Vertical Arrays Film for Solar Water Splitting - ACS Publications

Subscriber access provided by UNIV OF CALIFORNIA SAN DIEGO LIBRARIES

Article 3

Enhancement of the Photoelectrochemical Performance of WO Vertical Arrays Film for Solar Water Splitting by Gadolinium Doping Yang Liu, Jie Li, Wenzhang Li, Yahui Yang, Yaomin Li, and Qiyuan Chen J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b00966 • Publication Date (Web): 11 Jun 2015 Downloaded from http://pubs.acs.org on June 12, 2015

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 39

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Enhancement of the Photoelectrochemical Performance of WO3 Vertical Arrays Film for Solar Water Splitting by Gadolinium Doping Yang Liu1, Jie Li1, Wenzhang Li1, *, Yahui Yang2, *, Yaomin Li3, Qiyuan Chen1 1

School of Chemistry and Chemical Engineering, Central South University, Changsha

410083, China 2

College of Resources and Environment, Hunan Agricultural University, Changsha

410128, China 3

Department of Chemistry，University College London，20 Gordon Street，London，

WC1H 0AJ，UK

*

Corresponding author. Tel.: +86 731 8887 9616; fax: +86 731 8887 9616.

E-mail addresses: [email protected] ,[email protected]

1

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 39

Abstract In this study, WO3 nanoplates modified by doping with gadolinium (Gd) were synthesized by a hydrothermal method. The effect of the rare earth element on the photoelectrochemical (PEC) performance of WO3 one- or two-dimensional (2D) materials was examined. Scanning electron microscopy, transmission electron microscopy, and X-ray diffraction were employed to examine the crystal phase and morphology of the Gd-doped nanoplates thus prepared. Results of UV–vis spectroscopy,

valence-band

(VB)

X-ray

photoelectron

spectroscopy,

and

Mott–Schottky analyses indicated that the conduction band and VB potentials of WO3 shifted to negative values. Linear sweep voltammetry results indicated that the photocurrent density increased by 153% after modification by doping with Gd. In addition,

the

PEC

properties of

the

WO3

nanoplates

obtained

by

the

intensity-modulated photocurrent spectrum and incident photon-to-current conversion efficiency measurements indicated that modification by doping with Gd has a scintillating application in improving the PEC conversion properties of WO3-based 2D materials. 1. Introduction Photoelectrochemical (PEC) systems based on transition metal oxides, such as TiO2, ZnO, and WO3, have been attracting increasingly attention since the discovery of the photoinduced decomposition of water on TiO2 electrodes.1-8 Besides TiO2 and ZnO, WO3 is also a material that exhibits high reactivity, low toxicity, and excellent photocorrosion resistance. Moreover, the band gap of WO3 (2.5–2.8 eV)9-10 is 2


Page 3 of 39

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


narrower than those of TiO2 (3.2 eV for anatase)11 and ZnO (3.3 eV),12 which means a larger photoresponse region for WO3. In addition, the hole diffusion length of WO3 (~150 nm) is also longer than that of TiO2 (~10 nm).13-14 Even though WO3 has promising features, further improvements are required for making it efficient for use as photoelectric conversion materials. To recover the catalyst and improve the efficiency of water splitting and photocatalysis, WO3 is prepared as a film in a PEC cell system. Some methods have been employed for increasing the light-to-chemical (stored energy) conversion efficiency of WO3. On the other hand, the morphology of WO3 is a key factor affecting the efficiency of a PEC cell.5, 15 Some films having mesoporous as well as vertically aligned nanostructure array (nanowire, nanorod, and plate-like) morphologies have been fabricated by different processes for improving light harvesting and offering an effective channel for the directional transfer of electrons.5, 10, 16-17 To accelerate charge transfer, WO3 is coupled with some other materials such as TiO2, BiVO4, Fe2O3, CdS, and graphene.9, 18-23

At the same time, for improving PEC performance, doping WO3 with metals or

nonmetals is also a useful method for tuning the band gap energy.24-26 Recently, rare earth metals have demonstrated tremendous potential as dopants for enhancing the photocatalytic activity of TiO2, ZnO, and BiVO4. 27-29 Gadolinium (Gd)-doped titania nanoparticles (NPs) have been synthesized by a sol–gel method. After doping with Gd, a red-shift of the absorption edge was observed, resulting in the extension of the photoresponse region. The photocatalytic activity of the Gd-doped titania NPs was evaluated by the degradation of dyes such as rhodamine B: after 3



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

doping, the degradation of rhodamine B increased from 21.5% to 50.6%.27 The effect of Gd doping on the crystal structure of Gd-doped TiO2 was also discussed. Co-doping with rare earth metals and non-metals was also investigated,30-32 and the photocatalytic activity of TiO2 was further improved. Moreover, some of the Gd atoms of the dopant may distribute on the surface of materials in the form of gadolinia, which may inhibit the recombination of electrons and holes at the surface of the semiconductor.33-35 As aforementioned, the modification of semiconductor powder materials by doping with rare earth elements demonstrates excellent potential for enhancing their PEC performance. However, to the best of our knowledge, the effects of rare earth elements on WO3-containing one- or two-dimensional materials, as compared to semiconductor powder materials, have never been reported. In this paper, Gd-doped WO3 nanoplate arrays were synthesized by a hydrothermal method. The effect of doping with Gd on the PEC performance of WO3 was extensively studied by linear sweep voltammetry, electrochemical impedance spectroscopy (EIS), Mott–Schottky plots, as well as intensity-modulated photocurrent spectrum (IMPS) and incident photon-to-current conversion efficiency (IPCE) measurements.

2. Experimental section 2.1 Film Synthesis All chemicals were of analytical grade and used without further purification. Na2WO4 and Gd2O3 were used as the W and Gd precursors, respectively. The modified WO3 nanoplates doped with Gd (hereafter referred to as Gd-WO3) were 4


Page 4 of 39

Page 5 of 39

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


synthesized by a hydrothermal method. In a typical process, 0.231 g Na2WO4 and an appropriate amount of Gd2O3 were dispersed into 30 mL DI water. Second, 6 mL of 3M HCl was added to the solution. Third, after stirring for 10 min, 0.200 g (NH4)2C2O4 was added with additional stirring for 20 min, and the solution was diluted using 34 mL DI water. Fourth, the mixture was transferred to a 100 mL Teflon-lined stainless steel autoclave, which contained the fluorinedoped tin oxide FTO-glass substrate against the wall. Next, the autoclave was sealed and placed in an oven at 120 °C for 12 h. Then, after cooling to room temperature, the films were removed, washed with DI water and absolute alcohol, and dried at 60 °C for 0.5 h. Finally, the product was annealed at 500 °C for 1 h under air at a heating rate of 2 °C/min. Utilizing this approach, Gd-WO3 materials with different molar ratios were fabricated: 0.01:1, 0.02:1, 0.04:1, and 0.06:1, denoted as Gd-WO3 1%, Gd-WO3 2%, Gd-WO3 4%, and Gd-WO3 6%, respectively. For comparison, a WO3 nanoplate film without Gd2O3 was also prepared.

2.2 Characterization The crystalline phases of the as-prepared samples were characterized by X-ray diffraction (XRD, D/Max2250, Rigaku Corporation, Japan) using Cu Kа radiation at a diffraction angle (2θ) between 15° and 70°. Surface and cross-sectional morphologies of the films were investigated by scanning electron microscopy (SEM, JSM6700F, JEOL Company, Japan). The microstructures and lattice spacing were obtained using transmission electron microscopy (TEM, TECNAI G2 F20, FEI, 5



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Japan).

FTIR

spectra

were

recorded

on

a

Fourier

Page 6 of 39

transform

infrared

spectrophotometer (NICOLET, 6700, USA). X-ray photoelectron spectroscopy (XPS) measurements were conducted using an XPS instrument (ESCALAB 250Xi, Thermo Fisher-VG Scientific). UV–vis absorption spectra measurements were conducted using a diffuse-reflectance ultraviolet and visible spectrophotometer (UV–Vis, Shimadzu 2450).

2.3 Photoelectrochemical measurements A typical three-electrode system was used to measure the PEC properties using an electrochemical analyzer (Zennium, Zahner, Germany). The as-prepared films, platinum foil, and Ag/AgCl (saturated KCl) were used as the working electrode, counter electrode, and reference electrode, respectively. An aqueous solution of 0.2 M Na2SO4 was used as the electrolyte solution. The photocurrents were measured using linear sweep voltammograms in a potential range from −0.05 V to 1.4 V (vs. Ag/AgCl) with a scan rate of 20 mV/s under AM 1.5G illumination. Transient photocurrents were detected at 0.9 V (vs. Ag/AgCl). The EIS were measured at 0.8 V (vs. Ag/AgCl) with a perturbation amplitude of 10 mV and a frequency of 0.1 Hz–10 kHz. The Mott–Schottky plots were recorded at an AC frequency of 1 kHz. IMPS data were recorded using a Zahner CIMPS-2 system. A blue light-emitting diode (LED) driven by a PP210 frequency response analyzer was used as a lamp, which can provide both dc and ac components of illumination. The ac component of the current to the LED generated a modulation (10%) superimposed on the dc light 6


Page 7 of 39

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


intensity, which was adjusted to 247 mW/m2. Moreover, IPCE was conducted as a function of wavelength from 320 to 600 nm using a Si solar cell as the standard.

3. Results and discussion 3.1 Morphology and Structural Characterization XRD was employed to examine the structural properties of undoped and Gd-doped WO3 nanoplates grown on FTO, and Fig 1 shows the results. As shown in Fig 1a, several significant diffraction peaks (24.34°, 28.76°, 33.16°, 41.88°, 48.30° and 55.80°) were observed for WO3, characteristic of monoclinic WO3 (JCPDS no. 72-0677). Some peaks corresponding to tetragonal SnO2 were also observed (JCPDS no.46-1088). The main diffraction peaks of Gd-WO3 were indexed according to monoclinic WO3 (JCPDS no. 72-0677) and tetragonal SnO2 (JCPDS no.46-1088). In addition, in Fig 1b, two small diffraction peaks were observed at 47.11° and 53.46° for Gd-WO3, attributed to the (004) and (024) plane of monoclinic WO3 (JCPDS no. 72-0677), respectively; this results indicates that the dopants cause a slight lattice mismatch.36 The peaks of Gd-WO3 shifted to smaller angles with a slightly low intensity. On the one hand, the incorporation of Gd leads to the expansion of the WO3 lattice.37 On the other hand, the incorporation of Gd affects the crystallinity of WO3.38 SEM micrographs showing the morphological features of undoped and Gd-doped WO3 samples are shown in Fig 2. In Fig 2a, the FTO substrate was uniformly covered by the WO3 nanoplates, which were nearly perpendicular to the 7



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

substrate, and the thickness of nanoplates ranged from 250 to 500 nm. After doping with Gd, the morphology and crystal structure changed, as shown in Fig 2b. The thickness of the WO3 nanoplates changed to 320–580 nm. As can be seen in the cross-sectional view images of the films in Figs 2c and 2d, the nanoplate arrays grew vertically on FTO. The heights of the nanoplate arrays were 2.22 and 2.28 µm for WO3 and Gd-WO3, respectively. Electron-dispersive X-ray (EDX) analysis was employed to investigate the chemical compositions of the pure and doped WO3 nanoplates. The concentration of Gd in the doped sample obtained by EDX analysis was lower than that calculated by stoichiometry, possibly indicating that Gd atoms are not completely incorporated into WO3. To obtain more detailed information on the crystalline structure of WO3 and Gd-WO3 4%, TEM and selected-area electron diffraction (SAED) patterns were recorded (Fig 3). Fig 3a and 3b show low-magnification TEM images of the samples, where plate-like structures were observed. Figs 3c and 3d show the SAED patterns of the samples; after calcination, the WO3 nanoflakes exhibited polycrystalline configuration, while the SAED pattern of Gd-WO3 4% exhibited satellite diffraction spots, indicative of the distribution of defects and the distortion of WO3 lattice.39-40 Fig 4a shows the high-magnification TEM image of Gd-WO3 4%. The lattice fringes in the HRTEM micrographs indicate the crystalline nature of the Gd-WO3 4%. The fast Fourier transform patterns (FFT) of the two sections in Fig 4a are shown in Figs 4b and 4c. The profiles of lattice fringes in Figs 4b and 4c are also shown in Figs S1a and S1b, respectively. The lattice spacing of section I (red square) was clearly 8


Page 8 of 39

Page 9 of 39

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.334 nm, which corresponds to the (120) plane of monoclinic WO3 (JPCDS card: 72-0677). On the other hand, the lattice spacing of section II (blue square) was 0.225 nm, which corresponds to the (102) plane of hexagonal Gd2O3 (JPCDS card: 24-0430). In Fig 4c, the lattice planes became slightly blurred, and some defects (such as the region marked with the red circle in Fig 4c) were observed in the lattice of Gd-WO3 4%. The lattice fringe according to the red circle in Fig 4c was 0.419 nm, which suggests that the Gd atom displaces the W atom or occupies interstitial positions.26 Hence, some of the Gd atoms can enter the lattices of WO3, while the other Gd dopants can distribute on the grain boundary or on the surface of WO3 nanoplates in the form of gadolinia.34 As a type of material that exhibits no photoresponse under visible light, Gd2O3 may inhibit the recombination of electrons and holes at the interface of electrode and electrolyte as a passivation layer. To investigate the effect of Gd doping on the chemical composition and chemical environment of the WO3 nanoplates, the samples were subjected to XPS analysis (Fig 5). From the full spectra of pure WO3 and Gd doped WO3 in Fig 5a, it could be seen that the samples consist of W and O elements. The C 1s peak at 284.8 eV was assigned to adventitious carbon species from the XPS instrument, which was not marked in Fig 5a. As shown in Fig 5b, for undoped WO3, two peaks were observed at 35.9 and 38.05 eV, attributed to the characteristic W 4f7/2 and W 4f5/2, respectively. 41 In Fig 5c, the asymmetric O1s peak was fitted by two peaks: the peak at 530.71 eV (lattice oxygen) and 532.6 eV (surface hydroxyl oxygen). For Gd-WO3, the characteristic peaks of W 4f7/2 and W 4f5/2 were shifted to a lower binding energy 9



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

by approximately 0.05 eV, indicating that the WO3 lattice is occupied by Gd.42 High-resolution scanning XPS pattern of Gd 3d is shown in Fig 5d. The peak around 1188.19 eV was assigned to Gd 3d, suggesting that Gd in the +3 state is attributed to Gd2O3. From XPS measurements, an [O]/[W] value of 2.91 was estimated at the Gd-WO3 surface, which is lesser than that of undoped WO3 (3.05). Even though this ratio is non-stoichiometric, as it could be affected by adventitious oxygen present during XPS measurements, the oxygen content in the nanoplate film decreased after modification by doping with Gd. This decrease in the oxygen content is attributed to the fact that Gd replaces W after entering the lattices of WO3, which could increase the oxygen vacancy content; this observation is in agreement with results reported from other studies.36, 43 At the same time, a [Gd]/[W] value of 0.03 was obtained, which is higher than the value obtained from EDX analysis. Considering that the detection depth of EDX and XPS is different, it can be concluded that the surface of WO3 is enriched by some of the Gd atoms in the form of Gd2O3, which was observed in Fig 4a.

3.2 Optical Properties Fig 6 shows the FTIR spectra of WO3 and Gd-WO3 4% films. In Fig 6a, a weak peak was observed at approximately 1039 cm−1, attributed to the stretching mode of the W–O groups. A peak was also observed at 957 cm−1, attributed to the asymmetric vibration of the W–O bonds. Peaks were observed at 812 and 742 cm−1, also 10


Page 10 of 39

Page 11 of 39

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


attributed to the stretching mode of W−O−W in WO3.44 Simultaneously, broad absorption peaks were observed at 3440 and 1633 cm−1, attributed to the O–H stretching vibration arising from hydroxyl groups. Because of modification by doping with Gd, a slight shift was observed compared with the peaks (750, 814, 932, 1039, 1632 and 3421 cm−1) of WO3. In the FTIR spectra of the samples before calcination in Fig 6b, besides the similar peaks observed as in the spectrum of WO3, two more peaks were observed at 1330 and 1380 cm−1, attributed to H–O–H bending vibration. These peaks can be attributed to a small amount of absorbed H2O in Gd-WO3 4%.45-46 This observation is attributed to the fact that rare earth salts always form octahydrates, indicating that they exhibit a good affinity for hydroxyl groups or absorbed water.47 UV–vis diffuse reflectance spectroscopy was employed to investigate the light-absorbance properties of the samples (Fig 7). Bare WO3 can absorb light at wavelengths smaller than 460 nm, which corresponds to a band gap energy of 2.70 eV. In the case of Gd-WO3 4%, the onset absorption significantly shifted toward higher wavelength, from 460 to 470 nm, with a band gap energy of 2.64 eV. This observation is attributed to the fact that hybridization occurs between the O 2p and Gd 4f/5d orbitals after Gd is substituted or interstitially incorporated into the WO3 lattice.48

3.3 Photoelectric Properties Linear sweep voltammetry was employed to investigate the current densities of 11



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

the WO3 and various Gd-WO3 films in a potential range from −0.05 V to 1.4 V (vs. Ag/AgCl) at a scan rate of 20 mV/s. In Fig 8, the current densities of films were negligible when the light was chopped. The onset potential of the photocurrent shifted; a magnified image of the potential region around the onset potential is shown in Fig S2. As can be seen, the onset potential shifted to approximately −0.05 V for Gd-WO3 4% as compared to that for WO3. The photocurrents of all films increased with potential under illumination. All Gd-WO3 films exhibited higher PEC activity as compared to that of the bare WO3 film. At 1.0 V (vs. Ag/AgCl), the current densities of WO3, Gd-WO3 1%, Gd-WO3 2%, Gd-WO3 4%, and Gd-WO3 6% were 0.90, 1.49, 1.71, 2.28, and 2.09 mA/cm2, respectively. In other words, for Gd-WO3 4%, the photocurrent increased by 1.53 times as compared to that of undoped WO3. Meanwhile, the photocurrent density of the film increased with the concentration of the Gd dopant and the Gd2O3 content on the surface of nanoplates. However, the photocurrent density decreased for WO3 containing a high concentration of Gd dopant (6%). This decrease is attributed to the possible recombination of electron–hole pairs by the excess gadolinium species, and the excess Gd2O3 possibly inhibits the electron transfer at the interface between the photoanode and electrolyte. The corresponding photoconversion (light energy to chemical energy conversion) efficiencies were calculated.49 The maximum efficiency of the Gd-WO3 films was approximately 0.84%, which is 2.47 times that (referring to maximum efficiency) of the WO3 film. To verify the interface quality and the composite structure associated with 12


Page 12 of 39

Page 13 of 39

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


charge transfer, the EIS of the films were evaluated under visible light illumination. Fig 9 shows the Nyquist plots of the pure WO3 film and the Gd-WO3 film at an AC frequency varying from 10 kHz to 0.1 Hz, a potential of 0.6 V, and an AC voltage perturbation of

10 mV.

The

EIS exhibited semicircle

portions at an

intermediate-frequency region, attributed to electron transport and recombination at the interface between the electrode and electrolyte. The inset in Fig 9 shows the equivalent circuit of the films, which consist of series resistance (Rs), charge-transfer resistance (Rct), and a constant-phase element (CPE). 50 In the equivalent circuit of the EIS plots, Rs denotes the solution resistance, CPE is the constant-phase element for the electrolyte–electrode interface, and Rct is the charge transfer resistance across the interface between the electrode and the electrolyte. Rs is affected by the sheet resistance of the electrode and electrolyte, which was detected at high frequency around 10 kHz. The Rct value is in lieu of the diameter of the circle, which is inversely proportional to the efficiency of charge separation. Moreover, the CPE represents an integrated capacitor associated with all trap states. Table 1 lists the simulated impedance parameters. Rct decreased with increasing content of the Gd dopant. Under irradiation, a smaller value for the Rct is imperative to sustain the photocurrent as large numbers of the photogenerated minority charge carriers have to be efficiently injected into the electrolyte.51 This implies that increasing the dopant concentration is beneficial for forming a passivation layer of Gd2O3, and an appropriate Gd content is required for inhibiting the recombination of electrons and holes. However, excess Gd2O3 may hinder the charge transfer at the interface 13



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 39

between the electrode and electrolyte, which results in an increase of Rct for Gd-WO3 6%. To understand the recombination of the photogenerated electrons and holes in the samples, the transient time constant (τtr) was measured and calculated from the equations as follows 52-53: D = exp(−t/τtr)

(1)

D = (It − If)/(Ii − If)

(2)

In Fig 10, the photocurrents of WO3 and Gd-WO3 4% were approximately zero when the light was chopped, and the photocurrent exhibited decay under illumination, implying that the recombination of the photogenerated carriers occurs. From equations (1) and (2), a shorter transient time constant implies a larger extent of recombination. The τtr values of WO3 and Gd-WO3 4% were 7.03 and 8.19 s, respectively. Hence, it can be concluded that Gd2O3 covered on WO3 decreases the recombination of the photogenerated electrons and holes at the interface between the photoanode and electrolyte. To investigate the strong correlation between Gd doping and the enhancement of PEC performance, Mott–Schottky analysis was conducted. As shown in Fig 11, both samples exhibited a n-type characteristic with a positive slope. This result indicated that electrons are the majority carriers in semiconductors. The carrier densities were estimated from the slopes of the Mott–Schottky plots using the following equation. 1/C2 = (2/qε0εND)(E − EFB − kT/q)

(3) 14


Page 15 of 39

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Here, q, ε, and ND denote the electronic charge, dielectric constant of semiconductor, and donor concentration of semiconductors, respectively. E and EFB denote the applied and flat band potentials, respectively. The electron density of the bare WO3 film was 6.3 × 1020 /cm3, whereas after doping, the electron density increased to 3.42 × 1021 /cm3. This increase is attributed to the fact that the strong hybridization of the O 2p orbitals with the Gd 4f/5d states creates more charge carriers in the form of electrons or holes.48 An optimum carrier concentration is desired to achieve a good photoresponse. The flat band potentials of WO3 (0.245 V vs.Ag/AgCl) and Gd-WO3 (0.240 vs. Ag/AgCl) films were measured by the extrapolation of the linear parts of the Mott–Schottky plots to the potential axis. IMPS was used to investigate electron transport. Fig 12 shows the complex plane plot of the IMPS response. The electron transport time (τn) can be determined by the frequency at the imaginary maximum, according to the following formula. τn =1/(2πf(IMPS))

(4)

The electron transport times calculated for WO3 and Gd-WO3 4% were 1.49 and 2.35 ms, respectively. The longer electron transport time may be attributed to the thickness of the WO3 nanoplates,5 which increases the electron transfer length from the interface of the WO3/electrolyte to the interface of WO3/FTO. At the same time, excess doping may increase lattice mismatch, which does not benefit the transfer of electrons. Even though a longer transport time of Gd-WO3 does not benefit the PEC properties, the photocurrent of Gd-WO3 is higher than that of WO3. Therefore, the

15



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

electron transport ability exhibits no clear effect on the PEC performance in this system. To investigate the quantitative correlation of light absorption on the films, IPCE measurements were performed at a bias of 1.0 V (Fig 13). IPCE, which is expressed as the number of electrons in the external circuit produced by an incident photon at a given wavelength divided by the number of incident photons, can be viewed as a collective measure of the efﬁciencies (η) obtained from three microscopic processes: light harvesting, the separation of opposing charges, and the collection of the charges at the electrodes. In Fig 13, the WO3 film exhibited a strong photon response at 320–460 nm, with a maximum IPCE value of 23.00% at 350 nm. A considerably higher IPCE was observed for Gd-WO3 4% over the light response region at 320–470 nm, which exhibited a slight red shift as compared with that of bare WO3. The IPCE decreased to zero at wavelengths longer than 470 nm, in agreement with the above result obtained from diffuse-reflectance measurements. The highest IPCE of 61.88% was observed for Gd-WO3 4% at 350 nm, which is 1.69 times higher than that of WO3. Considering that the obvious difference between WO3 and Gd-WO3 is at the UV light region, the main factor for improving the PEC performance is the inhibition of the recombination of the photogenerated electrons and holes caused by the passivation layer of Gd2O3.

Schematic illustration To further understand the above experimental facts, a schematic is shown in Fig 16


Page 16 of 39

Page 17 of 39

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


14. As discussed, some of the Gd atoms can enter the lattices of WO3, while the other Gd atoms possibly distribute on the grain boundary or on the surface of WO3 nanoplates in the form of gadolinia.34 As the concentration of the Gd dopant increases, the Gd2O3 content also possibly increases, which can form an overlayer for decreasing the recombination of electrons and holes at the interface. First, for illustrating a possible mechanism, the band structure of the samples, which usually affects PEC performance, is discussed. The potentials for the top of the valence band (VB) were estimated by VB-XPS, which represents the XPS plots of samples at a binding energy of around 0 eV. 54 From Fig S3, the potential for the top of the VB for Gd-WO3 4% was 0.45 eV smaller than that for WO3. According to the equation Ec = Ef + kT ln(NC/ND), where Ec and NC denote the bottom energy level of the CB and effective states density of the CB energy levels, respectively. 34 The donor concentration ND increased after doping, while other parameters could be considered as no changes. Hence, the potential for bottom energy level of the CB for Gd-WO3 4% was also smaller than that for WO3, which results in a better reducing activity. It is consistent with the hypothesis that Gd entering the crystal lattices generates a higher reduction potential for the CB of electrons. 34 Furthermore, Gd dopants that distribute on the grain boundary or on the surface of WO3 nanoplates exist in the form of gadolinia, which can decrease the recombination of the electrons and holes at the interface between the photoanode and electrolyte. The recombination time of electron–hole pairs is extended, which was verified by the results obtained from the transient photocurrents. Even though the 17



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

results of IMPS suggest that the electron transport time increases after the addition of Gd dopants, which may be caused by the lattice mismatch, it exhibited a marginal effect on the PEC properties. Meanwhile, the results obtained from IPCE measurements indicated a better value in the whole photoresponse region, which affirms that Gd2O3 as the passivation layer significantly inhibits the recombination of electrons and holes.

Conclusion In summary, we reported the synthesis of Gd-doped WO3 nanoplates by a hydrothermal method. On the one hand, Gd enters the lattices of WO3 and changes the potential of the VB and CB of WO3. The addition of Gd dopants causes a red-shift of the absorption edge, resulting in the extension of the photoresponse region, and changes the potential of CB and VB of WO3, as compared to undoped WO3, to negative value. On the other hand, Gd dopants distribute on the surface of the WO3 nanoplates in the form of gadolinia, which serves as a passivation layer. The passivation layer can inhibit the recombination of electrons and holes, thereby leading to a longer lifetime of electron–hole pairs. The electron transport time slightly increased, suggesting that the transfer properties of the photogenerated electrons have not been modified. The results of PEC measurements indicate that WO3 modified by doping with Gd exhibits a higher photocurrent under light irradiation, and the IPCE exhibits a clear increase in the whole photoresponse region. Thus, we can conclude that the combination of doping as well as the passivation 18


Page 18 of 39

Page 19 of 39

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


layer leads to the improvement in the PEC performance of WO3 vertical array films.

Acknowledgements This study was supported by the National High Technology Research and Development Program of China (863 Program, Grant No. 2011AA050528), the National Nature Science Foundation of China (No. 51304253), the Hunan Provincial Natural Science Foundation of China (No. 13JJ6003), the Fundamental Research Funds for the Central Universities (No. 2012QNZT12), the Open-End Fund for the Valuable and Precision Instruments of Central South University.

ASSOCIATED CONTENT Supporting Information The profile of lattice fringes of sample, the onset potential of the photocurrents of different samples, VB-XPS plots. This information is available free of charge via the Internet at http://pubs.acs.org

19



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Reference 1.

Brewster, M. M.; Zhou, X.; Lu, M.-Y.; Gradecak, S. The Interplay of Structural and Optical

Properties in Individual ZnO Nanostructures. Nanoscale 2012, 4, 1455-1462. 2.

Ng, Y. H.; Iwase, A.; Kudo, A.; Amal, R. Reducing Graphene Oxide on a Visible-Light BiVO4

Photocatalyst for an Enhanced Photoelectrochemical Water Splitting. J. Phys. Chem. Lett. 2010, 1, 2607-2612. 3.

Li, J.; Liu, X.; Han, Q.; Yao, X.; Wang, X. Formation of WO3 Nanotube-based Bundles Directed

by NaHSO4 and Its Application in Water Treatment. J. Mater. Chem. A 2013, 1, 1246-1253. 4.

Zhang, H.; Duan, G.; Li, Y.; Xu, X.; Dai, Z.; Cai, W. Leaf-like Tungsten Oxide Nanoplatelets

Induced by Laser Ablation in Liquid and Subsequent Aging. Cryst. Growth Des. 2012, 12, 2646-2652. 5.

Su, J.; Feng, X.; Sloppy, J. D.; Guo, L.; Grimes, C. A. Vertically Aligned WO3 Nanowire Arrays

Grown Directly on Transparent Conducting Oxide Coated Glass: Synthesis and Photoelectrochemical Properties. Nano Lett. 2010, 11, 203-208. 6.

Kominami, H.; Yabutani, K.-i.; Yamamoto, T.; Kera, Y.; Ohtani, B. Synthesis of Highly Active

Tungsten(VI) Oxide Photocatalysts for Oxygen Evolution by Hydrothermal Treatment of Aqueous Tungstic Acid Solutions. J. Mater. Chem. 2001, 11, 3222-3227. 7.

Moniz, S. J. A.; Zhu, J.; Tang, J. 1D Co-Pi Modified BiVO4/ZnO Junction Cascade for Efficient

Photoelectrochemical Water Cleavage. Adv. Energy Mater. 2014, 4. 8.

Zhou, W.; Li, W.; Wang, J.-Q.; Qu, Y.; Yang, Y.; Xie, Y.; Zhang, K.; Wang, L.; Fu, H.; Zhao, D.

Ordered Mesoporous Black TiO2 as Highly Efficient Hydrogen Evolution Photocatalyst. J. Am. Chem. Soc. 2014, 136, 9280-9283. 9.

Rao, P. M.; Cai, L.; Liu, C.; Cho, I. S.; Lee, C. H.; Weisse, J. M.; Yang, P.; Zheng, X.

Simultaneously Efficient Light Absorption and Charge Separation in WO3/BiVO4 Core/Shell Nanowire Photoanode for Photoelectrochemical Water Oxidation. Nano Lett. 2014, 14, 1099-1105. 10. Zheng, J. Y.; Song, G.; Kim, C. W.; Kang, Y. S. Fabrication of (001)-Oriented Monoclinic WO3 Film on FTO Substrate. Nanoscale 2013, 5, 5279-5282. 11. Hoang, S.; Berglund, S. P.; Hahn, N. T.; Bard, A. J.; Mullins, C. B. Enhancing Visible Light Photo-oxidation of Water with TiO2 Nanowire Arrays via Cotreatment with H2 and NH3: Synergistic Effects between Ti3+ and N. J. Am. Chem. Soc. 2012, 134, 3659-3662. 12. AbdulAlmohsin, S.; Cui, J. B. Graphene-Enriched P3HT and Porphyrin-Modified ZnO Nanowire Arrays for Hybrid Solar Cell Applications. J. Phys. Chem. C 2012, 116, 9433-9438. 13. Salvador, P. Hole Diffusion Length in n-TiO2 Single Crystals and Sintered Electrodes: Photoelectrochemical Determination and Comparative Analysis. J. Appl. Phys. 1984, 55, 2977-2985. 14. Butler, M. A. Photoelectrolysis and Physical Properties of the Semiconducting Electrode WO3 J. Appl. Phys. 1977, 48, 1914-1920. 15. Li, W.; Li, J.; Wang, X.; Luo, S.; Xiao, J.; Chen, Q. Visible Light Photoelectrochemical Responsiveness of Self-Organized Nanoporous WO3 Films. Electrochim. Acta 2010, 56, 620-625. 16. Navarro, J. R. G.; Mayence, A.; Andrade, J.; Lerouge, F.; Chaput, F.; Oleynikov, P.; Bergström, L.; Parola, S.; Pawlicka, A. WO3 Nanorods Created by Self-Assembly of Highly Crystalline Nanowires under Hydrothermal Conditions. Langmuir 2014, 30, 10487-10492. 17. Yoon, S.; Kang, E.; Kim, J. K.; Lee, C. W.; Lee, J. Development of High-Performance Supercapacitor Electrodes Using Novel Ordered Mesoporous Tungsten Oxide Materials with High Electrical Conductivity. Chem. Commun. 2011, 47, 1021-1023. 20


Page 20 of 39

Page 21 of 39

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


18. Pan, J. H.; Lee, W. I. Preparation of Highly Ordered Cubic Mesoporous WO3/TiO2 Films and Their Photocatalytic Properties. Chem. Mater. 2006, 18, 847-853. 19. Su, J.; Guo, L.; Bao, N.; Grimes, C. A. Nanostructured WO3/BiVO4 Heterojunction Films for Efficient Photoelectrochemical Water Splitting. Nano Lett. 2011, 11, 1928-1933. 20. Zhu, J.; Li, W.; Li, J.; Li, Y.; Hu, H.; Yang, Y. Photoelectrochemical Activity of NiWO4/WO3 Heterojunction Photoanode under Visible Light Irradiation. Electrochim. Acta 2013, 112, 191-198. 21. Sivula, K.; Formal, F. L.; Grätzel, M. WO3−Fe2O3 Photoanodes for Water Splitting: A Host Scaffold, Guest Absorber Approach. Chem. Mater. 2009, 21, 2862-2867. 22. Liu, C.; Li, Y.; Li, W.; Zhu, J.; Li, J.; Chen, Q.; Yang, Y. CdS Quantum Dots Sensitized Platelike WO3 Photoelectrodes with a TiO2 Buffer-Layer. Mater. Lett. 2014, 120, 170-173. 23. Liu, Y.; Li, W.; Li, J.; Yang, Y.; Chen, Q. Enhancing Photoelectrochemical Performance with a Bilayer-Structured Film Consisting of Graphene-WO3 Nanocrystals and WO3 Vertically Plate-like Arrays as Photoanodes. RSC Adv. 2014, 4, 3219-3225. 24. Cole, B.; Marsen, B.; Miller, E.; Yan, Y.; To, B.; Jones, K.; Al-Jassim, M. Evaluation of Nitrogen Doping of Tungsten Oxide for Photoelectrochemical Water Splitting. J. Phys. Chem. C 2008, 112, 5213-5220. 25. Sun, Y.; Murphy, C. J.; Reyes-Gil, K. R.; Reyes-Garcia, E. A.; Thornton, J. M.; Morris, N. A.; Raftery, D. Photoelectrochemical and Structural Characterization of Carbon-Doped WO3 Films Prepared via Spray Pyrolysis. Int. J. Hydrogen Energy 2009, 34, 8476-8484. 26. Mu, W.; Xie, X.; Li, X.; Zhang, R.; Yu, Q.; Lv, K.; Wei, H.; Jian, Y. Characterizations of Nb-Doped WO3 Nanomaterials and Their Enhanced Photocatalytic Performance. RSC Adv. 2014, 4, 36064-36070. 27. Xu, J.; Ao, Y.; Fu, D.; Yuan, C. Synthesis of Gd-Doped TiO2 Nanoparticles Under Mild Condition and Their Photocatalytic Activity. Colloids Surf., A 2009, 334, 107-111. 28. Yayapao, O.; Thongtem, T.; Phuruangrat, A.; Thongtem, S. Sonochemical Synthesis of Dy-Doped ZnO Nanostructures and Their Photocatalytic Properties. J. Alloys Compd. 2013, 576, 72-79. 29. Xu, H.; Wu, C.; Li, H.; Chu, J.; Sun, G.; Xu, Y.; Yan, Y. Synthesis, Characterization and Photocatalytic Activities of Rare Earth-Loaded BiVO4 Catalysts. Appl. Surf. Sci. 2009, 256, 597-602. 30. Wang, Q.; Xu, S.; Shen, F. Preparation and Characterization of TiO2 Photocatalysts Co-Doped with Iron (III) and Lanthanum for the Degradation of Organic Pollutants. Appl. Surf. Sci. 2011, 257, 7671-7677. 31. Yu, T.; Tan, X.; Zhao, L.; Yin, Y.; Chen, P.; Wei, J. Characterization, Activity and Kinetics of A Visible Light Driven Photocatalyst: Cerium and Nitrogen Co-Doped TiO2 Nanoparticles. Chem. Eng. J. 2010, 157, 86-92. 32. Xing, M.-Y.; Qi, D.-Y.; Zhang, J.-L.; Chen, F. One-Step Hydrothermal Method to Prepare Carbon and Lanthanum Co-Doped TiO2 Nanocrystals with Exposed {001} Facets and Their High UV and Visible-Light Photocatalytic Activity. Chem. Eur. J. 2011, 17, 11432-11436. 33. Wang, Y.; Zhang, Y.-Y.; Tang, J.; Wu, H.; Xu, M.; Peng, Z.; Gong, X.-G.; Zheng, G. Simultaneous Etching and Doping of TiO2 Nanowire Arrays for Enhanced Photoelectrochemical Performance. ACS Nano 2013, 7, 9375-9383. 34. Zhao, D.; Peng, T.; Liu, M.; Lu, L.; Cai, P. Fabrication, Characterization and Photocatalytic Activity of Gd3+-Doped Titania Nanoparticles with Mesostructure. Microporous Mesoporous Mater. 2008, 114, 166-174. 21



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

35. Wang, T.; Luo, Z.; Li, C.; Gong, J. Controllable Fabrication of Nanostructured Materials for Photoelectrochemical Water Splitting via Atomic Layer Deposition. Chem. Soc. Rev. 2014, 43, 7469-7484. 36. Zhang, T.; Zhu, Z.; Chen, H.; Bai, Y.; Xiao, S.; Zheng, X.; Xue, Q.; Yang, S. Iron-Doping-Enhanced Photoelectrochemical Water Splitting Performance of Nanostructured WO3: A Combined Experimental and theoretical Study. Nanoscale 2015, 7, 2933-2940. 37. Tian, M.; Wang, H.; Sun, D.; Peng, W.; Tao, W. Visible Light Driven Nanocrystal Anatase TiO2 Doped by Ce From Sol–Gel Method and Its Photoelectrochemical Water Splitting Properties. Int. J. Hydrogen Energy 2014, 39, 13448-13453. 38. Lv, C.; Zhou, Y.; Li, H.; Dang, M.; Guo, C.; Ou, Y.; Xiao, B. Synthesis and Characterisation of Gd3+-Doped Mesoporous TiO2 Materials. Appl. Surf. Sci. 2011, 257, 5104-5108. 39. Li, M.-Y.; Shen, P. On the Nucleation and Paracrystal Interspacing of Zr-Doped Co3−δO4. Mater. Sci. Eng., B 2004, 111, 82-89. 40. Gaojie, X.; Qirong, P.; Zejun, D.; Li, Y.; Yuheng, Z. Microstructure, Localized Cu2+ Spins, and Transport Properties of Bi2Sr2Ca1-xYxCu2O8+δ. Phys. Rev. B 2000, 62, 9172-9178. 41. Cong, S.; Tian, Y.; Li, Q.; Zhao, Z.; Geng, F. Single-Crystalline Tungsten Oxide Quantum Dots for Fast Pseudocapacitor and Electrochromic Applications. Adv. Mater. 2014, 26, 4260-4267. 42. Choi, J., et al. Three-Dimensional Gd-doped TiO2 Fibrous Photoelectrodes for Efficient Visible Light-Driven Photocatalytic Performance. RSC Adv. 2014, 4, 11750-11757. 43. Wang, F.; Di Valentin, C.; Pacchioni, G. Rational Band Gap Engineering of WO3 Photocatalyst for Visible Light Water Splitting. ChemCatChem 2012, 4, 476-478. 44. Chen, Z.; Lv, H.; Zhu, X.; Li, D.; Zhang, S.; Chen, X.; Song, Y. Electropolymerization of Aniline onto Anodic WO3 Film: An Approach to Extend Polyaniline Electroactivity Beyond pH 7. J. Phys. Chem. C 2014, 118, 27449-27458. 45. Wang, M.-H.; Zhao, Z.-Y.; Liu, T.-T. Synthesis of Pr-Doped ZnO Nanoparticles by Sol–Gel Method and Varistor Properties Study. J. Alloys Compd. 2015, 621, 220-224. 46. Lin, J.; Lin, J.; Zhu, Y. Controlled Synthesis of the ZnWO4 Nanostructure and Effects on the Photocatalytic Performance. Inorg. Chem. 2007, 46, 8372-8378. 47. Tomaszewicz, E.; Leniec, G.; Kaczmarek, S. M. Re-Investigations of Thermal Decomposition of Gadolinium Sulfate Octahydrate. J. Therm. Anal. Calorim. 2010, 102, 875-881. 48. Subramanian, M.; Thakur, P.; Tanemura, M.; Hihara, T.; Ganesan, V.; Soga, T.; Chae, K. H.; Jayavel, R.; Jimbo, T. Intrinsic Ferromagnetism and Magnetic Anisotropy in Gd-doped ZnO Thin Films Synthesized by Pulsed Spray Pyrolysis Method. J. Appl. Phys. 2010, 108, 053904 (1-7). 49. Hsiao, P.-T.; Chen, L.-C.; Li, T.-L.; Teng, H. Vapor Treatment of Nanocrystalline WO3 Photoanodes for Enhanced Photoelectrochemical Performance in the Decomposition of Water. J. Mater. Chem. 2011, 21, 19402-19409. 50. Yourey, J. E.; Pyper, K. J.; Kurtz, J. B.; Bartlett, B. M. Chemical Stability of CuWO4 for Photoelectrochemical Water Oxidation. J. Phys. Chem. C 2013, 117, 8708-8718. 51. Cristino, V.; Caramori, S.; Argazzi, R.; Meda, L.; Marra, G. L.; Bignozzi, C. A. Efficient Photoelectrochemical Water Splitting by Anodically Grown WO3 Electrodes. Langmuir 2011, 27, 7276-7284. 52. Ibadurrohman, M.; Hellgardt, K. Photoelectrochemical Performance of Graphene-Modified TiO2 Photoanodes in the Presence of Glycerol as A Hole Scavenger. Int. J. Hydrogen Energy 2014, 39, 18204-18215. 22


Page 22 of 39

Page 23 of 39

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


53. Chen, K.; Feng, X.; Hu, R.; Li, Y.; Xie, K.; Li, Y.; Gu, H. Effect of Ag Nanoparticle Size on the Photoelectrochemical Properties of Ag Decorated TiO2 Nanotube Arrays. J. Alloys Compd. 2013, 554, 72-79. 54. Kako, T.; Meng, X.; Ye, J. Enhancement of Photocatalytic Activity for WO3 by Simple NaOH Loading. Appl. Catal., A 2014, 488, 183-188.

23



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figures Fig 1. (a) XRD pattern of the undoped and Gd-doped WO3 nanoplates, (b) partial enlarged view of (a) Fig 2. SEM images and enlarged SEM image of (a) WO3 nanoplates film and (b) Gd–WO3 nanoplates film, the cross-sectional images of (c) WO3 nanoplates film and (d) Gd–WO3 nanoplates film. Fig 3. TEM images of undoped and Gd-doped WO3 nanoplate: (a) undoped WO3 nanoplate, (b) Gd-doped nanoplate NPs, the corresponding SAED patterns: (c) undoped WO3 nanoplate and (d) Gd-doped nanoplate NPs Fig 4. (a) high-resolution transmission electron microscope of the Gd-WO3 4%, (b) and (c) the lattice fringes of corresponding section. Fig 5. XPS of WO3 and Gd- WO3 (a) full spectrum; (b) W 4f; (c) O 1s and (d) Gd 3d. Fig 6. FTIR of WO3 and Gd-WO3 4% (a) after and (b) before calcinations Fig 7. UV-vis diffused reflectance spectroscopy of WO3 and Gd-WO3 4% Fig 8. Linear sweep voltammetry curves and the photoconversion efficiencies of WO3 and various Gd-WO3 under dark or illumination Fig 9. The electrochemical impedance spectra of WO3 and various Gd-WO3 with the fitted equivalent circuit model shown in the inset. Fig 10. Transient photocurrent response WO3 and Gd-WO3 4% at 0.9 V (vs. Ag/AgCl) Fig 11. Mott-Schottky plots of WO3 and Gd-WO3 4% Fig 12. IMPS response of WO3 and Gd-WO3 4% Fig 13. Incident photo to current conversion efficiency of WO3 and Gd-WO3 4% Fig 14. Model for energy band structure of WO3 and Gd-WO3 Table 1. Series resistance (Rs) and charge transfer resistance (Rct) of WO3 and various Gd-WO3 Cover picture

24


Page 24 of 39

Page 25 of 39

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fig 1. (a) XRD pattern of the undoped and Gd-doped WO3 nanoplates, (b) partial enlarged view of (a)

25



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Fig 2. SEM images and enlarged SEM image of (a) WO3 nanoplates film and (b) Gd–WO3 4% nanoplates film, the cross-sectional images of (c) WO3 nanoplates film and (d) Gd–WO3 4% nanoplates film.

26


Page 26 of 39

Page 27 of 39

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fig 3. TEM images of undoped and Gd-doped WO3 nanoplate: (a) undoped WO3 nanoplate, (b) Gd-doped nanoplates, the corresponding SAED patterns: (c) undoped WO3 nanoplate and (d) Gd-doped nanoplates

27



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Fig 4. (a) high-resolution transmission electron microscope of the Gd-WO3 4%, (b) and (c) the lattice fringes of corresponding section.

28


Page 28 of 39

Page 29 of 39

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fig 5. XPS of WO3 and Gd- WO3 (a) full spectra; (b) W 4f; (c) O 1s and (d) Gd 3d.

29



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Fig 6. FTIR of WO3 and Gd-WO3 4% (a) after and (b) before calcination

30


Page 30 of 39

Page 31 of 39

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fig 7. UV-vis diffused reflectance spectroscopy of WO3 and Gd-WO3 4%

31



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Fig 8. Linear sweep voltammetry curves and the photoconversion efficiencies of WO3 and various Gd-WO3 under dark or illumination

32


Page 32 of 39

Page 33 of 39

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fig 9. The electrochemical impedance spectra of WO3 and various Gd-WO3 with the fitted equivalent circuit model shown in the inset.

Fig 10. Transient photocurrent response WO3 and Gd-WO3 4% at 0.9 V (vs. Ag/AgCl)

33



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Fig 11. Mott-Schottky plots of WO3 and Gd-WO3 4%

34


Page 34 of 39

Page 35 of 39

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fig 12. IMPS response of WO3 and Gd-WO3 4%

Fig 13. Incident photo to current conversion efficiency of WO3 and Gd-WO3 4%

35



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Fig 14. Model for energy band structure of WO3 and Gd-WO3

36


Page 36 of 39

Page 37 of 39

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1. Series resistance (Rs) and charge transfer resistance (Rct) of WO3 and various Gd-WO3

Rs (ohm) Rct (ohm)

WO3 26.70 270.40

Gd-WO3 1% 24.15 208.00

Gd-WO3 2% 25.16 125.30

Gd-WO3 4% 23.38 101.30

37


Gd-WO3 6% 27.85 103.40


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Cover picture

38


Page 38 of 39

Page 39 of 39

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


39


Vertical Arrays Film for Solar Water Splitting - ACS Publications

Recommend Documents