TiO2 nanocomposites

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Jul 17, 2014 - ment, they can alter the ecological balance in the environment as these ... that initiate redox reactions with molecular species adsorbed.
Environ Sci Pollut Res DOI 10.1007/s11356-014-3356-z

RESEARCH ARTICLE

Synthesis of mesoporous Mn/TiO2 nanocomposites and investigating the photocatalytic properties in aqueous systems Ekemena Oghenovoh Oseghe & Patrick Gathura Ndungu & Sreekanth Babu Jonnalagadda

Received: 16 May 2014 / Accepted: 17 July 2014 # Springer-Verlag Berlin Heidelberg 2014

Abstract Mesoporous 20 wt% Mn/TiO2 nanocomposites were synthesized adopting modified sol–gel method at different pH (pH=2, 7 and 11) conditions and calcined at 400 °C. Based on the characteristics of the 20 wt% Mn/TiO2 nanocomposites synthesized at pH 11, same procedure was adopted for the synthesis of different wt% Mn/TiO2. The nanocomposite samples and their surface properties were characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), mapping, inductively coupled plasma optical emission spectrometry (ICP-OES), Fourier transform infrared (FTIR), and fluorescence spectrometry. The nanocomposites existed in the anatase phase of TiO2 with no peak assigned to Mn on the diffractogram. The photocatalytic activities of the materials were evaluated by monitoring degradation of a model dye (methylene blue (MB)) in presence of visible light and ozone. The nanocomposite synthesized under neutral condition (pH=7) exhibited the best photocatalytic activity resulting from its relatively smaller crystal size (5.98 nm) and larger pore volume (0.30 cm3/g). One percentage of weight Mn/TiO2 showed 100 % decolouration of MB in the presence of O3 after 100 min.

Keywords Mn/TiO2 nanocomposite . pH water . Mesoporous . Photocatalytic ozonation . Visible light

Responsible editor: Philippe Garrigues E. O. Oseghe : P. G. Ndungu : S. B. Jonnalagadda (*) School of Chemistry and Physics, University of Kwa-Zulu Natal, Westville Campus, Private Bag X 54001, Durban 4000, South Africa e-mail: [email protected]

Introduction In recent times, water as a subset of the environment has been of serious concern. It stems from the fact that industries such as textile, plastics, and paper, and pulp, to name a few, generate streams of waste effluents containing significant amounts of organic pollutants such as organic dyes (Jiang et al.2012; Rauf et al. 2010; Wu et al. 2013). When these compounds are discharged to the larger aquatic system without prior treatment, they can alter the ecological balance in the environment as these molecules have carcinogenic and mutagenic properties towards aquatic organisms and humans as they bioaccumulate and biomagnifies up the food chain (Xu et al.2011). Currently, conventional methods such as adsorption and coagulation are being supplemented by advanced oxidation processes (AOPs) (photocatalysis, Fenton method, ozonalysis, sonolysis, photolysis) (Rivas et al. 2011). Of all these processes, photocatalysis seems to be most versatile and promising since UV or visible light source in the presence of the catalyst, would favour the oxidation of organic pollutants present in waste effluents (Rauf et al. 2011). Mesoporous TiO2 as a semiconductor, is known to have high surface area, excellent stability, can be regarded as nontoxic, and has a highly porous (2–50 nm in diameter) framework characteristics, and as such it is ideally suited as a photocatalytic material. This continuity in structure has some interesting physical–chemical implications, such as ease in the transfer of electrons within the material and from a practical viewpoint, it can aide in the recovery of catalyst material when compared to separate individual nanoparticles (Ismail and Bahnemann 2011). The photocatalytic activity of TiO2 is mainly dependent on the incident electromagnetic radiation having equal or higher photon energy than its band gap energy (3.2 eV). In general, the photocatalytic mechanism is built around the excitation of electrons from the valence band (VB) to the conduction band (CB) of the TiO2, leaving holes in the

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valence band. It is the electrons (e−) and holes (h+) generated that initiate redox reactions with molecular species adsorbed on the surface of the catalyst (TiO2) (Binas et al. 2012; Xue et al. 2008). However, its wide band gap energy and high electron–hole recombination rate limits its application under visible light conditions. Doping or surface modification by transition metals is considered one of the most efficient methods in reducing electron–hole recombination (Su et al. 2013). The metal on the surface of the photocatalyst can act as a trap site for the photo-generated electrons, which can prevent electron–hole recombination, and thereby improve photocatalytic activity al. (Binas et al. 2012; Leghari et al. 2011; Li et al. 2011). Li et al. (2011) developed a method for the synthesis of ordered anatase Mn–TiO2 nanospheres with controllable sizes in the range 200–300 nm (Li et al. 2011). The nanocomposite reportedly exhibited a high photocatalytic activity in degrading rhodamine B using visible light, and was ascribed to the electron interaction between Mn and TiO2. Zou et al. (2008) also reported on the preparation of a mesoporous structured MnO 2 –TiO 2 nanocrystal photocatalyst through a modified sol–gel process. The composite showed considerable visible-light photocatalytic activity for the degradation of methylene blue (MB), which was attributed to the modification of the electronic energy band structure of TiO2 by the uniform coating of MnO2. This extended the photocatalyst absorbance into the visible light region. Photocatalysed oxidation is a well-known advanced oxidation process, where hydroxyl radicals are formed from the combined effects of radiation, a catalyst and an oxidant. Oxidants such as H2O2, O2, and O3 over time have been employed to accelerate the process (Rodriguez et al. 2012). The use of O3 has received considerable attention due to its relatively high oxidant characteristics and its ability to generate hydoxyl radicals (OH•) (Aguinaco et al. 2012). O3 would either react with organic pollutants directly or indirectly via O3 decomposition and the formation of OH•. The drawback of ozonation is the slow rate of organic pollutant mineralization. As opposed to just ozonation, photocatalytic ozonation facilitates O3 decomposition and OH• formation leading to a fast and more effective mineralization of organic pollutants (Mahmoodi 2011). Maddila et al. (2013), reported on ozone catalysed degradation of trichlorophenol in the presence of Ce–Zr-loaded metal oxides. In this paper, we report on the effect of pH on the surface properties of mesoporous Mn–TiO2 nanocomposites with reduced band gap energy. The synergy between different wt% Mn–TiO2 synthesized under basic condition (based on its reduced band gap energy and better adsorptive characteristics) and O3 in the decolouration of methylene blue as a model dye is also reported.

Materials and method Catalyst synthesis For the synthesis of the photocatalytic materials, the sol–gel techniques developed by Aman et al. (2012) and Ismail and Bahnemann (2011) were adopted. The precursors used were titanium (IV) isopropoxide (TIP), purchased from SigmaAldrich (MW=284.22, 97 %), and manganese (II) acetate tetrahydrate, which was bought from Alfa-Aesar (MW= 245.08, Mn 22 %). The structure directing agent was a nonionic surfactant (Pluronic F-127, Sigma-Aldrich), and the solvent used consisted of a mixture of 1-propanol (SigmaAldrich, HPLC grade) and ultra-pure water (1:7.5 by volume). To synthesize the 20 wt% Mn–TiO2 samples at different pH conditions, 150 mL of ultra-pure water was mixed with a few drops of either 0.1 M HNO3 or 0.1 M NaOH to adjust the pH to values of 2, 7 or 11. Then 4.0 g of the surfactant was dissolved in 10 mL of 1-propanol, and then added to the pH adjusted water and allowed to stir for 10 min. Twenty grams of the titanium (IV) isopropoxide was mixed with 10 mL of 1-propanol, and then added slowly drop by drop to the surfactant solvent mixture. Finally, after 5 min, the required mass (5.009 g) of the manganese precursor was introduced into the solution, while stirring. After approximately 4.0 h, the solutions were centrifuged and the precipitates recovered. The samples were placed in an alumina boat and transferred to a horizontally aligned tube furnace. Samples were then dried at 100 °C (4.0 h), then heat treated at 200 °C for 1 h, and finally calcined at 400 °C for 4 h (ramp rate from 200–400 °C was 2 °C/min). The resulting samples were marked as TB400, TMA400, TMB400, and TMN400, where T = titanium, M = manganese, A = acidic condition (pH=2), B = basic condition (pH=11), N = neutral (pH=7), and 400 = calcination temperature in Celsius. Same procedure was adopted in the synthesis of 1, 5, and 10 wt% Mn–TiO2 under basic condition (pH=11). Catalyst characterization The surface morphology of the mesoporous materials were examined on a Leo 1450 Scanning Electron Microscope equipped with energy dispersive X-ray analyser (EDX). Detailed physical structural characteristics were observed with a JEOL JEM-1010 transmission electron microscope (TEM). The textural characteristics were determined by using a Micrometrics TriStar II 3030 instrument. Samples were degassed prior to textural analysis by using a Micrometrics Flow Prep (060) under N2 flow at 90 °C for 1 h, and then increasing the temperature to 200 °C, and leaving the samples to de-gas overnight. The phases of the materials were observed using powder X-ray diffraction (XRD) conducted on a Bruker D8 Advance instrument, equipped with a XRK 900 reaction chamber, a TCU 750 temperature control unit and a CuKα radiation (λ=0.15406 nm). UV-visible diffuse

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reflectance spectra were recorded with an Ocean Optics high resolution spectrometer (HR2000+) equipped with an integrating sphere accessory, using BaSO4 as a reference. Infrared spectra of the samples were recorded using Fourier transmission infrared (FTIR) spectrometer (PerkinElmer spectrum 100 series with universal ATR accessory). Inductively coupled plasma optical emission spectrometer (ICP-OES) (Optima 5300 DV), was used in analysing the concentration of the elements present in the samples after digesting in concentrated H2SO4. Dosimetry method was used in determining OH radical formation. In a typical procedure, 100 mg of the composite samples was added to 200 mL of the 0.1 mM terephthalic acid solution in 2 mM NaOH, and was then irradiated with visible light. Sampling was performed in every 20 min. Solution after centrifuging at 14,000 rpm was analysed on a Perkin Elmer LS 55 fluorescence spectrophotometer. The product of terephthalic acid hydroxylation, 2-hydroxyterephthalic acid, gave a peak at the wavelength of about 425 nm by the excitation with the wavelength of 310 nm. For photoluminescence analysis, the solid samples were excited at 310 nm on Perkin Elmer LS 55 fluorescence spectrophotometer. Thermogravimetric analysis of the samples was done under oxygen flow, 50 mL/min and heated from ambient temperature to 1,000 °C, 10 °C/min, using a Q seriesTM thermal analyser TGA (Q600). Photocatalytic and photocatalytic ozonation tests Photodegradation studies of methylene blue (MB) were carried out in a simple open quartz photochemical reactor using visible light. Before the light irradiation, 0.1 g of the photocatalyst was added to 200 mL of MB (5 mg/L) aqueous solution of pH 5.8, and then the mixture was thoroughly mixed by using an ultrasound bath (Selectech (PTY) LTD, UMC20, 400 W, 20 L), for 10 min, to disperse the powder. The mixture was then kept in the dark for 1 h, with continual stirring using a magnetic stirrer bar, to reach the adsorption–desorption equilibrium. The light source was a 500-W xenon lamp (CHFXQ500 W) with a UV filter that can cut off UV light with wavelengths shorter than 420 nm. At given time intervals (20 min), 1.0 mL aliquots were taken from the solution and immediately centrifuged at 14,000 rpm, the solution decanted, and the absorbance recorded on a UV-Visible spectrometer (Libra S6) at 665 nm. The determined absorbance was converted to concentration by using a calibration curve constructed from known concentrations of methylene blue. The same procedure was adopted in photocatalytic ozonation test but in the presence of O3 (0.05 M) generated through a Fischer Ozone 500 generator using a flow rate of 10 mL/min. In a typical experimental procedure, before the light irradiation, 0.1 g of the photocatalyst was added to 200 mL of MB (5 mg/L) aqueous solution (natural pH of 5.8), and then thoroughly dispersed by using an ultrasound bath (Selectech (PTY) LTD, UMC20, 400 W, 20 L), for 10 min. The mixture was then kept in the dark for 1 h, with continual

stirring using a magnetic stirrer bar, to reach the adsorption– desorption equilibrium. O3 was bubbled into the solution under stirring for 10 min prior to irradiation and was discontinued upon irradiation. Absorbance measurements of the sample after 20 min interval, was made on a UV-Visible spectrometer (Libra S6) at 665 nm. The measured absorbance was converted to concentration by using a calibration curve constructed from known concentrations of methylene blue.

Result and discussion Morphology Typical SEM images of the samples are presented in Fig. 1a–c. They all indicate agglomerated particles of irregular shapes. Also, the micrographs shows interparticle void which is typical for loose particle aggregates. Figure 1d, f illustrates the TEM and HRTEM images of TMB400 and TMA400, respectively. The TEM image of TMB400 shows that the nanocomposite consists of aggregates of particles, and the high resolution insert clearly shows the regular nano-sized channels (3.0 nm) that are expected within mesoporous materials. The wall thickness (2.6 nm) of the mesoporous composite materials (TMA400) can be observed in the HRTEM image (Fig. 1f). Elemental analysis To confirm the composition and concentration of the composite in the synthesized nanocomposites, ICP-OES analysis was done. Results from the ICP analysis (Table 1) show that samples made with a theoretical loading of 20 wt% under basic conditions (TMB400) had the least percentage of the dopant (9.94 wt% Mn/TiO2) and samples made under acidic conditions, TMA400, had the highest (13.58 wt% Mn/TiO2). Metal ions are held by electrostatic forces and exhibit an enhanced adsorption at low pH, hence the relatively higher percentage dopant exhibited by TMA400 (Ikhsan et al. 2004). With all samples with a target Mn wt% loading, the actual loading was lower, and this could be due to the slower hydrolysis kinetics of the manganese precursor when compared to the titania precursor, or a different hydrolysis mechanism with the two precursors. The slightly higher wt% loading with the TMA400 samples is attributed to the low pH, which seems to favour incorporation of the manganese. We comment on this result further when discussing the textural properties of the materials. XRD—structural analysis Figure 2 shows the XRD pattern of the synthesized nanocomposites calcined at 400 °C. All the samples show broad peaks that are typical for nano-sized materials, and the samples with

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a

b

c

d

e

f

Fig. 1 SEM (a TMA400, b TMB400, c TMN400); TEM (d TMB400); mapping (e TMB400); and HRTEM (f TMA400) micrographs of the composites

Mn, show relatively lower intensity titania peaks, and this can be attributed to Mn dopant. Peaks were observed at 2θ=25.3° (101), 37.9 (004), and 47.9 (200), which corresponds to the anatase phase of titania according to the JCPDS file no. 21– 1272. No diffraction line or phase for Mn was observed, which means Mn is either amorphous or incorporated into the anatase structure. The average crystallite size (D) was estimated with Scherrer’s equation. D¼

Kλ βcosθ

where K is a constant (dependent on crystal shape ≈ 0.9), λ is the X-ray wavelength (0.15418 nm), β is the full width at half maximum (FWHM) of the diffraction peak, and θ is the diffraction angle. The values of β and θ are taken for the crystal plane (101) of anatase phase. The crystallite sizes of the materials are shown in Table 1. From the pH-dependent composites, TMN400 had the smallest crystallite size while TMB400 exhibited a larger crystallite size, which implies that the composite sample synthesized under neutral pH inhibits the anatase grain growth more than other pH conditions. Alternatively, the amount of manganese incorporated into the titania framework may have inhibited the growth of the anatase grains; however,

Table 1 Summary of the data obtained from ICP analysis, textural characterisations, and XRD analysis on the various manganese-doped titania samples Samples

Ti (wt%)

BET average Crystal Lattice Unit cell Band Mn Surface area BET average Pore pore diameter volume particle size size (nm) parameter (Å) volume (Å)3 gap (eV) (wt%) (m2/g) (nm) (cm3/g) (nm)

TMA400

86.42

13.58

212.71

4.69

0.29

28.21

6.42

TMB400

90.06

9.94

203.00

5.01

0.25

29.56

6.74

TMN400

89.89

10.11

205.36

4.04

0.30

29.22

5.98

0 wt% Mn–TiO2

100.00

0.00

37.90

19.66

0.22

158.31

8.57

1 wt% Mn–TiO2

99.15

0.85

52.59

7.98

0.10

114.09

7.71

5 wt% Mn–TiO2

96.67

3.33

80.77

6.91

0.14

74.28

9.07

10 wt% Mn–TiO2

94.47

5.53

95.19

6.07

0.14

63.03

9.01

a=b=3.7762 c=9.4681 a=b=3.7808 c=9.4720 a=b=3.8014 c=9.4105 a=b=3.7945 c=9.5079 a=b=3.7945 c=9.4860 a=b=3.7956 c=9.4993 a=b=3.7922 c=9.4546

135.01

2.45

135.40

2.50

136.00

2.55

136.90

3.18

136.58

3.10

136.85

2.70

135.96

2.70

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(A)

2800

TMN400 TMA400 TMB400

A

2600

(B)

0 wt.% Mn-TiO2 1 wt.% Mn-TiO2 5 wt.% Mn-TiO2 10 wt.% Mn-TiO2

A

2400

Intensity

2200

A

2000

A

1800

A

A

A

A

A

1600

A

A

A A

A

A

A

1400 1200 1000 800

10

20

30

40

50

60

70

80

2Theta / degree

90 10

20

30

40

50

60

70

80

90

2Theta / degree

Fig. 2 XRD patterns of the a 20 wt% manganese-doped titania samples synthesized under acidic, neutral, and basic conditions and b the 0–10 wt% manganese-doped titania samples

the samples (TMA400) with largest amount of manganese had the second smallest crystallite size which shows that the Mn may not play a significant role in this regard. The sample synthesized under basic conditions, TMB400, had a slightly larger volume and crystallite size when compared to TMA400. This might be due to the relatively lower percent composition of the dopant (Mn) (Table 1). Transition metals at high pH values can precipitate, and this could explain the relatively low percentage composition of the dopant with sample TMB400 (Ikhsan et al. 2004). However, this effect is marginal since the difference with the neutral (TMN400) and basic samples (TMB400) is minimal. A good dispersion of Mn on TiO2 will cause an absence or drop in the intensity of the main titania peak, indicating a decrease in crystallinity (Xue et al. 2008). This could be the case with TMN400 having a relatively lower percentage of the dopant than TMA400 but exhibiting a lower peak intensity. The X-ray diffraction peaks of crystal plane (200) and (004) of anatase were selected to determine the lattice parameter of the composite materials. The lattice parameters are obtained by using the following equations: Bragg’s equation, nλ ¼ 2d sinθ 1 h2 þ k 2 l2 ¼ þ 2 2 2 a c d where n is an integer, θ is the angle between the incident ray and the scattering planes of (hkl), d is the distance between crystal planes of (hkl), λ is the wavelength of X-ray used, hkl is the crystal plane indices, and a and c are lattice parameters (in the anatase phase a=b≠c). The lattice parameters and the unit cell volume of all the samples are shown in Table 1. Dopants can act as a substitutional or interstitial impurity, and this can be determined by noting the changes observed

in the variation along the c-axis (Burns et al. 2004; Mohamed et al. 2007). Anatase contains four TiO2 molecules per unit cell having lattice parameters of a=3.78 Å and c=9.51 Å (Chauhan et al. 2012). Table 1 shows that the lattice parameter, a, of the acidic and basic samples does not change in comparison to the JCPDS value but the neutral TMN400 showed a slight increase. The low loading samples (1, 5, and 10 wt%), did not show any shift as the loading was increased, but were slightly larger than anatase. The c parameter for TMN400 is smaller than the value for anatase, and the values for the basic and acidic samples are similar but smaller than the value for anatase. This suggests the Mn acts as a substitutional dopant for the TMN400 (neutral synthesis conditions) and incorporates interstitially with the other doped materials that were synthesized under basic and acidic conditions (Mohamed et al. 2007). Textural characteristics Figure 3a shows the N2 adsorption–desorption isotherms and pore size distribution curves for the doped TiO2 synthesized using different pH. Based on IUPAC classifications, the materials exhibited type IV isotherms with an H2 type hysteresis loop (Sing et al. 1985). This implies that the pores within the materials are mainly within the mesoporous range (Luo et al. 2003). The pore size distribution is calculated from the BJH method (desorption curve). The materials showed pore size distribution (Fig. 3 inset) in the mesoporous size range (2.90– 4.30 nm) with TMN400 having a narrower pore size distribution, than the other doped samples synthesized under basic and acidic conditions. The BET surface area of TMA400 (Table 1) is slightly higher than the TMB400 and TMN400 samples, and from the ICP analysis, the TMA400 sample has a relatively higher percentage content of the dopant (Mn). N2 adsorption–

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0.18

0.08 0.06 0.04

3

0.02

-0.02 2

4

6

8

Pore diameter / nm

100 80 60

TMA400 TMB400 TMN400

40 20

0.0

0.2

0.4

0.6

0.8

1.0

Relative pressure (P/PO)

80

Pore volume / cm3/g.nm

100

0.10

0.00

120

1 wt.% Mn-TiO2

0.08

0.12

Quantity Adsorbed (cm /g STP)

140

0.14

Pore volume / cm³/g·nm

Quantity Adsorbed (cm³/g STP)

160

120

(a)

0.16

180

60

(b)

5 wt.% Mn-TiO2

0.07

10 wt.% Mn-TiO2

0.06 0.05 0.04 0.03 0.02 0.01 0.00 2

4

6

8

10

Pore diameter / nm

40 20 0

0.0

0.2

0.4 0.6 0.8 Relative pressure (P/PO)

1.0

Fig. 3 Nitrogen adsorption and desorption isotherms at 77 K for the a 20 wt% manganese-doped samples synthesized under acidic, neutral, and basic conditions and b 1–10 wt% manganese-doped samples

desorption isotherms and pore size distribution of the 1– 10 wt% Mn–TiO2, Fig. 3b, also exhibit type IV isotherm with H2 hysteresis loops. The insert (pore size distribution) also shows the pore sizes are distributed in the mesoporous region. The average pore diameter decreased and the pore volume decreased as the loading of Mn changed from 1–10 wt%. The increase in surface area with the 1–10 wt% samples can be due to the decrease in the average particle size (Table 1). This decrease in particle size does suggest that the Mn either coats the growing titania framework thus disrupting the formation of larger particles, a similar scenario has been suggested by other authors (Xue et al. 2008). Alternatively, the Mn is incorporated into the framework by substituting for the titanium centres and prevents the growth of the larger particles. The XRD results corroborate the scenario where the Mn substitutes for the titanium centres, and it is only apparent with TMA400, TMB400, and TMN400 samples which have an actual wt% of Mn greater than ~10 %, and a c parameter value smaller than the sample made with no Mn doping. A general mechanism for the formation of mesoporous systems, involves the template, pluronic F-127 in this work, forming micellular structures in the solvent system. Next, the precursor molecules surround the template, and then undergo hydrolysis and condensation steps. Thermal treatments to densify mesoporous walls and then remove the organic template are often applied (Ismail and Bahnemann 2011; Wan and Zhao 2007). The increase in surface area, increase in pore volume, and the decrease in pore diameter are results of the manganese dopant, as can be clearly seen when the wt% dopant increases from 1–10 to 20 wt% used for the different pH conditions. Titanium isopropoxide, the precursor used, undergoes rapid hydrolysis in aqueous solvent systems, and the lowering of the pH can control this process. Thus, it was not surprising when we observed that with the neutral and basic solutions precipitates formed very rapidly, and the acidic solution formed precipitates at a significantly slower rate. The

slower rate with the acidic solution can account for the slightly larger surface area, and marginal increase in manganese doping from the ICP results, but the neutral and basic systems have textural properties that are similar to the acidic conditions. The mechanism involved with the synthesis of mesoporous titania has been investigated by various authors. The mechanism involves the formation of nanocrystallites of titania that form aggregates with the template. In the previous studies, the formation of butanol from the hydrolysis of titanium butoxide resulted in a change of the micellular phases (Crepaldi et al. 2003; Nilsson et al. 2011). A similar process occurs with our system; however, the low surface area and larger pore size with the low loading manganese samples show the shift is not as dramatic as that reported by other others with butanol– water systems. Instead, at 20 wt% Mn doping, the release of acetate from the hydrolysis shifts the micellular phases significantly allowing for the formation of stable mesoporous structures. This suggests that the manganese dopant, at the higher wt%, plays a role in stabilizing and facilitating the formation of stable high surface area mesoporous systems, via the release of acetate, and does not depend on the pH conditions used. A possible mechanism that can explain these observations is that the titanium precursor undergoes hydrolysis to form amorphous nanoparticles that aggregate around the template (Xue et al. 2008). With the addition of the manganese acetate, the hydrolysis manganese precursor may result in some separate amorphous manganese nanoparticles or the manganese hydrolysis product coating the titanium-based amorphous nanoparticles. The XRD results did not show any separate manganese oxide phases, but instead, we observed that the manganese at the high wt% loading decreased the c parameter when compared to undoped titania, which shows the Mn substituted for some of the titanium centres.

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FTIR spectroscopy

Thermogravimetric analysis

The FTIR spectra of a commercial sample of TiO2 (Degussa P25), the nanocomposites synthesized using different pH conditions, and samples with 0–10 wt% manganese doping are presented in Fig. 4a, b. Peaks were observed in three main regions, specifically below 1,000, 1,200–1,800, and 3,000–3,600 cm−1. The broad band appearing between 3,269–3,304 cm−1 is due to the presence of water on the surface of the nanomaterials. A second peak that can be attributed to physisorbed water on the surface of the nanomaterials was observed at around 1,630 cm−1 (Bégin-Colin et al. 2009). It is interesting to note that the absorbance value of the broad band between 3,269– 3,304 cm−1 (attributed to the hydroxyl stretching mode from surface-sorbed water) decreases in the order of TMB400> TMN400>TMA400>Degussa P25. The same order is seen with hydroxyl bending mode observed at 1,630 cm−1. The low wt% samples did not show any trend with loading, which shows this observation, is likely due to pH conditions used rather than the introduction of manganese dopant. These results qualitatively show that TMB400 may have a stronger affinity for water molecules than the TMN400 or TMA400 samples. The hydroxyl groups plays an important role in water-based photocatalytic reactions, since they can capture any photoinduced holes that make it to the surface of the material, inhibiting electron–hole recombination, and forming hydroxyl radicals with high oxidation potentials l (Hou et al. 2008; Xu et al.2008). The bands below 1,000 cm−1 correspond to Ti–O– Ti vibrations (Neumann et al. 2005). Djaoued et al. (Djaoued et al. 2002) attributed bands appearing at 433 and 434 cm−1 to anatase phase and 491 cm−1 to rutile phase. Figure 4 shows bands appearing in the range of 422–449 cm−1, which correlates with the XRD results and proves all the materials are in the anatase phase with no rutile contributions.

Figure 5 shows results from the thermogravimetric analysis of TMA400, TMB400, and TMN400. The results showed that the as-prepared materials are quite stable with percentage weight loss ranging from 7.90–10.42 %, attributed to evaporation of physisorbed and chemisorbed surface H2O (Zhan et al. 2013). According to Dirk et al. (2010), thermogravimetric analysis can be used to quantify the amount of surface hydroxyl group. This is obtained by the following equation:

0.06

TMA400 TMB400 TMN400 Degussa P25

H2O

0.04

UV–vis diffuse reflectance spectroscopy Figure 6a presents the UV–vis diffuse reflectance spectra of the nanocomposite samples and the reference sample, Degussa P25. All of the doped samples synthesized under different pH conditions had their absorption edge extended into the visible light region, with their absorbance becoming stronger in the sequence of TMA400>TMB400>TMN400.

OH

0 wt.% Mn-TiO2 1 wt.% Mn-TiO2 5 wt.% Mn-TiO2 10 wt.% Mn-TiO2

(b)

0.08

Absorbance

0.06

0.03 0.02

OH 0.04

H2 O 0.02

0.01

0.00

0.00 -0.01

2  1000  W 3  Os

where NOH is the amount of surface hydroxyl group per square nanometer, W is the %weight loss of the materials in the temperature ranging between 180–1,000 °C, and O S is the BET surface area of the materials. Inserted table of Fig. 5 shows the calculated %weight loss and surface hydroxyl group of the materials with TMA400 having the least surface hydroxyl group and TMB400 possesses the highest. This correlates with the intensity of OH group of the materials as shown on the FTIR spectra (Fig. 4a)

0.10

(a)

0.05

Absorbance

N OH ¼

500

1000

1500

2000

2500 -1

Wavelength / cm

3000

3500

4000

-0.02

500

1000

1500

2000

2500

3000

3500

4000

Wavelength /cm-1

Fig. 4 FTIR spectra of the titania a synthesized at acidic, neutral, and basic pH with 20 wt% doping of manganese and b 0–10 wt% doping of manganese on titania

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band gap of TiO2 may be attributed to its crystallinity (Huang et al. 2000), or the presence of a new electronic states (Chauhan et al. 2012). Sample

Weight loss between 180-1000 °C (%)

OH density [n, OH/nm2]

TMA 400

2.2671

7.11

TMN 400

2.2554

7.32

TMB 400

2.5189

8.27

80

70

60

Photocatalytic activity Mn/TiO2 Typically, dyes can be used as a model organic pollutant to test the effectiveness of a newly developed photocatalyst. The main advantage is the ease of analysis, and the use of a modest spectrophotometer. To study the photocatalytic activity of the Mn/TiO2 nanocomposites, we chose methylene blue and carried out the experiments in a borosilicate reaction vessel. Figure 7 presents the data from the various photocatalytic experiments. The mesoporous samples (TMA400, TMN400, and TMB400) had excellent adsorptive properties in the absence of light, due to the large surface area, pore volume and pore size. From Fig. 7a, the largest amount of methylene blue adsorbed was with TMB400 which may have resulted from the relatively highest surface hydroxyl group as shown in the FTIR (Fig. 4a) and TGA (Fig. 5) results (Gunay et al. 2013; Song et al. 2012). Once the light energy was applied to the various photocatalysts, there was some further removal of dye from the water, due to degradation by the photocatalysts. The FTIR results qualitatively showed that TMB400 had a larger amount of surface water adsorbed onto its surface when compared to TMA400 and TMN400. Such a hydrophilic surface allows for the enhanced generation of hydroxyl radicals due to the capture of holes, by the physisorbed water, from electron–hole generation due to the incident photon energy. In the absence of any photocatalyst, there was only desorption and adsorption fluctuations on and off the surface of the reactor. In terms of overall removal of the dye, the mesoporous materials are much more effective than the benchmark catalysts, Degussa P25. However, when we examine the data in terms of photocatalysis and normalize with respect to the concentration after sorption equilibrium has been attained in the dark, we notice that photocatalytically, the mesoporous systems are actually less effective than the benchmark catalysts (Fig. 7b). Although P25 showed a better photocatalytic activity, the doped as-prepared catalysts still

TMA400 TMN400 TMB400

50 200

400

600

800

1000

Temperature / °C

Fig. 5 Thermogravimetric analysis of TMA400, TMN400, and TMB400

The lower wt% manganese-doped samples showed a similar redshift. The direct band gap energy was estimated by Tauc plot (Fig. 6) following the following equation: ðαhvÞ2 ¼ A hv−E g



where hv = photon energy, α = absorption coefficient (α= 4πk/λ; k is the absorption index or absorbance, λ is the wavelength in nm), Eg = energy band gap, A = constant. The value of band gap was determined by extrapolating the straight line portion of (αhv)2 on hv-axis as shown in the insert of Fig. 6b (Chauhan et al. 2012). Approximate direct band gap energies of the samples are obtained from the intercept of the tangent to the curves. All Mn-doped TiO2 exhibited a drop in band gap energy which should imply generation of electron–hole under visible light. Amongst the composite synthesized using different pH media, TMA400 had the lowest band gap energy (2.45 eV) while TMN400 had the highest of ≈ 2.55 eV. The same trend was observed in the Mn-doped TiO2 synthesized using basic media (1, 5, and 10 wt% Mn–TiO2). The drop in TiO2 band gap is due to the presence of the dopant. Chauhan et al. (2012) also observed a decrease in the band gap for Mn-doped TiO2. A shift in the Fig. 6 UV–vis diffuse reflectance spectra of the materials

1.4 1.2

Absorbance

0.006

Degussa P25 TMA400 TMB400 TMN400 0 wt.% Mn-TiO 2

(a)

1.0

0.005 0.004

1wt.% Mn-TiO 2

0.8

10 wt.% Mn-TiO 2

0.6

Degussa P25 TMA400 TMB400 TMN400 0 wt. % Mn-TiO 2

(b)

1 wt. % Mn-TiO 2 5 wt. % Mn-TiO 2

5 wt.% Mn-TiO 2

(hv)2

weight / %

90

0.003

10 wt. % Mn-TiO 2

0.002

0.4 0.2

0.001

0.0 400

500

600

700

Wavelength / nm

800

900

0.000 2.0

2.2

2.4

2.6

2.8

hv

3.0

3.2

3.4

Environ Sci Pollut Res 1.0

Adsorption in the dark

Photodegradation

(a)

(b)

1.00

0.9

0.95

0.8 0.90 0.7

C/C O

C\Co

Fig. 7 Photocatalytic activities of the materials in degrading MB a in the dark and under visible light, b under visible light after attaining adsorption–desorption equilibrium, and c different wt% Mn–TiO2 in the presence of O3

0.6 0.5 0.4

TMN400 TMA400 TMB400 P25 TB400 without catalyst

-60 -40 -20

0

0.85

P25 TB400 TMA400 TMB400 TMN400 Without catalyst

0.80 0.75 20

40

60

80 100 120 140

0

20

40

60

Time/ min

80

100

120

Time/ min

(c)

1.0

0.8

C/CO

Without catalyst and O3

0.6

O3 Degussa P25 0 wt. % Mn-TiO 2

0.4

1 wt. % Mn-TiO 2 5 wt. % Mn-TiO 2 10 wt. % Mn-TiO 2

0.2

Degussa P25 without O3 1 wt. % Mn-TiO 2 without O

3

0.0

0

20

40

60

80

100

Time /min

In previous studies, some authors have proposed that the degradation of methylene blue proceeded via a photosensitization pathway (Scheme 1). Upon absorbing visible light photons, the dye molecule was excited and electrons with high energy were transferred from the excited MB molecule to the conduction band of the anatase TiO2, and then to the conduction band of MnO2. During the transfer of electrons, any adsorbed O2 on the surface the materials was converted to the superoxide radical anion (O2•−) that oxidizes the MB (Chen et al. 2001; Xue et al. 2008). With our current materials, 50

Degussa P25 0 wt. % Mn-TiO2 0.25 wt. % Mn-TiO2

40

Concentration / 10-4 mM

exhibited better photocatalytic activity than the undoped as shown in Fig. 7b. The difference in photocatalytic activity between the P25 and the as-prepared materials may be due to different synthetic route. This implies better photocatalytic activity of doped P25 over the undoped if the same synthetic route is adopted. The photocatalytic activity of the mesoporous materials were TMN400>TMA400>TMB400 which might be due to their crystal sizes and pore volume that allows for movement of the dye solution (Bao et al. 2010), or the surface properties of the materials. This implies that the pH conditions during synthesis influences the characteristics, hence the photocatalytic activity of the materials. According to Meng et al. (2011), photocatalytic decomposition of dyes involves two steps—the adsorption of dye molecules and their degradation. The degradation mechanism can be a complicated and multistep process, with the formation of various intermediates. The amount and formation rate of hydroxyl radical by the materials (TMA400, TMB400, and TMN400) was carried out by using a dosimetry method (Kanazawa et al. 2011). Using this method, we could not detect any appreciable amounts of the hydroxyl radical. The experiments were repeated with the benchmark catalyst, undoped titania, and the lowest doped material (Fig. 8). The results clearly show that the manganese dopant suppresses the formation of hydroxyl radicals.

30

20

10

0 0

20

40

60

80

100

120

Time / mn

Fig. 8 Rate of hydroxyl ion formation using a dosimetry method, for the indicated catalysts

Environ Sci Pollut Res MB

-

O2

O2

MB*

e

Visible light

CB CB

MB VB

VB

TiO2

Mn

Scheme 1 Photocatalytic mechanism of TMA400, TMN400, and TMB400

a similar mechanism could be responsible for the degradation of the dye, and would mean the dopant maybe a recombination centre. According to Li et al. (2011), when manganese dopant content increases, it becomes the recombination centre for electron–hole pairs, reducing the separation efficiency of photo-generated charges, invariably reducing/preventing the formation of hydroxyl radical. This leads to a less photocatalytic activity, hence the low photocatalytic, but high adsorption characteristics due to high surface area, exhibited by TMA400, TMB400, and TMN400. To determine whether the manganese increased the electron–hole recombination rate, we conducted photoluminescence experiments on the TMA400, TMB400, and TMN400 samples. An increase in the intensity of the photoluminescence peak would indicate greater electron–hole recombination within the material (Yu et al. 2002). The TMA400, TMB400, and TMN400 showed intensities that were lower than the Degussa P25, which clearly shows that the dopant does not act as a recombination centre, at the high loadings (Fig. 9). As discussed above, the photosensitization mechanism suggests that oxygen radicals are the key species in degrading the dye. Thus, we used ozone to test whether the catalysts materials are effectively generating electron species that can eventually produce the reactive oxygen radicals. Figure 7c shows the photocatalytic ozonation of different wt% Mn– 6

TMB400 TMA400 TMN400 P25

5

Intensity

4 3 2 1 0 350

400

450

500

Wavelength / nm

Fig. 9 Photoluminescence spectra of the materials

550

TiO2 synthesized under basic condition (pH=11). From the FTIR results presented above, samples synthesized under basic conditions were shown to have a significant amount of physisorbed water on the surface the materials. Hydroxyl groups on the surface of a photocatalyst plays an important role in the photocatalytic reaction since they can capture the photo-induced holes from the surface of the material, inhibiting electron–hole recombination as well as forming hydroxyl radicals with high oxidation potential(Hou et al. 2008; Xu et al. 2008). O3 on the other hand is more easily reduced by photo-generated electron and in the long run can generate hydroxyl radicals (Mahmoodi 2011; Sun et al. 2013). The blank and the experiment with ozone alone are very similar, showing that the ozone itself is very ineffective for removing the dye from aqueous systems, within the timeframe of the experiment. It is interesting to note that the low 1 wt%Mn–TiO2 showed a better photocatalytic activity in the presence of O3 and the 5 and 10 wt% Mn–TiO2 showed fluctuations similar to the blank. This results from the hydroxyl group on the surface of the materials (Fig. 4b), which behaves as Bronstead acid sites (catalytic centres of metal oxides). Furthermore, an increase of Mn(II) dose slightly decreases the efficiency of the catalytic process. This phenomenon can be explained by the ability of high Mn(II) concentration to scavenge the hydroxyl radicals generated (Kasprzyk-Hordern et al. 2003). Thus at low manganese loading, ozone enhances the photocatalytic degradation of the dye, while at high loading, the ozone has no effect and seems to inhibit the little photocatalytic activity these samples have.

Conclusions Mesoporous Mn/TiO2 composite with reduced band gap energy ranging from 2.71–3.00 eV was synthesized by sol–gel method using different pH water. All the materials existed in the anatase phase and exhibited a relatively higher surface area than the commercial TiO2 (Degussa P25). The study also showed that pH water, aside having an effect on the percentage of dopants on TiO2, also influences the amount of OH functional groups on its surfaces, crystal sizes, and pore volumes. TMN400 showed a better photocatalytic activity under visible light due to its relatively smaller crystal size and larger pore volume. While 1 wt% Mn/TiO 2 showed 100 % decolouration of MB in the presence of O3. Acknowledgments The authors appreciates the School of Chemistry and Physics, University of KwaZulu-Natal, Westville Campus, Durban and iThemba LABS, Materials Research Department, South Africa for access to the facilities used for the research.

Environ Sci Pollut Res

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