A method for electrochemical growth of homogeneous nanocrystalline ZnO thin films at room temperature
Abstract
We report electrodeposition at room temperature (25◦C), in potentiostatic mode, from a cohesive ZnO nanocrystalline thin film of oxygenated zinc chloride bath. It is seen that immersion saturation by molecular oxygen precursors is a key parameter for growing oxides at low temperatures. After O2 Zn (OH) 2 is low, the solution is not saturated and the surface is more or less passivated by a thin layer such as an amorphous sheath. After bubbling strong and long molecular oxygen, the current density increases rapidly after an induction period of about 800 seconds. At the beginning of the current foot, ZnO crystallized seeds appear trapped in the initial amorphous layer. Film nucleation is a delayed process. The electrodes are then covered by a homogeneous ZnO film with a structure of several hundred nanometers in length consisting of nanocrystal with a size of about 17 nm. The photoluminescent spectrum of room-temperature films was dominated by strong UV emissions at 3.25 eV due to recombination from exonone. Emissions that appear to be centered at 2.36 eV, due to deep defects, are less strong than UV which indicate good structural quality of ZnO nanocrystalline films. Films have interesting properties to use as layer seeds for example.
Results
The formation of films on the surface of the electrode has been investigated for various intensity and duration of molecular oxygen bubbles before starting the experiment. Molecular oxygen has a dramatic effect on the shape of chronoamperograms recorded during the growth of the film at a constant applied potential. Fig. 1a shows that the settling current decreases rapidly when the flow of bubbles is low and short. The curve is then flat and the current exchanged is low. Similar behavior was reported elsewhere by one of the authors for films that grew at 22 atC and 28◦C from oxygen precursors [17]. This is more or less passivated during the 200s which followed electrochemical potential applications. Different forms of growth are found when low bubbles are extended to 2 hours. The cathodic current slowly increases which indicates that the deposited layer becomes more conductive with the deposition time (Fig. 1a). Several waves were observed after 3500 s. Increasing bubble flow has a more pronounced effect on current density. An induction period of around 800 seconds is found with a low current density, followed by a rapid increase in depositional currents. Two marked waves are observed again which might indicate some structural changes and / or morphological layers. Saturation of the solution with oxygen molecules appears later as a key parameter for film growth at RT and saturation can only be achieved with long and intense O2 bubbles.
This parameter is less critical in galvanostatic mode. On picture. 1b, we can see that the electrochemical potential decreases rapidly due to the increase in the electrode surface resistivity. After 100 seconds, the musim is reversed and the potential increases to reach a plateau at -1.17 V vs. SCE. The plateau potential is close to the formation boundary of metal zinc by the reduction of Zn (II) solution [17]. A strong and long bubble supports the transformation of the initial layer into a more conductive layer and a potential minimum is achieved beforehand (Fig. 1b, inset). We can note that the next plateau is very similar to the regime’s low and intense inflation. They are different in length: bubbling more intensely supports the appearance of the second transition which is characterized by an increase in electrochemical potential.
Five different films are prepared that are different from the preconditions of the settling bath, by electrochemical growth mode and by the total electrical charge exchanged (Q). Growth conditions are listed in Table 1. Figure. 2 displays the SEM display of films prepared in potentiatic mode. After a short bubble at low flow, the FTO electrode (Fig. 2a, inset) is closed by a membrane like a veil with a fibrous aspect (Fig. 2a). We can note that similar morphology was found after intense and brief inflating times, that is, before the current onset at 800 seconds. From the picture. 2b, the thickness of the deposit of sample A can be estimated at around 50 nm. No granules and nanocrystals can be observed. The layer performs poorly and its presence causes a low deposition current density as described above. The surface aspect changes dramatically when solution conditioning by oxygen molecules is bubbled at a low flow rate extended to 2 hours (Fig. 2c). The deposit is made of coarse grain with a size in the range of 100 nm. A better view of grains (Figure 2c, inset) shows that they are made of nanoparticles combined with a size of 15-20 nm. Deposits are more organized after the old O2 bubbles at intense flow rates (Fig. 2e and f). After deposition of the film at a constant potential for 1 hour, the electrode is homogeneously covered by entangled sheet tissue (Fig. 2e). A better view of the deposit indicates that the sheets are made of well-defined granules measuring around 15-20 nm (Fig. 2e, inset).
The entanglement observed in sample C can be related to reliefs drawn by fibrous structures in the film formed at the initial stage of deposition (Fig. 2a). This was confirmed by observations of sample D (Figure 2d) prepared under conditions similar to sample C, but growth was stopped after 20 minutes, i.e. at the foot of the current onset of density observed in Figs. 1a. Deposits have an intermediate aspect between A and C, which consists of a fibrous matrix where many small grains are trapped. Small grains can be interpreted as seeds that appear in the nucleation process pending. Figures 3a and c are views of sample E prepared at a constant current density after long and intense O2 bubbles. The sample zoom view E (Fig. 3c) is similar to sample C (Fig. 2e) even if the growth mode is different, being potentiostatic for sample C and galvanostatic for sample E. However, a general review of samples at low magnification clearly indicates that the film is not very homogeneous (Fig. 3a). It has poor quality with the presence of many coarse grains in the size of the micrometer. In contrast, sample C is prepared in potentiostatic mode, morphologically very homogeneous (Figure 3b). We can conclude that the potentiostatic procedure is far more preferable than the galvanostic procedure for obtaining a highly homogeneous layer and for application of seed layers for example.
The XRD pattern of various samples is dominated by substrate diffraction peaks (Fig. 4). Sample A does not show diffraction lines because of ZnO or Zn (OH) 2. Film that passives the surface is amorphous. The XRD patterns of samples B, C and E have additional diffraction peaks indexed with ZnO hexitic hexagonal structures. Globally, the intensity of each of the various ZnO diffraction lines is similar to the standard ZnO powder diagram, and no film texture is found. Their FWHM is significantly enlarged compared to the media peak as a result of the smaller size of the crystallite. The crystallite size (dm) is estimated from the Scherrer formula [17] and reported in Table 1. The mean value is around 20 nm for samples B and E. The values found for sample E correspond to those reported in Ref. [29] Intense O2 bubbles produce smaller average grain sizes at 17 nm which may be beneficial for seed layer applications. The DM calculated according to grain size was observed by high resolution SEM in Fig. 2 and 3.
Conclusion
We have explained the method for electrodeposition of homogeneous nanocrystalline ZnO thin films. The coating obtained is of very good quality for ZnO films grown from solution at room temperature. Saturation of the solution by oxygen molecules is a key parameter for growing oxides in RT. Film nucleation growth mechanism is determined. It was shown that the nucleation step was delayed by first forming the amorphous layer Zn (OH) and then ZnO seeds in the precipitated layer. The seed facilitates subsequent nanocrystalline ZnO growth.
The photoluminescence spectrum of film-room temperature was dominated by strong UV emissions at 3.23 eV due to recombination from eczema. The visible emission is centered at 2.36 eV, due to deep defects such as oxygen vacuum less strong. These deposits are important for seed layer preparation as well as for self-cleaning surfaces and superhidrophobic solar cell applications.