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Ti)
Journal of Photochemistry & Photobiology A: Chemistry 376 (2019) 196–205
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Visible light-driven photoelectrocatalytic degradation of acid yellow 17 using Sn3O4 flower-like thin films supported on Ti substrate (Sn3O4/TiO2/Ti)
T
A. Hudaa,b, P.H. Sumanb, L.D.M. Torquatob, Bianca F. Silvab, C.T. Handokoa, F. Guloa, , ⁎ M.V.B. Zanonib, M.O. Orlandib, ⁎
a b
Department of Environmental Science, Graduate Program, Sriwijaya University, Palembang, 30139, Indonesia Institute of Chemistry, São Paulo State University - UNESP, Araraquara, SP, 14800-060, Brazil
ARTICLE INFO
ABSTRACT
Keywords: Sn3O4 Microwave-assisted hydrothermal synthesis Visible-light photocatalysis Photoelectrocatalysis Dye degradation
This article reports a new method for preparing mixed valence tin oxide (Sn3O4) flower-like nanostructures using a microwave-assisted route. Thin-film Sn3O4/TiO2/Ti electrodes demonstrated highly efficient visible light driven photocatalytic degradation of monoazo acid yellow 17 (AY17) dye, reaching 95% color removal after 60 min at pH 2. Moreover, under low bias potential (0.5 V), the photoelectrocatalytic efficiency increased to 97% color removal and 83% removal of total organic carbon at a kinetic rate almost 3-fold higher than in photocatalysis. Liquid chromatography mass spectrometry was used to identify intermediate formation, and oxidation performance was proposed for photocatalytic and photoelectrocatalytic degradation with no organics identified after 120 min of treatment. The results indicate that Sn3O4/TiO2/Ti photoelectrodes offer a simple, green method for wastewater treatment employing visible light source.
1. Introduction Synthetic dyes are commonly used in various industrial sectors such as textiles, leathers, paints, papers, and plastics [1]. Annual production of organic dyes has reached approximately 750,000 tons, and 10–20% of this amount is released into bodies of water without proper treatments [2]. Most wastewater from textile dyes contains persistent organic pollutants and carcinogens, which are highly toxic, and mutagens causing serious environmental problems [3,4]. Furthermore, small amounts of dyes are easily visible in water and it reduces light penetration, subsequently affecting photosynthesis and potentially impacting biological activities in the aquatic ecosystem. However, dyes cannot be easily removed from wastewater since they are highly soluble. Removal from aqueous solution is a challenging task, specifically developing easy and efficient methods to ensure the safety of aquatic ecosystems, human lives, and the environment. The literature contains several reports describing semiconductors used in photocatalysts [5–8] and photoelectrocatalysis [9,10]. Photocatalysis (PC) and photoelectrocatalysis (PEC) have demonstrated great success in dye degradation due to the high generation of hydroxyl radicals (●OH) and holes (h+), which are nonselective and highly oxidizing species capable of mineralizing organic compounds [11,12]. The ⁎
main principle is irradiation of the semiconductor with photon energy exceeding its band gap energy (Eg), generating a photoelectron-hole pair (e−/h+). Moreover, under an additional bias potential greater than the flat band potential (Efb) of the semiconductor, there is less probability that the photo-charges will recombine, since there is an increase in band bending which results in the depletion of the photogenerated electron, and the photogenerated holes increased on the semiconductor surface [13]. As a result, the PEC process has been more efficient than PC in most cases, and this method has yielded excellent results in removing many organic pollutants through degradation and mineralization [14,15]. However, since most high-activated PC semiconductors have been reported to only absorb light in the ultraviolet (UV) range (which accounts for only 5% of the solar spectrum), a visible lightdriven PC semiconductor would be desirable to convert abundant photon energy from sunlight into chemical energy [8]. Sn3O4 is an intermediate phase in the tin oxide system which demonstrates n-type semiconductor behavior due to the presence of oxygen vacancies [16,17]. Sn3O4 has been reported to be an excellent photocatalyst material due to its band gap in the visible range (about 2.7 eV), which allows it to promote electrons to the conduction band under both visible light and UV radiation [18]. It has successfully been used as a visible light photocatalyst in the photodegradation of several
Corresponding authors. E-mail addresses: [email protected] (F. Gulo), [email protected] (M.O. Orlandi).
https://doi.org/10.1016/j.jphotochem.2019.01.039 Received 19 November 2018; Received in revised form 27 January 2019; Accepted 31 January 2019 Available online 19 February 2019 1010-6030/ © 2019 Elsevier B.V. All rights reserved.
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dyes such as methylene blue, methyl orange, 4-phenylazophenol, phenol, and rhodamine B [19–22]. However, Sn3O4 is a mixed valence phase of tin oxide which is difficult to synthesize [23,24]. Studies have shown that Sn3O4 can be successfully synthesized using carbothermal reduction [23] and hydrothermal [25] methods, but the morphology, purity, and physical properties depend on the selected route. Depositing Sn3O4 film on a titanium substrate offers some advantages. First, this system requires only a small amount of Sn3O4 compared to suspension, which is economically beneficial and more efficient. Second, Ti/TiO2 also attracts the photogenerated electrons and helps avoid the recombination process, improving quantum yield [8]. Finally, Ti/TiO2 increases the efficiency of photocatalytic activity by preventing agglomerations, which are common in the suspension process [26,27]. Agglomeration can decrease photocatalytic activity since it decreases the effective exposed area of the semiconductor and impedes light illumination on the active center of the semiconductor [28]. In this study, we report the synthesis of flower-like Sn3O4 structures using a microwave-assisted hydrothermal method at low temperatures (150 °C). To evaluate the photocatalytic activity of Sn3O4, we used titanium foils (Ti) to support Sn3O4 and form thin films. To the best of our knowledge, photocatalytic degradation based on thin Sn3O4 films has not yet been reported, and we used this approach to study the degradation of monoazo acid dye (acid yellow 17, AY17) under visible light illumination as a pollutant compound model. AY17 is a reactive dye commonly used in the detergent, soap, textile, printing, and cosmetic industries [29,30]. It has been reported to have mutagenic and tumorigenic effects and to be harmful to the respiratory system [30,31], and consequently presents a new application for the photocatalysis behavior of Sn3O4 thin films.
2.3. Characterization The synthesized materials were characterized using field emission scanning electron microscopy (FEG-SEM; JEOL, model JSM-7500 F), Xray diffraction (XRD; Rigaku, model D-Max 2500) using Cu-Kα radiation, and transmission electron microscopy (TEM; Philips, model CM200) equipped with an energy dispersive X-ray spectroscopy (EDS) detector. Zeta potential (ζ) measurements were taken with a zeta analyzer (Malvern Zetasizer Nano Series) dispersing Sn3O4 structures into a 0.1 mol L−1 Na2SO4 solution and adjusting the pH with 5 mmol L−1 NaOH and 5 mmol L−1 H2SO4 solutions. Photoactivity properties were evaluated by recording linear scan voltammetry from -0.5 V to 1.5 -V vs Standard Counter Electrode (SCE) at scan rate (v) of 10 mV s−1 in 0.1 mol L−1 Na2SO4 supporting electrolyte in a potentiostat/galvanostat (Autolab PGSTAT302 N). The dye degradation was performed at Sn3O4/TiO2/Ti electrode in 10 mg L−1 AY17 solution in 0.1 mol L−1 Na2SO4, pH 4.0 at 0.5 V bias potential for 150 min. To perform the tests of electrode stability, the amperometric measurements were performed recording 3 repetitions (3 cycles) of the photocurrent raised at controlled potential of 0.2 V, 0.5 V, 1.0 V and 1.5 V during 150 min of photoelectrolysis carried out at Sn3O4/TiO2/Ti electrode in 10 mg L−1 AY17 dye in 0.1 mol L−1 Na2SO4. 2.4. Photocatalytic degradation of monoazo acid yellow 17 (AY17) via photocatalysis and photoelectrocatalysis The photocatalytic (PC) and photoelectrocatalytic (PEC) processes took place in a 250 ml cylindrical homemade glass reactor (borosilicate glass, Amitel, São Paulo, Brazil) (Fig. 2) with a thermostatic bath (Quimis, Brazil), using a commercial 125 W high-pressure Hg lamp (Osram) as the light source. The lamp was inserted vertically in a quartz glass tube for the measurements using the UV and visible light (labeled UV light), and a commercial lead borosilicate glass tube was used as a filter of UV radiation when experiments were subjected to visible light. This kind of glass is opaque at wavelengths lower than 300 nm, with around 50% of transmittance at 350 nm and 90% of transmittance in the visible region [32]. The thin Sn3O4 film on Ti foil was employed as the working electrode (photoanode) and placed 1 cm from the lamp. For PEC degradation, a DSA electrode (De Nora Company) and Ag/AgCl (3 mol L−1) were used as the cathode and reference electrode, respectively. All experiments were performed using 150 mL of a 10 mg L-1 AY17 dye solution prepared in 0.1 M Na2SO4 as electrolyte. To find the best condition for AY17 degradation by the working electrode, some operational parameters such as pH, bias potential, and initial concentration were evaluated. The pH effects were investigated in both the PC and PEC processes, while the influence of bias potential and dye concentration were only investigated for PEC. The initial pH was adjusted by adding either 0.1 mol L-1 H2SO4 or 0.1 mol L-1 NaOH into the dye solution. In addition, photolysis (PL) was conducted as a control experiment by using only light illumination to investigate the photodegradation of AY17. To better understand the effects of dye concentration, the incident energy density was measured in the UVA range (320–400 nm) with a radiometer (PMA 2100, Solar Light Co., Glenside, PA, USA). Discoloration was assessed by monitoring the intensity of the maximum absorption peak of AY17 at 405 nm with a UV-Vis spectrophotometer (Cary 60 Agilent Technologies) using 3 mL aliquots taken at controlled times (min): 0, 30, 60, 90, 120, 150, and 180 for each process applied. The mineralization of organic matter was evaluated with a total organic carbon analyzer (TOC-VCPN, Shimadzu).
2. Experimental section 2.1. Preparation of flower-like Sn3O4 structures The flower-like Sn3O4 structures were produced using a microwaveassisted hydrothermal route. All chemicals used in the experiment were analytical grade. To obtain the desired structures, 0.9 g of SnF2 (SigmaAldrich, ≥ 99.9% purity) was dissolved into a mixture of distilled water and absolute ethanol (2:1 by volume) under vigorous stirring until a homogeneous white precursor solution was attained. The pH of the solution was then adjusted to 6 by slowly adding a 1 mol L−1 NaOH solution. The resulting mixture was transferred to a sealed Teflon-lined stainless-steel autoclave (Fig. 1) and hydrothermally treated at 150 °C for 2 h at a heating rate of 10 °C min−1 and then naturally cooled down to room temperature. The obtained suspension was centrifuged at 10,000 rpm and washed with distilled water and absolute ethanol several times. Finally, the material was dried at 70 °C for 24 h. 2.2. Preparation of thin-film Sn3O4/TiO2/Ti electrode The as-prepared Sn3O4 structures were deposited onto 3 x 4 cm titanium foils used as conductive substrates (Realum, Brazil). Before depositing, the titanium foils were polished and subsequently cleaned in a 10%HF/5%HNO3 solution to remove any residues. Next, the substrates were washed sequentially in acetone, isopropanol, and water, using sonication for 20 min at each step. The Sn3O4 film was then deposited onto the titanium substrate by spin coating using a mixture of 0.025 g of Sn3O4 and 0.026 g of polyvinyl butyral (PVB) dispersed in a 2.0 mL solution composed of toluene-methanol (72.4:27.6 v/v). After homogenization, 1.0 mL of 1-pentanol was added to the solution and stirred at 90 °C for 60 min to evaporate the toluene-methanol. The resulting viscous solution was then dropped onto the titanium substrates at 2000 rpm for 60 s. The films were finally annealed in air at 400 °C for 120 min to remove all organics.
2.5. Evaluation of AY17 degradation intermediates by LC–MS/MS For both the PC and PEC degradation processes, 5 mL aliquots were taken at predetermined times during the analysis (min): 0, 30, 60, 90, 120, 150, and 180, for PC and 3, 10, 15, 20, 30, 60, 120, and 150 for 197
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PEC. LC–MS/MS analysis of samples was done in a high-performance liquid chromatography device (1200 Agilent Technologies) coupled to a 3200 QTRAP mass spectrometer (Linear Ion Trap Quadrupole, AB SCIEX Instruments) operating in negative mode with TurboIonSpray ionization. The ionization parameters were obtained using a direct infusion of 10 μL min−1 of a solution containing 0.1 mg L−1 of AY17 in methanol:H2O (1:1, v/v). The working conditions were as follows: curtain gas: 20 psi; ion spray: -4500 V; gas 1: 50 psi; gas 2: 50 psi; temperature: 650 °C; declustering potential: -60.00 V; entrance potential: -6.50 V and interface heater: ON. Full scan experiments were performed from 150 to 950 Da. The different ions observed were selected and fragment ion experiments were also performed using collision energy ranging from 20 to 40 V. The liquid chromatography (LC) analysis was performed using a Phenomenex Kinetex PFP 5.0 μm (150 mm x 4.6 mm) column at 40 °C fitted to a PFP column guard. The mobile phase was water containing 0.1% of formic acid and acetonitrile in gradient elution from 5% to 70% of acetonitrile in 8 min, 2 min in 100% of acetonitrile and conditionate for 5 min in the initial condition. The flow rate used was 1.0 mL min−1 and the in-injection volume was 20 μL. Prior to LC–MS/MS analysis, the samples were cleaned using a solid phase extraction (SPE) Strata C18-E cartridge (60 mg. 3 mL) which was previously filled with 3 mL of methanol followed by 3 mL water. Next, 4 mL of sample was loaded through each cartridge, and 1 mL of water was added to remove the Na2SO4 electrolyte. The elution was carried out with 3 mL of methanol, and 1 mL with formic acid 0.1% (v/ v) of the eluted sample was used for further LC–MS/MS analysis.
Fig. 2. Reactor employed for AY17 degradation via photocatalysis and photoelectrocatalysis.
3. Results and discussion 3.1. Structural and morphological characterization The crystallinity and phase purity of the as-prepared materials synthesized via the microwave-assisted hydrothermal method were studied by X-ray diffraction (XRD). The diffractogram presented in Fig. 3 shows that these materials are composed only of structures grown in the triclinic structure of the Sn3O4 phase (JCPDS card #16-737). No residual contaminants or secondary phases were observed in XRD, meaning that the microwave-assisted hydrothermal method produces highly pure materials with well-controllable stoichiometry [28]. Peak intensities indicate that the structures do not exhibit any preferential orientation. The morphology and the crystallinity of the Sn3O4 structures were examined using FEG-SEM and TEM. The low-magnification SEM image in Fig. 4A shows that the materials are composed of flower-like structures ranging in size from 300 to 800 nm. Fig. 4B indicates that these flower-like structures consist of joined randomly- arranged leaves with smooth surfaces and thickness of approximately 10 nm. The bright field TEM image shown in Fig. 4C reveals that the aggregation of the leaves which forms the flowers results in an open structure, explaining the
Fig. 3. X-ray diffraction pattern of Sn3O4 material obtained after synthesis using the microwave-assisted hydrothermal method.
high surface area obtained by the BET method (170 m2 g−1). Fig. 4D shows a HRTEM image of a single leaf. The results indicate that each leaf is single-crystalline and the indexed interplanar distance (0.37 ± 0.01 nm) corresponds to the (101) plane of the Sn3O4 structure. The inset in Fig. 4D illustrates the SAED pattern from hundreds of flowers, confirming the Sn3O4 phase. The concentric rings are typical of polycrystalline materials, meaning that each leaf is randomly oriented during the formation of the material. 3.2. Photocatalytic activity of AY17 dye degradation Fig. 5A shows the discoloration of 10 mg L−1 AY17 in 0.1 M Na2SO4 at pH 2.0, 4.0, 6.0, and 9.0 using the Sn3O4/TiO2/Ti electrode through the photocatalysis reaction. The photodegradation of AY17 was
Fig. 1. Experimental setup used for the microwave-assisted hydrothermal process. 198
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Fig. 4. (A–B) Field emission scanning electron microscopy and (C–D) Transmission electron microscopy images of Sn3O4 flower-like structures. The inset in Fig. 4D shows the selected area electron diffraction pattern.
strongly affected by the initial pH of the dye solution; notably at pH 2.0, 95% of color was removed after 60 min of light irradiation. These findings can be explained by the zeta potential results shown in Fig. 5B. The zeta potential (mV) was shifted to the positive charge value in the acidic medium. Moreover, the isoelectric point (IEP) for Sn3O4 occurs around pH 2.5-3.0. Therefore, the positive charge of Sn3O4 at pH 2 could increase the electrostatic attraction between the Sn3O4 surface and the AY17 dye, favoring the adsorption process and consequently boosting the discoloration process. This means that at higher initial pH, the surface charge of Sn3O4 became negative promoting an electrostatic repulsion with the anionic sulfonate function of AY17 (pKa = 5.3) [30],
which is seen in decreased photodegradation efficiency at pH ≥ 6. Furthermore, this result was supported by the pseudo-first-order reaction kinetic constant which presented values of 3.76 × 10-2, 2.54 × 102 , 7.10 × 10-3, and 4.00 × 10-3 for pH 2.0, 4.0, 6.0, and 9.0, respectively. The results indicate that Sn3O4 can be a good alternative for removing color from AY17 solution when irradiated using visible light. Because of the good photocatalytic performance shown by the kinetic rate and the environmental justification for the experiment, pH 4 was selected for further measurements. As a control experiment, the photocatalytic degradation of AY17 dye solution using the annealed Ti substrate at pH 2 (optimized
Fig. 5. (A) The effects of initial pH in photocatalytic degradation of 10 mg L−1 monoazo acid yellow 17 dye in 0.1 M Na2SO4; and (B) zeta potential (ζ) of as-prepared Sn3O4. 199
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Fig. 6. (A) Linear sweep voltammetry curve for Sn3O4/TiO2/Ti in the dark, under ultraviolet light, and under visible light in 10 mg L−1 monoazo acid yellow 17 dye. Linear scan voltammetry, v = 10 mV s−1, in 0.1 mol L−1 Na2SO4; (B) The effects of bias potential in the photoelectrocatalytic degradation of 10 mg L−1 monoazo acid yellow 17 dye at pH 4; (C) Amperometry result on the effects of bias potential.
Fig. 7. PEC degradation of monoazo acid yellow 17 dye with different initial pH monitored by (A) absorbance at 405 nm; (B) change in pH solution; and (C) removal of total organic carbon.
Fig. 8. (A) Photoelectrocatalytic degradation of monoazo acid yellow 17 dye at four concentration levels (0.1 mol L−1 Na2SO4 and pH 4), monitored by absorbance at 405 nm; (B) The incident energy density using a 125 W high-pressure Hg-lamp with glass tube as the natural filter and 722 W/m2 as the initial incident energy density; and (C) Comparison of photodegradation of monoazo acid yellow 17 dye using several approaches.
condition) was conducted to verify the influence of intrinsic TiO2 film formed on the Ti substrate during the processing [33]. The results (not shown here) showed that annealed Ti foils yielded 18% of degradation after 180 min of light irradiation. The visible light illumination could not effectively activate the TiO2/Ti foils, resulting in low photocatalytic performance of TiO2/Ti substrate without the Sn3O4 semiconductor layer.
UV–vis light can be attributed to the characteristics of the Sn3O4, which absorbs both UV and visible light [18,19,21], although it can also have a minor contribution from the TiO2 on the surface of Ti [33], which effectively promotes electrons under UV light illumination. However, the high number of the electrons driven to the cathode initiates the gas formation (H2 evolution), which affects film stability and causes damage. For this reason, visible light irradiation was chosen for all subsequent experiments. Additionally, linear scan voltammograms indicate that the flat band potential for both visible and UV–vis light irradiation is around +0.25 V [34]. Fig. 6B shows the effect of applying the bias potential (Eap) (0.20, 0.50, 1.00, and 1.50 V) on the PEC degradation of 10 mg L−1 AY17 dye in 0.1 mol L−1 Na2SO4 using a Sn3O4/TiO2/Ti electrode irradiated with visible light. Although 97% of photodegradation occurs in the first 30 min at 0.50 V, this value reaches 99% after 150 min under irradiation. It is interesting to note that photodegradation is smaller when the bias is further increased, reaching about 50% of efficiency at 1.5 V after 60 min. The results indicate that there is a competition between the generation of the hydroxyl radical (which is necessary for dye discoloration) and the formation of oxygen at the higher applied potential on the Sn3O4/TiO2/Ti electrode. Moreover, the hydrogen flowing at the cathode could interfere in the generation of hydroxyl radicals. The chronoamperometric experiment was consequently conducted by
3.3. Photoelectrocatalytic (PEC) reaction of AY17 dye degradation Fig. 6A illustrates the typical photocurrent vs potential curves obtained for the oxidation of 10 mg L−1 AY17 dye in 0.1 mol L−1 Na2SO4 using the Sn3O4/TiO2/Ti working electrode in darkness, in visible light and under UV–vis irradiation. As expected, the photocurrent is negligible in the dark. A significant photocurrent was observed under UV and visible light illumination confirming that charge carriers (e-/h+) are generated by photoexcitation. A higher photocurrent response under UV–vis light exposure was seen in comparison to visible light, due to the higher photon energy. The borosilicate glass which was used as the natural UV light filter lead to a decrease of 40% in Hg lamp incident energy density (from 120 mW/cm2 to 72.2 mW/cm2) measured in the range of 320 nm–400 nm. According to the literature, the high photocurrent density in the 200
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Fig. 9. Stability of Sn3O4/TiO2/Ti in the photocatalytic and photoelectrocatalytic processes. Photodegradation of 10 mg L−1 monoazo acid yellow 17 dye in 0.1 Na2SO4 in the photocatalytic process, monitored by (A) UV–vis spectrophotometry and (B) removal of total organic carbon; photoelectrocatalytic process monitored by (C) UV–vis spectrophotometry, (D) removal of total organic carbon and (E) amperometry.
recording the change in photocurrent during the oxidation of 10 mg L−1 AY17 dye in 0.1 mol L−1 for 150 min at the controlled applied potential (Eap) of 0.20 V, 0.50 V, 1.00 V, and 1.5 V (Fig. 6C). For both cases, when the Eap was less than 0.5 V, the curve decreased rapidly and reached a steady state for a longer time. However, this behavior was not observed at 1.00 V and 1.50 V, most likely because of the change in oxygen via water oxidation (Eq. 1), which prevented the formation of hydroxyl radicals [34–36].
approximately 42% at pH 9.0. The results indicate that there was an elevated formation of hydroxyl radicals at low pH and the rate of oxidation is higher due to the increased adsorption of the selected dye on the Sn3O4/TiO2/Ti surface. They also show that the PEC process is a good strategy for removing color and promoting conversion of AY17 dye into CO2 and minerals. Considering that both the PC and PEC processes could present low mineralization efficiency in highly concentrated colored solutions such as dyes containing wastewater, further experiments were carried out to test PEC degradation of thin Sn3O4/TiO2/Ti film in four concentrations of AY17 (10, 20, 25, and 30 mg L−1), and the results are shown in Fig. 8A. It is clear that while 20 min of light irradiation was needed to reach ˜100% of color removal at 10 mg L−1 of AY17 dye (Kc = 7.66 × 10-2 min−1); the same process required 150 min of light irradiation to discolor a solution with 20 mg L−1 of AY17 dye (Kc = 3.29 × 10-2 min−1), which is the maximum concentration that could be completely removed. At concentrations of 25 and 30 mg L−1 of AY17 dye solution, the kinetic constant decreased to 4.50 × 10-3 min−1 and 4.30 × 10-3 min−1, respectively. Therefore, a high initial dye concentration presents a low kinetic rate of PEC degradation due to less light penetration [37]. Fig. 8B demonstrates the connection between dye concentration and the incident energy density in the photocatalytic reactor during each experiment. Note that increased dye concentration prevents photoexcitation of the Sn3O4/TiO2/Ti surface, thereby affecting the generation of hole-electron (e-/h+) pairs and consequently this surface’s photocatalytic performance [38]. Fig. 8C compares the percent photodegradation of 10 mg L−1 AY17 dye solution subjected to photolysis (PL), photocatalysis (PC), and photoelectrocatalysis (PEC). While direct PL through only visible light irradiation led to only 14% dye discoloration after 150 min, PC (visible light + Sn3O4 photoelectrode) reached 89% photodegradation after 60 min, and PEC (visible light + Eap = 0.5 V on Sn3O4/TiO2/Ti photoelectrode) was seen to be the most efficient process, with 96% photodegradation reached in the first 30 min. The entire process followed a pseudo-first-order reaction, and the values for kinetic rate (Kc) were 0.90 × 10-3 min−1, 2.54 × 10-3 min−1, and 7.66 × 10-2 min−1, respectively. Furthermore, the PEC degradation had the fastest kinetics constant (k), roughly 3-fold higher than the PC process and 85-fold higher than PL. For a better understanding of the stabilities of Sn3O4/TiO2/Ti, the
2H2O →O2 + 4H+ + 4e− and/or H2O + h+ →٠OH + H+
(1)
In addition, the gas evolution leached out the deposited Sn3O4 from the substrate, demonstrating the decreased stability of the photoanode under the higher applied potential. The stability of the thin Sn3O4 film can be seen in the amperometric results (Fig. 6C), with the rapid decrease in photocurrent indicating lower stability while a stable photocurrent curve shows higher film stability under the applied potential. It means that under bias higher than 0.50 V, the Sn3O4 film leached out and reduced the number of photogenerated holes, resulting in decreased generation of the hydroxyl radical. The 0.50 V bias potential was ultimately selected as the optimal condition for further PEC oxidation of the AY17 dye solution. Fig. 7A illustrates the effect of pH (2.0, 4.0, 6.0, and 9.0) on efficiency in the PEC process using the optimal bias potential (Eap = 0.50 V). Similar discoloration after 30 min of PEC electrolysis was seen in all the experiments. Moreover, the initial pH remained constant when PEC oxidation occurred at pH 2.0 and pH 4.0, but the value decreases dramatically when initial pH started at pH 6.0 and pH 9.0 (Fig. 7B). This behavior could indicate that the oxygen evolution via water oxidation could be competing in the process (Eq. 1) at the higher initial pH, decreasing the pH of the final solution due to the generation of protons during the PEC process. In order to test the performance of the Sn3O4/TiO2/Ti photoelectrode in the mineralization of 10 mg L−1 of AY17 dye in 0.1 mol L−1 Na2SO4 irradiated by visible light, the dissolved total carbon removal (TOC) was evaluated, and respective values are shown in Fig. 7C. TOC removal is optimally found at pH 2.0 and 4.0, reaching around 83%. At the higher pH values, mineralization decreased and reached 201
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Table 1 LC/MS data of the main intermediate identified during PC and PEC degradation of 10 mg L−1 AY17. Compound
Retention time (min)
[M-H]− m/za
Main fragment ions (MS/MS) m/z
Identified
AY17
6.03
505
305, 266, 171
PC and PEC
C1
7.33
441
305, 266
PC and PEC
C2
5.77
349
266, 252
PC
C3
7.37
367
239, 175
PC and PEC
C4
4.87
353
335, 265, 239
PC and PEC
C5 C6
7.18 7.40
348 241
239, 224 177, 161
PC and PEC PC and PEC
a b
Proposed structure
b
Detected in negative mode with z = 1. Structure not attributed.
working electrode was subjected to both the PC and PEC processes through a recycling test [39]. During PC degradation (Fig. 9A), the percentage of PC degradation did not decrease significantly after five continuous runs, demonstrating the high stability and reusability of the working electrode. After three cycles, however, TOC removal began to decrease mineralization efficiency slightly due to the formation of byproducts (Fig. 9B). The results were different in the PEC process,
showing a significant decrease in PEC degradation controlled by absorbance (Fig. 9C) and TOC removal (Fig. 9D) after the first cycle. This was supported by photocurrent density data, which showed that the number of photogenerated electrons drops significantly when time and cycles are increased (Fig. 9E). This is because the film was less stable when the bias was applied, releasing the Sn3O4 into the solution. It is important to note that the released Sn3O4 powder could easily be 202
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Fig. 10. Evolution of the intermediates (C1-C6) detected during monoazo acid yellow 17 dye degradation in (A) photocatalysis and (B) photoelectrocatalysis. Fig. 10 (C) Evolution of the intermediates in photoelectrocatalysis up to 30 min. (Monoazo acid yellow 17 dye, m/z 505); (C1, m/z 441); (C2, m/z 349); (C3, m/z 367); (C4, m/z 353); (C5, m/z 348); (AY17, m/z).
removed by centrifugation after the test and used for other experiments. The results prove that the film stability was relatively lower for the PEC process, although it is capable of high photodegradation and mineralization.
However, during AY17 degradation via PEC, only the intermediate C2 was not identified. The proposed structures, retention times, and main ion fragments detected in LC–MS/MS analysis of each intermediate are presented in Table 1. It should be mentioned that the proposed intermediate chemical structures were mainly based on mass to charge ratio and MS/MS fragmentation of each product [M−H]− identified. The intermediate identified as C1 (MW 442, m/z 441) was generated by desulfonation of benzenesulfonic acid in the AY17 molecule, where +SO3H was eliminated. Attack of the C1 azo group (-N = N-) by the hydroxyl radical promoted the loss of a phenol molecule, leading to the formation of a new compound designated C2 (MW 350, m/z 349). However, this intermediate was observed only in the PC process. The subsequent insertion of the hydroxyl into the C2 compound generates the intermediate C3 (MW 368, m/z 367). In PEC, C3 was
3.4. The main degradation intermediates of AY17 degradation High mineralization was observed in PEC degradation of AY17 dye. The main intermediates formed during degradation were analyzed and identified using LC–MS/MS by m/z transition detected in the negative mode after 150 min of the PC and PEC processes. The dye precursor AY17 was detected as m/z 505 ([M-2Na+1 H]−) at 6.30 min. Six main intermediates were detected during AY17 degradation via PC, with m/z 441 (C1), 349 (C2), 367 (C3), 353 (C4), 348 (C5) and 241 (C6).
Fig. 11. Proposed pathway for monoazo acid yellow 17 dye degradation in the photocatalytic and photoelectrocatalytic processes. 203
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probably generated by direct oxidation of the precursor molecule (C1 or even AY17), considering the greater efficiency of this process in generating the photoexcited electron-hole pairs (e−/h+) and consequently ٠OH radicals, which promotes faster oxidation of the complex organic molecules. The C4 product, identified as m/z 353, was probably generated after oxidation of the methyl group along with the loss of NH2NH−OH group. Meanwhile, the structure of the C5 intermediate was not completely elucidated, even after performing the MS/MS experiments. As a result, complementary measurements are ongoing to identify the related products obtained. The compound C6 (MW 242, m/z 241) was the least complex degradation product identified for both the PC and PEC processes after 180 min of reaction. The evolution of the intermediates was monitored throughout the PC and PEC experiments and is presented in Fig. 10. During the PC process, a continuous decrease in AY17 signal was observed up to 180 min of the experiment. This phenomenon is in accordance with the previous results presented in Fig. 5A. Despite the similar profiles obtained for evolution of the intermediate in both processes, dye degradation was significantly faster in the PEC process as can be observed by the complete decrease of the AY17 signal prior to 3 min of the experiment. Compared with the PC process, the evolution of intermediates throughout PEC was also significantly faster with these compounds identified during the first 30 min of the experiment. These results agree with the previous data shown in Figs. 7A and 7C, which confirmed removal of 98% of the color and 83% of TOC, respectively. Given the above considerations, identification of the degradation products of AY17 allowed us to propose a degradation pathway, which is presented in Fig. 11 below.
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Felix, H.L. Tuller, J.A. Varela, M.O. Orlandi, Comparative gas sensor response of SnO2, SnO and Sn3O4 nanobelts to NO2 and potential interferents, Sens. Actuators B Chem. 208 (March) (2015) 122–127. [24] A. Huda, R. Ichwani, C.T. Handoko, B. Yudono, M.D. Bustan, F. Gulo, Enhancing the visible-light photoresponse of SnO and SnO2 through the heterostructure formation using one-step hydrothermal route, Mater. Lett. (December) (2018). [25] J.-H. Zhao, et al., Synthesis mechanism of nanoporous Sn3O4 nanosheets by hydrothermal process without any additives, Chinese Phys. B 24 (June (6)) (2015) 066202. [26] G. Li, et al., Effect of the agglomeration of TiO2 nanoparticles on their photocatalytic performance in the aqueous phase, J. Colloid Interface Sci. 348 (August (2)) (2010) 342–347. [27] A.Y. Shan, T.I.M. Ghazi, S.A. Rashid, Immobilisation of titanium dioxide onto supporting materials in heterogeneous photocatalysis: a review, Appl. Catal. A Gen. 389 (December (1–2)) (2010) 1–8. [28] C.A. Zito, M.O. Orlandi, D.P. Volanti, Accelerated microwave-assisted hydrothermal/solvothermal processing: fundamentals, morphologies, and applications, J. Electroceramics 40 (June (4)) (2018) 271–292. [29] M. Ratnamala, M. Rahul, S. Sameer, M. Vaani, Omkar, T. Devdatt, Column studies for removal of acid yellow dye 17 from synthetic water using activated saw dust, Asian J. Chem. 29 (1) (2017) 191–195. [30] M.A. Ashraf, et al., Removal of acid yellow-17 dye from aqueous solution using ecofriendly biosorbent, Desalin. Water Treat. 51 (June (22–24)) (2013) 4530–4545. [31] J. Gao, Q. Zhang, K. Su, R. Chen, Y. Peng, Biosorption of Acid Yellow 17 from aqueous solution by non-living aerobic granular sludge, J. Hazard. Mater. 174 (February (1–3)) (2010) 215–225. [32] C.W. Dunnill, UV blocking glass: low cost filters for visible light photocatalytic assessment, Int. J. Photoenergy 2014 (2014) 1–5. [33] A.S. Martins, P.J.M. Cordeiro-Junior, G.G. Bessegato, J.F. Carneiro, M.V.B. 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4. Conclusion In summary, a highly purity hierarchical Sn3O4 flower-like nanostructure with well-controllable stoichiometry was successfully prepared using a simple microwave-assisted hydrothermal method. The as-prepared tin oxide flower-like structures were arranged into nano-sized single-crystalline leaves with smooth surfaces, which provided a high surface area. The Sn3O4 film deposited onto Ti foils attained 95% of color removal for 10 mg L−1 of AY17 solution after 60 min through the photocatalysis (PC) process at pH 2, using visible light irradiation. The results showed that the rate constant effectively increases by applying a low bias potential (0.5 V) (PEC), although this does affect film stability. Despite the lower stability, the thin Sn3O4 film is relatively stable after reusing 5 times in PC degradation and 3 times in PEC degradation, without significantly diminished photocatalytic performance. There is an optional wastewater treatment process in which PC and PEC provide high stability films and relatively low photodegradation and high photodegradation and lack of film stability, respectively. Furthermore, both processes permit a new wastewater treatment technology based on a simple, green method by utilizing visible light photocatalysts. Acknowledgements We would like to thank INCT – DATREM, FAPESP #2014/50945-4 and #2017/26219-0, CNPq #465571/2014-0, #443138/2016-8 and #150493/2017-7) for funding resources, and LMA-IQ-UNESP for providing electron microscopy facilities. This research collaboration was also supported by the PMDSU program of Kemenristekdikti, Indonesia through the PKPI-Sandwich Program 2017. References [1] R. Salehi, M. Arami, N.M. Mahmoodi, H. Bahrami, S. Khorramfar, Novel biocompatible composite (Chitosan–zinc oxide nanoparticle): preparation, characterization and dye adsorption properties, Colloids Surf. B Biointerfaces 80 (2010) 86–93.
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wastewater decolorization, Water Res. 98 (July) (2016) 39–46. [37] E. Fajriati, I. Mudasir, Wahyuni, Photocatalytic decolorization study of methyl orange by TiO2-Chitosan nanocomposites, Indones. J. Chem. 14 (3) (2014) 209–218. [38] Q. Zheng, C. Lee, Visible light photoelectrocatalytic degradation of methyl orange using anodized nanoporous WO3, Electrochim. Acta 115 (January) (2014) 140–145. [39] P. Wang, Y. Ao, C. Wang, J. Hou, J. Qian, Enhanced photoelectrocatalytic activity for dye degradation by graphene–titania composite film electrodes, J. Hazard. Mater. 223–224 (July) (2012) 79–83.
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Journal of Photochemistry & Photobiology A: Chemistry journal homepage: www.elsevier.com/locate/jphotochem
Visible light-driven photoelectrocatalytic degradation of acid yellow 17 using Sn3O4 flower-like thin films supported on Ti substrate (Sn3O4/TiO2/Ti)
T
A. Hudaa,b, P.H. Sumanb, L.D.M. Torquatob, Bianca F. Silvab, C.T. Handokoa, F. Guloa, , ⁎ M.V.B. Zanonib, M.O. Orlandib, ⁎
a b
Department of Environmental Science, Graduate Program, Sriwijaya University, Palembang, 30139, Indonesia Institute of Chemistry, São Paulo State University - UNESP, Araraquara, SP, 14800-060, Brazil
ARTICLE INFO
ABSTRACT
Keywords: Sn3O4 Microwave-assisted hydrothermal synthesis Visible-light photocatalysis Photoelectrocatalysis Dye degradation
This article reports a new method for preparing mixed valence tin oxide (Sn3O4) flower-like nanostructures using a microwave-assisted route. Thin-film Sn3O4/TiO2/Ti electrodes demonstrated highly efficient visible light driven photocatalytic degradation of monoazo acid yellow 17 (AY17) dye, reaching 95% color removal after 60 min at pH 2. Moreover, under low bias potential (0.5 V), the photoelectrocatalytic efficiency increased to 97% color removal and 83% removal of total organic carbon at a kinetic rate almost 3-fold higher than in photocatalysis. Liquid chromatography mass spectrometry was used to identify intermediate formation, and oxidation performance was proposed for photocatalytic and photoelectrocatalytic degradation with no organics identified after 120 min of treatment. The results indicate that Sn3O4/TiO2/Ti photoelectrodes offer a simple, green method for wastewater treatment employing visible light source.
1. Introduction Synthetic dyes are commonly used in various industrial sectors such as textiles, leathers, paints, papers, and plastics [1]. Annual production of organic dyes has reached approximately 750,000 tons, and 10–20% of this amount is released into bodies of water without proper treatments [2]. Most wastewater from textile dyes contains persistent organic pollutants and carcinogens, which are highly toxic, and mutagens causing serious environmental problems [3,4]. Furthermore, small amounts of dyes are easily visible in water and it reduces light penetration, subsequently affecting photosynthesis and potentially impacting biological activities in the aquatic ecosystem. However, dyes cannot be easily removed from wastewater since they are highly soluble. Removal from aqueous solution is a challenging task, specifically developing easy and efficient methods to ensure the safety of aquatic ecosystems, human lives, and the environment. The literature contains several reports describing semiconductors used in photocatalysts [5–8] and photoelectrocatalysis [9,10]. Photocatalysis (PC) and photoelectrocatalysis (PEC) have demonstrated great success in dye degradation due to the high generation of hydroxyl radicals (●OH) and holes (h+), which are nonselective and highly oxidizing species capable of mineralizing organic compounds [11,12]. The ⁎
main principle is irradiation of the semiconductor with photon energy exceeding its band gap energy (Eg), generating a photoelectron-hole pair (e−/h+). Moreover, under an additional bias potential greater than the flat band potential (Efb) of the semiconductor, there is less probability that the photo-charges will recombine, since there is an increase in band bending which results in the depletion of the photogenerated electron, and the photogenerated holes increased on the semiconductor surface [13]. As a result, the PEC process has been more efficient than PC in most cases, and this method has yielded excellent results in removing many organic pollutants through degradation and mineralization [14,15]. However, since most high-activated PC semiconductors have been reported to only absorb light in the ultraviolet (UV) range (which accounts for only 5% of the solar spectrum), a visible lightdriven PC semiconductor would be desirable to convert abundant photon energy from sunlight into chemical energy [8]. Sn3O4 is an intermediate phase in the tin oxide system which demonstrates n-type semiconductor behavior due to the presence of oxygen vacancies [16,17]. Sn3O4 has been reported to be an excellent photocatalyst material due to its band gap in the visible range (about 2.7 eV), which allows it to promote electrons to the conduction band under both visible light and UV radiation [18]. It has successfully been used as a visible light photocatalyst in the photodegradation of several
Corresponding authors. E-mail addresses: [email protected] (F. Gulo), [email protected] (M.O. Orlandi).
https://doi.org/10.1016/j.jphotochem.2019.01.039 Received 19 November 2018; Received in revised form 27 January 2019; Accepted 31 January 2019 Available online 19 February 2019 1010-6030/ © 2019 Elsevier B.V. All rights reserved.
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dyes such as methylene blue, methyl orange, 4-phenylazophenol, phenol, and rhodamine B [19–22]. However, Sn3O4 is a mixed valence phase of tin oxide which is difficult to synthesize [23,24]. Studies have shown that Sn3O4 can be successfully synthesized using carbothermal reduction [23] and hydrothermal [25] methods, but the morphology, purity, and physical properties depend on the selected route. Depositing Sn3O4 film on a titanium substrate offers some advantages. First, this system requires only a small amount of Sn3O4 compared to suspension, which is economically beneficial and more efficient. Second, Ti/TiO2 also attracts the photogenerated electrons and helps avoid the recombination process, improving quantum yield [8]. Finally, Ti/TiO2 increases the efficiency of photocatalytic activity by preventing agglomerations, which are common in the suspension process [26,27]. Agglomeration can decrease photocatalytic activity since it decreases the effective exposed area of the semiconductor and impedes light illumination on the active center of the semiconductor [28]. In this study, we report the synthesis of flower-like Sn3O4 structures using a microwave-assisted hydrothermal method at low temperatures (150 °C). To evaluate the photocatalytic activity of Sn3O4, we used titanium foils (Ti) to support Sn3O4 and form thin films. To the best of our knowledge, photocatalytic degradation based on thin Sn3O4 films has not yet been reported, and we used this approach to study the degradation of monoazo acid dye (acid yellow 17, AY17) under visible light illumination as a pollutant compound model. AY17 is a reactive dye commonly used in the detergent, soap, textile, printing, and cosmetic industries [29,30]. It has been reported to have mutagenic and tumorigenic effects and to be harmful to the respiratory system [30,31], and consequently presents a new application for the photocatalysis behavior of Sn3O4 thin films.
2.3. Characterization The synthesized materials were characterized using field emission scanning electron microscopy (FEG-SEM; JEOL, model JSM-7500 F), Xray diffraction (XRD; Rigaku, model D-Max 2500) using Cu-Kα radiation, and transmission electron microscopy (TEM; Philips, model CM200) equipped with an energy dispersive X-ray spectroscopy (EDS) detector. Zeta potential (ζ) measurements were taken with a zeta analyzer (Malvern Zetasizer Nano Series) dispersing Sn3O4 structures into a 0.1 mol L−1 Na2SO4 solution and adjusting the pH with 5 mmol L−1 NaOH and 5 mmol L−1 H2SO4 solutions. Photoactivity properties were evaluated by recording linear scan voltammetry from -0.5 V to 1.5 -V vs Standard Counter Electrode (SCE) at scan rate (v) of 10 mV s−1 in 0.1 mol L−1 Na2SO4 supporting electrolyte in a potentiostat/galvanostat (Autolab PGSTAT302 N). The dye degradation was performed at Sn3O4/TiO2/Ti electrode in 10 mg L−1 AY17 solution in 0.1 mol L−1 Na2SO4, pH 4.0 at 0.5 V bias potential for 150 min. To perform the tests of electrode stability, the amperometric measurements were performed recording 3 repetitions (3 cycles) of the photocurrent raised at controlled potential of 0.2 V, 0.5 V, 1.0 V and 1.5 V during 150 min of photoelectrolysis carried out at Sn3O4/TiO2/Ti electrode in 10 mg L−1 AY17 dye in 0.1 mol L−1 Na2SO4. 2.4. Photocatalytic degradation of monoazo acid yellow 17 (AY17) via photocatalysis and photoelectrocatalysis The photocatalytic (PC) and photoelectrocatalytic (PEC) processes took place in a 250 ml cylindrical homemade glass reactor (borosilicate glass, Amitel, São Paulo, Brazil) (Fig. 2) with a thermostatic bath (Quimis, Brazil), using a commercial 125 W high-pressure Hg lamp (Osram) as the light source. The lamp was inserted vertically in a quartz glass tube for the measurements using the UV and visible light (labeled UV light), and a commercial lead borosilicate glass tube was used as a filter of UV radiation when experiments were subjected to visible light. This kind of glass is opaque at wavelengths lower than 300 nm, with around 50% of transmittance at 350 nm and 90% of transmittance in the visible region [32]. The thin Sn3O4 film on Ti foil was employed as the working electrode (photoanode) and placed 1 cm from the lamp. For PEC degradation, a DSA electrode (De Nora Company) and Ag/AgCl (3 mol L−1) were used as the cathode and reference electrode, respectively. All experiments were performed using 150 mL of a 10 mg L-1 AY17 dye solution prepared in 0.1 M Na2SO4 as electrolyte. To find the best condition for AY17 degradation by the working electrode, some operational parameters such as pH, bias potential, and initial concentration were evaluated. The pH effects were investigated in both the PC and PEC processes, while the influence of bias potential and dye concentration were only investigated for PEC. The initial pH was adjusted by adding either 0.1 mol L-1 H2SO4 or 0.1 mol L-1 NaOH into the dye solution. In addition, photolysis (PL) was conducted as a control experiment by using only light illumination to investigate the photodegradation of AY17. To better understand the effects of dye concentration, the incident energy density was measured in the UVA range (320–400 nm) with a radiometer (PMA 2100, Solar Light Co., Glenside, PA, USA). Discoloration was assessed by monitoring the intensity of the maximum absorption peak of AY17 at 405 nm with a UV-Vis spectrophotometer (Cary 60 Agilent Technologies) using 3 mL aliquots taken at controlled times (min): 0, 30, 60, 90, 120, 150, and 180 for each process applied. The mineralization of organic matter was evaluated with a total organic carbon analyzer (TOC-VCPN, Shimadzu).
2. Experimental section 2.1. Preparation of flower-like Sn3O4 structures The flower-like Sn3O4 structures were produced using a microwaveassisted hydrothermal route. All chemicals used in the experiment were analytical grade. To obtain the desired structures, 0.9 g of SnF2 (SigmaAldrich, ≥ 99.9% purity) was dissolved into a mixture of distilled water and absolute ethanol (2:1 by volume) under vigorous stirring until a homogeneous white precursor solution was attained. The pH of the solution was then adjusted to 6 by slowly adding a 1 mol L−1 NaOH solution. The resulting mixture was transferred to a sealed Teflon-lined stainless-steel autoclave (Fig. 1) and hydrothermally treated at 150 °C for 2 h at a heating rate of 10 °C min−1 and then naturally cooled down to room temperature. The obtained suspension was centrifuged at 10,000 rpm and washed with distilled water and absolute ethanol several times. Finally, the material was dried at 70 °C for 24 h. 2.2. Preparation of thin-film Sn3O4/TiO2/Ti electrode The as-prepared Sn3O4 structures were deposited onto 3 x 4 cm titanium foils used as conductive substrates (Realum, Brazil). Before depositing, the titanium foils were polished and subsequently cleaned in a 10%HF/5%HNO3 solution to remove any residues. Next, the substrates were washed sequentially in acetone, isopropanol, and water, using sonication for 20 min at each step. The Sn3O4 film was then deposited onto the titanium substrate by spin coating using a mixture of 0.025 g of Sn3O4 and 0.026 g of polyvinyl butyral (PVB) dispersed in a 2.0 mL solution composed of toluene-methanol (72.4:27.6 v/v). After homogenization, 1.0 mL of 1-pentanol was added to the solution and stirred at 90 °C for 60 min to evaporate the toluene-methanol. The resulting viscous solution was then dropped onto the titanium substrates at 2000 rpm for 60 s. The films were finally annealed in air at 400 °C for 120 min to remove all organics.
2.5. Evaluation of AY17 degradation intermediates by LC–MS/MS For both the PC and PEC degradation processes, 5 mL aliquots were taken at predetermined times during the analysis (min): 0, 30, 60, 90, 120, 150, and 180, for PC and 3, 10, 15, 20, 30, 60, 120, and 150 for 197
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PEC. LC–MS/MS analysis of samples was done in a high-performance liquid chromatography device (1200 Agilent Technologies) coupled to a 3200 QTRAP mass spectrometer (Linear Ion Trap Quadrupole, AB SCIEX Instruments) operating in negative mode with TurboIonSpray ionization. The ionization parameters were obtained using a direct infusion of 10 μL min−1 of a solution containing 0.1 mg L−1 of AY17 in methanol:H2O (1:1, v/v). The working conditions were as follows: curtain gas: 20 psi; ion spray: -4500 V; gas 1: 50 psi; gas 2: 50 psi; temperature: 650 °C; declustering potential: -60.00 V; entrance potential: -6.50 V and interface heater: ON. Full scan experiments were performed from 150 to 950 Da. The different ions observed were selected and fragment ion experiments were also performed using collision energy ranging from 20 to 40 V. The liquid chromatography (LC) analysis was performed using a Phenomenex Kinetex PFP 5.0 μm (150 mm x 4.6 mm) column at 40 °C fitted to a PFP column guard. The mobile phase was water containing 0.1% of formic acid and acetonitrile in gradient elution from 5% to 70% of acetonitrile in 8 min, 2 min in 100% of acetonitrile and conditionate for 5 min in the initial condition. The flow rate used was 1.0 mL min−1 and the in-injection volume was 20 μL. Prior to LC–MS/MS analysis, the samples were cleaned using a solid phase extraction (SPE) Strata C18-E cartridge (60 mg. 3 mL) which was previously filled with 3 mL of methanol followed by 3 mL water. Next, 4 mL of sample was loaded through each cartridge, and 1 mL of water was added to remove the Na2SO4 electrolyte. The elution was carried out with 3 mL of methanol, and 1 mL with formic acid 0.1% (v/ v) of the eluted sample was used for further LC–MS/MS analysis.
Fig. 2. Reactor employed for AY17 degradation via photocatalysis and photoelectrocatalysis.
3. Results and discussion 3.1. Structural and morphological characterization The crystallinity and phase purity of the as-prepared materials synthesized via the microwave-assisted hydrothermal method were studied by X-ray diffraction (XRD). The diffractogram presented in Fig. 3 shows that these materials are composed only of structures grown in the triclinic structure of the Sn3O4 phase (JCPDS card #16-737). No residual contaminants or secondary phases were observed in XRD, meaning that the microwave-assisted hydrothermal method produces highly pure materials with well-controllable stoichiometry [28]. Peak intensities indicate that the structures do not exhibit any preferential orientation. The morphology and the crystallinity of the Sn3O4 structures were examined using FEG-SEM and TEM. The low-magnification SEM image in Fig. 4A shows that the materials are composed of flower-like structures ranging in size from 300 to 800 nm. Fig. 4B indicates that these flower-like structures consist of joined randomly- arranged leaves with smooth surfaces and thickness of approximately 10 nm. The bright field TEM image shown in Fig. 4C reveals that the aggregation of the leaves which forms the flowers results in an open structure, explaining the
Fig. 3. X-ray diffraction pattern of Sn3O4 material obtained after synthesis using the microwave-assisted hydrothermal method.
high surface area obtained by the BET method (170 m2 g−1). Fig. 4D shows a HRTEM image of a single leaf. The results indicate that each leaf is single-crystalline and the indexed interplanar distance (0.37 ± 0.01 nm) corresponds to the (101) plane of the Sn3O4 structure. The inset in Fig. 4D illustrates the SAED pattern from hundreds of flowers, confirming the Sn3O4 phase. The concentric rings are typical of polycrystalline materials, meaning that each leaf is randomly oriented during the formation of the material. 3.2. Photocatalytic activity of AY17 dye degradation Fig. 5A shows the discoloration of 10 mg L−1 AY17 in 0.1 M Na2SO4 at pH 2.0, 4.0, 6.0, and 9.0 using the Sn3O4/TiO2/Ti electrode through the photocatalysis reaction. The photodegradation of AY17 was
Fig. 1. Experimental setup used for the microwave-assisted hydrothermal process. 198
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Fig. 4. (A–B) Field emission scanning electron microscopy and (C–D) Transmission electron microscopy images of Sn3O4 flower-like structures. The inset in Fig. 4D shows the selected area electron diffraction pattern.
strongly affected by the initial pH of the dye solution; notably at pH 2.0, 95% of color was removed after 60 min of light irradiation. These findings can be explained by the zeta potential results shown in Fig. 5B. The zeta potential (mV) was shifted to the positive charge value in the acidic medium. Moreover, the isoelectric point (IEP) for Sn3O4 occurs around pH 2.5-3.0. Therefore, the positive charge of Sn3O4 at pH 2 could increase the electrostatic attraction between the Sn3O4 surface and the AY17 dye, favoring the adsorption process and consequently boosting the discoloration process. This means that at higher initial pH, the surface charge of Sn3O4 became negative promoting an electrostatic repulsion with the anionic sulfonate function of AY17 (pKa = 5.3) [30],
which is seen in decreased photodegradation efficiency at pH ≥ 6. Furthermore, this result was supported by the pseudo-first-order reaction kinetic constant which presented values of 3.76 × 10-2, 2.54 × 102 , 7.10 × 10-3, and 4.00 × 10-3 for pH 2.0, 4.0, 6.0, and 9.0, respectively. The results indicate that Sn3O4 can be a good alternative for removing color from AY17 solution when irradiated using visible light. Because of the good photocatalytic performance shown by the kinetic rate and the environmental justification for the experiment, pH 4 was selected for further measurements. As a control experiment, the photocatalytic degradation of AY17 dye solution using the annealed Ti substrate at pH 2 (optimized
Fig. 5. (A) The effects of initial pH in photocatalytic degradation of 10 mg L−1 monoazo acid yellow 17 dye in 0.1 M Na2SO4; and (B) zeta potential (ζ) of as-prepared Sn3O4. 199
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Fig. 6. (A) Linear sweep voltammetry curve for Sn3O4/TiO2/Ti in the dark, under ultraviolet light, and under visible light in 10 mg L−1 monoazo acid yellow 17 dye. Linear scan voltammetry, v = 10 mV s−1, in 0.1 mol L−1 Na2SO4; (B) The effects of bias potential in the photoelectrocatalytic degradation of 10 mg L−1 monoazo acid yellow 17 dye at pH 4; (C) Amperometry result on the effects of bias potential.
Fig. 7. PEC degradation of monoazo acid yellow 17 dye with different initial pH monitored by (A) absorbance at 405 nm; (B) change in pH solution; and (C) removal of total organic carbon.
Fig. 8. (A) Photoelectrocatalytic degradation of monoazo acid yellow 17 dye at four concentration levels (0.1 mol L−1 Na2SO4 and pH 4), monitored by absorbance at 405 nm; (B) The incident energy density using a 125 W high-pressure Hg-lamp with glass tube as the natural filter and 722 W/m2 as the initial incident energy density; and (C) Comparison of photodegradation of monoazo acid yellow 17 dye using several approaches.
condition) was conducted to verify the influence of intrinsic TiO2 film formed on the Ti substrate during the processing [33]. The results (not shown here) showed that annealed Ti foils yielded 18% of degradation after 180 min of light irradiation. The visible light illumination could not effectively activate the TiO2/Ti foils, resulting in low photocatalytic performance of TiO2/Ti substrate without the Sn3O4 semiconductor layer.
UV–vis light can be attributed to the characteristics of the Sn3O4, which absorbs both UV and visible light [18,19,21], although it can also have a minor contribution from the TiO2 on the surface of Ti [33], which effectively promotes electrons under UV light illumination. However, the high number of the electrons driven to the cathode initiates the gas formation (H2 evolution), which affects film stability and causes damage. For this reason, visible light irradiation was chosen for all subsequent experiments. Additionally, linear scan voltammograms indicate that the flat band potential for both visible and UV–vis light irradiation is around +0.25 V [34]. Fig. 6B shows the effect of applying the bias potential (Eap) (0.20, 0.50, 1.00, and 1.50 V) on the PEC degradation of 10 mg L−1 AY17 dye in 0.1 mol L−1 Na2SO4 using a Sn3O4/TiO2/Ti electrode irradiated with visible light. Although 97% of photodegradation occurs in the first 30 min at 0.50 V, this value reaches 99% after 150 min under irradiation. It is interesting to note that photodegradation is smaller when the bias is further increased, reaching about 50% of efficiency at 1.5 V after 60 min. The results indicate that there is a competition between the generation of the hydroxyl radical (which is necessary for dye discoloration) and the formation of oxygen at the higher applied potential on the Sn3O4/TiO2/Ti electrode. Moreover, the hydrogen flowing at the cathode could interfere in the generation of hydroxyl radicals. The chronoamperometric experiment was consequently conducted by
3.3. Photoelectrocatalytic (PEC) reaction of AY17 dye degradation Fig. 6A illustrates the typical photocurrent vs potential curves obtained for the oxidation of 10 mg L−1 AY17 dye in 0.1 mol L−1 Na2SO4 using the Sn3O4/TiO2/Ti working electrode in darkness, in visible light and under UV–vis irradiation. As expected, the photocurrent is negligible in the dark. A significant photocurrent was observed under UV and visible light illumination confirming that charge carriers (e-/h+) are generated by photoexcitation. A higher photocurrent response under UV–vis light exposure was seen in comparison to visible light, due to the higher photon energy. The borosilicate glass which was used as the natural UV light filter lead to a decrease of 40% in Hg lamp incident energy density (from 120 mW/cm2 to 72.2 mW/cm2) measured in the range of 320 nm–400 nm. According to the literature, the high photocurrent density in the 200
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Fig. 9. Stability of Sn3O4/TiO2/Ti in the photocatalytic and photoelectrocatalytic processes. Photodegradation of 10 mg L−1 monoazo acid yellow 17 dye in 0.1 Na2SO4 in the photocatalytic process, monitored by (A) UV–vis spectrophotometry and (B) removal of total organic carbon; photoelectrocatalytic process monitored by (C) UV–vis spectrophotometry, (D) removal of total organic carbon and (E) amperometry.
recording the change in photocurrent during the oxidation of 10 mg L−1 AY17 dye in 0.1 mol L−1 for 150 min at the controlled applied potential (Eap) of 0.20 V, 0.50 V, 1.00 V, and 1.5 V (Fig. 6C). For both cases, when the Eap was less than 0.5 V, the curve decreased rapidly and reached a steady state for a longer time. However, this behavior was not observed at 1.00 V and 1.50 V, most likely because of the change in oxygen via water oxidation (Eq. 1), which prevented the formation of hydroxyl radicals [34–36].
approximately 42% at pH 9.0. The results indicate that there was an elevated formation of hydroxyl radicals at low pH and the rate of oxidation is higher due to the increased adsorption of the selected dye on the Sn3O4/TiO2/Ti surface. They also show that the PEC process is a good strategy for removing color and promoting conversion of AY17 dye into CO2 and minerals. Considering that both the PC and PEC processes could present low mineralization efficiency in highly concentrated colored solutions such as dyes containing wastewater, further experiments were carried out to test PEC degradation of thin Sn3O4/TiO2/Ti film in four concentrations of AY17 (10, 20, 25, and 30 mg L−1), and the results are shown in Fig. 8A. It is clear that while 20 min of light irradiation was needed to reach ˜100% of color removal at 10 mg L−1 of AY17 dye (Kc = 7.66 × 10-2 min−1); the same process required 150 min of light irradiation to discolor a solution with 20 mg L−1 of AY17 dye (Kc = 3.29 × 10-2 min−1), which is the maximum concentration that could be completely removed. At concentrations of 25 and 30 mg L−1 of AY17 dye solution, the kinetic constant decreased to 4.50 × 10-3 min−1 and 4.30 × 10-3 min−1, respectively. Therefore, a high initial dye concentration presents a low kinetic rate of PEC degradation due to less light penetration [37]. Fig. 8B demonstrates the connection between dye concentration and the incident energy density in the photocatalytic reactor during each experiment. Note that increased dye concentration prevents photoexcitation of the Sn3O4/TiO2/Ti surface, thereby affecting the generation of hole-electron (e-/h+) pairs and consequently this surface’s photocatalytic performance [38]. Fig. 8C compares the percent photodegradation of 10 mg L−1 AY17 dye solution subjected to photolysis (PL), photocatalysis (PC), and photoelectrocatalysis (PEC). While direct PL through only visible light irradiation led to only 14% dye discoloration after 150 min, PC (visible light + Sn3O4 photoelectrode) reached 89% photodegradation after 60 min, and PEC (visible light + Eap = 0.5 V on Sn3O4/TiO2/Ti photoelectrode) was seen to be the most efficient process, with 96% photodegradation reached in the first 30 min. The entire process followed a pseudo-first-order reaction, and the values for kinetic rate (Kc) were 0.90 × 10-3 min−1, 2.54 × 10-3 min−1, and 7.66 × 10-2 min−1, respectively. Furthermore, the PEC degradation had the fastest kinetics constant (k), roughly 3-fold higher than the PC process and 85-fold higher than PL. For a better understanding of the stabilities of Sn3O4/TiO2/Ti, the
2H2O →O2 + 4H+ + 4e− and/or H2O + h+ →٠OH + H+
(1)
In addition, the gas evolution leached out the deposited Sn3O4 from the substrate, demonstrating the decreased stability of the photoanode under the higher applied potential. The stability of the thin Sn3O4 film can be seen in the amperometric results (Fig. 6C), with the rapid decrease in photocurrent indicating lower stability while a stable photocurrent curve shows higher film stability under the applied potential. It means that under bias higher than 0.50 V, the Sn3O4 film leached out and reduced the number of photogenerated holes, resulting in decreased generation of the hydroxyl radical. The 0.50 V bias potential was ultimately selected as the optimal condition for further PEC oxidation of the AY17 dye solution. Fig. 7A illustrates the effect of pH (2.0, 4.0, 6.0, and 9.0) on efficiency in the PEC process using the optimal bias potential (Eap = 0.50 V). Similar discoloration after 30 min of PEC electrolysis was seen in all the experiments. Moreover, the initial pH remained constant when PEC oxidation occurred at pH 2.0 and pH 4.0, but the value decreases dramatically when initial pH started at pH 6.0 and pH 9.0 (Fig. 7B). This behavior could indicate that the oxygen evolution via water oxidation could be competing in the process (Eq. 1) at the higher initial pH, decreasing the pH of the final solution due to the generation of protons during the PEC process. In order to test the performance of the Sn3O4/TiO2/Ti photoelectrode in the mineralization of 10 mg L−1 of AY17 dye in 0.1 mol L−1 Na2SO4 irradiated by visible light, the dissolved total carbon removal (TOC) was evaluated, and respective values are shown in Fig. 7C. TOC removal is optimally found at pH 2.0 and 4.0, reaching around 83%. At the higher pH values, mineralization decreased and reached 201
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Table 1 LC/MS data of the main intermediate identified during PC and PEC degradation of 10 mg L−1 AY17. Compound
Retention time (min)
[M-H]− m/za
Main fragment ions (MS/MS) m/z
Identified
AY17
6.03
505
305, 266, 171
PC and PEC
C1
7.33
441
305, 266
PC and PEC
C2
5.77
349
266, 252
PC
C3
7.37
367
239, 175
PC and PEC
C4
4.87
353
335, 265, 239
PC and PEC
C5 C6
7.18 7.40
348 241
239, 224 177, 161
PC and PEC PC and PEC
a b
Proposed structure
b
Detected in negative mode with z = 1. Structure not attributed.
working electrode was subjected to both the PC and PEC processes through a recycling test [39]. During PC degradation (Fig. 9A), the percentage of PC degradation did not decrease significantly after five continuous runs, demonstrating the high stability and reusability of the working electrode. After three cycles, however, TOC removal began to decrease mineralization efficiency slightly due to the formation of byproducts (Fig. 9B). The results were different in the PEC process,
showing a significant decrease in PEC degradation controlled by absorbance (Fig. 9C) and TOC removal (Fig. 9D) after the first cycle. This was supported by photocurrent density data, which showed that the number of photogenerated electrons drops significantly when time and cycles are increased (Fig. 9E). This is because the film was less stable when the bias was applied, releasing the Sn3O4 into the solution. It is important to note that the released Sn3O4 powder could easily be 202
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Fig. 10. Evolution of the intermediates (C1-C6) detected during monoazo acid yellow 17 dye degradation in (A) photocatalysis and (B) photoelectrocatalysis. Fig. 10 (C) Evolution of the intermediates in photoelectrocatalysis up to 30 min. (Monoazo acid yellow 17 dye, m/z 505); (C1, m/z 441); (C2, m/z 349); (C3, m/z 367); (C4, m/z 353); (C5, m/z 348); (AY17, m/z).
removed by centrifugation after the test and used for other experiments. The results prove that the film stability was relatively lower for the PEC process, although it is capable of high photodegradation and mineralization.
However, during AY17 degradation via PEC, only the intermediate C2 was not identified. The proposed structures, retention times, and main ion fragments detected in LC–MS/MS analysis of each intermediate are presented in Table 1. It should be mentioned that the proposed intermediate chemical structures were mainly based on mass to charge ratio and MS/MS fragmentation of each product [M−H]− identified. The intermediate identified as C1 (MW 442, m/z 441) was generated by desulfonation of benzenesulfonic acid in the AY17 molecule, where +SO3H was eliminated. Attack of the C1 azo group (-N = N-) by the hydroxyl radical promoted the loss of a phenol molecule, leading to the formation of a new compound designated C2 (MW 350, m/z 349). However, this intermediate was observed only in the PC process. The subsequent insertion of the hydroxyl into the C2 compound generates the intermediate C3 (MW 368, m/z 367). In PEC, C3 was
3.4. The main degradation intermediates of AY17 degradation High mineralization was observed in PEC degradation of AY17 dye. The main intermediates formed during degradation were analyzed and identified using LC–MS/MS by m/z transition detected in the negative mode after 150 min of the PC and PEC processes. The dye precursor AY17 was detected as m/z 505 ([M-2Na+1 H]−) at 6.30 min. Six main intermediates were detected during AY17 degradation via PC, with m/z 441 (C1), 349 (C2), 367 (C3), 353 (C4), 348 (C5) and 241 (C6).
Fig. 11. Proposed pathway for monoazo acid yellow 17 dye degradation in the photocatalytic and photoelectrocatalytic processes. 203
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probably generated by direct oxidation of the precursor molecule (C1 or even AY17), considering the greater efficiency of this process in generating the photoexcited electron-hole pairs (e−/h+) and consequently ٠OH radicals, which promotes faster oxidation of the complex organic molecules. The C4 product, identified as m/z 353, was probably generated after oxidation of the methyl group along with the loss of NH2NH−OH group. Meanwhile, the structure of the C5 intermediate was not completely elucidated, even after performing the MS/MS experiments. As a result, complementary measurements are ongoing to identify the related products obtained. The compound C6 (MW 242, m/z 241) was the least complex degradation product identified for both the PC and PEC processes after 180 min of reaction. The evolution of the intermediates was monitored throughout the PC and PEC experiments and is presented in Fig. 10. During the PC process, a continuous decrease in AY17 signal was observed up to 180 min of the experiment. This phenomenon is in accordance with the previous results presented in Fig. 5A. Despite the similar profiles obtained for evolution of the intermediate in both processes, dye degradation was significantly faster in the PEC process as can be observed by the complete decrease of the AY17 signal prior to 3 min of the experiment. Compared with the PC process, the evolution of intermediates throughout PEC was also significantly faster with these compounds identified during the first 30 min of the experiment. These results agree with the previous data shown in Figs. 7A and 7C, which confirmed removal of 98% of the color and 83% of TOC, respectively. Given the above considerations, identification of the degradation products of AY17 allowed us to propose a degradation pathway, which is presented in Fig. 11 below.
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4. Conclusion In summary, a highly purity hierarchical Sn3O4 flower-like nanostructure with well-controllable stoichiometry was successfully prepared using a simple microwave-assisted hydrothermal method. The as-prepared tin oxide flower-like structures were arranged into nano-sized single-crystalline leaves with smooth surfaces, which provided a high surface area. The Sn3O4 film deposited onto Ti foils attained 95% of color removal for 10 mg L−1 of AY17 solution after 60 min through the photocatalysis (PC) process at pH 2, using visible light irradiation. The results showed that the rate constant effectively increases by applying a low bias potential (0.5 V) (PEC), although this does affect film stability. Despite the lower stability, the thin Sn3O4 film is relatively stable after reusing 5 times in PC degradation and 3 times in PEC degradation, without significantly diminished photocatalytic performance. There is an optional wastewater treatment process in which PC and PEC provide high stability films and relatively low photodegradation and high photodegradation and lack of film stability, respectively. Furthermore, both processes permit a new wastewater treatment technology based on a simple, green method by utilizing visible light photocatalysts. Acknowledgements We would like to thank INCT – DATREM, FAPESP #2014/50945-4 and #2017/26219-0, CNPq #465571/2014-0, #443138/2016-8 and #150493/2017-7) for funding resources, and LMA-IQ-UNESP for providing electron microscopy facilities. This research collaboration was also supported by the PMDSU program of Kemenristekdikti, Indonesia through the PKPI-Sandwich Program 2017. References [1] R. Salehi, M. Arami, N.M. Mahmoodi, H. Bahrami, S. Khorramfar, Novel biocompatible composite (Chitosan–zinc oxide nanoparticle): preparation, characterization and dye adsorption properties, Colloids Surf. B Biointerfaces 80 (2010) 86–93.
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