Mimicking the Key Functions of Photosystem II in Artificial

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Cite This: J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Mimicking the Key Functions of Photosystem II in Artificial Photosynthesis for Photoelectrocatalytic Water Splitting Sheng Ye,†,‡ Chunmei Ding,‡ Ruotian Chen,‡ Fengtao Fan,‡ Ping Fu,‡ Heng Yin,‡ Xiuli Wang,‡ Zhiliang Wang,‡ Pingwu Du,† and Can Li*,†,‡ †

School of Chemistry and Materials Science, University of Science and Technology of China, Jinzhai Road 96, Hefei 230026, China State Key Laboratory of Catalysis, Dalian National Laboratory for Clean Energy, the Collaborative Innovation Center of Chemistry for Energy Materials (iChEM), Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Zhongshan Road 457, Dalian 116023, China

‡

S Supporting Information *

ABSTRACT: It has been anticipated that learning from nature photosynthesis is a rational and effective way to develop artificial photosynthesis system, but it is still a great challenge. Here, we assembled a photoelectrocatalytic system by mimicking the functions of photosystem II (PSII) with BiVO4 semiconductor as a light harvester protected by a layered double hydroxide (NiFeLDH) as a hole storage layer, a partially oxidized graphene (pGO) as biomimetic tyrosine for charge transfer, and molecular Co cubane as oxygen evolution complex. The integrated system exhibited an unprecedentedly low onset potential (0.17 V) and a high photocurrent (4.45 mA cm−2), with a 2.0% solar to hydrogen efficiency. Spectroscopic studies revealed that this photoelectrocatalytic system exhibited superiority in charge separation and transfer by benefiting from mimicking the key functions of PSII. The success of the biomimetic strategy opened up new ways for the rational design and assembly of artificial photosynthesis systems for efficient solar-to-fuel conversion.

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INTRODUCTION Photoelectrocatalytic (PEC) water splitting is becoming a promising way for producing chemical energy from solar energy.1−12 Essentially, PEC working mechanism is somewhat analogous to the light reaction systems of photosynthesis (PSII and PSI). Photosynthesis utilizes photons to drive the thermodynamically demanding water oxidation reaction at an oxygen evolving complex (OEC) in photosystem II (PSII), where the light absorbing, charge separation, and transfer occur efficiently.13−16 Surrounding the OEC (Mn4CaO5 cluster), there are a great many amino acid residues connected to the OEC by hydrogen bonding. A close tyrosine residue (TyrZ or YZ) acts as a charge mediator between the Mn4CaO5 catalyst and the light harvester, P680 chromophore.17,18 Upon obtaining a certain amount of collected light energy, P680 is photoexcited to produce a cation radical, P680+, which furnishes the oxidizing potential for water splitting with the redox-active TyrZ.19,20 Developing renewable energy with artificial photosynthesis highlights the importance of learning from nature photosynthesis.21−24 On one hand, despite various efforts made thus far to mimic the P680 chromophore using photosensitizers, such as organic dyes or inorganic nanocrystals,25−30 the construction of highly efficient and robust PEC systems remains challenging. On the other hand, many of water oxidation catalysts being structurally analogous to Mn4CaO5 cluster in PSII have been synthesized; however, they cannot show anticipated performance in photo(electro)catalytic systems.31−33 © XXXX American Chemical Society

In this work, we try to mimic the key constituents and functions of PSII and then construct a highly efficient artificial photosynthesis system for PEC water splitting (Figure S1). We employed BiVO4 semiconductor as the light harvester34−41 protected by a layered double hydroxide (NiFeLDH) as a hole storage layer, a partially oxidized graphene (pGO) as biomimetic TyrZ for charge transfer, and a new ligand-modified Co cubane with a phosphonate linkage as OEC. With this configuration, this PEC system achieves an ultralow onset potential (0.17 V), approaching the thermodynamically limited value, with a high photocurrent of 4.45 mA cm−2 at 1.23 V versus reversible hydrogen potential (RHE) and a high solar to hydrogen efficiency (STH > 2.0%).

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RESULTS AND DISCUSSION Figure 1a displays the PEC performances of a series of BiVO4based photoanodes. BiVO4 photoanode alone shows a low photocurrent, 1.3 mA cm−2 at 1.23 V versus RHE. As expected, Co4O4(O2CMe)4(PO3Et2) (CoPO3, Figures S2−S6) catalyst brings an enhancement in photocurrent and an improvement in onset potential by 140 mV compared with BiVO4 alone (Figures S6 and S7) due to the accelerated water oxidation kinetics. Moreover, CoPO3/BiVO4 photoanode demonstrates much higher photocurrent than CoPy/BiVO4 (Figure S8). This is Received: October 6, 2017 Published: January 17, 2018 A

DOI: 10.1021/jacs.7b10662 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Journal of the American Chemical Society

Figure 2. Interfacial charge-transfer resistance of artificial photosynthesis system. Tested (dot) and simulated (line) electrochemical impedance spectroscopy (EIS) of BiVO4, CoPO3/BiVO4, CoPO3/ pGO/BiVO4, and CoPO3/pGO/LDH/BiVO4 photoanodes at 0.8 V versus RHE under AM1.5G (1 sun) illumination. The equivalent circuit is in the inset.

most efficient interfacial charge transfer in the artificial photosynthesis system. To clearly characterize the charge separation and transfer process, we employed spatially resolved surface photovoltage spectroscopy (SR-SPS) and Kelvin probe force microscopy (KPFM).44 The amplitudes of varying contact potential difference (CPD) signals with tunable wavelength and chopped light were gained by a lock-in amplifier. Figure 3a shows that BiVO4 photoanode alone produces only a small surface photovoltage (SPV) due to the poor electron−hole separation efficiency in bare semiconductor. It is worth noting that SPV intensity is increased by up to seven-fold after adding CoPO3 catalyst, proving that the charge separation is remarkably improved and the recombination of accumulated holes with electrons is suppressed. More interestingly, the SPV value of CoPO3/pGO/BiVO4 almost doubles that of CoPO3/BiVO4 photoanode, indicating that the pGO layer promotes hole transfer, which is different from the function of reduced graphene oxide (RGO) as the electron transfer mediator.29,45,46 Very surprisingly, the SPV intensity of CoPO3/pGO/LDH/BiVO4 is up to an order of magnitude higher than that of bare BiVO4 photoanode, demonstrating a significant boost in charge separation for BiVO4 semiconductor, resulted from the successful construction of artificial photosynthesis system. Additionally, Figure 3b shows steady state CPD images for all the photoanodes before and after light irradiation. The z-axis represents the value of CPD, which reflects the concentration of photogenerated holes. All the photoanodes display an increase of CPD under illumination, indicating the migration of photogenerated holes toward the surface. As expected, the variations of ΔCPD signals show a similar pattern to the SPV. It is worth noting that the CoPO3/pGO/LDH/BiVO4 photoanode shows the greatest ΔCPD amplitude (560 mV), which is one order of magnitude higher than that of bare BiVO4 (45 mV). This significant increase in ΔCPD signal manifests the boost of hole transfer from BiVO4 to molecular catalyst by LDH and pGO layers. Overall, these results clearly indicate that this assembled photoanode exhibits a fantastic superiority in charge separation and transfer, benefiting from mimicking the key functions of PSII.

Figure 1. Photoelectrocatalytic water oxidation activity of artificial photosynthesis system. (a) Current−potential curves of BiVO4, CoPO3/BiVO4, CoPO3 /pGO/BiVO4, and CoPO3/pGO/LDH/ BiVO4 photoanodes. (b) Open circuit potential measurements under AM1.5G (1 sun) illumination.

attributed to the stronger bonding interaction than physical adsorption between the BiVO4 nanoparticles and the Co complex, which is more beneficial to charge transfer.8,42 It is noted that the CoPO3/pGO/BiVO4 photoanode shows superior performance to CoPO3/BiVO4 after loading the pGO interlayer. More strikingly, the integrated photoanode, CoPO3/pGO/ LDH/BiVO4, exhibits a high photocurrent, 4.45 mA cm−2, with an ultralow onset potential, 0.17 V. Meanwhile, the current− potential curve changes from a concave shape to a convex profile (higher fill factor) with much higher photocurrent. In addition, Figure 1b shows the open circuit potential (VOC) under AM1.5G 1 sun condition as a reference point for us to understand the shift of the onset potential.43 The bare BiVO4 electrode shows a more positive VOC (0.35 V) and loading CoPO3 catalyst decreases the VOC to 0.28 V. Interestingly, after loading pGO as an interlayer, the VOC is reduced to 0.23 V. Moreover, the integrated photoanode, CoPO3/pGO/LDH/BiVO4, delivers a lower VOC (0.20 V), indicating a greater driving force for water oxidation with more efficient charge separation in the photoanode. The measured VOC coincides with the onset potential of water oxidation (Figure 1a).43 Figure 2 shows the result of the electrochemical impedance spectroscopy (EIS) for the photoanodes. Rct represents the resistance of charge transfer across the electrolyte/semiconductor interface. The decrease of Rct after loading CoPO3 catalyst suggests a better interfacial charge transfer, which stems from the acceleration of surface water oxidation reaction in the presence of CoPO3 catalyst. It is worth noting that introducing the pGO layer further decreases the charge-transfer resistance. Clearly, the CoPO3/pGO/LDH/BiVO4 photoanode shows the smallest semicircle among all the photoanodes, implying the B


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To further clarify the roles of LDH and GO layers in the assembled photoanode, we performed a series of (photo)electrochemical and spectral measurements. Figure 4a shows the stability of BiVO4-based photoanodes for prolonged measurements. The CoPO3/pGO/BiVO4 photoanode shows higher performance than that of bare BiVO4 and CoPO3/BiVO4 photoanodes, but the photocurrent still decays rapidly to a low level due to the instability of BiVO4.47,48 It has been previously demonstrated that the application of a hole storage layer (HSL) is beneficial to the capture and storage of holes, preventing the photoanode from photocorrosion.49 Therefore, we employed a NiFeLDH layer as a HSL rather than an OEC to improve the stability of the integrated system (Figures S9 and S10).50 After loading of the LDH on the surface of BiVO4 photoanode, the CoPO3/pGO/LDH/BiVO4 photoanode shows no obvious decay after 10 h irradiation, with a Faradaic efficiency (ηF) up to 100% (Figure S11). Furthermore, molecular CoPO3 catalyst was also testified to be stable by X-ray photoelectron spectroscopy (XPS) after PEC measurements (Figure S12). Figure 4b shows the photocurrent response of the LDH/ BiVO4 photoanode compared with that of pristine BiVO4 under chopped light illumination. Evident transient cathodic/anodic peaks are observed under dark/light, illustrating that photogenerated holes are collected in the LDH layer, and thus lead to back electrons−holes recombination. The amount of stored holes was calculated from the recombination current (Figure 4b, inset),49 manifesting that the LDH layer functions as an effective HSL for hole capture and accumulation. More interestingly, the energy levels of the surface redox states determined from the derivative of the Mott−Schottky (MS) plot,49 shown in Figure 4c, are beneficial to the formation of a potential gradient for charge collection. Very significantly, after loading LDH on the BiVO4 surface, the dark CPD of LDH/BiVO4 photoanode

Figure 3. Directly probing charge separation and transfer process in artificial photosynthesis systems. (a) Spatially resolved surface photovoltage (SR-SPV) amplitude spectra at the nanoscale level under illumination from 400 to 600 nm. (b) Contact potential difference (CPD) images of BiVO4, CoPO3/BiVO4, CoPO3/pGO/BiVO4, and CoPO3/pGO/LDH/BiVO4 photoanodes in the dark and under illumination at 450 nm.

Figure 4. (a) Stability test of BiVO4, CoPO3/BiVO4, CoPO3/pGO/BiVO4, CoPO3/pGO/LDH/BiVO4 photoanodes under AM1.5G (1 sun) illumination. (b) Current−potential curve of LDH/BiVO4 photoanode under chopped sunlight. (c) Mott−Schottky (MS) plot measured on LDH film and the potentials of the redox states. (d) Contact potential difference (CPD) of LDH/BiVO4 photoanode in the dark and under illumination at 450 nm. C


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Figure 5. (a) Scheme of interaction between the OEC and the mediator. (b) Contact potential difference (CPD) of pGO/LDH/BiVO4 in the dark and under illumination at 450 nm. (c) Normalized transient absorption (TA) decays of LDH/BiVO4, CoPO3/LDH/BiVO4, and CoPO3/pGO/LDH/ BiVO4 photoanodes. Decay traces were recorded after UV laser (355 nm, 100 μJ cm−2, 0.9 Hz) excitation and probed at 540 nm. (d) Current−voltage characteristics of the devices. The conformation of devices is in the inset in panel d.

the VB of BiVO4 and the redox potential of CoIII/CoIV (Figure S17), is very crucial to charge transfer. Therefore, pGO can be regarded as a charge mediator transferring the holes from BiVO4 to CoPO3 catalyst. Very interestingly, Figure 5b shows that the dark CPD of pGO/LDH/BiVO4 cannot be recovered to the initial state after illumination with an increase of about 70 mV, which is different from the action of LDH/BiVO4 in Figure 4d. This not only demonstrates that holes can transfer from LDH to pGO, but also suggests that pGO plays a critical role in unidirectionally transporting holes, thus leading to the long-lived hole accumulation on pGO layer. Such kind of unidirectional conducting effect suppresses the recombination of holes with backward electrons. To investigate the impact of loading pGO layer on the kinetics of photogenerated carriers, we conducted the transient absorption spectra (TAS)52 and found that introducing CoPO3 catalyst leads to a decrease in the initial TA signal (Figure S18) and the decay half-time (t50%) from 140 to 74 μs in Figure 5c, which is ascribed to fast water oxidation reaction on CoPO3 catalyst and efficient utilization of holes. Moreover, loading pGO layer further decreases the hole lifetime (t50% = 23 μs), which again suggests that pGO plays a key role in promoting hole transfer. We further examined the charge-transfer ability of pGO by comparing the current−voltage characteristics of photovoltaic devices. As can be seen in Figure 5d, the device containing pGO/ ITO requires smaller external voltage for hole extraction/ injection than that containing ITO, indicating pGO can promote the hole transfer, in accordance with the action of pGO in the assembled photoanode. On the basis of these results, we propose that the key to the success of the artificial photosynthesis system undoubtedly lies in

overshoots the initial state by about 80 mV after illumination, shown in Figure 4d. This indicates that holes can be stored in the LDH layer when turning on the light. Nevertheless, the electrons can transfer across the LDH layer and recombine with the holes tardily after turning off the light, in agreement with the photoresponse (Figure 4b). We investigated the role of GO in charge transfer for the assembled photoanode. A series of GO with different degrees of oxidation was synthesized. Their oxygen containing groups, mainly composed of ketonic CO groups (531.9 ± 0.4 eV) and C−O (epoxide and hydroxyl) groups (533.5 ± 0.2 eV), can be deliberatively changed from 4.0 at. % to 32.3 at. % (Figures S13 and S14). It was found that pGO demonstrates superior performance, and further increasing the oxygen content leads to a significant decrease in water oxidation activity (Figure S15). The performance can be controlled by the characteristic functional groups of GO, which alters the surface property of graphene. We found that with increasing oxygen contents, the contact angle (CA) value gradually decreases (Figure S16), suggesting the improvement in hydrophilic property. However, the PEC performance is decreased after adding the deeply oxidized grapheme (dGO), indicating that the change of surface hydrophilicity is not the intrinsic reason for the difference of water oxidation activity. It is noted that both pGO and TyrZ as charge transfer mediators possess very important groups, −OH, which are hydrogen bonded to the CoPO3 and Mn4CaO5 complex, respectively (Figure 5a). More importantly, the −OH groups play a similar role in natural photosynthesis and our biomimetic PEC systems, functioning as a bridge to accept electrons from the OEC. In this case, O 2p orbital of pGO as the highest occupied molecular orbital (HOMO) level (1.85 eV),51 located between D


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in a 25 mL flask. The solution was stirred for 2 h. The products were filtrated and washed with ethanol and water and dried under vacuum for all night. Partially oxidized graphene (pGO) was prepared as follows. Graphene (25 mg) was added into piranha solution (7 mL of 98% H2SO4 and 3 mL of 30% H2O2) in a 25 mL flask. The solution was stirred under ambient condition for 5 h and diluted with a large amount of water. The products were filtrated and washed with ethanol and water and dried under vacuum all night. Preparation of BiVO4-Based Electrodes. BiVO4 film electrodes were prepared as previously reported.2 Note that the muffle furnace was replaced by the tube furnace. The pGO aqueous dispersion (1 mg mL−1) was deposited onto BiVO4 electrodes by spin coating (2500 rpm). Then the electrode was processed at 120 °C for 15 min. LDH was coated on the surface of BiVO4 electrode as previously reported.54,55 CoPO3/ pGO/BiVO4 and CoPO3/pGO/LDH/BiVO4 electrodes were prepared by adsorbing CoPO3 complex on the pGO/BiVO4 and pGO/LDH/ BiVO4 electrodes, respectively. Synthesis of Diethyl 4-Pyridylphosphonate. A mixed solution of 4-bromopyridine hydrochloride (2.0 g, 10.28 mmol), diethyl phosphate (2.21 g, 16 mmol), and tetrakis(triphenylphosphine)palladium (300 mg, 0.25 mmol) in 40 mL of anhydrous toluene containing 4 mL of triethylamine was refluxed under N2 atmosphere for 16 h. Then the reaction products were filtered, concentrated, and loaded into silica gel chromatography. The dichloromethane and methanol were employed as eluents to afford diethyl 4-pyridylphosphonate. Synthesis of Molecular Co Cubanes. Co cubane complex (CoPO3) was prepared with a diethyl 4-pyridylphosphonate linkage. Co(NO3)2·6H2O (1.45 g, 5 mmol) and CH3CO2Na·3H2O (1.35 g, 10 mmol) were mixed and heated in methanol (20 mL) at 75 °C, and diethyl 4-pyridylphosphonate (2.14 g, 5 mmol) was added to the mixed solution. Then 30% H2O2 (2.5 mL) was added dropwise and kept under reflux for 4 h. After cooling, the solution was reduced by rotary evaporation. The product was extracted by CH2Cl2, and the organic phase was dried with anhydrous Na2SO4. A dark-green compound was obtained by adding petroleum ether to organic phase. A mixed eluent of dichloromethane and methanol was employed to afford diethyl 4pyridylphosphonate. CoPy complex was synthesized in a similar way by replacing the diethyl 4-pyridylphosphonate with pyridine linkage. Sample Characterization. X-ray powder diffraction (XRD) was measured by a Rigaku D/Max-2500/PC powder diffractometer with Cu Kα radiation (5° min−1; operating current, 200 mA; operating voltage, 40 kV). The UV−visible diffuse reflectance spectra were measured by a JASCO V-550 spectrophotometer and obtained by Kubelka−Munk function. The scanning electron microscope (SEM) images were obtained by the Quanta 200 FEG SEM equipped with an energy dispersive S3 spectrometer (accelerating voltage 20 kV). X-ray photoelectron spectroscopy (XPS) measurement uses a spectrometer (VG ESCALAB MK2) with monochromatized Al−Kα as X-ray source. Contact potential difference (CPD) measurements were conducted on a commercial Bruker AFM under air atmosphere with an imaging mode of AM-KPFM. Kelvin probe force microscopy (KPFM) was tested under Lift mode (lift height 100 nm). The Si tip coated with Pt/Ir was employed as Kelvin tip (spring constant 1−5 N/m, resonant frequency 60−100 kHz). To acquire the surface photovoltage (SPV) spectroscopy, a monochromatic light split out from the light of Xenonarc lamp (500 W) through the monochromator (Zolix Omni-λ 500) was focused on the sample by lens with a fixed low grazing angle, which ensured sample under the measuring AFM tip was properly illuminated. To quantify the transient SPV signals, the varied surface potential signals were fed to a Stanford SR 830 lock-in amplifier and synchronized with the chopped signals. (Photo)electrochemical Procedures. The PEC measurements were performed in a three-electrode system. Pt was selected as a counter electrode, and saturated calomel electrode (SCE) was employed as reference electrode. The electrolyte was potassium borate solution (1.0 M, pH = 9.0). The light source was simulated AM1.5G (1 sun) illumination (Newport Sol 3A, Class AAA Solar simulator). The

its bioinspired attributes. In the CoPO3/pGO/LDH/BiVO4 photoanode, each component was indispensable, which was deposited in a particular sequence by mimicking the PSII (Figure S1). Especially, the introduction of interfacial materials, LDH and pGO, is very important. In PSII, TyrZ plays an important role in charge transfer between P680 light harvester and Mn4CaO5. The −OH groups of TyrZ function as a bridge in connection with CaMn4O5 complex. Likewise, the −OH groups of pGO interact with the phosphonate groups of molecular CoPO3 catalyst in our biomimetic PEC system. In other words, pGO was employed as a charge mediator between molecular catalyst (CoPO3) and the light harvester (BiVO4), which promotes the unidirectional hole transfer from BiVO4 to CoPO3. Additionally, LDH as the HSL captures the holes and protects the light absorber (BiVO4) from photocorrosion, which efficiently reduces the recombination of photogenerated carriers. These enlighten us on a deeper understanding of the importance of the interlayer, which is essential to further improve the water oxidation activity in an artificial photosynthesis system.53 Finally, the rational integration of the biomimetic units results in highly efficient charge separation and transfer in the assembled photoanode (Scheme 1), which delivers a 2.0% STH (Figure S19). We believe a Scheme 1. Schematic Representation of Integrated BiVO4 Photoanode Systema

a

Each component in the CoPO3/pGO/LDH/BiVO4 photoanode was deposited in an appointed sequence by mimicking the PSII.

promising future of exploiting more efficient artificial photosynthesis system can be realized through a bioinspired method.

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CONCLUSIONS In conclusion, we have demonstrated successfully a new strategy inspired by natural photosynthesis for PEC water oxidation. We employed LDH/BiVO4 as a light harvester, pGO layer as an interfacial conduction layer, and a ligand-modified cobalt−oxo cubane molecule as the OEC. The integrated photoanode exhibits an ultralow onset potential (0.17 V) with a high photocurrent, 4.45 mA cm−2 at 1.23 V and superior stability. These results indicate the surface modification of BiVO4 with LDH can reduce the electron/hole recombination and the molecular Co catalyst can accelerate the water oxidation rate. More importantly, pGO plays a crucial role of charge transfer in the biomimetic PEC system. Our findings provide a rational design and assembly of highly efficient artificial photosynthesis devices for solar energy conversion.

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EXPERIMENTAL SECTION

Chemicals. All reagents, such as graphene (G) and deeply oxidized graphene (dGO), were purchased from commercial sources. Preparation of GO with Different Oxidation Degrees. Minutely oxidized graphene (mGO) was prepared as follows. Twenty-five milligrams of graphene was added into 10 mL of 36% HCl solutions E


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potential was controlled by an electrochemical workstation (CH Instruments 760D). The photoelectrodes were illuminated from the back side unless otherwise mentioned. Amperometric measurements were carried out at 1.23 V versus RHE. Oxygen was determined online by a gas chromatograph (GC), during which a constant flow of Ar gas was bubbled through the closed cell. The recorded potentials versus SCE (E vs SCE) were converted against RHE using the Nernst equation below:

E vs RHE = E vs SCE + ESCE + 0.059 pH The current−voltage curves of Au−P3HT-ITO and Au−P3HTpGO/ITO devices were measured on a Keithley 2400 Source Measure Unit in dark condition. Mott−Schottky plot was measured by a potentiostat (Iviumstat, Ivium Technologies) in dark. The frequency of AC potential was 1 kHz, and the amplitude was 10 mV.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b10662. 1 H NMR spectra, HR-MS data, XPS data, UV−vis absorption spectra, PXRD patterns, HRSEM images, AFM images, EDX mapping, contact angles data, LSV and CV scans (PDF)

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AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Pingwu Du: 0000-0002-2715-0979 Can Li: 0000-0002-9301-7850 Author Contributions

S.Y. and C.D. contributed equally. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by 973 National Basic Research Program of the Ministry of Science and Technology of China (Grant No. 2014CB239400) and the National Natural Science Foundation of China (Grant Nos. 21633010 and 21603225).

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