Influence of mesopore distribution on photocatalytic behaviors of anatase TiO2 spherical nanostructures

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JIEC-2977; No. of Pages 7 Journal of Industrial and Engineering Chemistry xxx (2016) xxx–xxx

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Influence of mesopore distribution on photocatalytic behaviors of anatase TiO2 spherical nanostructures Hye Mee Yang, Soo-Jin Park * Department of Chemistry, Inha University, 100 Inharo, Incheon 402-751, South Korea

A R T I C L E I N F O

Article history: Received 27 January 2016 Received in revised form 16 April 2016 Accepted 2 July 2016 Available online xxx Keywords: Mesoporous TiO2 Solvothermal process Ammonia treatment Pore size Photocatalytic activity

A B S T R A C T

Mesoporous anatase TiO2 spheres were synthesized via sol–gel and solvothermal processes using ammonia. The crystallite size was shown to increase with the increasing amount of aqueous ammonia used, leading to a larger pore size. The mesopore volume of the materials reaches up to 0.308 cm3 g1. The TiO2 spheres showed improved catalytic activity in organic dye photodegradation owing to their high specific surface area, pore volume, and optimal pore size, which enable an increase of light absorption capability, as well as fast mass transfer by the optimal mesopore structure. ß 2016 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.

Introduction The depletion of clean water sources has become a worldwide issue due to increasingly rapid industrialization, population growth, long-term droughts, and lack of natural replenishment. Since wastewater constitutes one of the largest possible water resources, effective and economical water treatment technologies must be developed. Currently, photocatalysis attracts significant attention in the environmental cleaning field [1–7]. Photocatalytic activity depends on several factors, including surface properties, crystalline phase, particle size, and pore structure [8]. For the synthesis of a highly active photocatalyst, these factors need to be adjusted. Titanium dioxide (TiO2) has potential advantages as a photocatalyst due to its strong oxidizing power and high chemical inertness [9]. In addition, TiO2 is non-toxic, insoluble in water, cheap, and easy to maintain. Therefore, the assessment of TiO2 suitability for practical application is imperative [10]. For this assessment, controlled synthesis of TiO2 with welldefined structural architectures is important [11]. Various morphologies of TiO2 such as films, spheres, three-dimensional (3D) architectures, and tubes have been investigated using different synthetic strategies. TiO2 spheres have many characteristics that make them ideal candidates for photon-related

* Corresponding author. Tel.: +82 32 860 8438; fax: +82 32 860 8438. E-mail address: [email protected] (S.-J. Park).

applications, e.g. particle sizes comparable to optical wavelengths and relatively high refractive indexes. In order to optimize the performance of TiO2, it is desirable to combine surface modification and morphology. In photocatalysis, the pore structure is largely responsible for diffused photon transport. The ramified network of mesopores favors diffusion of molecules due to its molecular-sized channel openings and cavities, and its potential for molecule recognition owing to unique pore structures [12–19]. Therefore, TiO2 sphere to hierarchical structures with tunable architectures is an effective way to make mesoporous materials including high specific surface area. The mesostructures made in this way have high porosity, firm structure and intrinsic properties of nanoparticles including high surface areas and catalytic activities [20]. Various methods have been employed to synthesize TiO2 spheres, including wet chemical techniques such as sol–gel, precipitation, and hydrothermal/solvothermal processes [21,22]. The sol–gel method has many advantages, since the material composition and texture can be influenced by the proper combination of precursors and processing conditions. Solvothermal processes, using solvents at high pressure and mild temperature, also show promise for applications in nanotechnology [23]. The use of organic solvents in solvothermal synthesis facilitates the control of product structure and morphology due to their shapeand size-dependent properties. Recently, a combined sol–gel/ solvothermal process was applied to the preparation of titania with special microstructure and high surface area [24].

http://dx.doi.org/10.1016/j.jiec.2016.07.004 1226-086X/ß 2016 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.

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In this study, we have prepared mesoporous anatase TiO2 spheres by sol–gel and solvothermal processes utilizing ammonia treatment, and have shown them to be excellent photocatalysts. Their photocatalytic efficiency was investigated by monitoring the photodegradation of rhodamine B under simulated solar light irradiation. The effects of crystallite size, mesopore structure, and average mesopore size in relation to the ammonia treatment on photocatalytic behaviors are discussed in this work. Experimental Synthesis of TiO2 spheres Mesoporous TiO2 spheres were prepared by a two-step method combining sol–gel and solvothermal techniques. The compounds used in the synthesis were titanium(IV) isopropoxide (TTIP, 97%, Sigma–Aldrich), hexadecyltrimethylammonium bromide (CTAB, 98%, Sigma–Aldrich), absolute ethanol (99.9%, Duksan), aqueous ammonia (25%, Duksan), potassium chloride (99.0%, Samchun), and deionized water. A-TiO2 spheres were synthesized via a sol–gel process in presence of CTAB as a structure-directing agent. The monodispersity was controlled by adjusting the ionic strength of the solution with KCl [25]. 5.296 g of CTAB were dissolved in 800 mL of absolute ethanol, followed by addition of 3.20 mL of aqueous KCl (0.1 M). 17.6 mL of TTIP were dispersed in the above solution under vigorous stirring at room temperature. The resulting suspension was kept static at room temperature for 18 h. Subsequently, the TiO2 spheres were filtered and dried in air at ambient temperature. In order to obtain mesoporous anatase TiO2 spheres with highly crystalline frameworks, 1.6 g of the as-prepared A-TiO2 was dispersed in a mixture of ethanol (20 mL) and deionized water (10 mL). Different amounts of aqueous ammonia were subsequently added (0.0, 1.0, 2.0, 4.0, and 7.0 mL for M-TiO2, M1-TiO2, M2-TiO2, M4-TiO2, and M7-TiO2, respectively). The mixtures were sealed within a Teflon-lined autoclave and heated at 160 8C for 16 h. The solid products were collected by filtration and air-dried at room temperature. The resultant powders were calcined at 500 8C in air for 2 h to remove organic impurities.

Characterization The structural properties of the materials were examined by X-ray diffraction (XRD, D2 PHASER, Bruker). The average particle size was estimated using the Sherrer formula (1) [26]: d¼

ðKlÞ FWHMcos u

(1)

here d is the crystallite size, FWHM is the observed full width at half maximum, u is the Bragg angle, K is a constant that usually takes a value of 0.9, and l is the wavelength (Cu Ka, 0.1542 nm) of the X-ray diffraction radiation. The morphology was investigated using high-resolution scanning electron microscopy (HR-SEM, SU 8010, Hitachi) and field-emission transmission electron microscopy (FE-TEM, JEM2100F, JEOL). X-ray photoelectron spectroscopy (XPS, K-Alpha, Thermo Scientific) measurements were conducted using a 200 kV accelerating voltage and a monochromated Al Ka X-ray source. Fourier transform infrared spectroscopy (FTIR, VERTEX 80V, Bruker) was performed using KBr powder for sample preparation. Gas adsorption analysis (Model Belsorp Max instrument, BEL) was used to characterize specific surface area, total pore volume, mesopore volume, and mesopore size. The samples were degassed at 423 K for 10 h until a residual pressure of less than 0.01 kPa was obtained. The specific surface areas of the prepared samples were determined using the Brunauer–Emmett–Teller (BET) equation. The amount of nitrogen adsorbed at a relative pressure (PP01) of 0.99 was used to calculate the total pore volume. The mesopore volume, average diameter, and size distribution were calculated using the Barrett–Joyner–Halenda (BJH) equation. Diffuse reflection spectroscopy (DRS) was performed on a UV-vis spectrophotometer (S-3100, Scinco) with an SA-13.1 diffuse reflector, and the sample band gap was calculated using the Kubelka–Munk remission function corresponding to the spectrum. Photoluminescence (PL, Ram Boss, DONGWOO OPTRON) of the prepared TiO2 samples was measured using a He–Cd laser with a 325 nm excitation wavelength. Photocatalytic activity The photocatalytic activity of the prepared materials was evaluated by monitoring the degradation of aqueous rhodamine B

Fig. 1. HR-SEM images of A-TiO2 (a), M-TiO2 (b), M1-TiO2 (c), M2-TiO2 (d), M4-TiO2 (e), and M7-TiO2 (f).

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(RhB) solutions. Prior to being exposed to light, a mixture of 50 mL of RhB solution (10 ppm) and 20 mg of the catalyst was stirred in the dark for 30 min to ensure uniform dispersion and establish an adsorption-desorption equilibrium. The photocatalytic reaction was performed under simulated solar light, with irradiation times of 15, 30, 45, and 60 min. The residual RhB concentration was determined by measuring its absorption at 550 nm. In cyclic photodegradation experiments, the TiO2 photocatalyst was collected and dried. The recovered catalyst was used for five consecutive cycles performed under the same conditions. Results and discussion Characterization of TiO2 spheres Figs. 1 and 2 show the morphology and surface features of the as-synthesized TiO2 spheres, characterized by HR-SEM and FE-TEM. A-TiO2 is composed of spheres with 3.0 mm diameter possessing a smooth surface (Figs. 1(a) and 2(a)). The corresponding XRD pattern (Fig. 3(a)) indicates that A-TiO2 is amorphous in nature. M-TiO2 spheres obtained after the solvothermal process and calcination are 2.6 mm in diameter, indicating 14% shrinkage, and have relatively rough surfaces (Figs. 1(b) and 2(b)), compared to those seen in A-TiO2 (Fig. 1(a)). The well-resolved diffraction peaks in the XRD spectrum of M-TiO2 at 2u = 258, 378, 488, 538, 548, 628, 688, 698, and 748 correspond to the (1 0 1), (0 0 4), (2 0 0), (1 0 5), (2 1 1), (2 0 4), (1 1 6), (2 2 0), and (2 1 5) crystal planes, respectively, indicating the presence of the anatase structure (JCPDS card 21-1272) [27–29]. Scheme 1 depicts the M-TiO2 formation mechanism, which is a self-transformation of metastable aggregated species in A-TiO2 samples, accompanied by localized Ostwald ripening. During the initial stages of their formation, TiO2 species composed of the kinetically favored amorphous crystallites were aggregated, as confirmed by the XRD spectrum (Fig. 3(a)) [30]. At this stage, the as-synthesized ATiO2 species were not thermodynamically equilibrated and became metastable due to their high surface energy [31,32]. To decrease the total surface energy, these metastable species undergo localized Ostwald ripening. Consequently, the particles in the TiO2 spheres become larger, and the sphere diameter is decreased. Simultaneously, the mesopores are concomitantly formed from intercrystallite voids in the aggregated species

Fig. 3. XRD patterns of A-TiO2 (a), M-TiO2 (b), M1-TiO2 (c), M2-TiO2 (d), M4-TiO2 (e), and M7-TiO2 (f).

Scheme 1. Formation mechanism of mesoporous anatase TiO2 spheres.

[33,34]. As shown in Figs. 1 and 2, the mesoporous TiO2 spheres prepared using the solvothermal process with aqueous ammonia have rougher surfaces, larger particles, and smaller diameters. The images illustrate that the increased size of the TiO2 particles promotes the formation of larger pores on the sphere surface. Fig. 3 shows that the full-width at half-maximum (FWHM) of the anatase

Fig. 2. FE-TEM images of A-TiO2 (a), M-TiO2 (b), M1-TiO2 (c), M2-TiO2 (d), M4-TiO2 (e), and M7-TiO2 (f).

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Table 1 Summary of the physicochemical properties of the prepared-TiO2 spheres. Specimens

a

b

A-TiO2 M-TiO2 M1-TiO2 M2-TiO2 M4-TiO2 M7-TiO2

2 55 52 67 67 43

– 0.181 0.227 0.302 0.318 0.231

a b c d e

SBET (m2 g1)

VTotal (cm3 g1)

c

VMeso (cm3 g1)

d

e

Band gap energy (eV)

– 0.156 0.180 0.303 0.308 0.200

– 9.3 10.7 18.5 21.3 24.4

– 18.0 21.8 22.8 25.9 29.6

3.39 3.32 3.30 3.29 3.27 3.26

DP (nm)

DXRD(nm)

SBET, specific surface area calculated using Brunauer–‘Emmett–Teller equation at a relative pressure range of 0.03–0.22. VTotal, total pore volume estimated at a relative pressure pp1 0 ¼ 0:99. VMeso, mesopore volume determined from the Barrett-Joyner-Halenda (BJH) equation. Dp, average mesopore diameter calculated using the BJH equation. DXRD, crystallite diameter calculated using scherrer formula to the (1 0 1) anatase peak.

peaks decreases when aqueous ammonia is added, suggesting an increase in crystallite size. In this case, the anatase (1 0 1) reflection plane (2u = 258) was selected to estimate the crystallite sizes using the Scherrer formula, based on FWHM broadening. The average crystallite diameter for M-TiO2 is 18.0 nm (Table 1). Adding aqueous ammonia increased the crystallite size; the TiO2 spheres in the M1-TiO2, M2-TiO2, M4-TiO2, and M7-TiO2 consist of progressively larger crystallites with increasing amounts of aqueous ammonia added, because addition of aqueous ammonia promotes localized Ostwald ripening. Thus, it can be assumed that mesopore structure is developed through the solvothermal process with ammonia treatment [35,36]. The XPS analysis results in Fig. 4(a) demonstrate that Ti2p, O1s, and C1s core levels were clearly detected for both M-TiO2 and

Fig. 4. XPS spectra of the survey scan (a) and Ti2p (b) for M-TiO2 and M4-TiO2.

M4-TiO2; with the carbon mainly ascribed to hydrocarbons artifacts of XPS [35]. The results indicate that the addition of ammonia has no influence on nitrogen doping of TiO2 in the solvothermal process. The Ti2p core level (Fig. 4(b)) consists of two symmetrical deconvoluted peaks with binding energies of 495.3 and 464.9 eV, attributed to Ti2p3/2 and Ti2p1/2 spin-orbital splitting electron in the formation of Ti4+ [37]. Fig. 5 shows the textural properties of the as-synthesized samples, characterized using nitrogen gas adsorption-desorption (Table 1). For A-TiO2, the specific surface area and total pore volume were 2 m2 g1 and almost 0 cm3 g1, respectively, indicating a non-porous dense structure. Fig. 5(A) shows the nitrogen adsorption–desorption isotherms for all samples prepared. After the solvothermal process, classical type IV isotherms with hysteresis were observed in all cases. The results demonstrate the presence of mesoporous materials according to IUPAC classification [38]. The

Fig. 5. Nitrogen adsorption–desorption isotherms (A) and mesopore size distributions (B) of M-TiO2 (a), M1-TiO2 (b), M2-TiO2 (c), M4-TiO2 (d), and M7TiO2 (e). For clarity, the isotherms in (A) are offset by 200 cm3 g1 for M1-TiO2, 400 cm3 g1 for M2-TiO2, 600 cm3 g1 for M4-TiO2, and 800 cm3 g1 for M7-TiO2.

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mesopore size distribution (Fig. 5(B)) was obtained from the desorption branch of isotherms using the BJH equation. M-TiO2 has a higher specific surface area (55 m2 g1), total pore volume (0.181 cm3 g1), and mesopore volume (0.156 cm3 g1) than A-TiO2. It is hypothesized that intercrystallite voids were generated during the solvothermal process. Increasing the amount of aqueous ammonia used increased the specific surface area and pore volume of the TiO2 spheres, as well as the mesopore diameter. This instigated oriented crystal growth, giving rise to elongated particles, is a result of localized Ostwald ripening, promoted by the addition of ammonia. However, the specific surface area, total pore volume, and mesopore volume declined when more than 4 mL of aqueous ammonia were used, since the shrinkage of the TiO2 spheres along with enlargement of the TiO2 particles results in a corresponding decrease of the total volume, which ensures a well-developed pore structure, as illustrated by M7-TiO2 [25,39]. Diffuse reflectance and PL emission spectra were recorded to investigate the optical properties of the materials. Fig. 6(a) illustrates the UV–vis/DRS spectra of A-TiO2 spheres, M-TiO2 spheres, and TiO2 spheres prepared in presence of aqueous ammonia. After the solvothermal process, an increase in the possible absorbance wavelength range (red shift) is indicated. As found in the XRD analysis, the crystallite sizes increased with the addition of aqueous ammonia. The absorption band shifts toward the longer wavelength region as the crystallite sizes of the prepared samples increase. As shown in the inset of Fig. 6(a), bandgap energy was calculated using the Kubelka–Munk remission function corresponding to each spectrum. The band gap energy of the samples reduced from 3.39 to 3.26 eV with increasing crystallite size, as illustrated in Table 1. This phenomenon could

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be attributed to the quantum size effects of semiconductors, where the band-gap energy decreases as the crystallite size increases. However, there was scant distinction of the band-gap energy to rationalize photocatalytic activity improvement in M-TiO2 and TiO2 spheres after ammonia treatment, except for A-TiO2 [40,41]. PL measurements were used to detect the efficiency of charge carrier trapping, immigration, and transfer, and to investigate the fate of electron–hole pairs in semiconductors [42,43]. The PL emission spectra of M-TiO2 and M4-TiO2 are shown in Fig. 6(b). The samples show a broad emission band between 390 and 600 nm, revealing peaks that are nearly identical in shape and position [35,44]. This band was ascribed to bond-exciton emission due to the trapping of free excitons by titanate groups in the vicinity of defects [45]. Development of the pore structure by addition of ammonia retards surface recombination, thus lowering the intensity of the PL emission band [46]. Photodegradation of rhodamine B The photocatalytic activity of the TiO2 spheres was assessed by monitoring the degradation of RhB by UV–vis absorption spectroscopy. An aliquot of a 10 ppm RhB solution was added to each sample and irradiated by simulated solar light, followed by UV–vis absorption measurement. The photocatalytic degradation on TiO2 spheres was initiated by the generation of electrons and holes by irradiation, with the complete mechanism shown in Eqs. (2)–(7) [47,48]. þ

TiO2 þ solar light ! TiO2 þ ðe CB þ hVB Þðsolar light absorptionÞ (2) þ

TiO2 ðe CB þ hVB Þ ! heat þ TiO2 ðrecombinationÞ

(3)

 TiO2 ðe CB Þ þ O2 ! TiO2 þ O2

(4)

þ

(5)

TiO2 ðhVB Þ þ OH ! OH

þ

(6)

RhB þ OH ! degradationproducts

(7)

TiO2 ðhVB Þ þ H2 O ! Hþ þ OH

Initially, the RhB molecules are absorbed onto the TiO2 spheres to form a stable complex. When the RhB/TiO2 complex is illuminated, the TiO2 nanoparticles absorb light and generate electron/hole pairs, as shown in Eq. (2). The recombination of charge carriers with the release of heat possibly occurs, as shown in Eq. (3). The photo-generated electrons (e) are transferred to the catalyst surface and react with dissolved oxygen to form superoxide radicals (O2–), whereas the positively charged holes (h+) react with H2O and OH–to generate hydroxyl radicals (OH), as shown in Eqs. (4)–(6). The hydroxyl radicals are reactive enough to mineralize RhB, as shown in Eq. (7) [49]. Fig. 7 shows the first-order reaction kinetics used to model the change in RhB concentration with reaction time. The graph used to calculate the specific reaction rate constant utilizes the Langmuir– Hinshelwood rate Eq. (8) to quantitatively describe the change of RhB concentration as a function of reaction time. ln

Fig. 6. UV–vis/DRS spectra and plots of the Kubelka–Munk remission function corresponding to the spectra (inset) (a), and the photoluminescence (PL) spectra of the prepared samples (b).

c ¼ K app t c0

(8)

The terms c0 and c refer to the initial RhB concentration and the concentration after some elapsed photodegradation time t, respectively, while Kapp is the pseudo-first-order rate constant (min1). The rate constants and regression coefficients are listed in Table 2. The photocatalytic activity of the TiO2 sphere samples prepared solvothermally were better than that of A-TiO2, owing to the formation of mesoporous structures. Of all the samples prepared, M4-TiO2 exhibited the most impressive photodegradation efficiency

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processes. The amount of used aqueous ammonia had a crucial effect on the crystallite size, which increased concomitantly with the amount of ammonia added, leading to a well-developed mesopore structure. The photocatalytic activity of the TiO2 spheres was considerably enhanced by the solvothermal process and ammonia treatment. M4-TiO2 exhibited the best photocatalytic activity under simulated solar light irradiation, owing to its optimal pore structure. Higher pore volume and specific surface area improved the adsorption capacity, leading to faster charge separation and mass transfer in the photocatalytic reaction. Furthermore, the optimal pore size enabled effective molecular transport, leading to the enhanced diffusion of the RhB dye molecules to the active sites. Moreover, the prepared samples showed high durability and stability even after five repeated catalytic cycles. These results suggest that crystalline mesoporous TiO2 spheres are highly suitable for practical application in the purification of industrial wastewater containing non-degradable organic pollutants. Acknowledgment This work was supported by Nano-Convergence Foundation (www.nanotech2020.org) funded by the Ministry of Science, ICT and Future Planning (MSIP, Korea) & the Ministry of Trade, Industry and Energy (MOTIE, Korea) [Project Name: Development of Graphen-TiO2 flake for electromagnetic wave shield fiber]. References Fig. 7. Photodegradation behavior of rhodamine B under simulated solar light irradiation (a) and recycling of M4-TiO2 for RhB photodegradation (b).

Table 2 The rate constants and regression coefficients of rhodamine B photodegradation. Specimens A-TiO2 M-TiO2 M1-TiO2 M2-TiO2 M4-TiO2 M7-TiO2

Rate constants (min1) 2

0.01  10 0.20  102 0.28  102 0.66  102 1.00  102 0.53  102

R2 0.70 0.68 0.86 0.99 0.97 0.99

and rate. Higher pore volume and specific surface area permit more RhB dye molecules to enter the TiO2 spheres; therefore, enhancement of adsorption capacity by optimal mesoporous structures leads to faster mass transfer. Additionally, these mesoporous structures improves the light absorption capability and charge separation [50,51]. However, when compared with M2-TiO2, the increase in specific surface area and pore volume of M4-TiO2 is not enough to explain the improvement in efficiency of photocatalysis. The result can be rationalized by an optimal pore size that favors enhanced diffusion of the RhB dye molecules to the active sites due to effective molecular transport [52,53]. The reusability of catalytic materials is an essential issue for practical applications [54]. Hence, we utilized the same batch of the M4-TiO2 catalyst in five consecutive RhB photodegradation cycles to find that its photocatalytic activity was only marginally reduced, signifying high durability and stability [55]. Conclusions In summary, highly crystalline mesoporous TiO2 spheres were successfully prepared by employing sol–gel and solvothermal

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