materials Article
Structure and Photocatalytic Properties of Mn-Doped TiO2 Loaded on Wood-Based Activated Carbon Fiber Composites Xiaojun Ma *, Wanru Zhou and Yin Chen Department of Wood Science and Technology, Tianjin University of Science & Technology, Tianjin 300222, China;
[email protected] (W.Z.);
[email protected] (Y.C.) * Correspondence:
[email protected]; Tel.: +86-22-6060-0883 Academic Editor: Walid A. Daoud Received: 20 April 2017; Accepted: 2 June 2017; Published: 9 June 2017
Abstract: Mn-doped TiO2 loaded on wood-based activated carbon fiber (Mn/TiO2 -WACF) was prepared by sol–gel and impregnation method using MnSO4 ·H2 O as manganese source. The structure of Mn/TiO2 –WACF was characterized by SEM, XRD, FTIR, N2 adsorption and UV–Vis, and its photocatalytic activity for methylene blue degradation was investigated. Results show that Mn-doped TiO2 were loaded on the surface of wood-based activated carbon fiber with high-development pore structures. The crystallite sizes of Mn-doped TiO2 in composites were smaller than that of the undoped samples. With an increase of Mn doping content, Ti–O bending vibration intensity of Mn/TiO2 –WACF increased and then decreased. Moreover, Ti–O–Ti and Ti–O–Mn absorption peaks increased upon doping of Mn. Mn/TiO2 –WACF with low specific surface area, and pore volume was improved at 3.5–6.0 nm of mesopore distributions due to the Mn-doped TiO2 load. In addition, the UV–Vis showed that Mn/TiO2 –WACF (photodegradation rate of 96%) has higher photocatalytic activity than the undoped samples for methylene blue degradation under visible light irradiation. Keywords: Mn-doped TiO2 ; activated carbon fibers; wood; photocatalyst; characterization
1. Introduction As a well-known photocatalyst, TiO2 has attracted lots of interest over the past decades due to its chemical stability, thermal stability, high efficiency, nontoxicity, and low cost [1]. It is extensively applied to the purification of air, bactericidal action, anti-fog, self-cleaning, and degradation of organic pollutant compounds in wastewater. However, the band gap of TiO2 photocatalyst is 3.2 eV, therefore only under ultraviolet (UV) excitation light this semiconductor exhibits catalytic, and the ultraviolet content of sunlight has only 3% to 5%, which limits the use of solar energy. Electrons and holes easily recombine on the surface and interior of TiO2 particles, which reduces the photocatalytic activity of TiO2 [2–4]. In addition, the degradation rate of the suspension-type TiO2 photocatalyst is slower when the concentration of target pollutant is low. On the other hand, TiO2 powder is difficult to be recycled, as the material gets easily inactivated and coagulates, which limits the application of nano-TiO2 [5,6]. Therefore, the most important and challenging issue is to develop efficient visible light responsive photocatalysts by the modification of TiO2 and curing. At present, the main surface modifications used with TiO2 are compound semiconductor method [7–9], ion doping method [10–13] and precious metal deposition method [14,15], among which, ion doping of transition metal is an effective route. The studies suggest that a small amount of metal-ion-doped TiO2 can make TiO2 become potential wells to capture photogenerated electron–hole pairs; thus, the photo-generated electrons and holes are difficult to complex [16–19]. The focus of curing research is the choice of carrier and fixed process. Inorganic materials mainly used as carrier Materials 2017, 10, 631; doi:10.3390/ma10060631
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include glass, metal, ceramics, activated carbon and molecular sieves. Among them, the activated carbon fiber has become the main carrier of photocatalytic oxidation technology due to its good adsorption performance and photocatalytic synergistic effect [20]. With the growing awareness of environmental protection and the shortage of fossil resources, the sustainable development of biomass activated carbon fiber has gradually became an alternative photocatalyst carrier material and has shown excellent performance [21–23]. In the present study, using activated carbon fiber (wood-based activated carbon fiber, WACF) from liquefied wood as a carrier and Mn-doped TiO2 as photocatalyst, the photocatalytic composite materials (Mn/TiO2 –WACF) were prepared by the sol-gel and impregnation method. The effects of different Mn doping ratios on the microcrystalline structure, specific surface area and pore size distribution of the composites were investigated in detail. Additionally, the photocatalytic degradation of photocatalytic composite for methylene blue (MB) under visible light conditions is also discussed. 2. Experimental 2.1. Chemicals For the preparation of undoped and Mn-doped TiO2 photocatalytic composite materials, the materials used were ethanol (molecular mass (M) = 46.07, C2 H6 O), tetrabutyl titanate (M = 340.32, Ti(OC4 H9 )4 ), Manganese sulfate (M = 169.6, MnSO4·H2 O) and aceticacid (M = 60.05, C2 H4 O2 ). For photocatalytic degradation, methylene blue dye; MB (M = 319.85, C16 H18 N3 SCl) was used. All chemicals used were analytical reagent (AR) grade from Tianjin Jiangtian Chemical Co. Ltd. (Tianjin, China) and used without further purification. 2.2. Samples 10.2 g of Ti(OC4 H9 )4 was slowly poured into 90 mL of ethanol, and stirred with a magnetic stirrer for 30 min until colorless and transparent. Meanwhile, 3 mL of MnSO4 ·H2 O solution (17 g/L) was added to 2.2 mL of distilled water, 2 mL of acetic acid, and 60 mL of ethanol to from another solution. Then, the latter solution was slowly poured into the former solution under vigorous stirring until completely dissolved. Finally, the mixed solution was placed in a thermostatic water bath at 40 ◦ C for 2 h to obtain a yellowish emulsion colloid solution consisting of Mn–TiO2 gel. WACF (0.2 g) [24] was evenly immersed into the colloid solution. The solution was taken out after vibration and left to stand for 30 min. Then, the solution was placed into a vacuum tube-furnace for drying process under 105 ◦ C for 1 h. After heat treatment at 450 ◦ C for 30 min under different calcination temperatures through N2 (flow rate was 100 mL/min), Mn/TiO2 –WACF photocatalysis composite material was prepared. According to the molar ratio of Mn to Ti: 0:1, 1:600, 1:300, 1:100, and 1:50, the prepared samples were labeled as Ti–WACF, Mn/600Ti–WACF, Mn/300Ti–WACF, Mn/100Ti–WACF, and Mn/50Ti–WACF, respectively. 2.3. Measurements 2.3.1. Scanning Electron Microscopy Analysis JSM-7500F cold-field-emission scanning microscope (resolution at 3.0 nm and acceleration voltage at 1 kV) produced by JEOL (Tokyo, Japan) was adopted for appearance and morphology diagrams of the samples. After Mn/TiO2 –WACF photocatalysis, composite material samples were dried and bonded on the sample table to conduct surface vacuum metal spraying and observe their surface morphologies.
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2.3.2. XRD Analysis D/max2500 X-ray diffraction (XRD) was obtained on a RIGAKU instrument (Tokyo, Japan). Cu Kα X-ray was used, tube voltage was 40 kV, tube current was 100 mA, scanning angle scope 2θ was 20◦ –80◦ , and scanning speed was 8◦ min−1 . According to the Scherrer formula: D=
K1 λ β 1/2 cos θ
(1)
The average crystallite size D (nm) of TiO2 in photocatalysis samples is calculated. K1 is the shape factor of the crystalline with a value of 0.89, λ is the wavelength of X-ray with a value of 0.154 nm, β1/2 is full width at half maximum of diffraction peak (rad), and θ is the diffraction angle (◦ ). 2.3.3. FT-IR Analysis Nicolet 6700 Fourier transform infrared (FT-IR) spectrometer (Thermo Electron Corporation, Waltham, MA, USA) was used to analyze the samples. Samples were tested by 1:300 KBr disc technique. Absorbance spectra were acquired at 4 cm−1 resolution and signal-averaged over 32 scans in the scanning range of 400–4000 cm−1 . 2.3.4. Specific Surface Area and Aperture Analysis The surface area and the porosity of the samples were determined by N2 adsorption–desorption isotherm measured at 77 K in a Micromeritics ASAP-2020 apparatus (Micromeritics Instrument Corporation, Norcross, GA, USA). Before analysis, the samples were degassed at 350 ◦ C for 2 h. The specific surface area (SBET ) was calculated by the Brunauer–Emmett–Teller (BET) method using N2 adsorption isotherm data. The total pore volume (Vtot ) was evaluated by converting the amount of N2 adsorbed at a relative pressure of 0.995 to the volume of liquid adsorbate. The micropore area (Smicro ) and micropore volume (Vmicro ) were obtained by t-plot method. The mesopore area (Smeso ) and mesopore volume (Vmeso ) were calculated by Barrett–Joyner–Halenda method. Pore size distributions were calculated using Density Functional Theory (DFT) Plus Software (provided by Micromeritics Instrument Corporation, Norcross, GA, USA), which is based on the calculated adsorption isotherms for pores of different sizes. This program performs an inversion of the integral equation for the overall adsorption isotherm with respect to pore size distributions. 2.3.5. Photocatalysis Performance Test 33 mg of methylene blue was dissolved in 1000 mL of distilled water at 60 ◦ C and placed in a volumetric flask. Mn/TiO2 –WACF (10 mg) prepared under different calcination temperatures were accurately weighted and placed in a beaker with 100 mL of MB solution. The solutions were placed into a black case and magnetically stirrer for 40 min until adsorption equilibrium. A spectrophotometer (UV-1600 model produced by Shanghai MAPADA Instrument Co. Ltd. (Shanghai, China)) was used to measure and record absorbance A0 in preliminary test at 665 nm. Optical filters (transmission wavelength 400–800 nm) were covered on beakers filled with samples. Under the conditions of illumination and magnetic stirring, their absorbance was measured with a spectrophotometer every 40 min, and the decolorization ratio D was calculated according to Equation (2): D=
A0 − A × 100% A0
(2)
where A0 is the solution absorbance before illuminance, and A is the solution absorbance at a certain time.
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3. Results and Discussion 3.1. Morphological Characteristics SEM micrographies of the WACF are shown in Figure 1a, the surface of Mn/Ti–WACF had a layer of Mn-doped nano-TiO2 film with uniform thickness (Figure 1c). Mn-doped nano-TiO2 films were loaded on the surface of WACF, but some tilted and shed in the edges, mainly due to the inconsistency Materials 2017, 10, 631 4 of 10 of the fiber and modified photocatalyst shrinkage under high-temperature calcination. In addition, bare base materials existed on the surface of Mn/Ti–WACF (Figure 1d), which well-reserved abundant well-reserved abundant pore structures and provided advantages for had subsequent adsorption and pore structuresreaction. and provided advantages for subsequent adsorption and photocatalytic reaction. photocatalytic
(a)
(c)
(b)
(d)
Figure 1. Scanning electron micrographies of the surface of wood-based activated carbon fiber (WACF) Figure 1. Scanning electron micrographies of the surface of wood-based activated carbon fiber (WACF) (a,b) and Mn/TiO2–WACF (c,d). (a,b) and Mn/TiO2 –WACF (c,d).
3.2. XRD Analysis 3.2. XRD Analysis Figure 2 shows an X-ray diffractograms spectrum of the various samples. The diffraction peaks Figure 2 shows an X-ray diffractograms spectrum of the various samples. The diffraction peaks of of anatase TiO2 (101) (at 2θ = 25.22°) and (004) (at 2θ = 37.93°) were not cancelled after Mn doping, anatase TiO2 (101) (at 2θ = 25.22◦ ) and (004) (at 2θ = 37.93◦ ) were not cancelled after Mn doping, and no and no diffraction peaks associated with rutile and brookite (the crystal structure of TiO2 was diffraction peaks associated with rutile and brookite (the crystal structure of TiO2 was consistent with consistent with that of undoped Mn) was observed, suggesting that the crystal structure of TiO2 was that of undoped Mn) was observed, suggesting that the crystal structure of TiO2 was not affected by not affected by Mn doping. In addition, the intensity of these peaks decreased, and the main peak Mn doping. In addition, the intensity of these peaks decreased, and the main peak position shifted position shifted slightly with increasing Mn doping concentration, indicative of a decrease in slightly with increasing Mn doping concentration, indicative of a decrease in crystallization of TiO2 . crystallization of TiO2. This phenomenon also reveals that Mn ions successfully enter into the TiO2 This phenomenon also reveals that Mn ions successfully enter into the TiO2 crystal lattice, replacing part crystal lattice, replacing part of the Ti ions, destroying the symmetry of the crystal structure of TiO2, of the Ti ions, destroying the symmetry of the crystal structure of TiO2 , causing crystal lattice disorder causing crystal lattice disorder and increasing distortion, which results in defects in the lattice [25,26]. and increasing distortion, which results in defects in the lattice [25,26]. The full-width half maximum The full-width half maximum increased with increasing Mn doping concentrations, which indicates increased with increasing Mn doping concentrations, which indicates that doping Mn effectively that doping Mn effectively controlled the recrystallization or grain size of the TiO2 sintering during controlled the recrystallization or grain size of the TiO2 sintering during calcination. No impurity calcination. No impurity peak was associated with the Mn element in the XRD crystalline spectra of peak was associated with the Mn element in the XRD crystalline spectra of the samples due to scarce the samples due to scarce Mn doping. Mn doping.
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(116)(A) (220)(A)
(204)(A)
(105)(A) (211)(A)
(200)(A)
(004)(A)
Intensity (a.u.)
(215)(A)
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(101)(A)
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Mn/50Ti-WACF Mn/100Ti-WACF Mn/300Ti-WACF Mn/600Ti-WACF Ti-WACF
20
25
30
35
40
45
50
55
60
65
70
75
80
2θ (degree)
Figure 2. XRD diffractograms of loaded materials with different Mn doping concentrations.
Figure 2. XRD diffractograms of loaded materials with different Mn doping concentrations.
Table 1 presents the average crystallite size of modified nano-TiO2, which were calculated based
Table 1 presents the average size of 2modified , which were calculated based on Scherrer formula. The graincrystallite size of nano-TiO graduallynano-TiO increases 2with increasing Mn doping concentrations. TheThe crystallite sizes of (25.4–27.8 nm) of TiO2 afterincreases doping Mn were obviously Mn smaller on Scherrer formula. grain size nano-TiO with increasing doping 2 gradually than that of The undoped Mn, indicating that Mnnm) doping can 2inhibit the growth agglomeration of concentrations. crystallite sizes (25.4–27.8 of TiO after doping Mnand were obviously smaller TiO 2 grains. This small size promotes TiO2 to evenly and firmly wrap on the surface of WACF, than that of undoped Mn, indicating that Mn doping can inhibit the growth and agglomeration of TiO2 providing a superior condition for the photocatalytic reaction. grains. This small size promotes TiO2 to evenly and firmly wrap on the surface of WACF, providing a superior condition for thecrystallite photocatalytic reaction. Table 1. Average size of nano-TiO 2 doped with different Mn doping concentrations. Samples Table 1. Average Average crystallite size
Ti–WACF 36.4
Mn/600Ti–WACF 25.4
Mn/300Ti–WACF 26.6
Mn/100Ti–WACF 27.8
Mn/50Ti–WACF 27.5
crystallite size of nano-TiO2 doped with different Mn doping concentrations.
Samples
3.3. FTIR Spectroscopy Average crystallite size
Ti–WACF Mn/600Ti–WACF Mn/300Ti–WACF Mn/100Ti–WACF Mn/50Ti–WACF 36.4
25.4
26.6
27.8
27.5
FTIR spectra of TiO2 doped with different Mn doping concentration on WACF are shown in Figure 3. Stretching and bending vibration of hydroxyl groups of all loaded materials were in the 3.3. FTIR rangeSpectroscopy of 3200–3600 cm−1 and 1617–1635 cm−1, and the stretching vibration band intensity of Mn-doped samples was clearly than that of the undoped sample, indicating that TiO2 had a stronger FTIR spectra of TiOstronger 2 doped with different Mn doping concentration on WACF are shown in adsorption with Mn-doping. The absorption bands of loaded materials at 1444 cm−1 were associated Figure 3. Stretching and bending vibration of hydroxyl groups of all loaded materials were in the range to the stretching of C–H bonds and C–O–Ti. A smaller amount of Mn doping caused intensity of 3200–3600 cm−1 and 1617–1635 cm−1 , and the stretching vibration band intensity of Mn-doped enhancement of absorption bands, indicating that less Mn doping promotes TiO2 loading on the samples was clearly stronger than that of the undoped sample, indicating that TiO had a stronger WACF surface, as observed in the XRD measurements. Moreover, the absorption bands 2of all loaded 1 were associated adsorption with bands loaded materials at 1444 cm− materials in Mn-doping. the vicinity ofThe 618 absorption cm−1 belonged to theofcharacteristic absorption peaks of anatase TiO2 to the(consistent stretching of C–H bonds and C–O–Ti. A smaller amount of Mn doping caused intensity with the XRD test results), which represented the bending vibration of Ti–O, and the peak enhancement absorption bands, indicating Mn doping promotes the WACF intensity of was bounded by the Mn/Ti molarthat ratioless of 1:300. With the increaseTiO of Mn dopingon amount, 2 loading Ti–O intensity would initially increase and then decrease, theloaded intensity of surface, as bending observedvibration in the XRD measurements. Moreover, the absorption bandsand of all materials 1 belonged bigger than thatcharacteristic of pure TiO2. The latter had only Ti–O bending vibration, in theMn-doped vicinity ofmaterials 618 cm−was to the absorption peaks of anatase TiO (consistent 2 andXRD the former has Ti–O and represented Ti–O–Mn bond; this finding indicates Mn doping was intensity good for was with the test results), which the bending vibration of that Ti–O, and the peak TiO2 loading on WACF and for the formation of anatase TiO2. The absorption band of the undoped bounded by the Mn/Ti molar ratio of 1:300. With the increase of Mn doping amount, Ti–O bending and doped samples is over the range of 846–878 cm−1; the former had a weak band and only Ti–O–Ti vibration intensity would initially increase and then decrease, and the intensity of Mn-doped materials stretching vibration. The latter had stronger intensity bands due to the dual role of Ti–O–Ti and was bigger than that of pure TiO2 . The latter had only Ti–O bending vibration, and the former has Ti–O–Mn [27].
Ti–O and Ti–O–Mn bond; this finding indicates that Mn doping was good for TiO2 loading on WACF and for the formation of anatase TiO2 . The absorption band of the undoped and doped samples is over the range of 846–878 cm−1 ; the former had a weak band and only Ti–O–Ti stretching vibration. The latter had stronger intensity bands due to the dual role of Ti–O–Ti and Ti–O–Mn [27].
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Transmittance(%) (%) Transmittance
Mn/50Ti-WACF Mn/50Ti-WACF 25082493 25082493 Mn/100Ti-WACF Mn/100Ti-WACF
Mn/300Ti-WACF Mn/300Ti-WACF 2069 2024 2069 2024 Mn/600Ti-WACF Mn/600Ti-WACF
478 478 418 418
2851 2920 2851 2920 Ti-WACF Ti-WACF 2360 2340 2360 2340
1449 1449
3442 3442
4000 4000
3600 3600
878 846 878 846 628 1142 1006 628 1142 1006
1635 1635 1617 1617
3200 3200
2800 2800
2400 2400
2000 2000
1600 1600
1200 1200
800 800
400 400
-1 Wavenumber Wavenumber(cm (cm-1))
Figure 3. FTIR spectra of Mn/TiO Figure3. 3.FTIR FTIRspectra spectraof ofMn/TiO Mn/TiO222–WACF –WACFsamples differentMn Mndoping dopingconcentrations. concentrations. Figure samples with with different different Mn doping concentrations. −1 and In addition, the samples showed weak absorption band in the range of 2340–2360 cm and In addition, addition, the the samples samples showed showedaaaweak weakabsorption absorptionband bandin inthe therange rangeof of2340–2360 2340–2360cm cm −−1 1 and In −1 1006–1142 cm the P–H stretching vibration and the P–O–C bond; this finding was −1,, representing 1006–1142 cm representing the P–H stretching vibration and the P–O–C bond; this finding was 1006–1142 cm−1 , representing the P–H stretching vibration and the P–O–C bond; this finding was mainly due to the addition of the phosphoric acid catalyst during the liquefaction of the wood. mainly due due to to the the addition additionof ofthe thephosphoric phosphoricacid acidcatalyst catalystduring duringthe theliquefaction liquefactionof ofthe thewood. wood. mainly
3.4. Pore Volume and Aperture Analysis 3.4. Specific Specific Surface Surface Area, Area, Pore PoreVolume Volumeand andAperture ApertureAnalysis Analysis 3.4. Specific Surface Area, The adsorption/desorption and Mn/TiO The adsorption/desorption adsorption/desorption isotherms isotherms of of WACF WACF and andMn/TiO Mn/TiO222–WACF –WACF with different different Mn Mn doping doping The isotherms of WACF –WACF with with different Mn doping concentrations are shown in Figure 4. The figure shows that all samples belong to the typical I-type concentrations are shown in Figure 4. The figure shows that all samples belong to the typical I-type concentrations are shown in Figure 4. The figure shows that all samples belong to the typical I-type adsorption isotherm (Langmuir isotherm), and the pore structure is dominated by micropores. The adsorption isotherm pore structure is dominated by micropores. The adsorption isotherm (Langmuir (Langmuirisotherm), isotherm),and andthe the pore structure is dominated by micropores. curves of WACF and Mn/50Ti–WACF experienced obvious hangover boosting phenomena, and they curves of WACF and Mn/50Ti–WACF experienced obvious hangover boosting phenomena, and they The curves of WACF and Mn/50Ti–WACF experienced obvious hangover boosting phenomena, belonged to other I-A N belonged to I-B I-B type type material; the other materials materials belonged to to I-A type typetomaterial. material. The N22 adsorption adsorption and they belonged tomaterial; I-B type the material; the other belonged materials belonged I-A typeThe material. The N2 capacity of unloaded WACF sample was higher than that of Mn/TiO 2 –WACF sample. The hysteresis capacity of unloaded WACF sample was higher than that of Mn/TiO 2 –WACF sample. The hysteresis adsorption capacity of unloaded WACF sample was higher than that of Mn/TiO2 –WACF sample. of former and desorption processes was obvious than the latter, that of the the former adsorption adsorption andadsorption desorptionand processes was more more obvious latter, indicating indicating that The hysteresis of the former desorption processes wasthan morethe obvious than the latter, the former had larger mesopores and macropores than the latter. This finding indicates that the formerthat had mesopores macropores than the latter. Thislatter. finding that the the indicating thelarger former had largerand mesopores and macropores than the Thisindicates finding indicates photocatalyst was successfully loaded onto the fiber surface or filled with the pores of the load photocatalyst was successfully loaded onto the fiber surface or filled with the pores of the load that the photocatalyst was successfully loaded onto the fiber surface or filled with the pores of the material. Meanwhile, N quantity of material was lower than that of the material. Meanwhile, N22 adsorption adsorption quantity of Mn-doped Mn-doped material was the load material. Meanwhile, N2 adsorption quantity of Mn-doped material waslower lowerthan than that that of of the 2 became higher after Mn doping. undoped material, indicating that the load rate of TiO becamehigher higherafter afterMn Mndoping. doping. undoped material, material, indicating indicatingthat thatthe theload loadrate rateof ofTiO TiO22 became undoped 600 600
Volume(cm (cm33/g) /g) Volume
500 500 400 400 300 300 200 200 100 100 0.0 0.0
0.2 0.2
0.4 0.4
0.6 0.6
WACF WACF Ti-WACF Ti-WACF Mn/600Ti-WACF Mn/600Ti-WACF Mn/300Ti-WACF Mn/300Ti-WACF Mn/100Ti-WACF Mn/100Ti-WACF Mn/50Ti-WACF Mn/50Ti-WACF 0.8 0.8
1.0 1.0
Relative RelativePressure Pressure(p/p (p/p00))
Figure N adsorption/desorption samples. Figure4.4. 4.N N222adsorption/desorption adsorption/desorptionisotherm isothermof ofWACF WACFand andMn/TiO Mn/TiO222–WACF –WACF Figure isotherm of WACF and Mn/TiO –WACF samples. samples.
The The pore pore diameter diameter distribution distribution (DFT (DFT method) method) of of WACF WACF and and Mn/TiO Mn/TiO22–WACF –WACF are are shown shown in in The pore diameter distribution (DFT method) of was WACF and distributed Mn/TiO2 –WACF are0.4 shown in Figure 5. The curve shows that the pore size of WACF mainly between Figure 5. The curve shows that the pore size of WACF was mainly distributed between 0.4 nm nm and and Figure 5. The curve shows that the pore size of WACF was mainly nm distributed between 0.4 nm 3.5 3.5 nm, nm, and and that that Mn/TiO Mn/TiO22–WACF –WACF was was mainly mainly distributed distributed between between 0.4 0.4 nm and and 66 nm. nm. Therefore, Therefore, the the pore diameter distribution diagram of WACF and Mn/TiO 2 –WACF was similar, and no multimodal pore diameter distribution diagram of WACF and Mn/TiO2–WACF was similar, and no multimodal phenomenon phenomenon was was found. found. However, However, the the Mn/TiO Mn/TiO22–WACF –WACF had had aa small small amount amount of of mesopore mesopore between between
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and 3.5 nm, and that Mn/TiO2 –WACF was mainly distributed between 0.4 nm and 6 nm. Therefore, Materials 10, 631 distribution diagram of WACF and Mn/TiO2 –WACF was similar, and no multimodal 7 of 10 the pore2017, diameter
phenomenon was found. However, the Mn/TiO2 –WACF had a small amount of mesopore between 3.5 was almost almost useless useless because because the the former former was 3.5 and and 66 nm, nm, and and the the WACF WACF was was subjected subjected to to secondary secondary calcination thethe photocatalyst onto onto the latter The latter waslatter equivalent to the second calcinationafter afterloading loading photocatalyst the surface. latter surface. The was equivalent to activation by the evaporated photocatalyst to reduce, as much as possible, the TiO 2 photocatalyst, the second activation by the evaporated photocatalyst to reduce, as much as possible, the TiO2 resulting in pore diameter distribution of distribution carrier fiber of decline. addition, theInpore volume the photocatalyst, resulting in pore diameter carrierInfiber decline. addition, theofpore loaded material decreased that with of WACF, indicating that Mn-doped TiO2 volume of the loaded materialcompared decreasedwith compared that of WACF, indicating that Mn-doped photocatalyst can not only be wrapped on the fiber surface, but also it can be filled into the pores or TiO2 photocatalyst can not only be wrapped on the fiber surface, but also it can be filled into the attached to the hole wall. The pore volume of Mn-doped material was less than that of pure TiO 2-loaded pores or attached to the hole wall. The pore volume of Mn-doped material was less than that of material, showed that Mn showed doping that can Mn improve thecan TiO 2 loading rate. This result is pure TiO2which -loadedagain material, which again doping improve the TiO2 loading rate. consistent with the conclusion of adsorption isotherm. This result is consistent with the conclusion of adsorption isotherm. 4
Differential Pore Volume (cm3/g)
4
0.15
3
0.10 2
3
0.05
1
0
2
0.00
0.4
0.6
0.8
1.0
3
WACF Ti-WACF Mn/600Ti-WACF Mn/300Ti-WACF Mn/100Ti-WACF Mn/50Ti-WACF
1
4
5
6
0 0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
6.5
7.0
Pore Width (nm)
–WACF. Figure 5. Pore size distribution (density functional theory method) of WACF WACF and and Mn/TiO Mn/TiO22–WACF.
Specific of of thethe samples areare shown in Table 2. In Specific surface surface area, area, pore porevolume, volume,and andpore poreradius radius samples shown in Table 2. comparison with WACF and Ti–WACF, S BET , S micro , S meso , V tot , V micro and V meso of Mn/TiO 2 –WACF were In comparison with WACF and Ti–WACF, SBET , Smicro , Smeso , Vtot , Vmicro and Vmeso of Mn/TiO2 –WACF reduced. The surface area andarea poreand volume Mn/TiO as high as 1239 and 0.628 cm3/g, were reduced. The surface pore of volume of2–WACF Mn/TiOwere 2 –WACF were as high as 1239 and respectively. Moreover, with the increase of increase Mn doping content, of Mn/TiO 2–WACF 0.628 cm3 /g, respectively. Moreover, with the of Mn dopingPMic content, PMic of Mn/TiOincreased 2 –WACF first and then whereas Swhereas meso, Vmeso and, PVMe decreased first and then increased, indicating increased first decreased, and then decreased, Smeso and P decreased first and then increased, meso Me that Mn doping haddoping a certain on the micropores and mesopores. indicating that Mn hadeffect a certain effect on the micropores and mesopores. Table 2. Specific surface area, pore volume, and aperture parameters of WACF and Mn/TiO2–WACF. Sample
Sample
WACF
Vtot
Vmicro
Vmeso
PMic a
SBET
Smicro
Smeso
1802 1272 1418 1160852 764 980 691 1239803 966624
1272 852 764 691 803 624
530
0.875
0.581
0.294
66.4
530 408 408 309 309 205 205 322 322 342 342
0.875 0.710 0.710 0.602 0.602 0.477 0.477 0.628 0.628 0.556 0.556
0.581 0.384 0.384 0.348 0.348 0.317 0.317 0.364 0.364 0.284 0.284
0.294 0.272 0.272 0.230 0.230 0.137 0.137 0.230 0.230 0.272 0.272
66.4 54.1 54.1 57.8 57.8 66.5 66.5 58.0 58.0 51.1 51.1
a SBET (m2S 3/g) PMic tot 3/g) V(cm meso micro3/g) V micro(m2/g)Smeso /g) (m2/g) V(cm (cm (%) 2 2 2 3 3 3 (%) (m /g) (m /g) (m /g) (cm /g) (cm /g) (cm /g)
WACF Ti–WACF 1802 Ti–WACF Mn/600Ti–WACF1418 Mn/600Ti–WACF 1160 Mn/300Ti–WACF Mn/300Ti–WACF 980 Mn/100Ti–WACF1239 Mn/100Ti–WACF Mn/50Ti–WACF 966 Mn/50Ti–WACF
PMe b b P Me (%) (%) 33.6 33.6 38.3 38.3 38.2 38.2 28.7 28.7 36.6 36.6 48.9 48.9
Dc Dc (nm) (nm) 1.94 1.94 2.00 2.00 2.08 2.08 1.95 1.95 2.03 2.03 2.03 2.03
b Ratio of the mesopore volume to the total b Ratio of Ratio ofthe themicropore micropore volume tototal the pore totalvolume; pore volume; Ratio of volume to the the mesopore volume to the total pore volume; c Average pore diameter. pore volume; Average pore diameter.
aa
c
3.5. UV–Vis UV–Vis Analysis Analysis 3.5. The UV–Vis spectral spectral absorption curve of loaded materials with different Mn doping concentrations is shown in Figure 6. In the visible light region, the absorption difference of TiO2 material was not significant with different Mn:Ti molar ratios, i.e., 1:600, 1:300, and 1:100; these results are mainly due to the fact that the actual amount of Mn ions added was similar yet had greatly increased in comparison with pure TiO2-loaded materials (the maximum improvement was Mn/600Ti–WACF material). This finding indicated that Mn doping can change the light absorption properties of TiO2,
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significant with different Mn:Ti molar ratios, i.e., 1:600, 1:300, and 1:100; these results are mainly due to the fact that the actual amount of Mn ions added was similar yet had greatly increased in comparison with pure TiO2 -loaded materials (the maximum improvement was Mn/600Ti–WACF Materials 2017, 10, 631 8 of 10 Materials This 2017, 10, 631 10 , material). finding indicated that Mn doping can change the light absorption properties of8 of TiO 2 showed effect Mn doping wasnot notaffected affectedby bythe the loading loading process and and alsoalso showed thatthat thethe effect of of Mn doping was process and andthe thecarrier carrier and also showed that the effect of Mn doping was not affected by the loading process and the carrier fibers visible light absorbance of Mn Ti molar ratio 1:50 was slightly lower thanthat thatof fibers [28].[28]. TheThe visible light absorbance of Mn andand Ti molar ratio of of 1:50 was slightly lower than fibers [28]. The visible light absorbance of Mn and Ti molar ratio of 1:50 was slightly lower than that pure TiO 2, while the Mn doping amount was excessive and some of the impurity levels provided pureofTiO , while the Mn doping amount was excessive and some of the impurity levels provided the 2 of pure TiO2, while the Mn doping amount was excessive and some of the impurity levels provided the chance of electron–hole pairing in the photocatalytic reaction. chance of electron–hole pairing in the photocatalytic reaction. the chance of electron–hole pairing in the photocatalytic reaction.
Absorbance (a.u.) Absorbance (a.u.)
0.45 0.45
Mn/600Ti-WACF Mn/600Ti-WACF Mn/100Ti-WACF Mn/100Ti-WACF Mn/300Ti-WACF Mn/300Ti-WACF Ti-WACF Ti-WACF Mn/50Ti-WACF Mn/50Ti-WACF
0.40 0.40
0.35 0.35
0.30 0.30
0.25 0.25200 200
300 300
400 400
500 500
600 600
Wavelength (nm) Wavelength (nm)
700 700
Figure 6. UV–Vis spectra loaded materialswith withdifferent differentMn Mndoping doping concentrations. concentrations. Figure 6. UV–Vis spectra of of loaded materials Figure 6. UV–Vis spectra of loaded materials with different Mn doping concentrations.
3.6. Visible Photodegradation of Methylene Blue Solution 3.6. 3.6. Visible Photodegradation of Methylene Blue Visible Photodegradation of Methylene BlueSolution Solution The visible light degradation curves of MB solution with varying illumination time are shown TheThe visible light degradation curves timeare areshown shown visible light degradation curvesofofMB MBsolution solutionwith withvarying varying illumination illumination time in Figure 7. The Figure 7 shows that the degradation rate of the loading material increased with the in Figure 7. 7. The the degradation degradationrate rate loading material increased in Figure TheFigure Figure77shows shows that that the ofof thethe loading material increased with with the prolongation of the illumination time, and the degradation effect of the Mn-doped sample was better ofofthe illumination time, andand the the degradation effecteffect of theof Mn-doped samplesample was better the prolongation prolongation the illumination time, degradation the Mn-doped was than that of pure TiO2. The degradation of Mn/600Ti–WACF samples was the most satisfactory, which 2 . The degradation of Mn/600Ti–WACF samples was the most satisfactory, which than that of pure TiO better than of pure TiO2higher . The degradation Mn/600Ti–WACF samples was the most satisfactory, was up that to 96%, and 73% than that of of TiO 2. With the increase of Mn doping concentration, the waswas up up to 96%, andand 73%73% higher thanthan thatthat of TiO 2TiO . With the increase of Mn concentration, the which to 96%, higher of . With the increase ofdoping Mneffect doping 2 adsorption effect of the initial stage was enhanced, and the photocatalytic at concentration, later stage is effect of the initial stage was enhanced, and the photocatalytic effect atatlater stage is the adsorption adsorption effect of the initial stage was enhanced, and the photocatalytic effect later stage enhanced; thus, Mn doping can improve the adsorption performance of the TiO2-loaded material inis enhanced; thus, Mn doping can improve the adsorption performance of the TiO 2-loaded material in enhanced; thus, Mn dopingand canthe improve the adsorption the TiOthe material 2 -loaded the liquid environment concentration of targetperformance contaminantsofaround photocatalytic theliquid liquidenvironment environment and and the concentration concentration of of target target contaminants contaminants around the photocatalytic in the around material, providing favorablethe conditions for subsequent photodegradation ranges.the photocatalytic material, providing favorable conditions subsequentphotodegradation photodegradation ranges. ranges. material, providing favorable conditions forforsubsequent 100 100
Degradation rate (%) Degradation rate (%)
80 80
96% 96%
Ti-WACF Ti-WACF Mn/600Ti-WACF Mn/600Ti-WACF Mn/300Ti-WACF Mn/300Ti-WACF Mn/100Ti-WACF Mn/100Ti-WACF Mn/50Ti-WACF Mn/50Ti-WACF
78% 78% 75% 75%
60 60
41% 41%
40 40
23% 23%
20 20 0 0 0 0
40 40
80 80
120 120
160 160
Time (min) Time (min)
200 200
240 240
280 280
Figure 7. Degradation curve chart of methylene blue by samples with different Mn-doping Figure Degradationcurve curvechart chart ofof methylene methylene blue blue by by samples samples with with different Figure 7. 7.Degradation different Mn-doping Mn-doping concentrations under visible lights. concentrations under visible lights. concentrations under visible lights.
4. Conclusions 4. Conclusions Mn-doped TiO2 loaded on WACF (Mn/TiO2–WACF) was prepared by sol–gel and impregnation Mn-doped TiO2 loaded on WACF (Mn/TiO2–WACF) was prepared by sol–gel and impregnation method. The particle sizes of TiO2 in Mn/TiO2–WACF ranged from 25.4 to 27.8 nm, and were smaller method. The particle sizes of TiO2 in Mn/TiO2–WACF ranged from 25.4 to 27.8 nm, and were smaller
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4. Conclusions Mn-doped TiO2 loaded on WACF (Mn/TiO2 –WACF) was prepared by sol–gel and impregnation method. The particle sizes of TiO2 in Mn/TiO2 –WACF ranged from 25.4 to 27.8 nm, and were smaller than that of the undoped samples. With the increase of Mn doping content, Ti–O bending vibration intensity of Mn/TiO2 –WACF increased and then decreased. Moreover, Ti–O–Ti and Ti–O–Mn absorption peaks increased because of the doped Mn. The surface area and pore volume of Mn/TiO2 –WACF were as high as 1239 m2 /g and 0.628 cm3 /g, respectively. In addition, Mn/TiO2 –WACF can reach the highest photodegradation rate of 96% on MB under visible-light irradiation, which is higher by 73% than that of WACF loaded with TiO2 . Mn/TiO2 –WACF possesses relatively strong absorbency and photocatalytic activity after Mn doping. Acknowledgments: This work was supported by the National Natural Science Foundation of China (No. 31270607). Author Contributions: Xiaojun Ma conceived and designed the experiments; Yin Chen and Wanru Zhou performed the experiments and analyzed the data; Xiaojun Ma and Wanru Zhou wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.
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