Iron oxide nanocatalyst with titanium and silver nanoparticles: Synthesis, characterization and photocatalytic activity on the degradation of Rhodamine B dye

Nowadays, there is a growing concern about the environmental impacts of colored wastewater. Thus, the present work aims the synthesis, characterization and determination of photocatalytic activity of iron oxide (Fe2O3) nanocatalyst, evaluating the effect of hybridization with titanium (TiNPs-Fe2O3) and silver (AgNPs-Fe2O3) nanoparticles, on the degradation of Rhodamine B dye (RhB). Nanocatalysts were characterized by XRD, SEM, TEM, FTIR, N2 porosimetry (BET/BJH method), zeta potential and DRS. Photocatalytic tests were performed in a slurry reactor, with the nanocatalyst in suspension, using RhB as a target molecule, under ultraviolet (UV) and visible radiation. Therefore, the photocatalytic activity of the nanocatalysts (non-doped and hybridized) was evaluated in these ideal conditions, where the AgNPs-Fe2O3 sample showed the best photocatalytic activity with a degradation of 94.1% (k = 0.0222  min−1, under UV) and 58.36% (k = 0.007  min−1, under visible), while under the same conditions, the TiO2-P25 commercial catalyst showed a degradation of 61.5% (k = 0.0078  min−1) and 44.5% (k = 0.0044  min−1), respectively. According with the ideal conditions determined, reusability of the AgNPs-Fe2O3 nanocatalyst was measured, showing a short reduction (about 8%) of its photocatalytic activity after 5 cycles. Thus, the Fe2O3 nanocatalyst can be considered a promising catalyst in the heterogeneous photocatalysis for application in the degradation of organic dyes in aqueous solution.

Nanocatalyst has emerged as an alternative to increase catalytic efficiency, since it has advantages over commercial catalysts, such as higher specific surface area and porosity, making them with great potential application in heterogeneous photocatalysis 10 . One of the strategies used to increase the photocatalytic activity of nanocatalysts is the usage of hybridization with noble metal and metals, in order to reduce the recombination between photoelectrons/holes pairs and reducing the energy required to its photoactivation, allowing its application to visible radiation [11][12][13][14][15][16] . In addition, Rhodamine B dye (RhB) (C 28 H 31 N 2 O 3 Cl), a highly water soluble organic cation dye, belongings to the class of xanthenes, whose contact with humans can cause irritation to the skin, airways and eyes 17 . Moreover, it presents the chromophoric groups (−C=C −/− C=N−), as well as a characteristic carcinogenicity and neurotoxicity activity.
In this context, the present work aims the synthesis, characterization and determination of photocatalytic activity of Fe 2 O 3 nanocatalyst, hybridized with titanium nanoparticles (TiNPs) and silver nanoparticles (AgNPs) on the degradation of Rhodamine B (RhB) dye, under UV and visible radiation.

Materials and Methods
Synthesis of the fe 2 o 3 nanocatalyst. The synthesis of iron oxide nanocatalyst followed the chemical precipitation by sodium borohydride method, according to the literature 18 . Sodium borohydride (NaBH 4 , 0.2 mol L −1 , Neon, PA) and ferric chloride hexahydrate (FeCl 3 •H 2 O, 0.05 mol L −1 , Synth, PA) were mixed for 30 minutes, under magnetic stirring (250 rpm). After, the synthesized nanoparticles were vacuum filtered and washed with deionized water and diluted ethanol (~5%). Parameters such as pH (≈ 7), reagents concentrations, stirring speed, reaction time and temperature (23 ± 0.5 °C) were kept constant in order to avoid influence on the composition and properties of nanocatalyst.
Synthesis of hybridized nanocatalyst. For hybridization of Fe 2 O 3 nanocatalyst, the impregnation methodology was used with TiNPs and AgNPs, according to the literature 19 . Samples were magnetic stirring at room temperature for 90 min, after calcined at 450 °C (heating rate 10 °C min −1 ) for 4 hours. Finally, the granulometry was standardized with milling and sieving (# 12). Hybridized nanocatalysts with NPs were labeled as TiNPs-Fe 2 O 3 and AgNPs-Fe 2 O 3 , respectively. characterization of nanocatalyst. X-ray diffraction (XRD) was used to determine the crystallinity of the samples in a Bruker D2 Advance diffractometer with a copper tube (Kα = 1.5418 Ǻ) in the range of 5° to 70°, with tension acceleration and applied current of 30 kV and 30 mA, respectively.
For zeta potential (PZ), Malvern-Zetasizer ® model nanoZS (ZEN3600, UK) with closed capillary cells (DTS 1060) (Malvern, UK) was used to measure the zeta potential values of the samples. N 2 porosimetry was used to evaluate the textural properties of specific surface area (S BET ) and porosity (pore diameter -Dp and pore volume -Vp) using in an equipment Gemini VII 2375 Surface Area Analyzer Micromeritics ® and BET/BJH Methods (P Po −1 = 0.05-0.35).
Band gap energy was determined by diffuse reflectance spectroscopy at UV radiation (UV DRS) using a Varian Cary Scan Spectrophotometer with DRA-CA-301 accessory (Labsphere) coupled in the diffuse reflectance mode to determine the energy band gap by means of the Kubelka-Munk function with scans ranged from 200 to 600 nm.
Scanning electron microscopy (SEM) and Transmission electronic microscopy (TEM) were used to morphologically characterize the nanocatalysts (Fe 2 O 3 , TiNPs-Fe 2 O 3 and AgNPs-Fe 2 O 3 ) using a JSM5800 (JEOL) and JEM 1200 Exll (JEOL) microscope, respectively. Moreover, the samples were coated with a thin layer of conductive gold by a sputtering technique. photocatalytic activity. Photodegradation tests were carried out using a solution of RhB, as the target molecule, and nanocatalysts in suspension (slurry). Tests were performed in two stages: (a) dark stage (absence of radiation), where adsorption/desorption equilibrium of the target molecule occurred on the surface of the nanocatalyst, with a duration of 60 minutes, and (b) a photocatalytic reaction step (with visible or UV radiation) with duration of 120 minutes (collections at predetermined times of (0, 5, 15, 30, 45, 60, 75, 90, and 120 minutes). Then the samples were centrifuged (Cientec CT-5000R refrigerated centrifuge) for 20 minutes with a rotation of 5,000 rpm and finally diluted (1:10 v/v).
Moreover, the absorbance measurements of solutions collected during reactions were carried out in a double-beam spectrophotometer (Varian, Cary 100) with a halogen lamp at the wavelength characteristic of RhB (λ = 553 nm) Kinetic study of RhB degradation. In order to determine the specific reaction rate (k), a kinetic study of RhB dye degradation, under UV and visible radiation over time, was carried out according to the classic heterogeneous kinetic model (pseudo first-order model) (Eqs. 1 and 2) 20,21 :
The band gap of Fe 2 O 3 , TiNPs-Fe 2 O 3 and AgNPs-Fe 2 O 3 were found between 2.2; 2.0 and 1.8 eV, respectively. Thus, nanoparticles (NPs) promoted a reduction in conduction and valence bands, compared to Fe 2 O 3 nanocatalyst, generating a decrease in the energy required for photoactivation of the hybridized nanocatalysts and shifting the application of nanocatalysts to the visible region of radiation 25 .
Moreover, all samples showed a negative charge surface potential (−10.25 to −13.30 mV), according to zeta potential, indicating a charge compatibility, since RhB dye is characterized by its cationic nature 26,27 , increasing the RhB adsorption capacity on the catalytic surface and thus a possible better photocatalytic activity. Figure 1 shows the X-ray diffractograms of the synthesized samples (without and with NPs hybridization). According to Fig. 1 30,31 . Thus, the hybridization process with the NPs did not promote the formation of new peaks, as well as titanium and silver characteristic peaks were identified on Fe 2 O 3 sample, indicating a successful hybridization.
FTIR analysis was used as a qualitative analysis technique to determine the functional groups present in the synthesized materials, according to Fig. 2.
According to Fig. 2, iron oxide showed at 3394 cm −1 strip, is attributed to stretch vibrations (ν), while the 1620 cm −1 strip is attributed to flexural vibrations (δ) due to water adsorbed on the surface of the iron oxide nanoparticles 32 Fig. 3(d,e) show the TEM micrographs of the TiNPSs-Fe 2 O 3 and AgNPs-Fe 2 O 3 samples, respectively. Then, it was possible to identify a heterogeneous surface with a random distribution of the TiNPs and AgNPs, with the formation of small clusters of NPs. Then, it can be explained through of the zeta potential (surface charge), since using NPs, the ZP showed a smaller between the nanoparticles, causing a lower dispersion, in relation to Fe 2 O 3 nanocatalyst (with higher value of ZP) 33 . Thus, this greater dispersion of NPs tends to promote changes in the textural properties of nanocatalysts, such as an improve of the specific area and a greater number of active sites to conductive the RhB adsorption, directly affecting photocatalytic activity 34 . Therefore, TEM micrographs showed the presence of NPs (TiNPs and AgNPs) over Fe 2 O 3 in spherical shape with a diameter around 3 nm.
According to Fig. 4, the RhB concentration showed a negative effect, thus with increasing RhB concentration occurs a reduction of the photocatalytic activity on RhB degradation occurs due to the reduction in the number of active sites available for adsorption on surface of Fe 2 O 3 the nanocatalyst 35 . About the effect of pH, it has a positive effect on RhB degradation, since with the increase of the pH, the surface of Fe 2 O 3 nanocatalyst is more deprotonated, increasing the compatibility of charges between RhB and Fe 2 O 3 and thus increasing adsorption and photocatalytic activity 36 . The power of the semiconductor material to act as a sensitizer and to enhance the photodegradation of the RhB is based on their electronic structure with filled valence bond and empty conduction bond 37 . The semiconductor photooxidation instigated the photocatalysis of RhB in solution, leaving the catalyst surface with a strong oxidative potential of an electron-hole pair (h + VB ) (Eq. (4)), when photocatalyst was irradiated with higher energy than that of band gap energy (Eg), which allows the oxidation of the RhB molecule in a direct manner to the reactive intermediates (Eq. (5)).The hydroxyl radical (OH • ), the exceptionally strong and a non-selective oxidant which is formed either by decomposition of water (Eq. (6)) or by reaction of hole along with hydroxyl ion (OH − ) (Eq. (7)) is also responsible for degradation of phenol molecule. Leading to incomplete or complete mineralization of many organic molecules (Eq. (8)) 38 .  Table 3. According to Table 3, the AgNPS-Fe 2 O 3 nanocatalyst showed a photostability after five recycling processes, with a small decrease (about 8%) in the photocatalytic activity (58.36% to 53.69%), under visible radiation. www.nature.com/scientificreports www.nature.com/scientificreports/ conclusion According to the characterization and photocatalytic activity results, the hybridization process with NPs (TiNPs and AgNPs) caused positive changes in the properties of Fe 2 O 3 nanocatalyst for heterogeneous photocatalysis, such as: reduction of band gap energy (2.2 eV to 2.0 eV -TiNPs-Fe 2 O 3 and 1.8 eV -AgNPs-Fe 2 O 3 ); increase of surface area (158 m² g −1 to 304 m² g −1 -TiNPs-Fe 2 O 3 and 505 m² g −1 -AgNPs-Fe 2 O 3 ) and increase in photocatalytic activity under UV and visible radiation. Therefore, nanoparticle hybridization on nanocatalysts is a great option to improve the photocatalytic performance for degradation of organic pollutants (such as dyes) by heterogeneous photocatalysis.   Table 3. Effect of AgNPs-Fe 2 O 3 nanocatalyst reuse on degradation of the RhB.