Photo-induced processes on Nb2O5 synthesized by different procedures

Accepted Manuscript Title: Photo-induced processes on Nb2 O5 synthesized by different procedures Authors: C. Jaramillo-P´aez, F.J. S´anchez-Fern´andez, J.A. Nav´ıo, M.C. Hidalgo PII: DOI: Reference:

S1010-6030(18)30173-4 https://doi.org/10.1016/j.jphotochem.2018.03.040 JPC 11209

To appear in:

Journal of Photochemistry and Photobiology A: Chemistry

Received date: Revised date: Accepted date:

8-2-2018 27-3-2018 27-3-2018

Please cite this article as: C.Jaramillo-P´aez, F.J.S´anchez-Fern´andez, J.A.Nav´ıo, M.C.Hidalgo, Photo-induced processes on Nb2O5 synthesized by different procedures, Journal of Photochemistry and Photobiology A: Chemistry https://doi.org/10.1016/j.jphotochem.2018.03.040 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Photo-induced processes on Nb2O5 synthesized by different procedures C. Jaramillo-Páez1, 2, F.J. Sánchez-Fernández1, J.A. Navío1,*, M.C. Hidalgo1 1Instituto

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de Ciencia de Materiales de Sevilla, Centro Mixto Universidad de Sevilla-CSIC, Américo Vespucio 49, 41092 Sevilla, Spain 2Departamento de Química, Universidad del Tolima, Barrio Santa Elena, Ibagué, Colombia.

* Corresponding author: (J.A. Navío). E-mail address: [email protected]

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GRAPHICAL ABSTRACT

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Synthesis, structure and morphology of Nb2O5 prepared by different procedures. The effect of the preparation method of Nb2O5 influences the performance of the photocatalytic activity. The Ag+-RhB/Nb2O5 coupling increases the photocatalytic degradation of RhB by a synergistic procedure. Exploring the photocatalytic properties of Nb2O5 catalysts using a dye (RhB). Is Nb2O5 a good potential photocatalyst?

Abstract:

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The properties of Nb2O5 strongly depend on its synthesis procedure as well as the conditions of ulterior thermal treatment. We report the synthesis of Nb2O5 powders prepared by sol-gel precipitation method using niobium(V) ethoxide as precursor. Two chemical routes were chosen: the presence of tryethyl amine (TEA) as precipitant/template agent, or the oxidant peroxide method. In addition, microwave-assisted activation was also used. The as-prepared samples by the above procedures were amorphous. Structural changes upon heating from room temperature up to 800 ºC were investigated by X-ray powder diffraction technique combined with thermogravimetric analysis. The sequential thermal treatment up to 800 ºC promotes the crystallization of hexagonal phase to orthorhombic phase whereas the ulterior cooling to room temperature lead to a mixture of both phases. Samples calcined at selected temperatures of either 600 ºC or 800 ºC for 2 h, were characterized by XRD, SEM, N2-adsorption and diffuse reflectance spectroscopy (DRS). The synthetic approach routes as well as the combined microwave activation followed by ulterior thermal treatment lead to changes not only on particle size but also on the textural properties of the synthesized catalysts. The catalysts synthesized have been evaluated using Rhodamine B (RhB) as a substrate, under both UV and visible lighting conditions. None of the catalysts synthesized showed activity in the visible. Under UV-illumination conditions, some of the catalysts exhibited a relatively low photoactivity in the degradation of RhB, which is associated with a photosensitizing effect. However, the addition of Ag+ ions considerably increased the activity of all the catalysts in the degradation of RhB under UV-illumination conditions. A mechanism is proposed to explain the photo-induced processes obtained, leaving the door open to the possible implications of the observed results in relation to the interaction of RhB dye with noble metal nanoparticles such as silver.

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Keywords: Niobium(V) oxide; Ag-Nb2O5; Photocatalysis; Photoinduced processes ; Rhodamine B 1. Introduction

Niobium-containing materials have been prominent in recent decades because

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of their special applications in the high technology industries, especially in the aerospace scenario, metal super alloy and electro-electronics sectors, where they are widely used as microcapacitors [1]. In particular, niobium(V) oxide, Nb2O5, has been extensively studied due to its promising properties for different applications such as electrical [2,3], optical [4,5], catalytic [6,7] and thermal properties [8–10]. In recent years, it has been synthesized [11] in order to investigate its photocatalytic properties in exploring alternatives to TiO2, the

most widely used nanostructured semiconductor [12,13]. In fact, some niobium oxides are semiconductors [14] with energy values of band gap ranging between 3.1 to 4.0 eV [15,16] which, together with appropriate redox potential values for the valence and conduction bands, make these potential materials candidates for heterogeneous photocatalysis applications [17]. Several studies have been reported in the literature on the synthesis of niobium compounds with different structures and crystalline phases [18,19]. Such variations may

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give these materials different (photo)-catalytic activities. However, the volume of work devoted to the study of photocatalytic properties of niobium oxides is still

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low. Although there are some publications that have studied the photocatalytic

properties of this material either alone [17,20,21] or coupled with other materials [16,22–29], however doubts remain of its potential application as a photocatalyst. Doubts are established based on the wide band gap value of this

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semiconductor, in addition to the fact that, depending on the synthesis

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procedures, other factors such as structure, degree of crystallization, textural parameters, morphology, etc., could condition the photocatalytic properties of

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the material. In fact, the properties of Nb2O5 strongly depend on the synthesis

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procedure and the conditions of ulterior thermal treatment. For that reason, several researchers have been trying to change the physicochemical features of

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Nb2O5 by changing the synthesis procedure. Furthermore, Nb 2O5 presents a wide band gap, low toxicity potential good chemical and thermal stability and

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surface acidity [30]. In the present work we have explored the microwave assisted treatment in the pre-assisted template synthesis route of Nb2O5 as well as the use of H2O2 in the chemical-route of synthesis and its subsequent

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treatment assisted by microwaves. The use of microwave radiation as a heating source is a promising alternative in the synthesis of inorganic materials. When using this type of synthesis a considerable reduction of the reaction temperature

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and the processing time takes place, which causes a positive "side effect" since the growth of the particles during the reaction is generally reduced efficiently and the size of said particles. This is not only interesting in potential applications of nanometric materials due to its new functionalities, but also facilitates the study of the fundamental aspects of condensed matter physics at the nanoscale. There are generally no significant qualitative disadvantages found in microwave-synthesized materials in terms of crystallinity and physical

properties. In some cases, there are even quantitative improvements in the properties of the materials and interesting morphologies of the particles. Therefore, in this work, several types of crystalline Nb2O5 have synthesized by different chemical procedures followed by assisted microwave activation or thermal heating and selected photo-induced chemical processes of synthesized samples were investigated. For this purpose, Rhodamine B was chosen as the reference substrate, because it is a dye with a marked effect as a

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photosensitizer. In addition, there are numerous works where this dye is used to

evaluate the photocatalytic properties of different semiconductors. As a

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semiconductor solid, we have chosen Nb2O5, which is a wideband (WB) semiconductor and, as mentioned, is interested in its applications in diverse

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areas, including heterogeneous photocatalysis.

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2. Experimental

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2.1 Preparation of samples

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For the synthesis of materials, similar but not the same procedures as reported in reference [31] were followed, excepting the nature of the precipitating agent and the subsequent activation by microwave. Likewise in reference [31]

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ammonia aqueous solution was used whereas in the present work, either triethyl amine (TEA, labkem 99.5%) or aqueous H2O2 (Panreac 30% w/v)

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solutions were used as precipitating agent. The use of TEA is based on the template effect on the textural properties whereas the use of H 2O2 seems to

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have beneficial effect on the formation of oxo-hydroxo-peroxo brings like -O-NbOH-OO-Nb-, which has been demonstrate to have a marked effect on the crystallization temperature as well as on the crystal structure [32–34]. In most of the published works reporting the synthesis of Nb2O5, aqueous solutions of

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ammonia are used as a precipitating agent. However, in our work, we preferred to use triethyl amine not only as a precipitating agent but also as a template agent in order to evaluate the possibility of obtaining Nb 2O5 with different textural properties with respect to those obtained in other studies. As Nbprecursor, niobium(V) ethoxide Nb(OC2H5)5 (Alfa Aesar 99.99%), was used to develop two independent synthesis routes, as indicated in Flowchart 1. In route

A first, 10 g of niobium precursor was added to 100 mL of absolute ethanol under constant stirring, dropwise adding 10 mL of TEA leaving the system, under stirring for 2 hours, in a closed vessel with parafilm; after this process, two approximately equal amounts were separated what subsequently subjected to different treatments: one of them, was allowed to stir at 60 °C until almost evaporated, followed by continuous washing with 500 mL of a mixture of ethanol/water (50/50% V) and centrifugation and finally dried in an oven at 100

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°C overnight. The other separated portion, was subjected to a microwave

activation treatment (at 80 °C, 2 h), followed by cooling and subsequent

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evaporation at 60 °C with continued stirring; subsequently, washed, centrifuged

and dried under the same conditions as the previous sample. In route B, the samples were prepared following the same methodology as in route A, except instead of TEA in this case a solution of commercial H 2O2 was used in a molar

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ratio H2O2: Nb (OC2H5)5 = 10:1.

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The solids obtained, once dried, were subjected to characterization studies. The

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samples prepared by these procedures of route A will be denoted hereinafter as

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Nb2O5-P-(T) and Nb2O5-MW-(T), whereas the samples prepared by route B will be named as Nb2O5-H2O2-P-(T) and Nb2O5-H2O2-MW-(T) where "P" refers to the common sol-gel precipitation procedure, either by TEA ( route A) or by H2O2

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(route B), MW to microwave and T is the calcination temperature (in °C) for 2 h which are subjected to the samples, being T=30 as room temperature for the

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original samples without calcination.

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2.2 Characterization techniques X-ray diffraction (XRD) patterns were obtained on a Siemens D-501 diffractometer with Ni filter and graphite monochromator using Cu Kα radiation.

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Crystallite sizes were calculated from the line broadening of the main X-ray diffraction peaks, (001) and (100) for hexagonal phase or (001) and (180) for the orthorhombic phase by using the Scherrer equation. Peaks were fitted by using a Voigt function. Structural changes by thermal treatments of the prepared samples have been studied by XRD, using an acquisition of X-ray diffractograms obtained during

the process of continuous heating from room temperature to 800 °C and subsequent cooling to room temperature, in order to obtain information about structural changes produced after successive heating stages. For this purposes heating experiments were carried out in a HTK 1200 high-temperature chamber (Anton Paar). Experiments were performed under flowing pure air and at temperatures ranging from room temperature to 800 °C with a heating rate of 10

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°C·min−1. XRD diagrams of the samples were collected every 50 °C. Differential thermal and thermogravimetric analysis (DTA-TG) were carried out

using a high-temperature thermal analyzer Q600 SDT, at heating rate of β= 10

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ºC min-1 in pure flow air. Calcined alumina was used as inert reference material. Samples were gently packed into Pt crucibles and heated up to 800 ºC.

BET surface areas (SBET) of all samples were evaluated by N2-adsorption

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measurement with a Micromeritics ASAP 2010 instrument. Degasification of the

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samples was performed at 150 ºC for 30 min in He flow.

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The Diffuse Reflectance UV–vis Spectra (UV–vis DRS) were recorded on a Varian spectrometer model Cary 100 equipped with an integrating sphere and

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using BaSO4 as reference. Band-gaps values were calculated from the corresponding Kubelka–Munk functions, F(R∞), which are proportional to the

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absorption of radiation, by plotting (F(R∞) ×hν)1/2 against hν. The morphology for all the samples was studied by Field Scanning electron

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microscopy (FE-SEM) using a Hitachi S 4800 microscope; samples were dispersed in ethanol using an ultrasonicator and dropped on a copper grid.

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Transmission electron microscopy (TEM) was performed in a Philips CM 200 microscope. The samples for the microscopic analyzes were dispersed in

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ethanol using an ultrasonicator and dropped on a carbon grid.

2.3. Photo-assisted processes

The photo-assisted processes on the Niobium-based samples were first tested in the photocatalytic discoloration of Rhodamine B [Reagent Plus >99%] supplied by Sigma-Aldrich. From this section we will use the abbreviation of the

reagent used, that will hereafter be named along the text as RhB (Rhodamine B). "Rhodamine" is known as a group of coloring compounds derived from xanthene, toxic and water-soluble, is being used in some biotechnological applications. In particular, RhB is an amphoteric dye, although it is also classified as basic due to its positive total charge, which has a strong fluorescence. For this reason, it is a compound used in mixture with auramine as a histological technique in the detection of some organisms. Likewise, due to

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its emission of fluorescence close to 610 nm, it is also used as a laser dye, as a

bio-marker or as a water tracer. On the other hand, in the photocatalytic

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degradation processes of Rhodamine B, it has been shown that its effect (photo) -sensitizer notably improves the photocatalytic results [35,36].

Photo-assisted tests were carried out using a discontinuous batch system, this includes a 250 mL Pyrex reactor enveloped by an aluminum foil, filled with an

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aqueous suspension (100 mL) containing the single substrates (10 ppm of RhB)

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and the synthesized Nb2O5 material (1g/L).

Illumination conditions were performed, using an Osram Ultra-Vitalux lamp (300

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W) with a sun-like spectrum and a main line in the UVA range at 365 nm. UV-

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conditions were obtained by illuminated through a UV-transparent Plexiglas® top window (threshold absorption at 250 nm). The intensity of the incident UVA

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light on the solution was measured with a PMA 2200 UVA photometer (Solar Light Co.) being ca 90 W/m2 (UVA PMA2110 sensor; spectral response 320–

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400 nm). On the other hand, the intensity of light in the visible range measured in this case was 110 W/m2 (Photopic PMA2130 sensor; spectral response 400– 700 nm).

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In order to favor the adsorption–desorption equilibrium, prior to illumination the suspension was magnetically stirred for 20 min in the dark. Magnetic stirring and a constant pure oxygen flow of 20 L/h, as an oxidant, were used to produce

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a homogeneous suspension of the powder material in the solution. All photoassisted tests started at pH ca. 5.5 and the total reaction time was 120 min. Concentrations of RhB during the photo-assisted processes were analyzed by UV–visible spectroscopy, considering the main peak of dye in the visible range, located at 554 nm (for RhB). For this analysis, a UV–vis spectrometry with a Cary 100 (Varian) spectrometer was used. Aliquots (2 mL) were removed periodically during the experiments and filtered (Millipore Millex 25 0.45 mm

membrane filter) before test measurements. The initial reaction rates were calculated considering the first 15 minutes of reaction, and by using the equation r0=K C0/t were K is the reaction constant, obtained from the slope of the degradation profile graph (substrate concentration vs. reaction time), C0=starting concentration of the substrate [mg·L-1] and t is the time in seconds. We have proven that the initial concentration of the RhB-substrate do not suffer variations, either under direct photolysis or in the single presence of the

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Niobium-based materials. Double testing of selected samples ensured reproducibility of the measurements.

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Total organic carbon was followed by means of a TOC analyzer Shimadzu 500. Mineralization degrees (%) were evaluated by the TOC values upon 2 h of illumination, for all the photo-assisted processes studied, using the formula

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[1- (final TOC/initial TOC)]x100.

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3. Results and discussion

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3.1. Characterization

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To study the structural changes by thermal treatment, of the original Nb 2O5-P(30) and Nb2O5-MW-(30) samples, firstly the samples were heated from room

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temperature to 800 ºC and examined using XRD (Fig. 1A and 2A); subsequently, they were cooled from 800 ºC to room temperature and

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examined using XRD (Fig. 1B and 2B). At room temperature, both samples, Nb2O5-P-(30) and Nb2O5-MW-(30), were

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amorphous, exhibiting crystallization peaks above 550 °C. While the sample Nb2O5-P-(30) is amorphous at 550 °C and crystalline at 600 °C, the sample Nb2O5-MW-(30) is already crystalline at 550 °C. During dynamic heating, the Xray patterns shows peaks for both samples at the lowest heating temperature

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(550 °C-600 °C), which correspond to the XRD peaks of hexagonal structure for Nb2O5 (JCPDS 00-007-0061). However, dynamic heating progressively leads to a modification of the XRD peaks. At 800 °C, both samples exhibit XRD profiles that correspond, mainly, to the orthorhombic phase of Nb 2O5 (JCPDS 00-0300873).

It is interesting to note that positions of the main XRD peaks of the hexagonal and orthorhombic phases are so close to each other that it is difficult to establish differences between them. However, an amplification of the peak at 2ϴ=28º leads to establishing the differences between the two phases. Thus, for the hexagonal phase, the peak of the plane (100) appears at 28.58º while in this zone, for the orthorhombic phase, appear two peaks that correspond with the planes (180) and (200). These results are illustrated in Figures 1C for the

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sample Nb2O5-P-(T) and 2C for the sample Nb2O5-MW-(T). Another interesting

aspect to note is that the orthorhombic phase that is achieved at 800 °C, is

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stabilized while remaining during the cooling process. In fact, after cooling from 800 °C to room temperature preserves the orthorhombic phase, both for the sample Nb2O5-P-(T) and Nb2O5-MW-(T), after cooling to room temperature

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(Figures 1C and 2C respectively).

Independent portions of the as-prepared samples, Nb2O5-P-(30) and Nb2O5-

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MW-(30), were calcined for 2 h at 600 °C or 800 °C and once cooled the XRD

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results were obtained, shown in Figures 1D and 2D for each ones of the

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samples. As can be seen, the calcination ( in static conditions) at 600 °C for 2 h leads to the hexagonal phase of the Nb2O5 while the calcination at 800 °C 2 h leads to the formation of the orthorhombic phase; this result is the same for both

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samples, Nb2O5-P-(T) (Figure 1D) and Nb2O5-MW-(T) (Figure 2D). For the samples prepared in the presence of H2O2, both those obtained by

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precipitation (Nb2O5-H2O2-P) and those subjected to microwave activation treatment (Nb2O5-H2O2-MW), similar behaviors were obtained (see XRD

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profiles, Figures 3 and 4), both in heating/cooling procedures as in the calcination treatments (at 600 ºC and 800 ºC) for 2 h under static conditions. The comparative XRD results can be seen in Figures 3 and 4 for the samples

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Nb2O5-H2O2-P and Nb2O5-H2O2-MW, respectively. Figure 5 (A and B) shows DTA-TG results for the studied samples. Weight losses in TG up to 200 ºC are mainly associated to evolved water from the amorphous fresh materials. Differential thermal analysis (DTA) was used to study the system behavior during heating. The DTA curves for samples Nb2O5-P and Nb2O5-MW depicted in Fig. 5A displays two exothermic peaks centered at 298 ºC and 580 ºC. The first exothermic effect (at 298 ºC) is being

accompanied by an additional weight loss detected by TG. Those exothermic effects must be associated either to the total and fast elimination of more strongly bound water into the pores of the amorphous sample and/or to the elimination and combustion of residual species from the organometallic fragments of Nb-precursor and TEA. It is interesting to note that this effect of the first exothermic peak is not seen in the DTA/TG profiles of samples prepared in the presence of H2O2 (Figure 5B), possibly as a consequence of the

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oxidation of the organic residues by the oxidant H2O2 reactive, during the

preparation. On the other hand, the exothermic effects detected in samples

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subjected to microwave activation are less pronounced than that observed in samples not subjected to microwave activation. Possibly this effect is due to the

action of microwaves predispose to a structural pre-configuration facilitating its crystallization with a lower thermal activation. In any case, it can be concluded

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that the organic species were almost completely removed from the precipitated

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gel, either by the oxidation action of H2O2 or by heating in air above 250 ºC. This results is in accordance with the DTA/TGA and FT-IR studies reported by

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M. Ristić et al. [31].

Given the fact that X-ray diffraction results indicated that crystallization occurs in the samples above 550 °C, this allows us to assign that the exothermic peak

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detected at 550-600 °C is associated with a crystallization process. If the XRD results are compared between the samples Nb2O5-P and Nb2O5-H2O2 (Figures

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1A and 3A), it is evident that the sample not treated with H 2O2 (Nb2O5-P) is amorphous at 550 °C and crystalline at 600 °C (Figure 1A) while that prepared

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in the presence of H2O2, at the temperature of 550 °C is already crystalline (Figure 3A). If we assume this fact, it is evident that the presence of hydrogen peroxide in the synthesis medium, and foreseeable formation of hydroxoperoxos bonds, lowers the crystallization temperature, as previously mentioned

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[32–34].

Figure 6 shows the selected SEM-micrographs of the synthesized samples, named as Nb2O5-P-(T) and Nb2O5-MW-(T). In each figure, other micrographs are inserted at a higher magnification. According to the Figure 6A and 6C the samples Nb2O5-P-(600) and Nb2O5-MW-(600) mainly consist of irregular shape particles with heterogeneous size distributions; it can be found bigger particles

of average size 2.5 μm coexisting with other of smaller ones (1 μm). The calcination treatment at 800 °C 2 h, generates in the samples Nb2O5-P-(800) and Nb2O5-MW-(800) (Figures 6B and 6D) a quasi-homogeneous distribution in sizes of particles and forms presenting a high degree of sintering as can be seen in the inserted magnified figures. Figure 7 shows the selected SEM micrographs of the samples synthesized,

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designated as Nb2O5-H2O2-(T) and Nb2O5-H2O2-MW-(T). In both cases, the synthesis in the presence of H2O2 leads to samples, in which once calcined at 600 ºC, it is observed that there is a heterogeneous distribution in particle sizes

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and shapes. Whereas the big particles observed for samples, not treated in H2O2, are still present (3.0-5.0 μm) however it is observed now a highest population of small particles (0.5-1.0 μm). It is interesting to note that these

populations of smaller particles generate in the sample Nb 2O5-H2O2-(800) the

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formation of rods of ca. 1 μm in length, which are detailed in Figure 7B.

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However, the calcination treatment at 800 ºC in the sample Nb2O5-H2O2-MW-

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(800) leads to the formation of sintered quasi-spherical particle aggregates.

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All these results confirm that not only the synthesis procedure determines changes in the morphology of the samples but also the thermal treatments

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used.

The BET surface areas determined by the N2-adsorption-desorption method are

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compiled in Table 1 for samples calcined at either 600 ºC or 800 ºC for 2 h together with other physicochemical parameters. Looking at the samples

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calcined at 600 ºC 2 h, it can be seen (Table 1) that the use of H2O2 in the synthesis procedure leads to values of higher surface areas than those observed using a simple precipitation method in presence of TEA. What we can say without venturing into it, is that the chemistry of niobium is complex

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providing examples of peroxocomplexes formation in which up to 4 peroxo groups (O22-) bound to the metal [Nb(O22-]43- can be attached. Possibly, the differences in chemical nature between peroxides groups and the TEA, condition the process of nucleation and growth in some way that requires more in-depth studies. On the other hand, it is possible that the differences are associated with the so-called "solvent effect" where the differences between the dielectric constants (ε) of the medium can play a special role [37,38].

In addition, in the samples prepared by the peroxide route, an activation treatment with microwaves leads to an improvement in the value of the specific surface. Regarding the samples calcined at 800 ºC 2 h, there is evidence of a considerable loss of specific surface area (ca. 1 m 2/g), which corresponds to a drastic loss of the volume of total pores. It is interesting to note that although there are changes in the textural and

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morphological properties of the samples, however, the estimated band gap values do not undergo significant changes, being included in the range of

values estimated in other works [14–16]. The absorption band can be attributed

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to the electron transition from O 2p orbitals to Nb 4d orbitals by the following process: (Nb5+)-(O2-) + hv → (Nb4+)-(O-).

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3.2 Photo-assisted processes

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The photocatalytic properties of the samples obtained was evaluated by photodegradation kinetics from the plot of dye concentration as a function of

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exposure time under UV illumination. Figure 8 (A and B) collect the results of the photocatalytic evaluation using RhB as a probe molecule, using the samples

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synthesized in this work. Virtually all samples showed very low or negligible activity, at least in the first 30 minutes of UV illumination. However, the samples

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denominated as Nb2O5-H2O2-(600), Nb2O5-H2O2-MW-(600) and Nb2O5-H2O2MW-(800), showed a low but significant photoactivity in the UV whose evidence

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is highlighted by the arrows indicating the tendency (following the first three experimental points) of the initial reaction rates for those samples, whose values are also included in Table 1. After 30 min of UV-illumination, an amount of AgNO3, equivalent to a

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concentration of 0.01 M in the reaction system, was added to the photoreactor. This addition of Ag+ ions led to a rapid degradation of the RhB as shown in Figure 8A and 8B. This fact can be explained based on the predicted capture of photo-generated electrons by the Ag+ ions leading to a considerable decrease in the recombination of the charge carriers and to an increase in the process of degradation of the RhB through the photo-generated holes. A comparative

analysis of the results collected in Figure 8 (A and B) indicates that the set of samples calcined at 600 ºC are more effective than those calcined at 800 ºC, observing an optimum in the photocatalytic degradation of RhB for the catalyst Nb2O5-H2O2-MW-600 for which a total discoloration of the RhB (100% of conversion) is obtained in about only 20 min of UV-illumination in the presence of Ag+. For the set of samples calcined at 800 °C 2 h, the optimum is obtained for the sample Nb2O5-H2O2-MW-800 which exhibits a degree of conversion of

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the RhB of 100% after about 30 min of UV-illumination in the presence of ions Ag+. None of the samples synthesized showed activity in the visible region.

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The values of the initial reaction rates of the RhB discoloration for each

indicated catalyst with or without the addition of Ag+, are compiled in Table 1 together with the final mineralization degrees calculated from the TOC

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measurements, before and after Ag+-addition. Taking these values into account, it can be concluded that the most effective sample is Nb 2O5-H2O2-MW-600 in

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terms not only of the initial reaction rate but also in terms of the percentage of

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mineralization. However, if the comparison is made taking into account the

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values of the initial rates of degradation per unit of specific surface, it is the sample Nb2O5-H2O5-MW-800 which would be more effective in terms of considering this parameter but the mineralization percentage however is the

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lower one.

It is worth noting that the crystallite and particle sizes influence the

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photocatalytic properties of the material obtained. From one side, if the results of the crystallite size listed in Table 1 are observed, it can be inferred that the

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two samples that exhibit the best photocatalytic results (Nb2O5-H2O2-P-600 and Nb2O5-H2O2.MW-600) are precisely the ones that present values of smaller crystallite sizes and consequently higher specific surface values. On the other

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hand, the results of particle size distribution for the samples referred to above indicate that a larger population of small particles (0.5-1.0 μm) appears in the samples treated with hydrogen peroxide, which undoubtedly exerts a marked influence on the increase of specific surface. At this point, we want to indicate that we cannot establish a comparison between the photo-catalytic properties of our samples in relation to the properties of the samples obtained by M. Ristić et al [31]. The problem is that

the work referred to by M. Ristić et al [31] is a work that provides only information on the synthesis and characterization of an Nb 2O5 obtained using Nb(OC2H5)5 and ammonia. What we can establish is a comparison between the samples that in our work we denominate as Nb2O5-P-(T) and those of reference [31] named N1. The comparison is established under the consideration that in both cases the same niobium precursor but different precipitating agents are used. In this context, the profiles of the DTA/TGA curves for the indicated

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samples are very similar, except that in our case the exothermic peaks of the

sample of Nb2O5-P appear at slightly higher temperatures than in the case of

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the reference [31]. On the other hand, for our samples, the crystalline phase of

the sample Nb2O5-P-600 is the hexagonal phase (JCPDS 00-007-0061) while the sample denominated as N1-650 in the reference [31] is orthorhombic, although pseudohexagonal for the low temperature. On the other hand, the

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sizes of the crystallites are also different. This could indicate that the use of the

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precipitating agent/template exerts a certain role in the physico-chemical

A

parameters of the synthesized Nb2O5.

In any way, the photo-catalytic degradation of RhB in the presence of Ag+ is

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higher than those observed without Ag+; this is due to the enhancement in photo-degradation by trapping the photo-generated electrons by Ag+ species

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present in the medium.

In order to compare the results of the photo-catalytic evaluation of the sample

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Nb2O5-H2O2-MW-600 in the presence of Ag+ ions with those obtained with the commercial oxide TiO2 (Evonik, P25), widely used and established as reference

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photocatalyst, has been collected in Figure 9, the results of both in terms of the variations of the standardized concentrations (C/C0) vs. time of illumination (Figure 9A), and the results of the mineralization percentages of the RhB

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(Figure 9B). It can be seen that TiO2 (P25) is much more photoactive in the photo-assisted degradation of RhB than Nb2O5-H2O2-MW-600 (without adding Ag+ ions). However, the addition of Ag+ ions to the photocatalytic system of Nb2O5-H2O2-MW-600 considerably increases the activity thereof, the activities being very similar and with percentages of mineralization practically similar (Figure 9B). It should be noted that the commercial oxide TiO 2 (Evonik, P25)

has a BET specific surface area of about 50 m 2/g analogous to that of Nb2O5H2O2-MW-600 (Table 1). At the end of the photo-assisted degradation of RhB, using the optimized sample Nb2O5-H2O2-MW-600 after the addition of Ag+, the recovered solid (named as Ag/Nb2O5-H2O2-MW-600-R) was subjected to further studies. The morphology of the recovered metalized Ag/Nb2O5-H2O2-MW-600-R sample

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was performed with a TEM study. Figure 10A-B shows the crystals corresponding to the recovered sample with metallic-Ag surface deposited

particles. The respective EDS spectra for the recovered metalized Ag/Nb 2O5-

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H2O2-MW-600-R sample are shown in Figure 9C confirming the presence of Nb

and Ag elements (copper came from the grid of this sample holder). Figures 10A-B shows that this sample are formed by particles more or less ovoid or

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quasi-spherical in shape, of ca. 25 nm, with spots of metallic silver with good

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dispersion. A distribution mapping of elements (Figure 10D), revealed that there is a heterogeneous distribution of Nb and Ag in them, being the Ag deposits of

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variable size, between 2 and 8 nm in diameter.

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The analysis by XRF detected that this sample contains Ag (2.35%), Nb (68.11%) and O (29.54%), which denotes that during the photo-assisted

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process of discoloration of the RhB in the presence of Ag+ ions, the simultaneous photo-deposition of metallic silver has been produced. Likewise,

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in the X-ray diffractograms of the Ag/Nb2O5-H2O2-MW-600-R sample, metallic silver peaks were detected (data not shown).

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First at all, since there are several publications [28,29] reporting that doping with metals greatly improved the photo-catalytic efficiency of metal oxide photocatalysts, we explored the re-use of the recovered Ag/Nb2O5-H2O2-MW-600-R, on the photo-assisted discoloration of RhB under UV-illumination conditions,

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using this metallized sample. The results are reported in Figure 11 showing the kinetic profile of the first round compared to the initial experiment by adding Ag+ cations. Our re-use results allowed us to observe that during the consecutive cycles the initial rate of degradation of RhB decreased considerably, achieving values of conversions of 100% but with increasingly higher lighting periods in each consecutive cycle. For example, for the initial process a 100% conversion

is achieved in only 20 min of UV-illumination after the addition of Ag+; however, using the material recovered (Ag/Nb2O5-H2O2-MW-600-R), a UV-illumination time of more than 80 min is needed to achieve a 100% conversion (Cycle 1). In addition, in the successive cycles, it was observed the appearance of yellowgreenish colors with a visible luminescence in the liquid medium. This fact indicates that not only a progressive deactivation of the material must take place but also subsequent photo-induced processes. The deactivation process

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during the consecutive cycles of reuse, as well as the qualitatively observed luminiscense could be associated with the formation of complexes between the

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fragments of RhB and silver, since there is evidence of the formation of such complexes [39,40].

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3.3 Trying to understand the photo-assisted processes. A mechanism proposal

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A mechanism proposal can be established from the results obtained in the

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photo-assisted processes. Thus, it is evident that the samples obtained by the peroxidic route, Nb2O5-H2O2-P-600 and Nb2O5-H2O2-MW-600, exhibit a photo-

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catalytic activity in the degradation of RhB under UV-illumination conditions (Figure 9 A) with values of initial degradation rates of 0.85 x 10 -3 mg·L-1·s-1 and

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1.42 x 10-3 mg·L-1·s-1 respectively (Table 1). After the addition of Ag+ ions the photo-catalytic activity of the samples considerably increased giving superior

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initial degradation rates, and percentages of mineralization higher than 60%, which would indicate that the resulting degradation process is a discoloration

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due to the elimination of the chromophore group of the RhB. To initiate a surface photo-induced process photons must been absorbed either by the adsorbed molecule and/or by the solid surface itself. In the absence of Ag+ ions, the photo-induced process for discoloration of RhB would be

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associated with a mechanism of photo-sensitization of the dye itself, as illustrated in Scheme 1. In this case (absence of Ag+ ions) the photo-excitation of RhB to RhB* would lead to an injection of electrons in the valence layer of the semiconductor Nb2O5, which would lead to the formation of O2- species that in successive stages would generate hydroxyl radicals .OH that would facilitate the degradation of RhB. This process would be limited by the capacity of adsorption

of the RhB on the photocatalyst, which in turn would be limited by the values of the specific surfaces. In fact, it is the samples Nb2O5-H2O2-P-600 and Nb2O5H2O2-MW-600, which present higher values of specific surfaces of the full series of synthesized samples (Table 1). In order to rule out the photosensitizing effect of the dye used (RhB) we have carried out photocatalytic tests using phenol as a colorless organic

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substrate using the two most active samples of our work, Nb2O5-H2O2-P and Nb2O5-H2O2-MW, in the same experimental conditions. The initial

reaction rates obtained are 0.3x10-3 mg·L-1·s-1 and 0.4x10-3 mg·L-1·s-1

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respectively. These values are lower (about one third) than those obtained

for these samples when using RhB (ranging 0.85-1.42x10-3 mg·L-1·s-1) and in principle suggest a photosensitizing effect of the dye.

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On the other hand, if one introduces Ag+ ions into the solution, they work as

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(indirect)-oxidizing reagents. Thus, in the presence of Ag+ ions, a transfer of

act

as

electron

capping

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photogenerated electrons to the Ag+ species would be established that would agents,

thus

decreasing

the

hole-electron

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recombination and facilitating the degradation of RhB with the .OH radicals generated by hole-captures as illustrated in Scheme 1. Some authors suggest

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the use of scavengers to elucidate mechanisms in photocatalytic processes. In fact, in this sense, the addition of AgNO3 is already a scavenger. In this context,

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we have found striking at work [41], the presence of AgNO3 lead to a decrease in the percentage of degradation of MB (under visible illumination) with respect

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to the value obtained in the absence of AgNO3. However, in our case the addition of AgNO3 remarkably increases the photo-discoloration of RhB (Figure 8). Consequently, in our case, the mechanism of photocatalytic degradation of RhB under UV illumination requires the capture of electrons by an electron

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capping agent that facilitates oxidation. The re-use of the metallized sample, Ag/Nb2O5-H2O2-MW-600-R, would lead to a relative improvement in the degradation (discoloration) of the RhB (Figure 11), foreseeably as a consequence that the incorporation of metallic silver would act as sink of photo-generated electrons decreasing electron-hole recombination. However, the fact that the metallized photocatalyst, Ag/Nb 2O5-H2O2-MW-600-R,

loses photo-catalytic activity in successive cycles could indicate that for some reason, deactivation of the metallized sample is being generated. In principle, one could think of the successive loss of the silver incorporated in the first cycle. In order to deepen in this matter, a series of experiences have been made, through UV-Visible absorption spectroscopy. In a first experiment, the absorption spectrum was measured in the UV-vis of an aqueous solution of

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RhB in a concentration of 10 ppm, bubbling either oxygen or nitrogen in the dark, for 30 min; in neither case, significant variations in the maximum peak of

RhB absorption were observed after 30 min in the dark. For each case (oxygen

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or nitrogen atmospheres) the solutions were then illuminated in the UV, recording the UV-vis absorption spectrum at different time intervals for 80 min.

The results of these experiments are shown in figures inserted in Figure 12. As

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can be seen, the original spectrum of the aqueous solution of RhB undergoes no change in the dark under either oxygen or nitrogen flow or during UV-

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illumination after long periods of illumination, which confirms the photochemical

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stability of this dye and validates the non-photolysis. On the other hand, this

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result indicates that the catalyst under oxygen bubbling develops a photocatalytic action in the discoloration of the RhB, although very low (as shown in the results of Figure 8). In a second series of experiments, two independent

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portion of aqueous solutions of RhB were maintained under oxygen or nitrogen flow in the dark and then the same amount of Ag+ ions were added in

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concentration 0.01 M in the reaction medium, registering after 20 min in the dark and continuous flow of oxygen or nitrogen, the corresponding UV-vis

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spectrum, which are shown in Figure 12A (for oxygen bubbling) and 12B (for nitrogen bubbling). As can be seen, the addition of Ag+ ions to the medium leads to the appearance of a new absorption band below 250 nm, while the maximum absorption of RhB at 554 nm remains unchanged; below 250 nm, an

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absorption peak appears at 213 nm with a shoulder at 236 nm ascribed to the absorption spectrum of an aqueous solution of AgNO 3 (blank). Then, and in these same conditions, the solutions were illuminated in the UV recording the variations that occur in the UV-vis absorption spectrum. As can be seen in Figures 12A and 12B, the band observed below 250 nm remains practically unchanged while the absorption peak at 554 nm characteristic of the RhB

chromophore progressively decreases, as the UV illumination time increases, and the contribution of the band at 521 nm is becoming more evident. This fact could indicate that some species are established between the RhB and Ag+ which seems to be photo-active to chromophore breaking; this interaction could arises by a Lewis donor-acceptor bonding. In fact as indicated in the references [39,40] the photoluminescence and photocatalytic (RhB/Rhodamine fragmentssilver(I) complexes are evidenced and could be the cause of the deactivation of

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the metallized photo-catalyst after successive cycles of re-use. All these fact seems to indicate that, as we have previously postulated, there is an Ag +-RhB

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interaction that is sensitive to the action of ultraviolet light. It could be that in these conditions there is a transfer of the electrons from the excited state of RhB to Ag+ that would lead to the breakdown of the chromophore group of RhB. In any case, these processes are much slower in the discoloration of the RhB

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than those observed, under the same conditions (for instance bubbling oxygen),

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in the presence of the photocatalyst (Figure 8). These observations could have

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potential applications in systems such as those reported in reference [42,43].

Conclusions

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In this work some aspects of photo-induced processes on the surface of synthesized Nb2O5 have been reported.

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Considering the above results, it can be concluded that the synthesized photocatalysts have a low photocatalytic activity in the UV which can be

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associated to the fact of a high recombination of the carriers of photogenerated charges. In fact, the addition of Ag+ ions to the medium leads to a marked improvement in photocatalytic results due to the effect of Ag+ ions as electron

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capture agents.

Our blank experiments in the absence of photocatalyst indicate that a certain interaction between RhB and Ag+ cations is established which under UV light contributes to the breakage of the chromophore group of RhB. In any case, from the results of comparative photocatalytic activity, it can be deduced that the sample Nb2O5-H2O2-MW-600 was the most photoactive,

although if the comparative results of initial reaction rates per unit area are established, it is the sample Nb2O5-H2O2-MW-800 the most photoactive. From this, it can be concluded that the combined effect of a synthesis by peroxidic route followed by microwave activation, has positive effects on the photocatalytic properties of synthesized Nb2O5 in spite of that a wide band gap persist.

due to the high band gap value of this type of material.

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Under conditions of visible illumination, not evidence of photoactivity was found,

Finally, we must conclude that although Nb2O5 has been postulated as a good

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candidate in heterogeneous photocatalysis, nevertheless our results, indicate that a high value of the band gap, relatively low specific surfaces (in some cases) and a high recombination of charge carriers, as well as mixing phases,

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limit its use as photocatalyst against other commercial ones, such as TiO 2

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(Evonik, P25). In our opinion our results, beyond a practical application in the scenario of heterogeneous photocatalysis, could be valuable into the photo-

A

induced processes on the Nb2O5 surface particularly when a photo-sensitizer

Acknowledgement

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dye (as RhB) and noble metal nanoparticles are used.

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This work was supported by research fund from Project Ref. CTQ2015-64664C2-2-P (MINECO/FEDER UE). Research services of CITIUS University of

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Seville are also acknowledged. We thank the University of Tolima for economic

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support in the studies commission of César Augusto Jaramillo Páez.

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I N U SC R A M ED PT CC E A Flowchart 1. Schematic representation of the synthesis procedures

B)

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Figure 1. XRD patterns of the sample Nb2O5-P-(T). (A) X-ray diffraction diagrams collected at different temperatures ranging from room temperature to 800ºC and (B) subsequent cooling from 800ºC to room temperature; (C) selected and augmented zone (diring dynamic heating); (D) XRD patterns of the indicated catalysts calcined at the indicated temperatures 2 h (under static conditions).

B)

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Figure 2. XRD patterns of the sample Nb2O5-MW-(T). (A) X-ray diffraction diagrams collected at different temperatures ranging from room temperature to 800ºC and (B) subsequent cooling from 800ºC to room temperature; (C) selected and augmented zone (diring dynamic heating); (D) XRD patterns of the indicated catalysts calcined at the indicated temperatures 2 h (under static conditions).

B)

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Figure 3. XRD patterns of the sample Nb2O5-H2O2-(T). (A) X-ray diffraction diagrams collected at different temperatures ranging from room temperature to 800ºC and (B) subsequent cooling from 800ºC to room temperature; (C) selected and augmented zone (diring dynamic heating); (D) XRD patterns of the indicated catalysts calcined at the indicated temperatures 2 h (under static conditions).

B)

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Figure 4. XRD patterns of the sample Nb2O5-H2O2-MW-(T). (A) X-ray diffraction diagrams collected at different temperatures ranging from room temperature to 800ºC and (B) subsequent cooling from 800ºC to room temperature; (C) selected and augmented zone (diring dynamic heating); (D) XRD patterns of the indicated catalysts calcined at the indicated temperatures 2 h (under static conditions).

IP T SC R U N A M ED PT CC E A Figure 5. DTA-TG curves of the indicated samples.

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Figure 6. Selected micrographs of SEM of the synthesized samples, named as Nb2O5P-(T) and Nb2O5-MW-(T), calcined at the indicated temperatures 2 h. Details of the SEM micrographs are inserted at a higher magnification.

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Figure 7. Selected micrographs of SEM of the synthesized samples, named as Nb2O5H2O2-(T) and Nb2O5-H2O2-MW-(T), calcined at the indicated temperatures 2 h. Details of the SEM micrographs are inserted at a higher magnification.

IP T SC R U N A M ED PT CC E A Figure 8. Results of the evaluation of the photo-induced processes of the discoloration of the RhB with the indicated materials, calcined at (A) 600 ºC 2h and (B) 800 ºC 2 h, before and after de addition of Ag+.

IP T SC R U N A M ED PT CC E A Figure 9. Comparison of the results of the photo-assisted activity in the degradation of RhB with the indicated materials. (A) kinetic profiles an (B) Mineralization percentages.

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Figure 10. TEM-EDS study for the sample of Ag/Nb2O5-H2O2-MW-600-R

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Figure 11. Results of the reuse studies of the recovered Ag/Nb2O5-H2O2-MW-600-R, in the photo-assisted discoloration of the RhB under UV illumination conditions.

IP T SC R U N A M ED PT CC E A

Figure 12. (A) UV-vis spectra of an aqueous solution of AgNO3, RhB in the dark under continuous bubbling of O2 and RhB under UV illumination upon addition of Ag+; (B) UVvis spectra of an aqueous solution of AgNO3, RhB in the dark under continuous bubbling of N2 and RhB under UV illumination upon addition of Ag+.

I N U SC R

Table 1. Physico-chemical parameters, initial reaction rate and percentage of mineralization obtained with the indicated samples.

Nb2O5-MW-600

213.6

40.8

CC E

Nb2O5-H2O2-MW-600

001 (ort.)

Total pore volume *10-2 (cm3g-1)

Band Gap (eV)

180 (ort.)

Initial rate *10-3 (mg·L-1·s-1) Without adding Ag+

By adding Ag+

Mineralization (%) [1-(Final TOC/Initial TOC)]*100

4.8

2.04

3.47

0.65

7.02

45.2

168.8

36.6

2.8

1.26

3.54

0.43

7.12

50.6

31.7

15.2

40.5

11.29

3.44

0.85

6.97

61.2

15.4

51.4

16.66

3.48

1.42

9.77

77.0

PT

Nb2O5-H2O2-P-600

100 (hex.)

ED

Nb2O5-P-600

001 (hex.)

SBET (m2 g-1)

A

Crystallite size (nm)

M

Sample

30.1

165.4

66.6

≤1

0.75

0.48

6.03

45.4

Nb2O5-MW-800

180.1

75.2

≤1

0.39

0

5.93

50.1

Nb2O5-H2O2-P-800

94.2

75.2

≤1

0.58

0.25

6.93

54.5

Nb2O5-H2O2-MW-800

106.6

103.7

3.34

0.25

7.82

44.5

A

Nb2O5-P-800

3.8

IP T SC R U N A M

A

CC E

PT

ED

Scheme 1. A proposal of the photo-induced degradation mechanism of RhB in the presence of Nb2O5 and under lighting conditions in the UV (in the presence of Ag+).