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Nano-scaled Amorphous TiO2 Hollow Spheres: TiCl4 Liquid Droplets-based Hydrolysis Fabrication and Strong Hollow Structure-enhanced SERS Effects Haoming Bao, Hongwen Zhang, Guangqiang Liu, Yue Li, and Weiping Cai Langmuir, Just Accepted Manuscript • Publication Date (Web): 10 May 2017 Downloaded from http://pubs.acs.org on May 14, 2017
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Nano-scaled Amorphous TiO2 Hollow Spheres: TiCl4 Liquid Droplets-based Hydrolysis Fabrication and Strong Hollow Structure-enhanced SERS Effects Haoming Baoa, b, Hongwen Zhanga*, Guangqiang Liua, Yue Li a and Weiping Caia,b* a
Key Lab of Materials Physics, Anhui Key Lab of Nanomaterials and Nanotechnology,
Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, P.R. China b
University of Science and Technology of China, Hefei 230026, PR China
Abstract A very simple route is developed to fast fabricate nano-sized amorphous titanium dioxide (TiO2) hollow spheres just via dropping the pure four titanium chloride (TiCl4) liquid droplets into deionized water around room temperature. The TiO2 hollow spheres (THPs), with around 80 nm in mean diameter, can be formed within few seconds after dropping TiCl4 droplets into water. The shell layers of the obtained THPs are amorphous and porous in structure with porosity of 58% – 80% and show linear increase in the thickness with the THPs’ size. Further experiments have revealed that the reaction temperature, initial pH value and TiCl4 droplet’s size are crucial to formation, size, productivity and microstructure of the THPs. A model is proposed based on the fragmentation of liquid droplets, hydrolysis-induced formation and inward-growth of TiO2 shell layers, which can well describe the formation of the THPs. Importantly, such amorphous nanoscaled THPs have exhibited some strong hollow structure-enhanced performances. Typically, the THPs-built film shows the highest reflectivity in the visible region compared with the other structured TiO2 films. Especially, if it supports the Au nanoparticle’s film, the surface enhanced Raman scattering (SERS) effect is significantly enhanced by more than one order of magnitude. This work provides not only a simple and quick fabrication method for the THPs but also a new member for their family. KEYWORDS: Hydrolysis of TiCl4 liquid droplets; Fast synthesis; Amorphous TiO2 hollow nanospheres; Porous TiO2 shells; Hollow structure-enhanced performances. *
To whom all correspondence should be addressed E-mail:
[email protected];
[email protected], Fax: +86-551-65591434 1
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1. INTRODUCTION TiO2 has excellent physical and chemical properties, such as high thermal stability,1 super hydrophilicity,2 unique optical properties,3- 4 non-toxicity,5 and biocompatibility,6 and been extensively studied.7-11 Recently, the main researches in this area have been focused on fabrication of the micro/nanostructured TiO2 and its applications. The various shapes and structures of TiO2 have been fabricated successfully, such as nanorods,12 nanotubes,13 nanowires,14 films,15 and core-shell and hollow structured particles.16 Among them, the micro/nanostructured TiO2 hollow spheres (THPs) possess the lower density and the larger specific surface area, as well as the unique mechanical and optical properties,17-18 especially for those with small size and thin shell-layer,19-20 and hence have many applications in such as photoelectric conversion,21 lithium-ion batteries,22 photo catalysts23 and drug deliveries,24 etc. There have been many methods developed for fabrication of micro/nanostructured THPs. These methods can mainly be classified as (i) template method, which includes the hard templates (e.g., mono dispersed polymer latex, carbon and silica spheres, and metal nanoparticles)25-27
and
the
soft
templates
(e.g.,
micelles,
micro-emulsions,
macromolecules);28-29 (ii): the methods based on Ostwald ripening mechanism,30 and (iii): the methods based on Kirkendall effect.31 For instance, microscaled hollow spheres were synthesized by using the hard template polystyrene spheres (PSs)
25
or the soft templates
Tween 60 and polyethylene glycol 1000 (PEG1000);29 Anatase-phase THPs were prepared by hydrothermal treatment and Ostwald ripening process in an NH4F aqueous solution;30 Wang, et al proposed a strategy based on Kirkendall effect and synthesized the THPs.31 Totally, the hard template method typically involves preparation and surface modification of the templates,
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combination of precursors and the templates, and removal of the templates, etc. It is thus generally complex and time consuming. In addition, the collapse and deformation of the shell-layers would occur during removal of template,32 The soft templates are not stable in thermodynamics32-33 and easy to be affected by the external environment;9, 18 The latter two templates-free approaches simplify the preparation process to a certain extent, but need a variety of reaction reagents, involve some complex physical and chemical processes and hence are generally time-consuming. The THPs fabricated by these routes were mostly several hundred nanometers in size and larger than 50 nm in shell thickness, because they were built mostly by nanoparticle-blocks.21 Also, the THPs are mostly crystalline, which can provide a suitable band structure. It could be expected that the amorphous THPs tend to better chemical stability, strong adsorption properties and different optical properties from crystalline ones, as previously reported,34-36 and show some new or better performances. Till now, however, the fast and simple fabrication of the amorphous THPs with less than 100 nm in size has been expected and in challenge. Herein, we report, for the first time, a very simple route to fast synthesis of the nanoscaled THPs just by dropping the pure four titanium chloride (TiCl4) liquid droplets into deionized water around room temperature. The THPs could be formed within few seconds after dropping the TiCl4 droplets into water. Such hollow spheres are amorphous in phase structure, around 80 nm in mean diameter. The shell layer is of porous structure with ~ 0.5nm in pore size and 58% ~ 80% in porosity and show linear increase in the thickness with the THPs’ size. It has been found that formation, size, productivity and microstructure of the THPs are associated with the reaction temperature, initial pH value and TiCl4 droplet’s size.
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Importantly, such amorphous nanoscaled THPs have exhibited some strong hollow structure-enhanced performances. Typically, if they are used to support the Au nanoparticle’s film, the surface enhanced Raman scattering (SERS) effect of Au nanoparticles can be significantly enhanced. This study provides not only a new and quick fabrication route for the THPs but also a new member for their family. The details are reported in the article. 2. EXPERIMENTAL SECTION TiCl4 (99.99% pure, Alfa Aesar Corporation) is the only reagent to be used in this study. In the typical experiment, a flask with 10 mL deionized water was heated to 30oC, and then three droplets of TiCl4 liquid (3 µL for each droplet) were, in turn (5 sec. in interval), dropped by a pipette into the water with stirring, as schematically illustrated in Figure S1. After dropping, the reaction took place quickly and finished within few seconds. The products, dispersed in the deionized water, were obtained after subsequent centrifugal for three times. The morphology of the products was observed on a field emission scanning electron microscope (FESEM, FEI Sirion 200) equipped with an Oxford IE250X-Max50 energy disperse spectroscopy (EDS). Microstructural examinations were conducted on a JOEL JEM-2100 transmission electron microscope (TEM) operated at 120 kV. The isothermal nitrogen sorption measurements were carried out on a 100CX Ommishop 100CX. Raman spectra were measured on a confocal Raman spectrometer (Renishaw in Via Reflex) excited by the laser with 633 nm in wavelength, 0.17 mW in power. The integral time is 5 s and the spot size on the sample is 1µm in diameter. Optical spectra were recorded on a Shimadzu UV-3100 spectrometer.
3. RESULTS and DISCUSSION 4
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After dropping the TiCl4 droplets into water, the colorless water was turned to be a colloidal solution with milky color in a short time ( 8) are unbeneficial to formation of the homogenous THPs, as illustrated in Figs.6 (e, f). At pH=11, some big THPs (near 200nm in size) were observed in addition to the much smaller THPs (around 50nm in size). Further, at pH=12 no THP was formed but some sheet-like fragments and debris were observed [Fig.6 (f)]. 3.2.3.TiCl4 droplet’s size. The influence of the TiCl4 droplet’s size on the formation of THPs was examined by comparing the results from the different sized droplets (3µL, 5µL and 10µL in droplet volume). In the size range studied here, the droplet’s size has no obvious effect on the morphology of the formed THPs, except the mean THP’s size and its distribution width. Both the mean size and distribution width decrease slightly with the reducing droplet’s size, as demonstrated in Fig.1 (a) and Fig. 7. Finally, it should be emphasized that there are several factors, such as water temperature, initial TiCl4 droplet’s size and pH value, which can influence the mean size of THPs. Among them, the water temperature or the reaction temperature is the key factor for controlling the
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mean size, as shown Fig.S4. Also, it is effective to narrow the size distribution of THPs just by reducing the initial droplet’s size. 3.3. Formation of THPs.
Now, let us discuss the formation of THPs. When the liquid
TiCl4 was added into the water around room temperature, hydrolysis reaction would take place immediately according to the following reactions: 42-43 TiCl 4 + H 2 O ⇔ TiOH 3+ + H + + Cl −
(3)
TiOH 3+ ⇔ H + + TiO 2+
(4)
TiO 2+ + OH − ⇔ TiO2 + H +
(5)
TiCl4 reacted with H2O once contacting between them, and was hydrolyzed into TiOH3+ and hydrochloric acid [see reaction (3)]. Then the TiOH3+ ions would decompose into H+ and TiO2+ [reaction (4)]. Finally, the TiO2+ ions further hydrolyze to produce TiO2 molecules as well as H+ ions in the water according to reaction (5). Obviously, such hydrolysis reaction of TiCl4 would decrease pH value in the water. Here, if 9 uL TiCl4 was hydrolyzed completely in 10 mL water, the pH value could be calculated to be approximately 1.44 according to the above reactions, which is close to the experimental results [Fig.4(b)]. 3.3.1. Fragmentation of liquid droplets and formation of TiO2 shells. In our case, when an initial TiCl4 liquid droplet was dropped into the water around room temperature, hydrolysis reaction would occur tempestuously at the surface of the droplet. Such reaction would induce the ceaseless formation of TiO2 on the droplet’s surface during its falling in the water. The surface reaction-induced acting force on the droplet and gravitation would result in the fragmentation of the droplet, i.e. the original droplet would be fragmented into many smaller liquid beads, as illustrated in step (I) of Fig.8. The beads would be continuously fragmented
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into ultrafine drops. When the drops were small enough, depending on the violent degree of the hydrolysis reaction, further fragmentation was difficult due to their surface tension. At this moment, the TiO2 shell layers would be formed on the surface of the ultrafine drops due to the surface hydrolysis reaction, leading to the surface hardening of the drops, as demonstrated in step (II) of Fig.8. Because of the significant density difference between the solid TiO2 and the liquid TiCl4, the formed shell-layers had to be porous in structure (the details are seen in the next section). Such porous TiO2 shells would thus grow inward into the drops till complete consumption of the TiCl4, leading to the formation of the final THPs, as illustrated in steps (III) and ( IV) of Fig.8. As for the amorphous phase, it could be attributed to the fast formation and low temperature. 3.3.2. Dependence of the shell’s thickness on THP’s size. According to the formation process shown in Fig.8, we could quantitatively describe the dependence of the shell’s thickness on THP’s size. After the formation and in-ward growth of the TiO2 shell layers due to the hydrolysis reaction, a ultra-fine TiCl4 drop, which originates from the fragmentation of an initial droplet, is transformed in to a THP with the same diameter D as the drop and the shell thickness T. Based on the density difference of TiO2 and TiCl4, the relation between the diameter D and the shell thickness T of the THP can be expressed as, T=
ρTiCl 4 ⋅ M TiO 2 1 ⋅ 1 − 3 1 − ⋅ D 2 (1 − P) ⋅ ρTiO 2 ⋅ M TiCl 4
(6)
where P is the porosity of the shell layer, ρ and M are the density and molar weight, respectively. Letting ρTiCl 4 = 1.73 g / cm 3 ,
ρTiO 2 = 4.00 g / cm 3 , M TiCl 4 = 189.7 g / mole ,
M TiO 2 = 79.8 g / mole , Eq.(6) can be re-written as
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T=
1 0.1819 ⋅ 1 − 3 1 − ⋅ D 2 (1 − P)
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(6’)
Obviously, Eq.(6) or (6’) is in agreement with Eq.(1), showing the linear relation between the THP’s shell thickness and diameter, and the corresponding plot of T vs D passes through the origin, which is consistent with that shown in Fig.3. In this study, the ratio
T for all THPs falls into the range between 0.086 and 0.28, as D
shown in Fig.3. So, from Eq. (6’), we could know that the porosities of the shell layers are between 58% and 80% for all THPs. They were higher than the measured value (46%) which was underestimated due to the incomplete degassing treatment during the N2 sorption measurement. Such dispersion of the porosity should be attributed to the existence of difference in local hydrolysis reaction rates or formation conditions of THPs.
3.3.3. Explanation of the influencing factors. Further, according to the above formation process of the THPs, it is easy to understand the influences of the factors such as the water temperature, initial pH value and droplet’s size. Obviously, the higher the water temperature, the faster the hydrolysis reaction, which would lead to the higher fragmentation degree of the droplets. So, the final THPs’ size decreases with increase of the water temperature, as shown in Fig.5 and Fig.S4. Meanwhile, the higher the water temperature, reactions (3, 4, 5) would more tends to be complete, which leads to ever-increasing yields, as shown in the inset of [Fig.4(a)]. However, when the water temperature was too high (say, 90oC or higher), the sintering or fusion would occur on the interface among the THPs in contact with each other, leading to the aggregates of the THPs [Fig.5 (f)]. Contrarily, when the temperature was low enough (below 20oC), the reaction was relatively slow and quite incomplete. So the formed THPs were low in yield and big in size 12
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and hence easily broken [Fig.5 (a)]. For the influence of the pH values in the water during reaction, according to reactions (3-5), the acidic condition would decrease the hydrolysis reaction rate, which should be beneficial to formation of THPs. So we could obtain the THPs with good quality under acidic or weakly alkaline conditions (pH=1-8), as illustrated in Figs. 6(a-d). In this case, the pH value would mainly affect the reaction equilibrium and hence the yields. Under strong alkaline condition (say, pH=11), however, the hydrolysis reaction rate would be significantly increased due to the high concentration of reactant OH- [reaction (5)]. Such rapid reaction would make the TiCl4 droplets have less chance to break before formation of TiO2 shells. So the products contain a lot of big THPs, in addition to much smaller ones [Fig.6 (e)]. If the hydrolysis rate is further increased (pH=12), TiO2 shell-layer would be formed on the drops’ surface too fast to obtain the complete THPs (due to easily breaking) but some sheet-like fragments and debris [Fig.6 (f)] As for the influence of the TiCl4 liquid droplet’s size, in the size’s range studied here, change of the droplet’s size did not alter the conditions of THPs’ formation, leading to no obvious effect on the morphology of the formed THPs. On the other hand, due to its surface hydrolysis reaction and gravity, a bigger initial droplet would be fragmentized into more drops after entering the water, which would consume more OH- and decrease the local OHconcentration. It means that subsequent hydrolysis reaction rate for the drops would be lower, leading to the slight increase of the mean THP’s size and size distribution width with the rising droplet’s size, as shown in Fig.1(a) and Fig.7.
3.4. A new application: Hollow structure-enhanced SERS effect.
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the section Introduction, the hollow structured materials possess many special properties and hence extensive potential applications. Similarly, the amorphous nanoscaled THPs presented in this study have also exhibited some unique performances. Typically, the THPs-built film shows the highest reflectivity, up to 98% in the range of 400 nm to 800 nm, compared with the other solid TiO2 nanoparticles (NPs)-built films, as shown in Fig.S5. This could be attributed to the hollow structure. Further, if it supports the Au NPs’ film, the SERS effect of Au NPs was found to be significantly enhanced. Briefly, the THPs-supported Au NPs’ film, as a SERS substrate, was prepared by coating THPs on a Si wafer and then sputtering deposition of Au on the THPs’ layer, as described in details in Supporting Information. Fig. 9(a) schematically shows the structure of such THPs-supported Au NPs’ film. The THPs’ layer coated on the Si wafer was about 1.5 µm in thickness, and the Au NPs’ film on it was about 30 nm in equivalent thickness and 20-60 nm in NPs’ size. The surface and cross sectional morphologies are illustrated in Fig.9 (b, c). For comparison, the substrates of the rutile TiO2 or amorphous TiO2 NPs-supported Au NPs’ film, the pure Au NPs’ film and the pure THPs’ film were also fabricated by the same method, where the rutile TiO2 and amorphous TiO2 NPs’ were about 60 nm in mean size, as shown in Fig. S6. The Rhodamine 6G (R6G) was taken as the target (probe) molecules. The substrates were soaked in the R6G aqueous solution (10-6M) for 1.5 h before dying and Raman spectral measurements (the details are seen in the Supporting Information). Fig.10 shows the Raman spectra of R6G molecules on the different substrates. The pure Au NP’s film shows the typical SERS spectrum of R6G, as shown in curve (IV) of Fig.10. The rutile TiO2 and amorphous TiO2 NPs-supported Au NPs’ films show similar Raman
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spectrum in intensity for all peaks but around 6 times as high as that of the pure Au NPs’ film [see curves (II, III) and (IV) in Fig.10]. Further, for the THPs-supported Au NPs’ film, it shows the same spectral pattern as above but nearly 17 times higher peak intensities for all Raman peaks than those of the pure Au NPs’ film, exhibiting significantly enhanced SERS effect, as demonstrated in curves (I) and (IV) of Fig.10. Also, the Raman peak intensities for this THPs-supported Au NPs’ film are about 2.5 times as high as those of the solid TiO2 NPs’ films [curves (I) and (II, III) of Fig.10]. As for the pure THPs’ film, it shows no Raman peaks after soaking in the solution, as shown in curve (V) of Fig.10. These results indicate that the THPs could significantly enhance the SERS effect of the pure Au NPs’ film. Such Raman enhancement could be attributed to the high reflection of TiO2 NPs’ layers (Fig.S5) and the strong adsorption of R6G molecules on TiO2 NPs. In addition, there are many Au NPs formed within the TiO2 NPs’ layer during the sputtering deposition of Au film on it, as clearly shown in the inset of Fig.9 (c) (marked by arrows). These Au NPs would also excite the Raman signals of the surrounding R6G molecules adsorbed on TiO2 NPs. For the THPs-supported Au NPs’ film which shows the significantly higher Raman enhancement, it could mainly be associated with the special structure of the THPs. The THPs’ layer has much higher specific surface area than that of the solid TiO2 NPs’ layer due to the hollow structure, which would lead to the adsorption of more R6G molecules and hence much stronger Raman enhancement. Further study is needed to reveal such hollow structure -enhanced SERS effect.
4. CONCLUSION and REMARKS In summary, we have presented a very simple route to fast fabricate nano-sized THPs just via dropping the pure TiCl4 liquid droplets into the deionized water around room temperature. 15
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The THPs could be formed within few seconds after dropping TiCl4 droplets into water. The shell layers of THPs are amorphous and porous in structure. The shell’s thickness linearly increases with the THPs’ size. The porosity in the shells is between 58% ~ 80% for all THPs. It has been revealed that the reaction temperature, initial pH value and TiCl4 droplet’s size are crucial to the formation, size, productivity and microstructure of the THPs. The formation of such hollow nanospheres could be attributed to the hydrolysis of TiCl4 liquid-droplets, which involves a two-step process, or the fragmentation of liquid droplets due to the surface hydrolysis reaction and gravity effect, and the hydrolysis-induced formation and inward-growth of TiO2 shell layers. So, appropriately low hydrolysis rate, corresponding to the neutral or acidic condition and moderate temperature in water, is beneficial to the formation of THPs. Importantly, such amorphous nanoscaled THPs have exhibited some strong hollow structure-enhanced performances. Typically, the THPs-built film shows the highest reflectivity in the visible region compared with the other structured TiO2 films. And if it supports the Au NPs’ film, the SERS effect of the Au NPs’ film could be increased by more than one order of magnitude. It is expected that it could also increase the SERS effect of the other metal-substrates. Finally, compared with the previously reported methods (such as, Ostwald ripening-based, Kirkendall effect-based and template-based methods), which are generally complex and time consuming, or need many chemical reagents, the synthesis route presented in this work is template-free, quick, simple and easily controlled. Also, it does not need any reaction reagent except the TiCl4. The obtained THPs are smaller in size (
