Ag Nanoparticles Decorated Small-Sized AgTCNQF4 Nanorod

Article pubs.acs.org/JPCC

Ag Nanoparticles Decorated Small-Sized AgTCNQF4 Nanorod: Synthesis in Aqueous Solution and Its Photoinduced Charge Transfer Reactions Jing Wang, Weiqing Xu, Junjie Zhang, and Shuping Xu* State Key Laboratory of Supramolecular Structure and Materials, Jilin University, Changchun 130012, P. R. China S Supporting Information *

ABSTRACT: A kind of functional noble metal nanoparticles and metal/organic semiconductor composite nanomaterials, Ag nanoparticles (AgNPs, 6−10 nm in diameter) decorated small-sized Ag-tetracyano-p-tetrafluoroquinodimethane (AgTCNQF4) nanorods (150−400 nm in length and 60−100 nm in diameter), have been successfully synthesized through a rapid microemulsion reaction between TCNQF4 molecules and an AgNP colloid under a soft template of poly(ethylene glycol)-blockpoly(propylene glycol)-block-poly(ethylene glycol). The morphology, chemical structure, and elemental composition of the prepared AgNPs−AgTCNQF4 composite nanorods were studied by transmission electron microscopy, selected-area electron diffraction, and X-ray photoelectron spectroscopy. The real-time ultraviolet−visible spectroscopy assisted with two-dimensional correlation spectroscopic analysis was employed to explore the growth of AgNPs−AgTCNQF4 composite nanorods in microemulsion. These composite nanorods display the photoinduced charge transfer (CT) property from the monoanion (TCNQF4−) to dianion (TCNQF42−) selectively under 532 nm light irradiation. The larger content of AgNPs on the surface of AgTCNQF4 led to the higher conversion of dianion due to the plasmon-assisted photocatalysis. This photoelectric composite material is promising for the applications of light-writing data storage and photocatalysis.

1. INTRODUCTION TCNQ (TCNQ = 7,7,8,8-tetracyanoquinodimethane) and its fluorinated derivative TCNQF4 (TCNQF4 = 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane) are strong organic electron acceptors, which have been regarded as popular ligand candidates in coordination with the transition metal to form metal/organic semiconductor coordination polymers.1−5 Onedimensional charge-transfer (CT) metal/organic semiconductors, MTCNQ (M = Cu+ or Ag+), and its derivatives, MTCNQF4, have received much attention due to their unique optical and electrical bistable behaviors and memory effects.3−5 Since 2000, studies about the morphologies of MTCNQ(F4) have been extended from a simplex film to nanowires, nanorods, and microrods, etc. Table 1 summaries the sizes and morphologies of MTCNQ(F4) in previous studies. Not only the preparation methods, microcosmic morphologies, phases, and structures of MTCNQ(F4) but also their growth mechanisms have received an abundance of attention.16,18,22−25 At the same time, their applications in organic photo/ © 2014 American Chemical Society

electrochromic devices, data storage, memory devices, and field emission transistors have been developed.26−32 MTCNQ(F4) with a relatively large size to micron level are convenient to be fixed in microelectronic devices because of easy manipulation (see Table 1). Recently, MTCNQ(F4) complexes have been applied to a variety of new areas based on their CT reactions, such as catalysts,33 antibacterial agents,34 humidity sensors,35 and flexible optoelectronic devices.36 However, for the CT processes occurring on these metal/organic semiconductors, a relatively large size is adverse to the separation and transportation of electrons, which will result in a low catalysis activity and a weak photoelectric property. Metal/organic semiconductors combined with metal nanoparticles (MNPs) have been proved to be a feasible way to accelerate the charge separation and transport to MNP/ Received: July 13, 2014 Revised: September 23, 2014 Published: September 26, 2014 24752

dx.doi.org/10.1021/jp5069736 | J. Phys. Chem. C 2014, 118, 24752−24760

The Journal of Physical Chemistry C

Article

Table 1. Synthesis Methods, Morphologies, and Sizes of MTCNQ(F4) (M = Ag, Cu) in the Literatures species

a

preparation method

morphology

phase

sizea D, 100−600 nm; L, 10−20 μm6 D, 1−2 μm; L, 20−50 μm7 10−20 μm7 D, 100−200 nm; L, 10−50 μm8 D, 20−100 nm; L, 1−20 μm9 D, 130−180 nm10 D, ≈100 nm; L, >10 μm11 D, 40−70 nm10 L, 100 μm12 L, 20−40 μm13 D, 2 μm; L, 4 μm14 L, 20 μm15 D, 30−60 nm, L, 50−200 nm16 D, 50−120 nm; L, 180−560 nm16 D, 400 nm; L, 5 μm16 D, 180 nm; L, 30 μm17 D, 500 nm; L, 4.5 μm17 very long18 D, 30−200 nm; L, 1−200 μm19 L, ∼200 μm20 D, 1 μm; L, >10 μm21

AgTCNQ

solution process

nanowires microtubes

phase II

CuTCNQ AgTCNQ CuTCNQ AgTCNQ AgTCNQ CuTCNQ CuTCNQ CuTCNQ CuTCNQ AgTCNQ AgTCNQ

solution process vapor−solid chemical reaction vapor−solid chemical reaction two-phase method aqueous solution method two-phase method electrodeposition electrodeposition electrodeposition from pulse voltage electrodeposition in acetonitrile solution electrodeposition from an ionic liquid

microrods nanowires nanowires nanowires nanowires nanowires nanowires needle-shaped layer structure microtube nanowires nanorods

phase I phase II phase I

AgTCNQ CuTCNQ AgTCNQ AgTCNQF4 AgTCNQF4 AgTCNQF4

photocrystallization photocrystallization photocrystallization from ionic liquid vapor−solid chemical reaction electrodeposition solution process

nanowires nanowires microrods nanowires nanowires nanowires microtubes

phase I phase phase phase phase phase

I I, II I, II

phase I

D: diameter; L: length.

semiconductor interfaces.7,21 These composite materials are propitious to CT reactions. Our previous study showed that the CT reaction from TCNQF4− monoanion to TCNQF42− dianion could happen in an AuNPs decorated AgTCNQF4 microcrystal system but could not in a pure AgTCNQF4 system under the same conditions.21 However, to prepare the AuNPs decorated AgTCNQF4 microcrystals, the growth of AuNPs on AgTCNQF4 microcrystals via the galvanic replacement reaction consumed a number of AgTCNQF4 as precursors,7 which causes the damage and corrosion of AgTCNQF4 microcrystals and decreases productivity as well. In present study, we designed and synthesized a MNPs and metal/organic semiconductor composite nanomaterial, AgNPs decorated AgTCNQF4 nanorods (AgNPs−AgTCNQF4), via a no-damage, synchronous, and one-pot synthesis method in aqueous phase. To our knowledge, the as-prepared AgTCNQF4 owns the smallest size (150−400 nm in length and 60−100 nm in diameter) among such products prepared by the liquid phase reactions (see Table 1). We probed the formation process of AgNPs−AgTCNQF4 by the real-time ultraviolet−visible (UV− vis) spectroscopy assisted with the two-dimensional correlation spectroscopy (2DCOS) analysis. The photoinduced CT process within the AgNPs−AgTCNQF4 composite was also studied. Moreover, the role of AgNPs in the CT process was discussed. The results display that the AgNPs−AgTCNQF4 composite nanorods exhibit higher interfacial CT efficiency and the photoinduced transformation from AgTCNQF 4 to Ag2TCNQF4 can be accelerated under the plasmon-assisted photocatalysis.

Beijing Chemical Plant. The ultrapure water (18 MΩ) from a Millipore system was employed for all aqueous solutions. 2.2. Synthesis of AgNPs−AgTCNQF4 Nanorods. AgNP colloid was first prepared by using a previous method.37 4.0 mL of 0.1 M AgNO3 solution and 100 mL of water were poured into a 150 mL three-necked flask, then heated, and refluxed to slightly boiling. Next, 2.0 mL of 1% (w/v) trisodium citrate was added, and the mixture was refluxed for 40 min. After that, a cyaneous AgNP colloid was achieved, and it was cooled to room temperature before use. The AgNPs−AgTCNQF4 nanorods were prepared by a simple, fast, and one-pot method within a water, acetonitrile, and P123 mixed solution. 2.0 mL of a 1.0 mg/mL P123 aqueous solution and 50 μL of a 2.0 mM TCNQF4 acetonitrile solution were mixed in a 1 cm quartz cuvette under rapid stirring. P123 is a surfactant, and it formed micelles, coating the tiny drops of acetonitrile with TCNQF4 dissolved in. Then, 100 μL of AgNP colloid was immediately injected into above TCNQF4 microemulsions. The solution changed its color from yellow to blue in an instant, indicating the formation of AgNPs−AgTCNQF4 nanorods. Time-resolved UV−vis absorption spectra (Ocean Optics USB4000 spectrometer) with the interval of 50 ms and the exposure time of 400 ms were used to monitor the formation of AgNPs−AgTCNQF4 nanorods. Then, the solution containing AgNPs−AgTCNQF4 nanorods was centrifuged, and the precipitations were washed three times by water. Different amounts of AgNP precursors were tried to tune the reaction ratios of AgNPs and TCNQF4, and their produced precipitations were characterized by the transmission electron microscopy (TEM). 2.3. Photoinduced CT Reaction in AgNPs−AgTCNQF4 Nanorods. The photoinduced CT reaction on the dried AgNPs−AgTCNQF4 was carried out under the continuous irradiation of an optical fiber laser with a 532 nm wavelength at room temperature and air environment. We used a Y-type fiber probe in which one light path is either for the photoinduced catalysis reaction or for Raman excitation and the other light

2. EXPERIMENTAL SECTION 2.1. Chemicals. 7,7,8,8-Tetracyano-2,3,5,6-tetrafluoroquinodimethane (TCNQF4) and poly(ethylene glycol)-blockpoly(propylene glycol)-block-poly(ethylene glycol) (P123) were purchased from Sigma-Aldrich. Acetonitrile (for HPLC, ≥99.9%) was purchased from Aladdin Company. Silver nitrate (99.5%) and trisodium citrate (98%) and were obtained from 24753



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Figure 1. TEM images of AgNPs (A) and AgNPs decorated AgTCNQF4 nanorods which were prepared by mixing the TCNQF4 microemulsion (containing 50 μL of 2.0 mM TCNQF4 acetonitrile solution and 2.0 mL of 1.0 mg/mL P123 aqueous solution) with different volumes of the AgNP colloid. (B−E) correspond to the products using 50, 100, 150, and 200 μL of AgNP colloid, respectively. (F) An amplified TEM image of (E).

path works for Raman signal collection to a spectrometer (BWTEK). The power density to sample was 0.763 W/cm2. This CT reaction was completed within several minutes. 2.4. Characterizations. The TEM images, high-resolution TEM (HRTEM) images, and selected-area electron diffraction (SAED) pattern were all acquired with a JEM-2100F field emission transmission electron microscope (JEOL, Tokyo, Japan). Raman spectra excited under a 633 nm laser was recorded from a LabRam Aramis Raman microscope system (Horiba-Jobin Yvon, France) which equipped with a watercooled charge-coupled device (CCD) detector and a He−Ne ion laser. Raman spectra excited under a 532 nm laser (highpower semiconductor laser BWTEK, USA) were recorded by a portable BWTEK Raman spectrometer with a Y-type optical fiber probe. X-ray photoelectron spectra (XPS) were carried out using an VG ESCALAB MK II electron spectrometer; the excitation source was Mg Kα X-rays (hν = 1253.6 eV). 2.5. 2DCOS Treatment. The 2D correlation maps were calculated through a 2D Shige software which developed by Dr. Shigeaki Morita. The size of the perturbation steps was 0.5 s. In 2D correlation maps, the red regions indicate positive correlation intensities, whereas blue ones mean negative correlation intensities.

and then added the above solution into aqueous phase. Figure 1A shows a TEM image of a precursor, AgNPs, with a diameter of 50 ± 10 nm. The AgNP colloid reveals a cyaneous color (see its photograph as a1 in Figure 2A), and its plasmonic band is located at 417 nm (curve an in Figure 2A). The TCNQF4 microemulsion displays yellow (b1 in Figure 2A), and it has two absorption bands at 385 and 363 nm (curve b in Figure 2A), corresponding to neutral TCNQF4.38 When 100 μL of AgNP colloid was added into the TCNQF4 microemulsion, the mixture color suddenly changed into blue (c1 in Figure 2A). This reaction has been completed within 10 s. A UV−vis spectrum of the product (curve c in Figure 2A) shows two absorption peaks located at 345 and 607 nm. Based on our previous study,21 these two bands are attributed to the neutral AgTCNQF4 and the TCNQF4 anion radical of AgTCNQF4, respectively. Therefore, the reaction can be stated as follows (eqs 1−3):

3. RESULTS AND DISCUSSION 3.1. Characterization of AgNPs−AgTCNQF4 Nanorods. The AgNPs decorated small-sized AgTCNQF4 nanorods were synthesized via a CT reaction between an AgNP colloid and a TCNQF4 microemulsion. Since TCNQF4 is water insoluble, we first dissolved them in acetonitrile with the assistance of P123

Figures 1B−E show the TEM images of different AgNPs− AgTCNQF4 nanorods prepared by mixing the same volume of TCNQF4 with different volumes of AgNP colloid. It should be noted that in the preparation of AgTCNQF4 nanorods the added AgNP colloid was superfluous. When 50 μL of the AgNP colloid was added, a mass of rod-typed nanorods grew with the diameter of 60−100 nm and the length of 300 nm. It should be 24754

Ag 0 → Ag + + e−

(1)

TCNQF4 0 + e− → TCNQF4 −

(2)

Ag + + TCNQF4 − → AgTCNQF4

(3)



Article

Figure 2. (A) UV−vis absorption spectra of a AgNP colloid (a), a TCNQF4−acetonitrile solution (b), and a AgNPs−AgTCNQF4 nanorod contained solution (c). (a1−c1) are their corresponding photographs. (B) UV−vis absorption spectra of AgNPs−AgTCNQF4 which were prepared by mixing the TCNQF4 microemulsion (containing 50 μL of 2 mM TCNQF4 acetonitrile solution and 2.0 mL of 1.0 mg/mL P123 aqueous solution) and different volumes of AgNP colloid, and curves a−d correspond to the products using 50, 100, 150, and 200 μL of AgNP colloid, respectively. (C, D) HRTEM image with a SAEM pattern (inset) and the Ag 3d XPS spectrum of AgTCNQF4 nanorods.

separated components which are assigned to Ag 3d5/2 and 3d3/2 (Figure 2D). Among them, the peaks at 367.40 and 373.35 eV are attributed to Ag+ in AgTCNQF4, and those at 368.30 and 374.23 eV are assigned to Ag0 species.42,43 These data further confirm that the final product is composed of both AgNPs and AgTCNQF4. 3.2. Monitoring AgTCNQF4 Growth via Real-Time UV− Vis Absorption Spectroscopy. The formation of AgNPs− AgTCNQF4 nanorods in aqueous phase is a fast reaction. To investigate the growth of AgNPs−AgTCNQF4, we employed the real-time UV−vis absorption spectroscopy to monitor the formation of AgNPs−AgTCNQF4 nanorods in the microemulsion system. We quickly added AgNP colloid (100 μL) into the TCNQF4 microemulsion and recorded their UV−vis spectra (Figure 3A). At the beginning, two characteristic peaks of TCNQF4 at 385 and 363 nm due to the neutral TCNQF4 were observed. After the AgNP colloid was added, two peaks at 385 and 363 nm decreased while two broad bands in the 277− 422 and 550−800 nm regions increased with the reaction time. Figure 3B plots the UV−vis absorption peak intensities of the consuming TCNQF4 (at 385 nm) and the growing Ag− TCNQF4 (at 345 and 607 nm) along the reaction time. It can be found that the whole reaction is completed within 10 s. The AgTCNQF4 formation process can be divided into three distinct stages. Stage I (0−1500 ms) is a collision and aggregation period of AgNPs and TCNQF4. A number of AgNPs nucleated in this period. In stage II (1500−5000 ms), the peak intensities at 345 and 607 nm both increase, and on the contrary the peak intensity at 385 nm decreases rapidly. These reveal that it is a rapid electronic transfer reaction

noted that the formed AgTCNQF4 nanorods have the smallest size among the reported ones (in Table 1). It can also be observed from Figure 1B that there are several crystalline AgNPs with a size of ca. 6 nm, presenting darker contrast, on the surface of AgTCNQF4 nanorods. When the added amount of AgNP colloid increases to 100 μL, a lot of AgNPs with a little larger size (about 7.5 nm) are observed (Figure 1C). As the precursor of AgNP colloid further increases, two types of AgNPs with different diameters exist (Figures 1D,E). Smaller sized AgNPs (∼10 nm) are attached on the AgTCNQF4 nanorods (Figures 1D,F), while the bigger sized AgNPs with the average diameter of 50−60 nm, which are from the unreacted precursors, are disordered on the surface of AgTCNQF4 nanorods via the van der Waals interaction.39 Figure 2B show the UV−vis absorption spectra of AgNPs− AgTCNQF4 composites prepared at different reactant ratios of TCNQF4 and Ag. Curves a−d correspond to the AgNPs− AgTCNQF4 achieved with the increase of amount of the AgNP colloid, and it can be observed that the absorption band of the AgNP colloid gradually increases at 428 nm, indicating the existence of the excessive AgNPs. In order to further understand the structure and chemical composition of the composite nanorods, we used HRTEM, SAEM, and XPS to investigate them. Figure 2C displays a HRTEM image of an AgNPs−AgTCNQF4 nanorod. It displays a smooth morphology, proving a typical amorphous structure. The corresponding SAEM pattern reveals a characteristic diffusive ring pattern, showing that this nanorod is not a crystal structure. The chemical status of AgNPs−AgTCNQF4 nanorods were analyzed by XPS. The Ag 3d energy values show two 24755



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Figure 4. (A, B) Synchronous and asynchronous 2D correlation spectra. (C) Moving-window 2DCOS spectrum from the timeresolved spectra in Figure 3A. The spectral region is 260−500 nm.

Figure 4C shows the moving-window 2D correlation spectra (MW2DCOS) based on autocorrelation calculated from the time-resolved spectra in Figure 3A. It is plotted between the wavelength axis and the reaction time axis. The MW2DCOS can easily distinguish the changes of spectral intensity along the time direction. Two positive autocorrelation bands at 328 and 387 nm were observed along the time axis, which are assigned to AgTCNQF4 and TCNQF4 as we stated before. These two autocorrelation bands started to change at the different moment, indicating that the intensities of the 387 nm band changed faster than the 328 nm one. At 2500 ms, two correlation bands reached maxima at the same moment. Each peak maximum indicates the local maximum gradient of the spectral intensity changing along the time.46 3.3. Formation Mechanism of AgNPs−AgTCNQF4 Nanorods. Scheme 1 describes the growth mechanism of the AgNPs−AgTCNQF4 nanorods.47 Once the AgNP colloid and the TCNQF4 microemulsion are mixed in aqueous phase, the AgNPs and TCNQF4 collide and aggregate as soon as possible in the early state. The permeation of water happens on the microemulsion system, which makes AgNPs and TCNQF4 solution diffuse. AgNPs play as nucleus and TCNQF 4 molecules start to react with the AgNP selectively on the symmetric crystal plane (see Figure S1 in Supporting Information), generating nanolayered AgTCNQF4 on the AgNP nucleus and forming semirod-like AgTCNQF4. The electron affinity of TCNQF4 is 5.24 eV, which is higher than the work function of the Ag (4.26 eV) surface.24 Therefore, after the Ag nucleus is covered with the AgTCNQF4 nanolayer, electrons have to transfer from the Ag nucleus to the central region of AgTCNQF4 layer and combine with TCNQF4 to generate TCNQF4−. The born Ag+ ions migrate from the Ag nucleus to the surface of AgTCNQF4 and coordinate TCNQF4− ion to form an AgTCNQF4 multilayer structure. This efficient stacking makes the electron transfer and Ag+ ion

Figure 3. (A) Time-resolved UV−vis absorption spectra of the reaction solution when the AgNP colloid (100 μL) was added into TCNQF4 microemulsion (containing 50 μL of 2 mM TCNQF4 acetonitrile solution and 2.0 mL of 1.0 mg/mL P123 aqueous solution). (B) Plots of the intensities at 385 nm (TCNQF4, square), 345 nm (AgTCNF4, triangle), and 607 nm (AgTCNQF4, circle) with the reaction time.

happened between AgNPs and neutral TCNQF4 molecules in aqueous solution as described in eqs 1 and 2. At the same time, Ag+ ions and TCNQF4− anions were recombined into AgTCNQF4 as eq 3. Stage III (after 5000 ms) is an aging process in which the electronic transfer between Ag and TCNQF4 was completed and the AgTCNQF4 nanorods grew up to a certain size. In order to distinguish the overlapping peaks in this CT process, 2D correlation analysis was employed to plot the timeresolved spectra in the region of 260−500 nm along the time axis. Figure 4 shows the 2DCOS UV−vis spectra calculated from Figure 3A (260−500 nm) within the time range of 0−80 000 ms. The synchronous correlation map is shown in Figure 4A. There are two distinct autopeaks at 328 nm (assigned to neutral AgTCNQF4) and 387 nm (assigned to neutral TCNQF4) along the diagonal and one negative cross-peak (328, 387 nm) which suggests that the peak intensities at 328 and 387 nm present an opposite change trend along the time axis. The asynchronous map is derived from a position shift coupled with the intensity change.44 The asynchronous correlation map (Figure 4B) shows six cross-peak pairs, and they consist of a “butterfly pattern”. A negative cross-peak at (328, 387 nm) suggests that the intensities of absorption band at 387 nm changed before the ones at 328 nm. This indicates that the CT generated from TCNQF4 to TCNQF4− was earlier than the Ag+ ions and TCNQF4− anions recombination.45 24756



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Scheme 1. Schematic Diagram for the Formation of the AgNPs Decorated AgTCNQF4 Nanorods in Aqueous Solution with Surfactants

Figure 5. Time-dependent in situ Raman spectra of AgNPs−AgTCNQF4 excited by a 633 nm laser (A) and a 532 nm laser (B−E). The AgNPs− AgTCNQF4 samples for the photoinduced reactions were prepared by mixing 50 (A, B), 100 (C), 150 (D), and 200 μL (E) of the AgNP colloid with the same amount of TCNQF4 microemulsion. The color scales stand for the normalized Raman intensities by the strongest bands. (F) Reaction time for completing the photoinduced charge transfer reactions in AgNPs−AgTCNQF4 samples B−E.

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Scheme 2. Schematic Diagram of the Photoinduced CT in AgNPs−AgTCNQF4

peak at 1446 cm−1 (the νC−CN wing of AgTCNQF4) decreases, and the 1642 cm−1 peak (νCC ring) increases and becomes wider. Several new peaks are growing as well, e.g.. 1104, 1240, and 1390 cm−1. These data indicate that a photoinduced CT from TCNQF4− to TCNQF42− occurred in the small-sized AgTCNQF4 under the existence of AgNPs.21 Compared with AuNPs decorated large-sized AgTCNQF4 prepared via the galvanic replacement reaction, the photoinduced CT reaction in the small-sized AgTCNQF4 under the existence of AgNPs can complete in a much shorter time.21 The UV−vis spectra of the dried AgNPs−AgTCNQF4 composites with different AgNPs loading are shown in Figure S4 of the Supporting Information. This data shows that the plasmonic band of aggregated AgNPs is much close to 532 nm, which matches the excitation laser well. It means that under a 532 nm light field a localized surface plasmon resonance effect would be created, which excites more electrons and holes by charge carrier transfer and is advantageous to CT reaction. Moreover, we think the small size of the AgNPs−AgTCNQF4 nanorods is also beneficial for the interfacial CT between metal and semiconductor. More decorated AgNPs on AgTCNQF4 nanorods can also advance the photoinduced CT reaction from TCNQF4− to TCNQF42−. In the photoinduced CT reaction in different AgNPs−AgTCNQF4 nanorods which were prepared by different reaction ratios of AgNPs and TCNQF4, the timedependent in situ Raman spectra were recorded as shown in Figure 5B−E. With the laser irradiation time, the C−CN bond stretch at 1446 cm−1 (monoanion TCNQF4−) gradually decreases and then disappears, indicating the completion of the photoinduced CT reaction of the dianion TCNQF4− to dianion TCNQF42−. So, the threshold time for the observation of the intensities at 1446 cm−1 reaching zero is regarded as the finishing time for the photoinduced CT reaction. Figure 5F displays the finishing time of the photoinduced CT reaction are 110, 70, 30, and

Ag Nanoparticles Decorated Small-Sized AgTCNQF4 Nanorod

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