Diatom Frustules as a Biomineralized Scaffold for the Growth of

Aug 2, 2016 - Edward A. Lewis†, David J. Lewis†‡, Aleksander A. Tedstone‡, Georgia Kime§, Simon Hammersley§, Philip Dawson§, David J. Binks...
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Diatom Frustules as a Biomineralized Scaffold for the Growth of Molybdenum Disulfide Nanosheets Edward A. Lewis,† David J. Lewis,*,†,‡ Aleksander A. Tedstone,‡ Georgia Kime,§ Simon Hammersley,§ Philip Dawson,§ David J. Binks,§ Paul O’Brien,*,†,‡ and Sarah J. Haigh*,† †

School of Materials, University of Manchester, Oxford Road, Manchester M13 9PL, United Kingdom School of Chemistry, University of Manchester, Oxford Road, Manchester M13 9PL, United Kingdom § Photon Science Institute, University of Manchester, Oxford Road, Manchester M13 9PL, United Kingdom ‡

S Supporting Information *

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Molybdenum disulfide (MoS2) is a layered transition metal dichalcogenide that can be isolated as monolayer or few-layer thick sheets.24,25 Unlike the archetypal 2D material graphene,26 2D MoS2 sheets have a thickness dependent bandgap27 and the material has attracted attention for electronic,27 optoelectronic,28,29 catalytic,30 and tribological applications.31 Numerous routes to 2D MoS2 nanosheets have been developed, including micromechanical exfoliation,32 ultrasonic exfoliation,33,34 shear exfoliation,35 chemical vapor deposition,36 and hot-injection synthesis.37,38 We have recently explored single source molecular precursors for the synthesis of MoS2;37,39 MoS 2 sheets were produced by the hot-injection of Mo2O2S2(S2COEt)2 into oleylamine and these were combined with graphene to produce supercapacitor electrodes.37 Similarly tetrakis(diethyldithiocarbamato)molybdenum(IV) (MoL4),39−41 has been used to produce thin films of MoS2 doped with transition metals using aerosol-assisted chemical vapor deposition (AACVD).39 The air stability and solution processability of MoL4, combined with its clean decomposition, make MoL4 an attractive precursor to MoS2. It seems feasible that the precursor’s high solubility in organic solvents could be exploited to coat a wide range of substrates or templates with the precursor which can then be converted to MoS2. In this paper, we demonstrate such an approach to MoS2 decoration, using DE as a scaffold on which to grow few-layer MoS2 nanosheets. We call these hybrid structures DE-MoS2. In the final composite material, we find that the diatom exoskeleton is thoroughly clad with MoS2 nanosheets. The approach circumvents the need for assembly of a microporous scaffold and exploits nature’s ability to fabricate complex hierarchical structures that are expensive or impossible to produce using existing nanofabrication techniques.16 The simple method outlined here allows the assembly of nanosheet layers on micropatterned substrates of high surface area without the need for surface modification or linking agents.20,21 In this approach, DE is immersed in a tetrahydrofuran solution of MoL4 precursor, the majority of the solvent is then filtered off, leaving DE stained with the molybdenum(IV) complex. The resulting DE-MoL4 powder is allowed to dry

ature displays exquisite control of phase and morphology during biomineralization, and the resulting structures have been found to possess attractive functional properties, particularly in the area of photonics.1−4 Consequently, there is considerable scientific interest in replicating the intricate architectures found in nature,5−9 as well as enhancing the intrinsic properties of biomineralized structures’ by decoration with inorganic nanostructures.10−13 Diatoms are marine-dwelling eukaryotic unicellular algae,14 which produce biomineralized silica exoskeletons known as frustules. Diatom frustules comprise the majority of the biosilica found in oceans,14 and bulk quantities of the fossilized shells are recovered by dredging. This material is known as diatomaceous-earth (DE). It is widely available and inexpensive and finds use in a number of products including toothpaste and as a laboratory filter aid (under the trade name Celite). The intricate hierarchical nanostructures found in diatom frustules have attracted considerable interest for nanotechnology applications.15,16 Diatom frustules have the highest strengthto-weight ratio of all reported natural biomaterials,17 and their periodic nanoscale pores result in both high surface areas and unique photonic properties which enhance light uptake.15,18 In the living diatom, this aids photosynthesis but the functional structures persist in the deceased organisms’ biomineral shell.15,18,19 Biosilica is an inert insulator that provides an ideal scaffold or template for functional materials.13,15,20−22 By introducing semiconducting materials into the biomineralized architecture, it is potentially possible to exploit the frustules’ photonic properties for light harvesting or its large surface area for sensing and catalytic applications.15,19,20 For example, diatom frustules coated with TiO2 have been incorporated into dyesensitized solar cells, where the devices’ enhanced performance was attributed to improved light scattering by the diatoms.15 A surface sol−gel process has been used to coat diatom frustules with a ∼50 nm thick layer of SnO2; the resulting structures were used as NO gas detectors.20 Bao et al. have reported the conversion of the insulating silica of diatom frustules to semiconducting silicon by magnesiothermic reduction,23 after which the photocatalytic properties of the resulting structures was enhanced by subsequent deposition of CdS on the frustule’s surface.19 However, to-date, there have been no reports of the functionalization of biomineralized structure with two-dimensional (2D) semiconductor materials. © XXXX American Chemical Society

Received: April 29, 2016 Revised: July 28, 2016

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DOI: 10.1021/acs.chemmater.6b01738 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials completely before annealing at 450 °C under an inert atmosphere. Annealing results in the molecular precursor’s decomposition, most likely through a β elimination reaction involving a cyclic intermediate,42 to give DE-MoS2 (full experimental details and a discussion of decomposition mechanism can be found in the SI). Analysis of DE-MoS2 by Raman spectroscopy (Figure 1) showed the formation of MoS2, with the appearance of two

Figure 2. SEM images of DE-MoS2. (a−d) secondary electron (SE) images showing MoS2 decoration of the frustules’ surfaces. (e) Back scattered electron (BSE) image taken from the same area presented in image d.

Figure 1. (a) Raman spectrum of DE-MoS2 showing the characteristic MoS2 E2g and A1g optical phonons. This particular Raman spectrum was taken from the frustule shown in the optical image b (inset).

strong peaks centered at 382 and 408 cm−1 that are assigned to the E2g and A1g optical phonon modes, respectively. These features were not seen in either the unreacted MoL4 precursor or in the DE-MoL4 material before heat treatment. The Raman spectra suggest that the average thickness of MoS2 sheets is >4 layers (see the SI). Powder X-ray diffraction (XRD) also confirms the conversion of the precursor to crystalline MoS2, after heating at 450 °C for 30 min, as shown by the complete disappearance of the characteristic precursor peaks and the appearance of broad peaks corresponding to the MoS2 (002), (100), and (110) planes (see the SI). The micro- and nanoscale morphology of DE-MoS2 was investigated by scanning electron microscopy (SEM) imaging. Secondary electron (SE) SEM images revealed the nearly uniform presence of flake-like structures on the frustules’ surfaces (Figure 2). There is also evidence that some of the pores of the diatom are infilled with similar material (Figure 2d). Backscattered electron (BSE) SEM images show that the flakes observed in SE images are composed of a higher atomic number material than the surrounding frustule, which is predominantly composed of SiO2 (Figure 2d,e). Measurement of the surface-bound flakes visible in the SEM images reveals them to have a mean diameter of 132 ± 29 nm (see the SI). Aberration corrected high angle annular dark field scanning transmission electron microscopy (HAADF STEM) imaging was used to investigate the atomic structure of the MoS2 flakes (Figure 3). The flake-like structures observed in the SEM images are composed of hundreds of smaller MoS2 nanosheets. The sheets appear randomly oriented; within the same flake some sheets are side-on while others are found with their (001) basal plane parallel to the support (Figure 3c,d). Lateral nanosheet dimensions and thicknesses can be estimated based on measurement of side-on sheets,37 and we have determined an average lateral sheet size of 8.0 ± 3.9 nm and thicknesses ranging from monolayer to no more than 6 layers (see the SI). Fourier transforms taken from atomic resolution images typically show a high level of polycrystallinity supporting the observation from HAADF STEM images that

Figure 3. HAADF STEM images. (a) MoS2 nanosheets covering a silica pore in a frusule fragment. (b−d) MoS2 nanosheets extracted from DE-MoS2. Panel b shows a typical structure composed of smaller randomly oriented flakes. Panel c shows an atomic resolution image of a region of where the flake is oriented with the (001) basal plane orthogonal to the electron beam, a Fourier transform is inset to highlight the periodic lattice structure. Panel d is a higher magnification image of the region indicated by the dashed box in panel c with an atomic overlay, showing positions of Mo atoms (red) and S atoms (green).

the MoS2 structures grown on frustule surfaces are composed of multiple small flakes (further STEM images are available in the SI). Atomic resolution imaging of the flakes shows the hexagonal 2H-MoS2 crystal structure.37,39 Energy dispersive Xray (EDX) spectrum imaging was used to confirm the elemental distributions of Mo (Lα, 2.29 eV and Kα, 17.4 eV), S (Kα, 2.31 eV), and Si (Kα, 1.74 eV) on the surface of DE-MoS2 (Figure 4a−d). The strong Si elemental signal outlines the shape of the frustule, showing clearly where the boundary of the silica structure lies. The deconvoluted signals from Mo and S were found to be co-localized and covered the whole of the area where the Si was found, albeit at lower signal intensity. A periodic pattern of high intensity regions is observed in the Mo and S maps, these MoS2 rich regions B

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Chemistry of Materials

With over one hundred thousand known species of diatoms, and the potential for further genetic manipulation, a rich variety of frustule morphologies are available to explore.16 Furthermore, this approach could easily be adapted to other metal sulfides by changing the precursor chemistry.51−53 Single source precursor chemistry has previously been shown to allow control over the morphology of nanocrystal products,54 and combining multiple precursors offers a simple route to doped or alloyed nanostructures.39,55 This experimental flexibility provides enormous potential for control over both scaffold geometry and nanocrystal morphology and composition, which should allow extensive tuning of the resulting system’s properties and performance. In conclusion, we have demonstrated the synthesis of MoS2 nanosheets on diatom frustule scaffolds by solventless thermolysis of a single source precursor bound to the frustule’s surface. MoS2 is found to coat both the external surface and internal pores of the frustules. It grows as aggregates of small (∼8 nm), randomly oriented, nanosheets with thicknesses of less than 6 atomic layers. The use of the biomineralized scaffold allows hierarchical microporous structures to be produced with relatively few synthetic steps. The simple molecular precursor chemistry employed in this work potentially offers a robust and versatile route to the decoration of a wide range of complex structures with MoS2 nanosheets. This approach is potentially scalable and has advantages over established methods due to the absence of surface capping ligands.

Figure 4. (a−d) SEM EDX spectrum imaging of MoS2 decorated frustule, panel a shows the SE electron image and panels b, c, and d show the corresponding maps of Si Kα, S Kα, and Mo Lα X-ray counts. (e,f) STEM EDX spectrum imaging of a frustule pore; panel e shows maps of Si Kα, S Kα, and Mo Lα X-ray counts. (f) EDX spectrum line scan of Si Kα, S Kα, and Mo Lα X-ray emission taken from the region indicated by the arrow in panel e, dotted vertical lines mark the onset of significant intensity associated with Si Kα and Mo Lα X-ray emission.

correspond to pores in the frustule surface, confirming that the precursor is able to impregnate pores, leading to MoS2 decoration of both the interior and exterior surfaces of the diatom. Higher magnification EDX spectrum imaging was performed alongside HAADF STEM imaging: using a fragmented piece of frustule it was possible to map Mo, S, and Si distributions across a single pore edge (Figure 4e). A peak in the Mo and S X-ray counts before the onset of Si counts confirms that the interior of the pore is coated with MoS2. In the example shown, the MoS2 layer coating the interior of the pore is ∼50 nm thick (Figure 4f). Both thermogravimetric analysis and inductively coupled atomic emission spectroscopy suggest MoS2 loading of ∼1 wt %. Quantification of the EDX spectrum image for which the elemental maps are shown in Figure 4a−d, suggests a higher loading of ∼2 wt % MoS2. However, this latter figure should be treated with caution: Celite contains a broad range of diatom architectures, most likely with different surface area to volume ratios, and the complex geometry of structures prevents accurate X-ray absorption correction. The approach demonstrated here provides a promising route to new MoS2 based heterogeneous catalysts. The hierarchical nano- and microstructure of the DE-MoS2 material produced is promising for catalytic applications, where a high surface area is desirable. In addition, these randomly oriented nanosheets, display an exceptionally large number of edge sites and this is known to improve catalytic activity in the hydrogen evolution reaction among others.43−45 In addition to electrochemical energy storage and conversion applications, it is also conceivable that the synthesis route developed here could be used to produce biosensors.46,47 Importantly, the synthesis approach presented here, which uses simple heating of a dry powder, may prove easier to scale than hot-injection methods.48 The resulting MoS2 structures are free from organic ligands, such as oleylamine, which would otherwise need to be removed prior to use in catalysis or energy storage applications.37,49 UV−visible and photoluminescence (PL) spectroscopy of the samples are described in the Supporting Information, and demonstrate how the optical properties of the frustules can be controlled by the addition of the MoS2 nanosheets. The unique photonic properties of diatom frustules,18 in combination with the broadband absorption, good electron mobility, and tunable band gap of MoS2 (ranging from 1.2 eV in the bulk to 1.8 eV in monolayers), could also be exploited in optoelectronic devices.28,29,50

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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.chemmater.6b01738. Experimental details, XRD, PL, size analysis, and further STEM and EDX data (PDF).

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AUTHOR INFORMATION

Corresponding Authors

*D. J. Lewis. E-mail: [email protected]. Twitter: @David_J_L3wis. *P. O’Brien. E-mail: Paul.o’[email protected]. *S. J. Haigh. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS E.A.L. is funded by an EPSRC doctoral prize fellowship, grant number EP/M506436/1. G.K. is funded by EPSRC EP/ L01548X/1. P.O.B. acknowledges funding from EPSRC Grants EP/K010298/1 and EP/K039547/1. S.J.H. thanks EPSRC grants EP/K016946/1 and EP/G03737X/1 and Defence Threat Reduction Agency, grant HDTRA1-12-1-0013. We would like to thank Peter Budd and Rupesh Bhavsar for their help with surface area measurements, Rupesh is supported by EPSRC grant EP/M001342/1. The underlying experimental data for this manuscript are available online, DOI: 10.15127/ 1.302941.

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