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p-MnO nanocomposites for the photocatalytic degradation of anthracene
Journal of Photochemistry & Photobiology A: Chemistry 369 (2019) 85–96
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Synthesis and characterization of n-ZnO/p-MnO nanocomposites for the photocatalytic degradation of anthracene
T
Blanca L. Martínez-Vargasa,2, Marisela Cruz-Ramíreza, Jesús A. Díaz-Reala,1, J.L. Rodríguez-Lópezb, Francisco Javier Bacame-Valenzuelaa, Raúl Ortega-Borgesa, ⁎ Yolanda Reyes-Vidala,3, Luis Ortiz- Fradea, a Electrochemical Department, Centro de Investigación y Desarrollo Tecnológico en Electroquímica, Parque Tecnológico Querétaro Sanfandila SN, C. P. 760703, Pedro Escobedo, Querétaro, Mexico b Advanced Materials Department, Instituto Potosino de Investigación Científica y Tecnológica, A. C. Camino a la Presa San José, 2055, Lomas 4ª Secc. C.P. 78216, San Luís Potosí, Mexico
A R T I C LE I N FO
A B S T R A C T
Keywords: n-ZnO/p-MnO Nanocomposite Electrochemistry PAHs photocatalytic activity
n-ZnO/p-MnO nanocomposites with different percentages of manganese (0.5%, 1.1%, and 2.25%) with a semiconducting junction were prepared. Changes in Flat band potential (Efb) for ZnO due the different amounts of MnO was observed, meanwhile same donor density (Nd) was held in all materials. From chronoamperometric experiments under on-off illuminated conditions a transient time constant (τ), related to the electron transport in the electrodes were calculated, where higher values are observed in materials with high amounts of MnO. Photodegradation studies of anthracene in an ethanol:water (1:1, pH 12) solution were performed, showing that anthraquinone is the main product with no photodegrading of ethanol. The results suggest that the junction nZnO/p-MnO and materials with high transient time constant (τ), enhance the photocatalytic degradation. The best photocatalytic performance for the photodegradation of anthracene was obtained with the nanocomposite n-ZnO /p-MnO(Mn=2.25%).
1. Introduction In recent years, the degradation of recalcitrant pollutants using nontoxic, thermally and chemically stable semiconductor metal oxides as photocatalysts in aqueous systems has recently attracted much attention. Among those semiconductor metal oxides, n-type zinc oxide nanomaterials with a wide bandgap (Eg = 3.2 eV), have been recognized as excellent materials for photocatalytic processes due to their high photosensitivity, high catalytic activity, suitable band gap, low cost, and environmental friendliness [1–5]. However, enhancing the photocatalytic efficiency of ZnO nanocatalysts to meet the practical application requirements is still a challenge because due to a poor quantum yield caused by the fast recombination rate of photogenerated electronhole (e−-h+) pairs [6–11]. Many efforts have been made by several research groups to overcome this limitation by developing semiconductor-semiconductor and semiconductor-metal nanostructures
[12–14]. The coupling between ZnO and noble metals has shown better activity than simple ZnO for the degradation of different organic contaminants [9,15–19]. Nevertheless, the high cost of metals, such as Ag and Au, has motivated to look for alternatives. In a p-n semiconductorsemiconductor junction, an appropriate coupling of conduction bands (CB) and valence bands (VB) produce an electronic transport of photogenerated charge carriers that indirectly decreases the recombination. In this regard, n-ZnO has been prepared with p-type semiconductors, such as NiO and TiO, for photocatalytic applications [20–22]. However, other earth-abundant p-type semiconductor such as MnO with different technological applications [23,24], has not been used in combination with n-ZnO for photocatalytic degradation of recalcitrant compounds On the other hand, the polycyclic aromatic hydrocarbons (PAHs) pose an environmental problem, due to its carcinogenic and mutagenic activities [25,26]. The photocatalytic degradation of PAHs using
⁎
Corresponding author. E-mail address: [email protected] (L. Ortiz- Frade). Postdoctoral Position at The University of British Columbia, Clean Energy Research Centre 6250 Applied Science Lane, Vancouver, British Columbia, Canada, V6T 1Z4. 2 Postdoctoral Position at UA Ciencias Químicas, Universidad Autónoma de Zacatecas, Campus siglo XXI-Edificio 6, 98160, Zacatecas, México. 3 Catedrática CONACYT-CIDETEQ. 1
https://doi.org/10.1016/j.jphotochem.2018.10.010 Received 15 March 2018; Received in revised form 22 September 2018; Accepted 4 October 2018 Available online 05 October 2018 1010-6030/ © 2018 Elsevier B.V. All rights reserved.
Journal of Photochemistry & Photobiology A: Chemistry 369 (2019) 85–96
B.L. Martínez-Vargas et al.
experiments were carried out, with a JEOL-JET-TE 300 spectrometer. The chemical composition of surface oxides were determined by XPS using a PHI 5000 Versa Probe II system (Physical Electronics), with a monochromatic Al-Kα (1486.7 eV) as X-ray source with a voltage of 15 kV.
nanoparticles of ZnO and TiO2 has been proposed as a possible technology to solve the problem [27–30]. However, low efficiency and high recombination are their main disadvantages. The afore mentioned issues encouraged us to synthetize and fully characterize n-ZnO/p-MnO nanocomposites with a p-n conjunction, for the photocatalytic degradation of anthracene.
2.4. Preparation of photo-electrodes for electrochemical characterization 2. Experimental section The photo-electrodes were prepared by spin-coating method using a home-made device. Suspensions of ZnO or ZnO/MnO nanocomposite (15 mg mL−1 in ethanol) were added at 60 μL min−1 over a conducting glass substrate (SOLEMS, FTO 20 ohms) rotating at 1800 rpm. After the spin-coating, thermal annealing was performed at 200 °C (10 °C min−1, 1 h) under air to improve the mechanical stability of the samples. Electrical contact to the FTO was done using silver conductive paint (Radio Spares, 186–3593) to a copper wire, and isolated with epoxy resin (3 M, Scotch-weld DP-190).
2.1. Reagents Zinc acetate dihydrate, Zn(CH3COO)2⋅2H2O (J. T. Baker, 99%); manganese acetate tetrahydrate, Mn(CH3COO)2⋅4H2O (Strem Chemicals, 99%); sodium hydroxide, NaOH (J. T. Baker, 98.2%); absolute ethyl alcohol, CH3CH2OH (Aldrich 99%); acetone, CH3COCH3 (Aldrich 99%); hydrochloric acid, HCl (J. T. Baker; 37.3%); anthracene, C14H10 (Aldrich 99%); dichloromethane, and CH2Cl2 (J. T. Baker 99%) were used without further purification. Ultrapure water (18 M∧ cm−1) was obtained from a UV Millipore system.
2.5. Electrochemical characterization Electrochemical measurements were performed at 25 °C in a conventional three-electrode cell. Cyclic voltammetry (v = 50 mV s−1) and linear sweep voltammetry (v = 5 mV s−1) measurements were performed in a potential range from -0.2 to 1.2 V/RHE in the presence and absence of light. Photocurrent transient curves (v = 5 mV s−1) were obtained by light chopping (Lambda SC shutter controller) with on-off periods of 10 s. The effective irradiance power at electrode distance was 34 mW cm-2 using a 175 W Xenon lamp (Spectral Products ASB-XE175EX). For all the experiments, a 0.1 M NaOH solution (pH 12.9 in H2O) outgassed with N2 was used as supporting electrolyte. To minimize the ohmic drop and to avoid hole scavenger effects in charge transfer process under illumination experiments, no other solvent was added to the solution, despite photocatalytic experiments were carried out in the presence of ethanol see Section 2.6. A glassy carbon electrode and a reversible hydrogen electrode (RHE) served as counter and reference electrode, using an SP-300 Bio-logic potentiostat-galvanostat. The photo-electrochemical measurements were carried out at 25 °C in a three-electrode cell equipped with a quartz window to allow the UV light illumination of the entire portion of the film exposed to the electrolyte. The constant photocurrent measurements were recorded while the electrodes were polarized at 1.0 V/RHE and illuminated with a 175 W Xenon lamp (Spectral Products ASB-XE-175EX polychromatic source, with range of wavelength from 305 nm to 550 nm) coupled to a motorized monochromator (Horiba Jobin Ybon 0106-07-07). According to the literature the semiconducting properties of the photo-electrodes were estimated from Mott-Schottky plots using frequencies ranging from 100 kHz to 1 Hz, an amplitude of 10 mV and a potential range from -0.2 to 1.2 V/RHE [32,33]. The measurements were recorded in a conventional three-electrode cell, with a glassy carbon electrode and a reversible hydrogen electrode (RHE) served as the counter and reference electrode, respectively.
2.2. One-pot synthesis of colloidal n-ZnO/p-MnO nanocomposite ZnO/MnO nanocomposites in colloidal dispersions were obtained by a modification of a previously described method [31]. Briefly, different amounts of Zn(CH3COO)2⋅2H2O and Mn(CH3COO)2⋅4H2O, were used to synthesize nanocomposites with variable percentages of Mn, and are summarized in Table 1. The salts were dissolved in 100 mL of ethanol at room temperature, followed by the addition of 6.5 mL of 0.2 M NaOH in ethanol. The reaction mixture was heated for 2 h at 60 °C A similar method of preparation was used to synthesize ZnO. The obtained colloidal dispersion was characterized by UV–vis spectroscopy, with a diode array UV–vis spectrophotometer (Thermo Scientific Evolution Array). Emission measurements were carried out using a Horiba Jobin Yvon/Fluorolog spectrofluorimeter with an excitation wavelength of 365 nm. The size distribution of the nanoparticles was determined from HR-TEM images obtained using a Tecnai F30 with accelerating voltage of 300 kV FEG. 2.3. Isolation and characterization of n-ZnO/p-MnO powder The freshly prepared colloidal dispersion of ZnO/MnO nanocomposite in ethanol was concentrated by slow evaporation with airflow over the solution, using a commercial fan, until a precipitate was observed. The dispersion was centrifuged at 3500 rpm for 20 min, and the obtained powder was washed with ethanol and acetone. Finally, the sample was calcined at 275 °C for 1 h. The percentages of Mn present were measured by EDX analysis with a JEOL JSM-6510LV and with a spectrometer XRF-1800. The average particle size and crystal structure identification were determined from X-ray diffraction patterns, recorded with a Bruker-AXS D8 advanced diffractometer (Cu Kα= 1.5406 Å). The surface area was analyzed by the BranauerEmmett-Teller (BET) gas absorption method, using an autosorb iQ2 instrument. UV–vis diffuse reflectance spectra were recorded in the range from 200 to 800 nm, using a Varian Cary3 UV–vis spectrometer. To establish manganese species with unpaired electrons associated with geometry and oxidation state, electron paramagnetic resonance (EPR)
2.6. Photocatalytic experiments Photocatalytic experiments were performed at room temperature using a single 15 W UV lamp (emission at 365 nm) and a quartz cell with a path length of 1 cm. The distance between the cell surface and the lamp was 8 cm. The irradiation power (3.30 mW cm−2) was measured with a Radiometer Cole-Parmer UVX. A volume of 3.5 mL of 20 ppm anthracene in ethanol:water (1:1, pH 12) were added into the quartz cell. The experiments were carried out separately, in absence and in the presence of 1.5 mg of ZnO/MnO composite powder. The reaction solution was magnetically stirred in the darkness for five minutes to establish the adsorption/desorption equilibrium of anthracene on the photocatalyst surface. Photocatalysis of anthracene (20 ppm) exclusively in ethanol and ZnO were also evaluated. The total
Table 1 Amounts in mmol of reactants used to prepare nanocomposites with different manganese percentages. Nominal percentage of Mn
mmol of Zn(CH3COO)2 ⋅2H2O mmol of Mn(CH3COO)2⋅4H2O
1%
2.5%
5%
0.248 0.002
0.244 0.006
0.239 0.011
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time in photocatalytic experiments was 40 min. The reaction progress was monitored using UV–vis spectroscopy. The experiments were carried out without bubbling any gas prior each experiment. The concentration of oxygen in the mixture ethanol:water (1:1) it was considered as (0.1 mol L-1) according to the literature [34]. 2.7. Ethanol determination using UPLC-IR Analyses were carried out using a Waters (México) H-Class Acquity Ultra-Performance Liquid Chromatography System (UPLC) with cooling autosampler, quaternary solvent manager, Refractive Index (RI) detector (Model 2414) and oven for analytical column. A Biorad Aminex HPX-87H cation-exchange column (300 mm x 7.8 mm) was used to achieve the chromatographic determination, using a mobile phase of 0.008 N sulphuric acid at a flow rate (isocratic mode) of 0.4 ml min−1 and an injection volume of 10 μl. The temperatures of oven and RI cell were 30 °C and a 35 °C respectively. Peak heights were measured using Waters Empower3 chromatography software. Ethanol absolute:H2O solutions contained 0.7:0.7, 1.5:1.5, 3.1:3.1, 6.2:6.2, 12.5:12.5, 25:25 and 50:50% were used as reference solutions for calibration curve (three replicates). Ethanol was identified by comparison of retention times with ethanol (JT Baker). All reference solutions and samples (with semiconductors) were filtered using a 0.2 μm nylon membrane.
Fig. 1. UV–vis spectra of ZnO/MnO nanocomposite; a) 0% Mn, b) 1% Mn, c) 2.5% Mn and d) 5.0% Mn.
2.8. UPLC – PDA/Ms analysis Analyses were carried out using a H-Class Acquity UltraPerformance Liquid Chromatography System (UPLC) with cooling autosampler, quaternary solvent manager, oven for analytical column, extended wavelength photodiode array detector and QDa mass detector (Waters, Mexico). The QDa detector is a compact single quad mass detector equipped with an electrospray ionization (ESI) interface. A Waters UPLC BEH C18 column (50 mm x 2.1 mm i.d., 1.7 μm) was employed at 30 °C. Mobile phase A was 0.1% formic acid in HPLC water and mobile phase B was acetonitrile. A gradient method (see SI) was used and the injection volume was 10 μl. For mass detection, the QDa detector was operated in an electrospray positive ion mode and the cone voltage was set at 10 V. The desolvation temperature was settled at 600 °C. The MS Scan mode was used for a full mass spectrum between m/z 100 and 600, acquiring with a sample rate of 5 points/sec. In PDA detection, the system was employed by recording a multi-wavelength set in the wavelength range 210–400 nm. Anthraquinone and Anthracene (both from Sigma-Aldrich) in different reaction matrix were used as standard compounds. All samples and standards were filtered by 0.2 μm nylon membrane. Area of peaks were determinate using Waters Empower3 chromatography software by Waters.
Fig. 2. Emission spectra of ZnO/MnO nanocomposite a) 0% Mn, b) 1% Mn, c) 2.5% Mn and d) 5.0% Mn. The spectra were recorded in a range from 300 nm to 800 nm, with an excitation wavelength of 365 nm.
355 nm and 515 nm. The signal at 355 nm corresponds to the nearbandgap emission, and the broad emission at 515 nm is attributed to surface defects, such as oxygen and zinc vacancies [37–39]. In the case of ZnO/MnO nanocomposites, the green emission signal at 515 nm is dramatically quenched, indicating a reduction in the surface traps by the junction ZnO-MnO. In order to obtain information about the crystallographic phase in colloidal dispersion, TEM analysis was carried out. Fig. 3a shows selected HR-TEM images where typical dhkl distances for wurtzite (hexagonal) ZnO are observed. Fig. 3b shows the representative HR-TEM images of synthesized sample with a Mn content of 5.0%. The Fast Fourier Transform (FFT) pattern (inset in Fig. 3) indicates that the measured interplanar distance values are 0.28 nm and 0.2 nm, corresponding to the (100) plane of ZnO and the (002) plane of MnO, respectively. These studies clearly show the simultaneous presence of ZnO and MnO crystal lattices in the nanocomposite.
3. Results and discussion 3.1. Colloidal dispersions of ZnO/MnO nanocomposite To demonstrate the formation of nanocomposites in colloidal dispersion, electronic spectra for each synthesis condition was acquired. Fig. 1 shows UV–vis spectra of synthetized ZnO and ZnO/MnO. The band gap value for ZnO was 3.5 eV (355 nm), indicating a typical blue shift of 0.3 eV compared with the band gap of 3.2 eV for bulk material. In the case of ZnO/MnO nanocomposites, the band gap values for ZnO are smaller than the value of pure ZnO. The nanoparticle diameter were calculated according to the Weiner excitation model [35,36]. A value of 4.9 nm was obtained for ZnO, while for nanocomposites; regardless the amount of Mn, an average diameter value of 7.2 nm was calculated. Due to the low amount of Mn in the synthesis, it is expected that MnO is located over the surface of ZnO. Hence, emission spectroscopy was carried out to explore this idea. Fig. 2 shows the fluorescence spectra of ZnO and ZnO/MnO nanocomposites with an excitation wavelength at 365 nm. ZnO nanoparticles exhibit two spectral bands at
3.2. Solid state characterization of ZnO /MnO photocatalysts Once the formation of ZnO and MnO in colloidal dispersion was evidenced, powders of materials were isolated as described in the experimental section. Table 2 shows the nominal percentages of Mn in the synthetic conditions with corresponding real values measured by EDX and X-ray fluorescence analyses. It was found a good agreement between both quantification techniques. Using these values the 87
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Fig. 3. a) HR-TEM micrograph selected of ZnO nanoparticles, b) HR-TEM micrograph selected of ZnO/MnO(Mn=5.0%) nanocomposite. Table 2 Nominal percentages of Mn in synthetic conditions and percentages determined by EDX/XRF analyses. Nominal Mn%.
Mn% XRF Mn% EDX
1
2.5
5
0.4 0.5
1.3 1.1
2.1 2.25
nanocomposites are labeled from this point as ZnO/MnO(Mn=0.5%), ZnO/MnO(Mn=1.1%), and ZnO/MnO(Mn=2.25%). The diffractograms obtained for ZnO and MnO/ZnO nanocomposites are shown in Fig. 4. A comparison with the X-Ray diffraction pattern of commercial ZnO in wurtzite phase, reveals the same diffraction peaks and similar relative intensities. The diameter of the nanoparticles was calculated using the Debye-Scherrer equation, obtaining a value of 11.5 ± 0.2 nm for ZnO and values ranging from 13 to 15 nm for MnO/ZnO nanocomposites. It should be highlighted that due to the low amount of MnO in composites, no signal of this semiconductor was recorded. Nevertheless, other experiments demonstrated its presence. The surface area was calculated with the BET (Brunauer-EmmettTeller) method. Higher values for the synthesized materials are obtained in comparison with commercial ZnO, as is presented in Table 3. Fig. 5 shows typical adsorption-desorption isotherms that, according to IUPAC, corresponds to a mesoporous solid (type IV) with multilayers adsorption. Another characteristic observed in the isotherms is the difference between the adsorption and desorption curves, which suggests the development of hysteresis due to irregular capillary condensation [40]. The inset in Fig. 5, is in agreement with the low surface area and pore diameter for commercial ZnO. The magnetic environment of manganese in composites was explored by means of EPR. Fig. 6 shows the spectra of the MnO/ZnO nanocomposites with different Mn concentrations, exhibiting well-resolved hyperfine splitting, which is characteristic of Mn(II) ions. The latter suggests Mn-Mn interactions randomly distributed in the ZnO surface with no significant clustering [41,42]. From these spectra, g-
Fig. 4. XRD patterns of a) ZnO, MnO/ZnO with b) Mn = 0.5%, c) Mn = 1.1% and d) Mn = 2.25% and e) commercial ZnO, bottom of figure peaks from hexagonal wurtzite phase JCPDS 36-1451.
values of 1.997 and coupling constant (A) values of 72.2 G, were obtained in all cases. These values are in agreement with those reported for single-crystal of Mn (II) in hexagonal ZnO matrix, obtained at 77 K
88
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[42]. The obtained value of coupling constant is characteristic for Mn (II) in a tetrahedral environment, such as in MnO. Fig. 7A shows the XPS spectrum of ZnO/MnO(Mn=2.25%), where the main peaks for Zn 2p are recorded around 1020 eV. On the other hand, weak peaks in Mn 2p region appear close to 640 eV. Fig. 7B and Table 4 show an XPS analysis with Gaussian fitting for C 1 s, O 1 s, Zn 2p and Mn 2p regions. From the signal in the O 1 s region, the presence of MnO can be proposed. The broad signal centered at 531.37 eV corresponds to Zn(OH)+ formed on the surface [43–45]. Besides the main peak at 529.78 eV corresponds to ZnO [43,44,46]. Fig. 9c shows Zn 2p region, where the species ZnO, ZnCO3 and Zn(OH)+ are also identified [45]. On the other hand, from spectra in Mn 2p region, low resolution multiplet splitting corresponding to Mn 2p 3/2 state for the MnO can be identified over the surface of ZnO. It has been reported that the band gap of metal oxide nanoparticles can be modified through the incorporation of metallic ions, improving its photocatalytic activity towards the degradation. However, for the materials prepared in this work it is expected no incorporation of metal in the structure of ZnO. Hence, the band gap energy was determined by measuring diffuse reflectance spectra, assuming an indirect electronic transition. Figs. 8 and 9 shows the modified Kubelka-Munk function, [F (R)hv]1/2, plotted versus the incident photon energy, hv, [34]. The obtained values are very similar among the materials ZnOcom (Eg = 3.22 eV), ZnO (Eg = 3.19 eV), ZnO/MnO(Mn = 0.5%) (Eg = 3.19 eV), ZnO/MnO(Mn=1.1%) (Eg = 3.13 eV) and ZnO/ MnO(Mn = 2.25%)(Eg = 3.19 eV). These facts indicate that the expected trends on the photodegradation can be related to some other electronic properties. Therefore, electrochemical and photochemical studies were performed to understand the photocatalytic behavior.
Table 3 Values of pore diameter, superficial area and particle size of ZnO and MnO/ZnO nanocomposites. Material
Pore diameter (nm)
Superficial area (m2/g) BET
Particle size (nm) (DRX)
ZnO commercial ZnO ZnO/MnO(Mn=0.5%) ZnO/MnO(Mn=1.1%) ZnO/MnO(Mn=2.25%)
2.1 12.3 12.2 12.2 12.3
12.9 52.3 72.2 55.13 74.17
34.2 11.5 13.9 15.4 15.4
3.3. Electrochemical characterization
Fig. 5. Adsorption-desorption isotherm of a) ZnO and ZnO/MnO with b) Mn = 0.5%, c) Mn = 1.1% and d) Mn = 2.25%). Inset: Commercial ZnO sample.
3.3.1. Cyclic voltammetry It is important to point out that for the determination of semiconductor properties of materials, a steady state condition must be reached. Therefore, several potential scan cycles were applied to the electrodes until a constant electrochemical response. Fig. 9 a shows the cyclic voltammograms the FTO-supported ZnO at the third cycle, where the steady state was achieved. In the case of the synthesized ZnO (Fig. 9a) three reduction processes are observed, namely Ic (-0.08/ RHE), IIc (0.058 V/RHE), and IIIc (- 0.07 V/RHE). In the complete scan, another oxidation signal Ia at 0.218 V/RHE was also recorded. The reduction processes for metallic Zn must be discarded due to the predicted redox potential according to Pourbaix diagrams. Therefore, these processes are attributed to the filling of surface states, as reported for semiconductors with similar crystalline systems[47]. Additionally, from voltammogram of Fig. 9a, no faradaic processes are detected for potentials more positive than 0.5 V/RHE. For commercial ZnO, Fig. 9a, only a broad reduction signal I´c with a maximum current at 0.20 V/ RHE is observed. The difference in response for commercial and synthetized ZnO can be attributed changes in the energetic states at the surface of both materials. Fig. 9b shows a comparison of the cyclic voltammograms of synthesized ZnO and ZnO/MnO nanocomposites (Mn = 0.5%, 1.1 5 and 2.25%) deposited over FTO electrodes, where three zones are observed. The zone I corresponds to the filling of surface states of ZnO, with differences in peak potential and current density due to the amount of MnO. The zone II corresponds to a double layer capacitance for ZnO. On the other hand, in zone III, oxidation and reduction signals IIIa, IIIc, and IVa, associated to the filling of MnO states with double layer capacitance contribution are also observed. This is consistent with a more noticeable response in materials with greater amount of Mn.
Fig. 6. EPR spectra of ZnO/MnO nanocomposite with different Mn contents: a) 0.5%, b) 1.1% and c) 2.25%.
3.3.2. Mott-Schottky analysis In order to obtain Mott-Schottky plots, staircase potentio-electrochemical impedance spectroscopy was carried out with frequencies 89
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Fig. 7. A) XPS spectrum of ZnO/ MnO(Mn=2.25%) nanocomposite, B) XPS and Gaussian fits for: b) C 1 s, c) O 1 s, d) Zn 2p and e) Mn 2p of ZnO/ MnO(Mn=2.25%) nanocomposite.
1 2NA RT [E −Efb− ] = 2 Nd Fεε0 F CSC
ranging from 100 kHz to 1 Hz and sinusoidal amplitude of 10 mV. Fig. 10 shows 1/Csc2 vs. E plot for both ZnO samples using data at 1.002 kHz [48–50]. A positive slope, typical for the n-type semiconductors is observed. From this data, the onset potential (Eonset), donor density number (Nd) and flat-band potential (Efb) were calculated, using the Mott-Schottky Eq. (2):
(2) 23
−1
Where NA is the Avogadro´s number (6.023 × 10 mol ), F is the Faraday constant (9.65 × 104 Cmol−1), ε0 is the vacuum permittivity (8.8542 × 10-14 Fcm−1), ε is the dielectric constant of the semiconductor (8.5 for ZnO) [49,51], R is the gas constant (8.314 JK−1 mol−1), T is the absolute temperature (298 K), and E (V) is the 90
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applied potential. A summary of these values is presented in Table 5. A comparison of Nd values for all samples indicate no dopants in the ZnO lattice. On the other hand, a negative slope in Mott-Schottky plots for ZnO/MnO materials suggest the presence of p-MnO. The changes in slopes and flat-band potentials confirm the junction n-ZnO/p-MnO. It should be pointed out that Efb is comparable to the minimum potential required to reach zero band-bending and, therefore, to promote electron-hole pair separation. To this regard, pure ZnO materials did not show important differences; contrary to that observed in n-ZnO/p-MnO nanocomposite, where their value Efb becomes smaller with the increase of Mn percentage. In the case of p-MnO/n-ZnO nanocomposite (Mn 2.25%), two additional slopes from 0.8 to 1.0 V/ RHE are also observed, probably attributed to another p-n semiconductor junction. However, the evidence presented in this work did not allow us to propose the composition of the second p-n semiconductor junction.
Table 4 Identification of peaks and oxidation states for the studied regions in the highresolution XPS spectra. Peak
C 1s 1 2 3 O 1s 1 2 3 4 Zn 2p 1 2 3 Mn 2p 1 2 3 4 5 6
Position (eV)
FWHM (eV)
284.77 286.16 288.71
1.47 1.45 1.61
528.48 529.75 531.31 533.31
0.97 1.39 1.99 0.87
1020.88 1021.89 1022.83
1.60 1.60 1.60
637.91 639.35 40.17 641.11 642.09 643.84
1.01 1.21 1.21 1.21 1.21 1.21
% Gauss
X2 = 1.13 90 80 100 X2=1.46 100 97 97 90 X2=1.91 100 100 100 X2=1.0 52 52 52 52 52 52
Posible ID
Oxidation state
CeC CeO OeC=O
C4+ C4+ C4+
MneO ZneO ZneOH ZneCO3
O2− O2− O2− O2−
ZneO ZneOH ZneCO3
Zn2+ Zn2+ Zn2+
MneOH MneO Multiplet Multiplet Multiplet Multiplet
Mn2+ Mn2+ Mn2+ Mn2+ Mn2+ Mn2+
3.3.3. Photoelectrochemical study Fig. 11a and b show the voltammetric responses obtained in both dark and low illumination for all materials. For both ZnO, it is possible to observe typical photoanodic response due to the increase in current in positive direction under illumination. The recombination process and electron transport through the material can be evidenced from the transient curves (on-off illumination cycles) before the photocurrent plateau recorded in the I vs. t decay. However the time window used in the experimental conditions, suggest that the rate determining step is the electronic transport through the material. For synthetic ZnO, a longer time is registered in comparison with the commercial sample. In the nanocomposites, it can be observed that for a potentials near to 0.45 V/RHE, the I vs. t profile show a fast electron transport through the n-ZnO/p-MnO junction [49]. This electronic transport decreases as the polarization potential increases according to what several authors report have described [52,53]. The flat band potential was also calculated from photocurrent density (jph) onset. Very similar values were obtained to those in the Mott-Schottky analysis. At this point it is important to highlight that the voltammograms of ZnO/MnO (Mn = 0.5%, 1.1% and 2.25%) illuminated or under darkness presented oxidation signal around 0.1 V/RHE related to a reductive doping effect, that according to the literature can enhance the photocatalytic properties [54]. This effect practically not observed in samples in the absence of Mn.
Fig. 8. Schematic presentation of the Kubelka-Munk analysis of the absorption edge for determining the energy band Eg of, ZnO, ZnO/MnO nanocomposites and commercial ZnO.
3.3.4. Chronoamperometry response under illumination The potentiostatic responses under UV–vis light of the prepared electrodes are shown in Fig. 12. A constant polarization of 1.0 V/RHE
Fig. 9. A) Cyclic voltammograms at third cycle of a) synthesized ZnO and b) commercial ZnO. v = 50 mVs−1 in 0.1 M NaOH. B) Cyclic voltammograms for ZnO/MnO nanocomposite with Mn = 0.5%, 1.1% and 2.25% (third cycle). Recorded at v = 50 mVs−1 in 0.1 M NaOH. A RHE and a GCE electrode were used as reference and counter electrode, respectively. 91
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Fig. 10. Mott-Schottky plots for (A) ZnO and (B) n-ZnO/p-MnO nanocomposite, recorded in 0.1 M NaOH electrolyte, f = 1.002 Hz, ε = 8.5 (for all materials).
exponential decrease of the photocurrent with time until a stationary value is reached (Ist), and It is the current at time t, the photocurrent transient kinetics can be described by the following Eq. 4:
Table 5 Flat band potential (Efb), donor density (Nd) and onset potential determined from Mott-Schottky and cyclic voltammetry analysis for synthesized materials.
ZnO commercial n-ZnO n-ZnO / p-MnO(Mn=0.5%) n-ZnO / p-MnO(Mn=1.1%) n-ZnO/ p-MnO(Mn=2.25%) a b
Nc Nd x1021 cm−3
Efba V/RHE
Efbb V/RHE
0.242 0.243 0.357 0.321 0.287
0.076 0.069 0.176 0.135 0.120
0.07 0.07 0.18 0.14 0.12
t
R = e− τ
In which τ is the transient time constant [55–57] and, calculated from the reciprocal of slope of the plot ln(R) vs. time (Fig. 12c). We found that meanwhile the Mn content is increased in n-ZnO/p-MnO composites a high transient time constant(τ) is record, associated to slow electron transport in the film (see the numerical values in Fig. 12c). Low transient time constant(τ) for ZnO and commercial ZnO were obtained, see also Fig. 12c.
From Mott-Schottky slope. From jph onset cyclic voltammetry.
3.4. Evaluation of photocatalytic activity
was applied to the electrodes while they were irradiated with polychromatic illumination. From these curves, it is possible to calculate the time constant of the transient (τ) related to the electron mechanism in the film, calculated by the following analysis, from the Eq. 3:
R=
It −Ist Iin−Ist
(4)
The photocatalytic degradation of anthracene in the presence of nZnO/p-MnO nanocomposite (Mn = 0.5, 1.1%, and 2.25%) was investigated under UV light irradiation. Direct photolysis with no semiconductor materials was also performed. The degradation of anthracene was determined by measuring the anthracene concentration, using calibration curves constructed with absorbance values at 365 nm. Fig. 13 shows that the degradation of anthracene by photolytic oxidation is negligible, in contrast with the photocatalytic degradation using materials.
(3)
where Iin is the immediate photoresponse, consisting of an anodic spike caused by the separation of photogenerated electron-hole pairs at the semiconductor/electrolyte interface. The latter is followed by an
Fig. 11. a) Linear sweep voltammograms recorded for the prepared ZnO and commercial ZnO electrodes under darkness and b) Linear sweep voltammograms recorded for the prepared p-MnO/n-ZnO with (Mn = 0.5%, 1.1% and 2.25% electrodes under darkness, UV–vis light and transient curves. v = 5 mVs−1 with I = 34 mWcm-2. 92
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Fig. 12. Chronoamperometric (1.0 V/RHE) responses of a) commercial ZnO and synthesized ZnO, b) n- ZnO/ p-MnO (Mn = 0.5%, 1.1% and 2.25%) nanocomposites when UV–vis light was irradiated (on) and interrupted (off), c) comparison of transient times constants of all materials.
anthracene increases. It can be proposed that flat band potential and transient time constant, related electron transport in the film are key factors determining the photocatalytic activity. Additionally the photocatalytic activity of TiO2 P25 was evaluated, showing similar anthracene photodegradation than ZnO. All these facts demonstrated that a p-n junction between the two semiconductors increases the efficiency in the degradation of the organic molecule. All materials were used for several catalytic cycles and a decay around 25% in each cycle was recorded in all the cases, except for TiO2 P25 were a 10% was is observed. Despite the photocatalytic activity is a process were the semiconductor is in colloidal dispersion, and electrochemical experiments are related to film, we decide to use electrochemical parameters such as transient time constant (τ) related to the mechanism of electron transport in the materials to understand the photocatalytic performance of a semiconductor. The results suggest that the junction n-ZnO/p-MnO with high transient time constant (τ), enhance the photocatalytic degradation. The best photocatalytic performance for the photodegradation of anthracene was obtained with the nano-composite n-ZnO /pMnO(Mn=2.25%). Furthermore, the use of this parameter could be a simple approach to design new photocatalysts instead of expensive use time resolved techniques. From the literature, a photocatalytic mechanism is proposed. When ZnO is irradiated, hole-electron pair is produced (Eq. (1)), then an electron accumulation to MnO is proposed (Eq. (2)) [20–22]. The radical OH· is generated form the reaction with O2 or H2O with highly active h+ (Eqs. (3) and (4)). Also, the e− can reduce the oxygen, generating the superoxide radical (Eq. (4)), which reacts with water originating as well the radical OH· (Eq. (6)–(8)) [58].
Fig. 13. Photocatalytic degradation of anthracene (20 ppm) in ethanol:water (1:1) with commercial ZnO, ZnO and n-ZnO/p-MnO (Mn = 0.5%, 1.1% and 2.25%) composites at 40 min of UV illumination (irradiation at 365 nm, I0 = 3.30 mW cm−2). The direct photolysis (in the absence of semiconductor particles) is also included.
Commercial ZnO has the lowest photocatalytic degradation, only 62% at 40 min. Moreover ZnO nanoparticles degraded 74% for the same time. In contrast, n-ZnO/p-MnO nanocomposites have higher photodegradation efficiency. As Mn content increases, degradation of 93
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Scheme 1. Proposed photodegradation of anthracene in ethanol:H2O pH 12 in the presence and in the absence photocatalyst.
ZnO + hv → ZnO ( e− + h+) MnO +
h+
h+ e−
+
e−
OH−
→
MnO ( e−)
+ O2 →
⋅O2−
synthesis using simple acetate salts (Zn and Mn) as precursors. The photocatalytic activity of these nanocomposites increases due to the p-n junction between p-MnO and n-ZnO. The results suggest that the junction n-ZnO/p-MnO with high transient time constant (τ) obtained from electrochemical experiments, enhance the photocatalytic degradation. The best photocatalytic performance for the photodegradation of anthracene was obtained with the nano-composite n-ZnO /pMnO(Mn=2.25%).
(2) (3)
→ ⋅OH
+ H2 O → ⋅OH +
(1)
H+
⋅O2−
(4) (5)
HO2⋅+ OH−
(6)
HO2⋅+ HO2⋅ → H2 O2 + O2
(7)
Acknowledgements
H2 O2 + e− → OH− + ⋅OH
(8)
The authors gratefully acknowledge financial support from CONACYT grant numbers 106437, 216315, 258159,and CONACYTSENER-Sustentabilidad Energética246052. B. L. Martínez-Vargas and J. A. Díaz-Real acknowledge CONACyT for pH.D. scholarships. Also, the authors acknowledge Drs. Hector G. Silva Pereyra and Mariela Bravo Sańchez, both from the National Laboratory Research in Nanoscience and Nanotechnology (LINAN) at IPICYT, SLP, Mexico, for HR-TEM characterization and X-ray photoelectron spectra, respectively. An special ackowledege to Guadalupe Osorio-Monreal for all the help in this work.
+ H2 O →
In the case of ethanol, the analysis of UPLC indicate no transformation of this solvent in any photocatalytic condition (see SI). Hence ethanol can act as a hole scavenger in the process, with a constant effect in all samples, that cannot be avoid due to its presence makes anthracene soluble. Finally to propose some intermediate species of a photodegradation pathway UPLC analyses with a photodiode array and mass detector were carried out. The chromatograms, presented a first peak with a retention time in 4.1 min, related with the principal product of reaction. The absorption spectra of this peak are comparable for anthraquinone spectra. On the other hand, the mass spectrum of this peak shows a clear molecular ion at m/z of 209, matched favorably with mass of anthraquinone. A second peak with a retention time in 6.1 min, present a molecular ion at m/z of 179, similar to anthracene. Hence, the following photo-degradation pathway is proposed (Scheme 1).
Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jphotochem.2018.10. 010. References
4. Conclusions
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Synthesis and characterization of n-ZnO/p-MnO nanocomposites for the photocatalytic degradation of anthracene
T
Blanca L. Martínez-Vargasa,2, Marisela Cruz-Ramíreza, Jesús A. Díaz-Reala,1, J.L. Rodríguez-Lópezb, Francisco Javier Bacame-Valenzuelaa, Raúl Ortega-Borgesa, ⁎ Yolanda Reyes-Vidala,3, Luis Ortiz- Fradea, a Electrochemical Department, Centro de Investigación y Desarrollo Tecnológico en Electroquímica, Parque Tecnológico Querétaro Sanfandila SN, C. P. 760703, Pedro Escobedo, Querétaro, Mexico b Advanced Materials Department, Instituto Potosino de Investigación Científica y Tecnológica, A. C. Camino a la Presa San José, 2055, Lomas 4ª Secc. C.P. 78216, San Luís Potosí, Mexico
A R T I C LE I N FO
A B S T R A C T
Keywords: n-ZnO/p-MnO Nanocomposite Electrochemistry PAHs photocatalytic activity
n-ZnO/p-MnO nanocomposites with different percentages of manganese (0.5%, 1.1%, and 2.25%) with a semiconducting junction were prepared. Changes in Flat band potential (Efb) for ZnO due the different amounts of MnO was observed, meanwhile same donor density (Nd) was held in all materials. From chronoamperometric experiments under on-off illuminated conditions a transient time constant (τ), related to the electron transport in the electrodes were calculated, where higher values are observed in materials with high amounts of MnO. Photodegradation studies of anthracene in an ethanol:water (1:1, pH 12) solution were performed, showing that anthraquinone is the main product with no photodegrading of ethanol. The results suggest that the junction nZnO/p-MnO and materials with high transient time constant (τ), enhance the photocatalytic degradation. The best photocatalytic performance for the photodegradation of anthracene was obtained with the nanocomposite n-ZnO /p-MnO(Mn=2.25%).
1. Introduction In recent years, the degradation of recalcitrant pollutants using nontoxic, thermally and chemically stable semiconductor metal oxides as photocatalysts in aqueous systems has recently attracted much attention. Among those semiconductor metal oxides, n-type zinc oxide nanomaterials with a wide bandgap (Eg = 3.2 eV), have been recognized as excellent materials for photocatalytic processes due to their high photosensitivity, high catalytic activity, suitable band gap, low cost, and environmental friendliness [1–5]. However, enhancing the photocatalytic efficiency of ZnO nanocatalysts to meet the practical application requirements is still a challenge because due to a poor quantum yield caused by the fast recombination rate of photogenerated electronhole (e−-h+) pairs [6–11]. Many efforts have been made by several research groups to overcome this limitation by developing semiconductor-semiconductor and semiconductor-metal nanostructures
[12–14]. The coupling between ZnO and noble metals has shown better activity than simple ZnO for the degradation of different organic contaminants [9,15–19]. Nevertheless, the high cost of metals, such as Ag and Au, has motivated to look for alternatives. In a p-n semiconductorsemiconductor junction, an appropriate coupling of conduction bands (CB) and valence bands (VB) produce an electronic transport of photogenerated charge carriers that indirectly decreases the recombination. In this regard, n-ZnO has been prepared with p-type semiconductors, such as NiO and TiO, for photocatalytic applications [20–22]. However, other earth-abundant p-type semiconductor such as MnO with different technological applications [23,24], has not been used in combination with n-ZnO for photocatalytic degradation of recalcitrant compounds On the other hand, the polycyclic aromatic hydrocarbons (PAHs) pose an environmental problem, due to its carcinogenic and mutagenic activities [25,26]. The photocatalytic degradation of PAHs using
⁎
Corresponding author. E-mail address: [email protected] (L. Ortiz- Frade). Postdoctoral Position at The University of British Columbia, Clean Energy Research Centre 6250 Applied Science Lane, Vancouver, British Columbia, Canada, V6T 1Z4. 2 Postdoctoral Position at UA Ciencias Químicas, Universidad Autónoma de Zacatecas, Campus siglo XXI-Edificio 6, 98160, Zacatecas, México. 3 Catedrática CONACYT-CIDETEQ. 1
https://doi.org/10.1016/j.jphotochem.2018.10.010 Received 15 March 2018; Received in revised form 22 September 2018; Accepted 4 October 2018 Available online 05 October 2018 1010-6030/ © 2018 Elsevier B.V. All rights reserved.
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experiments were carried out, with a JEOL-JET-TE 300 spectrometer. The chemical composition of surface oxides were determined by XPS using a PHI 5000 Versa Probe II system (Physical Electronics), with a monochromatic Al-Kα (1486.7 eV) as X-ray source with a voltage of 15 kV.
nanoparticles of ZnO and TiO2 has been proposed as a possible technology to solve the problem [27–30]. However, low efficiency and high recombination are their main disadvantages. The afore mentioned issues encouraged us to synthetize and fully characterize n-ZnO/p-MnO nanocomposites with a p-n conjunction, for the photocatalytic degradation of anthracene.
2.4. Preparation of photo-electrodes for electrochemical characterization 2. Experimental section The photo-electrodes were prepared by spin-coating method using a home-made device. Suspensions of ZnO or ZnO/MnO nanocomposite (15 mg mL−1 in ethanol) were added at 60 μL min−1 over a conducting glass substrate (SOLEMS, FTO 20 ohms) rotating at 1800 rpm. After the spin-coating, thermal annealing was performed at 200 °C (10 °C min−1, 1 h) under air to improve the mechanical stability of the samples. Electrical contact to the FTO was done using silver conductive paint (Radio Spares, 186–3593) to a copper wire, and isolated with epoxy resin (3 M, Scotch-weld DP-190).
2.1. Reagents Zinc acetate dihydrate, Zn(CH3COO)2⋅2H2O (J. T. Baker, 99%); manganese acetate tetrahydrate, Mn(CH3COO)2⋅4H2O (Strem Chemicals, 99%); sodium hydroxide, NaOH (J. T. Baker, 98.2%); absolute ethyl alcohol, CH3CH2OH (Aldrich 99%); acetone, CH3COCH3 (Aldrich 99%); hydrochloric acid, HCl (J. T. Baker; 37.3%); anthracene, C14H10 (Aldrich 99%); dichloromethane, and CH2Cl2 (J. T. Baker 99%) were used without further purification. Ultrapure water (18 M∧ cm−1) was obtained from a UV Millipore system.
2.5. Electrochemical characterization Electrochemical measurements were performed at 25 °C in a conventional three-electrode cell. Cyclic voltammetry (v = 50 mV s−1) and linear sweep voltammetry (v = 5 mV s−1) measurements were performed in a potential range from -0.2 to 1.2 V/RHE in the presence and absence of light. Photocurrent transient curves (v = 5 mV s−1) were obtained by light chopping (Lambda SC shutter controller) with on-off periods of 10 s. The effective irradiance power at electrode distance was 34 mW cm-2 using a 175 W Xenon lamp (Spectral Products ASB-XE175EX). For all the experiments, a 0.1 M NaOH solution (pH 12.9 in H2O) outgassed with N2 was used as supporting electrolyte. To minimize the ohmic drop and to avoid hole scavenger effects in charge transfer process under illumination experiments, no other solvent was added to the solution, despite photocatalytic experiments were carried out in the presence of ethanol see Section 2.6. A glassy carbon electrode and a reversible hydrogen electrode (RHE) served as counter and reference electrode, using an SP-300 Bio-logic potentiostat-galvanostat. The photo-electrochemical measurements were carried out at 25 °C in a three-electrode cell equipped with a quartz window to allow the UV light illumination of the entire portion of the film exposed to the electrolyte. The constant photocurrent measurements were recorded while the electrodes were polarized at 1.0 V/RHE and illuminated with a 175 W Xenon lamp (Spectral Products ASB-XE-175EX polychromatic source, with range of wavelength from 305 nm to 550 nm) coupled to a motorized monochromator (Horiba Jobin Ybon 0106-07-07). According to the literature the semiconducting properties of the photo-electrodes were estimated from Mott-Schottky plots using frequencies ranging from 100 kHz to 1 Hz, an amplitude of 10 mV and a potential range from -0.2 to 1.2 V/RHE [32,33]. The measurements were recorded in a conventional three-electrode cell, with a glassy carbon electrode and a reversible hydrogen electrode (RHE) served as the counter and reference electrode, respectively.
2.2. One-pot synthesis of colloidal n-ZnO/p-MnO nanocomposite ZnO/MnO nanocomposites in colloidal dispersions were obtained by a modification of a previously described method [31]. Briefly, different amounts of Zn(CH3COO)2⋅2H2O and Mn(CH3COO)2⋅4H2O, were used to synthesize nanocomposites with variable percentages of Mn, and are summarized in Table 1. The salts were dissolved in 100 mL of ethanol at room temperature, followed by the addition of 6.5 mL of 0.2 M NaOH in ethanol. The reaction mixture was heated for 2 h at 60 °C A similar method of preparation was used to synthesize ZnO. The obtained colloidal dispersion was characterized by UV–vis spectroscopy, with a diode array UV–vis spectrophotometer (Thermo Scientific Evolution Array). Emission measurements were carried out using a Horiba Jobin Yvon/Fluorolog spectrofluorimeter with an excitation wavelength of 365 nm. The size distribution of the nanoparticles was determined from HR-TEM images obtained using a Tecnai F30 with accelerating voltage of 300 kV FEG. 2.3. Isolation and characterization of n-ZnO/p-MnO powder The freshly prepared colloidal dispersion of ZnO/MnO nanocomposite in ethanol was concentrated by slow evaporation with airflow over the solution, using a commercial fan, until a precipitate was observed. The dispersion was centrifuged at 3500 rpm for 20 min, and the obtained powder was washed with ethanol and acetone. Finally, the sample was calcined at 275 °C for 1 h. The percentages of Mn present were measured by EDX analysis with a JEOL JSM-6510LV and with a spectrometer XRF-1800. The average particle size and crystal structure identification were determined from X-ray diffraction patterns, recorded with a Bruker-AXS D8 advanced diffractometer (Cu Kα= 1.5406 Å). The surface area was analyzed by the BranauerEmmett-Teller (BET) gas absorption method, using an autosorb iQ2 instrument. UV–vis diffuse reflectance spectra were recorded in the range from 200 to 800 nm, using a Varian Cary3 UV–vis spectrometer. To establish manganese species with unpaired electrons associated with geometry and oxidation state, electron paramagnetic resonance (EPR)
2.6. Photocatalytic experiments Photocatalytic experiments were performed at room temperature using a single 15 W UV lamp (emission at 365 nm) and a quartz cell with a path length of 1 cm. The distance between the cell surface and the lamp was 8 cm. The irradiation power (3.30 mW cm−2) was measured with a Radiometer Cole-Parmer UVX. A volume of 3.5 mL of 20 ppm anthracene in ethanol:water (1:1, pH 12) were added into the quartz cell. The experiments were carried out separately, in absence and in the presence of 1.5 mg of ZnO/MnO composite powder. The reaction solution was magnetically stirred in the darkness for five minutes to establish the adsorption/desorption equilibrium of anthracene on the photocatalyst surface. Photocatalysis of anthracene (20 ppm) exclusively in ethanol and ZnO were also evaluated. The total
Table 1 Amounts in mmol of reactants used to prepare nanocomposites with different manganese percentages. Nominal percentage of Mn
mmol of Zn(CH3COO)2 ⋅2H2O mmol of Mn(CH3COO)2⋅4H2O
1%
2.5%
5%
0.248 0.002
0.244 0.006
0.239 0.011
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time in photocatalytic experiments was 40 min. The reaction progress was monitored using UV–vis spectroscopy. The experiments were carried out without bubbling any gas prior each experiment. The concentration of oxygen in the mixture ethanol:water (1:1) it was considered as (0.1 mol L-1) according to the literature [34]. 2.7. Ethanol determination using UPLC-IR Analyses were carried out using a Waters (México) H-Class Acquity Ultra-Performance Liquid Chromatography System (UPLC) with cooling autosampler, quaternary solvent manager, Refractive Index (RI) detector (Model 2414) and oven for analytical column. A Biorad Aminex HPX-87H cation-exchange column (300 mm x 7.8 mm) was used to achieve the chromatographic determination, using a mobile phase of 0.008 N sulphuric acid at a flow rate (isocratic mode) of 0.4 ml min−1 and an injection volume of 10 μl. The temperatures of oven and RI cell were 30 °C and a 35 °C respectively. Peak heights were measured using Waters Empower3 chromatography software. Ethanol absolute:H2O solutions contained 0.7:0.7, 1.5:1.5, 3.1:3.1, 6.2:6.2, 12.5:12.5, 25:25 and 50:50% were used as reference solutions for calibration curve (three replicates). Ethanol was identified by comparison of retention times with ethanol (JT Baker). All reference solutions and samples (with semiconductors) were filtered using a 0.2 μm nylon membrane.
Fig. 1. UV–vis spectra of ZnO/MnO nanocomposite; a) 0% Mn, b) 1% Mn, c) 2.5% Mn and d) 5.0% Mn.
2.8. UPLC – PDA/Ms analysis Analyses were carried out using a H-Class Acquity UltraPerformance Liquid Chromatography System (UPLC) with cooling autosampler, quaternary solvent manager, oven for analytical column, extended wavelength photodiode array detector and QDa mass detector (Waters, Mexico). The QDa detector is a compact single quad mass detector equipped with an electrospray ionization (ESI) interface. A Waters UPLC BEH C18 column (50 mm x 2.1 mm i.d., 1.7 μm) was employed at 30 °C. Mobile phase A was 0.1% formic acid in HPLC water and mobile phase B was acetonitrile. A gradient method (see SI) was used and the injection volume was 10 μl. For mass detection, the QDa detector was operated in an electrospray positive ion mode and the cone voltage was set at 10 V. The desolvation temperature was settled at 600 °C. The MS Scan mode was used for a full mass spectrum between m/z 100 and 600, acquiring with a sample rate of 5 points/sec. In PDA detection, the system was employed by recording a multi-wavelength set in the wavelength range 210–400 nm. Anthraquinone and Anthracene (both from Sigma-Aldrich) in different reaction matrix were used as standard compounds. All samples and standards were filtered by 0.2 μm nylon membrane. Area of peaks were determinate using Waters Empower3 chromatography software by Waters.
Fig. 2. Emission spectra of ZnO/MnO nanocomposite a) 0% Mn, b) 1% Mn, c) 2.5% Mn and d) 5.0% Mn. The spectra were recorded in a range from 300 nm to 800 nm, with an excitation wavelength of 365 nm.
355 nm and 515 nm. The signal at 355 nm corresponds to the nearbandgap emission, and the broad emission at 515 nm is attributed to surface defects, such as oxygen and zinc vacancies [37–39]. In the case of ZnO/MnO nanocomposites, the green emission signal at 515 nm is dramatically quenched, indicating a reduction in the surface traps by the junction ZnO-MnO. In order to obtain information about the crystallographic phase in colloidal dispersion, TEM analysis was carried out. Fig. 3a shows selected HR-TEM images where typical dhkl distances for wurtzite (hexagonal) ZnO are observed. Fig. 3b shows the representative HR-TEM images of synthesized sample with a Mn content of 5.0%. The Fast Fourier Transform (FFT) pattern (inset in Fig. 3) indicates that the measured interplanar distance values are 0.28 nm and 0.2 nm, corresponding to the (100) plane of ZnO and the (002) plane of MnO, respectively. These studies clearly show the simultaneous presence of ZnO and MnO crystal lattices in the nanocomposite.
3. Results and discussion 3.1. Colloidal dispersions of ZnO/MnO nanocomposite To demonstrate the formation of nanocomposites in colloidal dispersion, electronic spectra for each synthesis condition was acquired. Fig. 1 shows UV–vis spectra of synthetized ZnO and ZnO/MnO. The band gap value for ZnO was 3.5 eV (355 nm), indicating a typical blue shift of 0.3 eV compared with the band gap of 3.2 eV for bulk material. In the case of ZnO/MnO nanocomposites, the band gap values for ZnO are smaller than the value of pure ZnO. The nanoparticle diameter were calculated according to the Weiner excitation model [35,36]. A value of 4.9 nm was obtained for ZnO, while for nanocomposites; regardless the amount of Mn, an average diameter value of 7.2 nm was calculated. Due to the low amount of Mn in the synthesis, it is expected that MnO is located over the surface of ZnO. Hence, emission spectroscopy was carried out to explore this idea. Fig. 2 shows the fluorescence spectra of ZnO and ZnO/MnO nanocomposites with an excitation wavelength at 365 nm. ZnO nanoparticles exhibit two spectral bands at
3.2. Solid state characterization of ZnO /MnO photocatalysts Once the formation of ZnO and MnO in colloidal dispersion was evidenced, powders of materials were isolated as described in the experimental section. Table 2 shows the nominal percentages of Mn in the synthetic conditions with corresponding real values measured by EDX and X-ray fluorescence analyses. It was found a good agreement between both quantification techniques. Using these values the 87
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Fig. 3. a) HR-TEM micrograph selected of ZnO nanoparticles, b) HR-TEM micrograph selected of ZnO/MnO(Mn=5.0%) nanocomposite. Table 2 Nominal percentages of Mn in synthetic conditions and percentages determined by EDX/XRF analyses. Nominal Mn%.
Mn% XRF Mn% EDX
1
2.5
5
0.4 0.5
1.3 1.1
2.1 2.25
nanocomposites are labeled from this point as ZnO/MnO(Mn=0.5%), ZnO/MnO(Mn=1.1%), and ZnO/MnO(Mn=2.25%). The diffractograms obtained for ZnO and MnO/ZnO nanocomposites are shown in Fig. 4. A comparison with the X-Ray diffraction pattern of commercial ZnO in wurtzite phase, reveals the same diffraction peaks and similar relative intensities. The diameter of the nanoparticles was calculated using the Debye-Scherrer equation, obtaining a value of 11.5 ± 0.2 nm for ZnO and values ranging from 13 to 15 nm for MnO/ZnO nanocomposites. It should be highlighted that due to the low amount of MnO in composites, no signal of this semiconductor was recorded. Nevertheless, other experiments demonstrated its presence. The surface area was calculated with the BET (Brunauer-EmmettTeller) method. Higher values for the synthesized materials are obtained in comparison with commercial ZnO, as is presented in Table 3. Fig. 5 shows typical adsorption-desorption isotherms that, according to IUPAC, corresponds to a mesoporous solid (type IV) with multilayers adsorption. Another characteristic observed in the isotherms is the difference between the adsorption and desorption curves, which suggests the development of hysteresis due to irregular capillary condensation [40]. The inset in Fig. 5, is in agreement with the low surface area and pore diameter for commercial ZnO. The magnetic environment of manganese in composites was explored by means of EPR. Fig. 6 shows the spectra of the MnO/ZnO nanocomposites with different Mn concentrations, exhibiting well-resolved hyperfine splitting, which is characteristic of Mn(II) ions. The latter suggests Mn-Mn interactions randomly distributed in the ZnO surface with no significant clustering [41,42]. From these spectra, g-
Fig. 4. XRD patterns of a) ZnO, MnO/ZnO with b) Mn = 0.5%, c) Mn = 1.1% and d) Mn = 2.25% and e) commercial ZnO, bottom of figure peaks from hexagonal wurtzite phase JCPDS 36-1451.
values of 1.997 and coupling constant (A) values of 72.2 G, were obtained in all cases. These values are in agreement with those reported for single-crystal of Mn (II) in hexagonal ZnO matrix, obtained at 77 K
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[42]. The obtained value of coupling constant is characteristic for Mn (II) in a tetrahedral environment, such as in MnO. Fig. 7A shows the XPS spectrum of ZnO/MnO(Mn=2.25%), where the main peaks for Zn 2p are recorded around 1020 eV. On the other hand, weak peaks in Mn 2p region appear close to 640 eV. Fig. 7B and Table 4 show an XPS analysis with Gaussian fitting for C 1 s, O 1 s, Zn 2p and Mn 2p regions. From the signal in the O 1 s region, the presence of MnO can be proposed. The broad signal centered at 531.37 eV corresponds to Zn(OH)+ formed on the surface [43–45]. Besides the main peak at 529.78 eV corresponds to ZnO [43,44,46]. Fig. 9c shows Zn 2p region, where the species ZnO, ZnCO3 and Zn(OH)+ are also identified [45]. On the other hand, from spectra in Mn 2p region, low resolution multiplet splitting corresponding to Mn 2p 3/2 state for the MnO can be identified over the surface of ZnO. It has been reported that the band gap of metal oxide nanoparticles can be modified through the incorporation of metallic ions, improving its photocatalytic activity towards the degradation. However, for the materials prepared in this work it is expected no incorporation of metal in the structure of ZnO. Hence, the band gap energy was determined by measuring diffuse reflectance spectra, assuming an indirect electronic transition. Figs. 8 and 9 shows the modified Kubelka-Munk function, [F (R)hv]1/2, plotted versus the incident photon energy, hv, [34]. The obtained values are very similar among the materials ZnOcom (Eg = 3.22 eV), ZnO (Eg = 3.19 eV), ZnO/MnO(Mn = 0.5%) (Eg = 3.19 eV), ZnO/MnO(Mn=1.1%) (Eg = 3.13 eV) and ZnO/ MnO(Mn = 2.25%)(Eg = 3.19 eV). These facts indicate that the expected trends on the photodegradation can be related to some other electronic properties. Therefore, electrochemical and photochemical studies were performed to understand the photocatalytic behavior.
Table 3 Values of pore diameter, superficial area and particle size of ZnO and MnO/ZnO nanocomposites. Material
Pore diameter (nm)
Superficial area (m2/g) BET
Particle size (nm) (DRX)
ZnO commercial ZnO ZnO/MnO(Mn=0.5%) ZnO/MnO(Mn=1.1%) ZnO/MnO(Mn=2.25%)
2.1 12.3 12.2 12.2 12.3
12.9 52.3 72.2 55.13 74.17
34.2 11.5 13.9 15.4 15.4
3.3. Electrochemical characterization
Fig. 5. Adsorption-desorption isotherm of a) ZnO and ZnO/MnO with b) Mn = 0.5%, c) Mn = 1.1% and d) Mn = 2.25%). Inset: Commercial ZnO sample.
3.3.1. Cyclic voltammetry It is important to point out that for the determination of semiconductor properties of materials, a steady state condition must be reached. Therefore, several potential scan cycles were applied to the electrodes until a constant electrochemical response. Fig. 9 a shows the cyclic voltammograms the FTO-supported ZnO at the third cycle, where the steady state was achieved. In the case of the synthesized ZnO (Fig. 9a) three reduction processes are observed, namely Ic (-0.08/ RHE), IIc (0.058 V/RHE), and IIIc (- 0.07 V/RHE). In the complete scan, another oxidation signal Ia at 0.218 V/RHE was also recorded. The reduction processes for metallic Zn must be discarded due to the predicted redox potential according to Pourbaix diagrams. Therefore, these processes are attributed to the filling of surface states, as reported for semiconductors with similar crystalline systems[47]. Additionally, from voltammogram of Fig. 9a, no faradaic processes are detected for potentials more positive than 0.5 V/RHE. For commercial ZnO, Fig. 9a, only a broad reduction signal I´c with a maximum current at 0.20 V/ RHE is observed. The difference in response for commercial and synthetized ZnO can be attributed changes in the energetic states at the surface of both materials. Fig. 9b shows a comparison of the cyclic voltammograms of synthesized ZnO and ZnO/MnO nanocomposites (Mn = 0.5%, 1.1 5 and 2.25%) deposited over FTO electrodes, where three zones are observed. The zone I corresponds to the filling of surface states of ZnO, with differences in peak potential and current density due to the amount of MnO. The zone II corresponds to a double layer capacitance for ZnO. On the other hand, in zone III, oxidation and reduction signals IIIa, IIIc, and IVa, associated to the filling of MnO states with double layer capacitance contribution are also observed. This is consistent with a more noticeable response in materials with greater amount of Mn.
Fig. 6. EPR spectra of ZnO/MnO nanocomposite with different Mn contents: a) 0.5%, b) 1.1% and c) 2.25%.
3.3.2. Mott-Schottky analysis In order to obtain Mott-Schottky plots, staircase potentio-electrochemical impedance spectroscopy was carried out with frequencies 89
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Fig. 7. A) XPS spectrum of ZnO/ MnO(Mn=2.25%) nanocomposite, B) XPS and Gaussian fits for: b) C 1 s, c) O 1 s, d) Zn 2p and e) Mn 2p of ZnO/ MnO(Mn=2.25%) nanocomposite.
1 2NA RT [E −Efb− ] = 2 Nd Fεε0 F CSC
ranging from 100 kHz to 1 Hz and sinusoidal amplitude of 10 mV. Fig. 10 shows 1/Csc2 vs. E plot for both ZnO samples using data at 1.002 kHz [48–50]. A positive slope, typical for the n-type semiconductors is observed. From this data, the onset potential (Eonset), donor density number (Nd) and flat-band potential (Efb) were calculated, using the Mott-Schottky Eq. (2):
(2) 23
−1
Where NA is the Avogadro´s number (6.023 × 10 mol ), F is the Faraday constant (9.65 × 104 Cmol−1), ε0 is the vacuum permittivity (8.8542 × 10-14 Fcm−1), ε is the dielectric constant of the semiconductor (8.5 for ZnO) [49,51], R is the gas constant (8.314 JK−1 mol−1), T is the absolute temperature (298 K), and E (V) is the 90
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applied potential. A summary of these values is presented in Table 5. A comparison of Nd values for all samples indicate no dopants in the ZnO lattice. On the other hand, a negative slope in Mott-Schottky plots for ZnO/MnO materials suggest the presence of p-MnO. The changes in slopes and flat-band potentials confirm the junction n-ZnO/p-MnO. It should be pointed out that Efb is comparable to the minimum potential required to reach zero band-bending and, therefore, to promote electron-hole pair separation. To this regard, pure ZnO materials did not show important differences; contrary to that observed in n-ZnO/p-MnO nanocomposite, where their value Efb becomes smaller with the increase of Mn percentage. In the case of p-MnO/n-ZnO nanocomposite (Mn 2.25%), two additional slopes from 0.8 to 1.0 V/ RHE are also observed, probably attributed to another p-n semiconductor junction. However, the evidence presented in this work did not allow us to propose the composition of the second p-n semiconductor junction.
Table 4 Identification of peaks and oxidation states for the studied regions in the highresolution XPS spectra. Peak
C 1s 1 2 3 O 1s 1 2 3 4 Zn 2p 1 2 3 Mn 2p 1 2 3 4 5 6
Position (eV)
FWHM (eV)
284.77 286.16 288.71
1.47 1.45 1.61
528.48 529.75 531.31 533.31
0.97 1.39 1.99 0.87
1020.88 1021.89 1022.83
1.60 1.60 1.60
637.91 639.35 40.17 641.11 642.09 643.84
1.01 1.21 1.21 1.21 1.21 1.21
% Gauss
X2 = 1.13 90 80 100 X2=1.46 100 97 97 90 X2=1.91 100 100 100 X2=1.0 52 52 52 52 52 52
Posible ID
Oxidation state
CeC CeO OeC=O
C4+ C4+ C4+
MneO ZneO ZneOH ZneCO3
O2− O2− O2− O2−
ZneO ZneOH ZneCO3
Zn2+ Zn2+ Zn2+
MneOH MneO Multiplet Multiplet Multiplet Multiplet
Mn2+ Mn2+ Mn2+ Mn2+ Mn2+ Mn2+
3.3.3. Photoelectrochemical study Fig. 11a and b show the voltammetric responses obtained in both dark and low illumination for all materials. For both ZnO, it is possible to observe typical photoanodic response due to the increase in current in positive direction under illumination. The recombination process and electron transport through the material can be evidenced from the transient curves (on-off illumination cycles) before the photocurrent plateau recorded in the I vs. t decay. However the time window used in the experimental conditions, suggest that the rate determining step is the electronic transport through the material. For synthetic ZnO, a longer time is registered in comparison with the commercial sample. In the nanocomposites, it can be observed that for a potentials near to 0.45 V/RHE, the I vs. t profile show a fast electron transport through the n-ZnO/p-MnO junction [49]. This electronic transport decreases as the polarization potential increases according to what several authors report have described [52,53]. The flat band potential was also calculated from photocurrent density (jph) onset. Very similar values were obtained to those in the Mott-Schottky analysis. At this point it is important to highlight that the voltammograms of ZnO/MnO (Mn = 0.5%, 1.1% and 2.25%) illuminated or under darkness presented oxidation signal around 0.1 V/RHE related to a reductive doping effect, that according to the literature can enhance the photocatalytic properties [54]. This effect practically not observed in samples in the absence of Mn.
Fig. 8. Schematic presentation of the Kubelka-Munk analysis of the absorption edge for determining the energy band Eg of, ZnO, ZnO/MnO nanocomposites and commercial ZnO.
3.3.4. Chronoamperometry response under illumination The potentiostatic responses under UV–vis light of the prepared electrodes are shown in Fig. 12. A constant polarization of 1.0 V/RHE
Fig. 9. A) Cyclic voltammograms at third cycle of a) synthesized ZnO and b) commercial ZnO. v = 50 mVs−1 in 0.1 M NaOH. B) Cyclic voltammograms for ZnO/MnO nanocomposite with Mn = 0.5%, 1.1% and 2.25% (third cycle). Recorded at v = 50 mVs−1 in 0.1 M NaOH. A RHE and a GCE electrode were used as reference and counter electrode, respectively. 91
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Fig. 10. Mott-Schottky plots for (A) ZnO and (B) n-ZnO/p-MnO nanocomposite, recorded in 0.1 M NaOH electrolyte, f = 1.002 Hz, ε = 8.5 (for all materials).
exponential decrease of the photocurrent with time until a stationary value is reached (Ist), and It is the current at time t, the photocurrent transient kinetics can be described by the following Eq. 4:
Table 5 Flat band potential (Efb), donor density (Nd) and onset potential determined from Mott-Schottky and cyclic voltammetry analysis for synthesized materials.
ZnO commercial n-ZnO n-ZnO / p-MnO(Mn=0.5%) n-ZnO / p-MnO(Mn=1.1%) n-ZnO/ p-MnO(Mn=2.25%) a b
Nc Nd x1021 cm−3
Efba V/RHE
Efbb V/RHE
0.242 0.243 0.357 0.321 0.287
0.076 0.069 0.176 0.135 0.120
0.07 0.07 0.18 0.14 0.12
t
R = e− τ
In which τ is the transient time constant [55–57] and, calculated from the reciprocal of slope of the plot ln(R) vs. time (Fig. 12c). We found that meanwhile the Mn content is increased in n-ZnO/p-MnO composites a high transient time constant(τ) is record, associated to slow electron transport in the film (see the numerical values in Fig. 12c). Low transient time constant(τ) for ZnO and commercial ZnO were obtained, see also Fig. 12c.
From Mott-Schottky slope. From jph onset cyclic voltammetry.
3.4. Evaluation of photocatalytic activity
was applied to the electrodes while they were irradiated with polychromatic illumination. From these curves, it is possible to calculate the time constant of the transient (τ) related to the electron mechanism in the film, calculated by the following analysis, from the Eq. 3:
R=
It −Ist Iin−Ist
(4)
The photocatalytic degradation of anthracene in the presence of nZnO/p-MnO nanocomposite (Mn = 0.5, 1.1%, and 2.25%) was investigated under UV light irradiation. Direct photolysis with no semiconductor materials was also performed. The degradation of anthracene was determined by measuring the anthracene concentration, using calibration curves constructed with absorbance values at 365 nm. Fig. 13 shows that the degradation of anthracene by photolytic oxidation is negligible, in contrast with the photocatalytic degradation using materials.
(3)
where Iin is the immediate photoresponse, consisting of an anodic spike caused by the separation of photogenerated electron-hole pairs at the semiconductor/electrolyte interface. The latter is followed by an
Fig. 11. a) Linear sweep voltammograms recorded for the prepared ZnO and commercial ZnO electrodes under darkness and b) Linear sweep voltammograms recorded for the prepared p-MnO/n-ZnO with (Mn = 0.5%, 1.1% and 2.25% electrodes under darkness, UV–vis light and transient curves. v = 5 mVs−1 with I = 34 mWcm-2. 92
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Fig. 12. Chronoamperometric (1.0 V/RHE) responses of a) commercial ZnO and synthesized ZnO, b) n- ZnO/ p-MnO (Mn = 0.5%, 1.1% and 2.25%) nanocomposites when UV–vis light was irradiated (on) and interrupted (off), c) comparison of transient times constants of all materials.
anthracene increases. It can be proposed that flat band potential and transient time constant, related electron transport in the film are key factors determining the photocatalytic activity. Additionally the photocatalytic activity of TiO2 P25 was evaluated, showing similar anthracene photodegradation than ZnO. All these facts demonstrated that a p-n junction between the two semiconductors increases the efficiency in the degradation of the organic molecule. All materials were used for several catalytic cycles and a decay around 25% in each cycle was recorded in all the cases, except for TiO2 P25 were a 10% was is observed. Despite the photocatalytic activity is a process were the semiconductor is in colloidal dispersion, and electrochemical experiments are related to film, we decide to use electrochemical parameters such as transient time constant (τ) related to the mechanism of electron transport in the materials to understand the photocatalytic performance of a semiconductor. The results suggest that the junction n-ZnO/p-MnO with high transient time constant (τ), enhance the photocatalytic degradation. The best photocatalytic performance for the photodegradation of anthracene was obtained with the nano-composite n-ZnO /pMnO(Mn=2.25%). Furthermore, the use of this parameter could be a simple approach to design new photocatalysts instead of expensive use time resolved techniques. From the literature, a photocatalytic mechanism is proposed. When ZnO is irradiated, hole-electron pair is produced (Eq. (1)), then an electron accumulation to MnO is proposed (Eq. (2)) [20–22]. The radical OH· is generated form the reaction with O2 or H2O with highly active h+ (Eqs. (3) and (4)). Also, the e− can reduce the oxygen, generating the superoxide radical (Eq. (4)), which reacts with water originating as well the radical OH· (Eq. (6)–(8)) [58].
Fig. 13. Photocatalytic degradation of anthracene (20 ppm) in ethanol:water (1:1) with commercial ZnO, ZnO and n-ZnO/p-MnO (Mn = 0.5%, 1.1% and 2.25%) composites at 40 min of UV illumination (irradiation at 365 nm, I0 = 3.30 mW cm−2). The direct photolysis (in the absence of semiconductor particles) is also included.
Commercial ZnO has the lowest photocatalytic degradation, only 62% at 40 min. Moreover ZnO nanoparticles degraded 74% for the same time. In contrast, n-ZnO/p-MnO nanocomposites have higher photodegradation efficiency. As Mn content increases, degradation of 93
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Scheme 1. Proposed photodegradation of anthracene in ethanol:H2O pH 12 in the presence and in the absence photocatalyst.
ZnO + hv → ZnO ( e− + h+) MnO +
h+
h+ e−
+
e−
OH−
→
MnO ( e−)
+ O2 →
⋅O2−
synthesis using simple acetate salts (Zn and Mn) as precursors. The photocatalytic activity of these nanocomposites increases due to the p-n junction between p-MnO and n-ZnO. The results suggest that the junction n-ZnO/p-MnO with high transient time constant (τ) obtained from electrochemical experiments, enhance the photocatalytic degradation. The best photocatalytic performance for the photodegradation of anthracene was obtained with the nano-composite n-ZnO /pMnO(Mn=2.25%).
(2) (3)
→ ⋅OH
+ H2 O → ⋅OH +
(1)
H+
⋅O2−
(4) (5)
HO2⋅+ OH−
(6)
HO2⋅+ HO2⋅ → H2 O2 + O2
(7)
Acknowledgements
H2 O2 + e− → OH− + ⋅OH
(8)
The authors gratefully acknowledge financial support from CONACYT grant numbers 106437, 216315, 258159,and CONACYTSENER-Sustentabilidad Energética246052. B. L. Martínez-Vargas and J. A. Díaz-Real acknowledge CONACyT for pH.D. scholarships. Also, the authors acknowledge Drs. Hector G. Silva Pereyra and Mariela Bravo Sańchez, both from the National Laboratory Research in Nanoscience and Nanotechnology (LINAN) at IPICYT, SLP, Mexico, for HR-TEM characterization and X-ray photoelectron spectra, respectively. An special ackowledege to Guadalupe Osorio-Monreal for all the help in this work.
+ H2 O →
In the case of ethanol, the analysis of UPLC indicate no transformation of this solvent in any photocatalytic condition (see SI). Hence ethanol can act as a hole scavenger in the process, with a constant effect in all samples, that cannot be avoid due to its presence makes anthracene soluble. Finally to propose some intermediate species of a photodegradation pathway UPLC analyses with a photodiode array and mass detector were carried out. The chromatograms, presented a first peak with a retention time in 4.1 min, related with the principal product of reaction. The absorption spectra of this peak are comparable for anthraquinone spectra. On the other hand, the mass spectrum of this peak shows a clear molecular ion at m/z of 209, matched favorably with mass of anthraquinone. A second peak with a retention time in 6.1 min, present a molecular ion at m/z of 179, similar to anthracene. Hence, the following photo-degradation pathway is proposed (Scheme 1).
Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jphotochem.2018.10. 010. References
4. Conclusions
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