Cobalt-bilayer catalyst decorated Ta3N5 nanorod arrays as integrated electrodes for...

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http://dx.doi.org/10.1039/c3ee41854e

http://pubs.rsc.org/en/journals/journal/EE

Cobalt-bilayer Catalysts Decorated Ta3N5 Nanorod

Array as Integrated Electrodes for

Photoelectrochemical Water Oxidation

Jungang Hou, Chao Yang, Zheng Wang, Huijie Cheng, Shuqiang Jiao,* Hongmin Zhu*

State Key Laboratory of Advanced Metallurgy, School of Metallurgical and Ecological Engineering,

University of Science and Technology Beijing, Beijing 100083, China

Corresponding author: [email protected]; [email protected]

Abstract

Ta3N5 nanorod arrays were fabricated by nitridation of fluorine-containing tantalum oxide (F-Ta2O5)

nanorod arrays grown in situ on Ta substrates by a one-pot vapour-phase hydrothermal induced

self-assembly technique. In this protocol, the in-situ generation and the morphology of arrays

elaborately adjusted by reaction time, play a vital role in the formation of the F-Ta2O5 nanorod

arrays and a highly conductive interlayer between the nanorods and the substrate. Due to the shape

anisotropy, ordered hierarchical structure and high surface area, a high photoelectrochemical

activity was achieved by the optimum Ta3N5 nanorod photoelectrode with the photocurrent density

of 1.22 mA/cm2 under AM 1.5G irradiation at 1.23 V vs. RHE (reversible hydrogen electrode).

Furthermore, the higher and more stable photocurrent was demonstrated by combining the highly

active Ta3N5 nanorods with a stable Co3O4/Co(OH)2 (Co3O4/Co(II)) bilayer catalysts compared with

Co(II)/Ta3N5 and Co3O4/Ta3N5 photoelectrode, exhibiting that not only the onset potential is

negatively shifted but also the photocurrent and the stability are significantly improved, which is

correlated to an order of magnitude reduction in the resistance to charge transfer at the Ta3N5/H2O

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View Article OnlineDOI: 10.1039/C3EE41854E


interface. Specifically, about 92% of the initial stable photocurrent remains after long-term

irradiation at 1.23 V vs. RHE. At 1.23 V vs. RHE, the photocurrent density of Co3O4/Co(II)/Ta3N5

arrays reached 3.64 mA/cm2 under AM 1.5G simulated sunlight at 100 mW/cm2, and a maximum

IPCE of 39.5% was achieved at 440 nm. This combination of catalytic activity, stability, and

conformal decoration makes this a promising approach to improving the photoelectrochemical

performance of photoanodes in the general field of energy conversion.

KEYWORDS: Ta3N5 nanorod arrays; cobalt-bilayer catalyst; photoelectrochemical water splitting

Introduction

Harvesting energy directly from sunlight as nature accomplishes through photosynthesis is a very

attractive and desirable way to solve the energy challenge. Many efforts have been made to find

appropriate materials and systems that can utilize solar energy to produce chemical fuels. One of the

most viable options is the construction of a photoelectrochemical cell (PEC) using the

photoelectrode as the integrated device.[1, 2] Metal oxides are extensively studied as photoelectrodes

for PEC conversion of solar energy into chemical fuels.[3] However, due to the deeply located

energy potential of O 2p orbitals with the whole or majority of the valance band maximum,

visible-light-responsive metal oxide photoelectrodes often possess insufficient reduction potential,

and as a result an externally applied bias is inevitable. Meanwhile, non-oxide semiconductor with

smaller band gaps and appropriate energy levels for PEC performance has emerged as a model

material that has been studied extensively.[1-3] Compared to that of the oxygen 2p orbital, metal

nitrides due to the more negative potential of the nitrogen 2p orbital, often possess a narrow band

gap and can potentially encompass nearly the entire solar spectrum.[4]

Among these non-oxide semiconductors, (oxy)nitrides containing Ta5+ or Ti4+,[5-7] such as TaON,

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Ta3N5 and LaTiO2N, emerge as promising candidates for PEC overall water splitting.[8-10] Tantalum

nitride (Ta3N5) is one promising material that meets many of the requirements for the water splitting

even without external bias using a large portion of the solar spectrum (< 600 nm): (i) it has a

suitable band gap of 2.1 eV; (ii) it is feasible to conduct for water splitting due to an appropriate

band positions; and (iii) it is composed of nontoxic elements, making it environmentally

benign.[11-14] Recently, Ta3N5 photoelectrodes has attracted intensive interest, because the theoretical

maximum possible solar-to-hydrogen efficiency of a Ta3N5 photoelectrode in PEC water splitting is

predicted to be as high as 15.9% under AM 1.5 G irradiation.[15] Despite such promising attributes,

the overall water splitting efficiency of Ta3N5 photoanodes falls well short of the theoretical

maximum efficiency, which is possiblely limited by several main drawbacks: (i) the low carrier

mobility; and (ii) its poor stability due to self-photocorrosion in electrolyte solution.

The quantum efficiencies of photoelectrode for water splitting are primarily affected by the

structures of the photocatalyst and cocatalyst. Generally, the structure of a photocatalyst determines

the generation and transfer of carriers (electrons and holes) to the photocatalyst surface. The size,

morphology, surface chemistry, and crystal structure of photocatalysts often play a crucial role in

determining their photophysical and photocatalytic properties. Conventional photocatalysts are

typically employed in the form of powders. Nanostructured photoelectrode architectures have begun

to address current materials limitations. One-dimensional (1D) nanostructure arrays have the

apparent advantages of promoting the transport and separation of photoexcited charge carriers and

providing abundant surface reaction sites, which are crucial for obtaining high solar energy

conversion efficiency.[16-19] For examples, polycrystalline Ta3N5 nanotube arrays on the Ta substrate

were prepared by anodization of tantalum foil.[20-22] Ta3N5 nanorod arrays were produced via a

through-mask anodization method and a subsequent nitridation process using anodic alumina as the


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mask under the anodizing voltage of 650 V.[23] Although various 1D Ta3N5 nanostructure arrays can

be prepared, it is still a challenging for the synthesis of nitrides with a desired 1D nanostructure

array using low-cost and easily-scalable routes.

Moreover, the poor stability as the serious obstacle hinders the practical application of Ta3N5

photoelectrodes, thus requiring a suitable cocatalyst that plays a crucial role in promoting the

separation of electrons and holes and provides active sites for H2 and O2 evolution.[12,14] Another

characteristic limiting efficiency is sluggish oxygen evolution reaction kinetics at the surface of

Ta3N5 photoelectrodes, thus requiring a large applied potential to drive water oxidation. Although

the catalyst is not stable over time, IrO2 nanoparticles coupled to the surface of hematite reduced the

photocurrent onset potential by 0.2 V.[24] Electrodeposited films show on average a respectable 200

mV cathodic shift in the photocurrent onset potential, as well as stability due to a “self-healing”

mechanism in the presence of phosphate buffer.[25-28] The requirement for nanostructuring Ta3N5

presents a potential challenge for coupling many well-known water oxidation catalysts to the

surface. So far, various materials have been reported to act as effective cocatalysts for water

oxidation.[11-28] For examples, adding cobalt-based catalysts (cobalt ions, Co2+, the cobalt phosphate

catalyst,“Co-Pi”, or Co3O4) is equally interesting as they offer low-cost alternatives to the rare and

expensive Ir- and Ru-based catalysts yet provide a reduction in the improvement in photocurrent

stability.[11-31] Ultimately, stable and efficient photoanodes are desired, and methods to apply

uniform catalyst layers onto nanostructured photoanodes, such as Ta3N5, are therefore needed.

The requirement for nanostructuring Ta3N5 presents a potential challenge for coupling many

well-known water splitting catalysts to the surface. In this work, we report on the fabrication of

aligned Ta3N5 nanorod arrays using a general vapor-phase hydrothermal route and then demonstrate

the potential of using a facile decoration process to deposit a Co-based catalysts on the surface of


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Ta3N5 photoanodes with both flat and nanostructured geometries for solar-driven PEC water

splitting. Aligned fluorine-containing tantalum oxide (F-Ta2O5) nanorod arrays were first grown on

the Ta substrate via a facile vapor-phase hydrothermal process with the controllable reaction time.

After the subsequent nitridation process, the optimum Ta3N5 nanorod photoelectrode exhibited high

crystallinity, a highly conductive interlayer between the substrate, and high photocurrent density of

1.22 mA/cm2 under AM 1.5G simulated sunlight at 100 mW/cm2 with an applied potential of 1.23

V vs. RHE. Moreover, Co3O4/Co(II)/Ta3N5 nanorod arrays were prepared using a facile decoration

process for the deposition of conformal catalytic overlayers on nanostructures, facilitating the

surface chemistry further enables an identical catalyst overlayer to be deposited on Ta3N5

nanostructures with excellent reproducibility. Compared with Co(II)/Ta3N5 and Co3O4/Ta3N5

photoelectrode, the resulting Co3O4/Co(II)/Ta3N5 nanorod arrays exhibit that not only the onset

potential is negatively shifted but also the photocurrent and the stability are significantly improved.

At an applied potential of 1.23 V RHE, the photocurrent density of Co3O4/Co(II)/Ta3N5 arrays

reached 3.64 mA/cm2 under AM 1.5G simulated sunlight at 100 mW/cm2, and a maximum IPCE of

39.5% was achieved at 440 nm. Specifically, about 92% of the initial stable photocurrent remains

after 7200 s irradiation at 1.23 V vs. RHE. Thus, The PEC water splitting activity of the

cobalt-decorated Ta3N5 nanorods with the best durability against photocorrosion to date, is one of

the highest among all the photoanodes so far reported.

Experimental

Fabrication of Ta3N5 samples

Ta foil with a thickness of 0.25 mm (Alfa Aesar) was washed in ethanol, acetone, isopropanol

and deionized water each for 60 minutes before used. The clean Ta foil was suspended above a 0.15


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M HF aqueous solution in a Teflon-lined autoclave, which was then heated at 180~240 oC for 0~6 h

to grow F-containing Ta2O5 (F-Ta2O5) samples on the Ta foil. Under the different reaction time (1 h,

3 h and 6 h), the variable F-containing Ta2O5 samples exhibited the different shapes, such as

nanoparticle thin film, nanorod array and nanoflowers thin film, respectively. After heat-treatment,

the resultant F-Ta2O5 samples on the Ta foil under a gaseous atmosphere of NH3 with a flow of 20

mL min-1 at 10 °C min-1 and heated at 850 °C 3 h, transformed into the formation of Ta3N5

nanoparticle thin film, nanorod array and nanoflowers thin film, respectively.

Deposition of Co(OH)2 nanoparticles on Ta3N5 nanorod arrays

Ta3N5 nanorod arrays photoelectrodes was modified with Co(OH)2 nanoparticles via spin-coating

an alcoholic solution containing Co(CH3COO)2·4H2O (0.05 M, 99.9%, Aladdin), ethanolamine

(0.05 M, 99.0%, Aladdin) and an aqueous solution of NaOH (97.0%, Aladdin), onto the surface of

Ta3N5 nanorod arrays photoelectrodes at 2000 rpm for 10 s. By repeating the above deposition cycle,

the desired amount of Co(OH)2 nanoparticles can be loaded on the photoelectrodes.

Deposition of Co3O4 nanoparticles on Ta3N5 nanorod arrays

Deposition of Co3O4 nanoparticles on Ta3N5 nanorod arrays with the same molar ratio of Co

species of Co(OH)2/Ta3N5 photoelectrode was conducted via the facile hydrothermal route and the

spin-coating method. Firstly, Co3O4 nanoparticles were synthesized via the following process.

Co(CH3COO)2·4H2O (0.05 M, 99.9%, Aladdin) was dissolved in ethanol (15.0 mL), and then 25%

ammonium hydroxide (0.1 M, Aladdin) was added under vigorous stirring conditions. After stirring

for 30 min, the solution was transferred into an autoclave (30.0 mL), sealed, and maintained at

150 °C for 3 h. After this, the autoclave was naturally cooled to room temperature. The resulting

black solid products were washed with ethanol and the uniform Co3O4 nanoparticles were obtained.

Secondly, Co3O4/Ta3N5 nanorod arrays photoelectrodes was modified with Co3O4 nanoparticles via


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spin-coating an alcoholic solution containing Co3O4 nanoparticles onto the surface of Co3O4/Ta3N5

nanorod arrays photoelectrode at 2000 rpm for 10 s. By repeating the above deposition cycle, the

desired amount of Co3O4 nanoparticles can be loaded on the Ta3N5 photoelectrodes.

Deposition of Co3O4 nanoparticles on Co(OH)2/Ta3N5 nanorod arrays

Deposition of Co3O4 nanoparticles on Co(OH)2/Ta3N5 nanorod arrays was conducted via a

three-step process including the facile hydrothermal route and the spin-coating method. The total

molar amount of Co species as the same as that of individual Co(OH)2/Ta3N5 or Co3O4/Ta3N5

photoelectrode. Firstly, Co3O4 nanoparticles were synthesized via the following process.

Co(CH3COO)2·4H2O (0.05 M, 99.9%, Aladdin) was dissolved in ethanol (15.0 mL), and then 25%

ammonium hydroxide (0.1 M, Aladdin) was added under vigorous stirring conditions. After stirring

for 30 min, the solution was transferred into an autoclave (30.0 mL), sealed, and maintained at

150 °C for 3 h. After this, the autoclave was naturally cooled to room temperature. The resulting

black solid products were washed with ethanol and the uniform Co3O4 nanoparticles were obtained.

Secondly, Co3O4/Co(OH)2/Ta3N5 nanorod arrays photoelectrodes was further modified with Co3O4

nanoparticles via spin-coating an alcoholic solution containing Co(CH3COO)2 (0.05 M, 99.9%,

Aladdin), ethanolamine (0.05 M, 99.0%, Aladdin) and an aqueous solution of NaOH (0.1 M, 97.0%,

Aladdin) as well as as-prepared Co3O4 nanoparticles, onto the surface of Co(OH)2/Ta3N5 nanorod

arrays photoelectrodes at 2000 rpm for 10 s. By repeating the above deposition cycle, the desired

amount of Co3O4/Co(OH)2 bilayer nanoparticles can be loaded on the Ta3N5 photoelectrodes.

Thirdly, the obtained Co3O4/Co(OH)2 bilayer nanoparticles decorated Ta3N5 photoelectrodes has

been heated at 300 oC for 30 min in the dried gaseous atmosphere of N2 with a flow of 10 mL min-1.

Now, the as-prepared Co3O4/Co(OH)2/Ta3N5 photoelectrode (noted as Co3O4/Co(II)/Ta3N5) was

achieved.


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Characterization

The obtained products were characterized with powder X-ray diffraction (XRD, MAC Science

Co. Ltd Japan) using Cu Kα (λ = 0.1546 nm) and XRD patterns were obtained for 10-90o 2θ by step

scanning with a step size of 0.02o. The morphology and size of the resultant powders were

characterized by a Zeiss Ultra 55 field-emission scanning electron microscope (SEM) associated

with X-ray energy-dispersive spectrometer (EDX). Transmission electron microscopy (TEM)

images and the corresponding selected area electron diffraction (SAED) patterns were captured on

the transmission electron microscopy (TEM, JEM-2010) at an acceleration voltage of 200 kV. The

optical properties of the samples were analyzed by UV-vis diffuse reflectance spectroscopy (UV-vis

DRS) using a UV-vis spectrophotometer (UV-2550, Shimadzu). Fine BaSO4 powder is used as a

standard for baseline and the spectra are recorded in a range 190-900 nm. The chemical states of the

sample were determined by X-ray photoelectron spectroscopy (XPS) in a VG Multilab 2009 system

(UK) with a monochromatic Al Kα source and charge neutralizer.

Photoelectrochemical and impedance spectroscopic measurements

The photoelectrochemical water splitting was carried out in a three-electrode system, where the

prepared nanorod array films, Ag@AgCl electrode and a high surface area platinum mesh act as

working electrode, reference electrode and counter electrode, respectively, in the electrolyte of

aqueous 1 M NaOH solution under AM 1.5G illumination with a density of 100 mW cm-2

(Newport). A 300 W Xe lamp was utilized as a light source. The photocurrents and impedance

spectroscopic measurements were measured using a Solartron 1287 potentiostat with

electrochemical software. Impedance data were gathered using a 10 mV amplitude perturbation of

between 10000 and 0.01 Hz. Data were fit using Zview software (Scribner Associates). According

to the Nernst equation (ERHE = EAg/AgCl + 0.059pH + 0.196), the measured potentials vs the RHE


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scale can be obtained from the potentials vs Ag/AgCl. The oxygen evolution by

photoelectrochemical water splitting was conducted in the airtight reactor connected to a closed gas

circulation system. The amount of oxygen was determined by a gas chromatography (GC-3240)

equipped with TCD (molecular sieve 5 Å column, Ar carrier gas).

Results and discussion

Process for fabrication of variable tantalum-based samples is illustrated in Scheme 1. F-Ta2O5

nanorod arrays, perpendicularly aligned to the Ta substrate, was first prepared by a simple

vapor-phase hydrothermal treatment. The X-ray diffraction (XRD) pattern (Figure S1, ESI†)

indicates that the nanorods have an orthorhombic phase. A high resolution transmission electron

microscopy (TEM) image confirms the single-crystal nature of the nanorods and their preferential

growth along the [001] direction (Figure S2, ESI†). The elments Ta, O and F are all detected in the

samples by energy-dispersive X-ray spectroscopy (EDX, Figure S3, ESI†). To understand the

formation of the F-Ta2O5 nanorods, a time-dependent morphology evolution process in Figure 1

was recorded (Figure S2, ESI†). At the early stage (1 h) of the vapor-phase hydrothermal process, a

tantalum oxide thin film, consisting of particles with a size of less than 100 nm, was first formed

and some short nanorods began to grow. After 3 h, uniformly alligned F-Ta2O5 nanorod arrays were

obtained, which had been dissolved from the Ta substrate by the gaseous mixture of HF, H2O2 and

H2O at 240 oC. With the increasing reaction time up to 6 h, hierarchical F-Ta2O5 flower clusters

were developed by preferentially oriented nucleation and densely distributed over the surface of the

Ta substrate. A close-up of a flower revealed that this superstructure resembled a natural

chrysanthemum. Each flower was composed of a large number of F-Ta2O5 nanorod petals. After the

subsequent nitridation process, the Ta3N5 nanoparticles, alligned arrays and chrysanthemum-like


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hierarchical flower, as shown in Figure 1, were transformed from F-Ta2O5 samples. Figure 1 show a

typical top-view scanning electron microscopy (SEM) image of a Ta3N5 nanorod array. The

diameter of the nanorods is about 40 nm. The cross-sectional SEM image of the Ta3N5 nanorod

array (Figure S4, ESI†) reveals that the nanorods have a length of ∼600 nm and are vertically

oriented on the Ta substrate. Thus, in this protocol, the in-situ generation and the morphlogy of

arrays elaborately adjusted by reaction time, play vital roles in the formation of the Ta3N5 nanorod

arrays and a highly conductive interlayer between the nanorods and the substrate.

Scheme 1. Synthetic route of variable tantalum-based samples, such as F-Ta2O5 with different

morphology, Ta3N5, Co(II)/Ta3N5, Co3O4/Ta3N5, Co3O4/Co(II)/Ta3N5 nanorods array.


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Figure 1. (a,b) Top surface view of Ta3N5 nanorods array wtih different magnification, (c) Top

surface view of Ta3N5 nanoparticle thin film, (d) Top surface view of Ta3N5 nanoflower thin film.

PEC water oxidation measurement was carried out in a three-electrode system for the Ta3N5

based photoelectrodes as the working electrode with an exposed area of 2 cm2, a Pt foil as the

counter electrode, a Ag/AgCl as a reference electrode, and the different electrolyte. Hereafter, the

electrode potential is reported relative to the Ag/AgCl electrode, and the reversible hydrogen

electrode (RHE) potential is converted from the Ag/AgCl electrode. The photocurrent density

versus applied voltage scans of the samples were measured under visible light irradiation provided

by a 300 W Xenon lamp (AM1.5G). Due to the shape anisotropy, ordered hierarchical structure and

high surface area, a high photoelectrochemical activity of Ta3N5 nanorod arrays was achieved by

elaborately adjusted by reaction time. Figure 2 shows the photocurrent density-potential curves

dependent on morphology in Na2SO4 electrolyte (pH=6.5) under chopped light (AM 1.5G) and AM


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1.5G simulated sunlight at 100 mW/cm2. The obtained results clearly reveal the photocurrent

density of the Ta3N5 nanorod array photoelectrode is much higher than that of nanoparticle and

nanoflower thin film photoelectrode. Obviously, the Ta3N5 nanorod array photoelectrode yielded a

photocurrent density of 0.93 mA/cm2 at 1.23 V versus a reversible hydrogen electrode (VRHE),

which was ~3.0 times higher than that of a Ta3N5 nanoparticle thin film photoelectrode. More

importantly, the apparent enhancement of the photocurrent density was obtained at lower applied

potentials, demonstrateing the practical possibility of achieving overall water splitting using the

PEC strategy under a low bias. In addition, exploration experiments were conducted in most

commonly used Na2SO4 (0.5 M, pH = 6.5) and NaOH (1 M, pH = 13.6) under chopped light (AM

1.5G) and AM 1.5G simulated sunlight, in order to reveal the differences of PEC performances

initiated by the application of different electrolytes.[14,23] Under the same condition, the obtained

results clearly reveal the sharp differences in photocurrent collected in NaOH and Na2SO4. Higher

photocurrent values throughout the measured potential range are achieved in NaOH compared to

Na2SO4 at the same potentials, and at higher potentials the upgrade is more evident, as shown in

Figure 3. Thus, the PEC performance of the Ta3N5 photoelectrode is dependent upon the effect of

the morphology and electrolyte, indicating that the Ta3N5 photoelectrode exhibit the high

photocurrent density in NaOH electrolyte.


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Figure 2. (a) Photocurrent–potential curves measured under chopped light (AM 1.5G) and (b)

photocurrent densities measured under AM 1.5G solar light of Ta3N5 photoanodes with different

morphology in 0.5 M Na2SO4 electrolyte (pH = 6.5). All photocurrent-voltage data are collected

under AM 1.5G sunlight at 100 mW/cm2 with the scan rate of 30 mV/s.


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photocurrent densities measured under AM 1.5G solar light of Ta3N5 nanorod array photoanodes y

in 0.5 M Na2SO4 (pH = 6.5) and 1 M NaOH (pH = 13.6) as the variable electrolyte .


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photocurrent densities measured under AM 1.5G solar light of TaON nanorod array photoanodes

decorated by various catalysts (Co(II), Co3O4 and Co3O4/Co(II) bilayer catalysts) in 1 M NaOH (pH

= 13.6).


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Comprehensive consideration of various factors involving a photocatalyst and a cocatalyst is the

key to developing a photocatalytic material with a high quantum efficiency for water splitting.

Besides the nanostructure, surface modification with co-catalysts such as IrO2, Co-Pi, Co3O4 and

Co(OH)x nanoparticles is not only effectively promoting water splitting but also stabilizing nitride

photoanodes.[5-15] Especially, it is noted that the co-catalysts in the photocatalytic system deserve to

use cheap, earth-abundant metals in order to produce solar fuel on a large scale, such as

cobalt-based catalysts (Co(OH)2 or Co3O4). In this work, the Co(OH)2 and Co3O4 as co-catalysts

were employed for the decoration of the Ta3N5 nanorod arrays. Co(OH)2 nanoparticles were

deposited on Ta3N5 nanorod arrays by using a general spin-coating method. After completion of

homogeneous Co(OH)2 decoration, a thin layer of uniform Co3O4 nanoparticles was further

deposited on the Co(OH)2/Ta3N5 nanorod surface (Experimental section), resulting into the

formation of Co3O4/Co(OH)2/Ta3N5 (noted as Co3O4/Co(II)/Ta3N5) nanorod array. The optimum

amount of Co3O4 and Co(OH)2 nanoparticles for stable and efficient PEC water splitting can be

achieved by controlling the number of cycles. As we expect, Co3O4 and Co(OH)2 modified Ta3N5

electrodes show better PEC performances measured under chopped light (AM 1.5G) and AM 1.5G

simulated sunlight at 100 mW/cm2 for bare Ta3N5 electrode in the full tested potential range in

Figure 4. Especially, the photocurrent density of Co(II)/Ta3N5 and Co3O4/Ta3N5 nanorod array

photoelectrode is increased by about 2.8 and 2.5 folds, respectively, at an applied potential of 1.23

V RHE in NaOH electrolyte. Specifically, the photocurrent of Co(OH)2 loaded Ta3N5 photoanode is

higher than that of Co3O4 loaded Ta3N5 photoanode at full potential range. Furthermore, compared

with Co(II)/Ta3N5 and Co3O4/Ta3N5 photoelectrode, the higher and more stable photocurrent was

demonstrated by combining the highly active Ta3N5 nanorods with a stable Co3O4/Co(II)

co-catalysts exhibiting that not only the onset potential is negatively shifted but also the


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photocurrent and the stability are significantly improved. A higher photocurrent density means that

more photoinduced electrons could be transferred from the Co3O4/Co(II)/Ta3N5 arrays to the

counter electrode via the external circuit. With a further increase of bias potential, the photocurrent

density increased sharply. Although the change of internal electrostatic field is still existent, there is

almost no effect on the shape of the current−potential curve due to the relative minor effect, which

is in agreement with the previous reports.[32] At an applied potential of 1.23 V vs. RHE, the

photocurrent density of Co3O4/Co(II)/Ta3N5 arrays reached 3.64 mA/cm2 under AM 1.5G simulated

sunlight at 100 mW/cm2 in NaOH electrolyte. The photocurrent density of Co3O4/Co(II)/Ta3N5

nanorod array photoelectrode was ~6.3 times higher than that of a Ta3N5 nanoparticle thin film,

which is one of the highest among all the photoanodes so far reported.[11-28] Especially, the

maximum photocurrent of Co3O4/Co(II)/Ta3N5 arrays reaches as high as 6 mA/cm2 at 1.6 V vs.

RHE, which is higher than that of Co(II)/Ta3N5 and Co3O4/Ta3N5 arrays under 1.5 G irradiation.

To quantitatively evaluate the efficiency of PEC water splitting of the samples, the

photoconversion efficiency is calculated based on the equation,[33]

1240(%) 100ph

in

iIPCE

pλ

×= ×

×

where iph is the photocurrent (mA), λ is the wavelength (nm) of incident radiation, and Pin is the

incident light irradiance on the semiconductor electrode at the selected wavelength (mW). Figure 5

shows the wavelength dependence of IPCE for the Ta3N5 and Co3O4/Co(II)/Ta3N5 nanorods arrays,

indicating that the incident photon-to-current conversion efficiencies (IPCE) of Co3O4/Co(II)/Ta3N5

arrays photoelectrode is about 3 times of that of bare Ta3N5 nanorods arrays at the same potential.

On account of electrical bias facilitating the separation of the electron-hole pairs, IPCE values

increase with increasing externally applied bias. At an applied potential of 1.23 V vs. RHE, the

IPCEs for the Co3O4/Co(II)/Ta3N5 nanorods arrays were above 33% in the range of 400–520 nm.


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The IPCE increases with the potential suggesting that the applied potential plays an important role

in charge separation and/or collection. It is worth to pointing out that a maximum IPCE of 39.6%

was achieved at 440 nm for the Ta3N5 nanorods arrays decorated with low-cost Co3O4/Co(II)

bilayer co-catalysts. The photoresponse of the samples is coincident with the UV-visible diffusion

reflectance spectra (Figure S5, ESI†), which indicates that the anodic photocurrent is mainly

originated from the band gap transition of Ta3N5, while Co3O4/Co(II) co-catalysts in

Co3O4/Co(II)/Ta3N5 nanorods arrays electrode mainly plays a role of cocatalyst. Furhermore, the

amount of gases evolved under an applied potential of 1.23 V vs. RHE (Figure S6, ESI†),

determined that the photocurrent is indeed caused by the PEC water oxidation. These results

demonstrate that the recombination of photoexcited carriers is effectively inhibited through loading

Co3O4/Co(II) cocatalyst which can accelerate the water oxidation reaction and improve the charge

transfer processes.

Figure 5. Wavelength dependence of IPCE measured at 1.23 V vs. RHE for Ta3N5 nanorods array

and Co3O4/Co(II) bilayer catalysts decorated Ta3N5 nanorods array.


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Figure 6. Time courses for the photocurrent of bare Ta3N5 nanorods (black curve), Co(II)/Ta3N5

nanorod array (red curve), Co3O4/Ta3N5 nanorod array (blue curve), and Co3O4/Co(II)/Ta3N5

nanorod array (grey curve) measured in 1.0 M NaOH (pH = 13.6) at 1.23 V vs. RHE under AM

1.5G simulated sunlight at 100 mW/cm2 .

As mentioned above, the poor photocurrent stability is ascribed to the serious obstacle that

hinders the practical application of (oxy)nitrides photoelectrodes during water splitting reaction.[6-35]

After the enhancement of photocurrent through the optimized Ta3N5 nanorods arrays and NaOH

electrolyte, the major aim of this work is to improve the photostability of Ta3N5 photoelectrode. So,

the effect of Co3O4 and Co(OH)2 on the photostability of Ta3N5 nanorods arrays was also examined

by measuring and comparing photocurrent densities of bare Ta3N5, Co(II)/Ta3N5, Co3O4/Ta3N5 and

Co3O4/Co(II)/Ta3N5 photoelectrodes in NaOH solution at 1.23 V vs. RHE under AM1.5G

illumination (100 mW/cm2) for 7200 s. As shown in Figure 6, the photocurrent generated by the

bare Ta3N5 film decreases significantly within a few minutes, and the steady state photocurrent is


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negligibly low in few seconds, indicating that accumulation of photogenerated holes at the surface

of Ta3N5 due to poor kinetics for water oxidation and subsequent surface oxidative decomposition

of Ta3N5. It is well known that various cocatalysts, such as cobalt-based catalysts with low cost and

the rare and expensive Ir- and Ru-based catalysts, could play an pivot role in promoting the

separation of electrons and holes and improving the photocurrent stability.[5-28] Consequently, it is

evident that Co3O4 and Co(OH)2 as cocatalysts decorated Ta3N5 photoanode has presented the

highest PEC performance. With regard to the photostability, it can be seen that the photocurrent

decay curves for individual Co(II)/Ta3N5 and Co3O4/Ta3N5 photoanodes show substantial

enhancement in photostability. However, an unexpected phenomenological observation show that a

photocurrent density of 1.99 mA/cm2 for Co3O4/Ta3N5 photoanodes was maintained for 7200 s with

22% of decay, while a photocurrent density of 2.28 mA/cm2 for Co(II)/Ta3N5 photoanodes with

sustaining decrease was almost 62% of decay. This decline trend is similar with that of IrO2

modified Ta3N5 photoanode.[14] Noticeably, almost 92% of the initial stable photocurrent of ~2.78

mA/cm2 for Co3O4/Co(II)/Ta3N5 nanorod array photoanodes, remains after 7200 s irradiation at 1.23

V vs. RHE. Photostability of Ta3N5-based systems at this high level of photocurrent density for

several hours has been rarely achieved since the degree of photocorrosion is generally proportional

with the amount of photocurrent generated. This result demonstrates the exceptional promise of

Co3O4/Co(II) bilayer cocatalysts for improving photostability as well as photocurrent of Ta3N5.

Perfect coverage of the Ta3N5 surface by Co3O4/Co(II) bilayer cocatalysts apears to be necessary to

completely prevent the photocorrosion. To a certain extent, this modified approach using

Co3O4/Co(II) bilayer cocatalysts has resolved the above-mentioned limitations of the bare Ta3N5

photoanodes.


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Figure 7. High resolution transmission electron microscopy images of different nanorods array. (a)

bare Ta3N5 nanorods, (b) Co3O4/Co(II)/Ta3N5 nanorod array, (c) Co3O4/Ta3N5 nanorod array, and (d)

Co(II)/Ta3N5 nanorod array as well as (e) illustration of different nanorods with different

co-catalysts.

It is necessary to explore the distribution of Co3O4 and Co(OH)2 nanoparticles on the surface of

the Ta3N5 nanorods array photoanode for the improvement of PEC performance including

photocurrent and photostability. As indicated in Figure 7, the successful loading of Co3O4 and

Co(OH)2 nanoparticles on the nanorod surface was confirmed by HRTEM. The high resolution

TEM image (Figure 7a) recorded along the [002] zone axis shows a set of lattice fringes with

(e)


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spacings of 0.39 nm, corresponding to the and {020} plane of monoclinic Ta3N5, respectively. After

the decoration, as shown in Figure 7c, the HRTEM image of Co(OH)2 nanoparticles combined with

the selected area electron diffraction (SAED) present that the thin amorphous Co(OH)2

nanoparticles is homogeneously coating on the surface of Ta3N5 nanorods. Figure 7d depicts the

HRTEM image of the selected area, reveals a uniform distribution of Co3O4 nanoparticles on the

nanorods with very small article size, where the lattice fringe of 0.24 nm corresponded to the

refections from the (311) plane of Co3O4 (JCPDS 42-1467). As shown in Figure 7b, it is interesting

that the HRTEM image of the Ta3N5 nanorod modified with Co3O4/Co(II) cocatalyst shows that the

crystalline Co3O4 nanoparticles were uniformly depositing on the surface of the amorphous

Co(OH)2 layer and the Co3O4/Co(II) thin film including Co(OH)2 and Co3O4 nanoparticles was

decorated the surface of Ta3N5 nanorod. This facile deposition route provides a simple and flexible

method for driving selective self-assembly of the targeted interface, avoiding formation of islands

and nodules entirely, and that such uniform deposition would translate to improved

Co3O4/Co(II)/Ta3N5 PEC performance. In order to further identify the chemical states of Co, Ta and

N elements, the elemental composition of Co3O4/Co(II)/Ta3N5 nanorod was analyzed by X-ray

photoelectron spectroscopy (XPS, Figure S7, ESI†), exhibiting that a large amount of Co species

(2.3wt.%) exists on the nanorod surface. The compact interaction of Ta3N5 matrix and Co3O4/Co(II)

cocatalyst may ensure efficient transfer of photogenerated holes across the interfaces.

The interfacial properties between the electrodes (i.e., Ta3N5, Co(II)/Ta3N5, Co3O4/Ta3N5 and

Co3O4/Co(II)/Ta3N5 nanorods arrays) and the electrolyte were scrutinized by electrochemical

impedance spectroscopy (EIS) measurement that was carried out covering the frequency range of

104-0.1 Hz using an amplitude of 10 mV at the open circuit potential of the system. A semicircle

(i.e., the arch in the present study) in the Nyquist plot at high frequency represents the


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charge-transfer process, while the diameter of the semicircle reflects the charge-transfer resistance

(Figure 8). Clearly, the arches for Co3O4/Co(II)/Ta3N5 nanorods arrays under 100 mW·cm-2

illumination were much smaller than those for Ta3N5, Co(II)/Ta3N5 and Co3O4/Ta3N5 arrays

Figure 8. Nyquist plot measured at an applied potential of 1.23 V vs. RHE AM 1.5G simulated

sunlight at 100 mW/cm2 for bare Ta3N5 nanorods (black curve), Co(II)/Ta3N5 nanorod array (red

curve), Co3O4/Ta3N5 nanorod array (blue curve), and Co3O4/Co(II)/Ta3N5 nanorod array (grey curve)

and (b) Equivalent circuit used for water oxidation at Ta3N5 nanorod array photoelectrodes.

(b)


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implying that decoration with Co3O4/Co(II) bilayer catalysts significantly enhanced the

electronmobility by reducing the recombination of electron−hole pairs. As we know, however, that

the decrease in resistance of Co-Pi-coated hematite films is largely a result of the greater surface

area available for charge transfer to the electrolyte, due to the porous nature of the Co-Pi.[32] In this

work, the thin Co3O4/Co(II) bilayer catalysts is not expected to alter the surface area of the

photoanode, thus we can attribute the lower resistance to faster specific charge transfer kinetics (per

area). After the decoration of homogeneous Co3O4/Co(II) catalysts, the decrease in resistance, is

also evidence against a surface-state passivation mechansim since the resistance would be expected

to increase with the passivation of reactive surface states.[25,32]

Conlusions

In summary, we have developed a promising and efficient strategy of crafting uniform Co3O4/Co(II)

bilayer catalysts with very narrow particle size distribution loaded on highly alligned Ta3N5 nanorod

arrays by capitalizing on one-pot vapour-phase hydrothermal induced self-assembly technique. The

optimum Ta3N5 nanorod photoelectrode with the photocurrent density of 1.22 mA/cm2 under AM

1.5 G irradiation was achieved by the in-situ generation and the controllable morphlogy of arrays

elaborately adjusted by reaction time, using the F-Ta2O5 nanorod arrays with a highly conductive

interlayer between the nanorods and the substrate. Furthermore, the higher and more stable

photocurrent was demonstrated by combining the highly active Ta3N5 nanorods with a stable

Co3O4/Co(II) bilayer catalysts compared with Co(II)/Ta3N5 and Co3O4/Ta3N5 photoelectrode,

exhibiting that not only the onset potential is negatively shifted but also the photocurrent and the

stability are significantly improved. Specifically, about 92% of the initial stable photocurrent

remains after long-term irradiation at 1.23 V vs. RHE. At an applied potential of 1.23 V vs. RHE,


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the photocurrent density of Co3O4/Co(II)/Ta3N5 arrays reached 3.64 mA/cm2 under AM 1.5G

simulated sunlight at 100 mW/cm2, and a maximum IPCE of 39.5% was achieved at 440 nm. An

investigation into the role of thin Co3O4/Co(II) bilayer on Ta3N5 photoanodes was also conducted

with EIS. The combination of a reduction in the steady-state contrentration of oxidized surface

states, along with the reduction of charge transfer resistance from those surface states, demonstrates

that the charge transfer kinetics from the surface of the photoanode to the electrolyte were

accelerated. This combination of catalytic activity, stability, and conformal decoration makes this a

promising approach to improving the photoelectrochemical performance of photoanodes in the

general field of energy conversion.

Acknowledgements

This work was supported by National Science Foundation of China (No. 51102015, 21071014

and 51004008), National Basic Research Program of China (973 Program, No. 2013CB632404),

the Fundamental Research Funds for the Central Universities (No. FRF-AS-11-002A and

FRF-TP-12-023A), Research Fund for the Doctoral Program of Higher Education of China (No.

20110006120027), National High Technology Research and Development Program of China (863

Program, No. 2012AA062302), the Program for New Century Excellent Talents in University

(NCET-11-0577) and the 111 Project (B13004).

Supporting Information Available: XRD pattern, SEM and TEM image, EDX spectra, UV-visible

absorption spectra, and X-ray photoelectron spectroscopy.


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GRAPHIC FOR MANUSCRIPT


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Broader context

Photoelectrochemical (PEC) water splitting based on semiconductor materials is a promising

approach to harvest and store solar energy and may potentially supply the world's energy demand

with a clean and sustainable chemical fuel. Tantalum nitride (Ta3N5) is an attractive

photoanodematerial due to its high theoretical solar-to-hydrogen efficiency (15.9% under AM 1.5G

illumination), a suitable valence band position for water oxidation, excellent stability, and low

material cost. However, it is a challenge to design stable structures that can be used as anodes in the

photoelectrochemical cells. In this work, we create the first Co3O4/Co(II)/Ta3N5 nanorod arrays

photoanode with a better photoelectrochemical performance to perform water oxidation and

generate photocurrent. This strategy as a promising avenue provides new insights into the design

and tailoring of tantalum nitride to enhance the photoelectrochemical performance of photoanodes

in the general field of energy conversion.


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Cobalt-bilayer catalyst decorated Ta3N5 nanorod arrays as integrated electrodes for...

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Transcript of Cobalt-bilayer catalyst decorated Ta3N5 nanorod arrays as integrated electrodes for...