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Templated self-assembly of Au–TiO2 binary nanoparticles–nanotubes
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CCLET-2924; No. of Pages 5 Chinese Chemical Letters xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Chinese Chemical Letters journal homepage: www.elsevier.com/locate/cclet
Original article
Templated self-assembly of Au–TiO2 binary nanoparticles–nanotubes Yu-Xin Zhang a,b,*, Xiao-Dong Hao a, Zeng-Peng Diao a a b
College of Materials Science and Engineering, Chongqing University, Chongqing 400044, China National Key Laboratory of Fundamental Science of Micro/Nano-Devices and System Technology, Chongqing University, Chongqing 400044, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 30 December 2013 Received in revised form 16 February 2014 Accepted 18 March 2014 Available online xxx
In this work, we developed a templated self-assembly approach to fabricate self-supporting Au/TiO2 binary nanoparticles–nanotubes (NPNTs) for the first time. The stable Au/TiO2 nanoparticles colloids were pre-synthesized and then deposited onto an AAO template, following by a mild calcination process. Au/TiO2 binary NPNTs can be achieved after removing the AAO template by NaOH solution. In addition, Au/TiO2 NPNTs with different thicknesses and size distributions could be achieved by tailoring the process parameters, such as the molar ratio of AuNPs to TiO2NPs, deposition modes and calcinations conditions. Therefore, these findings made controllable formation of Au/TiO2 NPNTs attractive for promising fabrication methodologies of metal/metal oxides NPNTs. ß 2014 Yu-Xin Zhang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: Au/TiO2 colloids Nanoparticles–nanotubes Self-assembly AAO template
1. Introduction Nanoparticle–nanotubes (NPNTs), first proposed by Rubinstein and co-workers in 2003 [1], were paid much attention due to the combination of nanotube geometry with nanoparticle morphology, which could achieve high surface-to-volume ratio and the size effect. It was proposed that nanoparticles as attractive building blocks can be used to assemble three kinds of tubular structures, including single-component NPNTs, heterogeneous multicomponent NPNTs, and well-alloyed NPNTs [2]. Gold nanocrystals, the most stable noble nanoparticles as building blocks, have attracted considerable attention owing to their outstanding properties in catalytic [3–7], electrical [8], optical [9], and biomedical applications [10–12]. Besides, much research has been focused on self-assembly of AuNPs, such as nanowires [13– 15], nanochains [16,17], and nanotube [1,18]. But these onedimensional structures may lose the nanoparticle morphology, which inhibited their wide uses (e.g., nanocatalysts). Furthermore, Au/TiO2 nanocomposites have been extensively explored because of their unique properties of electrochromic [19], optical [20], surfaceenhanced Raman scattering [21], catalytic and photocatalytic activities [22–24]. It was reported that AuNPs deposited on the
* Corresponding author at: College of Materials Science and Engineering, Chongqing University, Chongqing 400044, China. E-mail address: [email protected] (Y.-X. Zhang).
surface of TiO2NPs would not only enhance the stability of gold nanocatalysts, but also improve the properties of both parts [25,26]. However, these kinds of nanocomposites were mostly confined to embedding AuNPs onto/into the TiO2 support with different morphology, such as nanospheres [25], nanofilm [26], and nanotube [27], which would more or less restrain their properties in the application system. It was therefore vital to develop suitable engineering methods for the development of Au/TiO2 self-assembled nanostructures and their potential applications. Previously, we have successfully synthesized well dispersed and stable Au/TiO2NPs colloids [28], and fabricated suspended AuNPs hybrid films inside TiO2 nanotubes [29]. To the best of our knowledge, there were few reports regarding the controllable fabrication of Au/TiO2NPNTs with nanoparticles morphology. In this study, by fine-tuning the deposition and calcination processes, the unique Au/TiO2NPNTs were prepared for the first time. 2. Experimental 2.1. Pretreatment of AAO membrane AAO template was selected based on the previous reports on the preparing nanotube nanostructures with AAO templates [30– 32]. A commercial AAO template was deposited in deionized water under vacuum for 30 min and dried under ambient condition. The AAO was treated by gently polishing its surface with sand paper (1000 mesh). For comparison, some AAO
http://dx.doi.org/10.1016/j.cclet.2014.03.038 1001-8417/ß 2014 Yu-Xin Zhang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Please cite this article in press as: Y.-X. Zhang, et al., Templated self-assembly of Au–TiO2 binary nanoparticles–nanotubes, Chin. Chem. Lett. (2014), http://dx.doi.org/10.1016/j.cclet.2014.03.038
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CCLET-2924; No. of Pages 5 Y.-X. Zhang et al. / Chinese Chemical Letters xxx (2014) xxx–xxx
2
Fig. 1. Schematic illustration of the formation of Au/TiO2NPNTs: step I, mixed colloids of as synthesized Au/TiO2NPs were simultaneously deposited onto the AAO template; step II, Au/TiO2NPs were firmly anchored on the AAO template during the calcination process; step III, removing of the AAO template by post-treatment with NaOH solution.
templates were pre-treated by diluted HCl to remove impurities and open the pores. 2.2. Preparation of AuNPs AuNPs were prepared according to Brust’s two-phase reaction procedure with some minor modifications [33,34]. Briefly, a hydrogen tetrachloroaurate trihydrate (HAuCl43H2O) aqueous solution (0.09 mmol) was added to a solution of tetraoctylammonium bromide (TOAB) in toluene (0.18 mmol), and the mixture was vigorously stirred. The yellow aqueous solution became colorless, and the toluene phase turned orange as a result of the transformation of [AuCl4] with TOAB cations. After stirring the solution mixed with 1-dodecanethiol (DT) and/or 3-mercaptopropionic acid (MPA) in toluene for 15 min at room temperature, a freshly prepared aqueous solution (0.9 mmol) of sodium borohydride (NaBH4) was added to the vigorously stirred solution. The resulting solution immediately turned from orange to deep brown
and continued to be stirred for 15 min. The organic phase (about 4.43 mL) was evaporated inside a vacuum container at room temperature and the dried products were kept in standard bottles for further use. 2.3. Preparation of TiO2NPs TiO2NPs were prepared according to a facile hydrothermal synthesis [28,35]. In a typical synthesis, tert-butylamine (0.1 mL) was dissolved in 10 mL of water, and the solution was transferred into a 50 mL teflon-lined stainless-steel autoclave. Subsequently, 0.15 g of titanium (IV) n-propoxide and 5 mL of oleic acid (OA) were dissolved in 10 mL of toluene in air and the solution was transferred to the autoclave without any stirring. The autoclave was sealed and maintained at 180 8C for 4 h, and air-cooled to room temperature. The crude solution of TiO2NPs were precipitated with methanol and further isolated by centrifugation and decantation.
Fig. 2. Typical TEM images of Au/TiO2NPNTs without (a, b) and with (c–f) calcination process at 773 K for 0.5 h. The synthesis details were as follows: The AAO template was put into a 0.2 mL of Au/TiO2NPs solution with a molar ratio of Au to TiO2 of 1:2.93 and an AuNPs concentration of 0.09 mol/L under vacuum condition till the solution was dried after evaporation; repeated the deposition process twice, then underwent different calcination processes before the removal of the AAO template. The AAO template was used without HCl pre-treatment.
Please cite this article in press as: Y.-X. Zhang, et al., Templated self-assembly of Au–TiO2 binary nanoparticles–nanotubes, Chin. Chem. Lett. (2014), http://dx.doi.org/10.1016/j.cclet.2014.03.038
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2.4. Preparation of mixed Au/TiO2NPs MPA-DT-capped AuNPs were added into TiO2NPs with a predesignated ratio in toluene. The mixed colloids could be kept for at least one year without any agglomerates, indicative of good stability of building blocks for self-assembled nanostructures. 2.5. Preparation of Au/TiO2NPNTs Typically, mixed colloids of Au-TiO2NPs were dropped onto the surface of AAO and deposited under vacuum till the solution was dried after complete evaporation. This process was repeated for several times, depending on the wall thickness of NPNTs. After calcining at 473–823 K for 30 min, the products were immersed in aqueous NaOH (5 mol/L) for 10 min to remove AAO template, and then subjected to the centrifugation/dispersion processes using ethanol and deionized water for several times. 3. Results and discussion Fig. 1 depicts a typical formation route for the fabrication of Au/ TiO2NPNTs. Firstly, a mixed solution of Au/TiO2 colloids with a designated concentration and molar ratio were deposited on the
3
AAO template (Step I). In this mixed solution, MPA served as a bridging agent between AuNPs and TiO2NPs, leading to a very stable mixed nanoparticles colloid [23,28]. Once these mixed colloids were deposited on the AAO template, MPA-DT capped AuNPs would readily adsorb on the surface of AAO due to the carboxylate head groups of MPA are chemisorbed on the Lewis acid sites of the AAO surface (Al3+ ions), resulting in the deposition of an Au/TiO2NPs film on the inner wall of the AAO template. After that, this Au/TiO2NPs-deposited AAO template would undergo a calcination process (Step II). At this stage, the surfactant was consumed and Au/TiO2NPs were anchored firmly on the inner wall of the AAO template. With the proper concentration of Au/TiO2NPs colloids and related deposition method, self-supporting tubular morphology of Au/TiO2NPs can be retained after the removal of the AAO template (Step III). Fig. 2 displays some representative TEM images of our Au/ TiO2NPNTs assembled from functionalized Au/TiO2NPs. The wall thickness can be controlled by the concentration of Au/TiO2NPs and the deposition cycles (Fig. 2b–d). For instance, the wall thickness of Au/TiO2NPNTs was enlarged from 30 nm to 52 nm after calcining at 773 K for 30 min, calculating out that the wall of Au/TiO2NPNTs was assembled by 7–9 layers according to the size of AuNPs. On the other hand, the particle sizes in NPNTs can also be
Fig. 3. Typical TEM images and the corresponding size distribution of Au/TiO2NPNTs with different HCl pretreatment time of AAO template: (a, b) 0.5 h; (c, d) 1 h; and (e, f) 8 h. The deposition method was the same: two drop of mix Au/TiO2NPs colloids was successively deposited onto the AAO template, then heating at 473 K for 5 h before the removal of AAO template. The mixed Au/TiO2NPs colloids with the molar ratio of Au to TiO2 was 1:2.93.
Please cite this article in press as: Y.-X. Zhang, et al., Templated self-assembly of Au–TiO2 binary nanoparticles–nanotubes, Chin. Chem. Lett. (2014), http://dx.doi.org/10.1016/j.cclet.2014.03.038
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CCLET-2924; No. of Pages 5 Y.-X. Zhang et al. / Chinese Chemical Letters xxx (2014) xxx–xxx
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1631
Transmittance (%)
b
1121
manipulated by the calcination conditions (e.g., processing temperature and time). Obviously, AuNPs grew from 2–3 nm to 4–7 nm due to Ostwald ripening and interparticle confined growth (Fig. 2a and b, see Fig. S1 in Supporting information). The contour of particle morphology remained visible even after relatively strong calcinations (e.g., 773 K for 30 min), indicating that the interparticle interactions of Au/TiO2NPs played a vital role to inhibit heavy homogeneous aggregation of nanocrystals. Meanwhile, the strong interparticle interactions of Au/TiO2NPs also facilitated the selfsupporting tubular morphology with closely packed nanoparticle shells and porous architectures. Besides, the HTTEM image and SEAD pattern in Fig. 2e and f confirm the formation of Au/ TiO2NPNTs in our experiments. In order to better understand the effect of calcination process in the formation of Au/TiO2NPNTs, an Au/TiO2NPs-depoisted AAO template was conducted to different heating processes (see Fig. S2 in Supporting information). As can be seen, the size of AuNPs was around 4 nm when the temperature was below 773 K, while it could be about 8 nm at 873 K. It is therefore absolutely vital to develop a mild calcination process to enhance the interactions among the Au/TiO2NPs by consuming some surfactant on the surface of NPs for controllable fabrication of a more stable and fixed Au/TiO2NPNTs. Apart from the effects of deposition and calcination processes, effect of HCl pre-treatment was also examined. Diluted HCl solution was most commonly selected to remove impurities and open pores on the surface of AAO template [36,37]. Surprisingly, the pretreatment using aqueous HCl would cause the aggregation of the otherwise stable AuNPs in our experiments, while TiO2NPs remained unchanged. Fig. 3 displays different Au/TiO2NPNTs after AAO template was conducted to different treatments with aqueous HCl. Briefly, after a long period of aqueous HCl treatment on AAO (longer than 1 h), AuNPs were about 12 nm, which was three times of those AuNPs treated by aqueous HCl for 30 min. We proposed that aqueous HCl pretreatment of AAO would increase the number of Lewis acid sites on the AAO surface (Al3+ ions), thus much more Au NPs were adsorbed on these sites initially. So they would grow into aggregated NPs when the surfactant was consumed during the calcination process. Thus, aqueous HCl pretreatment might not be favored in the formation of Au/TiO2NPNTs with controllable size distribution. In order to further probe the functionality evolution of the Au/ TiO2NPs, FTIR spectra are carried out and the results are shown in Fig. 4. The strong bands at 2856 and 2928 cm 1 can be designated
1465 1411 1283
a
4. Conclusion In summary, we have explored the formation processes of Au/ TiO2 NPNTs through templated self-assembly. It has been found that the thickness and size distribution of Au/TiO2 NPNTs could be tuned by altering the process parameters (deposition, calcination), and the self-assembly of metal NPs and metal oxides NPs into controllable tubular morphology could be achieved. Besides, we confirmed that the commonly used method for the pretreatment of AAO template with aqueous HCL was not favored in the formation of the fine-structure of Au/TiO2 NPNTs. Moreover, we noted that this type of Au/TiO2 NPNTs possessed many structural advantages and processing flexibilities: (1) AuNPs (or TiO2NPs) can be tuned with different sizes; (2) interparticle distribution depends on the concentration ratio of AuNPs to TiO2NPs; (3) wall thickness and porous properties can be engineered; (4) new properties can be obtained by introducing functional groups. All these control factors will determine the final porosity and functionality of the Au/TiO2 NPNTs in future applications (e.g., CO oxidation, photocatalysis, and biosensor). In principle, this controllable formation of Au/TiO2 NPNTs could be a very promising technology in the synthesis of binary nanoparticles–nanotubes. Acknowledgments The authors gratefully acknowledge the financial supports provided by National Natural Science Foundation of China (No. 51104194), Doctoral Fund of Ministry of Education of China (No. 20110191120014), No. 43 Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry and Fundamental Research Funds for the Central Universities (Nos. CDJZR13130035, CDJZR12248801 and CDJZR12135501, Chongqing University, China). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cclet.2014.03.038.
2954 2922 2951
3000
to the symmetric and asymmetric CH2 stretches of the hydrocarbon moiety, respectively, and a shoulder at 2960 cm 1 is attributed to the asymmetric stretch of the terminal CH3 groups in OA and DT. The band in the vicinity of 1631 cm 1 was attributed to the C5 5O stretch of the adsorbed oleic acid monomer. The lower C5 5O stretch frequency (compared with the value of 1775 cm 1 for the free acid) was assumed due to the fact that oleic acid monomer is coordinated to a Ti4+ Lewis site via the C5 5O oxygen. Besides, the bands at 1465, 1411, and 1283, 1121 cm 1 can be associated with the CH2 deformation (dCH2), the C–O–H in-plane bend (dC–O–H), and the C–OH stretch (nC–OH) and C–O (nC–O), respectively [38,39]. After calcinations, the bands corresponding to CH2, CH3, C–OH and C–O–H disappeared, indicating the thermal decomposition of organic groups had occurred. The remaining bands at 1631 and 1121 cm 1 confirmed the strong interaction between the carboxyl group and TiO2.
References
2500
2000
1500 -1
Wavenumber (cm )
1000
500
Fig. 4. FT-IR spectra of Au/TiO2NPs before (a) and after (b) calcining at 723 K for 0.5 h.
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Please cite this article in press as: Y.-X. Zhang, et al., Templated self-assembly of Au–TiO2 binary nanoparticles–nanotubes, Chin. Chem. Lett. (2014), http://dx.doi.org/10.1016/j.cclet.2014.03.038
CCLET-2924; No. of Pages 5 Chinese Chemical Letters xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Chinese Chemical Letters journal homepage: www.elsevier.com/locate/cclet
Original article
Templated self-assembly of Au–TiO2 binary nanoparticles–nanotubes Yu-Xin Zhang a,b,*, Xiao-Dong Hao a, Zeng-Peng Diao a a b
College of Materials Science and Engineering, Chongqing University, Chongqing 400044, China National Key Laboratory of Fundamental Science of Micro/Nano-Devices and System Technology, Chongqing University, Chongqing 400044, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 30 December 2013 Received in revised form 16 February 2014 Accepted 18 March 2014 Available online xxx
In this work, we developed a templated self-assembly approach to fabricate self-supporting Au/TiO2 binary nanoparticles–nanotubes (NPNTs) for the first time. The stable Au/TiO2 nanoparticles colloids were pre-synthesized and then deposited onto an AAO template, following by a mild calcination process. Au/TiO2 binary NPNTs can be achieved after removing the AAO template by NaOH solution. In addition, Au/TiO2 NPNTs with different thicknesses and size distributions could be achieved by tailoring the process parameters, such as the molar ratio of AuNPs to TiO2NPs, deposition modes and calcinations conditions. Therefore, these findings made controllable formation of Au/TiO2 NPNTs attractive for promising fabrication methodologies of metal/metal oxides NPNTs. ß 2014 Yu-Xin Zhang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: Au/TiO2 colloids Nanoparticles–nanotubes Self-assembly AAO template
1. Introduction Nanoparticle–nanotubes (NPNTs), first proposed by Rubinstein and co-workers in 2003 [1], were paid much attention due to the combination of nanotube geometry with nanoparticle morphology, which could achieve high surface-to-volume ratio and the size effect. It was proposed that nanoparticles as attractive building blocks can be used to assemble three kinds of tubular structures, including single-component NPNTs, heterogeneous multicomponent NPNTs, and well-alloyed NPNTs [2]. Gold nanocrystals, the most stable noble nanoparticles as building blocks, have attracted considerable attention owing to their outstanding properties in catalytic [3–7], electrical [8], optical [9], and biomedical applications [10–12]. Besides, much research has been focused on self-assembly of AuNPs, such as nanowires [13– 15], nanochains [16,17], and nanotube [1,18]. But these onedimensional structures may lose the nanoparticle morphology, which inhibited their wide uses (e.g., nanocatalysts). Furthermore, Au/TiO2 nanocomposites have been extensively explored because of their unique properties of electrochromic [19], optical [20], surfaceenhanced Raman scattering [21], catalytic and photocatalytic activities [22–24]. It was reported that AuNPs deposited on the
* Corresponding author at: College of Materials Science and Engineering, Chongqing University, Chongqing 400044, China. E-mail address: [email protected] (Y.-X. Zhang).
surface of TiO2NPs would not only enhance the stability of gold nanocatalysts, but also improve the properties of both parts [25,26]. However, these kinds of nanocomposites were mostly confined to embedding AuNPs onto/into the TiO2 support with different morphology, such as nanospheres [25], nanofilm [26], and nanotube [27], which would more or less restrain their properties in the application system. It was therefore vital to develop suitable engineering methods for the development of Au/TiO2 self-assembled nanostructures and their potential applications. Previously, we have successfully synthesized well dispersed and stable Au/TiO2NPs colloids [28], and fabricated suspended AuNPs hybrid films inside TiO2 nanotubes [29]. To the best of our knowledge, there were few reports regarding the controllable fabrication of Au/TiO2NPNTs with nanoparticles morphology. In this study, by fine-tuning the deposition and calcination processes, the unique Au/TiO2NPNTs were prepared for the first time. 2. Experimental 2.1. Pretreatment of AAO membrane AAO template was selected based on the previous reports on the preparing nanotube nanostructures with AAO templates [30– 32]. A commercial AAO template was deposited in deionized water under vacuum for 30 min and dried under ambient condition. The AAO was treated by gently polishing its surface with sand paper (1000 mesh). For comparison, some AAO
http://dx.doi.org/10.1016/j.cclet.2014.03.038 1001-8417/ß 2014 Yu-Xin Zhang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Please cite this article in press as: Y.-X. Zhang, et al., Templated self-assembly of Au–TiO2 binary nanoparticles–nanotubes, Chin. Chem. Lett. (2014), http://dx.doi.org/10.1016/j.cclet.2014.03.038
G Model
CCLET-2924; No. of Pages 5 Y.-X. Zhang et al. / Chinese Chemical Letters xxx (2014) xxx–xxx
2
Fig. 1. Schematic illustration of the formation of Au/TiO2NPNTs: step I, mixed colloids of as synthesized Au/TiO2NPs were simultaneously deposited onto the AAO template; step II, Au/TiO2NPs were firmly anchored on the AAO template during the calcination process; step III, removing of the AAO template by post-treatment with NaOH solution.
templates were pre-treated by diluted HCl to remove impurities and open the pores. 2.2. Preparation of AuNPs AuNPs were prepared according to Brust’s two-phase reaction procedure with some minor modifications [33,34]. Briefly, a hydrogen tetrachloroaurate trihydrate (HAuCl43H2O) aqueous solution (0.09 mmol) was added to a solution of tetraoctylammonium bromide (TOAB) in toluene (0.18 mmol), and the mixture was vigorously stirred. The yellow aqueous solution became colorless, and the toluene phase turned orange as a result of the transformation of [AuCl4] with TOAB cations. After stirring the solution mixed with 1-dodecanethiol (DT) and/or 3-mercaptopropionic acid (MPA) in toluene for 15 min at room temperature, a freshly prepared aqueous solution (0.9 mmol) of sodium borohydride (NaBH4) was added to the vigorously stirred solution. The resulting solution immediately turned from orange to deep brown
and continued to be stirred for 15 min. The organic phase (about 4.43 mL) was evaporated inside a vacuum container at room temperature and the dried products were kept in standard bottles for further use. 2.3. Preparation of TiO2NPs TiO2NPs were prepared according to a facile hydrothermal synthesis [28,35]. In a typical synthesis, tert-butylamine (0.1 mL) was dissolved in 10 mL of water, and the solution was transferred into a 50 mL teflon-lined stainless-steel autoclave. Subsequently, 0.15 g of titanium (IV) n-propoxide and 5 mL of oleic acid (OA) were dissolved in 10 mL of toluene in air and the solution was transferred to the autoclave without any stirring. The autoclave was sealed and maintained at 180 8C for 4 h, and air-cooled to room temperature. The crude solution of TiO2NPs were precipitated with methanol and further isolated by centrifugation and decantation.
Fig. 2. Typical TEM images of Au/TiO2NPNTs without (a, b) and with (c–f) calcination process at 773 K for 0.5 h. The synthesis details were as follows: The AAO template was put into a 0.2 mL of Au/TiO2NPs solution with a molar ratio of Au to TiO2 of 1:2.93 and an AuNPs concentration of 0.09 mol/L under vacuum condition till the solution was dried after evaporation; repeated the deposition process twice, then underwent different calcination processes before the removal of the AAO template. The AAO template was used without HCl pre-treatment.
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2.4. Preparation of mixed Au/TiO2NPs MPA-DT-capped AuNPs were added into TiO2NPs with a predesignated ratio in toluene. The mixed colloids could be kept for at least one year without any agglomerates, indicative of good stability of building blocks for self-assembled nanostructures. 2.5. Preparation of Au/TiO2NPNTs Typically, mixed colloids of Au-TiO2NPs were dropped onto the surface of AAO and deposited under vacuum till the solution was dried after complete evaporation. This process was repeated for several times, depending on the wall thickness of NPNTs. After calcining at 473–823 K for 30 min, the products were immersed in aqueous NaOH (5 mol/L) for 10 min to remove AAO template, and then subjected to the centrifugation/dispersion processes using ethanol and deionized water for several times. 3. Results and discussion Fig. 1 depicts a typical formation route for the fabrication of Au/ TiO2NPNTs. Firstly, a mixed solution of Au/TiO2 colloids with a designated concentration and molar ratio were deposited on the
3
AAO template (Step I). In this mixed solution, MPA served as a bridging agent between AuNPs and TiO2NPs, leading to a very stable mixed nanoparticles colloid [23,28]. Once these mixed colloids were deposited on the AAO template, MPA-DT capped AuNPs would readily adsorb on the surface of AAO due to the carboxylate head groups of MPA are chemisorbed on the Lewis acid sites of the AAO surface (Al3+ ions), resulting in the deposition of an Au/TiO2NPs film on the inner wall of the AAO template. After that, this Au/TiO2NPs-deposited AAO template would undergo a calcination process (Step II). At this stage, the surfactant was consumed and Au/TiO2NPs were anchored firmly on the inner wall of the AAO template. With the proper concentration of Au/TiO2NPs colloids and related deposition method, self-supporting tubular morphology of Au/TiO2NPs can be retained after the removal of the AAO template (Step III). Fig. 2 displays some representative TEM images of our Au/ TiO2NPNTs assembled from functionalized Au/TiO2NPs. The wall thickness can be controlled by the concentration of Au/TiO2NPs and the deposition cycles (Fig. 2b–d). For instance, the wall thickness of Au/TiO2NPNTs was enlarged from 30 nm to 52 nm after calcining at 773 K for 30 min, calculating out that the wall of Au/TiO2NPNTs was assembled by 7–9 layers according to the size of AuNPs. On the other hand, the particle sizes in NPNTs can also be
Fig. 3. Typical TEM images and the corresponding size distribution of Au/TiO2NPNTs with different HCl pretreatment time of AAO template: (a, b) 0.5 h; (c, d) 1 h; and (e, f) 8 h. The deposition method was the same: two drop of mix Au/TiO2NPs colloids was successively deposited onto the AAO template, then heating at 473 K for 5 h before the removal of AAO template. The mixed Au/TiO2NPs colloids with the molar ratio of Au to TiO2 was 1:2.93.
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4
1631
Transmittance (%)
b
1121
manipulated by the calcination conditions (e.g., processing temperature and time). Obviously, AuNPs grew from 2–3 nm to 4–7 nm due to Ostwald ripening and interparticle confined growth (Fig. 2a and b, see Fig. S1 in Supporting information). The contour of particle morphology remained visible even after relatively strong calcinations (e.g., 773 K for 30 min), indicating that the interparticle interactions of Au/TiO2NPs played a vital role to inhibit heavy homogeneous aggregation of nanocrystals. Meanwhile, the strong interparticle interactions of Au/TiO2NPs also facilitated the selfsupporting tubular morphology with closely packed nanoparticle shells and porous architectures. Besides, the HTTEM image and SEAD pattern in Fig. 2e and f confirm the formation of Au/ TiO2NPNTs in our experiments. In order to better understand the effect of calcination process in the formation of Au/TiO2NPNTs, an Au/TiO2NPs-depoisted AAO template was conducted to different heating processes (see Fig. S2 in Supporting information). As can be seen, the size of AuNPs was around 4 nm when the temperature was below 773 K, while it could be about 8 nm at 873 K. It is therefore absolutely vital to develop a mild calcination process to enhance the interactions among the Au/TiO2NPs by consuming some surfactant on the surface of NPs for controllable fabrication of a more stable and fixed Au/TiO2NPNTs. Apart from the effects of deposition and calcination processes, effect of HCl pre-treatment was also examined. Diluted HCl solution was most commonly selected to remove impurities and open pores on the surface of AAO template [36,37]. Surprisingly, the pretreatment using aqueous HCl would cause the aggregation of the otherwise stable AuNPs in our experiments, while TiO2NPs remained unchanged. Fig. 3 displays different Au/TiO2NPNTs after AAO template was conducted to different treatments with aqueous HCl. Briefly, after a long period of aqueous HCl treatment on AAO (longer than 1 h), AuNPs were about 12 nm, which was three times of those AuNPs treated by aqueous HCl for 30 min. We proposed that aqueous HCl pretreatment of AAO would increase the number of Lewis acid sites on the AAO surface (Al3+ ions), thus much more Au NPs were adsorbed on these sites initially. So they would grow into aggregated NPs when the surfactant was consumed during the calcination process. Thus, aqueous HCl pretreatment might not be favored in the formation of Au/TiO2NPNTs with controllable size distribution. In order to further probe the functionality evolution of the Au/ TiO2NPs, FTIR spectra are carried out and the results are shown in Fig. 4. The strong bands at 2856 and 2928 cm 1 can be designated
1465 1411 1283
a
4. Conclusion In summary, we have explored the formation processes of Au/ TiO2 NPNTs through templated self-assembly. It has been found that the thickness and size distribution of Au/TiO2 NPNTs could be tuned by altering the process parameters (deposition, calcination), and the self-assembly of metal NPs and metal oxides NPs into controllable tubular morphology could be achieved. Besides, we confirmed that the commonly used method for the pretreatment of AAO template with aqueous HCL was not favored in the formation of the fine-structure of Au/TiO2 NPNTs. Moreover, we noted that this type of Au/TiO2 NPNTs possessed many structural advantages and processing flexibilities: (1) AuNPs (or TiO2NPs) can be tuned with different sizes; (2) interparticle distribution depends on the concentration ratio of AuNPs to TiO2NPs; (3) wall thickness and porous properties can be engineered; (4) new properties can be obtained by introducing functional groups. All these control factors will determine the final porosity and functionality of the Au/TiO2 NPNTs in future applications (e.g., CO oxidation, photocatalysis, and biosensor). In principle, this controllable formation of Au/TiO2 NPNTs could be a very promising technology in the synthesis of binary nanoparticles–nanotubes. Acknowledgments The authors gratefully acknowledge the financial supports provided by National Natural Science Foundation of China (No. 51104194), Doctoral Fund of Ministry of Education of China (No. 20110191120014), No. 43 Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry and Fundamental Research Funds for the Central Universities (Nos. CDJZR13130035, CDJZR12248801 and CDJZR12135501, Chongqing University, China). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cclet.2014.03.038.
2954 2922 2951
3000
to the symmetric and asymmetric CH2 stretches of the hydrocarbon moiety, respectively, and a shoulder at 2960 cm 1 is attributed to the asymmetric stretch of the terminal CH3 groups in OA and DT. The band in the vicinity of 1631 cm 1 was attributed to the C5 5O stretch of the adsorbed oleic acid monomer. The lower C5 5O stretch frequency (compared with the value of 1775 cm 1 for the free acid) was assumed due to the fact that oleic acid monomer is coordinated to a Ti4+ Lewis site via the C5 5O oxygen. Besides, the bands at 1465, 1411, and 1283, 1121 cm 1 can be associated with the CH2 deformation (dCH2), the C–O–H in-plane bend (dC–O–H), and the C–OH stretch (nC–OH) and C–O (nC–O), respectively [38,39]. After calcinations, the bands corresponding to CH2, CH3, C–OH and C–O–H disappeared, indicating the thermal decomposition of organic groups had occurred. The remaining bands at 1631 and 1121 cm 1 confirmed the strong interaction between the carboxyl group and TiO2.
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2500
2000
1500 -1
Wavenumber (cm )
1000
500
Fig. 4. FT-IR spectra of Au/TiO2NPs before (a) and after (b) calcining at 723 K for 0.5 h.
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