Letter pubs.acs.org/NanoLett
Copper Nanowires: A Substitute for Noble Metals to Enhance Photocatalytic H2 Generation Shuning Xiao,† Peijue Liu,† Wei Zhu,† Guisheng Li,† Dieqing Zhang,*,† and Hexing Li*,†,‡ †
The Education Ministry Key Lab of Resource Chemistry, Shanghai Normal University, 100 Guilin Road, Shanghai 200234, China Shanghai University of Electric Power, 2588 Changyang Road, Shanghai 200090, China
‡
S Supporting Information *
ABSTRACT: Microwave-assisted hydrothermal approach was developed as a general strategy to decorate copper nanowires (CuNWs) with nanorods (NRs) or nanoparticles (NPs) of metal oxides, metal sulfides, and metal organic frameworks (MOFs). The microwave irradiation induced local “super hot” dots generated on the CuNWs surface, which initiated the adsorption and chemical reactions of the metal ions, accompanied by the growth and assembly of NPs building blocks along the metal nanowires’ surfaces. This solutionprocessed approach enables the NRs (NPs) @CuNWs hybrid structure to exhibit three unique characteristics: (1) high coverage density of NRs (NPs) per NWs with the morphology of NRs (NPs) directly growing from the CuNWs core, (2) intimate contact between CuNWs and NRs (NPs), and (3) flexible choices of material composition. Such hybrid structures also increased light absorption by light scattering. In general, the TiO2/CuNWs showed excellent photocatalytic activity for H2 generation. The corresponding hydrogen production rate is 5104 μmol h−1 g−1 with an apparent quantum yield (AQY) of 17.2%, a remarkably high AQY among the noble-metal free TiO2 photocatalysts. Such performance may be associated with the favorable geometry of the hybrid system, which is characterized by a large contact area between the photoactive materials (TiO2) and the H2 evolution cocatalyst (Cu), the fast and short diffusion paths of photogenerated electrons transferring from the TiO2 to the CuNWs. This study not only shows a possibility for the utilization of low cost copper nanowires as a substitute for noble metals in enhanced solar photocatalytic H2 generation but also exhibits a general strategy for fabricating other highly active H2 production photocatalysts by a facile microwave-assisted solution approach. KEYWORDS: microwave synthesis, photocatalysis, H2 generation, copper nanowires, TiO2
N
used as an alternative to expensive noble metals,11,12 especially for doping TiO2.13−16 Cu-doped composites containing dopants in the form of CuNPs typically exhibit a lower activity than that of composites with CuNWs. This difference may be due to the random dispersion of CuNPs in the TiO2 photocatalyst, which is not sufficient for electron transfer from TiO2 to Cu and, thus, disfavors the photocatalytic reaction.17−19 Learning from nature, plants grow via photosynthesis, in which light is harvested by leaves to generate photoelectrons, followed by electron transfer through chlorophyll. The TiO2/ CuNW hybrid photocatalyst contains TiO2 NRs assembled onto ultralong CuNWs. These TiO2 NRs can also harvest light to generate photoelectrons, with subsequent electron transfer through the CuNWs that might favor the photocatalytic processes. The assembled TiO2 nanorods promote light harvesting via multiple reflections to generate photoelectrons, and the one-dimensional CuNWs facilitate the transfer and gathering of the photoelectrons, leading to a high H 2
anocomposites have received increased attention due to their enhanced activity resulting from synergic effects.1−3 Most nanocomposites are typically fabricated by uniformly mixing different components via physical or chemical methods. Both theoretical predictions and experimental results have demonstrated that controlling the assembly of nanoparticle building blocks onto substrates generates fascinating properties due to the unique structures and morphologies that result,4−6 such as butterfly wings, fish scales, and lotus leaves, exhibiting cold resistance, selective light absorbance, and self-cleaning properties typically observed in nature. Herein, we developed a general strategy for decorating copper nanowires (CuNWs) with nanorods (NRs) or nanoparticles (NPs) consisting of metal oxides, metal sulfides, and metal organic frameworks (MOFs). Using TiO2 as an example, the performance of TiO2/ CuNW hybrids in photocatalytic H2 generation was also demonstrated. TiO2 has been widely used in the photocatalytic H2 generation reaction because it is inexpensive, nontoxic, and stable.7 Noble-metal dopants, such as Pt, Pd, and Rh,8,9 are frequently used as both the cocatalyst for H+ reduction and the electric conductor for separating photoelectrons from holes to inhibit their recombination.10 Recently, copper (Cu) has been © 2015 American Chemical Society
Received: January 8, 2015 Revised: July 7, 2015 Published: July 19, 2015 4853
DOI: 10.1021/acs.nanolett.5b00082 Nano Lett. 2015, 15, 4853−4858
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Nano Letters production rate via photocatalytic water reduction. The resulting H2 production rate can reach 5104 μmol h−1 g−1 with an apparent quantum yield (AQY) of 17.2%, a remarkably high AQY among the noble metal-free TiO2-based photocatalysts. As shown in Figure 1a−c, both the field emission scanning electron microscopy (FESEM) and transmission electron
Figure 2. (a) XRD patterns of the samples obtained from programmed microwave heating reactions (inset); (b−f) FESEM images of the samples tracking the progress of the microwave reaction at different heating times and temperatures: (b) heating for 6 min to 100 °C, heating for 10 min to 150 °C and then maintained for (c) 0 min, (d) 10 min, (e) 20 min, and (f) 30 min.
Figure 1. (a, b) FESEM images, (c) TEM image, and (d) HRTEM image with the attached SAED pattern of the C3 sample.
microscopy (TEM) images indicate that the CuNWs are uniformly coated with TiO2 nanorods that have an average diameter and length of 16 ± 5 nm and 120 ± 20 nm, respectively. The HRTEM image (Figure 1d) shows the close contact and obvious interface between TiO2 and Cu. The lattice fringe spacings of ca. 0.248 and 0.324 nm correspond to the (011) and (110) planes, respectively, of rutile TiO2, which agrees with the selected area diffraction (SAED) pattern. In addition, the lattice spacing of 0.255 nm corresponds to the (011) plane of the cubic phase Cu. The matching lattice between TiO2 (011) and Cu (011) favors the epitaxial growth of TiO2 nanorods on the CuNWs. The XRD patterns in Figure 2a demonstrate that the C3 sample produced well-resolved peaks at 2θ 13.7°, 18.0°, 20.6°, 27.2°, 28.3°, 32.0°, 34.5°, and 34.9° and at 2θ 43.2°, 50.3°, and 74.1° corresponding to the diffraction peaks of tetragonal rutile TiO2 (JC-PDF #21-1276) and cubic Cu (JC-PDF #03-1018), respectively. With an increase in either the microwave heating temperature or the hydrothermal time at 150 °C, the intensities of all peaks corresponding to rutile TiO2 gradually increased, indicating an increase in the crystallization degree. Meanwhile, the intensities of all of the Cu diffraction peaks gradually decreased due to the gradual coverage by TiO2, which was further confirmed by the FESEM images. As shown in Figure 2b, only separated dots were observed on the CuNWs subjected to microwave heating to 100 °C. As the microwave temperature or hydrothermal time were increased during microwave irradiation, the coverage density of the TiO2 nanorods on the CuNWs increased accompanied by an enhanced composite thickness (see Figure 2c−f). As a reference, we also synthesized rutile TiO2/CuNWs using an autoclave hydrothermal process at 150 °C (denoted as C3-HT). In contrast to C3, C3-HT exhibited poor crystallinity and a nonuniform morphology based on the XRD pattern and FESEM image (Supporting Information Figure S1), indicating
that the microwave-assisted process played a key role in assembling the TiO2 nanorods onto the CuNWs. First, the CuNWs exhibited enhanced adsorption toward the metal ions (Ti3+) due to the “skin effect”, which induced a negative electric current flowing primarily at the “skin” of the conductor under microwave irradiation.20 We carried out an adsorption test of Ti3+ in an aqueous solution containing CuNWs at different temperatures less than 60 °C for 10 min (Supporting Information Figure S2) because no significant hydrolysis of Ti3+ occurred in either the microwave or autoclave hydrothermal process under these conditions. As expected, the Ti3+ concentration in the solution containing CuNWs decreased much more rapidly when the solution was subjected to microwave heating compared to using the autoclave, indicating that microwave heating induced a stronger electrostatic adsorption toward Ti3+ due to the gathered electrons on the surface of the CuNWs.21 Second, microwave-induced local “super hot” dots can promote the hydrolysis of adsorbed Ti3+ and the crystallization of TiO2 on the surface of CuNWs due to its elevated temperature. These “super hot” dots resulted from Joule heating, which was generated by an eddy current in the electromagnetic field.21 Although the “super hot” dots are difficult to generate on a bulk metal subjected to microwave irradiation due to the small penetration depth of the electromagnetic field into metals, the ultralong CuNWs in the aqueous solution may still generate “super hot” dots on its surface. This behavior can be explained using a cylindroid model, as shown in eq 1
δ= 4854
2ρ (2πf )(μ0 μr )
(1) DOI: 10.1021/acs.nanolett.5b00082 Nano Lett. 2015, 15, 4853−4858
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Figure 3. Performances of different photocatalysts in photocatalytic H2 production under UV-LED irradiation (λ = 365 nm).
where δ, μ0, μr, ρ, and f refer to the limitation depth, magnetic constant, relative permeability of a specific material, electric resistance, and electromagnetic wave frequency, respectively. On the basis of the parameters of Cu (see Supporting Information), a relationship for the limitation depth as a function of the microwave frequency was obtained (Supporting Information Figure S3). From this relationship, a skin depth of 1.32 μm is the maximum external surface on which the 2.45 GHz microwave can induce a uniform current flow, contributing to the formation of hot spots on the surface of the CuNWs. Due to the appropriate diameter of the CuNWs (100 nm < 1.32 μm), the electromagnetic field can penetrate into the CuNWs and induce the eddy current on the surface of the CuNWs. This induced eddy current is responsible for the formation of Joule heating, which can create “super hot” dots on the surface of the CuNWs. Because the Joule heat on the CuNWs cannot be directly detected under microwave irradiation, it was determined by observing the changes in the solution temperature when the reaction system was subjected to microwave irradiation in the presence or absence of CuNWs. As shown in Supporting Information Figure S4, the solution containing CuNWs exhibited a substantially faster heating rate that resulted in a higher solution temperature compared to that of the solution without CuNWs. This temperature increase may be attributed to the Joule heating caused by the induced eddy current in the electromagnetic field, which was confirmed using a control experiment in which typical oil-bath heating was applied instead of microwave heating. The temperature of the solution in the presence or absence of CuNWs remained constant over a period of heating. These results further demonstrate that Joule heating was induced by microwave irradiation of the CuNWs. In highly efficient microwave heating, water exhibits excellent microwave absorbance, which may increase in the temperature. When the aqueous solution containing CuNWs was irradiated by microwaves, the heat could be easily transferred from the water to the CuNWs due to its excellent heat conductivity, which could also induce a rapid temperature increase on the CuNW surfaces leading to the formation of surface “super hot” dots. Due to the strong adsorption of Ti3+ and the local “super hot” dots on the CuNW surfaces under microwave irradiation, Ti3+ was rapidly adsorbed and hydrolyzed into TiO2 on the CuNWs. Then, the TiO2 nanorods formed an ordered assembly on the CuNWs, which was followed by crystallization and crystal growth. Meanwhile, the presence of Cl− and moderate acidity could direct the oriented growth of the rutile TiO2 crystal in the [001]
direction.22−25 By increasing the concentration of TiCl3 in the starting solution, growth of the (110) plane can be suppressed due to the large amount of Cl− adsorbed on the surface, which resulted in longer nanorods with an aspect ratio that changed from 4.9 to 21.9 (see FESEM and TEM images in Supporting Information Figure S5). However, the hydrolysis of Ti3+ into TiO2 occurred both in solution and on the CuNW surfaces by subjecting the solution to traditional autoclave hydrothermal heating. This result was confirmed by the results shown in Supporting Information Figure S1 in which the TiO 2 nanoparticles were randomly dispersed in solution and nonuniform TiO2 coverage of the CuNWs was observed. Other CuNW hybrids decorated with MOF-5, ZIF-8, and ZnS with good morphologies and intimate contact interfaces (see Supporting Information Figure S6) can also be fabricated via this microwave-assisted solution approach. Figure 3 shows the photocatalytic performances of different samples in 60 mL of water and 20 mL of methanol under irradiation by four UV-LED lamps with a characteristic wavelength of 365 nm. For comparison, Table 1 summarizes the structural parameters calculated from the N2 adsorption− desorption isotherms and the photocatalytic activities obtained from the results in Figure 3. From C1 to C3, the activity increased rapidly due to the increasing coverage of the CuNWs Table 1. Physical Properties and H2 Production Rates of the Samples sample C1 C2 C3 C4 C5 C6 P3 1.0 wt % Pt−P3 34 wt % Cu− P3MX 1.0 wt % Cu− P3 34 wt % Cu− P3 34 wt % CuRutile N3 4855
SBET (m3 g−1)
Vp (cm3 g−1)
H2 generation rate (μmol h−1g−1)
AQY (%)
32.6 38.6 42.5 24.5 20.7 19.6 38.0 27.1 31.2
0.10 0.13 0.14 0.040 0.050 0.050 0.12 0.030 0.090
1541 3147 5104 4263 2103 1440 Trace 1828 Trace
5.3 10.6 17.2 14.5 7.2 4.9 Trace 6.2 Trace
33.9
0.10
946
3.2
25.7
0.030
3924
13.4
26.1
0.20
309
1.1
18.0
0.060
1955
6.7
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Nano Letters by TiO2 nanorods. However, from C3 to C6, the activity decreased abruptly even though the coverage of the CuNWs by TiO2 nanorods increased further. The C3 sample exhibited the highest activity, corresponding to an H2 production rate of 5104 μmol h−1 g−1 and an AQY of 17.2%. The H2 generation rate of C3 was higher than those of other Cu-doped TiO2 photocatalysts synthesized according to previously reported methods,17−19 as shown in Supporting Information Figure S7, highlighting the promoting effect of the unique architecture of C3. The P3 sample obtained by completely removing the CuNWs from C3 exhibited almost no activity, and the mechanical mixture of CuNWs and P3 (referred as Cu− P3MX) also exhibited extremely low activity. The 34 wt % Cu− P3 sample, with the same content of Cu as C3, obtained by the H2 reduction of Cu(OH)2-P3 regained some of this photocatalytic activity, but it was still remarkably lower than the activity of C3. From the cycle performance (Supporting Information Figure S8) of the C3 and 34 wt % Cu−P3 samples, the enhancement of the H2 production rate in the first run was not remarkable. However, after a few cycles, the H2 evolution rate for the 34 wt % Cu−P3 sample decreased, and the H2 production rate was well maintained over the C3 sample. This result was primarily due to the loose contact between the CuNPs and the TiO2 nanorods in the 34 wt % Cu−P3 sample because the CuNPs were randomly dispersed on the external surface of the TiO2 nanorods, as shown in the SEM and TEM images (Supporting Information Figure S9). In contrast, the unique TiO2@CuNW structures of the C3 sample (nanorod-on-nanowire morphology) can facilitate electron transfer, light harvesting and adsorption of reactants, thus improving the H2 production rate and long-term recycling stability. The 1.0 wt % Cu−P3 sample exhibited a lower activity than the 34 wt % Cu−P3 sample due to a decrease in the Cu content. In addition, the 34 wt % Cu-Rutile sample obtained by H2 reduction of Cu(OH)2-Rutile (the rutile TiO2 resulted from P25 calcination at 600 °C for 4 h) also exhibited extremely low activity. Furthermore, the 1.0 wt % Pt−P3 sample exhibited a much higher activity than the 1.0 wt % Cu−P3 sample because H+ reduction to H2 occurred more favorably on Pt than that on Cu due to the lower overpotential of H+ on Pt (aH, Pt = 0.10 V) than that on Cu (aH, Cu = 0.87 V). This result is also consistent with the results from the electrocatalytic hydrogen evolution reaction (HER). As shown in Supporting Information Figure S10, Pt exhibited the highest HER activity, and the CuNWs exhibited a relatively lower HER activity than the well-known HER catalysts Ni−Mo26 or Ni2P.27 Although the CuNWs exhibited poor HER activity, they were still useful for photocatalytic H2 generation due to their good conductivity and stability, which was confirmed by the comparison of the photocatalytic activity of the C3 and 1.0 wt % Pt−P3 samples. Accordingly, the higher photocatalytic H2 production rate of C3 indicated that copper nanowires might be a good substitute for noble metals to enhance photocatalytic H2 generation. In general, H2 production from photocatalytic water reduction primarily proceeds via the reduction of H+ on active sites enriched with photoelectrons and the oxidation of a sacrificial agent (e.g., methanol) on active sites enriched with photoinduced holes. On the basis of the assembly of the TiO2 nanorods on the CuNWs, a plausible mechanism is proposed in Figure 4. First, TiO2 generates photoelectrons and holes under UV irradiation. The interface between the metal and semiconductor resulted in an oriented electron flow to the metals, which can effectively inhibit photoelectron−hole recombina-
Figure 4. Proposed photocatalytic reaction mechanism of CuNWs/ TiO2.
tion. The direction of electron transfer was primarily determined by their work functions. In the Cu/TiO2 samples, the higher work function of Cu (4.65 eV) compared with that of TiO2 (4.13 eV) ensured that the photoelectrons transferred from TiO2 to the CuNWs. Meanwhile, the strong Cu/TiO2 interaction with its large interface and the excellent electric conductivity of Cu created a Schottky barrier, which further facilitated the photoelectron transfers. Moreover, the CuNWs can also act as a cocatalyst for photocatalytic H2 generation, and the enriched photoelectrons on the CuNWs promoted H+ reduction to produce H2. The photogenerated holes remaining on the TiO2 nanorods were consumed by the oxidation of methanol as a sacrificial agent. Therefore, the light harvesting, the separation of photoelectrons from holes, and the cocatalyst played important roles in determining the H2 production efficiency. The results in Figure 5a indicate that the photocurrent first increased from C1 to C3 and then decreased from C3 to C6, which agrees with the change in their activities. As shown in Table 1, the BET specific surface area (SBET) increased from C1 to C3 due to the increasing coverage of TiO2 nanorods on the CuNW surfaces and the enhanced pore volume (VP), which enhance light harvesting and generate more photocarriers. From C3 to C6, the TiO2 nanorods overlapped on the CuNW surfaces, which resulted in an abrupt decrease in SBET and VP that reduced the light harvesting due to a shielding effect, leading to the generation of fewer photocarriers. As previously reported, the TiO2-based photocatalysts typically exhibit two PL peaks at approximately 380 and 560 nm (excitation wavelength = 280 nm). The peak at 560 nm corresponds to the dual-frequency peak. The relative intensity of the dualfrequency PL peak can serve as an indicator for evaluating the electron−hole recombination rate.28,29 As shown in Figure 5b, the intensity of the PL peak at approximately 560 nm decreased from C1 to C3, demonstrating that the photoelectron−hole recombination rate decreased due to the enhanced ordering degree of the assembled TiO2 nanorods favoring the photoelectron transfer through the CuNWs. From C3 to C6, the PL peak intensity increased, indicating an enhanced photoelectron−hole recombination rate due to the unfavorable overlap of the TiO2 nanorods for photoelectron transfer from TiO2 to the CuNWs, especially for the TiO2 located on the topoutside surface. The C3 sample exhibited both a strong light harvesting ability and low photoelectron−hole recombination, leading to the highest photocurrent response and thus the highest activity among the studied photocatalysts (i.e., C1 to C6). In addition, the apparent recombination rate (inferred from Supporting Information Figure S12) of 34 wt % Cu−P3 was low and comparable to that of the C3 sample. The SEM and TEM images of 34 wt % Cu−P3 are shown in Supporting Information Figure S9. The Cu dopants were present as small islands randomly dispersed on the external surface of the TiO2 nanorods rather than in a uniform thin film. The high 4856
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Figure 5. (a) Photocurrent responses in the light on−off process (UV-LED light, potential 0.5 V vs SCE) and (b) photoluminescence (PL) spectra with an excited light irradiation λ = 280 nm of sample C1−C6.
an orderly fashion on the CuNWs30 and resulted in an enhanced photoelectron−hole recombination rate and low photocurrent. In addition, each of the Cu nanoparticles were encapsulated by TiO2 nanorods, which hinders light harvesting and prevents the Cu cocatalyst from coming into contact with H+, leading to decreased H+ reduction to H2. It is important to note that C3, 1.0 wt % Cu−P3, and 34 wt % Cu−P3 exhibited nearly the same light harvesting abilities due to their similar morphologies and porous structures. The C3 sample exhibited lower electric impedance than the 34 wt % Cu−P3 sample because the CuNWs resulted in better electron transfer than the separated Cu nanoparticles. Therefore, the C3 sample exhibited a low photoelectron−hole recombination rate, high photocurrent, and enhanced photocatalytic activity. The 1.0 wt % Cu−P3 sample exhibited poor photoelectron transfer compared to that of the 34 wt % Cu−P3 sample, corresponding to a high photoelectron−hole recombination rate, low photocurrent, and decreased photocatalytic activity. This result can be understood by considering the decrease in the Cu content because Cu serves as both an excellent electric conductor and a cocatalyst. The 34 wt % Cu-Rutile sample also exhibited very low photocatalytic activity due to its poor light harvesting ability and photoelectron transfer, which resulted in a high photoelectron−hole recombination rate and a low photocurrent. In addition to having the highest photocatalytic activity, the C3 sample also exhibited strong durability, and this sample was repetitively used without a significant decrease in its H2 production rate (Supporting Information Figure S8), indicating its excellent stability against both structural damage and leaching of either the active sites or the cocatalyst. In conclusion, we developed a general microwave-assisted hydrothermal strategy for decorating CuNWs with highly crystalline NRs or NPs consisting of TiO2, ZnS, MOF-5, and ZIF-8. In general, TiO2/CuNWs exhibited a remarkably high AQY compared to noble-metal free TiO2 photocatalysts for photocatalytic H2 generation under UV light irradiation. The direct growth of TiO2 nanorods onto CuNWs may increase the contact interface, enhance light harvesting by multiple reflections and facilitate photoelectron transfer and enrichment, which further reduces photoelectron−hole recombination and promotes H+ reduction to H2. This study demonstrates the use of inexpensive copper nanowires as a substitute for noble metals in enhanced solar photocatalytic H2 generation and
dispersion of Cu nanoparticles could promote photoelectron transfer, leading to rapid photoelectron−hole separation. Therefore, this sample also exhibited a low photoelectron− hole recombination rate that was similar to that of C3. However, the Cu nanowires could transfer photoelectrons more easily over a relatively large area than the separated Cu nanoparticles, which may account for its slightly lower photoelectron−hole recombination rate than that of the 34 wt % Cu−P3 sample. To further understand the various photocatalytic performances, we measured the electrochemical impedance spectroscopy (EIS) response of the different photocatalyst electrodes (see Supporting Information Figure S11). The radius of the arc on the EIS Nyquist plot reflects the conductivity, charge transfer and carrier recombination kinetics during the photocatalytic process. The x axis and y axis represent the real (Z′) and the imaginary (Z″) parts of the impedance, respectively. The smaller semicircle corresponds to low electric impedance, and based on this impendence, the electric conductivity decreased in the following order: C3, 34 wt % Cu−P3, N3, 1.0 wt % Cu−P3, and P3. In addition, the PL spectra and photocurrent response were also determined (Supporting Information Figure S12 and S13). The P3 sample, which was prepared by removing the CuNWs from C3, possessed nearly the same light harvesting ability as that of the C3 sample due to their similar morphologies and porous structures. However, this sample exhibited the lowest photocurrent response due to its extremely high electric impedance in the absence of Cu as an electric conductor that lead to the highest photoelectron−hole recombination rate and the lowest photocurrent response, resulting in its low photocatalytic activity. The mechanical mixture consisting of P3 and 34 wt % CuNWs (Cu−P3MX) also exhibited very high electric impedance due to the poor interaction between Cu and TiO2, which disfavored the photoelectron transfer from TiO2 to Cu leading to an enhanced photoelectron−hole recombination rate and a reduced photocurrent. Therefore, this mixture exhibited very low photocatalytic activity. Although the N3 sample, which was synthesized using the same amount of CuNPs instead of CuNWs, was much more active than the P3 and Cu−P3MX samples, it still exhibited a much lower activity than that of the C3 sample because the TiO2 nanorods gathered on the Cu nanoparticles exhibited poor photoelectron transfer compared to the TiO2 nanorods that were assembled in 4857
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(19) Li, Y.; Wang, W.-N.; Zhan, Z.; Woo, M.-H.; Wu, C.-Y.; Biswas, P. Appl. Catal., B 2010, 100, 386−392. (20) Thostenson, E.; Chou, T.-W. Composites, Part A 1999, 30, 1055−1071. (21) Yoshikawa, N. J. Microw. Power Electromagn. Energy 2010, 44, 4. (22) Park, K.-S.; Min, K.-M.; Jin, Y.-H.; Seo, S.-D.; Lee, G.-H.; Shim, H.-W.; Kim, D.-W. J. Mater. Chem. 2012, 22, 15981−15986. (23) Guo, W.; Xu, C.; Wang, X.; Wang, S.; Pan, C.; Lin, C.; Wang, Z. L. J. Am. Chem. Soc. 2012, 134, 4437−4441. (24) Hosono, E.; Fujihara, S.; Kakiuchi, K.; Imai, H. J. Am. Chem. Soc. 2004, 126, 7790−7791. (25) Kumar, A.; Madaria, A. R.; Zhou, C. J. Phys. Chem. C 2010, 114, 7787−7792. (26) McKone, J.; Sadtler, B.; Lewis, N.; Gray, H. ACS Catal. 2013, 3, 166−169. (27) Popczun, E.; McKone, J.; Lewis, N.; Schaak, R. J. Am. Chem. Soc. 2013, 135, 9267−9270. (28) Bian, Z.; Zhu, J.; Cao, F.; Lu, Y.; Li, H. Chem. Commun. 2009, 25, 3789−3791. (29) Huo, Y.; Bian, Z.; Zhang, X.; Jin, Y.; Zhu, J.; Li, H. J. Phys. Chem. C 2008, 112, 6546−6550. (30) Li, G.; Wu, L.; Li, F.; Xu, P.; Zhang, D.; Li, H. Nanoscale 2013, 5, 2118−2125.
provides a general microwave-assisted solution strategy for fabricating other cascaded nanohybrids, such as NR- (NP-) decorated carbon fibers, to prepare new functional nanohybrids with fascinating properties.
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ASSOCIATED CONTENT
S Supporting Information *
Experimental details and additional figures. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.5b00082.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the NSFC (21477079, 21207090, 21261140333, and 21237003), Shanghai Government (13YZ054, 14ZR1430900, 15QA1403300), PCSIRT (IRT1269), the doctoral program of higher education (20123127120009), and SHNU (S30406).
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