Nanographene Oxide Functionalization with Organic and Hybrid

Subscriber access provided by CORNELL UNIVERSITY LIBRARY

Article

Nano-Graphene Oxide Functionalization with Organic and Hybrid Organic-Inorganic Polymers by Molecular Layer Deposition Aneeta Jaggernauth, Ricardo M Silva, Miguel Angelo Neto, Maria Jesús Hortiguela, Gil Gonçalves, Manoj Kumar Singh, Filipe José Oliveira, Rui Ferreira e Silva, and Mercedes Vila J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b07909 • Publication Date (Web): 04 Oct 2016 Downloaded from http://pubs.acs.org on October 10, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Nano-Graphene Oxide Functionalization with Organic and Hybrid Organic-Inorganic Polymers by Molecular Layer Deposition 1

1

1

2

2

Aneeta Jaggernauth , Ricardo M. Silva , Miguel A. Neto , Maria J. Hortigüela , Gil Gonçalves , Manoj K. Singh2, Filipe J. Oliveira1, Rui F. Silva1, Mercedes Vila2,3*. 1

Department of Materials and Ceramic Engineering, CICECO, University of Aveiro, Portugal 2 Department of Mechanical Engineering, TEMA-NRD, University of Aveiro, Portugal 3 Coating Technologies S.L.-CTECHnano, Tolosa Hiribidea 76, San Sebastian, Spain. [email protected]

Abstract

The functionalization of nano graphene oxide (GO) with polymers is desirable for increasing the interface compatibility of GO thereby enabling its use in a variety of applications such as biomedical and energy storage systems. Typically, wet chemistry processes are used to achieve polymer functionalization of GO, having limitations of high heterogeneity, time consumption, and difficult purification processes. Two approaches for dry functionalization of the nano-GO surface are therefore proposed, utilizing an atomic layer deposition (ALD) reactor; i) vaporization-condensation of polyethylene glycol amine (PEG-NH2) and ii) molecular layer deposition (MLD) of a polymer hybrid from trimethylaluminium (TMA) and ethylene glycol (EG). Carboxylic activated nano-GO (GO-COOH) powders were exposed to PEG at variable temperatures, determining that a minimum of 100 oC was sufficient for adsorption of the polymer. In addition, a layer by layer deposition (an MLD route) is proposed to impart control over the growth of a polymer hybrid onto the GO-COOH surface, and to

ACS Paragon Plus Environment

1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 31

enhance the efficiency of polymer deposition by sequentially supplying a passivation layer, –Al– bonds, for monomer attachment. FTIR and XPS results showed effective control on the growth of the hybrid polymer at the GO-COOH surface, achieved through optimization of ALD reactor experimental conditions.

1. Introduction Functionalization of GO with polymers is highly desirable on the development of new applications such as polymer composites, energy-related materials, sensors, and biomedical applications, due to its excellent electrical, mechanical, and thermal properties1. The surface functionalization of graphene with polymers can occur by the direct link of polymeric chains to the graphene surface or by the controlled growth of the polymeric chains at graphene surface by pre-laminar grafting of specific initiators and subsequent addition of desired monomers (ATRP or RAFT polymerization)2. All these reported approaches however are usually based on wet chemical processes which causes constraints on the synthesis process such as technological limitations related to the need for an inert atmosphere, non-uniform distribution of energy, a highly time consuming synthesis, complex purification processes resulting in significant losses of sample and more importantly, difficulty in scaling-up for commercial advantage3,4. The functionalization of graphene with PEG, usually designated PEGylation, has been highly explored over the last few years for the development of new strategies pertaining to therapeutic applications, as it improves GO’s pharmacokinetic and pharmacodynamic properties thereby preventing typical problems of a short circulating half-life, immunogenicity and low solubility5,6. Pegylation of GO is usually carried out with carbodiimide compounds, EDC 1-ethyl-3-(-3-dimethylaminopropyl) carbodiimide hydrochloride, a water soluble compound1,4, able to crosslink the carboxylic groups on


2

Page 3 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the surface of GO with primary amine groups on the backbone structure of PEG-NH2 resulting in the establishment of new amide bonds. The final structure carries no portion of the carbodiimide, so that they are considered to be zero-length carboxyl-to-amine crosslinkers. An important concern for this process is the time needed for carrying out this wet chemical activation and PEG functionalization of GO. Different studies have shown that it can easily take several days between reaction time and subsequent neutralization and purification processes, while losing certain amount of sample in the process4,7. Molecular layer deposition (MLD) is a variation of atomic layer deposition (ALD) to incorporate molecular layer control of uniform, continuous, conformal coatings. It is typical that ALD be performed for the creation of films of metal oxides, metal nitrides and other inorganic materials8, while MLD allows for the deposition of organic molecules to create organic-organic or organic-inorganic (hybrid) layered films9–13. Both techniques are carried out in a vacuum reactor and must achieve film growth by sequential and self-limiting surface reactions9. The deposition of films via MLD onto the surface of flat substrates is less challenging than that of powders, especially in the form of nanoparticles. The increased complexity of the shape, tendency to agglomerate and motion under vacuum conditions require the consideration and examination of some specific parameters. The benefit of coating nanoparticles of different shapes, including powders and tubes, is the modification of their surface properties, while maintaining those of the original materials9. This is evident in the case of GO and its nano-sized counterparts where, as was discussed above, its functionalization with PEG imparts biocompatibility and stability in aqueous and organic media, while maintaining the desired properties of the GO, in the case of its absorbance and fluorescence, for example.


3


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 31

The deposition of thin films onto powders and nanoparticles via MLD has been investigated using organic precursors10,12, as well as a combination of organic and inorganic precursors to obtain hybrid films8,9,14,15. Metal alkoxide polymeric films, called ‘metalcones’, are examples of organic-inorganic films grown from metal precursors and organic alcohols. Most popular are the ‘alucones’, grown from ethylene glycol (EG) and trimethylaluminium (TMA) precursors. The benefit of these films is the tenability of their properties resulting from hybrid effects, impacting properties such as density, elastic modulus, hardness and refractive index14. The time taken for deposition of these hybrid films attest to the efficiency and relative rapidity of MLD. Alucone films deposited onto flat Si substrates during cycle times of 242 s, realized a thickness of just under 400 Å after 100 cycles15. MLD grown inorganic films of polyamide nylon onto SiO2 powder was performed using a cycle (precursor Apurge-precursor B-purge) time of 700 s, realizing an estimated growth of 19 Å per cycle12. In addition to providing a faster approach to GO functionalization, ALD/MLD is known to result in complete precursor reactions as any unreacted precursor is removed during purging, along with any by-products, so that the resulting layer is quite uniform in composition. Thickness of the deposited layer can also be controlled by the number of deposition cycles16. Much of the research published about ALD onto graphene tends to achieve surface functionalization while preserving its electronic properties, or to create free-standing films once the graphene is etched away17–20. In the case of GO, to the best of our knowledge there are only a few works published; one describing an approach to decorate GO with Pt catalysts to increase the catalytic activity toward acid electrolytes21, and more recently, ALD of aluminium oxide dielectric onto plasma oxidized graphene22. ALD reviews have also highlighted


4

Page 5 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


depositions onto CNTs and graphene, with MLD being discussed in its value to metalcones and hybrid coatings, and GO barely being mentioned. The proposal here for the dry functionalization of GO with PEG via MLD will therefore be quite novel, allowing a one-step synthesis of functionalized nano-GO with PEG-derivatives, in large scale and without time consuming purification steps. 2 Experimental Two routes were performed for the dry functionalization of nano-GO powder in an atomic layer deposition (ALD) reactor: (i) Deposition via an aqueous solution of polyethylene glycol amine (PEG-NH2) towards the surface functional groups of the nano-GO until saturation of the available surface groups and (ii) hybrid polymer growth by molecular layer deposition (MLD) of trimethylaluminium (TMA) and ethylene glycol (EG) precursors (Figure 1).


5


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 31

Figure 1: Schematic illustration of the two different routes for the functionalization of activated nano-GO using an ALD reactor: (i) polyethylene glycol amine (PEG-NH2) deposition, which is expected to result in the attachment of PEG-NH2 chains to surface –COOH groups of activated GO via amide bond formation; and (ii) Hybrid polymer growth by molecular layer deposition (MLD) using TMA, which is expected to create a passivation layer of –Al-O-, onto which precursor B, EG, can be attached.

2.1 ALD reactor setup PEG-NH2 and the hybrid polymer depositions were carried out in a homemade crossflow ALD reactor. The ALD apparatus (Figure 2) consists of a 50 mm inside diameter stainless steel tube placed inside a furnace, equipped with pneumatic ALD valves, mass flow controller and a PC station for automatic operation. A one-stage rotary vane pump is used achieving a pressure of approximately 2.5 mbar during depositions.


6

Page 7 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2: Schematic of the ALD reactor used in the both routes of GO functionalization. The sample is placed at the centre of an inner flow tube, encased in a system comprising heating elements. Gas switching valves allow for the selective introduction of the required precursor. N2 is used as the carrier and purging gas, circulating in a vacuum environment.

2.2 Synthesis of carboxylic activated Nano-GO GO sheets were synthesized from graphite using a modified Hummers method and were subsequently broken down by ultrasonication until nano-GO was obtained23. The carboxylic activation of nano-GO (GO-COOH) was performed using an aqueous solution of chloroacetic acid and sodium hydroxide following the procedure previously reported24. GO-COOH powder was then obtained, after being extensively washed with ultrapure water by centrifugation and finally freeze dried. GO-COOH films were prepared by drop casting onto glass substrates, which were previously cleaned using different solvents in the following order: acetone, ethanol and water. GO-COOH suspension of concentration 1.8 mg/ml was dropped onto the substrates and dried in an oven at 80 oC for 15-20 minutes for increasing viscosity, and then air dried for about 20 minutes.


7


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 31

2.3 Dry Functionalization of GO-COOH using ALD reactor 2.3.1 PEG-NH2 solution preparation PEG-NH2 precursor was prepared by mixing 0.5 g amine-terminated PEG (Aldrich – Poly (ethylene glycol) bis(amine), Mw 2000) with 20 ml Milli-Q water for 30 minutes. It was then poured into the stainless steel precursor canister and attached to the ALD reactor. 2.3.2

Functionalization of GO-COOH films and powders

The prepared GO-COOH films, recognizing thicknesses between 6±1 µm and 9±1 µm, on glass substrates were placed into a ceramic sample holder and PEG-NH2 was injected into the chamber in pulses of 1 s. After each pulse, the samples remained exposed to the precursor for 2 s and the chamber purged for 120 s between pulses. This injection cycle procedure was repeated for the GO-COOH powder samples, with a mass ranging from 9 mg to 12 mg, where the PEG-NH2 pulse time was decreased to 0.5 s followed by an increase of the exposure time of 10 s while the purging step was kept constant. The deposition temperature was varied between 80 ºC and 130 ºC. Several samples, with different deposition temperatures and number of PEG-NH2 pulses, were synthesized. In order to avoid unwanted precursor condensation throughout the deposition, the delivery lines were heated to 100 ºC. The PEG-NH2 precursor canister was kept at 85 ºC. A nitrogen (N2) stream was used as a carrier and purge gas at a constant flow of 100 sccm. After deposition, both the GO-COOH films and powder samples were firstly washed in water to remove adsorbed PEG-NH2 from its surface, and then freeze dried to avoid the formation of agglomerates, prior to its characterization.


8

Page 9 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2.4 Functionalization of GO-COOH with hybrid polymer by MLD A hybrid polymer was deposited from trimethylaluminium (TMA) (257222 Aldrich 97%) and ethylene glycol (EG) reagents (03760 Fluka ≥98.0%), as metal precursor and counter reactant, respectively, and were kept in their canisters at RT and 85 oC, respectively. GO-COOH powder and piranha treated Si substrates (control substrates) were simultaneously pulsed with metal precursor (TMA) and counter-reactant (EG) for 0.5 s with an exposure time of 10 s during each step, and the chamber was purged for 120 s between pulses. The deposition temperature was fixed at 100 ºC, under 100 sccm of N2 flow supplied by the mass flow, and the delivery lines were heated to 100 ºC. Several samples with were synthesized, varying the number of cycles. Prior to characterization, all powder samples were washed in water and freeze dried, as mentioned above. 3. Characterization Si substrates exposed to MLD functionalization conditions were characterized by X-ray reflectivity (XRR) performed on a Philips X’Pert MRD X-ray diffractometer, with a copper radiation and a graphite monochromator, for the selection of pure Kα radiation. The X-ray tube was operated at 40 kV and 50 mA. The functionalized GO films on Si substrates were characterized by X-Ray photoelectron spectroscopy (XPS). XPS spectra were acquired in an Ultra-High Vacuum (UHV) system with a base pressure of 2x10–10 mbar. The system is equipped with a hemispherical electron energy analyzer (SPECS Phoibos 150), a delay-line detector and a monochromatic AlKα (1486.74 eV) X-ray source. High resolution spectra were recorded at normal emission take-off angle and with a pass-energy of 20 eV, which provides an overall instrumental peak broadening of 0.5 eV.


9


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 31

The resulting functionalized GO powders were characterized by Fourier transform infrared spectroscopy (FTIR) using Brucker Tensor 27 equipment, 256 scans in the range 400-4000 cm-1, with a resolution of 4 cm-1, on KBr pellets in transmission mode. Coated samples were further characterized by Scanning electron microscopy (SEM) and Energy Dispersive Spectroscopy (EDS) using a Hitachi SU 70 equipped with a Bruker Nano GmbH system using an XFlash 5010 detector. Thermogravimetric analysis (TGA) was carried out on a NETZSCH STA 449F3 TGA in a nitrogen (N2) environment from 30 oC to 600 oC at a rate of 5 oC/min. 4. Results and Discussion 4.1

Dry Functionalization of GO-COOH with PEG-NH2 in ALD reactor

In order to optimize the injection stage of the PEG-NH2 reactant into the ALD reactor chamber and to better facilitate deposition, nano-GO films prepared by drop-casting were used as support for the deposition. Ideally, several experimental parameters should be considered for the effective control of PEG-NH2 deposition at the surface of GO-COOH such as: the number of deposition cycles, exposure time and deposition temperature. In fact, it is expected that the number of deposition cycles and exposure time can be preponderant for the saturation of active sites of GO-COOH with PEG-NH2, although the deposition temperature should also have an important contribution by providing the thermal activation energy for the establishment of the new covalent amide bonds between the active carboxylic groups at GO-COOH surface and the free amine groups of PEG-NH2 (Figure 1, scheme i). In order to control the establishment of covalent bonds we performed a systematic study of the effect of the deposition temperature, i.e. 80, 90, 100, 120 and 130 ºC, on the PEGNH2 deposition onto the GO-COOH surface films (see Supporting Information). After the PEG-NH2 deposition, FTIR analysis is utilized to characterize the as-prepared


10

Page 11 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


samples, as shown in Figure S1. An increase in the deposition temperature to 100 °C shows an initial change in the C=O at 1630 cm-1 due to the amide I band and the appearance of C–N/N-H (Amide II) bands at 1589 cm–1 25, respectively, resulting from the establishment of new covalent amide bonds between the amine groups of PEG and the carboxylic groups. These bands become more intense as the deposition temperature increases from 100 oC to 130 oC, so that covalent functionalization of GO-COOH is more efficient with this increase. The surface morphology of GO-COOH before and after deposition is revealed in Figure S2.


11


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 31

Inspired by the promising results obtained for the nano-GO films with PEG-NH2, we also explored the effect of the deposition temperature and the number of PEG-NH2 pulses on the GO-COOH powders. In fact, the functionalization of powders is usually more difficult to control than in the case of films, so that the exposure time of the PEGNH2 was increased to provide more time for the reactant infiltration into the powder. In this context, the GO-COOH powder samples were exposed to 10 and 30 PEG-NH2 pulses at 100 ºC and 130 ºC (here denoted as 100PEG_10, 100PEG_30 and 130PEG_10, 130PEG_30, respectively). The chemical structural changes were monitored by FTIR (Figure 3).

Figure 3: FTIR spectra of PEG-NH2 functionalized GO analysed by variation in the number of pulses and temperature: (a) 10 and 30 pulses at 100 °C and (b) 10 and 30 pulses at 130 °C. Amine and amide deformation vibrations are present at 1716 cm-1 and 1630 cm-1, respectively, on the samples for deposition carried out at 100 oC for 30 pulses and at 130 oC for 10 and 30 pulses, confirming the attachment of the polymer to the GO-COOH surface via amide bonds.

Table 1 summarizes the results of the FTIR analysis. As was previously determined for the GO-COOH films (Figure S1), the peak around 1620 cm-1 (Figure 3(a)) on GOCOOH corresponds to C=C of skeleton graphene25–27. After deposition at 130 oC peaks are instead present around 1578 cm-1 contributed by the deformation vibrations of the amide II band or secondary amines, as well as a shoulder around 1630 cm-1, contributed


12

Page 13 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


by N-H bending of primary amines or amide II25,28, as bonds are formed between the NH2 PEG groups and COOH functional groups of GO. The broadening of the peak between 1630 cm-1 and 1578 cm-1 for the deposition at 100 oC over 30 cycles, suggests the changing surface of GO-COOH even under these conditions. The presence of secondary amines is significant as it demonstrates that a temperature of 130 oC over 10 or 30 cycles provides sufficient energy and time for definite attachment of the polymer as well as the possibility for further reactions between the attached polymer chains and new PEG-NH2 reactant, to form secondary amines. In addition, CH2 asymmetric and symmetric stretching vibrations around 2920 cm-1 and 2850 cm-1 (not seen in the scale used) of PEG chains are also present for all samples after deposition at either 100 oC or 130 oC, as well as C-H deformations at 1460 cm-1, and C-H wagging at 1390 cm-1 25,29– 31

, which is of a lower intensity than that seen on GO-COOH, since they are now due to

the C-H bonds of PEG-NH2. The absence of amide bond vibrations on the spectrum for 100PEG_10 sample however, suggests that C-H bonding maybe due to reduction of the GO-COOH rather than the attachment of PEG-NH2 via covalent bonding. Finally, the deposition and attachment of PEG-NH2 at 130 oC is confirmed by peaks at 1043 cm-1 and 707 cm-1, corresponding to C-O-C and N-H bonds, respectively25,28,29. C-O-C bonding is also present for sample 100PEG_30, owing to the presence of the PEG chain.


13


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 31

Table 1: FTIR comparative analysis for GO-COOH substrate, PEG-NH2, and nanocomposites resulting from the deposition performed at 100 oC and 130 oC for 10 and 30 cycles.

The spectra for 10 and 30 pulses of PEG-NH2 at either temperature (Figure 3), show differences which are explained by the increasing saturation of the available –COOH sites of GO with increasing number of pulses. This means that 10 pulses of PEG-NH2 deposition may not be sufficient for the saturation of all available sites, as the C-O-C peak is shifted from that contributed by GO-COOH to that contributed by the PEG-NH2 chain. This is corroborated by the widening of the peak around 1630 cm-1 for 100PEG_30 (Figure 3(a)), suggesting attachment of the polymer with this longer duration. It is not expected that the PEG-NH2 will attach to itself and so the purpose of increasing the number of pulses is to provide sufficient amount of reactant, so that all the available –COOH sites could be saturated, although the presence of secondary amines suggests the occurrence of some other reactions.

It can be concluded that

covalent amide bonding, evidenced by the presence of amide bands, is confirmed at 100 o

C after 30 pulses and at 130 oC after 10 and 30 pulses. It was initially expected that the

PEG-NH2 chain would attach to each available carboxyl surface site but in reality, it is


14

Page 15 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


more likely however, that these one-ended bonds occurred in addition to areas of double-ended polymer chain reactions and branching, especially in the cases where 2o amines may have been formed. It cannot be confirmed that the deposition created a continuous coating on the GO.

To better determine the extent of the deposition of PEG-NH2 at the surface of GOCOOH powders, TGA analysis was performed for the samples prepared under the different conditions. The difference in the TGA curves (Figure 4) clearly shows a difference in the powders when compared to PEG-NH2 and GO-COOH. TGA curves of 130PEG_10 and 100PEG_10 show continuous mass loss over the temperature tested. At 600 oC the total mass loss of the 130PEG_10 sample is 47% whereas that for 100PEG_10 is 64%. The both samples show mass loss in two steps, the first occurring around 149 oC coinciding with that of GO and due to its labile functional groups such as O-H and C-O, while the second starts at 247 oC and is due to the PEG-NH2 chains, at least for 130PEG_10, although a small mass percentage of strongly bonded groups on GO is also seen to be lost at this temperature. The mass loss of deposited PEG-NH2 occurs at lower temperatures, which is expected when stabilized by GO and other carbon materials, and determined by factors such as the interaction between melting PEG-NH2 and the GO surface and the PEG/GO quantity ratio31. The mass loss after 247 o

C is shown to be greater for 100PEG_10 than for 130PEG_10; 18% compared to 8%.

The FTIR results confirm that PEG-NH2 attachment via amide bonds is unlikely for the sample 100PEG_10 but TGA results demonstrate a much greater mass loss for this sample than that of the parent GO-COOH material as well as for 130PEG_10, where amide bonds are confirmed. The deposition performed at 100 oC for 10 pulses therefore


15


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 31

has the effect of greatly reducing the thermal stability of GO-COOH, while providing insufficient energy for the formation of amide bonding with PEG-NH2. The general lower mass loss in 130PEG_10, over the entire temperature range, is caused by the initial reduction of the GO-COOH at the higher temperature of deposition. In addition, deposition at this temperature could result in double reactions, as was suggested in other literature15, between two distinct GO carboxyl groups and both ends of one PEG chain, thereby imparting greater thermal stability to the 130PEG_10 sample, resulting in lower mass loss.

Figure 4: TGA curves for 100PEG_10 and 130PEG_10 for comparison with those of PEG-NH2 precursor and GO-COOH. 100PEG_10 recognizes decreased thermal stability due to the inability of the deposition conditions to initiate amide bond formation between substrate and precursor. 130PEG_10, on the other hand, shows a better thermal stability than GO-COOH due to the attachment of PEG-NH2 via amide bonds, with the possibility of branching and double reactions of the polymer chain also contributing to this result.

SEM characterization shown in Figure 5 demonstrates a similar morphology for activated GO ((a) and (b)) and sample 100PEG_30 ((c) and (d)). Indeed, similar results were observed for the functionalization of GO-COOH films with PEG-NH2 at different


16

Page 17 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


temperatures where no significant changes in morphology were indicated subsequent to deposition (Figure S2).

Figure 5: SEM images of GO-COOH powder (a) and (b) before and (c) and (d) after PEG-NH2 deposition. The morphology of the GO-COOH powder before and after deposition is demonstrated as being similar.

4.2

Organic-Inorganic Polymer Growth at GO-COOH surface by MLD

The reaction between GO-COOH and PEG-NH2, occurring in the ALD reactor is described by vaporization-condensation, and shows limited efficiency in the deposition of the polymer, as it relates to its uniformity. A layer by layer deposition (an MLD route), portrayed in Figure 1 (ii), imparts control over the growth of a hybrid polymer onto the surface of carboxyl activated GO and can possibly enhance the efficiency of polymer deposition by sequentially supplying a passivation layer for monomer attachment. This hybrid polymer MLD was therefore used as another method of achieving dry functionalization of GO-COOH. To obtain the best estimation of the initial surface changes, deposition was performed using continuous half cycles of TMA (TMA-pulse and purging step) followed by continuous half cycles of EG (EG-pulse and purging step) on the GO-COOH powder, as


17


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 31

it is expected that continuous pulses of the same precursor, in between purging, will more likely attach to available GO-COOH surface sites rather than to itself. FTIR results after 20 half cycles of each (TMA-EG_h20) are shown in Figure 6. The presence of EG groups are evident by peaks at 2925 cm-1 and 2852 cm-1 (not shown), demonstrative of CH2 stretching vibrations, while the appearance of a peak at 1460 cm1

, not seen on GO-COOH, is characteristic of C-H bonds of polyethylene15,30,31. The

presence of Al is seen at 620 cm-1 and 570 cm-1 32, suggesting the reaction of the – AlCH3 surface species with –OH of EG to produce Al-O bonds, previous studies have also measured a peak at 910 cm-1 for Al-O stretching15. Further, the appearance of the peaks at 1265 cm-1, 1160 cm-1 and 1112 cm-1 are due to C-O stretching25,29,31 of the EG or of the ester linkage between GO and Al, while that at 1050 cm-1 is due to primary alcohols, C-OH or C-O-C25,29 of EG. The peak at 1730 cm-1 confirms the presence of C=O and changes shape after deposition, shifting to higher frequencies due to the changing of carboxyl groups to ester linkages. The peak around 1635 cm-1, slightly shifted to the left subsequent to deposition, demonstrates the change in C=C vibrations because of the change in GO functional groups after deposition and the doublet at 1400 cm-1 and 1385 cm-1 suggest the presence of CH3 groups from unreacted precursor.


18

Page 19 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 6: FTIR spectra of GO nanocomposites resulting from MLD with TMA and EG. The shaded regions represent the band over which the indicated vibrations are found, while the dotted lines indicate those at just the one peak. Increasing numbers of TMAEG cycles result in the changing of the doublet peaks found between 1400 cm-1 and 1385 cm-1, suggesting complete reactions with the TMA precursor to remove its CH3 groups. C-O bands are found in the range 1112 cm-1 to 1265 cm-1, contributed by both the EG precursor and the GO powder. Al-O bonds from the TMA precursor is seen 910 cm-1, 620 cm-1 and 570 cm-1.

The proliferation of a polymer from the EG monomer is performed by MLD for alternating TMA-EG cycles, one full MLD cycle is describe by purge-TMA-purge-EG. FTIR spectra, shown in Figure 6, and the analysis of which is summarized in Table 2, confirm covalent bonding of the hybrid polymer to GO in this approach, showing absorbance peaks similar to those of the half cycle deposition. The characteristic EG peaks are seen at 2925 cm-1 and 2852 cm-1 (not shown), for aliphatic CH2 stretching vibrations,

and

at

1461

cm-1

and

1383

cm-1,

associated

with

branched

polyethylene15,30,31. The doublet at 1400 cm-1 and 1385 cm-1 seen with the half cycle deposition, disappears with increasing number of MLD cycles, demonstrating more complete TMA precursor reactions to remove CH3 groups. Al-O deformation vibrations are present at 620 cm-1 and 570 cm-1 32, and also at 910 cm-1 15. C-O stretching peaks


19


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 31

from the monomer or ester linkage groups are present at 1265 cm-1, 1160 cm-1 and 1112 cm-1 25,29,31, and primary alcohol groups at 1050 cm-1 25. C=O stretching is seen around 1730cm-1, and like the half cycle deposition, it changes shape after deposition and shifts to higher frequencies, due to the changing of carboxyl groups to ester linkages. The appearance of C-O from the EG monomer H-O-CH2-CH2-OH is preserved in the chain when chemisorbed to -Al-, possibly producing –(Al-O-CH2-CH2-O)n-H. Table 2: FTIR comparative analysis for nanocomposites produced by 20 MLD half cycles (TMA-EG_h20), and 20, 50 and 80 MLD full cycles; TMA-EG20, TMA-EG50 and TMA-EG80, respectively.

XRR analysis performed on Si substrates corroborates the increase in thickness of the polymer hybrid with the number of cycles. Approximate thicknesses of 5.0nm, 8.5nm and 12.5 nm were obtained after 20, 50 and 80 cycles, respectively, of TMA-EG MLD cycles (Figure 7 (a)). The half cycle deposition (TMA-EG_h20) yielded a film thickness of 6.5 nm on Si substrates.


20

Page 21 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 7: XRR results for Si samples with TMA-EG MLD; (a) XRR intensity curves for deposition on Si substrates; (b) variation in thickness and growth rate of hybrid polymer with number of cycles, showing error bars Thickness was measured from the intensity curves using a derivative equation of Snell’s law. The thickness increases with the number of cycles, while the growth rate decreases.

The analysis determined, across the range of the number of cycles investigated, an almost linear increase in the thickness of the adsorbed hybrid polymer and an exponential decrease in growth rate with increasing number of cycles (Figure 7(b)), with a maximum growth rate of 0.25 nm/cycle obtained for 20 cycles. Dameron et al15 recorded growth rates of 0.4 nm/cycle at 85 oC and 0.04 nm/cycle at 175 oC, recognizing a decrease in growth rate with increasing temperature, for TMA-EG MLD. The growth rate obtained here, 0.25 nm/cycle was obtained for depositions at 100oC, and is consistent with a lower growth rate at a higher temperature. The variation in growth rate could possibly be demonstrating the initial stages of substrate-enhanced growth33, whereby the deposition results in high growth rates at fewer cycles, followed by an exponential decrease as the number of cycles increases and an eventual almost constant growth rate. If this is the case, one explanation could be the presence of fewer bonding sites on the deposited layer than on the initial substrate33, in this case the treated Si substrate, which will happen if both –OH groups at the ends of one monomer


21


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 31

chain adsorb to two –Al– in the same layer, preventing further deposition at those sites, or that on deposition, -Al– adsorbs to 2 unique –O sites, reducing the previous 2 available sites to 1 for the next precursor pulse. The surface of an MLD deposited TMA-EG20 onto GO-COOH film was analysed by XPS. The C1s spectra for GO-COOH and TMA-EG20 are shown in Figure 8(a) (bottom and top, respectively). The C1s spectrum of GO-COOH shows a first peak (I) corresponding to the skeletal C-C structure, mixed sp2/sp3 carbon, around 285 eV34, a second peak (II) characteristic of C-O groups in graphene oxide, around 287 eV15, and two components at 288.3 and 289.1 eV (III, IV) assigned to carbonyl and carboxyl groups, respectively (C=O and O-C=O)34,35.

Figure 8: XPS spectra of (a) C1s peak for TMA-EG20, deposited on GO-COOH film (top) and GO-COOH (bottom) and (b) Al2p peak for TMA-EG20.The measured peaks of TMA-EG20; I, II and III, corresponds to hydrocarbons, C-O and C=O, respectively. The presence of the Al 2p peak confirms a reaction between the precursors.


22

Page 23 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The changes in the C1s spectrum for TMA-EG20 (Figure 8(a)) indicate the deposition of TMA-EG covering the GO film, even though a partial contribution from the GO film to the spectrum is not discarded. The main peak of the spectrum has a clear asymmetrical shape that requires two components to be fitted. The C1s component at the lowest energy, 285.8 eV (I’) is attributed to hydrocarbon species. This value ofbinding energy, although higher than usual for hydrocarbons, is close to the values observed for adventitious carbon contamination in the native oxide of aluminium (~286.2 eV)36. The C1s middle component (II’), with a binding energy of ∼287.2 eV, is attributed to C-O species15, as expected from the EG incorporation in the hybrid polymer (-O-CH2-CH2-O-Al). The smaller peak (III’) appearing around 290.2 eV is assigned to C=O and O-C=O species35. Besides a possible contribution from GOCOOH and atmospheric contamination to this peak, the presence of carbonyl groups in the hybrid polymer has been explained in the literature by its dehydrogenation into an enol form15, facilitated by the AlOx species, and the subsequent rearrangement into the keto tautomer. The Al2p spectrum of TMA-EG20 (Figure 8(b)) shows a quite symmetrical peak centred at 75.6 eV, similar to the values observed for Al2p signal from the native oxide of aluminium36. The Al-O signal from pure Al2O3 is typically around 74.6 eV but the shift of the Al2p peak to higher binding energies for higher oxidized forms such as AlO-C has been reported37, therefore being in good agreement with our results.

Thermal stability of the GO-TMA-EG samples is shown in the TGA curves of Figure 9. Mass loss under 100 oC is attributed to the loss of adsorbed water on the sample. Similar to the GO-PEG samples, the mass loss occurs in two steps. The first major mass loss is seen around 149 oC for GO-COOH, TMA-EG50 and TMA-EG80, while this is shown


23


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 31

around 158 oC for TMA-EG20, and is the result of the loss of GO functional groups such as carboxyl groups as well as the unreacted hydroxyl groups of EG. The higher degradation temperature of GO in TMA-EG20 is due to the strong bonds formed between GO and Al and quite possibly the ratio of hybrid polymer to GO, which is not a linear relationship31. The second major mass loss is seen around 281 oC, due to the deposited hybrid polymer. TMA-EG20 recognizes a mass loss of 9% here, while TMA-EG50 and TMA-EG80 both show a 5% loss, possibly because of the stable alumina formed between the polymer layers. TMA-EG50 and TMA-EG80 therefore retain more of their mass in the form of alumina, allowing the most susceptible portions of the EG chain to thermally degrade and vaporize. In addition, it was shown in XRR that the growth rate decreases with the number of cycles. If this is in fact due to double reactions and branching, a covering of the hybrid polymer rather than vertical, linear growth at surface sites, would increase the thermal stability of the composite.

Figure 9: TGA curves of GO-COOH and functionalized GO nanocomposites after 20, 50 and 80 TMA-EG MLD cycles. Thermal degradation of GO-COOH begins to occur at a relatively low temperature of 149 oC. The nanocomposite samples also show the loss of GO-COOH functional groups (first mass loss) as well as that of the polymer


24

Page 25 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


hybrid (second mass loss). The addition of more layers of Al is shown to increase the thermal stability of the nanocomposite.

Z-potential measurements, graphically portrayed in Figure 10, demonstrate a more positive zeta potential value after TMA/EG MLD, independent of the number of cycles, and having an average value of -29.35 ± 4.00 mV. Compared to the z-potential of GOCOOH at -41.8mV, the presence of the hybrid polymer creates a less negative potential, due to the presence of Al2O3 which generally carries a positive38,39 or low negative40 zpotential. Saravanan, et al41, carried out electrokinetic studies on alumina onto which polyethylene glycol (PEG), in concentrations of 0.1-100 ppm, was adsorbed, and found that this adsorption only slightly alters the zeta potential of alumina, but not appreciably altering its electrokinetic behaviour. This substantiates that the increase in z-potential is due mainly to the presence of Al2O3 in the hybrid.

Figure 10: Graphical representation of the zeta potential values of GO-COOH, and MLD deposited samples, shown with their respective number of cycles. The half cycle sample TMA-EG_h20 is also shown at 1 cycle since the half cycles are assumed to fully saturate all available surface sites rather than to react with its own functional groups. GO-COOH functional groups create a more negative zeta potential than the deposited organic-inorganic hybrid.


25


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 31

In the SEM images of the hybrid polymer deposited via 80 MLD cycles (TMA-EG80) (Figure 11) the appearance of fissures is observed. This can be due to the lack of interfacial contact between the GO and the passivation alumina layer, possibly resulting from the presence of the functional groups on the surface of GO preventing the maintenance of a continuous coating. Similar cracks were seen by Kim et al42 with alumina/graphene nanocomposites. In the present case, it could also be the result of the formation of a hardened polymer by EG and Al, which recognizes fissures due to thermal contraction upon cooling. These fissures are not observed in the SEM images of GO-PEG samples. The presence of Al was confirmed by EDS spectroscopy, Fig. S4.

Figure 11: Figure 12: SEM images of the deposited hybrid polymer composite after 80 MLD cycles (a) at 30 k and (b) 50 k magnitudes. The arrows added in (a) indicate the position of some fissures in the hybrid polymer, which are more clearly shown in (b). Conclusions The dry functionalization of GO-COOH was successfully accomplished by the MLD growth of a hybrid polymer using TMA and EG precursors. 100 oC is sufficient for this deposition with at least 20 TMA-EG cycles showing a change in the surface of GO, and depositing a minimum thickness of 5.0 nm. The formation of the expected uniform tethered chains of a hybrid polymer (-Al-O-CH2-CH2-O-)n have not been confirmed due to the presence of double reactions and branching, although FTIR confirmed the


26

Page 27 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


presence of -Al-O-, -C-H-, and -C-O- bonds being formed on GO-TMA-EG. In addition, XPS highlighted the existence of native oxide Al, and the increasing thickness of the hybrid polymer layer with number of cycles, confirming bond formation between EG and –Al-. The resulting hybrid polymer also resulted in an increase in the thermal stability of GO-COOH with an increasing number of cycles, according to TGA data. The route for dry functionalization of GO-COOH with PEG-NH2, via pulsed depositions in an ALD reactor, proved to be more challenging. FTIR demonstrated a changing GO surface with increasing number of cycles, with the main result being the presence of amide and -N-H- bonds suggesting the reaction between –COOH functional groups of GO and –NH terminating groups of PEG-NH2. A temperature of 130 oC resulted in pegylated GO within a relatively short time of about 5 hours, although control over the formation of double reactions will have to be investigated if the resulting nanoparticles are to be further activated.

Supporting Information Available FTIR spectra of activated GO-COOH films on glass substrates, amine-terminated PEG, and GO functionalized films at various temperatures (80, 90, 100, 120 and 130 ºC) and PEG-NH2 and GO-COOH spectra. SEM images of activated GO films before and after deposition. XRR intensity curves of for PEG deposition onto Si substrates. EDS spectrum of the elemental composition of activated GO after 80 MLD cycles of TMA and EG. Acknowledgements This work was developed within the scope of the project CICECO-Aveiro Institute of Materials, POCI-01-0145-FEDER-007679 (FCT Ref. UID /CTM /50011/2013),


27


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 31

financed by national funds through the FCT/MEC and when appropriate co-financed by FEDER under the PT2020 Partnership Agreement. Corresponding Author *M.Vila. Present address: Coating Technologies S.L. [email protected] Ph#: +34 943324603 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. References (1) (2) (3)

(4)

(5) (6) (7)

(8)

(9)

(10) (11) (12) (13)

(14)

(15)

Yang, K.; Feng, L.; Hong, H.; Cai, W.; Liu, Z. Preparation and Functionalization of Graphene Nanocomposites for Biomedical Applications. Nat Protoc 2013, 8, 2392–2403. Layek, R. K.; Nandi, A. K. A Review on Synthesis and Properties of Polymer Functionalized Graphene. Polym. (United Kingdom) 2013, 54, 5087–5103. Miao, W.; Shim, G.; Lee, S.; Lee, S.; Choe, Y. S.; Oh, Y. K. Safety and Tumor Tissue Accumulation of Pegylated Graphene Oxide Nanosheets for Co-Delivery of Anticancer Drug and Photosensitizer. Biomaterials 2013, 34, 3402–3410. Vila, M.; Portoles, M. T.; Marques, P. A.; Feito, M. J.; Matesanz, M. C.; RamirezSantillan, C.; Goncalves, G.; Cruz, S. M.; Nieto, A.; Vallet-Regi, M. Cell Uptake Survey of Pegylated Nanographene Oxide. Nanotechnology 2012, 23, 465103. Jokerst, J. V; Lobovkina, T.; Zare, R. N.; Gambhir, S. S. Nanoparticle PEGylation for Imaging and Therapy. Nanomedicine (Lond). 2011, 6, 715–728. Roberts, M. J.; Bentley, M. D.; Harris, J. M. Chemistry for Peptide and Protein PEGylation. Adv. Drug Deliv. Rev. 2012, 64, 116–127. Yang, K.; Zhang, S.; Zhang, G.; Sun, X.; Lee, S. T.; Liu, Z. Graphene in Mice: Ultrahigh in Vivo Tumor Uptake and Efficient Photothermal Therapy. Nano Lett 2010, 10, 3318– 3323. Yoon, B.; Seghete, D.; Cavanagh, A. S.; George, S. M. Molecular Layer Deposition of Hybrid Organic−Inorganic Alucone Polymer Films Using a Three-Step ABC Reaction Sequence. Chem. Mater. 2009, 21, 5365–5374. Liang, X.; King, D. M.; Li, P.; George, S. M.; Weimer, A. W. Nanocoating Hybrid Polymer Films on Large Quantities of Cohesive Nanoparticles by Molecular Layer Deposition. AlChE J. 2009, 55, 1030–1039. Yoshimura, T.; Tatsuura, S.; Sotoyama, W. Polymer Films Formed with Monolayer Growth Steps by Molecular Layer Deposition. Appl. Phys. Lett. 1991, 59, 482. Loscutoff, P. W.; Lee, H. B. R.; Bent, S. F. Deposition of Ultrathin Polythiourea Films by Molecular Layer Deposition. Chem. Mater. 2010, 22, 5563–5569. Du, Y.; George, S. M. Molecular Layer Deposition of Nylon 66 Films Examined Using in Situ FTIR Spectroscopy. J. Phys. Chem. C 2007, 111, 8509–8517. Adamczyk, N. M.; Dameron, a a; George, S. M. Molecular Layer Deposition of Poly ( PPhenylene Terephthalamide ) Films Using Terephthaloyl Chloride and PPhenylenediamine Molecular Layer Deposition of Poly ( P-Phenylene Terephthalamide ) Films Using Terephthaloyl Chloride and P-Phenylenediamine. Langmuir 2008, 2081– 2089. Lee, B. H.; Yoon, B.; Abdulagatov, A. I.; Hall, R. a.; George, S. M. Growth and Properties of Hybrid Organic-Inorganic Metalcone Films Using Molecular Layer Deposition Techniques. Adv. Funct. Mater. 2013, 23, 532–546. Dameron, A. A.; Seghete, D.; Burton, B. .; Davidson, S. D.; Cavanagh, A. S.; Bertrand,


28

Page 29 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(16) (17)

(18)

(19)

(20) (21)

(22)

(23)

(24)

(25) (26) (27)

(28)

(29)

(30)

(31)

(32)

(33) (34) (35)

(36)

J.; George, S. M. Molecular Layer Deposition of Alucone Polymer Films Using Trimethylaluminum and Ethylene Glycol. Chem. Mater. 2008, 20, 3315–3326. Leskela, M.; Ritala, M. Atomic Layer Deposition ( ALD ) : From Precursors to Thin Film Structures. Thin Solid Films 2002, 409, 138–146. Wang, L.; Travis, J. J.; Cavanagh, A. S.; Liu, X.; Koenig, S. P.; Huang, P. Y.; George, S. M.; Bunch, J. S. Ultrathin Oxide Films by Atomic Layer Deposition on Graphene. Nano Lett. 2012, 12, 3706–3710. McDonnell, S.; Pirkle, A.; Kim, J.; Colombo, L.; Wallace, R. M. Trimethyl-Aluminum and Ozone Interactions with Graphite in Atomic Layer Deposition of Al2O3. J. Appl. Phys. 2012, 112, 104110. Pirkle, A; McDonnell, S.; Lee, B.; Kim, J.; Colombo, L.; Wallace, R. M. The Effect of Graphite Surface Condition on the Composition of Al2O3 by Atomic Layer Deposition. Appl. Phys. Lett. 2010, 97, 82901. Marichy, C.; Pinna, N. Carbon-Nanostructures Coated/decorated by Atomic Layer Deposition: Growth and Applications. Coord. Chem. Rev. 2013, 257, 3232–3253. Hsieh, C. Te; Chen, W. Y.; Tzou, D. Y.; Roy, A. K.; Hsiao, H. T. Atomic Layer Deposition of Pt Nanocatalysts on Graphene Oxide Nanosheets for Electro-Oxidation of Formic Acid. Int. J. Hydrogen Energy 2012, 37, 17837–17843. Nourbakhsh, A.; Adelmann, C.; Song, Y.; Lee, S. Graphene Oxide Monolayers as Atomically Thin Seeding Layers for Atomic Layer Deposition of Metal Oxides. Nanoscale 2015, 7, 10781–10789. Gonçalves, G.; Vila, M.; Bdikin, I.; de Andrés, A.; Emami, N.; Ferreira, R. A. S.; Carlos, L. D.; Grácio, J.; Marques, P. A. A. P. Breakdown into Nanoscale of Graphene Oxide: Confined Hot Spot Atomic Reduction and Fragmentation. Sci. Rep. 2014, 4, 6735. Linares, J.; Matesanz, M. C.; Vila, M.; Feito, M. J.; Gonçalves, G.; Vallet-Regi, M.; Marques, P. A. A. P.; Portoles, M. T. Endocytic Mechanisms of Graphene Oxide Nanosheets in Osteoblasts, Hepatocytes and Macrophages. ACS Appl. Mater. Interfaces 2014, 6, 13697–13706. Socrates, G. Infrared and Raman Characteristic Group Frequencies; 2004. Peng, S.; Fan, X.; Li, S.; Zhang, J. Green Synthesis and Characterization of Graphite Oxide by Orthogonal Experiment. J. Chil. Chem. Soc. 2013, 58, 2213–2217. Marcano, D. C.; Kosynkin, D. V; Berlin, J. M.; Sinitskii, a; Sun, Z. Z.; Slesarev, a; Alemany, L. B.; Lu, W.; Tour, J. M. Improved Synthesis of Graphene Oxide. ACS Nano 2010, 4, 4806–4814. Kumar, D.; Rani, S. Synthesis of Amide Functionalized Graphene Oxide for Humidity Sensing Application ( Amide Functionalized Graphene Oxide for Humidity Sensing ). SENSORDEVICES 2015 Sixth Int. Conf. Sens. Device Technol. Appl. Synth. 2015, 142–145. Shameli, K.; Ahmad, M. Bin; Jazayeri, S. D.; Sedaghat, S.; Shabanzadeh, P.; Jahangirian, H.; Mahdavi, M.; Abdollahi, Y. Synthesis and Characterization of Polyethylene Glycol Mediated Silver Nanoparticles by the Green Method. Int. J. Mol. Sci. 2012, 13, 6639–6650. Ali Umar, M. I.; Yap, C. C.; Awang, R.; Mat Salleh, M.; Yahaya, M. The Effect of SpinCoated Polyethylene Glycol on the Electrical and Optical Properties of Graphene Film. Appl. Surf. Sci. 2014, 313, 883–887. Wang, C.; Feng, L.; Yang, H.; Xin, G.; Li, W.; Zheng, J.; Tian, W.; Li, X. Graphene Oxide Stabilized Polyethylene Glycol for Heat Storage. Phys. Chem. Chem. Phys. 2012, 14, 13233. Zhang, W. L.; Choi, H. J.; Leong, Y. K. Facile Fabrication of Graphene Oxide-Wrapped Alumina Particles and Their Electrorheological Characteristics. Mater. Chem. Phys. 2014, 145, 151–155. Puurunen, R. L. Surface Chemistry of Atomic Layer Deposition : A Case Study for the Trimethylaluminum / Water Process; 2005; Vol. 97. Akhter, S.; Zhou, X. L.; White, J. M. XPS Study of Polymer/organometallic Interaction: TMA on PVA Polymer. Appl. Surf. Sci. 1989, 37, 201–216. Li, L.; Yao, X.; Li, H.; Liu, Z.; Ma, W.; Liang, X. Thermal Stability of Oxygen-Containing Functional Groups on Activated Carbon Surfaces in a Thermal Oxidative Environment. J. Chem. Eng. Japan 2014, 47, 21–27. Crist, B. V. Handbooks of Monochromatic XPS Spectra—volume 2—commercially Pure Binary Oxides.; XPS International, LLC: Mountain View, 2005.


29


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(37)

(38) (39)

(40)

(41)

(42)

Page 30 of 31

A. Lushington, J. Liu, M.N. Bannis, B. Xiao, S. Lawes, R. Li, X. Sun, A. A Novel Approach in Controlling the Conductivity of Thin Films Using Molecular Layer Deposition. Appl. Surf. Sci. 2015. Sun, J.; Gao, L.; Li, W. Colloidal Processing of Carbon Nanotube / Alumina Composites. Chem. Mater. 2002, 14, 5169–5172. Estili, M.; Kawasaki, A. An Approach to Mass-Producing Individually Alumina-Decorated Multi-Walled Carbon Nanotubes with Optimized and Controlled Compositions. Scr. Mater. 2008, 58, 906–909. Taylor, A. J.; McClure, C. D.; Shipkowski, K. A.; Thompson, E. A.; Hussain, S.; Garantziotis, S.; Parsons, G. N.; Bonner, J. C. Atomic Layer Deposition Coating of Carbon Nanotubes with Aluminum Oxide Alters pro-Fibrogenic Cytokine Expression by Human Mononuclear Phagocytes in Vitro and Reduces Lung Fibrosis in Mice in Vivo. PLoS One 2014, 9, e106870. Saravanan, L.; Subramanian, S. Surface Chemical Studies on the Competitive Adsorption of Poly(ethylene Glycol) and Ammonium Poly(methacrylate) onto Alumina. J. Colloid Interface Sci. 2005, 284, 363–377. Kim, H. J.; Lee, S. M.; Oh, Y. S.; Yang, Y. H.; Lim, Y. S.; Yoon, D. H.; Lee, C.; Kim, J. Y.; Ruoff, R. S. Unoxidized Graphene/alumina Nanocomposite: Fracture- and WearResistance Effects of Graphene on Alumina Matrix. Sci. Rep. 2014, 4, 5176.


30

Page 31 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TOC Graphic


31

Nanographene Oxide Functionalization with Organic and Hybrid

Recommend Documents