Accepted Manuscript

Effects of pH and temperature on photocatalytic activity of PbTiO3 synthesized byhydrothermal method

Yongyu Li, Haijie Sun, Ning Wang, Wenxue Fang, Zhongjun Li

PII: S1293-2558(14)00190-3

DOI: 10.1016/j.solidstatesciences.2014.08.003

Reference: SSSCIE 4985

To appear in: Solid State Sciences

Received Date: 1 April 2014

Revised Date: 31 July 2014

Accepted Date: 7 August 2014

Please cite this article as: Y. Li, H. Sun, N. Wang, W. Fang, Z. Li, Effects of pH and temperature onphotocatalytic activity of PbTiO3 synthesized by hydrothermal method, Solid State Sciences (2014), doi:10.1016/j.solidstatesciences.2014.08.003.

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Effects of pH and temperature on photocatalytic activity of

PbTiO3 synthesized by hydrothermal method

Yongyu Li1,2, Haijie Sun2, Ning Wang1, Wenxue Fang1, Zhongjun Li1

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Effects of pH and temperature on photocatalytic activity of

PbTiO3 synthesized by hydrothermal method

Yongyu Li1,2, Haijie Sun2, Ning Wang1, Wenxue Fang1, Zhongjun Li1*

1 College of Chemistry and Molecular Engineering, Zhengzhou University,

zhengzhou 450001, RP China

2 Institute of Environmental & Catalytic Engineering, Department of Chemistry,

Zhengzhou Normal University, zhengzhou 450044, RP China

To whom correspondence should be addressed.

Phone/Fax: +86-0371-67783123. E-mail: lizhongjun@zzu.edu.cn

Abstract

PbTiO3 photocatalyst was synthesized successfully by facile hydrothermal

method. The effects of the hydrothermal reaction temperatures and the pH values of

the systems on the photocatalytic activities of PbTiO3 were investigated in detail. The

photocatalytic activities of samples were evaluated by the degradation of methyl

orange (MO) aqueous solution under simulated solar irradiation. The as-obtained

PbTiO3 sample exhibits anisotropical growth along the (0 0 1) plane, and its

photocatalytic activity is about 3 times higher than that of PbTiO3 prepared by

precipitation method. Moreover, the as-prepared PbTiO3 has high stability during

photocatalytic oxidation process, and does not cause secondary pollution.

Keywords: PbTiO3; photocatalysis; hydrothermal

1. Introduction

PbTiO3, a perovskite oxide, is an important material for its piezoelectricity,

ferroelectricity, and colossal magnetoresistivity, which makes it highly attractive for

fundamental research and various technical applications [1-4]. Recently,

perovskite-type oxide materials based on transition metals with d(0) and d(10)

electron configuration such as Nb(V), Ti(IV) and In( ) were reported as efficient

photocatalysts for overall water splitting with high quantum yields [5].

Pb2+-containing transition-metal oxides were found to be able to absorb visible light

due to their smaller bandgap sizes [6]. PbTiO3, a d(0) and d(10) metal oxide, can be

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used as an efficient visible-light photocatalyst for overall water splitting [7].

To date, PbTiO3 was prepared successfully by high reaction temperature methods

such as solid-state synthesis [8-10] and molten-salt flux techniques [7], which are

usually characterized by high cost, large particle sizes and low surface areas of the

products. It is believed that nanoscale materials perform higher photocatalytic

efficiency than their bulk counterparts due to the larger surface area and the faster

arrival to the reaction sites of the photogenerated electrons and holes. So developing

effective synthetic routes for PbTiO3 with nano-scale at low temperature is

significative to improve its photocatalytic performance. Compared with other methods,

hydrothermal synthesis is advantageous because the particle size and morphology can

be effective modulated, which also enables the growth of PbTiO3 with perovskite

structure far below the transition temperature [11]. Therefore, a facile hydrothermal

method carried out at a relatively low temperature was selected to demonstrate the

possibility of preparing PbTiO3 with nanostructures and to improve its photocatalytic

activity. The photocatalytic performance of products were evaluated by decomposing

methyl orange (MO) under simulated solar irradiation at room temperature. The

effects of hydrothermal temperatures and the suspension pH values on the

photocatalytic activities of products were investigated.

2. Experimental

2.1. Sample preparation

Tetrabutyl titanate ((C4H9O)4Ti) and lead nitrate Pb(NO3)2 were used as the

starting materials. All the reagents were of AR grade and were used without further

purification. In a typical synthesis, Pb(NO3)2 was dissolved in deionized water to form

solution (1 mol L−1), the pH value of the solution was adjusted by 3 mol L−1 NaOH

solution under vigorous stirring. The mixture was then stirred for 0.5 h in an ice bath.

The ethanol solution of (C4H9O)4Ti (1 mol L−1) was added dropwise. After being

further stirred for 1 h, the resulting precursor precipitate was transferred into a

Teflon-lined autoclave (100 mL) and heated at different temperature for 24 h under

autogenously pressure, and then air cooled to room temperature. The resulting

precipitates were filtrated, washed with ethanol and deionized water thoroughly and

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dried at 80 in air. To investigate the effects, the pH values were set at 12.95, 13.10,

13.25 and 13.40, and the hydrothermal temperatures were set at 80 , 120 , 160 ,

200 and 220 , respectively.

For comparison, PbTiO3 was also synthesized by precipitation method as

following: Pb(NO3)2 and Ti(OC4H9)4 as the raw materials, ammonia was used as the

precipitating agent to adjust the pH value to be 10.0. After stirred for 1 h, the

precursor precipitates were filtrated, washed and dried at 80 , and then calcinated at

500 for 2 h to obtain products.

2.2. Characterization

The X-ray diffraction (XRD) patterns of the samples were measured on an

X’Pert PRO X-ray powder diffractometer with Cu-Kα radiation (λ = 1.5418 A˚). The

microstructure of the sample was investigated by transmission electron microscopy

(TEM, JEOL JEM-2100). UV–vis diffuse reflectance spectrum (DRS) was obtained

using a UV–vis spectrometer (Shimadzu U-3010) by using BaSO4 as a reference.

Nitrogen adsorption–desorption isotherms were collected on a NOVA 1000e surface

area and porosity analyzer (Quantachrome, USA) at 77K after the samples had been

degassed in the flow of N2 at 150 for 1.5 h. The BET surface area was estimated

using desorption data.

2.3. Photocatalytic activity test

The photocatalytic performance of the as-prepared samples were characterized

by decomposing methyl orange (MO) under simulated solar irradiation at room

temperature. A 500 W Xe illuminator was used as an internal light source. The

photodegradation experiments were carried out with the samples (500 mg) suspended

in MO aqueous solution (500 mL, 10 mg L−1) with constant stirring. Prior to the

irradiation, the suspensions were magnetically stirred in the dark for 1 h to establish

the adsorption/desorption equilibrium. At the given time intervals, about 5mL of the

suspension was taken for analysis after centrifugation. The concentration of MO was

detected by measuring the absorption intensity at a wavelength of 464 nm. The

absorption intensity was converted to the MO concentration referring to a standard

curve which showed a linear relationship between the concentration and the

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absorption intensity.

3. Results and discussion

3.1. Characterization of PbTiO3

Fig.1a presents the XRD patterns of PbTiO3 samples at the pH of 13.25 with

different hydrothermal temperatures. As shown in the figure, the products obtained at

80 and 120 are amorphous, and diffraction peaks of PbTiO3 could not be

detected. When the temperature was increased to 160 , a series of sharp and clear

diffraction peaks appear, which could be assigned to tetragonal phase PbTiO3 (JCPDS

01-78-0298). No other impurities could be detected. As the hydrothermal temperature

was further increased to 220 , the diffraction peaks intensity of sample was

increased gradually, indicating that the crystallinities of samples improve. According

to the Scherrer formula, the crystallite sizes of PbTiO3 samples obtained at 160 ,

200 and 220 , calculated based on the data of (101) plane, are 53.2 nm, 67.6

nm and 78.7 nm, respectively.

Fig.1b shows the XRD patterns of PbTiO3 obtained at hydrothermal temperature

200 °C with different pH values. It can be seen that the crystallinity of products is low

at the pH value of 12.95. When the pH values are greater than 13, the diffraction

peaks match well with tetragonal phase PbTiO3 (JCPDS 01-78-0298), and no peaks of

any other phases or impurities were detected. The (001) peak intensity of samples

obtained at the pH of 13.25 was increased suddenly, even stronger than (101) peak

intensity, which may be due to anisotropic growth along the (001) facets [12].

According to the Scherrer formula, the sample crystallite sizes calculated based on

(101) plane are 51.3 nm, 67.6 nm and 67.7 nm when the pH value are 13.10, 13.25

and 13.40, respectively. It is obvious that the crystallite sizes of samples were

increased with increasing pH values.

Fig. 2 shows the TEM image and high-resolution TEM (HRTEM) image of the

PbTiO3 prepared at the pH of 13.25 and the hydrothermal temperature of 200 °C. The

as-prepared sample displays elliptical in shape, and its particle sizes could be

estimated to be about 250 – 400 nm, which is much larger than its crystallite size

(67.6 nm) obtained from the XRD data. The results indicate that there are multiple

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domain in the particles of the sample and the PbTiO3 particles observed by TEM are

mainly polycrystalline [13].

To illustrate the anisotropic growth along (001) plane of the as-prepared PbTiO3,

the HRTEM image and the standard XRD pattern of PbTiO3 (JCPDS 01-78-0298)

from powder diffraction file database are provided (Fig. 2b and Fig. 2c). In the

standard pattern (Fig. 2c), the intensity of the (0 0 1) peak is much weaker than that of

the (1 0 1) peak, which could be expressed as I (1 1 0) / I (1 0 2) = 0.25. In our case,

the intensity of the (0 0 1) peak is even beyond that of (1 0 2) peak, the value of I (0 0

1) / I (1 0 1) was increased to 1.59. The related HRTEM image of the as-prepared

sample (Fig. 2b) displays the clearly resolved crystalline domain, an uniform

interplanar spacing of 0.415 nm correspond well to the (001) plane of PbTiO3.

Furthermore, the angle labeled in the corresponding fast-Fourier transform (FFT)

pattern (inset in Fig. 2b) is 46.8◦, which is identical to the theoretical value obtains for

the angle between the (0 0 1) and (1 0 1) planes. The results prove that the PbTiO3

sample anisotropically grew along the (0 0 1) plane.

UV-Vis diffuse reflectance spectrum of the as-prepared PbTiO3 samples was

analyzed to investigate the optical absorption performance. As shown in Fig. 3, the

absorption edge of the PbTiO3 samples synthesized at the hydrothermal temperature

of 200 °C and the pH of 13.25 extends to the visible light region, ranging from 200

nm to 450 nm. The steep shape of the spectrum indicates that the visible light

absorption is ascribed not to the transition from the impurity level but to the band-gap

transition [14]. Its band gap could be derived using the equation αhν = A(hν - Eg)n/2,

where α, A, hν and Eg signify the absorption coefficient, proportionality constant,

photon energy and band gap energy, respectively [15]. Considering PbTiO3 to be an

indirect semiconductor (n = 4 is adopted), the plot of (αhν)1/2 versus hν affords the

bandgap of PbTiO3 as shown in the inset of Fig. 3, and then evaluate the band gap Eg

by extrapolating the tangent line to the hν axis intercept. The estimated band gap

energy of the resulting samples is about 2.75 eV, which is consistent with previous

reports [6,7].

The BET surface areas play a great role on the photocatalytical property of

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samples. The specific surface areas of samples were investigated by using nitrogen

adsorption and desorption isotherms. The surface areas of PbTiO3 prepared at the

hydrothermal temperature of 200 °C are 3.33, 2.56 and 2.52 m2 g-1 at the pH values of

13.10, 13.25 and 13.40, respectively. The BET surface areas were decreased gradually

with increasing pH value. However, the specific surface areas of samples prepared at

pH values of 13.25 and 13.40 have little difference, which is consistent with the

change of their crystallite sizes. The effects of the hydrothermal temperatures were

also investigated, the surface area of PbTiO3 prepared at pH value of 13.25 are 2.06,

2.56 and 1.74 m2 g-1 at 160, 200 and 220 °C, respectively. The specific surface areas

of samples were firstly increased and then decreased. When the temperature was

increased from 200 to 220 , the specific surface area were reduced, indicating

that the crystalline degree was increased with increasing temperature, and accordingly

specific surface area was decreased.

3.2. Photocatalytic performance

Fig. 4 shows the photocatalytic degradation of MO over PbTiO3 samples

prepared at different conditions. It can be seen from Fig. 4a that the photolysis of

methyl orange was very slow, and only about 3% was degraded after 180 min of

illumination. The degradation of MO with as-prepared PbTiO3 samples in the dark

condition gives similar results to the photolysis test, indicating that the adsorption of

MO on the PbTiO3 samples was limited after the adsorption–desorption equilibrium

was reached. However, about 91% of MO was decolored after 180 min of

illumination over the PbTiO3 samples, indicating that the product exhibits relatively

high photocatalytic activity. Although there might be photolysis of MO, the results of

above comparison signify that MO degradation in the present study was through a

photocatalytic process.

Fig. 4a presents the photocatalytic degradation of MO over PbTiO3 samples

prepared at the pH of 13.25 with different temperatures. It is obvious that the

hydrothermal temperatures had a great impact on photocatalytic properties of

as-prepared samples. As the hydrothermal temperature was increased from 120 to

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200 , the photocatalytic activities of products were increased gradually. It could be

ascribed to the increase of the photocatalyst crystallinity degree with increasing

temperature, which resulted in the reduction of the electron - hole recombination

centers. However, when the hydrothermal temperature was increased to 220 , the

photocatalytic activity of PbTiO3 was reduced inversely. It is due to the increase of

particle sizes with increasing temperature, which led to the decrease of specific

surface area and the active sites, so the photocatalytic activity of product was

decreased.

Fig. 4b shows the photocatalytic degradation of MO over PbTiO3 samples

prepared at 200 with different pH values. When the pH value was increased from

12.95 to 13.25, the photocatalytic activities of samples were increased, which can be

attributed to the improvement of the catalyst crystallinity according to the XRD

results. As the pH was further increased to 13.40, the photocatalytic activities of

products were decreased, which may be related to the decrease of the (001) peak

intensity, and it needs to be further studied. Generally, the optimal condition for the

preparation of PbTiO3 is the hydrothermal temperature of 200 and the reaction pH

value of 13.25.

For comparison, the photocatalytic activity of PbTiO3 prepared by precipitation

method was also tested. After 180 min of irradiation, the degradation efficiency of

MO was only 31%, which is much less than that of PbTiO3 prepared by hydrothermal

method under the same conditions.

The stability and reusability of catalyst is an important indicator for its practical

application. To investigate its stability, the photodegradation cycle experiments of

methyl orange over PbTiO3 samples were conducted, and the experimental results are

shown in Fig. 5. The experimental conditions are accordance with the

photodegradation experiment. It shows that the degradation efficiency of methyl

orange over PbTiO3 is still up 73% after five photocatalytic cycles. The photocatalytic

activity was not reduced considerably, indicating that the PbTiO3 sample has a high

stability during the photodegradation process of methyl orange. It is very important in

the application of the photocatalyst [16]. However, the photocatalytic activity of

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PbTiO3 was decreased to a certain degree through the recycling. This may be ascribed

to the presence of residual intermediates on the catalyst surface which reduced the

surface activity centers, and then made the photocatalytic activity of the catalyst

reduced [17]. Additionally, the XRD patterns of PbTiO3 samples after the cycle

experiments was analyzed and shown in Fig. 6. It reveals that the phase and structure

remained intact, implying that the as-prepared PbTiO3 has a high stability during

photocatalytic oxidation of model pollutant.

The research of PbTiO3 as photocatalyst on the degradation of organic matter is

scarce, and one of the reasons is that it may cause secondary pollution. So the

dissolubility of PbTiO3 in the photocatalytic process was investigated by inductively

coupled plasma atomic emission spectrometry (ICP-AES). The experimental process

is similar to the photocatalytic process. The light illumination time was set to 15 h,

and a sample was taken to analysis the amount of Pb2+ every 3h. After 1h dark

adsorption, the concentration of Pb2+ dissolved in solution was reached to 0.139

mg·L−1, and the concentration of Pb2+ was increased to 0.212 mg·L−1 after 3 h of

irradiation. But the concentration of Pb2+ was not increased much after 12 h of

irradiation, the final cumulative concentration of Pb2+ was 0.226 mg·L−1. The total

lead concentration dissolved in solution after 15 h of irradiation is lower than the limit

of 0.5 mg·L−1, which meets the National Emission Standards of China (GB

25466-2010). Therefore, PbTiO3 does not cause secondary pollution when it is

applied to wastewater treatment as a photocatalyst.

4. Conclusions

In summary, PbTiO3 photocatalyst was synthesized successfully by a facial

hydrothermal treatment. The hydrothermal reaction temperatures and the pH values of

systems have great influence on the photocatalytic activities of PbTiO3, which were

evaluated by the degradation of MO under simulated solar irradiation. According to

the photodegradation efficiencies, the optimal condition is the hydrothermal reaction

temperature of 200 and the pH value of 13.25. The PbTiO3 prepared at the

condition could degrade about 91% of MO after 180 min of illumination. In addition,

the sample is highly stable during photocatalytic oxidation process of model pollutant,

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and does not cause secondary pollution.

Acknowledgements

This work was supported by National Natural Science Foundation of China (NO.

21001096 and U1304204) and Foundation of He’nan Educational Committee

(NO.13A150348).

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Figure captions:

Fig. 1. XRD patterns of the PbTiO3 products prepared at different temperatures (a)

and different pH values (b).

Fig. 2. TEM image (a) and high-resolution TEM image (b) of the PbTiO3 prepared at

pH = 13.25 and hydrothermal temperature 200 °C, the inset of (b) is the

corresponding fast-Fourier transform (FFT) pattern. (c) XRD pattern of the PbTiO3

and standard XRD pattern of PbTiO3 (JCPDS no. 01-078-0298).

Fig. 3. The diffuse reflection spectrum of the PbTiO3 prepared at pH = 13.25 and

hydrothermal temperature 200 °C.

Fig. 4. Photocatalytic degradation of MO over PbTiO3 samples prepared at pH =

13.25 with different temperatures (a) and PbTiO3 samples prepared at 200 with

different pH values (b).

Fig. 5. Cycle degradation experiments of methyl orange over PbTiO3 samples.

Fig. 6. XRD patterns of PbTiO3 samples after the photocatalytic cycle experiments.

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Fig. 1.

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Fig. 2.

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Fig. 3.

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Fig. 4.

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Fig. 5.

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Fig. 6.

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Research highlights:

Effects of pH and temperature on photocatalytic activity of PbTiO3

synthesized by hydrothermal method

Yongyu Li1,2, Haijie Sun2, Ning Wang1, Wenxue Fang1, Zhongjun Li1*

◆PbTiO3 photocatalyst was synthesized by hydrothermal method.

◆The photocatalytic activity of PbTiO3 is about 3 times higher than that of the

PbTiO3 prepared by precipitation method.

◆The effects of the temperatures and the pH values of systems on the photocatalytic

activities of PbTiO3 were investigated.