V2O5 powders modified with different theoretical silver contents (1, 5, 10, 15, and 20 wt% as Ag2O) were obtained with acicular morphologies observed by scanning electron microscopy (SEM). Shcherbinaite crystalline phase is transformed into the Ag0.33V2O5 crystalline one with the incorporation and increase in silver content as was suggested by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analysis. With further increase in silver contents the Ag2O phase appears. Catalysts were active in photocatalytic degradation of malachite green dye under simulated solar light, which is one of the most remarkable facts of this work. It was found that V2O5-20Ag was the most active catalytic formulation and its activity was attributed to the mixture of coupled semiconductors that promotes the slight decrease in the rate of the electron-hole pair recombination.
In recent years, wastewaters from domestic and industrial uses have contributed to the environmental problem because they arrive to the soil and aquifers mantles polluting clean water. In order to address this issue, the effort of many researchers from several scientific disciplines around the world [
V2O5 powders from Fermont were treated by the surfactant assisted technique. This technique uses a micellar solution prepared using polyoxyethylene (23) lauryl ether (C12E23, Brij L23 30% w/v solution from Sigma Aldrich), dibutyl ether ([CH3(CH2)3]2O, DBE-reagent plus ≥99% from Sigma Aldrich), and AgNO3 (ACS Reagent, ≥99.0% from Sigma Aldrich). To obtain V2O5 photocatalyst, the commercial V2O5 was added to the micellar solution previously prepared with 30% Brij L23, 10% DBE, and 60% of water, solvothermally pressurized in a reactor at 60°C for 12 hours, with slow stirring. The obtained solids were filtered, washed with deionized water, and then thermally treated, raising temperature from room temperature to 400°C, at a heating rate of 5°C/min; afterwards, the temperature was isothermally maintained at 400°C for 3 hours in an air convection oven to eliminate the surfactant Brij L23; this sample was called hereinafter V2O5. The Ag-modified V2O5 photocatalysts were prepared by following the same synthesis procedure. Thus, the required amounts of V2O5 and AgNO3 precursors were added to the micellar solution of Brij L23 to obtain a theoretical content of 1, 5, 10, 15, and 20 wt.% of Ag2O. Photocatalysts will be referred hereinafter as V2O5-
The temperature of surfactant elimination to obtain the photocatalytic formulation was determined from thermogravimetry and differential scanning calorimeter analysis (TGA-DSC), performed by using a Netzsch, STA 449 F3 Jupiter equipment. The weight loss and the heat flow during decomposition were measured in flowing air (20 mL/min) and heating from room temperature to 600°C at a heating rate of 3°C/min. Molecular structure was studied through the infrared spectra (IR) acquired in a Bruker tensor 27 IR spectrometer with ATR accessory. Surface morphology was observed with scanning electron microscopy (SEM) by using a JEOL JSM 6510 LV microscope; additionally, compositional analysis was carried out with an acceleration voltage of 15 kV, using an EDS probe coupled to the same microscope. Raman spectra (RS) were acquired using an HR LabRam 800 system equipped with an Olympus BX40 confocal microscope; a Nd:YAG laser beam (532 nm) was focused by a 100x objective onto the sample surface. A cooled CCD camera was used to record the spectra, usually averaged for 100 accumulations of 10 seconds in order to improve the signal-noise ratio. All spectra were calibrated using the 521 cm−1 line of monocrystalline silicon. X-ray diffraction (XRD) patterns were obtained with a X’Pert PRO MPD Philips diffractometer (PANanalytical), using monochromatic CuK
The photocatalytic activity of the V2O5-
From TGA-DSC curves shown in Figure
TGA-DSC curves of V2O5 catalyst precursor.
The morphology of the V2O5-
Comparison of atomic percent of silver in V2O5-
Ag content | Ag content | |
---|---|---|
V2O5 | — | — |
V2O5-1Ag | 1.5 | 1.1 |
V2O5-5Ag | 2.9 | 1.3 |
V2O5-10Ag | 4.5 | 4.3 |
V2O5-15Ag | 4.5 | 5.1 |
V2O5-20Ag | 6.8 | 8.1 |
Scanning electron microscopy images at 3000x of (a) V2O5, (b) V2O5-5Ag, (c) V2O5-10Ag, and (d) V2O5-20Ag photocatalysts.
Textural properties, such as the specific surface area and total pore volume, were studied using the N2 physisorption technique. This was done to estimate the effect of the changes observed in the morphology of the V2O5-
Specific surface area (
Sample | | |
---|---|---|
| 3.3 | 0.005 |
| 3.4 | 0.010 |
V2O5-1Ag | 5.3 | 0.020 |
V2O5-5Ag | 11.0 | 0.023 |
V2O5-10Ag | 10.6 | 0.030 |
V2O5-15Ag | 11.9 | 0.025 |
V2O5-20Ag | 14.6 | 0.026 |
N2 adsorption-desorption isotherms of (a) V2O5, (b) V2O5-1Ag, (c) V2O5-5Ag, (d) V2O5-10Ag, (e) V2O5-15Ag, and (f) V2O5-20Ag.
Figure
XRD diffraction patterns of (a) V2O5, (b) V2O5-1Ag, (c) V2O5-5Ag, (d) V2O5-10Ag, (e) V2O5-15Ag, and (f) V2O5-20Ag photocatalysts.
To corroborate the microcrystalline structure observed by XRD, Raman spectra of unmodified and Ag-modified V2O5 photocatalysts were recorded. It can be seen in the spectra shown in Figure
Raman spectra of samples (a) V2O5, (b) V2O5-1Ag, (c) V2O5-5Ag, (d) V2O5-10Ag, (e) V2O5-15Ag, and (f) V2O5-20Ag.
Infrared spectra were recorded with two main purposes: firstly, to corroborate the surfactant elimination from the photocatalytic preparation, as can be seen in Figure
Infrared spectra of commercial V2O5 (a), V2O5 obtained through the surfactant assisted technique before calcination (b), and synthesized V2O5 after calcination (c).
IR spectra of (a) V2O5, (b) V2O5-1Ag, (c) V2O5-5Ag, (d) V2O5-10Ag, (e) V2O5-15Ag, and (f) V2O5-20Ag photocatalysts.
Figure
Elemental atomic contents for the V2O5-
Core level spectra of (a) V
It is well known that pentavalent vanadium has no d electrons and hence d-d transitions are not possible. Therefore, the observed bands in the electronic absorption spectrum are ascribed to charge transfer bands. The reflectance spectrum was processed with the Kubelka-Munk function and the optical band gap energy (
Band gap energy (
Catalyst | | Wavelength (nm) |
---|---|---|
V2O5 | 2.3 | 564 |
V2O5-1Ag | 2.2 | 566 |
V2O5-5Ag | 1.6 | 765 |
V2O5-10Ag | 1.4 | 867 |
V2O5-15Ag | 1.2 | 1000 |
V2O5-20Ag | 1.2 | 1033 |
Figure
Photoluminescence spectra of (a) commercial V2O5, (b) synthesized V2O5, and (c) V2O5-20Ag.
Photocatalytic activity was evaluated in the degradation of the malachite green dye. Figure
Kinetic rate constant (
Photocatalyst | % of degradation (UV-Vis) | % of degradation (TOC) | |
---|---|---|---|
V2O5 | 80.4 | 69.9 | |
V2O5-1Ag | 52.8 | 67.5 | |
V2O5-5Ag | 56.3 | 54.3 | |
V2O5-10Ag | 56.9 | 56.0 | |
V2O5-15Ag | 59.7 | 73.9 | |
V2O5-20Ag | 84.2 | 74.4 | |
Photocatalytic degradation of malachite green dye during the first 180 minutes of reaction time (a) without catalyst (photolysis) and the photocatalytic process using (b) commercial V2O5, (c) synthesized V2O5, (d) V2O5-1Ag, (e) V2O5-5Ag, (f) V2O5-10Ag, (g) V2O5-15Ag, and (h) V2O5-20Ag photocatalysts.
Ag-modified photocatalysts were obtained in an easy way with acicular morphologies and enhanced specific surface areas. The changes in morphology and textural properties are associated with the appearance of new phases such as Ag0.33V2O5 as well as Ag2O coexisting at higher Ag loads as was suggested by XRD and XPS. These new phases have a strong influence in the photocatalytic response of these systems. It was observed that Ag incorporation into the photocatalytic formulation narrows the band gap energy making these materials photoactive under sunlight, a natural and cheaper irradiation source. The obtained photocatalytic formulations are conformed by a mixture of crystalline phases that work as coupled semiconductors. The V2O5-20Ag catalyst was the most active for the MG degradation using simulated sunlight.
The authors declare that they have no competing interests.
The authors thank CONACyT for the financial support through CB-168827 and CB-240998 projects and CB-239648 project and also thank the academic staff of CCIQS Alejandra Núñez, Lizbeth Triana, Citlalit Martínez, and Dr. Uvaldo Hernández Balderas. E. Rodriguez Castellon and A. Infantes thank the financial support of the CTQ2015-68951-C3-3-R project (Ministerio de Economía y Competitividad) and FEDER funds.