Photoelectrochemical Oxygen Evolution Reaction Activity of

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Letter

Photoelectrochemical OER Activity of Amorphous Co-La Double Hydroxide-BiVO Fabricated by Pulse Plating Electrodeposition. 4

Manjeet Chhetri, Sunita Dey, and Chintamani Nagesa Ramachandra Rao ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b00247 • Publication Date (Web): 13 Apr 2017 Downloaded from http://pubs.acs.org on April 16, 2017

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Photoelectrochemical OER Activity of Amorphous Co-La Double Hydroxide-BiVO4 Fabricated by Pulse Plating Electrodeposition. Manjeet Chhetri, Sunita Dey, C. N. R. Rao * New Chemistry Unit, Physics and Chemistry of Materials Unit, International Centre for Materials Science (ICMS), Sheikh Saqr Laboratory, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur, P.O Bangalore-560064, India. Corresponding Author *C. N. R. Rao, [email protected].

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ABSTRACT:

Strategically integrating semiconducting films with an efficient water oxidation catalyst to reduce charge recombination

and

improve the photocurrent

is

the bottleneck

in

photoelectrochemical (PEC) water splitting. Amorphous catalysts, specially the mixed metal oxides/hydroxides show better stability and activity pertaining to their unique morphology. Facile synthesis of these catalysts reproducibly by a simple method has been problematic. In this article, we show, for the first time, the application of pulse plating to synthesize amorphous CoLa mixed double hydroxide (MDH) on BiVO4/FTO (FTO-Flourine doped Tin Oxide). The method provides better adhesion and uniform deposits with controlled composition and grain size and facilitates fast charge transport, while lowering the charge recombination at the interface of the electrolyte and the semiconductor. With respect to BiVO4, reduction in onset potential by 0.53 V as well as 2.7 and 33.4 times increment in photocurrent density (J) at a 1.23 V and lower potential 0.6 V respectively obtained by BiVO4/MDH is noteworthy. The results obtained here suggest the possibility of using BiVO4/MDH in PEC cells and photoelectrochemical diodes.

TOC GRAPHICS


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Photodecomposition of water1 into H2 and O2 is considered to be one of the important ways of tackling energy and environmental problems. Coupling solar energy with minimum electrical energy2 to amplify the production of H2 without compromising the cost is presently attempted. Semiconductors such as Fe2O3 3-4 and WO3 5 serve the purpose for water splitting, while n-type BiVO4 with its direct bandgap of ~2.4 eV, wherein the valence band maximum (VBM) is sufficiently

positive

relative

to

the

water

oxidation

potential,

is

effective

for

photoelectrochemical (PEC) water splitting. It also has its conduction band minimum (CBM) close to the H+/H2 redox potential which helps in attaining low onset overpotential for water splitting process. BiVO4 however gives a low water oxidation photocurrent because of the small photogenerated carrier separation efficiency and meager light harvesting ability as well as surface carrier transport efficiency6. These limitations can be minimized by the use of a suitable co-catalyst for water oxidation and introduction of heterojunctions7 in the photoanodes or by impurity doping8-9. A strategic combination of an oxygen evolution catalyst (OEC) with BiVO4 effectively increases the charge carrier concentration and reduces the recombination of photogenerated electrons and holes2. Most of the active OECs with high activity that have been reported are amorphous10-14. The short range order in amorphous OECs creates abundant active sites and their structural flexibility leads to long term photo-electrochemical stability and efficient shuttling of charges in the redox process during water oxidation15. Amorphous monometallic oxide/hydroxide compositions, however, exhibits slow water oxidation kinetics due to higher Tafel slopes and a significant overpotential16. It has been shown that a rational combination of heterogeneous 3d transition metals outperforms the activity shown by monometallic compositions17-18. In this regard, the use of mixed metal amorphous hydroxides and oxides seem to be beneficial since they favor charge


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transfer and structural modifications while retaining the stability11,

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16-19

. Only a handful of

amorphous mixed metal hydroxides/oxides have been exploited for photoelectrochemical (PEC) purposes on BiVO4 films possibly due to the difficulties in fabrication techniques. In particular, the application is mostly limited to Co, Ni, Fe and Mn based amorphous OECs with comparable ionic sizes. Incorporation of rare earth cations20-21 in transition metal based mixed oxides/LDHs is shown to enhance the catalytic efficiency but these materials have not been explored adequately for PEC water oxidation. The transition metal here acts as the redox active center while the rare earth ion induces the changes in crystallinity22. Although synthesis of some amorphous metal oxides and hydroxides has been achieved using photochemical metal-organic deposition (PMOD)4, electrodeposition23, photoelectrodeposition (PED)24, it remains a challenge to use these methods for every metal combinations. These approaches are excellent for synthesizing metal oxide OECs, but a proper control on the thickness and the amount of OECs on the semiconductor film remains limiting aspect due to factors like voltage protocols and synthesis conditions, hence not agreeably expandable to the mixed metal hydroxides for an array of metal combinations. In the present study, we demonstrate a convenient method to synthesize amorphous mixed metal hydroxides (MDH) of La and Co on BiVO4/FTO and investigate the PEC water splitting activity. Here, Co acts as an electron transfer agent while La causes an increase in the amorphous nature of catalyst leading to its stability. The study shows pulse plating for the electrochemical deposition of MDH to be an apt strategy to deposit on BiVO4 /FTO. This method is reproducible and can be extended to other combinations of metals some of which are presently being examined by us. The use of BiVO4/Co-La(OH)x MDH for PEC water splitting has not been extensively explored till now. Interestingly, this system exhibits excellent early photocurrent in


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comparison to the state of art photoanodes. The onset potential of 0.19 V and a photocurrent increment factor of 33.4 and 2.7 at 0.6 V and 1.23 V found by us suggest the importance of the BiVO4/Co-La MDH interface. Surface charge transfer efficiency (ηtrans) of ~75% and lower charge transfer resistance (Rct) in OER kinetics are also noteworthy, being superior to the monometallic hydroxides.

Figure 1. Morphology study of the electrodes. (a) FESEM image (top view) of the BiVO4-MDH showing the porosity in the electrode. The inset shows the cross sectional view of the electrode. FESEM images of (b) BiVO4-MDH and (d) and (e) pristine BiVO4. High magnification SEM images show the presence of additional patches of co-catalyst on (c) BiVO4-MDH in contrast with (f) BiVO4. BiVO4 was electrochemically synthesized by the procedure described earlier2 with some modification. Pulse plating was employed for the deposition of Co and La MDH on the BiVO4 film and on bare FTO. As a form of electrosynthesis, we selected two extremes of potential, -0.9 5 ACS Paragon Plus Environment

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V and -1.2 V vs Ag/AgCl (anodic and cathodic potential limits respectively), and fixed the current values of -0.005 A (anodic) and 0.01 A (cathodic). We used 4 segments in steps between these two current values with a hold time of 2.5 seconds at each value to control the composition, thickness, grain size and uniformity of the deposit (Fig. S1). These parameters were optimized after a series of control experiments on bare FTO and on BiVO4, with post analysis of absorbance and FESEM images, to control the amount and thickness of MDH on BiVO4. Control synthesis of only La(OH)x and Co(OH)x on BiVO4 was also carried out. The detailed synthetic method is discussed in the supporting information along with the characterizations data. For the control experiments, three catalysts Co-La MDH, Co(OH)x and La(OH)x were also deposited directly on the FTO substrate without BiVO4

Figure 2. (a) and (b) are the HRTEM images of monoclinic scheelite phase of BiVO4. (c) and (d) show the crystalline BiVO4 particles interconnected with each other in the film. The insets (c1) and (d1) are the SAED patterns highlighting the crystalline nature. (e) and (f) show the presence of both BiVO4 and amorphous MDH in the catalyst BiVO4-MDH catalyst. The layer of LaCo 6 ACS Paragon Plus Environment

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MDH is homogenously distributed over BiVO4. SAED as shown by the diffraction patterns shows the presence of both crystalline and amorphous BiVO4 and MDH respectively [see insets (e1), (e2) and (f1)]. The X-ray diffraction patterns of BiVO4-MDH on FTO (Fig. S2), showed reflection due to monoclinic scheelite structure of BiVO4 besides those of SnO2 (ICSD-33242, Space group = I112/b). The reflection corresponding to the hydroxides of La or Co was not found due to its amorphous nature. The layered deposition of nanoporous BiVO4 and MDH on FTO is revealed by the cross-sectional scanning electron microscope (FESEM) image (inset of Fig 1a). In the high magnification FESEM images, the appearance of rough patches distributed uniformly over the BiVO4 after the MDH deposition (Figures 1b,c,d and e) depict the deposition of MDH. The micro- and nano-structural difference between the two is revealed by TEM analysis. Figures 2 a and b confirm the crystalline nature of the as deposited BiVO4 on FTO (SAED pattern in insets c1 and d1). The HRTEM image and SAED pattern of BiVO4-MDH (Fig. 2e and the insets e1 and e2) confirm the deposition of amorphous MDH on BiVO4 crystallites. The well interconnected amorphous MDH particles and crystalline BiVO4 (Figures 2f and f1) over a wide area in the deposited film gives a proof of remarkable advantage of pulse plating technique employed by us. The SAED pattern show the presence of – crystalline BiVO4 (dots) and amorphous MDH (rings) (Figures 2 e1,f1,f2). LaCo MDH/BiVO4 was investigated by various spectroscopic techniques. The survey scan of the X-ray photoelectron spectrum (XPS) shows the presence of La and Co on the surface of BiVO4 (Fig. 3a). The La 3d signal corresponds to La(OH)325 and Co 2p signal along with satellite peaks (Figures 3b and 3c) shows the coexistence of Co2+ and Co3+ species (relative ratio of 1.90)26-27. The La to Co ratio is close to 1.1 for the deposition of 4 cycles of MDH on BiVO4 (ICP-OES and XPS). X-ray absorption near edge structure (XANES) region of


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the XAS spectra of BiVO4-MDH was collected at Co- K edge and compared with that of the reference material Co3O4 (Fig.S3). We emphasize mainly on the main edge region of XANES spectra here for the confirmation of oxidation states of Co.

(a)

(b)

BiVO4-MDH BiVO4

1200

1000

800

600

400

200

La3d3/2

0

860

Binding Energy (eV)

Co2p3/2

(c)

La3d5/2

Intensity (a.u.)

Intensity (a.u.)

Intensity (a.u.)

La, Co

855

850

845

840

satelite peaks Co2p1/2

805

835

800

795

790

785

780

775

Binding Energy (eV)

Binding Energy (eV)

(d)

(e) BiVO4-MDH BiVO4

400

440

480

520

560

M-OH

Transmittance (a.u.)

Raman intensity (a.u.)

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 8 of 20

600

υLa-OH, Co-OH

υCo-OH

M-OH

M-OH M-OH

Bi-O

V=O

BiVO4-MDH

VO43-

BiVO4

V-O V-O

200

400

600

800

1000

1200

400

-1

Wavenumber (cm )

600

800

1000

1200

1400

1600

1800

-1

Wavenumber (cm )

Figure 3. XPS of pristine BiVO4 and MDH deposited on BiVO4 (a) survey scan of the two catalysts showing the presence of La and Co on the surface of BiVO4. The Core level spectra of (b) La 3d and (c) Co 2p on the surface of BiVO4-MDH catalyst. (d) Raman spectra of BiVO4 and BiVO4-MDH showing significant broadening and shifting of various stretching and deformation modes of BiVO4. Inset shows the presence of La and Co hydroxides bond in magnified view in 400-600 nm regions. (e) FTIR spectrum of the two catalysts showing the difference and the presence of M-O and M-OH bonds in addition to the bands due to BiVO4.


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The normalized spectra at the main edge region for BiVO4-MDH matches exactly with that of the Co3O4 confirming the presence of the 2+ and 3+ states of Co in the film. The appearance of three extra bands at 476 cm-1, 519 cm-1 and 680 cm-1 in raman spectra due to the MDH deposition on BiVO4 is prominent from Fig. 3d. The first two bands can be attributed to O-M-O (M=La,Co) bending modes while the last is due to the Co-OH stretching vibration. Notably, these bands are also observed for MDH directly deposited on FTO without BiVO4 (Fig. S4). The characteristic intense band at 830 cm-1 due to the symmetric stretching of the VO43- tetrahedron become broad and appear at 826 cm-1 on MDH deposition. This could be caused by the elongation in V-O bond length as reported previously28-30 (Fig. 3d). The external modes (translational and rotational) of VO43- at 128 cm-1 and 214 cm-1 get shifted, become broad and weak on MDH deposition 26-27 implying that the metal hydroxide deposit is intimately bonded to the local structure of BiVO4. Additionally the FTIR spectrum of BiVO4 differs strikingly after MDH deposition (Fig. 3e). The IR bands at 477 and 530 cm-1 due to La-O and Co-OH respectively corroborate the conclusions from Raman spectra. ICP-OES analysis revealed ca. 2-3 wt% of MDH on BiVO4 (by Bi : Co or La ratio). The light absorption of various nanoporous electrodes was between 450-475 nm in accordance with its band gap of ~2.53 to 2.57 eV as given by tauc plot (Fig. S5). It must be noted here that the pulse plating technique is the key for the excellent morphology in contrast to conventional direct cathodic or anodic deposition. The photoelectrochemical water splitting activity of BiVO4-MDH was measured under AM 1.5G (100 mW cm-2) illumination in 0.5 M potassium phosphate (K-Pi) buffer (pH 7). The photocurrent density–potential (J-V) curves of the electrodes are shown in Fig. 4. BiVO4 alone has an onset potential ~0.72 V, while with MDH deposited on it, the onset potential shifts to a much negative value 0.19 V (Table S1).


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1.0

0.5

BiVO4-MDH BiVO4-Co(OH)x BiVO4-La(OH)x BiVO4

2.0

2

BiVO4-La(OH)x BiVO4 Dark

1.5

2.5

2.0

1.5

C urrent D ensity, J (m A /cm 2 )

(b)

BiVO4-MDH BiVO4-Co(OH)x

2.0

Current Density, J (mA/cm )

2

Current Density (mA/cm )

(a)

1.5

0.2

0.0

1.00

1.02

0.0

1.06

1.08

0.5

0.2

0.4

0.6

0.8

1.0

0.0

1.2

0.2

(c)

BiVO4-Co(OH)x

2.0

1.5

1.0

0.5

0.0

2

2

2.5

2.5

2.0

1.5

1.0 0.5

0.2

0.4

0.6

0.8

Potential (V) vs RHE

1.0

1.2

1.0

1.2

(e) BiVO4-La(OH)x

2.5

2.0

1.5

1.0

0.5

0.0

0.0 0.0

0.8

3.0

(d)

3.0


BiVO4-MDH

0.6


3.0

0.4



2

1.04


1.0

0.0

0.0


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0.0

0.2

0.4

0.6

0.8

1.0

1.2


0.0

0.2

0.4

0.6

0.8

1.0

1.2


Figure 4. (a) J-V curves of BiVO4 and different deposited OECs in a 0.5 M phosphate buffer (pH 7) under AM 1.5 G illumination. The dark current is shown as a dashed line. (b) J-V curves under chopped light illumination. Inset highlights expanded view of the photocurrent transients measured for 30 s pulse irradiation for BiVO4-Co(OH)x and BiVO4-MDH. Comparison of photocurrent for sulfite oxidation (dashed) and water oxidation (solid) for BiVO4 (black dashed) and (c) MDH, (d) Co(OH)x only and (e) La(OH)x only, with and without the presence of hole scavenger 1.0 M Na2SO3 . The photocurrent density for BiVO4-MDH is also significantly higher than bare BiVO4 and for mono hydroxide. The early onset potential and high photocurrent at a lower potential ( Co(OH)x > La(OH)x (Figs. 4 c-e). Thus, Co-La MDH utilizes, if not completely, the maximum fraction of the surfacereaching holes in the water oxidation. This difference in photocurrent between water and sulfite oxidation can also be due to the e-/h+ recombination at the BiVO4/OEC interface which can be verified by comparing the J-V curves for sulfite oxidation of BiVO4 with and without deposition of amorphous metal hydroxide layers (Figs. 4 c-e). As shown in Fig. 4 c, the J-V curves including the onset potential of BiVO4–MDH is equal with BiVO4, whereas the values obtained for BiVO4-Co(OH)x and BiVO4-La(OH)x are considerably lower. It is surmised that the presence of amorphous MDH ceases the BiVO4/OEC interfacial recombination completely which has not been achieved with the use of single hydroxides as co-catalysts (Figures 4 d and e). BiVO4/MDH shows a rapid increase in J in the lower potential region (< 0.6 V vs RHE), thereby giving an descent fill factor of 0.34 for water oxidation half reaction (Fig S6). The LSV under chopped


ACS Energy Letters

(a)

-800

(b) BiVO4 BiVO4-La(OH)x BiVO4-Co(OH)x BiVO4-MDH

-600

Img. Z'' (ohm)

-400

Rct

Rcon.

Rcat1 R cat2.

Q1

Q2

Q3

R sol.

Solution

-200 La/Co MDH BiVO4 FTO Glass

Araldite Ag Paste

Cu wire

0

0

250

500

750

1000

1250

Real Z' (ohm) 0.4

(c) η transfer

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0.6

0.8

1.0

1.2

0.8

0.8

0.7

0.7

0.6

0.6

0.5

0.5

0.4 0.3 0.2

0.4

BiVO4MDH BiVO4-Co(OH)x BiVO4-La(OH)x BiVO4

0.3 0.2

0.1 0.0 0.4

0.1 0.0 0.6

0.8

1.0

1.2


Figure 5. (a) Comparison of Nyquist plots of BiVO4 and deposited MDH on BiVO4 catalysts. (b) Equivalent circuit used to fit the EIS data obtained in (a). The EIS was measured at 0.7 V (vs. Ag/AgCl) in 0.5 M phosphate buffer (pH 7) under AM 1.5 G simulated solar illumination. (c) Comparison of surface charge transfer efficiency of the BiVO4 with modified catalysts. light irradiation shows transient photocurrent stability, which reflects that in comparison to the single hydroxides, MDH reduces surface recombination substantially (inset of Fig. 4b). We have attempted to gain insight into the improvement of PEC activity in BiVO4-MDH and the improved BiVO4/Co-La MDH interface by Electrochemical Impedance Spectroscopy (EIS) and ηtrans measurements. In order to probe the exact electrochemical processes occurring during the water oxidation process directly to the EIS data, all the condition were kept the same as that in case of LSV plots described earlier. The Nyquist plots obtained at 0.7 V (vs Ag/AgCl) for 12 ACS Paragon Plus Environment

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BiVO4, BiVO4-MDH, La- and Co-(OH)x, are presented in Fig. 5a which can be understood in terms of the equivalent circuit given in Fig. 5b. The proposed modified Randles’ circuit fits well (fitting error~0.01) with the experimental data points and help in estimating the charge transfer resistance (Rct) and the resistance for the transport of charge within the catalyst (Rcat). The values of Rct are 2x108 Ω, 9x109 Ω, 1537 Ω and 187.4 Ω for bare BiVO4, BiVO4-La(OH)x, BiVO4Co(OH)x, and BiVO4-MDH electrodes respectively. The value of Rcat also decreases considerably in case of BiVO4-MDH than BiVO4. The details are listed in Table-1.

Table 1. Details of EIS plot Sample

Ohm (Ω) Rct

Rcat1

Rcat2

BiVO4

2.2 x 108

384.3

2128

BiVO4 La(OH)x

9.02 x 109

161.8

514

BiVO4 -Co(OH)x

1357

173.2

419

BiVO4 - MDH

187.4

23.5

623

Charge transfer at the OEC/electrolyte and OEC/BiVO4 interface is more facile for BiVO4-MDH inducing fast transport of charge carrier throughout the film. The kinetics of charge separation and transport in the OEC/electrolyte interface was studied by measuring ηtrans. The ηtrans was determined by comparing the photocurrents for water and sulfite oxidation according to the following formula.

ߟ௧௥௔௡௦ = ௃

௃ಹమ ೀ

- Equation (1)

ಿೌమ ౏ೀయ


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According to Equation-1, ηtrans is plotted in Fig. 5c. We obtained ~75% charge transfer efficiency for BiVO4-MDH which could be due to the improved interface between BiVO4 and MDH signifying the importance of pulse plating. The onset potential for sulfite oxidation for BiVO4 (0.16 V ±0.02) is close to flat band potential (Efb). Similar Efb ca. 0.17 V for BiVO4-MDH was estimated from Mott-Schottky (MS) plots (Fig. S7). Therefore, the improved PEC performance of amorphous Co-La MDH is majorly due to its BiVO4/OEC interface engineering in order to reduce photogenerated surface charge recombination. The J-V plots obtained with annealed (500°C, 0.5 hr) BiVO4-MDH electrodes showed (>2) times decrease in photocurrent (Fig. S8), indicating that a more adaptive interface provided by the amorphousity of the structure plays a major role in photo electrolysis31-32. We also carried out additional experiments by depositing layer-by-layer La(OH)x and Co(OH)x on BiVO4 in order to manipulate the BiVO4/OER and OER/electrolyte interface and studying their effects in PEC water splitting and to complement the data obtained from EIS and Fig. 4c-e. Synthesis related to this is given in Fig. S1(e) and (f). The higher transient photostability of the photoanode when Co(OH)x forms an interface with the electrolyte in comparison to that for La(OH)x is evident from Fig. S10. There are two interesting differences- difference of ca. 0.8 mA/cm2 in the photo current and the transient photostability between the codeposited MDH and layer by layer deposition samples. We, therefore, propose that it is indeed the improved modified BiVO4/MDH interface obtained from the pulse plating technique that is key factor for the higher PEC activity. We have compared the activity of the present catalyst with the performance of mono and bi metallic oxides/hydroxides integrated with BiVO4 (Table S2). The onset potential of water oxidation, increment factor with respect to bare BiVO4 and the photocurrent obtained at lower potential (0.5 V) for BiVO4-MDH are comparable with the reported catalysts33-39 (Table S2). The


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activity of 4 cycles deposited MDH was found to be superior to other cycles (2 and 8 cycles, Fig. S9) and hence this was considered the optimized sample. The applied bias photon-to-current efficiency (ABPE), calculated from J-V curve considering 100% faradic efficiency, is given in Fig. 6a. The increase in ABPE of ca. 5 times with simultaneous decrease in applied potential from 1.04 V to 0.61 V for BiVO4-MDH in comparison with bare BiVO4 is noteworthy and makes it a potential system for PEC cells for solar driven water splitting. (b)

BiVO4-MDH, 4-cycles

0.4

(c)

2.2

Current Density (mA/cm 2 )

BiVO4

0.3

0.2

0.1

22

Experimentally produced Calculated from photocurrent

20

2.0

amount of O2 (µ moles )

(a)

ABPE (%)

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

ACS Energy Letters

18

BiVO4-MDH

1.8 1.6

16 14

0

1000

2000

3000

4000

2.2

12 10

2.0

BiVO4

1.8

8 6

1.6

4 0

0.0 0.0

0.2

0.4

0.6

0.8

1.0

1.2

1000

2000

3000

4000

Time (s)

30

40

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60

70

80

90 100 110 120

Time (mins)

Potential(V) vs RHE

Figure 6. (a) ABPE obtained in three electrode system. (b) J-t curve obtained under AM1.5G illumination comparing the stability of modified BiVO4 catalyst by MDH deposition. (c) Comparison of produced O2 with calculated one for BiVO4-MDH catalysts. The photostability of BiVO4-MDH and bare BiVO4 was examined form the J-t curve. A photocurrent density of 2.09 mA/cm2, obtained by applying 0.8 V between the working and counter electrodes for 4000 seconds (Fig. 6b) remained almost constant (93% retention) while in contrast BiVO4 loses 24% of its activity within 30 sec of irradiation. These characteristics make BiVO4/MDH good photoanode for the construction of a PEC cell. The Faradic efficiency calculated at 0.6 V was found to be greater than 80%. The amount of O2 evolved with the


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BiVO4-MDH catalyst estimated using O2 sensor (ocean optics) is presented alongside the amounts calculated from chronoamperometric measurements (Fig.6c). During the synthesis, pH-3.4 was optimized to avoid precipitation of hydroxides of La and Co at higher pH and to prevent dissolution of the deposit at lower pH during the synthesis. PEC studies were carried out at pH-7, so it may be possible that active component Co-pi may form during the linear sweep voltametry study15,21. The presence of Co-pi and the relative composition of Co and La before and after the PEC study has been investigated with additional characterization techniques. We did not find the presence of any phosphorus or any appreciable change in the composition of Co and La obtained from the electrodes after the PEC study in ICP, EDAX and XPS analysis (Fig. S11). We can, therefore, exclude the possibility of Co-pi formation as well as the dynamic “chemically changing” nature of electrode during PEC study here. Also, if that indeed were the case, then we should have obtained similar PEC activity for Co-La MDH and monometallic Co(OH)x hydroxides. This clearly is not the case. In conclusion, we have successfully employed pulse plating for electrodepositing amorphous Co-La(OH)x mixed double hydroxide over nanoporous BiVO4 with controlled layer thickness and improved adhesion. This approach helps in attaining an advantageous BiVO4/MDH interface to reduce photogenerated charge recombination and enhance their transport by controlling the grain size. PEC water oxidation reveals reduction in the onset potential by 0.53 V besides 33.4 times increment in J at a lower potential (0.6 V) achieved by BiVO4/MDH. The increased amorphous nature of the MDH and modified interface of BiVO4/OEC obtained by the present method helps in gaining better transient photostability and PEC activity. The charge transfer resistance of BiVO4 decreases in few orders of magnitudes with MDH deposition. Deposition of Co-La(OH)x based MDH not only enhances the ABPE of BiVO4 significantly 16 ACS Paragon Plus Environment

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even at a lower potential but also provides a remarkable resistance against photocorrosion establishing it as a promising candidate for photoelectrochemical water splitting. The synthetic method employed by us can be extended to other metal hydroxide/oxide combinations.

ASSOCIATED CONTENT Supporting Information Materials, Details of Synthesis, control experiments, Characterizations, PEC activity study, Additional data and explanation for-pulse plating, PXRD, XANES, Raman spectra, Tauc plot, Fill factor, Mott-Schottky plot, the comparison table. AUTHOR INFORMATION [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT MC thanks UGC-India and SD thanks Sheikh Saqr for fellowship. Authors thank the Department of Science and Technology, India (SR/NM/Z-07/2015) for the financial support and Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR) for managing the project. We acknowledge Synchrotron SOLEIL (Gif-sur Yvette, France) for provision of synchrotron radiation facilities at beamline SAMBA (proposal# 20160052).


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Photoelectrochemical Oxygen Evolution Reaction Activity of

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