Directed Nanoscale Self-assembly of Natural Photosystems on

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Directed Nanoscale Self-assembly of Natural Photosystems on Nitrogen-doped Carbon Nanotubes for Solar Energy Harvesting Insu Kim, Nyeongbeen Jo, Moon Young Yang, Jeonga Kim, Hwiseok Jun, Gil Yong Lee, Taeho Shin, Sang Ouk Kim, and Yoon Sung Nam ACS Appl. Bio Mater., Just Accepted Manuscript • DOI: 10.1021/acsabm.9b00120 • Publication Date (Web): 01 Apr 2019 Downloaded from http://pubs.acs.org on April 5, 2019

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ACS Applied Bio Materials

Directed Nanoscale Self-assembly of Natural Photosystems on Nitrogen-doped Carbon Nanotubes for Solar Energy Harvesting Insu Kim,† Nyeongbeen Jo,† Moon Young Yang,§ Jeonga Kim,† Hwiseok Jun,† Gil Yong Lee,†,‡ Taeho Shin,¶ Sang Ouk Kim,†,‡ and Yoon Sung Nam†,§,* †Department

of Materials Science and Engineering, ‡Multi-Dimensional Directed Nanoscale Assembly Center, and §KAIST Institute for NanoCentury, Korea Advanced Institute of Science and Technology, 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea. ¶Department of Chemistry, Chonbuk National University, 567 Baekje-daero, Deokjin-gu, Jeonju-si, Jeollabuk-do 54896, Republic of Korea *Address correspondence to [email protected]

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ABSTRACT Natural photosystems (PSs) have received much attention as a biological solar energy harvester because of their high quantum efficiency for energy transfer. However, the PSs hybridized with solid electrodes exhibit low light-harvesting efficiencies because of poor interface properties and random orientations of PSs, all of which interfere with efficient charge extraction and transfer. Herein, we report the linker-free, oriented self-assembly of natural PSs with nitrogen-doped carbon nanotubes (NCNTs) via electrostatic interaction. Protonated nitrogen-doped sites on the NCNTs facilitate spontaneous immobilization of the negatively charged stroma side of PSs, which provides a favorable orientation for electron transfer without electrically insulating polymer linkers. The resulting PS/NCNT hybrids exhibit a photocurrent density of 1.25 ± 0.08 A cm-2, which is much higher than that of PS/CNT hybrids stabilized with polyethylenimine (0.60 ± 0.01 A cm-2) and sodium dodecyl sulfate (0.14 ± 0.01 A cm-2), respectively. This work emphasizes the importance of the linker-free assembly of PSs into well-oriented hybrid structures to construct an efficient lightharvesting electrode.

KEYWORDS: Photosystems; Nitrogen-doped carbon nanotubes; Self-assembly; Electrostatic interaction; Light-harvesting



Introduction Photoelectrochemical conversion of solar energy is undoubtedly a promising route toward the hydrogen economy.1-2 It is now well recognized that efficient water splitting under benign conditions is a critical requirement for the sustainable production of solar fuels.3-4 Solar energy, the most abundant energy (> 120,000 terawatts) among renewable energy resources,5 has been extensively studied to convert sunlight into clean and stored energy.6-8 However, photoelectrochemical cells still have limited quantum efficiency and poor stability because of fast charge recombination and chemical degradation of photoelectrodes.9-11 Photosynthetic protein complexes, a biological solar energy harvester in nature, have received much attention as a biomimetic model for solar fuel production.12-14 The photosynthetic protein complexes comprising oxygen evolving cluster (OEC), plastoquinone (Pq), cytochrome (Cyt), plastocyanin (Pc), ferredoxin (Fd), NADP+ reductase, ATP synthase, photosystem (PS) I, and PS II are sophisticatedly assembled to absorb sunlight, generate excited charges, transfer the electrons to the reaction sites, and produce chemical fuels.15 In particular, the quantum efficiency of the natural light-harvesting antenna is nearly 100 %.16 Among the photosynthetic protein complexes, PS II and I are known as a key protein complex to light absorption, production of electron through water splitting, and fluent electron transfer.17,18 Many efforts have been made to harness the functionality of the PSs for solar energy harvesting.19,20 Previous works reported techniques to hybridize PSs with solid carbon, gold, and transparent conductive electrodes through physical adsorption,21 covalent bonding,22-23 hydrogen bonding,24 electrostatic attraction,25-26 and van der Waals interactions.27 In the absence of interfacial linker molecules, PSs are randomly associated with the electrodes.28 Unfortunately, disordered PS arrays hamper efficient electron transfer, which leads to fast charge recombination and a low quantum efficiency.29-30 With the aim to construct efficient 4 ACS Paragon Plus Environment

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light-harvesting systems with optimal electronic interactions, many attempts have been made to assemble PSs with the electrodes with a chemically favorable molecular configuration using polymers,8, 12 peptides,31-32 and DNA linkers.33 The interfacial linkers between the PSs and the electrodes need to ensure the specific orientation of PSs to promote the efficient generation of photocurrents. Besides, the linkers should not deteriorate the electronic interface structure. However, the generally-used linkers are electrically insulating, which increases the charge transfer resistance and thus compromises the final efficiency.34 The organic linkers are also easily degradable in harsh operating conditions, which reduces the durability of the PS-based photoelectrode. Here we introduce the linker-free self-assembly of PSs on nitrogen-doped carbon nanotubes (NCNTs) to construct an efficient photoanodic electrode. Our previous report demonstrated that the protonated N-doped sites of NCNTs facilitate linker-free electrostatic spontaneous binding of polyoxometalates with a chemically favorable orientation due to the balanced intermolecular interactions.34 In this work, PSs and NCNTs were directly selfassembled through electrostatic interactions to generate PS/NCNT hybrids without organic linkers. The well-oriented hybrid structure effectively suppresses charge recombination, leading to a high photocatalytic activity. Taking benefits of the linker-free and well-oriented structure, the PS/NCNT hybrids generated a high photocurrent density of 1.25 ± 0.08 A cm-2 at 0.3 V vs. Ag/AgCl under 1 sun illumination with a 450 nm long pass filter.



Results and Discussion Fresh spinach was filtered through four layers of cheesecloth to remove unblended leaves and ribs followed by mechanical squeezing. After the centrifugation of the squeezed and filtered spinach, as-extracted intact chloroplasts were obtained as a pellet (Figure 1a). Figure 1b shows the schematic illustration of chloroplast structures. To break the outer and inner membranes of chloroplasts and isolate photosystems from the lipid membrane of thylakoids, we treated the as-extracted chloroplasts with Triton X-100 as a surfactant with homogenization (Figure 1c). See details in the experimental section. The chemical functional groups of the asextracted and surfactant-treated chloroplasts were compared using Fourier transform infrared (FT-IR) analysis (Figure 1d). The peaks newly found in the surfactant-treated sample were assigned to the OEC at M-O coordination chelates (500-700 cm-1 and 830 cm-1),34 Pq at aromatic C=C stretching (1505 cm-1 and 1580 cm-1),35 and Pc at CH3SCH2- (775 cm-1), CH3-S(950 cm-1), -CH2-S-, CH2 wagging and C-N stretching (1240 cm-1), CH2-S-, CH2 deformation (1420 cm-1), C=N and C=C stretching (1450 cm-1), and N=C-N stretching (1630 cm-1),36,37 which are placed in the lipid membrane or the lumen, as shown in Figure 1c. The chemical structures of OEC, Pq, and Pc are shown in Figure S1 (Supporting Information). To remove small protein subunits such as Pq (750 Da), Pc (10.5 kDa), and Fd (10.95 kDa), we dialyzed the surfactant-treated sample using a 50 kDa molecular weight cut-off membrane against Trisbuffered saline at pH 7.8 at an ambient temperature.


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Figure 1. (a) Optical image of intact chloroplasts extracted from Spinach. (b) Schematic illustrations of chloroplast (b) and isolation of photosynthetic proteins from the lipid membrane of thylakoids (c). (d) FT-IR bands of as-extracted (orange) and surfactant-treated chloroplasts (blue). FT-IR bands of OEC (1), Pq (2), and Pc (3) were additionally detected in the surfactant-treated sample while not shown in the as-extracted chloroplasts. FT-IR spectroscopy was conducted using 1 wt-% sample in KBr pellets. The charge distribution over the entire surface of PSI and PSII is displayed in Figure 2a. The surfaces of PSI and PSII at the lumen side are negatively charged, enabling the spontaneous electrostatic hybridization of the PSs with protonated NCNTs (Figure 2b). Distinctive peaks for pyridinic N, pyrrolic N, quaternary N, and oxidized N appeared after N doping to the pristine CNTs (Figure S2). The surface charge of NCNT becomes positive below about pH 5 due to the protonation of lone pair electrons of N-dopants, enabling the electrostatic 7 ACS Paragon Plus Environment


assembly with the negatively charged lumen side of PSs (Figure S3).34 Transmission electron microscopy (TEM) image showed that the PSI and PSII were directly self-assembled on the NCNT surface into the ligand-free nano-assembled PS/NCNT hybrids (Figure 2c). Chemical structures of the PS/NCNT hybrids were characterized using UV-Vis absorption, FT-IR, and Raman spectroscopies. In the UV-vis absorption spectra, PS and PS/NCNT hybrids exhibit the absorption bands at 417 nm and 669 nm, which are attributed to the light absorption of PSI and PSII, respectively (Figure 3a). No absorption bands of NCNT exist in the same wavelength range. Various characteristic FT-IR bands of PS subunits in a range of 750-1600 cm-1 were observed in PS and PS/NCNT hybrids (Figure 1d), NCNT exhibits only stacking vibration of C=C in the aromatic ring band at 1380 cm-1 and C=C stretching at 1635 cm-1 (Figure S4a). These results indicate that PSs were successfully self-assembled with NCNT, and their intrinsic chemical structures were maintained during the hybridization process. In addition, the Raman intensity ratios of the D and G bands (ID/IG) of NCNT and PS/NCNT hybrids were compared (Figure 3b and S4b). The ID/IG ratio can be an indicator for the electrostatic complexation of NCNTs with negatively charged molecules as previously reported.34 The ID/IG ratios of pristine NCNT and the PS/NCNT hybrids were 1.13 and 0.92, respectively, confirming the electrostatic self-assembly of PSs on the protonated NCNT surface.


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Figure 2. Schematic illustrations of charge distribution of PSI and PSII in thylakoid membrane (a) and spontaneous electrostatic hybridization of PSI and PSII with protonated NCNTs (b). (c) TEM image of PS/NCNT hybrids. PSs were negatively stained by 1 wt-% uranyl acetate.

Figure 3. (a) UV–visible spectra of PS, NCNT, and PS/NCNT hybrids. (b) Raman spectra of NCNT and PS/NCNT hybrids. NCNT and hybrid exhibit D and G bands of defective disordered and crystalline graphitic sites at the NCNT surface, respectively.



The radiative photoluminescence (PL) decay of PSs was measured to examine the charge transfer in the assembled PS/NCNT hybrids (Figure 4a). For the comparison with the linker-mediated PS hybrids, we prepared polyethylenimine (PEI) and sodium dodecyl sulfate (SDS)-stabilized CNTs, denoted by ‘PEI-CNT’ and ‘SDS-CNT,’ respectively, as cationic and anionic linker-mediated electrostatic binding substrates for the hybridization of PSs under the identical experimental conditions as the hybridization of PS/NCNT. The PEI-CNT and SDSCNT exhibit the surface potential of 16.84 and -15.77 mV at pH 7, respectively. In the hybridized PS/NCNT, the PL intensity of PS at 680 nm was quenched by 94.9 % compared to the pristine PSs, and the excited electrons of PSs were drained to the NCNTs. However, in the case of the linker-mediated hybrids of PS/PEI-CNT and PS/SDS-CNT, the reductions of PL quenching were 87.9 % and 30.6 %, respectively, which indicates that polymer linker hinders electron transport to the CNT substrates. Electrochemical impedance spectroscopy (EIS) was carried out to compare the charge transfer resistances (Rct) of linker-free PS/NCNT and linkermediated hybrids of PS/PEI-CNT and PS/SDS-CNT. With an equivalent circuit model, the calculated Rct of the PS/NCNT (55.4 k was around 1.9 and 2.9 times smaller than the resistances of the PS/PEI-CNT (106 k and PS/SDS-CNT (161 k, respectively (Figure S5). The result confirms that the direct hybridization of PS/NCNT is beneficial for fast charge transfer while polymeric adhesive layers increase charge transfer resistance. Our previous study also showed that the PEI-stabilized CNTs exhibited twice higher charge transfer resistance than the NCNTs by hybridized with polyoxometalates for the electrolysis of water.34 No significant PL was observed for the Bare NCNT, PEI-CNTs, and SDS-CNTs. We also obtained time-resolved PL decay spectra of PS, PS/NCNT, PS/PEI-CNT, and PS/SDS-CNT to determine whether the immobilization of PSs to CNTs creates a new nonradiative energy transfer pathway, which reduces the PL lifetime (Figure 4a).38,39 The PL lifetime of PS, PS/NCNT hybrids, PS/PEI-CNT, and PS/SDS-CNT were 4.92 ns, 3.95 ns, 3.36 ns, and 4.92 ns, respectively. The faster PL decays of PS/NCNT hybrids and PS/PEI-CNT are 10 ACS Paragon Plus Environment

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ascribed to resonance energy transfer among densely assembled PSs, which serves as a relaxation pathway. The larger number of PSs were closely assembled because the surface charge density of PEI-CNT is higher than that of NCNT (5.84 mV and 16.84 mV for NCNT and PEI-CNT, respectively, in zeta potential measurements). To investigate the photoactivity for the PS/NCNT hybrids, we measured the photocurrent using chronoamperometry at 300 mV vs. Ag/AgCl using a three-electrode system as shown in Figure 4b. Chopped 1 sun illumination was used with a 450 nm long pass filter at a light intensity of 78 mW cm-2. Negligible photocurrent density (0.06 ± 0.01 A cm-2) was obtained from the bare PSs. Without appropriate substrates, PSs are randomly and loosely assembled with electrodes, which restricts efficient charge extraction to the electrodes and leads to low an anodic current density. The linker-mediated hybrids, PS/PEI-CNT and PS/SDS-CNT, exhibited photocurrent densities of 0.60 ± 0.01 A cm-2 and 0.14 ± 0.01 A cm-2, respectively. The PS/NCNT hybrids generated a much higher photocurrent density, 1.25 ± 0.08 A cm-2, which is approximately 20.8 times higher than that of the bare PS.

The photocurrent density of the PS/NCNT hybrids was comparable with the previous works of PS-based hybrids for light-harvesting (Table S1).



Figure 4. (a) PL intensity of PS, NCNT, PEI-CNT, SDS-CNT, PS/NCNT, PS/PEI-CNT, and PS/SDS-CNT hybrids with excitation at 430 nm. (b) Photocurrent of PS, PS/NCNT, PS/PEICNT, and PS/SDS-CNT hybrids at 300 mV vs. Ag/AgCl under chopped 1 sun illumination with a 450 nm long pass filter at a intensity of 78 mW cm-2. It is worth noting that our hybrids were spontaneously assembled by simple mixing without additional pre-treatments or surface activation of the NCNT substrates. Furthermore, the linker-free assembly of PSs with NCNTs is advantageous for the fast charge transport due to the absence of insulating organic linkers. Additionally, the well-oriented configuration of the PS/NCNT hybrids led to efficient charge extraction of excited photoelectrons from the PS reaction center to the electrode, as schematically described in Figure 5a. These features provide an efficient light-harvesting electrode with a higher photocurrent density. In contrast, the molecular linkers (PEI and SDS) increased charge transfer resistance, resulting in a lower photoactivity of the PS/linker/CNT hybrids (Figures 5b and 5c). In particular, the negatively charged functional group of SDS made the CNTs hybridized with the stroma side of PS, which estranges the reaction centers located at the lumen side from the CNT substrate, decreasing the efficiency of photoelectrochemical reactions.


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Figure 5. Schematic presentations for charge transport and recombination of PS/NCNT (a), PS/PEI-CNT (b), and PS/SDS-CNT hybrids (c).



Conclusion In summary, we have shown that the photosynthetic proteins can be directly self-assembled on NCNTs into the linker-free PS/NCNT hybrid nanomaterials, which exhibit excellent catalytic photoactivity. The electrostatic integration of PSs with NCNTs without organic linkers is critically beneficial for fast charge transport, providing light-harvesting with a relatively higher conversion efficiency. However, the photocurrent density in this work is still very lower compared to other semiconductor-based photoelectrodes. Further progress needs to be made to increase the total number of PSs deposited per unit electrode area possibly through threedimensional assembly (e.g., layer-by-layer deposition). We expect that the genetic modification of the photosynthetic proteins can also allow us to integrate electron shuttling proteins in natural photosynthesis with nanoscale materials to further facilitate fast and efficient electron transfer toward optimized light-harvesting applications.

Experimental Methods Extraction of PSs from spinach: Spinach oleracea was washed three times in the excess amount of cold water and remove ribs to collect clean spinach leaves. The leaves were mechanically squeezed using a commercially available automatic squeezer (HP-MWF12, Hurom, Gimhae, Republic of Korea) and filtered through four layers of cheesecloth to remove debris. About 300 g wet weight of spinach leaves became 150 mL of spinach juice. After centrifugation at 8,000 rcf for 5 min of spinach juice, the pellet was resuspended in the solubilization buffer (0.33 M sorbitol, 2 mM EDTA, 1 mM MgCl2, 1 mM MnCl2, 50 mM HEPES, and 1 % Triton X-100) by high speed homogenization (Wise Tis HG-15A homogenizer, Witeg Labortechnik GmbH, Wertheim, Germany) at 10,000 rpm for 5 min to break chloroplast membrane and isolate PSs from the lipid membrane of thylakoids. Insoluble impurities were removed by centrifugation at 12,000 rcf for 15 min. The supernatant was dialyzed against Tris14 ACS Paragon Plus Environment

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buffered saline at pH 7.8 in 50 kDa molecular weight cut-off dialysis tubing to isolate PSs and remove other small protein subunits. Hybridization of PSs with carbon nanotubes: Synthesis of NCNTs is described elsewhere.34 HiPco MWCNTs were dispersed in a 1 wt-% PEI and SDS aqueous solution and suspended by probe-type sonication for 30 min. Homogeneously dispersed PEI-CNT and SDSCNT were collected after precipitation by centrifugation at 16,000 rcf for 2 h. Five milligrams of NCNT, PEI-CNT, and SDS-CNT were added to a 15 mL aqueous solution. The aqueous solution of PSs (in Tris-buffered saline, pH 7.6, 5 mL, 1 mg mL-1) was mixed with the CNT suspensions with magnetic stirring at 250 rpm for 8 h, respectively. Unbound PSs in the aqueous supernatant were removed after centrifugation at 8,000 rcf for 15 min. PS/CNT hybrids were collected as pellets. Rinse the pellet three times by centrifugation at 8,000 rcf for 15 min in Tris-buffered saline (pH 7.6). (Photo)electrochemical analysis: Five milligrams of PS/NCNT, PS/PEI-CNT, and PS/SDS-CNT solutions were mixed with 50 μL of a 10 wt-% Nafion aqueous solution to prepare electrode inks. After sonication for 30 min, 20 μL of the inks were loaded onto a glassy carbon electrode (surface area = 0.07 cm2). Photoelectrochemical measurements were performed using a three-electrode system with an IVIUM potentiostat (IVIUM Technologies, Eindhoven, Netherlands). An Ag/AgCl (in 3 M NaCl) electrode and Pt wires were used as the reference and counter electrodes, respectively. Working buffer was 5 mM potassium phosphate buffer (pH 7) containing 100 mM KCl and 200 μM K4Fe(CN)6 for EIS and chronoamperometry. EIS was obtained at open circuit potential from 0.1 to 10 kHz. Photocurrent density was measured by chronoamperometry at 300 mV vs. Ag/AgCl under chopped illumination corresponding to 1 sun with a 450 nm long pass filter at a light intensity of 78 mW cm-2. Characterization and analysis: Optical images were obtained on an inverted microscope (LEICA DMI30000 B, Leica Microsystems, Wetzlar, Germany). Zeta potential was measured by ELSZ-1000 (Otsuka Electronics Co., Ltd., Osaka, Japan) in 50 mM NaCl aqueous solution 15 ACS Paragon Plus Environment


by adjusting pH 3 to 7.5 using 0.1 M HCl and NaOH. Field emission-TEM (FE-TEM, 200 kV) images were obtained on an FEI Tecnai F20 (Thermo Fisher Scientific, MA, USA). The characterization of solid powder PS, NCNT, and PS/NCNT hybrids was performed using FTIR (Bruker IF66/S and Hyperion 3000, Billerica, MA, USA) and Raman (ARAMIS, Horiba Jobin Yvon, Lille, France) spectroscopies. The absorbance was measured using UV−vis light absorption spectroscopy (UV-1800, Shimadzu Corp., Kyoto, Japan). PL intensities were analyzed using a fluorescence spectrophotometer (F-7000, Hitachi, Tokyo, Japan) with an excitation wavelength of 430 nm. All absorption and fluorescence analyses were measured using Tris-buffered saline (pH 7.6). Electrostatic potential of PSs was calculated and visualized by APBS40 and VMD41, respectively.


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Supporting Information. Chemical structures of OEC, Pq, and Pc. N 1s XPS spectra of pristine CNTs and NCNTs. Zeta potentials of NCNTs. FT-IR and Raman spectra of PS, NCNT, and PS/NCNT hybrids. EIS spectra of PS/NCNT, PS/PEI-CNT, and PS/SDS-CNT. Time-resolved PL decay spectra of PS, PS/NCNT, PS/PEI-CNT, and PS/SDS-CNT. A table providing a comparison of this study with previously reported light harvesting PS-based hybrids.

Acknowledgements We thank Mr. Hyun Seung Oh for his technical assistance on PL lifetime measurements. This work was supported by Nano•Material Technology Development Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2017M3A7B4052797) and the National Creative Research Initiative (CRI) Center for Multi-Dimensional Directed Nanoscale Assembly (2015R1A3A2033061). References 1. Bergmann, A.; Zaharieva, I.; Dau, H.; Strasser, P., Electrochemical Water Splitting by Layered and 3D Cross-linked Manganese Oxides: Correlating Structural Motifs and Catalytic Activity. Energy Environ. Sci. 2013, 6, 2745-2755. 2. Gao, M. R.; Sheng, W. C.; Zhuang, Z. B.; Fang, Q. R.; Gu, S.; Jiang, J.; Yan, Y. S., Efficient Water Oxidation Using Nanostructured alpha-Nickel-Hydroxide as an Electrocatalyst. J. Am. Chem. Soc. 2014, 136, 7077-7084. 3. Kanan, M. W.; Nocera, D. G., In Situ Formation of an Oxygen-evolving Catalyst in Neutral Water Containing Phosphate and Co2+. Science 2008, 321, 1072-1075. 4. Yin, Q. S.; Tan, J. M.; Besson, C.; Geletii, Y. V.; Musaev, D. G.; Kuznetsov, A. E.; Luo, Z.; Hardcastle, K. I.; Hill, C. L., A Fast Soluble Carbon-Free Molecular Water Oxidation Catalyst Based on Abundant Metals. Science 2010, 328, 342-345. 5. Lewis, N. S.; Nocera, D. G., Powering the planet: Chemical Challenges in Solar Energy Utilization. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 15729-15735. 6. Shi, Y. M.; Yu, Y. F.; Yu, Y.; Huang, Y.; Zhao, B. H.; Zhang, B., Boosting Photoelectrochemical Water Oxidation Activity and Stability of Mo-Doped BiVO4 through the Uniform Assembly Coating of NiFe-Phenolic Networks. ACS Energy Lett. 2018, 3, 1648-1654. 7. Zhang, R.; Fang, Y. Y.; Chen, T.; Qu, F. L.; Liu, Z.; Du, G.; Asiri, A. M.; Gao, T.; Sun, X. P., Enhanced Photoelectrochemical Water Oxidation Performance of Fe2O3 Nanorods Array by S Doping. ACS Sustain. Chem. Eng. 2017, 5, 7502-7506. 8. Li, G. L.; Feng, X. Y.; Fei, J. B.; Cai, P.; Li, J. A.; Huang, J. G.; Li, J. B., Interfacial Assembly of Photosystem II with Conducting Polymer Films toward Enhanced PhotoBioelectrochemical Cells. Adv. Mater. Interfaces 2017, 4, 1600619. 9. Moss, B.; Lim, K. K.; Beltram, A.; Moniz, S.; Tang, J. W.; Fornasiero, P.; Barnes, P.; Durrant, J.; Kafizas, A., Comparing Photoelectrochemical Water Oxidation, Recombination Kinetics and Charge Trapping in the Three Polymorphs of TiO2. Sci. Rep. 2017, 7, 2938. 17 ACS Paragon Plus Environment


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