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A Spatially Separated Organic-Inorganic Hybrid Photoelectrochemical Cell for Unassisted Overall Water Splitting Dawei Shao, Yahui Cheng, Jie He, Deqiang Feng, Lingcheng Zheng, Lijun Zheng, Xinghua Zhang, Jianping Xu, Weichao Wang, Wei-Hua Wang, Feng Lu, Hong Dong, Luyan Li, Hui Liu, Rongkun Zheng, and Hui Liu ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b01002 • Publication Date (Web): 29 Jun 2017 Downloaded from http://pubs.acs.org on June 29, 2017

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A Spatially Separated Organic-Inorganic Hybrid Photoelectrochemical

Cell

for

Unassisted

Overall Water Splitting Dawei Shao,† Yahui Cheng,*,† Jie He,† Deqiang Feng,† Lingcheng Zheng,† Lijun Zheng,† Xinghua Zhang,‡ Jianping Xu,§ Weichao Wang,† Weihua Wang,† Feng Lu,† Hong Dong,† Luyan Li,|| Hui Liu,¶ Rongkun Zheng,# and Hui Liu†

†

Department of Electronics and Key Laboratory of Photo-Electronic Thin Film Devices

and Technology of Tianjin, Nankai University, Tianjin 300350, China ‡

School of Material Science and Engineering, Hebei University of Technology, Tianjin

300130, China §

Institute of Material Physics, Key Laboratory of Display Materials and Photoelectric

Devices, Ministry of Education, Tianjin University of Technology, Tianjin 300384, China ||

School of Science, Shandong Jianzhu University, Jinan 250101, China

¶

Research Group of Quantum-Dot Materials & Devices, Institute of New-Energy

Materials, Tianjin University, Tianjin 300350, China #

School of Physics, the University of Sydney, NSW 2006, Australia 1 ACS Paragon Plus Environment

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ABSTRACT

The Z-scheme overall solar water splitting is a mimic of natural photosynthesis to convert solar energy into chemical fuels. Despite much effort, the available Z-scheme artificial systems are still rare and most of them rely on inorganic semiconductors. In this study, inspired by the characteristics of p-type and higher unoccupied molecular orbital level of organic semiconductors, a spatially separated organic-inorganic hybrid Z-schematic photoelectrochemical (PEC) cell for unassisted overall water splitting is developed by wiring

inorganic

ZnO

nanorod

arrays

photoanode

and

organic

poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid (P3HT:PCBM) photocathode in a tandem manner. In the short circuit configuration, the photocurrent density reaches an initial value of ~78 µA cm-2 and an average value of ~95 µA cm-2 under continuous UV-visible light irradiation, giving rise to the solar-to-fuel conversion efficiency of ~0.12%. The average gas evolution rates are 1.59 µmol h-1 for H2 and 0.75 µmol h-1 for O2 under zero bias condition without sacrificial agents, corresponding to the faradic efficiency of ~90%. These results demonstrate a possibility of introducing organic semiconductor into a Z-schematic system to realize unassisted overall solar water splitting, which enriches the practical Z-schematic photocatalytic systems.

KEYWORDS:

overall water splitting, photoelectrochemical, Z-scheme, organic

photocathode, organic-inorganic hybrid photoelectrochemical cell, tandem configuration

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1. INTRODUCTION Fossil fuels have been unable to satisfy the requirements of continuous industrial growth due to the over-exploitation and pollution. Alternatively, among the variety of renewable energies, hydrogen produced by photocatalytic or photoelectrochemical water splitting is an ideal and sustainable source because of its storability, transportability, and convertibility.1 For an efficient reaction of overall water splitting, photocatalysts are essential and their performance are strongly determined by the band structure. On one hand, the overall water splitting is an uphill procedure, which requires the photocatalyst have wide enough band gap (> 1.23 eV) to overcome the Gibbs free energy. Indeed an even larger band gap (> 2.0 eV) is usually needed due to the additional overpotential associated with the electron transfer and gas evolution steps.2 On the other hand, the photocatalyst should maintain good optical absorption capacity to efficiently utilize solar energy, corresponding to a narrow band gap. Obviously, these requirements are irreconcilable in a single light absorber system.3 Moreover, the proper conduction band minimum (CBM) and valence band maximum (VBM) for the redox reactions, as well as the good carrier separation capability, are also critical for overall water splitting. Therefore, it is necessary to explore a new type of photocatalytic system to satisfy all the requirements aforementioned. The Z-schematic photocatalytic system which imitates the green plant photosynthesis is one of the most promising candidates. An artificial all-solid-state Z-schematic photocatalytic system comprises one interfacial 3 ACS Paragon Plus Environment

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layer and two semiconductor photoelectrodes, i.e., a p-type photocathode and an n-type photoanode.4 The two photoelectrodes absorb different energy of the solar spectrum to drive the photocatalytic reaction, during which the majority carriers in the two photoelectrodes recombine at the ohmic interfacial layer and the minority carriers are used to carry out the oxidation or reduction half-reaction. As a result, the whole system holds strong optical absorption capability, carrier separation capability, and redox capability simultaneously. Up to now, the unassisted Z-schematic solar water splitting system is still rather rare and difficult to achieve, although significant research efforts have been devoted to study the n-type inorganic photoanodes, such as TiO2, ZnO, BiVO4, et al..5-7 So far, only a few of Z-schematic systems have been implemented.8-10 This is in part due to the fact that p-type inorganic semiconductors are relatively rare compared with n-type ones.11 Another reason is that most of inorganic oxide semiconductors, which are the widely used stable photoelectrode materials, usually have a CBM that is more positive than the reduction potential of water (H+/H2, 0 V vs. NHE). So, most of narrow band oxide semiconductors are not possible for the unassisted solar-driven water reduction as the photocathodes. Thus, it is highly desirable to explore alternative p-type semiconductors for water reduction half-reaction so as to develop competitive Z-schematic photocatalytic systems for overall water splitting. In contrast to inorganic semiconductors, the organic semiconductors are particularly attractive because most organic semiconductors are intrinsically in p-type,12 exemplified 4 ACS Paragon Plus Environment

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by a group of conjugated molecules including thiophene oligomers, benzodithiophene dimer, pentacene, phthalocyanines, poly(alkylthiophenes), etc..13 The highest occupied molecular orbital (HOMO) levels of these organic semiconductors are usually about 5 to 6 eV below vacuum level, whereas the lowest unoccupied molecular orbital (LUMO) levels of them are about 2 to 3 eV below vacuum level.14 These energy levels are very suitable for realizing the water reduction half-reaction. Moreover, the organic semiconductors have much more advantages, including their diversity, adjustable performance, high optical absorption coefficient, ease of processing, and solution-based manufacturing, etc.. Therefore organic semiconductors have great advantages and potential for water reduction as the photocathode. Nowadays, the applications of the organic semiconductors on photocatalysis have received more attention. Some organic semiconductors such as polyaniline, polypyrrole, poly(3-hexylthiophene) (P3HT), [6,6]-phenyl-C61-butyric acid (PCBM), g-C3N4, or their complex, have been employed to achieve the hydrogen evolution reactions or organic photodegradation reactions, as well as to form the full organic type-II heterostructures to achieve water splitting.15-20 However, unassisted solar-driven overall water splitting have not been achieved in most of organic systems. Moreover, the p-type character of organic semiconductors has not been explored to build the Z-schematic photocatalytic system. In particular, combining the p-type organic semiconductor to the mature inorganic photoanode to build the organic-inorganic hybrid system has not yet been reported. In the present work, we constructed an organic-inorganic hybrid Z-schematic PEC 5 ACS Paragon Plus Environment

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system with a tandem configuration by wiring a traditional n-type ZnO nanorods (NRs) array photoanode and a new type of organic P3HT:PCBM photocathode. The configuration of the prototype organic-inorganic hybrid PEC system is shown in Figure 1. In this system, the reduction and oxidation catalytic centers are spatially separated, which not only minimizes the undesirable back-reaction but also separates the photocatalytic products. Such a system can achieve unassisted solar-driven overall water-splitting without using any sacrificial agents in the solution. 2. EXPERIMENTAL SECTION 2.1. Preparation of ZnO NRs Photoanode. ZnO NRs array was prepared by a two-step method.21 In the first step, a substrate of FTO was wetted with a droplet of 0.05 M Zn(CH3COO)2 ethanol solution, rinsed with ethanol, and then blow-dried with nitrogen. The dropping step was repeated for 3-5 times to form Zn(CH3COO)2 crystallite film. Then the Zn(CH3COO)2 covered substrate was heated in the air at 350 °C for 20 min to yield a ZnO seed layer. The first whole process was repeated for 5 times. In the second step, the substrate covered with seed layer was face-down placed in a Teflon high pressure reaction autoclave (100 mL) containing the mixed solution of 0.025 M Zn(NO3)2 and 0.025 M hexamethylenetetramine (HMT), and then heated at 95 °C for 3 h to form the ZnO NRs. After the NRs growth process, the substrate was removed from the solution, rinsed with DI water, dried and then annealed in air at 450 °C for 3 h. Stable blue suspensions containing iridium oxide (IrOx·nH2O) nanoparticles (NPs) were obtained by hydrolyzing IrCl62- in NaOH at 90 °C to produce [Ir(OH)6]2- and then 6 ACS Paragon Plus Environment

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treating with HNO3 at 0 oC. The solution was adjusted to pH = 7 by adding 1.5 wt% NaOH solution.22 The ZnO NRs array was then immersed in the IrOx·nH2O suspensions overnight to absorb the IrOx NPs onto ZnO NRs. 2.2. Preparation of PEDOT:PSS/P3HT:PCBM/Pt Photocathode. The organic photocathode

was

entirely

solution-processed.

Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) was filtered and spin coated onto the FTO substrate in ambient air. After heating at 130 oC for 20 min in vacuum, the substrate was moved into glove box, where the P3HT:PCBM blend with the mass ratio of 1:1 was filtered with a 0.45 µm filter and spin coated onto the PEDOT:PSS layer. The P3HT:PCBM blend was then heated at 130 oC for 20 min in nitrogen atmosphere. At last, a layer of Pt was deposited onto P3HT:PCBM layer through ion beam sputtering method. 2.3. Characterization. Crystal structure and morphology of photoelectrodes were examined using X-ray diffraction (XRD, Rigaku D/max 2500), field emission scanning electron microscopy (FESEM, Hitachi SU 8010), and transmission electron microscopy (TEM, JEOL JEM-2100F), respectively. Optical absorption spectra were recorded using an UV-vis spectrophotometer (Shimadzu UV-2600) equipped with an integrating sphere. Photoluminescence (PL) spectra were measured at room temperature using a PL spectrophotometer (Jobin Yvon FluoroLog-3). Photoelectrochemical properties of the inorganic photoanode and the organic photocathode were individually measured using an electrochemical workstation 7 ACS Paragon Plus Environment

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(AMETEK VersaSTAT 4, Princeton) in a standard three-electrode configuration with a Pt wire as the counter electrode and Ag/AgCl as the reference electrode. The short-circuit measurements

of

the

externally

wire-linked

ZnO-IrOx

photoanode

and

PEDOT:PSS/P3HT:PCBM/Pt photocathode were performed using a two-electrode configuration, in which the electrochemical workstation remained short-circuited and behaved as an ammeter. The two photoelectrodes were in the same reactor and placed in a tandem configuration. Here the areas of both electrodes were 1 cm2. In all cases, 0.25 M Na2SO4 aqueous solution (pH = 6.8) was used as the electrolyte. The light source was a 300 W xenon lamp (NEeT, HSX-F/UV 300). The light intensity was maintained at 100 mW

cm-2.

All

potentials

have

been

referred

to

the

RHE

electrode:

E ( RHE) = E ( Ag/AgCl) + 0.210 + 0.059 × pH . In order to monitor the dissolution of photoanode, the concentration of Zn2+ ions in the electrolyte during photocatalytic reaction was measured using an inductively coupled plasma-optical emission spectrometer (ICP-OES, PerkinElmer Optima 8300). The photocatalytic activities of two short-circuited photoelectrodes were characterized by measuring the gaseous products in a photoreaction system. The system comprises a two-chamber system, a circulation system, vacuum pump, and a gas chromatograph (Shimadzu GC-2018) for inline measurements. Before reaction, the photoreaction system was thoroughly degassed by bubbling with argon gas for at least 30 minutes to expel the air in the reactor. The reaction was carried out in 0.25 M Na2SO4 aqueous solution buffered to pH = 6.8 with phosphate buffer under simulated sunlight of 100 mW cm-2. 8 ACS Paragon Plus Environment

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2.4. Calculation of the Faradic Efficiency and Energy Conversion Efficiency. To calculate the faradic efficiency (ηfaradic) of water splitting in the configuration of short-circuited electrodes, the following equation was applied:

ηfaradic =

2 × n H 2 ( mol) × 96485(C/mol) Q ( C)

× 100% ,

(1)

in which Q is the total amount of charge passed through the external circuit during a certain time period as measuring the evolved H2 gas. The solar-to-fuel conversion efficiency of the water splitting (η) was calculated using the following equation:

η=

1.23( V ) × I ( mA/cm2 ) × 100% , P( mW/cm2 )

(2)

in which I is the photocurrent density and P is the light intensity. 3. RESULTS AND DISCUSSION 3.1. Prototype of Photoelectrodes. As a typical n-type wide band gap oxide semiconductor, ZnO has been extensively studied because of its large excitation binding energy (60 meV), deep level defects and high electron mobility. Herein, ZnO was used as photoanode in the PEC system. ZnO NRs array with large specific surface area and good light trapping performance was grown vertically on the fluorine-doped tin oxide (FTO) glass substrate using a hydrothermal method. IrOx NPs were deposited onto the ZnO NRs as the cocatalyst to lower the overpotential of the oxygen generation. As shown in Figure 1, on the photocathode side, a hole transport layer of PEDOT:PSS 9 ACS Paragon Plus Environment

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was firstly deposited onto the FTO substrate through spin-coating method. Then a layer of P3HT:PCBM blend was spin coated onto the PEDOT:PSS layer as the photoactive layer. It should be noted that P3HT:PCBM is unstable in the oxygen and water environment. Besides, the P3HT:PCBM is hydrophobic,23 which prohibits the photogenerated carriers from transferring to water, and as a result inhibit the hydrogen production. In order to overcome these shortcomings, a layer of Pt with the nominal thickness of 3 nm was deposited onto the P3HT:PCBM layer through the ion beam sputtering method to protect the organic layer, to improve the hydrophilicity, and to reduce the overpotential of water reduction. The schematic energy diagram and the photogenerated carriers transfer process are presented in Figure 1. On the photoanode side, the band gap of wurtzite ZnO is about 3.3 eV, and its CBM and VBM are at -4.3 eV and -7.6 eV versus the vacuum level separately. On the photocathode side, the P3HT is a kind of p-type organic semiconductor with a band gap of 1.9 eV. The LUMO and HOMO levels of P3HT are at -3.0 eV and -4.9 eV, respectively.24 Since the typical binding energy of singlet exciton in P3HT is between 0.1 and 0.4 eV, the separation of exciton in P3HT is difficult.25 It has been reported that the bulk heterojunction (BHJ) composed of both organic donor and acceptor can separate the exciton effectively in the organic system. Under the optical irradiation, the photogenerated electron in organic donor transfers to organic acceptor to form exciton, and then the exciton is separated at the interface between the organic donor and acceptor in the BHJ.26 Therefore, here another organic semiconductor, PCBM, with the LUMO 10 ACS Paragon Plus Environment

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level of -4.3 eV and HOMO level of -6.0 eV27 was introduced to form a BHJ with P3HT to improve the carrier separation.28 This P3HT:PCBM BHJ has already been widely used in organic solar cell and organic-based PEC cell for high efficiency charge separation.18,26,29-32 The PEDOT:PSS (HOMO level: -5.0 eV), which can collect holes and block electrons, was used between the organic BHJ and the FTO substrate as a hole-transferring layer. The FTO glass functions as a charge recombination center with copper wires. Under the solar light irradiation, the UV light is absorbed by ZnO NRs photoanode to generate the electron-hole pairs. The photoexcited holes diffuse to IrOx/electrolyte interface to oxidize water into O2, while the photoexcited electrons transfer to the FTO/PEDOT:PSS interface of photocathode through the external copper wire. Meanwhile, the visible light passing through the photoanode is absorbed by the organic photocathode, exciting the excitons in the P3HT:PCBM BHJ. The photogenerated excitons diffuse to the P3HT:PCBM interface and dissociate into free electrons and holes. The photoexcited electrons then diffuse to the Pt layer and reduce protons in water to evolve H2 at the Pt/electrolyte interface, while the photoexcited holes diffuse through the PEDOT:PSS hole-transferring layer to FTO. The photogenerated electrons from photoanode and holes from photocathode recombine with each other at the FTO layer. As a result, the proof-of-concept organic-inorganic hybrid system with Z-scheme has been achieved, which can realize the unassisted overall solar water splitting with spatially separated H2 and O2 evolution. This carrier transport and recombination mechanism can 11 ACS Paragon Plus Environment

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be indirectly confirmed by photoluminescence (PL) spectra (Figure S1). Unassisted overall water splitting to simultaneously generate H2 and O2 is another strong evidence to prove the validity of this Z-scheme mechanism, which will be investigated in the following. 3.2. Characterization of Photoelectrodes. Figure 2a shows the X-ray diffraction (XRD) pattern of ZnO NRs array. The predominant diffraction peak at 34.4o can be assigned to the (002) diffraction peak of wurtzite ZnO (JCPDS No. 19-0191), indicating that the ZnO NRs grow along the [0001] preferred orientation. The peaks at 26.4o, 33.6o, 37.6o 51.4o, 54.4o, 61.5o and 65.5o are associated with FTO glass. Figure 2b,c gives the top-view and cross-sectional-view of scanning electron microscopy (SEM) images of the pristine ZnO NRs. It reveals that the ZnO NRs have a relatively uniform diameter of about 70 nm. The high resolution transmission electron microscope (HRTEM) images of the IrOx loaded ZnO NRs are shown in Figure 2d. It can be clearly seen that ZnO NR is in single crystalline. A clear lattice with the interplaner spacing of 0.26 nm can be observed, which corresponds to the (002) lattice planes of ZnO. The fast Foriour transform (FFT) pattern shown in the inset of Figure 2d confirms that the ZnO NRs grow along the [0001] direction, which is consistent with the XRD result. It can also be seen in Figure 2d that after loading the IrOx NPs, some smaller particles with the average size of about 1.5 nm can be observed. These IrOx NPs attaching on the surface of ZnO NRs will facilitate effective charge transferring during photocatalysis. Figure 2e shows the cross-sectional SEM image of the photocathode. It can be found 12 ACS Paragon Plus Environment

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that the interfaces between different layers are clear and flat. The thicknesses of PEDOT:PSS layer and P3HT:PCBM blend layer are about 100 nm and 150-200 nm, respectively. However, the Pt layer cannot be clearly identified since it is too thin. Figure 2f shows the UV-vis absorption spectra of different films. It displays that the ZnO NRs array mainly absorbs the UV light with a significant absorption starting from wavelength of 380 nm. After loading IrOx NPs, the absorption band edge of ZnO shows no shift, while the absorption intensity in the UV region reduces slightly. The organic PEDOT:PSS/P3HT:PCBM photocathode absorbs the visible light with an onset wavelength at about 650 nm. After depositing Pt film on P3HT:PCBM, the absorption spectrum shows only a slight decrease, indicating that Pt has no distinct impairment to optical absorption. When combining the ZnO-IrOx and PEDOT:PSS/P3HT:PCBM/Pt film together, the whole system exhibits obvious optical absorption in both UV-light and visible light region, indicating that the tandem system can effectively absorb the solar light over a wide wavelength range. 3.3. Photoelectrochemical Properties of Photoanodes and Photocathodes. Figure 3a,b shows the representative photocurrent density vs. potential (J – V) curves of photoelectrodes in 0.25 M Na2SO4 electrolyte solution buffered to pH 6.8. J – V curves are measured at a scan rate of 50 mV s-1. For the photoanode (Figure 3a), the dark scanning curve reveals a small background current density of even approach 0 µA cm-2 in the whole potential range. The pristine ZnO photoanode exhibits a photocurrent density of 680 µA cm-2 at +1.23 V vs. RHE, and the water oxidation photocurrent begins at about 13 ACS Paragon Plus Environment

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+0.40 V vs. RHE which is defined as the onset potential where the value of photocurrent density reaches 10 µA cm-2. Here, both the value of photocurrent density and the onset potential are comparable with that of reported bare ZnO photoanodes.33 Upon deposition of water oxidation cocatalyst IrOx, the onset potential of photoanode exhibits an obvious cathodic shift to about +0.25 V vs. RHE, which corresponds to the improved charge separation and the lowered overpotential of water oxidation.34-36 Besides, the photocurrent density of ZnO-IrOx photoanode at +1.23 V vs. RHE is similar to that of bare ZnO photoanode. Clearly, the deposition of IrOx NPs has led to a better PEC performance. Figure 3b shows J – V curves of the photocathode. Because of the hydrophobicity of the organic P3HT:PCBM film which suppresses the photogenerated carriers

from

transferring

to

water,

the

photocurrent

density

of

FTO/PEDOT:PSS/P3HT:PCBM is very low. The maximal value is 45 µA cm-2 obtained at 0 V vs. RHE and the onset potential is +0.55 V vs. RHE where the value of photocurrent density reaches 10 µA cm-2. After depositing a thin layer of Pt onto the top of the organic layer to form a FTO/PEDOT:PSS/P3HT:PCBM/Pt photocathode, the device presents higher photocurrent density. Its water reduction photocurrent starts at about +0.90 V vs. RHE and the saturation photocurrent density value is ~96 µA cm-2 starting at +0.40 V vs. RHE. For comparison, we also measured the J – V curves of the photocathode in absence of PCBM, as shown in Figure S2. It shows that the photocurrent density without PCBM is significantly lower than the photocurrent density when PCBM exists. These results indicate that the bulk heterojunction plays an important role in the 14 ACS Paragon Plus Environment

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separation of carriers. Figure 3c,d shows the transient photocurrent response curves of photoanode and photocathode obtained at the potential of 0 V vs. Ag/AgCl (i.e., +0.61 V vs. RHE) and 0 V vs. RHE, respectively. The photocurrent density for the ZnO-IrOx photoanode obtained at 0 V vs. Ag/AgCl is ~270 µA cm-2 which is higher than that of the bare ZnO photoanode (75 µA cm-2) and consistent with the results of J – V curves in Figure 3a. The improvement of the anode photocurrent is on account of the cathodic shifts of the onset potentials.37 On the other hand, the photocathode based on Pt cocatalyst layer yields a higher photocurrent density value of ~90 µA cm-2 at 0 V vs. RHE comparing to the initial photocurrent density value of 40 µA cm-2 of the bare P3HT:PCBM photocathode. In order to demonstrate the durability of the photoelectrodes, the photocurrent density versus time (J – t) curves were measured over a long period. As shown in Figure S3, at an applied potential of 0.61 V vs. RHE, the photocurrent density of the pure ZnO photoanode reduces with time, reaching a minimum of 13 µA cm-2 after 60000 s operation. The reduction in photocurrent is derived from the attack of the photo-generated holes on the Zn-O bond of ZnO and the disassociation of Zn2+ from ZnO surface.38 After the introduction of IrOx as cocatalyst, the stability of the photocurrent increased significantly. After 60000 s of irradiation, the photocurrent density remains at 250 µA cm-2. The improvement of the stability is mainly because that IrOx can rapidly transfer the photogenerated holes, so as to reduce corrosive effect of the photogenerated holes on the photoanode. The concentrations of Zn2+ ions in the electrolyte during 11 h 15 ACS Paragon Plus Environment

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PEC test were measured by ICP-OES. The results are shown in Figure S4. It is evident that the concentration of Zn2+ ion decreases when inducing IrOx cocatalyst, further proving that the stability of ZnO-IrOx photoanode is improved. The prolonged J – t curves of the photocathodes were tested at 0 V vs. RHE over a period of 60000 s and shown in Figure S5. For the FTO/PEDOT:PSS/P3HT:PCBM photoelectrode without the protection of Pt layer, the photocurrent density shows a sustained slow descent, from the initial value of 40 µA cm-2 to the value of 18 µA cm-2 after 60000 s. This reduction of photocurrent density results from the corrosion of photocathode, since the oxidative radical species can easily oxidize the polymers.26 When coating the organic photocathode with Pt layer, the photocurrent density is maintained at a higher value of around 90 µA cm-2 over 60000 s, indicating that the stability of the organic photocathode has been obviously improved due to the protection of Pt. In addition, the cross-sectional SEM image of the FTO/PEDOT:PSS/P3HT:PCBM/Pt photocathode after the 16 h PEC test is shown in Figure S6. It is clearly that the photocathode is not dissolved after the photocatalytic reaction, indicating that Pt plays a very good protective role. 3.4. Wire-Linked Photoelectrodes for Water Splitting. Figure 4a shows that the J – V curves of photoanode and photocathode intersect at 0.40 V vs. RHE and 92 µA cm-2, which implies that a nonzero of photocurrent could be achieved under the irradiation once wiring these two photoelectrodes directly, suggesting that the solar-driven water splitting for H2 and O2 evolution is possible in this tandem configuration without any 16 ACS Paragon Plus Environment

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external energy supply.39 To further verify this conjecture and reveal the effect of solar-driven water splitting of this PEC tandem system, the two photoelectrodes, i.e., the ZnO-IrOx photoanode and the PEDOT:PSS/P3HT:PCBM/Pt photocathode, are externally linked by copper wire and irradiated under simulated solar light (100 mW cm-2). The schematic configuration of the whole tandem PEC system is shown in Figure 1. The prolonged J – t curve is tested for 7200 s under short-circuit condition and shown in Figure 4b. It shows that in this short-circuit state, the photocurrent density firstly rise sharply, and then drops rapidly, exhibiting a spike. Then, the photocurrent density reaches a value of ~78 µA cm-2 under continuous illumination. It should be noticed that because of the existence of the possible extra loss mechanisms, this photocurrent density is lower than the J – V intersection of 92 µA cm-2 in Figure 4a. The photocurrent density is also confirmed through corresponding transient photocurrent response of this tandem configuration under chopped UV-vis light irradiation over 300 s (Figure 4c). The transient photocurrent density value is ~70 µA cm-2 which is consistent with that of the J – t curve in Figure 4b. This transient photocurrent density with the magnitude in the order of ~µA cm-2 is comparable to some inorganic tandem PEC systems such as CaFe2O4-TiO2 system and Rh-SrTiO3-BiVO4 system.9,10 In order to reveal the stability of the whole tandem PEC cell, the photocurrent curve was measured over a much longer term of 35000 s (Figure S7). It can be seen that the photocurrent density shows no evident decrease until 35000 s, indicating that the overall 17 ACS Paragon Plus Environment

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photocurrent density is stable after the modification of both the photoelectrodes. The average value of photocurrent density is about 95 µA cm-2 under continuous irradiation. Voltage generated under irradiation is another key parameter for describing the efficiency of the tandem PEC cell.39 The J – V curve of this wire-linked PEC cell is measured (Figure 5a). It demonstrates that the open-circuit voltage (Voc) of the PEC cell is about ~0.40 V which is good consistent with the intersection of the two J – V curves of individual photoelectrodes (Figure 4a). The short-circuit current density (at V = 0 V) is about 75 µA cm-2 consisting with the photocurrent density in Figure 4b,c, implying that overall solar-driven water splitting can be spontaneously performed without applying any external electric energy. As shown Figure 5b, a multi-step chronoamperometric curve for this tandem cell is measured with the applied voltage starting at -0.30 V and ending at +1.10 V with an increment of 0.10 V every 600 s. The results reveal that the photocurrent remains stable at each potential in the entire range and the current switches quite rapidly. The Voc obtained from the multi-step chronoamperometric curve is also 0.40 V and the short-circuit current density ( V = 0 V) is stable at 75 µA cm-2, which are in consistent with the results of J – V in Figure 4a.

3.5. Overall Water Splitting in Organic-Inorganic Hybrid PEC Cell. Figure 6 displays H2 and O2 gas evolution as a function of time during an 8 h testing period in neutral environments. In order to realize the separation of H2 and O2 to reduce the adverse reaction and the explosive risk, as well as to yield the maximum gas production, an ion-conductive membrane is used between PEDOT:PSS/P3HT:PCBM/Pt and


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ZnO-IrOx photoelectrodes compartments as shown in the inset of Figure 6. According to Figure 6, the average H2 and O2 production rates are 1.59 µmol h-1 and 0.75 µmol h-1, respectively, exhibiting a spontaneous reaction of water splitting. However, it should be noticed that within the first 2 h, the hydrogen-to-oxygen molar ratio is about 3.6:1, deviating from the normal ratio of 2:1 for water splitting. This may be because that a large part of generated O2 has dissolved in the water due to its higher solubility and lower amount.10 As time goes on, the hydrogen-to-oxygen molar ratio gradually approaches 2:1. On the basis of H2 production amount in 8 h and the mass of the polymers, the turn over numbers (TON) can be estimated. Here the TON was calculated from the following equations:

TON = mH 2 / mcat ,

(3)

where m H 2 is the molar amount of H2, and mcat is the total molar amount of P3HT:PCBM.40 The calculating procedure and the change of TON with time are presented in the Supporting Information (section 8 and Figure S8). It can be seen that there is no decay for the TON within 8 h. The total TON is up to 1030 for this tandem PEC cell over 8 h. These results further demonstrate that this organic-inorganic hybrid PEC cell has good durability. Besides, some additional tests were carried out to exclude the contribution of electrodes decomposition to H2 production and O2 production. On one hand, according to the ICP-OES results in Figure S4, it can be calculated out that in the presence of IrOx, after 9 h PEC reaction, the molar number of Zn2+ ions in the electrolyte is about 0.26 19 ACS Paragon Plus Environment

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µmol. So the amount of O2 produced by ZnO corrosion can be calculated according to the reaction ZnO + 2h + → Zn 2+ + 0.5O 2 .41

(4)

The obtained molar number of O2 is 0.13 µmol,38 which can be neglected compared with the O2 produced by photocatalytic water splitting (6.57 µmol for 8 h in Figure 6). On the other hand, the H2 production test on the P3HT:PCBM photocathode in water with the existence of triethanolamine as the hole sacrificial agent was carried out. The results are shown in Figure S9. In the 6 h H2 production test, there is only about 0.012 µmol H2 generated, which is far below the amount of H2 in 6 h in Figure 6 (9.5 µmol). So the H2 is not derived from the decomposition of the photocathode. All the above results exhibit that

both

the

contributions

of photoanode

decomposition

and

photocathode

decomposition to O2 production and H2 production can be neglected. According to Equation (1), using the average photocurrent density value of ~95 µA cm-2 within 35000 s measurements in the short circuit configuration under continuous UV-visible light irradiation (Figure S7), the average charge-to-chemical faradic efficiency is about ~90%. Besides, the calculated overall solar-to-fuel conversion efficiency based on the average photocurrent density is about 0.12% according to Equation (2). It should be noted that this solar-to-fuel conversion efficiency is still lower than some of the best inorganic Z-schematic photocatalytic systems, such as Si-TiO2 and IrOx/ZnS/CdS/TiO2-NiS/ZnS/CdSe/NiO systems.8,35 However, it has been comparable to that of the green plant photosynthesis (~0.2%).8,42 Importantly, these results have 20 ACS Paragon Plus Environment

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indicated that it is feasible to construct a Z-schematic photocatalytic system by using organic semiconductor as photocathode to achieve unassisted overall water splitting. In addition, these results have provided novel organic-inorganic hybrid Z-schematic photocatalytic prototypes, which will greatly enrich the Z-schematic photocatalytic systems by exploiting a variety of novel organic semiconductors as the photocathodes. It is worthy to point out that in this prototypical organic-inorganic hybrid PEC system, we only use the simplest structure to achieve the unassisted water splitting so that to eliminate the extra effect of modified material on gas generation and photocurrent improvement. Here only the most conventional cocatalysts, i.e., Pt and IrOx, are employed. Neither high performance surface modification, such as TiO2,43 MoS3,18 CdTe,44 CdSe,45 precious metals (Au,46 Ag47), nor elementary substance48 are employed to improve the photocurrent density. As a result, we have verified that through reasonable matching of organic and inorganic semiconductors, the obtained organic-inorganic hybrid PEC cell itself can successfully achieve photocatalytic overall water splitting. Besides, it should be noticed that in this tandem PEC cell, the final photocurrent density and solar-to-fuel conversion efficiency are mainly limited by the relatively modest photocurrent density (around 90 µA cm-2) of the organic photocathode. While through the systematic modification and optimization, both the value and stability of photocurrent density of the organic photocathode can be improved significantly.18,30 Take P3HT:PCBM BHJ for example. Through the surface modification of such as MoS3,29, TiO2,30-32 NiO-OH,26 reduced graphene oxide (RGO),26 or interfacial modification of 21 ACS Paragon Plus Environment

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metal,18 C60,18 or the introduction of hole-selective layer of MoS2 flakes,32 the photocatalytic stability of heterojunction can be greatly improved, and the photocurrent density can be raised to the level of mA cm-2 at 0 V vs. RHE. We thus strongly believe, through the reasonable preparation and optimization on the photoelectrodes, the photocurrent density of the organic-inorganic hybrid PEC cell in the short circuit configuration can be expected to reach the level of mA cm-2, and the solar-to-fuel conversion efficiency can be comparable to that of the inorganic Z-schematic systems, which would provide a new possibility for the photocatalytic overall water splitting. At last, it should be mentioned that although the overall water splitting for H2 and O2 generation has been achieved at zero external bias condition in the brand-new organic-inorganic hybrid Z-schematic PEC cell, there is still much work needed to be carried out in the future: the photostability in aqueous solution, the quality of the photoelectrodes especially the photocathode, the matching between two photoelectrodes, and the solar-to-fuel conversion efficiency are all needed to be further improved. Obviously, the Z-schematic photocatalytic systems based on organic semiconductors have very broad space and great potential in the future.

4. CONCLUSION In summary, a photoelectrochemical cell with an IrOx modified ZnO NRs photoanode and a PEDOT:PSS/P3HT:PCBM/Pt photocathode has been developed for spontaneous overall water splitting to produce H2 and O2 under UV-vis light irradiation. In the short circuit configuration of the organic-inorganic hybrid PEC system, the photocurrent 22 ACS Paragon Plus Environment

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density has reached an initial value of ~78 µA cm-2 and an average value of ~95 µA cm-2 under continuous illumination, giving rise to the solar-to-fuel conversion efficiency of ~0.12%. The average gas evolution rates of 1.59 µmol h-1 for H2 and 0.75 µmol h-1 for O2 at zero bias condition have been achieved with the faradic efficiency of ~90%. This spatially separated PEC system has effectively realized the separation of H2 and O2 to reduce the adverse reaction and the risk of explode, yielding the maximum gas production during the water splitting. More importantly, the design of spatially separated photoelectrodes has provided a pathway toward better solar-to-fuel conversion efficiency, as it allows newly discovered individual components to be readily plugged in. In short, this work demonstrates a feasible strategy to construct new-types of Z-schematic photocatalytic system for overall water splitting.

■ AUTHOR INFORMATION Corresponding Author *E-mail: [email protected].

Notes The authors declare no competing financial interest.

■ SUPPORTING INFORMATION Photoluminescence spectra, J – V curves of the photocathode without PCBM, prolonged J – t curves of the photoanodes, Zn2+ ions concentration in the electrolyte, prolonged J – t 23 ACS Paragon Plus Environment

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curves of the photocathodes, SEM image of photocathode after PEC test, prolonged J – t curve of the externally short-circuited PEC cell, turn over number (TON), H2 production test on photocathode.

■ ACKNOWLEDGEMENTS This research was supported by the National Natural Science Foundation of China (No. 51671108, 51571123), the National Basic Research Program of China (973 Program with No.

2014CB931703),

and

the

Tianjin

Natural

Science

Foundation

(No.

17JCZDJC37000).

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Figure 1.

Figure 1. Energy diagram of photoanode and photocathode in the solar-driven overall water splitting procedure.


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Figure 2.

Figure 2. (a) X-ray diffraction pattern of ZnO NRs array. (b) Top-view of SEM image of ZnO NRs array. (c) Cross-sectional-view of SEM image of ZnO NRs array. (d) TEM bright field image of IrOx modified ZnO NR. Circles in (d) denote the IrOx NPs. The inset of (d) is FFT pattern of the selected area (indicated by the square). (e) Cross-sectional SEM image of photocathode. (f) UV-vis absorption spectra of photoanode and photocathode.


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Figure 3.

Figure 3. (a) Photocurrent density of photoanode versus the applied voltage referenced to a reversible hydrogen electrode (RHE) in 0.25 M Na2SO4 electrolyte buffered to pH 6.8. (b) J – V curves of photocathode obtained under the same conditions as the photoanode curves. (c) J versus time (t) of photoanode at a bias potential of 0 V vs. Ag/AgCl under chopped light exposure. (d) J – t curves of photocathode at a bias potential of 0 V vs. RHE under chopped light exposure.


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Figure 4.

Figure

4.

(a)

J

–

V

curves

of

ZnO-IrOx

NRs

photoanode

and

PEDOT:PSS/P3HT:PCBM/Pt photocathode in 0.25 M Na2SO4 electrolyte. (b) J – t curve of

the

externally

short-circuited

PEDOT:PSS/P3HT:PCBM/Pt

and

ZnO-IrOx

photoelectrodes. (c) J – t curve of externally short-circuited PEDOT:PSS/P3HT:PCBM/Pt and ZnO-IrOx photoelectrodes under chopped light exposure.


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Figure 5.

Figure 5. (a) J – V curves of the PEC cell with ZnO-IrOx NRs photoanode and PEDOT:PSS/P3HT:PCBM/Pt photocathode. (b) Multi-step chronoamperometric curves of the tandem cell.


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Figure 6.

Figure 6. Evolution of H2 and O2 gases measured by gas chromatography. Inset is the structural schematic of wire-linked PEC cell for H2 and O2 production measurements.


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Figure 1. Energy diagram of photoanode and photocathode in the solar-driven overall water splitting procedure. 331x171mm (72 x 72 DPI)

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Figure 2. (a) X-ray diffraction pattern of ZnO NRs array. (b) Top-view of SEM image of ZnO NRs array. (c) Cross-sectional-view of SEM image of ZnO NRs array. (d) TEM bright field image of IrOx modified ZnO NR. Circles in (d) denote the IrOx NPs. The inset of (d) is FFT pattern of the selected area (indicated by the square). (e) Cross-sectional SEM image of photocathode. (f) UV-vis absorption spectra of photoanode and photocathode. 308x176mm (96 x 96 DPI)


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Figure 3. (a) Photocurrent density of photoanode versus the applied voltage referenced to a reversible hydrogen electrode (RHE) in 0.25 M Na2SO4 electrolyte buffered to pH 6.8. (b) J – V curves of photocathode obtained under the same conditions as the photoanode curves. (c) J versus time (t) of photoanode at a bias potential of 0 V vs. Ag/AgCl under chopped light exposure. (d) J – t curves of photocathode at a bias potential of 0 V vs. RHE under chopped light exposure. 656x498mm (96 x 96 DPI)


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Figure 4. (a) J – V curves of ZnO-IrOx NRs photoanode and PEDOT:PSS/P3HT:PCBM/Pt photocathode in 0.25 M Na2SO4 electrolyte. (b) J – t curve of the externally short-circuited PEDOT:PSS/P3HT:PCBM/Pt and ZnO-IrOx photoelectrodes. (c) J – t curve of externally short-circuited PEDOT:PSS/P3HT:PCBM/Pt and ZnOIrOx photoelectrodes under chopped light exposure. 555x193mm (96 x 96 DPI)


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Figure 5. (a) J – V curves of the PEC cell with ZnO-IrOx NRs photoanode and PEDOT:PSS/P3HT:PCBM/Pt photocathode. (b) Multi-step chronoamperometric curves of the tandem cell. 210x106mm (300 x 300 DPI)


ACS Catalysis

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Figure 6. Evolution of H2 and O2 gases measured by gas chromatography. Inset is the structural schematic of wire-linked PEC cell for H2 and O2 production measurements. 80x59mm (300 x 300 DPI)


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ACS Catalysis

For Table of Contents Only 82x44mm (300 x 300 DPI)


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