Photoinduced Conversion of Cu Nanoclusters Self-Assembly

Article pubs.acs.org/JPCC

Photoinduced Conversion of Cu Nanoclusters Self-Assembly Architectures from Ribbons to Spheres Lin Ai,† Yanchun Li,‡ Zhennan Wu,† Jiale Liu,† Yu Gao,† Yi Liu,† Zhongyuan Lu,‡ Hao Zhang,*,† and Bai Yang† †

State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012, P. R. China ‡ State Key Laboratory of Supramolecular Structure and Materials, Institute of Theoretical Chemistry, Jilin University, Changchun 130023, P. R. China S Supporting Information *

ABSTRACT: Two-dimensional (2D) nanomaterials have attracted much attention because of the unique layered structures and charming properties in many applications. However, with respect to stimulus-responsive 2D nanomaterials, the rigidity of most 2D nanostructures sheds doubt on achieving morphology response. In this paper, a photoresponsive 2D nanostructure is fabricated on the basis of the self-assembly of ultrasmall Cu nanoclusters (NCs) in colloidal solution. The Cu NCs are foremost decorated by the capping ligands with photoresponsive azobenzene (Azo) groups and by virtue of the flexibility of selfassembly techniques to produce nanoribbons. Because the ribbons are composed of individual NCs rather than a rigid whole, the ultraviolet (UV)-induced Cu NCs disassembly permits achieving morphology transformation. The disassembly of Cu ribbons is controlled by the Cu NCs rather than the surface ligands. However, the disassembled Cu NCs will reassemble into spheres if they are coated with Azo groups. The electrocatalytic performance of Cu self-assembly ribbons and spheres in oxygen reduction reaction is further compared. The ribbons show better catalytic activity than the spheres.

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INTRODUCTION Two-dimensional (2D) nanomaterials have attracted great attention in the past decade because of the unique layered structures and strong 1D confinement effect within a finite thickness.1−5 The fascinating properties of 2D nanomaterials derive many potentials in optoelectronic devices, catalysis, lithium batteries, theranostics, and other emerging areas.6−14 Since the intrinsic properties and apparent functionalities of 2D nanomaterials are led from their morphologies,15−18 the capability to perform morphology control under external stimulus is important for creating novel 2D structures and broadening the applications.19 According to the composition, 2D nanomaterials can be composed of carbon-based and non-carbon-based materials. The former include graphenes,20 polymers,21,22 organic molecules,23,24 peptides,25 and so forth. The latter include hexagonal boron nitride,26 semiconductors,27 transition metal dichalcogenides,2,8 metal oxides,28 germanane,29 silicene,30 phosphorene,31,32 and so forth. To date, the 2D nanomaterials with stimuli-responsive behavior are mainly limited to carbonbased materials, because the soft frameworks of these materials are sensitive toward external stimulus and it is easy to introduce stimuli-responsive components which are usually organic.33−35 For non-carbon-based materials, however, the rigid frameworks are insensitive to external stimulus and it is also difficult to © XXXX American Chemical Society

introduce organic stimuli-responsive components. The main limitation is attributed to the current preparation methods of non-carbon-based 2D nanomaterials, which conventionally involve molecular beam epitaxy,36 exfoliation,37−39 solutionphase synthesis,40−43 and so forth. The as-prepared 2D products are commonly crystalline. Because non-carbon-based 2D nanomaterials exhibit many charming propeties that carbon-based ones do not possess, the construction of stimuli-responsive 2D nanomaterials from non-carbon-based 2D nanomaterials is greatly welcome. To construct such a system, the rigidity of 2D frameworks should be properly controlled, accompanied by the introduction of stimuliresponsive components, which is difficult to realize on the basis of the conventional preparation of non-carbon-based 2D nanomaterials. Self-assembly, as a flexible technique for constructing higherlevel aggregated architectures from nano-objects in colloidal solutions or at interfaces, has been proved efficient to arrange their spatial distribution and subsequently reinforce the performance.44 One of the advantages of the self-assembly technique is the feasibility to fabricate a stimuli-responsive Received: July 1, 2016 Revised: October 8, 2016

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DOI: 10.1021/acs.jpcc.6b06600 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C system.45 Because the driving forces of self-assembly and the interactions between the as-assembled nano-objects are various weak interactions, such as hydrogen bonding, electrostatic, and van der Waals interactions,44,46 it is easy to achieve a stimuliresponsive system by formost modifying the capping ligands with temperature,34 light,19 pH,47 and redox48 sensitivities on nano-objects. Stimuli-responsive 1D and 3D assembles of inorganic nanoparticles (NPs) have been successfully achieved,49,50 but 2D ones are less reported yet. The main reason is the big size of NPs, which makes it difficult to achieve solution-dispersible 2D self-assembly architectures.51,52 Most recently, ultrasmall metal nanoclusters (NCs) with the diameter below 2 nm have been demonstrated as the building blocks for fabricating solution-dispersible 2D self-assembly architectures.53−56 Resulting from the protection of capping ligands, the NCs are separated in the self-assembly architectures and maintain the original properties. In particular, such 2D architectures are flexile rather than crystalline.1,15,57 Stimuliresponsive components can be easily introduced by designing the capping ligands. In this work, ultrasmall Cu NCs are capped with azobenzene (Azo)-modified alkylthiol ligands and employed as the building blocks to fabricate 2D self-assembly nanoribbons. Upon 365 nm ultraviolet (UV) light irradiation, the Cu ribbons disassemble because of the decrease of inter-NCs van der Waals interaction. The disassembled Cu NCs will reassemble into spheres as they are coated with Azo groups. Systematic investigations reveal that the photoresponsive morphology transformation greatly depends on the Azo content in the ligands, which influences the strength variation of inter-NCs van der Waals attraction after UV irradiation. Because the specific surface area of ribbons is higher than that of the spheres, the ribbons show better electrocatalytic activity than the spheres in oxygen reduction reaction (ORR).

Characterization. Transmission electron microscopy (TEM) was conducted using a Hitachi H-800 electron microscope at an acceleration voltage of 200 kV with a CCD camera. Atomic force microscope (AFM) tapping mode measurements were performed on a Nanoscope IIIa scanning probe microscope (Digital Instruments) using a rotated tapping mode etched silicon probe tip. An energy-dispersive X-ray spectroscopy (EDX) detector coupled with a scanning electron microscope (XL 30 ESEM FEG Scanning Electron Microscope, FEI Company) was used for elemental analysis. X-ray powder diffraction (XRD) investigation was performed on a Rigaku Xray diffractometer using Cu K radiation (λ = 1.5418 Å). X-ray photoelectron spectroscopy (XPS) was investigated using a VG ESCALAB MKII spectrometer with a Mg Kα excitation (1253.6 eV). Binding energy calibration was based on C 1s at 284.6 eV. FTIR spectra were performed with a Nicolet AVATAR 360 FTIR instrument. Thermal gravimetric analysis (TGA) and differential thermal gravity (DTG) measurements were performed on a Q500 Thermal Analyzer (New Castle TA Instruments) in flowing N2 with a heating rate of 10 °C/min. Mass spectra were measured on an ITQ 1100 (Thermo Fisher) mass spectrometer. Matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectra (MS) were recorded on a Bruker/Auto Reflex III mass spectrometer (Bremen, Germany) equipped. The ions were accelerated with pulsed ion extraction by a voltage of 19 kV and detected using a microchannel plate detector. Samples were dissolved in chloroform, and trans-2-3-[3-(4-tert-butylphenyl)-2-methyl-2propenylidene]malononitrile (DCTB) was used as the matrix material. UV−visible absorption spectra were obtained using a Shimadzu 3600 UV−vis−NIR spectrophotometer. 1H NMR spectra were recorded on a Bruker AVANCE 500 MHz spectrometer with tetramethylsilane as the internal standard. For the 1H NMR spectrum of cis form Cu NCs, the CDCl3 solution was irradiated with a 365 nm UV lamp for about 30 min to achieve the greatest amount of cis form prior to the NMR scan. The purity of the products was checked by TLC (Merck, silica gel 60). Scanning electron microscopy (SEM) images were acquired on a JEOL FESEM 6700F electron microscope. Dynamic light scattering (DLS) was operated on a Nano-ZS instrument, model ZEN 3600 (Malvern Instruments). Synthesis of Capping Ligands. Azo-(CH2)n-SH, n = 4, 6, 8, were synthesized from 4-(phenylazo)phenol according to the previous report with slight modification (Figure S1).58 Step 1. Esterification of the hydroxyl group. In a 50 mL three-neck flask, 4-(phenylazo)phenol (0.5 g, 2.5 mmol, 1 equiv) was dissolved in distilled DMF (25 mL). Subsequently, NaH (0.11 g, 3.75 mmol, 1.5 equiv) was added in batches under a N2 atmosphere, and the solution was stirred for 5 min. Then, Br(CH2)nBr, n = 4, 6, 8 (3.75 mmol, 1.5 equiv), was added to this solution. The solution was stirred for 5 h at room temperature. The reaction mixture was monitored by TLC analysis. Then, saturated ammonium chloride (25 mL) was added to the solution to stop the reaction and ethyl acetate was added, causing the solution to separate into two layers. The organic phase was separated from the solution and washed with water. After removing the water from the organic phase using MgSO4, the organic phase was evaporated to dryness to obtain compound 1 (Figure S1). The crude product was used for the next step directly without further purification. Step 2. Thioacetylation of the bromo group. In a 1000 mL three-neck flask, thioacetic acid (1.76 g, 23.2 mmol, 1.5 equiv) was dissolved in distilled DMF (60 mL). Subsequently, NaH

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EXPERIMENTAL SECTION Materials and General Techniques. 1-Dodecanethiol (DT, 98%) and sodium hydride (NaH, 60%) were purchased from Aladdin Chemistry Co. Ltd. Dibenzyl ether (BE, 98%) was purchased from Aldrich. Copper(II) 2,4-pentanedionate (CuAc2, 98%) was purchased from Alfa Aesar. Liquid paraffin (LP) was purchased from the Sinopharm Chemical Reagent Co. Ltd. 1-Butanethiol, 1,4-dibromobutane (Br(CH2)4Br), 1,6dibromohexane (Br(CH2)6Br), 1,8-dibromooctane (Br(CH2)8Br), 4-(phenylazo)phenol, and thioacetic acid (CH3COSH) were purchased from TCI. Acetone, ammonium chloride (NH 4 Cl), chloroform, dichloromethane, N,N-dimethylformamide (dried with molecular sieve), ethyl acetate, hexane, magnesium sulfate (MgSO4), methanol, potassium carbonate (K2CO3), potassium hydroxide (KOH), and sodium hydroxide (NaOH) were all commercially available products and used as received without further purification. Deuterated chloroform was obtained from Isotech. trans-2-3-[3-(4-tert-Butylphenyl)-2methyl-2-propenylidene]malononitrile (DCTB) was purchased from Fluka. Air- and moisture-sensitive reactions were performed under an inert atmosphere (N2 or Ar that had been passed through a Drierite tower). Glassware was oven-dried at 110 °C or flamedried, if necessary. The progress of each reaction was monitored by thin-layer chromatography (TLC) carried out on 0.25 mm E. Merck silica gel plates (60F-254). Baxter silica gel (6 nm) (300−400 mesh ASTM) was used for flash column chromatography. B


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The Journal of Physical Chemistry C

was repeated twice. Finally, the products were dispersed in 1 mL of chloroform. Photoisomerization. For photoinduced trans−cis isomerization, the solution samples were irradiated with 365 nm UV light from a WHF 203 UV lamp; the power intensity is 1, 10, and 18 mW/cm2, respectively. Samples (3.0 mL, with different concentrations) were sealed in a quartz cell, and irradiation was applied for a specific duration under stirring at a fixed distance of 10 cm. For cis−trans photoisomerization, the visible light irradiation was performed using a 200 W incandescent light bulb (450 nm, 1 mW/cm2). Electrocatalytic Activity Tests. The pretreatment of the bare glassy-carbon (BGC) rotating-disk electrode (RDE) (8.0 mm in diameter) was as follows: Prior to use, the electrodes were polished mechanically with aluminite powder under an abrasive paper to obtain a mirror-like surface, washed with ethanol and deionized water by sonication for 5 min, and allowed to dry in a desiccator. A 1 mg portion of each grinded sample was dispersed in 0.5 mL of solvent mixture of Nafion (5%) and deionized water (V/V = 1/9) by sonication, respectively. Electrochemical experiments were conducted using an AFMSRCE advanced electrochemical system from Pine Instrument Company. A conventional three-electrode cell was employed incorporating a working BGCRDE, an Ag/AgCl, KCl (3 M) electrode as reference electrode, and a Pt electrode as counter electrode. All potentials were measured and reported vs the Ag/AgCl, KCl (3 M) reference electrode. The experiments were carried out in N2-, O2-saturated 0.1 M KOH solution for the ORR. The potential range was cyclically scanned between −1.3 and +0.2 V at a scan rate of 50 mV/s after purging N2 or O2 for 30 min to ensure that the solution either deaerated or saturated with O2. Model and Simulation Method. Different ligand capped NCs are sketched by GaussView 5.0.8 and optimized by the VAMP module in Material Studio version 5.0. The stable Cu13 core structure and different ligand capped NCs are optimized by the AM1 method (Figures S6−S8). The geometry optimization of two kinds of monomers and dimers was performed using density functional theory (DFT) with the B3LYP hybrid functional and 6-31G (d, p) basis set. All the calculation was carried out using the Gaussian 09 (version D.01) package on a PowerLeader cluster (Figure 6). The Brownian dynamics (BD) simulation technique, a suitable and efficient method in treating the self-assembly of nano-objects, was used to simulate the transformation of selfassembly structures from trans- to cis-Cu13AzoBT10Ac2 (Figure S14). We adopted 10 AzoBT chains and 2 Ac chains connected to a center Cu core to form the composite system Cu13AzoBT10Ac2, where Cu13 represented 13 CG beads form a spherical rigid body.

(0.93 g, 23.2 mmol, 1.5 equiv) was added in batches under a N2 atmosphere, and the solution was stirred for 5 min. Then, distilled DMF (140 mL) containing crude compound 1 (15.5 mmol, 1 equiv, according to the theory yield last step) was added to this solution, and the solution was stirred for 5 h at room temperature. The reaction mixture was monitored by TLC analysis. Then, saturated ammonium chloride (200 mL) was added to the solution to stop the reaction and ethyl acetate was added, causing the solution to separate into two layers. The organic phase was separated from the solution and washed with a 1.0 M NaOH aqueous solution. After removing the water from the organic phase using MgSO4, the organic phase was evaporated to dryness to obtain compound 2 (Figure S1). The crude product was used for the next step directly without further purification. Step 3. Hydrolysis of the thioacetyl group. In a 1000 mL two-neck flask, crude compound 2 (13.8 mmol, 1 equiv, according to the theory yield last step) was dissolved in chloroform (50 mL). Subsequently, methanol (225 mL) and K2CO3 (4.75 g, 34.4 mmol, 2.5 equiv) were added. The solution was stirred for 5 h at room temperature. The reaction mixture was monitored by TLC analysis. Then, saturated ammonium chloride (225 mL) was added to the solution to stop the reaction and chloroform was added, causing the solution to separate into two layers. The organic phase was separated from the solution and washed with water. After removing the water from the organic phase using MgSO4, the organic phase was evaporated to dryness. The product 3 was subsequently dissolved in a small quantity of chloroform, and the target Azo-(CH2)n-SH, n = 4, 6, 8, were separated, respectively, by chromatography (filling material: silica gel 300−400 mesh; eluent solvent: hexane to remove the excess small polar reactant, 20:1 hexane:chloroform to give the products). The yield of Azo-(CH2)n-SH, n = 4, 6, 8, given following were calculated according to 4-(phenylazo)phenol. Compound 3, n = 4. Yield: 85%. 1H NMR (500 MHz, CDCl3): δ 7.93 (dd, J = 21.3, 8.2 Hz, 4H), 7.58−7.43 (m, 3H), 7.03 (d, J = 8.8 Hz, 2H), 4.10 (t, J = 6.2 Hz, 2H), 2.66 (dd, J = 14.7, 7.3 Hz, 2H), 2.05−1.80 (m, 4H). MS: calcd for C16H18N2OS: 286.39; found: 285.92. Compound 3, n = 6. Yield: 80%. 1H NMR (500 MHz, CDCl3): δ 7.93 (dd, J = 20.9, 7.9 Hz, 4H), 7.49 (m, 3H), 7.03 (d, J = 7.9 Hz, 2H), 4.08 (t, J = 6.0 Hz, 2H), 2.63−2.55 (m, 2H), 1.86 (dd, J = 13.1, 6.4 Hz, 2H), 1.70 (dd, J = 14.1, 7.1 Hz, 4H), 1.53−1.49 (m, 2H). MS: calcd for C18H22N2OS: 314.45; found: 313.64. Compound 3, n = 8. Yield: 77%. 1H NMR (500 MHz, CDCl3): δ 7.89 (dd, J = 19.2, 8.1 Hz, 4H), 7.52−7.40 (m, 3H), 7.00 (t, J = 5.9 Hz, 2H), 4.04 (dd, J = 8.6, 4.3 Hz, 2H), 2.69 (t, J = 7.3 Hz, 2H), 1.86−1.78 (m, 2H), 1.69 (m, 2H), 1.52−1.45 (m, 2H), 1.42−1.34 (m, 6H). MS: calcd for C20H26N2OS: 342.50; found: 341.61. Preparation of Cu NCs Self-Assembly Architectures. CuAc2 (10 mg, 0.038 mmol) was dissolved in a mixture of 1 mL of BE and 2.5 mL of LP at room temperature. AzoBT (AzoHT or AzoOT, 0.28 mmol), which acted as ligand cum reductant, was added and stirred at 120 °C for 20 min. Purification. After cooled down to room temperature, 1 mL of the orange suspension of the product was washed and precipitated by adding 1 mL of chloroform and 2 mL of acetone. After that, the products were separated by centrifugation at 6000 rpm for 5 min. The purification process

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RESULTS AND DISCUSSION The Azo-modified alkylthiols ligands are synthesized following the procedure published by Negsishi and co-workers with a little modification (Figure S1).58 Three Azo-modified ligands are synthesized by linking the Azo group onto the terminal of alkylthiol chains with 4, 6, and 8 −CH2−, respectively, which is defined as AzoBT, AzoHT, and AzoOT. The self-assembly nanoribbons of Cu NCs are fabricated in colloidal solution. In a typical experiment, CuAc2 (10 mg; 0.038 mmol) is dissolved in a mixture of BE (1 mL) and LP (2.5 mL). Subsequently, AzoBT (80 mg; 0.28 mmol), which acts as ligand cum reductant, is added. The resulting solution is heated at 120 °C C


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Figure 1. Low (a, e) and high (b, f) magnification TEM images, AFM (c, g) images, and SEM (d, h) images of nanoribbons (a−d) and nanospheres (e−h). Insets in (b) and (f): the TEM size distribution of Cu NCs in nanoribbons and nanospheres. AFM images present the average thickness of 37.5 ± 5.5 nm for the ribbons and average height of 196.2 ± 8.6 nm for the spheres. Inset in (h): the SEM size distribution of the spheres.

Figure 2. (a) Schematic illustration of the photoisomerization of Azo. (b) UV−visible absorption spectra of the NCs self-assembly ribbons as exposed to 365 nm UV light from 0 to 15 min. Inset in (b): optical photographs of the suspension. 1H NMR spectra (c) and small-angle XRD patterns (d) of the ribbons before and after 365 nm UV light irradiation. The power of the UV lamp is 10 mW/cm2.

S3, and S4). XPS of the Cu LMM peak in Figure S2e confirms the coexistence of Cu(0) and Cu(I) in Cu13AzoBT10Ac2 with 3 Cu(0) and 10 Cu(I), defining as Cu(0)3Cu(I)10AzoBT10Ac2. Because the Cu−S interaction is much stronger than that of Cu−O, the valence of Cu(I) corresponds to the copper atoms on the NC surface, which coordinate with AzoBT. The valence of Cu(0) corresponds to the atoms in the center or coordinating with Ac.59 The thermogravimetric analysis (TGA) curve of ribbons recorded in a N2 atmosphere shows almost no weight loss up to 250 °C (Figure S5), which demonstrates the good thermal stability of the ribbons. The formation of Cu NCs self-assembly ribbons is attributed to the dipole-induced asymmetric van der Waals (vdW) attraction.59

for 20 min under magnetic stirring. Figure 1a,d shows the transmission electron microscopy (TEM) and scanning electron microscopy (SEM) images of the as-assembled ribbons with an approximate width of 100 ± 12 nm and a length of about 2 um. The high magnification TEM image indicates that the building blocks of ribbons are individual NCs with an average diameter of 1.9 ± 0.5 nm (Figure 1b). Tapping-mode atomic force microscopy (AFM) in Figure 1c reveals that the ribbons are 2D stacked structures with the average thickness of 37.5 ± 5.5 nm. MALDI-TOF mass spectrometry, energydispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), and FTIR data reveal that the composition of Cu NCs within ribbons is Cu13AzoBT10Ac2 (Figures S2a,c, D


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supported by dynamic light scattering (DLS) measurments. As shown in Figure S9, the initial ribbons have two hydrodynamic diameters (DH) at 2326 and 106 nm, which, respectively, correspond to the length and the width. On the progress of morphology transformation, the two DH gradually coalesce into one. The final spheres shows a single DH of 270 nm. Owing to the existence of a hydration radius, the DH measured by DLS is a little larger than the diameter observed by TEM, AFM, and SEM, but firmly proves the morphology transformation from ribbons to spheres. Note that, although high irradiation intensity can accelerate the morphology transformation, the size and morphology of final spheres are regardless of the irradiation intensity (Figure S10). Namely, the external irradiation can only effect the kinetics processes of morphology transformation, but not the thermodynamics products. Once the spheres form, their size and morphology do not change as the irradiation duration is prolonged. The isomerization of the Azo groups during the morphology transformation is confirmed by UV−visible absorption spectra. Pure AzoBT shows two absorption peaks at 347 and 442 nm, which correspond to the π−π* and n−π* transitions, respectively.61 Under UV light irradiation, the absorption at 347 nm decreases while the absorption at 442 nm increases slightly.60 Subsequently, the spectrum can revert back to the original one under visible light irradiation (Figure S11a,b). On the basis of the absorption intensity at 347 nm, the reversibility of AzoBT trans−cis isomerization is evaluated as high as 95%. For the self-assembly ribbons of Cu NCs, a similar change of UV−visible absorption spectra is observed (Figure 2b). Under UV irradiation, the absorption peak of ribbons at 442 nm increases continuously with increasing the irradiation time, which implies the isomerization of Azo groups on the surface of Cu NCs. The isomerization ratio of Azo groups before and after UV light irradiation is calculated according to the integral area of proton NMR, which is as high as 98% and comparable to that of pure AzoBT in solution (Figure 2c). Accompanied with the spectral variation, the color of the ribbon suspension changes from turbid pale yellow to clear brown with weak Tyndall scattering (inset in Figure 2b and Figure S12), implying the morphology transformation from larger ribbons to smaller spheres. However, under visible light irradiation, only 5% Azo absorbance at 347 nm returns back (Figure S11c), showing the difficulty to achieve cis−trans conversion as locating Azo groups on the surface of Cu NCs. It should be mentioned that the peak positions of absorption spectra are fixed in the studied concentration range of ribbons, and no band shift is observed between the AzoBT solution and the selfassembly ribbons with the same AzoBT concentration (Figure S13). These indicate that there is no π−π interaction between Azo groups on the surface of Cu NCs.19 Further computer simulation on the transformation of Cu NCs self-assembly structures before and after UV irradiation is carried out on the basis of Brownian dynamics using a coarse grained model (Figure S14). The simulated result is consistent with the experimental observation. To confirm that Cu NCs are instinctively responsive to UV light, the self-assembly nanoribbons from 1-dodecanethiol (DT) capped Cu NCs with the diameter of 1.6 ± 0.3 nm are fabricated and irradiated with UV light. The length of DT is 1.74 nm, which is similar to that of AzoBT but without the Azo group. Instead of transforming to spheres with the diameter around 250 nm (Figure 1h), the ribbons disassemble and most NCs fuse into the nanoparticles with the diameter of 4.0 ± 1.5

In this context, the inter-NC dipolar interaction is anisotropic, thus facilitating the self-assembly of NCs with 1D orientation. The alkyl length and number density of the ligands also influence the strength of dipolar interaction, and, therefore, the morphology of the self-assembly structures. As calculated, the increase of alkyl length and the decrease of number density of ligands lead to the decrease of dipolar interaction, thus producing the self-assembly structures with a chunky morphology (Figures S6 and S7 and Calculation S1). As shown in Figure 1e, the nanoribbons can transfer to nanospheres under 365 nm UV irradiation at 10 mW/cm2 for 15 min. AFM and SEM images also confirm the generation of nanospheres with an approximate size of 250 ± 40 nm and the height of 196.2 ± 8.6 nm (Figure 1g,h). The TEM image further indicates that the spheres are built from the NCs with an average diameter of 1.9 ± 0.4 nm (Figure 1e,f). By combining MALDI-TOF MS, EDX, XPS, and FTIR measurements, the composition of NCs in spheres is suggested as Cu(0)3Cu(I)10AzoBT10Ac2 (Figures S2b,d,f, S3, and S4), which is identical to the NCs in ribbons. Computer-aided simulation of the conformation of the NCs before and after UV irradiation is shown in Figure S8, which exhibits the specific arrangement of Cu atoms in the NCs and the distribution of AzoBT on the NC surface. As one of the most studied photoresponsive groups, Azo undergoes isomerization between trans and cis conformation under UV and visible light irradiation.60 Such a structure switch causes the variation of molecular steric hindrance (Figure 2a), then alters the inter-NCs interactions and triggers morphology transformation of the as-assembled NCs architectures. To monitor the morphology transformation process, the suspension of nanoribbons is irradiated with 10 mW/cm2 365 nm UV light for different durations. Under TEM, the initial ribbons become curly upon 2 min irradiation (Figure 3a,b). Further irradiation generates hollow spheres and finally solid spheres (Figure 3c,d). This transformation process is also

Figure 3. TEM temporal morphology evolution of the NCs selfassembly ribbons in CHCl3 with 0 s (a), 2 min (b), 6 min (c), and 15 min (d) of 365 nm UV irradiation. The power of the UV lamp is 10 mW/cm2. E


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results further confirm that the morphology transformation relates to the photoisomerization of the Azo groups on Cu NCs. To analyze the driving force of the morphology transformation, the geometric framework of AzoBT-capped Cu NCs is established (Figures 5 and 6). According to the XRD data

nm (Figure S15), because all the UV irradiation is absorbed by the Cu NCs in the absence of Azo groups. This result proves that the ribbons-to-spheres transformation first involves the disassembly of Cu ribbons. This process is controlled by Cu NCs rather than the surface ligands. The disassembled Cu NCs will reassemble into spheres if they are coated with Azo groups. Otherwise, small NPs form, because of the large surface energy of individual Cu NCs. The influence of Azo content is studied by comparing the morphology transformation of the self-assembly ribbons constructed from the NCs capping with different molar ratios of 1-butanethiol (BT) and AzoBT (Figure 4). It can be seen

Figure 5. Schematic illustration of the geometric framework of AzoBT self-assembled on a flat Au substrate (a), and 1.9 nm Cu NCs (b). On the flat substrate, the average distance between two neighboring nitrogen double bonds of Azo is the same as that of neighboring Au−S bonds, which is calculated as 0.5 nm. On curved Cu NCs, the average distance of Azo increases to 1.21 nm according to the similar triangle theorem mathematically, 0.5/spacing = 0.95/(0.95 + 1.35), because of the increase of the adjacent angles of neighboring AzoBT.

and the Bragg equation (Figure 2d), the interspace between neighboring Cu NCs in the ribbons is calculated as 2.9 nm, which is consistent with the TEM observed 2.9 ± 1.6 nm (Figure S17a). By deducting the diameter of Cu NCs, the surface-to-surface distance between neighboring Cu NCs is calculated as 1.0 nm, which is shorter than the length of the trans-AzoBT chain but close to the long axis length of trans-Azo in AzoBT (Figures 2a and 6a). Because the short axis length of trans-Azo is only 0.44 nm, the average interspace between neighboring AzoBT on the surface of Cu NCs, which is 0.5 nm,61 is large enough to contain another trans-Azo from the AzoBT on the neighboring NCs (Figures 2a and 5). Therefore, it is reasonable to speculate that the AzoBT are not outstretched but twisted among Cu NCs. This consideration is strongly supported by FTIR measurements, which exhibit a broad half-width of the bands at 2925 and 2853 cm−1 corresponding to the −CH2− asymmetric stretching and symmetric stretching of the AzoBT in the ribbons (Figure S4).62 Since the most stable conformation for two plane transAzo groups is one perpendicular to another (Figure 6c), the intercalation and interaction of the AzoBT on neighboring NCs are illustrated in Figure 6e. After irradiation, the interspace between neighboring NCs increases to 4.3 nm (Figures 2d and S17b). The corresponding surface-to-surface distance increases to 2.4 nm, which is longer than the length of one cis-AzoBT but shorter than two (Figure 6b). As revealed by FTIR measurements, the −CH2− asymmetric and symmetric stretching of AzoBT in the spheres present a sharp half-width, which imply that the alkyl chains on NCs are in an ordered state (Figure S4).62 It can be concluded that the degree of overlapping between neighboring cis-AzoBT is reduced in comparison with that of trans-AzoBT (Figure 6d). The reason for such variation is ascribed to the enlarged steric

Figure 4. Low (a−c, g−i) and high (d−f, j−l) magnification TEM images of the NCs self-assembly architectures using BT and AzoBT mixture as the capping ligands before (a−f) and after (g−l) UV light irradiation. The BT-to-AzoBT molar ratio is 7:3 (a, d, g, j), 5:5 (b, e, h, k), and 3:7 (c, f, i, l).

from the TEM images that the formation of self-assembly ribbons is regardless of the ratio between BT and AzoBT. However, upon UV light irradiation, only the ribbons with a BT-to-AzoBT ratio lower than 5/5 can transfer to spheres. Otherwise, irregular structures composed of big NPs with the diameter of 8.0 ± 2.0 nm form. The reason is the same as that of the self-assembly ribbons from the NCs without Azo modification. Furthermore, the small-angle XRD patterns of the ribbons constructed from DT- and BT-coated Cu NCs before and after UV irradiation are compared with those from AzoBTcoated NCs (Figures S16 and 2d). DT and BT are the ligands without an Azo group, which does not possess photoresponsive behavior. Therefore, there is almost no change of the smallangle XRD patterns before and after UV irradiation. These F


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Figure 6. Energy-minimized structures and calculated length for trans (a) and cis isomers (b) of model compounds AzoBT. Energy-minimized dimer configuration for trans (c) and cis isomers (d) of AzoBT. Schematic illustration of the interaction of AzoBT on neighboring Cu NCs before (e) and after (f) UV light irradiation. Gray, blue, red, and orange spheres represent C, N, O, and S atoms. The big yellow sphere represents the Cu core composed of 13 Cu atoms.

proved by comparing the self-assembly architectures from the Azo-ligands with different lengths of alkyl chain (Figure 7). The self-assembly architectures are, respectively, constructed from three Azo-ligands, namely, AzoBT, AzoHT, and AzoOT. The diameter of the composed NCs is 1.9 ± 0.5 nm, 2.1 ± 0.3 nm, and 2.5 ± 0.6 nm, respectively. In these ligands, the Azo group is the rigid part, while the alkyl chain is the flexible part. According to the aforementioned discussion, the longer the

hindrance resulting from the isomerization of Azo groups from trans to cis conformation.60 Since the short axis length of cisAzo is 0.55 nm, which is larger than the interspace between neighboring AzoBT, the AzoBT is extruded out after UV irradiation (Figures 2a, 5, and 6f). In the current system, the isomerization of Azo groups on the NCs surface is allowed mainly by two reasons. First, as mentioned above, there is no intensive π−π interaction or other strong interactions to suppress isomerization.63 Second, the ultrasmall Cu NCs provide enough space for isomerization. It has been reported that no photoisomerization occurs when Azo-containing ligands are modified on a flat substrate, because the inter-Azo space is too small. As modifying Azo-containing ligands on spheres and in particular decreasing the size, the adjacent angles between two neighboring Azo groups will increase, thus increasing the free volume of each Azo groups.64 For the Cu NCs with the diameter of 1.9 ± 0.5 nm, the average distance between two neighboring nitrogen double bonds on the Azo groups is 1.21 nm (Figure 5), which is much larger than that on the flat substrate (0.5 nm). Since there is no intensive π−π or other interactions, the main interaction between the neighboring NCs should be the vdW interaction, which is derived from the capping ligands of AzoBT. On the basis of the geometrical configuration (Figure 6e,f), the vdW attractions between neighboring Cu NCs are calculated as 3.2 kBT, 0.07 kBT, and 0.22 kBT before and after UV light irradiation, and after visible light irradiation (Calculation S2), where kB is the Boltzmann constant and T is the absolute temperature. This result clearly indicates that the isomerization of AzoBT under UV irradiation greatly weakens the vdW attraction in between NCs, thus increasing the flexibility of NCs in the ribbons. Because spherical structures possess the lowest surface energy in colloidal solution, the disassembled NCs prefer to reassemble into spherical structures. The consideration that the morphology transformation relates to the inter-NC vdW attraction is further

Figure 7. Low (a−c, g−i) and high (d−f) magnification TEM images of the Cu NCs self-assembly architectures by using AzoBT- (a, d, g), AzoHT- (b, e, h), and AzoOT-capped NCs (c, f, i) as the building block before (a−f) and after (g−i) UV irradiation. G


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samples with the same concentration and volume on the electrode with the same area. In comparison with the spheres, the ribbons exhibit higher current density. It is known that the solvent-exposed surface area plays a crucial role in determining the catalytic performance of electrocatalysts. In the case of ribbons, most of Cu NCs are exposed to the surrounding environment becasue of the ultrathin feature, thus facilitating the contact between oxygen molecules and the activated Cu center. As a result, a high current density of 0.65 mA/cm2 is achieved at −0.9 V. For the spheres, however, most of the Cu NCs are tightly packed inside the spheres, which badly block the oxygen access to the Cu catalytic center. Consequently, the current density is only 0.42 mA/cm2 at −0.9 V. In addition, due to the larger solvent-exposed surface area, the smaller spheres with the diameter of 250 ± 40 nm exhibit higher current density than the bigger ones with the diameter of 650 ± 100 nm.

alkyl chain length is, the weaker the inter-NC vdW attraction is. As a result, the self-assembly ribbons from AzoHT are shorter than those from AzoBT, while the self-assembly architectures from AzoOT are quasi-spherical with the diameter about 500 nm. Undoubtedly, the self-assembly ribbons from AzoHT- and AzoOT-capped Cu NCs can transfer to spheres under UV light irradiation like AzoBT-capped ones, though the diameters are slightly bigger than those composed of AzoBT-capped Cu NCs. Because the compactness and regularity of NCs arrangement are decreased by the steric effect of cis-Azo,65 small-angle XRD results show an obvious increase of the interspace of NCs after UV light irradiation (Figure 2d). What is more, the sizes of the spheres after UV irradiation are controllable by altering the concentration of ribbons. High concentration of ribbons promotes the formation of spheres with large sizes (Figure S18). With the increase of concentration, the ribbons are more likely to intertwine with each other, thus producing bigger spheres. Note that visible light irradiation cannot lead to the reversion of Azo groups from cis back to trans as modified on Cu NCs. Therefore, the morphology transformation from NCs selfassembly ribbons to spheres is irreversible. The main reason is the difficulty to enhance the inter-NCs vdW attraction to a high level. The weak vdW attraction of 0.22 kBT is not enough to pull the transplaced NCs back. It is known that the Cu NCs are easy to be oxidized under ambient conditions. However, after forming self-assembly structures, the long-term stability is greatly improved. The ribbons are stable in the absence of UV irradiation, which maintain the original 2D morphology and absorption spectrum after 1 year storage in CHCl3 (Figure S19). The chemical composition of the Cu NCs stays as Cu13AzoBT10Ac2. Note that the good chemical stability does not influence the structure transformation from ribbons to spheres, because the ribbons are constructed from the self-assembly of individual Cu NCs and maintained through inter-NC weak interactions. This allows performing structure transformation by tuning the interNC weak interactions. Cu nanomaterials are conventionally employed as the electrocatalysts in ORR.59 Therefore, the electrocatalytic activity of Cu NCs self-assembly ribbons and spheres is further compared (Figure 8 and Figure S20). The amount of samples cured on the electrode surface is identical by coating the

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CONCLUSIONS In summary, we demonstrate the fabrication of photoresponsive 2D self-assembly architectures from ultrasmall Cu NCs. The NCs are foremost capped by the ligands with photoresponsive Azo groups and by virtue of the flexibility of the self-assembly technique to produce nanoribbons. Upon 365 nm UV light irradiation, the disassembly of the ribbons occurs, arising from UV-induced Cu NCs detaching from each other in solution. The disassembly process is controlled by the Cu NCs rather than the surface ligands. However, the disassembled Cu NCs are capable of reassembling into spheres if they are coated with Azo groups. Detailed studies reveal that the photoinduced variation of inter-NCs vdW interaction and, therefore, the morphology transformation can be tuned by adjusting the Azo content in the ligands. The electrocatalytic activity of the ribbons and spheres in the oxygen reduction reaction is further compared. The ribbons show better catalytic activity than the spheres. Although the morphology transformation from ribbons to spheres is still irreversible, the current studies undoubtedly open the door for further design of stimuli-responsive 2D nanomaterials.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b06600. Additional simulation, calculation, schematic illustration, composition analysis, size distribution, spectral comparison, optical photographs, and TEM images of Cu NCs and the self-assembly architectures (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel: +86 431 85159205. Fax: +86 431 85193423. E-mail: [email protected]. Author Contributions

H.Z. proposed and supervised the project. L.A., H.Z., Y.L., and B.Y. designed and performed the experiments and co-wrote the paper. Z.W., J.L., and Y.G. participated in most experiments. Z.L. and Y.L. designed and performed the computer simulations. All authors discussed the results and commented on the manuscript.

Figure 8. Cyclic voltammograms of the ribbons in O2- and N2saturated KOH solution, and 250 and 650 nm spheres in N2-saturated KOH solution for the ORR. The quantity of catalysts on electrodes is fixed at 2 mg/mL.

Notes

The authors declare no competing financial interest. H


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ACKNOWLEDGMENTS This work was supported by the National Key Research and Development Program of China (2016YFB0401701), NSFC (51425303, 21374042), the Natural Science Foundation of Jilin Province (20140101048JC), and the Special Project from MOST of China.

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Photoinduced Conversion of Cu Nanoclusters Self-Assembly

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