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Spontaneous photoelectric field-enhancement effect

prompts the low cost hierarchical growth of highly

ordered heteronanostructures for solar water splitting

Yankuan Wei, Jinzhan Su (), Xiaokang Wan, Liejin Guo, and Lionel Vayssieres ()

Nano Res., Just Accepted Manuscript • DOI: 10.1007/s12274-016-1050-9

http://www.thenanoresearch.com on Feb. 18, 2016

© Tsinghua University Press 2015

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Nano Research

DOI 10.1007/s12274-016-1050-9

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Graphical Table of contents:

A molecular co-catalyst is selectively photodeposited at the apex of a visible light-active semiconductor

nanopyramid by utilizing the spontaneous photoelectric field-enhancement effect to obtain enhanced

photocurrent under solar illumination in neutral aqueous solutions as a result of anisotropic photogenerated

charge carrier separation.

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Spontaneous photoelectric field-enhancement effect prompts the low cost hierarchical growth

of highly ordered heteronanostructures for solar water splitting

Yankuan Wei, Jinzhan Su*, Xiaokang Wan, Liejin Guo, Lionel Vayssieres*

International Research Center for Renewable Energy, State Key Laboratory for Multiphase Flow in

Power Engineering, School of Energy & Power Engineering, Xi’an Jiaotong University, Xi’an, P. R.

China.

* email: j.su@mail.xjtu.edu.cn (JZS); lionelv@xjtu.edu.cn (LV)

Abstract：A potentially universal new strategy for the large scale and low cost fabrication of visible

light-active highly ordered heteronanostructures based on the spontaneous photoelectric

field-enhancement effect inherent in pyramidal morphology is being demonstrated. These

hierarchical vertically oriented arrayed structures exhibit an active molecular co-catalyst at the apex

of a visible light-active large bandgap semiconductor for low cost solar water splitting in neutral

aqueous medium without sacrificial agent.

Keywords：Bismuth vanadate, co-catalysts, photocatalysis, solar water splitting, oxygen evolution

The crucial necessity of implementing renewable and sustainable sources of clean energy

in our societies has stimulated a plethora of fundamental and applied research studies on

artificial photosynthesis within the last decade [1]. A growing number of engineered

nanoarchitectures are being developed and tested for solar water splitting, which is one of

the very promising large scale strategy for a clean and sustainable hydrogen production.

For instance, conical/pyramidal-shaped nanostructured-arrays have gathered increasing

attention as very efficient architectures for enhanced visible-light management. Indeed,

nanopyramids have been utilized to enhance light absorption and improve solar cell

efficiencies due to substantial anti-reflecting and absorption enhancements [2]. It has also

been demonstrated that strong electrical fields can be induced on nanopyramids resulting in

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enhanced light absorption at the very tip [3] as well as the creation of enhanced local field

emission in nanopyramids with sharp tips and high tip density [4]. With a similar structure,

conical gold nano-arrays exhibit enhanced light absorption of less than 1% reflectivity over

a wide spectral range (450-900 nm) and wide incident angle range (0-70º) as a result of a

combination of diffractive scattering and localized plasmonic absorption [5]. Simulation

results [6] proved that both a nearly complete optical absorption and a superior carrier

collection efficiency can be achieved in ultrathin nanopyramidal iron oxide photoanodes

for water splitting. Furthermore, nanopyramidal-arrays do exhibit higher photocurrents

compared to compact thin films for water splitting applications as shown in our previous

report [7].

Among the typical promising low-cost oxide candidates (i.e. TiO2, α-Fe2O3, ZnO, WO3)

[8-11], BiVO4 has emerged as an efficient water-oxidation photocatalyst and has received

much attention due to its attractive bandgap and hole transport properties [12].

Quantum-confined BiVO4 without any co-catalyst is capable of an overall water splitting

[13]. However, its photocurrent stability is relatively poor with only 25% of the initial

photocurrent remaining after half an hour of illumination for thin films synthesized by

metal-organic decompositions [14]. This photocurrent decrease could originate from the

V5+ dissolution (into the solution) and oxidation products at the surface of the

photoelectrode, such as H2O2 or O2, acting as recombination centers. To overcome such

shortcomings and improve its activity, several approaches including surface modification

with metal ions [15], composites and heterojunctions [12, 16], and doping [17] have been

tested. For instance, composite structures such as reduced graphene oxide (rGO)/BiVO4

have shown enhanced photoactivity due to improved charge separation [18]. Also, when

large BiVO4 nanoparticles (100-500 nm) are paired with g-C3N4, water can be split into H2

and O2 in its ideal ratio of 2:1 rather efficiently [19]. A monoclinic-tetragonal

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heterostructured system was also constructed using Yttrium doping to stabilize the

tetragonal phase which also showed enhanced photocatalytic activities [20]. Furthermore,

photoanodes with two different oxygen evolution catalyst layers (amorphous

oxyhydroxides of Fe and Ni), have been fabricated yielding high photocurrent densities

[21]. For an improvement of the stability, ultrathin dual layers of TiO2 (~1 nm) and Ni

metal (7-9 nm) have been deposited onto polycrystalline thin films by sputtering [22].

Our general strategy to develop novel hierarchical architectures as photoelectrodes

combining low cost semiconductors and molecular co-catalysts is to utilize the spontaneous

photoelectric field enhancement inherent in the anisotropy of a pyramid-like morphology.

To demonstrate such an ability to create highly homogeneous and ordered visible

light-active heteronanostructures at large scale and low cost, bismuth vanadate

nanopyramid-arrays where their tips are capped by photodeposited nanoparticles of cobalt

phosphate (CoPi) at room temperature was selected as an example. Indeed, CoPi is a

well-known, simple, yet effective earth-abundant water-oxidation electrocatalyst [23]. As a

co-catalyst reducing the overpotential, it can self-assemble onto the surface of various

materials as an amorphous film, self-repair, and oxidize water in a variety of buffered

solutions [24] showing improved performance for water oxidation [25-28].

With this unique architecture, enhancing spatially the photogenerated charge carriers separation,

efficient photoelectrochemical water oxidation activity and stability could be achieved without

sacrificial agents. Typically, upon photochemical deposition of CoPi on semiconductors,

photogenerated holes diffuse to the surface and oxidize Co(II) to Co(III), resulting in the

precipitation of Co(III)-based oxygen evolution catalysts (OEC) at the semiconductor surface.

Without external bias, the photodeposition of OEC takes place spontaneously at the locations

where the holes are accumulating. According to simulation results, strong electrical fields are

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spontaneously generated at the very tip/apex of nanopyramidal/nanocone structures [3].

Consequently, a localized enhanced charge separation could be achieved and thus, specific

site-selective depositions of co-catalysts could be reached at the very tip of nanopyramids as

illustrated in Scheme 1a. Experimentally, such novel homogenous heteronanostructures consisting

of CoPi apex-capped BiVO4 nanopyramidal-arrays were synthesized in a simple two-step process

(Scheme 1b). First, the oriented growth of the BiVO4 arrays with sharp tips onto FTO substrates

(or other substrates) is achieved using a simple hydrothermal procedure reported by our group [7].

Typically, a spin-coated seed layer is deposited to assist the growth of the BiVO4

nanopyramid-arrays using the following precursor solution: 0.005 mole Bi(NO3)3•5H2O and 0.005

mole NH4VO3 are dissolved into 15 ml of 23.3 wt% HNO3 aqueous solution, followed by an

addition of 7.5 ml (5 g/100 ml) polyvinyl alcohol and 0.005 mole of citric acid. The as-prepared

solution was then spin-coated onto FTO substrates at 4000 rpm for 30 s, and then annealed in air at

450°C for 4 h. Subsequently, 0.002 mole of Bi(NO3)3•5H2O and 0.002 mole of NH4VO3 were

dissolved in 50 ml of 14% HNO3 aqueous solution to form a BiVO4 suspension. The pH of this

suspension was adjusted to 6.5 by adding NaHCO3. Finally, BiVO4-coated FTO substrates were

placed in the suspension and refluxed at 60°C under magnetic stirring for 6 h at 120 rpm. After

reflux, the substrates were removed, rinsed with deionized water, and dried under a mild stream of

nitrogen gas at room temperature. To increase crystallinity, the samples were subsequently

annealed at 400°C for 30 min in air. Secondly, CoPi co-catalyst spherical nanoparticles were

photo-deposited for 3 h on each tips of the nanopyramid-arrays at room temperature by immersing

the samples in a Petri dish containing an aqueous solution consisting of 0.4 mM CoCl2•6H2O and

0.1 M potassium phosphate solution (pH~7), and irradiated for 3 h under solar simulator operating

at 100 mW/cm2 illumination using a 500 W Xe lamp AM 1.5G filter. After deposition, samples

were rinsed thoroughly with deionized water, and dried under a mild nitrogen gas stream.

Figures 1a,b show scanning electron microscope (SEM) images of the BiVO4

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nanopyramid-arrays before and after CoPi photodeposition illustrating, as expected, that

homogeneous large arrays of CoPi apex-capped BiVO4 nanopyramidal arrays were successfully

fabricated. Indeed, one can clearly identify spherical ~100 nm nanoparticles exclusively grown

onto each tips of the nanopyramids. Figures 1c,d show transmission electron microscope (TEM)

images of their structure and interface. The nanopyramids are polycrystalline (d-spacing of 0.31

nm, which corresponds to the {112} planes of monoclinic scheelite BiVO4 (JCPDS: 01-075-1867)

while CoPi is amorphous. Figure 1e demonstrates the very high selectivity, effectiveness and

homogeneity of this approach as cobalt can indeed exclusively be found at the apex of the BiVO4

nanopyramids.

The obtained monoclinic structure is the preferable structure of BiVO4 as it shows much higher

activity for the photocatalytic water oxidation than tetragonal structure [29]. BiVO4 does exist as

the orthorhombic pucherite structure in nature [30] however, BiVO4 prepared in the laboratory are

only found in either scheelite or a zircon-type structures [31]. The scheelite structure can exhibit a

tetragonal or a monoclinic crystal system while the zircon-type structure only has a tetragonal

crystal system [31]. The monoclinic scheelite structure originates from the distortion of V-O and

Bi-O bonds, which removes the four-fold symmetry in the tetragonal system. As shown in Figure

2a, the monoclinic scheelite structure (space group 15, I2/b) is a layered structure with Bi-V-O

units stacked along the c axis which contains four unique lattice sites: Bi, V, O1, and O2. Each V

ion is coordinated by four O atoms in a tetrahedral site and each Bi ion is coordinated by eight O

atoms from eight different VO4 tetrahedral units. O1 is coordinated to one Bi and V, while O2 is

coordinated to two Bi and a single V. There are two different V–O bond lengths (1.77 Å and 1.69

Å) and four different Bi–O bond lengths (ca. 2.354 Å, 2.372 Å, 2.516 Å and 2.628 Å) existing in a

monoclinic scheelite structure [32]. This layered structure along c-axis contributes to the

successful growth of the BiVO4 nanopyramids. The Bi-polyhedra in the monoclinic scheelite

structure is much more distorted than that in the tetragonal scheelite structure and this distortion

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pronounces the local polarization which favorably affects the electron–hole separation.

All XRD peaks do correspond to the monoclinic BiVO4 (Figure 2b). The apex CoPi capping of

BiVO4 nanopyramid-arrays does not induce any noticeable changes in the XRD peak positions and

no specific peaks could be observed corresponding to any Co-based structures. By comparing the

relative intensities of the (112) and (004) reflections to that of the reference JCPDS compound, one

notices the peak intensity ratio of (004)/(112) is 1.25 times higher than the reference one, which

corresponds to a (004) preferential elongation of the BiVO4 arrays. Figures 2c,d show the x-ray

photoelectron spectroscopy (XPS) spectra, using Kα Al anode and calibrated using the C1s peak,

of the CoPi/BiVO4 samples. The characteristic peaks of Co 2p3/2 and Co 2p1/2 at 780.98 and 796.63

eV originating from Co(II) and Co(III) respectively confirm a successful deposition of CoPi.

However, compared to the reported electrodeposited CoPi XPS peaks [23], our sample shows Co

2p1/2 is ~0.9 eV shifted to higher binding energy, revealing an increase of the Co(II) (high

spin)/Co(III) ratio in our samples [33]. The Bi 4f7/2 and 4f5/2 peaks located at 164.3 and 159.0 eV

respectively confirm +III as the main oxidation state of bismuth ions in BiVO4 [34] and their

characteristic spin-orbit splitting is typical of Co(II) in Co-Pi [33] and Bi(III) in BiVO4 [35],

respectively.

The bandgap of the CoPi apex-capped and bare BiVO4 arrays are both found to be of 2.48~2.51

eV (Figure 3a). This suggests that the presence of CoPi does not change the bandgap but rather

enhances the light absorption in the 350-460 nm spectral region. The conduction band of

monoclinic BiVO4 is mainly composed of V3d, while the valence band is mainly composed of O2p

[36]. The hybridization of O2p with Bi6p shifts the top edge of the valence band upwards, resulting

in a slightly reduced bandgap compared to tetragonal BiVO4. A relatively large hole diffusion

length in monoclinic BiVO4 (100−200 nm) has been reported [36] and DFT calculations indicated

that holes are indeed weakly localized while photogenerated electrons are self-trapped due to a

structural localization [37]. At the apex of the nanopyramids, spontaneously generated electrical

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fields delocalized the trapped photogenerated electrons and enhanced charge separation, resulting

in specific site-selective depositions of co-catalysts at the very tip of nanopyramids.

Interestingly, it has been reported that in a typical CoPi photodeposition procedure, the

Co-based OECs were inherently deposited at the locations where the holes are most readily

available for solar O2 evolution [38]. One can therefore assume that the tips are indeed the

preferred activation sites for O2 evolution. With such a precise site-selected deposition, this

structure ensures an optimized use of the co-catalyst as well as an enhanced O2 evolution. To

demonstrate this assumption, fully covered CoPi BiVO4 nanopyramids were synthesized by

electrodeposition (CoPi/BiVO4-ED) and by photo-assisted electrodeposition (CoPi/BiVO4-P-ED),

see figure S1. Additionally, BiVO4 samples grown by Pulsed Laser Deposition (BiVO4-PLD) were

also used as benchmark. To compare their performance, photocurrent densities were measured

from back-side (substrate-to-electrode, SE) under 1 Sun (100 mW/cm2) illumination using a 500

W Xe lamp with AM 1.5G filter in neutral Na2SO4 aqueous solutions (0.5 M, pH 6.8) at room

temperature. As seen in Figure 3b, bare BiVO4 nanopyramids shows higher photocurrent than the

PLD BiVO4 sample. Moreover, the photocurrent of CoPi apex-capped BiVO4 increased by another

0.6 mA/cm2 compared to that of bare BiVO4, reaching 1.1 mA/cm2 at 1.23 V vs. RHE and a

maximum of 1.5 mA/cm2 at 1.8 V. These photocurrents are significantly higher than that of fully

Co-Pi covered BiVO4 nanopyramids (CoPi/BiVO4-ED, CoPi/BiVO4-P-ED) confirming the

optimized use of the co-catalyst (see figures S2-4 and table S1 for quantitative analysis and

additional data) with this simple photoelectric field-enhancement effect approach. Limited

majority carrier transports remain the major drawback of bismuth vanadate photocatalysts [39], i.e.

it is the electron (rather than the hole) mobility that limit the photocarrier transport and extraction.

Such results confirm this new strategy as a cost effective smart design approach for improved

properties. Indeed, preliminary results show that both, this novel purpose-built heteronanostructure

and its performance in neutral aqueous solutions are stable (Figures 3c,d).

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Conclusions

A new materials fabrication strategy has been demonstrated utilizing the spontaneous

photo-induced electric field enhancement effect inherent in pyramidal-like morphologies. Novel

visible light-active heteronanostructures consisting of oriented nanopyramid-arrays (of a large

bandgap semiconductor) with (molecular co-catalyst) nanoparticles grown exclusively on the apex

were successfully fabricated. These new hierarchical highly ordered and homogeneous structures

combining stable and earth abundant elements and simple fabrication techniques are setting a new

standard in smart (photocatalysts) design. This novel strategy was demonstrated here for the large

scale and low cost development and operation for heterojunction-based photoelectrodes which

offers great expectations for environmental remediation and water purifications as well as solar

water splitting in neutral solutions. Furthermore, given the rather universal capability of this

approach, innovative multifunctional heteronanostructures could easily be generated for various

classes of inorganic materials combining redox molecular chemistry, photochemistry and

hydrothermal synthesis as well as for the design of model systems for better fundamental

understanding of structure-property relationships.

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Scheme 1. (a) Theoretical mechanism of the highly selective growth of a nanoparticle at the apex

of a nanopyramid prompted by the spontaneous photoelectric field charge carrier separation

enhancement effect. (b) Experimental fabrication procedure of CoPi apex-capped BiVO4

nanopyramidal arrays.

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Figure 1. Electron microscopy images of bare (a) and CoPi apex-capped (b-d) nanopyramidal

BiVO4 with selected area diffraction (inset c), FFT (inset d) patterns and elemental energy

dispersive mapping (e).

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Figure 2. Crystal structure representation of monoclinic scheelite BiVO4 (Green: Bi, blue: V and

red: O). The two different type of oxygen atoms are marked as O1 and O2. (a)Thin film X-ray

diffraction (XRD) patterns of BiVO4 and Co-Pi apex-capped nanopyramidal BiVO4 samples (b).

XPS spectra of Co-Pi apex-capped nanopyramidal BiVO4 at the Co 2p (c) and Bi 4f (d) edges.

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Figure 3. (a) UV−vis diffuse reflectance spectra, (b) I-V photoelectrochemical responses of:

CoPi-apex nanopyramidal BiVO4 (1); CoPi nanopyramidal BiVO4 by P-ED-photo-assisted

electrodeposition (2); CoPi nanopyramidal BiVO4 by ED-electrodeposition (3); bare

nanopyramidal BiVO4 (4); and bare BiVO4 by PLD-pulsed laser deposition (5). (c) stability test

and (d) SEM image after the 1h- continuous illumination stability test using a 500 W Xe-lamp

solar simulator with an AM 1.5G filter (Oriel) light source (100mW cm-2) in a neutral Na2SO4

aqueous solution (0.5 M, pH 6.8) of bare (black) and CoPi-apex nanopyramidal BiVO4 (red). The

stability test was performed at 0.6 V vs. Ag/AgCl.

Acknowledgements

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This work was supported by the National Natural Science Foundation of China (No.5120218).

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