A Monolayer Polyoxometalate Superlattice

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A Monolayer Polyoxometalate Superlattice P eilei H e ,

B iao X u ,P eng-peng W ang ,H uiling L iu ,a nd X un W ang *

P . He, B. Xu, Dr. P.-P. Wang, H. Liu, Prof. X. Wang

D epartment of Chemistry T singhua University B eijing 100084 ,C hina E-mail: w angxun@https://www.sodocs.net/doc/6511666756.html, DOI: 10.1002/adma.201400856

which is obtained from the self-assembly of surfactant-encapsu-lated POMs by evaporation of solvents. (HDA) 3(TBA) 3P 2W 18O 62 clusters (HDA = hexadecyltrimethylammonium; TBA = tetrabutylammonium) synthesized based on our previously

reported method [ 21 ] were employed as building blocks. To make

the superlattice more processible, self-assembly was carried out on GO (to yield superlattice@GO). The superlattice@GO shows excellent electrocatalytic activity towards the reduction of hydrogen peroxide. Taking the fast response and wide linear range into consideration, the superlattice@GO can be a prom-ising hydrogen peroxide sensor. Additionally, the superlattice@GO showed enhanced photo-electrochemical properties. Both of these excellent properties can be attributed to the synergistic effects between the superlattice and GO. Therefore, the superla-ttice@GO can be considered as a functional material system.

[ 22 ] W e started our work with K 6P 2W 18O 62 as the POM and used HDA.Br and TBA.Br as surfactants to pro-duce (HDA) 3(TBA) 3P 2W 1

8O 62 (SEPs) by using a two-phase approach. [ 23 ] The elemental analysis (Table S1 in the Supporting

Information) and FTIR (Fourier-transform infrared) spectrum (Figure S1, Supporting Information) support the successful synthesis of SEPs. Copper acetate solution and ethyl benzoate were added into acetone in which the SEPs were dispersed. Then the mixed solution was dropped onto a grid and dried at room temperature (from 20 °C–30 °C) for 1 h. The morphology of the superlattice was determined by using TEM (transmis-sion electron microscopy; F igure 1 ; enlarged images are shown in Figure S2 in the Supporting Information). A low-magni? -cation TEM image of the sample shows large areas of square nanodisks (Figure 1 a ); whereas a closer examination (Figure 1 b ) shows a superlattice structure in the square nanodisks [as revealed by the fast Fourier-transformed (FFT) image, inset in Figure 1 b ]. The surfactants and POMs can be further sub-stituted. Superlattice structures can also be obtained (Figure 1 c ,d) when the HDA building block is substituted by TDA (tet-radecyltrimethylammonium) or ODA (octadecyltrimethylam-monium). Similarly, if K 6P 2W 18O 62 is replaced by other POMs (H 6P 2M o 18O 62 or K 7P 2W 17

V O 62 ), similar superlattice structures can also be achieved (Figure 1 e ,f). Furthermore, EDX (energy-disperse X-ray) spectroscopy analyses, coupled with TEM, indi-cate the presence of molybdenum (Figure S3a, Supporting Information) and vanadium (Figure S3b, Supporting Informa-tion) in the relevant superlattice structures (Figure 1 e ,f). We found that the concentration of copper ions played a key role in the self-assembly process. Some nanospheres formed in the absence of copper ions (Figure S4, Supporting Informa-tion). When the concentration of copper ions in the solution was kept at 0.4 m M , only cubes with a superlattice structure were obtained (Figure S5, Supporting Information), and only when the concentration of copper ions reached 2.4 m M could the monolayer superlattice be obtained. EDX mapping analysis

S uperlattices with highly uniform nanoparticles as building blocks have attracted increasing research interest owing to

their novel structures with tailored electronic, [ 1]magnetic, [ 2]

catalytic, [ 3] and optical properties, [ 4] which depend not only

on the properties of nanoparticles, but also on the electronic

and optical communication between them. [ 5] The advent of

numerous exciting studies of single-component [ 6] and multi-component [ 7] superlattices provides the possibility for controlled

design of novel nanostructured materials based on nanoparticle assembly. Most of the structures in such works are two-dimen-sional [ 8] or three-dimensional [ 6b ,7a ,9] superlattices which are pre-pared by means of a drying-mediated self-assembly process. As a type of 2D superlattice, monolayer superlattices could achieve

higher electronic mobility

[ 10 ] and much higher luminescence ef? ciency

[ 11 ] than multilayers. These excellent properties moti-vate us to study how to obtain a distinctive monolayer superlat-tice. Since most monolayer superlattice structures are stabilized

on tough substrates, such as single crystal Si, Ag,

[ 12 ]Au, [ 13 ]and so on, the application of monolayer superlattice structures will be restricted by rigid substrates, especially in ? elds that require high surface areas. To make the monolayer superlattice more processible, it would be preferable if the building blocks could be assembled on ? exible substrates, such as graphene or GO (graphene oxide). However, up to now most of the assembly of nanobuilding blocks on the surface of graphene or GO has been

disordered, [ 14 ] and the formation of periodic arrays or superlat-tice on ? exible graphene or GO still remains a great challenge. F undamentally, the choice of building blocks with a de? ned size and shape is crucial for optimizing the structure and func-tion of the resulting superlattice. POMs (polyoxometalates)

possess a variety of structures [ 15 ] and have abundant applica-tions in catalysis, [ 16 ]photochemistry, [ 17 ] and electrochemistry.

[ 18 ] POM clusters are usually in the range of 1–6 nm

[ 19 ] with single size distributions, and the solubility of POMs can be easily

changed by surfactant encapsulation.

[ 7b ] Therefore, surfactant-encapsulated POMs (SEPs)

[ 20 ] are excellent building blocks for the construction of superlattices. Recently, K ovalenko and

co-workers

[ 7b ] reported a family of superlattices comprising POM clusters and inorganic nanocrystals. However, monolayer superlattice assemblies containing POMs were not reported till now.

H ere we report the formation of a monolayer superlattice with

very high degrees of both translational and orientational order,

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con? rmed the homogeneous distribution of nitrogen, copper, and tungsten througout the superlattice structure (Figure S6, Supporting Information; the EDX mapping was collected on a nickel grid). Copper ions could induce the aggregation of mel-anin to form various aggregates at different copper concentra-tions. [ 24 ] In this case, it is apparent that copper ions induced

the self-assembly of the POMs. 31

P NMR (nuclear magnetic resonance) can provide a sensitive probe of metal ion incor-poration, [ 25 ] so it was used to classify the structure of POMs

in the SEPs and superlattice. No changes were observed in

the 31

P (NMR) spectra of the samples before and after self-assembly (Figure 1 g ). The FTIR spectra (Figure S1, Supporting Information) of the samples before and after self-assembly are almost the same except some peaks arising from ethyl ben-zoate (1713.58, 1608.27, and 1553.37 cm ?1

) and copper acetate (3449.16 and 3336.06 cm ?1 ). The powder X-ray diffraction pat-tern (Figure S7, Supporting Information) show a similar result

[the peaks from 10°–20° in the monolayer superlattice are from copper acetate hydrate (JCPDS No. 27–0145)]. These results establish that the copper ions have no effect on the structure of the POMs. However, the possible electrostatic interaction between copper ions and POMs may guide the assembly pro-cess of the superlattice, and Cu 2+ ions with coordination num-bers of 4 would probably induce the formation of monolayer

cubic superlattices with very high degrees of both translational and orientational order. Although the underlying mechanism needs to be further studied, monolayer POM superlattices with various compositions have been obtained by using this simple copper-ion-assisted evaporation method. H owever, it would be very dif? cult to explore the application of this kind of superlattice structure if all these self-assembly processes could only happen on macrosized carbon-coated ? lms. Therefore, we tried to make this monolayer superlat-tice more processible by carrying out the self-assembly process on GO. GO was chosen as substrate because of its good dis-persiblity and certain conductivity which may promote further applications. As shown in S cheme 1 , GO aqueous solution was ? rst dispersed on the substrate (silicon wafer or glass) and dried in a vacuum. Then the mixed solution for self-assembly was coated onto it. Afterwards, the solvent was evaporated in air. The GO with superlattice (superlattice@GO) was dispersed in water by ultrasound treatment. The formation of superlat-tice@GO was con? rmed by means of TEM images ( F igure 2 a and Figure S8, Supporting Information). Furthermore, dif-ferent building blocks show similar results with the superlat-tice structure dispersed homogeneously on GO (Figure S9, Supporting Information). The high-resolution TEM (HRTEM, Figure 2 b ) image clearly shows the superlattice with high degrees of both translational and orientational order. The scan-ning TEM (STEM) image (Figure 2 c ) con? rms the same result even more clearly. The low contrast of the superlattice struc-ture (Figure 2 a ) indicates that the thickness of the superlattice is very small. To determine the height of superlattice structure on GO, an atomic force microscopy (AFM) image was acquired on the GO (Figure 2 d ). The height of the superlattice structure was measured to be about 3.17 nm (inset in Figure 2 d ) which approximates to the sum of the size of a Wells–Dawson POM

(ca. 1 nm)

[ 26 ] with a HDA.Br molecule (ca. 2.2 nm), [ 27 ] so the formation of monolayer superlattice is con? rmed. In brief, the superlattice@GO heterostructure was achieved through putting the monlayer superlattice on the top of monolayer or few-layer GO. This is in accord with the basic principle of van der Waals

heterostructures. [ 28 ] The only difference to the traditional van

der Waals heterostructures is that the monolayer superlattice

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F igure 1. a ,b) Typical TEM images of the superlattice self-assembled from (H DA) 3(TBA) 3P 2W 18O 62

. The inset in (b) is the FFT pattern. TEM images of the superlattice self-assembled from c) (TDA) 3(TBA) 3P 2W 18O 62,d) (ODA) 3(TBA) 3P 2W 18O 62 , e) (H DA) 3(TBA) 3P 2M o 18O 62 , and f) (H DA) 3

(TBA) 3P 2W 17V O 61 . g) 31 P NMR characterizations for (H DA) 3(TBA) 3P 2W 18O 62

and superlattice. The insets in ? gure (a), (e), and (f) are the structures of the corresponding polyoxometalates. The C14 and C18 labels in (c) and (d) stand for the carbon number of the longest carbon chain in TDA.Br and ODA.Br.

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isn ’t an isolated atomic plane. The small-angle X-ray diffraction measurements of the superlattice on GO further veri? ed dif-ferent lattice spacings that correspond to different surfactants in the building blocks (Figure 2 e ). This result indicates that the lattice parameter can be easily adjusted by using surfactants with varying alkyl-chain lengths. Small-angle X-ray diffraction analysis was also performed for GO without superlattice and no peak from 2–5° was observed.

T he Wells–Dawson polyoxometalate has good electrocatalytic activity toward the reduction of hydrogen peroxide, and thus can be applied as an amperometric sensor for the detection of

hydrogen peroxide.

[ 29 ] The stability of Wells–Dawson polyoxo-metalates in superlattice@GO may be enhanced by encapsu-lating them in surfactants. The cyclic voltammetry (CV) curves of the superlattice@GO/glassy carbon electrode (GCE) in N 2

-saturated phosphate-buffered saline (PBS) with and without the addition of hydrogen peroxide are shown in Figure S10 in the Supporting Information. These results indicate that super-lattice@GO is stable in neutral solution. Almost all hydrogen

peroxide sensors with POMs operate only in acidic media

[ 29 ] which hinders their application in biological systems. As a hydrogen peroxide detector. By considering both sensitivity and operational stability, the electrode potential ?0.5 V vs. saturated calomel electrode (SCE) was selected as the optimized potential for H 2O 2 electrochemical detection. In the control experiments ( F igure 3 a ), the amperometric responses of H 2O 2

on a GO/GCE and superlattice/GCE were compared to that on superlat-tice@GO/GCE. The GO/GCE showed detectable but very small

current response which is consistent with the literature.

[ 30 ]The presence of the amperometric response shown by the superlat-tice/GCE was due to the redox process of the Wells–Dawson POM in the superlattice. When self-assembly was applied on the GO, a twofold current response could be clearly observed. This result shows the synergistic effect of superlattice and GO, which leads to an enhancement of sensitivity. With the addi-tion of aliquots of H 2O 2

, the currents at the superlattice@GO/GCE increased stepwise (Figure 3 b ). Well-de? ned steady-state current responses were obtained in less than 2 s at the applied potential as the H 2O 2 was added into the stirred PBS solution. The fast response for H 2O 2 electrochemical detection might be caused by the rapid diffusion of small molecules from the solu-tion to the superlattice@GO/GCE.

T he calibration plot of the hydrogen peroxide sensor shown in Figure 3 c indicates that the H 2O 2 sensor based on superlat-tice@GO exhibited a linear response range (LRR) from 5 × 10

?4 to 2 × 10 ?2 M with a linear regression equation (LRE) of I (μA) =

?4.70 C (m M )? 4.66 ( R = 0.99986), a limit of detection (LOD) of

12 μ M (S/N = 3), and a sensitivity of 66.53 μA cm ?2m M ?1.These

data are speci? c to the system used here.

P OMs can serve as water-oxidation catalysts and it is desir-able to use solar power to accomplish water oxidation. [ 16d ,31 ]

Hence, the water oxidation photocurrent response of the super-lattice@GO was also investigated. UV-vis transmittance spectra of the samples are shown in F igure 4 a . Firstly, the GO aqueous solution was dispersed well on ? uorine-doped tin oxide (FTO). Then the mixed solution for self-assembly was placed on the GO after the transmittance spectra of GO on FTO was measured. The transmittance of superlattice@GO on FTO decreased in the visible region compared with GO on FTO. This result could be attributed to the superlattice on GO; the superlattice@GO has signi? cant visible light absorption. Therefore, the photocur-rent response of the superlattice@GO was characterized under the illumination of visible light ( λ > 420 nm). Measurements of the photocurrent were carried out in a 0.02 M Na 2S O 4aqueous solution without additive. Figure 4 b shows plots of photocur-rent density against potential for GO, superlattice, and superlat-tice@GO. The measured potentials versus SCE were adjusted to the reversible hydrogen electrode (RHE) scale according

Adv. Mater. 2014, 26

, 4339–4344 S cheme 1. F ormation of monolayer superlattice on GO, with a building block in between.

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to the Nernst equation, E R HE = E S CE + 0.65 V. [ 32 ]

The superla-ttice@GO shows a much larger photocurrent density when illuminated by the visible light than the GO or superlattice. To further investigate the photoresponse of the superlattice@GO, the transient photocurrents of the samples (Figure 4 c ) were recorded during repeated ON/OFF illumination cycles at 1.23 V vs. RHE. Compared with the photocurrent density, the dark current densities cannot be neglected. When the dark current density was subtracted, the transient photocurrent

density of the superlattice@GO was about 2 μA cm

?2,which is larger than the sum of photocurrent density of superlattice

(0.052 μA cm ?2 ) and GO (0.13 μA cm ?2

). The superlattices with different surfactants (TDA, HDA, and ODA) of varying alkyl-chain length showed similar photocurrent responses (Figure S11, Supporting Information). To study the in? uence of the photooxidation of the surfactants, we tested the photo-current response of the pure surfactants on GO. The results (Figure S12, Supporting Information) show that the photocur-rent response of the surfactants (HTAB and TBAB) could be

neglected. Therefore, we think that the photocurrent is directly due to the photoinduced oxidation of water.

T hese results demonstrate a synergistic effect between superlattice and GO, which leads to an increase of the water-oxidation photocurrent density of superlattice@GO. This result

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F

igure 2. a ) Typical TEM image of the superlattices on GO self-assembled from the (HDA) 3(TBA) 3P 2W 1

8O 62 . b) HRTEM image of the superlattice on GO. The inset in (b) is the corresponding FFT pattern. c) STEM image of the superlattice on GO. d) AFM image and the height data corresponding to the points A to B. e) Small angle X-ray diffraction of the superlattices

on GO self-assembled from different building blocks.

F igure 3. a ) Amperometric response recorded at GO, superlattice, and

superlattice@GO with the addition of hydrogen peroxide at ?0.5 V vs. SCE in stirred nitrogen-saturated 0.1 M PBS solution (pH = 6.8) containing 0.1 M KCl. b) Amperometric responses of the superlattice@GO with injection of hydrogen peroxide. c) Corresponding calibration plot of steady-state currents against concentration of hydrogen peroxide.

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suggests that the superlattice@GO could be regarded as a functional material system. The conductivity of the SEPs, the building blocks of the superlattice, is intrinsically poor, but the existence of GO effectively accelerates charge transfer in

the photo-electrochemical cell. Thus the incorporation of GO in this functional system gives rise to improved performance of the superlattice@GO compared with superlattice alone.

I n conclusion, we have successfully achieved a monolayer superlattice containing Wells–Dawson POMs through a simple evaporation-induced self-assembly method. The self-assembly on the GO makes the monolayer superlattice more ? exible and processible, and the mechanism of action of the copper cations in the self-assembly requires further investigation. The superla-ttice@GO can be used as a hydrogen peroxide sensor with fast response and wide linear range. This superlattice@GO can also be applied as a new photoelectrode for the water photo-oxida-tion reaction. Both of these enhanced properties might result from a synergistic effect between the superlattice and GO. Our ? ndings may provide a promising avenue to integrating a monolayer superlattice with GO to show more new properties.

E xperimental Section

C hemicals : TDA.Br, H DA.Br, ODA.Br, and TBA.Br were purchased from Alfa Aesar. All other chemicals were purchased from Sinopharm Chemical Reagent Co. Ltd. (SCRC). K 6P 2W 18O 62·n H 2

O , [ 33 ]H 6P 2M o 18O 62·n H 2O , [ 34 ] and K 7P 2W 17V O 62·n H

2O [ 35 ]

were synthesized according to the literature. Graphene oxide was made by using a Hummers method. [ 36 ]

T ypical Procedure Leading to Surfactant-Encapsulated POMs [(HDA) 3(TBA) 3P 2

W 18O 62 for e xample]: H DA.Br (2.5 mmol) and TBA.Br (2.5 mmol) were added into CH Cl 3 (20 mL), and K 6P 2W 18O 6

2·n H 2O (0.8 mmol) was added into water (10 mL). After that, the organic phase was added into the solution of POMs with stirring. After 1 h, the mixture was centrifuged at 10000 rpm for 8 min. Then the precipitates were washed twice with deionized water. The precipitates were then dried at room temperature to obtain the product as powder. (TDA) 3(TBA) 3P 2W 18O 62,(ODA) 3(TBA) 3P 2W 18O 62,(H DA) 3(TBA) 3P 2M o 18O 62 , and (HDA) 3.5(TBA) 3.5P 2W 17V O 62

were made by using the same method. P reparation of Superlattices, Spheres, and Cubes : 4 mg SEPs were dissolved in 4 mL acetone by ultrasonic method. Then 300 μL copper acetate (0.02 M , acetic acid) and 650 μL ethyl benzoate were added into the solution in sequence. The mixed solution was dropped on a copper or nickel grid (We used a 100 μL micropipette to pipette 20 μL of mixed solution. Then a drop of mixed solution was dropped on the grids). The grids were dried at room temperature for 1 h, leaving a thin-? lm superlattice on the grid surface. The spheres and cubes were obtained by adjusting the content of copper acetate in the same method.

S elf-Assembly on GO : First, 0.05 mg mL

–1

GO solution was dispersed on the silicon wafer. When the silicon wafer was dried in a vacuum oven, the mixed solution mentioned above which was used to form superlattice was deposited on the GO ? lm. The solvents were allowed to evaporate slowly (about 1 h) at room temperature. Then superlattice@GO was released from the substrate and dispersed in water by ultrasonic processing of the silicon wafer in water. E lectrochemistry Experiments : CV, LSV (linear sweep voltammetry), and amperometric measurements were performed on a CH I650D electrochemical workstation. The photo-electrochemical cell was irradiated by using a PLS-SXE 300/300UV xenon lamp in the test. A standard three-electrode cell was used during the experiment. SCE was used as reference.

D etection of Hydrogen Peroxide : The aqueous solution of superlattice@GO (building block: (H DA) 3(TBA) 3P 2W 18O 62

, 25 μL) was dropped on the surface of a prepolished GC (glass carbon) disk electrode (3 mm in diameter). Then Na? on solution (0.5%, 5 μL) in ethanol was dropped on the sample. A platinum wire was used as counter electrode. The electrolyte, which consisted of a solution of 0.1 M PBS (pH 6.8) with 0.1 M KCl, was saturated with nitrogen for 30 min

Adv. Mater. 2014, 26

, 4339–4344 F igure 4. a ) Transmittance spectra of FTO (?), GO on FTO (?) and super-lattice@GO on FTO (?). b) Variation of photocurrent density versus

applied voltage and c) transient photocurrent density versus time plotted for GO (light gray line), superlattice (gray line), and superlattice@GO (black line) on FTO.

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before electrochemistry experiments. In the control experiments, GO

(0.1 mg mL –1

, 15 μL) corresponding to the mass of superlattice@GO was dropped onto the surface of a prepolished GC electrode before the test. Similarly, superlattice corresponding to the mass of superlattice@GO was dropped on the surface of a prepolished GC electrode before the test.

P hotocurrent Measurements : Photo-electrochemical performance was

tested by using a 300 W Xe lamp. First, the 400 μL (0.05 mg mL

–1)GO solution was dispersed on FTO, followed by dropping the mixed solution (building block: (H DA) 3(TBA) 3P 2W 18O 62 , 30 μL) for self-assembly. Na? on solution (0.5%, 5 μL) in ethanol was dropped on the sample. A platinum foil was used as counter electrode. 0.02 M Na 2S O 4 was used as electrolyte. In the control experiments, equal GO or mixed solution for self-assembly was dropped onto the FTO. The photocurrent response of surfactants (H TAB and TBAB) was measured in the same way. The mole number of surfactants is equal to the content of surfactants in the superlattice.

C haracterization : Elemental analyses of C, H , and N in the solid samples were carried out on a VarioEL (ElementarAnalysensysteme GmbH ). FTIR spectrometry was performed on a Nicolet AVATAR 360ESP FTIR. ICP Elemental analysis of Cu or W in the solid samples was determined by inductively coupled plasma atomic emission spectrometry (ICP-AES) (IRIS Intrepid II XSP, ThermoFisher). TEM was carried out on a H itachi H -7700 at 100 kV, and a Tecnai G2 F20 S-Twin H RTEM at 200 kV was used to characterize the sample. AFM was performed by using a Shimadzu SPM-9600 instrument. X-ray diffraction (XRD) characterization was carried out on a Bruker D8 X-ray diffractometer using Cu K α radiation ( λ = 0.15418 nm). The 31 P NMR spectrum was obtained by using a JEOL ECA-600 NMR spectrometer (600 MHz, acetone). Transmittance spectra were recorded by using a Shimadzu UV-3600 UV/Vis/NIR spectrophotometer. SEM characterizations were performed on FEI Sirion 200 scanning electron microscope by depositing samples on a silicon wafer.

S upporting Inf ormation

S

upporting Information is available from the Wiley Online Library or from the author.

A cknowledgements

T

his work was supported by NSFC (91127040, 21221062), and the State Key Project of Fundamental Research for Nanoscience and Nanotechnology (2011CB932402).

Received: F ebruary 23, 2014

Revised: M arch 19, 2014

Published online: April 29, 2014

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