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Al layered double hydroxides as photocatalysts
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Photodegradation of phenol using reconstructed Ce doped Zn/Al layered double hydroxides as photocatalysts M. Suárez-Quezada a , G. Romero-Ortiz a , V. Suárez b,1 , G. Morales-Mendoza b , L. Lartundo-Rojas c , E. Navarro-Cerón b , F. Tzompantzi b , S. Robles b , R. Gómez b , A. Mantilla a,∗ a
Instituto Politécnico Nacional, CICATA-Legaria, Av. Legaria No. 694, D.F. 11500, Mexico Universidad Autónoma Metropolitana-Iztapalapa, Departamento de Química, Av. San Rafael Atlixco No. 186, D.F. 09340, Mexico c Instituto Politécnico Nacional, Centro de Nanociencias y Micro y Nanotecnologías, Luis Enrique Erro s/n, Zacatenco, D.F. 07738, Mexico b
a r t i c l e
i n f o
Article history: Received 16 April 2015 Received in revised form 7 January 2016 Accepted 11 January 2016 Available online xxx Keywords: Layered double hydroxides Ce-doped ZnAl LDH LDH photocatalysts Ce oxidation states Phenol photodegradation Electron–hole pair separation
a b s t r a c t ZnAlCe layered double hydroxides (LDH) with different content of Ce (3.5, 5.0 and 10.0% mol) were successfully synthesized in one step by the co-precipitation technique. The partial incorporation of cerium into the layers of the material can be appreciated in the XRD diffraction patterns, showing some deformation in the crystallographic direction (1 1 0) of the ZnAlCe LDHs. In the samples calcined at 400 ◦ C, the UV–vis-DRS study showed a shift of the absorption edge toward the blue region of the spectra as a result of the cerium incorporation to the ZnAl LDH; the analysis of XPS confirms the co-existence of Ce4+ and Ce3+ in the ZnAlCe LDHs. The photodegradation and mineralization of phenol under UV irradiation was remarkably improved in the sample containing 5% mol of Ce. A mechanism where cerium in the layered material promotes the separation of the photogenerated electron–hole pairs is proposed. In this mechanism, Ce4+ acts as electron scavenger, facilitating the electron transfer toward adsorbed O2 and an accumulation of holes, increasing the generation of radicals OH• . © 2016 Elsevier B.V. All rights reserved.
1. Introduction Layered double hydroxides (LDHs) are compounds that consist of positive charged brucite-like layers. These compounds have the general formula [M2+ x M3+ 1−x (OH)2x Am·zH2 O], where M2+ and M3+ are divalent and trivalent metal ions, respectively, and Am− is an intercalate anion (CO3 2− the most common) which compensates the positive charge created by the partial substitution of M2+ by M3+ [1]. When a controlled thermal treatment is applied to LDHs, they become into the respective mixed oxides and may show high activity in photocatalytic reactions as the photodegradation of organic pollutants [2–7]. These annealed LDH materials have the capacity of recover its layered structure when they are put in contact with aqueous media, which is named memory effect. Because of their interesting photocatalytic properties, special attention has been given to ZnM LDHs materials (being M a trivalent metallic cation) in the literature [8–16]. Zn2+ induces attractive optical and
∗ Corresponding author. E-mail addresses: [email protected], [email protected] (A. Mantilla). 1 Catedrático CONACyT.
photocatalytic properties, when it is present as their different compounds, as metallic oxides or sulfides, as well as a doping agent with interesting results [17–22]. On the other hand, it has been reported that the incorporation or doping with rare earth elements (La+4 , Eu+4 , Ce4+ , etc.) enhances the photocatalytic efficiency of semiconductors, avoiding the hole-electron recombination during the photocatalytic reactions particularly in the zinc based materials, when these elements are incorporated as doping agents [23–26]. Several authors has been concerned in the incorporation of cerium in the surface of LDH once it was synthesized, using methods as impregnation, in order to improve the photocatalytic properties of them [27,28]. However, the integration of cerium into the LDH structure in a single step during its synthesis by co-precipitation, as well as the effect of the state of oxidation of cerium integrated in the structure of the Ce doped LDH thus obtained, have not been reported. The use of ions with 4f electronic configuration as doping agents in semiconductor oxides can lead to an important improvement of the photocatalytic activity [25]. Particularly, the Ce 4f levels have significant effect on the photo-induced generation and transfer of charge, as well as the decrease of electron–hole recombination. The electronic structures of the Ce3+ with 4f1 5d0 and the Ce4+ with
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4f0 5d0 can lead to obtaining different optical and photocatalytic properties in the host material [29]. Ce4+ can act as an effective electron scavenger to trap photogenerated electrons, and it can acts a stronger Lewis acid nature as well as a superior capability of trapping electrons than O2 •− [25,30,31]. On this regard, Ce diminishes the electron–hole pair recombination by trapping the photogenerated electrons, enhancing the generation of OH• radicals by the accumulation of the photogenerated holes [24]. With this in mind, in the present study the incorporation of cerium to ZnAl LDHs in a single step, during the synthesis by the coprecipitation method and the role of cerium in the photocatalytic behavior is reported. The solids were calcined and characterized by XRD, TGA-DSC analysis, nitrogen adsorption, UV–vis-DRS, photo-luminescence and XPS spectroscopies. The evaluation of the photocatalytic activity was made by irradiating with UV light an aqueous solution containing 40 ppm of phenol, a well-known pollutant used frequently as a model molecule. Fig. 1. XRD patterns of ZnAlCe LDHs synthesized by co-precipitation dried at 100 ◦ C.
2. Experimental 2.1. Preparation of catalysts ZnAlCe LDH’s were synthesized by the coprecipitation method using Zn(NO3 )2 . 6H2 O, Al(NO3 )3 . 9H2 O (Aldrich 99.9%) and Ce(NO3 )3 . 6H2 O (Aldrich 99.9%) as metal precursors and urea as precipitating agent. Aqueous solutions containing the metallic precursors in the adequate proportions to form the ZnAlCe LDHs (Zn/Al molar ratio = 2 and 3.5, 5, 10Ce% mol) were prepared, mixed and then urea was slowly added under stirring to the solution containing the precursors. After, the solution was put in reflux at 90 ◦ C for 36 h until a precipitate was formed. Afterwards, the synthesized material was washed using bi-distilled water; filtered and dried at 100 ◦ C for 12 h (dried sample). The obtained solids were annealed at 400 ◦ C for 12 h, using a heating rate of 1◦ /min. For identification, these calcined samples were labeled as ZnAl, ZnAlCe 3.5%, ZnAlCe 5% and ZnAlCe 10, for Ce contents of 0, 3.5, 5 and 10% mol, respectively. 2.2. X-ray diffraction
Fig. 2. Enlargement view for the peak of the reflection (110) of XRD in the ZnAlCe LDH materials.
XRD powder diffraction patterns of the ZnAlCe LDH materials were recorded at room temperature with a D-8 Advance (Bruker) diffractometer, using a CuK␣ source (l = 0.154 nm). The diffraction intensity as a function of the angle 2q was measured in the range 10–65◦ in 2q for fresh samples and 5–90◦ in 2q for annealed samples, with steps of 0.05◦ at 0.05 s−1 . Diffraction peaks identification in XRD patterns was made using the JCPDS database.
The working pressure in the spectrometer chamber during the analyses did not exceed 4 × 10−10 Torr. Survey scans were recorded with a 400 m spot size and a fixed energy pass of 160 eV, whereas narrow scans were collected at 60 eV energy pass analyzer. Carbon C 1s peak is usually a good way to detect and compensate the charge shift effect.
2.3. TGA-DSC analyses
2.6. Fluorescence spectroscopy
Thermal analysis were carried out using an ISI STI-i-1000 (simultaneous thermal analyzer), using a Pt pan in air flow and recorder from room temperature to 800 ◦ C, with a heating rate of 10 ◦ C/min.
Fluorescence spectra of the annealed samples were obtained with a luminescence spectrophotometer, SCINCO, model FluroMate FS-2. Emission spectra were obtained employing an excitation wavelength, emission and excitation slits of 10 nm, excitation filter of 320 nm and without emission filter. For a better resolution of the spectra, the photomultiplier (PMT) voltage used was 400 mV.
2.4. Band gap determination The band gap values for ZnAl and ZnAlCe LDHs were calculated with a UV–vis CARY 100 SCAN spectrophotometer, equipped with a diffuse reflectance integrating sphere attachment. 2.5. X-ray photoelectron spectroscopy X-ray photoelectron spectroscopy (XPS) spectra of the solids were collected using a K-Alpha Thermo Scientific spectrometer, with a monochromatic AlKa (1486 eV) radiation source and a hemispherical electron analyzer, having an energy resolution of 0.5 eV.
2.7. Nitrogen adsorption Specific surface area was determined from the nitrogen adsorption isotherms obtained with a Quantachrome NOVA 4200e equipment, using N2 as adsorbate and liquid nitrogen (−196 ◦ C) as cooler. The Brunauer–Emmet–Teller (BET) and Barrett–Joyner–Halenda (BJH) methods for specific surface area and mean pore size diameter calculations were used respectively
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Table 1 Textural properties, band gap energy, rate constant Kapp , and mineralization for the ZnAl and ZnAlCe LDH calcined at 400 ◦ C. Sample ◦
ZnAl 400 C ZnAlCe 3.5% 400 ◦ C ZnAlCe 5% 400 ◦ C ZnAlCe 10% 400 ◦ C
Specific surface area, (m2 /g)
Pore volume, (cm3 /g)
Band gap (eV)
Kapp (h−1 )
Mineralization (%)
147 157 167 105
0.286 0.405 0.576 0.196
3.62 (342 nm) 3.06 (405 nm) 3.23 (384 nm) 3.03 (409 nm)
221 × 10−3 434 × 10−3 426 × 10−3 218 × 10−3
78 81 95 82
2.8. Photocatalytic evaluation The photocatalytic evaluation of LDH calcined materials was carried out in a glass reactor having a jacket cooling, using 200 mL of a solution of phenol with a concentration of 40 ppm and 100 mg of catalyst. An UV Pen Ray lamp (254 nm and 4400 W/cm2 ), protected with a quartz tube was immersed in the solution as source of irradiation. The degradation of phenol was measured by taking aliquots of the solution, using a syringe with a nylon membrane in order to remove the suspended solid particles The evolution of the photodegradation was followed the phenol adsorption band at 269 nm with a Cary 100 UV–vis spectrophotometer. The mineralization of phenol was measured by the TOC determination, using Shimadzu LSN equipment.
3. Results and discussion 3.1. X-ray diffraction Fig. 1 shows the XRD patterns of dried ZnAlCe LDH materials. An excellent crystallinity and the presence of the typical peaks corresponding to the structure of layered double hydroxides can be observed, showing that the presence of Ce in the samples did not modify the crystalline layered packing. The intensity of the peaks increases with the cerium content, reaching a maximum in the sample ZnAlCe 5% and diminishing at higher content of cerium (ZnAlCe 10%). The reflection (0 0 3), which is correlated with the separation between the layers could be associated to an higher affinity with carbonates, when cerium is incorporated to the material. On the other hand, the intensity of the peak reflection shows the higher
crystallinity obtained in ZnAlCe 5% sample. Fig. 2 shows a zoom in the 59–63◦ 2q region, where the reflection corresponding to the crystallographic plane (1 1 0) can be analyzed. A shift of the (1 1 0) reflection peak can be observed in the samples containing Ce and it diminishes with the amount of Ce; this shift of the (1 1 0) peak allows to suggest the isomorphic incorporation of cations in LDHs materials, as it have been reported by several authors [2,3,8–10]. This decrease of the (1 1 0) reflection intensity can be due to some perturbation in the crystalline phase, which induces a strain of the lattice and hence, a displacement of the (1 1 0) signal toward lower values of 2q. In this case, the strain can be due to the incorporation of Ce3+ and Ce4+ , which have a bigger ionic radius than Zn2+ and Al3+ , (1.11, 1.01 Å 0.74 and 0.54, respectively) in the layers of the LDHs. The increase in the Ce content into the layers causes a displacement of the position of the atoms located on the perpendicular planes to the [1 1 0] crystallographic direction and the decrease the intensity of the (1 1 0) reflection peak as well as the crystallinity in this direction. Fig. 3 shows the XRD patterns of the LDH calcined at 400 ◦ C, where it can be observed the formation of an amorphous oxide phase. In sample ZnAlCe 10%, some peaks corresponding to layered structure were maintained after calcination, showing that Ce preserves the layered structure after calcination. In Fig. 4, a representative sequence of the XRD patterns obtained for the ZnAlCe 5% LDH dried at 100 ◦ C, annealed at 400 ◦ C and recovered after reaction is showed, where it is possible to note that the recovered sample shows the typical LDH structure (memory effect), which indicates that the predominant phase in aqueous media is the LDH structure highly hydroxylated [32].
Fig. 3. XRD patterns of ZnAlCe LDHs annealed at 400 ◦ C.
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Fig. 6. Uv–vis reflectance diffuse spectra (RDS) of the annealed samples: (a) 200–800 nm region, (b) enlarged spectra of 320–420 nm region.
Fig. 4. XRD patterns of ZnAlCe (5% Ce) LDH: dried at 100 ◦ C, annealed at 400 ◦ C and after reaction (reconstructed).
3.2. TGA-DSC analysis TGA-DSC analysis confirmed that the most important variations in the weight loss were around 400 ◦ C (Fig. 5). This behavior is consistent with that reported for LDH materials of the layered structure [5,11]. 3.3. Band gap determination The band gap values of the annealed LDHs reported in Table 1 are calculated according to the method reported elsewhere [33]. using the values obtained from the UV–vis analysis (Fig. 6a and b). According to these results, all the materials present band gap values in the UV region (3.03–3.23 eV), which are similar to those reported for ZnO. The absorption capacity in the region 200–270 nm changed with the presence of Ce, because of the transition of the 5d states of Ce [34]; however, this shift can be due to the insertion of orbital levels 4f, which can be available as intermediate states for the photoexcited electrons, from the valence band toward the 4f states. In the UV range 200–400 nm, only the 4f → 5d allowed transition electric dipoles are expected, overlapping with the absorption edge of the ZnAl LDH, and it can be misinterpreted as a shift in the band
gap [35,36]. Cerium is unique due to the strong 4f–5d hybridization, which allows the presence of an additional 4f local moment contribution to the electronic structure on the vicinity of the Fermi energy [37]. This means that the 4f states can be used as intermediate states between the valence and the conduction bands of the LDH material, contributing to the diminution of the energy necessary to excite electrons from the valence band to these 4f states. 3.4. X-ray photoelectron spectroscopy The ZnAlCe LDH samples were characterized by XPS technique, in order to determine the presence of Ce4+ and Ce3+ sub-species and their relationship to the conditions fixed during the thermal treatment. It is well known that the uncertainty on this is at least 0.2 eV, because there is increasing evidence that the adventitious carbon signal is not constant on all surfaces. Additionally, carbon is not the main component of the oxide surfaces and neither does it form continuous and homogenous layers over the oxides powder. Usually, ceramic materials and corrosion films present local charges hard to compensate using C 1s peak position. Since oxygen is the predominant element in this type of materials, O 1s peak is more suitable to detect and compensate the charge shift. Additionally, the values obtained using the O 1s peak for charge compensation are closer to the Ce4+ reported values, 882.6 ± 0.2 eV. Fig. 7 shows Ce3d doublet (Ce3d5/2 and Ce3d3/2). XPS spectra collected correspond to an average of three measurements in different points of each sample. The quantitative analysis of
Fig. 5. Thermal analysis of the ZnAl and ZnAlCe 5% selected samples.
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Fig. 8. Photocatalytic behavior of ZnAl and ZnAlCe LDHs calcined at 400 ◦ C in the photodegradation of phenol.
Fig. 7. XPS spectra of fresh and calcined ZnAl LDH materials with a Ce content of 5%.
Ce4+ and Ce3+ species contained in the powders was made by decomposition modeling of the signals in the characteristic spectra reported in the literature [38–41]. For this purpose, mixed Gaussian–Lorentzian functions, a non-linear square fitting algorithm and Shirley background subtraction were employed to deconvolute the Ce3d doublet spectra. The sub-bands labeled u and v represent the electronic configuration 3d10 4f1 initial corresponding to Ce3+ , and the sub-bands labeled u, u , u , v, v , and v represent the 3d10 4f◦ state of Ce4+ [36]. Finally, Relative Sensitivity Factors (RSF) were used to scale the raw peak areas for calculating the fraction of Ce4+ and Ce3+ in the samples annealed. The results indicate that after calcination of the dried samples the Ce4+ content increase until 89% and the Ce3+ decrease until 11%. The presence of Ce4+ and Ce3+ detected by XPS confirms the conclusions obtained from the XRD shift of the (1 1 0) reflection, this shift is due to the incorporation of a cation with different size to Zn2+ and Al3+ into the lattice
10% > ZnAl with photodegradation values of 90; 85; 70 and 63%, respectively. These results suggest that an optimum in the Ce content and hence in the 4f population is required for the phenol photodegradation. The kinetic parameters of the photoreaction, as a function of the Ce content were calculated and the apparent rate constant Kapp follows a first order kinetic, according to the Langmuir-Hinshelwood (L–H) model [42], Fig. 9. Similar photoactivities were found for 5 and 3.5% of Ce content, while lower rate constants were obtained for the photocatalysts with 10% of Ce and the ZnAl reference (Table 1). The role of Ce in the ZnAl structure is related to a photogenerated electron scavenger species, diminishing the electron hole recombination by efficient channeling [24] of photogenerated electrons (e− ) toward adsorbed O2 to form superoxide radical (O2 •− ) [25]. The results obtained in the XPS study of the calcined samples showed a Ce4+ /Ce3+ molar ratio = 8; this higher content of Ce4+ helps to capture the photogenerated electron, decreasing the electron–hole recombination process. The combination of oxidation states Ce3+ and Ce4+ into the photocatalysts structure (Ce3+ (photocatalyst) and Ce4+ (photocatalyst) ) could be the responsible of the increase in the separation of electrons and holes. Then, Ce4+ (photocatalyst) captures the photogenerated electrons, forming the metastable *Ce3+ (photocatalyst) specie. Ce4+ (photocatalyst) + e− → ∗Ce3+ (photocatalyst)
(1)
3.5. Photocatalytic activity Before the photocatalytic test, the suspension (powderorganic compound) was maintained in dark to achieve the adsorption–desorption equilibrium. Concerning to the capacity of adsorption, it was increasing with the presence of cerium in the samples, reaching a maximum at a cerium content of 5%; then, this capacity decreased in the sample with 10% of cerium, which present a lower value of adsorption than the sample with 5% of cerium (20 and 35% of adsorption, for 10 and 5%, respectively), as it can be noted in Fig. 8. In Table 1, the specific surface area values for the annealed samples are listed, showing a variation from 167 to 105 m2 /g. A good correlation between the specific surface area and the phenol adsorption was found. As it has been reported, the ions with 4f electron configuration like Ce [25] can form complex with various organic acids, which can explain the increase of the adsorption capacity in comparison with the ZnAl reference. After the lamp was turned on, we can see that the trend of photoactivity was as follows ZnAlCe 5% > ZnAlCe 3.5% > ZnAlCe
Fig. 9. First order kinetic constant for the phenol photodegradation on ZnAlCe LDH.
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Fig. 10. Proposed mechanism for the cerium presence in ZnAl LDH materials and its role as a charges separator.
Fig. 11. Decrease of fluorescence intensity as a function of the Ce content in the photoluminescence study of the annealed samples.
To recover its oxidation state, *Ce3+ (photocatalyst) transfer the electron toward O2 adsorbed on the surface. ∗Ce3+ (photocatalyst) + O2(ads) → Ce4+ (photocatalyst) + O2 •−
(2)
Ce3+
Meanwhile, (photocatalyst) captures the holes forming the metastable *Ce4+ (photocatalyst) specie. Ce3+ (photocatalyst) + h+ → ∗Ce4+ (photocatalyst)
(3)
can recover its oxidation state oxidizing the OH− anions adsorbed on the surface, forming the strong OH• radical oxidizing specie [23–26]. *Ce4+
(photocatalyst)
∗Ce4+ (photocatalyst) + OH(ads) − → Ce3+ (photocatalyst) + OH•
(4)
In Fig. 10, it can be seen our proposed mechanism: during the radiation, the photons absorbed on the solid can excite electrons from the oxygen or zinc occupied state, which form part of the layers, similarly as it occurs in ZnO. The photoexcited electron can move easily on the layer, producing a deficiency of electronic charge in the ground state of the oxygen or zinc atoms, which is compensated with electrons from the neighboring atoms producing the mobility of charge. This mechanism can be interpreted in a similar way than that used for the photogenerated holes h+ in semiconductor materials. Ce4+ into the layers of ZnAl LDH material can trap the photogenerated e− employing the 4f available states, changing momentarily to *Ce3+ , as it was described above. To recover the oxidation state of Ce4+ , the formed *Ce3+ transfers the captured electron toward adsorbed O2 on the surface, generating O2 •− radicals which can react with the water molecules and generate hydrogen peroxide H2 O2 . The continuous irradiation of the reaction system allows that this produced H2 O2 can rapidly be broken by the UV radiation, producing additional OH• radicals. The OH is well known as oxidant reactive specie, able to degrade the phenol molecule absorbed on the ZnAlCe LDH surface. On the other hand, the deficiency of electronic charge h+ can be transferred toward other neighbor atoms, and it can moves on the layers up to the surface to oxidize the adsorbed OH− anion producing hydroxyl radicals, which also subsequently oxidize the phenol molecule. To confirm the charge separation produced by the incorporation of Ce, the photo-luminescence spectra (PL) were obtained (Fig. 11). The fluorescence spectra demonstrate that the charge separation was increased by the presence of cerium by the decrease of the fluorescence band intensity. In order to evaluate the mineralization capacity of the ZnAlCe LDH materials, the TOC results obtained after 4 h of radiation are showed in Fig. 12. TOC values were calculated using the initial concentration after absorption in dark as the initial point (100%
Fig. 12. Mineralization of phenol in presence of ZnAlCe LDHs annealed at 400 ◦ C.
of carbon) in the solution. The LDH with highest photo catalytic activity for the mineralization of phenol was ZnAlCe 5%, achieving 95% of organic compound oxidation; ZnAl, ZnAlCe 3.5%, and ZnAlCe 10% samples only reached less than 80% of mineralization (Table 1). These results showed that the photoactivity of ZnAl LDHs can be notably improved by the isomorphic incorporation of cerium, reaching its optimal performance with the material containing 5% of cerium.
4. Conclusions The integration of cerium in the structure of ZnAl during the synthesis by coprecipitation produces a variation in the photocatalityc properties of these materials. The photocatalityc behavior in the photodegradation and mineralization of phenol was highly improved in sample with a content of cerium of 5%. The increase in the photoactivity in comparison with the ZnAl LDH materials could be explained by an increase of the electron–hole separation, due to the presence of Ce in its different oxidation states (Ce4+ and Ce3+ ), which acts as electron/hole receptors avoiding their recombination.
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Acknowledgments Authors acknowledge to 169157, 157156 and 2250-6 CONACyT projects and 20150621 SIP project for financial support. E Navarro-Cerón thanks to CONACyT for postdoctoral fellowship. M. Suárez-Quezada, G. Romero-Ortíz and G. Morales-Mendoza thank to CONACyT for scholarship support. We thank Erwin Gómez from IPN-CICATA Legaria, for XRD analysis. References [1] F. Cavani, F. Trifiró, A. Vaccari, Catal. Today 11 (1991) 173–301. [2] F. Tzompantzi, G. Mendoza, J.L. Rico, A. Mantilla, Catal. Today 220 (2014) 56–60. [3] A. Mantilla, F. Tzompantzi, J.L. Fernández, J.A.L. Díaz-Góngora, R. Gómez, Catal. Today 150 (2010) 353–357. [4] X. Xiang, L. Xie, Z. Li, F. Li, Chem. Eng. J. 221 (2013) 222–229. [5] E.M. Seftel, E. Popovici, M. Mertens, P. Cool, E.F. Vansant, Appl. Catal. B 84 (2008) 699–705. ˜ [6] F. Tzompantzi, A. Mantilla, F. Banuelos, J. Fernández, R. Gómez, Top. Catal. 54 (2011) 257–263. [7] G. Mendoza-Damián, F. Tzompantzi, A. Mantilla, A. Barrera, L. Lartundo-Rojas, J. Hazard. Mater. 263 P (2013) 67–72. [8] N. Ahmed, Y. Shibata, T. Taniguchi, Y. Izumi, J. Catal. 279 (2011) 123–135. [9] E. Martín del Campo, J.S. Valente, T. Pavón, R. Romero, A. Mantilla, R. Natividad, Ind. Eng. Chem. Res. 50 (2011) 11544–11552. [10] A. Mantilla, F. Tzompantzi, J.L. Fernández, J.A.I. Díaz-Góngora, G. Mendoza, R. Gómez, Catal. Today 148 (2009) 119–123. [11] E.M. Seftel, E. Popovici, M. Mertens, K. De Witte, G. Van Tendeloo, P. Cool, E.F. Vansant, Microporous Mesoporous Mater. 113 (2008) 296–304. [12] M. Parida, N. Baliarsingh, B. Sairam Patra, J. Das, J. Mol. Catal. A: Chem. 267 (2007) 202–208. [13] K. Parida, L. Mohapatra, Dalton Trans. 41 (2012) 1173–1178. [14] L. Mohapatra, K.M. Parida, Sep. Purif. Technol. 91 (2012) 73–80. [15] K.M. Parida, L. Mohapatra, Chem. Eng. J. 179 (2012) 131–139. [16] X. Wang, P. Wua, Y. Lu, Z. Huang, N. Zhu, C. Lin, Z. Dang, Sep. Purif. Technol. 132 (2014) 195–205. [17] S.M. Lam, J. Sin, A.Z. Abdullah, A.R. Mohamed, Desalin. Water Treat. 41 (2012) 131–169.
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Photodegradation of phenol using reconstructed Ce doped Zn/Al layered double hydroxides as photocatalysts M. Suárez-Quezada a , G. Romero-Ortiz a , V. Suárez b,1 , G. Morales-Mendoza b , L. Lartundo-Rojas c , E. Navarro-Cerón b , F. Tzompantzi b , S. Robles b , R. Gómez b , A. Mantilla a,∗ a
Instituto Politécnico Nacional, CICATA-Legaria, Av. Legaria No. 694, D.F. 11500, Mexico Universidad Autónoma Metropolitana-Iztapalapa, Departamento de Química, Av. San Rafael Atlixco No. 186, D.F. 09340, Mexico c Instituto Politécnico Nacional, Centro de Nanociencias y Micro y Nanotecnologías, Luis Enrique Erro s/n, Zacatenco, D.F. 07738, Mexico b
a r t i c l e
i n f o
Article history: Received 16 April 2015 Received in revised form 7 January 2016 Accepted 11 January 2016 Available online xxx Keywords: Layered double hydroxides Ce-doped ZnAl LDH LDH photocatalysts Ce oxidation states Phenol photodegradation Electron–hole pair separation
a b s t r a c t ZnAlCe layered double hydroxides (LDH) with different content of Ce (3.5, 5.0 and 10.0% mol) were successfully synthesized in one step by the co-precipitation technique. The partial incorporation of cerium into the layers of the material can be appreciated in the XRD diffraction patterns, showing some deformation in the crystallographic direction (1 1 0) of the ZnAlCe LDHs. In the samples calcined at 400 ◦ C, the UV–vis-DRS study showed a shift of the absorption edge toward the blue region of the spectra as a result of the cerium incorporation to the ZnAl LDH; the analysis of XPS confirms the co-existence of Ce4+ and Ce3+ in the ZnAlCe LDHs. The photodegradation and mineralization of phenol under UV irradiation was remarkably improved in the sample containing 5% mol of Ce. A mechanism where cerium in the layered material promotes the separation of the photogenerated electron–hole pairs is proposed. In this mechanism, Ce4+ acts as electron scavenger, facilitating the electron transfer toward adsorbed O2 and an accumulation of holes, increasing the generation of radicals OH• . © 2016 Elsevier B.V. All rights reserved.
1. Introduction Layered double hydroxides (LDHs) are compounds that consist of positive charged brucite-like layers. These compounds have the general formula [M2+ x M3+ 1−x (OH)2x Am·zH2 O], where M2+ and M3+ are divalent and trivalent metal ions, respectively, and Am− is an intercalate anion (CO3 2− the most common) which compensates the positive charge created by the partial substitution of M2+ by M3+ [1]. When a controlled thermal treatment is applied to LDHs, they become into the respective mixed oxides and may show high activity in photocatalytic reactions as the photodegradation of organic pollutants [2–7]. These annealed LDH materials have the capacity of recover its layered structure when they are put in contact with aqueous media, which is named memory effect. Because of their interesting photocatalytic properties, special attention has been given to ZnM LDHs materials (being M a trivalent metallic cation) in the literature [8–16]. Zn2+ induces attractive optical and
∗ Corresponding author. E-mail addresses: [email protected], [email protected] (A. Mantilla). 1 Catedrático CONACyT.
photocatalytic properties, when it is present as their different compounds, as metallic oxides or sulfides, as well as a doping agent with interesting results [17–22]. On the other hand, it has been reported that the incorporation or doping with rare earth elements (La+4 , Eu+4 , Ce4+ , etc.) enhances the photocatalytic efficiency of semiconductors, avoiding the hole-electron recombination during the photocatalytic reactions particularly in the zinc based materials, when these elements are incorporated as doping agents [23–26]. Several authors has been concerned in the incorporation of cerium in the surface of LDH once it was synthesized, using methods as impregnation, in order to improve the photocatalytic properties of them [27,28]. However, the integration of cerium into the LDH structure in a single step during its synthesis by co-precipitation, as well as the effect of the state of oxidation of cerium integrated in the structure of the Ce doped LDH thus obtained, have not been reported. The use of ions with 4f electronic configuration as doping agents in semiconductor oxides can lead to an important improvement of the photocatalytic activity [25]. Particularly, the Ce 4f levels have significant effect on the photo-induced generation and transfer of charge, as well as the decrease of electron–hole recombination. The electronic structures of the Ce3+ with 4f1 5d0 and the Ce4+ with
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4f0 5d0 can lead to obtaining different optical and photocatalytic properties in the host material [29]. Ce4+ can act as an effective electron scavenger to trap photogenerated electrons, and it can acts a stronger Lewis acid nature as well as a superior capability of trapping electrons than O2 •− [25,30,31]. On this regard, Ce diminishes the electron–hole pair recombination by trapping the photogenerated electrons, enhancing the generation of OH• radicals by the accumulation of the photogenerated holes [24]. With this in mind, in the present study the incorporation of cerium to ZnAl LDHs in a single step, during the synthesis by the coprecipitation method and the role of cerium in the photocatalytic behavior is reported. The solids were calcined and characterized by XRD, TGA-DSC analysis, nitrogen adsorption, UV–vis-DRS, photo-luminescence and XPS spectroscopies. The evaluation of the photocatalytic activity was made by irradiating with UV light an aqueous solution containing 40 ppm of phenol, a well-known pollutant used frequently as a model molecule. Fig. 1. XRD patterns of ZnAlCe LDHs synthesized by co-precipitation dried at 100 ◦ C.
2. Experimental 2.1. Preparation of catalysts ZnAlCe LDH’s were synthesized by the coprecipitation method using Zn(NO3 )2 . 6H2 O, Al(NO3 )3 . 9H2 O (Aldrich 99.9%) and Ce(NO3 )3 . 6H2 O (Aldrich 99.9%) as metal precursors and urea as precipitating agent. Aqueous solutions containing the metallic precursors in the adequate proportions to form the ZnAlCe LDHs (Zn/Al molar ratio = 2 and 3.5, 5, 10Ce% mol) were prepared, mixed and then urea was slowly added under stirring to the solution containing the precursors. After, the solution was put in reflux at 90 ◦ C for 36 h until a precipitate was formed. Afterwards, the synthesized material was washed using bi-distilled water; filtered and dried at 100 ◦ C for 12 h (dried sample). The obtained solids were annealed at 400 ◦ C for 12 h, using a heating rate of 1◦ /min. For identification, these calcined samples were labeled as ZnAl, ZnAlCe 3.5%, ZnAlCe 5% and ZnAlCe 10, for Ce contents of 0, 3.5, 5 and 10% mol, respectively. 2.2. X-ray diffraction
Fig. 2. Enlargement view for the peak of the reflection (110) of XRD in the ZnAlCe LDH materials.
XRD powder diffraction patterns of the ZnAlCe LDH materials were recorded at room temperature with a D-8 Advance (Bruker) diffractometer, using a CuK␣ source (l = 0.154 nm). The diffraction intensity as a function of the angle 2q was measured in the range 10–65◦ in 2q for fresh samples and 5–90◦ in 2q for annealed samples, with steps of 0.05◦ at 0.05 s−1 . Diffraction peaks identification in XRD patterns was made using the JCPDS database.
The working pressure in the spectrometer chamber during the analyses did not exceed 4 × 10−10 Torr. Survey scans were recorded with a 400 m spot size and a fixed energy pass of 160 eV, whereas narrow scans were collected at 60 eV energy pass analyzer. Carbon C 1s peak is usually a good way to detect and compensate the charge shift effect.
2.3. TGA-DSC analyses
2.6. Fluorescence spectroscopy
Thermal analysis were carried out using an ISI STI-i-1000 (simultaneous thermal analyzer), using a Pt pan in air flow and recorder from room temperature to 800 ◦ C, with a heating rate of 10 ◦ C/min.
Fluorescence spectra of the annealed samples were obtained with a luminescence spectrophotometer, SCINCO, model FluroMate FS-2. Emission spectra were obtained employing an excitation wavelength, emission and excitation slits of 10 nm, excitation filter of 320 nm and without emission filter. For a better resolution of the spectra, the photomultiplier (PMT) voltage used was 400 mV.
2.4. Band gap determination The band gap values for ZnAl and ZnAlCe LDHs were calculated with a UV–vis CARY 100 SCAN spectrophotometer, equipped with a diffuse reflectance integrating sphere attachment. 2.5. X-ray photoelectron spectroscopy X-ray photoelectron spectroscopy (XPS) spectra of the solids were collected using a K-Alpha Thermo Scientific spectrometer, with a monochromatic AlKa (1486 eV) radiation source and a hemispherical electron analyzer, having an energy resolution of 0.5 eV.
2.7. Nitrogen adsorption Specific surface area was determined from the nitrogen adsorption isotherms obtained with a Quantachrome NOVA 4200e equipment, using N2 as adsorbate and liquid nitrogen (−196 ◦ C) as cooler. The Brunauer–Emmet–Teller (BET) and Barrett–Joyner–Halenda (BJH) methods for specific surface area and mean pore size diameter calculations were used respectively
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Table 1 Textural properties, band gap energy, rate constant Kapp , and mineralization for the ZnAl and ZnAlCe LDH calcined at 400 ◦ C. Sample ◦
ZnAl 400 C ZnAlCe 3.5% 400 ◦ C ZnAlCe 5% 400 ◦ C ZnAlCe 10% 400 ◦ C
Specific surface area, (m2 /g)
Pore volume, (cm3 /g)
Band gap (eV)
Kapp (h−1 )
Mineralization (%)
147 157 167 105
0.286 0.405 0.576 0.196
3.62 (342 nm) 3.06 (405 nm) 3.23 (384 nm) 3.03 (409 nm)
221 × 10−3 434 × 10−3 426 × 10−3 218 × 10−3
78 81 95 82
2.8. Photocatalytic evaluation The photocatalytic evaluation of LDH calcined materials was carried out in a glass reactor having a jacket cooling, using 200 mL of a solution of phenol with a concentration of 40 ppm and 100 mg of catalyst. An UV Pen Ray lamp (254 nm and 4400 W/cm2 ), protected with a quartz tube was immersed in the solution as source of irradiation. The degradation of phenol was measured by taking aliquots of the solution, using a syringe with a nylon membrane in order to remove the suspended solid particles The evolution of the photodegradation was followed the phenol adsorption band at 269 nm with a Cary 100 UV–vis spectrophotometer. The mineralization of phenol was measured by the TOC determination, using Shimadzu LSN equipment.
3. Results and discussion 3.1. X-ray diffraction Fig. 1 shows the XRD patterns of dried ZnAlCe LDH materials. An excellent crystallinity and the presence of the typical peaks corresponding to the structure of layered double hydroxides can be observed, showing that the presence of Ce in the samples did not modify the crystalline layered packing. The intensity of the peaks increases with the cerium content, reaching a maximum in the sample ZnAlCe 5% and diminishing at higher content of cerium (ZnAlCe 10%). The reflection (0 0 3), which is correlated with the separation between the layers could be associated to an higher affinity with carbonates, when cerium is incorporated to the material. On the other hand, the intensity of the peak reflection shows the higher
crystallinity obtained in ZnAlCe 5% sample. Fig. 2 shows a zoom in the 59–63◦ 2q region, where the reflection corresponding to the crystallographic plane (1 1 0) can be analyzed. A shift of the (1 1 0) reflection peak can be observed in the samples containing Ce and it diminishes with the amount of Ce; this shift of the (1 1 0) peak allows to suggest the isomorphic incorporation of cations in LDHs materials, as it have been reported by several authors [2,3,8–10]. This decrease of the (1 1 0) reflection intensity can be due to some perturbation in the crystalline phase, which induces a strain of the lattice and hence, a displacement of the (1 1 0) signal toward lower values of 2q. In this case, the strain can be due to the incorporation of Ce3+ and Ce4+ , which have a bigger ionic radius than Zn2+ and Al3+ , (1.11, 1.01 Å 0.74 and 0.54, respectively) in the layers of the LDHs. The increase in the Ce content into the layers causes a displacement of the position of the atoms located on the perpendicular planes to the [1 1 0] crystallographic direction and the decrease the intensity of the (1 1 0) reflection peak as well as the crystallinity in this direction. Fig. 3 shows the XRD patterns of the LDH calcined at 400 ◦ C, where it can be observed the formation of an amorphous oxide phase. In sample ZnAlCe 10%, some peaks corresponding to layered structure were maintained after calcination, showing that Ce preserves the layered structure after calcination. In Fig. 4, a representative sequence of the XRD patterns obtained for the ZnAlCe 5% LDH dried at 100 ◦ C, annealed at 400 ◦ C and recovered after reaction is showed, where it is possible to note that the recovered sample shows the typical LDH structure (memory effect), which indicates that the predominant phase in aqueous media is the LDH structure highly hydroxylated [32].
Fig. 3. XRD patterns of ZnAlCe LDHs annealed at 400 ◦ C.
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Fig. 6. Uv–vis reflectance diffuse spectra (RDS) of the annealed samples: (a) 200–800 nm region, (b) enlarged spectra of 320–420 nm region.
Fig. 4. XRD patterns of ZnAlCe (5% Ce) LDH: dried at 100 ◦ C, annealed at 400 ◦ C and after reaction (reconstructed).
3.2. TGA-DSC analysis TGA-DSC analysis confirmed that the most important variations in the weight loss were around 400 ◦ C (Fig. 5). This behavior is consistent with that reported for LDH materials of the layered structure [5,11]. 3.3. Band gap determination The band gap values of the annealed LDHs reported in Table 1 are calculated according to the method reported elsewhere [33]. using the values obtained from the UV–vis analysis (Fig. 6a and b). According to these results, all the materials present band gap values in the UV region (3.03–3.23 eV), which are similar to those reported for ZnO. The absorption capacity in the region 200–270 nm changed with the presence of Ce, because of the transition of the 5d states of Ce [34]; however, this shift can be due to the insertion of orbital levels 4f, which can be available as intermediate states for the photoexcited electrons, from the valence band toward the 4f states. In the UV range 200–400 nm, only the 4f → 5d allowed transition electric dipoles are expected, overlapping with the absorption edge of the ZnAl LDH, and it can be misinterpreted as a shift in the band
gap [35,36]. Cerium is unique due to the strong 4f–5d hybridization, which allows the presence of an additional 4f local moment contribution to the electronic structure on the vicinity of the Fermi energy [37]. This means that the 4f states can be used as intermediate states between the valence and the conduction bands of the LDH material, contributing to the diminution of the energy necessary to excite electrons from the valence band to these 4f states. 3.4. X-ray photoelectron spectroscopy The ZnAlCe LDH samples were characterized by XPS technique, in order to determine the presence of Ce4+ and Ce3+ sub-species and their relationship to the conditions fixed during the thermal treatment. It is well known that the uncertainty on this is at least 0.2 eV, because there is increasing evidence that the adventitious carbon signal is not constant on all surfaces. Additionally, carbon is not the main component of the oxide surfaces and neither does it form continuous and homogenous layers over the oxides powder. Usually, ceramic materials and corrosion films present local charges hard to compensate using C 1s peak position. Since oxygen is the predominant element in this type of materials, O 1s peak is more suitable to detect and compensate the charge shift. Additionally, the values obtained using the O 1s peak for charge compensation are closer to the Ce4+ reported values, 882.6 ± 0.2 eV. Fig. 7 shows Ce3d doublet (Ce3d5/2 and Ce3d3/2). XPS spectra collected correspond to an average of three measurements in different points of each sample. The quantitative analysis of
Fig. 5. Thermal analysis of the ZnAl and ZnAlCe 5% selected samples.
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Fig. 8. Photocatalytic behavior of ZnAl and ZnAlCe LDHs calcined at 400 ◦ C in the photodegradation of phenol.
Fig. 7. XPS spectra of fresh and calcined ZnAl LDH materials with a Ce content of 5%.
Ce4+ and Ce3+ species contained in the powders was made by decomposition modeling of the signals in the characteristic spectra reported in the literature [38–41]. For this purpose, mixed Gaussian–Lorentzian functions, a non-linear square fitting algorithm and Shirley background subtraction were employed to deconvolute the Ce3d doublet spectra. The sub-bands labeled u and v represent the electronic configuration 3d10 4f1 initial corresponding to Ce3+ , and the sub-bands labeled u, u , u , v, v , and v represent the 3d10 4f◦ state of Ce4+ [36]. Finally, Relative Sensitivity Factors (RSF) were used to scale the raw peak areas for calculating the fraction of Ce4+ and Ce3+ in the samples annealed. The results indicate that after calcination of the dried samples the Ce4+ content increase until 89% and the Ce3+ decrease until 11%. The presence of Ce4+ and Ce3+ detected by XPS confirms the conclusions obtained from the XRD shift of the (1 1 0) reflection, this shift is due to the incorporation of a cation with different size to Zn2+ and Al3+ into the lattice
10% > ZnAl with photodegradation values of 90; 85; 70 and 63%, respectively. These results suggest that an optimum in the Ce content and hence in the 4f population is required for the phenol photodegradation. The kinetic parameters of the photoreaction, as a function of the Ce content were calculated and the apparent rate constant Kapp follows a first order kinetic, according to the Langmuir-Hinshelwood (L–H) model [42], Fig. 9. Similar photoactivities were found for 5 and 3.5% of Ce content, while lower rate constants were obtained for the photocatalysts with 10% of Ce and the ZnAl reference (Table 1). The role of Ce in the ZnAl structure is related to a photogenerated electron scavenger species, diminishing the electron hole recombination by efficient channeling [24] of photogenerated electrons (e− ) toward adsorbed O2 to form superoxide radical (O2 •− ) [25]. The results obtained in the XPS study of the calcined samples showed a Ce4+ /Ce3+ molar ratio = 8; this higher content of Ce4+ helps to capture the photogenerated electron, decreasing the electron–hole recombination process. The combination of oxidation states Ce3+ and Ce4+ into the photocatalysts structure (Ce3+ (photocatalyst) and Ce4+ (photocatalyst) ) could be the responsible of the increase in the separation of electrons and holes. Then, Ce4+ (photocatalyst) captures the photogenerated electrons, forming the metastable *Ce3+ (photocatalyst) specie. Ce4+ (photocatalyst) + e− → ∗Ce3+ (photocatalyst)
(1)
3.5. Photocatalytic activity Before the photocatalytic test, the suspension (powderorganic compound) was maintained in dark to achieve the adsorption–desorption equilibrium. Concerning to the capacity of adsorption, it was increasing with the presence of cerium in the samples, reaching a maximum at a cerium content of 5%; then, this capacity decreased in the sample with 10% of cerium, which present a lower value of adsorption than the sample with 5% of cerium (20 and 35% of adsorption, for 10 and 5%, respectively), as it can be noted in Fig. 8. In Table 1, the specific surface area values for the annealed samples are listed, showing a variation from 167 to 105 m2 /g. A good correlation between the specific surface area and the phenol adsorption was found. As it has been reported, the ions with 4f electron configuration like Ce [25] can form complex with various organic acids, which can explain the increase of the adsorption capacity in comparison with the ZnAl reference. After the lamp was turned on, we can see that the trend of photoactivity was as follows ZnAlCe 5% > ZnAlCe 3.5% > ZnAlCe
Fig. 9. First order kinetic constant for the phenol photodegradation on ZnAlCe LDH.
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Fig. 10. Proposed mechanism for the cerium presence in ZnAl LDH materials and its role as a charges separator.
Fig. 11. Decrease of fluorescence intensity as a function of the Ce content in the photoluminescence study of the annealed samples.
To recover its oxidation state, *Ce3+ (photocatalyst) transfer the electron toward O2 adsorbed on the surface. ∗Ce3+ (photocatalyst) + O2(ads) → Ce4+ (photocatalyst) + O2 •−
(2)
Ce3+
Meanwhile, (photocatalyst) captures the holes forming the metastable *Ce4+ (photocatalyst) specie. Ce3+ (photocatalyst) + h+ → ∗Ce4+ (photocatalyst)
(3)
can recover its oxidation state oxidizing the OH− anions adsorbed on the surface, forming the strong OH• radical oxidizing specie [23–26]. *Ce4+
(photocatalyst)
∗Ce4+ (photocatalyst) + OH(ads) − → Ce3+ (photocatalyst) + OH•
(4)
In Fig. 10, it can be seen our proposed mechanism: during the radiation, the photons absorbed on the solid can excite electrons from the oxygen or zinc occupied state, which form part of the layers, similarly as it occurs in ZnO. The photoexcited electron can move easily on the layer, producing a deficiency of electronic charge in the ground state of the oxygen or zinc atoms, which is compensated with electrons from the neighboring atoms producing the mobility of charge. This mechanism can be interpreted in a similar way than that used for the photogenerated holes h+ in semiconductor materials. Ce4+ into the layers of ZnAl LDH material can trap the photogenerated e− employing the 4f available states, changing momentarily to *Ce3+ , as it was described above. To recover the oxidation state of Ce4+ , the formed *Ce3+ transfers the captured electron toward adsorbed O2 on the surface, generating O2 •− radicals which can react with the water molecules and generate hydrogen peroxide H2 O2 . The continuous irradiation of the reaction system allows that this produced H2 O2 can rapidly be broken by the UV radiation, producing additional OH• radicals. The OH is well known as oxidant reactive specie, able to degrade the phenol molecule absorbed on the ZnAlCe LDH surface. On the other hand, the deficiency of electronic charge h+ can be transferred toward other neighbor atoms, and it can moves on the layers up to the surface to oxidize the adsorbed OH− anion producing hydroxyl radicals, which also subsequently oxidize the phenol molecule. To confirm the charge separation produced by the incorporation of Ce, the photo-luminescence spectra (PL) were obtained (Fig. 11). The fluorescence spectra demonstrate that the charge separation was increased by the presence of cerium by the decrease of the fluorescence band intensity. In order to evaluate the mineralization capacity of the ZnAlCe LDH materials, the TOC results obtained after 4 h of radiation are showed in Fig. 12. TOC values were calculated using the initial concentration after absorption in dark as the initial point (100%
Fig. 12. Mineralization of phenol in presence of ZnAlCe LDHs annealed at 400 ◦ C.
of carbon) in the solution. The LDH with highest photo catalytic activity for the mineralization of phenol was ZnAlCe 5%, achieving 95% of organic compound oxidation; ZnAl, ZnAlCe 3.5%, and ZnAlCe 10% samples only reached less than 80% of mineralization (Table 1). These results showed that the photoactivity of ZnAl LDHs can be notably improved by the isomorphic incorporation of cerium, reaching its optimal performance with the material containing 5% of cerium.
4. Conclusions The integration of cerium in the structure of ZnAl during the synthesis by coprecipitation produces a variation in the photocatalityc properties of these materials. The photocatalityc behavior in the photodegradation and mineralization of phenol was highly improved in sample with a content of cerium of 5%. The increase in the photoactivity in comparison with the ZnAl LDH materials could be explained by an increase of the electron–hole separation, due to the presence of Ce in its different oxidation states (Ce4+ and Ce3+ ), which acts as electron/hole receptors avoiding their recombination.
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Please cite this article in press as: M. Suárez-Quezada, et al., Photodegradation of phenol using reconstructed Ce doped Zn/Al layered double hydroxides as photocatalysts, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.01.009