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Mn-modified HfO2 nanoparticles with enhanced photocatalytic activity
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Mn-modified HfO2 nanoparticles with enhanced photocatalytic activity Luis A. Gonzáleza,∗, Saúl Gálvez-Barbozab, Efrain Vento-Lujanoa, José L. Rodríguez-Galiciaa, Luis A. García-Cerdac a
Centro de Investigación y de Estudios Avanzados del Instituto Politécnico Nacional, Av. Industrial Metalúrgica # 1062 Parque Industrial, C.P. 25900, Ramos Arizpe, Coahuila, Mexico Instituto Tecnológico de Ciudad Hidalgo, Av. Ing. Carlos Rojas Gutiérrez 2120, C.P. 61100, Ciudad Hidalgo, Michoacán, Mexico c Centro de Investigación en Química Aplicada, Departamento de Materiales Avanzados, Blvd. Enrique Reyna Hermosillo # 140, C.P. 25294, Saltillo, Coahuila, Mexico b
A R T I C LE I N FO
A B S T R A C T
Keywords: HfO2 Mn doping Optical property Photocatalyst Solar photodegradation Photocatalytic mechanism
Round shaped Mn-modified HfO2 nanoparticles were prepared by the Pechini type sol-gel method. The effects of Mn ions on the structure, particle growth, composition, optical properties and photocatalytic performance of HfO2 particles were investigated. The structure analysis revealed that the insertion of Mn2+, Mn3+, and Mn4+ ions inhibited the complete stabilization of tetragonal HfO2. Also, the decrease of particle size to values lower than 5 nm and the shift of optical band gap from 5.7 to 2.1 eV was obtained as effect of increasing the Mn content. HfO2 nanoparticles modified with 10 w% Mn exhibited the highest photocatalytic performance, reaching an efficiency of 91.93% in the decomposition of methylene blue, after 120 min of sunlight irradiation. The efficient trapping of photogenerated electrons on the surface of these nanoparticles generated •O2− radicals, which were the main oxidative species involved in the degradation of the dye. The improved photocatalytic performance of these nanoparticles is then attributable to their increased surface area, suitable photoactivation, and effective transport of photoexcited charge carriers. Based in studies of band gap, valence band position and active species, a mechanism of photodegradation for this photocatalyst was also proposed and discussed.
1. Introduction Metal oxide materials confined to nanoscale dimensions have become very important in recent decades because their size-dependent physical and chemical properties have encouraged the development of novel applications in electronics, medicine, energy and environment. Nanoparticle-assisted photocatalysis has been considered as a simple and sustainable approach for water and air purification. Particularly, the degradation of organic pollutants in water can be performed by strong oxidizing agents such as superoxide (.O2−) and hydroxyl radicals (.OH) formed near the photocatalysts surface. However, for this to occur, a photocatalyst particle must be able to absorb photons with an energy such that electrons and holes be generated and then, they be transported to the particle surface to participate in the degradation of the organic pollutants. Wide-band gap (> 3 eV) semiconductors such as TiO2 and ZnO have been suitably used as photocatalysts under ultraviolet (UV) irradiation [1–4]. Since solar energy is by far the largest available energy source on the earth, in last decades several strategies have been developed to improve the photocatalytic activity of wideband gap semiconductors under visible light. For instance, wide-band gap semiconductors have been intercalated with narrow band gap ∗
semiconductors to form composites [5–7]. However, the most used strategy is that related to the insertion of doping elements in the structure of these materials. The visible light driven photocatalytic activity of ZrO2, a semiconductor with band gap of 5.0 eV, has been significantly enhanced through its doping with Fe and Mn [8-9]. Although HfO2 has a crystalline structure similar to ZrO2, there is an important difference between their electronic structures. Specifically, HfO2 has a larger concentration of electron density on oxygen atoms than ZrO2 [10]. This is advantageous for photocatalysis purposes because such defect levels would serve as electron-trapping sites for accelerated transfer of photogenerated electrons. However, it has been reported that HfO2 has band gap energies of 5.53, 5.79, and 5.65 eV for the cubic (c-HfO2), tetragonal (t-HfO2), and monoclinic (m-HfO2) phases [11], respectively. Therefore, HfO2 cannot work as photocatalyst under visible light. For solar applications, the band gap narrowing to 3.90 eV was achieved in HfOx films [12]. However, to date, few approaches have been developed strategies for the use of HfO2 in photocatalysis applications. For instance, Jie et al. fabricated films of aligned HfO2 nanorods, nanosprings, and nanohelix with photocatalytic activity under UV radiation for hydrogen production from water splitting [13]. However, for HfO2 to operate as photocatalyst under visible
Corresponding author. E-mail address: [email protected] (L.A. González).
https://doi.org/10.1016/j.ceramint.2020.02.130 Received 24 May 2019; Received in revised form 13 February 2020; Accepted 13 February 2020 0272-8842/ © 2020 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Luis A. González, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2020.02.130
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light is necessary to extend its optical absorption to longer wavelengths. As a strategy, some authors obtained the decomposition of methylene blue (MB) dye through the photocatalytic activity of sputtered HfO2-xTiO2-x nanocolumn arrays [14]. Recently, the degradation of MB with visible light was achieved with HfO2/V2O5 nanocomposites fabricated by the precipitation method [15]. In order to produce the band gap narrowing of HfO2, we demonstrated the optical band gap shift from 5.67 to 4.02 eV by adding 10 w% Ce in t-HfO2 [16]. However, it has been shown that concentrations equal to or greater than 10 mol% Mn in HfO2 produce the effective enhancement of optical absorption with a broad band centered at 470 nm [17]. Besides, theoretical calculations have predicted the band gap (Eg) reduction when Hf ions are substituted by Mn ions in c-HfO2 [18]. These studies as well as the development of novel photocatalysts were the motivation for us to study the photocatalytic activity of Mn:HfO2 NPs under UV and simulated sunlight. Among the outstanding results of this paper, it is shown the preparation of Mn-modified HfO2 NPs, with sizes < 5 nm, by the Pechini type sol-gel method. This method was chosen, since it offers advantages such as low cost, simplicity, compositional control, and particles with high specific surface area. As known, metal oxides properties depend on the synthesis method. Therefore, it was required to investigate the effect of Mn ions in the structure, morphology and optical properties of the resulting HfO2 NPs. In addition to structural distortions, doping of HfO2 with Mn caused the band gap narrowing due to the insertion of new energy levels, both on the valence and conduction band. The improved photocatalytic performance of Mn:HfO2 NPs, for the decomposition of MB under UV and sunlight irradiation, is then associated to their small grain size, suitable photoactivation, and effective transport of photoexcited carriers towards the NPs surface.
excitation wavelength. The particle size and morphology of the samples were determined by a high-resolution transmission electron microscope (HRTEM) TITAN 80–300 FEI. Besides, an X-ray photoelectron spectroscopy system (Thermo Scientific Escalab 250Xi), equipped with a monochromatic AlKα X-ray source (1486.68 eV), was used to determine the chemical state of the samples. The optical band gap of the samples was estimated from diffuse reflectance spectra which were measured with an UV-VIS-NIR spectrophotometer Ocean Optics HR4000CG-UVNIR.
2. Experimental procedure
3. Results and discussion
2.1. Synthesis of Mn:HfO2 NPs
3.1. Crystalline structure analysis
The preparation of HfO2 NPs with 0, 2.5, 5, 7.5, 10 and 12.5 w% Mn content (labeled as HM0, HM2.5, HM5, HM7.5, HM10 and HM12.5), was performed by calcinating polymer resins from precursor solutions processed by the Pechini type sol-gel process. Hafnium chloride (HfCl4) and manganese nitrate tetrahydrate (Mn(NO3)3·4H2O) were used as metal precursors, whereas citric acid (C6H8O7) and ethylene glycol (C2H6O2) were used as chelating and cross-linking agents, respectively. All reagents were purchased from Sigma-Aldrich and used without further modification. The synthesis process used in this study is based on our methodology formerly implemented for the preparation of Ce:HfO2 NPs [19]. Typically, precursor solutions were prepared by dissolving citric acid in de-ionized water at room temperature under vigorous stirring. The required amount of HfCl4 and (Mn(NO3)3·4H2O) was added to the former solutions in order to have a molar ratio 1:1 of citric acid to metal ions. Next, the necessary volume of ethylene glycol was added to complete a solution with molar ratio 4:1 of ethylene glycol to citric acid. The precursor solutions were then heated at 80 °C, under constant stirring, until having two-thirds of the total volume. The resulting viscous solutions were heated at 130 °C for 24 h to form the complex polymer resins. Mn:HfO2 NPs were then obtained when polymer resins were calcinated in air for 2 h at 500 °C in an electric furnace.
Fig. 1 shows diffraction patterns of the synthesized Mn:HfO2 NPs. In accordance with the ICSD file card 01-078-5754, the diffraction peaks of the HM0 and HM2.5 samples are associated to m-HfO2. For Mn concentrations equal to or greater than 2.5 w%, diffraction peaks associated to t-HfO2 (ICSD No. 01-078-5756) start to arise and those
2.3. Evaluation of the photocatalytic activity The photocatalytic performance of the NPs was investigated for degradation of MB, a hazardous cationic dye found in the effluents of the textile industry. In a typical experiment, 0.190 mmol/L of the photocatalyst was added to 31.2 μmol/L of MB solution at pH 6.8 ± 0.2. Next, the resultant suspension was vigorously stirred for 30 min, in dark conditions, to accomplish the adsorption/desorption equilibrium. The degradation of MB dye occurred by irradiating the suspension, under constant stirring, with the luminous source of a solar simulator (Oriel LCS-100 small area Sol1A) for 120 min. This system was configured to study the dye degradation under full light irradiation in the spectral range from 280 to 800 nm and a simulated sunlight irradiation corresponding to the global standard spectrum AM1.5G. In order to determine the photodegradation rate of the dye, 2 ml aliquots were collected from each suspension every 20 min. Next, the NPs contained in the suspension aliquots were precipitated by centrifugation to finally determine the MB dye degradation in solution by UV–Vis spectrometry.
2.2. Characterization techniques The crystalline structure of the samples was analyzed from diffraction patterns obtained with a Siemens D-5000 diffractometer, with CuKα radiation (1.54056 Å), operated at 35 kV in the 2θ range from 10 to 80°. As a complementary technique, Raman spectra were obtained by a Jobin-Yvon Horiba XPLORA micro-Raman Spectrometer to identify the frequency modes associated to the crystalline phase of the samples. In this equipment, the 532 nm line of an Ar ion laser was used as
Fig. 1. XRD patterns of HfO2 and Mn modified HfO2 nanoparticles. 2
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Table 1 Crystalline phase, crystallite size and band gap of Mn:HfO2 nanoparticles. Sample
Crystalline phase
Crystallite size (nm)
Band gap (eV)
HM0 HM2.5 HM5
m-HfO2 m-HfO2 m-HfO2 t-HfO2 m-HfO2 t-HfO2 m-HfO2 t-HfO2 m-HfO2 t-HfO2
6.51 4.95 4.34 – 3.96 4.25 – 4.03 4.15
5.53 2.58 2.34
HM7.5 HM10 HM12.5
2.25 2.23 2.10
corresponding to m-HfO2 decreased. Observe that diffraction peaks of m-HfO2 are still identified in the pattern of the sample with highest Mn content (HM12.5). In principle, the substitution of Mn2+ in Hf4+ sites is expected due to the proximity of its ionic radii (Hf4+ 0.78 Å and Mn2+ 0.80 Å). Nevertheless, the oxygen deficiency is an essential factor for the stabilization of HfO2 high-temperature phases, as it was pointed out by Gao et al. when preparing Mn:HfO2 compounds by the conventional solid state reaction method in Ar [20]. The admixture of phases in HM12.5 can be then related to the oxidation of Mn2+ into Mn3+ and Mn4+ and the consequent decrease of oxygen vacancies. Since the ionic radii of Mn3+ (0.66 Å) and Mn4+ (0.60 Å) are significantly smaller than that of Mn2+, greater lattice distortions can be formed causing the tetragonal structure to be unstable. The average crystallite size of all samples was calculated with the Scherrer equation by using the most intense diffraction peaks, (−111) for m-HfO2 and (101) for t-HfO2. The decrease of crystallite size from 6.51 to 4.03 nm, as listed in Table 1, suggests that the inclusion of Mn ions cause lattice distortions that disrupt the crystal growth. Raman spectroscopy was used as a complementary technique to have a better understanding of the phase transitions in these samples. The group theory analysis [21,22], predicts 18 Raman active modes for the m-HfO2 phase (9Ag + 9Bg) and 6 Raman active modes for the t-HfO2 phase (A1g + 2B1g + 3Eg). Fig. 2(a) Fig. 3. XP spectra of the sample HM12.5: (a) Hf 4f, (b) Mn 2p and (c) O 1s.
shows the Raman spectrum of the HM0 sample exhibiting vibrational modes corresponding to m-HfO2 which are identified by the peaks at 105, 130, 142, 157, 216, 237, 249, 284, 316, 329, 377, 391, 491, 514, 545, 573, 634 and 667 cm−1 [23]. With the inclusion of Mn ions, some of these peaks are attenuated while new ones, associated to the γ1-form and metastable t'-HfO2, are visualized. The spectrum of the sample HM12.5 in Fig. 2(b) shows peaks centered at 263, 383, 567 and 619 cm−1 that can be associated to the γ1-form and peaks at 480 and 647 cm−1 that can be associated to vibrational modes of t'-HfO2. γ1 and γ2 are forms only identified by Raman scattering [24]. These defective structural forms, associated to point defects (oxygen vacancies and dopant cations), can be visualized between m-HfO2 and t'-HfO2. Hence, multiple state Mn ions delay the phase transformation to t-HfO2, displaying thus a broader range of coexistence between monoclinic and tetragonal phase. 3.2. Chemical states analysis XP spectroscopy was performed to analyze the chemical state of the elements present on the sample with highest Mn content (HM12.5). Fig. 3 (a) shows the Hf 4f spectrum fitted to two sets of double-peak components. The peaks located at 16.83 and 18.83 eV correspond to the doublet Hf4+ 4f7/2 and Hf4+ 4f5/2, respectively. The positions of these peaks are shifted towards higher binding energies, in comparison with those of the HM0 sample (not shown here). This energy shift can be attributed to the substitution of Hf ions by Mn ions in the HfO2 lattice. The second double-peak component at 15.35 and 17.51 eV can be
Fig. 2. Raman spectra of samples (a) HM0 and (b) HM12.5. 3
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Fig. 4. Analysis of the HM10 sample (a) TEM image, (b) histogram, (c) HR-TEM image, and (d) SAED pattern.
attributed to Hfx+ 4f7/2 and Hfx+ 4f5/2 (x < 4), respectively, which is identified as the formation of Hf suboxides due to the presence of oxygen vacancies [25]. The Mn 2p spectrum, shown in Fig. 3(b) has two localized peaks at 640 and 652 eV corresponding to Mn 2p3/2 and Mn 2p1/2, respectively. The peak-fit processing of the Mn 2p3/2 spectrum displays three different components located at binding energies of 639.9, 641.4 and 643.46 eV, which are respectively associated to Mn2+, Mn3+ and Mn4+ states, as expected from the microstructure analysis. The O1s XP spectrum shown in Fig. 3(c) was fitted to three oxygen components. The component OI at 528.6 eV is associated to the lattice oxygen, while the component OII at 530.12 eV is related to oxygen weakly adsorbed on the particle surface. The third component (OIII) at 531.48 eV is related to oxygen-deficient states of the HfO2 [26].
dislocations due to lattice strains, as indicated with the circle, which are responsible of the NPs surface defects. The selected-area electron diffraction (SAED) pattern, in Fig. 4(d), confirms the polycrystalline nature of this sample, with bright concentric rings which in this case are associated to four main reflection planes (111), (200), (220), and (311) of t-HfO2. 3.4. Optical properties Results from UV–Vis diffuse-reflectance spectroscopy confirm that the inclusion of Mn produced extrinsic defects in HfO2 which are related to the significant light absorption. This is evidenced by the remarkable red-shift of the optical absorption edge in the reflectance spectra of the samples HM7.5, HM10 and HM12.5, shown in Fig. 5. The reflectance data of all samples were used to estimate the optical band gap (Eg) values, under the assumption of direct optical transitions, by extrapolating the linear region of the plot of [F(R∞)*hv]2 versus hv to zero, as shown with the insets of Fig. 5. According to the Kubelka–Munk theory, F(R∞) is equivalent to the absorption coefficient and hv is the incident photon energy [27]. The Eg value of the HM0 sample was 5.53 eV, which is close to that reported in the literature for m-HfO2 (5.7 eV). The absorption threshold at 3.54 eV, observed in this curve, can be related to intermediate energy levels within the band gap due to the presence of oxygen vacancies [28]. With the inclusion of Mn2+, Mn3+ and Mn4+ ions, the overall band gap size decreased as consequence of extrinsic defects that create intermediate states within the band gap. This effect was greatly enhanced with the increase of Mn content. For instance, while the HM2.5 sample had an Eg of 2.58 eV, the HM12.5 sample had an Eg of 2.10 eV. Moreover, it is also observed a second absorption threshold related to intrinsic defects (oxygen
3.3. Morphology The morphology of the HM10 sample, which has the smallest crystallite size according to the XRD analysis, was investigated by highresolution TEM to determine the average size and shape of the particles. Fig. 4(a) shows a TEM micrograph of agglomerated round shaped particles with sizes below 5 nm. The particle size distribution is represented by the histogram shown in Fig. 4(b), where the average size was found to be 3.6 ± 0.5 nm. It is also observed that approximately 85% of the measured particle are in the range from 3 to 4.5 nm, which is in good agreement with the XRD analysis. The high-resolution TEM image of Fig. 4(c) depicts the lattice fringes of several particles confirming its crystalline feature. Lattice spacings of 0.29 and 0.25 nm associated to the (111) and (200) planes of the tetragonal phase were determined by measurements on the images of the particles shown in the respective figure. Moreover, this image also visualizes some 4
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Fig. 5. Reflectance spectra (inset: plot of (F(R∞)*hv)2 × 103) corresponding to (a) HM0, (b) HM7.5, (c) HM10 and (d) HM12.5 samples.
Fig. 7. (a) Degradation rate of MB dye by Mn:HfO2 NPs (b) Evolution of the UV–Vis absorption spectra corresponding to the MB degradation with HM10 NPs under simulated sunlight. Fig. 6. Valence-band XP spectra of samples HM0 and HM10.
study, a standard reference of MB concentration was determined through the proportional relationship between the absorbance (A), and the dye concentration (mol/L) in accordance to the Beer-Lambert law [29].
vacancies) induced by the presence of Mn ions with lower states. The calculated Eg values of all samples are listed in Table 1. In order to elucidate the origin of band gap narrowing, the analysis of density of states in the valence band (VB) in samples HM0 and HM10 was performed by XP spectroscopy. The spectrum of sample HM0, according to Fig. 6, shows that the VB edge is located approximately at 3.5 eV. Since the optical band gap of this sample was determined at 5.53 eV, the conduction band (CB) edge would be expected to appear around 2.03 eV. On the other hand, the VB edge of sample HM10 is located at approximately 1.5 eV. According to the calculations of band gap (2.23 eV), the CB edge would occur at 0.73 eV. In addition, it is observed that this spectrum presents a small shoulder with energy around 1.7–1.26 eV. This shoulder, indicated with a shaded circle in Fig. 6, may be responsible of the second absorption threshold observed in the optical measurements with UV–Vis spectroscopy. The density of states in VB near the Fermi level are associated to Mn 3d and O 2p states, while those states located below the bottom of the CB are associated to shallow trap states. These results indicate that the inclusion of Mn ions caused the band gap narrowing due to the insertion of new energy levels, both on the VB and CB of HfO2.
A = −log10
It = εbc I0
(1)
where I0 is the intensity of the incident beam, and It is the intensity of the emerging beam from the MB dye container. This is equivalent to the product of absorptivity (ε ), the length of the beam path (b) and the concentration of the dye solution (c). The photocatalytic activity of the Mn:HfO2 NPs was assessed by calculating the MB dye degradation rate (D) as
D=
CO − Ct × 100%, CO
(2)
where CO (mg/L) is the initial dye concentration and Ct (mg/L) is the dye concentration remaining in solution after an irradiation time t. The MB degradation percent, after 120 min of light irradiation, performed by the HM0 NPs and NPs with higher Mn contents (HM7.5, HM10 and HM12.5) is shown in the bar graph of Fig. 7(a). As expected, the MB dye solution with HM0 NPs had almost null degradation (5.2%). On the other hand, those MB solutions with HM10 NPs showed degradation rates of 97.05 and 91.93% when irradiated under full and sunlight spectra, respectively. Particularly, the evolution of MB dye degradation
3.5. Photocatalytic performance Prior to the photocatalytic activity evaluation of the samples under 5
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Table 2 MB degradation and k-values of the photocatalytic activity performed by the HM7.5, HM10 and HM12.5 samples. Sample
HM7.5 HM10 HM12.5
k-value ( × 10−3)
MB degradation (%) Full spectrum
Sunlight
Full spectrum
Sunlight
80.66 97.05 86.66
55 91.93 66.22
11.4 24.3 12.9
4.6 17.8 7.1
Fig. 8. MB dye decomposition under (a) full spectrum (b) solar spectrum.
with HM10 NPs is illustrated by measuring the attenuation of the absorption spectrum, displayed in Fig. 7(b), with absorption peak at 664 nm. The surface adsorption capacity of these NPs under dark conditions is evidenced by a decrease of 23.3% of the absorption peak. At the first 20 min of sunlight irradiation, the absorption spectrum decreased rapidly, indicating the appropriate photocatalytic activity of these NPs. With time, the amount of MB molecules decreased, and the absorption peak experienced a more gradual attenuation. The relative change of MB dye concentration (Ct/C0) under full light spectrum and sunlight irradiation is shown in Fig. 8. It is interesting to observe that, in dark conditions, the Mn:HfO2 NPs had greater dye adsorptions. This effect can be related to the improvement of electrostatic attraction, which is originated by the modification of surface charge and decrease of particle size (large surface area). Fig. 8 also shows that the dye concentration decreases monotonically with time when Mn:HfO2 NPs are irradiated both under full light spectrum and sunlight. The kinetics of photodegradation reaction was determined by calculating the firstorder kinetic constant (k) of the following equation
ln
CO = kt Ct
Fig. 9. (a) Photocatalytic performance of HM10 NPs under sunlight during five consecutive cycles (b) the effect of different scavengers on the degradation of MB dye with HM10 NPs.
degradation process of MB, 1 mM of different scavengers were added to aqueous systems containing HM10 NPs. Ethylene diaminetetraacetic acid (EDTA) was used to scavenge holes (h+), silver nitrate (AgNO3) was used to scavenge electrons (e−), p-benzoquinone (BQ) was used to scavenge super oxide anions (.O2−) and Isopropyl alcohol (IPA) was used to scavenge hydroxyl radicals (.OH). Fig. 9(b) shows the degradation efficiency, under sunlight irradiation, of the photocatalyst in presence of the different scavengers. In presence of IPA, the photocatalytic activity of the NPs was about 64.7%. Conversely, systems containing BQ, AgNO3 and EDTA-2Na exhibited MB degradations of 12.03, 20.31 and 30.35, respectively. Therefore, .O2− is the active species that mainly promotes the degradation of MB molecules, followed by e−, h+ and .OH. Based on the studies of band gap, VB position and active species, the
(3)
The k-values of the Mn:HfO2 NPs are listed in Table 2 where the HM10 NPs undoubtedly had superior degradation rates. HM10 NPs were carefully recovered by centrifugation and reused, under the same conditions, to evaluate their photocatalytic stability and recyclability. Fig. 9(a) shows the good performance of these NPs by reaching dye degradations of 91.93, 91.83, 91.6, 91.52, and 91.49% from the first, second, third, fourth and fifth cycle, respectively. These results confirm the recyclability and high stability of these photocatalytic NPs. In order to investigate the reactive species involved in the photocatalytic 6
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narrowing. The diffusion of charges and its suppressed recombination was suitably achieved due the small grain sizes (< 5 nm) and surface defects of these NPs. Moreover, the combination of these features promoted the adsorption of the MB dye on the NP surface. In summary, intermediate states formed by the insertion of Mn ions, facilitated the photogeneration of electron-hole pairs which were effectively transported to the NPs surface to degrade the adsorbed MB molecules. Declaration of competing interest None. Acknowledgements This work was supported by CONACYT through the becas program (grant number: 346872).
Fig. 10. Proposed mechanism of the photocatalytic activity of Mn:HfO2 NPs under UV or sunlight irradiation.
References
possible mechanism of photodegradation is illustrated in Fig. 10. Electron-hole pairs are stimulated by the absorption of photons with energy (hv) equal to or greater than the band gap. Mn:HfO2 + hv → e−(CB) + h+(VB)
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(4)
Since NPs are not able to create internal electric field, charges associated with point defects can only be transferred by the diffusion of charge carrying defects. The fastest transport of charge carriers to the surface can be then achieved by the short distance of the sample with smallest particle size (HM10). Charges are then captured by surface defects and adsorbed species so that the electron and hole recombination is suppressed. According to the proposed photocatalytic mechanism, the reduction potential of CB (−0.72 eV) of HM10 NPs is more negative than the reduction potential of O2/.O2− (−0.32 eV). Consequently, the accumulated electrons in CB can efficiently reduce O2 to .O2− radicals. O2 + e− →.O2− .
(5)
O2−
The radicals are able to oxidize the MB molecules to CO2 and H2O. On the other hand, holes accumulated in the VB are not suitable for the oxidation of OH− to .OH due to its less positive reduction potential. Therefore, holes are available to react directly with the adsorbed MB molecules [30]. Thus, the generation of highly active hydroxyl radicals could be only produced through the further reduction of superoxide radicals. .
O−2 + OH → .OH
(6)
.
Then, available OH radicals can effectively degrade the MB dye by oxidation processes. 4. Conclusions Mn-modified HfO2 NPs were successfully synthesized by the Pechini type sol-gel method and post annealing at 500 °C. We have demonstrated that the insertion of Mn ions have significant effects in the microstructure, particle size and optical properties of HfO2 NPs. The substitution of Hf4+ ions by Mn2+, Mn3+ and Mn4+ ions caused distortions to the HfO2 lattice leading to the formation of an admixture of m-HfO2 and t-HfO2. In consequence, the particle size decrease is associated to extrinsic and intrinsic defects that disrupted the crystal growth. Mid-gap states, introduced by the presence of Mn ions, promoted the shift of band gap energy from 5.7 to 2.1 eV and therefore, the enhancement of UV and visible light absorption. Results on the degradation of MB dye, showed that the HM10 NPs had the most efficient photocatalytic activity under sunlight irradiation. The improved photocatalytic performance of these NPs is related to the following aspects. The effective photogeneration of electron-hole pairs due to the improved light absorption which is associated to the optical band gap 7
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Part I, J. Opt. Soc. Am. 38 (1948) 448–457. [28] A.A. Rastorguev, V.I. Belyi, T.P. Smirnova, L.V. Yakovkina, M.V. Zamoryanskaya, V.A. Gritsenko, H. Wong, Luminescence of intrinsic and extrinsic defects in hafnium oxide films, Phys. Rev. B 76 (2007) 235315. [29] J.M. Hollas, Modern Spectroscopy, fourth ed., John Wiley & Sons Ltd, Chichester, England, 1987. [30] A. Houas, H. Lachheb, M. Ksibi, E. Elaloui, C. Guillard, J.-M. Herrmann, Photocatalytic degradation pathway of methylene blue in water, Appl. Catal. B Environ. 31 (2001) 145–157.
[24] M. Yashima, H. Takahashi, K. Ohtake, T. Hirose, M. Kakihana, H. Arashi, Y. Ikuma, Y. Suzuki, M. Yoshimura, Formation of metastable forms by quenching of the HfO2RO1.5 melts (R=Gd, Y and Yb), J. Phys. Chem. Solid. 57 (1996) 289–295. [25] W. Zhang, J.-Z. Kong, Z.-Y. Cao, A.-D. Li, L.-G. Wang, L. Zhu, X. Li, Y.-Q. Cao, D. Wu, Bipolar resistive switching characteristics of HfO2/TiO2/HfO2 trilayerstructure RRAM devices on Pt and TiN-coated substrates fabricated by atomic layer deposition, Nanoscale Res. Lett. 12 (2017) 393. [26] T. Guo, T. Tan, Z. Liu, Resistive switching behavior of HfO2 film with different Ti doping concentrations, J. Phys. D Appl. Phys. 49 (2016) 045103. [27] P. Kubelka, New contributions to the Optics of intensely light-scattering materials.
8
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Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Mn-modified HfO2 nanoparticles with enhanced photocatalytic activity Luis A. Gonzáleza,∗, Saúl Gálvez-Barbozab, Efrain Vento-Lujanoa, José L. Rodríguez-Galiciaa, Luis A. García-Cerdac a
Centro de Investigación y de Estudios Avanzados del Instituto Politécnico Nacional, Av. Industrial Metalúrgica # 1062 Parque Industrial, C.P. 25900, Ramos Arizpe, Coahuila, Mexico Instituto Tecnológico de Ciudad Hidalgo, Av. Ing. Carlos Rojas Gutiérrez 2120, C.P. 61100, Ciudad Hidalgo, Michoacán, Mexico c Centro de Investigación en Química Aplicada, Departamento de Materiales Avanzados, Blvd. Enrique Reyna Hermosillo # 140, C.P. 25294, Saltillo, Coahuila, Mexico b
A R T I C LE I N FO
A B S T R A C T
Keywords: HfO2 Mn doping Optical property Photocatalyst Solar photodegradation Photocatalytic mechanism
Round shaped Mn-modified HfO2 nanoparticles were prepared by the Pechini type sol-gel method. The effects of Mn ions on the structure, particle growth, composition, optical properties and photocatalytic performance of HfO2 particles were investigated. The structure analysis revealed that the insertion of Mn2+, Mn3+, and Mn4+ ions inhibited the complete stabilization of tetragonal HfO2. Also, the decrease of particle size to values lower than 5 nm and the shift of optical band gap from 5.7 to 2.1 eV was obtained as effect of increasing the Mn content. HfO2 nanoparticles modified with 10 w% Mn exhibited the highest photocatalytic performance, reaching an efficiency of 91.93% in the decomposition of methylene blue, after 120 min of sunlight irradiation. The efficient trapping of photogenerated electrons on the surface of these nanoparticles generated •O2− radicals, which were the main oxidative species involved in the degradation of the dye. The improved photocatalytic performance of these nanoparticles is then attributable to their increased surface area, suitable photoactivation, and effective transport of photoexcited charge carriers. Based in studies of band gap, valence band position and active species, a mechanism of photodegradation for this photocatalyst was also proposed and discussed.
1. Introduction Metal oxide materials confined to nanoscale dimensions have become very important in recent decades because their size-dependent physical and chemical properties have encouraged the development of novel applications in electronics, medicine, energy and environment. Nanoparticle-assisted photocatalysis has been considered as a simple and sustainable approach for water and air purification. Particularly, the degradation of organic pollutants in water can be performed by strong oxidizing agents such as superoxide (.O2−) and hydroxyl radicals (.OH) formed near the photocatalysts surface. However, for this to occur, a photocatalyst particle must be able to absorb photons with an energy such that electrons and holes be generated and then, they be transported to the particle surface to participate in the degradation of the organic pollutants. Wide-band gap (> 3 eV) semiconductors such as TiO2 and ZnO have been suitably used as photocatalysts under ultraviolet (UV) irradiation [1–4]. Since solar energy is by far the largest available energy source on the earth, in last decades several strategies have been developed to improve the photocatalytic activity of wideband gap semiconductors under visible light. For instance, wide-band gap semiconductors have been intercalated with narrow band gap ∗
semiconductors to form composites [5–7]. However, the most used strategy is that related to the insertion of doping elements in the structure of these materials. The visible light driven photocatalytic activity of ZrO2, a semiconductor with band gap of 5.0 eV, has been significantly enhanced through its doping with Fe and Mn [8-9]. Although HfO2 has a crystalline structure similar to ZrO2, there is an important difference between their electronic structures. Specifically, HfO2 has a larger concentration of electron density on oxygen atoms than ZrO2 [10]. This is advantageous for photocatalysis purposes because such defect levels would serve as electron-trapping sites for accelerated transfer of photogenerated electrons. However, it has been reported that HfO2 has band gap energies of 5.53, 5.79, and 5.65 eV for the cubic (c-HfO2), tetragonal (t-HfO2), and monoclinic (m-HfO2) phases [11], respectively. Therefore, HfO2 cannot work as photocatalyst under visible light. For solar applications, the band gap narrowing to 3.90 eV was achieved in HfOx films [12]. However, to date, few approaches have been developed strategies for the use of HfO2 in photocatalysis applications. For instance, Jie et al. fabricated films of aligned HfO2 nanorods, nanosprings, and nanohelix with photocatalytic activity under UV radiation for hydrogen production from water splitting [13]. However, for HfO2 to operate as photocatalyst under visible
Corresponding author. E-mail address: [email protected] (L.A. González).
https://doi.org/10.1016/j.ceramint.2020.02.130 Received 24 May 2019; Received in revised form 13 February 2020; Accepted 13 February 2020 0272-8842/ © 2020 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Luis A. González, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2020.02.130
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light is necessary to extend its optical absorption to longer wavelengths. As a strategy, some authors obtained the decomposition of methylene blue (MB) dye through the photocatalytic activity of sputtered HfO2-xTiO2-x nanocolumn arrays [14]. Recently, the degradation of MB with visible light was achieved with HfO2/V2O5 nanocomposites fabricated by the precipitation method [15]. In order to produce the band gap narrowing of HfO2, we demonstrated the optical band gap shift from 5.67 to 4.02 eV by adding 10 w% Ce in t-HfO2 [16]. However, it has been shown that concentrations equal to or greater than 10 mol% Mn in HfO2 produce the effective enhancement of optical absorption with a broad band centered at 470 nm [17]. Besides, theoretical calculations have predicted the band gap (Eg) reduction when Hf ions are substituted by Mn ions in c-HfO2 [18]. These studies as well as the development of novel photocatalysts were the motivation for us to study the photocatalytic activity of Mn:HfO2 NPs under UV and simulated sunlight. Among the outstanding results of this paper, it is shown the preparation of Mn-modified HfO2 NPs, with sizes < 5 nm, by the Pechini type sol-gel method. This method was chosen, since it offers advantages such as low cost, simplicity, compositional control, and particles with high specific surface area. As known, metal oxides properties depend on the synthesis method. Therefore, it was required to investigate the effect of Mn ions in the structure, morphology and optical properties of the resulting HfO2 NPs. In addition to structural distortions, doping of HfO2 with Mn caused the band gap narrowing due to the insertion of new energy levels, both on the valence and conduction band. The improved photocatalytic performance of Mn:HfO2 NPs, for the decomposition of MB under UV and sunlight irradiation, is then associated to their small grain size, suitable photoactivation, and effective transport of photoexcited carriers towards the NPs surface.
excitation wavelength. The particle size and morphology of the samples were determined by a high-resolution transmission electron microscope (HRTEM) TITAN 80–300 FEI. Besides, an X-ray photoelectron spectroscopy system (Thermo Scientific Escalab 250Xi), equipped with a monochromatic AlKα X-ray source (1486.68 eV), was used to determine the chemical state of the samples. The optical band gap of the samples was estimated from diffuse reflectance spectra which were measured with an UV-VIS-NIR spectrophotometer Ocean Optics HR4000CG-UVNIR.
2. Experimental procedure
3. Results and discussion
2.1. Synthesis of Mn:HfO2 NPs
3.1. Crystalline structure analysis
The preparation of HfO2 NPs with 0, 2.5, 5, 7.5, 10 and 12.5 w% Mn content (labeled as HM0, HM2.5, HM5, HM7.5, HM10 and HM12.5), was performed by calcinating polymer resins from precursor solutions processed by the Pechini type sol-gel process. Hafnium chloride (HfCl4) and manganese nitrate tetrahydrate (Mn(NO3)3·4H2O) were used as metal precursors, whereas citric acid (C6H8O7) and ethylene glycol (C2H6O2) were used as chelating and cross-linking agents, respectively. All reagents were purchased from Sigma-Aldrich and used without further modification. The synthesis process used in this study is based on our methodology formerly implemented for the preparation of Ce:HfO2 NPs [19]. Typically, precursor solutions were prepared by dissolving citric acid in de-ionized water at room temperature under vigorous stirring. The required amount of HfCl4 and (Mn(NO3)3·4H2O) was added to the former solutions in order to have a molar ratio 1:1 of citric acid to metal ions. Next, the necessary volume of ethylene glycol was added to complete a solution with molar ratio 4:1 of ethylene glycol to citric acid. The precursor solutions were then heated at 80 °C, under constant stirring, until having two-thirds of the total volume. The resulting viscous solutions were heated at 130 °C for 24 h to form the complex polymer resins. Mn:HfO2 NPs were then obtained when polymer resins were calcinated in air for 2 h at 500 °C in an electric furnace.
Fig. 1 shows diffraction patterns of the synthesized Mn:HfO2 NPs. In accordance with the ICSD file card 01-078-5754, the diffraction peaks of the HM0 and HM2.5 samples are associated to m-HfO2. For Mn concentrations equal to or greater than 2.5 w%, diffraction peaks associated to t-HfO2 (ICSD No. 01-078-5756) start to arise and those
2.3. Evaluation of the photocatalytic activity The photocatalytic performance of the NPs was investigated for degradation of MB, a hazardous cationic dye found in the effluents of the textile industry. In a typical experiment, 0.190 mmol/L of the photocatalyst was added to 31.2 μmol/L of MB solution at pH 6.8 ± 0.2. Next, the resultant suspension was vigorously stirred for 30 min, in dark conditions, to accomplish the adsorption/desorption equilibrium. The degradation of MB dye occurred by irradiating the suspension, under constant stirring, with the luminous source of a solar simulator (Oriel LCS-100 small area Sol1A) for 120 min. This system was configured to study the dye degradation under full light irradiation in the spectral range from 280 to 800 nm and a simulated sunlight irradiation corresponding to the global standard spectrum AM1.5G. In order to determine the photodegradation rate of the dye, 2 ml aliquots were collected from each suspension every 20 min. Next, the NPs contained in the suspension aliquots were precipitated by centrifugation to finally determine the MB dye degradation in solution by UV–Vis spectrometry.
2.2. Characterization techniques The crystalline structure of the samples was analyzed from diffraction patterns obtained with a Siemens D-5000 diffractometer, with CuKα radiation (1.54056 Å), operated at 35 kV in the 2θ range from 10 to 80°. As a complementary technique, Raman spectra were obtained by a Jobin-Yvon Horiba XPLORA micro-Raman Spectrometer to identify the frequency modes associated to the crystalline phase of the samples. In this equipment, the 532 nm line of an Ar ion laser was used as
Fig. 1. XRD patterns of HfO2 and Mn modified HfO2 nanoparticles. 2
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Table 1 Crystalline phase, crystallite size and band gap of Mn:HfO2 nanoparticles. Sample
Crystalline phase
Crystallite size (nm)
Band gap (eV)
HM0 HM2.5 HM5
m-HfO2 m-HfO2 m-HfO2 t-HfO2 m-HfO2 t-HfO2 m-HfO2 t-HfO2 m-HfO2 t-HfO2
6.51 4.95 4.34 – 3.96 4.25 – 4.03 4.15
5.53 2.58 2.34
HM7.5 HM10 HM12.5
2.25 2.23 2.10
corresponding to m-HfO2 decreased. Observe that diffraction peaks of m-HfO2 are still identified in the pattern of the sample with highest Mn content (HM12.5). In principle, the substitution of Mn2+ in Hf4+ sites is expected due to the proximity of its ionic radii (Hf4+ 0.78 Å and Mn2+ 0.80 Å). Nevertheless, the oxygen deficiency is an essential factor for the stabilization of HfO2 high-temperature phases, as it was pointed out by Gao et al. when preparing Mn:HfO2 compounds by the conventional solid state reaction method in Ar [20]. The admixture of phases in HM12.5 can be then related to the oxidation of Mn2+ into Mn3+ and Mn4+ and the consequent decrease of oxygen vacancies. Since the ionic radii of Mn3+ (0.66 Å) and Mn4+ (0.60 Å) are significantly smaller than that of Mn2+, greater lattice distortions can be formed causing the tetragonal structure to be unstable. The average crystallite size of all samples was calculated with the Scherrer equation by using the most intense diffraction peaks, (−111) for m-HfO2 and (101) for t-HfO2. The decrease of crystallite size from 6.51 to 4.03 nm, as listed in Table 1, suggests that the inclusion of Mn ions cause lattice distortions that disrupt the crystal growth. Raman spectroscopy was used as a complementary technique to have a better understanding of the phase transitions in these samples. The group theory analysis [21,22], predicts 18 Raman active modes for the m-HfO2 phase (9Ag + 9Bg) and 6 Raman active modes for the t-HfO2 phase (A1g + 2B1g + 3Eg). Fig. 2(a) Fig. 3. XP spectra of the sample HM12.5: (a) Hf 4f, (b) Mn 2p and (c) O 1s.
shows the Raman spectrum of the HM0 sample exhibiting vibrational modes corresponding to m-HfO2 which are identified by the peaks at 105, 130, 142, 157, 216, 237, 249, 284, 316, 329, 377, 391, 491, 514, 545, 573, 634 and 667 cm−1 [23]. With the inclusion of Mn ions, some of these peaks are attenuated while new ones, associated to the γ1-form and metastable t'-HfO2, are visualized. The spectrum of the sample HM12.5 in Fig. 2(b) shows peaks centered at 263, 383, 567 and 619 cm−1 that can be associated to the γ1-form and peaks at 480 and 647 cm−1 that can be associated to vibrational modes of t'-HfO2. γ1 and γ2 are forms only identified by Raman scattering [24]. These defective structural forms, associated to point defects (oxygen vacancies and dopant cations), can be visualized between m-HfO2 and t'-HfO2. Hence, multiple state Mn ions delay the phase transformation to t-HfO2, displaying thus a broader range of coexistence between monoclinic and tetragonal phase. 3.2. Chemical states analysis XP spectroscopy was performed to analyze the chemical state of the elements present on the sample with highest Mn content (HM12.5). Fig. 3 (a) shows the Hf 4f spectrum fitted to two sets of double-peak components. The peaks located at 16.83 and 18.83 eV correspond to the doublet Hf4+ 4f7/2 and Hf4+ 4f5/2, respectively. The positions of these peaks are shifted towards higher binding energies, in comparison with those of the HM0 sample (not shown here). This energy shift can be attributed to the substitution of Hf ions by Mn ions in the HfO2 lattice. The second double-peak component at 15.35 and 17.51 eV can be
Fig. 2. Raman spectra of samples (a) HM0 and (b) HM12.5. 3
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Fig. 4. Analysis of the HM10 sample (a) TEM image, (b) histogram, (c) HR-TEM image, and (d) SAED pattern.
attributed to Hfx+ 4f7/2 and Hfx+ 4f5/2 (x < 4), respectively, which is identified as the formation of Hf suboxides due to the presence of oxygen vacancies [25]. The Mn 2p spectrum, shown in Fig. 3(b) has two localized peaks at 640 and 652 eV corresponding to Mn 2p3/2 and Mn 2p1/2, respectively. The peak-fit processing of the Mn 2p3/2 spectrum displays three different components located at binding energies of 639.9, 641.4 and 643.46 eV, which are respectively associated to Mn2+, Mn3+ and Mn4+ states, as expected from the microstructure analysis. The O1s XP spectrum shown in Fig. 3(c) was fitted to three oxygen components. The component OI at 528.6 eV is associated to the lattice oxygen, while the component OII at 530.12 eV is related to oxygen weakly adsorbed on the particle surface. The third component (OIII) at 531.48 eV is related to oxygen-deficient states of the HfO2 [26].
dislocations due to lattice strains, as indicated with the circle, which are responsible of the NPs surface defects. The selected-area electron diffraction (SAED) pattern, in Fig. 4(d), confirms the polycrystalline nature of this sample, with bright concentric rings which in this case are associated to four main reflection planes (111), (200), (220), and (311) of t-HfO2. 3.4. Optical properties Results from UV–Vis diffuse-reflectance spectroscopy confirm that the inclusion of Mn produced extrinsic defects in HfO2 which are related to the significant light absorption. This is evidenced by the remarkable red-shift of the optical absorption edge in the reflectance spectra of the samples HM7.5, HM10 and HM12.5, shown in Fig. 5. The reflectance data of all samples were used to estimate the optical band gap (Eg) values, under the assumption of direct optical transitions, by extrapolating the linear region of the plot of [F(R∞)*hv]2 versus hv to zero, as shown with the insets of Fig. 5. According to the Kubelka–Munk theory, F(R∞) is equivalent to the absorption coefficient and hv is the incident photon energy [27]. The Eg value of the HM0 sample was 5.53 eV, which is close to that reported in the literature for m-HfO2 (5.7 eV). The absorption threshold at 3.54 eV, observed in this curve, can be related to intermediate energy levels within the band gap due to the presence of oxygen vacancies [28]. With the inclusion of Mn2+, Mn3+ and Mn4+ ions, the overall band gap size decreased as consequence of extrinsic defects that create intermediate states within the band gap. This effect was greatly enhanced with the increase of Mn content. For instance, while the HM2.5 sample had an Eg of 2.58 eV, the HM12.5 sample had an Eg of 2.10 eV. Moreover, it is also observed a second absorption threshold related to intrinsic defects (oxygen
3.3. Morphology The morphology of the HM10 sample, which has the smallest crystallite size according to the XRD analysis, was investigated by highresolution TEM to determine the average size and shape of the particles. Fig. 4(a) shows a TEM micrograph of agglomerated round shaped particles with sizes below 5 nm. The particle size distribution is represented by the histogram shown in Fig. 4(b), where the average size was found to be 3.6 ± 0.5 nm. It is also observed that approximately 85% of the measured particle are in the range from 3 to 4.5 nm, which is in good agreement with the XRD analysis. The high-resolution TEM image of Fig. 4(c) depicts the lattice fringes of several particles confirming its crystalline feature. Lattice spacings of 0.29 and 0.25 nm associated to the (111) and (200) planes of the tetragonal phase were determined by measurements on the images of the particles shown in the respective figure. Moreover, this image also visualizes some 4
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Fig. 5. Reflectance spectra (inset: plot of (F(R∞)*hv)2 × 103) corresponding to (a) HM0, (b) HM7.5, (c) HM10 and (d) HM12.5 samples.
Fig. 7. (a) Degradation rate of MB dye by Mn:HfO2 NPs (b) Evolution of the UV–Vis absorption spectra corresponding to the MB degradation with HM10 NPs under simulated sunlight. Fig. 6. Valence-band XP spectra of samples HM0 and HM10.
study, a standard reference of MB concentration was determined through the proportional relationship between the absorbance (A), and the dye concentration (mol/L) in accordance to the Beer-Lambert law [29].
vacancies) induced by the presence of Mn ions with lower states. The calculated Eg values of all samples are listed in Table 1. In order to elucidate the origin of band gap narrowing, the analysis of density of states in the valence band (VB) in samples HM0 and HM10 was performed by XP spectroscopy. The spectrum of sample HM0, according to Fig. 6, shows that the VB edge is located approximately at 3.5 eV. Since the optical band gap of this sample was determined at 5.53 eV, the conduction band (CB) edge would be expected to appear around 2.03 eV. On the other hand, the VB edge of sample HM10 is located at approximately 1.5 eV. According to the calculations of band gap (2.23 eV), the CB edge would occur at 0.73 eV. In addition, it is observed that this spectrum presents a small shoulder with energy around 1.7–1.26 eV. This shoulder, indicated with a shaded circle in Fig. 6, may be responsible of the second absorption threshold observed in the optical measurements with UV–Vis spectroscopy. The density of states in VB near the Fermi level are associated to Mn 3d and O 2p states, while those states located below the bottom of the CB are associated to shallow trap states. These results indicate that the inclusion of Mn ions caused the band gap narrowing due to the insertion of new energy levels, both on the VB and CB of HfO2.
A = −log10
It = εbc I0
(1)
where I0 is the intensity of the incident beam, and It is the intensity of the emerging beam from the MB dye container. This is equivalent to the product of absorptivity (ε ), the length of the beam path (b) and the concentration of the dye solution (c). The photocatalytic activity of the Mn:HfO2 NPs was assessed by calculating the MB dye degradation rate (D) as
D=
CO − Ct × 100%, CO
(2)
where CO (mg/L) is the initial dye concentration and Ct (mg/L) is the dye concentration remaining in solution after an irradiation time t. The MB degradation percent, after 120 min of light irradiation, performed by the HM0 NPs and NPs with higher Mn contents (HM7.5, HM10 and HM12.5) is shown in the bar graph of Fig. 7(a). As expected, the MB dye solution with HM0 NPs had almost null degradation (5.2%). On the other hand, those MB solutions with HM10 NPs showed degradation rates of 97.05 and 91.93% when irradiated under full and sunlight spectra, respectively. Particularly, the evolution of MB dye degradation
3.5. Photocatalytic performance Prior to the photocatalytic activity evaluation of the samples under 5
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Table 2 MB degradation and k-values of the photocatalytic activity performed by the HM7.5, HM10 and HM12.5 samples. Sample
HM7.5 HM10 HM12.5
k-value ( × 10−3)
MB degradation (%) Full spectrum
Sunlight
Full spectrum
Sunlight
80.66 97.05 86.66
55 91.93 66.22
11.4 24.3 12.9
4.6 17.8 7.1
Fig. 8. MB dye decomposition under (a) full spectrum (b) solar spectrum.
with HM10 NPs is illustrated by measuring the attenuation of the absorption spectrum, displayed in Fig. 7(b), with absorption peak at 664 nm. The surface adsorption capacity of these NPs under dark conditions is evidenced by a decrease of 23.3% of the absorption peak. At the first 20 min of sunlight irradiation, the absorption spectrum decreased rapidly, indicating the appropriate photocatalytic activity of these NPs. With time, the amount of MB molecules decreased, and the absorption peak experienced a more gradual attenuation. The relative change of MB dye concentration (Ct/C0) under full light spectrum and sunlight irradiation is shown in Fig. 8. It is interesting to observe that, in dark conditions, the Mn:HfO2 NPs had greater dye adsorptions. This effect can be related to the improvement of electrostatic attraction, which is originated by the modification of surface charge and decrease of particle size (large surface area). Fig. 8 also shows that the dye concentration decreases monotonically with time when Mn:HfO2 NPs are irradiated both under full light spectrum and sunlight. The kinetics of photodegradation reaction was determined by calculating the firstorder kinetic constant (k) of the following equation
ln
CO = kt Ct
Fig. 9. (a) Photocatalytic performance of HM10 NPs under sunlight during five consecutive cycles (b) the effect of different scavengers on the degradation of MB dye with HM10 NPs.
degradation process of MB, 1 mM of different scavengers were added to aqueous systems containing HM10 NPs. Ethylene diaminetetraacetic acid (EDTA) was used to scavenge holes (h+), silver nitrate (AgNO3) was used to scavenge electrons (e−), p-benzoquinone (BQ) was used to scavenge super oxide anions (.O2−) and Isopropyl alcohol (IPA) was used to scavenge hydroxyl radicals (.OH). Fig. 9(b) shows the degradation efficiency, under sunlight irradiation, of the photocatalyst in presence of the different scavengers. In presence of IPA, the photocatalytic activity of the NPs was about 64.7%. Conversely, systems containing BQ, AgNO3 and EDTA-2Na exhibited MB degradations of 12.03, 20.31 and 30.35, respectively. Therefore, .O2− is the active species that mainly promotes the degradation of MB molecules, followed by e−, h+ and .OH. Based on the studies of band gap, VB position and active species, the
(3)
The k-values of the Mn:HfO2 NPs are listed in Table 2 where the HM10 NPs undoubtedly had superior degradation rates. HM10 NPs were carefully recovered by centrifugation and reused, under the same conditions, to evaluate their photocatalytic stability and recyclability. Fig. 9(a) shows the good performance of these NPs by reaching dye degradations of 91.93, 91.83, 91.6, 91.52, and 91.49% from the first, second, third, fourth and fifth cycle, respectively. These results confirm the recyclability and high stability of these photocatalytic NPs. In order to investigate the reactive species involved in the photocatalytic 6
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narrowing. The diffusion of charges and its suppressed recombination was suitably achieved due the small grain sizes (< 5 nm) and surface defects of these NPs. Moreover, the combination of these features promoted the adsorption of the MB dye on the NP surface. In summary, intermediate states formed by the insertion of Mn ions, facilitated the photogeneration of electron-hole pairs which were effectively transported to the NPs surface to degrade the adsorbed MB molecules. Declaration of competing interest None. Acknowledgements This work was supported by CONACYT through the becas program (grant number: 346872).
Fig. 10. Proposed mechanism of the photocatalytic activity of Mn:HfO2 NPs under UV or sunlight irradiation.
References
possible mechanism of photodegradation is illustrated in Fig. 10. Electron-hole pairs are stimulated by the absorption of photons with energy (hv) equal to or greater than the band gap. Mn:HfO2 + hv → e−(CB) + h+(VB)
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(4)
Since NPs are not able to create internal electric field, charges associated with point defects can only be transferred by the diffusion of charge carrying defects. The fastest transport of charge carriers to the surface can be then achieved by the short distance of the sample with smallest particle size (HM10). Charges are then captured by surface defects and adsorbed species so that the electron and hole recombination is suppressed. According to the proposed photocatalytic mechanism, the reduction potential of CB (−0.72 eV) of HM10 NPs is more negative than the reduction potential of O2/.O2− (−0.32 eV). Consequently, the accumulated electrons in CB can efficiently reduce O2 to .O2− radicals. O2 + e− →.O2− .
(5)
O2−
The radicals are able to oxidize the MB molecules to CO2 and H2O. On the other hand, holes accumulated in the VB are not suitable for the oxidation of OH− to .OH due to its less positive reduction potential. Therefore, holes are available to react directly with the adsorbed MB molecules [30]. Thus, the generation of highly active hydroxyl radicals could be only produced through the further reduction of superoxide radicals. .
O−2 + OH → .OH
(6)
.
Then, available OH radicals can effectively degrade the MB dye by oxidation processes. 4. Conclusions Mn-modified HfO2 NPs were successfully synthesized by the Pechini type sol-gel method and post annealing at 500 °C. We have demonstrated that the insertion of Mn ions have significant effects in the microstructure, particle size and optical properties of HfO2 NPs. The substitution of Hf4+ ions by Mn2+, Mn3+ and Mn4+ ions caused distortions to the HfO2 lattice leading to the formation of an admixture of m-HfO2 and t-HfO2. In consequence, the particle size decrease is associated to extrinsic and intrinsic defects that disrupted the crystal growth. Mid-gap states, introduced by the presence of Mn ions, promoted the shift of band gap energy from 5.7 to 2.1 eV and therefore, the enhancement of UV and visible light absorption. Results on the degradation of MB dye, showed that the HM10 NPs had the most efficient photocatalytic activity under sunlight irradiation. The improved photocatalytic performance of these NPs is related to the following aspects. The effective photogeneration of electron-hole pairs due to the improved light absorption which is associated to the optical band gap 7
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