Aqueous-based synthesis of mesoporous TiO2 and ...

Available online at www.sciencedirect.com

CERAMICS INTERNATIONAL

Ceramics International 42 (2016) 2488–2496 www.elsevier.com/locate/ceramint

Aqueous-based synthesis of mesoporous TiO2 and Ag–TiO2 nanopowders for efficient photodegradation of methylene blue Ipsita Hazra Chowdhury, Sourav Ghosh, Milan Kanti Naskarn Sol–Gel Division, CSIR-Central Glass and Ceramic Research Institute, Kolkata 700032, India Received 14 August 2015; received in revised form 9 October 2015; accepted 9 October 2015 Available online 23 October 2015

Abstract Mesoporous anatase TiO2 was synthesized by a hydrothermal method at 180 1C/24 h using titanium (IV) oxysulfate (TIOS), urea and sodium dodecyl sulfate (SDS) under aqueous medium. The Ag nanoparticle doped anatase TiO2 (Ag–TiO2) was prepared by an impregnation method using water dispersible Ag obtained from green carambola extract at room temperature. The particles were characterized by X-ray diffraction (XRD), thermo gravimetric (TG) and differential thermal analysis (DTA), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), N2 adsorption–desorption study, and transmission electron microscopy (TEM). The photocatalytic degradation of methylene blue (MB) was studied in the presence of TiO2 and Ag–TiO2 particles under UV and visible light. The initial enhancement of MB dye degradation in the presence of Ag–TiO2 was due to its ability to trap electrons inhibiting electron–hole recombination. Pure anatase TiO2 having higher surface area and pore volume also influenced predominantly in reducing the recombination of electrons–holes pair, reflecting higher efficiency for photodegradation of MB after a certain period of exposed time. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: A. Powders; B. Microstructure; B. Porosity; D. TiO2; E. Functional application

1. Introduction Titania is becoming an important material with versatile applications as pigments, capacitors, solar cells, catalyst, photocatalysts etc [1]. It has been extensively studied as an effective photocatalyst for photocatalytic degradation of organic and inorganic pollutants in waste water [2]. Due to its unique properties like chemical inertness, non-toxicity and photostability, titania finds a wide range of applications [3]. However, the major constraints for TiO2 photocatalyst are its low quantum efficiency, wide band gap (3.2 eV) energy, and relatively high electron–hole recombination rate [4]. Therefore, doping with metals in TiO2 is becoming essential to increase the life time of the charge carrier as well as band gap tuning to a desired level [5]. Metal doping affects the physico-chemical properties like crystallinity, optical, textural and surface properties etc of TiO2 toward its applications. The above n

Corresponding author. Tel.: þ91 33 24733496x3516. E-mail address: [email protected] (M.K. Naskar).

http://dx.doi.org/10.1016/j.ceramint.2015.10.049 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

properties are influenced by different synthetic methods [6,7]. Silver doped TiO2 is very much attractive for better photocatalytic acivity in terms of enhancement of electron–hole separation by acting as electron traps, extending light absorption into the visible range and modifying surface properties of photocatalysts. Different methods have been reported for the synthesis of TiO2 nanparticles, such as co-precipitation [8], mechanochemical [9], hydrothermal [10,11], sovothermal [12], sol–gel [13], microemulsion [14], microwave [15] etc. Mesoporous TiO2 was synthesized by hydrothermal method using titanium butoxide in the presence of organic solvent to control rapid hydrolysis of titania precursor [16,17]. Kolen'ko et al. [18] hydrothermally synthesized nanocrystalline mesoporous TiO2 from aqueous solution of titanyl oxalate acid via many processing steps. Mesoporous hierarchical TiO2 nanostructures were prepared by the hydrothermal method using TiO2 powder in the presence of H2O2 and NaOH [19] via controlled multistep process. To avoid the use of highly hydrolysable titanium butoxide as precursor, and to minimize processing

I.H. Chowdhury et al. / Ceramics International 42 (2016) 2488–2496

step, in the present study, mesoporous TiO2 nanoparticles with higher surface area were synthesized by a single step hydrothermal method at 180 1C/24 h in the presence of aqueous based precursors of titanium oxysulfate, urea, sodium dodecyl sulfate and water. For the preparation of Ag doped TiO2 nanocomposites, photoreduction treatment [20,21] is applied for the reduction of Ag þ ions to metallic Ag using UV light. Zhang et al. [22] used L-Tyrosine as reducing agent for Ag doping in TiO2 microspheres. In the present method, for doping Ag into TiO2, an impregnation method was adopted using AgNO3 and green carambola fruit extract at pH 10 under stirring for 12 h at room temperature in dark, in the absence of any photoreduction technique and high cost reducing agent. In this method, polyols and ascorbic acids present in the carambola extract was exploited for the reduction of Ag þ to Ago at room temperature. They also behaved as capping agents for well dispersion of Ag nanoparticles [23] during impregnation into TiO2. In the present work, to the best of our knowledge, we first report Ag doped TiO2 nanoparticles using carambola extract, and the influence of dual properties i.e., electron trapping efficiency of Ag in Ag–TiO2, and higher surface area and pore size of undoped anatase TiO2 toward photocatalytic efficiency of the synthesized powders for the degradation of methylene blue (MB) dye. The photocatalytic performance of Ag–TiO2 obtained from pure ascorbic acid, and also P25 (Evonik) was also studied for comparison. 2. Experiment 2.1. Materials Titanium (IV) oxysulfate (TIOS) (15 wt% solution in dilute sulfuric acid), P25 (Evonik), ascorbic acid were purchased from Sigma-Aldrich. Sodium dodecyl sulfate (SDS), silver nitrate (AgNO3), urea and sodium hydroxide were purchased from Merck, India. Deionized (DI) and Millipore water was used throughout the experiment. Green carambola was purchased from local market. 2.2. Preparation of TiO2 nanoparticles In a typical experiment, 10 mmol urea and 2 mmol SDS were dissolved in 25 mL DI water followed by addition of 5 mmol TIOS (0.78 ml) into the former solution under stirring condition. After stirring for 30 min, the above solution was transferred into a 50 mL Teflon-lined autoclave, followed by hydrothermal treatment at 180 1C for 24 h. After the reaction, the particles were collected by centrifugation, and washed with DI water and acetone, followed by drying at 70 1C for 4 h. The dried as-prepared particles were calcined at 500 1C with a heating rate of 2 1C min 1 and dwell time of 2 h. 2.3. Preparation of Ag doped TiO2 nanoparticles In a typical experiment, 0.2 mmol TiO2 powder was dispersed in 50 mL DI water under sonication followed by

2489

adding 0.1 mmol AgNO3 into the dispersed solution. 10 mL of carambola extract was added into the above mix solution followed by dropwise addition of 1 M NaOH solution up to pH 10 under stirring condition. The whole reaction mixture was kept in dark under constant stirring for 12 h. The Ag impregnated TiO2 nanoparticles were then separated by centrifugation, washed with Millipore water for several times to remove the biomolecules, followed by drying in air. 2.4. Characterization Powder X-ray diffraction (XRD) studies of the samples were performed by a Philips X'Pert Pro PW 3050/60 powder diffractometer using Ni-filtered Cu-Kα radiation (λ ¼ 0.15418 nm) operated at 40 kV and 30 mA. The thermal behaviors of the uncalcined (as-prepared) particles were studied by thermogravimetry (TG) and differential thermal analysis (DTA) with (Netzsch STA 449C, Germany) from room temperature to 1000 1C in air atmosphere at the heating rate of 10 1C/min. Raman measurements were performed by a STR500 Raman Spectrometer (Cornes Technology Make) using an excitation wavelength of 514 nm with a power of 50 mW. Scattered light was analyzed using a spectrometer grating with a spectral resolution of o 1 cm 1. X-ray photoelectron spectroscopy (XPS) measurements were carried out in a PHI 5000 Versaprobe II Scanning XPS microprobe (ULVAC-PHI, USA). The spectra were recorded with monochromatic AlKα (hν ¼ 1486.6 eV) radiation with an overall energy resolution of 0.7 eV. Nitrogen adsorption–desorption measurements were conducted at 77 K with a Quantachrome (ASIQ MP) instrument. The powders were outgassed in vacuum at 250 1C for 4 h prior to the measurement. The surface area was obtained using the Brunauer–Emmett–Teller (BET) method within the relative pressure (P/Po) range of 0.05–0.20 and the pore size distribution was calculated by Barrett–Joyner–Halenda (BJH) method. The nitrogen adsorption volume at the relative pressure (P/Po) of 0.99 was used to determine the pore volume. The morphology of the particles was examined by the transmission electron microscopy (TEM), using a Tecnai G2 30ST (FEI) instrument operating at 300 kV. Elemental composition of the sample was analyzed with energy dispersive analysis of X-ray spectroscopy (EDS) coupled to TEM. The Na and Ag content in the samples was estimated by inductively coupled plasmaatomic emission spectroscopy (ICP-AES) (Model: Spectro Ciros Vision, Germany). UV–visible spectra were recorded using a UV–vis–NIR spectrophotometer (UV-3101PC, Shimadzu) in the wavelength range of 200–800 nm. 2.5. Photocatalytic study For a comparative study, the photocatalytic activity of synthesized TiO2, Ag–TiO2 (obtained from carambola), Ag– TiO2 (obtained from ascorbic acid) and reference powder (P25, Evonik) was studied for the degradation of MB solution under UV (λ ¼ 365 nm) and visible (λ¼ 465 nm) light irradiation each at room temperature in the photoreactor. In a typical photocatalytic test, 2 mg of the sample was mixed with 12 mL


(ii) (i) 10

20

30

40 50 2θ (degree)

60

70

80

Fig. 1. XRD patterns of (i) TiO2 and (ii) Ag–TiO2 particles. (●): Anatase TiO2; (▲): Ag.

100

3. Results and discussion

3.1.1. XRD analysis XRD patterns of (i) TiO2 and (ii) Ag–TiO2 particles are shown in Fig. 1. The appearance of diffraction peaks at around 25.091, 37.651, 48.021, 53.891, 55.071, 62.381, 68.071, 70.071 and 751 corresponding to the crystal planes of (101), (004), (200), (105), (211), (204), (220) and (215), respectively is due to the presence of anatase TiO2 (JCPDS no. 21-1272) in both the samples: (i) TiO2 and (ii) Ag–TiO2. In Ag–TiO2 samples, in addition of anatase phase, the characteristic peaks of metallic Ag (JCPDS no. 04-0783) at 2θ values of around 38.011, 44.261, 64.021 and 77.361 corroborating to the crystal planes of (111), (200), (220) and (311), respectively were noticed. 3.1.2. Thermal analysis The DTA–TGA curves of as-prepared TiO2 particles are shown in Fig. 2. In the TGA curves, the mass loss occurred in three stages, i.e. up to 300 1C with mass loss of about 10%, in the temperature range of 300–500 1C with mass loss of about 16%, and in the temperature range of 500–1000 1C with mass loss of about 9%. In the DTA curve, the endothermic peaks at about 132 1C and 285 1C were due to the dehydration of physically adsorbed water and release of hydroxyl groups, respectively, accompanying with a first stage mass loss of about 10% upto 300 1C in the TGA curve. The exothermic peaks at around 335 1C and 464 1C accompanying with endothermic peak at around 415 1C corroborated to the decomposition of surfactants (SDS) and release of inorganic moieties from the precursor (TIOS). It was reflected in the second stage mass loss of about 16% in the temperature range 300–500 1C. The third stage mass loss of about 9% in the temperature range of 500–1000 1C was ascribed to the

Exo

80

Endo

3.1. Characterization of TiO2 and Ag–TiO2 particles

90

70

Residual Weight (%)

of 10 5 M MB dye solution followed by stirring for about 60 min in dark to achieve adsorption equilibrium. Aliquots were taken out, filtered by Millipore filter paper (pore dia 0.22 mm). The filtrates were analyzed using a UV–visible spectrophotometer. For visible light irradiation, the dye solution was placed in a rectangular box (68 cm 25 cm 36 cm). At the inside top of the box two tubes, each with a power of 18 W, were attached. The solution was irradiated from a distance of 11 cm with visible light (λ ¼ 465 nm) at room temperature in the photoreactor. For UV light irradiation, the dye solution was placed in a rectangular box (36 cm 30 cm 44 cm) fitted with eight tubes with a power source of 6 W each placed at the inside top of the box. The solution was irradiated from a distance of 11 cm with UV light (λ¼ 365 nm) at room temperature in the photoreactor. After a certain interval of irradiation, aliquots were collected, filtered and monitored by UV–visible spectrophotometer. The decrease in absorption intensity at λmax ¼ 664 nm indicates the photocatalytic degradation of the MB.

Intensity (a.u.)

2490

60 0

200

400

600

800

1000

Temperature ( o C) Fig. 2. DTA–TGA curves of as-prepared TiO2 particles.

decomposition of residual inorganic and organic moieties from the sample.

3.1.3. Raman analysis Fig. 3 shows the Raman spectra of (i) TiO2 and (ii) Ag–TiO2 particles. For pure TiO2 sample, the appearance of sharp and intense peak at 144 cm 1 and a very weak peak at around 196 cm 1 assigning to Eg mode of vibration was the characteristic of anatase TiO2 [24]. In the higher frequency region, the other peaks at around 400, 516 and 639 cm 1 corresponding to B1g, A1g and Eg modes also confirmed the presence of anatase TiO2 [25]. In the presence of Ag in TiO2, the intensity of the corresponding Raman peaks due to anatase TiO2 was significantly decreased. However, two broad peaks at around 1348 and 1568 cm 1 were due to COO stretching mode [26] of adsorbed biomolecules containing ascorbic acid present in Ag nanoparticles. It is worth mentioning that Ag nanoparticles were obtained from green carambola fruit extract. Thus, the cause of decrease in intensity of anatase TiO2 in the presence of Ag was due to the presence of adsorbed biomolecules.


3.1.4. XPS analysis The surface properties of TiO2 and Ag–TiO2 particles were obtained from XPS (X-Ray Photoelectron Spectroscopy). Fig. 4a reveals the XPS data of (i) TiO2 and (ii) Ag–TiO2 samples in the full spectrum. The peaks associated with Ti (3p, 3s, 2p, 2s), O (1s), and C (1s) were noticed in TiO2 and Ag–TiO2 samples in addition to Ag 3d spectrum in the latter sample. Fig. 4b shows the high resolution XPS curve of Ag 3d region. The binding energy of Ag 3d5/2 and Ag 3d3/2 centered at 367.44 and 373.48 eV, respectively with the separation of peak at 6.04 eV indicated the presence of metallic Ag in Ag–TiO2 sample [27,28]. 3.1.5. Textural properties Fig. 5 shows the nitrogen adsorption–desorption isotherms of (a) TiO2 and (b) Ag–TiO2 particles. It displays type IV isotherm according to IUPAC classification, which indicated mesoporous characteristic of the sample. The appearance of type H-3 hysteresis loop indicated the formation of

1348

Eg

B1g A1g Eg

(ii) (i) 0

500

1000

1500

2000

-1

Raman Shift (cm ) Fig. 3. Raman spectra of (i) TiO2 and (ii) Ag–TiO2 particles.

O1S

asymmetric, interconnected, slit-like mesoporosity in the samples [29]. It is to be noted that the uptake of nitrogen increased steeply above the relative pressure of 0.7. The corresponding BJH pore size distributions derived from desorption data of the isotherms are shown in the insets of Fig. 5. For TiO2 samples, the pore size distributions were wider than those of Ag–TiO2 samples. It was due to partial filled-in of the pores with the Ag nanoparticles in the samples. The textural properties (BET surface area, total pore volume and pore diameter) of TiO2 and Ag–TiO2 samples are shown in Table 1. It indicates that BET surface area, total pore volume and pore size of TiO2 samples were higher than those of Ag– TiO2 samples. It demonstrated that the presence of Ag nanoparticles within the pores lowered the textural properties of the samples. 3.1.6. TEM studies Fig. 6 shows the TEM images of (a) TiO2 and (b) Ag–TiO2 particles. The corresponding high magnified images are shown by arrow marks. It indicates the aggregation of nanosized particles of dimension 10–20 nm. The selected area electron diffraction (SAED) patterns, HR-TEM and energy dispersive X-ray spectroscopy (EDS) are shown in Fig. 7a–c for TiO2 samples, while the corresponding images for Ag–TiO2 samples are exhibited in Fig. 7d–f, respectively. The SAED patterns of the samples indicated the polycrystalline nature of the particles (Fig. 7a and d). The bright spots of the concentric diffraction rings corresponding to (211), (200), (004) and (101) matched well with the anatase TiO2 planes obtained from XRD. The HR-TEM of the TiO2 samples show d-spacing of 0.35 nm corresponding to the (101) lattice planes of anatase TiO2 planes (Fig. 7b and e). In Ag–TiO2 samples, the d-spacing of 0.23 nm corresponding to (111) plane of Ag demonstrated the presence of Ag nanoparticles (shown with dotted circle) (Fig. 7e). The presence of Ti, O and Ag in the samples was revealed by energy dispersive X-ray spectroscopy (EDS) (Fig. 7c and f). The atomic ratio of Ti and O almost matched with that in TiO2. The presence of 0.16 at% Ag was noticed in Ag–TiO2 samples

Ag (3d 5/2)

TiO2 NPs Ag doped TiO2 NPs

OKLL

Intensity (a.u.)

Ti2S

Ti2P

Ag (3d 3/2)

Ag3d

C1S

(ii) Ti3STi3P

Intensity (a.u.)

Intensity (a.u.)

1568

Eg

2491

(i) 1000

800

600

400

Binding Energy (eV)

200

0

380 378 376 374 372 370 368 366 364 362

Binding Energy (eV)

Fig. 4. (a) XPS data of (i) TiO2 and (ii) Ag–TiO2 samples in full spectrum, and (b) high resolution XPS curve of Ag 3d region.


350

Volume adsorbed (cm g STP)

350 300

-1

3

-1

Volume adsorbed (cm g STP)

2492

250

3

250

300

200 150 100 50 0 0.0

0.2

0.4

0.6

0.8

1.0

Relative pressure(p/p0)

200 150 100 50 0 0.0

0.2

0.4

0.6

0.8

1.0

Relative pressure(p/p0)

Fig. 5. Nitrogen adsorption–desorption isotherms of (a) TiO2 and (b) Ag–TiO2 particles. Insets show the corresponding pore size distributions.

Table 1 Textural properties of TiO2 and Ag–TiO2 samples. Sample

SBET (m2 g 1)a

Vp-Total (cm3 g 1)b

DBJH (nm)c

TiO2 Ag–TiO2

158 146

0.53 0.50

24.4 9

a

BET surface area. b Total pore volume. c Pore diameter by BJH desorption.

(Fig. 7f). However, ICP-AES analysis shows 13.5 wt% Ag in Ag–TiO2 sample. 3.1.7. Optical absorption study Different materials absorb radiation at different wavelengths. The yield of photo generated electron–hole pair depends on the intensity of incident photons with energy exceeding or equaling the band gap energy. Fig. 8a shows the optical absorption spectra of pure TiO2 and Ag–TiO2 samples. It is clear that for Ag–TiO2 sample, the absorption peak shifted towards longer wavelength (red-shift) compared to that of TiO2 sample. This red-shift of absorption peak for Ag–TiO2 was due to reduction of band gap energy and recombination rate. The optical band gap energy (Eg) was calculated using the Tauc equation: ð1Þ ðαhνÞ1=n ¼ A hν E g where α, hν, A and n are the absorption coefficient, photon energy, proportionality constant depending on the transition probability, n is a constant depending on the nature of transition, i.e, direct allowed (n ¼ 1/2), indirect allowed (n¼ 2), direct forbidden (n¼ 3/2) and indirect forbidden (n¼ 3). In the present case, n ¼ 2 for indirect allowed band gap [30]. Fig. 8b shows the plots of (αhν)1/2 vs. hν for pure TiO2 and Ag–TiO2. The band gap energy was calculated by extrapolating a tangent to the x-axis. The estimated band gaps for pure TiO2 and Ag–TiO2 were found to be 3.18 and 1.60 eV,

respectively. It signified that Ag doping in TiO2 matrix decreased the band gap energy, which allow the photoirradaiation under visible light and delay in recombination rate with enhancement of photocatalytic activity. 3.1.8. Photocatalytic activity Before irradiation with UV and visible light, adsorption ability of the samples onto the MB dye (kept in the dark for 60 min for homogenization) was studied. The adsorption ability of pure TiO2 (15%) was found to be slightly higher than that of Ag–TiO2 (13.6%) samples because of higher surface area of the former. The photocatalytic activity of synthesized TiO2, Ag–TiO2 (obtained from carambola), Ag–TiO2 (obtained from ascorbic acid) and reference powder (P25, Evonik) was studied by the degradation of MB solution under UV (Fig. 9a–d) and visible (Fig. 9e–h) light irradiation. It is noticed that under UV light, photodegradation of MB occurred with faster rate for P25 (Fig. 9d) compared to that for synthesized TiO2 (Fig. 9a); however they reached at about constant value (85–86%) after 60 min of irradiation. The relatively high photocatalytic efficiency of P25 could be due to the presence of both anatase and rutile phase of TiO2 (4:1) rendering suppressed recombination of photogenerated electron and holes [31–33]; the interface between the two phases enhanced the photocatalytic activity of P25. However, under visible light, synthesized TiO2 (Fig. 9e) and P25 (Fig. 9h) show less amount of MB degradation (14–18%) due to high band gap energy of TiO2 resulting low photoexcitation. Interestingly, for Ag–TiO2 (obtained from carambola) under UV (Fig. 9b) and visible (Fig. 9f) light, a significant increase in MB photodegradation (about 50–55%) occurred within 2 min followed by slow degradation. It is to be noted that Ag–TiO2 (obtained from carambola) sample shows comparable degradation of MB in the presence of both UV and visible light. Compared to Ag–TiO2 (obtained from carambola), the photodegradation of MB in the presence of synthesized TiO2 under UV light (Fig. 9a) changed steadily, and after a certain time of irradiation, i.e., 50 min, its efficiency was becoming slightly higher, and exhibited 85% degradation


2493

Fig. 6. TEM images of (a) TiO2 and (b) Ag–TiO2 particles.

after 60 min of irradiation. It is worth mentioning that a very less amount of Na impurity (0.026%) in the synthesized TiO2 and Ag–TiO2 (obtained from carambola) samples acted as recombination center, which could reduce slightly the photocatalytic efficiency [33,34]. As the Na content in both the samples was found to be same, the influence of trace amount of Na should be the same and negligible for both the samples. The photodegradation of MB under UV and visible light in the presence of Ag–TiO2 (obtained from ascorbic acid) (Fig. 9c and g) was lower than that in the presence of Ag–TiO2 (obtained from carambola) exhibiting 55% and 71% degradation, respectively after 60 min of irradiation. It signified that carambola extract is very much effective for the reduction of Ag þ and stabilization of metallic Ag in Ag–TiO2 due to higher capping ability of biomolecules [23], resembling higher photocatalytic efficiency. The mechanism of photoexcitation and dye decomposition using TiO2 and Ag–TiO2 photocatalyst upon irradiation with light was explained [20]. The photocatalyst upon irradiation

with light of suitable wavelength renders migration of electrons from the valance band to the conduction band, generating electron deficient holes in the valence band. Thus, formation of electron–hole pairs occurs. The holes and electrons generate superoxide radical anions (O2 ) and hydroxy radicals (.OH). The O2 react with H2O to furnish OH and HOO having powerful oxidizing ability. On reacting with these powerful oxidizing agents, the dye gets oxidized and decolorized. The furnished electron is able to regenerate the photocatalyst. The efficiency of the dye degradation is dependent on the concentration of oxygen molecules which either scavenge the conduction band electron or prevent the electron–hole recombination. The initial enhancement of MB dye degradation in the presence of Ag–TiO2 could be due to its ability to trap electrons inhibiting effectively the recombination of excited electrons and holes on the surface of TiO2. However, after a certain time its efficiency decreased. Surface area of the samples has a role to monitor the efficiency of dye degradation. Mesoporous semiconductors of high surface area not only

2494


3500

(211) (200) (004)

Ti

Intensity (Counts)

3000

(101)

d=0.35nm (101)

At% Ti : 28.63 O : 71.37

O

2500 2000 1500

C

1000

Cu

Ti

Cu

500

Cu

0 0

2000

4000

6000

8000

Energy (eV) 1200

TiO2 (211) /TiO2 (200) /Ag (004) / TiO2 (101) /TiO2

Intensity (Counts)

Ag d=0.23nm(111) d=0.35nm(101)

Ti

1000

At% Ti : 32.20 O : 67.64 Ag : 0.16

O C

800 600 400

Cu

200

Ti

Cu

Cu

Ag

0 0

2000

4000

6000

8000

10000

Energy (eV) Fig. 7. The selected area electron diffraction (SAED) patterns, HR-TEM and energy dispersive X-ray spectroscopy (EDS) for (a, b, c) TiO2 and the corresponding images for (d, e, f) Ag–TiO2.

3.5 2.0

(i) TiO2

3.0

1.0

0.5

1/2 (ev cm -1 )1/2

1.5

2.5

(α hν )

Absorbance (a.u)

(ii) Ag-TiO2

1.0

(i) TiO2 (ii) Ag-TiO 2

2.0 1.5

0.5

0.0

1.6 ev

3.18 ev

0.0 200

400

600

800

1000

1

2

3

4

5

6

7

hν (ev)

Wavelength (nm)

Fig. 8. (a) Diffused reflectance spectra of (i) TiO2 and (ii) Ag–TiO2, and (b) Variation of (αhν)1/2 vs. photon energy of (i) TiO2 and (ii) Ag–TiO2.

restricted recombination of photogenarated charge carriers, but they provided more surface reactive sites for photocatalytic dye degradation process. Interestingly, after a certain time, compared to Ag–TiO2, TiO2 having higher surface area and pore size influenced predominantly in reducing the recombination of electrons–holes pair, which affected the efficiency of photocatalyst. It is inferred that dual properties i.e., electron trapping efficiency of Ag in Ag–TiO2, and higher surface area and pore size of undoped anatase TiO2 played a significant role to increase the efficiency for MB degradation.

4. Conclusions In summary, we have demonstrated a facile hydrothermal synthesis of mesoporous anatase TiO2, and Ag–TiO2 by the impregnation method using water-dispersible Ag obtained from green carambola extract at room temperature. BET surface area, pore volume and pore size of TiO2 decreased with Ag doping. The HR-TEM of the TiO2 samples show dspacing of 0.35 nm corresponding to the (101) lattice planes of anatase TiO2 planes, while in Ag–TiO2 samples, the d-spacing


% of methylene blue

100

h e

80 60

g c

40

f b a d

20 0 0

10

20

30 40 Time (min)

50

60

Fig. 9. The % degradation of MB with time under (a–d) UV and (e–h) visible light irradiation: (a, e) synthesized TiO2, (b, f) Ag–TiO2 (obtained from carambola extract), (c, g) Ag–TiO2 (obtained from ascorbic acid), and (d, h) P25, Evonik.

of 0.23 nm corresponding to (111) plane of Ag confirmed the presence of Ag nanoparticles. The initial higher efficiency of photocatalytic degradation of MB in the presence of Ag–TiO2 was due to electron trapping of Ag to enhance electron–hole separation, followed by decrease of its efficiency after a certain period of time caused due to lower surface area and pore size compared to pure anatase TiO2. Therefore, dual properties i.e., electron trapping efficiency of Ag in Ag–TiO2, and higher surface area and pore size of undoped anatase TiO2 played a significant role to increase the efficiency for MB degradation. Acknowledgment The authors would like to thank the Director of this institute for his kind permission to publish this paper. They acknowledge the help rendered by Nano-structured Materials Division and Material Characterization Division for material characterization. The authors I.H.C. and S.G. are thankful to UGC and CSIR, Government of India for their fellowships. The work was funded by DST-SERB Project (Grant no. SR/S3/ME/ 0035/2012), Government of India (No. GAP 0616). References [1] J.C. Colmenares, M.A. Aramendia, A. Marinas, J.M. Marinas, F. J. Urbano, Synthesis, characterization and photocatylitic activity of different metal-doped titania systems, Appl. Catal. A: Gen. 306 (2006) 120–127. [2] S.I. Mogal, V.G. Gandhi, M. Mishra, S. Tripathi, T. Shripathi, P.A. Joshi, D.O. Shah, Single-step synthesis of silver-doped titanium dioxide: influence of silver on structural, textural, and photocatalytic properties, Ind. Eng. Chem. Res. 53 (2014) 5749–5758. [3] K. Hashimoto, H. Irie, A. Fujishima, TiO2 photocatalysis: a historical overview and future prospects, Jpn. J. Appl. Phys. 44 (2005) 8269–8285. [4] M.S. Lee, S.S. Hong, M. Mohseni, Synthesis of photocatalytic nanosized TiO2–Ag particles with sol–gel method using reduction agent, J. Mol. Catal. A: Chem. 242 (2005) 135–140. [5] R. Nainani, P. Thakur, M. Chaskar, Synthesis of silver doped TiO2 nanoparticles for the improved photocatalytic degradation of methyl orange, J. Mater. Sci. Eng. B 2 (2012) 52–58.

2495

[6] S. Senthilkumar, K. Porkodi, R. Vidyalakshmi, Photodegradation of a textile dye catalyzed by sol–gel derived nanocrystalline TiO2 via ultrasonic irradiation, J. Photochem. Photobiol. A 170 (2005) 225–232. [7] W.J. Li, E.W. Shi, T. Fukuda, Particle size of powders under hydrothermal conditions, Cryst. Res. Technol. 38 (2003) 847–858. [8] X. Quan, T. Huaiqin, Z. Qinghua, S. Xuemei, Preparation of Lanthanumdoped TiO2 photocatalysts by co-precipitation, J. Mater. Sci. 42 (2007) 6287–6296. [9] S. Yin, Q. Zhang, F. Saito, T. Sato, Preparation of visible light-activated titania photocatalyst by mechanochemical method, Chem. Lett. 32 (2003) 358–359. [10] K.M. Reddy, D. Guin, S.V. Manorama, A.R. Reddy, Selective synthesis of nanosized TiO2 by hydrothermal route: Characterization, structure property relation, and photochemical application, J. Mater. Res. 19 (2004) 2567–2575. [11] S. Yang, L. Gao, Preparation of titanium dioxide nanocrystallite with high photocatalytic activities, J. Am. Ceram. Soc. 88 (2005) 968–970. [12] R.A. Roca, E.R. Leite, Size and shape tailoring of titania nanoparticles synthesized by solvothermal route in different solvents, J. Am. Ceram. Soc. 96 (2013) 96–102. [13] R. Mohammadi, D.J. Fray, A. Mohammadi, Sol–gel nanostructured titanium dioxide: controlling the crystal structure, crystallite size, phase transformation, packing and ordering, Microporous Mesoporous Mater. 112 (2008) 392–402. [14] M.H. Zori, Synthesis of TiO2 nanoparticles by microemulsion/heat treated method and photodegradation of methylene blue, J. Inorg. Organomet. Polym. 21 (2011) 81–90. [15] M.B. Suwarnkar, R.S. Dhabbe, A.N. Kadam, K.M. Garadkar, Enhanced photocatalytic activity of Ag doped nanoparticles synthesized by a microwave assisted method, Ceram. Intern. 40 (2014) 5489–5496. [16] J. Jitputti, S. Pavasupree, Y. Suzuki, S. Yoshikawa, Synthesis and photocatalytic activity for water-splitting reaction of nanocrystalline mesoporous titania prepared by hydrothermal method, J. Solid State Chem. 180 (2007) 1743–1749. [17] T. Jiang, Q. Zhao, M. Li, H. Yin, Preparation of mesoporous titania solid superacid and its catalytic property, J. Hazard. Mater. 159 (2008) 204–209. [18] Y.V. Kolen’ko, V.D. Maximov, A.V. Garshev, P.E. Meskin, N. N. Oleynikov, B.R. Churagulov, Hydrothermal synthesis of nanocrystalline and mesoporous titania from aqueous complex titanyl oxalate acid solutions, Chem. Phys. Lett. 388 (2004) 411–415. [19] C. Wang, L. Yin, L. Zhang, Y. Qi, N. Lun, N. Liu, Large scale synthesis and gas-sensing properties of anatase TiO2 three-dimensional hierarchical nanostructures, Langmuir 26 (2010) 12841–12848. [20] N. Sobana, M. Muruganadham, M. Swaminathan, Nano-Ag particles doped TiO2 for efficient photodegradation of direct azo dyes, J. Mol. Catal A: Chem. 258 (2006) 124–132. [21] L. Yang, X. Xiang, W. Ruan, J. Yang, B. Zhao, W. Xu, J.R. Lombardi, Charge-transfer-induced surface-enhanced Raman scattering on Ag–TiO2 nanocomposites, J. Phys. Chem. C. 113 (2009) 16226–16231. [22] L. Zhang, L. Chen, L.G. Chen, G. Zhu, A facile synthesis of flowershaped TiO2/Ag microspheres and application in photocatalyst, RSC Adv. 4 (2014) 54463–54468. [23] I.H. Chowdhury, S. Ghosh, M. Roy, M.K. Naskar, Green synthesis of water-dispersible silver nanoparticles at room temperature using green carambola (star fruit) extract, J. Sol-Gel Sci. Technol. 73 (2015) 199–207. [24] K. Page, R.G. Palgrave, I.P. Parkin, M. Wilson, S.L.P. Savin, A. V. Chadwick, Titania and silver-titania composite films on glass-potent antimicrobial coatings, J. Mater. Sci. 17 (2007) 95–104. [25] W. Ma, Z. Lu, M. Zhang, Investigation of structural transformations in nanophase titanium dioxide by Raman Spectroscopy, Appl. Phys. A. 66 (1988) 621–627. [26] N.T. Nolan, M.K. Seery, S.J. Hinder, L.F. Healy, S.C. Pillai, A systematic study of the effect of silver on the chelation of formic acid to a titanium precursor and the resulting effect on the anatase to rutile transformation of TiO2, J. Phys. Chem. C 114 (2010) 13026–13034.

2496


[27] Handbook X-ray Photoelectron Spectroscopy, in: C.D. Wagner, W. M. Riggs, L.E. Davis, J.F. Moulder, G.E. Muilen-berg (Eds.), PerkinElmer, Eden Prairie, MN, 1979. [28] J. Thiel, L. Pakstis, S. Buzby, M. Raffi, C. Ni, D.J. Pochan, S.I. Shah, Antibacterial properties of silver-doped titania, Small 3 (2007) 799–803. [29] M. Roy, S. Ghosh, M.K. Naskar, Ligand-assisted soft-chemical synthesis of seft-assembled different shaped mesoporous Co3O4: efficient visible light photocatalysts, Phys. Chem. Chem. Phys. 17 (2015) 10160–10169. [30] A.B. Murphy, Band-gap determination from diffuse reflectance measurements of semiconductor films, and application to photo electrochemical water-splitting, Sol. Energy Mater. Sol. Cells 91 (2007) 1326–1337. [31] A. Hu, R. Liang, X. Zhang, S. Kurdi, D. Luong, H. Huang, P. Peng, E. Marzbanrad, K.D. Oakes, Y. Zhou, M.R. Servos, Enhanced

photocatalytic degradation of dyes by TiO2 nanobelts with hierarchical structures, J. Photochem. Photobiol.: A Chem. 256 (2013) 7–15. [32] Z. Wang, W. Cai, X. Hong, X. Zhao, F. Xu, C. Cai, Photocatalytic degradation of phenol in aqueous nitrogen-doped TiO2 suspensions with various light sources, Appl. Catal. B: Environ. 57 (2005) 223–231. [33] Z. Zheng, H. Liu, J. Ye, J. Zhao, E.R. Waclawik, H. Zhu, Structure and contribution to photocatalytic activity of the interfaces in nanofibres with mixed anatase and TiO2(B) phases, J. Mol. Cata. A: Chem. 316 (2010) 75–82. [34] H. Tada, M. Tanaka, Dependence of TiO2 photocatalytic activity upon its film thickness, Langmuir 13 (1997) 360–364.