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Photocatalytic Anatase TiO Thin Films on Polymer Optical Fiber using Atmospheric-Pressure Plasma Kamal Baba, Simon Bulou, Patrick Choquet, and Nicolas D. Boscher ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b01398 • Publication Date (Web): 31 Mar 2017 Downloaded from http://pubs.acs.org on April 1, 2017

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ACS Applied Materials & Interfaces

Photocatalytic Anatase TiO2 Thin Films on Polymer

Optical

Fiber

using

Atmospheric-

Pressure Plasma Kamal Baba, Simon Bulou, Patrick Choquet and Nicolas D. Boscher* Luxembourg Institute of Science and Technology, Materials Research and Technology Department, L-4362 Esch-sur-Alzette, Luxembourg. KEYWORDS: Anatase TiO2, temperature sensitive substrate, side glowing polymer optical fiber, atmospheric pressure plasma deposition, photocatalysis

ABSTRACT: Due to the undeniable industrial advantages of low-temperature atmosphericpressure plasma processes, such as low cost, low temperature, easy implementation and inline process capabilities, they have become the most promising ‘next generation’ candidate system for replacing thermal chemical vapor deposition or wet chemical processes for the deposition of functional coatings. In the work detailed in this article, photocatalytic anatase TiO2 thin films were deposited at a low temperature on polymer optical fibers using an atmospheric-pressure plasma process. This method overcomes the challenge of forming

ACS Paragon Plus Environment

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crystalline transition-metal oxide coatings on polymer substrates by using a dry and upscalable method. The careful selection of the plasma source and the titanium precursor, i.e. titanium ethoxide with a short alkoxy group, allowed the deposition of well-adherent, dense and crystalline TiO2 coatings at low substrate temperature. Raman and XRD investigations showed that the addition of oxygen to the precursor’s carrier gas resulted in a further increase of the film’s crystallinity. Furthermore, the films deposited in the presence of oxygen exhibited a better photocatalytic activity towards methylene blue degradation, assumedly due to their higher amount of photoactive {101} facets.

INTRODUCTION Titanium dioxide (TiO2), preferentially in the anatase phase, has become arguably the most studied wide-band gap semiconductor photocatalyst since the discovery of its photocatalytic water splitting activity by Fujishima and Honda in 1972.1 Under UV irradiation, electrons are raised to the conduction band and holes created in the valence band of anatase TiO2. The electron-hole pairs created can then participate in various oxidation and reduction reactions at the surface of anatase TiO2. Anatase TiO2, both in nanomaterial and thin film forms, has thus been successfully employed as photocatalyst material for the oxidation of organic compounds2 or antimicrobial purposes for water and air decontamination.3 To better separate the aqueous or gaseous media from the nano-sized photocatalysts, a fixed-bed system has been developed, in which the photocatalysts are immobilized on the walls of the reactor or on the supported substrates around the light source.4,5 However, the


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effective reactive area between photocatalysts and contaminants in these reactors is smaller than that of classical suspended-bed systems. One of the solutions to increase the reactive area was the use of a large surface area substrate, i.e. active carbon, zeolite and hollow microbeads.6–8 However, since the turbidity of the medium is increased with the suspended particles, a shadowing effect appeared leading to a decrease of the light-depth penetration and therefore drastically lowering the photocatalytic reaction rate.4,9 The idea of using optical fibers (OFs) as both a means of light transmission and support for photocatalysts was originally proposed and theoretically evaluated by Ollis and Marinangeli in the late 1970s and early 1980s.10–12 Several authors have developed, characterized, and modeled optical fiber reactor (OFR) systems for water treatment13–15 and others have demonstrated the feasibility of TiO2-coated OFs for the photocatalytic oxidation of organic compounds.16–18 Plastic or polymer optical fibers (POFs), and more particularly side-emitting POFs, can be also used as photocatalyst support for OFRs. Usually, polymethylmethacrylate (PMMA) and fluoropolymers are used as core and cladding polymers, respectively. In contrast to quartz or silica, used as the core material for glass optical fiber, PMMA is very ductile, cheap and facilitates the production of optical fiber with a large core size.19 Joo et al.20 compared POFs and quartz OFs (QOFs) as lighttransmitting media and substrates for their potential use in photocatalytic environmental purification systems. They concluded that the use of POFs is preferred to QOFs due to several significant advantages, such as their ease of handling, lower cost and relatively reasonable light attenuation at the wavelength near 400 nm. Guillard et al.21,22 reported in several works the possibility of air treatment and water decontamination using new photocatalytic textiles developed by Brochier® Technologies Company. This photocatalytic


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textile is based on POFs (PMMA/fluorinated polymer), textile fibers (polyester) and photocatalytic TiO2 coating, The POFs ware mechanically micro-textured to permit a side emission of UV light. Over the past 40 years, many techniques have been investigated to form anatase TiO2 thin films with effective photocatalytic properties, including sputtering,23 sol-gel,2 atomic layer deposition (ALD)24 and chemical vapor deposition (CVD).25,26 To date, sol-gel is the method of choice for the deposition of TiO2 thin films on OFs. Nevertheless, such an approach has either required long-winded steps to directly form anatase TiO2 thin films or involves an annealing treatment at 200-450°C.16,18,27–29 The use of sol-gel processes with novel precursors, such as ethylene glycol-modified titanate (EGMT), was reported as a promising low-temperature route towards the deposition of crystalline TiO2 thin film for photocatalytic30 and solar cells31 applications. However, annealing steps at a relatively low temperature (90-130°C) were required to form crystalline TiO2 films. In recent years, alternative plasma-enhanced CVD (PECVD) processes operating at low or intermediate temperatures have been investigated to overcome the existing drawbacks of classical deposition methods. Anatase TiO2 thin films have notably been grown from titanium isopropoxide (TTIP) at temperatures as low as 150°C using an inductively coupled radio frequency (RF) plasma operated under vacuum.32 Anatase TiO2 structures have also been grown using various atmospheric pressure PECVD setups. Both atmospheric-pressure dielectric barrier discharge (AP-DBD)33,34 and atmospheric pressure plasma jet35 fed with titanium tetrachloride (TiCl4) have been demonstrated to form nanocrystalline anatase TiO2 powders at temperatures as low as 150°C. Atmospheric pressure plasma torches have also been reported to successfully lead to the low-temperature deposition (i.e. 165°C) of


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anatase TiO2 nanocrystallites when using TTIP36 or titanium bis(acetylacetonate) diisopropoxide (TIPO).37 However, the deposited films appeared loose and powdery due to excessive gas-phase reactions promoted by the high reactivity of the titania precursors combined with the highly reactive plasma environment.36 Hence, significantly higher substrate temperatures (i.e. 215°C) were required to form dense and highly photocatalytic TiO2 coatings.37 Among the PECVD methods, surfatrons are a well-known microwave (MW) plasma source that allows the coupling of electromagnetic wave energy to the plasma via surface waves. They can produce a high plasma density at relatively low microwave (MW) power.38–40 Furthermore, they provide a very high degree of ionization in the active region of plasma with an electron density which can reach values of up to 1020-1021 m-3 and high discharge stability with low fluctuation of plasma parameters,41 which are highly compatible with the formation and deposition of crystalline transition metal oxides. On the other hand, the surfatron geometry allows the introduction of a dielectric substrate in its cavity (Figure S1 and Figure S2). If centered far from the surface discharge, temperaturesensitive substrates, such as POFs, can even be introduced in the MW cavity and readily coated. In the present work, we investigated for the first time the atmospheric-pressure and low-temperature PECVD of crystalline TiO2 films on POFs. Titanium ethoxide (TEOT) was selected as the titanium precursor.42 In the first part of this work, crystalline TiO2 thin films are deposited on POFs, and in the second section, the deposition parameters are tuned with the aim of improving the photocatalytic properties of the films evaluated from the degradation of methylene blue (MB) and monitored by UV-Vis spectrophotometry. The structural and morphological properties of the deposited thin films were investigated with


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Raman spectroscopy, X-ray diffraction (XRD) and scanning electron microscopy (SEM). Chemical composition was investigated with X-ray photoelectron spectroscopy (XPS) and energy dispersive X-ray spectroscopy (EDX).

RESULTS AND DISCUSSION

Atmospheric-pressure and low-temperature plasma deposition of anatase TiO2 coatings on polymer optical fibers Our strategy towards the atmospheric-pressure and low-temperature deposition of photocatalytic anatase TiO2 coatings on polymer optical fibers relies on the use of a microwave plasma discharge fed with argon and generated in a quartz tube through a surfatron device. The selected atmospheric-pressure microwave plasma discharge provides a surface discharge inducing a high degree of ionization and high electron density on the inner surface of the quartz tube (Figure S1).43 In contrast, the central part of the tube is far less reactive, with temperatures compatible with the use of polymer substrates. Thus, 1D polymer substrates, i.e. POFs, were introduced and passed without alteration in the central zone of the quartz tube (Figure 1). Subsequently, the POFs were coated with a whitish coating thanks to the injection of a TiO2 precursor, i.e. TEOT, carried to the postdischarge zone using a pure argon stream.


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Figure 1. Schema of the microwave plasma setup for the atmospheric-pressure and lowtemperature roll-to-roll deposition of anatase TiO2 on polymer optical fibers.

SEM observation of the coated POFs reveals the deposition of a homogeneous thin film featuring spherical shape particles with a diameter ranging between 30 and 150 nm (Figure 2a). The film thickness is about 200 nm for a 120 s treatment time (Figure 2b). EDS analysis of the film (Figure 2c and d), indicates that the surface is composed of titanium (ca. 18%) and oxygen (ca. 52%), originating from the deposited film and a non-negligible carbon content (ca. 29%), most possibly arising from the coated POFs such as later confirmed by XPS analysis. The presence of platinum peaks is attributed to the metallization of the sample. Raman spectroscopy analysis of the TiO2 thin film grown on the PMMA optical fiber highlights the formation of the anatase TiO2 phase (Figure 2e). The three peaks, at 146, 397 and 637 cm-1, corresponding to the Eg (ν6), B1g (ν4) and Eg (ν1) optical vibration modes,


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respectively, are characteristics of the anatase phase of TiO2.44,45 Peaks observed at 290, 385, 732 and 749 cm-1 are all attributed to the PMMA optical fiber.

Figure 2. (a) Top view and (b) cross-section SEM images of a TiO2 thin film deposited from TEOT on to optical fibers using a microwave plasma discharge fed with argon. For ease of the cross-section preparation, silica core optical fibers with a polyimide cladding were employed for the cross-section sample. (c) Chemical composition (d) EDS spectra and (e) Raman spectra of anatase TiO2 thin films deposited from TEOT on to PMMA optical fibers using a microwave plasma discharge fed with argon. The Raman spectrum of an uncoated PMMA optical fiber is also plotted as a reference. (f) Photography of a TiO2-coated side glowing polymer optical fiber, connected to a white visible light source.

Effect of O2 on the morphology, structure and composition of the TiO2 coatings The addition of oxygen to plasma processes is known to influence the composition, structure and properties of transition metal oxide thin films.46–49 In order to investigate the


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benefit of oxygen addition and further evidence the potential of the proposed approach, various admixtures of oxygen to the argon carrier gas were studied (i.e. 80%/20%, 50%/50%, 20%/80% and 0%/100% argon/oxygen carrier gas mixtures). Our investigations further revealed that the thin films’ chemical and physical properties were extremely similar for all films prepared where oxygen was present in the carrier gas. Therefore, for the clarity of the present paper, only the samples prepared from a 100%/0% (pure argon) and 50%/50% (mixed Ar/O2) argon/oxygen carrier gas mixture are reported. In the present section, the deposition experiments were pursued on more conventional 2D substrates, i.e. small silicon wafer pieces, to allow the various characterizations undertaken and ensure the accuracy of the photocatalytic properties evaluation. Irrespective of the oxygen concentration in the argon carrier gas, the coatings formed exclusively on the central zone of the silicon wafers, which corresponds to the position where the POFs were previously placed. The films, which exhibit a whitish and smooth appearance, are adherent to the substrate and homogenous over a length of 5 ± 1 mm in the direction parallel to the reactor’s longitudinal axis (Figure S3). SEM analyses of the films deposited on Si substrate for the two different TEOT carrier gas compositions investigated, i.e. pure Ar and mixed Ar/O2 (Figure 3), reveal surface morphologies very similar to that observed for the TiO2 thin film previously deposited on polymer optical fiber, with numerous spherical shape particles with a diameter ranging between 30 and 150 nm (Figure 3 a and c). Similarly, the film thickness, measuring between 500 and 700 nm on the SEM cross-section observations (Figure 3 b and d), was not significantly affected by the O2 addition to the TEOT carrier gas flow.


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Figure 3. Top-view and cross-section SEM images of TiO2 thin films deposited on Si substrate as a function of the TEOT carrier gas composition: (a and b) pure Ar and (c and d) mixed Ar/O2.

XPS analyses of the films do not reveal any significant disparities in their chemical composition. Elemental composition in the bulk of the films, measured after surface etching by Ar+ (Table 1) show that the films contain oxygen and titanium with an O/Ti ratio of 1.8. Overall, less than 3 at.% of carbon was detected in the bulk of the coatings.

Table 1. Relative atomic composition in the bulk of the TiO2 coatings after 500 s Ar+ etching as a function of the TEOT carrier gas composition. Carrier gas composition

O1s [at.%]

Ti 2p [at.%]

C1s [at.%]

Pure Ar

62

35

3

Mixed Ar/O2

63

35

2

For the XPS analyses performed without etching, the binding energies associated with the Ti 2p peaks at 458.9 and 464.7 eV, were identified as Ti–O bonds of Ti4+ in TiO2 (Figure 4). The separation between the Ti 2p3/2 and Ti 2p1/2 peaks is 5.7 eV for all films. No Ti3+


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peak was present in the surface samples. The O 1s peak (Figure 4) comprises two contributions at 530.4 and 532.1 eV. The major one at 530.4 eV is attributed to oxygen in TiO2, while the second one at 532.1 eV is attributed to the presence of carbon contamination (C–O) at the surface of the samples.50 These values, as well as the fact that titanium is present with only one valence state (Ti4+), are characteristic of a stoichiometric surface. After argon etching, the presence of a shoulder on the low binding energies side of the Ti 2p peaks, characteristic of the presence of Ti3+, is observed (Figure S6).51 This observation is not surprising, since TiO2 is known to become non-stoichiometric upon argon ion bombardment. This is a result of the removal of oxygen from the surface, caused by the preferential sputtering phenomenon.51

Figure 4. High-resolution spectra of the Ti 2p (left), O 1s (middle) and C 1s core levels (right) for the XPS surface analysis of the TiO2 coatings elaborated from two TEOT carrier gas compositions: pure argon and mixed Ar/O2.

Figure 5a shows the Raman spectra of TiO2 thin films grown on Si substrate with a pure Ar and mixed Ar/O2 as TEOT carrier gas composition. Three peaks at 146, 397 and 637 cm-


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characteristic of the anatase phase of TiO2.44,45 are observed for both of the films. These

peaks correspond to the Eg (ν6), B1g (ν4) and Eg (ν1) optical vibration modes of anatase TiO2, respectively. The Raman spectra clearly confirm that anatase TiO2 is the only crystalline phase in the films.

Figure 5. (a) Raman spectra of TiO2 thin films on Si substrate as a function of TEOT carrier gas composition. (b) XRD patterns of the TiO2 thin films on Si substrate for different carrier gas composition. The peak marked by a circle (○) originates from the aluminum substrate holder of the XRD equipment.


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The XRD patterns of TiO2 films deposited on Si substrate with or without the addition of O2 to the TEOT carrier gas flow are presented in Figure 5b. The peaks observed at 2θ = 25.3°, 37.8° and 48.1°, correspond to the anatase lattice planes (101), (004) and (200), respectively. The film deposited with the pure argon carrier gas are strongly orientated along the (004) axis. The addition of oxygen to the carrier gas results in an increase of the (101) peak intensity, indicating a preferential crystal growth along the (101) plane. The TiO2 crystallite size, estimated from the (101) peak using the Scherrer formula, was not observed to be significantly influenced by the O2/Ar ratio. The crystallite size is 19 ± 1 nm. The high resolution XRD spectra of the (101) peak and its fitting can be consulted in the supporting information (Figure S7). The addition of O2 to the argon post-discharge obviously influenced the growth of the TiO2 thin films. It is well-known that an atmospheric-pressure argon plasma jet can interact with species intentionally injected in its stream (such as in the present work) or originating from the surrounding open-air atmosphere. As a consequence, reactive species such as OH, O, excited N2, NH and Ar metastable can be formed.52 More particularly, some reactive oxygen species (ROS) can be produced from the O2 introduced in the post discharge by energy transfer and/or Penning effects.53,54 Indeed, exited oxygens such as O(5P) and O(3P) can be generated by energy transfer from argon metastable (Arm) in the post-discharge (O2 + Arm → O (3P) + O (3P) + Ar;

O2 + Arm → O (3P) + O (1D) + Ar

and

O2 + Arm → O (3P) + O (1S) + Ar).52–54 In addition, O2+ and O+ can be created due to the charge transfer mechanisms (O2 + Ar+ → Ar (1S0) + O2+ and O (3P) + Ar+ → Ar (1S0) + O+) thanks to the Penning ionization of argon (Ar (4sj) + Ar (4si) → Ar (1S0) + Ar+ + e-).53 The


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formation of ROS is believed to promote the crystal growth of the titanium dioxide thin films.

The effect of O2 on the photocatalytic properties of the TiO2 coatings The photocatalytic performances of the TiO2 coatings elaborated from different carrier gas compositions were evaluated by following the degradation of MB under UV light. The degradation curves are available in the supporting information (Figure S8), while the degradation rates of MB are presented in Figure 6a. The curves of ln(C0/C) versus irradiation time are linear (Figure S8b), indicating a good correlation to pseudo first order kinetic law (ln(C0/C) = kt).55 The apparent degradation rate constants of the TiO2 thin films k (h-1) were extracted from the plotting of ln(C0/C) versus the irradiation time. As shown in Figure 6a, the degradation rate of MB is significantly higher when oxygen is added to the carrier gas (i.e. 0.055 h-1). In contrast, the degradation rate of the TiO2 coating deposited from pure argon is 4 times weaker (i.e. 0.013 h-1). These differences are more likely due to the change of crystal growth orientation after adding oxygen to the carrier gas, as shown in the XRD patterns (Figure 5b). Indeed, several works reported that anatase TiO2 thin films with predominant {001} facets exhibit lower reactivity in photooxidation reactions for OH radical generation and photoreduction reactions than those with predominant {101} facets.56–58 It has also been concluded that the true photoreactivity order of crystalline facets of anatase is {010} > {101} > {001}.56,59 Since the anatase TiO2 thin film deposited with pure argon as the carrier gas has a preferential crystal growth along the (004) plan, this coating contains fewer photoreactive {101} facets than the TiO2 coating deposited with Ar/O2 as the carrier gas and by consequence, is less photocatalytically active.


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Figure 6. First order rate constant calculated from the linear plot of ln(C0/C) versus the irradiation time for the photodegradation of methylene blue over TiO2 thin films deposited on (a) Si wafers from different carrier gas compositions (pure Ar and mixed Ar/O2) and (b) polymer optical fibers (POFs) from a mixed Ar/O2 carrier gas composition.

Photocatalytic anatase TiO2-coated polymer optical fibers To undoubtedly demonstrate the potentiality of the developed AP-PECVD for the coating of POFs and the elaboration of OFRs for the photocatalytic removal of pollutants, the MB degradation over TiO2-coated POFs was carried out under UV-A light in a quartz cuvette containing 3.5 mL of the MB solution and 3 samples of 2 cm long TiO2-coated POFs (ФPOF = 2 mm). A mixed Ar/O2 carrier gas composition, corresponding to the 50%/50% Ar/O2 admixture was employed to coat the POFs. In consistence with the results presented in the previous sections, the physico-chemical characterizations of the TiO2-coated POF samples revealed the formation of anatase TiO2. For the photocatalytic performance evaluation of the TiO2-coated POFs, the UV-A irradiation was delivered from outside the


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POFs and through the MB solution and the quartz cuvette walls. The UV-A light intensity on the surface of POFs is evaluated to 2 mW·cm-2, while the total photocatalytic active TiO2 surface exposed to the UV-A light is estimated to be 0.6 cm2 (data are normalized to a surface of 1 cm2 for the sake of comparison with the TiO2 coatings deposited on Si substrates). As depicted in Figure 6b, the TiO2-coated POFs exhibit good photocatalytic activity with a degradation rate constant evaluated at 0.046 h-1. One should note that in addition to involving smaller surfaces (i.e. 0.6 cm2 vs 1 cm2), the MB degradation experiments from TiO2-coated POFs involved a greater amount of MB solution (i.e. 3.5 mL) than the TiO2-coated Si wafers ones reported above (i.e. 2 mL). Thus, taking into account both the sample surfaces and MB solution volumes ratios, the photocatalytic performance of the TiO2-coated POFs (i.e. 0.046 h-1) is highly comparable to the normalized value obtained for the TiO2-coated Si wafers (i.e. 0.055 h-1). In comparison to other works, this degradation rate is similar to the degradation rate of 4-chlorophenol obtained by Hofstadler et al.13 using TiO2-coated glass fibers. On the other hand, Ji et al.60 obtained a lower MB degradation rate with a 500 nm thick TiO2-coated optical fiber (i.e. 0.012 h-1). However, the degradation rate was enhanced more than ten times when the coating thickness was increased above 1 µm. In conclusion, the excellent photocatalytic activity of the TiO2-coated POFs authenticate the suitability of the proposed PECVD approach for the fast and atmospheric-pressure deposition of photocatalytic anatase TiO2 on temperaturesensitive substrates.

Among the numerous routes towards the deposition of anatase TiO2 thin films, the APPECVD method developed is particularly suitable for the treatment of polymer optical


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fibers and the preparation of OFRs for the photocatalytic decontamination of water or air. The careful selection of the TiO2 precursor, i.e. TEOT, ensured the deposition of dense and crystalline TiO2 thin films in contrast to the more conventional TiO2 CVD precursors, i.e. TTIP and TiCl4. Indeed, when employing TTIP with the setup described in the present work, powdery and poorly crystalline coatings with low adherence and low photocatalytic degradation rates were obtained (Figure S8). Such observation is consistent with previous PECVD results, which reported the formation of dense and smooth coatings when using TEOT and rough coatings composed of nanoparticles when using TTIP.61 Previous works also reported that TiO2 thin film growth mechanisms, responsible for the resulting chemistry, crystallinity and morphology of the films, are influenced by the length of the precursor’s alkoxy groups. Notably, the dissociation rate of a titania precursor with short alkoxy groups is known for being faster than those possessing long alkoxy groups.61 TEOT, with shorter alkoxy groups than TTIP, is expected to react more efficiently and generate a higher concentration of reactive species in the gas phase. These reactive species enhance the amount of absorbed species at the substrate surface forming many nucleation sites that promote the growth of denser and more crystalline coatings with smoother surfaces. In contrast, TTIP, with longer alkoxy groups and a slower dissociation rate, induces a lower amount of reactive species upon exposure to the plasma. Therefore, the few nuclei formed during the early stage of the reaction coexist on the substrate surface with unreacted precursor molecules that negatively affect the crystallinity and morphology of the films. One should also note that in the present work, the thin films produced from TEOT exhibited much higher growth rates (i.e. 1.25 nm·s-1) than those grown from TTIP (i.e. < 0.2 nm·s-1) under identical deposition conditions, including the precursor delivery rates (i.e.


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10 μL·min−1). Such an observation is consistent with the previous remarks but also with the work of Ritala et al.62 that reported an evident inverse relationship between the growth rates of TiO2 films and the size of precursor molecules. In addition, it was found that the films grown from the alkoxides were more crystalline than those deposited from TiCl4. This result was correlated to an ordering effect of the alkoxide ligands bridged between titanium cations on the surface of the growing film.62

Since most of the OFs are poorly transparent in UV light, a further improvement currently under investigation is the deposition of doped TiO226 and plasmonic TiO2 thin films for visible light photocatalytic activity. Such developments would allow the widespread use of POFs for the elaboration of OFRs that make a better use of the solar spectrum. It would also provide a viable alternative to quartz OFs (QOFs) that require the mechanical or chemical removal of their cladding,14,16,18 inducing brittleness63 in the QOFs and limiting the OFR size. One should also mention that our strategy towards the atmospheric-pressure and lowtemperature deposition of photocatalytic anatase TiO2 thin films is not specific to the coating of OFs and could be implemented on other temperature-sensitive 1D substrates, e.g. polymer or textile fibers. In addition, the described method, highly compatible with industrial processes, may be applicable to the low-temperature synthesis and deposition of other crystalline transition metal oxide thin films and reach other functionalities, such as electrical conductivity or electrochromic (WO3) or thermochromic coatings (VO2).

CONCLUSION


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Anatase TiO2 thin films were successfully deposited at atmospheric pressure and low temperature on polymer substrates using a single-step plasma deposition process involving the use of a microwave plasma discharge fed with argon. In addition to affording a fast and dry low-temperature route towards the formation of crystalline oxide coatings, our inherently scalable AP-PECVD process is particularly suitable for the roll-to-roll coating of wire substrates. The use of a titanium precursor with a short alkoxy group length, i.e. titanium ethoxide, carefully injected in the post-discharge zone allows the deposition of well-adherent, dense and crystalline TiO2 coatings. Irrespective of the carrier gas composition, i.e. pure argon and argon/oxygen mixture, the films had a stoichiometry close to TiO2 and exhibited the characteristic Raman and XRD spectra of anatase. When O2 was added to the carrier gas, the crystal growth of the films switched from a (004) to a (101) preferential orientation. As a consequence, the films deposited in the presence of O2 in the carrier gas possessed a better photocatalytic activity towards the degradation of MB due to the higher amount of photoactive {101} facets in those films. Finally, the suitability of the described AP-PECVD method was demonstrated for the coating of polymer optical fibers towards the preparation of a new generation of optical fiber reactors for the photocatalytic decontamination of water and air.

EXPERIMENTAL SECTION Plasma-enhanced chemical vapor deposition and materials. The plasma discharge was generated at atmospheric pressure inside a quartz tube of 5 and 7 mm inner and outer diameter, respectively (Figure S2). The surface-wave discharge is produced by applying a high-frequency electric field to a gas (Argon by Air Liquide, 99.999%) flowing in a


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discharge tube by means of a surfatron wave launcher. The MW generator with a frequency of 2.45 GHz is operated in continuous mode with a power of 200 W. The wave launcher (Surfatron 80 by SAIREM) is powered via a coaxial cable and cooled by water. The discharge tube is cooled by compressed air and its end is at a distance of 3 cm from the launching gap. The flow of discharge gas, i.e. argon, is maintained at 10 standard liters per minute (slm) by a MKS mass flow controller. TEOT (Sigma-Aldrich, 80%) was used as the titanium precursor. Since this chemical compound is viscous at room temperature, it was diluted in hexane (Sigma-Aldrich, ≥ 97%) to 0.5 M. In order to limit the excessive formation of powders in the gas phase, which is detrimental for the thin film deposition, the precursor flow rate is fixed at 10 μL·min−1 and carried toward an ultrasonic nebulizing nozzle operating at 120 kHz (Sono-Tek) with a syringe pump. The mist formed at the outlet of the nozzle, comprising droplets with diameters ranging between 10 and 20 μm, is carried with a 1 slm gas flow and injected in the post-discharge zone at a distance of 6 cm from the launching gap (Figure 1). In an attempt to improve the photocatalytic properties of the TiO2 thin films, the 1 slm carrier gas composition was tuned from pure argon to a pure oxygen (Air Liquide, 99.999%) mixture. Both side glowing PMMA (2 mm diameter; Luxylum) (Figure 2f) and silica core with polyimide cladding (0.8 mm diameter; CeramOptec) optical fibers and double side-polished silicon wafers (2 × 2 cm) were employed as substrates. For the OF substrates, 1 m-long pieces were placed from the left to the right side of the experimental setup and through the discharge tube in a horizontal position. In the present work, all the deposition experiments were performed in static mode for a total duration of 2 min. The characterized area of the


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coated OFs was located in the post-discharge region (from 5 to 8 cm from the launching gap) and close to the precursor injection outlet (i.e. 6 cm from the launching gap) during the deposition experiments. Silicon substrates were cleaned using absolute ethanol (97%) and dried with nitrogen prior to use. During the deposition experiments, the Si substrate was placed in the deposition chamber in the same horizontal position as OF substrates, at a distance of 6 cm from launching gap. The deposition time for the experiments carried out on Si wafers was 10 min. Thin film characterization. Scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectrometry (EDX) (Hitachi SU-70 FE-SEM), was employed to characterize the thickness, morphology and chemical composition of the coatings. Prior to SEM observations, the nonconductive sample was sputter-coated with 5 nm of platinum to prevent image charging and distortion. XPS analyses were performed on a Kratos AxisUltra DLD instrument using a monochromatic Al Kα X-ray source (hυ = 1486.6 eV) at pass energy of 20 eV. AES was performed on a ThermoVG Microlab 350 operating at 20 kV, 3 nA and 30°. An argon sputtering operating at 2 keV and 2 mA was used for approximately 500 s in a rastering mode in order to remove surface contamination and gain information on the elemental composition in the bulk of the coating. The peak positions of the samples were referenced with respect to carbon (C 1s) at 285.4 eV. The Raman spectra were recorded with a Renishaw inVia micro-Raman spectrometer at an excitation wavelength of 532 nm with a laser power of approximately 0.44 mW focused on a 1 µm2 spot. X-ray diffraction (XRD) measurements were carried out at a grazing angle of 5° with a Bruker D8 instrument using CuKα radiation (λ = 1.54 Å) operating at 40 mA and 20 kV. Crystallite sizes of TiO2 were calculated using the Scherrer equation:


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D = kλ/βcosθ

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(1)

where D is the crystallite size, λ the wavelength of X-ray radiation, K the shape factor constant usually taken as 0.9, β the peak width at half-maximum height, and θ is the diffraction angle. Photocatalytic activity measurements. The photocatalytic activity of TiO2 films under UV-A light was evaluated by measuring the photodecomposition of methylene blue (MB) in aqueous solution (C16H18N3S–Cl–3H2O; 0.05 wt.% in H2O, Sigma-Aldrich) with an initial concentration of 10 µmol·L−1 and a volume of 2 mL. For the ease of the study, the photocatalytic activity of the produced thin films was evaluated from a series of coated silicon wafers. The 1 cm2 samples were immersed in the dye solution in a 12 well plate (each sample in a different well). The well plate containing the samples and the dye was stirred using an orbital shaker (250 rpm) under irradiation from a 16 W black light lamp (Herolab; 365 nm, 2 mW·cm−2). Before light irradiation, the solutions were stirred in the dark for 1 h to ensure the establishment of an adsorption–desorption equilibrium. Photocatalytic degradation was monitored by measuring the absorption spectra of the MB solution at 664 nm with a UV-Vis spectrophotometer (Tecan Infinite M1000 Pro). To confirm the potential of the proposed approach, the same procedure was applied to POFs coated with the best-identified TEOT carrier gas composition.

ASSOCIATED CONTENT Supporting Information


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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.xxXxx0x Microwave plasma discharge and surfatron device; Photography of the TiO2 coating; Thin films growth rates estimation; Thin films characterizations; Photocatalytic activity of the TiO2 thin films; Morphology of a TiO2 thin film elaborated from Titanium isopropoxide (TTIP).

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] ORCID Kamal Baba: 0000-0002-8405-035X Nicolas D. Boscher: 0000-0003-3693-6866

ACKNOWLEDGMENT The authors would like to gratefully acknowledge the financial support of the Luxembourgish “Fonds National de la Recherche” through the CORE project PLASMONWIRE (C13/MS/5894615). Jérôme Guillot, Yves Fleming, Miguel QuesadaGonzalez, Jean-Baptiste Chemin and Lindsey Auguin from LIST are also acknowledged for their contribution.


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Photobiol. A Chem. 2015, 307–308, 88–98.


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Table of Contents /Abstract Graphic


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