Polyaniline/TiO2 composites: green photocatalysic synthesis and application in wastewater remediation

In recent years, polyaniline (PANI) composites and nanocomposites with metal and metal-oxide materials have received growing attention for electrochemical and photoelectrochemical applications (Gu 2013). Among them, PANI/TiO2 composites are probably the most interesting systems due to synergistic effects between the conductive polymer and the oxide photocatalyst in terms of photogenerated charge separation and photocatalytic efficiency (Bae 2011). Moreover, polyaniline has been reported to possess favourable sorption properties, which can be exploited for pollutant remediation (Alcaraz-Espinoza 2015, Janaki 2012). PANI/TiO2 composites are thus promising candidates for wastewater treatment combining different pollutant remediation approaches. Polyaniline is classically synthesised via oxidative polymerization (Tran 2011), which involves noxious reagents (aniline and peroxydisulfates) and leads to toxic and carcinogenic byproducts (such as benzidine and trans-azobenzidine). In recent years, greener alternatives have been reported, such as a synthetic process starting from aniline dimer ((4-aminophenil)aniline) and using Fe3+ as catalyst and H2O2 as oxidant (Della Pina 2018). Unfortunately, this alternative procedure does not offer any control over the polymer morphology, leading to compact materials with low surface area and, as a consequence, poor dye-sorption capability. Very recently, we proposed a new photocatalytically induced green synthesis leading to stable polyaniline/TiO2 composites with porous morphology, wide surface area, high crystallinity and, most important, excellent dye removal performance and reusability (Cionti 2018). The reaction is carried out in two steps: at first, the aniline dimer is dissolved in a HCl aqueous solution and TiO2 is added while starting UV irradiation. In the second step, H2O2 is added in the dark, leading to the final product. In this work, we shed light on the photocatalytic nature of the synthetic mechanism, highlighting the different roles of TiO2 and of H2O2 on the composite structural and morphological features as well as on the composite performance for pollutant abatement. The reaction mechanism was investigated by a combination of spectrometric techniques, radical scavenger tests, and surface characterizations (Fig.1). By sampling the reaction mixture at different irradiation times, we demonstrated that under UV irradiation the growth of the oligomers occurs at the TiO2 particle surfaces. The same experiment carried out without UV irradiation showed the intrinsic photocatalytic nature of the process: in the dark, only short oligomers without appropriate chain conjugation were produced. However, even after prolonged UV irradiation, the final green product could be obtained only upon addition of H2O2, showing that, while oligomer formation is initiated by radicals produced by TiO2 photocatalysis, small amounts of an oxidant (H2O2) are still needed for the polymer chain growth. The role of the H2O2 amount proved to be especially crucial with respect to the composite properties. Increasing the H2O2 amount together with that of TiO2 led to composites with low surface area and reduced dye removal capability (Fig.2 a) due to a faster polymerization step. On the other hand, when only the photocatalyst amount was increased, neither the product morphology, nor its dye-removal ability were affected. This enables to increase the TiO2 content within the composite with the aim of enhancing its photocatalytic performance. In this respect, the composite stability was tested in water under prolonged UV irradiation, showing that the material optical, structural and morphological properties remained unchanged. The composite was tested towards the removal of anionic azo dyes in aqueous solution, evaluating the effect of the matrix composition and the composite reusability (Fig.2 b), showing promising results

Triply green polyaniline: UV irradiation-induced synthesis of highly porous PANI/TiO2 composite and its application in dye removal

An environmentally benign procedure for the preparation of polyaniline/TiO2 composites is presented. The UV irradiation-induced synthesis leads to materials with good crystallinity and tailored morphology, showing promising sorption and recycle properties in dye removal tests. A reaction mechanism is proposed on the basis of LC-MS and FT-IR investigations

Photocatalytic and oxidative synthetic pathways for highly efficient PANI-TIO2 nanocomposites as organic and inorganic pollutant sorbents

Polyaniline (PANI)-materials have recently been proposed for environmental remediation applications thanks to PANI stability and sorption properties. As an alternative to conventional PANI oxidative syntheses, which involve toxic carcinogenic compounds, an eco-friendly procedure was here adopted starting from benign reactants (aniline-dimer and H2O2) and initiated by ultraviolet (UV)-irradiated TiO2. To unlock the full potential of this procedure, we investigated the roles of TiO2 and H2O2 in the nanocomposites synthesis, with the aim of tailoring the properties of the final material to the desired application. The nanocomposites prepared by varying the TiO2:H2O2:aniline-dimer molar ratios were characterized for their thermal, optical, morphological, structural and surface properties. The reaction mechanism was investigated via mass analyses and X-ray photoelectron spectroscopy. The nanocomposites were tested on both methyl orange and hexavalent chromium removal. A fast dye-sorption was achieved also in the presence of interferents and the recovery of the dye was obtained upon eco-friendly conditions. An efficient Cr(VI) abatement was obtained also after consecutive tests and without any regeneration treatment. The fine understanding of the reaction mechanism allowed us to interpret the pollutant-removal performances of the different materials, leading to tailored nanocomposites in terms of maximum sorption and reduction capability upon consecutive tests even in simulated drinking water

PANI-TiO2 composites: the mechanism behind a green process

Polyaniline (PANI) is an important member of the family of organic conductive polymers, which holds potential in numerous fields, such as electronics, optics and photovoltaics [1]. In recent years, PANI has received increasing attention for application in wastewater treatment due to its sorption properties enabling the removal of a broad range of pollutants [2,3]. Polyaniline is classically synthesized by oxidative polymerization [4], which involves noxious reagents (aniline as starting compound and persulfates as oxidant) and gives rise to toxic and carcinogenic by-products (such as benzidine and trans-azobenzene). A great deal of effort has been devoted to find alternative green routes: in particular, some of us reported a benign synthesis based on aniline dimer ((4-aminophenyl)aniline), H2O2 as oxidant and Fe3+ as catalyst [5]. However, this procedure yields no control on the polymer morphology, leading to a compact PANI with low surface area and poor dye sorption capability. We have recently developed an alternative green synthesis based on TiO2 photocatalysis, enabling a morphological control of the polymer [6]. In this work, the reaction mechanism has been investigated in depth via LC-MS, FT-IR and z-potential analyses. In the first stage, carried out under UV irradiation, the growth of oligomers from the aniline dimer takes place on the TiO2 particle surfaces, activated by the photocatalytic generated radicals. In the second step, the addition of H2O2 (80% less than in the Fe-catalyzed synthesis) activates the polymer growth, giving rise to the final product. By separating the oligomerization and polymerization steps, polymer composites with high crystallinity and porous morphology could be prepared. The dye sorption capability of the samples was tested toward methyl orange as model for anionic azo dyes: promising results were obtained both in terms of dye removal and product reusability. The creation of PANI-TiO2 composites opens the door to future applications exploiting the complementary properties of the two materials, such as pollutant removal processes based on combined sorption and photocatalytic degradation

Bouncing Droplets: A Hands-On Activity To Demonstrate the Properties and Applications of Superhydrophobic Surface Coatings

Here we report a hands-on activity addressed to master\u2019s students in Physical Chemistry and Materials Science courses on the properties and applications of superhydrophobic surfaces. This simple and intuitive experience can also be used to teach undergraduate and high school students, thanks to its application-oriented approach and tangible results. Superhydrophobicity was achieved by the functionalization of oxide powders with alkylsilanes and their subsequent deposition on a glass substrate. The film\u2019s superhydrophobicity was assessed by different application tests and compared with the behaviors of model hydrophobic, hydrophilic, and superhydrophilic surfaces. Its antistain properties were tested with both model dye solutions and everyday liquids. Films were fouled with graphite and dye powders to compare the self-cleaning capabilities of the different surfaces, and single droplet transport was achieved by adding magnetic particles. This engaging and adaptable experience introduces students to basic concepts of surface science in an intuitive and tangible way

UV-induced synthesis of polyaniline-TiO2 hybrids: a mechanistic study

Polyaniline, an important member of the conductive polymer family, has received increasing attention due to its peculiar pH-dependent properties which open the way to a wide spectrum of applications, ranging from electronics and optics to photovoltaic1,2. PANI is traditionally synthesized via oxidative polymerization3, a process which involves toxic reagents (aniline and persulfates) and leads to carcinogenic byproducts. Aiming to more environmentally friendly procedures, other synthetic strategies have been developed during the years: in particular, some of us4 have reported a green synthesis involving aniline dimer ((4-aminophenyl)aniline) as starting compound, H2O2 as oxidant and Fe3+ as catalyst, thus yielding H2O as only coproduct. Unfortunately, with this eco-friendly process, there is no control on the polymer morphology and the result is a compact PANI with low dye-sorption capabilities. We have recently proposed a new green synthesis in which PANI growth is activated by TiO2 photocatalysis giving rise to PANI-TiO2 hybrid systems5. The reaction is carried out in two steps: the photocatalytically induced oligomerization of aniline dimer at the TiO2 surface, and the polymerization step initiated by H2O2 addition. In this work, the reaction mechanism was investigated via radical scavenger tests and by a combination of LCMS, FTIR, XPS and \u3b6-potential measurements. UV light is essential to initiate the reaction, as without irradiation only short oligomers with poor chain-conjugation are formed (Fig.1). Overall, this synthetic method leads to composites stable under UV irradiation in usage conditions, with high specific surface areas (crucial for sorption properties) and enhanced crystallinity, which is beneficial for PANI conductivity. The role of synthetic parameters like reagent ratios and temperature was also investigated; while incrementing the H2O2 amount leads to poorer crystallinity and lower surface area, the TiO2 content in the hybrid can be increased without affecting its morphology and performance. All samples were tested towards the removal of model dye pollutants. The reusability of the nanocomposite and the influence of common interferents were investigated, also via tests in simulated drinking water. References [1] C.O. Baker, X. Huang, W. Nelson, R.B. Kaner, Chem. Soc. Rev., 46, 2017, 1510. [2] F. Cui, Y. Huang, L. Xu, Y. Zhao, J. Lian, J Bao, H Li, Chem. Commun., 54, 2018, 4160. [3] H.D. Tran, J.M. D\u2019Arcy, Y. Wang, P.J. Beltramo, V.A. Strong, R.B. Kaner, J. Mater. Chem., 21, 2011, 3534 [4] C. Della Pina, M. A. De Gregorio, L. Clerici, P. Dellavedova, E. Falletta, J. Hazard. Mater., 2018, 344, 308 [5] C. Cionti, C. Della Pina, D. Meroni, E. Falletta, S. Ardizzone, Chem. Commun., 2018, 54, 1070

Tailoring the structural and electronic features of N-DOPED TiO2/SnO2 photocatalysts for pollutant degradaton

TiO2/SnO2 composites hold promise in numerous cutting-edge research fields, such as gas sensors (Chen 2015), fuel cells (An 2013) and photocatalysis (Toloman 2019). Thanks to the near isomorphism of TiO2 rutile (P42/mnm, a=4.5937 \uc5, c=2.9587 \uc5) and SnO2 cassiterite (P42/mnm, a=4.7382 \uc5, c=3.1871 \uc5), stable heterojunctions between the two semiconductors can be obtained. Furthermore, SnO2 and TiO2 present different work functions (4.4 and 4.2 eV, respectively). The Fermi energy of TiO2 is higher than that of SnO2, which can promote electron transfer from the TiO2 conduction band to the SnO2 one, with the ensuing formation of an interface contact potential (Floriano 2014). These electron transfer phenomena can be exploited to promote the separation of charge carriers in photocatalytic applications, leading to more efficient materials. Extending the lifetime of photo-generated charges is a crucial aspect particularly for N-doped TiO2, as in these materials visible-light activity is generally achieved at the expense of faster charge carriers recombination (Asahi 2014; Rimoldi 2015). We have recently shown that addition of Sn species can promote the photocatalytic activity under solar irradiation of N-doped TiO2 (Rimoldi 2018). In this work, N-doped TiO2/SnO2 materials are investigated as photocatalyst for the light activated degradation of both water and gas phase pollutants. N-doped TiO2/SnO2 and TiO2/SnO2 samples were synthesized in a broad range of Sn:Ti molar ratios (up to 20%) adopting three different methods (mechanical mixing, co-synthesis and seeded growth) followed by calcination at 400\ub0C. Commercial and ad hoc synthesized TiO2 and SnO2 samples were also analyzed as references. The structural, morphological, electronic and surface features of each set of samples were investigated in detail. The local structure, defectivity and charge transfer phenomena were also studied via in situ X-ray absorption spectroscopy experiments. High resolution X-ray diffraction and X-ray absorption curves at the Sn L1-edge and Ti K-edge showed that the adopted synthetic strategies control the microstructure in terms of both phase composition and concentration of defects, which in turn imply significant changes in both the long- and short-range structures. In particular, the coprecipitation route gives rise to notable differences with respect to the other two methods, showing no peaks due to SnO2 cassiterite and promoted growth of TiO2 rutile (Fig. 1b) as well as a different Sn coordination geometry (Fig. 1a). Furthermore, the operando measurements provided evidence of electron transfer mechanisms taking place upon UV light irradiation. The distinctive character of samples from coprecipitation route is also appreciable in terms of light absorption features (Fig. 2), where the copromotion with Sn and N gives rise to synergistic effects. Samples were tested under UV and solar irradiation towards the remediation of an emerging pollutant (tetracycline) in water and of a model VOC (ethanol) in the gas phase. Both the molecule disappearance, the reaction intermediate and final mineralization were monitored, showing promising results in terms of photocatalytic activity of the copromoted samples (Fig. 2). The photocatalytic performances were correlated with the structural and spectroscopic results also on the grounds of the reaction pathways