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Pt core–shell particles by electroless metal plating
Colloids and Surfaces A: Physicochem. Eng. Aspects 448 (2014) 88–92
Contents lists available at ScienceDirect
Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Fabrication of TiO2 /Pt core–shell particles by electroless metal plating Yoshio Kobayashi a,∗ , Yuya Ishii a , Hideyuki Yamane a , Ken-ichi Watanabe b , Hidekazu Koda b , Hiroshi Kunigami b , Hideki Kunigami b a Department of Biomolecular Functional Engineering, College of Engineering, Ibaraki University, 4-12-1 Naka-narusawa-cho, Hitachi, Ibaraki, 316-8511, Japan b K. K. Shinko Kagaku Kogyosho, 1544-19 Mashimori, Koshigaya, Saitama, 343-0012, Japan
h i g h l i g h t s
g r a p h i c a l
a b s t r a c t
• The present work proposes a simple method for fabricating TiO2 /Pt core–shell particles. • TiO2 particles prepared by a sol–gel method were surface-modified with poly(diallyldimethylammonium chloride). • Pt fine particles were pre-deposited on the surface-modified TiO2 particles by reducing Pt ions in the presence of the TiO2 particles. • Pt shells were formed on the Pt-predeposited TiO2 particles by electroless metal plating method.
a r t i c l e
i n f o
Article history: Received 15 December 2013 Received in revised form 5 February 2014 Accepted 7 February 2014 Available online 16 February 2014 Keywords: TiO2 Pt Core–shell Particles Electroless metal plating
a b s t r a c t An electroless metal plating method was used to form Pt shells on sub-micrometer-sized titania particles fabricated by a sol–gel method. The electroless metal plating method comprised three steps: (1) surface-modification of titania particles with poly(diallyldimethylammonium chloride) (PDADMAC) (TiO2 –PDADMAC), (2) pre-deposition of Pt nuclei or Pt fine particles on the titania particles by reducing Pt ions in the presence of TiO2 –PDADMAC particles (TiO2 –Pt) and (3) growth of the pre-deposited Pt by immersing the TiO2 –Pt particles in a Pt-plating solution. TEM observation and X-ray diffractometry revealed that surface modification with PDADMAC promoted the pre-deposition of Pt and that crystalline Pt shells with a thickness of approximately 25 nm were successfully produced on the titania particles using initial concentrations of 0.8 × 10−3 M TiO2 and 0.375 × 10−3 M Pt in the Pt-plating solution. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Composites of plural materials are of special interest because they exhibit multiple functions derived from their individual components. Among the numerous structures that have been proposed for functional materials to date, core–shell structures, in which a
∗ Corresponding author. Tel.: +81 294 38 5052; fax: +81 294 38 5078. E-mail address: [email protected] (Y. Kobayashi). http://dx.doi.org/10.1016/j.colsurfa.2014.02.018 0927-7757/© 2014 Elsevier B.V. All rights reserved.
core particle is coated with a metallic shell, have attracted significant attention owing to their unique properties and potential applications in fields such as catalysis, optics and biotechnology [1–5]. Core–shell structures are also interesting economically because a small amount of expensive material can be used to cover a core of inexpensive material [5]. Several methods for producing core–shell particles have been developed [2,4,6–10]. Many of these methods address the fabrication of particles composed of cores of metal oxides and shells of noble metals, such as Ag and Au [4,6–9]. The fabrication methods
Y. Kobayashi et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 448 (2014) 88–92
consist of multiple steps: synthesis of the core particles, surfacemodification of the particles to improve the affinity between the particle surface and metals, deposition of metallic nanoparticles on the particle surface, and formation of a metallic shell through further deposition and growth of the deposited metallic nanoparticles. In addition to Ag and Au, Pt is also an interesting noble metal because Pt nanomaterials exhibit extraordinary chemical and physical properties that can be harnessed for sensors, catalysts, and magnetic materials [11–17]. Despite this interest in Pt nanomaterials, few studies have explored the fabrication of Pt shells on metal oxide particles. Kim et al. developed a laser-induced method for fabricating Pt shells that display enhanced catalytic and Raman properties [2]. Liu et al. fabricated Fe2 O3 /Pt core–shell particles by reducing Pt ions in the presence of Fe2 O3 particles and investigated their electrochemical activity in methanol oxidation [10]. In these methods, Pt nanoparticles were deposited on the surfaces of support metal oxide particles by reducing Pt ions in the presence of the support particles, and the formation of a Pt shell was achieved through the further deposition of Pt nanoparticles. Thus, in these methods, the Pt nanoparticles were first fabricated in solution rather than on the support particles. This approach can produce aggregates of Pt nanoparticles during or prior to deposition, reducing the yield of the core–shell particles. Electroless metal plating techniques can plate metallic films onto insulating support materials [18–20]. These techniques comprise three steps. First, the support surface is sensitized by the adsorption of a sensitizer. Second, the surface is activated through the pre-deposition of metallic nanoparticles. Third, metal is deposited and plated onto the pre-deposited metallic nuclei. Importantly, electroless metal plating techniques can produce core–shell particles at high yield: because metal deposition following the generation of metal nuclei occurs only on pre-deposited metal surfaces, all the generated metal nuclei will be consumed to produce core–shell particles. This study aimed to develop a method for fabricating TiO2 /Pt core–shell particles using electroless metal plating. In a preliminary experiment using a conventional plating method, the TiO2 particles were damaged during sensitization and activation because the pH values of the solutions used for sensitization and activation were too low for the TiO2 particles to be chemically stable. Therefore, the conventional metal plating method was modified in the present work: Pt nuclei were pre-deposited by reducing Pt ions in the presence of TiO2 particles (TiO2 –Pt), and the growth of the pre-deposited Pt and formation of a Pt shell were achieved using a Pt-plating solution. Importantly, this modification of the conventional plating method will simplify the fabrication of TiO2 /Pt core–shell particles because the processes of sensitization and activation of the support surface in the conventional metal plating method are replaced with a single pre-deposition of Pt nanoparticles.
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hexachloroplatinate(IV) hexahydrate (H2 PtCl6 ·6H2 O) (98.5%) was used as a Pt source. For the preparation of the Pt-pre-deposited TiO2 particle colloid solution, citric acid (cit) monohydrate (>99.5%) and sodium borohydride (NaBH4 ) (>92.0%) were used as a stabilizer and a reducing reagent, respectively. For the preparation of the TiO2 /Pt core–shell particle colloid solution, hydrazine monohydrate (>98.0%) was used as a reducing agent. Aqueous solutions of sodium hydroxide (NaOH) (1 M) and hydrochloric acid (HCl) (35.0–37.0%) were used to adjust the pH of the particle colloid solutions for the -potential measurements of the particles. All chemicals besides PDADMAC were purchased from Wako Pure Chemicals, Ltd. and used as received. The water used for solution preparation was ion-exchanged and distilled using a Shimadzu SWAC-500. 2.2. Preparation
2. Experimental
A TiO2 particle colloid solution was prepared by the hydrolysis and condensation of TTIP. First, a mixture of methylamine, water, acetonitrile and 2-propanol was added to TTIP dissolved in 2-propanol at 30 ◦ C. The mixture was then allowed to react for 2 h. The initial concentrations of TTIP, methylamine and water were 30 × 10−3 , 5 × 10−3 and 0.3 M, respectively, in a co-solvent of 72.4% (v/v) acetonitrile/2-propanol. Following the synthesis of the TiO2 colloids, the solvent was replaced with water by centrifuging the solution at 15,000 rpm, removing the solvent, adding water and shaking on a vortex mixer. Subsequently, a PDADMAC aqueous solution was added to the TiO2 colloids for surface-modification (TiO2 –PDADMAC). The reaction temperature was 35 ◦ C. After 24 h, the solvent of the TiO2 –PDADMAC colloids was replaced with water using the same process used for the as-prepared TiO2 colloids. The pre-deposition of Pt nanoparticles was performed by reducing Pt ions in the presence of TiO2 –PDADMAC particles. Cit, H2 PtCl6 , NaOH and NaBH4 aqueous solutions at 35 ◦ C (TiO2 –Pt) were sequentially added to the TiO2 –PDADMAC colloid solution. The NaOH addition was performed to adjust the pH to approximately 6.8 because NaBH4 exhibits optimum reducing activity in neutral and basic pH ranges. The concentrations of TiO2 , Pt, NaBH4 and cit in the final solution were 7.5 × 10−3 , 1.6 × 10−3 , 1.92 × 10−3 and 2.0 × 10−3 M, respectively. After 15 min, the solvent of the TiO2 –Pt particles was replaced with water using the same process outlined above for the as-prepared TiO2 colloids. The formation of TiO2 /Pt core–shell particles was achieved using an electroless metal plating method. Following a previous study on electroless Pt plating [21], a Pt plating solution was prepared by dissolving H2 PtCl6 ·6H2 O in a 4% HCl aqueous solution to give a Pt concentration of 1.6 × 10−3 M. The Pt plating solution was added to the TiO2 –Pt colloid solution at 5 ◦ C. After 1 h, hydrazine was added to the mixture to initiate Pt deposition (TiO2 /Pt). The concentrations of TiO2 , Pt and hydrazine in the final solution were 0.375 × 10−3 −7.5 × 10−3 , 0.8 × 10−3 and 1.0 × 10−3 , respectively. After 24 h, the solvent of the TiO2 /Pt colloids was replaced with water using the same process as outlined above for the as-prepared TiO2 colloids.
2.1. Chemicals
2.3. Characterization
The starting reagent for the titania particles was titanium tetraisopropoxide (TTIP, 95%). A mixture of 2-propanol (99.7%) and acetonitrile (99.5%) was used as a solvent. The catalyst used for the sol–gel reaction of TTIP was methylamine (MA, 40% aqueous solution). A poly(diallyldimethylammonium chloride) solution (PDADMAC) (average Mw : 100,000–200,000, 20 wt% in H2 O, Sigma-Aldrich) was used for the surface-modification of the TiO2 particles. The PDADMAC was dialyzed using a cellulose tube (pore size: 5.0 nm, As-One Co., Osaka, Japan) prior to use. Hydrogen
TEM was used to investigate the morphology of the particles and performed with a JEOL JEM-2000FX II microscope operating at 200 kV. To prepare samples for TEM, the colloid solution was dropped on a collodion-coated copper grid, and the dispersion medium was evaporated in air followed by in vacuo. The particle powder was characterized by X-ray diffractometry (XRD). The volume-average particle sizes were determined by measuring dozens of particle diameters in the TEM images. The introduction of PDADMAC on the particle surfaces was qualitatively confirmed
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Fig. 1. Photograph of the TiO2 colloid solution (a), and TEM image of the TiO2 particles in the colloid solution (b).
using -potential measurements. To measure the -potential of the particles, electrophoretic light scattering (ELS) was performed with a Brookhaven Zeta Plus zeta potential analyzer. Either aqueous HCl or aqueous NaOH was added to the colloid solution to vary the pH for the ELS measurement. Powder samples, which were obtained by pulverizing particles with a mortar, were used for the XRD measurements. The XRD measurements were performed with a Rigaku Ultima IV X-ray diffractometer at 40 kV and 30 mA using Cu K␣ radiation. 3. Results and discussion 3.1. TiO2 particles Fig. 1(a) shows a photograph of the as-prepared particle colloid solution. The TTIP solution became opaque following the addition of methylamine/H2 O/acetonitrile/2-propanol, indicating the formation of TiO2 particles. No particle sedimentation was observed over a period of several hours, suggesting that the TiO2 particles were colloidally stable. Fig. 1(b) shows a TEM image of the as-prepared particles. The particles were quasi-spherical with an average diameter of 119.3 ± 22.1 nm. Because a TiO2 particle colloid solution was prepared for each fabrication of the TiO2 –Pt particles or the TiO2 /Pt core–shell particles, the average diameters of TiO2 particles may be different for each fabrication. However, the average diameter would be distributed within the statistical error. Fig. 2(a) displays the -potential of the TiO2 particles as a function of solution pH. The particles had an isoelectric point (i.e.p.) of 3.2. The i.e.p. was smaller than those previously reported for TiO2 particles fabricated by the sol–gel method (5.1–6.7) [22–24]. Hydrolyzed species such as Ti–O− , which were derived from the TTIP remaining on the TiO2 particles because of an incomplete sol–gel reaction, lowered the i.e.p. of the particles. Fig. 3(a) shows the XRD pattern of the particles. A structure-less pattern was observed, indicating that the as-prepared TiO2 particles were either amorphous or comprised crystallites that were too fine to be detected by XRD.
Fig. 2. -Potentials as a function of pH for (a) TiO2 particles and (b) TiO2 /PDADMAC particles.
3.3. TiO2 –Pt particles Fig. 4(A) and (B) show photographs of the TiO2 –Pt colloid solutions. Both colloid solutions remained colloidally stable following Pt pre-deposition. The colloid solution prepared from the TiO2 particles lacking PDADMAC appeared slightly brownish and opaque. In contrast, with the use of the TiO2 –PDADMAC particles, the TiO2 –Pt particle colloid solution assumed a color of dark black. These observations suggest that Pt readily formed composites with the TiO2 particles–PDADMAC. Fig. 4(a) and (b) show TEM images of the TiO2 –Pt particles. The darker and lighter regions of the particles correspond to Pt and TiO2 , respectively. Small amounts of Pt nanoparticles were attached to the unmodified TiO2 particles with as average diameter of 113.8 ± 19.6 nm. In contrast, numerous Pt nanoparticles coated the surfaces of the TiO2 –PDADMAC particles with as average diameter of 121.3 ± 18.2 nm. With the respect to the surface-modification with PDADMAC, since the average diameters of the unmodified TiO2 particles and the modified TiO2 particles were almost the same within the statistical error, the PDADMAC was considered to form quite thin layers
3.2. TiO2 –PDADMAC particles The TiO2 particles were colloidally stable even after surfacemodification with PDADMAC. Fig. 2(b) shows the -potential of the TiO2 –PDADMAC particles as a function of the pH of the solution. The i.e.p. of the TiO2 –PDADMAC particles was 7.0, higher than that of the TiO2 particles. The higher i.e.p. was most likely due to the cationic group of PDADMAC. Accordingly, this result indicated that the particle surfaces were successfully modified with PDADMAC.
Fig. 3. XRD patterns of (a) TiO2 particles and (b) TiO2 /Pt particles. Sample (b) is the same as sample (a) in Fig. 5.
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Fig. 4. Photographs of the TiO2 –Pt particle colloid solutions. For samples (A) and (B), the support particles used were TiO2 particles that were either unmodified or modified with PDADMAC, respectively. (a) and (b) Show TEM images of the TiO2 –Pt particles in colloid solutions (A) and (B), respectively.
on the TiO2 particle surface. According to Ashayer et al.’s work fabricating SiO2 /Au core–shell particles, the surface-modification of SiO2 particles with PDADMAC promotes the formation of a core–shell structure because the positively charged sites in PDADMAC electrostatically attract negatively charged Au particles [7]. A similar electrostatic mechanism may also be at play in the present work; the cationic surfaces of the TiO2 –PDADMAC particles most likely adsorbed many anionic PtCl6 2− ions electrostatically. This adsorption generated Pt nuclei close to the sites of adsorption. As a result, the Pt nanoparticles were efficiently attached to the surfaces of the TiO2 –PDADMAC particles. 3.4. TiO2 /Pt particles Fig. 5(A–C) show photographs of the TiO2 /Pt colloid solutions prepared using various TiO2 concentrations. All the colloid solutions were black in color. A black sediment appeared for a TiO2 concentration of 0.375 × 10−3 M, though no sediment was observed for TiO2 concentrations of 1.5 × 10−3 and 7.5 × 10−3 M. Fig. 5(a–c) show TEM images of the TiO2 /Pt particles. The morphology of the support particles was constant even after the electroless plating process, indicating that the electroless plating process did not damage the particle morphology. For the TiO2 /Pt particles fabricated at a low TiO2 concentration of 0.375 × 10−3 M, Pt shells with a thickness of approximately 25 nm formed successfully on the TiO2 –Pt particle surfaces, surrounding the TiO2 –Pt particles, and no independent Pt nanoparticles were produced. This result indicates that the majority of the Pt nuclei generated in the electroless plating process were consumed for the formation of Pt shells. Fig. 3(a) shows the XRD pattern of the TiO2 /Pt particles. Several peaks were detected at 39.9, 46.3 and 67.8 degrees. These were attributed to the (1 1 1), (2 0 0) and (2 2 0) planes of cubic metallic Pt (JCPDS card no. 4-0802), respectively. The average crystal size of the Pt, as estimated from the line broadening of the 46.3 degree peak in the XRD spectrum according to the Scherrer equation, was 8.4 nm. Because
Fig. 5. Photographs of the TiO2 /Pt particle colloid solutions prepared at the Pt concentrations of (A) 0.375 × 10−3 , (B) 1.5 × 10−3 and (C) 7.5 × 10−3 M. The support particles used were from sample (b) in Fig. 4. (a)–(c) Show TEM images of the TiO2 /Pt particles in colloid solutions (A), (B) and (C), respectively.
the shell thickness observed with TEM was larger than the crystal size, the Pt shells consisted of polycrystallites. This result indicates that the Pt ions were successfully reduced and converted to shells composed of metallic Pt polycrystallites using the present method. The Pt shell thickness was approximately 6 nm at the TiO2 concentration of 1.5 × 10−3 M, that is, the Pt shells were made thinner by increasing the TiO2 concentration to 1.5 × 10−3 M. The increase in the TiO2 concentration decreased the amount of generated Pt nuclei relative to the number of TiO2 particles, because the initial Pt concentrations were kept constant in the two TiO2 /Pt colloid solutions. As a result, a decrease in the amount of Pt shells occurred at higher TiO2 concentrations. This phenomenon completely prevented the formation of Pt shells at the Pt concentration as high as 7.5 × 10−3 M, though the Pt shells with a thickness of approximately 3 nm were observed in some particles. For the TiO2 concentration of 0.375 × 10−3 M, a black sediment formed, as shown in Fig. 5(a-1). The complete coating of the TiO2 particles with Pt shells concealed the TiO2 surface. Particles with a TiO2 surface were more colloidally stable in aqueous solution than those with a Pt surface because TiO2 surfaces are more hydrophilic than Pt surfaces. Accordingly, the observation of a black sediment indicated the successful formation of core–shell structures comprising a TiO2 core and a Pt shell.
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4. Conclusions Pt nanoparticles were uniformly pre-deposited on the surfaces of sub-micrometer-sized titania particles modified with PDADMAC by the direct reduction of Pt ions in the presence of titania particles. The shells, which were composed of Pt crystallites, were fabricated on the surfaces of the TiO2 –Pt particles using an electroless metal plating technique in which the TiO2 –Pt particles act as a substrate. The Pt shell thickness decreased with increasing TiO2 concentration, allowing the Pt shell thickness to be tailored. Conditions of 0.8 × 10−3 M TiO2 and 0.375 × 10−3 M Pt provided TiO2 /Pt particles with a Pt shell thickness of ca. 25 nm and a Pt crystal size of 8.4 nm. A study on application of TiO2 /Pt particles to catalyst is being prepared for their practical use. Acknowledgments This work was partially supported by K. K. Shinko Kagaku Kogyosho. We express our thanks to Prof. T. Noguchi at the College of Science of Ibaraki University, Japan for assistance with the TEM observations. References [1] S. Kalele, S.W. Gosavi, J. Urban, S.K. Kulkarni, Nanoshell particles: synthesis, properties and applications, Curr. Sci. 91 (2006) 1038–1052. [2] M.R. Kim, J.-Y. Kim, S.J. Kim, D.-J. Jang, Laser-induced fabrication of platinum nanoshells having enhanced catalytic and Raman properties, Appl. Catal., A 393 (2011) 317–322. [3] H.-W. Kim, K.-M. Kang, H.-Y. Kwak, J.H. Kim, Preparation of supported Ni catalysts on various metal oxides with core/shell structures and their tests for the steam reforming of methane, Chem. Eng. J. 168 (2011) 775– 783. [4] S.N. Abdollahi, M. Naderi, G. Amoabediny, Synthesis and physicochemical characterization of tunable silica–gold nanoshells via seed growth method, Colloids Surf., A 414 (2012) 345–351. [5] B.J. Jankiewicz, D. Jamiola, J. Choma, M. Jaroniec, Silica–metal core–shell nanostructures, Adv. Colloid Interface Sci. 170 (2012) 28–47. [6] J.C. Flores, V. Torres, M. Popa, D. Crespo, J.M. Calderon-Moreno, Variations in morphologies of silver nanoshells on silica spheres, Colloids Surf., A 330 (2008) 86–90. [7] R. Ashayer, S.H. Mannan, S. Sajjadi, Synthesis and characterization of gold nanoshells using poly(diallyldimethyl ammonium chloride), Colloids Surf., A 329 (2008) 134–141.
[8] J. Choma, D. Jamioła, P. Nyga, M. Jaroniec, Synthesis of rod-like silica–gold core–shell structures, Colloids Surf., A 393 (2012) 37–41. [9] N. Zhang, F. Xue, X. Yu, H. Zhou, E. Ding, Metal Fe3+ ions assisted synthesis of highly monodisperse Ag/SiO2 nanohybrids and their antibacterial activity, J. Alloys Compd. 550 (2013) 209–215. [10] Y.-T. Liu, Q.-B. Yuan, D.-H. Duan, Z.-L. Zhang, X.-G. Hao, G.-Q. Wei, S.-B. Liu, Electrochemical activity and stability of core–shell Fe2 O3 /Pt nanoparticles for methanol oxidation, J. Power Sources 243 (2013) 622–629. [11] N.F. Atta, M.F. El-Kady, Novel poly(3-methylthiophene)/Pd, Pt nanoparticle sensor: synthesis, characterization and its application to the simultaneous analysis of dopamine and ascorbic acid in biological fluids, Sens. Actuators, B 145 (2010) 299–310. [12] E.J. Yoo, T. Okada, T. Akita, M. Kohyama, I. Honma, J. Nakamura, Sub-nano-Pt cluster supported on graphene nanosheets for CO tolerant catalysts in polymer electrolyte fuel cells, J. Power Sources 196 (2011) 110–115. [13] Y. Benguedouar, N. Keghouche, J. Belloni, Structural and magnetic properties of Ni–Pt nanoalloys supported on silica, Mater. Sci. Eng., B 177 (2012) 27–33. [14] K. Fuku, M. Goto, T. Sakano, T. Kamegawa, K. Mori, H. Yamashita, Efficient degradation of CO and acetaldehyde using nano-sized Pt catalysts supported on CeO2 and CeO2 /ZSM-5 composite, Catal. Today 201 (2013) 57–61. [15] F.A. Harraz, A.M. Salem, B.A. Mohamed, A. Kandil, I.A. Ibrahim, Electrochemically deposited cobalt/platinum (Co/Pt) film into porous silicon: structural investigation and magnetic properties, Appl. Surf. Sci. 264 (2013) 391–398. [16] S. Vahdatifar, A.A. Khodadadi, Y. Mortazavi, Effects of nanoadditives on stability of Pt/SnO2 as a sensing materialfor detection of CO, Sens. Actuators, B 191 (2014) 421–430. [17] T. S¸ims¸ek, S¸. Özcan, Effective magnetic anisotropy enhancement of FePt nanocrystals through shape control, J. Magn. Magn. Mater. 351 (2014) 47–51. [18] V.K. Bulasara, H. Thakuria, R. Uppaluri, M.K. Purkait, Effect of process parameters on electroless plating and nickel–ceramic composite membrane characteristics, Desalination 268 (2011) 195–203. [19] K.L. Yung, Y. Xu, C.L. Kang, B.Q. Jiang, Electroless deposition of biocompatible Ag/W on quartz for the purpose of variotherm micro molding, J. Mater. Process. Technol. 213 (2013) 136–142. [20] A. Tarditi, C. Gerboni, L. Cornaglia, PdAu membranes supported onto pofvacuum-assisted ZrO2 -modified porous stainless steel substrates, J. Membr. Sci. 428 (2013) 1–10. [21] C.-M. Chen, C.-H. Chen, S.-J. Cherng, T.-C. Wei, Electroless deposition of platinum on indium tin oxide glass as the counter electrode for dye-sensitized solar cells, Mater. Chem. Phys. 124 (2010) 173–178. [22] M. Kosmulski, Compilation of PZC and IEP of sparingly soluble metal oxides and hydroxides from literature, Adv. Colloid Interface Sci. 152 (2009) 14–25. [23] M. Kosmulski, The pH-dependent surface charging and points of zero charge: V. Update, J. Colloid Interface Sci. 353 (2011) 1–15. [24] M. Sahu, K. Suttiponparnit, S. Suvachittanont, T. Charinpanitkul, P. Biswas, Characterization of doped TiO2 nanoparticle dispersions, Chem. Eng. Sci. 66 (2011) 3482–3490.
Contents lists available at ScienceDirect
Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Fabrication of TiO2 /Pt core–shell particles by electroless metal plating Yoshio Kobayashi a,∗ , Yuya Ishii a , Hideyuki Yamane a , Ken-ichi Watanabe b , Hidekazu Koda b , Hiroshi Kunigami b , Hideki Kunigami b a Department of Biomolecular Functional Engineering, College of Engineering, Ibaraki University, 4-12-1 Naka-narusawa-cho, Hitachi, Ibaraki, 316-8511, Japan b K. K. Shinko Kagaku Kogyosho, 1544-19 Mashimori, Koshigaya, Saitama, 343-0012, Japan
h i g h l i g h t s
g r a p h i c a l
a b s t r a c t
• The present work proposes a simple method for fabricating TiO2 /Pt core–shell particles. • TiO2 particles prepared by a sol–gel method were surface-modified with poly(diallyldimethylammonium chloride). • Pt fine particles were pre-deposited on the surface-modified TiO2 particles by reducing Pt ions in the presence of the TiO2 particles. • Pt shells were formed on the Pt-predeposited TiO2 particles by electroless metal plating method.
a r t i c l e
i n f o
Article history: Received 15 December 2013 Received in revised form 5 February 2014 Accepted 7 February 2014 Available online 16 February 2014 Keywords: TiO2 Pt Core–shell Particles Electroless metal plating
a b s t r a c t An electroless metal plating method was used to form Pt shells on sub-micrometer-sized titania particles fabricated by a sol–gel method. The electroless metal plating method comprised three steps: (1) surface-modification of titania particles with poly(diallyldimethylammonium chloride) (PDADMAC) (TiO2 –PDADMAC), (2) pre-deposition of Pt nuclei or Pt fine particles on the titania particles by reducing Pt ions in the presence of TiO2 –PDADMAC particles (TiO2 –Pt) and (3) growth of the pre-deposited Pt by immersing the TiO2 –Pt particles in a Pt-plating solution. TEM observation and X-ray diffractometry revealed that surface modification with PDADMAC promoted the pre-deposition of Pt and that crystalline Pt shells with a thickness of approximately 25 nm were successfully produced on the titania particles using initial concentrations of 0.8 × 10−3 M TiO2 and 0.375 × 10−3 M Pt in the Pt-plating solution. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Composites of plural materials are of special interest because they exhibit multiple functions derived from their individual components. Among the numerous structures that have been proposed for functional materials to date, core–shell structures, in which a
∗ Corresponding author. Tel.: +81 294 38 5052; fax: +81 294 38 5078. E-mail address: [email protected] (Y. Kobayashi). http://dx.doi.org/10.1016/j.colsurfa.2014.02.018 0927-7757/© 2014 Elsevier B.V. All rights reserved.
core particle is coated with a metallic shell, have attracted significant attention owing to their unique properties and potential applications in fields such as catalysis, optics and biotechnology [1–5]. Core–shell structures are also interesting economically because a small amount of expensive material can be used to cover a core of inexpensive material [5]. Several methods for producing core–shell particles have been developed [2,4,6–10]. Many of these methods address the fabrication of particles composed of cores of metal oxides and shells of noble metals, such as Ag and Au [4,6–9]. The fabrication methods
Y. Kobayashi et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 448 (2014) 88–92
consist of multiple steps: synthesis of the core particles, surfacemodification of the particles to improve the affinity between the particle surface and metals, deposition of metallic nanoparticles on the particle surface, and formation of a metallic shell through further deposition and growth of the deposited metallic nanoparticles. In addition to Ag and Au, Pt is also an interesting noble metal because Pt nanomaterials exhibit extraordinary chemical and physical properties that can be harnessed for sensors, catalysts, and magnetic materials [11–17]. Despite this interest in Pt nanomaterials, few studies have explored the fabrication of Pt shells on metal oxide particles. Kim et al. developed a laser-induced method for fabricating Pt shells that display enhanced catalytic and Raman properties [2]. Liu et al. fabricated Fe2 O3 /Pt core–shell particles by reducing Pt ions in the presence of Fe2 O3 particles and investigated their electrochemical activity in methanol oxidation [10]. In these methods, Pt nanoparticles were deposited on the surfaces of support metal oxide particles by reducing Pt ions in the presence of the support particles, and the formation of a Pt shell was achieved through the further deposition of Pt nanoparticles. Thus, in these methods, the Pt nanoparticles were first fabricated in solution rather than on the support particles. This approach can produce aggregates of Pt nanoparticles during or prior to deposition, reducing the yield of the core–shell particles. Electroless metal plating techniques can plate metallic films onto insulating support materials [18–20]. These techniques comprise three steps. First, the support surface is sensitized by the adsorption of a sensitizer. Second, the surface is activated through the pre-deposition of metallic nanoparticles. Third, metal is deposited and plated onto the pre-deposited metallic nuclei. Importantly, electroless metal plating techniques can produce core–shell particles at high yield: because metal deposition following the generation of metal nuclei occurs only on pre-deposited metal surfaces, all the generated metal nuclei will be consumed to produce core–shell particles. This study aimed to develop a method for fabricating TiO2 /Pt core–shell particles using electroless metal plating. In a preliminary experiment using a conventional plating method, the TiO2 particles were damaged during sensitization and activation because the pH values of the solutions used for sensitization and activation were too low for the TiO2 particles to be chemically stable. Therefore, the conventional metal plating method was modified in the present work: Pt nuclei were pre-deposited by reducing Pt ions in the presence of TiO2 particles (TiO2 –Pt), and the growth of the pre-deposited Pt and formation of a Pt shell were achieved using a Pt-plating solution. Importantly, this modification of the conventional plating method will simplify the fabrication of TiO2 /Pt core–shell particles because the processes of sensitization and activation of the support surface in the conventional metal plating method are replaced with a single pre-deposition of Pt nanoparticles.
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hexachloroplatinate(IV) hexahydrate (H2 PtCl6 ·6H2 O) (98.5%) was used as a Pt source. For the preparation of the Pt-pre-deposited TiO2 particle colloid solution, citric acid (cit) monohydrate (>99.5%) and sodium borohydride (NaBH4 ) (>92.0%) were used as a stabilizer and a reducing reagent, respectively. For the preparation of the TiO2 /Pt core–shell particle colloid solution, hydrazine monohydrate (>98.0%) was used as a reducing agent. Aqueous solutions of sodium hydroxide (NaOH) (1 M) and hydrochloric acid (HCl) (35.0–37.0%) were used to adjust the pH of the particle colloid solutions for the -potential measurements of the particles. All chemicals besides PDADMAC were purchased from Wako Pure Chemicals, Ltd. and used as received. The water used for solution preparation was ion-exchanged and distilled using a Shimadzu SWAC-500. 2.2. Preparation
2. Experimental
A TiO2 particle colloid solution was prepared by the hydrolysis and condensation of TTIP. First, a mixture of methylamine, water, acetonitrile and 2-propanol was added to TTIP dissolved in 2-propanol at 30 ◦ C. The mixture was then allowed to react for 2 h. The initial concentrations of TTIP, methylamine and water were 30 × 10−3 , 5 × 10−3 and 0.3 M, respectively, in a co-solvent of 72.4% (v/v) acetonitrile/2-propanol. Following the synthesis of the TiO2 colloids, the solvent was replaced with water by centrifuging the solution at 15,000 rpm, removing the solvent, adding water and shaking on a vortex mixer. Subsequently, a PDADMAC aqueous solution was added to the TiO2 colloids for surface-modification (TiO2 –PDADMAC). The reaction temperature was 35 ◦ C. After 24 h, the solvent of the TiO2 –PDADMAC colloids was replaced with water using the same process used for the as-prepared TiO2 colloids. The pre-deposition of Pt nanoparticles was performed by reducing Pt ions in the presence of TiO2 –PDADMAC particles. Cit, H2 PtCl6 , NaOH and NaBH4 aqueous solutions at 35 ◦ C (TiO2 –Pt) were sequentially added to the TiO2 –PDADMAC colloid solution. The NaOH addition was performed to adjust the pH to approximately 6.8 because NaBH4 exhibits optimum reducing activity in neutral and basic pH ranges. The concentrations of TiO2 , Pt, NaBH4 and cit in the final solution were 7.5 × 10−3 , 1.6 × 10−3 , 1.92 × 10−3 and 2.0 × 10−3 M, respectively. After 15 min, the solvent of the TiO2 –Pt particles was replaced with water using the same process outlined above for the as-prepared TiO2 colloids. The formation of TiO2 /Pt core–shell particles was achieved using an electroless metal plating method. Following a previous study on electroless Pt plating [21], a Pt plating solution was prepared by dissolving H2 PtCl6 ·6H2 O in a 4% HCl aqueous solution to give a Pt concentration of 1.6 × 10−3 M. The Pt plating solution was added to the TiO2 –Pt colloid solution at 5 ◦ C. After 1 h, hydrazine was added to the mixture to initiate Pt deposition (TiO2 /Pt). The concentrations of TiO2 , Pt and hydrazine in the final solution were 0.375 × 10−3 −7.5 × 10−3 , 0.8 × 10−3 and 1.0 × 10−3 , respectively. After 24 h, the solvent of the TiO2 /Pt colloids was replaced with water using the same process as outlined above for the as-prepared TiO2 colloids.
2.1. Chemicals
2.3. Characterization
The starting reagent for the titania particles was titanium tetraisopropoxide (TTIP, 95%). A mixture of 2-propanol (99.7%) and acetonitrile (99.5%) was used as a solvent. The catalyst used for the sol–gel reaction of TTIP was methylamine (MA, 40% aqueous solution). A poly(diallyldimethylammonium chloride) solution (PDADMAC) (average Mw : 100,000–200,000, 20 wt% in H2 O, Sigma-Aldrich) was used for the surface-modification of the TiO2 particles. The PDADMAC was dialyzed using a cellulose tube (pore size: 5.0 nm, As-One Co., Osaka, Japan) prior to use. Hydrogen
TEM was used to investigate the morphology of the particles and performed with a JEOL JEM-2000FX II microscope operating at 200 kV. To prepare samples for TEM, the colloid solution was dropped on a collodion-coated copper grid, and the dispersion medium was evaporated in air followed by in vacuo. The particle powder was characterized by X-ray diffractometry (XRD). The volume-average particle sizes were determined by measuring dozens of particle diameters in the TEM images. The introduction of PDADMAC on the particle surfaces was qualitatively confirmed
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Fig. 1. Photograph of the TiO2 colloid solution (a), and TEM image of the TiO2 particles in the colloid solution (b).
using -potential measurements. To measure the -potential of the particles, electrophoretic light scattering (ELS) was performed with a Brookhaven Zeta Plus zeta potential analyzer. Either aqueous HCl or aqueous NaOH was added to the colloid solution to vary the pH for the ELS measurement. Powder samples, which were obtained by pulverizing particles with a mortar, were used for the XRD measurements. The XRD measurements were performed with a Rigaku Ultima IV X-ray diffractometer at 40 kV and 30 mA using Cu K␣ radiation. 3. Results and discussion 3.1. TiO2 particles Fig. 1(a) shows a photograph of the as-prepared particle colloid solution. The TTIP solution became opaque following the addition of methylamine/H2 O/acetonitrile/2-propanol, indicating the formation of TiO2 particles. No particle sedimentation was observed over a period of several hours, suggesting that the TiO2 particles were colloidally stable. Fig. 1(b) shows a TEM image of the as-prepared particles. The particles were quasi-spherical with an average diameter of 119.3 ± 22.1 nm. Because a TiO2 particle colloid solution was prepared for each fabrication of the TiO2 –Pt particles or the TiO2 /Pt core–shell particles, the average diameters of TiO2 particles may be different for each fabrication. However, the average diameter would be distributed within the statistical error. Fig. 2(a) displays the -potential of the TiO2 particles as a function of solution pH. The particles had an isoelectric point (i.e.p.) of 3.2. The i.e.p. was smaller than those previously reported for TiO2 particles fabricated by the sol–gel method (5.1–6.7) [22–24]. Hydrolyzed species such as Ti–O− , which were derived from the TTIP remaining on the TiO2 particles because of an incomplete sol–gel reaction, lowered the i.e.p. of the particles. Fig. 3(a) shows the XRD pattern of the particles. A structure-less pattern was observed, indicating that the as-prepared TiO2 particles were either amorphous or comprised crystallites that were too fine to be detected by XRD.
Fig. 2. -Potentials as a function of pH for (a) TiO2 particles and (b) TiO2 /PDADMAC particles.
3.3. TiO2 –Pt particles Fig. 4(A) and (B) show photographs of the TiO2 –Pt colloid solutions. Both colloid solutions remained colloidally stable following Pt pre-deposition. The colloid solution prepared from the TiO2 particles lacking PDADMAC appeared slightly brownish and opaque. In contrast, with the use of the TiO2 –PDADMAC particles, the TiO2 –Pt particle colloid solution assumed a color of dark black. These observations suggest that Pt readily formed composites with the TiO2 particles–PDADMAC. Fig. 4(a) and (b) show TEM images of the TiO2 –Pt particles. The darker and lighter regions of the particles correspond to Pt and TiO2 , respectively. Small amounts of Pt nanoparticles were attached to the unmodified TiO2 particles with as average diameter of 113.8 ± 19.6 nm. In contrast, numerous Pt nanoparticles coated the surfaces of the TiO2 –PDADMAC particles with as average diameter of 121.3 ± 18.2 nm. With the respect to the surface-modification with PDADMAC, since the average diameters of the unmodified TiO2 particles and the modified TiO2 particles were almost the same within the statistical error, the PDADMAC was considered to form quite thin layers
3.2. TiO2 –PDADMAC particles The TiO2 particles were colloidally stable even after surfacemodification with PDADMAC. Fig. 2(b) shows the -potential of the TiO2 –PDADMAC particles as a function of the pH of the solution. The i.e.p. of the TiO2 –PDADMAC particles was 7.0, higher than that of the TiO2 particles. The higher i.e.p. was most likely due to the cationic group of PDADMAC. Accordingly, this result indicated that the particle surfaces were successfully modified with PDADMAC.
Fig. 3. XRD patterns of (a) TiO2 particles and (b) TiO2 /Pt particles. Sample (b) is the same as sample (a) in Fig. 5.
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Fig. 4. Photographs of the TiO2 –Pt particle colloid solutions. For samples (A) and (B), the support particles used were TiO2 particles that were either unmodified or modified with PDADMAC, respectively. (a) and (b) Show TEM images of the TiO2 –Pt particles in colloid solutions (A) and (B), respectively.
on the TiO2 particle surface. According to Ashayer et al.’s work fabricating SiO2 /Au core–shell particles, the surface-modification of SiO2 particles with PDADMAC promotes the formation of a core–shell structure because the positively charged sites in PDADMAC electrostatically attract negatively charged Au particles [7]. A similar electrostatic mechanism may also be at play in the present work; the cationic surfaces of the TiO2 –PDADMAC particles most likely adsorbed many anionic PtCl6 2− ions electrostatically. This adsorption generated Pt nuclei close to the sites of adsorption. As a result, the Pt nanoparticles were efficiently attached to the surfaces of the TiO2 –PDADMAC particles. 3.4. TiO2 /Pt particles Fig. 5(A–C) show photographs of the TiO2 /Pt colloid solutions prepared using various TiO2 concentrations. All the colloid solutions were black in color. A black sediment appeared for a TiO2 concentration of 0.375 × 10−3 M, though no sediment was observed for TiO2 concentrations of 1.5 × 10−3 and 7.5 × 10−3 M. Fig. 5(a–c) show TEM images of the TiO2 /Pt particles. The morphology of the support particles was constant even after the electroless plating process, indicating that the electroless plating process did not damage the particle morphology. For the TiO2 /Pt particles fabricated at a low TiO2 concentration of 0.375 × 10−3 M, Pt shells with a thickness of approximately 25 nm formed successfully on the TiO2 –Pt particle surfaces, surrounding the TiO2 –Pt particles, and no independent Pt nanoparticles were produced. This result indicates that the majority of the Pt nuclei generated in the electroless plating process were consumed for the formation of Pt shells. Fig. 3(a) shows the XRD pattern of the TiO2 /Pt particles. Several peaks were detected at 39.9, 46.3 and 67.8 degrees. These were attributed to the (1 1 1), (2 0 0) and (2 2 0) planes of cubic metallic Pt (JCPDS card no. 4-0802), respectively. The average crystal size of the Pt, as estimated from the line broadening of the 46.3 degree peak in the XRD spectrum according to the Scherrer equation, was 8.4 nm. Because
Fig. 5. Photographs of the TiO2 /Pt particle colloid solutions prepared at the Pt concentrations of (A) 0.375 × 10−3 , (B) 1.5 × 10−3 and (C) 7.5 × 10−3 M. The support particles used were from sample (b) in Fig. 4. (a)–(c) Show TEM images of the TiO2 /Pt particles in colloid solutions (A), (B) and (C), respectively.
the shell thickness observed with TEM was larger than the crystal size, the Pt shells consisted of polycrystallites. This result indicates that the Pt ions were successfully reduced and converted to shells composed of metallic Pt polycrystallites using the present method. The Pt shell thickness was approximately 6 nm at the TiO2 concentration of 1.5 × 10−3 M, that is, the Pt shells were made thinner by increasing the TiO2 concentration to 1.5 × 10−3 M. The increase in the TiO2 concentration decreased the amount of generated Pt nuclei relative to the number of TiO2 particles, because the initial Pt concentrations were kept constant in the two TiO2 /Pt colloid solutions. As a result, a decrease in the amount of Pt shells occurred at higher TiO2 concentrations. This phenomenon completely prevented the formation of Pt shells at the Pt concentration as high as 7.5 × 10−3 M, though the Pt shells with a thickness of approximately 3 nm were observed in some particles. For the TiO2 concentration of 0.375 × 10−3 M, a black sediment formed, as shown in Fig. 5(a-1). The complete coating of the TiO2 particles with Pt shells concealed the TiO2 surface. Particles with a TiO2 surface were more colloidally stable in aqueous solution than those with a Pt surface because TiO2 surfaces are more hydrophilic than Pt surfaces. Accordingly, the observation of a black sediment indicated the successful formation of core–shell structures comprising a TiO2 core and a Pt shell.
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4. Conclusions Pt nanoparticles were uniformly pre-deposited on the surfaces of sub-micrometer-sized titania particles modified with PDADMAC by the direct reduction of Pt ions in the presence of titania particles. The shells, which were composed of Pt crystallites, were fabricated on the surfaces of the TiO2 –Pt particles using an electroless metal plating technique in which the TiO2 –Pt particles act as a substrate. The Pt shell thickness decreased with increasing TiO2 concentration, allowing the Pt shell thickness to be tailored. Conditions of 0.8 × 10−3 M TiO2 and 0.375 × 10−3 M Pt provided TiO2 /Pt particles with a Pt shell thickness of ca. 25 nm and a Pt crystal size of 8.4 nm. A study on application of TiO2 /Pt particles to catalyst is being prepared for their practical use. Acknowledgments This work was partially supported by K. K. Shinko Kagaku Kogyosho. We express our thanks to Prof. T. Noguchi at the College of Science of Ibaraki University, Japan for assistance with the TEM observations. References [1] S. Kalele, S.W. Gosavi, J. Urban, S.K. Kulkarni, Nanoshell particles: synthesis, properties and applications, Curr. Sci. 91 (2006) 1038–1052. [2] M.R. Kim, J.-Y. Kim, S.J. Kim, D.-J. Jang, Laser-induced fabrication of platinum nanoshells having enhanced catalytic and Raman properties, Appl. Catal., A 393 (2011) 317–322. [3] H.-W. Kim, K.-M. Kang, H.-Y. Kwak, J.H. Kim, Preparation of supported Ni catalysts on various metal oxides with core/shell structures and their tests for the steam reforming of methane, Chem. Eng. J. 168 (2011) 775– 783. [4] S.N. Abdollahi, M. Naderi, G. Amoabediny, Synthesis and physicochemical characterization of tunable silica–gold nanoshells via seed growth method, Colloids Surf., A 414 (2012) 345–351. [5] B.J. Jankiewicz, D. Jamiola, J. Choma, M. Jaroniec, Silica–metal core–shell nanostructures, Adv. Colloid Interface Sci. 170 (2012) 28–47. [6] J.C. Flores, V. Torres, M. Popa, D. Crespo, J.M. Calderon-Moreno, Variations in morphologies of silver nanoshells on silica spheres, Colloids Surf., A 330 (2008) 86–90. [7] R. Ashayer, S.H. Mannan, S. Sajjadi, Synthesis and characterization of gold nanoshells using poly(diallyldimethyl ammonium chloride), Colloids Surf., A 329 (2008) 134–141.
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