# H2Ti3O7 Nanotubes Decorated with Silver Nanoparticles for Photocatalytic Degradation of Atenolol

Authors: Mariana Hinojosa-Reyes; Roberto Camposeco-Solís; Facundo Ruiz; Nereyda Niño Martínez; Vicente Rodríguez González; M. E. Compeán-Jasso
Journal: Journal of Nanomaterials
 (2017)

---

## Abstract

The photocatalytic degradation/adsorption process of theβ-blocker atenolol (ATL) under UV irradiation is described using two types of silver decorated catalysts: silver/titania and silver/titanates. The silver ions were reduced on the surface of TiO2-P25-Degussa using gallic acid. Silver/titanates were prepared by a microwave-assisted hydrothermal method using the silver/titania as the starting material to obtain the hydrogen titanate (H2Ti3O7) structure with tubular morphology. These materials were characterized by X-ray diffraction, UV-Vis spectroscopy, N2 physisorption, temperature programmed reduction, TEM, and FTIR spectroscopy. During the photocatalytic process, the ATL molecules were completely converted to amino-diol byproducts. It is the first time that these materials have been applied during the photocatalytic process in the degradation of pharmaceuticals products. The success of the silver nanoparticles (2 nm) consists of the homogeneous distribution over the surface of titanate nanotubes inhibiting the hole/electron recombination promoting the oxidation process. The Ag@H2Ti3O7 with a concentration of silver as 1.0% shows the highest adsorption/degradation of ATL than the Ag@TiO2 and the P25-Degussa. The great performance in the reuse test consists in the strong attachment of the silver nanoparticles on the titanium surface that inhibits the silver lixiviation during the photocatalytic tests.


---

## Body

## 1. Introduction

Titanium dioxide is a material widely used due to the stability of its chemical structure, low toxicity, low cost, and physicochemical properties [1]. Due to these properties, it has been applied in anticorrosion problems, self-cleaning coatings, paints for solar cells, and catalysis applications [2]. Tubular structures have received particular interest since the carbon nanotubes were discovered; they are very interesting structures because of their high surface/volume ratios and they are size-dependent; these properties are enhanced when TiO2 nanotubes are doped with other elements or functionalizing their surface with metallic nanoparticles.The TiO2 nanotubes are a semiconductor with a large surface area with respect to TiO2 crystals, which have been applied in dye-sensitized solar cells, water photoelectrolysis, photo-degradation of hazardous materials, and catalysis, among other applications [3]. Powdered titanium dioxide has been efficiently used to eliminate a large variety of organic pollutants present in water; among the catalysis applications it has been applied during the photocatalytic degradation of organic compounds; recent studies have reported the removal of dyes by combining adsorption and photocatalysis processes [4]. Emerging Contaminants (EC) are present in a wide range of pharmaceuticals and personal care products (PPCPs) among which are cosmetics, antimicrobials, antibiotics, analgesics, antidepressants, hypertensives (β-blockers), and many other chemicals that are widely used on a daily basis for various purposes [5]. Within the hypertensives β-blockers we select the β1 receptor antagonist, atenolol (ATL), due to it having been used primarily for the treatment of cardiovascular diseases for more than thirty years. Due to its extensive usage and limited human metabolism, ATL was widely detected in sewage effluents and surface waters, usually with a concentration ranging from ngL−1 to μgL−1. Previous researchers have demonstrated that ordinary wastewater treatments cannot remove ATL effectively. Furthermore, many studies have provided evidence that ATL could inhibit the growth of human embryonic cells and was ecotoxic to freshwater species. Therefore, it is essential to develop advanced treatment technologies for ensuring effective elimination of ATL in wastewaters before releasing into natural waters [6]. Treatment plants are not able to degrade residues of this nature, and, as a result, they are introduced into the aquatic environment [7]; over time these chemical residues cause many affectations on aquatic organisms and generate human diseases.An alternative technology and cost-effective solution to deal with this kind of contaminants is the use of the advanced oxidation processes (AOPs) that are based in the hydroxyl radicals able to oxidize pollutants into harmless end-products, which could result in a full mineralization (CO2, H2O, and inorganic ions) [8].In this work, we report for the first time the photocatalytic degradation/adsorption of atenolol in aqueous phase using TiO2 nanotubes (H2Ti3O7) modified with silver nanoparticles. Ag/titanium materials have particular attention for their low cost and the superior properties of silver [9]. On the other hand, the main advantage of TiO2 nanotubes is their high surface area that enhances the photocatalytic/adsorption processes; at the same time, the silver presence inhibits the hole/electron recombination that enhances the photoactivity regarding the well-known P25-Degussa.


## 2. Experimental Section

### 2.1. Catalysts Preparation

The Ag@TiO2 catalysts were prepared by dissolving 2.5 g of TiO2-P25-Degussa in 250 mL of deionized water and sonicated for 15 min; then AgNO3 (99.3%, Fermont) 1 × 10-3M was added to reach three different concentrations: 0.5, 1.0, and 2.5% wt; then gallic acid (97.5–102.5%, Sigma-Aldrich) 6 × 10−4 M was added. The pH of the reaction media was adjusted to 9 by adding NH4OH, and the obtained sludge was kept under magnetic stirring for 1 hour. HNO3 was incorporated to neutralize the solution; four hours later, two phases were formed (liquid-solid); the solid phase was removed and dried for four hours at 80°C.The synthesis of Ag@TiO2 nanotubes was carried out by a microwave-assisted method (Eyela MWO-1000 Wave Magic). 1.5 g of the previously obtained samples (Ag@TiO2) was placed into a 150 mL Teflon vessel with 60 mL of NaOH 10 N. The reaction mixture was placed in a microwave tube and microwaved for 4 hours at a power output of 195 W and a continuous stirring of 600 rpm. The temperature reached was 140°C. The solid was washed with distilled water until the pH was neutralized and finally the solid was dried at 100°C for 2 hours.Samples were identified as Ag@TiO2-X for the case of silver/titania and Ag@H2Ti3O7-X for the case silver/titanates;X indicates the silver concentration in % wt.


### 2.2. Characterization Techniques

The Ag@TiO2 and Ag@H2Ti3O7 catalysts were characterized by X-ray diffraction in order to identify the crystalline phases using an X-ray diffractometer SmartLab RIGAKU with CuKα radiation (1.5404 Å). Diffuse reflectance UV-Vis spectra of the photocatalysts were obtained using a Cary 5000 (UV-Vis-NIR) spectrophotometer; spectralon Teflon (from Agilent) was used as a reference blank and the band gap energy was determined by using the Kubelka-Munk method. The physical adsorption of N2 at −196°C was carried out using a NOVA 3200e, Surface Area & Pore Size Analyzer by Quantachrome Instruments. Samples were previously outgassed at 200°C during 14 hours. The Brunauer-Emmett-Teller method (BET method) was used to calculate the specific surface area. Temperature Programmed Reduction (TPR) was performed in a ChemBET TPR/TPD chemisorption analyzer by Quantachrome Instruments using an interval of temperature ranging from Room Temperature (RT) to 600°C with a rate of 10°C min−1 and flow of 30 mL min−1 of H2 (10%)/Ar gas. Morphology of the Ag catalysts was obtained by Scanning Electronic Microcopy using a JEOL JSM-6510 microscope at an accelerating voltage of 30 kV.Infrared (IR) spectra were obtained using a Shimadzu IRTracer-100 spectrophotometer equipped with a Praying Mantis for DRIFT spectroscopy and a low/high-temperature reaction chamber by Harrick. In each experiment, approximately 25 mg of dried sample was packed in the sample holder. IR measurements were carried out, under a constant flow of N2 (30 mL/min), from RT to 600°C in steps of 100°C.Diffuse reflectance UV-Vis spectra of the catalysts were obtained using a CARY 5000 (UV-VIS-NIR) spectrophotometer equipped with a Praying Mantis and a high-temperature reaction chamber (Harrick) and it was recorded during the activation thermal treatment, from 25°C and every 50°C until reaching 600°C; in each experiment, approximately 25 mg of the dried sample was packed in the sample holder under constant N2 flow (30 mL min−1).The samples were examined by Transmission Electron Microscopy (TEM) in a Tecnai FEI 300 operated at 300 kV. The samples were suspended in isopropanol and then sonicated for 5 min. Finally, the samples were mounted in a Cu TEM grid. The particle size distribution histograms for the catalysts were obtained from the measurements of about 200 particles. The average particle diameter (ds) was calculated using the following formula: ds=∑nidi/∑ni, where ni is the number of particles of diameter di.


### 2.3. Photocatalytic Evaluation

A standard solution of atenolol (>98%, Sigma) with 10 ppm was used to evaluate the photocatalytic activity of the Ag photocatalysts. Photocatalytic experiments were carried out in triplicate in a reactor with 200 mL of ATL at 10 ppm in vigorous stirring under 90 minutes of dark adsorption; after that, theUV365nm irradiation was turned on. The full time of reaction with irradiation was 210 minutes. Aliquots were taken since the start of the reaction and subsequently every 30 minutes; then they were filtered with a Nylon membrane of 0.45 μm by Pall.Firstly, the catalyst concentration was optimized using the following: 175, 250, and 375 mg L−1, corresponding to 35, 50, and 75 mg of catalyst into 200 mL of contaminant solution. The monitoring of the reaction consists in the measurement of the intensity band at 223 nm characteristic of the ATL molecule using an UV-Vis Ocean Optics USB4000 spectrophotometer. To monitor the Total Organic Carbon (TOC), samples were taken from the reaction at beginning, after adsorption and the end of the photocatalytic reaction, using a TOC-VCPN Total Organic Carbon Analyzer by Shimadzu. The Ag lixiviation during the photocatalytic reactions was measured by using an atomic absorption spectrophotometer AA 7000 by Shimadzu. A cyclic test was carried out with the Ag@H2Ti3O7 1.0 catalyst to evaluate the reusability and stability of the material.


## 2.1. Catalysts Preparation

The Ag@TiO2 catalysts were prepared by dissolving 2.5 g of TiO2-P25-Degussa in 250 mL of deionized water and sonicated for 15 min; then AgNO3 (99.3%, Fermont) 1 × 10-3M was added to reach three different concentrations: 0.5, 1.0, and 2.5% wt; then gallic acid (97.5–102.5%, Sigma-Aldrich) 6 × 10−4 M was added. The pH of the reaction media was adjusted to 9 by adding NH4OH, and the obtained sludge was kept under magnetic stirring for 1 hour. HNO3 was incorporated to neutralize the solution; four hours later, two phases were formed (liquid-solid); the solid phase was removed and dried for four hours at 80°C.The synthesis of Ag@TiO2 nanotubes was carried out by a microwave-assisted method (Eyela MWO-1000 Wave Magic). 1.5 g of the previously obtained samples (Ag@TiO2) was placed into a 150 mL Teflon vessel with 60 mL of NaOH 10 N. The reaction mixture was placed in a microwave tube and microwaved for 4 hours at a power output of 195 W and a continuous stirring of 600 rpm. The temperature reached was 140°C. The solid was washed with distilled water until the pH was neutralized and finally the solid was dried at 100°C for 2 hours.Samples were identified as Ag@TiO2-X for the case of silver/titania and Ag@H2Ti3O7-X for the case silver/titanates;X indicates the silver concentration in % wt.


## 2.2. Characterization Techniques

The Ag@TiO2 and Ag@H2Ti3O7 catalysts were characterized by X-ray diffraction in order to identify the crystalline phases using an X-ray diffractometer SmartLab RIGAKU with CuKα radiation (1.5404 Å). Diffuse reflectance UV-Vis spectra of the photocatalysts were obtained using a Cary 5000 (UV-Vis-NIR) spectrophotometer; spectralon Teflon (from Agilent) was used as a reference blank and the band gap energy was determined by using the Kubelka-Munk method. The physical adsorption of N2 at −196°C was carried out using a NOVA 3200e, Surface Area & Pore Size Analyzer by Quantachrome Instruments. Samples were previously outgassed at 200°C during 14 hours. The Brunauer-Emmett-Teller method (BET method) was used to calculate the specific surface area. Temperature Programmed Reduction (TPR) was performed in a ChemBET TPR/TPD chemisorption analyzer by Quantachrome Instruments using an interval of temperature ranging from Room Temperature (RT) to 600°C with a rate of 10°C min−1 and flow of 30 mL min−1 of H2 (10%)/Ar gas. Morphology of the Ag catalysts was obtained by Scanning Electronic Microcopy using a JEOL JSM-6510 microscope at an accelerating voltage of 30 kV.Infrared (IR) spectra were obtained using a Shimadzu IRTracer-100 spectrophotometer equipped with a Praying Mantis for DRIFT spectroscopy and a low/high-temperature reaction chamber by Harrick. In each experiment, approximately 25 mg of dried sample was packed in the sample holder. IR measurements were carried out, under a constant flow of N2 (30 mL/min), from RT to 600°C in steps of 100°C.Diffuse reflectance UV-Vis spectra of the catalysts were obtained using a CARY 5000 (UV-VIS-NIR) spectrophotometer equipped with a Praying Mantis and a high-temperature reaction chamber (Harrick) and it was recorded during the activation thermal treatment, from 25°C and every 50°C until reaching 600°C; in each experiment, approximately 25 mg of the dried sample was packed in the sample holder under constant N2 flow (30 mL min−1).The samples were examined by Transmission Electron Microscopy (TEM) in a Tecnai FEI 300 operated at 300 kV. The samples were suspended in isopropanol and then sonicated for 5 min. Finally, the samples were mounted in a Cu TEM grid. The particle size distribution histograms for the catalysts were obtained from the measurements of about 200 particles. The average particle diameter (ds) was calculated using the following formula: ds=∑nidi/∑ni, where ni is the number of particles of diameter di.


## 2.3. Photocatalytic Evaluation

A standard solution of atenolol (>98%, Sigma) with 10 ppm was used to evaluate the photocatalytic activity of the Ag photocatalysts. Photocatalytic experiments were carried out in triplicate in a reactor with 200 mL of ATL at 10 ppm in vigorous stirring under 90 minutes of dark adsorption; after that, theUV365nm irradiation was turned on. The full time of reaction with irradiation was 210 minutes. Aliquots were taken since the start of the reaction and subsequently every 30 minutes; then they were filtered with a Nylon membrane of 0.45 μm by Pall.Firstly, the catalyst concentration was optimized using the following: 175, 250, and 375 mg L−1, corresponding to 35, 50, and 75 mg of catalyst into 200 mL of contaminant solution. The monitoring of the reaction consists in the measurement of the intensity band at 223 nm characteristic of the ATL molecule using an UV-Vis Ocean Optics USB4000 spectrophotometer. To monitor the Total Organic Carbon (TOC), samples were taken from the reaction at beginning, after adsorption and the end of the photocatalytic reaction, using a TOC-VCPN Total Organic Carbon Analyzer by Shimadzu. The Ag lixiviation during the photocatalytic reactions was measured by using an atomic absorption spectrophotometer AA 7000 by Shimadzu. A cyclic test was carried out with the Ag@H2Ti3O7 1.0 catalyst to evaluate the reusability and stability of the material.


## 3. Results and Discussion

### 3.1. Physicochemical Properties

#### 3.1.1. X-Ray Diffraction

The X-ray diffractograms are shown in ‎Figure1. At the top the Ag@TiO2 1.0 is presented; it is possible to see only the pattern corresponding to P25-Degussa with the presence of anatase (JCPDS 04-002-2678) and rutile (JCPDS 70-7347) crystalline phases. At the bottom, the Ag@H2Ti3O7 diffractograms are presented. We can observe the total conversion from anatase/rutile to H2Ti3O7 (JCPDS 47-0651) titanate nanotubes [10] after microwave-assisted method was used [11]. In the same way, the presence of peaks related to metallic silver or silver oxides is not present; this could be due to a low silver concentration or the homogeneous silver nanoparticles dispersion. From ‎Figure 1 at the bottom, in the case of Ag@H2Ti3O7 1.0 sample, it is evident that the peak corresponding to the (020) planes is less intense than the other samples; it is due to a less concentration of sodium ions. These sodium ions come from the basic conditions (NaOH). This change may arise due to the complete exchange of sodium ions by protons after extensive washing with acid [12].Figure 1
X-ray diffractograms of the silver catalysts. At the top is of the P25-Degussa and at the bottom is of the Ag@H2Ti3O7.


#### 3.1.2. UV-Vis Spectroscopy

The UV-Vis spectra of the Ag catalysts are shown in ‎Figure2. Figure 2(a) corresponds to materials synthesized by reduction method. In the first instance, it is observed the silver surface plasmon resonance (SPR) which is shifted to higher wavelength according to the silver concentration from 0.5, 1.0 and 2.5% wt. corresponding to 500, 480, and 459 nm, respectively; also, a higher intensity of the SPR band was detected in the sample with higher silver content. It has been reported a value of 480 nm for Ag nanoparticles and its value decreases for small nanoparticles [13, 14]. Figure 2(b) corresponds to titanate nanotubes; in this case, the SPR is less evident. This situation could be attributed to silver nanoparticles that are found into nanotube and a lower concentration is on the surface of its structure.Figure 2
UV-Vis spectra of the silver catalysts of (a) Ag@TiO2 and (b) Ag@H2Ti3O7.
(a)
(b)Band gap values of the catalysts were determined by using the Kubelka-Munk method; these values are reported in Table1. For the case of Ag@TiO2 materials, the band gap energy (Eg) has values of 2.48, 2.41, and 2.25 eV; it is possible to observe that Eg is inversely proportional to silver concentration. For the case of samples prepared by reduction followed by hydrothermal method, the band Eg shows values in the range of 2.7 to 2.8 eV. It is important to mention that Eg of the P25 is notably affected by the silver concentration since P25-Degussa has a well-known value of 3.2 eV [15].Table 1
Physicochemical properties of Ag materials.
Sample
Surface area (m2 g−1)
E
g (eV)
Crystalline phases
Ag@TiO2 0.5
52.1
2.48
Anatase, rutile
Ag@TiO2 1.0
51.4
2.41
Anatase, rutile
Ag@TiO2 2.5
52.9
2.25
Anatase, rutile
Ag@H2Ti3O7 0.5
287.1
2.76
H2Ti3O7
Ag@ H2Ti3O7 1.0
297.1
2.87
H2Ti3O7
Ag@ H2Ti3O7 2.5
188.2
2.73
H2Ti3O7An additional UV-Vis-NIR DRS with temperature increasing experiment was carried out to elucidate the behavior of the Ag@H2Ti3O7 sample at different temperatures in the range of 25 to 400°C; see ‎Figure 3. In this context, a dramatic change in the behavior in the range of temperatures 200 to 300°C is appreciated; around these temperatures, collapsing of the nanotube structure happens [16]. In addition, from a temperature of 25°C, a surface resonance plasmon at 450 nm corresponding to silver nanoparticles is observed; as the temperature increases, this band increases its intensity and moves to higher wavelengths. According to the inset of ‎Figure 3 is corroborated the presence of the Ag nanoparticles inside the nanotube structure, and according to temperature increases, these Ag nanoparticles get out to the surface. Therefore always the SPR is present in all the temperatures range; see inset of ‎Figure 3.Figure 3
UV-Vis-NIR DRS collected spectra for the Ag@H2Ti3O7 1.0 sample with temperature increasing from 25 to 400°C. Inset corresponding to the SPR of the AgNPs from RT to 200°C.


#### 3.1.3. Nitrogen Physisorption

The nitrogen physisorption isotherms are shown in ‎Figure4. The Ag@H2Ti3O7 samples present high surface area values: 287, 297, and 188 m2 g−1 for Ag concentrations of 0.5, 1.0, and 2.5% wt, respectively (see ‎Table 1). The Ag@TiO2 1.0 presents the highest surface area. According to IUPAC classification, nanotubes have type IV isotherms and H3 hysteresis loop [17]. This kind of hysteresis is associated with aggregates of plate-like particles giving rise to slit-shaped pores [17]. Pore size distribution, of the three samples, was analyzed using the Barrett–Joyner–Halenda (BJH) method; see insets of ‎Figure 4. It is important to notice that the Ag@H2Ti3O7 1.0 sample presents two pore size distributions, one centered at around 4 nm and another one at around 4 nm. The other samples, with a silver concentration of 0.5 and 2.5, have the pore size distribution centered at approximately 4 nm. For the Ag@H2Ti3O7 1.0 sample, we found it has the less concentration of sodium ions (after washing); this fact could explain why this sample has a bimodal pore size distribution. We think that the 4 nm pores correspond to the inside diameters of titanate nanotubes and the other larger pores (8 nm) are the space between the nanotubes in the nanotubes-bundles [18].Figure 4
Adsorption-desorption nitrogen isotherms of the Ag@H2Ti3O7 catalysts. Insets show the size pore distribution.For comparison the Ag@TiO2 materials also were measured and have values in order of 52 m2g−1 (see ‎Table 1), almost the same value corresponding to well-known bare TiO2-P25-Degussa [18].


#### 3.1.4. Temperature Programmed Reduction

The thermograms corresponding to the temperature reduction of the silver photocatalysts prepared by hydrothermal assisted method are shown in ‎Figure5. Specifically for the high silver concentration sample (Ag@H2Ti3O7 2.5), the thermograms have a well-defined peak that begins at 149° and finishes at 190°C with a maximum at 170°C; this peak corresponds to the reduction of Ag1+ to Ag0. For the sample Ag@H2Ti3O7 0.5, this peak is not so evident, but it still presents a band between 150° and 350°C, which also corresponds to the reduction of silver, but due to the low silver concentration and its interaction with titanium support, it is shifted to higher temperatures. The Ag@TiO2 2.5 result is presented as a reference and it shows a peak around 225 and 400°C that also corresponds to the reduction of Ag1+ to Ag0. At temperatures above of 400°C, it is possible to observe how the reduction of the support starts (Ti4+ to Ti3+) [19]. It is important to mention that the titanium reduction temperature varies according to the nanoparticles concentration [20]; at the higher temperature, it is possible to observe a lower interaction with silver nanoparticles.Figure 5
Temperature programmed reduction graph of the silver-titanium catalysts.In ‎Figure6(a) is shown an STEM-HAADF image of the Ag@H2Ti3O7 1.0 sample. In this image, we could observe a well-defined morphology of the nanotubes-H2Ti3O7 structure. This image corroborates the homogeneous silver deposition on the nanotube structure. Silver is found forming spherical nanoparticles (white dots), homogeneously distributed in space and size. The Ag@TiO2 material consists in agglomerates of spherical nanometric particles of TiO2 (P25-Degussa). Once the Ag@TiO2 samples were treated by the hydrothermal method, there is a complete conversion from spherical nanoparticles to large nanotubes corresponding to H2Ti3O7 stoichiometry according to XRD information.Figure 6
(a) STEM-HAADF image for Ag@H2Ti3O7 1.0 sample. Frequency histograms for (b) Ag nanoparticles size and (c) nanotube diameter size.
(a)
(b)
(c)Frequency histograms of size and diameter are reported for Ag nanoparticles and nanotubes; see ‎Figures6(b) and 6(c); where the Ag nanoparticles average size is 2.23 nm; therefore, the reduction method guarantees homogeneous silver size dispersion. According to the frequency histogram for diameter nanotubes, the average diameter is 6.6 nm.According to literature, for a first time is reported the least size of Ag nanoparticles supported on TiO2 nanotubes, since the nanoparticles size reported is on the order of 5 or higher than 10 nm [21–23]. With respect to the measurements of nanotubes, we obtained a lesser diameter size than what was reported in the literature, which is an average of 8–10 [21], 10–20 [22], and 60–70 nm [23]. These nanotubes applied firstly in the photocatalytic degradation of dye or model molecules in this research are successfully used during the photocatalytic adsorption/degradation of the pharmaceutical atenolol.The hydroxyl groups present in the H2Ti3O7 surface are shown in ‎Figure 7. The part (a) corresponds to nanotube decorated with silver and shows two bands located at 3653 and 3715 cm−1, while for the bare H2Ti3O7 nanotube two bands are shown located at 3658 and 3717 cm−1; see ‎Figure 7 part (b). These bands are assigned to hydrogen bonded to adjacent OH groups in terminal positions due to the stretching modes of Ti-OH [24, 25]. By comparison, the H2Ti3O7 bands have a major intensity regarding the Ag@H2Ti3O7 material; this is directly associated with Ag nanoparticles that are anchored over these OH groups.Figure 7
FTIR spectra of (a) Ag@H2Ti3O7 1.0 and (b) H2Ti3O7.
(a)
(b)According to structural and surface properties of Ag-nanotubes, these have been applied successfully during the photocatalytic degradation/adsorption of atenolol.


### 3.2. Photocatalytic Results

The previously synthesized Ag@H2Ti3O7 nanotubes were tested during the photocatalytic degradation of ATL. First, an optimization of the catalyst was made using the concentrations of 175, 250, and 375 mg L−1. Results indicate that optimal load is 250 or 375 mg L−1, but for practical purposes, it was chosen a concentration of 250 mg L−1. With this load is reached a final degradation of 80%, which corresponds to the same adsorbed quantity during the dark conditions; see ‎Figure 8(a). The evaluation of the rest of materials with this load indicates a low degradation/adsorption of 50% for Ag@H2Ti3O7 0.5 and 10% for Ag@H2Ti3O7 2.5; see ‎Figure 8(b). The best photocatalytic degradation is due to the Ag@H2Ti3O7 1.0 sample and the catalyst with bigger pore diameter and with the highest surface area. As already mentioned, the highest surface area is correlated with minor sodium ions concentration presents in the H2Ti3O7 structure. Therefore, a minor sodium ions concentration represents an enhancement during the adsorption/degradation of the ATL molecule.Figure 8
(a) Optimization of the silver load of photocatalyst using Ag@H2Ti3O7 1.0., (b) 50 mg of the photocatalyst, and (c) comparative behavior regarding TiO2 references and photolysis for the 200 mL of ATL degradation at 10 ppm and pH normal.
(a)
(b)
(c)For comparative purposes, ‎Figure8(c) shows the behavior of the P25-Degussato compare with the Ag@TiO2 1.0, Ag@H2Ti3O7 1.0, and it respective photolysis. Photolysis shows a negligible degradation. The P25-Degussa and Ag@TiO2 1.0 show a minimal adsorption under dark conditions of around 3%. Once the UV light is turned on, the Ag@TiO2 1.0 shows a final degradation of 20%, while P25-Degussa is around 30% of the ATL molecule. Although it is well known that silver nanoparticles avoid the hole-electron recombination, in this case, the commercial TiO2 shows a greater behavior than the silver modified catalyst; this is due to gallic acid precedent from the synthesis that was not removed and could cover the active sites of the TiO2. At the same time the Ag@H2Ti3O7 1.0 shows 80% of adsorption and once the ATL is adsorbed onto the nanotube surface is followed the formation of the respective byproducts.


#### 3.2.1. TOC Analysis

The TOC analysis for the degradation of ATL molecule using the nanotubes decorated with silver reveals a maximum mineralization of 50% for the Ag@H2Ti3O7 1.0 catalyst, 25% for the Ag@H2Ti3O7 0.5, and less than 5% for the Ag@H2Ti3O7 2.5 material (‎Figure 9). According to UV-Vis spectroscopy measurements that indicate a maximum degradation of 80%, for the Ag@H2Ti3O7 1.0 catalyst, only a 50% of the ATL is mineralized into CO2 and water; the remaining 30% is converted into a simple molecule, the byproduct amino-diol [26, 27]. The photocatalytic process has a high efficiency when there is a higher percentage of adsorption. Thus, the novelty of the silver decorated nanotubes synthesized in this research has a good performance during the degradation of atenolol molecule.Figure 9
TOC (mg/L) measurements during the photocatalytic degradation of ATL using the Ag@H2Ti3O7 catalysts at pH 7.


#### 3.2.2. Silver Lixiviation Test

The stability of the Ag@H2Ti3O7 catalysts was evaluated through its lixiviation by atomic absorption; aliquots were taken at the end of the photocatalytic reaction (4 hours). ‎Table 2 reveals the concentration of Ag (ppm) and the corresponding percentage. The aliquot taken for the Ag@H2Ti3O7 0.5 sample shows a final Ag concentration of 0.0053 ppm equivalent to 0.4% of full lixiviation. The Ag@H2Ti3O7 1.0 sample shows a full lixiviation of 1.2% and the Ag@H2Ti3O7 2.5 shows a full lixiviation of 6.4%. The obtained percentages are negligible and, with this result, we can affirm that hydrothermal assisted method guarantees catalysts with silver nanoparticles strongly attached to the TiO2 nanotubes.Table 2
Silver lixiviation test during the photocatalytic degradation of ATL at 10 ppm and pH normal.
Sample
Ag concentration (ppm)
Lixiviation (%)
Ag@H2Ti3O7 0.5
0.0053
0.4
Ag@H2Ti3O7 1.0
0.0282
1.2
Ag@H2Ti3O7 2.5
0.0805
6.4


#### 3.2.3. Reusability Test

The behavior of the ATL degradation using the Ag@H2Ti3O7 1.0 can be seen in ‎Figure 10. After 30 min of adsorption during dark conditions, a decrease of the UV band is observed occurring at 225 nm and 276 nm. After that, a minimal degradation is observed at the end of adsorption process; during the 60 and 90 minutes of photocatalytic degradation an additional band at 248 nm is observed; this band is assigned to the byproducts formed during the photocatalytic process and according to literature corresponds to the formation of an amino-diol [26, 27]. Amino-diol is a byproduct of ATL fragmentation due to the participation of hydroxyl radicals (OH∙) and irradiation UV effect during the photocatalytic degradation reaction. It is important to mention that an amino-diol is the simplest and less contaminant molecule compared to initial ATL.Figure 10
Reusability test (three cycles) during the photocatalytic degradation of ATL using the Ag@H2Ti3O7 1.0 catalyst.To corroborate the stability of the most active Ag@H2Ti3O7 titanate, a reusability test of three cycles was made (see ‎Figure 10). In the first cycle is observed an immediate adsorption around 50% and we can see the characteristic ATL band at 225 nm. After the dark absorption process and when the UV lamp is turned on, the 225 nm remains almost constant until the first 90 min of photocatalytic reaction and it starts to appear a shoulder at 248 nm corresponding to amino-diol byproduct.Once the first cycle was completed, the catalyst was recovered, dried, and used again in a second cycle (‎Figure10). In this graph, a lower adsorption is observed around 20%, and immediately after that the lamp turns on; it starts the appearance of shoulder corresponding to the byproducts. In the third cycle, the initial dark adsorption is negligible.Therefore, with this experiment could be verified the stability and reusability of the Ag@H2Ti3O7 1.0 catalyst. According to that when the catalyst is reused, the ATL conversion to amino-diol is accelerated. This is a good result due to amino-diol being simpler and less contaminant than ATL initial molecule.


## 3.1. Physicochemical Properties

### 3.1.1. X-Ray Diffraction

The X-ray diffractograms are shown in ‎Figure1. At the top the Ag@TiO2 1.0 is presented; it is possible to see only the pattern corresponding to P25-Degussa with the presence of anatase (JCPDS 04-002-2678) and rutile (JCPDS 70-7347) crystalline phases. At the bottom, the Ag@H2Ti3O7 diffractograms are presented. We can observe the total conversion from anatase/rutile to H2Ti3O7 (JCPDS 47-0651) titanate nanotubes [10] after microwave-assisted method was used [11]. In the same way, the presence of peaks related to metallic silver or silver oxides is not present; this could be due to a low silver concentration or the homogeneous silver nanoparticles dispersion. From ‎Figure 1 at the bottom, in the case of Ag@H2Ti3O7 1.0 sample, it is evident that the peak corresponding to the (020) planes is less intense than the other samples; it is due to a less concentration of sodium ions. These sodium ions come from the basic conditions (NaOH). This change may arise due to the complete exchange of sodium ions by protons after extensive washing with acid [12].Figure 1
X-ray diffractograms of the silver catalysts. At the top is of the P25-Degussa and at the bottom is of the Ag@H2Ti3O7.


### 3.1.2. UV-Vis Spectroscopy

The UV-Vis spectra of the Ag catalysts are shown in ‎Figure2. Figure 2(a) corresponds to materials synthesized by reduction method. In the first instance, it is observed the silver surface plasmon resonance (SPR) which is shifted to higher wavelength according to the silver concentration from 0.5, 1.0 and 2.5% wt. corresponding to 500, 480, and 459 nm, respectively; also, a higher intensity of the SPR band was detected in the sample with higher silver content. It has been reported a value of 480 nm for Ag nanoparticles and its value decreases for small nanoparticles [13, 14]. Figure 2(b) corresponds to titanate nanotubes; in this case, the SPR is less evident. This situation could be attributed to silver nanoparticles that are found into nanotube and a lower concentration is on the surface of its structure.Figure 2
UV-Vis spectra of the silver catalysts of (a) Ag@TiO2 and (b) Ag@H2Ti3O7.
(a)
(b)Band gap values of the catalysts were determined by using the Kubelka-Munk method; these values are reported in Table1. For the case of Ag@TiO2 materials, the band gap energy (Eg) has values of 2.48, 2.41, and 2.25 eV; it is possible to observe that Eg is inversely proportional to silver concentration. For the case of samples prepared by reduction followed by hydrothermal method, the band Eg shows values in the range of 2.7 to 2.8 eV. It is important to mention that Eg of the P25 is notably affected by the silver concentration since P25-Degussa has a well-known value of 3.2 eV [15].Table 1
Physicochemical properties of Ag materials.
Sample
Surface area (m2 g−1)
E
g (eV)
Crystalline phases
Ag@TiO2 0.5
52.1
2.48
Anatase, rutile
Ag@TiO2 1.0
51.4
2.41
Anatase, rutile
Ag@TiO2 2.5
52.9
2.25
Anatase, rutile
Ag@H2Ti3O7 0.5
287.1
2.76
H2Ti3O7
Ag@ H2Ti3O7 1.0
297.1
2.87
H2Ti3O7
Ag@ H2Ti3O7 2.5
188.2
2.73
H2Ti3O7An additional UV-Vis-NIR DRS with temperature increasing experiment was carried out to elucidate the behavior of the Ag@H2Ti3O7 sample at different temperatures in the range of 25 to 400°C; see ‎Figure 3. In this context, a dramatic change in the behavior in the range of temperatures 200 to 300°C is appreciated; around these temperatures, collapsing of the nanotube structure happens [16]. In addition, from a temperature of 25°C, a surface resonance plasmon at 450 nm corresponding to silver nanoparticles is observed; as the temperature increases, this band increases its intensity and moves to higher wavelengths. According to the inset of ‎Figure 3 is corroborated the presence of the Ag nanoparticles inside the nanotube structure, and according to temperature increases, these Ag nanoparticles get out to the surface. Therefore always the SPR is present in all the temperatures range; see inset of ‎Figure 3.Figure 3
UV-Vis-NIR DRS collected spectra for the Ag@H2Ti3O7 1.0 sample with temperature increasing from 25 to 400°C. Inset corresponding to the SPR of the AgNPs from RT to 200°C.


### 3.1.3. Nitrogen Physisorption

The nitrogen physisorption isotherms are shown in ‎Figure4. The Ag@H2Ti3O7 samples present high surface area values: 287, 297, and 188 m2 g−1 for Ag concentrations of 0.5, 1.0, and 2.5% wt, respectively (see ‎Table 1). The Ag@TiO2 1.0 presents the highest surface area. According to IUPAC classification, nanotubes have type IV isotherms and H3 hysteresis loop [17]. This kind of hysteresis is associated with aggregates of plate-like particles giving rise to slit-shaped pores [17]. Pore size distribution, of the three samples, was analyzed using the Barrett–Joyner–Halenda (BJH) method; see insets of ‎Figure 4. It is important to notice that the Ag@H2Ti3O7 1.0 sample presents two pore size distributions, one centered at around 4 nm and another one at around 4 nm. The other samples, with a silver concentration of 0.5 and 2.5, have the pore size distribution centered at approximately 4 nm. For the Ag@H2Ti3O7 1.0 sample, we found it has the less concentration of sodium ions (after washing); this fact could explain why this sample has a bimodal pore size distribution. We think that the 4 nm pores correspond to the inside diameters of titanate nanotubes and the other larger pores (8 nm) are the space between the nanotubes in the nanotubes-bundles [18].Figure 4
Adsorption-desorption nitrogen isotherms of the Ag@H2Ti3O7 catalysts. Insets show the size pore distribution.For comparison the Ag@TiO2 materials also were measured and have values in order of 52 m2g−1 (see ‎Table 1), almost the same value corresponding to well-known bare TiO2-P25-Degussa [18].


### 3.1.4. Temperature Programmed Reduction

The thermograms corresponding to the temperature reduction of the silver photocatalysts prepared by hydrothermal assisted method are shown in ‎Figure5. Specifically for the high silver concentration sample (Ag@H2Ti3O7 2.5), the thermograms have a well-defined peak that begins at 149° and finishes at 190°C with a maximum at 170°C; this peak corresponds to the reduction of Ag1+ to Ag0. For the sample Ag@H2Ti3O7 0.5, this peak is not so evident, but it still presents a band between 150° and 350°C, which also corresponds to the reduction of silver, but due to the low silver concentration and its interaction with titanium support, it is shifted to higher temperatures. The Ag@TiO2 2.5 result is presented as a reference and it shows a peak around 225 and 400°C that also corresponds to the reduction of Ag1+ to Ag0. At temperatures above of 400°C, it is possible to observe how the reduction of the support starts (Ti4+ to Ti3+) [19]. It is important to mention that the titanium reduction temperature varies according to the nanoparticles concentration [20]; at the higher temperature, it is possible to observe a lower interaction with silver nanoparticles.Figure 5
Temperature programmed reduction graph of the silver-titanium catalysts.In ‎Figure6(a) is shown an STEM-HAADF image of the Ag@H2Ti3O7 1.0 sample. In this image, we could observe a well-defined morphology of the nanotubes-H2Ti3O7 structure. This image corroborates the homogeneous silver deposition on the nanotube structure. Silver is found forming spherical nanoparticles (white dots), homogeneously distributed in space and size. The Ag@TiO2 material consists in agglomerates of spherical nanometric particles of TiO2 (P25-Degussa). Once the Ag@TiO2 samples were treated by the hydrothermal method, there is a complete conversion from spherical nanoparticles to large nanotubes corresponding to H2Ti3O7 stoichiometry according to XRD information.Figure 6
(a) STEM-HAADF image for Ag@H2Ti3O7 1.0 sample. Frequency histograms for (b) Ag nanoparticles size and (c) nanotube diameter size.
(a)
(b)
(c)Frequency histograms of size and diameter are reported for Ag nanoparticles and nanotubes; see ‎Figures6(b) and 6(c); where the Ag nanoparticles average size is 2.23 nm; therefore, the reduction method guarantees homogeneous silver size dispersion. According to the frequency histogram for diameter nanotubes, the average diameter is 6.6 nm.According to literature, for a first time is reported the least size of Ag nanoparticles supported on TiO2 nanotubes, since the nanoparticles size reported is on the order of 5 or higher than 10 nm [21–23]. With respect to the measurements of nanotubes, we obtained a lesser diameter size than what was reported in the literature, which is an average of 8–10 [21], 10–20 [22], and 60–70 nm [23]. These nanotubes applied firstly in the photocatalytic degradation of dye or model molecules in this research are successfully used during the photocatalytic adsorption/degradation of the pharmaceutical atenolol.The hydroxyl groups present in the H2Ti3O7 surface are shown in ‎Figure 7. The part (a) corresponds to nanotube decorated with silver and shows two bands located at 3653 and 3715 cm−1, while for the bare H2Ti3O7 nanotube two bands are shown located at 3658 and 3717 cm−1; see ‎Figure 7 part (b). These bands are assigned to hydrogen bonded to adjacent OH groups in terminal positions due to the stretching modes of Ti-OH [24, 25]. By comparison, the H2Ti3O7 bands have a major intensity regarding the Ag@H2Ti3O7 material; this is directly associated with Ag nanoparticles that are anchored over these OH groups.Figure 7
FTIR spectra of (a) Ag@H2Ti3O7 1.0 and (b) H2Ti3O7.
(a)
(b)According to structural and surface properties of Ag-nanotubes, these have been applied successfully during the photocatalytic degradation/adsorption of atenolol.


## 3.1.1. X-Ray Diffraction

The X-ray diffractograms are shown in ‎Figure1. At the top the Ag@TiO2 1.0 is presented; it is possible to see only the pattern corresponding to P25-Degussa with the presence of anatase (JCPDS 04-002-2678) and rutile (JCPDS 70-7347) crystalline phases. At the bottom, the Ag@H2Ti3O7 diffractograms are presented. We can observe the total conversion from anatase/rutile to H2Ti3O7 (JCPDS 47-0651) titanate nanotubes [10] after microwave-assisted method was used [11]. In the same way, the presence of peaks related to metallic silver or silver oxides is not present; this could be due to a low silver concentration or the homogeneous silver nanoparticles dispersion. From ‎Figure 1 at the bottom, in the case of Ag@H2Ti3O7 1.0 sample, it is evident that the peak corresponding to the (020) planes is less intense than the other samples; it is due to a less concentration of sodium ions. These sodium ions come from the basic conditions (NaOH). This change may arise due to the complete exchange of sodium ions by protons after extensive washing with acid [12].Figure 1
X-ray diffractograms of the silver catalysts. At the top is of the P25-Degussa and at the bottom is of the Ag@H2Ti3O7.


## 3.1.2. UV-Vis Spectroscopy

The UV-Vis spectra of the Ag catalysts are shown in ‎Figure2. Figure 2(a) corresponds to materials synthesized by reduction method. In the first instance, it is observed the silver surface plasmon resonance (SPR) which is shifted to higher wavelength according to the silver concentration from 0.5, 1.0 and 2.5% wt. corresponding to 500, 480, and 459 nm, respectively; also, a higher intensity of the SPR band was detected in the sample with higher silver content. It has been reported a value of 480 nm for Ag nanoparticles and its value decreases for small nanoparticles [13, 14]. Figure 2(b) corresponds to titanate nanotubes; in this case, the SPR is less evident. This situation could be attributed to silver nanoparticles that are found into nanotube and a lower concentration is on the surface of its structure.Figure 2
UV-Vis spectra of the silver catalysts of (a) Ag@TiO2 and (b) Ag@H2Ti3O7.
(a)
(b)Band gap values of the catalysts were determined by using the Kubelka-Munk method; these values are reported in Table1. For the case of Ag@TiO2 materials, the band gap energy (Eg) has values of 2.48, 2.41, and 2.25 eV; it is possible to observe that Eg is inversely proportional to silver concentration. For the case of samples prepared by reduction followed by hydrothermal method, the band Eg shows values in the range of 2.7 to 2.8 eV. It is important to mention that Eg of the P25 is notably affected by the silver concentration since P25-Degussa has a well-known value of 3.2 eV [15].Table 1
Physicochemical properties of Ag materials.
Sample
Surface area (m2 g−1)
E
g (eV)
Crystalline phases
Ag@TiO2 0.5
52.1
2.48
Anatase, rutile
Ag@TiO2 1.0
51.4
2.41
Anatase, rutile
Ag@TiO2 2.5
52.9
2.25
Anatase, rutile
Ag@H2Ti3O7 0.5
287.1
2.76
H2Ti3O7
Ag@ H2Ti3O7 1.0
297.1
2.87
H2Ti3O7
Ag@ H2Ti3O7 2.5
188.2
2.73
H2Ti3O7An additional UV-Vis-NIR DRS with temperature increasing experiment was carried out to elucidate the behavior of the Ag@H2Ti3O7 sample at different temperatures in the range of 25 to 400°C; see ‎Figure 3. In this context, a dramatic change in the behavior in the range of temperatures 200 to 300°C is appreciated; around these temperatures, collapsing of the nanotube structure happens [16]. In addition, from a temperature of 25°C, a surface resonance plasmon at 450 nm corresponding to silver nanoparticles is observed; as the temperature increases, this band increases its intensity and moves to higher wavelengths. According to the inset of ‎Figure 3 is corroborated the presence of the Ag nanoparticles inside the nanotube structure, and according to temperature increases, these Ag nanoparticles get out to the surface. Therefore always the SPR is present in all the temperatures range; see inset of ‎Figure 3.Figure 3
UV-Vis-NIR DRS collected spectra for the Ag@H2Ti3O7 1.0 sample with temperature increasing from 25 to 400°C. Inset corresponding to the SPR of the AgNPs from RT to 200°C.


## 3.1.3. Nitrogen Physisorption

The nitrogen physisorption isotherms are shown in ‎Figure4. The Ag@H2Ti3O7 samples present high surface area values: 287, 297, and 188 m2 g−1 for Ag concentrations of 0.5, 1.0, and 2.5% wt, respectively (see ‎Table 1). The Ag@TiO2 1.0 presents the highest surface area. According to IUPAC classification, nanotubes have type IV isotherms and H3 hysteresis loop [17]. This kind of hysteresis is associated with aggregates of plate-like particles giving rise to slit-shaped pores [17]. Pore size distribution, of the three samples, was analyzed using the Barrett–Joyner–Halenda (BJH) method; see insets of ‎Figure 4. It is important to notice that the Ag@H2Ti3O7 1.0 sample presents two pore size distributions, one centered at around 4 nm and another one at around 4 nm. The other samples, with a silver concentration of 0.5 and 2.5, have the pore size distribution centered at approximately 4 nm. For the Ag@H2Ti3O7 1.0 sample, we found it has the less concentration of sodium ions (after washing); this fact could explain why this sample has a bimodal pore size distribution. We think that the 4 nm pores correspond to the inside diameters of titanate nanotubes and the other larger pores (8 nm) are the space between the nanotubes in the nanotubes-bundles [18].Figure 4
Adsorption-desorption nitrogen isotherms of the Ag@H2Ti3O7 catalysts. Insets show the size pore distribution.For comparison the Ag@TiO2 materials also were measured and have values in order of 52 m2g−1 (see ‎Table 1), almost the same value corresponding to well-known bare TiO2-P25-Degussa [18].


## 3.1.4. Temperature Programmed Reduction

The thermograms corresponding to the temperature reduction of the silver photocatalysts prepared by hydrothermal assisted method are shown in ‎Figure5. Specifically for the high silver concentration sample (Ag@H2Ti3O7 2.5), the thermograms have a well-defined peak that begins at 149° and finishes at 190°C with a maximum at 170°C; this peak corresponds to the reduction of Ag1+ to Ag0. For the sample Ag@H2Ti3O7 0.5, this peak is not so evident, but it still presents a band between 150° and 350°C, which also corresponds to the reduction of silver, but due to the low silver concentration and its interaction with titanium support, it is shifted to higher temperatures. The Ag@TiO2 2.5 result is presented as a reference and it shows a peak around 225 and 400°C that also corresponds to the reduction of Ag1+ to Ag0. At temperatures above of 400°C, it is possible to observe how the reduction of the support starts (Ti4+ to Ti3+) [19]. It is important to mention that the titanium reduction temperature varies according to the nanoparticles concentration [20]; at the higher temperature, it is possible to observe a lower interaction with silver nanoparticles.Figure 5
Temperature programmed reduction graph of the silver-titanium catalysts.In ‎Figure6(a) is shown an STEM-HAADF image of the Ag@H2Ti3O7 1.0 sample. In this image, we could observe a well-defined morphology of the nanotubes-H2Ti3O7 structure. This image corroborates the homogeneous silver deposition on the nanotube structure. Silver is found forming spherical nanoparticles (white dots), homogeneously distributed in space and size. The Ag@TiO2 material consists in agglomerates of spherical nanometric particles of TiO2 (P25-Degussa). Once the Ag@TiO2 samples were treated by the hydrothermal method, there is a complete conversion from spherical nanoparticles to large nanotubes corresponding to H2Ti3O7 stoichiometry according to XRD information.Figure 6
(a) STEM-HAADF image for Ag@H2Ti3O7 1.0 sample. Frequency histograms for (b) Ag nanoparticles size and (c) nanotube diameter size.
(a)
(b)
(c)Frequency histograms of size and diameter are reported for Ag nanoparticles and nanotubes; see ‎Figures6(b) and 6(c); where the Ag nanoparticles average size is 2.23 nm; therefore, the reduction method guarantees homogeneous silver size dispersion. According to the frequency histogram for diameter nanotubes, the average diameter is 6.6 nm.According to literature, for a first time is reported the least size of Ag nanoparticles supported on TiO2 nanotubes, since the nanoparticles size reported is on the order of 5 or higher than 10 nm [21–23]. With respect to the measurements of nanotubes, we obtained a lesser diameter size than what was reported in the literature, which is an average of 8–10 [21], 10–20 [22], and 60–70 nm [23]. These nanotubes applied firstly in the photocatalytic degradation of dye or model molecules in this research are successfully used during the photocatalytic adsorption/degradation of the pharmaceutical atenolol.The hydroxyl groups present in the H2Ti3O7 surface are shown in ‎Figure 7. The part (a) corresponds to nanotube decorated with silver and shows two bands located at 3653 and 3715 cm−1, while for the bare H2Ti3O7 nanotube two bands are shown located at 3658 and 3717 cm−1; see ‎Figure 7 part (b). These bands are assigned to hydrogen bonded to adjacent OH groups in terminal positions due to the stretching modes of Ti-OH [24, 25]. By comparison, the H2Ti3O7 bands have a major intensity regarding the Ag@H2Ti3O7 material; this is directly associated with Ag nanoparticles that are anchored over these OH groups.Figure 7
FTIR spectra of (a) Ag@H2Ti3O7 1.0 and (b) H2Ti3O7.
(a)
(b)According to structural and surface properties of Ag-nanotubes, these have been applied successfully during the photocatalytic degradation/adsorption of atenolol.


## 3.2. Photocatalytic Results

The previously synthesized Ag@H2Ti3O7 nanotubes were tested during the photocatalytic degradation of ATL. First, an optimization of the catalyst was made using the concentrations of 175, 250, and 375 mg L−1. Results indicate that optimal load is 250 or 375 mg L−1, but for practical purposes, it was chosen a concentration of 250 mg L−1. With this load is reached a final degradation of 80%, which corresponds to the same adsorbed quantity during the dark conditions; see ‎Figure 8(a). The evaluation of the rest of materials with this load indicates a low degradation/adsorption of 50% for Ag@H2Ti3O7 0.5 and 10% for Ag@H2Ti3O7 2.5; see ‎Figure 8(b). The best photocatalytic degradation is due to the Ag@H2Ti3O7 1.0 sample and the catalyst with bigger pore diameter and with the highest surface area. As already mentioned, the highest surface area is correlated with minor sodium ions concentration presents in the H2Ti3O7 structure. Therefore, a minor sodium ions concentration represents an enhancement during the adsorption/degradation of the ATL molecule.Figure 8
(a) Optimization of the silver load of photocatalyst using Ag@H2Ti3O7 1.0., (b) 50 mg of the photocatalyst, and (c) comparative behavior regarding TiO2 references and photolysis for the 200 mL of ATL degradation at 10 ppm and pH normal.
(a)
(b)
(c)For comparative purposes, ‎Figure8(c) shows the behavior of the P25-Degussato compare with the Ag@TiO2 1.0, Ag@H2Ti3O7 1.0, and it respective photolysis. Photolysis shows a negligible degradation. The P25-Degussa and Ag@TiO2 1.0 show a minimal adsorption under dark conditions of around 3%. Once the UV light is turned on, the Ag@TiO2 1.0 shows a final degradation of 20%, while P25-Degussa is around 30% of the ATL molecule. Although it is well known that silver nanoparticles avoid the hole-electron recombination, in this case, the commercial TiO2 shows a greater behavior than the silver modified catalyst; this is due to gallic acid precedent from the synthesis that was not removed and could cover the active sites of the TiO2. At the same time the Ag@H2Ti3O7 1.0 shows 80% of adsorption and once the ATL is adsorbed onto the nanotube surface is followed the formation of the respective byproducts.


### 3.2.1. TOC Analysis

The TOC analysis for the degradation of ATL molecule using the nanotubes decorated with silver reveals a maximum mineralization of 50% for the Ag@H2Ti3O7 1.0 catalyst, 25% for the Ag@H2Ti3O7 0.5, and less than 5% for the Ag@H2Ti3O7 2.5 material (‎Figure 9). According to UV-Vis spectroscopy measurements that indicate a maximum degradation of 80%, for the Ag@H2Ti3O7 1.0 catalyst, only a 50% of the ATL is mineralized into CO2 and water; the remaining 30% is converted into a simple molecule, the byproduct amino-diol [26, 27]. The photocatalytic process has a high efficiency when there is a higher percentage of adsorption. Thus, the novelty of the silver decorated nanotubes synthesized in this research has a good performance during the degradation of atenolol molecule.Figure 9
TOC (mg/L) measurements during the photocatalytic degradation of ATL using the Ag@H2Ti3O7 catalysts at pH 7.


### 3.2.2. Silver Lixiviation Test

The stability of the Ag@H2Ti3O7 catalysts was evaluated through its lixiviation by atomic absorption; aliquots were taken at the end of the photocatalytic reaction (4 hours). ‎Table 2 reveals the concentration of Ag (ppm) and the corresponding percentage. The aliquot taken for the Ag@H2Ti3O7 0.5 sample shows a final Ag concentration of 0.0053 ppm equivalent to 0.4% of full lixiviation. The Ag@H2Ti3O7 1.0 sample shows a full lixiviation of 1.2% and the Ag@H2Ti3O7 2.5 shows a full lixiviation of 6.4%. The obtained percentages are negligible and, with this result, we can affirm that hydrothermal assisted method guarantees catalysts with silver nanoparticles strongly attached to the TiO2 nanotubes.Table 2
Silver lixiviation test during the photocatalytic degradation of ATL at 10 ppm and pH normal.
Sample
Ag concentration (ppm)
Lixiviation (%)
Ag@H2Ti3O7 0.5
0.0053
0.4
Ag@H2Ti3O7 1.0
0.0282
1.2
Ag@H2Ti3O7 2.5
0.0805
6.4


### 3.2.3. Reusability Test

The behavior of the ATL degradation using the Ag@H2Ti3O7 1.0 can be seen in ‎Figure 10. After 30 min of adsorption during dark conditions, a decrease of the UV band is observed occurring at 225 nm and 276 nm. After that, a minimal degradation is observed at the end of adsorption process; during the 60 and 90 minutes of photocatalytic degradation an additional band at 248 nm is observed; this band is assigned to the byproducts formed during the photocatalytic process and according to literature corresponds to the formation of an amino-diol [26, 27]. Amino-diol is a byproduct of ATL fragmentation due to the participation of hydroxyl radicals (OH∙) and irradiation UV effect during the photocatalytic degradation reaction. It is important to mention that an amino-diol is the simplest and less contaminant molecule compared to initial ATL.Figure 10
Reusability test (three cycles) during the photocatalytic degradation of ATL using the Ag@H2Ti3O7 1.0 catalyst.To corroborate the stability of the most active Ag@H2Ti3O7 titanate, a reusability test of three cycles was made (see ‎Figure 10). In the first cycle is observed an immediate adsorption around 50% and we can see the characteristic ATL band at 225 nm. After the dark absorption process and when the UV lamp is turned on, the 225 nm remains almost constant until the first 90 min of photocatalytic reaction and it starts to appear a shoulder at 248 nm corresponding to amino-diol byproduct.Once the first cycle was completed, the catalyst was recovered, dried, and used again in a second cycle (‎Figure10). In this graph, a lower adsorption is observed around 20%, and immediately after that the lamp turns on; it starts the appearance of shoulder corresponding to the byproducts. In the third cycle, the initial dark adsorption is negligible.Therefore, with this experiment could be verified the stability and reusability of the Ag@H2Ti3O7 1.0 catalyst. According to that when the catalyst is reused, the ATL conversion to amino-diol is accelerated. This is a good result due to amino-diol being simpler and less contaminant than ATL initial molecule.


## 3.2.1. TOC Analysis

The TOC analysis for the degradation of ATL molecule using the nanotubes decorated with silver reveals a maximum mineralization of 50% for the Ag@H2Ti3O7 1.0 catalyst, 25% for the Ag@H2Ti3O7 0.5, and less than 5% for the Ag@H2Ti3O7 2.5 material (‎Figure 9). According to UV-Vis spectroscopy measurements that indicate a maximum degradation of 80%, for the Ag@H2Ti3O7 1.0 catalyst, only a 50% of the ATL is mineralized into CO2 and water; the remaining 30% is converted into a simple molecule, the byproduct amino-diol [26, 27]. The photocatalytic process has a high efficiency when there is a higher percentage of adsorption. Thus, the novelty of the silver decorated nanotubes synthesized in this research has a good performance during the degradation of atenolol molecule.Figure 9
TOC (mg/L) measurements during the photocatalytic degradation of ATL using the Ag@H2Ti3O7 catalysts at pH 7.


## 3.2.2. Silver Lixiviation Test

The stability of the Ag@H2Ti3O7 catalysts was evaluated through its lixiviation by atomic absorption; aliquots were taken at the end of the photocatalytic reaction (4 hours). ‎Table 2 reveals the concentration of Ag (ppm) and the corresponding percentage. The aliquot taken for the Ag@H2Ti3O7 0.5 sample shows a final Ag concentration of 0.0053 ppm equivalent to 0.4% of full lixiviation. The Ag@H2Ti3O7 1.0 sample shows a full lixiviation of 1.2% and the Ag@H2Ti3O7 2.5 shows a full lixiviation of 6.4%. The obtained percentages are negligible and, with this result, we can affirm that hydrothermal assisted method guarantees catalysts with silver nanoparticles strongly attached to the TiO2 nanotubes.Table 2
Silver lixiviation test during the photocatalytic degradation of ATL at 10 ppm and pH normal.
Sample
Ag concentration (ppm)
Lixiviation (%)
Ag@H2Ti3O7 0.5
0.0053
0.4
Ag@H2Ti3O7 1.0
0.0282
1.2
Ag@H2Ti3O7 2.5
0.0805
6.4


## 3.2.3. Reusability Test

The behavior of the ATL degradation using the Ag@H2Ti3O7 1.0 can be seen in ‎Figure 10. After 30 min of adsorption during dark conditions, a decrease of the UV band is observed occurring at 225 nm and 276 nm. After that, a minimal degradation is observed at the end of adsorption process; during the 60 and 90 minutes of photocatalytic degradation an additional band at 248 nm is observed; this band is assigned to the byproducts formed during the photocatalytic process and according to literature corresponds to the formation of an amino-diol [26, 27]. Amino-diol is a byproduct of ATL fragmentation due to the participation of hydroxyl radicals (OH∙) and irradiation UV effect during the photocatalytic degradation reaction. It is important to mention that an amino-diol is the simplest and less contaminant molecule compared to initial ATL.Figure 10
Reusability test (three cycles) during the photocatalytic degradation of ATL using the Ag@H2Ti3O7 1.0 catalyst.To corroborate the stability of the most active Ag@H2Ti3O7 titanate, a reusability test of three cycles was made (see ‎Figure 10). In the first cycle is observed an immediate adsorption around 50% and we can see the characteristic ATL band at 225 nm. After the dark absorption process and when the UV lamp is turned on, the 225 nm remains almost constant until the first 90 min of photocatalytic reaction and it starts to appear a shoulder at 248 nm corresponding to amino-diol byproduct.Once the first cycle was completed, the catalyst was recovered, dried, and used again in a second cycle (‎Figure10). In this graph, a lower adsorption is observed around 20%, and immediately after that the lamp turns on; it starts the appearance of shoulder corresponding to the byproducts. In the third cycle, the initial dark adsorption is negligible.Therefore, with this experiment could be verified the stability and reusability of the Ag@H2Ti3O7 1.0 catalyst. According to that when the catalyst is reused, the ATL conversion to amino-diol is accelerated. This is a good result due to amino-diol being simpler and less contaminant than ATL initial molecule.


## 4. Conclusions

The silver/titanates nanotubes (Ag@H2Ti3O7) show a high efficiency of adsorption at around 80% for β-blocker ATL, part of the ATL molecules adsorbed onto the silver/titanates surface area transformed to amino-diol and 50% of the ATL is mineralized into CO2 and water. The photocatalytic behavior of this kind of material could be due to the high increase of its surface area; in this case, it is almost 300 m2 g−1; this increase in the surface area is induced for the hydrothermal assisted microwave method and also due to the fewer sodium ions concentration. Also, this behavior could be attributed to the presence of silver nanoparticles and they are responsible for avoiding the recombination of the electron/hole pair. The silver/titanates results reported in this work indicate that they are good candidates for use in wastewater treatment plants to eliminate PPCPs and they could avoid human and marine species diseases.



---

*Source: 9610419-2017-06-27.xml*