Preparation and photocatalytic activity of SnO2

Preparation and photocatalytic activity of SnO2

Author’s Accepted Manuscript Preparation and photocatalytic activity of SnO2 Yu Zhiyong, Qiu Ruiying, Li Huanrong, Wang Zhiyin, Ma Xiaohong, Dong Chao...

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Author’s Accepted Manuscript Preparation and photocatalytic activity of SnO2 Yu Zhiyong, Qiu Ruiying, Li Huanrong, Wang Zhiyin, Ma Xiaohong, Dong Chaonan

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S0167-577X(15)31054-5 http://dx.doi.org/10.1016/j.matlet.2015.12.100 MLBLUE20060

To appear in: Materials Letters Received date: 18 November 2015 Revised date: 22 December 2015 Accepted date: 23 December 2015 Cite this article as: Yu Zhiyong, Qiu Ruiying, Li Huanrong, Wang Zhiyin, Ma Xiaohong and Dong Chaonan, Preparation and photocatalytic activity of SnO2 Materials Letters, http://dx.doi.org/10.1016/j.matlet.2015.12.100 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Preparation and photocatalytic activity of SnO2 Yu Zhiyong a*, Qiu Ruiying a, Li Huanrong a*, Wang Zhiyin b, Ma Xiaohong c, Dong Chaonan a a

Department of Chemistry, Renmin University of China, Beijing 100872, China

b

School of Chemical and Environmental Sciences, Shaanxi University of Technology, Hanzhong 723001, China

c

State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing100190, China

ABSTRACT SnO2 is prepared by the hydrolysis and the automatic oxidation of SnCl2 in water, it shows the photocatalytic activity for the degradation of methyl orange in water under the UV light illumination. In the above process, methyl orange concentration decreases quickly, the total organic carbon (TOC) decreases slowly; inorganic ions (SO42-, NO3-, NH4+) can be formed; the pH value in the systems decreases gradually; a small quantity of HO· can be generated. In order to estimate the roles of active species during the above process, isopropanol, ammonium oxalate, and 1,4-benzoquinone, as the scavengers for HO·, h+, O2·- are introduced into the systems respectively. Isopropanol and (NH4)2C2O4 are effective scavengers for active species HO· and h+ respectively; but 1,4-benzoquinone is not a satisfactory scavenger to capture O2·- at least in this work. At last, SnO2 is characterized by N2 sorption, DRS, XRD, SEM and TEM. Keywords: Photodegradation; SnO2; methyl orange; semiconductor; photocatalysis. ---------------------------------------------------------* Corresponding author. Fax: +86-10-62516614. E-mail: [email protected] (Yu Zhiyong).

1. Introduction SnO2 has gained more and more attention due to its gas sensing property. SnO2 gas sensors are in high demand for many applications including environmental monitoring, prevention of leakage and incomplete combustion, it can be used to detect the toxic gases (e.g. CO, SO2, NOx ) and flammable gases ( e.g. C2H2, H2 ) [1]. But, on the other hand, SnO2 can also be used as a photocatalyst to degradate organic pollutants in the heterogeneous system. Heterogeneous photocatalysis is one of effective methods to treat wasterwater with large amounts of azo dyes, active HO· can be generated during this process [2,3]. In the labs, SnO2 can be prepared by the following methods: (1). K2S2O8 oxidizes SnCl2 in water under UV light [4]; (2). Hydrolysis of SnCl4 in NaOH solution [5]; (3). Sonochemical and hydrothermal synthesis from SnCl2 [6]; (4). Cysteine reacts with SnCl4 in water to get SnS2, SnS2 is calcinated to get SnO2 [7]; (5). SnSO4 is dissolved in H2SO4 solution, then placed under UV lamp [8], and so on. In this work, at first, we prepare SnO2 by the simple method----hydrolysis and automatic oxidation of SnCl2, then we study its photocatalytic activity on the degradation of methyl orange (MO) in water under the UV light illumination (decrease of MO concentration and TOC). At the same time, we measure the inorganic ions and pH change in the system. In order to estimate the roles of active species (HO·, h+, O2·-) during the above processes, isopropanol, ammonium oxalate, and 1,4-benzoquinone as the scavengers are introduced into the systems respectively. At last, we characterize SnO2 by some techniques.

2 2. Experimental 2.1. Chemicals and materials SnCl2·2H2O, NaOH, methyl orange (MO, C14H14N3NaO3S, MW = 327.33 g/mol, its molecular structural formula seeing (a) below), terephthalic acid (HOOCC6H4COOH, TA), isopropanol (CH3CHOHCH3, IPA), ammonium oxalate ((NH4)2C2O4, AO), and 1,4-benzoquinone (BQ) are analytical reagents and used without further purification; the 30W UV lamp (253.7 nm; Kongjun Houqin, Beijing, China); distilled water is used throughout this work; 0.22 µm filter membrane (micro PES, made in Membrana company, Germany). NaO3S

N N

N(CH3)2

(a)

2.2. SnO2 preparation A given amount of SnCl2·2H2O is dissolved in enough distilled water, we can get the precipitate, the precipitate is purified with enough distilled water for some times, and then drying in air naturally. 2.3. Methyl orange adsorption test 100 mg SnO2 and 100 mL of 0.10 mM methyl orange solution are put into a beaker (the total volume is 150 mL) together, the mixture (suspension) is stirred in the dark. At the different time, samples are taken and filtered by the filter membrane, then methyl orange concentrations are measured with the UV-Vis spectrophotometer (Varian Cary 50 series) at the absorption peak (λ = 464 nm). 2.4. Methyl orange photodegradation process (1). 100 mg SnO2 and 100 mL of 0.10 mM MO solution are put into a beaker (the total volume is 150 mL) together, the mixture (suspension) is then put under the 30W UV lamp vertically, the distance between the UV lamp and the surface of the suspension in the beaker is 4.2 cm. The suspension is stirred in the dark for 30 minutes. Afterwards, we turn on the UV lamp, the photoreaction begins. At the different illumination time, samples are taken and filtered by the filter membrane for the measurement of MO concentration. (2). Repeat the step (1), samples are taken at the different illumination time and filtered by the filter membrane for the measurement of the total organic carbon (TOC) by the TOC analyzer (TOC–VCPH, Shimadzu). In order to get accurate quantitative determination of TOC in solutions, we use methyl orange with high concentration (0.10 mM) than that commonly found in the textile effluents [2,3]. (3). Repeat the step (1) four times, at 3-rd or 6-th or 9-th or 12-th hour, respectively, we turn off the UV lamp, 60 mL samples are taken and filtered by the filter membrane for the measurement of pH value; 40 mL samples are taken and filtered by the filter membrane for the measurement of inorganic ions (NO3-; NO2-; NH4+; SO42-). (4). 100 mg SnO2 and 100 mL of (0.04 mM MO + 0.40 mM scavenger) (scavenger = AO, IPA, BQ) solution are put into a beaker (the total volume is 150 mL) together, the following operations are the same as the step (1). 2.5. Measurements of pH value, inorganic ions and HO· The details are the same as those in the Ref. [9]. 2.6. Characterization of SnO2 The details are the same as those in the Ref. [9].

3

3. Results and Discussion 3.1. SnO2 preparation According to the following equation: 2SnCl2 + 2H2O + O2 = 2SnO2 ↓ + 4HCl, the hydrolysis and the automatic oxidation of Sn2+ in water take place simultaneously, we can get SnO2 in the form of precipitates, which is not soluble in water, so we can wash the precipitate with enough distilled water, then let SnO2 dry in air naturally. At last, we get light yellow SnO2 powder. 3.2. Adsorption test Adsorption behavior is a key step for the heterogeneous photoreaction, we carry out the adsorption tests at first. Co means the initial methyl orange (MO) concentration, C means MO concentration at the different time. As far as the system of (100 mg SnO2 + 100 mL of 0.10 mM MO) is concerned, when the adsorption time is at 0, 5-th, 10-th, 20-th, 30-th minute, the C/Co is 1.00, 0.9986, 0.9853, 0.9853, 0.9853, respectively, which means that the adsorption equilibrium can be set up within 30 minutes. 3.3. Decrease of methyl orange concentration and total organic carbon Fig.1(A) shows the UV--Vis spectra change of methyl orange (MO) solution in the system of (100 mg SnO2 + 100 mL of 0.10 mM MO) under the UV light illumination. During the photocatalytic degradation process, the adsorption bands of methyl orange in the visible region decrease with the illumination time gradually (at t = 1-6 h), finally disappear (at t = 9 h, 12 h), which indicates the destruction of its chromophoric structure (namely azo bond, –N=N–) [2,3]. Fig.1(B) corresponds to the photocatalytic degradation kinetics of MO, it is easy to find that the ratio of C/Co decreases with the illumination time quickly, but the ratio of TOC/TOCo decreases with the illumination time slowly. At t = 12-th hour, the C/Co is 0.0074, the TOC/TOCo is 0.7922; which means that methyl orange molecules can not be photomineralized fully, the intermediates are stable in the system. 1.5

1 a 0.8

1

b

0.5

g

0 264

364

f

b c d e

464

ratio

Abs.

0.6 A

0.4

B a

0.2 0 564

wavelength (nm)

664

0

3

6 time (h)

9

12

4

9

8 D 6

a

6

pH

C (mg/L)

C

4

c 3 2 b 0

0 0

3

6

9

12

0

3

6

time (h)

12

time (h)

1

120 d

E 0.8

F

c

Intensity

C/Co

9

0.6 0.4

c

0.2

80 b 40 a

b

a

0

0 0

15

30

45

60

time (min)

360

400

440

480

520

560

wavelength (nm)

Fig.1. (A). UV--Vis spectral change of MO in the system of (100 mg SnO2 + 100 mL of 0.10 mM MO) as a function of the irradiation time. (a) t = 0 h, (b-g) illumination at t = 1, 2, 4, 6, 9, 12 h, respectively. (B). Photocatalytic degradation kinetics of MO in the system of (100 mg SnO2 + 100 mL of 0.10 mM MO). (a). C/Co, (b). TOC/TOCo. (C) The relation between the concentrations of inorganic ions generated in the system of (100 mg SnO2 + 100 mL of 0.10 mM MO) and the illumination time. (a) SO42-, (b) NO3-, (c) NH4+. (D) pH change in the filtrates from the system of (100 mg SnO2 + 100 mL 0.10 mM MO) with the illumination time. (E) Photocatalytic degradation of methyl orange (MO) in the different systems with the various scavengers. (a) 100 mg SnO2 + 100 mL of 0.04 mM MO; (b) 100 mg SnO2 + 100 mL of (0.04 mM MO + 0.40 mM IPA); (c) 100 mg SnO2 + 100 mL of (0.04 mM MO + 0.40 mM AO). (F) Fluorescent spectra of the filtrates from the system of (100 mg SnO2 + 100 mL 5×10-4 M TA) under the UV light illumination. (a) t = 0.5 h, (b) t = 1 h, (c) t = 2 h, (d) t = 3 h.

3.4. Generation of inorganic ions and pH change During the above process, the inorganic ions are formed. Fig.1 (C) shows the relation between the concentrations of the inorganic ions and the illumination time in the above system. From the Fig.1 (C), we know that NO2- is not formed; and that the concentration of NO3- is low and changes slightly with the prolongation of the illumination time; and that the concentrations of SO42- and NH4+ increase quickly with the prolongation of the illumination time. At the end of the photocatalytic degradation reaction, namely at t = 12-th hour, the concentrations of NO2-, NO3-, SO42-, NH4+ are 0, 0.16 mg/L, 8.3 mg/L, 3.6 mg/L, respectively. At this moment, as for the element nitrogen, [NO2-] + [NO3-] + [NH4+] < 0.3 mM (1 mol methyl orange molecule contains 3 mol N atoms), the reasons include but not limited to: (i) the formation

5 of nitrogen-containing organic intermediate(s), (ii) the formation of molecular nitrogen (N2) [10]. It is worthwhile to note that the generation of NH4+ during the photocatalytic degradation of methyl orange signifies a concurrent reduction reaction [13]. In addition, as for the element sulfur, [SO42-] < 0.1 mM (1 mol methyl orange molecule contains 1 mol S atom), the reasons include but not limited to: (i) the formation of sulfur-containing organic intermediate(s), (ii) the formation of HS- or SO32- rather than SO42directly [11,12]. Dissolved CO2 makes the distilled water more acidic, the pH value of the distilled water in our lab is 6.0, the pH value of 0.10 mM methyl orange is 6.2. Fig.1 (D) shows the pH value change with the illumination time in the above system. From the beginning (t = 0) to the 3-rd hour, the pH value in the system decreases quickly; from t = 3-rd hour to t = 12-th hour, the pH value in the system decreases very slowly. At t = 12-th hour, the pH value in this system is 4.1, which means that the strong acids (H 2SO4, HNO3) are generated during the photocatalytic degradation process. 3.5. Detection of active species by scavengers In the above process, it is reasonable to think that at least one kind of active species (HO·, h+, O2·-, and so on) is formed. In order to estimate the roles of active species during the above process, isopropanol (IPA), ammonium oxalate (AO), and 1,4-benzoquinone (BQ) as the scavengers for HO·, h+, O2·- are introduced into the systems respectively [13-16]. From Fig.1 (E), it is easy to find that IPA has little effect on the photocatalytic activity of the SnO2, which suggests that HO· does not play an important role for the photocatalytic degradation of methyl orange. On the contrary, AO has the obvious inhibition on the photocatalytic degradation of methyl orange, which suggests that h+ plays a relatively important role for the photocatalytic degradation of methyl orange. In fact, a small quantity of HO· can be generated in the above process, we can measure HO· by terephthalic acid (TA) indirectly. TA reacts with HO· to generate the highly fluorescent product, 2hydroxyterephthalic acid (TAOH), which emits photoluminescence at 426 nm on the excitation of its own absorption band (λ = 315 nm) [17,18]. The intensity of the peak attributed to TAOH is known to be proportional to the amount of HO· generated. Fig.1(F) shows the fluorescence spectra of the system of (100 mg SnO2 + 100 mL of 5×10-4 M TA) under the UV illumination. With the prolongation of the illumination time, the fluorescent intensity increases slowly. Now, we analyse the role of 1,4-benzoquinone (BQ) as the scavenger for O2·-. Fig.2(A) shows the photocatalytic degradation of methyl orange (0.04 mM) mediated by SnO2. At t = 0, the absorbance of curve (a) in Fig.2(A) is 1.00, then the absorbance of the system decreases gradually with the illumination time. The color of 0.40 mM BQ in water is light yellow, under the experimental condition, there is a chemical reaction to take place for BQ, the evidence is that the absorbance of BQ solution varies with the illumination time, seeing Fig.2(B). At t = 0, the absorbance of curve (a) in Fig.2(B) is very little, at t = 30 min, the absorbance of curve (c) in Fig.2(B) is the highest, then the absorbance of the system decreases very slowly, at t = 60 min, the absorbance of curve (e) in Fig.2(B) is more than that at t = 0 (curve (a)). Fig.2(C) shows the photocatalytic degradation of methyl orange (MO) mediated by SnO2 in the presence of BQ. At t = 0, the absorbance of curve (a) in Fig.2(C) is more than 1.00, at t = 15 min, the absorbance of curve (b) in Fig.2(C) is the highest, then the absorbance of the system decreases slowly. Therefore, here, it

6 is impossible for us to use 1,4-benzoquinine (BQ) to estimate the role of O2·-. Based on the above fact, we say that BQ is not a satisfactory scavenger to capture O2·-, at least in this work [9]. 2

2 B

A

Abs.

1.5 Abs.

at 464 nm from up to down c,d,e,b,a

1.6 a

1

1.2 0.8

b

0.5 c

0.4 d

e

0 264

364

0 464

564

664

wavelength (nm)

264

364

464

564

664

wavelength (nm)

2 C at 464 nm from up to down b,c,a,d,e

Abs.

1.5 1 0.5 0 264

364

464

564

664

wavelength (nm) Fig.2. Photocatalytic degradation of methyl orange (0.04 mM) and 1,4-benzoquinine (BQ) in different systems. (A) 100 mg SnO2 + 100 mL 0.04 mM MO; (B) 100 mL 0.40 mM BQ; (C) 100 mg SnO2 + 100 mL of (0.04 mM MO + 0.40 mM BQ). (a) t=0, (b) t = 15 min, (c) t = 30 min, (d) t = 45 min, (e) t = 60min.

3.6. XPS analysis The XPS allows the determination for the surface composition of very thin outermost surface layer around 2 nm with very high surface sensitivity. In order to analyze the chemical composition of the photocatalyst before and after use, we have carried out the XPS measurement, the related data are listed in Table 1, the element C can be ascribed to the residual carbon from the precursor, whose peak position (C 1s) is near 285.00 eV. Fig.3 shows the XPS survey of SnO2 before and after use, the peak positions of Sn 3d, O 1s are near 487.0eV and 531.0 eV respectively. At the illumination time t = 12-th hour, there is element N accumulation on the surface of SnO2, whose peak positions is near 400.0eV, which indicate that the surface of the SnO2 sample is covered by low amounts of adsorbed methyl orange molecules or / and intermediates produced during the dye photodecomposition. Table 1 XPS peak position of SnO2 ------------------------------------------------------------------------------------

7 Peak Binding energy (eV) FWHM (eV) Surface composition(at.%) -----------------------------------------------------------------------------------SnO2 (before use) C 1s

284.79

2.07

39.9

O 1s

531.25

2.32

40.94

Sn 3d

487.18

1.67

19.16

SnO2 (100 mg SnO2 +100 mL 0.1 mM MO, illumination t = 12h) C 1s

284.92

1.98

47.74

O 1s

531.23

2.51

36.29

Sn 3d

487.22

1.65

14.88

N 1s

399.27

0.26

1.09

-----------------------------------------------------------------------------------

100

100 Sn 3d

B

A

Intensity (X10^4)

Intensity (X10^4)

Sn 3d 75 O 1s 50

25

75

O 1s 50

C 1s

25

C 1s

0

0 0

200

400

600

800

1000 1200

Binding energy (eV)

0

200

400

600

800

1000 1200

Binding energy (eV)

Fig.3. (A) XPS survey spectra of SnO2 before use, (B) XPS survey spectra of SnO2 separated from the system of (100 mg SnO2 + 100 mL 0.10 mM MO) at the illumination t = 12-th hour.

3.7. Characterization of SnO2 The nitrogen adsorption-desorption isotherm of SnO2 is shown in Fig.4(A). It can be seen that the isotherm profile can be categorized as the type IV with a small hysteresis loop observed at a relative pressure of 0.5-1.0, which indicates the presence of a mesoporous structure in SnO2. Fig.4(B) showa the Barrett-Joyner-Halenda (BJH) pore size distribution of SnO2 obtained from the adsorption branch, the mesopores are mainly centered at about 2 nm and 6 nm. The specific surface area of SnO2 is 49.4 m2 /g [8,19,20]. Fig.4 (C) shows the diffuse reflectance spectrum (DRS) of SnO2. From 200 nm to 280 nm, the absorbance of SnO2 is very high, from 280 nm to 450 nm, the absorbance of SnO2 decreases quickly, from 450 nm to 800 nm, the absorbance of SnO2 is low and flat. Fig.4 (D) show the XRD pattern of SnO2, XRD is used to investigate the phase structures of the materials, we can easily find that the prepared SnO2 powder does not display any crystallized structure. Fig.4 (E) shows the SEM images of SnO2, we can find

8 that the morphology of SnO2 is not regular; Fig.4 (F) shows the TEM images of SnO2, we can find that SnO2 consists of agglomerates of particles. 0.02 B

A

dV/dD (mL/g/nm)

Vol. abs.(mL/g) (STP)

60

40

20

0.016 0.012 0.008 0.004 0

0 0

0.2

0.4

0.6

0.8

0

1

5

10

15

20

25

pore size (nm)

Relative pressure (P/Po )

1.2

D

C

Intensity (a.u.)

0.9

Abs.

30

0.6 0.3 0 200

300

400

500

600

700

800

5

35

50

65

80

2θ (°)

wavelength (nm)

E

20

F

Fig.4. (A) N2 adsorption-desorption isotherm of SnO2, (B) BJH pore size distribution of SnO2, (C) Diffuse reflectance spectra (DRS) of SnO2, (D) XRD pattern of SnO2, (E) SEM image of SnO2, (F) TEM image of SnO2.

4. Conclusions SnO2 is prepared by the hydrolysis and the automatic oxidation of SnCl2 in water, it can be used as a photocatalyst for the degradation of methyl orange in water under the UV light illumination. During the

9 above process, metnyl orange concentration decreases quickly, but the total organic carbon (TOC) decreases slowly; the inorganic ions (SO42-, NO3-, NH4+) can be formed; the pH value in the system decreases gradually; a small quantity of HO· can be generated. Isopropanol and (NH4)2C2O4 are effective scavengers for active species HO· and h+ respectively; but 1,4-benzoquinone is not a satisfactory scavenger to capture O2·- at least in this work.

Acknowledgment This work is supported by The Basic Research Funds in Renmin University of China from the Central Government (No. 12XNLL03).

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► SnO2 is prepared by the hydrolysis and the automatic oxidation of SnCl2 in water. ► SnO2 shows the photocatalytic activities for the degradation of methyl orange. ► Inorganic ions (SO42-, NO3-, NH4+) and HO· can be formed during the above process. ► 1,4-benzoquinone is not a satisfactory scavenger to capture O2·- in this work.