TiO2 Photonic Crystal Sensitized with Mn3O4 Nanoparticles and

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TiO Photonic Crystal Sensitized with MnO Nanoparticles and Porphine Manganese(III) as Efficient Photoanode for Photoelectrochemical Water Splitting Jie Huang, Dongmei Chu, Kezhen Li, Xia Li, Aijuan Liu, Chunyong Zhang, Yukou Du, and Ping Yang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b05985 • Publication Date (Web): 07 Dec 2017 Downloaded from http://pubs.acs.org on December 7, 2017

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

TiO2 Photonic Crystal Sensitized with Mn3O4 Nanoparticles and Porphine Manganese(III) as Efficient Photoanode for Photoelectrochemical Water Splitting

Jie Huang,a,b Dongmei Chu,a Kezhen Li,a Xia Li,a Aijuan Liu,a Chunyong Zhang,a Yukou Du,a Ping Yang*a

a

College of Chemistry, Chemical Engineering and Materials Science, Soochow University,

Suzhou, 215123, China b

Key Laboratory of Nano-Bio Interface, Division of Nanobiomedicine, CAS Center for

Excellence in Nanoscience, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou 215123, China

ACS Paragon Plus Environment

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ABSTRACT We report the synthesis and photoelectrochemical characterization of a novel composite consisted of Mn3O4 nanoparticles, porphine manganese(III) (PMA) and TiO2 photonic crystal (TPC). The prepared composite (Mn3O4/PMA/TPC) was used for fabricating the photoanode of a photoelectrochemical tandem cell. The obtained Mn3O4/PMA/TPC composite was characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), UV-vis diffuse reflectance spectroscopy (DRS). The results demonstrated that PMA and Mn3O4 nanoparticles had been loaded in the hole of TPC successfully. The photoelectrochemical characterization of Mn3O4/PMA/TPC electrode revealed an enhanced light harvesting and effective electron-holes separation. The photoelectrochemical tandem cell, of which Mn3O4/PMA/TPC electrode acted as a photoanode and a Pt plate as counter electrode, was used to evaluate the feasibility for water splitting to produce H2 and O2 under a 300 W solar simulator irradiation. The gases evolved from the system when the applied voltage was 1.0 V (vs. RHE). The evolution amount of hydrogen and oxygen can reach to 12.2 µmol and 4.4 µmol, respectively, under 4 h simulated solar-light irradiation. The possible mechanism of the surface modification effects was proposed. The results suggest that Mn3O4 nanoparticles and PMA modified TPC can act as efficient catalyst for fabricating photoanode in a photoelectrochemical tandem cell.


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1. INTRODUCTION Since the first report of photoelectrochemical decomposition of water into hydrogen and oxygen by the n-type semiconductor TiO2-Pt system,1 splitting water into hydrogen and oxygen under solar light irradiation has become one of the most promising strategies to meet sustainable energy demands of the human society. Much attention has been paid to photoelectrochemical (PEC) cells constructed by a separate anode and cathode, where oxygen and hydrogen can be generated, respectively, from the respective cells.2,3 Due to its excellent stability, non-toxic, and high catalytic performance, TiO2 has good application prospect in the photocatalytic field.4-6 However, some intrinsic defects, such as wide band gap and low light utilization ratio of TiO2, restrict its practical application.7,8 Porphine and its derivatives are promising materials for photochemical energy conversion and storage because of their excellent light-harvesting ability in the visible light region of the solar spectrum with high extinction coefficients.9 Semiconductors sensitized with porphine and its derivatives have been found to utilize the visible light region of the solar radiation efficiently.10,11 In general, photogenerated charge separation and transfer is strongly affected by the surface structural features of the materials.12-14 The macroporous layer of TiO2 is usually sintered on a transparent conducting substrate, which provides a large surface area for sensitizer adsorption.15-17 It has become widely recognized that TiO2 photonic crystal (TPC) is the outstanding representative of macroporous TiO2 materials.18 TPC with honeycomb structure can significantly increase the interaction between light and TiO2, as well as enhance the light absorption and photoactivity of TiO2.19 Mallouk et al.20 demonstrated that TPC could enhance the visible light absorption of dye sensitized solar cells. Tetreault and his colleagues21 confirmed that TPC was advantageous to the


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dye loading, electrolyte filling and charge collection. Li et al.22 found that TPC had higher photocatalytic efficiency than that of TiO2 nanoparticles (NPs). It has long been known that manganese oxide is essential for oxygen evolution in the photosynthetic process of green plants by using water as the electron source.23 In artificial photosynthesis, hausmannite (Mn3O4) as one of the mixed valence manganese oxides has been employed as a cocatalyst to build novel semiconductor composites for photocatalytic oxygen production.24 These composite photocatalysts exhibited higher performance since Mn3O4 NPs in the composites acted as O2 evolution centers.25 Moreover, Lin et al.26 indicated that Mn3O4 NPs combined with dye could improve the photoelectric conversion efficiency of solar cells. In this paper, we designed and fabricated a photocatalytic electrode composed of Mn3O4 NPs and porphine manganese (PMA) sensitized TiO2 photonic crystal (Mn3O4/PMA/TPC) as shown in Figure 1. PMA bridged the connection of Mn3O4 NPs and TPC. The PMA molecule has four sulfonic groups. Two sulfonic groups connected with TPC, and the other two connected with Mn3O4 NPs. Here, PMA adsorbed on TiO2 acts as a sensitizer to harvest incident light, TPC acts as an excellent semiconductor to adjust electron transfer, and Mn3O4 NPs act as cocatalyst to improve the photocatalytic activity for gas evolution. The prepared Mn3O4/PMA/TPC electrode was used as a photoanode to assemble a photoelectrochemical tandem cell with a Pt sheet as counter electrode. Through examination of the optoelectronic and photoelectrochemical properties of Mn3O4/PMA/TPC photoanode, we evaluate the feasibility of the tandem cell for overall water splitting under simulated solar-light irradiation without any sacrificial agent. The evolution amount of hydrogen and oxygen can reach to 12.2 µmol and 4.4 µmol, respectively, under 4 h simulated solar-light irradiation with 1.0 V (vs. RHE) bias. This investigation might


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provide a simple paradigm for realizing photocatalytic overall water splitting to produce H2 and O2. 2. EXPERIMENTAL SECTION 2.1 Materials. Tetrabutyl titanate, isopropyl alcohol, polyvinyl pyrrolidone (PVP, MW=10000), nitric acid (HNO3), manganese acetate tetrahydrate (Mn(Ac)2·4H2O), anhydrous ethanol, and polystyrene sphere (PSS, particle size = 1 µm) were purchased from J&K Company. 5,10,15,20-tetrakis(4-sulfonatophenul)-porphine manganese(III) chloride (PMA) was purchased from Sigma. All materials were used without further purification. 2.2 Preparation of Mn3O4 NPs. Mn3O4 NPs were prepared according to the published method.27 In brief, 0.3767g of Mn(Ac)2·4H2O was dissolved in 15.0 mL anhydrous ethanol. The resulting solution was transferred into a 30 mL Teflon-lined stainless steel autoclave, and treated at 180 °C for 11 h. After the reaction, the autoclave was allowed to cool to room temperature. The precipitate was washed with anhydrous ethanol several times, and dispersed in anhydrous ethanol for future use. 2.3 Preparation of TiO2 colloid. The TiO2 colloid was prepared by sol-gel method.28 In a typical experiment, 1 mL of tetrabutyl titanate was dissolved in 20 mL of isopropyl alcohol. Then, the solution was added dropwise to 50 mL of PVP solution (6×10-3 g mL-1, pH~1.5, adjusted by 1 M HNO3). The mixture was stirred for 6 h to obtain milky white colloidal solution of TiO2. 2.4 Preparation of PSS colloidal crystal template. Fluorine-doped tin oxide (FTO) glass was ultrasonic cleaned with acetone and anhydrous ethanol for 15 min respectively, and further ultrasonic cleaned with Piranha solution (VH2SO4:VH2O2=3:1) for 1 h. After that, the FTO glass was washed with water for three times, resulting in FTO glass with hydrophilic surface. The


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FTO glass with hydrophilic surface was soaked in water for future use. There is no obvious change of the sheet resistivity of the FTO glass before and after hydrophilic treatment, which means the surface property of FTO glass is stable after hydrophilic treatment. The cleaned FTO glass was firstly covered with a TiO2 blocking layer by spin-coating method, then, was treated at 80 °C for 30 min for preventing the contact between the redox mediator in the electrolyte and the FTO plate.29 The pretreated FTO glass was put vertically in a beaker containing with 0.25% PSS suspension. The beaker with FTO glass was put in a constant temperature humidity chamber and kept at 60 °C, 60% humidity for 24 h. During the process, the PSS were self-assembled on the surface of FTO glass. In order to increase the interaction between the PSS and the PSS with the FTO plate, the FTO glass covered with PSS was treated at 80 °C for 30 min,30 resulting in PSS colloidal crystal template. 2.5 Preparation of TPC electrode. The FTO glass covered with PSS colloidal crystal template was put vertically in the TiO2 colloidal solution (0.014 mg mL-1) for 30 min. Then the FTO glass was lifted vertically at the speed of 2.5 mm s-1. TiO2 colloid was deposited on surface of PSS colloidal crystal through the effect of capillary force. After that, the obtained FTO glass was dried at room temperature, and then treated in air at 450 °C for 4 h to remove PSS template,31 obtaining the TPC electrode. 2.6 Preparation of Mn3O4/PMA/TPC electrode. The TPC electrode was first immersed in the ethanol solution of PMA for 1 h and then in Mn3O4 ethanol solution for 30 min, respectively. After that, the TPC electrode was dried at 80 °C for 30 min, and let it naturally cool down to room temperature, obtaining Mn3O4/PMA/TPC electrode. 2.7 Characterization of materials. Morphological features of Mn3O4 NPs were characterized by TEM (Tecnai G2 F20 S-Twin). The morphological features of PSS colloidal


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crystal template, TPC electrode and Mn3O4/PMA/TPC electrode were characterized by SEM (Quanta 400 FEG). UV-vis diffuse reflectance spectrometry of the samples was recorded with UV-vis spectrophotometer (UV3010, Hitachi). Fourier transform infrared (FTIR) spectra of samples (KBr pellet) were collected using a Thermo Nicolet 6700 FTIR spectrometer. Raman spectra were obtained using a confocal microprobe Raman system (HR 800) equipped with a holographic notch filter and a CCD detector. A long working distance 50× objective was used to collect the Raman scattering signal. The size of the laser spot is 1.7 mm. The excitation wavelength was 633 nm from a He-Ne laser. 2.8 Photoelectrochemical measurement. The measurements of photoelectrochemical experiments were carried out on a CHI 660D potentiostat/galvanostat electrochemical analyzer in a three-electrode system consisted of a saturated calomel electrode (SCE) as a reference electrode, a working electrode and a platinum wire as counter electrode. The prepared TPC electrode, PMA/TPC electrode and Mn3O4/PMA/TPC electrode were used as working electrode. The electrolyte was 0.2 M of phosphate buffer solution (PBS, pH~7.0). The working electrode was irradiated by a GY-10 xenon lamp (150 W) during the measurement. All of the measurements were carried out at room temperature. The potential vs. SCE was converted to the RHE scale using the following equation:32 E RHE = E SCE + 0.059 pH + E 0 SCE

where ERHE is the converted potential vs. RHE, ESCE is the experiment value of potential vs. SCE, E0SCE = 0.2415 V at 25 °C. 2.9 Photocatalytic reaction. The photocatalytic reaction was carried out in a self-made three-electrode PEC device (Scheme 1). The Mn3O4/PMA/TPC electrode and a Pt plate were placed in two individual cells as photoanode and counter electrode, respectively, and an SCE as a


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reference electrode was put in a sub-chamber connecting with the working electrode cell. The cells were interconnected with a Nafion membrane to allow ion exchange. The electrodes were immersed in 0.5 M Na2SO4 (pH was adjusted to 1 with H2SO4) and were connected with a potentiostat. The photoanode and counter electrode were connected under short circuit. The system was deaerated by bubbling argon into the Na2SO4 solution for 1 h before the reaction. The working electrode was irradiated by a solar simulator (Ultra-vitalux, 15 mW cm-2) at 25 °C and atmospheric pressure. The wavelength range of the light source is in 280-980 nm. The distance between the working electrode and the lamp was about 20 cm. The gases evolved during the reaction were analyzed by an online gas chromatograph (GC 1690) equipped with a thermal conductivity detector and 5 Å molecular sieve columns using argon as a carrier gas.

3. RESULTS AND DISCUSSION 3.1. Characterization of the samples. TEM image shows that the average size of Mn3O4 NPs prepared by solvothermal method was 13 nm (Figure 2A and 2B). The high resolution TEM (HRTEM) image (inset of Figure 2A) indicates the obtained Mn3O4 NPs with a lattice parameter of 0.5 nm, which can be attributed to the (101) lattice fringes of Mn3O4 crystals.27 The energydispersive X-ray spectrometry (EDX) pattern (Figure 2C) shows the presence of Mn and O, demonstrating the elementary composition of Mn3O4 NPs. SEM image of PSS colloidal template is shown in Figure 3A. From the image, we can see that PSS in ca. 1 µm diameter immobilized on the FTO plate are arranged in a two-dimensional hexagonal close packing without vacancies or defects. The surface of TPC electrode with highly order structure confirms that the inverse holes are faithful replicas of the original PSS templates (Figure 3B). This honeycomb structure not only benefits for further loading sensitizers and


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cocatalyst NPs but also enhances the absorption ability of visible light.19 The pore size of TPC hole (450 nm) is only 45% of that of PSS (1 µm), which is due to the squeezing of TiO2 colloid to PSS during calcining.18 As shown in the inset of Figure 3B, the hole of TPC is consisted of TiO2 NPs. After TPC electrode was successively immersed in PMA solution and Mn3O4 colloidal solution, respectively, PMA was adsorbed on the surface of the holes of TPC electrode, and Mn3O4 NPs were chelated with sulfonic acid groups of PMA, thus, obtaining Mn3O4/PMA/TPC electrode (Figure 3C). The EDX pattern (Figure 3D) demonstrates the presence of Mn, Cl and S elements in the prepared Mn3O4/PMA/TPC electrode, indicating that PMA sensitizer and Mn3O4 NPs have been successfully loaded in the holes of TPC. 3.2. Investigation of optical and photoelectrochemical properties. Figure 4 demonstrates UV-vis diffuse reflectance spectroscopies (DRS) and UV-vis absorbance spectra of the electrode materials. The TiO2 NPs film has relatively high diffuse reflection capacities in the wavelength range between 400 and 450 nm, while a rapid decrease in diffuse reflection capacities is observed as the wavelength increases from 450 to 800 nm (Figure 4A). However, TPC electrode has higher diffuse reflection capabilities in the visible regions (from 400 to 800 nm) (Figure 4A), indicating that the incident light is significantly scattered within these macro-porous TPC electrodes.33 The high light scattering of TPC will increase the light harvesting efficiency.31,33 The multi-reflection of the incident light in air-filled pore of the TiO2 matrix can effectively enhance the optical path way.31,33 Figure 4B shows that PMA demonstrates a strong absorbance at 480 nm, which can be assigned to charge transfer mixed with π to π* transition of PMA moiety,34 and the weak absorbance in the range of 560-800 nm is attributed to the Q bands of the

a2u (π ) → eg* (π ) transition of PMA.34,35 From Figure 4B, we can also see that PMA/TPC and Mn3O4/PMA/TPC electrode exhibit the same two typical electronic absorptions to free PMA.


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However, the low energy Q band with maxima at 650 nm of PMA in the composite shows a 28 nm red-shift with respect to that of free PMA, which is attributed to the strong electronic coupling and significant electronic interaction between the PMA moiety and TiO2.36 Moreover, the Q band of PMA/TPC and Mn3O4/PMA/TPC electrode show much higher absorbance intensity than that of free PMA. This is because the high light scattering of TPC can improve the light absorbance of the dye, especially in the weak absorbance region.31,37 This phenomenon has been already observed by other research groups.20 Furthermore, the Mn3O4/PMA/TPC composite demonstrates further enhanced optical absorbance in UV-vis region, which can be attributed to the result of the interaction between TPC, MPA and Mn3O4 NPs.38 To confirm the connection between PMA and TPA, as well as the connection between PMA and Mn3O4 NPs, we performed the FTIR and Raman investigations about PMA, PMA modified TPC (PMA/TPC) and PMA modified Mn3O4 NPs (PMA/Mn3O4). Figure S1A presents the FTIR spectra of the samples. For PMA/TPC and PMA/Mn3O4, most typical characteristic peaks for TPC, Mn3O4, and PMA can be found in the spectra. The absorption band in the region of 450750 cm-1 can be appointed to the Ti-O stretching of TiO2,39 the peaks at 600 cm-1 are associated with the coupling mode between Mn-O stretching modes of octahedral sites.27 The shapes of the infrared absorption bands at 739, and 811 cm-1 derived from the C-H bending vibration of PMA, while the characteristic absorption band at 1630 cm-1 originated from the skeletal vibration of the porphyrins.40 The peak at 1009 cm-1 is assigned to the Mn-N vibration of PMA.41 Significantly, the characteristic peaks for sulfonic groups at 1037 and 1120 cm-1 blue shifted to 1044 and 1133 cm-1 for both PMA/TPC and PMA/Mn3O4. These results demonstrated the interaction between PMA and TPC, as well as between PMA and Mn3O4 NPs.40,42


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Figure S1B presents the Raman spectra of the samples. The Raman spectrum of TPC shows the characteristics peak at 636 cm-1, which is assigned to the fundamental vibrational modes of TiO2 with the symmetries of A1g.43 After PMA was adsorbed on TPC (PMA/TPC), the characteristic band of TPC shifted slightly to 640 cm-1, which can be attributed to the interaction between PMA and TPC.44 Moreover, Raman spectrum of PMA/TPC also includes peaks at 1368, 1237 and 1030 cm-1, donating to the porphyrin ring, Cm-phenyl, and Pyrrole vibration of PMA, respectively.45 These Raman spectra further prove that PMA was connected with TPC. The linear scan voltammetry (LSV) of TPC electrode under dark condition (Figure 5A, curve a) shows a cathodic current at 0.15 V (vs. RHE) bias. The cathodic current decreases when the applied potential increases from 0.15 V to 0.3 V (vs. RHE). At E > 0.3 V (vs. RHE), the current changes to an anodic one and it ascends gradually to a plateau (0.002 mA cm−2) at E = 0.35 V (vs. RHE). However, LSV of TPC electrode under illumination (Figure 5A, curve b) shows the anodic current at 0.15 V (vs. RHE) bias. The anodic current increases quickly when the applied potential increases from 0.15 V to 0.35 V (vs. RHE). At E > 0.35 V (vs. RHE), the current increases slowly, and shows a significantly higher current than that at the same applied potential under dark condition, indicating the good photoresponse of TPC electrode. For the PMA/TPC electrode, the anodic photocurrent density is 0.05 mA cm−2 at 0.35 V (vs. RHE) bias under the same illumination condition (Figure 5B, curve b), which is more than 2.5 times higher than that of the pristine TPC electrode (0.021 mA cm−2). The higher photocurrent density of the PMA/TPC electrode may be attributed to the nice light absorption of PMA and high light scattering effect of TPC.33 For the Mn3O4/PMA/TPC electrode, the photocurrent density increases to 0.12 mA cm−2 at 0.35 V (vs. RHE) bias under the same illumination condition (Figure 5C, curve b). Then the photocurrent density continues to increase to 0.21 mA cm−2 at 0.6


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V (vs. RHE) bias, which may be ascribed to that Mn3O4 NPs in the composites can enhance charge transfer efficiently, improving the photoresponse of Mn3O4/PMA/TPC electrode.38,46 From LSV, the relationship between the applied bias potential and the photocurrent can be found, the applied bias > 0.6 V (vs. RHE) is selected to measure the gas evolution of Mn3O4/PMA/TPC electrode. The results of photocurrent measurements of the samples are shown in Figure 5D. The photocurrent response of the bare FTO electrode is negligible under the performed conditions. TPC electrode demonstrates a weak photocurrent density (ca. 20 µA cm-2) due to its weak absorption to solar simulator irradiation. The photocurrent response of PMA/TPC electrode reaches to 27 µA cm-2, owing to the strong optical absorption of PMA and high light scattering effect of TPC, as well as efficient photoexcited electron transfer from PMA to TPC. For the Mn3O4/PMA/TPC electrode, the anodic photocurrent density increases to 60 µA cm-2, which is ca. 2.2 times as high as that of PMA/TPC electrode. The significant improvement of photocurrent response of Mn3O4/PMA/TPC electrode ascribes to good optical absorption of the Mn3O4/PMA/TPC composite, efficient charge transfer and separation among Mn3O4 NPs, PMA and TPC.38,46 3.3.

Photocatalytic

gas

evolution.

The

photocatalytic

performances

of

the

photoelectrochemical tandem cell composed of the as-prepared Mn3O4/PMA/TPC electrode as photoanode and Pt plate as counter electrode for overall water splitting were investigated under a solar simulator irradiation (light intensity of 15 mW/cm2). Figure 6A shows that no significant gases can be detected with the applied potential at 0.9 V (vs. RHE). The hydrogen and oxygen can be observed unambiguously when the applied potential is at 1.0 V (vs. RHE). The total amount of H2 produced over Pt electrode and O2 produced over Mn3O4/MPA/TPC electrode with


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1.5 h light irradiation is 0.11 µmol and 0.04 µmol, respectively. The amount of the evolved gases increases with increasing applied potential from 1.0 V to 1.3 V (vs. RHE), which is accordance with the results of LVS results. The result of the photocatalytic gases evolution from Mn3O4/PMA/TPC electrode under a constant applied potential at 1.0 V (vs. RHE) is shown in Figure 6B. In 4 h light irradiation, the amount of hydrogen and oxygen evolved from Pt electrode and Mn3O4/PMA/TPC electrode are 12.2 µmol and 4.4 µmol, respectively. For comparison, we investigated the gas evolution of the photoelectrochemical tandem cell under illumination of AM 1.5 G with light intensity of 100 mW/cm2 (Figure S2). Under light irradiation with higher intensity, the total amount of H2 produced over Pt electrode and O2 produced over Mn3O4/MPA/TPC electrode with 1.5 h light irradiation can reach to 12.9 µmol and 4.6 µmol (applied potential at 1.0 V (vs. RHE)), respectively. In 4 h light irradiation, the amount of hydrogen and oxygen evolved from Pt electrode and Mn3O4/MPA/TPC electrode increased to 48.6 µmol and 17.7 µmol (applied potential at 1.0 V (vs. RHE)), respectively. The gas evolution is higher than the measurements under light illumination with 15 mW/cm2 light intensity, because gas evolution is related with the intensity of illumination light under certain conditions.47 These results demonstrate that Mn3O4/PMA/TPC could be a promising photo-anodic catalyst for fabricating photoanode electrode to split water. Kim’s group reported a novel TiO2-based anode (TiO2@rGO@Au) for photoelectrochemical catalytic water splitting. The anode demonstrated a remarkable PEC performance. However, the noble metal Au was used as cocatalyst and the photocatalytic reactions was performed at relative higher applied potential.48 We measured the morphology and Raman spectra of Mn3O4/PMA/TPC electrode after photoelectrochemical experiment (Figure S3). The SEM image shows that Mn3O4 NPs were still in the hole of TPC (Figure S3A). And as shown by Raman spectra, the peaks at 640, 657 and 1368 cm-1, which are


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assigned to TiO2,43 Mn3O427 and PMA,45 respectively, could be also obtained (Figure S3B). These results demonstrated the stability of Mn3O4/PMA/TPC electrode. Moreover, we chose Na2SO4 solution (pH=1) instead of PBS for photoelectrochemical water splitting, because Na2SO4 solution (pH=1) is good for gas generation (more gas evolution and lower applied potential) than PBS solution, which may attribute to the ionic conductivity of Nafion in the acidic electrolyte is higher than that in PBS.49 The proposed water-splitting mechanism for the tandem photoelectrochemical device is illustrated in Scheme 2. Under the artificial solar light irradiation, Mn3O4/PMA/TPC electrode can be excited and the photo-induced electrons and holes be produced. The photoexcited electrons transfer to the Pt electrode through the external circuit under applied bias, where protons are reduced to H2. The Mn3O4 NPs, which are chelated with sulfonic acid groups of PMA, capture photogenerated holes. The accumulated oxidation potential on the NPs oxidizes water to produce O2 and protons. The Nafion film allows protons exchange between photoanode and cathode compartments to sustain charge balance during the reaction. Okada et al.50 have demonstrated that Na+ has some negative effect on the proton transport through Nafion membrane. The researchers also reported that ionic mobility in Nifion membrane was (1.49± 0.03)×10-7 m2 V-1 s-1 and (2.7±0.1)×10-8 m2 V-1 s-1 for H+ and Na+, respectively, which means that H+ moves about five times faster than Na+ in the membrane. However, in order to achieve better PEC performance, the negative effect of Na+ of electrolytes should be eliminated in the future researches.48,51

4. CONCLUSIONS


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In summary, we successfully prepared Mn3O4/PMA/TPC composite for fabricating anodic electrode, which shows excellent photoelectrochemical properties than that of TPC and PMA/TPC. The photocatalytic production of H2 and O2 by splitting water under simulated solar light illumination is realized from the photoelectrochemical tandem cell composed of the asprepared Mn3O4/PMA/TPC photoanode and Pt counter electrode. The light scattering of TPC enhances the optical absorption of PMA, and Mn3O4 NPs efficiently promote the separation of photogenerated electron-hole pairs, thereby increasing the photocurrent and cell efficiency. This study indicates that Mn3O4/PMA/TPC is a promising organic-inorganic composite for fabricating the photoanode of photoelectrochemical cell for overall water splitting.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. FTIR and Raman spectra of samples; SEM image and Raman spectrum of electrode after photoelectrochemical experiment.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (P. Yang). Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENTS The authors are grateful for the financial support of this research by the National Natural Science Foundation of China (21373143), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and the Project of Scientific and Technologic Infrastructure of Suzhou (SZS201207).


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Figure captions

Figure. 1 Schematic illustration of Mn3O4/PMA/TPC electrode.


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25

A Percentage (%)

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


Mn 0.5 nm

20

C

B

15 10 5 0

8

10

12

14

16

18

Particle size (nm)

Figure. 2 TEM image (A), size distribution (B), and EDX pattern (C) of Mn3O4 NPs. The inset of (A) is the HRTEM image of Mn3O4 NPs.


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A

B

C

D

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Figure. 3 SEM images of PSS colloidal crystal film (A), TPC electrode surface (B), and Mn3O4/PMA/TPC electrode surface (C). And (D) the EDX pattern of Mn3O4/PMA/TPC electrode surface (white box in (C)). The inset of (B) is the single hole of TPC.


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9

4

A

TiO2 NPs TPC

3 2 1 0 200

400 600 Wavelength (nm)

800

B Absorbance (a.u.)

Diffuse reflectance (%)

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


PMA PMA/TPC Mn3O4/PMA/TPC

6

3

0 200

400 600 Wavelength (nm)

800

Figure. 4 (A) UV-vis diffuse reflectance spectra of TiO2 NPs film and TPC electrode. (B) UVvis absorbance spectra of PMA, PMA/TPC electrode and Mn3O4/PMA/TPC electrode.


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0.02

A

(b)

0.021

(a)

0.00

0.002

-0.02 0.3 0.15

Current Density (mA cm-2)

0.04

0.65 1.15 Potential (V vs. RHE)

0.05

0.04

(a)

0.00

0.35

0.15


80

C

D

(b)

0.21 0.12

0.1 (a)

0.0 -0.1 0.15

J (mA cm-2)

0.2

(b)

B

-0.04

0.35

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0.08

0.3 Current Density (mA cm-2)

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

Current Density (mA cm-2)


60

blank TPC PMA/TPC Mn3O4/PMA/TPC

40 20

0.35

0.6


0 30

90

150 210 Time (s)

270

Figure. 5 The linear sweep voltammetry of TPC electrode (A), PMA/TPC electrode (B) and Mn3O4/PMA/TPC electrode (C) without (curve (a)) or with (curve (b)) light illumination (150 W), respectively, and scan rate = 10 mV s−1, in a three-electrode system with Pt as counter electrode and a saturated calomel electrode as reference electrode, operated in 0.2 M pH~7.0 PBS. And (D) photocurrent responses of FTO-electrode (blank), TPC electrode, PMA/TPC electrode, and Mn3O4/PMA/TPC electrode, to the chopped UV-vis light irradiation in 0.2 M pH~7.0 PBS.


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3.5 3.0

12

A

2.5

H2 O2

Gas evolution (µmol)

Gas evolution (µmol)

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


2.0 1.5 1.0 0.5 0.0 0.9

1.0 1.1 1.2 1.3 Potential (V vs. RHE)

1.4

B H2 O2

10 8 6 4 2 0

0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 Time (h)

Figure. 6 (A) The gas evolution of PEC device with Mn3O4/PMA/TPC electrode as the photoanode under different applied potential with light illumination (15 mW/cm2). (B) The gas evolution of PEC device with Mn3O4/PMA/TPC electrode as the photoanode under 1.0 V (vs. RHE) applied potential with light illumination (15 mW/cm2).


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Scheme 1. Appearance of the PEC.


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Scheme 2. The proposed photogenerated charges transporting process and the process of water oxidation on Mn3O4/PMA/TPC photoanode.


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TOC Graphic


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TiO2 Photonic Crystal Sensitized with Mn3O4 Nanoparticles and

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