Exploring the Confinement Effect of Carbon Nanotubes on the

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Exploring the confinement effect of carbon nanotubes on the electrochemical properties of Prussian blue nanoparticles Ti-Wei Chen, Zhong-Qiu Li, Kang Wang, Feng-Bin Wang, and Xing-Hua Xia Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b03690 • Publication Date (Web): 22 May 2018 Downloaded from http://pubs.acs.org on May 22, 2018

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Exploring the confinement effect of carbon nanotubes on the electrochemical properties of Prussian blue nanoparticles Ti-Wei Chen,‡ Zhong-Qiu Li,‡ Kang Wang, Feng-Bin Wang, Xing-Hua Xia* State Key Laboratory of Analytical Chemistry for Life Science, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China KEYWORDS: carbon nanotubes; Prussian blue nanoparticles; confinement effect; electrocatalysis

ABSTRACT: A novel and efficient photochemical method has been proposed for the encapsulation of Prussian blue nanoparticles (PBNPs) inside the channels of carbon nanotubes (PB-in-CNTs) in an acidic ferrocyanide solution under UV/Vis illumination and the confinement effect of CNTs on the electrochemical properties of PBNPs is systematically explored. PB-inCNTs shows a faster electron transfer process, an enhanced electrocatalytic activity toward the reduction of H2O2 and an increased anti-base ability compared to PBNPs loaded outside of CNTs (PB-out-CNTs). In addition, PB-in-CNTs show an increased electrochemical reversibility and an unexpected stable pH tolerance of the catalytic activity with the decreasing of CNT diameters. The improved electrochemical properties of PB-in-CNTs are attributed to the modified electronic

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properties and dimensions of PBNPs induced by the confinement effect of CNTs. This work provides further insight into the confinement effect on the properties of nanomaterials, and will inspire extensive relevant investigations in the development of novel composites or excellent catalysts.

INTRODUCTION Carbon nanotubes (CNTs) have attracted great attention owing to their unique structural, electronic, and mechanical properties.1-3 By utilization of their highly specific surface area and excellent conductivity, CNTs have been extensively served as supporting matrix to disperse and stabilize various transition metal and metal oxide catalysts4 and the resulting composites exhibit superior catalytic or electrocatalytic properties.5-8 Carbon nanotubes can be considered as rolled-up graphene sheets. The formed curvature deforms the distribution of π bonding electrons, resulting in different electron distribution inside and outside of the CNTs as predicted by theoretical calculations.9 This fact implies that assembly of catalysts on the inner or outer surface of CNTs may result in distinguished properties due to the different interactions between the catalysts and CNTs, which has been confirmed experimentally recently.10-14 If the catalysts are confined inside the CNTs, the confinement effect will affect the distribution of reactants and products, the mass transport and then the reaction kinetics, and render the catalysts displaying unique physical and chemical properties when compared to their bulk systems. Thus, by intercalating diverse guest materials into the inner channels of CNTs, we can make nanoreactors or create novel nanocatalysts and nanocomposites with improved physical and chemical properties.


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Actually, ever since the CNTs were first discovered by Iijima in 1991,15 theoretical and experimental studies concerning the filling of CNTs with foreign matters have been proposed continuously.16-22 Up to now, large amount of reports on intercalation of metals,18 inorganic salts,19 metallic oxides,20 endohedral metallofullerenes,21 fullerene C60 molecule22 and organic molecules23 into CNTs have appeared. The encapsulating techniques include in situ arc discharge, 24-25

gas phase deposition26-27 and liquid phase filling methods.8, 28-29 Among them, the liquid phase

filling method, which depends on the capillarity between the pore walls of CNTs and guest matters, is a simple approach to the intercalation of various metals and metallic oxides or halides.30 Recently, Bao and his colleagues improved the wet intercalating process, and thus, various metals or metallic oxide nanocatalysts can be uniformly dispersed inside the small-sized carbon nanotubes. 10-14 They demonstrated that the nanocatalysts confined in the interior of CNTs showed much better catalytic activity and selectivity in comparison with the ones located on the exterior of CNTs. They ascribed the improved properties to the unique confinement effect of CNTs and the different electronic interaction between the catalysts and the concave and convex surfaces of CNTs. These intriguing findings have stimulated considerable scientific and technological interests and expectations of constructing novel nanocomposites. However, the filling methods proposed in literature require rigorous and tedious experimental operations in the opening and filling stages of CNTs, such as oxidization in strong acid medium and high temperature treatment under vacuum. These methods cannot avoid the deposition of a relatively large portion of foreign matters at the outside surface of CNTs. Thus, additional removal processes are usually required if ones want to study the confinement effect. As a result, develop innovative mild filling method to intercalate guest matters with special and desirable properties into CNTs remains challenging. In addition, we notice that there is hardly any exploration of the confinement effect on the


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electrochemical properties of encapsulated materials in CNTs.31-33 Therefore, filling electroactive materials in CNTs is expected to provide a novel and powerful platform to explore the interesting confinement effect. Xie et al. reported that Prussian blue nanoparticles (PBNPs) encapsulated in the interior of CNT by wet filling process exhibited high stability in long-term measurements and limited mass transfer during fast-scan cyclic voltammetry.31, 33 However, they did not give deep interpretation of these phenomena. The confinement effects of CNTs on the morphology and electrochemical properties of PBNPs encapsulated in the nanochannels of CNTs have not been studied yet. Herein, we report a simple and efficient photochemical method for selectively filling PBNPs into the inner channels of carbon nanotubes under relatively mild experimental conditions. The filling approach is based upon the capillary action of hollow CNT cavities. A high filling yield can be achieved while the deposition of PBNPs on the exterior surface of CNTs is effectively avoided. The PBNPs loaded inside (PB-in-CNTs) and outside the CNTs (PB-out-CNTs) are systematically analyzed by field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and electrochemical testing techniques. Experimental results show that the nanocomposites with PBNPs confined inside the CNTs show distinctively different electrochemical properties resulting from the modification of electronic properties and morphology of PBNPs which are caused by the unique confinement effect of CNTs. RESULTS AND DISCUSSION Preparation and Characterization of PB-in-CNTs and PB-out-CNTs. CNTs with inner diameter of 200~300 nm, defined as “CNTs-300”, were synthesized by a template-based wet method.34 Commercial anodic aluminum oxide (AAO) membrane with 60 μm thickness and 200


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nm pore diameter was first immersed into a 0.5 M glucose aqueous solution for 30 min. Subsequently, the AAO membrane together with glucose solution was transferred into an autoclave and heated at 180 oC for 5 h for proceeding thermal polymerization of glucose. After cooling to room temperature, the polymerized product at the surface of AAO templates was carefully scraped, followed by water washing for several times. Then, the carbon polymerimpregnated AAO template was carbonized at 900 oC under argon atmosphere for 3 h, resulting in CNTs embedded in AAO template. CNTs with inner diameter of 50 nm, defined as “CNTs-50”, were synthesized by a templatebased dry method. 35-36 Highly ordered AAO nanochannel array with an approximately 50 nm pore size was prepared by a two-step anodic oxidation of aluminum sheet (0.1 mm thick with 99.99% purity) at a constant potential of 40 V in 0.5 M oxalic acid aqueous solution at 20 oC. After the anodization, the electrolyte was replaced with a mixture of HClO4 (70%)/ethanol (volume ratio of 1:1) and electrochemical detachment of AAO from the aluminum substrate was carried out by applying a 5 s voltage pulse of 50 V, resulting in a free-standing through-hole AAO membrane as reported in our previous work.35-36 Subsequently, the template was put in a right angle copper net to keep it vertical during the following chemical vapor deposition (CVD) procedure, which facilitated the permeation of carbon source gas into the nanochannels. The reactor temperature was increased to 900 oC under argon flow (flow rate: 50 sccm). When the temperature stabilized at 900 o

C, a 5 sccm ethylene flow was initiated. Thermal decomposition of ethylene occurred in the

nanochannels, resulting in the formation of CNTs. After deposition for 10 min, the ethylene flow terminated, and the furnace was cooled to room temperature under argon flow. The AAO template became curled and black, which indicated successful preparation of CNTs in the AAO template.


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Figure 1. Schematic illustration of the fabrication process of PB-in-CNTs and PB-out-CNTs.

To prepare PB inside the CNTs, a novel and facile photochemical method was used. As Figure 1 shows, the CNT-embedded AAO template was dipped into a 10 mL precursor of 1 mM K3Fe(CN)6, 1 mM FeCl3 and 0.1 M KCl aqueous solution in a small beaker. After adjusting the solution pH to 1.6 with hydrochloric acid, the small beaker was placed in a vacuum desiccator with the pressure maintained at -0.1 MPa. Due to the underpressure and the capillary force, the precursor solution can be transported into the inner channels of CNTs. Subsequently, the membrane was irradiated with 365 nm ultraviolet light for 24 h to transform the precursor into PBNPs as described in our previous report.37 Then, PB-in-CNTs were obtained after chemical removal of the AAO template in 20% HF solution overnight. PB-in-CNTs-300 and PB-in-CNTs50 were successfully synthesized using different sized CNTs. For the preparation of PBNPs decorated at the outer surface of CNTs, the AAO template was first removed chemically (Figure 1), and then the obtained CNTs were dispersed in 10 mL precursor of 1 mM K3Fe(CN)6, 1 mM FeCl3 and 0.1 M KCl aqueous solution in a small beaker


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under ultrasonication. After irradiation of the system under 365 nm light for 24 h, PBNPs deposited on the outer surface of CNTs were obtained. In this case, intercalation of PBNPs in the inner channels of CNTs can be avoided. By employing different sized CNTs, PB-out-CNTs-300 and PB-out-CNTs-50 were prepared.

Figure 2. TEM images of the CNTs-300 (A and B) and CNTs-50 (C and D) at different magnifications.

The typical FESEM and TEM images of the as-prepared CNTs at different magnifications are shown in Figure S1-S2 and Figure 2. As Figure 2A-B shows, CNTs-300 have a nanotube structure with an uniform diameter (about 300 nm) and open ends, and possess a smooth wall surface and a thin wall thickness (about 11 nm). These features are critical for the successful filling of PB precursor. CNTs-50, similar to CNTs-300, have an open-ended 65 nm tube structures, but with a coarser wall surface and a thicker wall thickness (17 nm) (Figure 2C-D). Both the CNTs-300 and


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CNTs-50 show larger diameters than their original templates, that might be resulted from the effect of thermal expansion together with inhomogeneous reaction conditions during the synthesis processes. Raman spectroscopy was used to characterize the graphitization degree of the as-synthesized CNTs. As shown in Figure S3A, the Raman spectrum of CNTs-300 shows a peak at 1600 cm-1 which is assigned to the vibration of sp2-bonded carbon atoms (G band), and a peak at 1354 cm-1 which is attributed to the vibration model of sp3-hybridized carbon (D band). This result indicates the graphitic structure of the prepared CNTs-300. The observed intensity ratio (ID/IG) of 0.78 reveals that the CNTs-300 possess defects in the graphene structure. The Id/Ig ratio for CNTs-50 is 1.07 (Figure S3B), which is a litter larger than that for CNTs-300, demonstrating that the CNTs50 have a lower graphitization degree. The open-ended CNTs embedded in AAO template are used directly as nanoreactors for the preparation of PBNPs intercalated in the cavities of CNTs using photochemical reaction.37 In this experiment, PBNPs are synthesized in an acidic ferrocyanide solution under UV/Vis illumination. During this process, ferricyanide ion (Fe(CN)63-) is reduced into ferrocyanide ion (Fe(CN)64-) through a photochemical reaction, and free ferric ion (Fe3+) is obtained from the acidolysis of Fe(CN)63-, and then, Fe(CN)64- coordinates with the released Fe3+ to form PB. After chemical removal of the AAO templates, PB-in-CNTs nanocomposites can be obtained. As shown by the TEM images in Figure 3A, PBNPs with diameters between 40-50 nm are uniformly intercalated inside the nanotubes in PB-in-CNTs-300. There is no aggregation of PBNPs observed which may be caused by the confinement effect of CNTs. Similar results are obtained for PB-in-CNTs-50 (Figure 3B). Most PBNPs dispersed in the inner channel of CNTs exhibit a diameter less than 10


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nm, which is smaller than that in PB-in-CNTs-300. Besides, due to the smaller size and high density of PBNPs in 50 nm CNTs, partial aggregation cannot be avoided.

Figure 3. TEM images of PB-in-CNTs and PB-out-CNTs with tube diameters of 300 nm and 50 nm: PB-in-CNTs-300 (A), PB-in-CNTs-50 (B), PB-out-CNTs-300 (C), and PB-out-CNTs-50 (D).

For comparison, PBNPs with the same composition dispersed on the outer surface of CNTs (PBout-CNTs-300 and PB-out-CNTs-50) were also synthesized. As can be seen from the TEM images of PB-out-CNTs-300 (Figure 3C), cubic PB nanoparticles with a diameter of about 200 nm are anchored tightly on the outer surface of CNTs. The similar structure and morphology of PB nanoparticles in PB-out-CNTs-50 (Figure 3D) are observed and the size of PBNPs located on the outer surface is approximately 20 nm.


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Figure 4. (A) PXRD patterns of bulk PB (a), PB-out-CNTs-300 (b) and PB-in-CNTs-300 (c). (B) FTIR spectra of CNTs-300 (a), bulk PB (b), PB-out-CNTs-300 (c) and PB-in-CNTs-300 (d).

Powder X-ray diffraction (PXRD) measurements and Fourier transform infrared (FTIR) spectra further proved the successful filling of PBNPs into CNTs. As shown in Figure 4A, the diffraction peaks of PB-in-CNTs-300 at 2θ =17.35°, 24.81°, 35.32°, 43.39° correspond to the (200), (220), (400), (420) crystalline planes of fcc PB, respectively. Similar results are found in PB-out-CNTs300 and bulk PB. These results indicate the formation of PBNP crystals in the channels of CNTs. In the FTIR spectra (Figure 4B), it is obvious that there is no adsorption in the 1000-3500 cm-1 region in the case of CNTs. In contrast, the IR spectra of bulk PB, PB-in-CNTs-300 and PB-outCNTs-300 all present a characteristic adsorption peak at 2081 cm-1 which is ascribed to the CN stretching vibration of PB. This again confirms the formation of PBNPs in the channels of CNTs.


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Figure 5. Cyclic voltammograms of PB-out-CNTs-300 (a) and PB-in-CNTs-300 (b) modified GC electrodes in 0.1 M K2SO4 (A) and 0.1 M PBS solution (B) at a scan rate of 50 mV/s, respectively. The currents are normalized by the cathodic peak current for the reduction of PB to PW in base electrolyte.

Electrochemical behaviors of PB-in-CNTs. The electrochemistry behaviors of PB intercalated in CNTs and its electrocatalytic activity toward hydrogen peroxide reduction were studied. Figure 5 shows the cyclic voltammograms (CVs) of PB-in-CNTs-300 and PB-out-CNTs-300 modified GC electrodes in a mixture of 0.1 M K2SO4 and 0.1 M phosphate buffer solution (PBS). In order to compare easily the electrochemical properties of the two nanocomposites, the measured currents are normalized. It can be seen that both the nanocomposites display the typical redox properties of PB. The pair of peaks located at around 0.15 V corresponds to the transition between PB and Prussian white (PW), while the pair of peaks centered at 0.82 V corresponds to the transition between PB and Prussian yellow (PY). In addition, from Figure 5A we can see that the cathodic peak potential (Pc ) for the reduction of PB to PW for PB-in-CNTs is distinctly positively-shifted by 62 mV and exhibits a more reversible process with a smaller peak potential difference (ΔEp, Pa-


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Pc) compared with that for PB-out-CNTs. This indicates that a faster charge transfer occurs for PB-in-CNTs. Such phenomena become more obvious for the PB nanoparticles confined in smaller carbon nanotubes. In the case of PB-in-CNTs-50, the anodic peak potential shifts negatively accompanying the positive shift of the cathodic peak potential for the reduction of PB to PW as compared to the ones for PB-out-CNTs-50 (Figure S4A). The peak potential difference for the PBPW transition of PB-in-CNTs-50 is much smaller than that of PB-in-CNTs-300, demonstrating a more reversible electrochemical process. Such phenomena do not depend on the composition of the electrolytes since similar behavior in PBS is also observed (Figure 5B and Figure S4B). The distinct electrochemical behaviors of PB filled inside CNTs can be attributed to the confinement effect of CNTs. A similar effect was recently reported for the thermal reduction of Fe3O4 nanowires and Fe2O3 nanoparticles encapsulated in CNTs at different temperature.11, 38 It has been reported that when the sp2 hybridization electron structure of the graphene walls is curved and deformed, it causes the deviation of π-electron density from planarity leading to the π-electron density shifting from the concave inner surface to the convex outer surface of CNTs. This results in an electron-deficient interior surface and an electron-enriched outer surface.9, 39-42 Furthermore, theoretical and experimental studies have demonstrated that a small charge transfer between transition metal atoms and various kinds of CNTs can occur.11, 43-45 Thus, it is not hard to speculate that the electronic interaction between PBNPs and the inner and outer surface of CNTs may be different. Based on the above analysis, we believe that the d-orbital electron of Fe in PB can partially compensate the electron deficiency of concave inner surface of CNTs. Apparently, this destabilizes the oxidizing form PBNPs and facilitates the reduction of PB into PW, which allows PB-in-CNTs to show an excellent electrochemical performance compared to PB-out-CNTs.


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Furthermore, cyclic voltammograms of PB-in-CNTs-300 modified GC electrode under different scan rate were investigated. As seen from Figure S5, when the scan rate is kept in a slow range of 10-50 mV/s, the anodic and cathodic peak currents are proportional to the scan rate directly, indicating a surface controlled electrochemical process. However, when the scan rate increases further to the range of 40-600 mV/s, these two peak currents are proportional to the square root of scan rate, which suggests a diffusion controlled process of potassium ion in the zeolite structure of PBNPs which is similar to the PB modified electrode without carbon nanotubes.46 Besides, the transfer coefficient (α) and electron transfer rate constant (kS) of PB-in-CNTs are calculated according to the Laviron theory (see supporting information, section S1), and they are calculated as 0.704 and 1.46 s-1. The large value of kS indicates that the electron transfer for PB confined in carbon nanotubes is relatively fast, which is ascribed to the confinement effect of CNTs. Since PB is a typical electrocatalyst for H2O2 reduction, the catalytic activity of the PB-in-CNTs300 and PB-out-CNTs-300 were studied using cyclic voltammetry. As shown in Figure 6, PB-inCNTs-300 exhibits more positive onset potential (0.3 V) for H2O2 reduction when compared to PB-out-CNTs-300 (0.2 V), which indicates a better electrocatalytic activity of PB-in-CNTs-300 toward H2O2 reduction than that of PB-out-CNTs-300. This significantly enhanced electrocatalytic activity may be attributed to the excellent electrochemical property of intercalated PBNPs in CNTs, resulting from the electronic interaction between PBNPs and inner surface of CNTs induced by confinement effect. The difference in electrocatalytic activity of PBNPs inside and outside of 50 nm carbon nanotubes toward H2O2 reduction is not so obvious (Figure S6), which is not yet understood. In addition, it indicates that the anodic peak for the transformation from PW to PB in the case of PB-out-CNTs-300 disappears, which demonstrates that the chemical oxidation of PW by H2O2 is a fast process as compared to the transformation from PB to PW and the diffusion of


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H2O2 to the electrode surface presents an unlimited process. However, this anodic peak for the transformation from PW to PB in the case of PB-in-CNTs-300 still appears, which might indicate that the diffusion of H2O2 in CNTs presents a limited process.

Figure 6. Cyclic voltammograms of PB-out-CNTs-300 (a) and PB-in-CNTs-300 (b) modified GC electrodes in a 0.1 M PBS in the presence of 10 mM H2O2 at a scan rate of 50 mV/s. The currents are normalized by the cathodic peak current for the reduction of PB to PW in base electrolyte.

Effect of tube diameter on the electrochemical property of PB-in-CNTs. It is well known that the morphology, distribution as well as particle size of catalysts have an important influence on its performance. For PBNPs encapsulated in CNTs, the size of tube diameter may change its morphology, granular size and dispersion and thus modify its electrochemical property and catalytic activity. To investigate this effect, CNTs with two different diameters (300 nm and 50 nm) were synthesized and filled with PBNPs (PB-in-CNTs-300 and PB-in-CNTs-50). A comparison of CVs for these two nanocomposites in 0.1 M K2SO4 and 10 mM H2O2 (in PBS) solution is given in Figure 7. Figure 7A compares the electrochemical properties of PB-in-CNTs with different diameters. The oxidation peak potential (0.16 V) from PW to PB for PB-in-CNTs-


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50 is negatively shifted by 20 mV as compared to that of PB-in-CNTs-300 (0.18 V). It demonstrates that the electrochemical reversibility of PB-in-CNTs is increased with the decreasing of the tube diameters, indicating a faster electron transfer rate of PBNPs filled in smaller CNTs. TEM images in Figure 3 show that the particle size and morphology of PBNPs encapsulated in CNTs vary with the size of CNTs because of the space restriction. Furthermore, the electron density difference between inside and outside surface of CNTs should be enhanced when the inner diameter of CNTs becomes smaller, which results in a stronger electronic interaction between the concave inner surface of CNTs and the encapsulated PBNPs in smaller nanotubes. As a result, we believe that the electrochemical behavior of PB confined inside CNTs varies with the inner diameter of CNTs.

Figure 7. Cyclic voltammograms of PB-in-CNTs with tube diameter of 300 nm (a) and 50 nm (b) modified GC electrodes in 0.1 M K2SO4 solely (A) and 0.1 M PBS + 10 mM H2O2 (B) at a scan rate of 50 mV/s. The currents are normalized by the cathodic peak current for the reduction of PB to PW in base electrolyte.


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Moreover, by carefully comparing Figure 7A with Figure 5 and Figure S4, we can find that the size of carbon nanotube diameters has a greater impact on the oxidation conversion from PW to PB than the corresponding reduction conversion from PB to PW. As we have explained above, this distinct electrochemical property of PB can be attributed to confinement effect of CNTs due to the strong interactions between the electron-donating PB and electron-deficient interior surface of CNTs. Figure 7B depicts the CVs of PB-in-CNTs-300 and PB-in-CNTs-50 modified GC electrodes in 0.1 M PBS solution containing 10 mM H2O2 at a scan rate of 50 mV/s, which indicates that the diameter of CNTs has no significant effect on the electrocatalytic activity of PB toward H2O2 reduction, even though PB-in-CNTs displays a significantly enhanced electrocatalytic activity for the reduction of H2O2 than PB-out-CNTs.

Figure 8. Cyclic voltammograms of PB-out-CNT-300 (A), and PB-in-CNTs-300 (B) modified GC electrodes in 0.1 M K2SO4 solution at a scan rate of 50 mV/s at pH: 5.25, 7.40, 8.90, 11.29 and 12.72.


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pH tolerance of PB-in-CNTs. It has been reported that PB shows a very low stability in alkaline pH medium and decomposes easily.47 This is probably due to the strong interaction between ferric ions and hydroxyl ions (OH-) which forms Fe(OH)3 at pH higher than 6.4. As a result, Prussian blue-based biosensors are restricted in their practical use. Therefore, looking for a method to increase the operational stability of PB is of great importance. To evaluate the stability of PB-inCNTs, voltammetric experiments in alkaline solutions were conducted. Figure 8 shows the CVs of PB-out-CNTs-300 and PB-in-CNTs-300 modified GC electrodes in K2SO4 solution at different pH values. By calculating the decrease of the peak currents for PB under different conditions, we can see that PB-in-CNTs-300 shows much better stability in alkaline solutions, which is due to the protective effect of the tube wall of CNTs on the encapsulated PBNPs. As shown in Figure 8B, PB-in-CNTs-300 still keeps a good electrochemical performance even in alkaline electrolyte with pH as high as 11.29. The peak current at 0.2 V for the oxidation of PW to PB for PB-in-CNTs-300 falls only 20% from solution pH 5.25 to 11.29. While this peak current for PB-out-CNTs-300 decreases rapidly as solution pH increases. At pH 12.72, the characteristic redox peaks of PB-outCNTs-300 almost no longer exist (Figure 8A). Similar results are obtained with PB-in-CNTs-50 and PB-out-CNTs-50 (Figure S7). These results indicate that PBNPs encapsulated in CNTs possess an increased anti-base capability, which greatly expands the application of PB-based materials. Conclusions In summary, a relatively simple, controllable, and effective photochemical strategy of filling PBNPs into the channels of CNTs via a convenient photochemical process is proposed. We


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explored the confinement effect on the electrochemical property and electrocatalytic activity of PB-in-CNT. The results demonstrate that PB-in-CNTs shows a faster electron transfer rate and an enhanced electrocatalytic activity toward the reduction of H2O2 as compared to PB-out-CNTs. This can be attributed to the strong electronic interactions between the electron-donating PB and electron-deficient interior surface of CNTs. In addition, PB-in-CNT shows an excellent anti-base ability due to the protective effect of CNTs. Furthermore, effect of tube diameter on the electrochemical property of PB filled in CNTs is investigated. A faster electron transfer rate of PBNPs filled in smaller CNTs is obtained because of the stronger electronic interactions between the encapsulated PBNPs and the concave inner surface of CNTs. However, the size of tube diameter of CNTs does not show significant effect on the electrocatalytic activity of PB toward H2O2 reduction, and the reason is yet to be studied. The present results are believed to further deepen the understanding of unusual physical and chemical performances of nanomaterials in confined environments and stimulate extensive investigations on the confinement effect of CNTs on the magnetic, electrical, optical, catalytic, biological and mechanical properties of encapsulated nanomaterials. However, further experimental and theoretical studies on the mechanism of confinement effect on the electrochemical properties of this intriguing and promising nanocomposite are highly desirable. Finally, the present method is promising for designing and synthesizing novel functional nanocomposites with special functions and excellent performances. Experimental Section Materials. Commercial anodic aluminum oxide membrane (thickness 60μm; pore diameter 200 nm) was purchased from Whatman. Aluminum sheets (thickness 0.1 mm; purity 99.99%) were


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obtained from Xinjiang Zhonghe Limited Corp. (China). Analytical grade reagents of glucose, oxalic acid, sulfuric acid, phosphoric acid, perchloric acid, 36% hydrochloric acid aqueous solution, potassium hydroxide, potassium ferricyanide, ferric chloride, potassium chloride, hydrogen fluoride, potassium sulphate, monopotassium phosphate, dipotassium phosphate, 30% hydrogen peroxide aqueous solution and ethanol were purchased from Sinopharm Chemical Reagent Co., Ltd. (China). All solutions were prepared daily with deionized water (18.2 MΩ•cm1

, Milli Q). Characterizations. The morphology of the CNTs was characterized using a field emission

scanning electron microscopy (FESEM, Hitachi, Japan, S-4800, 10 kV) and the morphology of PB-in-CNTs and PB-out-CNTs was imaged on a transmission electron microscope (TEM, JEM2100) at an accelerating voltage of 200 kV. Raman scattering measurements were performed on a FT-Raman spectrometer (Renishaw InVia) using a 514 nm laser source. FT-IR spectra were recorded on a Bruker (Germany) FT-IR spectrometer over a range from 400 to 4500 cm-1. X-ray diffraction (XRD) characterization was performed with a XRD-6000 X-ray diffractometer (Shimadzu, Japan) using Cu Kαradiation at a scanning speed of 4° min-1. Electrochemical measurements were carried out on a CHI 830B Electrochemical Workstation (CH instruments, Shanghai, China) using a conventional three-electrode system at room temperature. A modified glassy carbon (GC) disc electrode (3 mm in diameter), a platinum wire and a saturated calomel electrode (SCE) were used as the working, counter, and reference electrodes, respectively. Electrode preparation and modification. The GC electrode was well polished with 1, 0.3, and 0.05 μm alumina slurry sequentially and then cleaned ultrasonically in absolute ethanol and ultrapure water sequentially for 3 min. The cleaned GC electrode was dried with nitrogen steam


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before modification. The electrode modification was performed by casting 10 μL PB-in-CNTs or PB-out-CNTs suspension onto the pretreated bare GC electrode using a micropipette tip and dried in a vacuum desiccator at room temperature. The modified electrodes were directly used for electrochemical measurements.

ASSOCIATED CONTENT Supporting Information. SEM images, Raman spectra, CV curves. This material is available free of charge on the ACS publications website at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] Author Contributions ‡These authors contributed equally to this work. ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (21635004, 21675079, 21627806).


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