Composites formed from tungsten trioxide and graphene oxide for the next generation of electrochromic interfaces

Journal Pre-proof Composites formed from tungsten trioxide and graphene oxide for the next generation of electrochromic interfaces Valentina Dinca, Qian Liu, Simona Brajnicov, Anca Bonciu, Angela Vlad, Cerasela Zoica Dinu PII:

S2452-2139(19)30122-6

DOI:

https://doi.org/10.1016/j.coco.2019.11.015

Reference:

COCO 284

To appear in:

Composites Communications

Received Date: 5 August 2019 Revised Date:

1 November 2019

Accepted Date: 25 November 2019

Please cite this article as: V. Dinca, Q. Liu, S. Brajnicov, A. Bonciu, A. Vlad, C.Z. Dinu, Composites formed from tungsten trioxide and graphene oxide for the next generation of electrochromic interfaces, Composites Communications (2019), doi: https://doi.org/10.1016/j.coco.2019.11.015. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

Composites formed from tungsten trioxide and graphene oxide for the next generation of electrochromic interfaces Valentina Dincaa,*,#, Qian Liub,#, Simona Brajnicova, Anca Bonciua, Angela Vlada and Cerasela Zoica Dinub,* a

National Institute for Lasers, Plasma and Radiation Physics, Atomistilor 409, Magurele, Bucharest, 077125, Romania

b

Department of Chemical and Biomedical Engineering, West Virginia University, Morgantown, WV, 20506, USA

# The authors have contributed equally.

*Corresponding Author: Cerasela Zoica Dinu, Ph.D. Department of Chemical and Biomedical Engineering West Virginia University Benjamin M. Statler College of Engineering and Mineral Resources PO Box 6102 Morgantown, WV, 26506, USA E-mail: [email protected] Tel.: +1-304-293-9338 Fax: +1-304-293-4139

Abstract Thin films of electrochromic (EC) materials are being used as energy saving platforms. However, manufacturing high quality of such films while ensuring their superior stability and high coloration availability, as well as their persistency in diverse conditions of temperature and/or illumination, is challenging. Herein we proposed to create the next generation of composites that display EC characteristics using a user-controlled manufacturing strategy based on Matrix Assisted Pulsed Laser Evaporation (MAPLE). In our approach, tungsten trioxide and graphene oxide were employed as starting materials with MAPLE controlling material deposition conditions and overall composites’ thickness. The resulting EC composites were characterized for their chemical and physical properties using Fourier Transform Infrared Spectroscopy and energy dispersive spectroscopy, and optical and atomic force microscopy respectively, with changes in their electrochemical characteristics being evaluated by cyclic voltammetry and related to film thickness through the use of spectrophotometry. It was found that MAPLE deposition conditions allow for formation of composites that display enhanced electron transport capabilities and high energy efficiency at their interfaces; further, it was found that MAPLE allowed for uniform films production, of controlled thickness as well as eliminated possible impurities normally associated with methods used for EC films production such as sputtering, sol–gel or electrodeposition, and chemical wet deposition respectively. Our work could potentially provide a user-controlled synthesis and manufacturing strategy for the formation of next generation of EC thin films that have minimum interfacial defects while possessing maximum conversion

efficiency

to

thus

influence

and/or

dictate

their

energy

saving

profiles.

Keywords Electrochromic (EC) materials, thin films, Matrix Assisted Pulsed Laser Evaporation (MAPLE)

1. Introduction Based on modulation of either their transmitted or reflected radiation, molybdenum trioxide (MoO3), iridium and titanium dioxide (IrO2 and TiO2) were proposed as the next generation of electrochromic (EC) materials to be used for fabrication of “smart” windows,1,2 anti-glare car mirrors,3,

4

and state-of-charge

indicators.5 Their fast and adaptable color change characteristics6 upon intensively and uniformly adsorbing or reflecting light,7,

8

was also proposed for integration in creating the next generation of energy saving

platforms. For this, the EC materials need to be incorporated into thin film composites to actively control the sunlight flux and thus dictate the specific energy profile of the envisioned platform.9-11 Thin films have been previously obtained by sputtering,12 thermal evaporation deposition,13 hydrothermal synthesis,14 spray coating,15 sol-gel16 and electrochemical methods,17 just to name a few. The color change responses of such films was shown to correlate with the diffusion coefficient of ions in the constituting material/s as well as with the overall film’s thickness.18-20 Research also showed that, for instance, sputter deposition could lead to formation of films with a better adhesion to the substrate than the ones created using an evaporation method.21 Moreover, analysis showed that interfacing EC nanoparticles with graphene deposited using electrochemical approaches22 could lead to EC-active hybrid structures with superior stability and higher coloration efficiency than controls formed only from EC nanoparticles.23 However, when considering EC thin film formation, the above methods are many times unreliable because they either require expensive instruments and strict synthesis conditions, or they cannot produce high quality films of controlled thickness and uniformity. Furthermore, such methods are poorly scalable, time consuming and costly when feasible large-scale manufacturing is being considered. The next generation of EC thin films should be composite in nature and allow for efficient control of charge transfer capabilities at their interfaces. Furthermore, the next generation of EC films should possess reversible optical characteristics as well as exhibit superior stability and high coloration efficiency, all under diverse conditions of temperature and/or illumination.

Herein we hypothesized that the characteristics of the next generation of scalable EC composites are function of interface reactions of starting materials and could be defined under user-controlled manufacturing strategies. To demonstrate our hypothesis, we have selected two starting materials, namely graphene oxide (GO) and tungsten trioxide (WO3) and a Matrix Assisted Laser Evaporation (MAPLE) method. The choice of graphene as a starting material was based on previous research that showed that its electrical,24, 25 thermal26, 27 and chemical28, 29 properties, its unique band structure and high carrier mobility30, 31

respectively, allow for graphene-based thin films growth either by chemical vapor deposition combined

with sputtering deposition,32,

33

or by sol-gel method,34 to lead to structures of high transparency, with

flexible electronic properties.35 Furthermore, analysis showed that such structures’ opto-electronic properties could be tuned based on film thickness36 and doping characteristics.37 Similarly, the choice of WO3 was based on its known potential to serve as a highly active and stable EC system,38 with high optical and absorption capabilities.39 Specifically, previous research showed that when WO3 was doped with inorganic and organic dopants such as Fe2O3,40 TiO241 or chitosan,42 it exhibited EC-like properties which were function of the level of oxygen deficiency created by such doping. Lastly, previous research showed that MAPLE provides an efficient method for fabrication of a variety of thin films from organic,43 to biomaterials,44-46 semiconductors,47 or composite films48,

49

respectively, all in one step, under user-

controlled strategies and in mild conditions.50 By employing materials dissolved or suspended within a liquid medium, MAPLE was also extended towards obtaining more complex coatings such as gradient films assemblies51 or shellac thin films composites.52 We envision that the proposed strategy could help define and create the next flexible platform that allows thin and ultrathin EC composites generation, all with controlled morphologies, to dictate electrical and conductive interfacial properties53 under local doping and local charge densities respectively, and thus to potentially enhance EC thin films implementation as energy saving platforms with extension to optoelectronics and catalytic conversion.54

2. Materials and Methods 2.1 Target Preparation Graphene oxide (GO) and tungsten trioxide (WO3) were purchased from Sigma-Aldrich and used for preparing their respective targets. For this, GO and WO3 were dispersed in double distilled water (Di-water) to 0.5 weight % concentrations; solutions were sonicated for 6- 7 h on a Sharpertek Digital Ultrasonic cleaner XP PRO and subsequently kept overnight at room temperature. Prior individual targets’ formation, the solutions were again sonicated for 15 min each and rapidly frozen drop-by-drop in a liquid N2 cooled copper container. The container was subsequently mounted on a cryogenic holder, inside a deposition chamber (Neocera spherical vacuum chamber of 12" diameter). The targets were maintained frozen using liquid N2 and a circulating tubing systems. The temperature was monitored with the use of two thermocouples placed directly onto the target holder. 2.2 Substrates Preparation Double polished Si (100) transparent in infrared (Neyco) and glass coverslips (Marienfeld) were used as substrates. Prior to any experiments, the substrates were carefully cleaned in an ultrasonic bath, in three consecutive steps. The first step was performed in acetone (99.5% purity, Chimreactiv SRL), second in ethyl alcohol (99.5% purity, Chimreactiv SRL), and third in Di-water; each one of the steps had a 10 min duration. Resulting cleaned substrates were blown-dried under N2 and used immediately. 2.3 Matrix Assisted Pulsed Laser Evaporation (MAPLE) for thin films preparation A “Surelite II” pulsed Nd: YAG laser system (Continuum Company) of 5-7 ns pulse duration, operating at 266 nm under a 10 Hz repetition rate, was used to irradiate the frozen targets. Deposition as layers or single component, i.e., layer-by-layer of GO or WO3, mixtures of GO and WO3- all (50/50 weight %), or individual GO and WO3 components respectively were considered. The laser fluence was 0.6 J/cm2.

In order to avoid local overheating of the target upon multiple pulses irradiation, the target was rotated at 20 rpm using a motion-feed driven motor. Substrates were positioned parallel to the target at a fixed distance of 3.5 cm and were intended to collect any material expelled upon laser’s interaction with the target. Substrates were kept at ambient temperature during the deposition. Any volatile species were removed from the deposition chamber by vacuum pumps while maintaining the background pressure at 12×10−3 Pa through the help of an integrated TPU 170 turbomolecular pump (Pfeiffer-Balzers). The number of pulses was varied from 36 to 108 kPulses, with 72 kPulses being used for GO, 54 for WO3, and 72 for GO/WO3 and their mixtures, respectively. Thin films thicknesses were measured using a UV−visible spectrophotometer (UV 2600, Shimadzu) equipped with an integrating sphere operating in reflection mode. 2.4 Drop Cast Control Films Control films were prepared by drop casting; specifically, 100 µl of GO (0.5 weight % concentrations), WO3 (0.5 weight % concentrations), or GO and WO3 mixture (50/50% in volume with each component at 0.5% solution) respectively, were placed each on polished Si or glass substrates and left to dry at room temperature, overnight. 2.5 Physicochemical Characterization of the Films Morphological characterizations of the samples (thin films obtained by MAPLE and drop casts used as controls) were performed by Optical Microscopy (OM), Atomic Force Microscopy55 and Scanning Electron Microscopy56 respectively. For OM, the images were acquired using an Axiovert 200 Microscope coupled to a Carl Zeiss AxioCam MRm camera. AFM (XE 100 AFM, Park systems) measurements were performed in non-contact mode, while SEM investigations were carried out on a JSM-531 Inspect S Electron Scanning Microscope (FEI Company) at 25 kV. Chemical evaluation of the deposited thin films or controls was performed using Fourier Transform Infrared Spectroscopy (FTIR; Jasco FT/IR-6300 type) and Energy Dispersive Spectroscopy (EDAX, Element 2CB detector on a JSM-531 Inspect S Electron Scanning

Microscope, FEI Company) respectively. All FTIR measurements were carried out in absorption mode, in the 400-7800 cm−1 range, and at a resolution of 4 cm-1. The FTIR spectra were represented as an average of 1024 scans using the Rosenfeld apodization function. EDAX was operated at 25 kV. 2.6 Electrochemical Investigation Cyclic voltammetry (CV) curves were recorded on a VersaSTAT 3 potentiostat/galvonostat (Princeton Applied Research) using a three-electrode system consisting of a saturated Ag/AgCl used as the reference electrode, an Ag wire used as the counter electrode, with the sample itself serving as the working electrode respectively. For the measurements, the individual sample was fixed with conductive copper tape (Ted Pella, INC.). The potential was cycled from -1.0 to 1.0 V (vs. Ag/AgCl) at a scan rate of 50 mV/s in a 20 mL of 0.1 M H2SO4 (ACROS Organics) aqueous solution, with cycles repeated until the recorded CV curves became stable. The samples were subsequently cleaned with Di-water, dried and stored at room temperature. 2.7 Statistical Analysis All experiments were repeated at least 3 times for all samples; AFM, SEM and EDAX measurements were performed on at least 3 different areas of each individual sample, for a total of at least 9 independent evaluations. All tables are presented as the average with (+/-) standard deviation values. All graphs are presented as the mean value of the number of replicates with (+/-) standard error (SE) bars.

3. Results and Discussion 3.1 Thin Films Formation We hypothesized that Matrix Assisted Pulsed Laser Evaporation (MAPLE) is a viable technique to form controlled interfaces of electrochromic (EC) composites. Using graphene oxide (GO) and tungsten trioxide (i.e., GO and WO3) as starting materials and MAPLE technique, we envisioned that possible impurities normally associated with methods such as sputtering,57 sol–gel or electrodeposition,58,

59

and

chemical wet deposition60 traditionally used for EC composite’s formation could be eliminated. To demonstrate our hypothesis and thus the feasibility of the proposed approach, we first deposited alternating, layer-by-layer thin film composites of GO and WO3 (i.e., GO/WO3) and compared them with composites deposited from starting GO and WO3 mixtures (i.e., GO+WO3). MAPLE deposited thin films of single components, i.e., GO and WO3 respectively, as well as drop casts of such components served as controls. All samples were characterized for their morphology using Optical Microscopy (OM) and Scanning Electron Microscopy 56,56 while Atomic Force Microscopy 55 performed in contact mode allowed for samples’ surface roughness evaluation.61 Our analysis showed that composites of GO/WO3 and GO+WO3 can be successfully obtained by MAPLE (Figure 1). Results revealed that such composites differ in their overall morphology and homogeneity (both from each other as well as from their individual controls), with the differences being due to their starting materials combinations/type and the deposition conditions being used (Supporting Material Figure S1) . Specifically, control MAPLE depositions of single GO component (Figure 1a) led to formation of thin films with visible random aggregates and flakes with average sizes of 2 µm (+/-200 nm). This was in contrast with the drop cast sample (Supporting Material Figure S2a, with cross-section shown in Figure S2b) that showed uniform repartitions of GO flakes with sizes from submicron all the way up to averages of 10 µm (+/-1µm). AFM roughness analysis also confirmed the formation of non-uniform, islands-like conformations (Figure 1a inset) and an overall root mean square roughness of about 68 nm for such sample,

while OM showed non-regular distributions in the films, i.e., MAPLE deposited ones as well as the control drop casts, Supporting Material Figure S3. MAPLE deposited WO3 thin films displayed a rather smooth geometry with isolated islands (1 µm+/150 nm average size; Figure 1b). This was in contrast with the drop cast sample of WO3 that revealed large micron sized particles (average size of 20 µm (+/-3µm), Supporting Material Figure S2c with cross-section anaysis shown in Figure S2d). The reduced sizes and smoother profiles of the MAPLE deposited WO3 films were also confirmed by AFM (Figure 1b inset; mean roughness was 13 nm) and OM (Supporting Material Figure S3c and S3d respectively), again, all relative to the drop cast samples. Alternating the individual components, i.e., GO and WO3, led to layer-by-layer thin films that displayed larger surface aggregates (average 1200 nm (+/-150 nm); Figure 1c) than the ones formed from the GO+WO3 mixture (average surface aggregates of 800 nm +/-100 n; Figure 1d). Only few GO flakes with lateral dimensions of up to 2.7 µm were observed for the mixed composites; this was in contrast with the films formed using the layer-by-layer method that displayed GO sheets with lateral dimensions of up to 5.2 µm. The smoother profiles were further confirmed by AFM analysis, with results showing an average roughness of about 38 nm for the GO/WO3 layer-by-layer composites (Figure 1c inset) and 33 nm for the GO+WO3 composite (Figure 1d inset) respectively. This was again in contrast with the results obtained for the drop cast films of both GO+WO3 (Figure 1e) and GO/WO3 (Figure 1f), which both contained randomly distributed and stacked layers of GO and WO3 of different thickness, as well as large micron-sized particle aggregates (20 µm+/-2 µm). Lastly, supporting OM (Supporting Material Figure S3e and Figure S3f respectively) also confirmed the smoother profiles obtained by MAPLE for both layer-by-layer and mixture composites, all relative to the ones obtained by drop casting.

Figure 1: Thin films of GO (a) and WO3 (b) obtained by MAPLE. Insets with individual sample’ roughness analysis as determined by AFM. MAPLE deposition of layer-by-layer (c) and mixtures (d) of GO and WO3 composites. AFM insets of selected areas of such samples with surface roughness of 38 nm for the layer-bylayer and 33 nm for the mixture respectively. Drop casts of layer-by-layer (e) and mixture (f) of GO and WO3 composites.

3.2 Mechanisms Associated with EC Composites Formation The uniform geometry and low roughness of the MAPLE deposited composites when compared to the drop cast controls was hypothesized to be due to the laser-induced transfer process of each component or mixture of components. Specifically, considering that the solvent used for samples’ preparation was water, a medium with low absorbance at 266 nm,62-64 it was expected that its local water sublimation will influence the overall transfer and deposition of any components being disolved in,65, coalescence processes.48,

67

66

as well as to induce local

Moreover, it was expected that the expanding water-vapor interface created

around the individual component or mixtures would influence the dynamics of the transfer process in itself, with the laser plume-induced species to be expelled from the target to depend on the individual target’s configuration, density and its local conformation/morphology at the irradiation site.43, 68 Our results further support the idea that there are two distinct regimes of transfer and material’s fragmentation respectively, with such regimes being more proeminent for WO3 relative to the GO, most likely due to the different local bond strength and level of stiffness of the materials themselves. Indeed, the absorbance of GO at 266 nm was shown to be ~0.9069 while absorbance of WO3 at the same wavelength is known to be ~1.23,70 which is ~137% of that of GO. Previous research has already showed that the materials adsorbing more energy will have an increased transfer kinetics at their interfaces because of their increased resulting thermal characteristics and desorption capabilities respectively.50 Such assumptions are further supported by reports that showed that GO has high levels of stiffness71 due to its hexagonally arranged sp2 bonded carbon atoms; this is in contrast with the lower stiffness and reduced thermal conductivity of WO3.72 Lastly, within the laser plume-induced species, it was expected that local heating and subsequent heat dissipation combined with water sublimation at the site of laser interaction and transfer will break the integrity of the individual components (i.e., GO or/ and WO3) found in the direct path of the laser beam, with the degree of fragmentation to influence the “amount” and “integrity” of the thin film subsequently deposited through a direct dependency on both the number of applied pulses used for transfer66, 73 as well as local distribution of such pulses onto the sample.65 These assumptions are supported by our analysis that

showed the presence of coalescence grains onto the films deposited by MAPLE, as well as a decrease of films’ surface roughness from ~68 to 13 nm, all relative to the roughness or conformation of individual components respectively (Figure 1 a-d insets). Moroeever, our results are comparable with previously reported ones, with the differences being presumably due to the different deposition conditions (changes in pressure) and target concentrations being used.64,

74

Specifically, an increase in pressure was previously

shown to occur during the MAPLE deposition process and was confirmed to slowdown the plume species being deposited because of the enhanced collisions in between individual components being transferred to ultimately influence samples’ morphologies.64

3.3 Elemental, Chemical and Electrochemical Characterization of the Composites The higher absorbance of the WO3 relative to that of GO (both in the mixtures as well as when as individual component) and the distinct regimes of transfer and material fragmentation indicated above were expected to further influence the elemental distributions within individual thin films. Indeed, analysis performed using Energy Dispersive Spectroscopy (EDAX; Figure 2a for GO/WO3 and Figure 2b for GO+WO3, with Supporting Material Table S1 for GO and Supporting Material Table S2 for WO3 thin films respectively) showed that the starting elements, i.e., C, O for GO and GO-containing samples and C, O, W for WO3 and WO3-containing samples respectively, were present. However, for the MAPLE deposited GO or WO3 films there were increases in the C contents, all relative to their drop cast counterparts. Further, the MAPLE deposited GO or WO3 films showed a decrease in their O contents relative to their drop cast counterparts (Supporting Material Figure S4). Analysis also revealed that W content decreased for the MAPLE deposited WO3 and layer-by-layer deposited composites when compared to drop cast WO3 control or MAPLE deposited GO+WO3 composite respectively. Specifically, the mass ratio of C:O was estimated to be 1:2 and 1:1 for the MAPLE deposited GO+WO3 and GO/WO3 composites relative to the drop cast GO+WO3 and GO/WO3 respectively. The observed increase in C content for both of the GO+WO3 or

GO/WO3 deposited composites relative to the drop cast samples was presumably due to the laser interaction with the respective materials and resulting induced processes, i.e., reduction and/ or oxidation. Indeed, previous research64 showed that when GO experiences fragmentation, molten clusters and droplets can form as result of a induced deoxygenating process. Also, the higher W content was presumably due to the films having a higher oxidation state when compared to their starting material (i.e., transition from the W+5 to W+6).75 Indeed, previous analysis showed that during the laser-induced heat dissipation at a target surface, partial reduction of W could occur, with the O being released to lead to the formation of nonstoichiometric WO3,76 a thermodynamically stable specie. Fourier Transform Infrared Spectroscopy (FTIR) confirmed chemical compositions of the samples, with results showing that the main vibrational signature of GO and WO3 as single component, mixed and multi-layered composites were changing with sample’s characteristic and deposition conditions respectively (i.e., MAPLE versus drop cast; Figure 3c). Specifically, the spectrum of GO showed typical adsorption peaks at 980, 1228 and 1780 cm-1 as associated with C-O (alkoxy), C-O (epoxy) and C=O stretching vibrations respectively.77 The peak at 1630 cm-1 was presumably due to the δO-H bending or ʋC=C stretching vibrations.78 The peak at 1725 cm-1 was presumably due to the ʋC=O stretching vibration of the carboxylic and/or carbonyl moiety functional groups, while the peak at 2840 cm-1 could be due to the ʋC-H stretching vibration.79 The large bands observed from 3000-3700 cm-1 was assigned to ʋC-H and ʋO-H stretching vibrations respectively and was presumably due to the presence of water as solvent.74,

80, 81

Similarly, when analyzing the IR vibrations of WO3, broad absorption peaks in the range 500–1000 cm-1 characteristic of the different ʋ(O-W-O) stretching vibrations were induced by the laser plume.82, 83 Below 1000 cm-1, the FTIR spectrum showed a broad band around 676 cm-1 which was attributed to the stretching of O-W modes, with the peaks of WO3 between 750 and 900 cm-1 being assigned to the asymmetric stretching vibration of the O-W-O.84, 85

Figure 2: a) Elemental composition of MAPLE deposited layer-by-layer (a) or mixtures (b) samples. FTIR analysis of the composites deposited by MAPLE helped identify single elements (black line-GO, red lineWO3) or mixed and multi-layered structures (blue line mixed GO+WO3 and orange line layer-by-layer GO/WO3) and composing corresponding chemical species.

FTIR spectra of GO+WO3 and WO3/GO composites revealed the presence of both GO and WO3 signatures, including the broad band in the range 3000-3700 cm-1 previously assigned to the O-H stretching vibrations of the adsorbed water.74, 80, 81 A widening of the band due to the stretching of the C–OH group was visible in the GO+WO3 thin films. The peak located at 1625 cm-1 could be either indexed to the O-H bending mode of the adsorbed water or ascribed to the O surface atoms and/or of carbonate groups of the thin films being formed.74 Comparison of the FTIR spectrum of GO, WO3 and GO+WO3 and GO/WO3 showed no additional peaks or losses of any peaks, thus indicating that no changes in the chemical compositions of the thin films occurred. This confirmed the “quality control” of MAPLE method over sputtering,57 electrodeposition,58,

59

or wet chemical methods60 known to introduce impurities at EC

deposition. Cyclic voltammetry (CV) measurements allowed direct qualitative assessment of the intrinsic capacitance of the deposited thin films. Analyses were performed from -1.0 to 1.0 V, at a scan rate of 50 mV/s (Figure 3b and c respectively). WO3 electrode displayed a typical cathodic peak at -0.2 V and an anodic peak at 0 V.86 For the GO+WO3 and GO/WO3 composites there were however shifts in the cathodic peaks as well as an increase in peak height which was more pronounced for the GO/WO3 relative to the GO+WO3 sample. This indicated that GO’s combination with WO3 led to changes in the overall electrochemical properties of the thin films used as electrodes, all dependend on the sample characteristics. Specifically, the cathodic peak of GO+WO3 composite appeared at -0.25 V, while the cathodic peak of the GO/WO3 composite appeared at -1.9 V. This suggests that the layer-by-layer thin films had a lower reduction potential relative to the films obatined from mixtures. The cathodic peak shift of GO/WO3 electrode to a positive potential also indicated that the layer-by-layer deposition was benefiting from a different mass transport process at its interface thus creating a different electronic environment than GO+WO3 mixture. This was presumably due to the fragmentation noted in Figure 1, with such fragmentation to potentially lead to embedment of WO3 (as the higher adsorbing material69) into the overall

GO structure. Indeed, this was supported by the shift of the cathodic peak to a more negative value for the GO+WO3 mixture, indicating that the reduction of WO3 was more difficult at this interface. A larger shift of the CV curve was observed for the GO+WO3 electrode relative to the WO3 electrode alone and was likely due to the uniform mixing of the GO and WO3 in the composite, as supported by analysis of sample’s morphology and lower roughness in Figure 1. The effect was not as pronounced for the GO/WO3 electrode; in particular, its lower surface uniformity and higher roughness led to a smaller shift in the CV curve relative to that observed for the WO3 electrode. Our analysis is supported by previous literature that showed that combining semiconductor materials with GO leads to reduced conductivity of a functional electrode87 due to the changes in electron transfer properties at the semiconductor-GO interface, however with such changes to depend on the morphological characteristics of the samples. Further, our results are in agreement with previous study showing similar electrochemical behavior of an electrochemically fabricated WO3 electrode cathodically polarized.88 Lastly, our analysis are comparable with the ones reported by previous studies, namely electrochemical deposition, 88

hydrothermal synthesis,89 glancing-angle magnetron sputtering,90 reactive sputter deposition,91

electrodeposition41 and spin coating,92 respectively (Supporting Materials Table S3), thus confirming that the unique topography of hierarchical structures does not only influence the charge transport at interfaces but further, enhances cyclic stability of an electrode.56 Considering that due to its increased thermal energy50 WO3 has an increased transfer kinetics at this interface relative to GO, and considering that the above CV analysis showed that combining WO3 with GO leads to changes in the conductivity of the resulting thin film composite relative to films obtained from individual component, we also evaluated whether such CV changes correlate with films thickness’s profiles. We aimed to establish whether film thickness could be an indicator of an electrochemical behavior to thus predict an overall coloration efficiency of the designed EC thin films. This hypothesis was formulated based on previous studies93,

94

that showed that design strategies could potentially allow for creation of larger

surface areas with improved structural stability and conductivity, and further, are the prerequisites for an electrochemically active electrode.95

Figure 3: a) CV curves and b) Thickness analysis of thin films prepared by MAPLE, namely GO, GO+WO3, WO3 and GO/WO3 respectively.

Film thickness was analyzed using photo reflectance spectroscopy and is shown in Figure 3b. Analysis confirmed that only slightly thicker films were obtained for the GO+WO3 relative to the GO/WO3 composite. This was presumably due to the fragmentation noted in Figure 1, with such fragmentation leading to embedment of the WO3 into the overall GO structure (also confirmed through cross-section analysis in Supporting Materials Figure S1) and thus as “easiness” in the deposition uniformity and efficiency. With GO absorbance at 266 nm being ~0.9069 and WO3 adsorbance at the same wavelength of ~1.23,70 it is expected that local “plume” embedment of the two components will also be proned to creating O deficiencies in the WO3-based thin films, similar to the ones noted in reports of WO3 doped with inorganic and organic dopants such as Fe2O3,40 TiO241 or chitosan.42 We envision that above deficiencies could potentially be used as predictors of CV performance and could be corroborated by local changes in the morphology or density of the WO3 distribution within MAPLE deposited thin films to influence electron-hole recombination induced processes.95 We also envision that controlling the MAPLE deposition conditions does not only allow for manipulation of the films characteristics (e.g., thickness) but further, could allow for user-designed EC capacity for the formation of next generation of EC thin films that have minimum interfacial defects while possessing maximum conversion efficiency. This assumption if supported by previous reports showing that WO3-based thin film of 500 nm thickness exhibit the maximum coloration efficiency (37.3 cm2/C),96 with other complementary studies revealing that the response time and coloration efficiency are thickness and roughness dependent and thicker films display significantly faster responses in coloration as well as higher durability and increased life cycle than thinner ones.97

4. Conclusions We demonstrated that MAPLE could be used to obtain controlled thin film composites of reduced roughness of WO3 and GO, or combination there are. We also demonstrated that control of both the deposition condition and sample characteristics influence the local transfer of each one of the starting

materials and further, their overall interfacing in composite’s formation. Our analysis showed that changes in local thin film morphology, as well as changes in the physicochemical characteristics and performance of the obtained EC composites are directly correlated with the ability of the composite to allow energy transfer at its interface. The above demonstrated laser induced transfer could make the combination of WO3 and GO not only easy to control but furthermore, easily implementable by ensuring local transfer interfaces to dictate composite materials applicability in EC or as energy saving platforms. In particular, it is envisioned that large scale integration of such thin films could lead to user-controlled local O doping capabilities which are expected to then influence thin films performance and thus help expand their implementation as energy saving platforms.

Conflicts of interest There are no conflicts to declare. Acknowledgements This work was funded by the National Science Foundation (NSF) grant 1454230. The authors acknowledge the

use

of

Shared

Facilities

at

West

Virginia

University.

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Supporting Material

Figure S1: Cross-section of GO and WO3 thin films deposited by MAPLE. Layer-by-layer (c; GO/ WO3) and mixtures (d; GO+WO3) are also shown. Scale bar: 5 µm.

Figure S1: a) Drop cast of GO and (b) WO3 coatings, with their respective control cross sections shown in (c) and (d) respectively.

Figure S3: a) Optical microscopy images of MAPLE deposited thin films of GO. b) Optical microscopy images of drop-casted thin films of GO. c) Optical microscopy images of MAPLE deposited thin films of WO3. d) Optical microscopy images of drop-casted thin films of WO3. e) Optical microscopy images of drop cast layer by layer and mixture (f) of GO and WO3.

Figure S4: Histogram showing the oxygen percentage comparison between the single element (WO3 ), mixed and multilayered structures (mixed WO3 and GO and multilayered coating WO3/GO) obtained by drop cast and the ones obtained by MAPLE.

Table S1:

MAPLE deposited GO

Control GO

AVERAGE

SD

%RSD

C

82.81

1.42

1.71

O

2.46

0.08

3.08

Si

14.74

1.47

9.97

C

63.13

0.93

1.48

O

36.47

1.16

3.18

Si

0.40

0.30

76.73

Elemental composition of MAPLE deposited films is drastically different than that of the drop-casted samples. This is presumably due to the degree of fragmentation resulted upon laser plume irradiation of the individual targets.

Table S2:

MAPLE deposited WO3

Control WO3

AVERAGE

SD

%RSD

C

17.66

0.11

0.61

O

44.82

0.39

0.87

Si

32.91

0.41

1.23

W

4.61

0.13

2.84

C

14.17

0.4

2.8

O

70.62

0.22

0.31

Si

2.24

0.16

6.91

W

12.96

0.2

1.58

Elemental composition of MAPLE deposited films is drastically different than that of the drop-casted samples. This is presumably due to the degree of fragmentation resulted upon laser plume irradiation of the individual targets.

Table S3.

Materials

Maximum

anodic Fabrication technique

peak current density (mAcm-2) GO+WO3

7.81

MAPLE

this study

GO/WO3

8.44

MAPLE

this study

WO3

9.09

MAPLE

this study

WO3

0.30

electrochemical

88

deposition WO3

0.358

hydrothermal synthesis

89

WO3

0.56

glancing-angle

90

magnetron sputtering WO3/Ag/W/WO3

0.6

reactive

sputter

91

deposition WO3

0.45

electrodeposition

41

WO3

0.89

spin coating

92

Electrochromic properties of the as-fabricated MAPLE GO+WO3, GO/WO3 and WO3 thin films relative to films fabricated using other techniques; corresponding references are also included.

Highlights -

Tungsten trioxide and graphene oxide were used to form electrochromic composites with controlled thickness and uniformity.

-

Matrix Assisted Pulsed Laser Evaporation (MAPLE) allowed composites deposition in a user-controlled fashion and with no impurities.

-

Demonstrated laser induced transfer led to energy transfer interfaces that dictated composite characteristics and applicability for energy saving platforms.

Conflicts of interest There are no conflicts to declare.