Monoclinic WO3 Phase Junction for

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K2SO4-Assisted hexagonal/monoclinic WO3 phase junction for efficient photocatalytic degradation of RhB Yao Lu, Jing Zhang, Fangfang Wang, Xuebing Chen, Zhaochi Feng, and Can Li ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00168 • Publication Date (Web): 01 May 2018 Downloaded from http://pubs.acs.org on May 2, 2018

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ACS Applied Energy Materials

K2SO4-Assisted Hexagonal/Monoclinic WO3 Phase Junction for Efficient Photocatalytic Degradation of RhB Yao Lu a, Jing Zhang a *, Fangfang Wang a, Xuebing Chen a, Zhaochi Feng b, Can Li b a

College of Chemistry, Chemical Engineering and Environmental Engineering, Liaoning Shihua

University, Fushun 113001, Liaoning, China b

State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of

Sciences, Dalian, Liaoning 116023, China *To whom correspondence should be addressed. E-mail: [email protected] Tel.: +86-24-56863390

ACS Paragon Plus Environment

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ACS Applied Energy Materials 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

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Abstract

Fabrication of phase junction in the photocatalyst is one of the efficient strategies for the enhancement of the photocatalytic activity. However, researches on the relation between phase composition and photocatalytic property of WO3 are limited because the barely controllable phase transition process from monoclinic to hexagonal phase. A facile sol-gel synthesis of composition tunable hexagonal/monoclinic-WO3 (h/m-WO3) phase junction with K2SO4 as stabilizing agent is developed. X-ray powder diffraction (XRD), Scanning Electron Microscopy (SEM), UV-Raman, High-resolution Transmission Electron Microscopy (HR-TEM), and UV-vis diffusion reflectance spectroscopy (DRS) are employed to investigate the structures, morphologies, crystalline phases, phase composition, and optical properties of the as-prepared samples. Contents of hexagonal phase in the WO3 samples can be precisely adjusted in a wide range from 0 to 71.1 wt% by regulating K2SO4 amount, calcination temperature, and calcination time. Degradation of rhodamine B (RhB) of samples indicates that reaction rate depends significantly on contents of hexagonal/monoclinic phase in the WO3 samples. A 7.4 times enhancement in the reaction rate is observed for the h/m-WO3 sample with 71.1 wt% h-WO3 than the pure m-WO3. The increased photocatalytic activity is attributed to the formation of phase junction between h-WO3 and m-WO3, which exhibits high efficiency of the separation and transfer of photoexcited electron-hole, as evidence from electrochemical impedance spectra (EIS) results. This work provides a new insight into the fabrication of a phase composition designable h/m-WO3 phase junction with high photocatalytic performance, which benefits the application of WO3 in the environmental protection issues.

Keywords: Tungsten oxide; Phase junction; Sol-gel method; Photocatalysis; Rhodamine B


2

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1．．Introduction Tungsten oxides and tungsten bronzes have attracted much attention during the last few decades owing to their promising photocatalytic properties for the degradation of organic pollutants

1-2

and for O2 evolution

3-5

. However, short lifetimes of photo-induced electron-hole

pairs resulting from fast recombine of the charge carriers in WO3 deteriorate the photocatalytic performance. To solve this problem, a variety of strategies have been performed to improve the charge carriers separation efficienty of WO3, for example, via suitable morphology design doping

18-19

, and defect engineering

20

15-17

,

. Ye et al. 15 found that hierarchical hollow WO3 shells

exhibited much higher photocatalytic acrivity because it possessed larger surface areas, higher light transmission, and much more active sites. Zhang et al.

20

indicated that a proper

concentration of oxygen vacancies introduced in WO3 hierarchical nanostructures by air treatment at certain temperatures could trap and transfer electrons, thus decreasing electro-hole recombination rate and improving the conductivity, so that greatly improved photocatalytic activity. Additionally, forming a semiconductor heterojunction has been proved an effective method to promote the separation of electron-hole pairs and thus the photocatalytic activity of photocatalyst 21-23, 6-14. Since our findings on outstanding photocatalytic property of anatase/rutile TiO2 24, a new concept of phase junctions has been demonstrated to be an effective strategy for promoting charge separation in photocatalysis. The phase junction in Ga2O3 25, CdS 26, Bi2O3 27-28 has been proven to improve the separation efficiency and then boost the photocatalytic activity. WO3 exists mainly in five polymorphs that are monoclinic, triclinic, orthorhombic, and tetragonal along with a hexagonal phase

29

. Monoclinic (m-WO3) and hexagonal (h-WO3) are the

commonly phases used in photocatalysis, and h-WO3 transformed into m-WO3 with increasing


3


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the temperature because m-WO3 is more stable than h-WO3. However, an interesting phase transition, i.e. m-WO3 → h-WO3 was observed in our previous work30. Moreover, the hexagonal/monoclinic WO3 phase junction (h/m-WO3) was fabricated by controlling m-WO3→ h-WO3 transformation. An about 2.2 times enhancement in the photodegradation rate for RhB is observed for the h/m-WO3 sample than the pure m-WO3. It is well known that the multi-phase mixture ratio plays an important role in the photocatalytic performance for the phase junction photocatalysts

25-26, 31-32

. Thus, the

investigations for the phase tuning in the h/m-WO3 phase junctions and the effect of phase composition in the h/m-WO3 phase junctions on its photocatalytic performance are prerequisites for designing and construction of advanced WO3 photocatalysts. However, in our work mentioned above30, a problem is that the h-WO3 content in h/m-WO3 phase junction, which is fabricated by controlling m-WO3→ h-WO3 transformation, can only be tuned from 0 to 21 wt%. This is mainly because the structure of h-WO3 formed from monoclinic is difficult to maintain 33 and it easily transformed back into m-WO3 with increasing the calcination temperature

30, 34

.

Moreover, no relevant studies have focused on the m-WO3 → h-WO3 phase transformation process. It has been shown recently that the structure of h-WO3 cannot be maintained without some stabilizing ions or molecules in the hexagonal channels 33, 35-48, which means that the hexagonal channels of h-WO3 are not completely empty. Thus, the strictly stoichiometric h-WO3 does not exist, while it is just an idealized compound, and h-WO3 sample is in fact hexagonal tungsten bronzes with very low residual ion or molecule content. In this work, Na+ and K+ were selected as stabilizing ions to stabilize the structure of h-WO3 in order to tune the phase composition in h/m-WO3 phase junction fabricated by controlling m-


4

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WO3 → h-WO3 transition. The stabilization effect of Na+ and K+ for h-WO3 and the mechanism of maintaining the structure of h-WO3 were investigated based on the XRD, UV Raman spectroscopy, SEM, and XPS observation. The control of the stabilizing ions in hexagonal channels of h-WO3 is supposed to be the key point for maintaining the structure of h-WO3, and thus for the phase tuning in h/m-WO3 phase junction. As compared with Na+, most K+ exists in the hexagonal channel to maintain the structure of h-WO3. Based on these above results, we successfully developed a method to tune the phase composition of WO3 in wide range by phase transformation from m-WO3 to h-WO3, in which K2SO4 as stabilizing agent. Furthermore, the photocatalytic properties of the WO3 with different crystalline phases were studied. It is found that the formation of phase junction with suitable phase composition is important for the photoinduced electron-hole separation, which verified by electrochemical impedance spectroscopy (EIS) measurements. It should be noted that the photocatalytic activity of the h/m-WO3 sample with 28.9 wt% m-WO3 (71.1 wt% h-WO3) has improved 7.3 times than that of pure m-WO3. To the best of our knowledge, this is the first report on a controllable synthesis of phase junction in WO3, and discussion the relation between the composition of phase junction and photocatalytic properties. 2. Materials and Methods 2.1 Materials Ammonium tungstate hydrate (APT), potassium sulfate (K2SO4), citric acid monohydrate (C6H8O7·H2O, CA), ethylene glycol (C2H6O2, EG), and rhodamine B (RhB, 99%), were purchased from Sinopharm Chemical Reagent Co. Ltd. All the above chemicals were of analytical grade and directly used without further purification. 2.2 Preparation of K2SO4 (or Na2SO4) modified WO3 sample and WO3 sample


5


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Using sol-gel method 49, 10 g of APT powder and a certain amount of K2SO4 (1 wt%, for example) were dispersed into 190 mL of distilled water under continuous stirring at 90 °C. After 4.1440 g of CA was added, the suspension liquid above turn into clarified solution. 10 mL of EG was added to the above solution followed by concentrating at 90 oC to form wet gel. Finally, the K2SO4 modified WO3 sample Dry gel (was labeled as mK-WO3-Dry gel, where m represents the mass percentages of K/W (m=0.5, 1, 1.5, 2 wt%)) was obtained by drying wet gel in oven at 120 o

C for 24 h, and followed by a calcination process at 350 oC for 1.5 h. After pestle, this mK-

WO3-Dry gel was calcined at 700-900 °C for 2-54 h in air. The obtained samples were labeled as mK-WO3-T-t, where T and t represent the calcination temperature (°C) and calcination time (h) used to obtain the samples, respectively. For comparison, WO3 Dry gel sample (was labeled as WO3-Dry gel) was prepared via same procedure without adding K2SO4. After WO3-Dry gel was calcined at 400~900 °C for 4 h in air, the sample was denoted as WO3-T-4 h, where T represents the calcination temperature (°C). 1% Na2SO4 modified WO3 sample Dry gel (was labeled as 1%Na-WO3-Dry gel) was also prepared via same procedure with adding K2SO4 in this study. 1%Na-WO3-T-t sample was obtained by calcinating 1%Na-WO3-Dry gel from 700 oC to 900 oC for 4 h. 2.3 Characterization In order to analyze the formation process of WO3, thermogravimetry and differential thermal analysis (TG–DTA) was performed using a TG/DTA-6300 thermal analyzer at the rate of 10 °C·min-1 in an air flow of 60 mL·min−1. The crystalline phase and phase composition of the as-prepared sample were characterized by X-ray diffraction (XRD) on a Rigaku MiniFlex diffractometer with a Cu Kα radiation source (λ=1.5418 Å) at a scan rate of 2o·min-1 in the 2θ range from 10 to 40 o. The X-ray photoelectron spectroscopy (XPS) investigations were


6

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performed by Thermofisher ESCALAB 250Xi using monochromated Al Ka as the excitation source. The morphologies of the samples were investigated using scanning electron microscopy (SEM, Quanta 200 F) and high-resolution transmission electron microscopy (HETRM, JEOL, JEM– 2100). The phase composition on the surface of the sample was measured by UV Raman spectra at room temperature with Jobin-Yvon T64000 triple-stage spectrograph with spectral resolution of 2 cm-1. The laser line at 325 nm of a He-Cd laser was used as an exciting source. UV-vis diffuse reflectance spectra (UV-vis DRS) were collected on a JASCOV-550 UV-vis spectrophotometer in the range of 200 to 800 nm. The BET surface area was determined by N2 adsorption–desorption isotherms using a NOVA 4200e surface area. Electrochemical Impedance Spectroscopy (EIS) measurements were performed with a VSP300 electrochemical workstation. The spectra were collected using the conventional threeelectrode system consisting of a modified fluorine doped tin oxide (FTO) coated glass as the working electrode, saturated calomel electrode (SCE) as the reference electrode, and platinum wire as the counter electrodes. The EIS measurements were proceeded between 100 kHz and 0.1 Hz at open circuit voltage in a 0.1 M KCl solution with 5 mM Fe(CN)63-/Fe(CN)64- with an amplitude of 10 mV. The modified FTO working electrode was prepared by a simple method as follows: 10 mg of each photocatalyst sample was dispersed in a solution containing 0.4 mL of ultrapure water, 0.6 mL ethylene glycol (EG) and 50 µL 5% Nafion solution to make a homogeneous suspension. Then 100 µL of the slurry was dripped on FTO glass (dimensions: 1 cm × 1 cm), and dried the modified FTO working electrode in an oven at 60 oC for 8 hours prior to the measurement 50. 2.4 Photocatalytic reaction


7


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A set of photocatalytic degradation experiments were performed with the following procedure: photodegradation RhB was carried out in a 250 ml Pyrex reactor filled with de-ionized water (60 ml) containing RhB (10 mg/L) and the WO3 samples. The suspension was stirred for 30 min in the dark to obtain adsorption–desorption equilibrium of the dye before illumination. After irradiation with a 300 W Xenon lamp with 420nm filter (PLS-SXE300UV, Beijing perfectlight technology co. LTD), a 4 ml aliquot was taken every 30 min and immediately centrifuged. The RhB concentration in the clear solution was analyzed by optical characteristic absorption (UVmini-1240 spectrophotometer, SHIMADZU Dentschland Gmbh) at a wavelength of 553 nm for a RhB solution. The blank experiments without illumination and without catalysts in the RhB solution under the same conditions are also compared in this study. The blank study shows that mere photolysis can be ignored for RhB. The degree of mineralization was monitored through analysis of the total organic carbon (TOC) content in the irradiated solution using a SHIMADZU TOC- VSCH analyzer. 3. Results and Discussion Fig. S1 shows the TG-DTA curves of WO3-Dry gel, which can be separated into four weight loss steps, two endothermic and two exothermic heat effects during decomposition in air. These results show that the WO3-Dry gel decomposition takes place during four distinguished steps, which are attributed to dehydration, decomposition of CA and EG, and NH3 evolution 49. The weight loss of the WO3-Dry gel is remarkable until 610 oC, which indicates removal of impurities. From the XRD results of WO3-T-4 h samples (as shown in Fig. S2), the unidentified impurities are found in the WO3-600 oC-4 h. This result is consistent with the data of TG-DTA. Therefore, the calcination temperature for WO3-Dry gel, mK-WO3-Dry gel, and mNa-WO3-Dry gel samples is selected from 700 oC to 900 oC.


8

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K2SO4 and Na2SO4 were used as stabilizing agents for the h-WO3, respectively to investigate the effect of K+ or Na+ on the phase composition of WO3. A series of experiments were performed by controlling the calcination temperature from 700 oC to 900 oC with a K/W or Na/W mass percentage of 1 wt%. XRD results and the corresponding crystalline phase compositions of 1%K-WO3-T-4 h and 1%Na-WO3-T-4 h were shown in Fig. 1. For comparison, the XRD patterns of pure WO3 sample were added in Fig. 1. When the WO3 sample was calcined at 700 ºC, some strong peaks at 2θ= 23.1°, 23.6°, 24.4°, and 34.2°, which is the typical monoclinic WO3 peaks were observed from Fig. 1. Additionally, two weak peaks at 2θ =14.0° and 28.2°ascribed to hexagonal WO3 were also observed. The Xray diffraction peaks of monoclinic and hexagonal phase (marked with “▼” and “•”) agree well with the information in JCPDS card NO. 83-0950 and 75-2187, respectively and no other unidentified peaks are observed. These results indicate that both monoclinic (m-WO3) and hexagonal (h-WO3) phases coexist in the WO3-700 oC-4 h samples, while m-WO3 is the main crystalline phase. XRD patterns of WO3-800 oC-4 h and WO3-900 oC-4 h almost unchanged in addition to the intensity of h-WO3 peaks slightly increase with increasing the calcination temperature. This implies high temperature promotes the formation of the h-WO3 from m-WO3, which is similar as the prvious result from Martínez-de la Cruz 51. It is obvious that both monoclinic (m-WO3) and hexagonal (h-WO3) phases coexist in the 1%K-WO3-700 oC-4 h sample (Fig. 1). The intensities of typical peaks of h-WO3 increase greatly and h-WO3 became the main crystalline phase with increasing the temperature to 800 oC. The evolution of the typical peaks of h-WO3 was due to the transformation from m-WO3 to hWO3. However, the intensities of typical peaks of h-WO3 decrease in the 1%K-WO3-900 oC-4 h sample because h-WO3 transformed back to m-WO3, which is agreement with the results from


9


our previous results 30. Additionally, no potassium oxide-related diffraction peaks were observed in all the 1%K-WO3-T-4 h samples. As seen from Fig. 1, all the 1%Na-WO3-T-4 h samples feature both m-WO3 with small amounts of h-WO3. However, impurity identified as Na2W4O13 can be detected from Fig. 1 in all of the 1%Na-WO3-T-4 samples with Na2SO4 as stabilizing agents. (a)

●:h-WO3 ▼:m-WO3 ○:Na2W4O13

(b) o

▼ 1%Na-WO3-900 C-4 h ▼ ▼ ● ▼▼ 1%K-WO3-900 oC-4 h

●

●:h-WO3 ▼:m-WO3 ○:Na2W4O13 ● ●○

WO3-900 oC-4 h 1%Na-WO3-800 oC-4 h 1%K-WO3-800 oC-4 h WO3-800 oC-4 h

○

○

○

1%Na-WO3-700 oC-4 h

▼▼

/5

Intensity (a.u.)

Intensity (a.u.)

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 10 of 39

○ /20

○ /2

1%K-WO3-700 oC-4 h

/5

WO3-700 oC-4 h

10

15

20

25

30

35

40

13

14

15

2-Theta (degree)

27

28

29

30

2-Theta (degree)

Fig. 1 (a) XRD and (b) magnified patterns of WO3-T-4 h, 1%K-WO3-T-4 h, and 1%Na-WO3-T-4 h samples. ●= h-WO3, ▼= m-WO3 crystal structure, ○=Na2W4O13 The weight ratio (wt%) of m-WO3 and h-WO3 phases (Wm, Wh) in the above WO3 samples were calculated by RIR method 52-55, based on the XRD patterns, which can be estimated using the following formula: =

(1)

⁄ ⁄

=

⁄ ⁄

= 1 −

(2)

where Im and Ih are the intensities of the strongest lines of the X-ray diffraction patterns of mWO3 and h-WO3, respectively. The RIR values of m-WO3 (RIRmon= 5.66, JCPDS NO. 83-0950)


10

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and h-WO3 (RIRhexa= 8.33, JCPDS NO. 75-2187) phases were obtained by fitting XRD results using the MDI JADE software. 100

WO3-T-4 h 1%K-WO3-T-4 h 1%Na-WO3-T-4 h

80 h-WO3 Content (%)

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


60 40 20 0 700

800

900 o

Calcination Temperature ( C)

Fig. 2 The h-WO3 content of WO3-T-4 h, 1%K-WO3-T-4 h, and 1%Na-WO3-T-4 h samples. Fig. 2 compares the h-WO3 content in the WO3-T-4 h, 1%K-WO3-T-4 h, and 1%Na-WO3-T-4 h samples. As shown in Fig. 2, the content of h-WO3 varied between 2.0 and 9.3 wt% in the WO3-T-4 h samples when the calcination temperature is increased from 700 oC to 900 oC. It can be seen that it is difficult to regulate the phase content of h-WO3 in WO3-T-4 h samples without stabilizing ions. The contents of h-WO3 in 1%Na-WO3-T-4 h samples were only tuned within the relatively narrow range of 2.0-31.4 wt%. It is interesting to note that the h-WO3 content can be tuned between 2.0 and 71.1 wt% in the 1%K-WO3-T-4 h samples. Thus, it can be concluded that K+ has a significant advantage in stabilizing h-WO3, and the content of h-WO3 was easily tuned in the K+ modified WO3 as compared with pure WO3 samples and 1%Na-WO3-T-4 h samples.


11


(a)

1%K-WO3-800 oC-4 h

K 2p

(b)

Na 1s

K 2p3/2 295.82 eV

296

Na 1s 1071.35 eV

294

1074

292

1071

o O 1s 1%Na-WO3-800 C-4 h 530.24 eV

(d) W 4f

W 4f5/2 37.66 eV

O 1s

1%K-WO3-800 oC-4 h

O 1s

WO3

532

530

528

Binding Energy (eV)

526

Intensity (a.u.)

O 1s 531.31 eV

534

1068

1065

Binding Energy (eV)

Binding Energy (eV) (c) O 1s

1%Na-WO3-800 oC-4 h

Intensity (a.u.)

Intensity (a.u.)

K 2p3/2 293.03 eV

Intensity (a.u.)

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 12 of 39

1%Na-WO3-800 oC-4 h W 4f7/2 35.53 eV

1%K-WO3-800 oC-4 h

W 4f

WO3

W 4f

39

38

37

36

35

34

33

Binding Energy (eV)

Fig. 3 XPS spectra of (a) K 2p in 1%K-WO3-800 oC-4 h, (b) Na 1s in 1%Na-WO3-800 oC-4 h, and the comparison of (c) O 1s and (d) W 4f in the 1%K-WO3-800 oC-4 h, 1%Na-WO3-800 oC-4 h, and WO3 samples. In order to finger out the stabilizing mechanism for h-WO3 of two ions (K+, Na+), XPS was performed to examine the composition of the samples by measuring the binding energy spectra of K 2p, W 4f , and O 1s electrons in 1%K-WO3-800 oC-4 h, Na 1s, W 4f, and O1s electrons in


12

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1%Na-WO3-800 oC-4 h. Fig. 3 shows the XPS spectra of K 2p peak in 1%K-WO3-800 oC-4 h, Na 1s in the 1%Na-WO3-800 oC-4 h, and the comparison of O 1s and W 4f in the above two samples. The XPS spectra of O 1s and W 4f in pure WO3 sample were added in Fig 3 for comparison. As shown in Fig.3a, the fitted peaks with a binding energy at 293.0 and 295.8 eV are assigned to K+ combined with O2- of WO3 56-57. Similarly, the observation of a Na+ 1s peak at 1071.3 eV (Fig.3b) results from Na+ combined with O2- of WO3 58. For the fitting spectrum of O 1s (Fig. 3c), two fitting peaks of O 1s at 530.2 and 531.3 eV, which are in the same position in the three samples, can be assigned to crystal lattice oxygen O2of WO3 and adsorbed H2O or hydroxyl groups on the surface of WO330, 59, respectively. As shown in Fig. 3d, two peaks at 35.5 and 37.7 eV, are attributed to the spin-orbit splitting W 4f7/2 and W 4f

5/2

of WO3 30, 60 respectively are observed in these three samples. It can be observed

that K+ and Na+ ions do not change the peak positions of the O 1s and W 4f in WO3. Therefore, we indicated that the doped K+ or Na+ show no obvious effect on the chemical surrounding of WO3. Besides of that, no shift of the diffraction peaks of 1%K-WO3-T-4 h and 1%Na-WO3-T-4 h can be seen from XRD results (Fig. 1). These results show that the existence of K+ or Na+ stabilizing ions does not distort the structure of WO3. Table 1 Comparison of the atomic number ratios of K or Na and W for 1%K-WO3-800 oC-4 h and 1%Na-WO3-800 oC-4 h sample calculated from the XPS results and real experimental conditions.

Samples

Atomic ratios (%) measured by XPS

Defined atomic ratios (%)

K/W

Na/W

K/W

Na/W

1%K-WO3-800 oC-4 h

2.6

-

4.7

-

1%Na-WO3-800 oC-4 h

-

17.4

-

8.0


13


Page 14 of 39

Furthermore, the surface molar ratios of K/W in 1%K-WO3-800 oC-4 h and Na/W in 1%NaWO3-800 oC-4 h samples were estimated and displayed in Table 1, based on XPS results, which is working as an efficient surface analysis technique to measure element content within a few angstrom depth beneath the sample surface. As shown in Table 1, content of K+ on the surface of 1%K-WO3-800 oC-4 h is much lower than defined value, while Na+ content on the surface of 1%Na-WO3-800 oC-4 h is much higher than defined value. These results indicate that most of K+ may distribute in the bulk of WO3 while most of Na+ may distribute on the surface of WO3.

Fig. 4 Scheme for the proposed mechanism of the position of (a) K+ and (b) Na+ in the structure of hexagonal-WO3. We attributed the main reason of the above results to the different atomic diameters between K+ (0.133 nm 61) and Na+ (0.098 nm 61). In h-WO3, because of the special arrangement of cornersharing WO6 octahedral, hexagonal channels (dia.=0.2321nm) are formed along the structure and


14

Page 15 of 39

hexagonal channels are surrounded by trigonal ones (dia.=0.0767nm) (Fig. 4). According to Li’s result 62, stabilizing ions (NH4+ or Na+) for hexagonal phase WO3 synthesized by hydrothermal method is thought to be located in hexagonal channels. K+ or Na+ ions are supposed to mainly locate in hexagonal channels in h-WO3 structures. Na+ with smaller diameter may diffuse more easily from hexagonal channels to adjacent distorted trigonal channels (Fig. 4), because of its reduced binding energy during calcination process (Table S1), and then transfer to the surface of WO3 when the temperature goes up, resulting in the collapse of hexagonal structure and then transformed from hexagonal phase into monoclinic phase. However, for the K+ ions with larger diameter, it tends to locate in hexagonal channels to maintain h-WO3, until the energy is higher to induce the structure distortion enough through the increase of calcination temperature or calcination time. Thus, K+ is proved to be the optimum stabilizing ion for h-WO3, as compared with Na+. ●:h-WO3 ▼:m-WO3

(b) 100

◇:impurity tungsten bronze o 2%K-WO3-800 C-4 h ▼ ◇

●

▼▼◇ ◇▼

●

◇

● ▼ ▼ o

1.5%K-WO3-800 C-4 h

o

1%K-WO3-800 C-4 h

o

Content of h-WO3 (%)

(a)

Intensity (a.u.)

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


80 60 40 20

0.5%K-WO3-800 C-4 h

10

15

20

25

30

35

4014 15

0

0.5

2-Theta (degree)

1.0

1.5

2.0

Concentration of K2SO4 (%)

Fig. 5 (a) XRD patterns and (b) crystalline phase compositions of mK-WO3-800 oC-4 h samples. ●= h-WO3, ▼= m-WO3 crystal structure, ◇=impurity tungsten bronze.


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We further investigated the effect of the amount of K2SO4 on the crystalline phase and phase composition of WO3. Fig. 5a and 5b shows the XRD patterns and phase compositions of mKWO3-800 oC-4 h samples, respectively. As shown in Fig. 5a, all the mK-WO3-800 oC-4 h samples are characterized as mixture of monoclinic and hexagonal phases. The content of hexagonal phase in the mK-WO3-800 oC-4 h samples (m=0.5, 1, 1.5, 2 wt%) is 29.0, 71.1, 68.3 and 51.6 wt%, respectively. It can be seen that the content of hexagonal phases in mK-WO3-800 o

C-4 h samples firstly increase and then decrease with increasing the concentration of K2SO4,

and reach a maximum with the K2SO4 amount of 1 and 1.5 wt%. However, a peak at 2θ=14.2°, which can be denoted as impurity tungsten bronze was observed when the amount of the K2SO4 is more than 1 wt%, and the peak intensity increases with increasing of the K2SO4 amount. Thus, 1 wt% of K2SO4 was selected as the optimal amount to tune the amount of hexagonal or monoclinic phase in the WO3.

o

1%K-WO3-800 C-51 h

(b) 100 ●

o

1%K-WO3-800 C-48 h

●

●

o

1%K-WO3-800 C-36 h o

1%K-WO3-800 C-24 h o

1%K-WO3-800 C-4 h o

◇

10

15

1%K-WO3-800 C-2 h

20

25

30

35

◇

4014 15

80 Content of h-WO3 (%)

(a)

●:h-WO3 ▼:m-WO3 ◇:impurity tungsten bronze o ▼ ▼▼1%K-WO3-800▼ C-54 h ▼

Intensity (a.u.)

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 16 of 39

60 40 20 0 2

2-Theta (degree)

4

24

36

48

51

54

Calcination Time (h)

Fig. 6 (a) XRD pattern and (b) crystalline phase compositions of 1%K-WO3-800 oC-t ranging from 2 to 54 h.


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Calcination-time-dependent experiments were performed in the time range from 2 to 54 h with temperature of 800 oC and a K2SO4 amount of 1 wt%, so as to tune the phase composition in the 1%K-WO3-800 oC-t samples. As shown in Fig. 6a and 6b, mixed phases of h-WO3 and m-WO3 are formed after calcination time of 2 h. The diffraction peaks of h-WO3 become predominant and the content of h-WO3 is 71.1wt% and 59.5 wt% with increasing the calcination time to 4 h and 24 h. After calcination for 36 h, the diffraction peaks of h-WO3 gradually diminish in intensity. When the calcination time was longer than 54 h, h-WO3 completely transformed into m-WO3 and only the m-WO3 was observed from Fig. 6a. Thus, a controllable synthesis of WO3 with tuned crystalline phase can be performed by tuning the calcination time range from 4 to 54 h. Raman spectroscopy is an sensitive method for detecting phase changes 63, thus UV Raman spectra was conducted for the 1%K-WO3-800 oC-4 h and 1%K-WO3-800 oC-54 h samples (Fig. S3). It can be seen that the Raman bands of h-WO3 at 108, 253, 320, 645, 690 cm-1 64 and typical Raman bands of m-WO3 at 134, 273, 715, 807 cm-1 64 coexist in the 1%K-WO3-800 oC-4 h sample, which indicates the formation of the mixed phases. There is only monoclinic phase of WO3 in the 1%K-WO3-800 oC-54 h sample. These results are consistent with the results from XRD. Moreover, no Raman bands corresponding to oxides of potassium are observed, suggesting that there was no appreciable chemical reaction between the K ion and WO3. Morphology of the 1%K-WO3-800 oC-t sample was characterized by SEM measurement (shown in Fig. 7). From Fig. 7, all the 1%K-WO3-800 oC-t (t< 54 h) sample with mixed phases exhibit obviously irregular microstructures with bulk shape and rod-like shape particles, while 1%K-WO3-800 oC-54 h shows only bulk shape microstructures. Moreover, obvious aggregation phenomena are observed in all the samples.


17


Page 18 of 39

Fig. 7 SEM images of 1%K-WO3-800 oC-t samples (a) 2 h, (b) 4 h, (c) 24 h, (d) 36 h, (e) 48 h, (f) 54 h.

Fig. 8 Reaction rate constant of photocatalytic degradation of RhB (Kapp) under visible light irradiation and crystal compositions of 1%K-WO3-800 oC-t samples.


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To investigate the effect of the crystalline phase and phase composition of WO3 on its photocatalytic activity, the photocatalytic activities of 1%K-WO3-800 oC-t samples with different contents of monoclinic/hexagonal were evaluated under visible light irradiation. Fig. S4 and 8 presents the curves of RhB concentration changes with irradiation time on 1%K-WO3-800 o

C-t samples. Only 27.8% of RhB is degraded by 1%K-WO3-54 h with pure monoclinic phase

(Fig. S4). Clearly, greatly enhanced in photocatalytic efficiency or rate constant are observed for 1%K-WO3-800 oC-t (t

Monoclinic WO3 Phase Junction for

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