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Self-Assembled Heteroepitaxial AuNPs/SrTiO: Influence of AuNPs Size on SrTiO Band Gap Tuning for Visible Light-Driven Photocatalyst 3

Kok Hong Tan, Hing Wah Lee, Jhih-Wei Chen, Chang Fu Dee, Burhanuddin Yeop Majlis, Ai Kah Soh, Chung Lin Wu, Siang-Piao Chai, and Wei Sea Chang J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 06 Jun 2017 Downloaded from http://pubs.acs.org on June 8, 2017

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The Journal of Physical Chemistry

Self-Assembled Heteroepitaxial AuNPs/SrTiO3: Influence of AuNPs Size on SrTiO3 Band Gap Tuning for Visible Light-Driven Photocatalyst Kok Hong Tan,† Hing Wah Lee,‡ Jhih-Wei Chen, § Chang Fu Dee, ǁ Burhanuddin Yeop Majlis, ǁ Ai Kah Soh, † Chung-Lin Wu, § Siang-Piao Chai,┴ Wei Sea Chang,†,＊ †

Mechanical Engineering Discipline, School of Engineering, Monash University Malaysia,

Bandar Sunway, Selangor 47500, Malaysia. ‡

Nanoelectronics Lab, MIMOS Berhad, 57000 Technology Park Malaysia, Malaysia.

§

Department of Physics, National Cheng Kung University,Tainan 70101, Taiwan.

ǁ

Institute of Microengineering and Nanoelectronics, Universiti Kebangsaan Malaysia, Bangi,

Selangor 43600, Malaysia. ┴

Chemical Engineering Discipline, School of Engineering, Monash University Malaysia, Bandar

Sunway, Selangor 47500, Malaysia. ＊

Corresponding author, Tel.: +60-3-5514-5677, Fax: +60-3-5514-6207

E-mail: [email protected]

1 ACS Paragon Plus Environment


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ABSTRACT: Self-assembled heteroepitaxial offers tremendous opportunity to tailor optical and charge transport properties in noble metal-semiconductor interface. Here, we incorporated gold nanoparticles (AuNPs) onto the {001} facets of semiconductor strontium titanate, SrTiO3 (STO) by means of heteroepitaxial approach to investigate the band gap tuning and its effect of photoresponse. We demonstrate that Fermi energy level of the system can be tuned by controlling the AuNPs size. X-ray photoelectron spectroscopy (XPS) shows that the energy difference between Sr3d and Au4f core levels measured in the AuNPs/STO (100) heterojunction increases from 47.90 eV to 49.26 eV with decreasing AuNPs size from 65 nm to 16 nm, respectively. Hence, the Fermi energy level was shifted towards conductive band of STO (100) and the system charge transfer efficiency was improved. It was also found that smaller AuNPs sizes exhibiting higher photoactivity as the result of band gap narrowing effect. Photoactivity was improved by broadening the catalyst absorption spectrum to the visible light region. This study provides a basic understanding of the photoelectrochemistry of metal-semiconductor heterostructure for visible light-energy conversion.


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1. INTRODUCTION Semiconductors have been widely used as photocatalysts due to their promising applications in solar energy conversion and environmental purification.1-4 However, the large band gap in most of the semiconductors can only possess UV photoresponse, leading to insufficient efficiency in charge separation which limits the photoactivity performance.5-8 This is because UV accounts for 6 % of the incoming solar energy; whereas about 50 % of the incoming solar energy is visible radiation.9 Much effort has been devoted to improve the visible light absorption ability of large band gap semiconductors. Such efforts include the combining noble metal nanoparticles with large band gap semiconductor as well as band gap tuning for full spectrum harvesting.10 It has been reported that noble metal nanoparticles have a broad absorption band in the visible range due to surface plasmon resonance (SPR) absorption.11-16 Alternatively, the incorporation of noble metal nanoparticles on the surface of semiconductors will tune the band gap of semiconductors to facilitate charge carrier separation and thus improve the interfacial charge transfer kinetics.1719

Recently, it was discovered that crystal facets play an important role in the control of

photoexcited electron-hole separation.20-22 Li et. al.23 has shown that oxidation and reduction can occur with the presence of multiple crystal facets in polycrystalline semiconductor photocatalyst. It was suggested that multiple crystal facets exhibit competitive redox reaction on the different crystal facets, resulting in poor photoactivity performance. Thus, understanding the correlation between photoactivity behavior and the crystal interfaces has become an essential aspect as the light absorption strongly depends on the atomic arrangement of the exposed crystal facets.24 In this study, {100} facets of strontium titanate (STO (100)) was selected due to its high chemical stability.25 Furthermore, it has been pointed out that STO (100) is favorable for water splitting due to its large band gap energy is enough for dissociation of a water molecular (1.23



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eV) in which the conduction band edge of STO (100) is located at more negative potential than the reduction potential of water with the aim of hydrogen production and the valence band of the STO (100) is more positive than the oxidation potential of water to form oxygen. 26, 27 Meanwhile, gold nanoparticles (AuNPs) was chosen as the nobel metal nanoparticles since AuNPs can increase the efficiency of photocatalyst by increasing the absorption ability of large band gap semiconductor over visible region of spectrum range.12, 28 To develop visible light responsive photocatalyst, heteroepitaxial approach consisting AuNPs (111) on STO (100) was used as a model system to manipulate optical and electronic properties via band gap engineering in order to explore the charge separation of the photo-excited charge carrier and transport mechanism in the system. The band gap engineering is revealed by X-ray photoelectron spectroscopy (XPS). We demonstrated that Fermi energy level of STO (100) can be tuned by incorporating AuNPs onto STO surface by means of heteroepitaxy approach. 2. EXPERIMENTAL DETAILS 2.1 Synthesis of AuNPs/STO (100). The STO (100) substrate of 5.0L mm x 5.0W mm x 0.5T mm in dimensions was used. Au film of various thicknesses 2 nm, 4 nm and 6 nm were deposited onto the STO (100) 5.0L mm x 5.0W mm faces by means of physical vapour deposition (PVD) method. Thermal dewetting method was used to produce AuNPs with size of 16 nm, 32 nm and 65 nm at 1 atm oxygen pressure for 10 minutes. This method was chosen as it is the simplest and low cost technique to produce AuNPs.29 During the annealing process, the surface energy of the Au film and the interface energy between the Au film and STO (100) decrease. Subsequently, the Au film starts to crack and agglomerate to form droplets.30 This process can still occur by surface diffusion even in the solid state of Au is lower than the melting temperature of the Au


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film.31 Hence, the AuNPs sizes can be easily controlled by the initial Au film thickness and annealing temperature. 2.2 Characterization. The crystalline orientation of the AuNPs/STO (100) was studied using Bruker D8 Discover X-Ray Diffraction (XRD), outfitted with CuKα as the radiation source. All XRD data were collected at 2θ ranges with a scan rate of 0.02o/step and step size of 0.02o/minutes. The crystal quality of the AuNPs was observed via FEI-Tecnai F20, 200kV S/TEM. The absorption spectrum of the AuNPs/STO (100) was analyzed using Agilent Cary 100 UV-Visible Spectrophotometer from wavelength 350 nm to 800 nm. Photoluminescence (PL) with 325 nm wavelength and 40x NUV objective were used to determine the charge recombination of different sizes of AuNPs on STO (100) substrate. The energy band alignment of the AuNPs/STO (100) heterostructure was determined by XPS. The XPS measurements were carried out at room temperature on a thermo Scientific K-Alpha system, equipped with a monochromatized Al Kα X-ray source of 1486.6 eV at the National Synchrotron Radiation Research Center, Taiwan. The surface topography of the AuNPs/STO (100) was analyzed using Bruker Multimode 8 Atomic Force Microscopy (AFM) with a scan size of 3 µm x 3 µm. For CAFM measurement, conductive 0.7 wt% Niobium-doped STO (0.7 % Nb-STO) (100) substrates were used to obtain current-voltage (I-V) measurement. 2.3 Photoresponse. The electrochemical experiments were performed with a potentiostat (CHI6005E Electrochemical Analyzer) connected to a three-electrode system containing working electrode (photocatalyst with 0.7 % Nb-STO (100) as substrate), a saturated Ag/AgCl reference electrode and a platinum wire (Pt) counter electrode. The three electrodes were immersed into an electrochemical cell containing 75 mL of 0.5 M sodium sulfate (Na2SO4) solution. An external excitation source of 500 W xenon light source (CHF-XM-500W) was



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employed to record the electrochemical performance of the catalyst under illumination. The light source intensity was measured to be 100 mW cm-2. Chronoamperometric (CA) was measured by using three different filters such as AM 1.5G (300 nm to 800 nm), >400 nm wavelength and UV (200 nm to 400 nm) filters in order to modify the emission spectrum from the xenon light source to determine the photoactivity of AuNPs/Nb-STO (100) under different spectral ranges. Electrochemical Impedance Spectroscopy (EIS) was carried out under AM 1.5G illumination to investigate the charge transfer resistance features of the AuNPs/Nb-STO (100). RESULTS AND DISCUSSION 3.1 Structure Characterization. Figure 1 shows the morphology and particle size distribution of the 16 nm, 32 nm and 65 nm AuNPs resulting from Au films of 2 nm, 4 nm and 6 nm, respectively, after the annealing treatment. The AuNPs of 16 nm, 32 nm and 65 nm were used to study the effect of AuNPs size on the photoactivity. Figure 2 (a) shows the XRD θ-2θ scan of the Au/STO (100) and AuNPs/STO (100). Only the AuNPs (111) peak at 2θ ≅ 37.8o was detected. The peaks at 22.3o, 46.0o and 72.1o are corresponded to the STO substrate. HR-TEM analysis was employed to obtain the crystallographic structure of AuNPs/STO (100). The cross-sectional HR-TEM image of AuNPs/STO (100) in Figure 2 (b) shows heterostructure interface between crystalline AuNPs and STO (100). Figure 2 (c) shows the zoom in HR-TEM image of AuNP with an interplanar distance of 0.23 nm that corresponds to AuNP (111) lattice plane, implying that the AuNPs are self-assembled onto the STO (100). The AuNP (111) lattice plane is consistent with the XRD results. Thus, the HR-TEM results corroborated by XRD measurements exhibited a 3-D orientation relationship of AuNPs (111)||STO (100), in which AuNPs (111) are embedded in the STO (100) substrate, as shown in Figure 2 (d).


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Figure 1. AFM images of the surface morphology of various AuNPs size (a) 16 nm, (b) 32 nm and (c) 65 nm loaded on STO (100) and the respective AuNPs size distribution.



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Figure 2. (a) XRD results for STO (100), Au/STO (100) and AuNPs/STO (100); (b) Enlarged HR-TEM cross section image of the AuNPs/STO interface and the corresponding cross sectional TEM image in the inset; (c) Zoom in HR-TEM image of AuNPs/STO; (d) Schematic representation of a self-assembled heteroepitaxial AuNPs/STO (100).

3.2 Optical and Electrical Properties. Figure 3 (a) shows the absorption spectra of the STO (100) at below 390 nm in the UV region, while the AuNPs exhibited absorption peak in the visible region from 500 nm to 700 nm. The result suggests that the presence of AuNPs has contributed significantly to the photoactivity in the visible region since AuNPs absorb visible 8 ACS Paragon Plus Environment

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light. It is interesting to note that the absorption peak of AuNPs shows a red shift from 589 nm to 636 nm with increasing AuNPs size from 16 nm to 65 nm, respectively. Similar finding was found by other researchers where, AuNPs produce collective coherent electron oscillation resulting from light stimulation will be confined at the surface generating an intense electrical field upon the irradiation.11 However, the enlargement of AuNPs size disallowed light to polarize the AuNPs homogeneously. The scattering effect is assumed to be dominant over absorption of light, leading to red shifting and broadening of the SPR absorption band.32 Thus, the peak position was shifted to longer wavelength region. Figure 3 (b) exhibits Tauc plot of AuNPs/STO (100) from 2.90 eV to 3.20 eV, confirming that the AuNPs has induced band shifting in STO (100) from UV to visible region. The band gap for raw STO (100) is 3.10 eV. With increasing AuNPs sizes from 16 nm to 65 nm, the band gap for AuNPs/ STO (100) has shifted to 3.07 eV, 3.08 eV and 3.09 eV, respectively.



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Figure 3. (a) UV-Vis absorption spectrum for STO (100) and AuNPs/STO (100) with different AuNPs sizes; (b) Tauc plot of STO (100) and AuNPs/STO (100) with a photon energy range of 3.00-3.15 eV; (c) PL spectra of STO (100) with various AuNPs size taken under the irradiation of 325 nm laser at room temperature. 10 ACS Paragon Plus Environment

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PL technique was carried out to investigate the electronic structure and photochemical properties of the AuNPs/STO (100). As shown in Figure 3 (c), the AuNPs/STO (100) exhibited a strong and wide PL signal at the range of 380 nm to 650 nm. During photo-excited process, the electrons in the valence band of semiconductors were excited and moved to conduction band when exposed to an energy equivalent to or higher than their energy band gap. The photogenerated electrons were unstable, thus leading to the electron–hole recombination in which the energy of recombination was then released as luminescence emission. This suggests that higher electron–hole recombination rate gives higher PL intensity, resulting in poor photoactivity. It was shown that the PL signals of STO (100) are much stronger than AuNPs. This finding is consistent with other reported work in which no new PL spectrum was formed for deposition of noble metals on semiconductors, instead the intensity of PL was influenced by the nanoparticle sizes.33 The two peaks located at 430 nm and 500 nm are corresponded to the band edge free excitons and the recombination of photogenerated holes with singly ionized charge states of the intrinsic defects such as oxygen vacancies (bound excitons) of the STO (100), respectively.33 Besides, our result also shows that the smaller AuNPs tends to capture more electrons compare to bigger AuNPs. That is, the smaller AuNPs can present a slower electronhole recombination and hence affect the overall photocatalyst reaction. This is evident in PL intensity of AuNPs/STO (100), for example, as shown in Figure 3 (c), indicative of minimal PL intensity in response to the photoexcited process. High resolution XPS was carried out to explore the charge transport and the energy band potential between AuNPs/STO (100) interfaces. The XPS spectrum of the Au4f and Sr3d for 16 nm, 32 nm and 65 nm AuNPs/STO (100) were displayed in Figure 4. The peak position of



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Au4f7/2 for all AuNPs/STO (100) was located at 84.0 eV demonstrates that AuNPs are metallic, which was consistent with our HR-TEM and XRD results. It was observed that the binding energy of Sr3d shifted from 131.90 eV to 133.26 eV, toward a higher binding energy with decreasing AuNPs size. By measuring the core level values and the valence band maximum (VBM) binding energy, the valence band offset (∆ ) can be determined by using Kraut’s method,34 and presented in following equation: ∆ = ∆ − E − E + E − E − E is the energy difference between Sr3d and Au4f core levels where ∆ = E − E and E − E are measured in the heterojunction AuNPs/STO (100), while E

the valence band maximum and Fermi energies with reference to the core level positions of STO (100) and AuNPs, respectively. Thus, Schottky barrier height (SBH) of the AuNPs/ STO (100) can be calculated based on the following equation:7 φ = ∆ + where φ is the SBH, ∆ is the conduction band offset of STO (100) and is the built-in potential. The detailed calculations demonstrating the band structure change of the AuNPs/STO (100) were presented in Supporting Information. The values of energy bands were summarized in Table 1. It was found that decreasing the AuNPs size leads to an increase in ∆ from 47.90 eV to 49.26 eV, corresponding to shift of the Fermi energy level towards conductive band of STO (100) and hence a better charge transfer efficiency.


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Figure 4. The Au4f and Sr3d XPS spectra of AuNPs/STO (100) with different AuNPs sizes.

Table 1. Energy band values of AuNPs/STO (100) heterojunction.

C-AFM was conducted in order to further explain the charge transport between AuNPs/Nb-STO (100) interfaces. As depicted in Figure 5 (a), all the AuNPs/Nb-STO (100) samples exhibited a 13 ACS Paragon Plus Environment


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non-linear I-V curve. This implies that these AuNPs/Nb-STO (100) interfaces give rise to rectifying Schottky diode-like behavior. It was known that the Fermi energy level of AuNPs (+0.45 V versus NHE) is more positive than the conduction band level of Nb-STO (-0.84V versus NHE).5, 35 When AuNPs were deposited on Nb-STO (100), the Fermi energy level of NbSTO (100) will shift upwards until equilibrium is achieved and the Schottky barrier is formed. When a bias voltage is applied on the AuNPs, a Schottky diode-like behavior was detected due to the nanometric localization of the current across the metal-semiconductor interface. A general trend observed during C-AFM characterization is the increase in biased voltage as AuNPs size increases. This was demonstrated by the electrical responses collected from 16 nm, 32 nm and 65 nm AuNPs/Nb-STO (100). The biased voltage applied for 16 nm, 32 nm and 65 nm AuNPs were 2.4 V, 3.4 V and 4.8 V, respectively. It was observed that as the I-V curves are directly related to the AuNPs size of the system. The I-V have large positive curve with increasing AuNPs size. Such I-V characteristic was due to high bias voltage is required for electrons to flow from AuNPs to Nb-STO (100) as the SBH increased with increasing AuNPs size. C-AFM current mappings for 16 nm and 65 nm AuNPs are shown in Figure 5 (b1) and Figure 5 (b2), respectively. A comparison of the current mappings indicates that bright area is more conductive, for example, the 16 nm AuNPs is more electrically active than 65 nm AuNPs, and is thus the possible mechanism to modify SBH.


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Figure 5. (a) I-V curve for 16 nm, 32 nm and 65 nm AuNPs/Nb-STO (100) obtained by C-AFM; (b) C-AFM current mapping taken with a sample bias of 4.0 V applied to (b1) 16 nm and (b2) 65 nm AuNPs and the corresponding AFM topography for (b3) 16 nm and (b4) 65 nm AuNPs/NbSTO (100) of the same sample.

3.3 Photoresponse Characterization. Figure 6 (a-c) shows the CA of AuNPs/Nb-STO (100) recorded in the dark and under light illumination with three different filters such as AM 1.5G, >400 nm wavelength and UV filters, respectively. Nb-STO is a large band gap semiconductor exhibiting photoresponse only in the UV light region. However, this limitation can be improved by loading with AuNPs as AuNPs have the ability to enlarge optical absorption to the extent of visible light region.12 This is a key factor to narrow the band gap energy of the Nb-STO (100) which may help explain the increase in PEC performance as more visible light absorption is expected to occur in the visible light region.



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Figure 6. Chronoamperometric of Nb-STO (100) and AuNPs/Nb-STO (100) recorded in 0.5 M aqueous Na2SO4 solution in the dark and under light illumination: (a) AM1.5G filter, (b) >400 nm filter and (c) UV filter; (d) The effect of AuNPs size on STO band gap, area covered, the energy difference between Sr3d and Au4f core levels (∆ ) as well as the photocurrent of AuNPs/Nb-STO (100) under >400 nm filter.

However, the average surface area of the contact between AuNPs and Nb-STO (100) was found to dependent on the average AuNPs size. The average contact area covered by 16 nm and 65 nm AuNPs are, in fact, grows from 7.0 % to 19.5 % based on the 3 µm x 3 µm AFM images shown in Figure 1. Despite the average contact area for 65 nm AuNPs/Nb-STO (100) is larger than the


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16 nm AuNPs/Nb-STO (100), the latter gives rise to higher photocurrent (~2.2 x 10-7 A/cm2) under visible light irradiation (>400 nm). Interestingly, there is no photoresponse for raw NbSTO (100) under visible light irradiation (>400 nm), as shown in Figure 6 (b). The photoresponse of the 16 nm, 32 nm and 65 nm AuNPs/Nb-STO (100) might explain the fact that the large band gap of the Nb-STO (100) has been tuned by the respective AuNPs towards lower energy; that is, the narrowing band gap can greatly increase the visible light absorption and photoactivity. Furthermore, the Fermi energy level of the system shifted towards conductive band of STO (100) with decreasing AuNPs size, based on the XPS result in Figure 4. Therefore, 16 nm AuNPs/Nb-STO (100) with band gap energy of 3.07 eV and ∆ of 49.26 eV exhibits good visible light-driven photoactivity compared to the two other AuNPs sizes. It should be noted that electron-hole pairs were generated during light illumination. The process of illumination will lead to a continued increase in the concentration of electron-hole pairs. At the same time there is an increased in the numbers of electrons recombined with holes. When the light is turned off, the hole current falls abruptly to zero. As the result, the current in the external circuit changes sign as electrons continue to flow and recombine with the remaining holes.36 The correlation between STO band gap, area covered by AuNPs, photocurrent and ∆ resulting from various AuNPs size was summarized in Figure 6 (d). In contrast, raw Nb-STO (100) having 3.10 eV band gap energy possess good photoresponse under UV irradiation due to their large surface area exposed to UV without coverage of AuNPs, as shown in Figure 6 (c). Increasing the AuNPs size from 16 nm to 65 nm led to a decrease in the UV absorption from 5.5 x 10-5 A/cm2 to 0.1 x 10-5 A/cm2. Such a significant decrease in photoresponse under UV irradiation suggests that the photoresponse is primarily depended on the exposed surface areas of {001} facets of NbSTO.



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To clearly illustrate the charge transfer process at AuNPs/Nb-STO (100) and electrolyte interface, the EIS measurements were performed. Figure 7 (a) shows the EIS results presented in the form of Nyquist plot. The equivalent circuit model of charge transfer in the AuNPs/Nb-STO (100) system is shown in Figure 7 (b). In the equivalent circuit model, R1 represents the solution resistance. R2 and C1corresponds to the charge transfer resistance and capacitance induced by AuNPs, respectively. R3 represent the charge transfer resistance and CPE1 is constant phase element (CPE) of the particle spacing within the AuNPs. R4 is the charge transfer resistance of Nb-STO (100), while CPE2 is responsible for the capacitance induced by Nb-STO (100). The substitution of CPE with conventional capacitor was to obtain a better Nyquist plot fitting line. The replacement of the double layer capacitance was due to limitation of the AuNPs/Nb-STO (100) surface, such as roughness and porosity that lead to imperfect capacitive behavior.37-39 The fitting results of the equivalent circuit model were summarized in Table 2.

Figure 7. (a) Nyquist plot of AuNPs/Nb-STO (100); (b) Equivalent circuit model of the AuNPs/Nb-STO (100).


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Table 2. The fitting results from the Electrochemical Impedance Spectroscopy measurements.

The proposed mechanism of AuNPs/Nb-STO (100) was shown in Figure 8. Figure 8 (a) shows that the Nb-STO (100) valence electrons can only be excited from valence band to conduction band when the light photons absorbed by the valence electrons is equal to or greater than the band gap of Nb-STO (100) for photoactivity. The large band gap of Nb-STO (100) at 3.10 eV corresponds to the UV spectrum, thus allowing Nb-STO (100) possesses only UV photoresponse. However, when AuNPs were deposited on Nb-STO (100), the Fermi energy level of the system will realign until the equilibrium state is achieved due to the redistribution of the free electrons across the AuNPs/Nb-STO (100) interfaces. As pointed out based on Figure 3 (b), the absorption spectra of the AuNPs/STO (100) interface demonstrated band gap shifting, hence a shift to smaller energy; for example, the band gap of Nb-STO (100) has been narrowed down by 16 nm AuNPs from 3.10 eV to 3.07 eV. As a result, the absorption wavelength at the AuNPs/Nb-STO (100) interface is observed to shift from UV to visible region. From this point of view, the valence electrons of AuNPs/Nb-STO (100) will be excited into the conduction band by visible light illumination (>400 nm), as shown in Figure 6 (b). Comparing to the conduction band of Nb-STO (100), the work function of Au is lower with the value of 5.1 eV,40 hence the excited electrons flow from Nb-STO (100) to AuNPs. This has led to a better electron-hole separation and thus increased the photoactivty performance. 19 ACS Paragon Plus Environment


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Figure 8. Schematic of the proposed mechanism of charge transfer of the (a) exposed surface area of Nb-STO (100) and (b) AuNPs/Nb-STO (100) interface.

Furthermore, the photoactivity of AuNPs/Nb-STO (100) was found to increase with decreased AuNPs size, which could be attributed to the Fermi energy level shifting towards conductive band of STO (100) and hence a better charge transfer efficiency; this can be seen in the XPS result in Figures 4. As a result, smaller AuNPs, for example 16 nm AuNPs, gives high photoactivity as compared to other photocatalyst with larger AuNPs. These results support for the importance of charge transfer efficiency at AuNPs/Nb-STO (100) interface in photoactivity 20 ACS Paragon Plus Environment

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which can be manipulated by controlling AuNPs size. With all the above findings, the effect of AuNPs size on the overall energy potential of the photocatalyst was established. 3. CONCLUSIONS We tailored the optical and electrical properties of AuNPs/STO (100) interfaces for photoactivity by controlling the AuNPs size. The gathering evidence to support the effective interfacial charge separation by AuNPs, from UV-Vis, PL, XPS, C-AFM and PEC has contributed to the enhanced photoactivity of the AuNPs/STO system. 16 nm AuNPs/STO (100) exhibited high photoactivity under three different filters AM 1.5G, UV and >400 nm wavelength filters. The high catalytic performance of 16 nm AuNPs/STO (100) was attributed to the three main factors working in conjunction: (i) smaller AuNPs size improves the visible light absorption by narrowing the band gap of STO (100), (ii) smaller AuNPs performs superior shift in core level compared to larger AuNPs and thus shift Fermi energy level closer to conductive band of STO (100) and increase the charge transfer efficiency, and (iii) smaller AuNPs can act as a better electron trapping center to decrease the electron-hole recombination rate. This study indicates that smaller AuNPs are the potential mediating role for designing effective charge transfer in semiconductor photocatalyst for visible light energy conversion. AUTHOR INFORMATION Corresponding Author ＊

E-mail: [email protected]

Author Contributions The manuscript was written through the contributions of all authors. All authors have given approval to the final version of the manuscript. Notes



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The authors declare no competing financial interest. ACKNOWLEDGMENTS This work is supported by the Ministry of Higher Education (MOHE) Malaysia under the FRGS grant (FRGS/1/2014/SG06/MUSM/03/1). ASSOCIATED CONTENT Supporting Information Available This information is available free of charge via the Internet at http://pubs.acs.org. ABBREVIATIONS AuNPs, gold nanoparticles; Nb-STO, Niobium-doped strontium titanate; STO, strontium titanate. REFERENCES (1)

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TOC Graphic


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SrTiO

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