Synergistic Effect of Dual Particle-Size AuNPs on TiO2 for Efficient Photocatalytic Hydrogen Evolution

Design of efficient catalysts for photocatalytic water-splitting hydrogen evolution is of fundamental importance for the production of sustainable clean energy. In this study, a dual particle-size AuNPs/TiO2 composite containing both small (16.9 ± 5.5 nm) and large (45.0 ± 9.8 nm) AuNPs was synthesized by annealing two different sized AuNPs onto TiO2 nanosheets. Dual particle-size AuNPs/TiO2 composites of 2.1 wt% catalyze photocatalytic H2 evolution 281 times faster than pure TiO2. Control experiments indicate the observed rate increase for the 2.1 wt% dual particle-size AuNPs/TiO2 composites is larger than 2.1 wt% small AuNPs/TiO2 composites, or 2.1 wt% large AuNPs/TiO2 composites in isolation. The observed photocatalytic enhancement can be attributed to the synergistic effect of dual particle-size AuNPs on TiO2. Specifically, small-sized AuNPs can act as an electron sink to generate more electron-hole pairs, while the surface plasmon resonance (SPR) effect of large-sized AuNPs concurrently injects hot electrons into the TiO2 conduction band to enhance charge transfer. In addition, a gold-dicyanodiamine composite (GDC)-directed synthesis of 2.1 wt% dual particle-size AuNPs/TiO2 composites was also completed. Notably, a photocatalytic efficiency enhancement was observed that was comparable to the previously prepared 2.1 wt% dual particle-size AuNPs/TiO2 composites. Taken together, the synergistic effects of dual particle-size AuNPs on TiO2 can be potentially used as a foundation to develop semiconductor photocatalyst heterojunction with enhanced photocatalytic activity.


Introduction
The development of clean and renewable fuels continues to attract significant attention due to the environmental concern caused by global consumption of fossil fuels. Electrochemical photolysis of water to generate hydrogen (H 2 ) using a TiO 2 electrode was first reported in 1972 [1]. Since that time, photocatalytic H 2 production has become one of the most promising routes to secure H 2 as an alternative energy source [2]. Use of TiO 2 as a photocatalyst was often driven by its low cost and strong oxidizing capacity; however, the high recombination rate of photoinduced electron-hole pairs significantly limited the photocatalytic activity of TiO 2 . Approaches for improving the photocatalytic performance of TiO 2 have included: (a) heteroatom doping [3]; (b) textural design [4]; and (c) heterojunction formation with metals or other semiconductors [5,6]. Among these Teflon-lined autoclave. The reaction mixture was maintained at 150 • C for 4 h. The resulting solid was rinsed with a mixture of DI water and ethanol (volume ratio: 1/2) and subsequently freeze-dried.

Fabrication of Small (16.9 ± 5.5 nm) AuNPs/TiO 2 Photocatalysts
A freshly prepared aqueous NaBH 4 solution (10 mL, 0.5 M) was rapidly added into an aqueous HAuCl 4 (0.5 mmol, 190 mL) solution under vigorous stirring. The resulting solution was stirred at room temperature (ca. 22 • C) for 2.5 h. The as-obtained AuNPs solution was combined with TiO 2 (100 mg), vortexed to ensure thorough mixing, and freeze-dried. Annealing was completed at 550 • C in a tube furnace for 6 h in air with a ramp of 5 • C min −1 . The wt% values of small AuNPs relative to TiO 2 were 0.02, 0.05, 0.1, 1.0, 2.0, and 2.1 wt% respectively.

Preparation of Dual Particle-Size AuNPs/TiO 2 Photocatalysts
TiO 2 (100 mg) was added to a 10 mL aqueous solution containing both 2.0 wt% large AuNPs and 0.1 wt% small AuNPs. The resulting solution was sonicated until thoroughly mixed, freeze-dried, and annealed in a tube furnace at 550 • C for 6 h in air with a ramp of 5 • C min −1 .

Preparation of GDC-Directed Dual Particle-Size AuNPs/TiO 2 Photocatalysts
An aqueous solution containing HAuCl 4 (0.02 M) and dicyanodiamine (0.006 M) was heated at 60 • C for 30 min to synthesize gold-dicyanodiamine composites (GDCs). The resulting solid was rinsed multiple times with DI water. The 2.1 wt% dual particle-size AuNPs/TiO 2 was also achieved by mixing appropriate amounts of GDCs and TiO 2 nanosheets, and annealing them in a tube furnace at 550 • C in air for 6 h with a ramp of 5 • C min −1 .

Photocatalytic Activity Evaluation
Photocatalytic activity was evaluated by H 2 generation from water splitting. The photocatalytic H 2 evolution reactions were carried out in a Pyrex reactor under vacuum. The temperature of the reactant solution was kept at 6 • C by flowing cooling water. Each as-prepared photocatalyst (10 mg) was dispersed in a mixture of 80 mL DI water and 20 mL triethanolamine solution. Subsequently, the resulting suspensions were illuminated by a 300 W xenon lamp (CEL-HXF 300, CEAULIGHT, Beijing, China, 350-780 nm). Gas chromatography with a thermal conductivity detector (TCD) was employed for H 2 production analysis using N 2 as carrier gas.

Electrochemical Characterization
Transient photocurrent and electrochemical impedance spectroscopy (EIS) were conducted in Na 2 SO 4 (0.1 M) using an electrochemical instrument (Gamry Interface 1010, Warminster, PA, USA) with a traditional three-electrode system under light irradiation (300 W Xe-lamp). The applied photocurrent bias was 0.9 V. The working electrode was prepared by dispersing each photocatalyst (5 mg) in 1 mL of ethanol (containing 25 µL of nafion solution) and dropped on FTO glass. Ag/AgCl and Pt electrodes were used as the reference electrode and counter electrode, respectively.

Results and Discussion
Large AuNPs/TiO 2 and small AuNPs/TiO 2 photocatalysts with different AuNPs weight ratios were synthesized to determine the optimum weight percent of AuNPs for photocatalysis. TEM was utilized to evaluate the microstructures of the prepared photocatalysts. The large AuNPs/TiO 2 composite (Figure 1a,b) had an average Au nanosphere size of 45.0 ± 9.8 nm ( Figure S1), while the small AuNPs/TiO 2 composite (Figure 1d,e) had an average Au nanosphere size of 16.9 ± 5.5 nm ( Figure S2). Both composites appeared to be well distributed onto the TiO 2 matrix. The 0.235 nm lattice spacing observed in these composites can be ascribed to the d spacing of Au (111) crystal plane, while the 0.352 nm lattice spacing belongs to the TiO 2 (101) crystal plane (Figure 1c,f) [24,25].

Electrochemical Characterization
Transient photocurrent and electrochemical impedance spectroscopy (EIS) were conducted in Na2SO4 (0.1 M) using an electrochemical instrument (Gamry Interface 1010, Warminster, PA, USA) with a traditional three-electrode system under light irradiation (300 W Xe-lamp). The applied photocurrent bias was 0.9 V. The working electrode was prepared by dispersing each photocatalyst (5 mg) in 1 mL of ethanol (containing 25 μL of nafion solution) and dropped on FTO glass. Ag/AgCl and Pt electrodes were used as the reference electrode and counter electrode, respectively.

Results and Discussion
Large AuNPs/TiO2 and small AuNPs/TiO2 photocatalysts with different AuNPs weight ratios were synthesized to determine the optimum weight percent of AuNPs for photocatalysis. TEM was utilized to evaluate the microstructures of the prepared photocatalysts. The large AuNPs/TiO2 composite (Figure 1a,b) had an average Au nanosphere size of 45.0 ± 9.8 nm ( Figure S1), while the small AuNPs/TiO2 composite (Figure 1d,e) had an average Au nanosphere size of 16.9 ± 5.5 nm ( Figure S2). Both composites appeared to be well distributed onto the TiO2 matrix. The 0.235 nm lattice spacing observed in these composites can be ascribed to the d spacing of Au (111) crystal plane, while the 0.352 nm lattice spacing belongs to the TiO2 (101) crystal plane (Figure 1c,f) [24,25]. Photocatalytic performance of all synthesized photocatalysts was evaluated by measuring H2 evolution rates from water splitting. Each photocatalyst was evaluated for 3 h to minimize experimental errors. The H2 evolution rate using large AuNPs/TiO2 composites increased up to 2 wt% AuNPs and then decreased at 3.5 wt% (Figure 2a). The highest H2 evolution rate of 4006 μmol h −1 g −1 (2 wt% AuNPs) was approximately 215 times greater than that of TiO2 (18.67 μmol h −1 g −1 ) under the same conditions. Small AuNPs/TiO2 composites exhibited increasing H2 evolution up to 1 wt% AuNPs; however, no significant increase was observed above 0.1 wt% (Figure 2b). The 0.1 wt% small AuNPs/TiO2 composites exhibited H2 production with a generation rate of 3340 μmol h −1 g −1 , which was approximately 179 times greater than that of pure TiO2.  Photocatalytic performance of all synthesized photocatalysts was evaluated by measuring H 2 evolution rates from water splitting. Each photocatalyst was evaluated for 3 h to minimize experimental errors. The H 2 evolution rate using large AuNPs/TiO 2 composites increased up to 2 wt% AuNPs and then decreased at 3.5 wt% (Figure 2a). The highest H 2 evolution rate of 4006 µmol h −1 g −1 (2 wt% AuNPs) was approximately 215 times greater than that of TiO 2 (18.67 µmol h −1 g −1 ) under the same conditions. Small AuNPs/TiO 2 composites exhibited increasing H 2 evolution up to 1 wt% AuNPs; however, no significant increase was observed above 0.1 wt% (Figure 2b). The 0.1 wt% small AuNPs/TiO 2 composites exhibited H 2 production with a generation rate of 3340 µmol h −1 g −1 , which was approximately 179 times greater than that of pure TiO 2 .
Based on the observations above, we hypothesized that a dual particle-size AuNPs/TiO 2 composite (2 wt% large AuNPs + 0.1 wt% small AuNPs) could exhibit a synergistic effect that would further increase the rate of H 2 evolution. Indeed, a hydrogen evolution rate of 5245 µmol h −1 g −1 was higher than the rate observed with either single particle-size AuNPs/TiO 2 composite (Figure 2c). Control experiments with 2.1 wt% small AuNPs/TiO 2 or 2.1 wt% large AuNPs/TiO 2 failed to achieve the H 2 evolution rate observed for the 2.1 wt% dual particle-size AuNPs/TiO 2 composite (Figure 2d). These results indicate the fabrication of both small and large AuNPs populations onto TiO 2 appears to lead to a synergistic effect for photocatalytic H 2 evolution. Based on the observations above, we hypothesized that a dual particle-size AuNPs/TiO2 composite (2 wt% large AuNPs + 0.1 wt% small AuNPs) could exhibit a synergistic effect that would further increase the rate of H2 evolution. Indeed, a hydrogen evolution rate of 5245 μmol h −1 g −1 was higher than the rate observed with either single particle-size AuNPs/TiO2 composite (Figure 2c). Control experiments with 2.1 wt% small AuNPs/TiO2 or 2.1 wt% large AuNPs/TiO2 failed to achieve the H2 evolution rate observed for the 2.1 wt% dual particle-size AuNPs/TiO2 composite ( Figure 2d). These results indicate the fabrication of both small and large AuNPs populations onto TiO2 appears to lead to a synergistic effect for photocatalytic H2 evolution. The successful fabrication of a dual particle-size 2.1 wt% AuNPs/TiO2 composite was confirmed by TEM and HRTEM images ( Figure 3). Two distinguishable AuNP size distributions, of 17.3 ± 7.5 nm and 46.3 ± 6.1 nm, were found by measuring 100 individual AuNPs ( Figure S3). Significantly, these particle-size distributions were very similar to those observed with the single particle-size composites described above. The Au (111) (0.235 nm) and TiO2 (101) (0.352 nm) crystal planes seen in Figure 3c indicated that both Au and TiO2 were successfully incorporated in the composite structure. The crystal phase of pure TiO2, 0.1 wt% small AuNPs/TiO2, 2 wt% large AuNPs/TiO2, and 2.1 wt% dual particle-size AuNPs/TiO2 were investigated using powder XRD. All XRD patterns showed several characteristic peaks at 25   The successful fabrication of a dual particle-size 2.1 wt% AuNPs/TiO 2 composite was confirmed by TEM and HRTEM images ( Figure 3). Two distinguishable AuNP size distributions, of 17.3 ± 7.5 nm and 46.3 ± 6.1 nm, were found by measuring 100 individual AuNPs ( Figure S3). Significantly, these particle-size distributions were very similar to those observed with the single particle-size composites described above. The Au (111) (0.235 nm) and TiO 2 (101) (0.352 nm) crystal planes seen in Figure 3c indicated that both Au and TiO 2 were successfully incorporated in the composite structure. The crystal phase of pure TiO 2 , 0.1 wt% small AuNPs/TiO 2 , 2 wt% large AuNPs/TiO 2 , and 2.1 wt% dual particle-size AuNPs/TiO 2 were investigated using powder XRD. All XRD patterns showed several characteristic peaks at 25.  [26,27]. Small AuNPs/TiO 2 ; however, exhibited no significant difference from TiO 2 . We hypothesize this is due to the low content (0.1 wt%) of small AuNPs on TiO 2 . Nanomaterials 2019, 9, x 6 of 14 significant difference from TiO2. We hypothesize this is due to the low content (0.1 wt%) of small AuNPs on TiO2. (d) X-ray diffraction (XRD) patterns of pure TiO2, small AuNPs/TiO2, large AuNPs/TiO2, and dual particle-size AuNPs/TiO2 hybrids.
UV-vis diffuse reflectance spectra (DRS) and Photoluminescence (PL) spectra were collected to gain insight into the relationship between optical and photocatalytic properties of the synthesized materials. As shown in Figure 4a, the photocatalysts containing AuNPs exhibited a light absorption range significantly beyond the 390 nm absorption edge of pure TiO2 [28][29][30][31]. In addition, a typical absorption peak around 580 nm can be observed from all three AuNPs/TiO2 hybrids, corresponding to the SPR of AuNPs [32]. PL spectra displayed fluorescence quenching from all three AuNPs/TiO2 hybrids (Figure 4b). These results suggest the deposition of AuNPs onto TiO2 can efficiently restrain the electron-hole recombination [33,34], which leads to faster transfer of electrons between the TiO2 and the AuNPs. In addition, the time-resolved fluorescence decay spectra of all samples were also obtained (Figure 4c). The results suggest all four photocatalysts possessed similar fluorescence lifetimes; however, changes to the band gap of TiO2 were observed after introducing different-sized AuNPs ( Figure S4), indicating coupling AuNPs onto TiO2 could increase the utilization of light [35]. The large AuNPs/TiO2 and dual particle-size AuNPs/TiO2 exhibited higher absorption intensity than that of small AuNPs/TiO2. This observation can be attributed to the stronger electron-trapping effect UV-vis diffuse reflectance spectra (DRS) and Photoluminescence (PL) spectra were collected to gain insight into the relationship between optical and photocatalytic properties of the synthesized materials. As shown in Figure 4a, the photocatalysts containing AuNPs exhibited a light absorption range significantly beyond the 390 nm absorption edge of pure TiO 2 [28][29][30][31]. In addition, a typical absorption peak around 580 nm can be observed from all three AuNPs/TiO 2 hybrids, corresponding to the SPR of AuNPs [32]. PL spectra displayed fluorescence quenching from all three AuNPs/TiO 2 hybrids (Figure 4b). These results suggest the deposition of AuNPs onto TiO 2 can efficiently restrain the electron-hole recombination [33,34], which leads to faster transfer of electrons between the TiO 2 and the AuNPs. In addition, the time-resolved fluorescence decay spectra of all samples were also obtained (Figure 4c). The results suggest all four photocatalysts possessed similar fluorescence lifetimes; however, changes to the band gap of TiO 2 were observed after introducing different-sized AuNPs ( Figure S4), indicating coupling AuNPs onto TiO 2 could increase the utilization of light [35]. The large AuNPs/TiO 2 and dual particle-size AuNPs/TiO 2 exhibited higher absorption intensity than that of small AuNPs/TiO 2 . This observation can be attributed to the stronger electron-trapping effect caused by the SPR of large AuNPs populations. Additionally, obvious reduction in PL emission intensity was observed from both the large and dual particle-size AuNPs/TiO 2 , likely resulting from the stronger electron-hole separation effect in these two photocatalysts. Both observations are consistent with the photocatalytic testing results, in which the large AuNPs/TiO 2 and dual particle-size AuNPs/TiO 2 composites exhibit higher photocatalytic efficiency than that of the small AuNPs/TiO 2 composites. However, no major optical property difference was observed between large AuNPs/TiO 2 and dual particle-sized AuNPs/TiO 2 . As such, the improved photocatalytic properties of the dual particle-size AuNPs/TiO 2 must emerge from synergistic interactions between the two AuNPs populations on the TiO 2 surface. Nanomaterials 2019, 9, x 7 of 14 the stronger electron-hole separation effect in these two photocatalysts. Both observations are consistent with the photocatalytic testing results, in which the large AuNPs/TiO2 and dual particlesize AuNPs/TiO2 composites exhibit higher photocatalytic efficiency than that of the small AuNPs/TiO2 composites. However, no major optical property difference was observed between large AuNPs/TiO2 and dual particle-sized AuNPs/TiO2. As such, the improved photocatalytic properties of the dual particle-size AuNPs/TiO2 must emerge from synergistic interactions between the two AuNPs populations on the TiO2 surface. Electrochemical measurements of photocurrent responses were collected for all four samples to further probe the enhanced photocatalytic activity of the dual particle-size AuNPs/TiO2 composite (Figure 5a). Both 2 wt% large AuNPs/TiO2 and 0.1 wt% small AuNPs/TiO2 composites demonstrated enhanced photocurrent density than that of pure TiO2, implying a higher separation efficiency of photogenerated charge carriers from large AuNPs/TiO2 and small AuNPs/TiO2 [36]. Significantly, the photocurrent density observed from 2.1 wt% dual particle-size AuNPs/TiO2 was almost three times greater than the photocurrent observed from the 2 wt% large AuNPs/TiO2 sample. This result strongly suggests that the co-existence of dual particle-size AuNPs on TiO2 streamlines charge separation and photogenerated electron transfer. EIS measurements were also carried out under light irradiation (350-780 nm) to provide additional evidence in support of this observation. EIS Nyquist plots of pure TiO2, 0.1 wt% small AuNPs/TiO2, 2 wt% large AuNPs/TiO2, and 2.1 wt% dual particlesize AuNPs/TiO2 displayed gradually decreased semicircle radius (Figure 5b). Since the resistance of charge transfer was directly proportional to the semicircle radius of the Nyquist plot [36,37], the introduction of 2.1 wt% dual particle-size AuNPs onto TiO2 could enhance the charge transfer separation and inhibit the recombination of photogenerated electron-hole pairs. Taken together, it appears the population of small AuNPs dispersed on TiO2 can readily capture electrons generated from the conduction band of TiO2; this phenomenon leads to the generation of more electron-hole pairs (narrower band gap) and much easier electron migration [16,38]. Secondly, the large AuNPs offer intensive local electric fields through SPR [39]. Hot electrons formed can be injected into the conduction band of TiO2, which greatly facilitates electron transfer and light use efficiency.  Electrochemical measurements of photocurrent responses were collected for all four samples to further probe the enhanced photocatalytic activity of the dual particle-size AuNPs/TiO 2 composite (Figure 5a). Both 2 wt% large AuNPs/TiO 2 and 0.1 wt% small AuNPs/TiO 2 composites demonstrated enhanced photocurrent density than that of pure TiO 2 , implying a higher separation efficiency of photogenerated charge carriers from large AuNPs/TiO 2 and small AuNPs/TiO 2 [36]. Significantly, the photocurrent density observed from 2.1 wt% dual particle-size AuNPs/TiO 2 was almost three times greater than the photocurrent observed from the 2 wt% large AuNPs/TiO 2 sample. This result strongly suggests that the co-existence of dual particle-size AuNPs on TiO 2 streamlines charge separation and photogenerated electron transfer. EIS measurements were also carried out under light irradiation (350-780 nm) to provide additional evidence in support of this observation. EIS Nyquist plots of pure TiO 2 , 0.1 wt% small AuNPs/TiO 2 , 2 wt% large AuNPs/TiO 2 , and 2.1 wt% dual particle-size AuNPs/TiO 2 displayed gradually decreased semicircle radius (Figure 5b). Since the resistance of charge transfer was directly proportional to the semicircle radius of the Nyquist plot [36,37], the introduction of 2.1 wt% dual particle-size AuNPs onto TiO 2 could enhance the charge transfer separation and inhibit the recombination of photogenerated electron-hole pairs. Taken together, it appears the population of small AuNPs dispersed on TiO 2 can readily capture electrons generated from the conduction band of TiO 2 ; this phenomenon leads to the generation of more electron-hole pairs (narrower band gap) and much easier electron migration [16,38]. Secondly, the large AuNPs offer intensive local electric fields through SPR [39]. Hot electrons formed can be injected into the conduction band of TiO 2 , which greatly facilitates electron transfer and light use efficiency.  Figure S5 shows the N2 adsorption-desorption isotherms of all four samples. All isotherms featured typical type-IV curves; however, the pure TiO2 sample was found to have a specific surface area of 17.09 m 2 g −1 . The small AuNPs/TiO2, large AuNPs/TiO2, and dual particle-size AuNPs/TiO2 exhibited BET surface areas of 58.82 m 2 g −1 , 43.23 m 2 g −1 , and 11.77 m 2 g −1 , respectively. Surface area results suggest the photocatalytic activity enhancement of dual particle-sized AuNPs/TiO2 did not originate from a change in surface area [40]. XPS measurements were also employed to analyze the surface electron states in the dual particle-size AuNPs/TiO2 composites. The survey XPS spectrum of the dual particle-size AuNPs/TiO2 and pure TiO2 is shown in Figure 6a. The weak Au signal was mainly due to the low content of AuNPs on the TiO2 surface. The peak position of Ti 2p (458.6 and 464.3 eV) in AuNPs/TiO2 composites shows no shift compared to pure TiO2 (Figure 6b). A 0.3 eV negative shift in the O 1s peaks at 532.9 and 530.2 eV (Figure 6c), as well as a 0.8 eV negative shift from standard Au 4f 0 5/2 and Au 4f 0 7/2 (Figure 6d), were observed in the dual particle-size AuNPs/TiO2 composites. These perturbations were consistent with an increase in electron density for Au and O 2− . XPS results can be taken as direct evidence for the formation of a Schottky barrier between the AuNPs and TiO2 [36,41,42].  Figure S5 shows the N 2 adsorption-desorption isotherms of all four samples. All isotherms featured typical type-IV curves; however, the pure TiO 2 sample was found to have a specific surface area of 17.09 m 2 g −1 . The small AuNPs/TiO 2 , large AuNPs/TiO 2 , and dual particle-size AuNPs/TiO 2 exhibited BET surface areas of 58.82 m 2 g −1 , 43.23 m 2 g −1 , and 11.77 m 2 g −1 , respectively. Surface area results suggest the photocatalytic activity enhancement of dual particle-sized AuNPs/TiO 2 did not originate from a change in surface area [40]. XPS measurements were also employed to analyze the surface electron states in the dual particle-size AuNPs/TiO 2 composites. The survey XPS spectrum of the dual particle-size AuNPs/TiO 2 and pure TiO 2 is shown in Figure 6a. The weak Au signal was mainly due to the low content of AuNPs on the TiO 2 surface. The peak position of Ti 2p (458.6 and 464.3 eV) in AuNPs/TiO 2 composites shows no shift compared to pure TiO 2 (Figure 6b). A 0.3 eV negative shift in the O 1s peaks at 532.9 and 530.2 eV (Figure 6c), as well as a 0.8 eV negative shift from standard Au 4f 0 5/2 and Au 4f 0 7/2 (Figure 6d), were observed in the dual particle-size AuNPs/TiO 2 composites. These perturbations were consistent with an increase in electron density for Au and O 2− . XPS results can be taken as direct evidence for the formation of a Schottky barrier between the AuNPs and TiO 2 [36,41,42]. Based on the results described above, we wondered if analogous H 2 evolution rates could be observed for dual-particle sized AuNPs/TiO 2 composites prepared by a different synthetic method. Such a result would provide evidence that dual-particle fabrication may be beneficial for AuNPs/TiO 2 composites regardless of the chosen synthetic strategy. With this in mind, we prepared a 2.1 wt% dual-sized AuNPs/TiO 2 composite by introducing GDCs onto the TiO 2 matrix. TEM images of the as-prepared GDCs revealed a uniform spherical structure (diameter~450 nm) with homogenous incorporation of 3-5 nm AuNPs ( Figure S6a). After annealing GDCs to TiO 2 , both large-sized AuNPs and small-sized AuNPs were observed on the TiO 2 surface (Figure 7). AuNP size distributions were determined to be 45.7 ± 2.9 nm and 16.3 ± 2.8 nm by measuring 100 individual AuNPs ( Figure S7). Notably, these two size distributions were very similar to the size distributions of AuNPs on the previously synthesized dual particle-size AuNPs/TiO 2 composites. XRD experiments ( Figure S8) revealed characteristic Au peaks of (111), (200), (220), and (311) crystal planes, which confirms the successful incorporation of AuNPs onto the TiO 2 matrix. Photocatalytic H 2 experiments using GDC-directed dual particle-size AuNPs/TiO 2 photocatalysts revealed a H 2 evolution rate slightly higher (6150 µmol h −1 g −1 ) than that observed from the previously prepared dual particle-size AuNPs/TiO 2 photocatalysts (Figure 8a). All additional characterization experiments indicated both dual particle-size AuNPs/TiO 2 composites have analogous electrochemical properties (Figure 8b,c). BET surface area measurements of the 2.1 wt% GDC-directed AuNPs/TiO 2 composite indicated no alternation of specific surface area after AuNPs incorporation, similar to that of 2.1 wt% dual particle-size AuNPs/TiO 2 ( Figure S9). Binding energy shifts and intensity changes observed from XPS spectra ( Figure S10) showed strong interactions between AuNPs and TiO 2 in the 2.1 wt% GDC-directed AuNPs/TiO 2 composite for photocatalytic enhancement, consistent with the result obtained from the 2.1 wt% dual particle-size AuNPs/TiO 2 photocatalysts. Nanomaterials 2019, 9, x 9 of 14 Figure 6. (a) Survey X-ray photoelectron spectroscopy (XPS) spectrum and high-resolution spectra of (b) Ti 2p, (c) O 1s, and (d) Au 4f for pure TiO2 and dual particle-size AuNPs/TiO2 photocatalysts.  Figure 6. (a) Survey X-ray photoelectron spectroscopy (XPS) spectrum and high-resolution spectra of (b) Ti 2p, (c) O 1s, and (d) Au 4f for pure TiO 2 and dual particle-size AuNPs/TiO 2 photocatalysts.
A feasible mechanism for the enhanced H 2 evolution performance from both dual particle-size AuNPs/TiO 2 composites is presented in Figure 9. Initially, photo-excited electron-hole pairs are generated from the conduction and valence band of TiO 2 under light irradiation. The generated electrons are then readily captured and transferred to AuNPs [34,43], which contributes to the formation of a Schottky barrier between AuNPs and TiO 2 . Specifically, small-sized AuNPs dispersed on TiO 2 can promote the capture of electrons generated in the TiO 2 conduction band, which forms more electron-hole pairs and facilitate the charge transfer [44,45]. Concurrently, an intensive local electric field near large-sized AuNPs can be formed though SPR [15]. Consequently, the hot electrons can be injected into the conduction band of TiO 2 and significantly boost the charge transfer [19]. As a result, the captured electrons in small sized AuNPs, as well as the transferred electrons from large sized AuNPs to TiO 2 , can both synergistically promote the reduction of H 2 O to H 2 (holes are consumed by sacrificial agent TEOA). Based on the results described above, we wondered if analogous H2 evolution rates could be observed for dual-particle sized AuNPs/TiO2 composites prepared by a different synthetic method. Such a result would provide evidence that dual-particle fabrication may be beneficial for AuNPs/TiO2 composites regardless of the chosen synthetic strategy. With this in mind, we prepared a 2.1 wt% dual-sized AuNPs/TiO2 composite by introducing GDCs onto the TiO2 matrix. TEM images of the asprepared GDCs revealed a uniform spherical structure (diameter ~450 nm) with homogenous incorporation of 3-5 nm AuNPs ( Figure S6a). After annealing GDCs to TiO2, both large-sized AuNPs and small-sized AuNPs were observed on the TiO2 surface ( Figure 7). AuNP size distributions were determined to be 45.7 ± 2.9 nm and 16.3 ± 2.8 nm by measuring 100 individual AuNPs ( Figure S7). Notably, these two size distributions were very similar to the size distributions of AuNPs on the previously synthesized dual particle-size AuNPs/TiO2 composites. XRD experiments ( Figure S8) revealed characteristic Au peaks of (111), (200), (220), and (311) crystal planes, which confirms the successful incorporation of AuNPs onto the TiO2 matrix. Photocatalytic H2 experiments using GDCdirected dual particle-size AuNPs/TiO2 photocatalysts revealed a H2 evolution rate slightly higher (6150 μmol h −1 g −1 ) than that observed from the previously prepared dual particle-size AuNPs/TiO2 photocatalysts (Figure 8a). All additional characterization experiments indicated both dual particlesize AuNPs/TiO2 composites have analogous electrochemical properties (Figure 8b,c). BET surface area measurements of the 2.1 wt% GDC-directed AuNPs/TiO2 composite indicated no alternation of specific surface area after AuNPs incorporation, similar to that of 2.1 wt% dual particle-size AuNPs/TiO2 ( Figure S9). Binding energy shifts and intensity changes observed from XPS spectra ( Figure S10) showed strong interactions between AuNPs and TiO2 in the 2.1 wt% GDC-directed AuNPs/TiO2 composite for photocatalytic enhancement, consistent with the result obtained from the 2.1 wt% dual particle-size AuNPs/TiO2 photocatalysts. A feasible mechanism for the enhanced H2 evolution performance from both dual particle-size AuNPs/TiO2 composites is presented in Figure 9. Initially, photo-excited electron-hole pairs are generated from the conduction and valence band of TiO2 under light irradiation. The generated electrons are then readily captured and transferred to AuNPs [34,43], which contributes to the formation of a Schottky barrier between AuNPs and TiO2. Specifically, small-sized AuNPs dispersed on TiO2 can promote the capture of electrons generated in the TiO2 conduction band, which forms more electron-hole pairs and facilitate the charge transfer [44,45]. Concurrently, an intensive local electric field near large-sized AuNPs can be formed though SPR [15]. Consequently, the hot electrons can be injected into the conduction band of TiO2 and significantly boost the charge transfer [19]. As a result, the captured electrons in small sized AuNPs, as well as the transferred electrons from large sized AuNPs to TiO2, can both synergistically promote the reduction of H2O to H2 (holes are consumed by sacrificial agent TEOA).   A feasible mechanism for the enhanced H2 evolution performance from both dual particle-size AuNPs/TiO2 composites is presented in Figure 9. Initially, photo-excited electron-hole pairs are generated from the conduction and valence band of TiO2 under light irradiation. The generated electrons are then readily captured and transferred to AuNPs [34,43], which contributes to the formation of a Schottky barrier between AuNPs and TiO2. Specifically, small-sized AuNPs dispersed on TiO2 can promote the capture of electrons generated in the TiO2 conduction band, which forms more electron-hole pairs and facilitate the charge transfer [44,45]. Concurrently, an intensive local electric field near large-sized AuNPs can be formed though SPR [15]. Consequently, the hot electrons can be injected into the conduction band of TiO2 and significantly boost the charge transfer [19]. As a result, the captured electrons in small sized AuNPs, as well as the transferred electrons from large sized AuNPs to TiO2, can both synergistically promote the reduction of H2O to H2 (holes are consumed by sacrificial agent TEOA). Figure 9. Schematic diagram for possible mechanism of dual particle-size AuNPs/TiO2 composites for H2 evolution.

Conclusions
In summary, we successfully synthesized a 2.1 wt% dual particle-size AuNPs/TiO2 composite and investigated the synergistic effects of the two different AuNPs size distributions on photocatalytic H2 production enhancement. The 2.1 wt% dual particle-size AuNPs/TiO2 composites not only display a H2 evolution efficiency 281 times greater than TiO2 alone, but also outperform the   Figure 9. Schematic diagram for possible mechanism of dual particle-size AuNPs/TiO 2 composites for H 2 evolution.

Conclusions
In summary, we successfully synthesized a 2.1 wt% dual particle-size AuNPs/TiO 2 composite and investigated the synergistic effects of the two different AuNPs size distributions on photocatalytic H 2 production enhancement. The 2.1 wt% dual particle-size AuNPs/TiO 2 composites not only display a H 2 evolution efficiency 281 times greater than TiO 2 alone, but also outperform the AuNPs/TiO 2 composites containing small or large AuNPs at a total concentration of 2.1 wt% AuNPs. These synergistic effects appear to be attributed to increased electron-hole pairs promoted by small sized AuNPs, as well as the enhanced electron transfer facilitated through the SPR of large AuNPs. Significantly, a comparable photocatalytic efficiency enhancement can be also observed from dual particle-size AuNPs/TiO 2 photocatalysts synthesized through a different GDC-directed synthetic method. The synergistic effects observed here from dual particle-size AuNPs composites may open a new pathway for designing other efficient photocatalysts in the future.

Conflicts of Interest:
The authors declare no conflict of interest.