Enhanced Photocatalytic Production of H2O2 by Nafion Coatings on S

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C: Surfaces, Interfaces, Porous Materials, and Catalysis 2

2

Enhanced Photocatalytic Production of HO by Nafion Coatings on S,N-Codoped Graphene Quantum Dots Modified TiO 2

Longhui Zheng, Jingzhen Zhang, Yun Hang Hu, and Mingce Long J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b02311 • Publication Date (Web): 09 May 2019 Downloaded from http://pubs.acs.org on May 9, 2019

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The Journal of Physical Chemistry

Enhanced Photocatalytic Production of H2O2 by Nafion Coatings on S,N-Codoped Graphene Quantum Dots Modified TiO2 Longhui Zheng†, Jingzhen Zhang†, Yun Hang Hu†, Mingce Long*†,‡

†School

of Environmental Science and Engineering, Shanghai Jiao Tong University, 800

Dongchuan Road, Shanghai 200240, China.

‡Key

Laboratory of Thin Film and Microfabrication Technology (Ministry of Education),

Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China.

1 ACS Paragon Plus Environment

The Journal of Physical Chemistry 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

ABSTRACT

Photocatalytic production of H2O2 requires to simultaneously promote the formation and suppress the decomposition of H2O2. This work explored a promising strategy of Nafion (perfluorinated polymer with sulfonate groups) coatings to enhance photocatalytic H2O2 production. The presence of Nafion layer on the S,N codoped graphene quantum dots (SNG) modified TiO2 was characterized by TEM, FTIR and XPS measurements. The incorporation of Nafion coatings significantly improves photocatalytic production of H2O2 (373 µM/h under simulated sunlight irradiation), which is about 70% higher than that without Nafion coatings. Both accelerated formation (34.8 µM/min) and dramatically inhibited decomposition (0.003 min-1) of H2O2 contribute to the efficient production of H2O2. Moreover, the ratios of H2O2 formation rate in the presence and absence of Nafion layers are more significantly improved at the neutral pH, which are 1.2 and 1.7 at pH 3 and 6.5, respectively. Nafion coatings show the abilities to induce strongly negative charged hydrophobic surface, which can enhanced local proton activity and oxygen concentration on the catalyst surface. The enhanced production of H2O2 by Nafion coatings can be attributed to the unaltered two-electron oxygen reduction reaction (ORR) pathway, the promoted charge transfer, the enhanced proton activity and oxygen diffusion, and the blocked formation of surface peroxide complexes. This work provides an insight for modulation of proton-coupled electron transfer (PCET) dominated ORR in photocatalysis through surface modification of Nafion coatings.


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1. INTRODUCTION Semiconductor photocatalysis has been found broad applications in environmental remediation,1–4 antibacterial,5,6 fuel production,7–11 nitrogen fixation,12,13 and so on.14 Among these, photocatalytic production of hydrogen peroxide (H2O2) has attracted increasing attention due to the advantages in utilization of water and oxygen as the source material and using solar light as the energy.15–26 The mechanism for photocatalytic H2O2 production includes two steps: (1) upon irradiation electrons in the valence band are excited to conduction band; and then free photogenerated charges migrate to the surface of catalysts; (2) the holes oxidize electron donor (such as alcohols) to provide protons (Eq. 1), and the electrons are trapped by oxygen to produce H2O2 through the proton coupled electron transfer (PCET) process via a one-electron (Eqs. 2 and 3) or two-electron (Eq. 4) pathway. 𝑅 ― 𝐶𝐻2𝑂𝐻 + 2 ℎ + →𝑅 ― 𝐶𝐻𝑂 + 2 𝐻 + 𝑂2 + 𝑒 ― →𝑂2∙ ―

𝐻+

𝐻𝑂2∙

(1) (2)

𝐻𝑂2∙ + 𝐻 + + 𝑒 ― →𝐻2𝑂2 𝑂2 +2𝑒 ― +2𝐻 + →𝐻2𝑂2

(3) (4)

TiO2 based photocatalysts have been extensively studied for the production of H2O2 over the past years, due to such merits of chemical stability and low cost.18,21,26 Although many new photocatalytic materials with novel compositions and structures are sought to attain wider light spectrum response and higher efficiency,17,27,28 TiO2 based photocatalysts are still the optimal candidates based on the consideration of practical application. However, two challenges restrict the photocatalytic H2O2 production on TiO2: the dominated inefficient single electron oxygen reduction reaction (ORR), and the simultaneous decomposition of H2O2 by forming peroxide



complexes (≡Ti-OOH).29–32 Therefore, efforts to achieve highly selective two-electron reduction of O2 and inhibition of photodecomposition of H2O2 are demanding. Surface modification of TiO2 has been explored to improve photocatalytic performance,33–36 leading to significant enhancement in H2O2 production as well.37–39 Surface loadings of noble metal nanoparticles, like bimodal Au nanoparticles and Au-Ag bimetallic alloy,37,39 can promote the separation of photogenerated electron-hole pairs, induce visible light driven activities through localized surface plasmon resonance mechanism (LSPR), and suppress adsorption and decomposition of H2O2. A rutile TiO2 modified by bimodal size Au nanoparticles displayed efficient H2O2 production under visible light irradiation when using HCOOH as the electron donor, which was attributed to the interfacial electron transfer between the different sizes of Au nanoparticles.39 Simultaneously, surface complexation with cations (like Zn(II)) or anions (like fluorine) was developed to inhibit H2O2 decomposition by suppressing the formation of peroxide complexes.40,41 However, most of these surface modification have insufficient selectivity in twoelectron ORR pathway to accelerate H2O2 production. Recently, studies indicated that the photocatalytic ORR pathways can be modulated by surface modification with polyprotic acids,32 carbon nanomaterials,31 and a combination of noble metal and oxoanions.42 Although the coupling of oxoanions (like phosphate, carbonate or borate) with other surface modification is a promising strategy to enhance H2O2 production, the adsorption capability of these ions onto TiO2 is highly pH-dependent.43 Nafion, an anionic perfluorosulfonic polymer, has widely been used as a proton conducting and cation-exchange membrane in fuel cells, owing to its good ionic conductivity, and chemical and thermal resistance.44–46 It has also been applied in the catalyst layer of ORR cathodes to reduce the charge transfer resistance, and ionic and gas transport limitation.47 Vorha et al. firstly found


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Nafion coated TiO2 displayed enhanced activity,48 and then Nafion modification has been investigated in the field of photocatalysis.29,49–54 Compared with anion or cation modification, Nafion coating is more favorable to tune surface properties of photocatalysts due to its stronger binding and less pH dependence. The main roles of Nafion modification for the improved photocatalytic performance have been attributed to three parts. Firstly, surface of photocatalysts becomes negative charged as a result of anionic sulfonate groups (–SO3–) in the Nafion layer, so as to tune the selectivity in adsorption of organic substrates.49,51 Secondly, the coupling interaction between Nafion and photocatalysts induces cathodic shifts of flat-band potentials, and results in a relatively stronger reductive power of photogenerated electrons.29,52 Thirdly, Nafion loading can facilitate photogenerated charge transfer due to the decreased impedance across solid/liquid interface.52 Generally, oxygen transport and oxygen reduction through a PCET process are two crucial steps for photocatalytic H2O2 production.31,32 Considering the favorable effects of Nafion, it could be a promising candidate for TiO2 modification to promote oxygen reduction and enhance H2O2 production in photocatalysis. In this paper, based on the previous developed SNG modified TiO2 (SNG/TiO2), the Nafion coating SNG/TiO2 was synthesized, which dramatically enhanced H2O2 production, especially at neutral pH conditions. Furthermore, the mechanism on the enhanced performance was discussed and the significant role of Nafion was highlighted. 2. EXPERIMENTAL SECTION 2.1. Materials. Commercial Degussa P25 TiO2 was used in all the experiments. 5 wt% Nafion dispersions was purchased from DuPont without purification. N,N-diethyl-p-phenylenediamine (DPD, 97%), 2-propanol and sodium perchlorate (NaClO4) were obtained from Sigma Aldrich. 5,5-Dimethy-lpyrroline N-oxide (DMPO) and horseradish peroxidase (POD, RZ > 1.5) were provided by



Adamas Co., Ltd., China and Sangon Biotech Co., Ltd., China, respectively. KNO3, HNO3, NaOH, thiourea and Citric acid were obtained from Sinopharm Group Co. Ltd. 2.2. Syntheses of Photocatalysts. The SNG solution was prepared according to the literature method.55 Specifically, a 7.5 ml clear solution of 0.315 g citric acid and 0.345 g thiourea was transferred to a 30 mL Teflon-lined stainless steel autoclave, then the temperature was raised to 160 °C and maintained for 4 hours. SNG solution was obtained by centrifugation for 20 min at 6000 rpm. The composite of SNG/TiO2 was synthesized through a previously reported method.31 Briefly, 1 g of TiO2 powder was added to a 10 ml SNG solution. The mixture was stirred for 6 hours. Then the impregnated solid was collected by filtration, and dried at 60 °C for 24 hours. The Nf-SNG/TiO2 was obtained using an impregnation method.49 SNG/TiO2 (0.1 g) was dispersed in 9.9 mL H2O by ultrasound for 20 minutes. Then, 0.1 mL of Nafion dispersions (5 wt%) was added to the solution with intensively stirring overnight. Then, the Nf-SNG/TiO2 powder were obtained by water washing and drying in a vacuum oven at 30℃ for 24 hours. 2.3. Characterizations. The morphology of catalysts was observed by a high resolution-transmission electron microscopy (HR-TEM, Tecnai G2 F20, FEI). The infrared spectra were recorded on a Fourier transform infrared (FTIR) spectrometer (Nicolet 6700). X-ray photoelectron spectroscopy (XPS) spectra were carried out by using a monochromatic Al Kα source (1486.6 eV) on an Axis Ultra DLD spectrometer (Kratos Analytical-A Shimadzu, Japan), which the overall instrument resolution was 0.48 eV. The carbon contents of SNG in the catalysts were measured by using an Elementar vario MACRO cube analyzer (Elementar Analysensysteme GmbH, Germany). The zeta potentials were determined using a zeta potential analyzer (Malvern Zetasizer 2000).


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The quasi-Fermi levels (EFL) of catalysts were measured by adding 20 mg catalysts and 10 mg methylviologen dichloride ((MV)Cl2, > 98%, TCI) into a 40 mL 0.1 M KNO3 aqueous solution. A platinum plate and a saturated calomel electrode were used as a working electrode and a reference electrode, respectively. The suspension was continuously stirred and irradiated with a 500 W Xe lamp. Then the photovoltages were recorded at different pHs which were adjusted by HNO3 or NaOH and N2 was drummed into the suspension throughout the experiment. The electron spin resonance (ESR) for DMPO–O2•− signals were measured on an ESR spectrometer (MS 5000, Magent Tech, Germany) by using DMPO as the spin trap. The center field is 3506 G, sweep width is 200 G, sampling time is 0.015 s, microwave frequency is 9.815 GHz and microwave power is 15 mW. In a typical procedure, 0.5 g/L catalysts were added into 20 mM DMPO methanol solution and the suspension was irradiated for 25 min under simulated sunlight. Then about 0.5 ml sample was filtered through a 0.22 μm PES filter, and immediately transferred to a flat quartz ESR cell for ESR measurements 2.4. Photocatalytic Performance. Photocatalytic H2O2 production tests were carried out by adding 25 mg of catalysts into a mixture of water (47 ml) and 2-propanol (3 ml) in a quartz cuvette. The pH values of the suspensions were adjusted to 3.0 by 1 M HClO4. The suspensions were irradiated by visible light (500 W Xenon lamp, λ ≥ 420 nm) or simulated sunlight (500 W Xenon lamp, λ ≥ 300 nm) under oxygen saturation conditions. Photocatalytic decomposition of H2O2 were carried out in the same reactor system. 25 mg catalysts were added into a 50 mL H2O2 suspension at 0.7 mM. To achieve adsorption–desorption equilibrium, the suspension was kept in the dark for 30 min. Then it was illuminated by visible light or simulated sunlight. At appropriate intervals, the samples was taken out, and filtered through a 0.22 μm nylon filter. H2O2 concentration was measured by a DPD-POD



method.30,56 Briefly, 1 mL of sample aliquots added into a mixture of 3 mL of phosphate buffer (0.5 M, pH = 6), 5.9 mL of water, 0.05 mL of DPD (10 mg/mL) and 0.05 mL of POD (1 mg/mL). After 50 seconds, the H2O2 concentration was measured at 551 nm on a UV-vis spectrophotometer (T6-New Century, Purkinje General). 2.5. Electrochemical and ORR Tests. CHI 760E electrochemical workstation was used for electrochemical tests. A Pt wire and an Ag/AgCl electrode were used as the counter and reference electrodes, respectively. The electrolyte was a 0.1 M NaClO4 solution at pH 3.5. The surface area of the working electrode was 1 cm2. The working electrode was prepared by scraping suspensions of catalysts on indium tin oxide glass and dried at 80 °C for 3 hours. N2 was purged continuously during the measurements. To confirm that the formation of surface peroxide complexes is blocked by a Nafion layer, the TiO2 and Nf-TiO2 electrodes were immersed into a 0.7 mM H2O2 solution for 30 min; then the electrodes were dried in a vacuum oven at 30 °C for 2 hours, and then were applied for photocurrent measurements. The AC amplitude and frequency of the electrochemical impedance spectroscopy (EIS) were 5 mV and 0.1-105 kHz, respectively, and the open circuit voltage is +0.2 V (vs Ag/AgCl). The 500 W xenon lamp was employed as the light source for photocurrent tests. Rotating disc electrode (RDE) tests were taken on a modulated speed rotator (MSR) and glassy carbon disc electrode that were purchased from Pine Co. Ltd. The slurry was prepared by mixing catalysts (2 mg) and ethanol (1 ml). Then, 40 µL dispersion was applied on glass carbon RDE with silver conductive adhesive and dried. The linear sweep voltammetry (LSV) was carried out in O2 saturated NaClO4 electrolyte (0.1 M) at pH 3.5, the scan rate was 50 mV s−1, and the range of rotating speeds was 0 ~2000 rpm. 3. RESULTS AND DISCUSSION


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3.1. Characterizations of Catalysts.

Figure 1. (a) TEM images of Nf-SNG/TiO2; (b) FTIR spectra of different catalysts. Inset is the HR-TEM image of Nf-SNG/TiO2.

The TEM image in Figure 1a shows the morphology of Nf-SNG/TiO2 nanoparticles. The sizes of TiO2 nanoparticles are about 20-30 nm. Many small dots (partially highlighted in the figure) in 1-2 nm sizes are uniformly deposited on the surface of nanoparticles, indicating the immobilization of SNG on TiO2. In the inset image of Figure 1a, the notable lattice spacing of 0.35 nm corresponds to (101) crystal planes of anatase phase TiO2, which accounts of about 75% in the composition of Degussa P25. The conformal coating of Nafion layer on the nanoparticles can be discerned as a thickness of 1-2 nm amorphous margin, whose thickness depends on the percentage of Nafion in the sample.52 This illustrates the successful modification of TiO2 by Nafion and SNG. FTIR spectra was used to identify the functional groups existing in SNG or Nafion in the catalysts. As shown in Figure 1b, comparing to the spectrum of TiO2, the spectrum of SNG/TiO2 displays new bands that can be attributed to SNG: the band centering at 1575 cm-1 corresponds to C=C groups that originated from dehydration of citric acid; the bands at around 1401 cm-1 and 9 ACS Paragon Plus Environment


1052 cm-1 are ascribed to the bending vibrations of C-N and C-O, respectively;55 the presence of C=S that comes from the polycondensation of thiourea is indicated by the band at around 1083 cm-1.55 In the spectra of Nf-TiO2 and Nf-SNG/TiO2, the emerged two bands at about 983 cm-1 and 1232 cm-1 can be assigned to CFR-CF3 and CF2 bending vibration, respectively. In addition, the new band at 1061 cm-1 is ascribed to the stretching vibration of S-O groups in the anionic sulfonate groups in Nafion molecules.52 Above evidence proves the presence of a Nafion layer on the surface of SNG/TiO2 nanoparticles, which also results in a notable hydrophobic behavior of Nf-SNG/TiO2 powder due to the presence of hydrophobic perfluoro-backbone (–(CF2CF2)n–CFCF2–).57 XPS measurements was conducted to analyze the chemical states and elementary compositions of Nf-SNG-TiO2, and results are shown in Figure 2 and Figure S1. The highresolution C 1s spectrum (Figure 2a) can be deconvoluted into four bands. The main peak centering at 284.8 eV is ascribed to the sp2 C (C–C or C=C) bonds in graphene structure.58,59 The other three peaks at 286.2, 288.8 and 291.6 eV can be assigned to O–C, C=O and CF2 groups, respectively.45 The two peaks at 399.8 and 401.5 eV in the N 1s spectrum (Figure 2b) are attributed to the pyrrolic N (C-N-C) and graphitic N or N–H bonds, respectively.60,61 The S 2p spectrum displays three peaks (Figure 2c), corresponding to thiophene like structure in SNG at 163.1 eV, S=O bonding at 168.3 eV and the sulfur in sulfonate groups of Nafion at 168.9 eV, respectively.52,62 The sulfur dopants in graphene structure can promote ORR by inducing high charge density.63 The high-resolution F 1s spectrum (Figure 2d) shows one band centering at 688.6 eV, which can be attributed to the signal of F atoms from the Nafion layer.64 This is in consistent with the indication of CF2 groups in the C1s XPS spectrum and above FTIR spectra.


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Figure 2. High resolution XPS spectra of C 1s (a), N 1s (b), S 2p (c), and F 1s (d) in NfSNG/TiO2. 3.2. Photocatalytic Performance for H2O2 Production. The production of H2O2 in the suspensions of various photocatalysts under both visible light and simulated sunlight were investigated. Figure 3a shows that the production of H2O2 over the pristine TiO2 under visible light is very low, and this can be understood by the wide band gap of TiO2. Nafion modification cannot alter the band gap of TiO2.29 Thus, Nf-TiO2 displays poor activity for H2O2 production under visible light irradiation (Figure S2c). However, surface modification of TiO2 by the dual doped carbon quantum dots (SNG) dramatically increased H2O2 yield under visible light. At the optimized dosage of SNG (0.5 wt%), about 82.8 μM H2O2 is detected after 120 min irradiation, which is 6.8 times of that for pristine TiO2. This can be ascribed to the photosensitization of SNG: upon visible light irradiation, electrons in SNG can be excited and transfer to the conduction band (CB) of TiO2, and subsequently be trapped by oxygen. The 11 ACS Paragon Plus Environment


H2O2 yield can be much improved after introduction of a Nafion coating on SNG/TiO2. The H2O2 concentration in the Nf-SNG/TiO2 suspension with 3.5% Nafion content is the highest, reaching 141 μM after 120 min irradiation, which is around 1.7 times of the values for SNG/TiO2.

Figure 3. Photocatalytic generation of H2O2 on various catalysts under visible light (a) and simulated sunlight (b) irradiation (A: TiO2, B: Nf-TiO2, C: SNG/TiO2, D: 2.5% Nf-SNG/TiO2, E: 3.5% Nf-SNG/TiO2, F: 5% Nf-SNG/TiO2, G: 7.5% Nf-SNG/TiO2).

Different from the negligible influence of Nafion on bare TiO2 under visible light, Nf-TiO2 displays significant improved H2O2 production under simulated sunlight (Figure S2d). The H2O2 concentration increases with the Nafion dosage. However, further increase of Nafion dosage above 5 wt% cannot enhance H2O2 production. This can be ascribed to that the excessive coverage of surface modifiers on the catalysts can block active adsorption sites of TiO2 and reduce the accessibility of hole scavengers. At the optimized dosage of Nafion (5 wt%), more than 327 μM H2O2 is produced in the Nf-TiO2 suspension after 120 min irradiation, which is 2.4 times of that in the pristine TiO2 system. Under simulated sunlight, SNG modification also enhanced H2O2 production at the optimal dosage of SNG (Figure 3b). The improved performance of SNG/TiO2 was mainly attributed to the significantly enhanced formation rate of H2O2. The role of SNG has been previously revealed, which can provide two kinds of active sites for oxygen adsorption and 12 ACS Paragon Plus Environment

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proton relays, and accordingly enable a highly selective two-electron photocatalytic reduction of oxygen.31 In addition, as the modification of Nafion and SNG at the optimal ratio on TiO2, H2O2 production is further improved up to 745.5 μM after 120 min irradiation, which is 5.5 and 1.7 folds as that for bare TiO2 and SNG/TiO2. Moreover, H2O2 production in SNG/TiO2 suspension displays a downward trend when the reaction proceeds to two hours. This can be attributed to the decomposition of H2O2 that is signified at a higher H2O2 concentration. However, the downward trend almost disappears for samples with Nafion modification. This suggested that Nafion can serve as a barrier to suppress the decomposition of H2O2, and result in H2O2 production continuously increased upon prolonged illumination.

Figure 4. Photocatalytic decomposition of H2O2 on various catalysts under visible light (a) and simulated sunlight (b) irradiation.

The decomposition of H2O2 under visible light and simulated sunlight were measured. As shown in Figure 4a and 4b, TiO2 exhibits the highest H2O2 decomposition rates under both visible light and simulated sunlight. The decomposition of H2O2 can be attributed to the formed peroxide complexes (≡Ti–OOH), which can be directly oxidized by holes under UV irradiation or indirectly decomposed through a sensitization of the complexes under visible light.32,65 The modified TiO2 catalysts exhibits suppressed decomposition of H2O2. Under simulated sunlight, SNG modification 13 ACS Paragon Plus Environment


can moderately inhibit H2O2 decomposition, which can be attributed to the suppressed formation of the complexes ≡Ti-OOH on the surface of TiO2. However, the photocatalytic decomposition of H2O2 on Nf-TiO2 and Nf-SNG/TiO2 decreases more significantly, which are as low as 0.007 and 0.003 min-1, respectively. This can be attributed to the fact that Nafion covers the surface groups of Ti-OH on TiO2, which reduces H2O2 adsorption and suppresses the formation of complexation groups. Figure S3 shows the photocurrent densities of TiO2 and Nf-TiO2 electrodes in the presence or absence of H2O2 under visible light irradiation ( ≥ 420 nm). Both electrodes produce no photocurrent in the absence of H2O2 due to the wide band gap of TiO2. In the presence of 0.7 mM H2O2, obvious photocurrent (16.5 A/cm2) is generated on TiO2 electrode, indicating the formation of the visible light responsive surface peroxide complexes (≡Ti-OOH).65 However, negligible photocurrent can be observed on the Nf-TiO2 electrode even with H2O2, indicating that the Nafion layer block the formation of the surface peroxide complexes.

Figure 5. (a) The formation (kf) and (b) decomposition rate constants (kd) of H2O2 for various catalysts under visible light and simulated sunlight irradiation.

Kinetic analyses were carried out to understanding the formation and decomposition rates of H2O2 on different catalysts. The generation of H2O2 accords with the zero-order reaction kinetics and the decomposition of H2O2 conforms to the first-order reaction kinetics. According to the 14 ACS Paragon Plus Environment

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formation (kf) and decomposition rate constants (kd) of H2O2, the concentration of H2O2 is obtained from the Eq. 5, where kf and kd are H2O2 formation and decomposition rate constants, respectively.

[H2O2] = (kf kd) [1 ― exp ( ― kdt)]

(5)

Under both light sources, bare TiO2 displays the lowest kf and largest kd, resulting in the least H2O2 production (Figure 5). The Nf-SNG/TiO2 shows the highest catalytic performance. As mentioned above, the decomposition rate of H2O2 is significantly reduced in the presence of Nafion, and the kd value is about 7% and 4% of that for bare TiO2 and SNG/TiO2 under simulated sunlight. This can be ascribed to that the side chains (–OCF2CF(CF3)OCF2CF2–SO3−) of Nafion attach to the surface of TiO2,50 which can inhibit the adsorption and decomposition of H2O2. Besides the ability to inhibit H2O2 decomposition, Nafion can also enhance the formation of H2O2. The H2O2 formation rate constant (kf) of Nf-SNG/TiO2 reaches up to the highest value of 34.8 μM/min under simulated sunlight, which is 1.2 times of that for SNG/TiO2. The combination of formation and decomposition rate of H2O2 after Nafion coating contributes to the significantly enhanced H2O2 production.

Figure 6. (a) The H2O2 formation rate constants (kf) and (b) decomposition rate constants (kd) at various pHs.



The effects of pH on the rate of H2O2 formation (kf) and decomposition (kd) were investigated. As shown in Figure 6, the kd increases with the increase of pH, indicating H2O2 photodecomposition is accelerated, which can be understood by the enhanced adsorption of H2O2 on TiO2 at high pHs.65 The value of kf decreases as the pH increases, which indicates that H2O2 production is the pH dependent PCET process.66 However, H2O2 production in the Nf-TiO2 and Nf-SNG/TiO2 suspensions is less sensitive to proton concentrations. Even at pH 6.5, the kf value for Nf-SNG/TiO2 is as high as 17.68 M/min. According to Figure S4, about 397.45 M H2O2 is produced after 120 min irradiation. However, negligible H2O2 can be detected in TiO2 suspension at this pH. Moreover, the ratio of kf for Nf-SNG/TiO2 to SNG/TiO2 is about 1.2 at pH 3, which increased to 1.7 at pH 6.5. This indicates that at neutral conditions, the favorable effect of Nafion on the formation of H2O2 is significant, and remarkable H2O2 production over Nf-SNG/TiO2 can be observed even at neutral conditions. The stability and repeatability of photocatalytic H2O2 production were evaluated by a cycle test (Figure S5). After four cycles under simulated sunlight irradiation, the catalytic activity of Nf-SNG/TiO2 does not decrease, indicating that Nf-SNG/TiO2 has a high stability in photocatalytic H2O2 production. 3.3. Mechanism on the Enhanced Photocatalytic H2O2 Production. The photocatalytic production of H2O2 strongly depends on the free surface photogenerated electrons, and available protons and oxygen. Generally, Nafion coatings on TiO2 can result in a relatively hydrophobic surface, because the hydrophilic acidic chains of Nafion attach to the surface of TiO2, and the hydrophobic polytetrafluoroethylene backbones (–(CF2CF2)nCFCF2–, n = 6–10) dangle into the bulk solution. It can be expected that the Nafion layer on the catalysts accelerates oxygen diffusion from solution to the catalyst surface, and increases the available oxygen for electron scavenging. On the other hand, the available protons on the catalyst surface


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increase in the presence of Nafion layers, which can be explained by zeta potentials. Figure 7 shows zeta potentials of suspended catalysts as a function of pHs. The isoelectric point (pHzpc) of pristine TiO2 is about 6.5, which is consistent with previous reports.54 The composite SNG/TiO2 shows decreased zeta potentials, which can be ascribed to the abundant negatively charged amino and carboxylic groups in the SNG component.67 In the presence of Nafion, zeta potentials of catalysts become negative over the all pH range (pH=1–13). The negative zeta potential can be attributed to the replacement of positively charged surface groups (≡Ti-OH2+) by anionic sulfonate groups of Nafion molecules.49,50,54 The zeta potentials of Nf-TiO2 and Nf-SNG/TiO2 at pH 6.5 are as low as –48 mV and –60 mV, suggesting that the catalysts with Nafion coatings are strongly negative charged. The cation-exchange and proton conductive abilities of Nafion layers can favor to concentrate protons at the surface region of the catalysts, and accordingly improve the local activity of protons within the Nafion layers.

Figure 7. Zeta potentials of suspended catalysts as a function of pHs.



Figure 8. Photocurrent of (1) TiO2, (2) Nf-TiO2, (3) SNG/TiO2 and (4) Nf-SNG/TiO2 under (a) visible light (λ ≥ 420 nm) and (b) simulated sunlight irradiation at +0.2 V vs. Ag/AgCl.

Furthermore, the number of free surface photogenerated electrons is crucial in photocatalytic ORR processes, which can be indicated by photocurrents generated from the film electrodes of the catalysts. The photocurrents for TiO2, Nf-TiO2, SNG/TiO2 and Nf-SNG/TiO2 electrodes were compared. As shown in Figure 8, after introduction of Nafion layer, the visible light photocurrent density of SNG/TiO2 increases from 52.3 to 80.3 A/cm2. Under simulated sunlight, the photocurrent density of Nf-SNG/TiO2 increases to 137.7 A/cm2, about 1.4 times of that of SNG/TiO2. However, the Nafion coating does not obviously improve photocurrent of pristine TiO2, while dramatically improve that of SNG/TiO2. This is consistent with previous reports: Nafion modification can greatly enhance sensitized photocurrent from RhB or HA modified TiO2,29,49 while result in negligible photocurrent changes for pristine TiO2.49 The enhanced photocurrent and the facilitated separation of photogenerated carriers can be attributed to a coupling interaction between Nafion and catalysts or surface adsorbates, because the deprotonated Nafion possesses a large dipole moment at the interface to favor the hole migration.52


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Figure 9. (a) Electrochemical impedance spectroscopy (EIS) plots for samples. (b) Photovoltage vs. suspension pH value measured for suspended catalysts in the presence of MV2+.

The EIS spectra for catalyst electrodes were measured to estimate impedance changes and charge transfer resistances (Rct) at the electrode /electrolyte interface (Figure 9a). TiO2 exhibits an almost straight line at lower frequencies, which is a characteristic of the diffusion limiting step in the electrochemical process. Furthermore, Nf-SNG/TiO2 displays the lowest value of Rct, indicating the promoted charge separation and transfer in the Nafion-coated SNG/TiO2 electrode. This can be attributed to the enhanced electrolytic conductivity by Nafion loading.46 To understand the effect of Nafion modification on the quasi-Fermi levels of catalysts, an indication of the positions where electrons transfer out for oxygen reduction, the photovoltage vs. suspension pH plots in the presence of methylviologen dichloride were measured (Figure 9b). The quasi-Fermi levels can be calculated according to Eq. 6, 𝐸𝐹𝐿(𝑝𝐻) = 𝐸0 ―𝑘(𝑝𝐻 ― 𝑝𝐻0)

(6)

in which E0 is the standard reduction potential of the redox couple (e.g. −0.45 V for MV2+/ MV˙+), and k is generally 0.059 V/pH for semiconductors. The EFL at pH 7 is obtained as –0.57, –0.61, – 0.40 and –0.49 V vs. NHE for TiO2, Nf-TiO2, SNG/TiO2 and Nf-SNG/TiO2, respectively. The strong coupling of Nafion and SNG/TiO2 results in the downward shift of quasi-Fermi levels of 19 ACS Paragon Plus Environment


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photocatalysts, which would accelerate electron transfer from conduction band of TiO2 to SNG, as well as the surface oxygen reduction occurring on SNG.

Figure 10. (a) Koutecky-Levich plots of the four film electrodes at −0.75 V vs. Ag/AgCl; (b) ESR spectra of photocatalytic systems after 25 min illumination.

Electron transfer number of ORR was measured by linear-sweep voltammetry (LSV) on a rotating-disk electrode (RDE) in an O2-saturated electrolyte, and results are shown in Figure S6. The cathodic currents in N2 purged conditions are very low, indicating the high stability of all electrodes. In the O2 saturated electrolytes, the cathodic current density can represent the ORR rate on the surface of catalysts. Both Nafion and SNG facilitate electrons transfer from TiO2 to O2 and enhance ORR rates. The cathodic current density increases with the increase of the rotation speeds, and Nf-SNG/TiO2 displays the highest ORR rate. By linear fitting of the reciprocal rotating speed versus reciprocal current density collected at different potentials, the overall number of the transferred electrons (n) and the kinetic current density (ik) in ORR were quantitatively estimated according to the slope and intercept of the linearly fitted plots obtained by equation 3, 1 i

1

= ik +

1 2 1 ― 0.62𝑛𝐹𝐷30𝜐 6𝐶0

× 1/𝜔1/2


(3)

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wherein i is the measured current density, F is the Faraday constant (F=96485 C mol−1), Co is the bulk saturated concentration of oxygen, ω is the angular velocity of the disc (ω = 2Nπ, where N is the linear rotation speed), Do is the diffusion coefficient of oxygen, and v is the kinetic viscosity of the electrolyte. In the present experiments, Do=1.93×10−5 cm2 s−1, v=0.0109 cm2 s−1, Co=1.26×10−3 M. 32 Figure 10a shows the Koutecky-Levich plots of the four film electrodes at − 0.75 V vs. Ag/AgCl. The n values for bare TiO2 and Nf-TiO2 are 0.91 and 0.89, respectively. This indicates ORR on both catalysts is a single-electron dominated pathway, and incorporation of a Nafion layer does not alter the ORR mechanism. However, the n values for SNG/TiO2 and Nf-SNG/TiO2 increase to 2.36 and 2.01, respectively. This suggests that SNG modification enables a twoelectron dominated ORR pathway. This has been previously attributed to the special charge density of the atoms on the sulfur and nitrogen co-doping graphene quantum dots: the carbon and sulfur atoms with positive charge density serve as potential active sites for ORR by increasing molecular oxygen adsorption, and those carbon and nitrogen atoms with negative charge density can play as the proton relay sites to promote PCET processes.32 The overall number of transferred electrons at the different voltages of the first reduction plateau for Nf-SNG/TiO2 is shown in Figure S6e. The number of electrons transferred per oxygen molecule in the ORR process is 1.9–2.3 at potentials ranging from –0.73 to –0.85 V, confirming a two-electron dominated procedures for oxygen reduction. The two-electron ORR on SNG/TiO2 and Nf-SNG/TiO2 can be further supported by electron spin resonance (ESR) results. The ESR spectra using DMPO as the spin trap are carried out to detect the presence of O2•−, the inevitable intermediate in single-electron oxygen reduction pathway. As shown in Figure 10b, the typical signals of DMPO–O2•− is notable in the TiO2



suspension after illumination under simulated sunlight for 25 min, indicating that oxygen reduction on TiO2 is dominated by a single electron pathway. However, such signals are barely identifiable in the illuminated SNG/TiO2 and Nf-SNG/TiO2 suspensions, suggesting the absence of O2•− and a multi-electrons ORR pathway taking place in these systems.

Scheme 1. The role of Nafion layer for photocatalytic H2O2 production on Nf-SNG/TiO2.

Hence, photocatalytic production of H2O2 on Nf-SNG/TiO2 is a two-electron dominated ORR pathway. As described in Scheme 1, The role of Nafion for the enhanced H2O2 production on NfSNG/TiO2 includes that: (1) the Nafion adlayer can block TiO2 surface to generate ≡ 𝑇𝑖 ― 𝑂𝑂𝐻 complexes, and suppress the decomposition of H2O2; (2) the decreased charge transfer resistances and cathodic shifts of fermi level can boost charge transfer and inhibit recombination; (3) the hydrophobic Nafion layer enhances the transport of protons and oxygen, resulting in a proton and oxygen concentrated region near the surface of catalyst, and accordingly enhance the local activity of protons and promote ORR and H2O2 production at neutral pH conditions. 4. CONCLUSIONS In summary, incorporation of a Nafion layer on the sulfur and nitrogen dual doped graphene quantum dots modified TiO2 can dramatically improve the performance of H2O2 production. 22 ACS Paragon Plus Environment

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Nafion coatings do not change the two-electron ORR pathway on SNG/TiO2, but accelerate the formation and inhibit the decomposition of H2O2. Besides blocking the formation of surface peroxide complexes, the important role of Nafion can be attributed to the increase of oxygen and proton transport, as well as the promoted charge transfer. Moreover, the enhanced local activity of protons on the surface of the catalyst enables a significant H2O2 production at neutral pHs. The results demonstrate that modification with Nafion coatings could be a promising strategy to realize applicable photocatalytic H2O2 production, and to modulate ORR in the PECT dominated photocatalysis.

ASSOCIATED CONTENT Supporting Information. XPS spectra, Photocatalytic generation of H2O2 on catalysts, Comparison on visible light photocurrents, Stability of Nf-SNG/TiO2 and Linear-sweep voltammetry. AUTHOR INFORMATION Corresponding Author Mail address: School of Environmental Science and Engineering, Shanghai Jiao Tong University, 800 Dong Chuan Road, Shanghai 200240, People’s Republic of China Tel: 86-21-54747354; Fax: 86-21-54740825; E-mail: [email protected] ORCID Mingce Long：http://orcid.org/0000-0002-5168-8330 Notes The authors declare no conflict of interest. ACKNOWLEDGMENTS



Financial supports from the Natural Science Foundation of China (No. 21876108) and Shanghai Municipal International Cooperation Foundation (No. 18230742900) are gratefully acknowledged. We gratefully acknowledge the support in XPS and FTIR measurements by Dr. Limin Sun of the Instrumental Analysis Center and Ms. Xiaofang Hu of School of Environmental Science and Engineering of Shanghai Jiao Tong University.

REFERENCES (1) Jo, W. K.; Kim, Y. G.; Tonda, S. Hierarchical Flower-like NiAl-layered Double Hydroxide Microspheres Encapsulated with Black Cu-doped TiO2 Nanoparticles: Highly Efficient VisibleLight-driven Composite Photocatalysts for Environmental Remediation. J. Hazard. Mater. 2018, 357, 19–29. (2) Skliri, E.; Miao, J.; Xie, J.; Liu, G.; Salim, T.; Liu, B.; Zhang, Q.; Armatas, G. S. Assembly and Photochemical Properties of Mesoporous Networks of Spinel Ferrite Nanoparticles for Environmental Photocatalytic Remediation. Appl. Catal. B: Environ. 2018, 227, 330–339. (3) Long, M.; Zheng, L.; Tan, B.; Shu, H. Photocatalytic Self-cleaning Cotton Fabrics with Platinum (IV) Chloride Modified TiO2 and N-TiO2 Coatings. Appl. Surf. Sci. 2016, 386, 434–441. (4) Zhang, J.; Mao, X.; Xiao, W.; Zhuang, Y. Photocatalytic Degradation of Sulfamethazine by Graphitic Carbon Nitride-Modified Zinc Molybdate: Effects of Synthesis Method on Performance, Degradation Kinetics, and Mechanism. Chin. J. Catal. 2017, 38, 2009–2020. (5) Jalvo, B.; Faraldos, M.; Bahamonde, A.; Rosal, R. Antibacterial Surfaces Prepared by Electrospray Coating of Photocatalytic Nanoparticles. Chem. Eng. J. 2018, 334, 1108–1118. (6) Xu, Y.; Wen, W.; Wu, J. M. Titania Nanowires Functionalized Polyester Fabrics with Enhanced Photocatalytic and Antibacterial Performances. J. Hazard. Mater. 2018, 343, 285–297.


Page 24 of 33

Page 25 of 33 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


(7) Yu, H.; Liu, W.; Wang, X.; Wang F. Promoting the Interfacial H2-evolution Reaction of Metallic Ag by Ag2S Cocatalyst: A Case Study of TiO2/Ag-Ag2S Photocatalyst. Appl. Catal. B: Environ. 2018, 225, 415–423. (8) Ran, J.; Guo, W.; Wang, H.; Zhu, B.; Yu, J.; Qiao, S. Z. Metal-free 2D/2D Phosphorene/gC3N4 Vander Waals Heterojunction for Highly Enhanced Visible-light Photocatalytic H2 Production. Adv. Mater. 2018, 30, 1800128–1800133. (9) Pachfule, P.; Acharjya, A.; Roeser, J.; Langenhahn, T.; Schwarze, M.; Schomacker, R.; Thomas, A.; Schmidt, J. Diacetylene Functionalized Covalent Organic Framework (COF) for Photocatalytic Hydrogen Generation. J. Am. Chem. Soc. 2018, 140, 1423–1427. (10) Wei, L.; Yu, C.; Zhang, Q.; Liu, H.; Wang, Y. TiO2-based Heterojunction Photocatalysts for Photocatalytic Reduction of CO2 into Solar Fuels. J. Mater. Chem. A 2018, 6, 22411–22436. (11) Jiang, J.; Cao, S.; Hu, C.; Chen, C. A Comparison Study of Alkali Metal-Doped g-C3N4 for Visible-Light Photocatalytic Hydrogen Evolution. Chin. J. Catal. 2017, 38, 1981–1989. (12) Comer, B. M.; Liu, Y. H.; Dixit, M. B.; Hatzell, K. B.; Ye, Y.; Crumlin, E. J.; Hatzell, M. C.; Medford, A. J. The Role of Adventitious Carbon in Photo-catalytic Nitrogen Fixation by Titania. J. Am. Chem. Soc. 2018, 140, 15157–15160. (13) Wang, S.; Ichihara, F.; Pang, H.; Chen, H.; Ye, J. Nitrogen Fixation Reaction Derived from Nanostructured Catalytic Materials. Adv. Funct. Mater. 2018, 28, 1803309–1803334. (14) Ran, J.; Jaroniec, M.; Qiao, S. Z. Cocatalysts in Semiconductor-based Photocatalytic CO2 Reduction: Achievements, Challenges, and Opportunities. Adv. Mater. 2018, 30, 1704649– 1704679.



(15) Wei, Z.; Liu, M.; Zhang, Z.; Yao, W.; Tan, H.; Zhu, Y. Efficient Visible-light-driven Selective Oxygen Reduction to Hydrogen Peroxide by Oxygen-enriched Graphitic Carbon Nitride Polymers. Energy Environ. Sci. 2018, 11, 2581–2589. (16) Shi, L.; Yang, L.; Zhou, W.; Liu, Y.; Yin, L.; Hai, X.; Song, H.; Ye, J. Photoassisted Construction of Holey Defective g-C3N4 Photocatalysts for Efficient Visible-light-driven H2O2 Production. Small 2018, 14, 1703142–1703150. (17) Shiraishi, Y.; Kanazawa, S.; Kofuji, Y.; Sakamoto, H.; Ichikawa, S.; Tanaka, S.; Hirai, T. Sunlight-driven Hydrogen Peroxide Production from Water and Molecular Oxygen by Metal-free P8hotocatalysts. Angew. Chem. Int. Ed. 2014, 53, 13454–13459. (18) Harbour, J. R.; Tromp, J.; Hair, M. L. Photogeneration of Hydrogen Peroxide in Aqueous TiO2 Dispersions. Can. J. Chem. 1984, 63, 204–208. (19) Campos-Martin, J. M.; Blanco-Brieva, G.; Fierro, J. L. Hydrogen Peroxide Synthesis: An Outlook Beyond the Anthraquinone Process. Angew. Chem. Int. Ed. 2006, 45, 6962–6984. (20) Fukuzumi, S.; Lee, Y.-M.; Nam, W. Solar-driven Production of Hydrogen Peroxide from Water and Dioxygen. Chem. Eur. J. 2018, 24, 5016 – 5031. (21) Moon, G. H.; Kim, W.; Bokare, A. D.; Sung, N. E.; Choi, W. Solar Production of H2O2 on Reduced Graphene Oxide-TiO2 Hybrid Photocatalysts Consisting of Earth-abundant Elements Only. Energy Environ. Sci. 2014, 7, 4023–4028. (22) Hu, S.; Qu, X.; Li, P.; Wang, F.; Li, Q.; Song, L.; Zhao, Y.; Kang, X. Photocatalytic Oxygen Reduction to Hydrogen Peroxide over Copper Doped Graphitic Carbon Nitride Hollow Microsphere: The Effect of Cu(I)-N Active Sites. Chem. Eng. J. 2018, 334, 410-418.


Page 26 of 33

Page 27 of 33 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


(23) Peng, Q.; Zhao, H.; Qian, Lin.; Wang, Y.; Zhao, G. Design of a Neutral Photo-electro-Fenton System with 3D-ordered Macroporous Fe2O3/carbon Aerogel Cathode: High Activity and Low Energy Consumption. Appl. Catal. B: Environ. 2015, 174, 157–166. (24) Moon, G.; Fujitsuka, M.; Kim, S.; Majima, T.; Wang, X.; Choi, W. Eco-Friendly Photochemical Production of H2O2 through O2 Reduction over Carbon Nitride Frameworks Incorporated with Multiple Heteroelements. ACS Catal. 2017, 7, 2886–2895. (25) Zhao, H.; Chen, Y.; Peng, Q.; Wang, Q.; Zhao, G. Catalytic activity of MOF(2Fe/Co)/carbon aerogel for improving H2O2 and •OH generation in solar photo–electro–Fenton process. Appl. Catal. B: Environ. 2017, 203, 127–137. (26) Long, M.; Zheng, L. Engineering Vacancies for Solar Photocatalytic Applications. Chin. J. Catal. 2017, 38, 617–624. (27) Mase, K.; Yoneda, M.; Yamada, Y.; Fukuzumi, S. Efficient Photocatalytic Production of Hydrogen Peroxide from Water and Dioxygen with Bismuth Vanadate and A Cobalt(II) Chlorin Complex. ACS Energy Lett. 2016, 1, 913–919. (28) Gogoi, S.; Karak, N. Solar-driven Hydrogen Peroxide Production Using Polymer-supported Carbon Dots as Heterogeneous Catalyst. Nano-Micro Lett. 2017, 9, 40. (29) Zheng, L. H.; Yu, X. J.; Long, M. C.; Li, Q. L. Humic Acid-mediated Visible-light Degradation of Phenol on Phosphate-modified and Nafion-modified TiO2 Surfaces. Chin. J. Catal. 2017, 38, 2076–2084. (30) Wang, C.; Long, M.; Tan, B.; Zheng, L.; Cai, J.; Fu, J. Facilitated Photoinduced Electron Storage and Two-electron Reduction of Oxygen by Reduced Graphene Oxide in rGO/TiO2/WO3 Composites. Electrochim. Acta 2017, 250, 108–116.



(31) Zheng, L. H.; Su, H. R.; Zhang, J. Z.; Walekar, L. S.; Molamahmood, H. V.; Zhou, B. X.; Long, M. C.; Hu, Y. H. Highly Selective Photocatalytic Production of H2O2 on Sulfur and Nitrogen for Co-doped Graphene Quantum Dots Tuned TiO2. Appl. Catal. B: Environ. 2018, 239, 475–484. (32) Sheng, H.; Ji, H. W.; Ma, W. H.; Chen, C. C.; Zhao, J. C. Direct Four-electron Reduction of O2 to H2O on TiO2 Surfaces by Pendant Proton Relay. Angew. Chem. Int. Ed. 2013, 52, 9686– 9690. (33) Park, H.; Park, Y.; Kim, W.; Choi, W. Surface Modification of TiO2 Photocatalyst for Environmental Applications. J. Photochem. Photobiol. C: Photochem. Rev. 2013, 15, 1–20. (34) Kong, L.; Zhang, X.; Wang, C.; Wan, F.; Li, L. Synergic Effects of CuxO Electron Transfer Co-catalyst and Valence Band Edge Control over TiO2 for Efficient Visible-light Photocatalysis. Chin. J. Catal. 2017, 38, 2120–2131. (35) Qi, K.; Cheng, B.; Yu, J.; Ho, W. A Review on TiO2-based Z-scheme Photocatalysts. Chin. J. Catal. 2017, 38, 1936–1955. (36) Liang, H.; Meng, Q.; Wang, X.; Zhang, H.; Wang, J. Nanoplasmonically Engineered Interfaces on Amorphous TiO2 for Highly Efficient Photocatalysis in Hydrogen Evolution. ACS Appl. Mater. Interfaces 2018, 10, 14145−14152 (37) Teranishi, M.; Hoshino, R.; Naya, S.; Tada, H. Gold-nanoparticle-loaded Carbonate-Modified Titanium(IV) Oxide Surface: Visible-light-driven Formation of Hydrogen Peroxide from Oxygen. Angew. Chem. Int. Ed. 2016, 55, 12773–12777. (38) Tomasz Baran, S. W.; Minguzzi, A.; Rondinini, S.; Vertova, A. Achieving Efficient H2O2 Production by A Visible-light Absorbing, Highly Stable Photosensitized TiO2. Appl. Catal. B: Environ. 2019, 244, 303–312.


Page 28 of 33

Page 29 of 33 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


(39) Tsukamoto, D.; Shiro, A.; Shiraishi, Y.; Sugano, Y.; Ichikawa, S.; Tanaka, S.; Hirai, T. Photocatalytic H2O2 Production from Ethanol/O2 System Using TiO2 Loaded with Au–Ag Bimetallic Alloy Nanoparticles. ACS Catal. 2012, 2, 599–603. (40) Maurino, V.; Minero, C.; Mariella, G.; Pelizzetti, E. Sustained Production of H2O2 on Irradiated TiO2-fluoride Systems. Chem. Commun. 2005, 20, 2627–2629. (41) Maurino, V.; Minero, C.; Pelizzetti, E.; Mariella, G.; Arbezzano, A.; Rubertelli, F. Influence of Zn(II) Adsorption on the Photocatalytic Activity and the Production of H2O2 Over Irradiated TiO2. Res. Chem. Intermed. 2007, 33, 319–332. (42) Xiong, X.; Zhang, X.; Liu, S.; Zhao, J.; Xu, Y. Sustained Production of H2O2 in Alkaline Water Solution Using Borate and Phosphate-modified Au/TiO2 Photocatalysts. Photochem. Photobiol. Sci. 2018, 17, 1018–1022. (43) Long, M. C.; Brame, J.; Qin, F.; Bao, J. M.; Li, Q. L.; Alvarez, P. J. J. Phosphate Changes Effect of Humic Acids on TiO2 Photocatalysis: from Inhibition to Mitigation of Electron-hole Recombination. Environ. Sci. Technol. 2017, 51, 514–521. (44) Sondheimer, S. J.; Bunce, N. J.; Fyfe, C. A. Structure and Chemistry of Nafion-H: A Fluorinated Sulfonic Acid Polymer. J. Macromol. Sci. 1986, 26, 353–413. (45) Huang, C. D.; Tan, K. S.; Lin, H. Y.; Tan, K. L. XRD and XPS Analysis of the Degradation of the Polymer Electrolyte in H2O2 Fuel Cell. Chem. Phys. Lett. 2003, 371, 80–85. (46) Seger, B.; Kamat, P. V. Fuel Cell Geared in Reverse: Photocatalytic Hydrogen Production Using A TiO2/Nafion/Pt Membrane Assembly with No Applied Bias. J. Phys. Chem. C 2009, 113, 18946–18952. (47) Antolini, E.; Giorgi, L.; Pozio, A.; Passalacqua, E. Influence of Nafion Loading In the Catalyst Layer of Gas-diffusion Electrodes for PEFC. J. Power Sources 1999, 77, 136–142.



(48) Vohra, M. S.; Tanaka, K. Enhanced Photocatalytic Activity of Nafion-coated TiO2. Environ. Sci. Technol. 2001, 35, 411–415. (49) Park, H.; Choi, W. Photocatalytic Reactivities of Nafion-coated TiO2 for the Degradation of Charged Organic Compounds under UV or Visible Light. J. Phys. Chem. B 2005, 109, 11667– 11674. (50) Wang, W. Y.; Ku, Y. Effect of Solution pH on the Adsorption and Photocatalytic Reaction Behaviors of Dyes Using TiO2 and Nafion-coated TiO2. Colloids Surf. A 2007, 302, 261–268. (51) Chen, K. T.; Lu, C. S.; Chang, T. H.; Lai, Y. Y.; Chang, T. H.; Wu, C. W.; Chen, C. C. Comparison of Photodegradative Efficiencies and Mechanisms of Victoria Blue R Assisted by Nafion-coated and Fluorinated TiO2 Photocatalysts. J. Hazard. Mater. 2010, 174, 598–609. (52) Zheng, X. L.; Song, J. P.; Ling, T.; Hu, Z. P.; Yin, P. F.; Davey, K.; Du, X. W.; Qiao, S. Z. Strongly Coupled Nafion Molecules and Ordered Porous CdS Networks for Enhanced VisibleLight Photoelectrochemical Hydrogen Evolution. Adv. Mater. 2016, 28, 4935–4942. (53) Arun, V.; Sankaran, K. R. Nafion Coated TiO2 and CuO Doped TiO2 Modified Glassy Carbon and Platinum Electrodes: Preparation, Characterization and Application of Enhancement of Electrochemical Sensitivity for Azines. J. Electroanal. Chem. 2016, 769, 35–41. (54) Park, H.; Choi, W. Visible-light-sensitized Production of Hydrogen Using Perfluorosulfonate Polymer-coated TiO2 Nanoparticles: An Alternative Approach to Sensitizer Anchoring. Langmuir 2006, 22, 2906–2911. (55) Qu, D.; Zheng, M.; Du, P.; Zhou, Y.; Zhang, L.; Li, D.; Tan, H.; Zhao, Z.; Xie, Z.; Sun, Z. Highly Luminescent S, N co-doped Graphene Quantum Dots with Broad Visible Absorption Bands for Visible Light Photocatalysts. Nanoscale 2013, 5, 12272–12277.


Page 30 of 33

Page 31 of 33 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


(56) Bader, H.; Sturzenegger, V.; Hoigné, J. Photometric Method for the Determination of Low Concentrations of Hydrogen Peroxide by the Peroxidase Catalyzed Oxidation of N,N-diethyl-pphenylenediamine (DPD). Water Res. 1988, 22, 1109–1115. (57) Kim, M. A.; Lee, W. Y. Amperometric Phenol Biosensor Based on Sol-gel Silicate/Nafion Composite Film. Anal. Chim. Acta 2003, 479, 143–150. (58) Wu, Z. L.; Gao, M. X.; Wang, T. T.; Wan, X. Y.; Zheng, L. L.; Huang, C. Z. A General Quantitative pH Sensor Developed with Dicyandiamide N-doped High Quantum Yield Graphene Quantum Dots. Nanoscale 2014, 6, 3868–3874. (59) Xiao, X.; Wang, T.; Bai, J.; Li, F.; Ma, T.; Chen, Y. Enhancing the Selectivity of H2O2 Electrogeneration by Steric Hindrance Effect. ACS Appl. Mater. Interfaces 2018, 10, 42534−42541. (60) Fei, H.; Dong, J.; Arellano-Jimenez, M. J.; Ye, G.; Dong Kim, N.; Samuel, E. L.; Peng, Z.; Zhu, Z.; Qin, F.; Bao, J.; Yacaman, M. J.; Ajayan, P. M.; Chen, D.; Tour, J. M. Atomic Cobalt on Nitrogen-doped Graphene for Hydrogen Generation. Nature commun. 2015, 6, 8668–8675. (61) Walekar, L. S.; Hu, P.; Liao, F.; Guo, X.; Long, M. Turn-on fluorimetric and colorimetric probe for hydrogen peroxide based on the in-situ formation of silver ions from a composite made from N-doped carbon quantum dots and silver nanoparticles. Microchim Acta 2018, 185, 31. (62) Metsik, J.; Timusk, M.; Kaambre, T.; Mandar, H.; Umalas, M.; Kuus, A.; Puust, L.; Utt, K.; Sildos, I.; Maeorg, U. Stability of Poly(3,4-ethylenedioxythiophene) Thin Films Prepared by Vapor Phase Polymerization. Polym. Degrad. Stabil. 2016, 126, 170–178. (63) Zhang, L.; Niu, J.; Li, M.; Xia, Z. Catalytic Mechanisms of Sulfur-doped Graphene as Efficient Oxygen Reduction Reaction Catalysts for Fuel Cells. J. Phys. Chem. C 2014, 118, 3545– 3553.



(64) Andersen, S. M.; Dhiman, R.; Skou, E. Chemistry of Carbon Polymer Composite ElectrodeAn X-ray Photoelectron Spectroscopy Study. J. Power Sources 2015, 274, 1217–1223. (65) Li, X. Z.; Chen, C. C.; Zhao, J. C. Mechanism of Photodecomposition of H2O2 on TiO2 Surfaces under Visible Light Irradiation. Langmuir 2001, 17, 4118–4122. (66) Zhao, H.; Qian, L.; Guan, X.; Wu, D.; Zhao, G. Continuous Bulk FeCuC Aerogel with Ultradispersed Metal Nanoparticles: An Efficient 3D Heterogeneous Electro-Fenton Cathode over a Wide Range of pH 3-9. Environ. Sci. Technol. 2016, 50, 5225–5233. (67) Zhang, M.; Bai, L. L.; Shang, W. H.; Xie, W. J.; Ma, H.; Fu, Y. Y.; Fang, D. C.; Sun, H.; Fan, L. Z.; Han, M.; Liu, C. M.; Yang, S. H. Facile Synthesis of Water-soluble, Highly Fluorescent Graphene Quantum Dots as A Robust Biological Label for Stem Cells. J. Mater. Chem. 2012, 22, 7461–7467.


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TOC Graphic


Enhanced Photocatalytic Production of H2O2 by Nafion Coatings on S

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