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titanium dioxide nanosheets with small gold nanoparticles for efficient photocatalytic hydrogen evolution
Journal of Colloid and Interface Science 555 (2019) 94–103
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Regular Article
Constructing functionalized plasmonic gold/titanium dioxide nanosheets with small gold nanoparticles for efficient photocatalytic hydrogen evolution Lei Cheng a, Dainan Zhang a, Yulong Liao a, Fei Li b,c, Huaiwu Zhang a, Quanjun Xiang a,⇑ a State Key Laboratory of Electronic Thin Film and Integrated Devices, School of Electronic Science and Engineering, University of Electronic Science and Technology of China, Chengdu 610054, PR China b Electronic Materials Research Lab, Key Lab of Education Ministry/International Center for Dielectric Research, School of Electronic and Information Engineering, Xi’an Jiaotong University, Xi’an 710049, PR China c State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an 710049, PR China
g r a p h i c a l a b s t r a c t The as-prepared Au/TiO2 nanosheets with decorating small plasmonic Au NPs through urea reduction method exhibit enhanced photocatalytic activity of hydrogen generation.
a r t i c l e
i n f o
Article history: Received 21 May 2019 Revised 8 July 2019 Accepted 23 July 2019 Available online 23 July 2019 Keywords: TiO2 nanosheets Plasmonic Au NPs Photocatalytic hydrogen evolution Urea reduction method
⇑ Corresponding author. E-mail address: [email protected] (Q. Xiang). https://doi.org/10.1016/j.jcis.2019.07.060 0021-9797/Ó 2019 Elsevier Inc. All rights reserved.
a b s t r a c t Small plasmonic Au nanoparticles (NPs)-decorated with TiO2 nanosheets were fabricated to improve the photocatalytic performance. The Au/TiO2 nanosheets with Au NPs of different sizes ranging from 3 nm to 28 nm were prepared by using hydrothermally obtained TiO2 nanosheets as substrate via urea and light reduction method. During synthesis, the obtained Au NPs through urea reduction treatment in different calcination temperatures possessed smaller size (3–13 nm) than those of the light reduction method (28 nm). The introduced Au NPs were tightly loaded on the surface of TiO2 nanosheets through in situ growth reduction process of chloroauric acid. The emergence of smaller Au NPs promoted the photocatalytic performance over Au/TiO2 nanosheets. The as-prepared Au/TiO2 nanosheets with small Au NP sizes of 3–5 nm showed the highest photocatalytic rate of hydrogen production (230 mmolh1) under xenon lamp illumination, exceeding more than twice that of Au/TiO2 nanosheets with loading of larger Au NPs (28 nm). The favorable constituents and combination of Au/TiO2 nanosheets provided large surface adsorptive sites for reactant adsorption, introduced plasmonic effects and formed Schottky barrier junction via surface plasmon resonance. The Schottky barrier height was lower due to the presence of smaller Au NPs, thereby enhancing the charge separation through the Schottky transfer hub to neighboring TiO2 nanosheets. The synergistic effect between the plasmonic hot carrier-driven Au NPs and TiO2 nanosheets
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was discussed. The photocatalytic mechanism was also proposed for the fabrication of visible lightrestricted photocatalysts with smaller Au NPs. Ó 2019 Elsevier Inc. All rights reserved.
1. Introduction TiO2, as a cost-effective and environmentally friendly photocatalytic material, has been regarded as an emerging solar-fuel-driven alternative since the pioneering study of photoelectrocatalytic water splitting over TiO2 electrodes [1–4]. Studies have shown that TiO2 has considerable capability for various solar-driven applications, such as optoelectronic devices [5], solar-fuel generation [6–8], dye degradation and sewage treatment [9–11]. In particular, controllable anatase TiO2 nanosheets with exposed (0 0 1) facets have demonstrated photocatalytic efficiency higher than those with domain (1 0 1) facets [12–16]. Previous research has also illustrated that anatase TiO2 nanosheets with highly exposed (0 0 1) facets were more domain reactive in enhancing photocatalytic performance than those with thermodynamically stable (1 0 1) facets [10]. However, the intrinsic drawback of high energy band gap (3.2 eV) forms the visible light-restricted TiO2 photocatalyst and severely hinders the functionality to the range of visible light response [17–21]. Several efforts, such as noble metallic loading [9,22,23], cocatalyst coupling [24] and morphological organization [7,25], have been widely explored in recent years for effectively maximizing the solar light-harvesting ability and boosting photocatalytic performance over TiO2 photocatalysts [26,27]. Recently, incorporating Au nanoparticles (NPs) with TiO2 has been proven favorable for lengthening the absorption range of TiO2 photocatalysts from UV to wide solar spectrum [22,23,28]. On the one hand, the semiconductor–metal photocatalysts can form a Schottky junction, which acts a charge carrier hub for trapping and transferring the photogenerated electrons [29–34]. On the other hand, the exhibited surface plasmon resonance (SPR) by plasmonic Au NPs courses highly thermal- and photocatalytically activity, playing an effective role for exclusive photocatalytic reactions [4,35,36]. Studying the specific impact between Au NPs and TiO2 photocatalysts is of significance for improving photocatalytic activity over Au/TiO2 system. The various existing parameters of Au NPs, such as morphology, distribution and size, play an important role in physicochemical properties, and further determine the photocatalytic performance [7,37]. Particularly, the different sizes of Au NPs show vital influence on the Schottky nanoscale and electrical properties when coupling with semiconductors [23,38–41]. For example, Lee et al. investigated the electrical properties of Au NPs on n-type TiO2 by colloidal self-assembled nanopatterning [38]. The formed Schottky barrier height was lower than that of the conventional Au/TiO2 nanodiode with large Au NPs. Moreover, Moon et al. reported the plasmonic hot carrierdriven Au NPs/TiO2 nanotube arrays for enhanced oxygen evolution with decorated Au NPs (5–30 nm) [40]. However, the functionalized Au/TiO2 system for high efficiency of photocatalytic hydrogen generation is still limited. The effective synthesis of Au/TiO2 nanosheets with small Au NPs size and the dimensional interaction of Au NPs have been rarely reported yet. Therefore, the specific photocatalytic mechanism involving the different sizes of Au NPs versus photocatalytic properties could be the attractive prospect for manufacturing highly efficient Au/TiO2 photocatalysts and widely realizing their practical applications. In this work, small plasmonic Au NP-decorated TiO2 nanosheets with exposed (0 0 1) facets were designed for efficient photocatalytic hydrogen evolution. A series of Au/TiO2 nanosheets with corresponding Au NP size of 3–13 and 28 nm were prepared by
using hydrothermally synthesized TiO2 nanosheets as supporting base through the urea and light reduction method, respectively. The results show that plasmonic Au/TiO2 nanosheets with smaller Au NPs have higher photocatalytic rate of hydrogen production. This contribution of enhanced photocatalytic performance was coursed by positive activity of plasmonic hot carrier-driven Au NPs and actively synergistic effect over the formed interface Schottky junction. By constructing a series of Au/TiO2 nanosheets with coupling different sizes of plasmonic Au NPs, a new insight into the photocatalytic mechanism involving the size of Au NPs versus photocatalytic properties was provided for obtaining desirable photocatalytic hydrogen production. This approach could be momentous to developing plasmonic Au hot carriers and contribute to wide application of visible light-restricted TiO2 and other photocatalysts limited by photoresponse. 2. Experimental 2.1. Materials The chemicals mainly include tetrabutyl titanate (C16H36O4Ti), hydrofluoric acid (HF, 40%), chloroauric acid (HAuCl44H2O), urea (H2NCONH2), and ethanol. The chemical reagents used in hydrogen production and electrochemical tests mainly include glycerin, ethanol, Na2SO4 (99.0%) and polyethylene glycol (PEG2000). All the aforementioned reagents, which were obtained from Sinopharm Chemical Reagent Co., Ltd. and used without further purification, were of analytical grade. 2.2. Synthesis of the TiO2 nanosheets Anatase TiO2 nanosheets were prepared by hydrothermal method [10]. In a typical synthesis, C16H36O4Ti (25 mL) was mixed with HF aqueous solution (3 mL) in a 100 mL autoclave with an inner Teflon lining following the typical hydrothermal treatment at 180 °C for 24 h. The obtained precipitate was collected by centrifugation with distilled water and ethanol for three times and then dried at 60 °C for 12 h. The obtained sample was denoted as TiO2. 2.3. Synthesis of the Au/TiO2 nanosheet composites A series of Au/TiO2 nanosheet composites with different sizes of Au NPs (1.0 wt%) were prepared by urea reduction method [41]. Typically, 1 g of as-prepared TiO2 nanosheets were added into the 100 mL mixed solution containing HAuCl44H2O (1.05 103 mol∙L1) and H2NCONH2 (0.42 mol∙L1) with the initial pH = 2. The suspension was then slowly stirred at 80 °C for 16 h to facilitate urea decomposition with pH = 7. The obtained precipitate was collected by centrifugation (12,000 r∙min1, 10 min) after stirring with 100 mL distilled water at 50 °C for four times and then dried at 100 °C for 2 h. Afterward, the appropriately dried precipitate was calcined at 100, 150, 180, 200, and 250 °C for 4 h, obtaining a series of Au/TiO2 nanosheet composites with different Au NP sizes labeled as AT-100, AT-150, AT-180, AT-200 and AT-250, respectively. For comparison, Au/TiO2 nanosheets loaded with large size Au NPs were prepared through the light reduction method. A total of 1 g of as-prepared TiO2 nanosheets were added into the
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Table 1 Detailed experimental conditions for the as-prepared samples. Sample
Synthetic method
Temp (°C)
AT-100 AT-150 AT-180 AT-200 AT-250 LR
Urea reduction Urea reduction Urea reduction Urea reduction Urea reduction Light reduction
100 150 180 200 250 Room temp
100 mL mixed solution containing HAuCl44H2O (1.05 103 mol∙L1) and ethanol/water (1:1) at room temperature. Subsequently, the solution was sonicated for 5 min and vigorously stirred under xenon lamp illumination for 1 h. The obtained precipitate was collected by centrifugation and then dried at room temperature for 12 h, which is denoted as LR. All samples involving detailed experimental conditions were listed in Table 1. 2.4. Characterization Powder X-ray diffraction (XRD) patterns were obtained through a Bruker D8 Advance X-ray diffractometer using Cu Ka as a ray source at a scan rate of 10°/min and scan range (2h) of 10°–85°. The UV-vis diffuse reflection spectra (UV-vis DRS) of the sample was measured by a UV-2550 UV–Vis spectrophotometer. Transmission electron microscopy (TEM) images were captured using a Hitachi-7650 model transmission electron microscope (HITACHI, Japan) with an accelerating voltage of 120 kV, and high-resolution TEM (HRTEM) images were observed using a JEM-2100F (JEOL, Japan) electron microscope at an accelerating voltage of 200 kV. Scanning electron microscopy (SEM) images were recorded through an S-4800 (Hitachi, Japan). Raman spectra were conducted using Renishaw InVia micro-Raman spectrometer with an excitation source of 514.5 nm Ar+ laser. Scanning transmission electron microscopy (STEM) images and EDS mapping were observed using Tecnai G2 F20 S-TWIN, FEI. X-ray photoelectron spectroscopy (XPS) analysis was tested on a Leybold Heraeus-Shenyang SKL-12 X-ray spectrometer equipped with a VG CLAM 4 MCD electron energy analyzer with Mg Ka (hv = 1253.6 eV) as excitation light at 10 kV and 15 mA. The photoluminescence (PL) spectra were detected using a FLS920 (Edinburgh Instruments, UK) at room temperature. 2.5. Photocatalytic activity measurement The photocatalytic activity for hydrogen production was performed in a 100 mL Pyrex flask using 350 W xenon lamp as a light source (model PLS-SXE300, Beijing Perfectlight Technology Co., Ltd.). Typically, 50 mg of as-prepared sample was dispersed in 80 mL aqueous solution containing 10% vol of glycerol. Before irradiation, the system was purged by bubbling nitrogen for 30 min to remove the dissolved oxygen and then sealed with a silicone rubber septum. The Pyrex flask was placed at a distance of 12 cm from the photocatalytic reactor and under irradiation for 1 h. The illumination area and focused intensity were controlled at approximately 17 cm2 and 0.63 W (d = 12 cm), respectively. Hydrogen production was measured by extracting 0.4 mL of gas through a gas chromatograph (GC-14C Shimadzu, Japan, TCD, N2 carrier gas). For the apparent quantum efficiency (AQE) test under the wavelengths of 365 nm (PLS-LED100, Beijing Perfectlight Technology Co., Ltd.), the light-emitting diode was put 1 cm away from the reactor to excite the photocatalytic hydrogen reaction. The illumination area and focused intensity were tested at approximately 7 cm2 and 20 mW (d = 1 cm), respectively. The AQE was measured through the following equation:
Mass ratio (g) TiO2
Au
Colour
Particle size (nm)
1 1 1 1 1 1
0.01 0.01 0.01 0.01 0.01 0.01
Grayish purple Light purple purple Dark purple Dark purple Fuchsia
3 5 8 10 13 28
number of reacted electrons 100 number of incident photons number of evolved H2 molecules 2 ¼ 100 number of incident photons
AQE ½% ¼
ð1Þ
2.6. Photoelectrochemical measurement Transient photocurrent responses and electrochemical impedance spectra (EIS) were measured using an electrochemical workstation (CHI660E; Shanghai Chenhua Co., Ltd., China) in a threeelectrode system. Ag/AgCl (saturated KCl) was used as the reference electrode, platinum wire was used as the counter electrode, 0.5 mol∙L1 Na2SO4 aqueous solution was used as electrolyte, and 350 W xenon lamp was used as the light source. The transient photocurrent was captured with a 0.5 eV starting voltage, and a light source was turned off at intervals of 60 s. The EIS was recorded with the AC voltage amplitude of 10 mV at 0.5 V and frequency range of 0.01–105 Hz. The working electrode was made by the scraper method. A total of 30 mg of the prepared sample and 30 mg of polyethylene glycol were mixed in a mortar, and 0.5 mL of ethanol was added to grind into a slurry. The slurry was evenly coated on the FTO conductive glass with active area and film thickness of approximately 1 cm2 and 0.01 mm, respectively, and then dried in an oven at 100 °C for 1 h for obtaining a working electrode. 3. Results and discussion The Au/TiO2 nanosheet composites with different Au NPs sizes were prepared by using two kinds of synthetic strategies: urea reduction method and light reduction method (see Scheme 1). The loading of Au NPs on the TiO2 nanosheets via urea reduction method had a smaller size than that prepared by light reduction method. Figs. 1a and S1 show the typical XRD patterns of the asobtained samples prepared by urea reduction method (samples AT-100, AT-150, AT-180, AT-200, and AT-250) and light reduction method (sample LR) as well as pure TiO2 nanosheets (sample TiO2). All samples display evident characteristic diffraction peaks at 25.3°, 37.8°, 48.0°, 54.9° and 62.7°, corresponding to (1 0 1), (0 0 4), (2 0 0), (2 1 1) and (2 0 4) planes of the anatase TiO2 (JCPDS No. 21-1272). No characteristic diffraction peaks of Au NPs were identified due to their low content and low diffraction intensity. The similar intensity and width at characteristic diffraction peaks of anatase TiO2 nanosheets imply that the lattice structure was maintained with the introduction of Au NPs. Compared with the pure TiO2 nanosheets, sample AT-100 had higher crystallinity and better crystallographic structure (Fig. S1), which implies that the samples decorated with small plasmonic Au NPs through urea reduction method have a high-phase purity and crystallinity. Fig. 1b shows the UV-vis DRS of pure TiO2, AT-100, AT-150, AT-180, AT-200, AT-250 and LR, wherein all samples show a significant absorption edge at 380 nm, which is associated with the intrinsic band gap of anatase TiO2 (3.2 eV) in the literature [12]. This result implies that the lattice of TiO2 was maintained because
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Scheme 1. Illustration of the synthetic process for the Au/TiO2 nanosheet composites with different Au NP sizes through urea and light reduction method.
Fig. 1. (a) XRD patterns of the as-obtained samples prepared by urea reduction method (samples AT-100, AT-150, AT-180, AT-200, and AT-250) and light reduction method (sample LR); (b) UV-Vis DRS spectra of pure TiO2 nanosheets (sample TiO2), AT-100, AT-150, AT-180, AT-200, and AT-250 and LR; (c) XPS survey spectra of the pure TiO2 nanosheets (sample TiO2), AT-150, and AT-180; and (d) the corresponding high-resolution XPS spectra of Au 4f for AT-150 and AT-180 samples.
the Au NPs were deposited rather than incorporated on the TiO2 supporting substrate. Compared with pure TiO2, the AT-100, AT150, AT-180, AT-200, AT-250 and LR composite samples exhibited strong absorption edges in the visible light region within
500–700 nm mainly due to the plasmon resonance effect of Au NPs. Further comparison indicates that Au/TiO2 composites coupled with various Au NP sizes of 3–13 nm showed higher absorption centers and intensities than those of 28 nm in the visible
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region of 600 nm. This finding indicates that different sizes of Au NPs course the mutative photoresponsive activity, which plays an effective role for exclusive solar light-harvesting ability of TiO2 photocatalysts. The XPS patterns illustrate the chemical composition and elemental status of the studied samples as depicted in Fig. 1c. The data indicate the presence of Ti, O, F and C in the pure TiO2, AT-150 and AT-180, with the emergence of characteristic peaks at binding energies of 458 (Ti 2p), 529 (O 1s), 684 (F 1s) and 284 eV (C 1s), respectively. The Ti and O elements are derived from TiO2 nanosheets, and C 1s and F 1s correspond to the background signals of C and the surface adsorbed F on TiO2 nanosheets,
respectively [42]. The typical high-resolution XPS spectrum of Au 4f in AT-150 and AT-180 samples is shown in Fig. 1d. The amount of Au in AT-150 and AT-180 were 1.6 and 1.9 at %, respectively. Two prominent peaks can be found at the binding energies of 82.7 and 86.4 eV, which are assigned to the characteristic values of Au element. The 3.7 eV binding energy difference between Au 4f5/2 and Au 4f7/2 is also attributed to the typical metal state form of Au element [43,44]. These XPS results confirm the successful synthesis of Au NPs and the strong interaction between Au NPs and TiO2 nanosheet substrates. Fig. 2a and b show typical TEM and HRTEM images of the AT-150 sample synthesized through the urea reduction method
Fig. 2. (a) TEM and (b) HRTEM images of the as-obtained AT-150 (synthesized through the urea reduction method by calcining at 150 °C for 4 h); (c) TEM and (b) HRTEM images of LR (prepared via light reduction method); (e) SEM image and (f) side-view crystalline structure of AT-180.
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by calcining at 150 °C for 4 h. As shown in Fig. 2a, the rectangularlike TiO2 nanosheets with an average size of 50 nm were evidently observed, and the small Au NPs were clearly dispersed on the surface of layered TiO2 support. These rectangular-like TiO2 nanosheets with high crystallinity exhibit well-defined lattice fringes (Fig. 2b). The crystal lattice fringes of TiO2 nanosheets with the d values of 0.35 nm correspond well to the (1 0 1) lattice planes of anatase TiO2 [42,45]. The small nanoparticles with an average size of 5 nm were recorded, and the d spacing of 0.236 nm can be assigned to the (1 1 1) lattice planes of Au NPs [46]. The SEM image, STEM image and corresponding elemental mapping results of AT-150 sample also show TiO2 nanosheets with regular rectangular-like shape and the loading of small Au NPs (Figs. 2e and S4), corresponding to the results of TEM image. Moreover, as shown in Fig. 3, the AT-100, AT-180, AT-200, and AT-250 samples demonstrate the small Au NPs sizes within 3–13 nm loaded on the TiO2 nanosheets. Form the inserted maps in Fig. 3, we can see that the distribution state of Au NPs is concentrated relatively, of which the difference of Au NPs sizes is around 4 nm. The inserted HRTEM images in Fig. 3 show that crystal lattice fringes of TiO2 nanosheets with the d values of 0.35 and 0.235 nm correspond well to the (1 0 1) and (0 0 1) lattice planes of anatase TiO2, respectively. The results indicate the successful synthesis of Au/TiO2 nanosheet composites and the suitability of anatase TiO2 nanosheets with highly exposed domain-reactive facets as supporting substrate for loading with Au NPs (Fig. 2f). As shown in
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Fig. 2c and d, the LR sample synthesized via light reduction method demonstrates the agglomerated TiO2 nanosheets, which crystallized as evidenced from the TiO2 (1 0 1) (d = 0.35 nm) crystal lattice fringes [12]. Moreover, the LR sample has larger Au NP size of 28 nm compared with that of the urea reduction strategy, suggesting that urea reduction is beneficial for the synthesis of Au NPs with small size. Fig. 4a shows the Raman spectra of the pure TiO2, sample AT200, and sample LR. The several representative binding energy peaks at 148, 399, 518, and 639 cm1 due to Eg(1), B1g(1), A1g + B1g(2) and Eg(2) vibration modes of anatase TiO2 were respectively found, which implies the existence of controllable anatase TiO2 [24]. Moreover, no characteristic Raman spectrum of Au NPs was identified possibly due to the weak crystallinity and low mass content of Au NP loading. The separation and transfer efficiency of photocatalytic carriers over the as-obtained samples were elucidated using the PL analysis (Fig. 4b). All the samples show a near bandedge emission around 425 nm, corresponding to band-to-band emission of TiO2 and the transition of electrons from the oxygen vacancies to TiO2 valance band. In contrast to the sample LR prepared by the light reduction method, the samples AT-100, AT-180 and AT-200 synthesized via the urea reduction method demonstrate increased PL intensity, which may be attributed to a defect-state luminescence phenomenon when coupling smaller Au NPs [10,47]. Moreover, the transient photocurrent responses and EIS were performed in 0.5 M Na2SO4 aqueous solution and
Fig. 3. TEM and HRTEM images of the as-obtained (a) AT-100, (b) AT-180, (c) AT-200, and (d) AT-250 (The inserted maps are the particle size distribution of Au NPs).
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Fig. 4. (a) Raman spectra of the pure TiO2 nanosheets (sample TiO2), sample AT-200, and sample LR; (b) PL spectra of the as-obtained samples prepared by urea reduction method (samples AT-100, AT-180, and AT-250) and light reduction method (sample LR); (c) Transient photocurrent responses and (d) Nyquist plots of as-obtained samples (samples AT-100, AT-150, AT-180, AT-200 and AT-250 and LR) in 0.5 m Na2SO4 aqueous solution under visible light irradiation at 0.5 V vs Ag/AgCl.
used to confirm the photocurrent reproducibility and charge transfer resistance over the studied samples. Fig. 4c depicts the similarity of transient photocurrent density of the AT-100 and AT-150, which was both higher than that of other samples, indicating the improvement of photogenerated charge separation over the Au/TiO2 nanosheets when loading Au NPs with 3–5 nm. The typical EIS exhibits the charge transfer resistance of the studied samples. A small arc radius in the EIS spectra generally indicates high charge transfer efficiency. As shown in Fig. 4d, the AT-150 sample prepared by urea reduction method exhibits similar semicircular arcs to AT-100 sample. The small arc radius of AT-100 and AT150 samples illustrates the high efficiency of charge transportation, corresponding to the results of transient photocurrent density. Based on the experimental results, the samples with 3– 5 nm synthesized by the urea reduction method have certain advantages in promoting photo-generated carrier transfer and prolonging the lifetime of carriers compared with those of the large Au NP size. Fig. 5a and b illustrate the photocatalytic hydrogen production activity of the as-obtained samples prepared by urea reduction method (samples AT-100, AT-150, AT-180, AT-200 and AT-250) and light reduction method (sample LR) under xenon lamp illumination and monochromatic light irradiation, respectively. Sample AT-150 with Au NP sizes of 5 nm exhibits high photocatalytic performance with hydrogen production rate at 230 mmolh1 under xenon illumination and the corresponding apparent quantum efficiency reached up to 15.94% under the wavelength of 365 nm. With the increase in Au NP size from 8 nm to 28 nm, the activity
of photocatalytic hydrogen production over Au/TiO2 nanosheets was significantly reduced from 170 mmolh1 to 105.3 mmolh1. In addition, compared to the pure TiO2, the photocatalytic hydrogen evolution of all Au/TiO2 system with loading Au NPs was significantly improved. This finding indicates that the small size of Au NPs has a positive role in promoting the photocatalytic activity of Au/TiO2 system under xenon lamp illumination. A series of experiments for photocatalytic hydrogen production was investigated through monochromatic light irradiation to optimize the activity over Au/TiO2 nanosheets with coupling different size of plasmonic Au NPs (Fig. 5c) under the wavelength of 450 nm, 490 nm, 585 nm and 630 nm. The hydrogen production rate of sample AT-100 with smaller Au NPs size (3 nm) was higher than the other samples under the wavelength of 450 nm, 490 nm and 585 nm, respectively. However, the sample AT-100 shows the lower photocatalytic activity compared to other samples under the light of 630 nm, which may be attributed to the weak plasmon resonance effect of Au NPs under visible light irradiation at 630 nm or interference from other factors that needs to be further studied. Fig. S5 shows the STEM and TEM images of the as-obtained AT-150 after photocatalytic hydrogen evolution, the microscopic morphology of the sample has not changed, which further illustrates the photocatalytic stability of the Au/TiO2 system. Upon visible light irradiation of Au/TiO2 system, the oscillating electric field was formed on the surface of Au NPs. This oscillation effect could course deviation of electric field from the nuclear framework. The SPR was excited based on the restoring force between electrons and nuclei, which could course in changes of
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Fig. 5. Photocatalytic activity of hydrogen production over the samples prepared by urea reduction method (samples AT-100, AT-150, AT-180, AT-200, and AT-250) and light reduction method (sample LR) in 80 mL aqueous solution containing 10% vol of glycerol under (a) and (b) visible light irradiation and (c) monochromatic light irradiation; (d) Schematic of plasmon oscillation for Au NPs showing the deviation of electric field from the nuclei; and (e) Schematic of the Au/TiO2 nanosheets loaded with different sizes of Au NPs for the transport of plasmonic hot electrons induced by the SPR to the CB of neighboring TiO2 nanosheets.
electron density and charge distribution, thus forming a collective oscillation of plasmonic hot carriers (Fig. 5d) [23,36,48]. These hot electrons induced by the SPR can be transferred to the conduction band (CB) of neighboring TiO2 nanosheets based on the higher Fermi level of Au compared with that of the Schottky barrier height [36], thus accelerating photocatalytic hydrogen production. The various sizes of Au NPs would course the different effects on the electric field polarization of SPR, resulting in the changeable efficiency of the transfer and separation of plasmonic hot electrons. The desirable photocatalytic activity over the Au/TiO2 system was increased in the present study when smaller Au NPs were loaded. This finding can be attributed to the reduced Schottky barrier height and accelerated plasmonic hot electrons of the small Au NPs. A schematic diagram of the Schottky barrier height and binding energy level at the interface over Au/TiO2 nanosheets loaded with different sizes of Au NPs is shown in Fig. 5e. Plasmonic hot
electrons induced by SPR of Au NPs formed Schottky junction between the TiO2 nanosheets and Au NPs. This Schottky junction structure acts as transfer hub to carry and transfer the plasmonic hot electrons with sufficient energy to the CB of neighboring TiO2 nanosheets, thus promoting the separation of ground state and excited electrons on the surface of Au NPs. The local electric field coursed by SPR at the interface between Au NPs and TiO2 nanosheets has variation in nanoscale, which depends on the size of Au NPs. As the size decreases, the Schottky barrier height exhibits lower nanoscale junction as well as the reduced binding energy of Au NPs, resulting in an increased electronic cloud injection into the CB of TiO2 nanosheets. The Au/TiO2 nanosheets with 5 nm Au NPs possess lower binding energy level near the Fermi level in comparison with that of loading Au NPs with 28 nm, leading to the significant contribution in enhancing the transport of plasmonic hot carriers under the Schottky barrier junction.
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Fig. 6. Mechanism for the photocatalytic process of hydrogen evolution with the electron transfer and energy band structure in the synergistic Au/TiO2 nanosheets under the UV and visible light illumination.
By controlling the size of Au NPs in 5 nm, the efficiency of photocatalytic hydrogen production over the Au/TiO2 nanosheets would be significantly increased, which is also consistent with the results of experimental hydrogen production (Fig. 5a and b). Fig. 6 shows the proposed photocatalytic mechanism for the promoted photocatalytic hydrogen generation over the Au/TiO2 nanosheets with Au NP size at 3–5 nm. The spectrum of xenon lamps is in a wavelength range of 320–780 nm. Therefore, under the UV-light irradiation, TiO2 nanosheets were excited by the absorbed photon, facilitating the photogeneration of electrons at the valence band (VB). Under the action of band energy, the photogenerated electrons on the VB transfer to the CB of TiO2 nanosheets, thus forming electron and hole pairs. Au NPs could act as the electronic receiver of photogenerated electrons, accelerating the separation of electron–hole pairs, thus further enhancing the photocatalytic performance of hydrogen production. Moreover, the as-prepared anatase TiO2 nanosheets exposed domain-reactive (0 0 1) facets with high reactivity and low thermodynamic stability [12–15]. The photogenerated electrons remaining on the CB of TiO2 nanosheets can also directly generate hydrogen by reversing oxidation–reduction reaction with water molecules adsorbed on the surface, altogether enhancing the photocatalytic activity over the Au/TiO2 nanosheets. Owing to the local electric field of Au NPs coursed by SPR, the plasmonic hot carriers would be excited when the light energy is more than 2.3 eV (k = 546 nm) [39,49]. Therefore, the SPR in Au NPs was excited due to the restoring force between electrons and nuclei under visible light irradiation, resulting in the collective oscillation of plasmonic hot carrier. These plasmonic hot electrons can be transferred to the CB of TiO2 nanosheets by preferred Schottky barrier height, which acts as transfer hub to promote the separation of ground state and excited electrons on the Au NP surface. The smaller Au NPs possess low nanoscale junction and form lower binding energy level near Fermi level, demonstrating the important contribution of Au NPs in injecting the plasmonic hot carriers to TiO2 nanosheets (Fig. 5e). The hot electrons received by the CB of TiO2 nanosheets participate in the photocatalytic hydrogen generation of water splitting for the Au/TiO2 nanosheets. Based on the preceding discussion, the introduction of SPR over Au NPs maximizes the light response range for visible light-responsive TiO2 photocatalytic system. The positive activity of plasmonic hot carrier-driven Au NPs and actively
synergistic effect over the formed interface Schottky junction play significant roles in the necessary photocatalytic hydrogen generation under the xenon lamp illumination.
4. Conclusions The functionalized plasmonic Au/TiO2 nanosheets with small Au NP sizes of 3–13 nm and 28 nm were prepared by using hydrothermally synthesized TiO2 nanosheets as supporting base via the urea and light reduction method, respectively. The experimental results reveal that the as-obtained small Au NPs show a positive effect on promoting the photocatalytic performance of the Au/TiO2 nanosheets. The as-prepared plasmonic Au/TiO2 nanosheets with Au NP size of 3–5 nm had the highest photocatalytic rate of hydrogen production at 230 mmolh1. The positive activity of plasmonic hot carrier-driven Au NPs and actively synergistic effect over the reduced interface Schottky junction favored the transfer and separation of photocatalytic charge carriers through the Schottky transfer hub to neighboring TiO2 nanosheets, thus enhancing the photocatalytic activity of hydrogen generation under the xenon lamp illumination. This work paves a promising way to comprehensively understand actively synergistic effects of smaller Au NPs versus highly photocatalytic capability over the Au/TiO2 nanosheets, which could be extended for designing other visible light-restricted photocatalysts with decorated small Au NPs. Acknowledgments This work was partially supported by the National Natural Science Foundation of China under Grant Nos. 51672099 and 21403079, Sichuan Science and Technology Program under No. 2019JDRC0027, and Fundamental Research Funds for the Central Universities under No. 2017-QR-25.
Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcis.2019.07.060.
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Journal of Colloid and Interface Science journal homepage: www.elsevier.com/locate/jcis
Regular Article
Constructing functionalized plasmonic gold/titanium dioxide nanosheets with small gold nanoparticles for efficient photocatalytic hydrogen evolution Lei Cheng a, Dainan Zhang a, Yulong Liao a, Fei Li b,c, Huaiwu Zhang a, Quanjun Xiang a,⇑ a State Key Laboratory of Electronic Thin Film and Integrated Devices, School of Electronic Science and Engineering, University of Electronic Science and Technology of China, Chengdu 610054, PR China b Electronic Materials Research Lab, Key Lab of Education Ministry/International Center for Dielectric Research, School of Electronic and Information Engineering, Xi’an Jiaotong University, Xi’an 710049, PR China c State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an 710049, PR China
g r a p h i c a l a b s t r a c t The as-prepared Au/TiO2 nanosheets with decorating small plasmonic Au NPs through urea reduction method exhibit enhanced photocatalytic activity of hydrogen generation.
a r t i c l e
i n f o
Article history: Received 21 May 2019 Revised 8 July 2019 Accepted 23 July 2019 Available online 23 July 2019 Keywords: TiO2 nanosheets Plasmonic Au NPs Photocatalytic hydrogen evolution Urea reduction method
⇑ Corresponding author. E-mail address: [email protected] (Q. Xiang). https://doi.org/10.1016/j.jcis.2019.07.060 0021-9797/Ó 2019 Elsevier Inc. All rights reserved.
a b s t r a c t Small plasmonic Au nanoparticles (NPs)-decorated with TiO2 nanosheets were fabricated to improve the photocatalytic performance. The Au/TiO2 nanosheets with Au NPs of different sizes ranging from 3 nm to 28 nm were prepared by using hydrothermally obtained TiO2 nanosheets as substrate via urea and light reduction method. During synthesis, the obtained Au NPs through urea reduction treatment in different calcination temperatures possessed smaller size (3–13 nm) than those of the light reduction method (28 nm). The introduced Au NPs were tightly loaded on the surface of TiO2 nanosheets through in situ growth reduction process of chloroauric acid. The emergence of smaller Au NPs promoted the photocatalytic performance over Au/TiO2 nanosheets. The as-prepared Au/TiO2 nanosheets with small Au NP sizes of 3–5 nm showed the highest photocatalytic rate of hydrogen production (230 mmolh1) under xenon lamp illumination, exceeding more than twice that of Au/TiO2 nanosheets with loading of larger Au NPs (28 nm). The favorable constituents and combination of Au/TiO2 nanosheets provided large surface adsorptive sites for reactant adsorption, introduced plasmonic effects and formed Schottky barrier junction via surface plasmon resonance. The Schottky barrier height was lower due to the presence of smaller Au NPs, thereby enhancing the charge separation through the Schottky transfer hub to neighboring TiO2 nanosheets. The synergistic effect between the plasmonic hot carrier-driven Au NPs and TiO2 nanosheets
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was discussed. The photocatalytic mechanism was also proposed for the fabrication of visible lightrestricted photocatalysts with smaller Au NPs. Ó 2019 Elsevier Inc. All rights reserved.
1. Introduction TiO2, as a cost-effective and environmentally friendly photocatalytic material, has been regarded as an emerging solar-fuel-driven alternative since the pioneering study of photoelectrocatalytic water splitting over TiO2 electrodes [1–4]. Studies have shown that TiO2 has considerable capability for various solar-driven applications, such as optoelectronic devices [5], solar-fuel generation [6–8], dye degradation and sewage treatment [9–11]. In particular, controllable anatase TiO2 nanosheets with exposed (0 0 1) facets have demonstrated photocatalytic efficiency higher than those with domain (1 0 1) facets [12–16]. Previous research has also illustrated that anatase TiO2 nanosheets with highly exposed (0 0 1) facets were more domain reactive in enhancing photocatalytic performance than those with thermodynamically stable (1 0 1) facets [10]. However, the intrinsic drawback of high energy band gap (3.2 eV) forms the visible light-restricted TiO2 photocatalyst and severely hinders the functionality to the range of visible light response [17–21]. Several efforts, such as noble metallic loading [9,22,23], cocatalyst coupling [24] and morphological organization [7,25], have been widely explored in recent years for effectively maximizing the solar light-harvesting ability and boosting photocatalytic performance over TiO2 photocatalysts [26,27]. Recently, incorporating Au nanoparticles (NPs) with TiO2 has been proven favorable for lengthening the absorption range of TiO2 photocatalysts from UV to wide solar spectrum [22,23,28]. On the one hand, the semiconductor–metal photocatalysts can form a Schottky junction, which acts a charge carrier hub for trapping and transferring the photogenerated electrons [29–34]. On the other hand, the exhibited surface plasmon resonance (SPR) by plasmonic Au NPs courses highly thermal- and photocatalytically activity, playing an effective role for exclusive photocatalytic reactions [4,35,36]. Studying the specific impact between Au NPs and TiO2 photocatalysts is of significance for improving photocatalytic activity over Au/TiO2 system. The various existing parameters of Au NPs, such as morphology, distribution and size, play an important role in physicochemical properties, and further determine the photocatalytic performance [7,37]. Particularly, the different sizes of Au NPs show vital influence on the Schottky nanoscale and electrical properties when coupling with semiconductors [23,38–41]. For example, Lee et al. investigated the electrical properties of Au NPs on n-type TiO2 by colloidal self-assembled nanopatterning [38]. The formed Schottky barrier height was lower than that of the conventional Au/TiO2 nanodiode with large Au NPs. Moreover, Moon et al. reported the plasmonic hot carrierdriven Au NPs/TiO2 nanotube arrays for enhanced oxygen evolution with decorated Au NPs (5–30 nm) [40]. However, the functionalized Au/TiO2 system for high efficiency of photocatalytic hydrogen generation is still limited. The effective synthesis of Au/TiO2 nanosheets with small Au NPs size and the dimensional interaction of Au NPs have been rarely reported yet. Therefore, the specific photocatalytic mechanism involving the different sizes of Au NPs versus photocatalytic properties could be the attractive prospect for manufacturing highly efficient Au/TiO2 photocatalysts and widely realizing their practical applications. In this work, small plasmonic Au NP-decorated TiO2 nanosheets with exposed (0 0 1) facets were designed for efficient photocatalytic hydrogen evolution. A series of Au/TiO2 nanosheets with corresponding Au NP size of 3–13 and 28 nm were prepared by
using hydrothermally synthesized TiO2 nanosheets as supporting base through the urea and light reduction method, respectively. The results show that plasmonic Au/TiO2 nanosheets with smaller Au NPs have higher photocatalytic rate of hydrogen production. This contribution of enhanced photocatalytic performance was coursed by positive activity of plasmonic hot carrier-driven Au NPs and actively synergistic effect over the formed interface Schottky junction. By constructing a series of Au/TiO2 nanosheets with coupling different sizes of plasmonic Au NPs, a new insight into the photocatalytic mechanism involving the size of Au NPs versus photocatalytic properties was provided for obtaining desirable photocatalytic hydrogen production. This approach could be momentous to developing plasmonic Au hot carriers and contribute to wide application of visible light-restricted TiO2 and other photocatalysts limited by photoresponse. 2. Experimental 2.1. Materials The chemicals mainly include tetrabutyl titanate (C16H36O4Ti), hydrofluoric acid (HF, 40%), chloroauric acid (HAuCl44H2O), urea (H2NCONH2), and ethanol. The chemical reagents used in hydrogen production and electrochemical tests mainly include glycerin, ethanol, Na2SO4 (99.0%) and polyethylene glycol (PEG2000). All the aforementioned reagents, which were obtained from Sinopharm Chemical Reagent Co., Ltd. and used without further purification, were of analytical grade. 2.2. Synthesis of the TiO2 nanosheets Anatase TiO2 nanosheets were prepared by hydrothermal method [10]. In a typical synthesis, C16H36O4Ti (25 mL) was mixed with HF aqueous solution (3 mL) in a 100 mL autoclave with an inner Teflon lining following the typical hydrothermal treatment at 180 °C for 24 h. The obtained precipitate was collected by centrifugation with distilled water and ethanol for three times and then dried at 60 °C for 12 h. The obtained sample was denoted as TiO2. 2.3. Synthesis of the Au/TiO2 nanosheet composites A series of Au/TiO2 nanosheet composites with different sizes of Au NPs (1.0 wt%) were prepared by urea reduction method [41]. Typically, 1 g of as-prepared TiO2 nanosheets were added into the 100 mL mixed solution containing HAuCl44H2O (1.05 103 mol∙L1) and H2NCONH2 (0.42 mol∙L1) with the initial pH = 2. The suspension was then slowly stirred at 80 °C for 16 h to facilitate urea decomposition with pH = 7. The obtained precipitate was collected by centrifugation (12,000 r∙min1, 10 min) after stirring with 100 mL distilled water at 50 °C for four times and then dried at 100 °C for 2 h. Afterward, the appropriately dried precipitate was calcined at 100, 150, 180, 200, and 250 °C for 4 h, obtaining a series of Au/TiO2 nanosheet composites with different Au NP sizes labeled as AT-100, AT-150, AT-180, AT-200 and AT-250, respectively. For comparison, Au/TiO2 nanosheets loaded with large size Au NPs were prepared through the light reduction method. A total of 1 g of as-prepared TiO2 nanosheets were added into the
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Table 1 Detailed experimental conditions for the as-prepared samples. Sample
Synthetic method
Temp (°C)
AT-100 AT-150 AT-180 AT-200 AT-250 LR
Urea reduction Urea reduction Urea reduction Urea reduction Urea reduction Light reduction
100 150 180 200 250 Room temp
100 mL mixed solution containing HAuCl44H2O (1.05 103 mol∙L1) and ethanol/water (1:1) at room temperature. Subsequently, the solution was sonicated for 5 min and vigorously stirred under xenon lamp illumination for 1 h. The obtained precipitate was collected by centrifugation and then dried at room temperature for 12 h, which is denoted as LR. All samples involving detailed experimental conditions were listed in Table 1. 2.4. Characterization Powder X-ray diffraction (XRD) patterns were obtained through a Bruker D8 Advance X-ray diffractometer using Cu Ka as a ray source at a scan rate of 10°/min and scan range (2h) of 10°–85°. The UV-vis diffuse reflection spectra (UV-vis DRS) of the sample was measured by a UV-2550 UV–Vis spectrophotometer. Transmission electron microscopy (TEM) images were captured using a Hitachi-7650 model transmission electron microscope (HITACHI, Japan) with an accelerating voltage of 120 kV, and high-resolution TEM (HRTEM) images were observed using a JEM-2100F (JEOL, Japan) electron microscope at an accelerating voltage of 200 kV. Scanning electron microscopy (SEM) images were recorded through an S-4800 (Hitachi, Japan). Raman spectra were conducted using Renishaw InVia micro-Raman spectrometer with an excitation source of 514.5 nm Ar+ laser. Scanning transmission electron microscopy (STEM) images and EDS mapping were observed using Tecnai G2 F20 S-TWIN, FEI. X-ray photoelectron spectroscopy (XPS) analysis was tested on a Leybold Heraeus-Shenyang SKL-12 X-ray spectrometer equipped with a VG CLAM 4 MCD electron energy analyzer with Mg Ka (hv = 1253.6 eV) as excitation light at 10 kV and 15 mA. The photoluminescence (PL) spectra were detected using a FLS920 (Edinburgh Instruments, UK) at room temperature. 2.5. Photocatalytic activity measurement The photocatalytic activity for hydrogen production was performed in a 100 mL Pyrex flask using 350 W xenon lamp as a light source (model PLS-SXE300, Beijing Perfectlight Technology Co., Ltd.). Typically, 50 mg of as-prepared sample was dispersed in 80 mL aqueous solution containing 10% vol of glycerol. Before irradiation, the system was purged by bubbling nitrogen for 30 min to remove the dissolved oxygen and then sealed with a silicone rubber septum. The Pyrex flask was placed at a distance of 12 cm from the photocatalytic reactor and under irradiation for 1 h. The illumination area and focused intensity were controlled at approximately 17 cm2 and 0.63 W (d = 12 cm), respectively. Hydrogen production was measured by extracting 0.4 mL of gas through a gas chromatograph (GC-14C Shimadzu, Japan, TCD, N2 carrier gas). For the apparent quantum efficiency (AQE) test under the wavelengths of 365 nm (PLS-LED100, Beijing Perfectlight Technology Co., Ltd.), the light-emitting diode was put 1 cm away from the reactor to excite the photocatalytic hydrogen reaction. The illumination area and focused intensity were tested at approximately 7 cm2 and 20 mW (d = 1 cm), respectively. The AQE was measured through the following equation:
Mass ratio (g) TiO2
Au
Colour
Particle size (nm)
1 1 1 1 1 1
0.01 0.01 0.01 0.01 0.01 0.01
Grayish purple Light purple purple Dark purple Dark purple Fuchsia
3 5 8 10 13 28
number of reacted electrons 100 number of incident photons number of evolved H2 molecules 2 ¼ 100 number of incident photons
AQE ½% ¼
ð1Þ
2.6. Photoelectrochemical measurement Transient photocurrent responses and electrochemical impedance spectra (EIS) were measured using an electrochemical workstation (CHI660E; Shanghai Chenhua Co., Ltd., China) in a threeelectrode system. Ag/AgCl (saturated KCl) was used as the reference electrode, platinum wire was used as the counter electrode, 0.5 mol∙L1 Na2SO4 aqueous solution was used as electrolyte, and 350 W xenon lamp was used as the light source. The transient photocurrent was captured with a 0.5 eV starting voltage, and a light source was turned off at intervals of 60 s. The EIS was recorded with the AC voltage amplitude of 10 mV at 0.5 V and frequency range of 0.01–105 Hz. The working electrode was made by the scraper method. A total of 30 mg of the prepared sample and 30 mg of polyethylene glycol were mixed in a mortar, and 0.5 mL of ethanol was added to grind into a slurry. The slurry was evenly coated on the FTO conductive glass with active area and film thickness of approximately 1 cm2 and 0.01 mm, respectively, and then dried in an oven at 100 °C for 1 h for obtaining a working electrode. 3. Results and discussion The Au/TiO2 nanosheet composites with different Au NPs sizes were prepared by using two kinds of synthetic strategies: urea reduction method and light reduction method (see Scheme 1). The loading of Au NPs on the TiO2 nanosheets via urea reduction method had a smaller size than that prepared by light reduction method. Figs. 1a and S1 show the typical XRD patterns of the asobtained samples prepared by urea reduction method (samples AT-100, AT-150, AT-180, AT-200, and AT-250) and light reduction method (sample LR) as well as pure TiO2 nanosheets (sample TiO2). All samples display evident characteristic diffraction peaks at 25.3°, 37.8°, 48.0°, 54.9° and 62.7°, corresponding to (1 0 1), (0 0 4), (2 0 0), (2 1 1) and (2 0 4) planes of the anatase TiO2 (JCPDS No. 21-1272). No characteristic diffraction peaks of Au NPs were identified due to their low content and low diffraction intensity. The similar intensity and width at characteristic diffraction peaks of anatase TiO2 nanosheets imply that the lattice structure was maintained with the introduction of Au NPs. Compared with the pure TiO2 nanosheets, sample AT-100 had higher crystallinity and better crystallographic structure (Fig. S1), which implies that the samples decorated with small plasmonic Au NPs through urea reduction method have a high-phase purity and crystallinity. Fig. 1b shows the UV-vis DRS of pure TiO2, AT-100, AT-150, AT-180, AT-200, AT-250 and LR, wherein all samples show a significant absorption edge at 380 nm, which is associated with the intrinsic band gap of anatase TiO2 (3.2 eV) in the literature [12]. This result implies that the lattice of TiO2 was maintained because
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Scheme 1. Illustration of the synthetic process for the Au/TiO2 nanosheet composites with different Au NP sizes through urea and light reduction method.
Fig. 1. (a) XRD patterns of the as-obtained samples prepared by urea reduction method (samples AT-100, AT-150, AT-180, AT-200, and AT-250) and light reduction method (sample LR); (b) UV-Vis DRS spectra of pure TiO2 nanosheets (sample TiO2), AT-100, AT-150, AT-180, AT-200, and AT-250 and LR; (c) XPS survey spectra of the pure TiO2 nanosheets (sample TiO2), AT-150, and AT-180; and (d) the corresponding high-resolution XPS spectra of Au 4f for AT-150 and AT-180 samples.
the Au NPs were deposited rather than incorporated on the TiO2 supporting substrate. Compared with pure TiO2, the AT-100, AT150, AT-180, AT-200, AT-250 and LR composite samples exhibited strong absorption edges in the visible light region within
500–700 nm mainly due to the plasmon resonance effect of Au NPs. Further comparison indicates that Au/TiO2 composites coupled with various Au NP sizes of 3–13 nm showed higher absorption centers and intensities than those of 28 nm in the visible
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region of 600 nm. This finding indicates that different sizes of Au NPs course the mutative photoresponsive activity, which plays an effective role for exclusive solar light-harvesting ability of TiO2 photocatalysts. The XPS patterns illustrate the chemical composition and elemental status of the studied samples as depicted in Fig. 1c. The data indicate the presence of Ti, O, F and C in the pure TiO2, AT-150 and AT-180, with the emergence of characteristic peaks at binding energies of 458 (Ti 2p), 529 (O 1s), 684 (F 1s) and 284 eV (C 1s), respectively. The Ti and O elements are derived from TiO2 nanosheets, and C 1s and F 1s correspond to the background signals of C and the surface adsorbed F on TiO2 nanosheets,
respectively [42]. The typical high-resolution XPS spectrum of Au 4f in AT-150 and AT-180 samples is shown in Fig. 1d. The amount of Au in AT-150 and AT-180 were 1.6 and 1.9 at %, respectively. Two prominent peaks can be found at the binding energies of 82.7 and 86.4 eV, which are assigned to the characteristic values of Au element. The 3.7 eV binding energy difference between Au 4f5/2 and Au 4f7/2 is also attributed to the typical metal state form of Au element [43,44]. These XPS results confirm the successful synthesis of Au NPs and the strong interaction between Au NPs and TiO2 nanosheet substrates. Fig. 2a and b show typical TEM and HRTEM images of the AT-150 sample synthesized through the urea reduction method
Fig. 2. (a) TEM and (b) HRTEM images of the as-obtained AT-150 (synthesized through the urea reduction method by calcining at 150 °C for 4 h); (c) TEM and (b) HRTEM images of LR (prepared via light reduction method); (e) SEM image and (f) side-view crystalline structure of AT-180.
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by calcining at 150 °C for 4 h. As shown in Fig. 2a, the rectangularlike TiO2 nanosheets with an average size of 50 nm were evidently observed, and the small Au NPs were clearly dispersed on the surface of layered TiO2 support. These rectangular-like TiO2 nanosheets with high crystallinity exhibit well-defined lattice fringes (Fig. 2b). The crystal lattice fringes of TiO2 nanosheets with the d values of 0.35 nm correspond well to the (1 0 1) lattice planes of anatase TiO2 [42,45]. The small nanoparticles with an average size of 5 nm were recorded, and the d spacing of 0.236 nm can be assigned to the (1 1 1) lattice planes of Au NPs [46]. The SEM image, STEM image and corresponding elemental mapping results of AT-150 sample also show TiO2 nanosheets with regular rectangular-like shape and the loading of small Au NPs (Figs. 2e and S4), corresponding to the results of TEM image. Moreover, as shown in Fig. 3, the AT-100, AT-180, AT-200, and AT-250 samples demonstrate the small Au NPs sizes within 3–13 nm loaded on the TiO2 nanosheets. Form the inserted maps in Fig. 3, we can see that the distribution state of Au NPs is concentrated relatively, of which the difference of Au NPs sizes is around 4 nm. The inserted HRTEM images in Fig. 3 show that crystal lattice fringes of TiO2 nanosheets with the d values of 0.35 and 0.235 nm correspond well to the (1 0 1) and (0 0 1) lattice planes of anatase TiO2, respectively. The results indicate the successful synthesis of Au/TiO2 nanosheet composites and the suitability of anatase TiO2 nanosheets with highly exposed domain-reactive facets as supporting substrate for loading with Au NPs (Fig. 2f). As shown in
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Fig. 2c and d, the LR sample synthesized via light reduction method demonstrates the agglomerated TiO2 nanosheets, which crystallized as evidenced from the TiO2 (1 0 1) (d = 0.35 nm) crystal lattice fringes [12]. Moreover, the LR sample has larger Au NP size of 28 nm compared with that of the urea reduction strategy, suggesting that urea reduction is beneficial for the synthesis of Au NPs with small size. Fig. 4a shows the Raman spectra of the pure TiO2, sample AT200, and sample LR. The several representative binding energy peaks at 148, 399, 518, and 639 cm1 due to Eg(1), B1g(1), A1g + B1g(2) and Eg(2) vibration modes of anatase TiO2 were respectively found, which implies the existence of controllable anatase TiO2 [24]. Moreover, no characteristic Raman spectrum of Au NPs was identified possibly due to the weak crystallinity and low mass content of Au NP loading. The separation and transfer efficiency of photocatalytic carriers over the as-obtained samples were elucidated using the PL analysis (Fig. 4b). All the samples show a near bandedge emission around 425 nm, corresponding to band-to-band emission of TiO2 and the transition of electrons from the oxygen vacancies to TiO2 valance band. In contrast to the sample LR prepared by the light reduction method, the samples AT-100, AT-180 and AT-200 synthesized via the urea reduction method demonstrate increased PL intensity, which may be attributed to a defect-state luminescence phenomenon when coupling smaller Au NPs [10,47]. Moreover, the transient photocurrent responses and EIS were performed in 0.5 M Na2SO4 aqueous solution and
Fig. 3. TEM and HRTEM images of the as-obtained (a) AT-100, (b) AT-180, (c) AT-200, and (d) AT-250 (The inserted maps are the particle size distribution of Au NPs).
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Fig. 4. (a) Raman spectra of the pure TiO2 nanosheets (sample TiO2), sample AT-200, and sample LR; (b) PL spectra of the as-obtained samples prepared by urea reduction method (samples AT-100, AT-180, and AT-250) and light reduction method (sample LR); (c) Transient photocurrent responses and (d) Nyquist plots of as-obtained samples (samples AT-100, AT-150, AT-180, AT-200 and AT-250 and LR) in 0.5 m Na2SO4 aqueous solution under visible light irradiation at 0.5 V vs Ag/AgCl.
used to confirm the photocurrent reproducibility and charge transfer resistance over the studied samples. Fig. 4c depicts the similarity of transient photocurrent density of the AT-100 and AT-150, which was both higher than that of other samples, indicating the improvement of photogenerated charge separation over the Au/TiO2 nanosheets when loading Au NPs with 3–5 nm. The typical EIS exhibits the charge transfer resistance of the studied samples. A small arc radius in the EIS spectra generally indicates high charge transfer efficiency. As shown in Fig. 4d, the AT-150 sample prepared by urea reduction method exhibits similar semicircular arcs to AT-100 sample. The small arc radius of AT-100 and AT150 samples illustrates the high efficiency of charge transportation, corresponding to the results of transient photocurrent density. Based on the experimental results, the samples with 3– 5 nm synthesized by the urea reduction method have certain advantages in promoting photo-generated carrier transfer and prolonging the lifetime of carriers compared with those of the large Au NP size. Fig. 5a and b illustrate the photocatalytic hydrogen production activity of the as-obtained samples prepared by urea reduction method (samples AT-100, AT-150, AT-180, AT-200 and AT-250) and light reduction method (sample LR) under xenon lamp illumination and monochromatic light irradiation, respectively. Sample AT-150 with Au NP sizes of 5 nm exhibits high photocatalytic performance with hydrogen production rate at 230 mmolh1 under xenon illumination and the corresponding apparent quantum efficiency reached up to 15.94% under the wavelength of 365 nm. With the increase in Au NP size from 8 nm to 28 nm, the activity
of photocatalytic hydrogen production over Au/TiO2 nanosheets was significantly reduced from 170 mmolh1 to 105.3 mmolh1. In addition, compared to the pure TiO2, the photocatalytic hydrogen evolution of all Au/TiO2 system with loading Au NPs was significantly improved. This finding indicates that the small size of Au NPs has a positive role in promoting the photocatalytic activity of Au/TiO2 system under xenon lamp illumination. A series of experiments for photocatalytic hydrogen production was investigated through monochromatic light irradiation to optimize the activity over Au/TiO2 nanosheets with coupling different size of plasmonic Au NPs (Fig. 5c) under the wavelength of 450 nm, 490 nm, 585 nm and 630 nm. The hydrogen production rate of sample AT-100 with smaller Au NPs size (3 nm) was higher than the other samples under the wavelength of 450 nm, 490 nm and 585 nm, respectively. However, the sample AT-100 shows the lower photocatalytic activity compared to other samples under the light of 630 nm, which may be attributed to the weak plasmon resonance effect of Au NPs under visible light irradiation at 630 nm or interference from other factors that needs to be further studied. Fig. S5 shows the STEM and TEM images of the as-obtained AT-150 after photocatalytic hydrogen evolution, the microscopic morphology of the sample has not changed, which further illustrates the photocatalytic stability of the Au/TiO2 system. Upon visible light irradiation of Au/TiO2 system, the oscillating electric field was formed on the surface of Au NPs. This oscillation effect could course deviation of electric field from the nuclear framework. The SPR was excited based on the restoring force between electrons and nuclei, which could course in changes of
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Fig. 5. Photocatalytic activity of hydrogen production over the samples prepared by urea reduction method (samples AT-100, AT-150, AT-180, AT-200, and AT-250) and light reduction method (sample LR) in 80 mL aqueous solution containing 10% vol of glycerol under (a) and (b) visible light irradiation and (c) monochromatic light irradiation; (d) Schematic of plasmon oscillation for Au NPs showing the deviation of electric field from the nuclei; and (e) Schematic of the Au/TiO2 nanosheets loaded with different sizes of Au NPs for the transport of plasmonic hot electrons induced by the SPR to the CB of neighboring TiO2 nanosheets.
electron density and charge distribution, thus forming a collective oscillation of plasmonic hot carriers (Fig. 5d) [23,36,48]. These hot electrons induced by the SPR can be transferred to the conduction band (CB) of neighboring TiO2 nanosheets based on the higher Fermi level of Au compared with that of the Schottky barrier height [36], thus accelerating photocatalytic hydrogen production. The various sizes of Au NPs would course the different effects on the electric field polarization of SPR, resulting in the changeable efficiency of the transfer and separation of plasmonic hot electrons. The desirable photocatalytic activity over the Au/TiO2 system was increased in the present study when smaller Au NPs were loaded. This finding can be attributed to the reduced Schottky barrier height and accelerated plasmonic hot electrons of the small Au NPs. A schematic diagram of the Schottky barrier height and binding energy level at the interface over Au/TiO2 nanosheets loaded with different sizes of Au NPs is shown in Fig. 5e. Plasmonic hot
electrons induced by SPR of Au NPs formed Schottky junction between the TiO2 nanosheets and Au NPs. This Schottky junction structure acts as transfer hub to carry and transfer the plasmonic hot electrons with sufficient energy to the CB of neighboring TiO2 nanosheets, thus promoting the separation of ground state and excited electrons on the surface of Au NPs. The local electric field coursed by SPR at the interface between Au NPs and TiO2 nanosheets has variation in nanoscale, which depends on the size of Au NPs. As the size decreases, the Schottky barrier height exhibits lower nanoscale junction as well as the reduced binding energy of Au NPs, resulting in an increased electronic cloud injection into the CB of TiO2 nanosheets. The Au/TiO2 nanosheets with 5 nm Au NPs possess lower binding energy level near the Fermi level in comparison with that of loading Au NPs with 28 nm, leading to the significant contribution in enhancing the transport of plasmonic hot carriers under the Schottky barrier junction.
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Fig. 6. Mechanism for the photocatalytic process of hydrogen evolution with the electron transfer and energy band structure in the synergistic Au/TiO2 nanosheets under the UV and visible light illumination.
By controlling the size of Au NPs in 5 nm, the efficiency of photocatalytic hydrogen production over the Au/TiO2 nanosheets would be significantly increased, which is also consistent with the results of experimental hydrogen production (Fig. 5a and b). Fig. 6 shows the proposed photocatalytic mechanism for the promoted photocatalytic hydrogen generation over the Au/TiO2 nanosheets with Au NP size at 3–5 nm. The spectrum of xenon lamps is in a wavelength range of 320–780 nm. Therefore, under the UV-light irradiation, TiO2 nanosheets were excited by the absorbed photon, facilitating the photogeneration of electrons at the valence band (VB). Under the action of band energy, the photogenerated electrons on the VB transfer to the CB of TiO2 nanosheets, thus forming electron and hole pairs. Au NPs could act as the electronic receiver of photogenerated electrons, accelerating the separation of electron–hole pairs, thus further enhancing the photocatalytic performance of hydrogen production. Moreover, the as-prepared anatase TiO2 nanosheets exposed domain-reactive (0 0 1) facets with high reactivity and low thermodynamic stability [12–15]. The photogenerated electrons remaining on the CB of TiO2 nanosheets can also directly generate hydrogen by reversing oxidation–reduction reaction with water molecules adsorbed on the surface, altogether enhancing the photocatalytic activity over the Au/TiO2 nanosheets. Owing to the local electric field of Au NPs coursed by SPR, the plasmonic hot carriers would be excited when the light energy is more than 2.3 eV (k = 546 nm) [39,49]. Therefore, the SPR in Au NPs was excited due to the restoring force between electrons and nuclei under visible light irradiation, resulting in the collective oscillation of plasmonic hot carrier. These plasmonic hot electrons can be transferred to the CB of TiO2 nanosheets by preferred Schottky barrier height, which acts as transfer hub to promote the separation of ground state and excited electrons on the Au NP surface. The smaller Au NPs possess low nanoscale junction and form lower binding energy level near Fermi level, demonstrating the important contribution of Au NPs in injecting the plasmonic hot carriers to TiO2 nanosheets (Fig. 5e). The hot electrons received by the CB of TiO2 nanosheets participate in the photocatalytic hydrogen generation of water splitting for the Au/TiO2 nanosheets. Based on the preceding discussion, the introduction of SPR over Au NPs maximizes the light response range for visible light-responsive TiO2 photocatalytic system. The positive activity of plasmonic hot carrier-driven Au NPs and actively
synergistic effect over the formed interface Schottky junction play significant roles in the necessary photocatalytic hydrogen generation under the xenon lamp illumination.
4. Conclusions The functionalized plasmonic Au/TiO2 nanosheets with small Au NP sizes of 3–13 nm and 28 nm were prepared by using hydrothermally synthesized TiO2 nanosheets as supporting base via the urea and light reduction method, respectively. The experimental results reveal that the as-obtained small Au NPs show a positive effect on promoting the photocatalytic performance of the Au/TiO2 nanosheets. The as-prepared plasmonic Au/TiO2 nanosheets with Au NP size of 3–5 nm had the highest photocatalytic rate of hydrogen production at 230 mmolh1. The positive activity of plasmonic hot carrier-driven Au NPs and actively synergistic effect over the reduced interface Schottky junction favored the transfer and separation of photocatalytic charge carriers through the Schottky transfer hub to neighboring TiO2 nanosheets, thus enhancing the photocatalytic activity of hydrogen generation under the xenon lamp illumination. This work paves a promising way to comprehensively understand actively synergistic effects of smaller Au NPs versus highly photocatalytic capability over the Au/TiO2 nanosheets, which could be extended for designing other visible light-restricted photocatalysts with decorated small Au NPs. Acknowledgments This work was partially supported by the National Natural Science Foundation of China under Grant Nos. 51672099 and 21403079, Sichuan Science and Technology Program under No. 2019JDRC0027, and Fundamental Research Funds for the Central Universities under No. 2017-QR-25.
Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcis.2019.07.060.
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