Research Article | Open Access
Direct Synthesis of Highly Dispersible PACMA-Capped TiO2 Nanoparticles and Its Adsorption Properties towards Pb(II)
A simple low-temperature one-step synthetic method of a hybrid material involving TiO2 nanoparticles modified by an organic polymer is here reported. TiO2 nanoparticles were grown by hydrolysis of hexafluorotitanate using boric acid as a fluoride scavenger. TiO2 synthesis was performed in the presence of poly(acrylic acid-co-maleic acid) (PACMA). This procedure yields a crystalline TiO2 nanopowder capped with PACMA, termed PACMA@TiO2, according to X-ray diffraction and infrared spectroscopy characterization methods. Elemental analysis denotes the presence in the powder of a small amount of ammonium. Transmission and scanning electron microscopies show that the material is constituted by needles of ca. 200 nm in length, fused into star-like particles. Selected area electron diffraction analysis indicates that the particles are aggregated and only partially organized. The dried powdered material can easily be dispersed in water. The colloidal suspension obtained is highly stable, and its potential application in heavy metal adsorption is demonstrated with aqueous Pb(II), followed by using inductively coupled plasma optical emission spectrometry.
Liquid phase deposition (LPD) is a wet process used for the formation of metal oxide thin films and powders at relatively low temperatures. The method is based on the hydrolysis of metallic fluorocomplexes and controlled by the addition of a fluoride scavenger, such as boric acid, which shifts the chemical equilibrium towards the precipitation of the corresponding metal oxide [1, 2]. LPD has been used to precipitate crystalline titanium dioxide (TiO2) thin films homogeneously on different substrates. The method has been thoroughly studied by several groups, with special emphasis in determining factors that control nucleation and crystallinity [3–5]. This synthetic route allows the deposition of TiO2 on labile organic substrates, for example, polyimide resins  and other polymeric films , since a thermal posttreatment is not required to produce crystalline materials. The deposition of hybrid TiO2 films, incorporating compounds like alkylbenzene sulfonate surfactants  or copper phthalocyanine , is also feasible. In a different example, hybrid methylene blue/TiO2 nanocomposite thin films have been used to fabricate light-activated oxygen indicators . LPD has also been successfully used for the deposition of CNTs/TiO2 composite films with photocatalytic activity . Deki et al. have shown that the addition of polyethylene glycol during the growth of TiO2  or SnO2  by LPD allows the synthesis of monodisperse nanoparticles (3–5 nm). Likewise, polyvinylpyrrolidone has been used to tune the grain size of TiO2 films deposited by LPD . In all these examples, the used polymers are not incorporated to the final product, whereas in other examples, the adsorption of the polymer onto the oxide surface is a key factor to control the properties of the material. For instance, the addition of acrylic acid on the surface of TiO2 nanoparticles increased the stability in water . Methyl methacrylate has also been grown directly on the surface of 6-palmitate ascorbate-modified nanoparticles to make nanocomposites . Inorganic nanoparticle-polymer hybrid composites have numerous applications . An example is the formulation of easily processable dielectrics [18, 19] that usually require the modification of the oxide surface with coupling agents to favor their interaction with the polymer matrix, thus limiting nanoparticle aggregation . Different polymers, including poly(acrylic acid) derivatives, are used to improve the stability of aqueous dispersions prior to deposition, as well as the adherence and homogeneity of deposited coatings [21, 22]. In these materials, the adsorbed polymer shell enhances nanoparticle dispersibility through steric and/or electrostatic effects. The adsorption properties are also modified.
In this work, poly(acrylic acid-co-maleic acid) (PACMA), a polyacid with great affinity to oxide nanoparticles improving its dispersibility [23, 24], was selected to control the growth of TiO2 nanoparticles by LPD in a one-step route. The resulting end product, termed PACMA@TiO2, precipitates in the form of nanopowder. Moreover, PACMA@TiO2 is highly stable after water dispersion, which allows its use as a solid adsorbent for water pollutants. In this context, adsorption processes are being widely used by various researchers for the removal of heavy metals from waste streams. Indeed, pollution of water is a key problem worldwide, therefore optimizing processes that allow purifying industrial effluents very desirable. Specifically for Pb(II), the use of solid adsorbents for their elimination has many economic advantages . It has been shown that pristine TiO2 has some affinity for Pb(II) [26, 27], which can be improved by the control of particle morphology  and surface modification  or by using synergistic composites of TiO2 and cellulose . As a potential application, this study also analyzes the PACMA@TiO2 adsorption of Pb(II) from the water.
2. Materials and Methods
Ammonium hexafluorotitanate (AHFT), boric acid, poly(acrylic acid-co-maleic acid) (PACMA, averaged molecular weight 3000 Dalton, 50 wt.% in H2O), and Pb(II) nitrate were purchased from Sigma-Aldrich and used without further purification. Water purified with a Milli-Q system (Millipore, conductivity < 0.05 μS/cm) was employed.
2.2. Preparation of the PACMA@TiO2 Powders
A solution of 660 mg of AHFT in 40 mL of water was mixed with another aqueous solution involving 623 mg of boric acid and 125 mg of PACMA in 40 mL. The mixture was a clear solution that resulted in a white precipitate after heating at 80°C for 3.5 h. The precipitate was separated by centrifugation, washed with fresh water, and dried at 60°C in an air oven. The estimated yield was 60%, considering that according to elemental analysis, the sample contains an 82 wt.% of TiO2.
2.3. Characterization Techniques
Powder X-ray diffraction (PXRD) patterns were measured with a Siemens D5000 apparatus using the CuKα radiation. Patterns were recorded from 2θ = 5 to 50°, with a step scan of 0.02° counting for 1 s at each step. Sample morphology was observed with a scanning electron microscope (SEM) (Zeiss Merlin). Transmission electron microscopy (TEM) images were taken with a JEOL 2011 microscope operating at 200 kV. Selected area electron diffraction analysis was used to get information about crystallinity and aggregate order. The presence of adsorbed organics, including residual reagents, was ascertained by attenuated total reflectance Fourier-transform infrared (ATR-FTIR) spectroscopy (Bruker, Tensor model equipped with an MKII Golden Gate). Measurements of BET surface area were performed in a Micromeritics ASAP 2000 using low-temperature N2 adsorption-desorption isotherms. Samples were previously degassed at 373 K during 18 h under vacuum. For UV-vis measurements, the powder was suspended in water aided by ultrasound and the spectrum was recorded over the wavelength range 200–800 nm using a Varian Cary UV-V spectrophotometer. DLS measurements were carried out in a Zetasizer Nano ZS (Malvern). Pollutant adsorption tests were performed in 50 mL aliquots taken from a mother solution containing 24 mg/L of Pb(II), prepared from Pb(NO3)2. Different amounts of PACMA@TiO2 powder were added to these aliquots and left standing overnight. Stirring was not necessary to disperse the adsorbent. Finally, the different dispersions were centrifuged and each sample was analyzed using inductively coupled plasma optical emission spectrometry (ICP-OES, Optima 4300DV, Perkin Elmer) to determine the Pb(II) content.
3. Results and Discussion
LPD processes are mostly used to prepare thin films through heterogeneous nucleation on surfaces. In a different approach, in this work, the LPD route is used to obtain a fine precipitate. Homogeneous nucleation is enabled by the presence in the solution of a dissolved PACMA polymer, which has affinity to the growing TiO2 crystallites. PACMA is thus adsorbed on the TiO2 surface, discontinuing the growth and increasing the stability of the small nanometric particles in the aqueous growing media. Heterogeneous precipitation in the reactor walls was not observed. The precipitate was separated from the liquid phase by filtration before solid state characterization. The PXRD pattern recorded for the PACMA@TiO2 sample matches that of the TiO2 anatase phase (JCPDS no. 21-1272) (Figure 1). As usual in TiO2 prepared by LPD, the relative intensity of the peaks indicates some preferential growth along the (004) direction. The widening in the low intensity peaks is due to the small crystal size, a fact that was confirmed by TEM characterization. TEM micrographs also showed the anisotropic shape of the precipitated nanoparticles in the form of needles. Primary crystals suffer some oriented agglomeration forming star-like macrostructures (Figure 2). The polycrystalline structure is also observed by selected area electron diffraction (insert in Figure 2), which displayed elongated spots arising from different Bragg reflections arising from individual needles fused together following some preferential crystal orientation. SEM pictures corroborate those observations (Figure 2). The star-like morphology would complicate the formation of compact aggregates. The estimated value of the BET surface area is 63 m2/g. The isotherm is type II (Figure 3), with a hysteresis H1 loop appearing at high relative pressure (>0.9), which is associated to pores created by agglomeration of uniformly sized particles.
ATR-FTIR spectrum (Figure 4) indicates that the material incorporates a small percentage of organic products (bands at 1715, 1630, and 1423 cm−1). This behavior was expected considering that polycarboxylic acids have high affinity for the TiO2 surface. The presence of two bands of different intensities assigned to asymmetric C=O stretching indicates that only some of the carboxylic acid groups are protonated (band at 1715 cm−1), while most are deprotonated (band at 1630 cm−1). The band at 1423 cm−1 could be assigned to symmetric C=O stretching, but its position and relative intensity suggest that the sample contains also the NH4+ cation adsorbed from the precursor solution. In fact, the broad absorption band around 3200–3100 cm−1 is also compatible with the presence of NH4+ cations (N-H stretching). Other contributions to this band are adsorbed water and the hydroxide anion. The measured elemental analysis (3.49% C; 1.57% H; 0.61% N) includes N in the composition, thus confirming the presence of residual ammonium in the composite, as it is the unique possible source of nitrogen. Additionally, elemental analysis results suggest the precipitation of PACMA@TiO2 powder with a composition of 9 wt.% of polymer and a 9 wt.% of water, with one adsorbed ammonium each per three carboxylate groups. The percentage of water agrees with the weight loss measured during sample activation at 100°C under vacuum.
The obtained PACMA@TiO2 solid powder was easily redispersed in water, and the obtained suspension was stable for weeks. In the synthetized product, the polymer coating provides both steric stabilization and charge stabilization of the nanoparticles. Dynamic light scattering characterization gave an averaged size of 200 nm for the dispersed powder (Figure 5), a value in accordance with SEM characterization. UV-vis spectra, obtained from the light transmitted through several aqueous suspensions of PACMA@TiO2 at different concentrations (Figure 6), showed the abrupt increase in the absorption below 380 nm typical of semiconductor crystalline TiO2 .
As a potential application for the synthetized material as a solid adsorbent, its adsorption capacity was explored by choosing Pb(II) as a heavy metal pollutant to be removed from the water. The amount of Pb(II) adsorbed at equilibrium was determined in aqueous suspensions with different concentrations of PACMA@TiO2. Any pH controlling additives were added. A 200 mg/L suspension of the composite was able to reduce the Pb(II) concentration from 24 to 3.5 ppm. This means that 100 mg of Pb(II) can be captured per gram of PACMA@TiO2 adsorbent. By triplicating the amount of adsorbent in the aqueous dispersion, the quantity of Pb(II) remaining in the solution was reduced to less than 1 ppm. As a control, it was demonstrated that, under similar experimental conditions, 200 mg of commercial TiO2 Degussa P25 adsorbed a significantly small amount of Pb(II), reducing the concentration from 24 to only 14 ppm. The adsorption results obtained with the PACMA@TiO2 powder are comparable to others obtained with TiO2 materials precipitated by much more intricate processes .
Polymer-functionalized TiO2 nanoparticles were synthesized in a single step by LPD. The inorganic particles were capped with a partially deprotonated PACMA shell, with accompanying ammonium cations. The composite consisted of elongated particles of 200 nm that agglomerated in star-like macrostructures. The dried powder material was easily dispersible in water and can be used to adsorb remarkable amounts of Pb(II) from aqueous solutions, making it potentially valuable for environmental remediation applications.
Data used to support the findings of this study are included in the article.
Conflicts of Interest
The authors declare that there is no conflict of interest regarding the publication of this paper.
This work was partially financed by the Spanish National Plan of Research CTQ2014-56324-CO2-P1 and CTQ2017-83632 projects. C. D./ICMAB acknowledges the financial support from the Spanish MEC, through the “Severo Ochoa” Programme for Centres of Excellence in R&D (SEV-2015-0496). M. S-S. also acknowledges the Universitat Autònoma de Barcelona for his predoctoral grant.
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