Nanostructure and photocatalytic properties of TiO2 films deposited at low temperature by pulsed PECVD

Abstract The nanostructure and photocatalytic properties of TiO2 thin films deposited by PECVD on silicon substrates were investigated. The films were grown at low temperature (


Introduction
TiO 2 thin films have attracted intensive research as photocatalysts for their advantages in chemical stability, non-toxicity, commercial availability at a low cost and high photocatalytic activity [1][2][3]. The TiO 2 properties are due to its crystalline form which is mainly encountered as three phases including tetragonal anatase, tetragonal rutile and orthorhombic brookite [4].
TiO 2 exhibits different band gap energies for its different phases, viz. anatase (3.2 eV), rutile (3 eV) and brookite (3.1 eV) [5][6][7][8]. In addition to the effect of TiO 2 crystallization, some other essential factors such as grain size, preferred grain orientation, presence of structure defects and capability to absorb hydroxyl groups in the initial phase of excitation may also affect the photocatalytic activity [9][10][11]. Powders suspended in polluted solution due to their higher specific area may lead to more efficient photocatalytic properties, however films do not require filtering to be recovered after the volume to be depolluted has been treated. These films can be prepared by using different methods including sol-gel method [12][13][14][15][16][17][18][19], which is the most often used to synthesize TiO 2 thin films for photocatalytic applications. However other strategies were also largely investigated, such as liquid phase deposition [20], magnetron sputtering [21][22][23], atomic layer deposition [24], spray pyrolysis [25], chemical vapor deposition [26], metalorganic chemical vapor deposition [27] and plasma enhanced chemical vapor deposition (PECVD) [28][29][30][31][32]. Each method may lead to the growth of particular structure and film morphology, which may be quite distinctive and very difficult to repeat with another method. These particularities, on the other hand, may have a substantial effect on the overall photocatalytic activity of the material.
In the literature, titanium dioxide in anatase is generally reported to be obtained at high temperature, in the range of 400-600 °C [33]. However, plasma assisted deposition (magnetron sputtering, PECVD) is known to allow decreasing the deposition temperature of many materials due to the bombardment of the growing film by energetic neutral and charged species. Nevertheless, in the case of TiO 2 , the deposition of anatase thin films by magnetron sputtering or PECVD below 150 °C was reported by only few authors [23,34].
In our previous studies, PECVD processes were used to yield some nanocolumnar structures with different phases including amorphous, anatase and rutile [35][36]. Actually these processes exhibit favorable features such as low deposition temperature, good film adhesion, high purity, good step coverage and easy control of reaction parameters [28,37]. Enhancement of chemical reactions by means of low temperature plasma has been known and used for many years. In particular, some new nanostructured materials that cannot be obtained under equilibrium conditions could be prepared using chemical reaction under non equilibrium conditions [38][39][40][41], and in the area of property-altering surface coatings [42][43].
For photocatalytic applications, in order to reduce the substrate temperature during deposition while maintaining a sufficient energy input to obtain TiO 2 in crystallized form, the PECVD deposition can be controlled in pulsed plasma mode. In this mode, the electrical power is periodically applied to the gas, so that two times are then defined: the discharge time, when the power is applied for a time denoted T on , and the post-discharge time, when the power is switched off for a time denoted T off . Varying the plasma-on time to the period time has two important effects [44][45]: (1) to vary the mean power coupled to the plasma and hence the deposition temperature; (2) to allow the control of the ion to neutral flux ratio, which strongly influences the nanostructure and crystallinity.
In this work, we investigate the influence of the duty cycle (DC), defined as the discharge time to the period ratio, on the structural and photocatalytic properties of the TiO 2 films deposited on silicon substrates in pulsed PECVD mode. The latter are also compared to the film deposited in continuous mode. The aim is both to gain insight into the influence of pulsed parameters on the structure of the deposited films by transmission electron microscopy (TEM), scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray diffraction (XRD), Raman spectroscopy and to deposit TiO 2 films with high photocatalytic activity at low temperature (typically below 100 °C).

Experimental
In this study, TiO 2 films were deposited on silicon (100) substrate (Si, supplied by Codex Ltd.], used as the titanium precursor, was injected into the diffusion chamber through a dispersal ring above the substrate holder. The flow rates of pure TTIP and oxygen were fixed at 0.24 sccm and 16 sccm, respectively, which corresponds to a 98.5:1.5 O 2 /TTIP mixture, while the total working pressure was fixed at 0.4 Pa. The O 2 /TTIP plasmas can be operated either in continuous (CW) or pulsed mode. In this study the frequency was fixed at 1 kHz and the duty cycle (DC) was varied from 10 to 75% by tuning the T on time. The thickness lies between 250 and 300 nm for all deposited films. The substrate temperature was measured using temperature sensitive labels stuck on the back side of the Si substrates [46]. It was measured to be around 120 °C in continuous (CW) standard plasma conditions and was shown to be decreased in the pulsed mode down to 80 °C for DC=50% and below 50 °C for DC = 25%. The deposition parameters are summarized in Table 1. The film morphology was observed by SEM using a JEOL-type JSM 7600F, operating at 10 kV. TEM analysis was performed using a Hitachi H9000-NAR microscope (LaB 6 filament, Besides, the FTIR absorption in the range 2800-3000 cm -1 is linked to the quantity of SA by a conversion factor , which gives an equivalency between a number of molecules of SA per cm 2 and an integrated FTIR area value of 1 cm -1 . A conversion factor value of  = 9.7×10 15 molecules of SA cm -2 per cm -1 of FTIR absorption is used in this article [48]. This allows us to calculate an UV light activity R SA defined as the number of molecules cm -2 of SA degraded per second. SEM micrographs including side-views and top-views of the TiO 2 films deposited in continuous mode and in pulsed mode for different duty cycles are displayed in Fig. 1. All the films exhibit columnar structures, but it seems that the column size can be decreased as the duty cycle is reduced which can be identified by TEM in details. The CW mode leads to a typical morphology on Si substrate in agreement with previous studies [35], which is the gradient columnar structure including a dense sublayer at the interface and a disordered columnar top layer. The morphologies for the films grown in the pulsed mode with DC ≥ 40% retain the same gradient columnar structure, but the degree of homogeneity in the growth direction is improved as DC is decreased. Moreover, when DC ˂ 40%, the columns appear more organized and the whole morphology seems to be homogenous along the growth direction.

Film morphology
The evolution of cross-sectional morphology is also reflected in the surface topography (see

RMS = 4.0 nm
The TEM image (in Fig. 3) of a 300 nm-thick TiO 2 film deposited in the CW mode on Si substrate clearly shows a highly crystallized film with a columnar morphology. The interface between the Si (100) substrate with a ~4 nm native amorphous SiO 2 layer and the polycrystalline TiO 2 film was identified in Fig. 3(a). The SAED pattern (Fig. 3(b)) is typical for the anatase phase. The presence of a dense layer topped with a columnar crystallized film is quite consistent with SEM pictures which indicated the presence of a dense film topped with a gradient layer [35]. The TEM study of the TiO 2 film deposited in the pulsed mode (DC = 50%) displayed in Fig.   4 shows that using pulsed plasma allows decreasing the deposition temperature (measured to be less than 80 °C) while conserving anatase crystallites (in Fig. 4(b)). Compared to the CW mode, a similar columnar morphology was observed, but a slight decrease in the column diameter from ~30 to ~15 nm was observed for the pulsed mode.
Pulsing the plasma decreases the mean energy transferred to the substrate over a whole period (T on + T off ) (due to a decrease in the heat and ion fluxes during T off ), which affects the surface Si of the growing film and causes a reorganization of the surface atoms. Thus, the reduction of plasma deposition time by decreasing the duty cycle for pulsed mode leads to a more homogenous columnar morphology with a decreased column size and a decrease in the surface roughness.    [50]. However, in the case of DC = 25%, the characteristic peaks of anatase are very weak in intensity, but the peak at 144 cm -1 is clearly observed which suggests that the film may consist in a small amount of anatase dispersed in an amorphous matrix. For the films deposited at DC = 10%, the Raman spectra did not show any indication of crystallized TiO 2 .

Film crystallization
In the case of CW mode, ions in the plasma can be continuously generated to participate in the crystallization of the growing film with a simultaneous release of energy which is helpful for the anatase formation. However, in the pulsed mode, the flux and energy of the ion bombardment are no longer generated in post-discharge, which may lead to a reduction in the crystallization rate by increasing the plasma-off time. In addition, the probable phenomenon of hydrolysis of TTIP during post-discharge can also hamper the crystallization, as this reaction is thus carried out in thermodynamic equilibrium and tends to create amorphous TiO 2 [51]. Therefore, a reduction of the amount of the crystallized anatase phase is observed when the duty cycle is decreased. This statement can indicate that the growth of anatase crystalline nanocolumns is directly linked to the deposition period during the plasma on time, independently of the substrate temperature.

Photocatalytic activity
According to the above discussion concerning the influence of pulsed plasmas on the TiO 2 film morphology and crystallization, some interesting points about the anatase content, columns size and organization, surface state were demonstrated, which may affect the photocatalytic properties of these TiO 2 films. In this work, the photocatalytic activity of thin films was evaluated by monitoring of the degradation of stearic acid under UV light.  photocatalytic performance (with the same measuring device) of bare silicon substrate, PECVD-grown rutile films [35] and 2 μm TiO 2 film obtained by magnetron sputtering (PVD) at 250 °C [52]. One can note that the silicon substrate has no photocatalytic activity (as expected), whereas the films presented in this work all exhibit a decrease of the SA quantity when illuminated by UV light.
In order to compare with available literature data, the formal quantum efficiency (FQE) for SA is calculated [48] and presented in Fig. 7(b). The FQE is calculated as the number of SA photodegraded by the incident photons (see eq. 1). The slope of the degradation curve of SA, R SA and FQE for the films deposited under different conditions are shown in Table 2.
(eq. 1) TiO 2 layers deposited for DC ≥ 50% contain a significant amount of anatase and present the highest photocatalytic activity. Although the TiO 2 layer grown at DC = 40% has a quite rough top surface, the crystalline anatase content detected by Raman scattering and XRD is high enough to lead to a lower but still significant photocatalytic activity. In the cases of DC = 25 and 10%, TiO 2 layers have a reduced but still significant conversion rate which is higher than the PVD-deposited anatase at 250 °C [52]. Indeed, the R SA of the film obtained at DC = 10%, at a temperature < 50 °C is comparable to other films obtained using an atmospheric pressure PECVD process with substrate temperature around 100 °C [54]. Moreover, the rutile TiO 2 film deposited by PECVD shows a rather poor photodegradation efficiency.
This study has demonstrated that it is possible to deposit TiO 2 layers with high photocatalytic activity at temperatures below 100 °C (50% ≤ DC ≤ 75%). This therefore opens up interesting prospects for the deposition of photocatalytic layers, for example on flexible polymer substrates.

Conclusions
Highly efficient photocatalytic anatase nanocolumnar TiO 2 thin films were successfully deposited by pulsed-PECVD process at relatively low temperature less than 100 °C on Si substrate. The effect of plasma-off time by tuning the duty cycle of pulsed mode was investigated on TiO 2 thin films in view of improving their photocatalytic properties. The efficient photocatalytic property of such a film depends on the presence of the anatase phase at its surface and also its roughness and microstructure. At a duty cycle between 40% and 100%, the bombardment of the particles at the substrate is strong enough for inducing mobility of surface particles, promoting the formation of crystalline anatase TiO 2 films with disordered columnar morphologies. The photocatalytic activity for these films is very high.
As DC is decreased below 40%, the accumulation of impact energy and heat is insufficient, leading to the formation of a surface including only a small amount of anatase embedded in an amorphous film, which has relatively smooth surface and homogeneous columns along the growth direction. Thus the photocatalytic activity is reduced, but is still significant regarding the low substrate temperature.