PDF Enhancing photoelectrochemical performance of the Bi2MoO6 ... - NJU

(1)

PAPER

Cite this:Nanoscale, 2020,12, 18446

Received 9th April 2020, Accepted 25th July 2020 DOI: 10.1039/d0nr02809f rsc.li/nanoscale

Enhancing photoelectrochemical performance of the Bi

₂

MoO

₆

photoanode by ferroelectric polarization regulation †

Zhongshuai Xie,âZongyang Cui,âJiafeng Shi,âCheng Lin, âKan Zhang, â Guoliang Yuan *âand Jun-Ming Liu ^b

Photoelectrochemical water splitting provides a promising strategy for converting solar energy into chemical fuels and has attracted extensive interest. Herein, Bi2MoO6nanopillars with large surface areas were fabricated on an ITO-coated glass substrate and their photoelectrochemical properties are enhanced through appropriate manipulation of ferroelectric polarization. The Bi2MoO6photoanode with polarization orientation toward ITO shows an enhanced photocurrent density of 250μA cm⁻²at 1.23 Vvs.

reversible hydrogen electrode, which is 28% higher than that of pristine Bi2MoO6 nanopillars without macroscopic polarization. The corresponding depolarization electricfield benefits the separation of light- excited electron–hole pairs, thus minimizing the recombination of charge carriers and further enhancing the photocurrent current density. Our work offers a new strategy of Bi2MoO6-based photoelectrochemical devices with great potential of application in the conversion of solar energy into chemical fuels.

1. Introduction

Over the past few years, photocatalytic water splitting has attracted much attention owing to its potential to produce clean and sustainable energy.¹ However, the conversion efficiency of photocatalysts has often been limited by poor light absorption and low charge separation as well as unsatis- factory light stability.²To overcome these challenges, strategies such as band gap engineering, micro/nanoengineering, co- catalyst engineering, surface engineering and interface engineering have been explored to ameliorate the drawbacks of the currently developed photocatalysts and photoelectrochemical cells.^2,3 Compared with pure photocatalysts, photoelectrochemical cells have much higher energy conversion efficiency because the external electric field can effectively separate light- excited electron–hole pairs, manipulate redox reactions and ease the separation of gaseous production.⁴ As the most important components of photoelectrochemical cells, highly efficient and stable semiconductor photoelectrodes have been extensively studied. Nevertheless, due to the limitations of chemical synthesis technology and material feasibility, these

photoelectrodes have still limitations to a certain extent in terms of energy conversion eﬃciency which needs to be improved by these methods.

In some photocatalysts, the space charge region and built- in electrical field (Ein) near the interface are the major driving forces for the separation of photogenerated electron–hole pairs.^5–7Generally, a complicated strategy is employed to form the heterojunction; however, the space charge region usually exists near the interface and only takes up a small part of the entire photocatalyst. In contrast to theEinnear the interface, the depolarization electric field of ferroelectric polarization (EFE) extends across the entire volume of the film or bulk material, eﬀec- tively separating the electron–hole pairs and driving them toward their respective electrodes.^8,9Being similar to the external electric field of photoelectrochemical cells, this EFE can also adjust the chemical potential and surface band bending of photoelectrodes based on ferroelectric materials, thereby allowing tailoring the photoelectrochemical performance.^10–12 Due to these fascinating features, ferroelectric metal oxides have been intensively studied either as the sole harvesting materials, such as BiFeO3,^11,13,14 Bi2FeCrO6,^15,16KNbO3,¹⁰BaTiO3,¹⁷or in hybrid systems, as exem- plified by the Au/BiFeO3 heterostructure,¹⁸ BiFeO3/BiVO4 photoanode¹⁹ and TiO2/BaTiO3 photoanode,²⁰ which have greatly enhanced photocatalytic or photoelectrochemical activities through appropriate manipulation of ferroelectric polarization.

Bi2MoO6is the simplest (n= 1) member of the large family of layered perovskite-related compounds with the general formula (Bi2O2)(A_n−1BnO3n+1).²¹At room temperature, Bi2MoO6

†Electronic supplementary information (ESI) available. See DOI: 10.1039/

d0nr02809f

aSchool of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing 210094, P. R. China. E-mail: [email protected]

bNational Laboratory of Solid State Microstructures, Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, P. R. China

Published on 04 August 2020. Downloaded by NANJING UNIVERSITY on 10/10/2020 10:18:30 AM.

View Article Online

View Journal | View Issue

(2)

has the P21ab Aurivillius structure which allows the macroscopic ferroelectric polarization in theory.^22,23 In addition, Bi2MoO6is one of the most studied materials in photocatalytic and photoelectrochemical fields, exhibiting remarkable photocatalytic and photochemical properties in dye degradation and water splitting, respectively.^24,25 However, the rapid combination of electron–hole pairs generated by light irradiation is still one of the major limitations of Bi2MoO6for energy conversion eﬃciency.^24,26 Various modification strategies such as morphology control,²⁷ doping^28,29 and semiconductor combination^26,30,31have been developed to improve the photocatalytic or photoelectrochemical activities of Bi2MoO6. However, to our knowledge, there have been no reports on the macroscopic polarization of Bi2MoO6observed in experiments and the eﬀect of ferroelectric regulation on photoelectrochemical properties until now.

In this work, Bi2MoO6nanopillars with large surface areas were prepared on an ITO-coated glass substrate. The ferroelectric polarization orientation toward ITO greatly enhances the photoelectrochemical properties of the Bi2MoO6 photoanode.

EFEleads to the eﬃcient separation of photogenerated carriers and a 28% enhanced photocurrent density,i.e.250μA cm⁻²at 1.23 Vvs.reversible hydrogen electrode (RHE).

2. Experimental

2.1. Material preparation

The Bi2MoO6nanopillars were grown on the ITO-coated glass substrate via a pulsed laser deposition technique using a Bi2MoO6target. A Bi2MoO6target with 5% Bi excessively com- pensating for the evaporation of the Bi element was sintered at 670 °C by conventional solid state reactions. Prior to the deposition of Bi2MoO6nanopillars, the ITO-coated glass substrates were washed in acetone, ethanol and deionized water each for 30 min to remove impurities. The deposition chamber was initially evacuated to a base pressure of 10⁻³Pa. The distance between the target and the substrate was set to 6.5 cm. The deposition process was carried out at a substrate temperature of 480 °C and an oxygen pressure of 15 Pa, with a KrF excimer laser operated at a laser repetition rate of 2 Hz and a laser energy of 45 mJ. After the deposition, the samples were post- annealed at 480 °C in situ for 30 min in 1000 Pa of O2

atmosphere.

2.2. Characterization

Cross-sectional images of the samples were observed with a field emission scanning electron microscope (SEM, FEI Quanta 250F). The XRD patterns were characterized with a Bruker D8 diﬀractometer equipped with a Cu-Kα radiation source. The crystal structure was also characterized by trans- mission electron microscopy (TEM, FEI Tecnai G2 F20 S-Twin).

X-ray photoelectron spectroscopy (XPS) spectra were recorded with an X-ray photoelectron spectrometer (Thermo Scientific K-Alpha+). The ultraviolet–visible (UV–Vis) spectrum was recorded using a UV–Vis spectrophotometer (Shimadzu Co.).

Polarization versus electric field (P–E) hysteresis loops were measured at 1000 Hz with a commercial ferroelectric tester (Radiant precision multiferroic II). The morphology and piezo- electric force microscopy (PFM) images were observed with a commercial atomic force microscope (AFM, Bruker Multimode 8) and the conductive tip (Bruker MESP-RC).

2.3. Photoelectrochemical measurements

All photoelectrochemical measurements were performed on an electrochemical workstation (CH Instruments Inc., CHI 660E) in a three-electrode configuration using a Bi₂MoO₆ sample as the working electrode, a Pt foil as the counter electrode, and saturated Ag/AgCl as the reference electrode. Na₂SO₄solution ( pH = 6.8) with 0.5 M concentration was used as the electrolyte. AM 1.5 G simulated solar light illumination (100 mW cm⁻²) was obtained from a 350 xenon lamp solar simulator coupled with an AM 1.5 G filter (NBet). All electrodes were illu- minated from the front side with an active area of 1 cm². Before the measurements, the light intensity was calibrated with a light power meter. The conversion between the poten- tialsvs.Ag/AgCl andvs.RHE was performed using the follow- ing equations:

Eðvs:RHEÞ ¼Eðvs:Ag=AgClÞ þE_Ag=AgClþ0:0591pH where E_Ag/AgCl is 0.1976 V at 25 °C. During a typical J–V measurement, linear sweep voltammetry (LSV) was conducted at a scan rate of 30 mV s⁻¹. H₂ gas was collected with an air- tight three-electrode photoelectrochemical cell in the 0.5 M Na₂SO₄ and 0.2 M Na₂SO₃ aqueous solution where Na₂SO₃is used as the hole scavenger, and then the H₂ amount was measured with a gas chromatograph (PerkinElmer Clarus 580 GC). Electrochemical impedance spectroscopy (EIS) was performed with an AC voltage amplitude of 5 mV and a frequency range of 0.1–100 kHz under AM 1.5 G illumination. The Mott– Schottky plots were recorded at 1 kHz in the dark.

3. Results and discussion

Fig. 1a shows the schematic diagram of the geometry and crystal structure of Bi2MoO6 nanopillars. Bi2MoO6 unit cells are composed of [Bi2O2]²⁺slices linked with a corner-sharing structure of MoO6 octahedra.^21,24 The layered configuration facilitates good electron conductivity and influences the visible light photo-absorption.²⁴ When a dense Bi2MoO6 target was sputtered by the pulsed laser with a focused spot diameter of

∼0.5 mm, the Bi2MoO6film with a flat surface was also grown on the ITO-coated glass substrate at 480 °C. When the focused spot diameter increases from∼0.5 mm to∼2 mm while other conditions are unchangeable, the generated ions have a weaker kinetic energy and a shorter diﬀusion length on the substrate which just allows the island growth mode at 480 °C.

A decrease in the kinetic energy of the ablated particles from the target plays a significant role in forming nanopillars. In addition, increasing the oxygen pressure and target–substrate distance can also create favorable conditions for the growth of

Nanoscale Paper

(3)

nanopillars.^32,33 The corresponding crystal structure is investigated by X-ray diﬀraction (Fig. 1b). All the diﬀraction peaks of the Bi2MoO6 nanopillars can be readily indexed to γ-Bi2MoO6(JCPDS No. 21-0101),^27,34indicating the high purity of the prepared nanopillars. Most importantly, [200] oriented crystal grains account for the majority according to the XRD patterns.

These Bi2MoO6nanopillars have a special columnar structure with a large specific surface which is beneficial for enhancing photoelectrochemical performance. Fig. 1c and d display the SEM images of the as-prepared Bi2MoO6nanopillars, pro- viding a direct view of the section and surface morphology.

The Bi2MoO6nanopillars with a height of 290 nm and a diameter of 100–200 nm are uniformly distributed on the ITO- coated glass substrate. Furthermore, the columnar structure of these Bi2MoO6 nanopillars is also observed in the cross-sectional TEM image (Fig. S1†). The root-mean-square roughness (Rq) of the Bi2MoO6nanopillars is 31.8 nm, while theRqof the ITO film and the flat Bi2MoO6 film is 3.9 nm and 9.5 nm according to the surface image characterized by AFM (Fig. S2†). The larger specific surface areas of this morphology can greatly increase the electrochemically active area. At the same time, the nanopillars with a columnar structure shorten the distance of charge transport and reduce the recombination of electron–hole pairs, which is beneficial to water splitting.

The energy-band gap (Eg) of Bi2MoO6was analyzed according to the UV–Vis diﬀuse reflectance spectrum (Fig. 2a) and was calculated according to the Tauc plot:³⁵

αhν¼AðhυEgÞⁿ⁼²

where α, hν, and A are the absorption coeﬃcient, photon energy, and a constant, respectively. Meanwhile, n is determined by the type of optical transition in the semiconductor (n = 1 for a direct transition,n = 4 for an indirect transition). For Bi2MoO6, then value is 1 and Eg is approxi- mately 2.78 eV which are close to previously reported values.^34,36

The chemical states of the elements in the Bi2MoO6nano- pillars are identified according to XPS spectra. The XPS survey spectrum (Fig. S3†) reveals the coexistence of Bi, Mo and O elements, and there are no other impurity peaks. The high- resolution XPS spectra of each element are shown in Fig. 2b–d.

Two strong peaks in the Bi 4f region at 159.0 eV and 164.3 eV are assigned to Bi 4f7/2and Bi 4f5/2, confirming the presence of Bi³⁺ species (Fig. 2b).²⁶ Besides, there are two characteristic peaks at 232.4 eV and 235.5 eV (Fig. 2c), corresponding to Mo 3d5/2 and Mo 3d3/2 states of Mo⁶⁺ in Bi2MoO6, respectively.

Furthermore, the two peaks at 529.9 eV and 531.9 eV for the O 1s spectra belong to lattice oxygen and oxygen vacancies in the Fig. 1 (a) Schematic diagram of the Bi2MoO6nanopillars on the ITO-coated glass substrate. (b) XRD patterns, (c) cross-sectional view and (d) the top-view SEM images of the Bi2MoO6nanopillars.

(4)

host lattice, respectively (Fig. 2d).^37,38The diﬀerences between the Fermi level (EF) and the valence band maximum (Ev) were determined by the intercepts of the extrapolated lines for the spectra near the Fermi edge,³⁹which is obtained as 1.88 eV (Fig. 2e). The band structure of Bi2MoO6is achieved in Fig. 2f which indicates the n type semiconductor being consistent with previous reports.^25,36

Both P–E hysteresis loops and PFM images confirm the ferroelectric polarization switching of Bi2MoO6 nanopillars.

As shown in Fig. 3a, a remanent polarization (Pr) of 1.2 μC cm⁻² is observed in the P–E loop of the (0l0) Bi2MoO6

single-crystal film on the Nb:SrTiO3 substrate, and an unsa- turated P–E loop is observed in Bi2MoO6 nanopillars due to a larger leakage current especially at grain boundaries. This study proves that Bi2MoO6 is a typical ferroelectric semiconductor. Furthermore, the morphology and ferroelectricity of a 3 × 3 μm² region are characterized using a AFM with PFM mode.⁴⁰ Bi2MoO6 nanopillars are the columnar crystal grains with average diameters of 150–200 nm according to the three-dimensional AFM morphology (Fig. 3b). The polarization toward ITO (downward polarization, dark color) was introduced by the tip with 10 V scanning upon a 2 × 2 μm² region (Fig. 3c), and then the polarization against ITO (upward polarization, bright color) was introduced again by the tip with−10 V bias scanning upon the central 1 × 1μm² region (Fig. 3d). The stable polarizations toward and against the ITO bottom electrode prove the ferroelectric polarization switching of Bi2MoO6 nanopillars again. The ferroelectricity

originates from the non-centrosymmetric crystal structure.

Bi2MoO6 belongs to the family of bismuth-layered compounds (i.e. Aurivillius family) which is composed of bismuth-oxygen (Bi2O2) and perovskite-like octahedral layers.²¹ Ferroelectric ordering in most multi-layer Aurivillius phases typically leads to a transition from a tetragonal (4/

mmm) to a polar orthorhombic structure.²³ The distorted MoO6 octahedra are related to the existence of spontaneous polarization in the crystal. Bi2MoO6 is an orthorhombic ferroelectric phase at room temperature and it transforms into an orthorhombic paraelectric phase at a Curie temperature of 604 °C according to previous studies.^23,41,42

The polarization toward ITO significantly enhances the photocurrent density to 250 μA cm⁻² at 1.23 V vs. RHE.

Photocurrent density versus potential (J–V) curves of the Bi2MoO6photoanode were recorded to study the role of ferroelectric polarization in photoelectrochemical water splitting.

As shown in Fig. 4a, the very lowJin the dark indicates a small amount of electrocatalytic water splitting even at 1.6 Vvs.RHE which far exceeds the water splitting potential. The as-grown sample exhibits aJof∼195μA cm⁻²at 1.23 Vvs.RHE with an onset potentialðVon^* Þof 0.63 V, whereV_on^* is the potential at the intersection point of potential-axis and the tangent at the maximum slope of photocurrent.^19,43 This J is 129% higher than theJof the flat Bi2MoO6film under the same characteristic conditions (Fig. S4†). To fully pole the Bi2MoO6 nanopillars for macroscopic polarization, a piece of dust-free cloth impregnated with ionic liquid (1-ethyl-3-methylimidazolium Fig. 2 (a) UV–Vis diﬀuse reﬂectance spectra of the Bi2MoO6nanopillars. XPS spectra of (b) Bi 4f, (c) Mo 3d, (d) O 1s and (e) valence band. (f ) Energy-band diagram.

Nanoscale Paper

(5)

bis(trifluoromethanesulfonyl)imide) was used to contact the surface of Bi2MoO6nanopillars, and then a flat piece of aluminum foil on the ionic liquid was used as the top electrode to provide a uniform electric field (Fig. S5†).⁴⁴ The 5 V pulsed voltage was applied on aluminum foil for 5 min to fully pole Bi2MoO6nanopillars. Polarization switching is realized by 5 V according to the corresponding PFM image (Fig. S6†). After that, the sample was ultrasonically cleaned in ethanol and deionized water to remove the ionic liquid for further photoelectrochemical measurements. There is an enhancedJof∼250μA cm⁻²for the Bi2MoO6nanopillar photoanode with the macroscopic polarization toward ITO ( positive poling), which is 28%

enhancement compared with the as-grown Bi2MoO6 nanopillars without macroscopic polarization. Such J is much higher than those of the photocurrent density of Bi2MoO6

photoanodes reported in previous literature (Table S1†).

Besides, a negative shift in the V_on^* 65 mV suggests that polarization switching changes band alignment and tunes the charge separation as well as the charge transfer eﬃciency. The produced H2gas was analyzed using gas chromatography. The as-grown Bi2MoO6 photoanode and that with polarization toward ITO produced the H2gas with a rate of 4.08μmol cm⁻² and 5.61 μmol cm⁻² per hour which further confirmed the improved photoelectrochemical performance of the photo-

anode by appropriate manipulation of the ferroelectric polarization (Fig. S7†). However, the Bi2MoO6 photoanode with the polarization against ITO (negative poling) exhibits aJof 182μA cm⁻² which is slightly lower than that of the origin Bi2MoO6

nanopillars. In addition, the minor standard deviation of photocurrent shows good stability and repeatability for diﬀerent Bi2MoO6 nanopillar photoanodes which were prepared, subjected to polarization poling and characterized under the same conditions (Fig. S8†). Fig. 4b shows the J–V curves under chopped light illumination. The rapid photoresponse was observed during the light on/oﬀ state. The obvious spikes at low potential (<1 V) are ascribed to the rapid recombination of photogenerated carriers under light illumination. The chronoamperometry (J–t) measurements under chopped light illumination at 1.23 Vvs.RHE were performed to further study the instantaneous photoresponse (Fig. 4c), whose results are consistent with those of the J–V measurements. In a word, the J–V measurements show that a 28%

increase of photocurrent has been realized by tuning the ferroelectric polarization.

To gain more insight into the suppression of charge recombination, the transient photocurrent curve was recorded by chronoamperometry to investigate charge recombination (inset in Fig. 4d). A normalized parameter (i.e., D) is intro- Fig. 3 (a)P–Ehysteresis loops of Bi₂MoO₆nanopillars and Bi₂MoO₆ﬁlm on the Nb:SrTiO₃substrate. (b) AFM 3D morphology of Bi₂MoO₆nanopillars. (c and d) Out of plane PFM images of the domain structure in the Bi₂MoO₆nanopillars at diﬀerent bias voltages.

(6)

duced from the J–time (J–t) curve to determine the charge recombination,^16,45

D¼ ItIst

IinIst

where It, Ist and Iin are the time-dependent, the steady-state photocurrent and the initial photocurrent, respectively. The normalized plots of ln(D) as a function of time are displayed in Fig. 4d. The transient time constant (i.e., Time) is defined as the time at ln(D) =−1. Time is estimated as 13 s, 32 s and 9 s for the as-grown sample and that with polarization toward ITO and against ITO, which confirms the suppression of charge recombination when polarization is toward ITO. Furthermore, the charge transfer eﬃciency (ηtrans) of origin Bi2MoO6 nanopillars is just 38% at 1 V (vs.RHE), and that of the Bi2MoO6

with polarization toward ITO significantly increases to 56%

(Fig. S9†). The increase of Time andηtransproves that theEFE

of the macroscopic polarization toward ITO remarkably enhances the separation eﬃciency of light-excited electron– hole pairs. However, all the samples still suﬀer photocorrosion to some extent under long-term illumination due to the surface-accumulated holes, and the photoelectrochemical photocurrent decreases about 34–38% at 1.23 Vvs.RHE within

8000 s (Fig. S10†). Nevertheless, the photocurrent of the sample with polarization toward ITO at 1.23 Vvs.RHE is still 30% higher than that of the pristine sample, indicating the good stability of ferroelectric polarization during this photoelectrochemical process.

The dependence of the band structure on polarization can elucidate the enhanced charge separation of Bi2MoO6 nanopillars discussed above (Fig. 5). At first, when Bi2MoO6 is immersed in the electrolyte, the light-excited electrons transfer from the Bi2MoO6 to the electrolyte due to the difference between the Bi2MoO6 work function and electrochemical potential of the electrolyte, leading to upward band bending and a significant space charge region width on the surface of Bi2MoO6, as shown in Fig. 5a. After Bi2MoO6 nanopillars are positively poled, the macroscopic ferroelectric polarization toward ITO induces negative ( positive) bound charges at the Bi2MoO6/electrolyte (Bi2MoO6/ITO) interface, as shown in Fig. 5b. As a result, the highEFEincreases the magnitude of upward (downward) band bending toward the Bi2MoO6/electrolyte (Bi2MoO6/ITO) interface, producing a wider space charge region at the Bi2MoO6-electrolyte and Bi2MoO6-ITO interface side. Due to the favorable band bending, the photogenerated electron–hole pairs in the bulk Bi2MoO6will be sep- Fig. 4 (a)J–Vcurves of the as grown Bi2MoO6photoanode and those with polarization toward and against ITO. (b)J–Vcurves under chopped light illumination. (c)J–tcurves of photoanodes at 1.23 Vvs.RHE. (d) Normalized plots of theJ–tdependence for the as-grown (orange color) Bi2MoO6photoanode and those with polarization toward ITO (red color) and against ITO (blue color), where the inset shows the correspondingJ–t curves of the photoanode at 1.23 Vvs.RHE and on/offmeans light switch on/off.

Nanoscale Paper

(7)

arated eﬀectively by theEFE, leading to an enhanced photoelectrochemical performance. However, the situation is reversed when the polarization against ITO is induced in Bi2MoO6

nanopillars (Fig. 5c). The downward band bending at the Bi2MoO6/electrolyte interface and upward band bending at the Bi2MoO6/ITO interface will hinder the transfer of photogenerated electrons to the ITO bottom electrode and photogenerated holes to the electrolyte, which is against the water oxi- dation reaction in the electrolyte.

Mott–Schottky plots support the charge transfer mecha- nism and validate the eﬀect of ferroelectric polarization on the improved photoelectrochemical performance of the Bi2MoO6

photoanode. The flat-band potential (Vfb) is obtained from the Mott–Schottky equation,⁴⁶

1 C²¼ 2

eεε0Nd ðVVfbÞ kT e

where Crepresents capacitance at the semiconductor/electrolyte interface,eis the electronic charge,εois the permittivity of vacuum,εis dielectric constant of Bi2MoO6(i.e., 37.5),⁴⁷Ndis the majority charge-carrier concentration in the semiconductor, V is the applied potential, Vfb is the flat-band potential,kis Boltzmann’s constant, andTis the temperature.

TheVfbis determined from thexintercept by extrapolating the linear fit of the 1/C²vs. Vplot. A positive slope in the 1/C²vs. V plots confirms the n-type conductivity of Bi2MoO6, which is consistent with the previous band structure according to the XPS result. Considering thatkT/eis calculated to be 0.026 V at 300 K, theVfbof the Bi2MoO6 photoanode is calculated to be 0.42 V, 0.32 V and 0.44 V (vs.RHE) for the as-grown Bi2MoO6

nanopillars and those with polarization toward ITO and against ITO, respectively (Fig. 6a). A negative shift in the flat- band potential for the sample with polarization toward ITO

confirms the enhancement in the upward band bending. Such band bending evolution increases the space charge region width, enhances the separation eﬃciency of the photogenerated charge carriers and accelerates the transfer of the charge carriers for the photoelectrochemical water-splitting reaction.

On the other hand, the positive shift of Vfb suggests a reduction of band bending and the downward band bending will hinder the transfer of carriers in the photoanode.⁴⁸ In addition, theNdis estimated to be 1.75 × 10¹⁹cm⁻³according to the slope of the linear fit of the Mott–Schottky plot for the as-grown sample. For the Bi2MoO6 nanopillars with polarization toward ITO, theNdis estimated to be 6.40 × 10¹⁹ cm⁻³. Oxygen vacancies introduced by the charge injection may be one of the reasons for the increasing carrier concentration.⁴⁹Such increase can shift the Fermi level of Bi2MoO6

toward the conduction band, introduce a more significant band bending at the Bi2MoO6/electrolyte interface and further facilitate charge separation and transportation at the interface.⁵⁰

Electrochemical impedance spectroscopy supports that ferroelectric polarization tunes the charge-transfer eﬃciency at the Bi2MoO6/electrolyte interface. There is a reduced charge transfer resistance at the interface between Bi2MoO6 and the electrolyte for the Bi2MoO6 nanopillars with polarization toward ITO due to the shorter radius of the circular arc compared to the as-grown Bi2MoO6 nanopillars without macroscopic polarization.^10,48,51However, the larger charge transfer resistance in the sample with polarization against ITO will degrade the photoelectrochemical performance which is consistent with photoelectrochemical measurements. The results indicate that the polarization toward ITO and the corres- pondingEFEcan indeed enhance the interface charge transfer eﬃciency of the Bi2MoO6nanopillars.

Fig. 5 Schematic diagrams of the energy band structure of ferroelectric Bi₂MoO₆(a) without macroscopic polarization, and with macroscopic polarization (b) toward and (c) against the ITO bottom electrode.

(8)

4. Conclusions

In summary, Bi2MoO6 nanopillars with large surface areas were grown on the ITO-coated glass substrate by a pulsed laser deposition technique through optimizing growth conditions.

The ferroelectric polarization switching of Bi2MoO6 nanopillars is confirmed byP–Eloops and PFM images. The macroscopic polarization orientation toward the ITO electrode was successfully induced by a special poling method with an ionic liquid as the top electrode. Such polarization increases the magnitude of band bending, thus promoting the separation efficiency of light-excited electron–hole pairs and enhancing holes’ extraction, and offers more carriers for the water oxi- dation reaction. As a result, the Bi2MoO6nanopillars with the polarization toward ITO show a 28% enhanced photocurrent density of 250μA cm⁻² at 1.23 Vvs.RHE compared with the origin nanopillars without macroscopic polarization. This work gives inspiration for the development of new and highly efficient photoelectrochemical devices with ferroelectric nanopillars.

Con ﬂ icts of interest

There are no conflicts of interest to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (51790492, 51721001 and 61874055) and the National Key Research and Development Program of China (2016YFA0300101).

References

1 X. T. Xu, L. Pan, X. Zhang, L. Wang and J. J. Zou,Adv. Sci., 2019,6, 1801505.

2 X. Li, J. G. Yu, J. X. Low, Y. P. Fang, J. Xiao and X. B. Chen, J. Mater. Chem. A, 2015,3, 2485–2534.

3 T. Hisatomi, J. Kubota and K. Domen, Chem. Soc. Rev., 2014,43, 7520–7535.

4 H. Wu, H. L. Tan, C. Y. Toe, J. Scott, L. Wang, R. Amal and Y. H. Ng,Adv. Mater., 2020,32, 1904717.

5 K. Zhang, B. J. Jin, C. Park, Y. Cho, X. F. Song, X. J. Shi, S. L. Zhang, W. Kim, H. B. Zeng and J. H. Park, Nat.

Commun., 2019,10, 2001.

6 X. Cheng, Y. Zhang and Y. P. Bi, Nano Energy, 2019, 57, 542–548.

7 L. Li, P. A. Salvador and G. S. Rohrer,Nanoscale, 2014,6, 24–42.

8 S.-W. Cheong, D. Talbayev, V. Kiryukhin and A. Saxena,npj Quantum Mater., 2018,3, 1–7.

9 F. Chen, H. W. Huang, L. Guo, Y. H. Zhang and T. Y. Ma, Angew. Chem., Int. Ed., 2019,58, 10061–10073.

10 S. Li, J. M. Zhang, B.-P. Zhang, W. Huang, C. Harnagea, R. Nechache, L. Zhu, S. W. Zhang, Y.-H. Lin, L. Ni, Y.-H. Sang, H. Liu and F. Rosei,Nano Energy, 2017,35, 92–100.

11 J. Song, T. L. Kim, J. Lee, S. Y. Cho, J. Cha, S. Y. Jeong, H. An, W. S. Kim, Y.-S. Jung, J. Park, G. Y. Jung, D.-Y. Kim, J. Y. Jo, S. D. Bu, H. W. Jang and S. Lee,Nano Res., 2017,11, 642–655.

12 C. Y. Wang, D. W. Cao, F. G. Zheng, W. Dong, L. Fang, X. D. Su and M. R. Shen,Chem. Commun., 2013,49, 3769–

3771.

13 W. Ji, K. Yao, Y.-F. Lim, Y. C. Liang and A. Suwardi,Appl.

Phys. Lett., 2013,103, 062901.

14 D. W. Cao, Z. J. Wang, N. Nasori, L. Y. Wen, Y. Mi and Y. Lei,Angew. Chem., Int. Ed., 2014,53, 11027–11031.

15 S. Li, B. AlOtaibi, W. Huang, Z. T. Mi, N. Serpone, R. Nechache and F. Rosei,Small, 2015,11, 4018–4026.

16 W. Huang, C. Harnagea, X. Tong, D. Benetti, S. H. Sun, M. Chaker, F. Rosei and R. Nechache, ACS Appl. Mater.

Interfaces, 2019,11, 13185–13193.

17 W. J. Song, P. A. Salvador and G. S. Rohrer,ACS Appl. Mater.

Interfaces, 2018,10, 41450–41457.

Fig. 6 (a) Mott–Schottky plots and (b) EIS Nyquist plots of the Bi₂MoO₆photoanode under diﬀerent polarization conditions.

Nanoscale Paper

(9)

18 Y. L. Huang, W. S. Chang, C. N. Van, H. J. Liu, K. A. Tsai, J. W. Chen, H. H. Kuo, W. Y. Tzeng, Y. C. Chen, C. L. Wu, C. W. Luo, Y. J. Hsu and Y. H. Chu, Nanoscale, 2016, 8, 15795–15801.

19 J. L. Xie, C. X. Guo, P. P. Yang, X. D. Wang, D. Y. Liu and C. M. Li,Nano Energy, 2017,31, 28–36.

20 W. G. Yang, Y. H. Yu, M. B. Starr, X. Yin, Z. D. Li, A. Kvit, S. F. Wang, P. Zhao and X. D. Wang,Nano Lett., 2015,15, 7574–7580.

21 Q. Campbell, D. Fisher and I. Dabo,Phys. Rev. Mater., 2019, 3, 015404.

22 M. Maczka, P. T. Freire, C. Luz-Lima, W. Paraguassu, J. Hanuza and J. Mendes Filho,J. Phys.: Condens. Matter, 2010,22, 015901.

23 V. I. Voronkova, E. P. Kharitonova and O. G. Rudnitskaya, J. Alloys Compd., 2009,487, 274–279.

24 H. B. Yu, L. B. Jiang, H. Wang, B. B. Huang, X. Z. Yuan, J. H. Huang, J. Zhang and G. M. Zeng, Small, 2019, 15, e1901008.

25 Y. H. Peng, Q. H. Liu, J. Q. Zhang, Y. Zhang, M. Geng and J. Q. Yu,J. Phys. Chem. C, 2018,122, 3738–3747.

26 G. P. Zhang, D. Y. Chen, N. J. Li, Q. F. Xu, H. Li, J. H. He and J. M. Lu,Appl. Catal., B, 2019,250, 313–324.

27 M. H. Wu, Y. X. Wang, Y. Xu, J. Ming, M. Zhou, R. Xu, Q. Fu and Y. Lei, ACS Appl. Mater. Interfaces, 2017, 9, 23647–23653.

28 W. Q. Li, X. G. Ding, H. T. Wu and H. Yang,Appl. Surf. Sci., 2018,447, 636–647.

29 X. Ding, W. K. Ho, J. Shang and L. Z. Zhang,Appl. Catal., B, 2016,182, 316–325.

30 Y. Ma, Y. L. Jia, L. N. Wang, M. Yang, Y. P. Bi and Y. X. Qi, Phys. Chem. Chem. Phys., 2016,18, 5091–5094.

31 L. Wang, R. Y. Wang, Y. Zhou, Q. Shen, J. H. Ye, C. P. Wu and Z. G. Zou,Catal. Today, 2019,335, 262–268.

32 S. Y. Kim, J. H. Lee, J. J. Kim and Y. W. Heo,J. Nanosci.

Nanotechnol., 2014,14, 9020–9024.

33 H. Verma, D. Mukherjee, S. Witanachchi, P. Mukherjee and M. Batzill,J. Cryst. Growth, 2010,312, 2012–2018.

34 X. L. Xue, R. P. Chen, C. Z. Yan, Y. Hu, W. J. Zhang, S. Y. Yang, L. B. Ma, G. Y. Zhu and Z. Jin,Nanoscale, 2019, 11, 10439–10445.

35 S. N. Lou, R. Amal, J. Scott and Y. H. Ng,ACS Appl. Energy Mater., 2018,1, 3955–3964.

36 Z. Jia, F. Lyu, L. C. Zhang, S. Zeng, S. X. Liang, Y. Y. Li and J. Lu,Sci. Rep., 2019,9, 7636.

37 Y. Zheng, T. F. Zhou, X. D. Zhao, W. K. Pang, H. Gao, S. Li, Z. Zhou, H. K. Liu and Z. P. Guo, Adv. Mater., 2017,29, 1700396.

38 L. Ge, Q. Liu, D. Jiang, L. J. Ding, Z. R. Wen, Y. S. Guo, C. F. Ding and K. Wang, Biosens. Bioelectron., 2019, 135, 145–152.

39 L. T. Quynh, C. N. Van, Y. Bitla, J.-W. Chen, T. H. Do, W.-Y. Tzeng, S.-C. Liao, K.-A. Tsai, Y.-C. Chen, C.-L. Wu, C.-H. Lai, C.-W. Luo, Y.-J. Hsu and Y.-H. Chu,Adv. Energy Mater., 2016,6, 1600686.

40 A. Gruverman and S. V. Kalinin, J. Mater. Sci., 2006, 41, 107–116.

41 V. I. Voronkova, E. P. Kharitonova and O. G. Rudnitskaya, Inorg. Mater., 2011,47, 183–191.

42 R. Jiménez, A. Castro and B. Jiménez, Appl. Phys. Lett., 2003,83, 3350–3352.

43 D. K. Zhong, M. Cornuz, K. Sivula, M. Grätzel and D. R. Gamelin,Energy Environ. Sci., 2011,4, 1759–1764.

44 A. Herklotz, E.-J. Guo, A. T. Wong, T. L. Meyer, S. Dai, T. Z. Ward, H. N. Lee and M. R. Fitzsimmons,Nano Lett., 2017,17, 1665–1669.

45 Y. C. Liu, Z. Kang, S. C. Zhang, Y. Li, H. L. Wu, J. Wu, P. W. Wu, Z. Zhang, Q. L. Liao and Y. Zhang,Inorg. Chem.

Front., 2018,5, 1533–1539.

46 J. Huang, Y. Wang, X. Q. Liu, Y. C. Li, X. Q. Hu, B. He, Z. Shu, Z. Li and Y. L. Zhao,Nano Energy, 2019,59, 33–40.

47 J. F. Yang, Z. Y. Ling and Y. Zhuang,J. Mater. Sci.: Mater.

Electron., 2014,25, 1856–1862.

48 S. Singh and N. Khare,Appl. Phys. Lett., 2017,110, 152902.

49 G. M. Wang, Y. Yang, Y. C. Ling, H. Y. Wang, X. H. Lu, Y.-C. Pu, J. Z. Zhang, Y. X. Tong and Y. Li,J. Mater. Chem. A, 2016,4, 2849–2855.

50 X. Wu, H. X. Li, X. Y. Wang, L. Y. Jiang, J. H. Xi, G. Du and Z. G. Ji,J. Alloys Compd., 2019,783, 643–651.

51 Z. R. Liu, L. W. Wang, X. Yu, J. Zhang, R. Q. Yang, X. D. Zhang, Y. C. Ji, M. Q. Wu, L. Deng, L. L. Li and Z. L. Wang,Adv. Funct. Mater., 2019,29, 1807279.