (10pp)
doi:10.1088/0957-4484/25/16/165401
Plasmonic Ag deposited TiO2 nano-sheet film for enhanced photocatalytic hydrogen production by water splitting Enzhou Liu1,5 , Limin Kang1,5 , Yuhao Yang1 , Tao Sun2 , Xiaoyun Hu3 , Changjun Zhu4 , Hanchen Liu4 , Qiuping Wang4 , Xinghua Li3 and Jun Fan1 1
School of Chemical Engineering, Northwest University, No 229 Taibai North Road, Xi’an, Shaanxi 710069, People’s Republic of China 2 School of Chemistry and Chemical Engineering, Nanjing University, Hankou Road, Nanjing 210093, People’s Republic of China 3 Department of Physics, Northwest University, No 229 Taibai North Road, Xi’an, Shaanxi 710069, People’s Republic of China 4 School of Science, Xi’an Polytechnic University, No 19 Jinhua South Road, Xi’an, Shaanxi 710048, People’s Republic of China E-mail: [email protected] Received 16 October 2013, revised 22 January 2014 Accepted for publication 19 February 2014 Published 26 March 2014
Abstract
TiO2 nano-sheet film (TiO2 NSF) was prepared by a hydrothermal method. Ag nanoparticles (NPs) were then deposited on the surface of TiO2 NSF (Ag/TiO2 NSF) under microwave-assisted chemical reduction. The prepared samples were characterized by x-ray diffraction (XRD), scanning electron microscopy (SEM), UV–visible (UV–vis) absorption spectroscopy, x-ray photoelectron spectroscopy (XPS), photoluminescence (PL) spectroscopy, and Raman scattering spectroscopy. The results revealed that the Ag NPs were well dispersed on the anatase/rutile mixed-phase TiO2 nano-sheet surface with a metallic state. The visible light absorption and Raman scattering of TiO2 were enhanced by Ag NPs based on its surface plasmon resonance effect. Besides, Ag NPs could also effectively restrain the recombination of photogenerated electrons and holes. Photocatalytic water splitting was conducted on the films to obtain hydrogen, and the experimental results indicated that plasmonic Ag NPs could greatly enhance the photocatalytic activity of TiO2 due to the synergistic effect between electron transfer and surface plasmon resonance enhanced absorption. The hydrogen yield obtained from the optimal sample reached 8.1 µmol cm−2 and the corresponding energy efficiency was about 0.47%, which was 8.5 times higher than that of pure TiO2 film. Additionally, the formation mechanism of TiO2 nano-sheet film is preliminarily discussed. Keywords: titanium dioxide photocatalysis, plasmonic, Ag nanoparticles, nano-sheet film, photocatalytic water splitting S Online supplementary data available from stacks.iop.org/Nano/25/165401/mmedia (Some figures may appear in colour only in the online journal) 1. Introduction
semiconductor-based photocatalysis has attracted great attention in the field of solar energy conversion. One particular focus has been on photocatalytic water splitting [2–6], which is a promising way to produce renewable hydrogen fuel from solar energy. In the past four decades, over 130 inorganic
Since the photoinduced decomposition of water on a TiO2 electrode was discovered by Fujishima and Honda [1], 5 Co-first authors.
0957-4484/14/165401+10$33.00
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c 2014 IOP Publishing Ltd
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materials and their derivatives have been employed as catalysts for water splitting [7], and many investigations have focused on TiO2 -based photocatalysts due to their relatively high reactivity, chemical stability, low cost, and environmental friendly features [8–14]. However, TiO2 has a relatively wide band gap of 3.2 eV that requires ultraviolet (UV) light for activation, and UV light constitutes only about 4% of the sunlight spectrum. Besides, TiO2 has a high recombination rate of photoexcited electron–hole pairs during the photocatalysis process, leading to a lower photocatalytic efficiency. Therefore, extending the photoresponse of TiO2 to the visible light range and improving the separation of photoexcited electron–hole pairs are the two big challenges for utilization of light to photogenerate H2 from water [6, 15–17]. Many efforts have been made to develop visible light active TiO2 with high photocatalytic efficiency, such as surface photosensitization [18, 19], metal or nonmetal doping [20–22], semiconductor combination [23–25], and noble metal deposition [26–30]. Previous studies have shown that noble metal deposition has proved to be an effective way to restrain the recombination of the photogenerated electron–hole pairs. Recently, studies have shown that noble metal nanoparticles (NPs), such as Ag and Au, can also improve the visible light photoactivity of TiO2 , based on their surface plasmon resonance (SPR) effect [31–33], which results from the collective oscillation of electrons at the surface of metal NPs induced by the appropriate light irradiation [34–36]. When the oscillation frequency of the magnetic field associated with the light is in accordance with that of free electrons, the SPR effect occurs, and light energy is coupled into the metal NPs simultaneously, resulting in photocatalytic activity enhancement. This was first observed in the photocatalytic decomposition of methylene blue using Ag(core)/SiO2 (shell)/TiO2 film by Awazu’s team [37]. They named this new phenomenon ‘plasmonic photocatalysis’. Later, Yang et al demonstrated an enhanced photocurrent of iron oxide photoanodes by coating the semiconductor thin film on Au nanopillars [38], and the enhancement was attributed to increased optical absorption originating from both surface plasmon resonances and photonic-mode light trapping within the nanostructured topography. Furthermore, their study showed that the above optical engineering methods are fully compatible with, and independent of, efforts on material quality optimization, which together can lead to improved performance of solar-driven water splitting. Intensive research activity has recently been devoted to SPR effect enhanced TiO2 photocatalysis [39–52]. Chen et al have demonstrated that Au/TiO2 NPs exhibited a higher H2 production rate from water splitting than that of pure TiO2 [46], because electrons excited under UV light in TiO2 can transfer to Au NPs, and the SPR effect induced by appropriate visible light can boost the energy of trapped electrons, leading to a better photocatalytic activity. Our previous study has shown that Ag NPs could improve the photocatalytic performance of TiO2 nanotube arrays too, based on the SPR effect in photocatalytic water splitting [47]. Further study revealed that SPR absorption by Ag or Au NPs can generate plasmon-induced photoexcited electrons, which can migrate to the conduction band of TiO2
and induce the photocatalytic reaction under visible light irradiation. Zhou and his colleagues demonstrated that Au–TiO2 nanocomposites showed a better photocatalytic activity on Rhodamine-B degradation, arising from SPR absorption of Au NPs under visible light irradiation [48]. SPR-induced visible light active SiO2 @TiO2 –Au, Ag/AgCl/TiO2 , and Au/TiO2 nanotubes have been successfully prepared by Kim’s, Yu’s, and Yang’s research groups, respectively [49–51]. Nevertheless, understanding the plasmonic photocatalysis mechanism is still a challenging task, and more experimental evidence is required for a clear understanding of SPR effect enhanced photocatalysis. Furthermore, compared with film photocatalysts, nanoparticle photocatalysts are not convenient to use and are hard to recycle. Therefore, photocatalysts with film structure could be good candidates for the large-scale practical utilization of plasmonic photocatalysts in water splitting. In this work, Ag NP deposited TiO2 nano-sheet films (Ag/TiO2 NSFs) with different Ag concentrations were successfully prepared by the combination of a hydrothermal method and a microwave-assisted chemical reduction process. UV–visible (UV–vis) absorption and Raman spectroscopy were used to investigate the SPR phenomenon, and photoluminescence (PL) spectroscopy was employed to reveal the electron–hole recombination properties. In addition, the photocatalytic performance was evaluated by photocatalytic water splitting to hydrogen, and a transfer-enhancement synergistic mechanism is proposed to explain the experimental results under different reaction conditions. 2. Experimental details 2.1. Preparation of TiO2 nano-sheet film (TiO2 NSF)
All the chemicals were of analytical grade and were used without further purification. TiO2 NSF was prepared by a hydrothermal method. First, a titanium (Ti) sheet (99.5% purity, 0.5 mm thickness, and 2.1 mm × 4.2 mm) was chemically polished in a mixture solution (HF–HNO3 –H2 O = 1:4:5 by volume) for 15 min, followed by ultrasonication in water to remove residues. Subsequently, it was sealed in a 100 ml Teflon-lined stainless steel vessel containing 50 ml NaOH aqueous solution (1.0 mol l−1 ), which was heated at 180 ◦ C for 24 h and then cooled to room temperature naturally. Thereafter, it was cleaned with water and acid-treated by soaking in HCl aqueous solution (0.25 wt%) at room temperature for 24 h, then washed with water again and dried at room temperature. Finally, TiO2 NSF was obtained by annealing the dried sheet at 450 ◦ C for 2 h with a heating rate of 3 ◦ C min−1 . Discussion of the formation process of TiO2 NSF is given in the supporting information (available at stacks.iop.org/Nano/25/165401/mmedia). 2.2. Preparation of Ag deposited TiO2 nano-sheet film (Ag/TiO2 NSF)
Ag/TiO2 NSF was obtained by a microwave-assisted chemical reduction process, which was inspired by Moon’s work [53]. 0.5 g of polyvinylpyrrolidone (PVP, K30) was dissolved in 20 ml diethylene glycol (DEG) at room temperature and stirred 2
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for 4 h. Then an appropriate amount of NaH2 PO2 ·H2 O was added into the solution, which was kept stirring for 30 min. Subsequently, 20 ml of AgNO3 aqueous solution was poured into the above solution, which was kept stirring for 5 min; then TiO2 NSF was put into above mixture solution and it was exposed to microwave irradiation for 5 min at 140 ◦ C in a microwave system (MDS-8, Sineo Microwave Chemical Technology Co. Ltd, Shanghai, China). Finally, Ag/TiO2 NSF was obtained after washing the sheet with ethanol and water several times. The obtained samples were referred to as x Ag/TiO2 NSF, with x representing the millimolar (mM) concentration of Ag+ in the reaction solution; x varied from 0.3 to 2.1 in intervals of 0.3 in our experiment. The samples were thus denoted as 0.3 Ag/TiO2 NSF, 0.6 Ag/TiO2 NSF, 0.9 Ag/TiO2 NSF, 1.2 Ag/TiO2 NSF, 1.5 Ag/TiO2 NSF, 1.8 Ag/TiO2 NSF, and 2.1 Ag/TiO2 NSF, respectively. For comparison, pure TiO2 NSF was created under the same conditions as described above, without Ag deposition. In addition, the molar ratio of NaH2 PO2 ·H2 O and AgNO3 was 10:1, and PVP and NaH2 PO2 ·H2 O were used as the capping molecule and reducing agent, respectively, according to the literature [53].
Figure 1. XRD patterns of the samples: (a) pure TiO2 NSF, (b) 0.3
Ag/TiO2 NSF, (c) 0.6 Ag/TiO2 NSF, (d) 0.9 Ag/TiO2 NSF, (e) 1.2 Ag/TiO2 NSF, (f) 1.5 Ag/TiO2 NSF, (g) 1.8 Ag/TiO2 NSF, and (h) 2.1 Ag/TiO2 NSF.
with a thermal conductivity detector and a 5 Å molecular sieve packed column. N2 was used as the carrier gas at a flow rate of 30 ml min−1 . The photocatalytic activity of the samples was determined by the quantitatively detected hydrogen yield using an external standard in the same concentration range. The hydrogen yield and energy efficiency were calculated based on equations (1) and (2) below. The detailed calculation process is given in the supporting information (see figure S2 available at stacks.iop.org/Nano/25/165401/mmedia).
2.3. Characterization
The morphologies of the samples were observed using scanning electron microscopy (SEM, JEOL JSM-6390A) with energy-dispersive x-ray (EDS) analysis. Powder x-ray diffraction (XRD) patterns were identified by a Shimadzu XRD6000 powder diffractometer (Cu Kα radiation). The absorption spectra of the samples were recorded on a Shimadzu UV-3600 UV/vis/NIR spectrophotometer with an integrating sphere detector, and BaSO4 was used as a reference. Besides, x-ray photoelectron spectroscopy (XPS) measurements were obtained with a Kratos AXIS NOVA spectrometer, Raman spectra were collected on a Raman spectrometer (Renishaw inVia Reflex), and photoluminescence (PL) spectra were obtained using a florescence spectrophotometer (Hitachi F-7000).
YH2 = (CH2 × Vgas )/Scat E e = 1HH2 × (CH2 × Vgas )/E a ,
(1) (2)
where YH2 is the hydrogen yield (µmol cm−2 ), CH2 is the hydrogen concentration (µmol l−1 ), Vgas is the gas volume above the reaction solution (l), Scat is the surface area of the film (cm2 ), E e is the energy efficiency, 1HH2 is the heat of combustion of hydrogen (kJ mol−1 ), and E a is the energy of photons absorbed by the film under ideal conditions (J) (see figure S2 available at stacks.iop.org/Nano/25/165401/ mmedia).
2.4. Photocatalytic water splitting
The apparatus used for photocatalytic water splitting has been described in our previous study [47]. The experiment was carried out in a quartz photoreactor with a volume of 150 ml. A 16 W high-pressure Hg lamp was employed as the UV light source with a wavelength of 254 nm, and a 500 W Xenon lamp was used as the visible light source under the assistance of a 400 nm cut-off optical filter; both of the lamps were placed outside the reactor facing the photocatalytic film, and the distance between the lamps and the film was 30 cm. The system was shielded by a black box during the reaction to prevent interference from outside light and to protect us from the UV light. The film was fixed in the reactor and immersed in 80 ml 10 vol.% ethanol/H2 O solution, and high-purity N2 was bubbled through the system for 30 min to eliminate dissolved oxygen before irradiation. During the experiment, a needle-type probe was inserted into the reactor to withdraw the generated gas, which was analyzed using a gas chromatograph (Fuli GC-9790II, Zhe Jiang, China) equipped
3. Results and discussion 3.1. Characterization of samples
The XRD patterns of the samples are presented in figure 1. The characteristic diffraction peaks of anatase and rutile TiO2 are observed in all the samples after annealing at 450 ◦ C. Although the amount of rutile phase in the samples is very small, this anatase/rutile mixed phase is suitable for the photocatalytic reaction [44]. The intense diffraction peaks at 35.1◦ , 38.4◦ , 40.2◦ , 53.0◦ , 62.9◦ , 70.6◦ , and 76.2◦ are attributed to the Ti substrate under the nano films. For the Ag decorated samples, the XRD diffraction peaks of Ag/TiO2 NSFs are almost the same as those of pure TiO2 NSF after Ag deposition, indicating that the mixed-phase TiO2 is well maintained, and two additional peaks related to metallic Ag are observed at 3
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Figure 2. (a) SEM images of pure TiO2 NSF and (b) the magnification of image (a).
Figure 3. SEM images of (a) 0.3 Ag/TiO2 NSF, (b) 0.9 Ag/TiO2 NSF, (c) 1.5 Ag/TiO2 NSF, and (d) 2.1 Ag/TiO2 NSF.
44.3◦ and 64.4◦ in the samples with higher Ag concentration. The diffraction peaks of Ag are not obvious in 0.3 Ag/TiO2 NSF, which may be attributed to the fact that the content of Ag is so low that it could not be detected. The existence of Ag is also proved by the SEM, EDS, and XPS analyses below. In order to give an overall impression on the prepared samples, photographs are shown in figure S3 (see supporting information available at stacks.iop.org/Nano/25/165401/ mmedia). It is apparent that TiO2 NSF with different Ag concentrations exhibits different colors under natural sunlight: the color of the film changes from gray to dark brown as the Ag concentration increases. This may be related to the light absorption properties of the samples (see figure 5). The structure of the films is shown in figure 2. The overall appearance of the TiO2 NSF is like a carpet with a rough surface (figure 2(a)), which consists of a large number of continuous distributed nano-sheets (figure 2(b)), so we named it TiO2 nano-sheet film. It has a large surface area and high volume fraction of atoms on the surface, which can provide more active sites for photoreaction. Besides, it is worth noting
that the prepared film exhibits a good stability, and it is relative hard to detach from the Ti substrate, which is beneficial for the practical utilization of film photocatalysts. Figure 3 illustrates the morphology of Ag deposited TiO2 NSFs. It is apparent that Ag NPs are dispersed uniformly with spherical morphology on the films rather than being particularly aggregated. The size of Ag NPs changes from 20 to 50 nm, leading to a broad adsorption in the visible light range (see figure 5). As shown in figure 3, the density of Ag NPs on the surface of 0.3 Ag/TiO2 NSF is lower than that on the surface of 0.9 Ag/TiO2 and 1.5 Ag/TiO2 NSF (figures 2(b) and (c)); this may be due to the lower concentration of Ag+ in reaction solution. However, the higher concentration of Ag+ may lead to the aggregation of Ag NPs on the surface of 2.1 Ag/TiO2 NSF (figure 3(d)). Energydispersive x-ray (EDS) spectroscopy was used to analyze the elemental composition of 1.5 Ag/TiO2 NSF, and the result is shown in figure 4. Ti, O and Ag are detected from the film surface, suggesting the existence of Ag particles. XPS spectra give the evidence that Ag+ can be reduced to metallic silver 4
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Figure 4. EDS patterns of 1.5 Ag/TiO2 NSF; the inset image is a back-scattered electron (BSE) image of the sample.
Ag NPs on the nano-sheet surface. The absorption spectra of Ag NPs in different samples are obtained through subtracting the contribution of pure TiO2 absorption from the absorption of Ag/TiO2 . As shown in figure 5(b), there is an obvious absorption between 400 and 500 nm with two absorption peaks, which is possibly due to the wide size distribution of Ag NPs as shown in figure 3, because the absorption spectrum depends on the nanoparticle size and shape, etc [34, 35, 54, 55]. The enhanced light absorption in the visible light region can therefore improve the activity of TiO2 by increasing the quantities of photogenerated electrons and holes or by enhancing the energy of trapped electrons; this is discussed later. To determine the electron–hole recombination properties, the photoluminescence (PL) emission spectrum is used to study the trapping and migration efficiency of charge carriers and to understand the behavior of the electron–hole pairs. Figure 6 shows the PL emission spectra of the prepared samples under excitation at 220 nm. It can be observed that the emission spectra of Ag/TiO2 composites appear to be similar to those of pure TiO2 NSF. The emission peaks around 396 and 467 nm are due to the free-excitation emission of the band gap and the charge-transfer transition from Ti3+ to the oxygen anion in a TiO6 octahedral complex, respectively [56, 57]. Under the same intensity of excitation irradiation, the emission intensity of Ag/TiO2 composites is lower than that of pure TiO2 NSF, suggesting that the recombination of photogenerated electrons and holes is suppressed effectively. That is because the metal work function of Ag is higher than that of TiO2 , so the photogenerated electrons will transfer from the conduction band of TiO2 to Ag NPs until the Fermi level equilibrium is achieved, forming a Schottky barrier on their contact surface. The electron transfer can reduce the recombination chance of electrons and holes [29], leading
during the microwave-assisted chemical reduction process (see figure S4 available at stacks.iop.org/Nano/25/165401/ mmedia). The above results indicate that the microwave-assisted chemical reduction method is an appropriate way to deposit Ag NPs under the assistance of a PVP–DEG system, which is usually used as a reaction environment for the fabrication of metal nanoparticles in hydrothermal conditions [53]. In this study, microwave irradiation was employed because of its rapid and uniform volumetric heating property, and Ag NPs with good dispersibility were successfully deposited on TiO2 NSF in a relative short time. Besides, it is important that the film can maintain its original morphology after the Ag loading process. UV–vis absorption spectra of the samples were measured to study their optical properties. As shown in figure 5(a), all the samples exhibit strong absorption in the range below 400 nm. This is attributed to the characteristic absorption of TiO2 from band–band electron transitions. Besides, the pure TiO2 films also show visible light absorption property, and the absorption increases on increasing the visible light wavelength. This is different from the absorption property of TiO2 nanoparticles reported in the literature (see figure S5 available at stacks. iop.org/Nano/25/165401/mmedia). This phenomenon may be related to its unique nanostructure. Compared with pure TiO2 film, Ag/TiO2 composites show broad absorption in the visible region arising from the SPR effect of Ag NPs, and the surface plasmon absorption gives the valid evidence that Ag NPs exist in the samples too. Further observation indicates that the intensity of surface plasmon absorption increases with increasing Ag amount, which is probably due to the greater number of Ag NPs deposited on the TiO2 surface. However, the absorption intensity of 2.1 Ag/TiO2 NSF is lower than that of 1.8 Ag/TiO2 NSF. This may be related to the aggregation of 5
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Figure 6. PL emission spectra of the samples under excitation at
220 nm; the emission before and after Ag deposition appears in the same color.
Raman spectroscopy was employed to further investigate the SPR effect based on the surface-enhanced Raman scattering (SERS) from Ag NPs. SERS is not only available for ions and molecular, but can also be extended to nanomaterials, such as TiO2 [58, 59]; it can detect the phases of TiO2 . Figure 7 shows the Raman scattering spectra of pure TiO2 and Ag deposited TiO2 NSFs. The anatase TiO2 belongs to the D4h point group, and its characteristic Raman active modes are at 144 cm−1 (Eg ), 196 cm−1 (Eg ), 395 cm−1 (B1g ), 515 cm−1 (B1g ), and 637 cm−1 (Eg ) [60]. Thus all five peaks observed in figure 7(a) can be ascribed to the allowed vibration frequency of the anatase phase, and the frequency of the rutile phase is not observed because of its lower content in the mixed phase (figure 1). However, the scattering peak of pure TiO2 NSF at 144 cm−1 exhibits a slight shift to higher frequency in Ag deposited TiO2 NSFs (figure 7(b)). The interface structure between Ag and TiO2 may play a crucial role in the peak shift [22, 47, 58]. The above characteristic Raman scattering peaks of anatase TiO2 are enhanced after deposition of Ag NPs, and the scattering intensity increases with increasing Ag amount. The enhancement is due to SERS from Ag, because the local electronic field near the surface of Ag NPs is greatly enhanced due to the SPR effect, and TiO2 in close vicinity to Ag NPs can strongly receive the influence of the local field, thereby giving a strong Raman signal. From the above results, it is believed that the SPR effect of Ag NPs indeed has an influence on TiO2 , and this influence may be beneficial to TiO2 in photocatalytic reactions.
Figure 5. (a) UV–vis absorption spectra of the samples; the absorption before and after Ag deposition appears in the same color. (b) UV–vis absorption spectra of Ag NPs in different samples, obtained by subtracting the contribution of pure TiO2 absorption from the absorption of Ag deposited TiO2 NSF samples.
to lower emission intensity. Based on the above analysis, the deposition of Ag NPs can reduce the recombination chance of electrons and holes. However, there is an optimal deposition amount. When the amount of Ag is relatively lower, it has a weak suppression for the recombination. The lowest recombination rate is obtained when the concentration of Ag+ is 1.5 mmol l−1 in reaction solution. On increasing the concentration of Ag+ , Ag NPs can aggregate on the surface of the nano-sheet (figure 3(d)) and also adversely work as recombination centers for the electrons and holes, reducing the suppression ability. This is one of the reasons why 1.5 Ag/TiO2 displays the highest photocatalytic activity for photocatalytic water splitting. Further observation indicates that deposition of Ag NPs will result in a new emission peak, around 373 nm in figure 6, because the emission around 373 nm is not observed in the emission spectra of TiO2 NSF before Ag deposition. The peak around 373 nm emerges in all modified samples and it belongs to the emission of the band gap transition in our previous studies [20, 22]. It can be seen from figure 6 that the relative intensity of the emission around 373 nm increases with increasing Ag amount, which is probably due to the greater number of Ag NPs deposited on the TiO2 surface, indicating relatively more recombination for emission around 373 nm.
3.2. Photocatalytic activities of H2 production
The main target of this work is to detect H2 production through water splitting over TiO2 catalysts. Yang and Strataki et al discovered that using ethanol as the sacrificial agent gave a higher H2 production efficiency [61, 62]. Thus ethanol was chosen as the sacrificial agent in our work. Photocatalytic water splitting was conducted on nano-sheet films under different light irradiation. Figure 8 shows the H2 yield of the samples after 3 h reaction under UV and visible light (λ > 400 nm) 6
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Figure 8. Hydrogen yield of photocatalytic water splitting under UV
and visible light (λ > 400 nm) irradiation for 3 h.
the reaction was carried out; the results show that Ag is not oxidized after the 12 h irradiation (see figures S6 and S7 available at stacks.iop.org/Nano/25/165401/mmedia). To further understand the role of Ag NPs during the photoreaction, the photocatalytic water splitting was conducted using pure TiO2 and 1.5 Ag/TiO2 composite under UV or visible light (λ > 400 nm) irradiation, respectively. Figure 9(a) presents the relationship between the irradiation time and the H2 yield under UV light. It can be seen that H2 yield increases gradually with increasing the light irradiation time; the H2 yield of 1.5 Ag/TiO2 NSF is 5.6 times higher than that of pure TiO2 NSF after 3 h irradiation, and the corresponding H2 yield is 4.2 and 0.75 µmol cm−2 , respectively. The enhanced photocatalytic activity is attributed to the charge-transfer property of Ag NPs: the electrons of TiO2 excited under UV light can transfer to the Ag NPs and reduce the recombination chance of electrons and holes, facilitating the photoreaction process. For the experiment under visible light (λ > 400 nm) irradiation, the results are presented in figure 9(b). No hydrogen is detected when pure TiO2 NSF is used as the catalyst, but 1.5 Ag/TiO2 NSF is found to be active for H2 production, which should be attributed to the SPR effect of Ag NPs. Under visible light irradiation, the SPR absorption induced photoexcited electrons in Ag NPs can move through the Ag/TiO2 composite interface into the conduction band of TiO2 , and then induce the photoreaction, which has been proved in the literature [38, 40, 41, 43, 48]. However, the H2 yield can only reach 1.6 µmol cm−2 under UV light irradiation (figure 9(b)), this is lower than that of under UV light irradiation (4.2 µmol cm−2 , figure 9(a)), or UV and visible light irradiation (8.1 µmol cm−2 , figure 8), suggesting that SPR absorption of Ag NPs is not an effective way to induce a photocatalytic reaction. It can be concluded from above results that the photocatalytic activity enhancement of TiO2 should be ascribed to the role of electron trapping and SPR absorption properties of Ag NPs. On the one hand, the UV light activity of TiO2 is improved based on the electron trapping of Ag NPs, which is demonstrated by the PL spectra in figure 6. On the other
Figure 7. Raman scattering spectra of the samples.
irradiation. Compared with the pure TiO2 film, Ag deposited TiO2 films exhibit higher photocatalytic activities, and the H2 yield increases with the increase of Ag content, suggesting that Ag NPs have a great effect on the photocatalytic activity of the samples. However, this increase is no longer significant when the content of Ag NPs is higher on the surface of TiO2 films, such as with 1.8 and 2.1 Ag/TiO2 NSFs. Although 1.8 Ag/TiO2 NSF has a more efficient utilization of light than 1.5 Ag/TiO2 NSF (figure 5), Ag NPs adversely work as the recombination center of photogenerated charges when they are over deposited, as we mentioned in discussion of the PL emission spectrum (figure 6), leading to a lower quantum efficiency. It can be concluded that photogenerated electron recombination should be considered when increase the visible light absorption of TiO2 . Further analysis shows that the H2 yield of 1.5 Ag/TiO2 NSF is 8.5 times higher than that of pure TiO2 NSF under UV and visible light (λ > 400 nm) irradiation; the corresponding H2 yield is 8.1 and 0.95 µmol cm−2 , respectively. The energy efficiency was about 0.47% on the 1.5 Ag/TiO2 composite. In addition, the H2 yield on TiO2 NSF treated without Ag deposition (see section 2.2) is 0.92 µmol cm−2 under UV and visible light (λ > 400 nm) irradiation for 3 h, indicating that the Ag deposition process has no obvious effect on the activity of TiO2 NSF. The long-time stability of 1.5 Ag/TiO2 NSF for H2 evolution was investigated and an XPS analysis after 7
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Figure 10. The transfer-enhancement synergistic mechanism for
photocatalytic water splitting over Ag/TiO2 NSF.
on the Ag/TiO2 nanocomposite. Upon UV light irradiation, the electrons of TiO2 are excited to the conduction band and the holes are left behind in the valence band. Then the electrons migrate to the Ag NPs and accumulate, forming a Schottky barrier between the Ag NPs and the TiO2 NSF. Simultaneously, the SPR effect is induced by the visible light, forming a strong local electronic field to enhance the energy of trapped electrons, making them transfer and react with electron acceptors more easily. The synergetic effect between charge transfer and SPR absorption of Ag NPs is achieved, leading to the boosted photocatalytic performance on the Ag/TiO2 surface. The plausible reaction pathways of H2 production from water splitting are summarized in the following equations (equations (3)–(9)). During the process of water splitting, holes in valence band of TiO2 can react with H2 O molecules to form H+ and ·OH (equation (6)). Subsequently, ·OH can be consumed through chemical reaction with CH3 CH2 OH (equation (7)), accelerating the decomposition of H2 O to H+ and ·OH, leading to more H+ product and increasing the rate of H2 production. On the other hand, the electrons with enhanced energy react with H+ at the surface of Ag NPs. H+ can receive an electron to form an H atom, and then two H atoms combine with each other to form an H2 molecule. It is inferred that the higher energy created by the SPR effect can promote trapped electrons being involved in the reaction, facilitating H2 generation. In this system, ethanol serves as a sacrificial reagent, which also helps to inhibit the recombination of photoinduced electrons and holes and thus to increase the efficiency of the entire process [63].
Figure 9. Photocatalytic activity of pure TiO2 and 1.5 Ag/TiO2 NSF
under (a) UV light irradiation and (b) visible light (λ > 400 nm) irradiation.
hand, visible light activity of TiO2 is induced by the SPR absorption of Ag NPs, because TiO2 cannot be activated under visible light irradiation, which is proved by the experimental result in figure 9(b) and the SPR absorption in figure 5. The experimental results show that the H2 yield (8.1 µmol cm−2 ) obtained under UV and visible light irradiations is higher than the sum of H2 yield (5.8 µmol cm−2 ) when UV and visible light irradiations are used separately. When UV light is used as the irradiation source, the H2 yield is 4.2 µmol cm−2 after 3 h irradiation; it can reach 8.1 µmol cm−2 when visible light irradiation is added, suggesting that visible light irradiation can greatly enhance the photocatalytic activity of TiO2 , which is due to the SPR effect of Ag NPs. However, the H2 yield is only 1.6 µmol cm−2 under visible light irradiation for 3 h, indicating that the SPR absorption of excited electrons in Ag NPs can induce the photocatalytic water splitting, but with a lower efficiency.
− TiO2 + hv → h + vb + ecb − ecb
− → eAg
− − eAg + hv(visible) → eSPR + h+ vb + H2 O → ·OH + H
(3) (4) (5)
(6) CH3 CH2 OH + ·OH → CH3 COOH + H+ (7) − + eSPR + H → H (8) H + H → H2 (9) − where h + is the hole in the valence band, e is the electron in vb cb − the conduction band, eAg is the electron trapped by Ag NPs, − eSPR is the electron enhanced by the SPR effect.
3.3. Mechanism of photocatalytic water splitting
Based on the above analysis, a transfer-enhancement synergetic photocatalytic mechanism is proposed in figure 10 to help understand the photocatalytic process that takes place 8
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4. Conclusions
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Ag deposited TiO2 nano-sheet film was prepared by a microwave-assisted chemical reduction method. The visible light absorption and Raman scattering of TiO2 are enhanced by Ag NPs due to their SPR effect. In addition, Ag NPs can also restrain the recombination of photogenerated electron– hole pairs and improve the charge-transfer efficiency. The experimental results indicate that SPR absorption of Ag NPs in the visible region can greatly boost the energy of trapped electrons through the strong local electron field of the SPR effect, making them transfer and react with electron acceptors easily. The 1.5 Ag/TiO2 NSF composite exhibits the best activity for H2 production due to the synergistic effect between the electron transfer and SPR absorption; the H2 yield of 1.5 Ag/TiO2 NSF is 8.5 times higher than that of pure TiO2 NSF under UV and visible light (λ > 400 nm) irradiation. This study suggests a promising method to develop SPR effect enhanced photocatalysis, which may have a profound implication on the future utilization of solar light. Acknowledgments
This work was supported by the National Natural Science Foundation of China (Nos 21306150, 21176199 and 51372201), the Shaanxi Provincial Research Foundation for Basic Research, China (Nos 2013JQ2003, 2011JM1001 and 2012JM1020), the Scientific Research and Industrialization Cultivation Foundations of the Education Department of Shaanxi Provincial Government, China (Nos 2013JK0693 and 2011JG05), the Scientific Research Foundation of Northwest University (No 12NW19), and the Scientific Research Foundation of Northwest University (No PR12216). References [1] Fujishima A and Honda K 1972 Nature 238 37–8 [2] Wang X, Maeda K, Thomas A, Takanabe K, Xin G, Carlsson J M, Domen K and Antonietti M 2008 Nature Mater. 8 76–80 [3] Chen X, Shen S, Guo L and Mao S S 2010 Chem. Rev. 110 6503–70 [4] Ingram D B and Linic S 2011 J. Am. Chem. Soc. 133 5202–5 [5] Daskalaki V M, Panagiotopoulou P and Kondarides D I 2011 Chem. Eng. J. 170 433–9 [6] Dholam R, Patel N, Adami M and Miotello A 2009 Int. J. Hydrog. Energy 34 5337–46 [7] Osterloh F E 2007 Chem. Mater. 20 35–54 [8] Hoffmann M R, Martin S T, Choi W and Bahnemann D W 1995 Chem. Rev. 95 69–96 [9] Khan S U M, Al-Shahry M and Ingler W B 2002 Science 297 2243–5 [10] Xiang Q, Yu J and Jaroniec M 2012 J. Am. Chem. Soc. 134 6575–8 [11] Alenzi N, Liao W S, Cremer P S, Sanchez-Torres V, Wood T K, Ehlig-Economides C and Cheng Z 2010 Int. J. Hydrog. Energy 35 11768–75 [12] Ni M, Leung M K H, Leung D Y C and Sumathy K 2007 Renew. Sustain. Energy Rev. 11 401–25 9
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[43] Kim H, Choi C, Khamwannah J, Noh S Y, Zhang Y, Seong T Y and Jin S 2013 J. Renew. Sustain. Energy 5 053104 [44] Wen Y, Liu B, Zeng W and Wang Y 2013 Nanoscale 5 9739–46 [45] Liu E, Kang L, Wu F, Sun T, Hu X, Yang Y, Liu H and Fan J 2013 Plasmonics 9 61–70 [46] Chen J J, Wu J C S, Wu P C and Tsai D P 2011 J. Phys. Chem. C 115 210–6 [47] Wu F, Hu X, Fan J, Liu E, Sun T, Kang L, Hou W, Zhu C and Liu H 2013 Plasmonics 8 501–8 [48] Zhou M, Zhang J, Cheng B and Yu H 2012 Int. J. Photoenergy 2012 532843 [49] Kochuveedu S T, Kim D P and Kim D H 2012 J. Phys. Chem. C 116 2500–6 [50] Yu J, Dai G and Huang B 2009 J. Phys. Chem. C 113 16394–401 [51] Liang Y Q, Cui Z D, Zhu S L, Liu Y and Yang X J 2011 J. Catal. 278 276–87 [52] Mankidy B D, Joseph B and Gupta V K 2013 Nanotechnology 24 405402 [53] Park B K, Jeong S, Kim D, Moon J, Lim S and Kim J S 2007 J. Colloid Interface Sci. 311 417–24
[54] Amendola V, Bakr O M and Stellacci F 2010 Plasmonics 5 85–97 [55] Dai Z G, Xiao X H, Zhang Y P, Ren F, Wu W, Zhang S F, Zhou J, Mei F and Jiang C Z 2012 Nanotechnology 23 335701 [56] Zhang Y X, Li G H, Jin Y X, Zhang Y, Zhang J and Zhang L D 2002 Chem. Phys. Lett. 365 300–4 [57] Yu J, Yu H, Ao C H, Lee S C, Yu J C and Ho W 2006 Thin Solid Films 496 273–80 [58] Kremenovi´c A, Antic B, Blanuˇsa J, Comor M, Colomban P, Mazerolles L and Bozin E S 2011 J. Phys. Chem. C 115 4395–403 [59] Swamy V, Kuznetsov A, Dubrovinsky L S, Caruso R A, Shchukin D G and Muddle B C 2005 Phys. Rev. B 71 184302 [60] Naumenko D, Snitka V, Snopok B, Arpiainen S and Lipsanen H 2012 Nanotechnology 23 465703 [61] Yang Y Z, Chang C H and Idriss H 2006 Appl. Catal. B 67 217–22 [62] Strataki N, Bekiari V, Kondarides D I and Lianos P 2007 Appl. Catal. B 77 184–9 [63] Chen X and Mao S S 2007 Chem. Rev. 107 2891–959
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Plasmonic Ag deposited TiO2 nano-sheet film for enhanced photocatalytic hydrogen production by water splitting
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Nanotechnology Nanotechnology 25 (2014) 165401 (10pp)
doi:10.1088/0957-4484/25/16/165401
Plasmonic Ag deposited TiO2 nano-sheet film for enhanced photocatalytic hydrogen production by water splitting Enzhou Liu1,5 , Limin Kang1,5 , Yuhao Yang1 , Tao Sun2 , Xiaoyun Hu3 , Changjun Zhu4 , Hanchen Liu4 , Qiuping Wang4 , Xinghua Li3 and Jun Fan1 1
School of Chemical Engineering, Northwest University, No 229 Taibai North Road, Xi’an, Shaanxi 710069, People’s Republic of China 2 School of Chemistry and Chemical Engineering, Nanjing University, Hankou Road, Nanjing 210093, People’s Republic of China 3 Department of Physics, Northwest University, No 229 Taibai North Road, Xi’an, Shaanxi 710069, People’s Republic of China 4 School of Science, Xi’an Polytechnic University, No 19 Jinhua South Road, Xi’an, Shaanxi 710048, People’s Republic of China E-mail: [email protected] Received 16 October 2013, revised 22 January 2014 Accepted for publication 19 February 2014 Published 26 March 2014
Abstract
TiO2 nano-sheet film (TiO2 NSF) was prepared by a hydrothermal method. Ag nanoparticles (NPs) were then deposited on the surface of TiO2 NSF (Ag/TiO2 NSF) under microwave-assisted chemical reduction. The prepared samples were characterized by x-ray diffraction (XRD), scanning electron microscopy (SEM), UV–visible (UV–vis) absorption spectroscopy, x-ray photoelectron spectroscopy (XPS), photoluminescence (PL) spectroscopy, and Raman scattering spectroscopy. The results revealed that the Ag NPs were well dispersed on the anatase/rutile mixed-phase TiO2 nano-sheet surface with a metallic state. The visible light absorption and Raman scattering of TiO2 were enhanced by Ag NPs based on its surface plasmon resonance effect. Besides, Ag NPs could also effectively restrain the recombination of photogenerated electrons and holes. Photocatalytic water splitting was conducted on the films to obtain hydrogen, and the experimental results indicated that plasmonic Ag NPs could greatly enhance the photocatalytic activity of TiO2 due to the synergistic effect between electron transfer and surface plasmon resonance enhanced absorption. The hydrogen yield obtained from the optimal sample reached 8.1 µmol cm−2 and the corresponding energy efficiency was about 0.47%, which was 8.5 times higher than that of pure TiO2 film. Additionally, the formation mechanism of TiO2 nano-sheet film is preliminarily discussed. Keywords: titanium dioxide photocatalysis, plasmonic, Ag nanoparticles, nano-sheet film, photocatalytic water splitting S Online supplementary data available from stacks.iop.org/Nano/25/165401/mmedia (Some figures may appear in colour only in the online journal) 1. Introduction
semiconductor-based photocatalysis has attracted great attention in the field of solar energy conversion. One particular focus has been on photocatalytic water splitting [2–6], which is a promising way to produce renewable hydrogen fuel from solar energy. In the past four decades, over 130 inorganic
Since the photoinduced decomposition of water on a TiO2 electrode was discovered by Fujishima and Honda [1], 5 Co-first authors.
0957-4484/14/165401+10$33.00
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materials and their derivatives have been employed as catalysts for water splitting [7], and many investigations have focused on TiO2 -based photocatalysts due to their relatively high reactivity, chemical stability, low cost, and environmental friendly features [8–14]. However, TiO2 has a relatively wide band gap of 3.2 eV that requires ultraviolet (UV) light for activation, and UV light constitutes only about 4% of the sunlight spectrum. Besides, TiO2 has a high recombination rate of photoexcited electron–hole pairs during the photocatalysis process, leading to a lower photocatalytic efficiency. Therefore, extending the photoresponse of TiO2 to the visible light range and improving the separation of photoexcited electron–hole pairs are the two big challenges for utilization of light to photogenerate H2 from water [6, 15–17]. Many efforts have been made to develop visible light active TiO2 with high photocatalytic efficiency, such as surface photosensitization [18, 19], metal or nonmetal doping [20–22], semiconductor combination [23–25], and noble metal deposition [26–30]. Previous studies have shown that noble metal deposition has proved to be an effective way to restrain the recombination of the photogenerated electron–hole pairs. Recently, studies have shown that noble metal nanoparticles (NPs), such as Ag and Au, can also improve the visible light photoactivity of TiO2 , based on their surface plasmon resonance (SPR) effect [31–33], which results from the collective oscillation of electrons at the surface of metal NPs induced by the appropriate light irradiation [34–36]. When the oscillation frequency of the magnetic field associated with the light is in accordance with that of free electrons, the SPR effect occurs, and light energy is coupled into the metal NPs simultaneously, resulting in photocatalytic activity enhancement. This was first observed in the photocatalytic decomposition of methylene blue using Ag(core)/SiO2 (shell)/TiO2 film by Awazu’s team [37]. They named this new phenomenon ‘plasmonic photocatalysis’. Later, Yang et al demonstrated an enhanced photocurrent of iron oxide photoanodes by coating the semiconductor thin film on Au nanopillars [38], and the enhancement was attributed to increased optical absorption originating from both surface plasmon resonances and photonic-mode light trapping within the nanostructured topography. Furthermore, their study showed that the above optical engineering methods are fully compatible with, and independent of, efforts on material quality optimization, which together can lead to improved performance of solar-driven water splitting. Intensive research activity has recently been devoted to SPR effect enhanced TiO2 photocatalysis [39–52]. Chen et al have demonstrated that Au/TiO2 NPs exhibited a higher H2 production rate from water splitting than that of pure TiO2 [46], because electrons excited under UV light in TiO2 can transfer to Au NPs, and the SPR effect induced by appropriate visible light can boost the energy of trapped electrons, leading to a better photocatalytic activity. Our previous study has shown that Ag NPs could improve the photocatalytic performance of TiO2 nanotube arrays too, based on the SPR effect in photocatalytic water splitting [47]. Further study revealed that SPR absorption by Ag or Au NPs can generate plasmon-induced photoexcited electrons, which can migrate to the conduction band of TiO2
and induce the photocatalytic reaction under visible light irradiation. Zhou and his colleagues demonstrated that Au–TiO2 nanocomposites showed a better photocatalytic activity on Rhodamine-B degradation, arising from SPR absorption of Au NPs under visible light irradiation [48]. SPR-induced visible light active SiO2 @TiO2 –Au, Ag/AgCl/TiO2 , and Au/TiO2 nanotubes have been successfully prepared by Kim’s, Yu’s, and Yang’s research groups, respectively [49–51]. Nevertheless, understanding the plasmonic photocatalysis mechanism is still a challenging task, and more experimental evidence is required for a clear understanding of SPR effect enhanced photocatalysis. Furthermore, compared with film photocatalysts, nanoparticle photocatalysts are not convenient to use and are hard to recycle. Therefore, photocatalysts with film structure could be good candidates for the large-scale practical utilization of plasmonic photocatalysts in water splitting. In this work, Ag NP deposited TiO2 nano-sheet films (Ag/TiO2 NSFs) with different Ag concentrations were successfully prepared by the combination of a hydrothermal method and a microwave-assisted chemical reduction process. UV–visible (UV–vis) absorption and Raman spectroscopy were used to investigate the SPR phenomenon, and photoluminescence (PL) spectroscopy was employed to reveal the electron–hole recombination properties. In addition, the photocatalytic performance was evaluated by photocatalytic water splitting to hydrogen, and a transfer-enhancement synergistic mechanism is proposed to explain the experimental results under different reaction conditions. 2. Experimental details 2.1. Preparation of TiO2 nano-sheet film (TiO2 NSF)
All the chemicals were of analytical grade and were used without further purification. TiO2 NSF was prepared by a hydrothermal method. First, a titanium (Ti) sheet (99.5% purity, 0.5 mm thickness, and 2.1 mm × 4.2 mm) was chemically polished in a mixture solution (HF–HNO3 –H2 O = 1:4:5 by volume) for 15 min, followed by ultrasonication in water to remove residues. Subsequently, it was sealed in a 100 ml Teflon-lined stainless steel vessel containing 50 ml NaOH aqueous solution (1.0 mol l−1 ), which was heated at 180 ◦ C for 24 h and then cooled to room temperature naturally. Thereafter, it was cleaned with water and acid-treated by soaking in HCl aqueous solution (0.25 wt%) at room temperature for 24 h, then washed with water again and dried at room temperature. Finally, TiO2 NSF was obtained by annealing the dried sheet at 450 ◦ C for 2 h with a heating rate of 3 ◦ C min−1 . Discussion of the formation process of TiO2 NSF is given in the supporting information (available at stacks.iop.org/Nano/25/165401/mmedia). 2.2. Preparation of Ag deposited TiO2 nano-sheet film (Ag/TiO2 NSF)
Ag/TiO2 NSF was obtained by a microwave-assisted chemical reduction process, which was inspired by Moon’s work [53]. 0.5 g of polyvinylpyrrolidone (PVP, K30) was dissolved in 20 ml diethylene glycol (DEG) at room temperature and stirred 2
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for 4 h. Then an appropriate amount of NaH2 PO2 ·H2 O was added into the solution, which was kept stirring for 30 min. Subsequently, 20 ml of AgNO3 aqueous solution was poured into the above solution, which was kept stirring for 5 min; then TiO2 NSF was put into above mixture solution and it was exposed to microwave irradiation for 5 min at 140 ◦ C in a microwave system (MDS-8, Sineo Microwave Chemical Technology Co. Ltd, Shanghai, China). Finally, Ag/TiO2 NSF was obtained after washing the sheet with ethanol and water several times. The obtained samples were referred to as x Ag/TiO2 NSF, with x representing the millimolar (mM) concentration of Ag+ in the reaction solution; x varied from 0.3 to 2.1 in intervals of 0.3 in our experiment. The samples were thus denoted as 0.3 Ag/TiO2 NSF, 0.6 Ag/TiO2 NSF, 0.9 Ag/TiO2 NSF, 1.2 Ag/TiO2 NSF, 1.5 Ag/TiO2 NSF, 1.8 Ag/TiO2 NSF, and 2.1 Ag/TiO2 NSF, respectively. For comparison, pure TiO2 NSF was created under the same conditions as described above, without Ag deposition. In addition, the molar ratio of NaH2 PO2 ·H2 O and AgNO3 was 10:1, and PVP and NaH2 PO2 ·H2 O were used as the capping molecule and reducing agent, respectively, according to the literature [53].
Figure 1. XRD patterns of the samples: (a) pure TiO2 NSF, (b) 0.3
Ag/TiO2 NSF, (c) 0.6 Ag/TiO2 NSF, (d) 0.9 Ag/TiO2 NSF, (e) 1.2 Ag/TiO2 NSF, (f) 1.5 Ag/TiO2 NSF, (g) 1.8 Ag/TiO2 NSF, and (h) 2.1 Ag/TiO2 NSF.
with a thermal conductivity detector and a 5 Å molecular sieve packed column. N2 was used as the carrier gas at a flow rate of 30 ml min−1 . The photocatalytic activity of the samples was determined by the quantitatively detected hydrogen yield using an external standard in the same concentration range. The hydrogen yield and energy efficiency were calculated based on equations (1) and (2) below. The detailed calculation process is given in the supporting information (see figure S2 available at stacks.iop.org/Nano/25/165401/mmedia).
2.3. Characterization
The morphologies of the samples were observed using scanning electron microscopy (SEM, JEOL JSM-6390A) with energy-dispersive x-ray (EDS) analysis. Powder x-ray diffraction (XRD) patterns were identified by a Shimadzu XRD6000 powder diffractometer (Cu Kα radiation). The absorption spectra of the samples were recorded on a Shimadzu UV-3600 UV/vis/NIR spectrophotometer with an integrating sphere detector, and BaSO4 was used as a reference. Besides, x-ray photoelectron spectroscopy (XPS) measurements were obtained with a Kratos AXIS NOVA spectrometer, Raman spectra were collected on a Raman spectrometer (Renishaw inVia Reflex), and photoluminescence (PL) spectra were obtained using a florescence spectrophotometer (Hitachi F-7000).
YH2 = (CH2 × Vgas )/Scat E e = 1HH2 × (CH2 × Vgas )/E a ,
(1) (2)
where YH2 is the hydrogen yield (µmol cm−2 ), CH2 is the hydrogen concentration (µmol l−1 ), Vgas is the gas volume above the reaction solution (l), Scat is the surface area of the film (cm2 ), E e is the energy efficiency, 1HH2 is the heat of combustion of hydrogen (kJ mol−1 ), and E a is the energy of photons absorbed by the film under ideal conditions (J) (see figure S2 available at stacks.iop.org/Nano/25/165401/ mmedia).
2.4. Photocatalytic water splitting
The apparatus used for photocatalytic water splitting has been described in our previous study [47]. The experiment was carried out in a quartz photoreactor with a volume of 150 ml. A 16 W high-pressure Hg lamp was employed as the UV light source with a wavelength of 254 nm, and a 500 W Xenon lamp was used as the visible light source under the assistance of a 400 nm cut-off optical filter; both of the lamps were placed outside the reactor facing the photocatalytic film, and the distance between the lamps and the film was 30 cm. The system was shielded by a black box during the reaction to prevent interference from outside light and to protect us from the UV light. The film was fixed in the reactor and immersed in 80 ml 10 vol.% ethanol/H2 O solution, and high-purity N2 was bubbled through the system for 30 min to eliminate dissolved oxygen before irradiation. During the experiment, a needle-type probe was inserted into the reactor to withdraw the generated gas, which was analyzed using a gas chromatograph (Fuli GC-9790II, Zhe Jiang, China) equipped
3. Results and discussion 3.1. Characterization of samples
The XRD patterns of the samples are presented in figure 1. The characteristic diffraction peaks of anatase and rutile TiO2 are observed in all the samples after annealing at 450 ◦ C. Although the amount of rutile phase in the samples is very small, this anatase/rutile mixed phase is suitable for the photocatalytic reaction [44]. The intense diffraction peaks at 35.1◦ , 38.4◦ , 40.2◦ , 53.0◦ , 62.9◦ , 70.6◦ , and 76.2◦ are attributed to the Ti substrate under the nano films. For the Ag decorated samples, the XRD diffraction peaks of Ag/TiO2 NSFs are almost the same as those of pure TiO2 NSF after Ag deposition, indicating that the mixed-phase TiO2 is well maintained, and two additional peaks related to metallic Ag are observed at 3
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Figure 2. (a) SEM images of pure TiO2 NSF and (b) the magnification of image (a).
Figure 3. SEM images of (a) 0.3 Ag/TiO2 NSF, (b) 0.9 Ag/TiO2 NSF, (c) 1.5 Ag/TiO2 NSF, and (d) 2.1 Ag/TiO2 NSF.
44.3◦ and 64.4◦ in the samples with higher Ag concentration. The diffraction peaks of Ag are not obvious in 0.3 Ag/TiO2 NSF, which may be attributed to the fact that the content of Ag is so low that it could not be detected. The existence of Ag is also proved by the SEM, EDS, and XPS analyses below. In order to give an overall impression on the prepared samples, photographs are shown in figure S3 (see supporting information available at stacks.iop.org/Nano/25/165401/ mmedia). It is apparent that TiO2 NSF with different Ag concentrations exhibits different colors under natural sunlight: the color of the film changes from gray to dark brown as the Ag concentration increases. This may be related to the light absorption properties of the samples (see figure 5). The structure of the films is shown in figure 2. The overall appearance of the TiO2 NSF is like a carpet with a rough surface (figure 2(a)), which consists of a large number of continuous distributed nano-sheets (figure 2(b)), so we named it TiO2 nano-sheet film. It has a large surface area and high volume fraction of atoms on the surface, which can provide more active sites for photoreaction. Besides, it is worth noting
that the prepared film exhibits a good stability, and it is relative hard to detach from the Ti substrate, which is beneficial for the practical utilization of film photocatalysts. Figure 3 illustrates the morphology of Ag deposited TiO2 NSFs. It is apparent that Ag NPs are dispersed uniformly with spherical morphology on the films rather than being particularly aggregated. The size of Ag NPs changes from 20 to 50 nm, leading to a broad adsorption in the visible light range (see figure 5). As shown in figure 3, the density of Ag NPs on the surface of 0.3 Ag/TiO2 NSF is lower than that on the surface of 0.9 Ag/TiO2 and 1.5 Ag/TiO2 NSF (figures 2(b) and (c)); this may be due to the lower concentration of Ag+ in reaction solution. However, the higher concentration of Ag+ may lead to the aggregation of Ag NPs on the surface of 2.1 Ag/TiO2 NSF (figure 3(d)). Energydispersive x-ray (EDS) spectroscopy was used to analyze the elemental composition of 1.5 Ag/TiO2 NSF, and the result is shown in figure 4. Ti, O and Ag are detected from the film surface, suggesting the existence of Ag particles. XPS spectra give the evidence that Ag+ can be reduced to metallic silver 4
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Figure 4. EDS patterns of 1.5 Ag/TiO2 NSF; the inset image is a back-scattered electron (BSE) image of the sample.
Ag NPs on the nano-sheet surface. The absorption spectra of Ag NPs in different samples are obtained through subtracting the contribution of pure TiO2 absorption from the absorption of Ag/TiO2 . As shown in figure 5(b), there is an obvious absorption between 400 and 500 nm with two absorption peaks, which is possibly due to the wide size distribution of Ag NPs as shown in figure 3, because the absorption spectrum depends on the nanoparticle size and shape, etc [34, 35, 54, 55]. The enhanced light absorption in the visible light region can therefore improve the activity of TiO2 by increasing the quantities of photogenerated electrons and holes or by enhancing the energy of trapped electrons; this is discussed later. To determine the electron–hole recombination properties, the photoluminescence (PL) emission spectrum is used to study the trapping and migration efficiency of charge carriers and to understand the behavior of the electron–hole pairs. Figure 6 shows the PL emission spectra of the prepared samples under excitation at 220 nm. It can be observed that the emission spectra of Ag/TiO2 composites appear to be similar to those of pure TiO2 NSF. The emission peaks around 396 and 467 nm are due to the free-excitation emission of the band gap and the charge-transfer transition from Ti3+ to the oxygen anion in a TiO6 octahedral complex, respectively [56, 57]. Under the same intensity of excitation irradiation, the emission intensity of Ag/TiO2 composites is lower than that of pure TiO2 NSF, suggesting that the recombination of photogenerated electrons and holes is suppressed effectively. That is because the metal work function of Ag is higher than that of TiO2 , so the photogenerated electrons will transfer from the conduction band of TiO2 to Ag NPs until the Fermi level equilibrium is achieved, forming a Schottky barrier on their contact surface. The electron transfer can reduce the recombination chance of electrons and holes [29], leading
during the microwave-assisted chemical reduction process (see figure S4 available at stacks.iop.org/Nano/25/165401/ mmedia). The above results indicate that the microwave-assisted chemical reduction method is an appropriate way to deposit Ag NPs under the assistance of a PVP–DEG system, which is usually used as a reaction environment for the fabrication of metal nanoparticles in hydrothermal conditions [53]. In this study, microwave irradiation was employed because of its rapid and uniform volumetric heating property, and Ag NPs with good dispersibility were successfully deposited on TiO2 NSF in a relative short time. Besides, it is important that the film can maintain its original morphology after the Ag loading process. UV–vis absorption spectra of the samples were measured to study their optical properties. As shown in figure 5(a), all the samples exhibit strong absorption in the range below 400 nm. This is attributed to the characteristic absorption of TiO2 from band–band electron transitions. Besides, the pure TiO2 films also show visible light absorption property, and the absorption increases on increasing the visible light wavelength. This is different from the absorption property of TiO2 nanoparticles reported in the literature (see figure S5 available at stacks. iop.org/Nano/25/165401/mmedia). This phenomenon may be related to its unique nanostructure. Compared with pure TiO2 film, Ag/TiO2 composites show broad absorption in the visible region arising from the SPR effect of Ag NPs, and the surface plasmon absorption gives the valid evidence that Ag NPs exist in the samples too. Further observation indicates that the intensity of surface plasmon absorption increases with increasing Ag amount, which is probably due to the greater number of Ag NPs deposited on the TiO2 surface. However, the absorption intensity of 2.1 Ag/TiO2 NSF is lower than that of 1.8 Ag/TiO2 NSF. This may be related to the aggregation of 5
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Figure 6. PL emission spectra of the samples under excitation at
220 nm; the emission before and after Ag deposition appears in the same color.
Raman spectroscopy was employed to further investigate the SPR effect based on the surface-enhanced Raman scattering (SERS) from Ag NPs. SERS is not only available for ions and molecular, but can also be extended to nanomaterials, such as TiO2 [58, 59]; it can detect the phases of TiO2 . Figure 7 shows the Raman scattering spectra of pure TiO2 and Ag deposited TiO2 NSFs. The anatase TiO2 belongs to the D4h point group, and its characteristic Raman active modes are at 144 cm−1 (Eg ), 196 cm−1 (Eg ), 395 cm−1 (B1g ), 515 cm−1 (B1g ), and 637 cm−1 (Eg ) [60]. Thus all five peaks observed in figure 7(a) can be ascribed to the allowed vibration frequency of the anatase phase, and the frequency of the rutile phase is not observed because of its lower content in the mixed phase (figure 1). However, the scattering peak of pure TiO2 NSF at 144 cm−1 exhibits a slight shift to higher frequency in Ag deposited TiO2 NSFs (figure 7(b)). The interface structure between Ag and TiO2 may play a crucial role in the peak shift [22, 47, 58]. The above characteristic Raman scattering peaks of anatase TiO2 are enhanced after deposition of Ag NPs, and the scattering intensity increases with increasing Ag amount. The enhancement is due to SERS from Ag, because the local electronic field near the surface of Ag NPs is greatly enhanced due to the SPR effect, and TiO2 in close vicinity to Ag NPs can strongly receive the influence of the local field, thereby giving a strong Raman signal. From the above results, it is believed that the SPR effect of Ag NPs indeed has an influence on TiO2 , and this influence may be beneficial to TiO2 in photocatalytic reactions.
Figure 5. (a) UV–vis absorption spectra of the samples; the absorption before and after Ag deposition appears in the same color. (b) UV–vis absorption spectra of Ag NPs in different samples, obtained by subtracting the contribution of pure TiO2 absorption from the absorption of Ag deposited TiO2 NSF samples.
to lower emission intensity. Based on the above analysis, the deposition of Ag NPs can reduce the recombination chance of electrons and holes. However, there is an optimal deposition amount. When the amount of Ag is relatively lower, it has a weak suppression for the recombination. The lowest recombination rate is obtained when the concentration of Ag+ is 1.5 mmol l−1 in reaction solution. On increasing the concentration of Ag+ , Ag NPs can aggregate on the surface of the nano-sheet (figure 3(d)) and also adversely work as recombination centers for the electrons and holes, reducing the suppression ability. This is one of the reasons why 1.5 Ag/TiO2 displays the highest photocatalytic activity for photocatalytic water splitting. Further observation indicates that deposition of Ag NPs will result in a new emission peak, around 373 nm in figure 6, because the emission around 373 nm is not observed in the emission spectra of TiO2 NSF before Ag deposition. The peak around 373 nm emerges in all modified samples and it belongs to the emission of the band gap transition in our previous studies [20, 22]. It can be seen from figure 6 that the relative intensity of the emission around 373 nm increases with increasing Ag amount, which is probably due to the greater number of Ag NPs deposited on the TiO2 surface, indicating relatively more recombination for emission around 373 nm.
3.2. Photocatalytic activities of H2 production
The main target of this work is to detect H2 production through water splitting over TiO2 catalysts. Yang and Strataki et al discovered that using ethanol as the sacrificial agent gave a higher H2 production efficiency [61, 62]. Thus ethanol was chosen as the sacrificial agent in our work. Photocatalytic water splitting was conducted on nano-sheet films under different light irradiation. Figure 8 shows the H2 yield of the samples after 3 h reaction under UV and visible light (λ > 400 nm) 6
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Figure 8. Hydrogen yield of photocatalytic water splitting under UV
and visible light (λ > 400 nm) irradiation for 3 h.
the reaction was carried out; the results show that Ag is not oxidized after the 12 h irradiation (see figures S6 and S7 available at stacks.iop.org/Nano/25/165401/mmedia). To further understand the role of Ag NPs during the photoreaction, the photocatalytic water splitting was conducted using pure TiO2 and 1.5 Ag/TiO2 composite under UV or visible light (λ > 400 nm) irradiation, respectively. Figure 9(a) presents the relationship between the irradiation time and the H2 yield under UV light. It can be seen that H2 yield increases gradually with increasing the light irradiation time; the H2 yield of 1.5 Ag/TiO2 NSF is 5.6 times higher than that of pure TiO2 NSF after 3 h irradiation, and the corresponding H2 yield is 4.2 and 0.75 µmol cm−2 , respectively. The enhanced photocatalytic activity is attributed to the charge-transfer property of Ag NPs: the electrons of TiO2 excited under UV light can transfer to the Ag NPs and reduce the recombination chance of electrons and holes, facilitating the photoreaction process. For the experiment under visible light (λ > 400 nm) irradiation, the results are presented in figure 9(b). No hydrogen is detected when pure TiO2 NSF is used as the catalyst, but 1.5 Ag/TiO2 NSF is found to be active for H2 production, which should be attributed to the SPR effect of Ag NPs. Under visible light irradiation, the SPR absorption induced photoexcited electrons in Ag NPs can move through the Ag/TiO2 composite interface into the conduction band of TiO2 , and then induce the photoreaction, which has been proved in the literature [38, 40, 41, 43, 48]. However, the H2 yield can only reach 1.6 µmol cm−2 under UV light irradiation (figure 9(b)), this is lower than that of under UV light irradiation (4.2 µmol cm−2 , figure 9(a)), or UV and visible light irradiation (8.1 µmol cm−2 , figure 8), suggesting that SPR absorption of Ag NPs is not an effective way to induce a photocatalytic reaction. It can be concluded from above results that the photocatalytic activity enhancement of TiO2 should be ascribed to the role of electron trapping and SPR absorption properties of Ag NPs. On the one hand, the UV light activity of TiO2 is improved based on the electron trapping of Ag NPs, which is demonstrated by the PL spectra in figure 6. On the other
Figure 7. Raman scattering spectra of the samples.
irradiation. Compared with the pure TiO2 film, Ag deposited TiO2 films exhibit higher photocatalytic activities, and the H2 yield increases with the increase of Ag content, suggesting that Ag NPs have a great effect on the photocatalytic activity of the samples. However, this increase is no longer significant when the content of Ag NPs is higher on the surface of TiO2 films, such as with 1.8 and 2.1 Ag/TiO2 NSFs. Although 1.8 Ag/TiO2 NSF has a more efficient utilization of light than 1.5 Ag/TiO2 NSF (figure 5), Ag NPs adversely work as the recombination center of photogenerated charges when they are over deposited, as we mentioned in discussion of the PL emission spectrum (figure 6), leading to a lower quantum efficiency. It can be concluded that photogenerated electron recombination should be considered when increase the visible light absorption of TiO2 . Further analysis shows that the H2 yield of 1.5 Ag/TiO2 NSF is 8.5 times higher than that of pure TiO2 NSF under UV and visible light (λ > 400 nm) irradiation; the corresponding H2 yield is 8.1 and 0.95 µmol cm−2 , respectively. The energy efficiency was about 0.47% on the 1.5 Ag/TiO2 composite. In addition, the H2 yield on TiO2 NSF treated without Ag deposition (see section 2.2) is 0.92 µmol cm−2 under UV and visible light (λ > 400 nm) irradiation for 3 h, indicating that the Ag deposition process has no obvious effect on the activity of TiO2 NSF. The long-time stability of 1.5 Ag/TiO2 NSF for H2 evolution was investigated and an XPS analysis after 7
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Figure 10. The transfer-enhancement synergistic mechanism for
photocatalytic water splitting over Ag/TiO2 NSF.
on the Ag/TiO2 nanocomposite. Upon UV light irradiation, the electrons of TiO2 are excited to the conduction band and the holes are left behind in the valence band. Then the electrons migrate to the Ag NPs and accumulate, forming a Schottky barrier between the Ag NPs and the TiO2 NSF. Simultaneously, the SPR effect is induced by the visible light, forming a strong local electronic field to enhance the energy of trapped electrons, making them transfer and react with electron acceptors more easily. The synergetic effect between charge transfer and SPR absorption of Ag NPs is achieved, leading to the boosted photocatalytic performance on the Ag/TiO2 surface. The plausible reaction pathways of H2 production from water splitting are summarized in the following equations (equations (3)–(9)). During the process of water splitting, holes in valence band of TiO2 can react with H2 O molecules to form H+ and ·OH (equation (6)). Subsequently, ·OH can be consumed through chemical reaction with CH3 CH2 OH (equation (7)), accelerating the decomposition of H2 O to H+ and ·OH, leading to more H+ product and increasing the rate of H2 production. On the other hand, the electrons with enhanced energy react with H+ at the surface of Ag NPs. H+ can receive an electron to form an H atom, and then two H atoms combine with each other to form an H2 molecule. It is inferred that the higher energy created by the SPR effect can promote trapped electrons being involved in the reaction, facilitating H2 generation. In this system, ethanol serves as a sacrificial reagent, which also helps to inhibit the recombination of photoinduced electrons and holes and thus to increase the efficiency of the entire process [63].
Figure 9. Photocatalytic activity of pure TiO2 and 1.5 Ag/TiO2 NSF
under (a) UV light irradiation and (b) visible light (λ > 400 nm) irradiation.
hand, visible light activity of TiO2 is induced by the SPR absorption of Ag NPs, because TiO2 cannot be activated under visible light irradiation, which is proved by the experimental result in figure 9(b) and the SPR absorption in figure 5. The experimental results show that the H2 yield (8.1 µmol cm−2 ) obtained under UV and visible light irradiations is higher than the sum of H2 yield (5.8 µmol cm−2 ) when UV and visible light irradiations are used separately. When UV light is used as the irradiation source, the H2 yield is 4.2 µmol cm−2 after 3 h irradiation; it can reach 8.1 µmol cm−2 when visible light irradiation is added, suggesting that visible light irradiation can greatly enhance the photocatalytic activity of TiO2 , which is due to the SPR effect of Ag NPs. However, the H2 yield is only 1.6 µmol cm−2 under visible light irradiation for 3 h, indicating that the SPR absorption of excited electrons in Ag NPs can induce the photocatalytic water splitting, but with a lower efficiency.
− TiO2 + hv → h + vb + ecb − ecb
− → eAg
− − eAg + hv(visible) → eSPR + h+ vb + H2 O → ·OH + H
(3) (4) (5)
(6) CH3 CH2 OH + ·OH → CH3 COOH + H+ (7) − + eSPR + H → H (8) H + H → H2 (9) − where h + is the hole in the valence band, e is the electron in vb cb − the conduction band, eAg is the electron trapped by Ag NPs, − eSPR is the electron enhanced by the SPR effect.
3.3. Mechanism of photocatalytic water splitting
Based on the above analysis, a transfer-enhancement synergetic photocatalytic mechanism is proposed in figure 10 to help understand the photocatalytic process that takes place 8
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4. Conclusions
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Ag deposited TiO2 nano-sheet film was prepared by a microwave-assisted chemical reduction method. The visible light absorption and Raman scattering of TiO2 are enhanced by Ag NPs due to their SPR effect. In addition, Ag NPs can also restrain the recombination of photogenerated electron– hole pairs and improve the charge-transfer efficiency. The experimental results indicate that SPR absorption of Ag NPs in the visible region can greatly boost the energy of trapped electrons through the strong local electron field of the SPR effect, making them transfer and react with electron acceptors easily. The 1.5 Ag/TiO2 NSF composite exhibits the best activity for H2 production due to the synergistic effect between the electron transfer and SPR absorption; the H2 yield of 1.5 Ag/TiO2 NSF is 8.5 times higher than that of pure TiO2 NSF under UV and visible light (λ > 400 nm) irradiation. This study suggests a promising method to develop SPR effect enhanced photocatalysis, which may have a profound implication on the future utilization of solar light. Acknowledgments
This work was supported by the National Natural Science Foundation of China (Nos 21306150, 21176199 and 51372201), the Shaanxi Provincial Research Foundation for Basic Research, China (Nos 2013JQ2003, 2011JM1001 and 2012JM1020), the Scientific Research and Industrialization Cultivation Foundations of the Education Department of Shaanxi Provincial Government, China (Nos 2013JK0693 and 2011JG05), the Scientific Research Foundation of Northwest University (No 12NW19), and the Scientific Research Foundation of Northwest University (No PR12216). References [1] Fujishima A and Honda K 1972 Nature 238 37–8 [2] Wang X, Maeda K, Thomas A, Takanabe K, Xin G, Carlsson J M, Domen K and Antonietti M 2008 Nature Mater. 8 76–80 [3] Chen X, Shen S, Guo L and Mao S S 2010 Chem. Rev. 110 6503–70 [4] Ingram D B and Linic S 2011 J. Am. Chem. Soc. 133 5202–5 [5] Daskalaki V M, Panagiotopoulou P and Kondarides D I 2011 Chem. Eng. J. 170 433–9 [6] Dholam R, Patel N, Adami M and Miotello A 2009 Int. J. Hydrog. Energy 34 5337–46 [7] Osterloh F E 2007 Chem. Mater. 20 35–54 [8] Hoffmann M R, Martin S T, Choi W and Bahnemann D W 1995 Chem. Rev. 95 69–96 [9] Khan S U M, Al-Shahry M and Ingler W B 2002 Science 297 2243–5 [10] Xiang Q, Yu J and Jaroniec M 2012 J. Am. Chem. Soc. 134 6575–8 [11] Alenzi N, Liao W S, Cremer P S, Sanchez-Torres V, Wood T K, Ehlig-Economides C and Cheng Z 2010 Int. J. Hydrog. Energy 35 11768–75 [12] Ni M, Leung M K H, Leung D Y C and Sumathy K 2007 Renew. Sustain. Energy Rev. 11 401–25 9
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