Electrochemically Synthesized Tungsten Trioxide Nanostructures for Photoelectrochemical Water Splitting

Home , Tungsten trioxide

Colloids and Surfaces A: Physicochem. Eng. Aspects 484 (2015) 297–303

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Colloids and Surfaces A: Physicochemical and

Engineering Aspects

journal homepage: www.elsevier.com/locate/colsurfa

Electrochemically synthesized tungsten trioxide nanostructures for

photoelectrochemical water splitting: Inﬂuence of heat treatment on

physicochemical properties, photocurrent densities and electron shuttling

a a,b,∗ a a,b

Tao Zhu , Meng Nan Chong , Yi Wen Phuan , Eng-Seng Chan

School of Engineering, Chemical Engineering Discipline, Monash University Malaysia, Jalan Lagoon Selatan, Bandar Sunway, Selangor Darul Ehsan 47500, Malaysia

Sustainable Water Alliance, Advanced Engineering Platform, Monash University Malaysia, Jalan Lagoon Selatan, Bandar Sunway, Selangor Darul Ehsan

47500, Malaysia

h i g h l i g h t s g r a p h i c a l a b s t r a c t

•

We investigated the inﬂuence of heat The as-deposited amorphous WO3 ﬁlms were subjected to heat treatment at different annealing tem-

treatment on physicochemical prop- peratures, in order to transform from amorphous into polycrystalline WO3 nanostructures. A clear and

erties, photocurrent densities and distinctive phase transition from amorphou to monoclinic WO3 phases was also observed with elevated

electron shuttling ability of nanos- heat treatment by XRD. And the surface morphologies of nanostructured WO3 thin ﬁlms were seen to

tructured tungsten trioxide (WO3) undergo four major physical transformation stages during the heat treatment process. Further FTIR analy-

thin ﬁlms prepared via electrochem- sis reafﬁrmed that the appearance and disappearance of key functional groups during the heat treatment

ical deposition method. process are perfectly linked to the thermal-induced phase transition in polycrystalline WO3 nanostruc-

•

Surface morphologies of nanostructures. EIS results showed that the separation of photogenerated charge carriers in nanostructured WO3

◦

tured WO3 thin films undergo four thin films was greatly enhanced when the films were annealed at 600 C with a recombination time of

major physical transformation stages 3.65 ms. Finally the synthesized nanostructured WO3 thin ﬁlms also achieved the highest photocurrent

◦

during the heat treatment process. density of up to 35 ␮A/cm2 at 600 C, potentially due to the improved intrinsic electron shuttling ability

•

The appearance and disappearance after the elevated heat treatment process.

of key functional groups during heat

treatment are perfectly linked to the

thermal-induced phase transition in

polycrystalline WO3 nanostructures.

•

Separation of photogenerated charge

carriers in nanostructured WO3 thin

ﬁlms was greatly enhanced when the

◦

ﬁlms were annealed at 600 C with a

recombination time of 3.65 ms.

•

The highest photocurrent density

of up to 35 ␮A/cm was achieved

for nanostructured WO3 thin ﬁlm

◦

annealed at 600 C, potentially due to

the improved intrinsic electron shut-

tling ability after heat treatment.

∗

Corresponding author. Fax: +603 5514 6207.

E-mail address: [email protected] (M.N. Chong).

http://dx.doi.org/10.1016/j.colsurfa.2015.08.016

298 T. Zhu et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 484 (2015) 297–303

a r t i c l e i n f o a b s t r a c t

Article history: The primary aim of this study was to investigate the inﬂuence of heat treatment on the physicochemical

Received 22 May 2015

properties, photocurrent densities and capacity for electron shuttling of nanostructured tungsten triox-

Received in revised form 7 August 2015

ide (WO3) thin ﬁlms prepared via an electrochemical deposition method. The as-deposited amorphous

Accepted 9 August 2015

WO3 ﬁlms were further subjected to heat treatment at various annealing temperatures to transform the

Available online 12 August 2015

amorphous material into polycrystalline WO3 nanostructures. X-ray diffraction (XRD) spectra indicated

the existence of polycrystalline WO3 nanostructures on the surfaces of the ﬂuorine-doped tin oxide (FTO)

Keywords:

electrodes. Through XRD analysis, a clear and distinctive phase transition from amorphous to monoclinic

Nanostructured thin ﬁlms

WO3 was observed with elevated heat treatment. This WO3 phase transition was further examined by

Electrochemical deposition

ﬁeld emission-scanning electron microscopy (FE-SEM) imaging whereby the surface morphologies of the

Phase transition

Annealing treatment nanostructured WO3 thin ﬁlms were observed to progress through four major physical transformation

Electrochemical impedance spectroscopy stages during the heat treatment process. Further Fourier transform infrared (FTIR) spectroscopy anal-

ysis reafﬁrmed that the appearance and disappearance of key functional groups during heat treatment

coincided with the thermally induced phase transitions of the polycrystalline WO3 nanostructures. In

addition, the inﬂuence of heat treatment on the intrinsic electron shuttling ability of nanostructured WO3

thin ﬁlms synthesised at different annealing temperatures was studied using electrochemical impedance

spectroscopy (EIS). EIS results indicated that the separation of photogenerated charge carriers in the

◦

nanostructured WO3 thin ﬁlms was greatly enhanced when the ﬁlms were annealed at 600 C, exhibit-

ing recombination times of 3.65 ms. This was thermodynamically linked to the increase in average WO3

crystallite size and the reduction of WO3 grain boundaries during the thermally induced phase transition,

which led to the suppression of the electron-hole pair recombination rate.

1. Introduction

morphology and structural effects that determine the overall efﬁ-

ciency of PEC-assisted water splitting.

Tungsten trioxide (WO3) is an n-type semiconductor material

It is known that a number of synthesis factors affect the sur-

that has found application as a new generation photocatalyst owing

face morphology and structure of thin nanostructured WO3 ﬁlms

to its unique chemical characteristics, particularly its high resis-

synthesised over photoelectrodes. These include, for example, pre-

tance in most aqueous media [1]. In addition, the band gap of the

cursor solution concentration, pH, applied potential, and heat

semiconductor WO3 photocatalyst (2.6 eV) is lower than that of the

treatment. In our previous communication, we reviewed the effects

gold standard for semiconductor photocatalysts, titanium dioxide

of different synthesis factors on the sizes, compositions and thick-

(TiO2, 3.2 eV), meaning that WO3 can absorb 12% into the visible

nesses of electrochemically synthesised, nanostructured WO3 thin

light spectrum [2]. Theoretically, the valence band position of semi-

ﬁlms [13]. Among the various electrochemical synthesis factors,

conductor WO3 is energetic enough to induce oxygen evolution

treatment at elevated annealing temperatures is known to have

during water electrolysis; therefore, it is commonly regarded as

a significant influence on the eventual WO3 thin films in terms

an oxygen-evolution catalyst (OEC). Based on this aspect, many

of surface morphology, crystal structure and phase transition. For

scientists have previously focused on the utilisation of semicon-

instance, Liu et al. [14] explained that low photoactivity might

ductor WO3 as the OEC in standard photoelectrochemical (PEC)

result from poor crystallization in the annealed WO3 ﬁlms. They

cells used for solar energy conversion during water splitting [3–6].

found that the photoactivity of WO3 was enhanced when the

Other studies have reported that surface modiﬁcations of WO3 ◦

annealing temperature was increased to 450 C, linking this to bet-

photocatalysts extended their applications to include the oxida-

ter crystallinity and a decrease in WO3 surface defects. Ng et al. [15]

tive decomposition of organic pollutants, in addition to the norm

found that the morphologies and degrees of crystallinity observed

oxygen evolution during the water electrolysis process [7–9].

in WO3 electrodes annealed at different temperatures varied with

Thermodynamically, the splitting of water is an uphill process

the measured photocurrent densities. In another study, Hong et al.

in which water molecules either gain or lose electrons [10]. In PEC-

[16] found that the average crystallite size in polycrystalline WO3

assisted water splitting, the redox reaction is directly related to the

ﬁlms increased with increasing annealing temperature.

positions of the valence band (VB) and conduction band (CB) of

To date, there is no systematic study on the inﬂuence of heat

the semiconductor photocatalyst employed [11]. The CB position

treatment on electrochemically synthesized, nanostructured WO3

of the WO3 photocatalyst makes it unsuitable for use as a cathodic

thin ﬁlms. Thus, the main aim of this study was to investigate the

hydrogen-evolution catalyst (HEC); however, the comparatively

inﬂuence of heat treatment on the physicochemical properties,

ideal position of the WO3VB illustrates its suitability for use at the

electron shuttling and photocurrent densities of nanostructured

anodic OEC [12]. Thus, the mismatch in the position of the CB of

WO3 thin ﬁlms prepared via electrochemical deposition. For the

the WO3 photocatalyst seriously impairs its potential for hydrogen

ﬁrst time, the inﬂuence of heat treatment on the intrinsic electron

evolution within the PEC-assisted water splitting process. How-

shuttling in nanostructured WO3 thin ﬁlms synthesised at differ-

ever, the water splitting process also be strongly inﬂuenced by

ent annealing temperatures was examined using electrochemical

the surface and bulk structures of electrode containing the pho-

impedance spectroscopy (EIS). The successfully synthesized WO3

toelectrocatalyst thin ﬁlm, because this process occurs primarily in

thin ﬁlms were characterized using an advanced suite of methods

aqueous solutions. The electrode surface can provide active sites

including X-ray diffraction (XRD), ﬁeld emission-scanning elec-

for the absorption of water molecules; the bulk material is respon-

tron microscopy (FE-SEM), Fourier transform infrared spectroscopy

sible for the prevention of recombination of photogenerated charge

(FTIR), electrochemical impedance spectroscopy (EIS) and pho-

carriers in the semiconductor WO3 photocatalyst. Therefore, care-

tocurrent density measurements. This work constitutes a more

ful synthesis of a thin, nanostructured WO3 ﬁlm over the anodic

fundamental approach toward understanding the inﬂuence of heat

electrode surfaces can regulate both the surface

T. Zhu et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 484 (2015) 297–303 299

treatment on electrochemically synthesized, nanostructured WO3

thin ﬁlms and how the observed differences help to improve PEC-

assisted water splitting efﬁciency.

2. Materials and methods

2.1. Preparation of precursor solution

In this study, all chemicals were used as received without further

puriﬁcation. Hydrogen peroxide (H2O2, 30%) was obtained from

HmbG Chemicals, USA. Tungsten (W) powder with a particle size

of 325 mesh was purchased from ChemSoln, USA. Platinum (Pt)

black (purity ≥99.97%) with particle size ≤20 ␮m was supplied by

ChemSoln, USA. All other miscellaneous chemicals were purchased

from Merck, USA. In this study, the preparation of the precursor

solution for electrochemical deposition was performed as previ-

Fig. 1. The deposition current density vs. time proﬁle during the synthesis of nanos-

ously reported [4]. Initially, the precursor solution was prepared tructured WO3 thin ﬁlms on an FTO electrode via electrochemical deposition. Inset:

FE-SEM images of (a) control blank FTO electrode and (b) as-deposited amorphous

by dissolving 1.8 g of W powder in 50 mL of H2O2 over the course

WO3 ﬁlms on FTO electrode.

of 24 h. Excess H2O2 was then decomposed through the addition

of a small amount of Pt black. The solution was then heated at the

◦ −1 −1

optimum temperature (60 C) until no gas bubbles were evident the range of 500–4000 cm at a resolution of 4 cm . The PEC

[17]. This was followed by diluting the precursor solution to 50 mM properties of the samples were measured at room temperature

via the addition of 150 mL of 50/50 (% v/v) of water/2-propanol. in a dark box using the same PGSTAT2014 Applied Potentiostat

The function of 2-propanol is to extend the stability of the pre- unit and FTO glass slide as the WE. However, the peroxy-tungstic

cursor solution by preventing the precipitation of an amorphous acid (PTA) electrolyte solution was replaced by a 0.1 mol/L sodium

WO3-based hydrated phase [18]. acetate (CH3COONa) aqueous solution for the measurement of PEC

properties. During the measurement of PEC properties, a 100 W

2.2. Preparation of nanostructured WO3 thin ﬁlms halogen lamp restricted at a frequency of 0.05 Hz was used as the

light source, with a light-source-to-sample distance of 10 cm. Lin-

The initial amorphous WO3 thin ﬁlms were prepared via an ear potentiodynamic voltammetry was applied at a scan rate of

electrochemical deposition route utilising a precursor solution 5 mV/s with a step size of 1 mV.

2−

containing W2O11 anions. The electrochemical deposition of

nanostructured WO3 thin ﬁlms was performed at room temper- 3. Results and discussion

ature using a conventional three-electrode electrochemical cell

system, PGSTAT204 Applied Potentiostat (Metrohm, Netherlands). 3.1. Determination of deposition current density

An FTO glass slide (ChemSoln, USA; 14 /sq; 2.5 cm × 1.5 cm) was

used as the working electrode (WE) after cleaning with acetone Electrochemical deposition is a cost-effective method for the

and extra-pure water, while Pt was used as the counter electrode synthesis of nanostructured semiconductor thin ﬁlms, as it does not

(CE) and Ag/AgCl (4 M KCl) as the reference electrode (RE). In this require expensive instrumentation and allows for precise control

study, all measured potentials were made with reference to the of ﬁlm thickness. Previously, Wei et al. [19] reported electrochem-

Ag/AgCl (4 M KCl) electrode. During the electrochemical deposition ical deposition to be an extremely promising synthetic method in

process, the immersed FTO area was held constant at 2 cm 1.5 cm. terms of mass production potential and in its ability to produce

The applied potential between the WE and RE was ﬁxed at - nanostructured semiconductor thin ﬁlms with large surface area-

0.45 V, and controlled by a PGSTAT204 Applied Potentiostat unit to-volume ratios. Generally, the two following mechanisms are

[18]. After the electrodeposition process, the as-deposited amor- commonly employed in the electrochemical deposition method:

phous WO3 ﬁlms on FTO were rinsed using distilled water, dried (1) direct reduction of metals onto the WE and (2) increase in

in clean air and subsequently heat-treated in the furnace (Carbo- interfacial pH and local supersaturation, followed by metal oxide

lite 301, UK). The as-deposited amorphous WO3 ﬁlms were further precipitation [13]. The latter mechanism is based on the generation

subjected to heat treatments at different annealing temperatures, of a localised high-pH region near the electrode surface to induce

◦ ◦

ranging from 100 C to 600 C, to transform the amorphous material electrochemical deposition of metal ions. The electrochemical

into polycrystalline WO3 nanostructures. The heat treatment was deposition mechanism at work in the formation of nanostructured

◦

carried out at an increasing rate of 10 C/min to the desired anneal- WO3 thin ﬁlms can be depicted by these two reaction steps (Eqs.

ing temperature and remained constant for 20 min from thereof. (1) and (2)):

After heat treatment, the samples were cooled down at a cooling

2− +

◦ 2W 10H2O2 → W2O11 + 2H + 9H2O (1)

rate of 2.5 C/min under continuous airﬂow. All experiments were

repeated in triplicate where deemed appropriate and necessary.

2− + −

+ + +

2.3. Characterization of nanostructured WO3 thin ﬁlms W2O11 (2 x)H xe

→ 2WO3 + (2 + x)/2H2O + (8 − x)/4O2 (2)

XRD measurements were collected at room temperature using

2− 2−

Cu K␣ radiation ( = 1.54 Å) with a potential of 40 kV and a cur- In these reactions, W2O11 is [(O2)2W(O)OW(O)(O2)2] and (O2)

rent of 30 mA (Philips PW1830, Netherlands). Microstructural and denotes a peroxide ligand [18].

chemical analysis was performed by FE-SEM on uncoated samples Fig. 1 shows the deposition current density vs. time proﬁle at

with an accelerating voltage of 5 kV (FEI Nova NanoSEM, USA). constant potential (−0.45 V vs. SCE). As seen in Fig. 1, the deposition

The FTIR spectra of the samples were obtained on a Bruker Inc. current density decreases rapidly with an increase in deposition

Vector 22 (Bruker, Germany) coupled with an ATR accessory in time, reaching a plateau after approximately 250 s. In this instance,

300 T. Zhu et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 484 (2015) 297–303

◦

Fig. 2. FE-SEM images of nanostructured WO3 thin ﬁlms formed at different annealing temperatures (100–600 C).

the deposition current density is related to the sheet resistance molecules and the structural transformation of WO3 that accom-

of the WE and the electrical conductivity of the precursor solu- panies the loss of water molecules as vapor [14]. Thereafter, the

tion. The high initial current density is due to the relatively higher average WO3 crystallite size decreased with increasing annealing

◦ ◦

conductivity of the working FTO electrode and the precursor solu- temperature, up to 300 C. For the sample treated at 400 C, smaller

tion with a high initial W concentration. Amorphous WO3 ﬁlms WO3 crystals were found to have agglomerated in the formation

form on the surface of working FTO electrode according to Eqs. of larger, irregularly sized WO3 crystals with an average crystal-

(1) and (2). Growth in the thickness of the WO3 ﬁlm will result agglomerate size of 70 nm. When the sample was heat-treated at

◦

in a more highly resistant working FTO electrode, causing the 500 C, the FE-SEM image indicated the presence of physical cracks

rapid reduction and stagnation of the deposition current density on the surface of the annealed WO3 thin ﬁlm. These cracks were

at approximately 1.65 mA/cm . In addition, the dynamic growth of absent in the as-deposited amorphous WO3 ﬁlm, indicating that

WO3 grains decreases the available surface area of the working FTO they were thermally generated during heat treatment or cooling.

electrode, which in turn reduces the deposition current density. A similar observation of the presence of physical cracks was previ-

Another reason for the decreasing trend in the deposition current ously reported in study by Rahman et al. [21]. They concluded that

density vs. time proﬁle may be due to the diffusion of ions in the the presence of physical cracks led to higher photoactivity, as they

solution [20]. The reaction rate at the surface of the FTO working generated more active surface sites for contact between the WE and

electrode quickly depletes the W concentration from the initial pre- the aqueous solution at the interface. The ﬁnal stage of heat treat-

2− ◦

cursor solution faster than the ions (W2O11 ) can be replenished ment (600 C) resulted in the disappearance of the physical cracks,

by diffusion. However, the time required for stabilisation of the producing closely uniform and homogeneous nucleus-shaped WO3

deposition current density is largely a function of the WO3 grains crystals with sizes of 86 nm.

size. More speciﬁcally, it is the time required for the impingement

and percolation processes to take place. Once the impingement pro-

3.3. X-ray diffraction

cess occurs, the working FTO electrode is completely covered by a

layer of WO3 grains, which stabilises the deposition current density.

Fig. 3 shows the XRD spectra of the nanostructured WO thin

FE-SEM images of the control blank FTO, as well as the as-deposited 3

ﬁlms after heat treatment at various annealing temperatures rang-

amorphous WO ﬁlms, are shown in Fig. 1. The FE-SEM image of the

3 ◦ ◦

ing from 100 C to 60 C. From Fig. 3, the XRD spectrum of the

semiconductor WO3 ﬁlm displayed irregular particle size, possibly

as-deposited WO ﬁlm shows no distinctive or sharp diffraction

owing to its amorphous nature as reported by Kwong et al. [18] 3

peaks, in accord with its amorphous nature [15]. Similarly, when

◦

the sample was heat-treated at 300 C, neither distinctive nor sharp

3.2. Field emission – scanning electron microscopy diffraction peaks were evident, indicating retention of the mate-

rial’s amorphous nature during the loss of surface-adsorbed and

◦ ◦

The as-deposited amorphous WO3 ﬁlms were subjected to heat chemisorbed water molecules between 100 C and 300 C. Heat

◦ ◦

treatment at different annealing temperatures, ranging from 100 C treatment of the WO3 ﬁlm at 400 C induced the formation of dis-

◦ ◦ ◦ ◦

to 600 C. Fig. 2 shows the surface morphologies, crystal sizes and tinctive and sharp diffraction peaks at 23.3 , 23.8 and 24.6 , which

structures of the nanostructured WO3 thin ﬁlms formed at different were indexed to the (0 0 2), (0 2 0) and (2 0 0) planes, respectively.

◦

annealing temperatures. At 400 C, the amorphous WO3 ﬁlm undergoes a phase transi-

As seen in Fig. 2, the surface morphologies of the nanostructured tion to the monoclinic WO3 phase according to JCPDS data (JCPDS

WO3 thin ﬁlms were signiﬁcantly altered by the heat treatment. 43-1035), in agreement with previous observations by Ng et al.

◦

The ﬁlms experienced four major physical transformation stages: [14]. In the heat treatment of amorphous WO3 ﬁlms above 400 C

◦ ◦

(1) loss of surface-adsorbed and chemisorbed water molecules; (i.e., 500 C and 600 C), similar photoactive phases of monoclinic

(2) agglomeration of WO3 crystals; (3) physical cracking and; (4) WO3 (Figure 3) were observed, in contrast to previous studies that

formation of closely uniform and homogeneous nucleus-shaped reported mixed and contradictory results regarding the presence

WO3 crystals. During the ﬁrst transformation stage, an average of photoactive WO3 phases after heat-treatment at annealing tem-

◦

WO3 crystallite size of 30 nm was observed after heat treatment peratures above 400 C [16]. For example, Kalanur [2] showed the

◦

of the amorphous ﬁlm at 100 C. As outlined, such a change could presence of a hexagonal WO3 crystal structure after heat treatment

◦

be attributed to both the oxidation of surface-adsorbed water above 400 C, indicating that heat treatment might not be the sole

T. Zhu et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 484 (2015) 297–303 301

Fig. 5. Photocurrent density proﬁles of nanostructured WO3 thin ﬁlms annealed at

◦ ◦

temperatures ranging from 400 C to 600 C.

was subjected to heat treatment, the amorphous structure was

transformed into other photoactive WO3 structures. In this study, it

was found that when the amorphous WO3 ﬁlm was heat-treated at

Fig. 3. XRD spectra of nanostructured WO3 thin ﬁlms heat-treated at annealing ◦ −

◦ ◦ 1

100 C, a distinct W-O-W peak was observed at 893 cm with the

temperatures ranging from 100 C to 600 C, with one blank FTO as the control

−1

experiment. simultaneous disappearance of the O H peak at 1663 cm , signify-

ing an oxidation processes. Simultaneously, both surface-adsorbed

and chemisorbed water molecules are lost to the surroundings

as water vapour, leading to the disappearance of the O H peak

−1

at 1663 cm with increasing heat treatment. No O H peak was

−1

observed near 1663 cm when the as-deposited amorphous WO3

ﬁlms were heat-treated at annealing temperatures ranging from

◦ ◦

200 C to 600 C. From the FTIR spectra, one of the prominent char-

acteristics of the heat-treated WO3 thin ﬁlms was the presence of

−1

distinct W O W peaks at 893 cm , attributed to the presence

of WO3 crystallites [24]. This observation, in accord with obser-

vations made from the XRD spectra, conﬁrmed the transition of

an amorphous WO3-based hydrated phase into a crystalline WO3

phase.

3.5. Photocurrent density measurement

Fig. 5 shows the dependency of photocurrent density on

the applied potential shown by the nanostructured WO3 thin

Fig. 4. FTIR spectra of the nanostructured WO3 thin ﬁlms annealed at temperatures ﬁlms in 0.1 M of aqueous CH3COONa solution. In general, it was

◦ ◦

ranging from 100 C to 600 C. A control blank FTO spectrum is also included.

observed that the photocurrent density increased with increas-

ing applied potential, owing to the increased band bending at the

contributing factor in the creation of photoactive phases and the WO3/electrolyte interfaces that leads to a higher mobility of the

eventual WO3 crystal structures. In this study, it was observed that photogenerated excitons [5]. This phenomenon will facilitate the

the degree of crystallinity in the monoclinic WO3 phase increased generation and separation of excitons, leading to a higher photocur-

◦ ◦

with increasing heat treatment to 500 C and 600 C. This was evi- rent density and eventually a higher overall PEC-assisted water

denced from the distinctive, sharper diffraction peaks present for splitting efﬁciency. In this study, the highest photocurrent den-

◦ ◦ 2

the thin ﬁlms samples treated at 500 C and 600 C [15]. There was a sity was measured to be 35 ␮A/cm for the nanostructured WO3

◦

simultaneous increase in peak intensity and decrease in peak width thin ﬁlm annealed at 600 C. This was followed by the thin ﬁlms

◦ ◦ ◦ ◦

for the thin ﬁlms treated at 500 C and 600 C, compared to the annealed at 500 C and 400 C, where the photocurrent densities

◦ 2 2

ﬁlm annealed at 400 C. Moreover, the detection of characteristic were 25 ␮A/cm and 15 ␮A/cm , respectively. The photocurrent

◦ ◦ ◦

XRD peaks along the plane of the thin ﬁlms treated at 500 C and densities for samples annealed at 100 C to 300 C could not be

◦

600 C indicated preferential growth along the (2 0 0) and (0 0 2) obtained, possibly due to the instability of nanostructured WO3

orientations of the WO3 crystals, respectively. thin ﬁlms during or after the photocurrent density measurements.

Inverse photocurrent density proﬁles were observed for the WO3

◦ ◦

3.4. Fourier transform infrared spectroscopy thin ﬁlms treated from 100 C to 300 C (data not shown). The pri-

mary reason for the stability of the nanostructured WO3 thin ﬁlms

Fig. 4 shows the FTIR spectra of the nanostructured WO3 thin could be due to the formation of the different photoactive WO3

◦

ﬁlms annealed at different temperatures ranging from 100 C to phases at different annealing temperatures, as evidenced from the

◦

600 C. The characteristic WO3 peak, corresponding to a W O W XRD and FTIR spectra.

−1

bond, is normally observed 893 cm [22]. From Fig. 4, the as- From all the characterization studies, it can be concluded that

deposited amorphous WO3 ﬁlm shows a wide, broadened peak the photocurrent density and stability of the nanostructured WO3

−1 −1

between 500 cm and 1000 cm without the distinctive W O W thin ﬁlms are strongly dependent on the existence of photoac-

−1

peak near 893 cm . This is due to the amorphous nature of the tive WO3 phases, as well as their average crystallite size, surface

as-deposited WO3 ﬁlm, which is made up of non-aligned W- uniformity and degree of crystallinity. It is worth mentioning that

◦

nanoclusters [23]. When the as-deposited amorphous WO3 ﬁlm the nanostructured WO3 thin ﬁlm annealed at 500 C with surface

302 T. Zhu et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 484 (2015) 297–303

◦ ◦

Fig. 6. EIS spectra of the nanostructured WO3 thin ﬁlms annealed at temperatures ranging from 400 C to 600 C. (a) Nyquist plot; (b) Enlargement of high-frequency region

and; (c) Bode phase plot.

Table 1

physical cracks (see Fig. 2) shows a lower photocurrent density than

◦ Kinetic parameters for the recombination reaction in nanostructured WO3 thin ﬁlms

the WO3 thin ﬁlm treated at 600 C. This ﬁnding contradicts a pre- ◦ ◦

annealed from 400 C to 600 C.

vious study by Rahman et al. [21], where higher photoactivity was

Samples R () R () ␶(ms)

attributed to the presence of physical cracks, which allow more ct w

◦

extensive WO3/electrolyte interactions. So as to more fully probe WO3 (400 C) 150.38 51.51 0.31

◦

this disagreement, EIS measurements were taken to provide infor- WO3 (500 C) 120.36 31.51 2.31

◦

WO3 (600 C) 78.23 20.56 3.65

mation on the inﬂuence of heat treatment on the intrinsic electron

shuttling ability of the nanostructured WO3 thin ﬁlms, particularly

◦

in materials treated at 500 C.

electrolyte/WO3 electrode [26]. The small-semicircle in the high-

frequency region was ﬁtted to a value of transport resistance (Rw),

3.6. Electrochemical impedance spectroscopy which was ascribed to the accumulation/transport of injected elec-

trons and the charge transfer across the WO3/WE interface [26].

EIS is a useful characterization method for the analysis of Table 1 shows the variation in the measured Rct and Rw values with

◦ ◦

efﬁcient photogenerated-charge-transport processes. It is a steady- increasing heat treatment from 400 C to 600 C.

state method used to measure the current response to the As seen in Table 1, the nanostructured WO3 thin ﬁlm annealed at

◦

application of an AC voltage as a function of frequency. EIS has 600 C had lower charge-transfer and transport resistances than the

◦ ◦

previously been used to study the charge-transport kinetics in PEC- thin ﬁlms treated at 400 C and 500 C. This facile electron transfer

assisted water splitting processes [25–27]. We have performed EIS indicated that the intrinsic electron shuttling ability was optimum

◦

measurements on the nanostructured WO3 thin ﬁlms annealed at for the ﬁlm treated at 600 C. The higher charge transfer and trans-

◦ ◦ ◦

temperatures ranging from 100 C to 600 C. Due to the unstable port resistances measured for the WO3 thin ﬁlm annealed at 500 C

◦ ◦

nature of the WO3 thin ﬁlms formed from 100 C to 300 C (as can be linked to the presence of physical cracks and the existence of

discussed in Section 3.5), EIS measurements were not performed a relatively disordered ﬁlm structure, resulting in loose mechanical

for these samples. Fig. 6(a) shows the Nyquist plot of the EIS data adhesion between the ﬁlm and the FTO working electrode.

◦ ◦

from the thin ﬁlms annealed from 400 C to 600 C under con- Poor mechanical adhesion between the nanostructured WO3

stant illumination. Fig. 6(b) shows the Bode phase plot for the thin ﬁlm and the FTO working electrode might lead to higher resis-

enlargement of high-frequency region in Fig. 6(a). The Nyquist plot tance and lower photocurrent density. The characteristic frequency

in Fig. 6(a) exhibits two-semicircles, including a large-semicircle peaks (1–103 Hz) for the nanostructured WO3 thin ﬁlms annealed

◦ ◦

at low frequency and a small-semicircle at high frequency. The at 500 C and 600 C in the Bode phase plot are shown in Fig. 6(c).

large-semicircle in the Nyquist plot was ﬁtted to a charge-transfer As seen in Fig. 6(c), the characteristic frequency peaks for

◦ ◦ ◦

resistance (Rct) value, which was assigned to the interfaces of the thin ﬁlms annealed at 400 C, 500 C and 600 C have

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and is given by the following equation [29]:

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◦

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◦ 2228–2246.

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◦

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◦ properties of nanostructured hematite thin ﬁlms prepared via

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◦

in the nanostructured WO3 thin ﬁlm annealed at 600 C. From the 371–380.

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photocurrent ﬂow at the molecular level. It is anticipated that this

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study will provide a fundamental platform in the electrochemical- powders, Curr. Opin. Solid State Mater. Sci. 11 (1–2) (2007) 19–27.

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synthesis of nanostructured WO3 thin ﬁlms with further surface

WO3 supported on sulphated tin oxide catalysts, Arabian J. Chem. (2015),

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Acknowledgments

[26] Z. Xu, X. Li, J. Li, L. Wu, Q. Zeng, Z. Zhou, Effect of CoOOH loading on the

photoelectrocatalytic performance of WO3 nanorod array ﬁlm, Appl. Surface

Sci. 284 (2013) 285–290.

The authors are grateful to the ﬁnancial support provided by

[27] M. Zhang, C. Yang, W. Pu, Y. Tan, K. Yang, J. Zhang, Liquid phase deposition of

the eScience fund (Project NO: 03-02-10-SF0121) from Ministry

WO3/TiO2 heterojunction ﬁlms with high photoelectrocatalytic activity under

of Science, Technology and Innovation (MOSTI), Malaysia. Simi- visible light irradiation, Electrochim. Acta 148 (0) (2014) 180–186.

lar gratitude also goes to the Advanced Engineering Platform and [29] H. Xu, X. Tao, D.-T. Wang, Y.-Z. Zheng, J.-F. Chen, Enhanced efﬁciency in

dye-sensitized solar cells based on TiO2 nanocrystal/nanotube double-layered

School of Engineering, Monash University Malaysia.

ﬁlms, Electrochimica Acta 7 (2010) 2280–2285.

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