Correlation between Multiple Growth Stages and Photocatalysis of

Article pubs.acs.org/JPCC

Correlation between Multiple Growth Stages and Photocatalysis of SrTiO3 Nanocrystals Hongquan Zhan,†,‡,§ Zhi-Gang Chen,*,§ Jianle Zhuang,† Xianfeng Yang,† Qili Wu,† Xiangping Jiang,‡ Chaolun Liang,† Mingmei Wu,*,† and Jin Zou*,§,∥ †

MOE Key Laboratory of Bioinorganic and Synthetic Chemistry/State Key Laboratory of Optoelectronic Materials and Technologies/Key Laboratory of Environment and Energy Chemistry of Guangdong Higher Education Institutes, School of Chemistry and Chemical Engineering, Instrumental Analysis and Research Center, Sun Yat-Sen (Zhongshan) University, Guangzhou 510275, P. R. China ‡ Department of Material Science and Engineering, Jingdezhen Ceramic Institute, Jingdezhen 333001, P. R. China § Materials Engineering and ∥Centre for Microscopy and Microanalysis, The University of Queensland, St. Lucia, QLD 4072, Australia ABSTRACT: The relationship between growth kinetic and photocatalysis of SrTiO3 nanoparticles is investigated by close correlation of their growth behaviors under the different stages. The detailed structural characterizations show that SrTiO3 crystal growth does not follow the classic route. A new growth mechanism, including the first formation of SrTiO3 mesoporous sphere followed by the single crystal growth via oriented attachment, size shrinking, and Ostwald ripening, is proposed. The photocatalytic ability of the as-grown SrTiO3 products at different growth stages, checked by the degradation of methyl orange, shows different properties. The results indicate that the size, morphology, and defects of the resultant SrTiO3 products, tailored during the different growth stages, were highly responsible for the photocatalytic activity. Especially, the semicrystalline SrTiO3 mesoporous spheres produced by oriented attachment at the initial stage show the highest photocatalytic ability. describe the growth of STO. Calderone et al.29 synthesized single crystal cubic STO particles with a controlled size and shape formed via OA. Moreira et al.30 also discovered the nucleation and crystal growth processes of STO via OA with a microwave-assisted hydrothermal method. Although these studies proposed the formation mechanism of STO nanoparticle using OA, the relationship between the property and growth mechanism is still not clear. The Ostwald ripening (OR) mechanism, a classical growth pathway which involves the growth of larger particles at the expense of smaller ones,31−33 often coexists with OA during nanosynthesis. Both mechanisms may operate simultaneously with the dominant pathway changing and in turn affect the crystal growth process.10 However, few works about the property−structure relationship directed by OA and OR have been reported, although such growth mechanisms were used to realize the crystal growth of BaTiO3 and CaTiO3.34,35 As an isostructural compound of CaTiO3 and BaTiO3, STO may have a similar crystal growth behavior, which deserves fundamental understanding. Especially, the detailed explanation of the correlation

1. INTRODUCTION Strontium titanate (SrTiO3, STO) is an important material for applications in catalysis1−3 and electronics.4,5 Detailed crystal growth procedure and morphological control have significant influences on its physicochemical properties because of the structure-dependent property which can be tailored through controlling growth parameters and procedure.6−10 For example, Souza et al.11 reported that the photoluminescence of STO could be modified through controlling the growth steps. Recently, a variety of STO nanostructures, such as nanosheets,12 mesocrystals,13,14 mesoporous structures,15−18 hollow spheres,19 superstructures,20,21 and hybrid structures,22−25 have been widely studied. Their structures have been well addressed; however, the correlation between their structures, properties, and growth mechanism(s) has not been clearly revealed. In-depth understanding of crystal growth mechanisms provides guidance for the structure- and property-control during the STO synthesis. Several mechanisms have been proposed to understand the growth of STO nanocrystals. The in situ transformation and dissolution−precipitation mechanism are well accepted to understand the growth of STO nanocrystals from titania sources in hydrothermal conditions.26,27 A new mechanism, oriented attachment (OA), in which larger crystals formed through crystallographically assembling smaller nanocrystals,6,28 was recently used to © XXXX American Chemical Society

Received: December 14, 2014 Revised: January 29, 2015

A

DOI: 10.1021/jp512448p J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

source was positioned inside a cylindrical Pyrex vessel. This vessel was surrounded by a recirculating water jacket (Pyrex) to effectively preclude the IR part of the light and cool the lamp. Prior to irradiation, the suspension of the photocatalyst in MO aqueous solution was stirred in the dark for 30 min to secure an adsorption/desorption equilibrium. At the given irradiation time intervals, 3.0 mL of reaction suspension was sampled and separated by centrifugation. The absorption spectra of the centrifuged reaction solution were measured on a 721 spectrophotometer (Shanghai Yuefeng CO. Ltd.). The concentration of MO was determined by monitoring the change in the absorbance at 464 nm.

between the structure−photocatalysis relationships under different growth mechanism has not been elucidated. In this study, we present the crystal growth stage of perovskite-type STO from Sr(NO3)2 and tetrabutyl titanate in aqueous solution of poly(ethylene glycol)-200 (PEG-200) and propose a new multistep crystallization process by monitoring crystal size change and microstructure evolution. The growth kinetics, crystalline structure, and relevant photocatalysis of STO nanoparticles during the growth are investigated, and the property−structure relationship is determined.

2. EXPERIMENTAL SECTION Synthesis. Tetrabutyl titanate [titanium n-butoxide, Ti(OC4H9)4, TNB] with a concentration of 0.088 M (analytical grade) was slowly added to PEG-200, and 223 mg of Sr(NO3)2 (analytical grade) was dissolved in 3.2 mL of double-distilled water that was preboiled for 15 min to remove any dissolved CO2. The TNB/PEG solution was then slowly added into the aqueous solution of Sr(NO3)2 under stirring. The Sr:Ti ratio of the mixture was fixed as 1.2 in order to ensure a complete conversion of the titanium precursor into STO. A NaOH solution was in turn added to keep the batch at a designated pH value of ca. 11. The suspension was finally transferred into a 20 mL Teflon-lined autoclave for the consequence hydrothermal reaction. The hydrothermal reaction was conducted at 140 °C in an oven for different durations, ranging from 30 min to 25 days. After the reaction, the autoclave was removed from the oven and quickly cooled down to ambient temperature. Soluble impurities were removed by repeatedly washing the synthesized products with hot double-distilled water. The products were recovered via filtration and washed extensively with distilled water, dilute acetic acid, and finally with ethanol in order to remove any adsorbed impurities, such as the excess strontium components. The final synthesized products were ultimately placed into a desiccator after overnight drying at 60 °C. Characterization. The structural characteristic of assynthesized products was performed using X-ray diffraction (XRD) on a Rigaku D/MAX 2200 VPC diffractometer, operating at 40 kV and 20 mA, with steps of 0.02° at 10° min−1 in a 2θ range from 20° to 80°. Scanning electron microscopy (SEM, FEI Quanta 400 Thermal FE environment SEM) and transmission electron microscopy (TEM, JEM2010HR electron microscope equipped with a Gatan GIF system) were used to determine the morphological, structural, and chemical characteristics of synthesized produces. The Brunauer−Emmett−Teller (BET) nitrogen physisorption experiments were carried out on a Micromeritics ASAP 2010 system. The pore size distributions of synthesized produces were determined by using the Barrett−Joyner−Halenda (BJH) algorithm according to the desorption data of the N2 isotherms. For PL measurements, using a Renishaw micro-Raman model inVia Reflex spectrograph and the excitation wavelength of 325 nm, the spectrum range was extended to 400−650 nm. The UV−vis absorption was recorded using Lamda850 spectrophotometer in total reflection mode by the integration sphere in the region of 250−700 nm. Photocatalytic Activity Evaluation. The photocatalytic activity for the degradation of methyl orange (MO) was performed in a Pyrex reactor in which 200 mg of the photocatalysts was suspended in 200 mL of MO aqueous solution (10 mg/L). A 250 W mercury lamp (Beijing Perfect CO. Ltd., the wavelength within the range of 365 nm) as a light

3. RESULTS AND DISCUSSION Figure 1a is XRD pattern of products synthesized at different stages. As can be seen, no diffraction peaks can be observed in

Figure 1. (a) XRD patterns of the amorphous and STO products synthesized at 140 °C with different reaction durations. (b) Calculated crystallite sizes as a function of reaction duration, in which three stages are seen. The inset is an enlarged plot for the first stage.

the products synthesized at room temperature (RT) for 5.0 h, suggesting that the sample is amorphous. Weak diffraction peaks appeared after a reaction of 0.5 h, indicating the initiation of the crystalline phase. With increasing the reaction duration, the intensities of the diffraction peak increase, and they can be B


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The Journal of Physical Chemistry C exclusively indexed to the cubic structured STO with a = 3.905 Å and a space group of Pm3m (JCPDS card no. 35-0734).30 The increased intensity and sharpness of the diffraction peaks indicate a continuous increase of the crystallinity and crystal sizes. The sizes of synthesized products can be calculated using the Scherrer equation from the full width at half-maximum (fwhm) of 110* diffraction peaks.36 The calculated crystallite sizes are plotted in Figure 1b as a function of the reaction duration. As can be seen, the curve is different from the previous descriptions,26,29,30 suggesting that the growth behavior of our synthesized products may experience an alternative route. According to the calculated crystallite size, the entire process can be divided into three stages (A−B, B−C, and C−D), which will be discussed later. To understand the morphological and structural characteristics of synthesized products, SEM and TEM investigations were performed. Figures 2a and 2b are typical SEM and TEM

HRTEM image taken from a typical colloidal particle and shows that its majority is still amorphous with a few crystallite nanodomains (with a size of ca. 5 nm, similar to the value deduced from the XRD data) separately developed inside the particle. It is of interest to note that these nanodomains have similar crystal orientation, suggesting that they have already undergone continuous rotation, interaction, and self-adjusted their orientations. Such a similar phenomenon of crystallization has also been observed during the crystal growth of BaTiO3.34 In the initial stage, the STO formation reaction burst and produced high supersaturation. The primary amorphous particles of STO quickly transformed to aggregate by mesoscale assembly under PEG. The surfactant played an important role in both the controlling hydrolysis of Ti(OC4H9)4 and the selfassembly processes.37 By complexing the ions, PEG can block or retard the growth path of single ions, making self-assembly effects more significant than the classical crystallization route. With the reaction going on, the nanocrystallines emerged and attached together. To understand the growth processing of STO, the products synthesized at different growth stages (A−B, B−C, and C−D), such as 2, 24, 72, and 600 h, are investigated in detail by TEM and SAED. Figures 3a1 and 3a2 are typical TEM and HRTEM images of the products synthesized for 2 h, and the insets are their corresponding SAED patterns. As can be seen, the particles have a rough surface with a single crystalline characteristic (refer to the inset SAED), and a pore structure can be clearly seen, which suggests that small nanoparticles loosely attached together with the growth going on. Extending the hydrothermal reaction duration to 24 habout the end of the first stage (refer to Figures 3b1 and 3b2)the nanocrystals attach more compactly and tend to join together to form a single crystalline, as demonstrated in the highly crystalline SAED patterns and HRTEM image. With increasing the reaction duration to 72 h, Figure 3c1 shows that a close cubic morphology started to appear in the particle, in which the surface of the particle becomes glazed and the size is shrunk. In this process, the nanoparticles are rearranged, fused, and associated with shrinkage according to the Frenkel shrink mechanism.38 Figure 3c2 is a HRTEM image and shows the lattice fringes at the surface are completely continuous and the original 5 nm primary particles are almost being consumed completely at this stage. When reaction time reaches to 600 h, Figure 3d1 shows that the crystal has completely become the perfect cubic shape following the classic morphology. The inset SAED pattern and HRTEM (refer to Figure 3d2) show that the colloidal particles have become single crystals. As shown in Figure 3, it is of interest to note that the intensities of the diffraction spots on the SAED patterns increase notably, and the SAED pattern shows a single-crystal feature even when the colloidal particles are truly aggregates of smaller crystals at the beginning from 0.5 to 24 h. Moreover, the diffraction spots in the SAED pattern become stronger and the single-crystal areas become larger, although a large number of defects can be detected in the HRTEM at the beginning. The pores are filled with further crystallization in the synthesized products with prolonging the reaction duration, eventually to form a cubic crystal after the reaction duration reaches to 600 h, in which the lattice defects in particles are reduced substantially, via the OR mechanism. As can been seen in Figures 3b2 and 3c2, the crystallinity between cores and surfaces of STO600 is different. The surface region of the particle has become single-crystalline, while the core is still in a state of aggregated nanocrystallites

Figure 2. (a) SEM, (b) TEM, and (c) HRTEM images of products prepared at stage A (synthesized at 140 °C for 0.5 h). The inset in (b) is the SAED pattern taken from the particles marked by the white circle in (b). The HRTEM image shown in (c) is taken from white rectangle inside (b). The inset in (c) is the corresponding FFT pattern.

images taken from products synthesized at stage A (synthesized around 0.5 h), in which spherical shaped colloidal particles with a size of ca. 50 nm can be seen. The inset of Figure 2b is the selected area electron diffraction (SAED) pattern and shows weak diffraction spots, suggesting the poor semicrystallization of these colloidal particles. Figure 2c is a corresponding C


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Figure 4. Nitrogen adsorption/desorption isotherms and BJH pore size distribution plots (inset) of the products synthesized at 140 °C with different reaction durations.

from 0.5 to 24 h (Table 1). These results show that the semicrystalline STO mesoporous spheres produced at the Table 1. BET of Products Synthesized at 140 °C with Different Reaction Durations sample

RT5.0h

0.5h

2.0h

24.0h

BET(m2/g)

148.23

220.36

193.49

119.45

initial stage possess not only the largest surface area and smaller particle size but also a mesoporous structure. Taking all investigations mentioned above into account, the three stages (A−B, B−C, and C−D), reflecting the kinetics of STO process, can be explained with different growth mechanisms. As can be seen from the inset in Figure 1b, the first stage (A−B) shows that the crystal grows rapidly up to ca. 30 nm and then tends to be much slower. From the HRTEM images, it can be seen that the first stage of the STO crystal growth from 0.5 to 24 h was dominated by an oriented aggregation process. For such a growth process, the kinetics can be described by39

Figure 3. TEM (1) and HRTEM (2) images of the typical individual colloidal particles synthesized at 140 °C for different reaction durations: (a) 2 h, (b) 24 h, (c) 72 h, (d) 600 h. Insets in a1, b1, c1, and d1 are the corresponding SAED patterns. The edge area of each particle marked by a rectangle was further investigated by HRTEM, as shown in a2, b2, c2, and d2. Insets in a2, b2, c2, and d2 are the corresponding FFT patterns, obtained from their individual HRTEM images.

d = d0

1 + mk1t 1 + k1t

(1)

where d is the average crystallite size at the time t, d0 is the initial average crystallite size at the starting point, m is defined as the aggregation factor which represents the degree of crystal combination, and k1 represents the crystal growth rate constant. From XRD and HRTEM image, d0 is ca. 5 nm at point A. The final average crystallite size dB is ca. 30 nm at point B; therefore, m = dB/d0 = 6. The first stage of crystal growth can be fitted well to eq 1. The curve of the crystallite size change shown in the inset of Figure 1b remarkably matches the experiment results, indicating the availability of eq 1, which can actually address crystal growth with an OA process. The deduced parameters are summarized in the first column of Table 2. As shown in Figure 1b, the crystallite size decreases at the second stage (from B to C), implying a possible shrinkage of particles during the growth process. Such a reduction may be attributed to the shrinkage of particles when they fused together from OA to OR and reduce the porosity produced

with some defects. The mass density of the core is much lower than that in the shell. Such phenomena may support a reversed crystal growth route from the surface to the core, which is similar to the case of CaTiO3.35 To evaluate the porous structure of the synthesized STO, the isothermal curve and BJH pore-size distribution are used, and the results are shown in Figure 4. Both of them reveal the mesoporous characteristics of the nanoparticles. As can be seen, the STO synthesized for 0.5 h exhibited the narrow mesopore size distribution centered at 4.5 nm. The sorption data also indicated the systematic decrease of BET surface area from 220.36 to 119.45 m2/g with increasing the reaction duration D


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orientated aggregation of STO nanocrystallites, (2) size shrinking and filling of the pores by further crystallization, and (3) perfection of crystals via OR. To understand the defect state of the synthesized STO, the photoluminescence (PL) spectra are investigated, and the result is shown in Figure 5a. As can be seen, three bands, including a blue luminescence band peaked at ca. 455 nm, a green luminescence band peaked at ca. 530 nm, and a green-yellow luminescence band peaked at ca. 565 nm are observed. It is well-known that PL properties are attributed to the imperfections degree of the crystal structure,41,42 and difference bands represent different types of structure defects, such as dislocation or atomic vacancies.8,11 The blue emission can be ascribed to shallow effects and the green-yellow emissions to deep effects.43 According to the PL results shown in Figure 5a, products synthesized for 0.5 h show the highest peak intensity. Subsequently, with increasing the synthesis duration, PL intensity decreases significantly although the band gap energy did not change obviously. The decrease is fast at the initial stage and become slow at the later stage. The decrease in PL emission is believed to be related to the higher density of charge carriers or defects resulting from OA in the products synthesized in the shorter duration. With increasing the synthesis duration, the defects concentration decreases, and consequently, the number of charge carriers (vacancies and electrons trapped) also decreases, resulting in reducing the PL emission of the synthesized products.8,11 As a consequence, we conclude that our PL behaviors are directly related to the crystal growth process. For shortened synthesis durations, the crystals are small and have more defects, which contribute to the increase of the PL emission. When the synthesis duration is increased, the nanoparticles grow perfectly and the reduced defects decrease the PL emission.

Table 2. Simulated Data for the Three Stages of Crystal Growth of STO Nanoparticles at 140 °C first stage

second stage −1

third stage −1

d0 (nm)

k1 (h )

dB (nm)

k2 (h )

dC (nm)

k3 (h−1/3)

n

5.24

1.16

31.56

0.089

28.37

0.65

2.56

during OA. These phenomena have been reported in BaTiO3,34 TiO2,39 and Pt nanocrystals.40 A kinetic model can then be simulated as follows:39 d = dB − k 2(t − t B)

(2)

where tB is beginning time of the second stage (ca. 24 h at 140 °C) and dB is the initial crystallite size at the beginning of the second stage (ca. 30 nm). The obtained values are listed in the middle column of Table 2. The datum of k2 is obtained to be 0.089. Such a small value of k2 suggests a slow change of the crystallite size with the reaction duration. In the third stage, the size increase is reinitiated, but the increase rate is much slower than that at the first stage, which can be simulated by the kinetic equation of OR:39 d = k 3(t − tC)1/ n + dC

(3)

where k3 is a temperature-dependent reaction rate constant, n is an exponent, and dC is the crystal size at starting time point of tC; herein tC = 72 h at 140 °C for point C in Figure 1b. Table 2 and Figure 1b show that the fitted data match well with the experimental results. According to OR mechanism, n ≈ 3 suggests that the kinetics of the third stage are controlled by the volume diffusion of ions in the matrix.39 On the basis of the above structure changes and kinetic analysis, the growth of STO exhibits three distinct stages: (1)

Figure 5. (a) PL spectra, (b) UV−vis diffuse reflectance spectra, and (c) photocatalytic activity of STO samples synthesized at 140 °C for different reaction durations. (d) Comparison of photocatalytic activities of the typical STO samples. E


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from inside particles to their surfaces easy because the photogenerated holes may easily find their way through the defects to travel to the interface and react with the organic molecules.15,42 Therefore, the defects present in our STO samples (such as structural distortions described by PL and HRTEM) also play a significant role in enhancing the photocatalytic activity. Therefore, this mesoporous structure is believed to have better light absorption, pollutant absorption, and carrier separation, which result in an improving performance. On the basis of the above results and discussion, the photocatalytic activity of our STO nanoparticles is found to directly relate to the growth process. To illustrate their relationship, we combined the growth kinetics and structural evolution with the catalytic ability in one scheme, as shown in Figure 6, and can be explained as follows. At initial growth

To understand the band absorption of the samples from different growth stage, the UV−vis diffuse reflectance spectra are used, as shown in Figure 5b. The shape and absorption peak of the diffuse reflectance are slightly different at the process of crystal growth. Two absorption bands at a peak ca. 295 nm and a shoulder ca. 350 nm are similar to the absorption characteristics of standard STO.1 In the initial stage, the absorption peak ca. 295 nm showed a blue-shift with respect to the spectra of the products grown for longer duration from 72 to 600 h, at 300, 306, and 309 nm for products grown for 72, 240, and 600 h, respectively. This is probably attributed to the fact that the crystal sizes at the initial stage are smaller. The two absorption bands tend to be sharper with the growth procedure, indicating more crystalline samples tend to grow large. In the later stage from 72 to 600 h, the absorption bands around 300 nm show an obvious red-shift while increasing the crystallite size. The corresponding band gap of STO are described in Table 3, which became narrow with the synthesis Table 3. Indirect Band Gap of STO Nanoparticle at 140 °C for Different Times sample

24 h

72 h

240 h

600 h

Eg (eV)

3.63

3.44

3.43

3.37

duration. (The values of band gap were obtained by the Wood and Tauc method.11) These results show that the light absorption of STO nanoparticles is significantly enhanced with the synthesis going on at the later stage. The photocatalytic degradation of MO in aqueous solution under ultraviolet light irradiation (λ = 360 nm) is depicted in Figures 5c and 5d. As can be seen, STO nanoparticles at different stages exhibit obvious different photocatalytic activities: The amorphous sample shows a significant ability to degrade MO, and the other samples with different crystallization show the variation of degradation ability. In the first stage (0.5−24 h), the degradation ability to MO decreases with increasing the reaction duration, showing an opposite trend with crystallite size evolution. The photocatalytic activity of the STO samples grown in the early stage reduces rapidly with prolonging the growth duration, while reduces slowly in the later period. In the second stage (24−72 h), the photocatalytic activity of samples shows a trace difference with different growth durations. In the third stage (72−600 h), the degradation of MO tends to be accelerated steadily with increasing the growth duration. Interestingly, the semicrystalline sample of 0.5 h shows the better photocatalytic ability than the completed-crystalline or amorphous. To understand this reason, we note that the photocatalytic process under irradiation depends strongly upon two important factors: (i) absorption of light followed by separation of the e/h pairs and (ii) adsorption of organic pollutants.44 The semicrystalline STO sample synthesized at 0.5 h possess not only the largest surface area and smaller crystallite size but also a specific pore structure (refer to Figures 2 and 4 and Table 1), all facilitating the increase in adsorption capacity. With the smaller size of the crystalline, the generated e−/h+ pairs may easily migrate to the particle surfaces before undergoing charge-carrier recombinations inside particles. Moreover, the special mesoporous sphere structure of STO nanoparticles enables the efficient charge separation through interparticle charge transfer.15−18 The defects are considered to facilitate the separation of the e−/h+ pairs that make the transfer

Figure 6. Schematic illustration showing the correlation between the evolution of crystal growth and photocatalysis and kinetics of STO samples in the presence of PEG.

stage, the STO nanoparicles have the enhanced photocatalytic activity due to their smaller crystallite size, porous structure, large BET, and effective defects. With the crystal growth continuing, these structural advantages gradually decreased, and the photocatalytic activity was also quickly weakened. In the later stage, the photocatalytic performance increased slowly due to the red-shift of the absorption bands of increased-size STO particles.

4. CONCLUSIONS The crystal growth of STO nanocrystals has been recognized to be nonclassical crystal growth, followed by a self-assembly process with an OA mechanism which results in mesoporous sphere and a slow crystal growth related to classical OR which results in cubic-like crystal. This overall crystal growth can further be divided into three stages with different kinetic behaviors and different crystallite size or morphology. Three mathematic equations were used to address the three-stage crystal growth, which match perfectly with the experimental data. The photocatalytic activity of the synthesized STO nanoparticles at the different stages was investigated. The highest photocatalytic activity found in the semicrystalline samples was due to the combined effect of the crystallite size, specific surface area, porous structure, and lattice defects. This study provides a pathway to produce high-performance photocatalysis. F


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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Z.-G.C.). *E-mail: [email protected] (M.M.W.). *E-mail: [email protected] (J.Z.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This work was financially supported by NSFC (21271190, 51262009, U1301242), Research Fund for the Doctoral Program of Higher Education of China (RFDP) (No. 20130171130001), the Government of Guangdong Province for NSF (S2012020011113) and industry (2012B09000026), the State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University (2012-01), Australian Research Council, and the visiting scholar program of the young teacher development plan in the university of Jiangxi Province.

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Correlation between Multiple Growth Stages and Photocatalysis of

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