SubstrateâElectrode Interface Engineering by an Electron-Transport

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Substrate-electrode Interface Engineering by an Electron Transport Layer in Hematite Photoanode Chunmei Ding, Zhiliang Wang, Jingying Shi, Tingting Yao, Ailong Li, Pengli Yan, Baokun Huang, and Can Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b12818 • Publication Date (Web): 01 Mar 2016 Downloaded from http://pubs.acs.org on March 8, 2016

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ACS Applied Materials & Interfaces

Substrate-electrode Interface Engineering by an Electron Transport Layer in Hematite Photoanode Chunmei Ding, Zhiliang Wang, Jingying Shi, Tingting Yao, Ailong Li, Pengli Yan, Baokun Huang, Can Li* State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences; Dalian National Laboratory for Clean Energy, Dalian, 116023, China.

KEYWORDS: hematite; Fe2O3; Li; WO3; electron transport layer; interface;

ABSTRACT: The photoelectrochemical (PEC) water oxidation efficiency of photoanodes is largely limited by interfacial charge transfer processes. Herein, a metal oxide electron transport layer (ETL) was introduced at the substrate-electrode interface. Hematite photoanodes prepared on Li+ or WO3 modified substrates deliver higher photocurrent. It is inferred that a Li doped Fe2O3 (Li:Fe2O3) layer with lower flat band potential than the bulk is formed. Li:Fe2O3 and WO3 are proved to function as an expressway for electron extraction. Via introducing ETL, both the charge separation and injection efficiencies are improved. The lifetime of photogenerated electrons is prolonged by 3 times, and the ratio of surface charge transfer and recombination rate is enhanced by 5 times with Li:Fe2O3 and 125 times with WO3 ETL at 1.23 V vs. RHE. This result indicates the expedited electron extraction from photoanode to the substrate can suppress not only the recombination at the back contact interface but also those at the surface, which results higher water oxidation efficiency. 1

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1 INTRODUCTION Photoelectrochemical (PEC) water splitting produces hydrogen using solar energy in an environmentally friendly way.1-3 To realize efficient PEC water splitting, photoanodes with wide light absorption range such as Fe2O3,4-5 BiVO46-8, Ta3N59-10 and Si11-12 have been paid much attention. Hematite is one of the most exceptional photoanode candidates as it is stable, nontoxic, earth-abundant and has a high theoretical solar-to-hydrogen efficiency of 16.5%.1 However, its efficiency is still low as its low electron mobility,13 short hole diffusion length (2-4 nm)14 and high electron-hole recombination rate15-16. Recombination of charge carriers particularly at the interfaces of electrode-electrolyte, bulk crystal boundaries and electrode-substrate impede the charge transfer and hamper the efficiency of hematite. Many strategies have been proved effective for suppressing the recombination at the electrode-electrolyte interface and that inside the bulk, such as cocatalyst loading17-20, surface passivating,21-25 doping26-29 and morphology control30-33. To improve the properties of the substrate-electrode interface, researchers endeavor to mitigate the so called “dead layer” which is in low crystallinity with many trap states34 via modifying the substrate with underlayers such as SnO2,35 SiO2,35-36 Ga2O3,37-38 Nb2O538-39 and TiO239 which can improve the crystallinity. Nevertheless, these strategies are only effective for ultrathin hematite films (< 50 nm) prepared without high temperature treatment. Besides, the conduction bands of these underlayers are higher than Fe2O3, so there is no driving force and are unfavorable for electron transfer especially for thick films with many defects. Thick electrode can absorb more light and may deliver higher efficiency as long as the charge carriers are used for surface reaction effectively, but little attention has been paid on the 2


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substrate-electrode interface engineering where electrons are expected to move to the conductive substrate as quickly as possible. To facilitate the electron transport, carbon-based materials have been employed.40 And in solar cells, the issue of electron extraction has been successfully resolved by introducing cathodic electron conducting layer.41-43 However, it is still a common problem of photoanode in PEC that the electron collection process is sluggish and the flowing back of accumulated electrons to the surface will result in “back electron-hole recombination”.15, 44 Herein, we focus on the engineering of substrate-electrode interface of thick hematite photoanodes with thickness of above 300 nm. An layer of WO3 or Li doped Fe2O3 (Li:Fe2O3), both with lower flat band potential (Ef) than the bulk electrode, was introduced and proved to play a role of electron transport layer (ETL). The PEC performances of the obtained hematite photoanodes are obviously improved, probably due to the expedited electron extraction to the substrate at the back contact interface. And the effects of ETL can extend through the thick electrode to the surface resulting greatly suppressed surface recombination. 2 EXPERIMENTAL SECTION 2.1 Fabrication of electrodes: The Fe2O3 photoanode was fabricated by chemical vapor deposition and annealing method.45 Typically, 0.25g of ferrocene was sintered at 550 °C in a furnace for 2 h in a crucible (30 mL) covered by a FTO (4 cm × 4 cm) glass. The central part of the sample was cut and annealed at 750 °C in air for 15 min. Fe2O3 electrodes doped with Li in the bulk was fabricated by mixing Li(acac)2 and ferrocene as precursor. WO3 and TiO2 electrodes were fabricated by direct current reactive magnetron sputtering system (Kurt J. Lesker Company PVD75) with a W (> 99.95%) and Ti target (> 99.995%) respectively (See supporting information for details). 3



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2.2 Modification of FTO with ETL: FTO substrates were modified with Li salts by spin coating (1500 rpm for 40 s) 250 μL Li salts solution (50 mM for Fe2O3 electrodes, 2 mM for WO3 and TiO2 electrodes). In the control experiments, the FTO substrate was modified by methanol without Li salts. For the deposition of WO3 ETL, 250 μL 50 mM (NH4)6H2W12O40 dissolved in CH3OH/H2O (v/v = 7 : 1) mixed solvents was spin coated on FTO and then treated at 500 oC in air for 30 min. In the control experiments, the FTO substrate was modified by the solvent without (NH4)6H2W12O40 and thermal treated samely. 2.3 Characterizations: Samples were characterized by X-ray diffraction (XRD) on a Rigaku D/Max-2500/PC powder diffractometer using Cu Kα radiation (40 kV × 200 mA). The UV-visible diffuse reflectance spectra were recorded on a UV-visible spectrophotometer (JASCO V-550). Raman spectra were measured on a Renishaw Raman spectrometer with a 532 nm laser line as an exciting source. The morphologies of the electrodes were performed by a Quanta 200 FEG scanning electron microscope (SEM) equipped with an energy dispersive spectrometer (accelerating voltage of 20 kV). The work functions were determined from Scanning Kelvin probe microscopy (SKPM) images obtained by a Veeco Nanoscope III-D AFM. The sheet resistances are measured by four-point probe method via a Hall instrument (HL5500 LN2 CRYOSTAT, nanometrics). 2.4 Electrochemical measurements: The linear sweep voltammetric (LSV) currents, electrochemical impedance spectroscopy (EIS), incident photon-to-current efficiency (IPCE), Mott-Schottky (MS) and open circuit potential (OCP) measurements were performed in a three-electrode setup with Pt counter electrode (2 cm × 3 cm) and SCE reference electrode. All potentials were converted to potentials vs. RHE (E vs. RHE = E vs. SCE + 0.244 V + 0.059 V × pH). 4


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And all electrochemical measurements were conducted with a potentiostat (Iviumstat, Ivium Technologies) otherwise mentioned. IPCE was measured under monochromatic light irradiation provided by a Xe lamp and filtered by a monochromator (CROWNTECH QEM24-D 1/4 m Double). The light intensity was calibrated by a Si diode. Electrolytes were purged with Ar during the measurements. All the photoanodes were irradiated from the front side. Intensity modulated photocurrent spectrum (IMPS) measurements were conducted by a potentiostat (IM6ex, Zahner Company) in the same three-electrode configuration as above. Intensity modulated light was provided by a light-emitting diode (LED) controlled by a LED driver (PP211) that allowed superimposition of sinusoidal modulation (~10%) on a dc illumination level. The wavelength of light is 430 nm with an average intensity of 5 mW cm-2, and the modulation amplitude of lamp voltage is 20 mV. The photocurrent as a function of frequency (from 10 KHz to 100 mHz) and the photocurrent after the light was turned on was recorded at different potentials. 3 RESULTS AND DISSCUSSION 3.1 Effects of Li modified interfacial layer on photoanodes Figure 1a shows that with LiNO3 modified FTO substrate, the LSV photocurrent of Fe2O3 photoanode is enhanced by about 1.8 times at 1.23 V vs. RHE (optimization of the concentration of LiNO3 is given in Figure S1a). Besides, such an enhancement phenomenon is also observed for FTO modified by other Li salts (Figure S1b), suggesting the promotion effect of Li ions. What makes us more confident is that for other photoanode materials such as WO3 and TiO2, similar results were also obtained (Figure 1b, c, and Figure S1c, d). The corresponding IPCE and EIS results further evidenced the much improved PEC performance of the photoanodes (Figure S2). In contrast, when 5



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introducing Li into the bulk phase of Fe2O3, the photoresponse is decreased and the onset potential is shifted to more positive potential (Figure 1d), that is because doping Li into n-type Fe2O3 will bring more electron-acceptor defects and lower the Ef of Fe2O3. This indicates. Li ions can function positively only when they exist at the substrate-electrode interface. There are negligible differences of the morphology, light absorption, Raman spectra and XRD patterns between FTO-LiNO3-Fe2O3 and FTO-Fe2O3 electrodes (Figure S3, S4). Besides, the sheet resistance is slightly increased and the average work function of FTO is increased negligilbly after the modification with Li (Table 1), which is unfavorable for electron injection from the photoanode to the substrate. Therefore, we can safely eliminate the possibility that the changes of the substrate itself could be the reason for the enhancement of photocurrent. However, the XRD patterns (Figure 2a) of FTO-Fe2O3 and FTO-LiNO3-Fe2O3 photoanodes show that there is an decrease of the value of 2θ after introducing Li indicating the reduction of the crystal plane distance and the shrinking of the crystal lattice based on Scherrer equation, while the shift of FTO signals is negligible. Besides, the MS plots also show higher charge carriers density and a small positive shift of the Ef by about 50 mV as the Ef of the whole electrode is dragged down by the Li:Fe2O3 layer (Figure 2b). Moreover, when FTO were modified by other alkaline metal ions including Na+, K+, Mg2+, Ca2+, Sr2+, and Ba2+, the obtained electrode shows significantly reduced photocurrent (Figure S5) probably as the radius of those ions are much larger than Li. Based on the facts above, it is deduced that there is an layer of Li:Fe2O3 which has lower Ef than the bulk phase and is favorable for the extraction and transport of electrons from the bulk electrode to the substrate.

6


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(a)

-2

J /(mA cm )

0.6 1 FTO-LiNO3-Fe2O3

0.4

2 FTO-Fe2O3

1

0.3

1 FTO-LiNO3-WO3

1 2

2 0.2

0.2 0.1 0.0

0.4

0.8

1.2

1 FTO-LiNO3-TiO2 -2

(b) 2 FTO-WO3

0.0

J /(mA cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.10 2 FTO-TiO 2

1.2

0.05

1.6

(d)

1 blank 1 0.2 2 bulk-Li-1% 3 bulk-Li-5% 2

1 2 3

0.1 0.0

0.00 0.0

0.8

1.6 (c)

0.5 1.0 1.5 Potential vs. RHE/V

0.4

0.8 1.2 Potential vs. RHE/V

1.6

Figure 1 LSV scans of (a) Fe2O3, (b) WO3 (c) TiO2 photoanodes on blank and LiNO3 modified FTO substrates, and (d) pristine Fe2O3 and bulk Li doped Fe2O3 photoanodes under chopped light illumination. Electrolyte: 1 M LiOH (pH 12) for Fe2O3 and TiO2, 0.5 M Li borate (borate ions 0.5 M, pH 7) for WO3; Light source: AM 1.5G sunlight simulator (100 mW cm-2). Table 1 Sheet resistances (R) and work functions of FTO after modification with Li and calcination Samples

R / (Ω sq-1)a

R / (Ω sq-1)b

work function b / eV

FTO-blank

13.2

13.1

4.59

FTO-LiNO3

16.5

17.6

4.55

FTO-LiAc

16.4

17.4

4.53

FTO-LiCl

17.3

17.7

4.51

FTO-LiOH

17.9

17.3

4.51

a

: calcined in air at 500 oC for 2 h; b: calcined in air at 500 oC for 2 h and 700 oC for 5 min. 7



(a) 1 FTO-Fe2O3 Fe2O3

-2

1

FTO

2

o

0.1 

35 36 37 38 2 theta/

o

FTO-Fe2O3 2

FTO-LiNO3-Fe2O3

1

-2



10

2 FTO-LiNO3-Fe2O3

(b)

3 4

o

0.1



C /(10 F cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0 0.2

64

0.4 0.6 Potential vs. RHE/V

0.8

Figure 2 (a) XRD patterns and (b) MS plots of FTO-Fe2O3 and FTO-LiNO3-Fe2O3 photoanodes 3.2 Effects of WO3 interfacial layer The results above indicate that the substrate-electrode interfacial layer is critical for the transport of electrons. To further confirm this deduction, a foreign layer of WO3 which possesses lower Ef than Fe2O3 and has excellent electron conducting property judging from the value of the electron diffusion length46-47 was introduced between FTO and Fe2O3 electrode. There have been examples of WO3/Fe2O3 heterojunction electrodes in which WO3 functions as host scaffold and Fe2O3 as guest absorber.48-49 Herein, we deposited small amount of WO3 on FTO substrates by spin coating a precursor solution followed by calcination. From Figure 3a, the photocurrent of FTO-WO3-Fe2O3 photoanode is much higher than the electrode without WO3 (denoted as FTO’-Fe2O3). It is notable that the transient peak currents when the light is turned on/off are greatly reduced. Such transient currents were measured (Figure S6) and the corresponding charge and discharge currents were calculated by subtracting the peak current by the steady state current when the light was turned on/off (Figure 3b). Obviously, the charge and discharge currents are much lower for FTO-WO3-Fe2O3 photoanode, indicating that the recombination of photogenerated carriers is greatly 8


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inhibited. Besides, the corresponding IPCE value is higher and the charge transfer resistance across the electrode-electrolyte interface obtained from EIS analysis is smaller (Figure S7). There are no signals of WO3 in the XRD patterns and Raman spectra (Figure S8) as the amount is small, but the presence and effects of WO3 can be confirmed by the decrease of light absorption, the shifting of Ef

1 FTO-WO3-Fe2O3 -2

0.4

-2

(a)

0.6

J(peak-steady) / (mA cm )

to more positive potential, and the increase of the charge carrier density (Figure S9).

J /(mA cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

2 FTO-Fe2O3 2

0.2 0.0

(b)

0.08

charge

0.00 discharge FTO'-Fe2O3

-0.04

0.4 0.6 0.8 1.0 1.2 1.4 1.6 Potential vs. RHE/V

FTO-WO3-Fe2O3

0.8 1.0 1.2 1.4 Potential vs. RHE/V

1.6

Figure 3 (a) LSV scans of FTO’-Fe2O3 and FTO-WO3-Fe2O3 electrodes and (b) the corresponding charge and discharge currents calculated by subtracting the amperometric transient peak current and the steady state current after about 1 min under chopped light illumination. Electrolyte: 1 M LiOH (pH 12); Light source: AM 1.5G sunlight simulator (100 mW cm-2). 3.3 Analysis of surface recombination by transient OCP If the extraction of photogenerated electrons to the substrate is not fast enough, they may be trapped or flow back to the surface region or be trapped. These electrons will cause surface recombination as shown in Figure 4a in two ways: reaction with the adsorbed photooxidized OH• and high valence Fe species etc. via route (1) or recombination with the holes in surface region of the electrode which is called “back e--h+ recombination”15 through routes (2). So we further analyzed the

9



normalized transient decay profile of OCP. As the time scale of OCP decay is slow (s ~ min), thus it mainly gives the information about the surface recombination mentioned above rather than the bulk recombination via route (3) which is very fast (< 1 ms)15, 50-51. As Figure 4b, c shows, with Li:Fe2O3 or WO3 ETL, the decay of OCP after the light is turned off is much more slower with the half lifetime increased by 2.5 times and 15.5 times, respectively (SI-5, Figure S10), indicating the retarding of surface recombination. (a) e -e -e -e - e - e (1) slow surface recombination, s ~ min

(3) (2)

ETL

FTO

(1)

h+ h+

O (2) back e--h+ recombination, 10~100 ms R (3) direct bulk recombination, < 1 ms O: high valence Fe species, adsorbed O2, OH• etc.

h+ h+h+h+

R: reduced species

Photoanode (b)

5s

17s

0.4 1 0.3

2

0.2 1 FTO-Fe2O3

0.1

2 FTO-LiNO3-Fe2O3

(c)

0.5 Normalized OCP

0.5 Normalized OCP

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.0

14.8 s

245.3 s

0.4 1

0.3

2

0.2 1 FTO'-Fe2O3

0.1

2 FTO-WO3-Fe2O3

0.0

0

5

10 Time/s

15

20

0

60

120 180 Time/s

240

300

Figure 4 (a) A schematic description of the recombination processes in a Fe2O3 photoanode, and the normalized transient decay profiles of OCP of (b) FTO-Fe2O3, FTO-LiNO3-Fe2O3, (c) FTO’-Fe2O3 and FTO-WO3-Fe2O3 electrodes,. Electrolyte: 1 M LiOH (pH 12); Light source: AM 1.5G sunlight simulator (100 mW cm-2).

10


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3.4 Charge separation and injection efficiencies

 (injection) / %

(a)

80

1 FTO-LiNO3-Fe2O3

60

2 FTO-Fe2O3

(b)

1 80 2 60

40

40

20

20

1

1 FTO-WO3-Fe2O3 2 FTO'-Fe2O3

2

0

0 0.8

1.0

1.2

1.4

0.8

(c)

 (seperation) / %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1 FTO-LiNO3-Fe2O3

6 2 FTO-Fe O 2 3

1.0

1.2

1 3.5 1 FTO-WO3-Fe2O3 3.0 2 FTO'-Fe O 2 3 2 2.5

(d)

1

2

2.0

4

1.5 0.8 1.0 1.2 Potential vs. RHE/V


Figure 5 Charge (a, b) injection and (c, d) separation efficiencies of FTO-Fe2O3, FTO-LiNO3-Fe2O3, FTO’-Fe2O3 and FTO-WO3-Fe2O3 electrodes To clarify the roles of WO3 and Li:Fe2O3 interfacial layers, we measured the charge separation and injection efficiencies (Figure 5, SI-6 and Figure S11). It is interesting to note that they are both increased. This manifests that the substrate-hematite interfacial layer which we called ETL can not only enhance the charge separation at the back contact interface, but also can function through such a thickness of more than 300 nm to the surface of the electrode. Even when the thickness is further increased, the effect of ETL is still obvious (Figure S12). The rapid extraction of electrons to substrate results in a rather low concentration of electrons in the electrode surface region, and thus the probability of surface recombination is reduced. In other words, the charge separation and

11



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injection processes exist side by side and play a part together, and they can be synergetically enhanced with ETL. 3.5 IMPS analysis of carrier kinetics To follow the charge transient behaviors, we further conducted IMPS measurement (SI-7). When modulating the light intensity with a wide width of frequency, the photocurrent responses accordingly in the frequency domain. IMPS has been widely used in dye-sensitized solar cells and is capable of analyzing the dynamic charge transport processes.52-55 The IMPS signals in different frequency domains can be ascribed to various PEC process with different rate constants. In high frequency region, surface recombination processes with low rate constant can be avoided. The signals displays one semicircle in the fourth quadrant (Figure 6a, b) and the frequency at the minimum (fmin) can be related to the diffusion lifetime of photogenerated electrons (τd) which reflects the average time of photogenerated electrons transferring from the electrode to the back contact56-57 (SI-7). The obtained τd of FTO-LiNO3-Fe2O3 is about 3 times of that of FTO-Fe2O3, and it is similar with FTO-WO3-Fe2O3 and FTO’-Fe2O3 (Figure 6c, d). Longer τd means less recombination during the transfer of electrons to the substrate and is favorable for obtaining higher photocurrent, so that is one reason for the photocurrent enhancement with Li:Fe2O3 or WO3 layer. This verifies that the WO3 and Li:Fe2O3 layer can indeed expedite the transfer of electrons to the substrate. Besides, the low-frequency intercept (Jinter) corresponds to the additional dc photocurrent generated by the intensity increment.58 The Jinter of FTO-LiNO3-Fe2O3 is 7.5 times of that of FTO-Fe2O3, and the value of FTO-WO3-Fe2O3 is about twice of FTO’-Fe2O3, because faster electron transport can facilitate charge-collection and thus increase the dc photocurrent. 12


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fmax Jinter fmin

-0.2

0.0

-6

-6

0.0

(b)

Imag/(10 A)

Imag/(10 A)

(a)

-0.1 FTO-Fe2O3

FTO-Fe2O3

-0.4

FTO-LiNO3-Fe2O3

0.0

-0.2

0.4 -6 Real/(10 A)

0.8

FTO'-WO3-Fe2O3

0.1

(c) FTO-LiNO3-Fe2O3

2

0.2 0.3 0.4 -6 Real/(10 A) FTO-WO3-Fe2O3

4

d / ms

d / ms

FTO-Fe2O3

0.5 (d)

5

1

FTO'-Fe2O3

3 2 1 0

0 1.0

1.0

1.1 1.2 1.3 1.4 1.5 Potential vs. RHE/V (e)

10

1.1 1.2 1.3 1.4 1.5 Potential vs. RHE/V (f)

386 FTO-WO3-Fe2O3

FTO-LiNO3-Fe2O3

FTO'-Fe2O3

384

kt / k r

FTO-Fe2O3

kt / kr

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


5

382 50

125 times

5 times

25 0

0 1.0


1.0

1.4


1.4

Figure 6 (a, b) IMPS plots of FTO-Fe2O3 (solid circle), FTO-LiNO3-Fe2O3 (open circle), FTO’-Fe2O3 (solid triangle) and FTO-WO3-Fe2O3 (open triangle) electrodes at different potentials, (c, d) the average lifetime of photogenerated electrons (τd), and (e, f) the ratio of rate constant of charge transfer (kt) and recombination (kr). Light source: 430 nm LED.

13



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Moreover, the arc in low frequency region can mainly be ascribed to charge transfer and surface recombination processes via routes (1) and (2) shown in Figure 4a57-58. This can be confirmed by the disappearance of the first quadrant arc and obvious increase of τd in the presence of hole scavenger H2O2 (Figure S13). And the rate constant of charge transfer (kt) and surface recombination (kr) can be derived from the frequency at the radio maximum (fmax) and the corresponding amperometric photocurrent decay profile (SI-7, Figure S14 and S15). When the kt/kr ratio is increased, there will be less surface recombination and higher photocurrent may be obtained. As shown in Figure 6e and f, without ETL, the ratio of kt/kr is around one or even less, which means surface charge recombination is predominant. However, with ETL, the kt/kr ratio at 1.23 V of FTO-LiNO3-Fe2O3 is about 5 times of that of FTO-Fe2O3, and the value of FTO-WO3-Fe2O3 is 125 times of that of FTO’-Fe2O3. This clearly proves that the charge transfer is stimulated and the recombination is suppressed with ETL, resulting higher photocurrent. The IMPS results echo well the PEC analysis above. Thus, it is reasonable to arrive at a conclusion that the promoted electron extraction at the substrate-electrode interface via the ETL can greatly reduce surface recombination processes, which permits more holes being reserved and transferred to the electrode surface for water oxidation. 4 CONCLUSIONS The substrate-electrode interface of a hematite photoanode was engineered by ETL. The PEC performance of hematite is obviously enhanced with Li modified FTO, which is also effective for TiO2 and WO3 photoanodes. It is inferred that a Li:Fe2O3 layer with lower Ef is formed, which is favorable for the electron extraction from hematite to the substrate. With WO3 modified FTO, hematite also delivers higher photocurrent and the charge/discharge transient currents are reduced. 14


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Li:Fe2O3 and WO3 interfacial layers are certified to play a role of ETL, the function of which can even be extended through the thick electrode to the surface. The decay of transient OCP is much more slower with ETL, suggesting the inhibition of surface recombination. And the charge separation and injection efficiencies are both improved with ETL. In addition, IMPS results show the lifetime of photogenerated electrons is prolonged by about 3 times, and the ratio of surface charge transfer and recombination rate is enhanced by 125 and 5 times with WO3 and Li:Fe2O3 respectively. All these indicate the accelerated electron extraction from the electrode to the substrate can synergetically enhance the charge separation of the electrode and the surface injection of holes to water oxidation. SUPPORTING INFORMATION Details about chemicals and materials, preparation of electrodes, physical and electrochemical characterizations including analysis of LSV, MS, EIS, OCP, IPCE, IMPS, charge separation and injection efficiencies are given in the supporting information. ACKNOWLEDGEMENTS This work was financially supported by 973 National Basic Research Program of the Ministry of Science and Technology of China (No. 2014CB239400), the National Natural Science Foundation of China (No. 21090340, 21373210).

AUTHOR INFORMATION * Corresponding author: E-mail: [email protected]; Tel: 86-411-84379070; Fax: 86-411-84694447; Homepage: http:// www.canli.dicp.ac.cn 15



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NOTES: The authors declare no competing financial interest. REFERENCES (1) Chen, Z. B.; Jaramillo, T. F.; Deutsch, T. G.; Kleiman-Shwarsctein, A.; Forman, A. J.; Gaillard, N.; Garland, R.; Takanabe, K.; Heske, C.; Sunkara, M.; Others. Accelerating Materials Development for Photoelectrochemical Hydrogen Production: Standards for Methods, Definitions, and Reporting Protocols. J. Mater. Res. 2010, 25, 3-16. (2) Li, Z.; Luo, W.; Zhang, M.; Feng, J.; Zou, Z. Photoelectrochemical Cells for Solar Hydrogen Production: Current State of Promising Photoelectrodes, Methods to Improve Their Properties, and Outlook. Energy Environ. Sci. 2013, 6, 347-370. (3) Walter, M. G.; Warren, E. L.; McKone, J. R.; Boettcher, S. W.; Mi, Q.; Santori, E. A.; Lewis, N. S. Solar Water Splitting Cells. Chem. Rev. 2010, 110, 6446-6473. (4) Cao, D.; Luo, W.; Feng, J.; Zhao, X.; Li, Z.; Zou, Z. Cathodic Shift of Onset Potential for Water Oxidation on a Ti4+ Doped Fe2O3 Photoanode by Suppressing Back Reaction. Energy Environ. Sci. 2014, 7, 752-759. (5) Lin, Y. J.; Yuan, G. B.; Sheehan, S.; Zhou, S.; Wang, D. W. Hematite-based Solar Water Splitting: Challenges and Opportunities. Energy Environ. Sci. 2011, 4, 4862-4869. (6) Abdi, F. F.; Han, L.; Smets, A. H. M.; Zeman, M.; Dam, B.; van de Krol, R. Efficient Solar Water Splitting by Enhanced Charge Separation in a Bismuth Vanadate-silicon Tandem Photoelectrode. Nat. Commun. 2013, 4, 2195. 16


Page 17 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(7) Park, Y.; McDonald, K. J.; Choi, K. Progress in Bismuth Vanadate Photoanodes for Use in Solar Water Oxidation. Chem. Soc. Rev 2013, 42, 2321-2337. (8) Jia, Q.; Iwashina, K.; Kudo, A. Facile Fabrication of an Efficient BiVO4 Thin Film Electrode for Water Splitting under Visible Light Irradiation. Proc. Natl. Acad. Sci. U. S. A. 2012, 109, 11564-11569. (9) Liu, G. J.; Shi, J. Y.; Zhang, F. X.; Chen, Z.; Han, J. F.; Ding, C. M.; Chen, S. S.; Wang, Z. L.; Han, H. X.; Li, C. A Tantalum Nitride Photoanode Modified with a Hole-Storage Layer for Highly Stable Solar Water Splitting. Angew. Chem. Int. Ed. 2014, 53, 7295-7299. (10) Li, Y.; Zhang, L.; Torres-Pardo, A.; González-Calbet, J. M.; Ma, Y.; Oleynikov, P.; Terasaki, O.; Asahina, S.; Shima, M.; Cha, D.; Zhao, L.; Takanabe, K.; Kubota, J.; Domen, K.; Veerman, A. Cobalt Phosphate-modified Barium-doped Tantalum Nitride Nanorod Photoanode with 1.5% Solar Energy Conversion Efficiency. Nat. Commun. 2013, 4, 2566. (11) Hu, S.; Shaner, M. R.; Beardslee, J. A.; Lichterman, M.; Brunschwig, B. S.; Lewis, N. S. Amorphous TiO2 Coatings Stabilize Si, GaAs, and GaP Photoanodes for Efficient Water Oxidation. Science 2014, 344, 1005-1009. (12) Kenney, M. J.; Gong, M.; Li, Y.; Wu, J. Z.; Feng, J.; Lanza, M.; Dai, H. High-Performance Silicon Photoanodes Passivated with Ultrathin Nickel Films for Water Oxidation. Science 2013, 342, 836-840.

17



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 25

(13) Rosso, K. M.; Smith, D.; Dupuis, M. An ab Initio Model of Electron Transport in Hematite (Alpha-Fe2O3) Basal Planes. J. Chem. Phys. 2003, 118, 6455-6466. (14) Liao, P. L.; Carter, E. A. Hole Transport in Pure and Doped Hematite. J. Appl. Phys. 2012, 112, 1-14. (15) Le Formal, F.; Pendlebury, S. R.; Cornuz, M.; Tilley, S. D.; Gratzel, M.; Durrant, J. R. Back Electron-Hole Recombination in Hematite Photoanodes for Water Splitting. J. Am. Chem. Soc. 2014, 136, 2564-2574. (16) Huang, Z. Q.; Lin, Y. J.; Xiang, X.; Rodriguez-Cordoba, W.; McDonald, K. J.; Hagen, K. S.; Choi, K. S.; Brunschwig, B. S.; Musaev, D. G.; Hill, C. L.; Wang, D. W.; Lian, T. Q. In Situ Probe of Photocarrier Dynamics in Water-splitting Hematite (Alpha-Fe2O3) Electrodes. Energy Environ. Sci. 2012, 5, 8923-8926. (17) Yu, Q.; Meng, X. G.; Wang, T.; Li, P.; Ye, J. H. Hematite Films Decorated with Nanostructured Ferric Oxyhydroxide as Photoanodes for Efficient and Stable Photoelectrochemical Water Splitting. Adv. Funct. Mater. 2015, 25, 2686-2692. (18) McDonald, K. J.; Choi, K. S. Photodeposition of Co-Based Oxygen Evolution Catalysts on Alpha-Fe2O3 Photoanodes. Chem. Mater. 2011, 23, 1686-1693. (19) Zhong, D. K.; Sun, J. W.; Inumaru, H.; Gamelin, D. R. Solar Water Oxidation by Composite Catalyst/alpha-Fe2O3 Photoanodes. J. Am. Chem. Soc. 2009, 131, 6086-6087.

18


Page 19 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(20) Tilley, S. D.; Cornuz, M.; Sivula, K.; Gratzel, M. Light-Induced Water Splitting with Hematite: Improved Nanostructure and Iridium Oxide Catalysis. Angew. Chem. Int. Ed. 2010, 49, 6405-6408. (21) Yang, X. G.; Liu, R.; Du, C.; Dai, P. C.; Zheng, Z.; Wang, D. W. Improving Hematite-based Photoelectrochemical Water Splitting with Ultrathin TiO2 by Atomic Layer Deposition. ACS Appl. Mater. Interfaces 2014, 6, 12005-12011. (22) Hisatomi, T.; Le Formal, F.; Cornuz, M.; Brillet, J.; Tetreault, N.; Sivula, K.; Gratzel, M. Cathodic Shift in Onset Potential of Solar Oxygen Evolution on Hematite by 13-group Oxide Overlayers. Energy Environ. Sci. 2011, 4, 2512-2515. (23) Le Formal, F.; T E Treault, N.; Cornuz, M.; Moehl, T.; Gr A Tzel, M.; Sivula, K. Passivating Surface States on Water Splitting Hematite Photoanodes with Alumina Overlayers. Chem. Sci. 2011, 2, 737-743. (24) Li, X.; Bassi, P. S.; Pablo P. Boix; Fang, Y.; Wong, L. H. Revealing the Role of TiO2 Surface Treatment of Hematite Nanorods Photoanodes for Solar Water Splitting. ACS Appl. Mater. Interfaces 2015, 7, 16960-16966. (25) Ahmed, M. G.; Kretschme, I. E.; Kandiel, T. A.; Ahmed, A. Y.; Rashwan, F. A.; Bahnemann, D. W. A Facile Surface Passivation of Hematite Photoanodes with TiO2 Overlayers for Efficient Solar Water Splitting. ACS Appl. Mater. Interfaces 2015, 7, 24053-24062.

19



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 25

(26) Pu, A. W.; Deng, J. J.; Li, M.; Gao, J.; Zhang, H.; Hao, Y. Y.; Zhong, J.; Sun, X. H. Coupling Ti-doping and Oxygen Vacancies in Hematite Nanostructures for Solar Water Oxidation with High Efficiency. J. Mater. Chem. A 2014, 2, 2491-2497. (27) Kim, J. Y.; Magesh, G.; Youn, D. H.; Jang, J.; Kubota, J.; Domen, K.; Lee, J. S. Single-crystalline, Wormlike Hematite Photoanodes for Efficient Solar Water Splitting. Sci. Rep. 2013, 3, 2681. (28) Chemelewski, W. D.; Hahn, N. T.; Mullins, C. B. Effect of Si Doping and Porosity on Hematite's (Alpha-Fe2O3) Photoelectrochemical Water Oxidation Performance. J. Phys. Chem. C 2012, 116, 5256-5262. (29) Zhang, M. L.; Luo, W. J.; Li, Z. S.; Yu, T.; Zou, Z. G. Improved Photoelectrochemical Responses of Si and Ti Codoped Alpha-Fe2O3 Photoanode Films. Appl. Phys. Lett. 2010, 97, 1-3. (30) Warren, S. C.; Voïtchovsky, K.; Dotan, H.; Leroy, C. M.; Cornuz, M.; Stellacci, F.; Hébert, C.; Rothschild, A.; Grätzel, M. Identifying Champion Nanostructures for Solar Water-splitting. Nat. Mater. 2013, 12, 842-849. (31) Jun, H.; Im, B.; Kim, J. Y.; Im, Y. O.; Jang, J. W.; Kim, E. S.; Kim, J. Y.; Kang, H. J.; Hong, S. J.; Lee, J. S. Photoelectrochemical Water Splitting over Ordered Honeycomb Hematite Electrodes Stabilized by Alumina Shielding. Energy Environ. Sci. 2012, 5, 6375-6382.

20


Page 21 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(32) Bora, D. K.; Braun, A.; Erni, R.; Fortunato, G.; Graule, T.; Constable, E. C. Hydrothermal Treatment of a Hematite Film Leads to Highly Oriented Faceted Nanostructures with Enhanced Photocurrents. Chem. Mater. 2011, 23, 2051-2061. (33) Kim, J. Y.; Youn, D. H.; Kim, J. H.; Kim, H. G.; Lee, J. S. Nanostructure-Preserved Hematite Thin Film for Efficient Solar Water Splitting. ACS Appl. Mater. Interfaces 2015, 7, 11412-14123. (34) Cesar, I.; Sivula, K.; Kay, A.; Zboril, R.; Graetzel, M. Influence of Feature Size, Film Thickness, and Silicon Doping on the Performance of Nanostructured Hematite Photoanodes for Solar Water Splitting. J. Mater. Chem. C 2009, 113, 772-782. (35) Liang, Y.; Enache, C. S.; van de Krol, R. Photoelectrochemical Characterization of Sprayed α-Fe2O3 Thin Films: Influence of Si Doping and SnO2 Interfacial Layer. Int. J. Photoenergy 2008, 2008, 739864. (36) Le Formal, F.; Gratzel, M.; Sivula, K. Controlling Photoactivity in Ultrathin Hematite Films for Solar Water-Splitting. Adv. Funct. Mater. 2010, 20, 1099-1107. (37) Hisatomi, T.; Brillet, J.; Cornuz, M.; Le Formal, F.; Tetreault, N.; Sivula, K.; Gratzel, M. A Ga2O3 Underlayer as an Isomorphic Template for Ultrathin Hematite Films Toward Efficient Photoelectrochemical Water Splitting. Faraday Discuss. 2012, 155, 223-232. (38) Zandi, O.; Beardslee, J. A.; Hamann, T. Substrate Dependent Water Splitting with Ultrathin α-Fe2O3 Electrodes. J. Phys. Chem. C 2014, 118, 16494-16503.

21



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 25

(39) Hisatomi, T.; Dotan, H.; Stefik, M.; Sivula, K.; Rothschild, A.; Gratzel, M.; Mathews, N. Enhancement in the Performance of Ultrathin Hematite Photoanode for Water Splitting by an Oxide Underlayer. Adv. Mater. 2012, 24, 2699-2702. (40) Kim, J. Y.; Jun, H.; Hong, S. J.; Kim, H. G.; Lee, J. S. Charge Transfer in Iron Oxide Photoanode Modified with Carbon Nanotubes for Photoelectrochemical Water Oxidation: An Electrochemical Impedance Study. Int. J. Hydrogen Energy 2011, 36, 9462-9468. (41) Fu, P.; LinquanHuang; WeiYu; DongYang; GuijiLiu; Zhou, L.; JianZhang; CanLi. Efficiency Improved for Inverted Polymer Solar Cells with Electrostatically Self-assembled BenMeIm-Cl Ionic Liquid Layer as Cathode Interface Layer. Nano Energy 2015, 13, 275-282. (42) Sun, Y. M.; Seo, J. H.; Takacs, C. J.; Seifter, J.; Heeger, A. J. Inverted Polymer Solar Cells Integrated with a Low-Temperature-Annealed Sol-Gel-Derived ZnO Film as an Electron Transport Layer. Adv. Mater. 2011, 23, 1679-1683. (43) You, J. B.; Chen, C. C.; Dou, L. T.; Murase, S.; Duan, H. S.; Hawks, S. A.; Xu, T.; Son, H. J.; Yu, L. P.; Li, G.; Yang, Y. Metal Oxide Nanoparticles as an Electron-Transport Layer in High-Performance and Stable Inverted Polymer Solar Cells. Adv. Mater. 2012, 24, 5267-5272. (44) Ma, Y.; Pendlebury, S. R.; Reynal, A.; Le Formal, F.; Durrant, J. R. Dynamics of Photogenerated Holes in Undoped BiVO4 Photoanodes for Solar Water Oxidation. Chem. Sci. 2014, 5, 2964-2973.

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Page 23 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(45) Deng, J. J.; Lv, X. X.; Gao, J.; Pu, A. W.; Li, M.; Sun, X. H.; Zhong, J. Facile Synthesis of Carbon-coated Hematite Nanostructures for Solar Water Splitting. Energy Environ. Sci. 2013, 6, 1965-1970. (46) Su, J. Z.; Guo, L. J.; Bao, N. Z.; Grimes, C. A. Nanostructured WO3/BiVO4 Heterojunction Films for Efficient Photoelectrochemical Water Splitting. Nano Lett. 2011, 11, 1928-1933. (47) Hong, S. J.; Lee, S.; Jang, J. S.; Lee, J. S. Heterojunction BiVO4/WO3 Electrodes for Enhanced Photoactivity of Water Oxidation. Energy Environ. Sci. 2011, 4, 1781-1787. (48) Sivula, K.; Le Formal, F.; Gratzel, M. WO3-Fe2O3 Photoanodes for Water Splitting: a Host Scaffold, Guest Absorber Approach. Chem. Mater. 2009, 21, 2862-2867. (49)

Jin,

T.;

Diao,

P.;

Wu,

Q.;

Di

Xu;

Hu,

D.;

Xie,

Y.;

Zhang,

M.

WO3

Nanoneedles/a-Fe2O3/cobalt Phosphate Composite Photoanode for Efficient Photoelectrochemical Water Splitting. Appl. Catal., B 2014, 148-149, 304-310. (50) Kim, H. I.; Monllor-Satoca, D.; Kim, W.; Choi, W. N-doped TiO2 nanotubes Coated with a Thin TaOxNy Layer for Photoelectrochemical Water Splitting: Dual Bulk and Surface Modification of Photoanodes. Energy Environ. Sci. 2015, 8, 247-257. (51) Kim, J.; Monllor-Satoca, D.; Choi, W. Simultaneous Production of Hydrogen with the Degradation of Organic Pollutants Using TiO2 Photocatalyst Modified with Dual Surface Components. Energy Environ. Sci. 2012, 5, 7647-7656.

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Page 24 of 25

(52) Ponomarev, E. A.; Peter, L. M. A Generalized Theory of Intensity Modulated Photocurrent Spectroscopy (IMPS). J Electroanal. Chem. 1995, 396, 219-226. (53) Dloczik, L.; Ileperuma, O.; Lauermann, I.; Peter, L. M.; Ponomarev, E. A.; Redmond, G.; Shaw, N. J.; Uhlendorf, I. Dynamic Response of Dye-Sensitized Nanocrystalline Solar Cells: Characterization by Intensity-Modulated Photocurrent Spectroscopy. J. Phys. Chem. B 1997, 101, 10281-10289. (54) Kruger, J.; Plass, R.; Gratzel, M.; Cameron, P. J.; Peter, L. M. Charge Transport and Back Reaction in Solid-State Dye-Sensitized Solar Cells: A Study Using Intensity-Modulated Photovoltage and Photocurrent Spectroscopy. J. Phys. Chem. B 2003, 107, 7536-7539. (55) de Jongh, P. E.; Vanmaekelbergh, D. Investigation of the Electronic Transport Properties of Nanocrystalline Particulate TiO2 Electrodes by Intensity-Modulated Photocurrent Spectroscopy. J. Phys. Chem. B 1997, 101, 2716-2722. (56) Su, J.; Guo, L.; Bao, N.; Grimes, C. A. Nanostructured WO3/BiVO4 Heterojunction Films for Efficient Photoelectrochemical Water Splitting. Nano Lett. 2011, 11, 1928-1933. (57) Peter, L. M.; Wijayantha, K. G. U.; Tahir, A. A. Kinetics of Light-driven Oxygen Evolution at a-Fe2O3 Electrodes. Faraday Discuss. 2012, 155, 309-322. (58) Peter, L. M.; Ponomarev, E. A.; Franco, G.; Shaw, N. J. Aspects of the Photoelectrochemistry of Nanocrystalline Systems. Electrochim. Acta 1995, 45, 549-560.

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TABLE OF CONTENTS Electron Transport Layer e -e - e -e e - e-

e - e- e-

Suppressed Surface Recombination FTO

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h+ Photoanode

h+ h+h+h+

O2 H 2O

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SubstrateâElectrode Interface Engineering by an Electron-Transport

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