Solid-Phase Photocatalysts: Physical Vapor Deposition of Au

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Solid-Phase Photocatalysts: Physical Vapor Deposition of Au Nanoislands on Porous TiO Films for Millimolar HO Production within a Few Minutes 2

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Kihyeun Kim, Jiyoon Park, Hyeonghun Kim, Gun Young Jung, and Min-Gon Kim ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b02269 • Publication Date (Web): 03 Sep 2019 Downloaded from pubs.acs.org on September 3, 2019

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Solid-Phase Photocatalysts: Physical Vapor Deposition of Au Nanoislands on Porous TiO2 Films for Millimolar H2O2 Production within a Few Minutes Kihyeun Kim†, Jiyoon Park‡, Hyeonghun Kim‡, Gun Young Jung‡, and Min-Gon Kim†* †

Department of Chemistry and ‡School of Materials Science and Engineering, Gwangju Institute

of Science and Technology (GIST), Gwangju 61005, Republic of Korea *E-mail: [email protected] (M.-G.K.).

ACS Paragon Plus Environment

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ABSTRACT The production of hydrogen peroxide (H2O2) using photocatalytic nanoparticles is an emerging field with applications in organic synthesis, biosensors, and fuel cells. Colloidal photocatalysts are, however, limited in their applications due to poor efficiency and stability. In this study, millimolar production of H2O2 was achieved within 5 min using solid-phase photocatalysts—Au nanoislands (NIs) on porous TiO2 films—without supplying O2 or stirring the solution. Au/TiO2 showed an almost 80-fold greater productivity than TiO2, which can be explained by considering two factors. First, the physical-vapor-deposited Au resulted in the formation of Au NIs of various sizes on TiO2, whose work functions were size-dependent. Thus, the combination of small Au NIs, TiO2, and large Au NIs allowed the introduction of potential gradients, and thus the inherent potential barriers at the small Au NI/TiO2 junctions would be reduced, thereby minimizing the recombination of electron–hole pairs. Second, the porous TiO2 films may effectively scatter UV light, leading to enhanced electron–hole pair generation. The inherent properties of the solidphase photocatalysts could also circumvent stability issues caused by aggregation. These solidphase photocatalysts should facilitate the development of high-efficiency H2O2 generation and promote technology based on H2O2-mediated processes.

KEYWORDS: hydrogen peroxide, solid-phase photocatalysts, titanium dioxide, size-dependent work function, heterojunction photocatalysts


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Table of Contents graphic 2H+ + O2 Large Au island H2O2

5.1

2H+ + O2

e e

Small Au island TiO2

4.2

e

5.1

e EF

h h

HA HAox

H2O2

AuSmall TiO2

AuLarge

HA: hole acceptor


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INTRODUCTION Hydrogen peroxide (H2O2) plays an important role in various fields, such as organic synthesis, enzyme-mediated reactions for biosensors, and is an ideal alternative to H2 as an energy carrier in fuel cells.1–3 In addition to the conventionally used anthraquinone method, which has safety issues and involves complicated procedures,3 photo-induced H2O2 production is possible using semiconductor materials. This method has attracted significant attention because it is a direct synthetic method using only water, molecular oxygen, and light.3,4 Among the various semiconductor materials reported to date, colloidal titanium dioxide (TiO2) particles have been intensively investigated for the production of H2O2 using light due to their high photoreactivity and stability, even in harsh environments.5–7 However, photogenerated H2O2 has thus far only been produced in micromolar amounts using TiO2, due to the decomposition of H2O2 and the recombination of photogenerated electron–hole pairs in TiO2.6–8 One method to address the above problems is the use of Au-loaded TiO2 nanoparticles (NPs), which have been reported to enhance the photocatalytic production of H2O2,9,10 via the formation of an internal potential gradient at the metal/semiconductor junctions (e.g. a Schottky junction), thus preventing the recombination of photogenerated electron–hole pairs.11,12 Loading an Au/Ag alloy onto the TiO2 NPs should lower the Schottky barrier at the junction, resulting in further enhancement of H2O2 production to the mM level.12 Additional heterojunctions and multiple junctions of materials on TiO2 NPs have subsequently been explored to maximize the efficiency of H2O2 production by adjusting the alignment of the work functions and Fermi levels of the materials.13–16 However, a number of challenges remain in the field of photocatalytic H2O2 generation and its applications. First, the production of mM levels of H2O2 is a lengthy process that takes


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over an hour;9,12–15 this represents an obstacle to H2O2-mediated applications such as biosensors and fuel cells. Therefore, photocatalysts that enable the rapid production of H2O2 are required. Second, colloidal photocatalysts, contained within solution media, used for H2O2 generation have limitations in portable applications like point-of-care biosensors,17 because solutions are not portable; in addition, the colloids are easily aggregated under non-optimum conditions.18 Thus, photocatalysts that exhibit high performance and stability under a range of conditions are necessary to broaden the scope of H2O2-mediated applications. Herein, we report solid-phase photocatalytic films based on the physical vapor deposition of Au nanoislands (NIs) onto porous TiO2 films for millimolar H2O2 generation within a few minutes. Au physical vapor deposition produced Au NIs with a range of sizes on the TiO2 surface. These NIs were probably responsible for the fast H2O2 production, due to the formation of potential gradients via the combination of small Au NIs, TiO2, and large Au NIs. Moreover, the inherent potential barriers at small Au NI/TiO2 junctions were lowered. These suggested mechanisms were attributed to the size-dependent work function of the metal NIs. In addition, the porous structure of the TiO2 films can result in strong UV light scattering, leading to increased generation of electron–hole pairs in TiO2. In turn, these phenomena may promote H2O2 generation. Furthermore, the solid-phase photocatalytic films do not undergo aggregation under any conditions, and are thus highly advantageous for portable applications, making the method readily applicable to various fields.

RESULTS AND DISCUSSION Figure 1a illustrates the procedure employed to prepare the solid-phase photocatalytic films composed of Au NIs on porous TiO2. TiO2 paste diluted in 2-methoxyethanol was spin-coated


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onto a glass substrate, which was then soft-baked and sintered in air, at 125 and 550 °C, respectively, to form porous TiO2 films. The metal NIs were then formed on the TiO2 films via physical vapor deposition using evaporators. The crystalline phase of the sintered TiO2 film was revealed to be anatase by X-ray diffraction (XRD) analysis (Figure S1 in the supporting information). Scanning electron microscopy (SEM) was employed to confirm the porous structures and thicknesses of the sintered TiO2 films (Figures 1b,c, respectively). The TiO2 NPs appeared to form a porous film with a thickness ~140 nm after the sintering process. Figures 1d,e show the transmission electron microscopy (TEM) images of the Au NIs formed by physical vapor deposition. To observe the Au NIs loaded on the TiO2 films, the Au/TiO2 films were detached from the glass substrates and placed on a TEM grid. The TEM results demonstrated that Au NIs of various sizes (2–20 nm) were formed on the surfaces of the TiO2 films, as shown in Figures 1d,e. Crosssectional TEM images of Au NIs on TiO2 films were also obtained to characterize the number of Au NIs per unit area of TiO2, which was found to be 0.012 per nm2 (Figure S2). For these measurements, the Au NI/TiO2 specimen was prepared on Si wafer, rather than glass, by focused ion beam (FIB) milling; the use of the Si wafer was because conducting substrates is highly suitable for the FIB milling. In addition, plasmonic light absorption was observed at 560 nm due to the absorbance by the Au NIs on the TiO2 films (Figure S3). To evaluate the photocatalytic performance of the prepared metal/TiO2 films in H2O2 production, tetramethylbenzidine (TMB) was used as a model probe molecule. TMB is colorless but turns blue upon reacting with peroxidase in the presence of H2O2. The observed absorbance at 650 nm (Figure S4) was attributed to the peroxidase/TMB/H2O2 reaction. Calibration curves were obtained by injecting known concentrations of H2O2 in 10-mM citrate buffer (pH 3.8) into a


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mixed solution of peroxidase (5 mU mL−1 and 50 mU mL−1) and TMB (0.1 mg mL−1) to determine the photo-generated H2O2 concentration; these measurements have been described in further detail in the supporting information (Figure S5). Figure 2a shows the results of colorimetric reactions using a bare porous TiO2 film and Au-, Ag-, Pt-, and Pd-loaded porous TiO2 films. A citrate buffer (10 mM, pH 3.8) containing 5% ethanol was dropped onto the films, which were subsequently exposed to UV irradiation (365 nm) for 1 min. It should be noted that all the photocatalytic H2O2 production experiments reported in this work were conducted using the 10-mM, pH 3.8 citrate buffer solution containing 5% ethanol unless otherwise specified. TMB (0.1 mg mL−1) and peroxidase (5 mU mL−1 for Au/TiO2 or 50 mU mL−1 for bare TiO2 and Ag, Pt, and Pd/TiO2) were used for H2O2 quantification. Au/TiO2 demonstrated superior performance compared to the other metals, with a 56.6-fold higher performance compared to the bare TiO2 film at 1 min. To confirm that the TMB-mediated colorimetric reaction indeed occurred due to the production of H2O2, the same experiment was performed using the Au/TiO2 substrate without peroxidase. As shown in Figure S6, no colordeveloping reaction occurred in the absence of peroxidase. Thus, it can be concluded that the colorimetric reaction occurred due to the presence of photocatalytically generated H2O2. In addition, it was confirmed that the citrate salts were essentially incapable of acting as hole acceptors but promoted H2O2 production in the presence of ethanol (Figure S7). There have been some reports that salts in buffer solutions could prevent H2O2 decomposition at the surfaces of photocatalysts.19,20 The role of salts in buffer solutions in the presence of ethanol is currently being explored and characterized in detail in our laboratory: we have observed that the H2O2 production and decomposition rates differed depending on the type of buffer solutions containing


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5% ethanol (Figure S8). Notably, H2O2 production in buffer solutions could be highly advantageous for various applications in biosensors and biological systems. Figure 2b shows the effect of UV irradiation time on the photocatalytic production of H2O2. The bare porous TiO2 films produced only a few micromoles of H2O2 over 30 min of UV irradiation; an enlarged version of the bare TiO2 plot is shown in Figure S9. As for the solidphase Au/TiO2 film, the concentration of photocatalytically produced H2O2 reached the mM level. The H2O2 concentration increased with irradiation time up to 20 min, and then plateaued from 20 to 30 min; the H2O2 production yields were enhanced by factors of 56.6, 88.4, 86.5, and 80.6 compared to bare TiO2 at 1 min, 10 min, 20 min, and 30 min, respectively. After 30 min, the H2O2 concentration decreased (Figure S10a), which was expected because the H2O2 decomposition rate (originating from H2O2 + 2ecb- + 2H+ → 2H2O and/or H2O2 + 2hvb+ → O2 + 2H+)21 surpassed the H2O2 production rate (originating from O2 + 2ecb- + 2H+ → H2O2 or/and 2H2O + 2hvb+ → H2O2 + 2H+; former is more likely to happen)21,22. In detail, insufficient supply of O2 seems to

have been responsible for the decreased H2O2 production rather than lack of ethanol (the hole acceptor) because the tendency to decrease after 30 min was observed regardless of ethanol concentration, as shown in Figure S10b for concentrations in the range of 1–10% v/v; the effects of the lack of ethanol are observed when the ethanol concentration drops to 0.1% v/v. As for the supply of O2, H2O2 production was increased under O2 ambient condition, compared to air ambient condition (Figure S10c). Furthermore, the amount of aldehyde produced via ethanol oxidation, which corresponds to H2O2 production, was also observed (Figure S11); a proposed reaction pathway for H2O2 production is demonstrated in Figure S12.12 Although it is difficult to directly compare the performances of the solid-phase photocatalysts with the colloidal ones, it should be noted that the solid-phase Au NI/TiO2 photocatalyst in this work produced mM level


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of H2O2 within 5 min without O2 injection or stirring the reaction solution, and an initial H2O2 production rate of 30 µmol mg–1 h–1 was achieved; the results are comparable to those obtained for other photocatalysts in previously reported studies (Table S1). Owing to the films’ ability to rapidly produce H2O2 in significant quantities without requiring complicated set-ups or equipment, the films are well-suited for use in H2O2-mediated applications. In 2012, Tsukamoto et al. reported that Au/TiO2 NPs exhibited approximately three-fold higher performance than TiO2 NPs in the production of H2O2 in deionized water (DI water).12 By contrast, in this study, a significantly greater performance enhancement (~80×) was obtained. Although all the principal experiments on photocatalytic H2O2 production in this study were conducted using 10-mM citrate buffer (pH 3.8) containing 5% ethanol, the use of DI water containing 5% ethanol apparently showed a similar degree of enhancement, comparing the use of Au NI/TiO2 with that of TiO2, as shown in Figure S13. Thus, regardless of the reaction solution, the reasons for the enhancement when using Au NI/TiO2 are probably the two key mechanisms shown in Figures 3a,b. First, the variously sized Au NIs (2–20 nm) effectively formed potential gradients through the TiO2, and the inherent potential barriers at small Au NI/TiO2 junctions decreased, as shown in Figure 3a. According to previous reports, the work functions of metal NPs vary depending on their size.23–27 For example, Zhang et al. reported that smaller Au NPs displayed lower work functions than larger Au NPs on n-Si.26 Therefore, the junctions between the typically n-type TiO2 used herein and the large and small Au NIs should have, respectively, high and low potentials, due to the size-dependent differences in the work functions of the Au NIs. Thus, potential gradients would be formed via the combination of small Au NIs, TiO2, and large Au NIs; in addition, the inherent potential barriers at the small Au NI/TiO2 junctions would be lowered. When potential gradients are generated in metal (i)/semiconductor/metal (ii) systems


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(where metal (i) and metal (ii) have different work functions), photogenerated electrons can overcome the inherent potential barriers at the junctions more effectively due to the potential gradients,28 allowing photogenerated electrons to be easily extracted from the photocatalysts. Second, the densely organized porous Au/TiO2 film exhibited strong UV light scattering (Figure 3b),5,29,30 which could result in enhanced light absorption, and in an increased electron–hole pair population. Thus, these two key mechanistic explanations can account for the superior H2O2 production performance of the solid-phase Au/TiO2 films described herein. Solid-phase photocatalysts have significant advantages in terms of their reusability, achieved typically by a simple washing process; by comparison, colloidal photocatalysts require centrifuge equipment to reuse the particles.18 Their potential reusability facilitates the growth of the use of solid-phase photocatalysts in practical photocatalysis-mediated applications. The reusability of the studied photocatalytic films was therefore evaluated; the films were washed by spraying them with fresh deionized water. The photocatalytic films were found to have stable and reproducible performances when evaluated over multiple reuse cycles (Figure 4). In addition, the films are inherently incapable of aggregation thanks to their solid-state nature. Hence, the ability to reuse the photocatalytic films without an energy-consuming process and without unwanted aggregation could be useful for their practical application to various fields, such as organic synthesis, biosensors, and fuel cells.

CONCLUSIONS In conclusion, we have demonstrated the superior catalytic behavior of solid-phase heterojunction photocatalysts for H2O2 production based on physical vapor deposition of Au onto porous TiO2 films. These films were shown to efficiently produce mM levels of H2O2 within


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5 min, which represented an approximately 80-fold enhancement in performance with respect to that of the bare TiO2 films. These results were obtained in the absence of O2-saturated conditions and without stirring the reaction solution. This superior performance was attributed to two factors. First, Au NIs of various sizes were formed on the porous TiO2 surfaces, effectively generating potential gradients within the TiO2 and lowering the inherent potential barrier at small Au NI/TiO2 junctions, owing to the size-dependent work function of metal NIs on semiconductor materials. Second, the porous structures of the Au NI-loaded TiO2 films resulted in a high degree of UV light scattering. The approach of using metal Nis with size-dependent work functions and a solid-phase platform could pave the way for their use in high-efficiency H2O2 production, as well as in H2O2-mediated applications including organic synthesis, biosensors, and fuel cells. As an example of these applications, work is currently in progress to apply these photocatalysts to the development of portable biosensors for point-of-care testing. To this end, the effect of salts in the buffer solutions on the properties of Au/TiO2 for photocatalytic H2O2 production is also being examined; appropriate compositions of reaction solutions for specific applications are being explored.

EXPERIMENTAL SECTION Materials. TiO2 paste (18NR-T titania paste) was purchased from Greatcell Solar, Australia. Citric acid (ACS reagent, ≥99.5%), trisodium citrate dihydrate (compliant with USP testing specifications), 3,3ʹ,5,5ʹ-tetramethylbenzidine (≥99%), 2-methoxyethanol (anhydrous, 99.8%), and sulfuric acid (ACS reagent, 95.0–98.0%) were obtained from Sigma-Aldrich. Peroxidase (PEO-301, 110 U mg−1) was purchased from Toyobo Co., Ltd. (OSK, Japan). Hydrogen peroxide (35% w/w aq. soln., stab.) was purchased from Alfa Aesar. Ethanol (HPLC


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grade) and phosphate-buffered saline (PBS, pH 7.4) were purchased from Duksan (Ansan, South Korea) and Biosesang (Seongnam, South Korea), respectively. Fabrication procedure for the photocatalytic films. Glass slides (1.5 cm × 1.0 cm) were cleaned using piranha solution (H2SO4:H2O2 = 3:1) for 30 min in a 65 °C oven and washed using DI water. TiO2 paste diluted with 2-methoxyethanol in a weight ratio of 1:5 was spincoated onto the glass slides at 4,000 rpm for 30 s. To evaporate the solvent, the TiO2-pastecoated glass slides were heated on a hot plate at 125 °C for 5 min. Sintering was then performed in a furnace at 550 °C for 1 h under air to eliminate any organic components and coarse TiO2 nanoparticles, yielding porous TiO2 films on the glass substrate. To form metal NPs on the porous TiO2 films, a 3-nm-thick layer of Au, Ag, Pd, or Pt was deposited using a thermal or ebeam evaporator at a rate of 0.2 Å s−1 (thermal evaporation was used for Au and Ag, and e-beam evaporation for Pd and Pt). Instruments. The porous bare TiO2 and Au-loaded TiO2 films were characterized using X-ray diffraction analysis (XRD, D/Max-2500, Rigaku), field-emission scanning electron microscopy (FE-SEM, Hitachi S-4700, Japan), and transmission electron microscopy (TEM, Tecnai G2 F30 S-Twin, FEI, OR, USA). The Au/TiO2 films were weighed using a microbalance (Mettler Toledo, XP6 Microbalance, USA). Following the colorimetric reaction, absorbance was measured using a Cytation 5 instrument (BioTek Instruments, Inc., USA). Quantitative analysis of the photocatalytically generated H2O2. Citrate buffer (50 μL, 10 mM, pH 3.8) containing 5% (v/v) ethanol was dropped onto the metal/TiO2 films (1.5 cm × 1.0 cm), and H2O2 was then produced upon irradiating the films using a 365-nm UV lamp (UVITEC, UK). The light flux of the UV lamp was 3.15 mW cm−2 (320–380 nm), measured using a digital radiometer (GILTRON GT-510, Taiwan), and the UV light irradiated the entire


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area of the films from 1 cm above the films. During UV exposure, the film was exposed to ambient air (open system), and the relative humidity was maintained at 100% to prevent evaporation of the solution. The buffer containing the produced H2O2 (20 µL) was extracted and injected into a 384-well plate containing a mixture of TMB (20 μL, 0.1 mg mL−1 in 10 mM citrate buffer; pH 3.8) and peroxidase (20 μL, 50 mU mL−1 in PBS for bare TiO2, Ag/TiO2, Pt/TiO2, and Pd/TiO2 or 5 mU mL−1 in PBS for Au/TiO2) for quantitative analysis. In addition, the peroxidase (50 mU mL−1 in PBS) was used to quantify low concentrations of H2O2 (≤0.1 mM) produced by Au/TiO2, i.e., H2O2 produced using only DI water or buffer solution without ethanol. The absorbance of the colored products was dependent on the H2O2 concentration, as measured using the Cytation 5 instrument. (To obtain reliable results, the starting time of the absorbance measurement was recorded because the TMB reaction was time-dependent.) All experiments, including the UV irradiation and colorimetric reactions, were conducted at room temperature.


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ORCID Gun Young Jung: 0000-0003-1163-8651 Min-Gon Kim: 0000-0002-3525-0048 Author Contributions The manuscript was written through contributions from all the authors. All the authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. Supporting Information. The supporting information is available free of charge via the Internet at http://pubs.acs.org. XRD analysis of sintered TiO2 film, absorbance spectra of TiO2 and Au NI/TiO2 films, absorbance after TMB reaction, calibration curves for reference, colorimetric TMB reaction without peroxidase (a control experiment to prove the H2O2-mediated TMB reaction), H2O2 production using bare TiO2 films, H2O2 production under various conditions, and aldehyde production using Ai NI/TiO2 films, and comparison of the performances of our works and others. (PDF)


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ACKNOWLEDGMENT This work was financially supported by grants from the Global Research Laboratory (GRL) Program (NRF-2013K1A1A2A02050616) and the Mid-career Researcher Program (NRF2017R1A2B3010816) through a National Research Foundation grant funded by the Ministry of Science, ICT, and Future Planning.


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ACS Catalysis

(a)

Metal nanoislands

TiO2 film

TiO2 NP

Glass Spin coating of TiO2 paste

(b)

Air annealing

(c)

Metal deposition

(d)

(e)

Au nanoislands

TiO2

140 nm

200 nm

200 nm

Figure 1. (a) Schematic illustration of the fabrication process for the solid-phase photocatalytic films. The metal NIs were deposited using a thermal or e-beam evaporation; the deposition thickness was 3 nm, as observed by thickness monitors. (b) and (c) Top and cross-sectional SEM images, respectively, of the bare porous TiO2 films after annealing. (d) and (e) TEM images of Au NPs on porous the TiO2 surfaces; the relatively dark areas represent Au NIs.


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(b)

0.40 0.36 0.32 0.28 0.24 0.20 0.16 0.12 0.08 0.04 0.00

H2O2 concentration (mM)

(a)

iO 2 /T Pd iO 2 /T Pt O 2 i /T Ag iO 2 /T

Au

O2 Ti

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


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1.8 1.5 1.2 0.9

Bare TiO2 film

0.6

Au/TiO2 film

0.3 0.0 0

5 10 15 20 25 30 UV irradiation time (min)

Figure 2. (a) H2O2 production performances of the various solid-phase photocatalysts under UV illumination for 1 min. (b) Time-dependent H2O2 production using bare TiO2 and Au/TiO2 films.


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ACS Catalysis

(a)

2H+ + O2 Large Au island

H2O2

5.1 eV

2H+ + O2

e e

Small Au island TiO2

4.2 eV

5.1 eV

e e

EF h

h

HA HAox

(b) UV irradiation Scattered light

H2O2

Small Au nanoisland

TiO2

Large Au nanoisland

HA: hole acceptor

Figure 3. Proposed mechanism for high-performance H2O2 production using the physical-vapordeposited Au on porous TiO2 films. (a) Size effect of the Au NI-loaded TiO2 on H2O2 generation. (b) UV light scattering from the porous TiO2 structure.


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


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0.5 0.4 0.3 0.2 0.1 0.0

1 2 3 4 5 6 7 8 9 10 Number of cycles

Figure 4. Reusability of the Au/TiO2 films. The UV irradiation time was 1 min, and the solidphase substrate was washed using DI water between each reuse cycle.


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Solid-Phase Photocatalysts: Physical Vapor Deposition of Au

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