Improving Photocatalytic Performance and Recyclability by

Article pubs.acs.org/JPCC

Improving Photocatalytic Performance and Recyclability by Development of Er-Doped and Er/Pr-Codoped TiO2/Poly(vinylidene difluoride)−Trifluoroethylene Composite Membranes P. M. Martins,† V. Gomez,‡ A. C. Lopes,† C. J. Tavares,† G. Botelho,§ S. Irusta,‡ and S. Lanceros-Mendez*,† †

Centro/Departamento de Física and §Departamento de Química, Universidade do Minho, 4710-057 Braga, Portugal ‡ Department of Chemical Engineering, Nanoscience Institute of Aragon, University of Zaragoza, 50018 Zaragoza, Spain ABSTRACT: Photocatalysis has become an attractive process to remove contaminants from aquatic environments, with TiO2 being the most widely used photocatalyst. In spite of the advantages of the process, two main problems still have to be overcome: reutilization/recycling of TiO2 nanoparticles, which is a time-consuming and expensive process, and the fast recombination rate of the electron−hole pairs. This work reports on the photocatalytic activity of rare earth metal doped (erbium, Er) and codoped (erbium and praseodymium, Er/Pr) TiO2 nanoparticles immobilized in a poly(vinylidene difluoride)−trifluoroethylene (PVDF−TrFE) copolymer membrane as a suitable strategy to overcome the aforementioned limitations. It is shown that doped and codoped nanoparticles were successfully immobilized into the PVDF−TrFE membranes, with a controllable degree of porosity. A high surface area (273 m2/g) was attained for these nanoparticles. The low band gap (2.63 eV) of these TiO2-modified nanoparticles, coupled with a highly porous structure (∼75%) of the membrane microstructure, synergistically envisages the best photocatalytic performance by degrading 98% of a solution of methylene blue after 100 min of exposure to UV. one of the most widely used photocatalysts.14 There are three main crystal phases of titania (anatase, brookite, and rutile) presenting different photocatalytic activities, with anatase being the more interesting one for photocatalytic applications.7 There are nevertheless some drawbacks regarding photocatalytic applications of bare TiO2 powders in suspension.7 The use of TiO2 nanoparticles with sizes from 4 to 30 nm leads to the formation of aggregates, with the consequent loss of surface area and photocatalytic efficiency.15 Additionally, the nonporous structure of TiO2 nanoparticles leads to a lower adsorption of organics on its surface.16 Further known drawbacks associated with use of these nanophotocatalysts are, on one hand, reutilization/recycling of TiO2 nanoparticles, which is time-consuming and requires expensive separation/ filtering processes;11 and on the other hand, recombination between photogenerated electrons and holes, which is often the largest hindrance in photocatalytic efficiency.17 In order to overcome the inability to recuperate, recycle, and reuse catalysts, immobilization has attracted wide attention. TiO2 immobilization on inactive supports such as glass, polymer, zeolite, silica, and ceramic18 has been addressed. In the last years some few works have reported the production of

1. INTRODUCTION Water pollution is becoming a serious concern and complex issue, as so many and different contaminants are released into the water environment with disregard of their fate.1,2 Environmental concern is thus focusing on uprising micropollutants such as pharmaceuticals, personal care products, antibiotics, and pathogens, which are present in increasing amounts in the water environment.3 Many of these chemicals are potentially harmful for human life and are unable to be treated by conventional biological processes.4 In this sense, it is important to develop energy-efficient and sustainable processes, such as photocatalysis,5−7 to decompose or mitigate such environmental pollutants.8 Photocatalysis has thus received much attention, and a variety of products and materials have been applied in water purification9.10 In general, photocatalysis occurs when UV irradiation generates electron−hole pairs that will react with H2O, OH−, and O2 to produce highly oxidizing species, usually the hydroxyl radical (•OH), superoxide radical anions (O2• −), and hydrogen peroxide (H2O2). These species will decompose organics into CO2, H2O, and other harmless compounds.11 Catalysts are required in this process,12 and among several photocatalysts (Fe2O3, ZnO, ZnS, CdS, WO3, and SrTiO3), TiO2 has been proven to be the most suitable one, due to its significant oxidizing properties under UV irradiation,7 superhydrophilicity,13 chemical stability in a large pH range, nontoxicity, and durability. TiO2 has thus become © XXXX American Chemical Society

Received: September 14, 2014 Revised: November 7, 2014

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dx.doi.org/10.1021/jp509294v | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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Similarly, the codoped TiO2 nanoparticles were produced by adding praseodymium(III) nitrate hexahydrate (Sigma−Aldrich) and erbium(III) nitrate pentahydrate, both with 0.005 atomic ratios (0.5% Er/0.5% Pr). After 5 min under magnetic stirring, 5 mL of deionized water was added, and finally the mixture was poured into a Teflon-lined steel autoclave that was then sealed and heated in a microwave oven for 15 min at 120 °C. After synthesis, the nanoparticle suspension was centrifuged at 9000 rpm, the supernatant was discarded, and the nanoparticles were resuspended in ethanol under sonication for 3 min. This procedure was repeated twice before the particles were finally dried overnight in an oven at 80 °C. 2.2. Er-TiO2/PVDF−TrFE and Er/Pr-TiO2/PVDF−TrFE Nanocomposite Production. Poly(vinylidene difluoride)co-trifluoroethylene, PVDF−TrFE (Solvay 70:30) composite membranes with 5 wt % synthesized nanoparticles were prepared by solvent casting. The nanoparticles were added to 9.5 mL of N,N-dimethylformamide (DMF, from Merck), and with the aim to achieve good dispersion of the particles, the solution was placed in an ultrasound bath for 4 h. After that, 1 g of copolymer was added to the solution, achieving a concentration of 10% wt % polymer, and kept under magnetic stirring until complete dissolution occurred. Finally, the solution was placed in a glass Petri dish and the DMF solvent was evaporated at room temperature. 2.3. Nanoparticle and Membrane Characterization. The crystal structure, purity, and degree of crystallinity of the nanoparticles was evaluated by X-ray diffraction measurements in a Rygaku/Max System RU 300. The porosity of the assynthesized nanoparticles was measured by nitrogen adsorption at 77 K in a Micromeritics TriStar analyzer (Micromeritics, Norcross GA). Samples (0.5 g) were outgassed at 26.7 Pa and 350 °C for 6 h before performing the adsorption experiments were performed. Surface area was determined by use of the Brunauer−Emmett−Teller (BET) equation.41 Pore size distribution was determined by use of the Barrett−Joyner− Halenda (BJH) data reduction scheme in the desorption branch of the isotherms.42 Optical properties of the nanoparticle powder were measured by UV−vis absorption spectroscopy with a Jasco V-670 spectrophotometer coupled with a solid sampling system (integrating sphere). In order to estimate the indirect band gap, the spectra were obtained in reflectance (R) mode and the absorbance was calculated via the Kubelka−Munk equation (eq 1):

porous nanocomposites with photocatalytic activity, such as TiO 2 /polyaniline, 19 CoFe 2 O 4 /polyaniline, 20 TiO 2 /poly(ethylene glycol),21 TiO2/polystyrene,22 and TiO2/silica,23 among others. In addition, the immobilization into a microor mesoporous structure can prevent fast corrosion of the catalyst, as the exposed area is lower than in nanoparticles in suspension.24 However, when photocatalysts are immobilized in those materials, a significant decrease in photocatalytic activity and selectivity occurs due to a substantial reduction of active surface area-to-volume ratios, plus inefficient light harvesting related to the high content of inactive matrixes.25 In this respect, we propose the use of poly(vinylidene difluoride)−trifluoroethylene, PVDF−TrFE, as a substrate to immobilize TiO2 nanopowders. This copolymer is produced in the form of membrane by solvent evaporation with controlled degree of porosity and pore size.26−28 Moreover, fluoropolymers such as PVDF−TrFE exhibit excellent radiation, chemical, and thermal resistance due to stable C−F bonds in the main polymer chain, making them suitable as catalyst carriers.29 Another major limitation of semiconductor photocatalysis is their reduced spectral activation. TiO2 is characterized by a wide band gap (3.2 eV for anatase) and is photocatalytically active only under UV irradiation ( 1% > 3%), supporting the aforementioned interpretation of doping hindering TiO2 crystal growth. Concerning the codoped sample, the size is similar to the 0.5% Er-TiO2, indicating that dopant content is more relevant than dopant type in determining nanoparticle size. Regarding the ζ potential data (inset in Figure 3), the produced nanoparticles are more stable at pH above 8 or below 4, in which the ζ potential values are below −30 and above +30, respectively. Within the mentioned ζ potential range, the peripheral charge values of nanoparticles are higher, contributing to nanoparticle repulsion and thus greater stability, avoiding aggregation and precipitation. Additionally, the results are consistent with the literature,51 which indicates that at pH = 6.8 the TiO2 net charge is 0, at pH < 6.8 TiO2 is positively charged, and at pH > 6.8 TiO2 is negatively charged. Noticeably, pristine, doped, and codoped TiO2 nanoparticles do not show different ζ potential behavior, which is attributed to the similar charge of the ionic species involved: Ti4+, Er3+, and Pr3+. ζ potential measurements are relevant for the intended applications as the adsorption of molecules is pHdependent:52,53 if TiO2 nanoparticles and the pollutant/dye exhibit opposite charges, they will attract each other, promoting the adsorption and enhanced photoactivity. Specific surface area is one of the most important properties concerning photocatalytic activity, as photocatalytic reactions occur on the surface of the nanocatalyst.39 In this sense, when surface area increases, the number of active sites increases proportionally and therefore a high surface area of the nanocatalyst is required for photocatalytic purposes. Table 2 shows that high specific surface areas are obtained for all nanoparticles. Several works reporting on different methods to produce pristine and doped TiO2 nanoparticles present surface areas below 200 m2/g.35 In this work, the microwave-assisted

where D corresponds to the crystal size, λ is the wavelength of incident X-ray radiation (Cu Kα; λ = 0.154 056 nm), θ is the diffraction angle, and β is the full width at half-maximum.46 The estimated average size values (listed in Table 1) are in Table 1. Nanoparticle Sizes Estimated by Scherrer Equation and Size Intensity by DLS Backscattering (173°) at 25 °C XRD sample

crystallite size (nm)

TiO2 0.5% Er-TiO2 1% Er-TiO2 3% Er-TiO2 0.5% Er/0.5% Pr-TiO2

7.04 6.48 6.43 6.31 6.42

DLS size (d, nm) 28 21 18 13 21

± ± ± ± ±

5 3 6 4 5

PDI 0.32 0.35 0.31 0.21 0.28

± ± ± ± ±

0.01 0.03 0.04 0.01 0.04

agreement with those estimated from the TEM images: pristine TiO2 nanoparticles show a higher crystal size (≅7.0 nm) than the doped (6.4, 6.5, and 6.3 nm) and codoped (6.42 nm) samples. It is worth mentioning that polydispersion and anisotropy of the nanoparticles may be responsible for less accurate results for nanoparticle sizes estimated by the Scherrer equation, despite the considered shape factor of 0.89, as peak broadening is not only caused by the small crystallite size.47 In spite of small differences between the produced nanoparticles, the results indicate that the higher the amount of dopant, the smaller the nanoparticle, which is consistent with previous reports addressing such trends in hindrance of growth of titanium dioxide crystallites imposed by the dopant.48,49 As-synthesized nanoparticles show hydrodynamic diameters ranging from 13 to 28 nm (Figure 3) and average PDI values from 0.32 to 0.21, as measured by DLS (Table 1). All the samples show similar diameters, supporting the reproducibility of the microwave-assisted method for TiO2 nanoparticle synthesis.35 It should be noted that sizes estimated from the TEM images (average size ∼12 nm) and those calculated from the XRD diffractograms indicate smaller nanoparticles relative to DLS, which may be explained by nanoparticle agglomeration in the aqueous medium. Agglomerated nanoparticles show a smaller diffusion rate and higher light scaterring than isolated D



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As-produced powder was analyzed by UV−vis absorption spectroscopy. The bare TiO2 nanoparticles revealed low light absorbance in the visible region (Figure 4a) related to the low band gap (3.11 eV). Conversely, the doped and codoped samples revealed absorption peaks in the visible region. The three characteristic Er peaks located at 490, 523, and 654 nm correspond to the transitions from 4I15/2 to 4F7/2, 2H11/2, and 4 F9/2, respectively.38 Particularly, the 4f electron transitions on Er3+ promotes the separation of photogenerated electron−hole pairs, which in turn yield a higher photocatalytic activity of the catalyst. Concerning the Pr absorption peaks, the results show a spectral band from 400 to 500 nm, associated with the f → f transition, and an absorption peak at 600 nm due to the 1D2 →3H4 transition.58 Furthermore, the peak intensitirs, namely, the 523 and 654 nm peaks assigned to Er, are also proportional to the dopant concentration used (Figure 4b). Beyond the aforementioned peaks in the visible spectrum, the doped and codoped samples show a red shift (toward the visible region) near the UV-to-visible transition (around 400 nm) (Figure 5). This results show that the Er and Pr rare earth

Table 2. Dopant Content, BET Surface Area, and BJH Pore Size for All Produced Nanoparticles sample

dopant content (mg)

BET surface area (m2/g)

BJH pore size (nm)

TiO2 0.5% Er-TiO2 1% Er-TiO2 3% Er-TiO2 0.5% Er/0.5% PrTiO2

14.7 29.4 88.2 14.7/14.7

239 164 159 127 273

3.8 3.6 4.8 5.6 2.8

method enables the production of bare TiO2 nanoparticles with larger surface areas: 239 m2/g. Low dopant loading has the benefit of increasing largely the surface area of the as-synthesized TiO2 nanoparticles. This probably occurs due to the change in anatase structure caused by the substitution of Ti4+ for a larger ionic radius (Er3+ or Pr3+). However, Er-doped nanoparticles show a smaller surface area than as-synthesized pristine or Er/Pr-codoped TiO2 nanoparticles. A plausible explanation may be assigned to the high ionic radius of Er3+, which blocks some of the TiO2 pores, yielding a surface area reduction. These results are different from reports on similar particles for different dopants.35,54 However, it is worthy of notice that, in the latter case, the method used by those authors for nanoparticle synthesis is different and the microwave-assisted method, used in the present work, is tailored to optimize the surface area of pure TiO2 nanoparticles. Nevertheless, the surface areas of 0.5% (164 m2/g), 1% (159 m2/g), and 3% (127 m2/g) Er-doped nanoparticles are higher than that of commercial P25 TiO2 nanoparticles (50 m2/g).55 Regarding pore size distribution, determined via the BJH data reduction scheme, the results indicate values of 2.8, 3.6, 4.8, and 5.6 nm for 0.5% Er/0.5% Pr-TiO2, 0.5% Er-TiO2, 1% Er-TiO2, and 3% Er-TiO2, respectively, resulting in a significant modification of TiO2 nanoparticle surface area depending on the dopant loading. These results are consistent with previous works56,57 using microwave-assisted technique for different dopants, which showed by high-resolution TEM analysis the presence of pores ranging from 2 to 4 nm. Additionally, such results are also in good agreement with the BET surface area aforementioned results, as lower pore sizes contribute to higher surface area.

Figure 5. UV−vis spectra for TiO2 and 0.5%, 1%, and 3% Er-TiO2 samples in the transition from UV to visible radiation range.

metal dopants modified the overall absorption spectra of the NP, allowing absorbance enhancement in the visible region and therefore enhanced photocatalytic performance of the doped and codoped TiO2 nanoparticles when irradiated with visible light.37 It should be noted that similar results have been

Figure 4. (a, left) UV−visible spectra for pristine, Er-doped, and Er/Pr-codoped TiO2 samples. Dotted red rectangles and black circles identify the absorbance peaks in the visible region of erbium (Er3+) and praseodymium (Pr3+), respectively. (b, right) Absorption peaks for TiO2 doped with Er at different concentrations. E



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obtained with Fe3+-doped TiO2,59 silver and nitrogen,60 and cerium and neodymium codoping.48 The indirect band gap was determined from the Kubelka− Munk equation by plotting [F(R)hν]1/2 versus hν (Figure 6).61

similar spherical pores and size, independent of the chosen nanoparticle filler. Regarding the filler nanoparticles, good dispersion is achieved within the polymer matrix, with few nanoparticle aggregates inside inside the walls of the pores of the nanocomposite. The degree of porosity, around 74%, of the produced samples is summarized in Table 4, showing that there is no significant difference between them, as the variations are within experimental error. These data indicates that the nanoparticle filler type and content has no influence on the phase diagram and crystallization dynamic of the polymer, which are mainly governed by the polymer/solvent initial ratio and solvent evaporation temperature.28 Additionally, the pore interconnectivity, which is an essential factor for mass transfer across the membrane, is rendered by the spaces between the polymer microspheres that form the pore walls.28 It has been already shown that a large pore channel enhances light utilization and increase of mass transfer of reactants for photocatalysis applications.55 These facts prove the importance of the porous structure of the polymer for photocatalytic activity of the nanocomposite, as the efficiency depends on how they allow good interaction of the dye with the TiO2 surface, a crucial step for the photocatalysis as mentioned above. PVDF and vinylidene copolymers can crystallize in different phases, corresponding to different chain conformations.26 The α-phase presents a nonpolar structure with a TGTG′ chain conformation, while the β- and γ-phases present a polar structure with all-trans planar zigzag TTT and T3GT3G′ chain conformation, respectively. Polymer crystalline phase is deeply influenced by the processing technique and conditions or filler inclusions such as nanoparticles or clays.26 PVDF−TrFE (70:30) crystallizes in the β-phase regardless of solvent evaporation rates or nanoparticle content.63,64 Thus, FTIR-ATR measurements were performed for all prepared samples (Figure 8). Characteristic bands of polymer β-phase appears at 840 and 1279 cm−1, independent of the filler presence and content. No trace of the nonpolar α-phase, characterized, among others, by the FTIR band at 766 cm−1, or of any other polymorph is found in the composites. This fact broadens the range of applicability of these membranes, as the photocatalytic performance can be, hereafter, associated with the use of the membranes as sensors and/or actuators.26,65 3.3. Photocatalytic Activity Evaluation. To study the photocatalytic activity of the produced nanocomposites, roomtemperature degradation of methylene blue (MB) in aqueous solution under UV irradiation was evaluated. These results are shown in Figure 9. In order to validate comparisons of the photocatalytic assays, all the produced nanocomposites present similar thickness (∼550 μm) and porosity (∼73%). In addition, all photocatalytic assays were performed with the same amount of nanocomposite (0.4 g), volume (13 mL), and concentration of MB (1 × 10 −5 M) solution. In other words, the different photocatalytic activities of the nanocomposites are due to the nanoparticles’ distinctive properties. All nanocomposites were able to degrade the MB molecule, as indicated by the decrease in the absorbance characteristic peak (663 nm) with increasing UV irradiation time. The initial absorbance (at t = 0) is different for the tested samples due to the initial experimental conditions: samples were placed for 30 min in the MB solution in the dark without UV irradiation. Such data are in agreement with the reported importance of pollutant adsorption for photocatalytic activity. Not only the dopant concentration but

Figure 6. Determination of band gap of pristine, Er-doped, and Er/Prco-doped TiO2 nanoparticles at [F(R)]1/2 = 0.

All the doped and codoped samples show lower band-gap values than the pristine TiO2 nanoparticles (3.05 eV); the Erdoped particles show similar band gaps of 2.92 eV for 0.5% and 1% Er and 2.85 eV for 3% Er. As previously reported,35,59 a significant reduction of the band-gap values is observed with increasing amounts of dopant in the samples, in agreement with the obtained results. The band-gap reduction is related to the red shift observed in Er-doped samples as compared to the absorbance limit of pristine TiO2 in the visible region as mentioned before. Noticeably, through the microwave-assisted technique, even as-produced pristine TiO2 nanoparticles show lower band-gap values than Evonik P25 nanoparticles (Eg = 3.2 eV)36 (Table 3). Table 3. Band-Gap Values for All Produced Nanoparticles at [F(R)]1/2 = 0 sample

band gap (eV)

TiO2 0.5% Er-TiO2 1% Er-TiO2 3% Er-TiO2 0.5% Er/0.5% Pr-TiO2

3.05 2.92 2.92 2.85 2.63

It is interesting to note that the codoping with two rare earth metals (Er and Pr) resulted in further reduction of band-gap values (Eg = 2.63 eV), supporting the synergetic effect of the two dopants on band-gap reduction. The presented results suggest the presence of intermediate states between valence band and conduction band due to the presence of the dopant species.30,62 3.2. Nanocomposite Characterization. Nanocomposite membranes of PVDF−TrFE incorporating TiO2 nanoparticles were produced by solvent evaporation at room temperature from a homogeneous solution in DMF. SEM surface and crosssection images (Figure 7) show the polymer microstructure of the 0.5% Er-TiO2/PVDF−TrFE sample. The pore size distribution is shown in the inset of Figure 7b, where pore sizes between 30 and 110 μm can be observed, with prevalence in the range 50−80 μm. All produced nanocomposites show F



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Figure 7. (a) Surface and (b) cross-section SEM images of the porous 0.5% Er-TiO2/PVDF−TrFE nanocomposite membranes.

also the choice of dopant affects MB adsorption .66 In fact, the paramount role of adsorption is to overcome the fast (nanoseconds) recombination of electron−hole pair, and generally optimum adsorption results in enhanced photocatalytic activity. Moreover, for better adsorption of substances to the catalyst surface (where photocatalysis take place), an improved interfacial charge transfer occurs.36,66 Hence, the data clearly indicate that the codoped 0.5% Er/0.5% Pr-TiO2/ PVDF−TrFE nanocomposite is more efficient in adsorbing MB at the surface of the nanoparticles than the other samples, as will be discussed later. Some works show the relevance of adsorption for photocatalysis by use of rare earth elements as dopants, sustaining that adsorption is deeply connected to the type of dopant for specific molecules.52,67,68 The results depicted in Figure 9 show that the nanocomposites with incorporated doped or codoped nanoparticles present enhanced photocatalytic activities when compared to the nanocomposite containing bare TiO2 nanocatalyst. Pure PVDF−TrFE membrane was also tested and did not reveal any photocatalytic activity (results not shown). The obtained results correlated with the enhanced BET surface area of doped and codoped nanoparticles with respect to pristine ones and also to the adsorption rates of MB to the TiO2 nanoparticle surface. The three nanocomposites containing Er-doped nanoparticles show slightly similar photocatalytic performance, although it is apparent that the sample with the higher amount of Er (3%) has a lower reaction rate and degradation efficiency of MB after 100 min (Table 5), when compared to those with lower Er concentration (0.5% and 1%). This can be ascribed to the existence of an optimum amount of dopant; larger amounts lead to an increase of intermediate states in the band gap, which

Table 4. Degree of Porosity of Nanocomposites with Pristine, Doped, and Codoped TiO2 Nanoparticles sample TiO2/PVDF−TrFE Er0.5%-TiO2/PVDF−TrFE Er1%-TiO2/PVDF−TrFE Er3%-TiO2/PVDF−TrFE 0.5% Er/0.5% Pr-TiO2/PVDF−TrFE a

degree of porositya (%) 73 72 73 75 74

± ± ± ± ±

6 7 8 6 5

Determined via pycnometer.

Figure 8. FTIR spectra of TiO2/PVDF−TrFE composites with TiO2 nanoparticles with different dopant content.

Table 5. First-Order Rate Constant (k) and Degradation Level of Methylene Blue Aqueous Solution (10−5 M, pH 6.8) for All Produced Nanocomposites sample TiO2/PVDF−TrFE 0.5% Er-TiO2/PVDF− TrFE 1% Er-TiO2/PVDF−TrFE 3% Er-TiO2/PVDF−TrFE 0.5% Er/0.5% Pr-TiO2/ PVDF−TrFE

Figure 9. UV−vis absorption variation at 663 nm of a methylene blue (10−5 M) aqueous solution as a function of time for nanocomposite membranes.

G

reaction rate (1/min)

degradation of MB solution after 100 min (%)

0.012 0.023

81 96

0.023 0.021 0.023

97 95 98



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nanocomposites present similar spectra during the irradiation time and similar transparency at the end of the assays.

inevitably act as recombination centers and thus decrease the photocatalytic efficiency of TiO2 nanoparticles. Similar findings have been reported for Ag doping, where exceeding 0.25 mol % covers the surface of TiO2, decreasing the amount of photogenerated charge carriers and reducing photocatalytic activity.69 In a similar way, a high dopant level of selenium diminishes photocatalytic activity.30 This fact is attributed to the increased recombination rate of charge carriers due to a decrease in the average distance between trap sites, caused by an increasing amount of dopant in a single particle.70 Additionally, from the tested nanocomposites, the codoped sample reveals a higher adsorption rate at t = 0, a high reaction rate (k = 0.023 min−1), and the highest BET surface area (273 m2/g). Moreover, after 100 min under UV irradiation the MB degradation is approximately 98%, which represents the highest degradation value among the tested samples. These results correlated well with those from the presented surface area data, indicating that the codoped sample possesses the highest value of trapping sites, promoting high adsorption rate and photocatalytic activity. These results are consistent with other works employing B and N codoped nanoparticles in MB photocatalysis71 and support the synergetic effect of codoping in photocatalytic performance. Moreover, when the reports on photocatalyst activity in suspension are considered, it is worth noting that the photocatalytic performance herein reported in immobilized systems is comparable to that of systems employing nano/microparticles in suspension,35 representing a further step for photocatalytic applications due to the simpler removal and recycling. This efficiency is also deeply related to the nanocomposite microstructure, with degrees of porosity ranging from 72% to 75% and the macroporous structure of the polymer that enables good mass transfer, promoting interaction between MB and the porous inside wall loaded with nanoparticles and proper light harvesting, suitable for photocatalysis. Figure 10 shows evolution of the absorbance spectra over 110 min at 10-min intervals for the 3% Er-TiO2/PVDF−TrFE

4. CONCLUSIONS Doped and codoped nanoparticles (∼10 nm in size) were synthesized and proven to enable crystallite size reduction, enhanced adsorption rate of MB to the photocatalyst, separation of the electron−hole pair, and red shift in the absorbance spectra, which allows visible light utilization. Afterward, novel photocatalytic nanocomposites with controlled porous microstructure were also successfully produced. From all the tested nanocomposites, the one with codoped Er/ Pr nanoparticles showed the highest photocatalytic activity in the degradation of MB. These results can be ascribed to a favorable combination of physical−chemical properties of the nanoparticles, such as high adsorption rate of MB by dopants (Er and Pr) in the catalyst surface, high surface area (273 m2/ g), and efficient prevention of the electron−hole pair recombination promoted by the dopant species. Furthermore, the polymer membrane also plays a crucial role in efficient photocatalytic performance, allowing for a highly porous structure (74%) and macropore architecture, suitable for good radiation harvesting and interaction between MB solution and immobilized nanoparticles on the inside wall of the pores. The proposed system proved to be suitable for photocatalytic applications, as it allows one to overcome the major hindrances of suspension systems (recuperation and visible light utilization). Regardless of catalyst immobilization, the photocatalytic activities of the nanocomposites, in MB degradation, exhibit remarkable reaction rates, close to the values presented for suspension systems.

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AUTHOR INFORMATION

Corresponding Author

*E-mail lanceros@fisica.uminho.pt. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Fundo Europeu de Desenvolvimento Regional (FEDER) through the COMPETE Program and by the Portuguese Foundation for Science and Technology (FCT) in the framework of the Strategic Project PEST-C/FIS/ UI607/2013, PEST-C/QUI/UIO686/2013 and the project Matepro −Optimizing Materials and Processes, ref NORTE07-0124-FEDER-000037, cofunded by the Programa Operacional Regional do Norte (ON.2 − O Novo Norte), under Quadro de Referência Estratégico Nacional (QREN), through the Fundo Europeu de Desenvolvimento Regional (FEDER). P.M.M. and A.C.L. thank the FCT for Grants SFRH/BD/ 98616/2013 and SFRH/BD/62507/2009, respectively.

Figure 10. Degradation of 10−5 M methylene blue in aqueous solution by 3% Er-TiO2/PVDF−TrFE nanocomposite.

sample, indicating that the degradation of MB occurs with a small peak shift to values below the monitored 663 nm peak. This shift is perhaps associated with N-demethylation of MB by TiO2 nanoparticles or the formation of N-demethylated intermediates,72 Additionally, there are no MB byproduct peaks that interfere with photocatalytic activity probes at 663 nm. Furthermore, it is observed that the MB solution has become completely colorless after 90 min of irradiation. All the tested

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ABBREVIATIONS PVDF−TrFE, poly(vinylidene difluoride)−trifluoroethylene; MB, methylene blue H



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