Res Chem Intermed DOI 10.1007/s11164-015-2286-9

Synthesis of PbxCr12xMoO4 oxides using microwave process and their photocatalytic activity under visible light irradiation Young In Song1 • Seong-Soo Hong1

Received: 7 June 2015 / Accepted: 15 September 2015  Springer Science+Business Media Dordrecht 2015

Abstract Lead molybdate (PbMoO4) and chromium-substituted lead molybdate (PbCr1-xMoxO4) were successfully synthesized using a microwave-assisted method and characterized by XRD, Raman spectroscopy, SEM, PL, and DRS. We also investigated the photocatalytic activity of these materials for the decomposition of rhodamine B under UV and visible light irradiation. The XRD and Raman results revealed the successful synthesis of 51–59 nm, well-crystallized PbMoO4 crystals with the microwave-assisted hydrothermal method. The DRS spectra of PbMo1-xCrxO4 catalysts showed new intensive absorption bands in the visible light region. The PbMoO4 catalysts showed the lowest photocatalytic activity and the activity was increased with an increase of chromium substitution content under visible light irradiation. The PL peaks appeared at about 540–580 nm for all catalysts and the excitonic PL signal was proportional to the photocatalytic activity for the decomposition of rhodamine B. Keywords Chromium-substituted lead molybdate (PbCr1-xMoxO4)  Microwaveassisted hydrothermal process  Photocatalytic decomposition of rhodamine B

Introduction PbMoO4 and PbWO4 materials with a scheelite structure have wide potential and practical applications in many fields such as, photoluminescence (PL), solid-state optical masers, optical fibers, scintillator materials, humidity sensor, magnetic materials, and catalysts [1, 2]. PbMoO4 and SrMoO4 crystallize in the scheelite structure, which belong to the tetragonal space group I41/a [1]. PbMoO4 has been & Seong-Soo Hong [email protected] 1

Department of Chemical Engineering, Pukyong National University, 365 Shinsun-ro, Nam-ku, Busan 608-739, Korea

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reported as a photocatalyst for the splitting of water [3]. However, PbMoO4 can only absorb UV light (k \ 387 nm), which is only 3–5 % of solar light due to its wide band gap of 3.2 eV. To maximize the use of sunlight, a new type of photocatalyst with a scheelite structure has been exploited as a highly effective photocatalysts in the visible region in recent years [4]. The doping or replacement of a foreign element into active photocatalysts with wide band gaps in order to make a donor or an acceptor level in the forbidden band is one of the ways to develop new visible-light-driven photocatalysts. There are many reports that the doped photocatalysts and semiconductor electrodes, such as TiO2 [5, 6], SrTiO3 [7] and ZnS [8], respond to visible light. In particular, the effects of doping of Cr3? ions into TiO2 and SrTiO3 on the photocurrent of semiconductor electrodes and the photocatalytic decomposition of organic compounds have been widely studied [5–8]. The band structure of PbMoO4 was previously suggested to be composed of Mo4d [conduction band (CB)] and hybridization of O2p and Pb6s [valence band (VB)] [9]. When Cr ion was substituted into PbMoO4 material, there was an isolated accepter level derived from Cr3d orbitals between the conduction band and valence band. Conventional processes of synthesis suffer many shortcomings leading to defects, such as high calcination temperature for a long time, no control of particle size and a lack of any purity phase. In order to solve these problems, the microwave-assisted method can be applied for the preparation of mixed metal oxides. Microwave heating has become popular in the crystallization of materials such as zeolites and other inorganic materials [10], due to its many benefits, including rapid synthesis, phase selectivity, narrow particle size distribution, and facile morphology control_ENREF_26 compared to conventional solvothermal or hydrothermal synthesis methods. Our previous research showed that perovskite-type oxides prepared by a microwave-assisted process exhibited relatively high photocatalytic activities for the decomposition of organic dyes [11, 12]. In this study, nanosized PbMoO4 and PbMo1-xCrxO4 particles were prepared using a microwave-assisted hydrothermal process. The synthesized materials were characterized using X-ray diffraction (XRD), diffuse reflectance spectroscopy (DRS), photoluminescence (PL), Raman spectroscopy, and scanning electron microscopy (SEM). We have also investigated the photocatalytic activity of these materials for the decomposition of rhodamine B (RhB) under visible light irradiation.

Experimental All starting materials were used as purchased without any further purification. PbMoO4 and chromium-substituted PbMoO4, PbMo1-xCrxO4, were synthesized by a microwave-assisted hydrothermal process. Pb(NO3)24H2O (Samchun, 99 %), (NH4)6Mo7O244H2O (Junsei, 99 %) and K2Cr2O7 (Junsei, 99 %) in a stoichiometric ratio were used as the starting materials without further purification. The starting materials were added into double-walled digestion vessels consisting of an inner liner and cover made up of Teflon and an outer strength vessel shell of Ultem

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Synthesis of PbxCr1-xMoO4 oxides using microwave process…

polyetherimide. Then, 20 ml of ethylene glycol was added. Under stirring, 4 mol/l of NaOH solution was used to adjust pH. Microwave-assisted synthesis was performed using a modified 900-W and 2.45-GHz home microwave digestion system (LG Electronics Co.). The irradiation time of the microwave was fixed 60 min. The resulting solid powders were collected, washed with deionized water and ethanol, and then dried at 120 C for 12 h. The crystal structures of the prepared PbMo1-xCrxO4 materials were examined by XRD with Cu-Ka radiation (Rigaku Co. Model DMax). DRS was performed on a Varian Cary 100 using polytetrafluoroethylene (PTFE) as a standard. The morphology of the samples was characterized by SEM (HITACHI S-2400, Japan). The micro-Raman (MR) spectra of the products were obtained with a Raman spectrometer (Dimension-pl-Raman, USA) with radiation of 532 nm from an argon ion laser. PL spectra were examined using a fluorescence spectrophotometer (KIMMON KOHA, Japan) with a Xe lamp (power 350 W) as the light source at room temperature. The photocatalytic reactions in the visible region were performed with a 300-W Xe-arc lamp (Oriel) and a 410-nm cut-off filter. Light was passed through a 10-cm IR water filter and then focused onto a 150-ml Pyrex container with a quartz window. The reactor was filled with 100 ml of an aqueous dispersion in which the concentrations of the photocatalyst and rhodamine B (RhB) were 30 and 20 mg/l, respectively. The temperature of the solution was controlled at room temperature. Before the light was turned on, the solution was continuously stirred in the dark for 60 min to ensure adsorption–desorption equilibrium between the photocatalyst surface and organic molecules. At the given interval time, about 3 ml of the suspension was collected, and then filtered through a 0.22-lm membrane filter to get a clear solution for collections of the spectra. The quantitative determination of RhB was performed using a UV–Vis spectrophotometer (Mecasys Optizen Pop) at k = 554 nm.

Results and discussion Figure 1 shows the XRD patterns of PbMoO4, PbMo1-xCrxO4, and PbCrO4 catalysts prepared using the microwave-assisted process. The XRD patterns of PbMo1-xCrxO4 catalysts were in good agreement with the pattern of PbMoO4 catalyst as shown in Fig. 1a. Their XRD patterns revealed that the products can be indexed as the tetragonal structure with space group I41/a, in agreement with the respective JCPDS No. 08-0475 for PbMoO4. However, the XRD peaks of PbCrO4 catalyst revealed the different pattern compared to PbMo1-xCrxO4 catalyst. The diffraction peaks shifted to a lower angle side of PbMoO4 without broadening as the value of x increased, as shown in Fig. 1b. The successive shift in the XRD pattern indicated that the obtained crystals were not mixtures of PbMoO4 (scheelite-type structure) and PbCrO4 (non-scheelite-type structure) phases but pure scheelite-type phase of chromium-substituted PbMo1-xCrxO4 catalysts. The approximate crystallite sizes of PbMO4 and PbMo1-xCrxO4 catalysts were calculated from the (112) peak of the XRD patterns with Scherrer’s equation [13].

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(a)

(b) (112)

g) g) (204)

(104) (200)

(303)

(101)

f)

Intensity (a.u.)

Intensity (a.u.)

f)

e)

d)

e) d)

c)

c)

b) b) a) a)

20

40

60

80

27.0 27.2 27.4 27.6 27.8 28.0 28.2 28.4

2 Theta (degree)

2 Theta (degree)

Fig. 1 XRD patterns of PbMo1-xCrxO4 catalysts prepared using microwave-assisted process: (a) x = 0, (b) x = 0.0025, (c) x = 0.005, (d) x = 0.01, (e) x = 0.02, (f) x = 0.05, (g) x = 1

D ¼ 0:9k=b cos h where D is the crystallite size in nm, k is the radiation wavelength (0.15405 nm for Cu-Ka), b is the corrected half-width, and h is the diffraction peak angle. The average crystallite sizes of all samples were calculated by Scherrer’s equation and are shown in Table 1. The prepared PbMo1-xCrxO4 samples by the microwaveassisted process showed similar crystallite sizes of about 52 nm regardless of chromium ratio. However, the PbMoO4 sample showed the largest crystallite size of about 59 nm. Figure 2 shows the Raman spectra in the range from 150 to 1000 cm-1 of the PbMo1-xCrxO4 catalysts prepared using microwave-assisted hydrothermal process. The PbMo1-xCrxO4 catalysts revealed the same Raman peak with the PbMoO4 catalyst reported before [14]. The Raman peak at 869.5 cm-1 was assigned to the symmetric stretching vibration mode m1 (Ag) of the [MoO4] clusters in the PbMoO4 crystal [15]. The peaks at 767.0 and 741.2 cm-1 corresponded to the anti-symmetric stretching m3 (Bg) and m3 (Eg) vibration modes, respectively. Two modes at 347.6 and 317.2 cm-1 were interpreted as the weaker m4 (Bg) and stronger m2 (Ag) of the

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Synthesis of PbxCr1-xMoO4 oxides using microwave process… Table 1 The physical properties and photocatalytic activity of PbMoO4 and PbMo1-xCrxO4 catalysts prepared using microwave-assisted process Catalyst

k0 (910-3, min-1)a

Particle size (nm)

Under UV light

Under visible light

PbMoO4

59

17.9

PbMo0.9975Cr0.0025O4

51

11.1

7.7

PbMo0.995Cr0.005O4

52

9.6

12.6

PbMo0.99Cr0.01O4

52

4.9

15.4

PbMo0.98Cr0.02O4

52

4.6

17.6

PbMo0.95Cr0.05O4

52

–

21.3

Apparent first-order constant (kapp) of photocatalytic degradation of Rhodamine B

intensity (a.u.)

a

4.3

e) d)

c) b) a) 200

400

600

800

1000

1200

-1

Raman shift (cm ) Fig. 2 Raman spectra of PbMo1-xCrxO4 catalysts prepared using microwave-assisted process: (a) x = 0.0025, (b) x = 0.005, (c) x = 0.01, (d) x = 0.02, (e) x = 0.05

regular [MoO4]2- tetrahedrons, respectively. The Ag mode at 166.5 cm-1 for PbMoO4 is much weaker than other modes. These results were consistent with other reports about PbMoO4 [15, 16]. No peaks in the XRD or Raman spectra from other impurities were detected. Therefore, it is reasonable to conclude that scheelite-type phase of chromium-substituted PbMo1-xCrxO4 crystals were successfully synthesized with the microwave-assisted hydrothermal method. The light absorption properties of the photocatalysts were examined by DRS. Figure 3 shows the DRS spectra of PbMoO4 and chromium-substituted PbMo1-xCrxO4 catalysts prepared using microwave-assisted process. As shown in Fig. 3, PbMoO4 sample displayed absorption spectrum in the UV light region. However, the spectra of PbMo1-xCrxO4 catalysts showed new intensive absorption bands in

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ABS (a.u.)

f) e) d) c) b)

a)

200

400

600

800

Wavelength (nm) Fig. 3 UV–Vis diffuse reflectance spectra of PbMo1-xCrxO4 catalysts prepared using microwaveassisted process: (a) x = 0, (b) x = 0.0025, (c) x = 0.005, (d) x = 0.01, (e) x = 0.02, (f) x = 0.05

the visible light region in addition to the band gap absorption band of PbMoO4 in the ultraviolet region. This result indicates that all the PbMo1-xCrxO4 catalysts are potential visible-light-driven photocatalysts. In addition, the absorption band moves to the higher wavelength according to the increase of substituted chromium ion content and the photocatalytic activity is also increased with an increase of substituted chromium ion content (Fig. 6). It is well known that the band structure of PbMoO4 was previously suggested to be composed of Mo4d [conduction band (CB)] and hybridization of O2p and Pb6s [valence band (VB)] [9]. In PbMo1-xCrxO4 catalysts, there was an isolated accepter level derived from Cr3d orbitals between the conduction band and valence band. It was thought that the substitution of chromium ion to keep the charge balance and to make electron-donor and electron-accepter levels in a forbidden band is one of the effective ways to develop active photocatalysts with visible light response [17]. The morphology of the PbMo1-xCrxO4 samples was examined by the SEM analysis. Figure 4 shows some selected SEM micrographs of the samples synthesized by the microwave-assisted hydrothermal method. It can be observed that the particles agglomerate when x value is 0.0025 (Fig. 4a). With the x value increasing to 0.005 and 0.01, the morphologies keep unchanged (Fig. 4b, c). When the x value increases to 0.02 and 0.05, the regular 18-facet polyhedron can be clearly observed (Fig. 4d, e). This result suggests that the substitution of chromium ion enhanced the crystallinity of scheelite-type PbMoO4 samples. It is well known that photocatalytic oxidation of organic pollutants follows Langmuir–Hinshelwood kinetics [11, 12], where the rate is proportional to the coverage h:

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Synthesis of PbxCr1-xMoO4 oxides using microwave process…

(a)

(b)

(c)

(d)

(e)

Fig. 4 SEM images of PbMo1-xCrxO4 catalysts prepared using microwave-assisted process: (a) x = 0.0025, (b) x = 0.005, (c) x = 0.01, (d) x = 0.02, (e) x = 0.05

r¼

dc KC ¼ kh ¼ k dt 1 þ KC

ð1Þ

where k is the true rate constant, which is dependent upon various parameters such as the mass of the catalyst, the flux efficiency, and the oxygen coverage. K is the adsorption coefficient of the reactant, and C is the reactant concentration. When C is very small, the product KC is negligible with respect to unity and under these conditions, Eq. (1) describes a first-order kinetic reaction. Setting the parameters in

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Eq. (1) to the initial conditions of the photocatalytic procedure, t = 0, the concentration can be given as C = C0, which results in Eq. (2).   C  ln ð2Þ ¼ kapp t C0 where kapp is the apparent first-order reaction constant. The photocatalytic activity for the decomposition of RhB over PbMoO4 and PbMo1-xCrxO4 catalysts prepared using the microwave-assisted process under UV light irradiation is shown in Fig. 5 and Table 1. When a blank test was carried out in the absence of the photocatalyst, about 2 % of the RhB was decomposed after 2 h by the photolysis reaction. As shown in Fig. 5, the PbMoO4 catalysts showed the highest photocatalytic activity and the activity was decreased with an increase of chromium substitution content. Figure 6 and Table 1 show the photocatalytic activity for the decomposition of RhB over PbMoO4 and PbMo1-xCrxO4 catalysts under visible light irradiation. As shown in Fig. 6, it was obtained the different results that the PbMoO4 catalysts showed the lowest photocatalytic activity and the activity was increased with an increase of chromium substitution content. It is well known that the photocatalytic activity is related to photo-absorption [18]. When the photocatalytic decomposition of RhB was carried out under visible light irradiation, the amount of photo absorption in the visible region plays an important role in the photocatalytic activity. As shown in Fig. 3, the absorption spectrum moved to higher wavelength according to the order of photocatalytic activity of catalysts. Especially, the pure PbMoO4 catalyst showed a very small absorption spectrum in the visible light region and a very poor photocatalytic activity. Figure 7 shows the PL spectra of PbMo1-xCrxO4 catalysts prepared using the microwave-assisted process. The PbMo1-xCrxO4 samples showed obvious excitonic

1.0

PbMoO4 PbMo0.9975 Cr0.0025 O4 PbMo0.995 Cr0.005 O4 PbMo0.99 Cr0.01 O4 PbMo0.98Cr0.02 O4

0.8

C/C o

0.6

0.4

0.2

0.0

0

20

40

60

80

100

120

Time (min) Fig. 5 Photocatalytic decomposition of rhodamine B over PbMoO4 and PbMo1-xCrxO4 catalysts prepared using microwave-assisted process under UV light irradiation

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Synthesis of PbxCr1-xMoO4 oxides using microwave process… 1.0

PbMoO4 PbMo0.9975 Cr0.0025 O4 PbMo0.995 Cr0.005 O4

0.8

PbMo0.99 Cr0.01 O4 PbMo0.98 Cr0.02 O4 PbMO0.95 Cr0.05 O4

C/C o

0.6

0.4

0.2

0.0 0

20

40

60

80

100

120

Time (min) Fig. 6 Photocatalytic decomposition of rhodamine B over PbMoO4 and PbMo1-xCrxO4 catalysts prepared using microwave-assisted process under visible light irradiation

ABS (a.u.)

e)

350

d) c) b) a)

400

450

500

550

600

Wavelength (nm) Fig. 7 PL spectra of PbMo1-xCrxO4 catalysts prepared using microwave-assisted process: (a) x = 0.0025, (b) x = 0.005, (c) x = 0.01, (d) x = 0.02, (e) x = 0.05

PL signals with similar shape regardless of the chromium substitution content. However, the excitonic PL peak moved to a higher wavelength region and the intensity of peaks increased with an increase of chromium substitution content. PbMoO4 materials usually exhibited strong and wide PL signals at the range of 500 to 600 nm with an excited wavelength of 300 nm. For the PbMo1-xCrxO4 catalysts, one obvious PL peak appeared from 540 to 580 nm. The emission spectrum of the metal molybdates can be ascribed to the charge-transfer transitions within the [MoO4] clusters [19]. The stronger the excitonic PL signal, the higher the

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content of the surface oxygen vacancies and defects. In addition, during the process of the photocatalytic reaction, oxygen vacancies and defects can become the centers to capture photo-induced electrons and thereby inhibit the recombination of photoinduced electrons and holes. Moreover, oxygen vacancies can promote the adsorption of oxygen, leading to the formation of a strong interaction between the photo-induced electrons bound by oxygen vacancies and the adsorbed oxygen. This result indicates that the binding for the photo-induced electrons of oxygen vacancies can simultaneously capture the photo-induced electrons of adsorbed oxygen and the oxygen radical group. Therefore, oxygen vacancies and defects favor photocatalytic reactions in which oxygen is active to promote the oxidation of organic substances. This suggests that the stronger the PL intensity, the larger the amount of oxygen vacancies and defects, and the higher the photocatalytic activity. As shown in Fig. 7, the photocatalytic activity on the decomposition of RhB shows the same order with the intensity of PL peaks of PbMo1-xCrxO4 catalysts.

Conclusions In this study, we investigated the photocatalytic activities of lead molybdate (PbMoO4) and chromium-substituted lead molybdate (PbCr1-xMoxO4) scheelitetype oxides prepared using the microwave-assisted hydrothermal process for the decomposition of rhodamine B. They were characterized by XRD, Raman spectroscopy, SEM, PL, and DRS. The XRD and Raman results revealed the successful synthesis of 51–59 nm, well-crystallized PbMoO4 crystals with the microwave-assisted hydrothermal method. The DRS spectra of PbMo1-xCrxO4 catalysts showed new intensive absorption bands in the visible light region in addition to the band gap absorption band of PbMoO4 in the ultraviolet region. The PbMoO4 catalysts showed the lowest photocatalytic activity and the activity was increased with an increase of chromium substitution content under visible light irradiation. The PL peaks appeared at about 540–580 nm for all catalysts and the excitonic PL signal was proportional to the photocatalytic activity for the decomposition of rhodamine B. Acknowledgments This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (No. 2011-0006722).

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