Effect of Silver Nanoclusters on Physical and Electrical Properties of Cerium Oxide Thin Film

Abstract

An in situ X-ray photoelectron spectroscopy (XPS) investigation has revealed that silver metal clusters (Ag-NCs) can enhance the redox property of cerium oxide (CeO₂) at relatively lower temperatures by oxidizing Ag-NCs to Ag²⁺ and Ag³⁺ states. Strong metal support interaction (SMSI) effect at the interface is indicated by a specific interaction seen in high-resolution transmission electron microscopy (HRTEM) images of CeO₂/Ag-NCs, confirming Ag-NC encapsulation by deposited CeO₂ layer after heating. Through chemisorption processes, the SMSI effect aids in the release of oxygen from the ceria surface by making a bond of Ag, O, and Ce. Consequently, employing silver as a supporting novel metal improves the redox behaviour of CeO₂ at nearly 100°C. The band gap of cerium is modified because of this interaction as shown by UV-vis spectroscopy, influencing the electronic charge transport property of ceria. The current-voltage (I-V) characteristics in silver cluster supported ceria thin film verify the significant increase in current under visible light illumination as compared to the current in dark conditions. This renders that Ag-NCs supported CeO₂ is suitable for photocatalysis and the capacitance-voltage (C-V) characteristic confirms the enhanced storage capacity of Ag-NCs /CeO₂-based metal-oxide-semiconductor (MOS) devices qualifies it for use as non-volatile memory (NVM) devices.

1. Introduction

Cerium metal is one of the most interesting element in Lanthanum group of periodic table as it shows different physical properties in oxidized form. It is well known that cerium metal ([Xe]$4{\mathrm{f}}^{1}5{\mathrm{d}}^{1}6{\mathrm{s}}^2$; four outer valance electrons) is reactive in air and changes to ${\mathrm{CeO}}_2$ with electronic configuration [Xe]. Depending on the presence of valance electrons the electronic structure of cerium vary as a result the cerium present in mixed valance state. This property makes this material an important place in industry for various applications as heterogeneous catalyst for oxidizing hazard gasses [1], water gas splitting [2], in memristors for application in meromorphic computing [3] also as a photodiode [4]. Minimization of redox reaction temperature of cerium oxide to make it useful as heterogeneous catalyst is one of the most challenging work now a days. Several studies have done where it was clearly indicated that the chemical state of ceria is most important factor to improve the catalytic behavior of ceria [5]. In general the bulk cerium oxide started redox reaction after heating at ${600}^{°}\mathrm{C}$ [6].

The most important nature of cerium oxide is that its oxygen adsorption and desorption property based on the experimental environment. Researchers were trying to go in a conclusion of that procedure by many ways but not succeed till now. Most of researchers are believe that cerium oxide changes its oxidation state by following Mars-van Krevelen (MvK) mechanism when one oxygen ion remove from the surface of ceria then two of neighboring ${\mathrm{Ce}}^{4+}$ ions take the electrons and form ${\mathrm{Ce}}^{3+}$ ions to make charge neutrality [7]. To occur this reaction we need to control some parameters during the formation of ceria.

Introducing metals, the redox reaction temperature improved and occur at relatively lower temperature has been studied by many researchers [8, 9, 10, 11, 12]. Mostly the novel metals give the better result in that case compare to other metals. The growth condition of the sample determines the interaction between ceria and the corresponding metals. Many research on different structure of supported metals as single atom [13], cluster of metals [14], doping with ceria [15] has been studied for getting better redox reaction at relatively lower temperature. In general cerium oxide having higher oxygen storage and release capacity when it present in mixed valance state (presence of ${\mathrm{Ce}}^{3+}$ and ${\mathrm{Ce}}^{4+}$ state together). Introducing metals, an increment in the oxygen storage capacity within ceria occur due to the interaction at both the surface and interface between metals and ceria. This interaction is known as the strong metal support interaction (SMSI) which was first observed by Tauster et al. on ${\mathrm{TiO}}_2$ [16]. In 1990, research on increasing oxygen storage capability of ceria by introducing rare earth metal was described by Miki et al. and Cho by considering the two types of lattice defects within ceria [17]. The SMSI effect at the interface takes an important role in redox property of ceria.

The interaction and the corresponding structural change of cerium oxide with the supported metals are different in case of bulk as compare to the thin film of ceria. As it is already known that the formation of a thin film of a material from bulk can change the dimension of the material i.e. the material completely changes from 3D to 2D structure in nature. As a result the physical properties of the 2D structure is different from 3D one which affects direct on the electron transport of the material. Therefore, people are growing their interest on controlling the redox reaction of cerium oxide based thin films to make it applicable in form of devices. Considering the supported metal, as single atom and cluster or nanoparticles of a metal shows different properties. In some metals the cluster formation of metal changes its electronic structure.

Considering this situation, we have investigated the interaction between novel metal (silver Ag) cluster with cerium oxide thin film by varying temperature from 25 to ${250}^{°}\mathrm{C}$. Using this combination we have achieved the redox reaction temperature at ${100}^{°}\mathrm{C}$ as verified from the results of in-situ XPS experiment. Based on the results we have prepared one metal-oxide-semiconductor (MOS) structured based device to get to know the effect of Ag-metal on the electrical transport of ceria to application in non-volatile memory devices. Presence of silver metal clusters within cerium oxide also affects the optical band gap and from current-voltage study of Ag-NCs supported ceria an optical response of the device we have achieved under visible light for application in photocatalysis.

2. Experimental details

We have prepared the cerium oxide thin film by rf magnetron sputtering technique using 99.999% pure cerium oxide target. The p-type silicon (100) was used as a substrate for this purpose. We have used common RCA cleaning method [18] to clean the substrate. During sputtering we have used rf power of 45 W with 5mTorr of deposition pressure. The deposition rate is maintained as 1 nm/min. Deposited cerium oxide having thickness of 10 nm on which the silver (Ag) metals in form of nanocluster of size 4 nm was deposited using NC200U cluster deposition UHV system which is equipped with a quadrupole mass filter and x-ray photoelectron spectroscopy (XPS) technique. To compare the silver cluster supported ceria thin film combination we have also deposited pristine ceria thin film using same deposition parameters. The physical characteristics of pristine and Ag-nanocluster/${\mathrm{CeO}}_2$ have performed by differential scanning calorimetry (DSC), cross sectional TEM study, in-situ XPS, UV-vis spectroscopy and grazing incidence X-ray diffraction (GIXRD) technique.

To form a MOS structure of this thin film combination we have again deposited cerium oxide capping layer of thickness 40 nm on Si/ ${\mathrm{CeO}}_2$/Ag-NCs thin film. For electrode deposition we have patterned on the film using photomask of 100 $\mu$ diameter with a separation of 25 $\mu$ using photolithography technique. We have then used e-beam evaporator to deposit titanium metal of thickness 300 nm. The capacitance-voltage and current-voltage characteristics was performed using Agilent E4980A LCR meter and Keithley 2450 source-meter, respectively in signatron microprobe station with a heating facility of 25–${300}^{°}\mathrm{C}$.

3. Experimental results and discussions

The effect of heating on pristine ceria and silver cluster supported ceria was verified by performing the DSC study is presented in Figure 1(a). In case of pristine ceria thin film the endothermic peak appeared at 150 and ${200}^{°}\mathrm{C}$ because of the involving of 2nd order phase transition of ceria. In silver cluster supported ceria thin film the exothermic peak appeared at ${200}^{°}\mathrm{C}$ indicating a phase transition with a corresponding endothermic peak at ${595}^{°}\mathrm{C}$ because of melting of Ag-cluster. As it is known that bulk silver having melting temperature at ${961}^{°}\mathrm{C}$ i.e. the formation of silver clusters makes a reduced melting temperature. Figure 1(b) and (c) are the SEM images of pristine ceria and after depositing Ag-NCs on ceria thin film.

Figure 1.

(a) Is the DSC curve of pristine ceria and silver cluster supported ceria thin film, (b) is the SEM image of as deposited ceria thin film and (c) is the SEM image after depositing Ag-clusters on ceria.

GIXRD and TEM study has been performed at room temperature and after annealing under vacuum at ${250}^{°}\mathrm{C}$ [19]. From GIXRD study of pristine ceria and silver supported ceria the presence of silver is confirmed within the sample. A clear difference in the size of deposited silver clusters was observed after heating at ${250}^{°}\mathrm{C}$ as well as an encapsulation of clusters by ceria surface layer was also observed from cross-sectional TEM study. The HRTEM images clearly verifies a layer of ceria fully cover the deposited silver clusters during heat treatment whereas the as deposited silver clusters are attached on the deposited cerium oxide thin film at room temperature i.e. without performing any annealing.

Based on the DSC study the in-situ XPS of both the samples has been performed and presented in Figures 2–5. From the XPS curve of Cerium 3d in absence of presence of silver metal clusters it is clearly observe that the improvement of the redox property of cerium oxide in silver cluster supported cerium oxide thin film. There is no effect of temperature on the redox property of ceria in case of pristine ceria whereas an improved redox property at relatively lower temperature as ${100}^{°}\mathrm{C}$ is observed from in-situ XPS study for silver supported ceria thin film. A detailed study on pristine ceria and silver supported ceria thin film is presented in [19] with detailed analysis and description. Figure 4 is the XPS spectra of Ag3d where the reversible change of oxidation states of silver clusters was verified with increasing temperature. The interaction of cerium with silver via oxygen due to the chemisorption process is the reason of oxidation of silver which can help to make the reduction of ceria at relatively lower temperature. A lower binding energy shift of oxygen XPS spectra in Ag-Nc/${\mathrm{CeO}}_2$ sample as observed in Figure 5 also indicates an involvement of ${\mathrm{Ce}}^{3+}$ states within the sample.

Figure 2.

Ce3d XPS spectra with temperature for pristine ceria thin film.

Figure 3.

Ce3d XPS spectra with temperature for Ag-NCs/ceria thin film.

Figure 4.

Ag3d XPS spectra with temperature for Ag-NCs/ceria thin film.

Figure 5.

O1S XPS spectra with temperature for Ag-NCs/ceria thin film.

The summary of the experiment is that the presence of silver affects the redox property of ceria in form of thin film after heat treatment. And the required temperature is relatively low in that case which can make this combination useful for application in low temperature catalytic activity of ceria.

3.1 Effect of light on novel metals

Over the past few decades the use of metal nanoparticles (NPs) to utilize light absorption grabbing the attention. As absorption of light is an interesting property of metal NPs and by introducing this NPs within oxides it can be used as a photocatalysis which is known as metal-induced photocatalysis (MIP) [20]. The effect of light absorptions of metal NPs under illumination of visible light is one of the challenging work in photocatalyst study. The localized surface plasmon resonance (LSPR) and the interband transition mainly the reason for the generation of MIP process in metal NPs [21]. Mostly for novel metal NPs the LSPR absorption occur near the visible light region. The metal oxides mostly ${\mathrm{TiO}}_2$ and ${\mathrm{CeO}}_2$ having band gap energy fallen into UV-region those are mostly inactive under visible light where the electron-hole pair generate only after UV illumination. By introducing novel metals within these two oxides the visible light activity of these oxides enhance. Jiang et al. observed that by introducing Ag in ${\mathrm{TiO}}_2$ the photocatalytic activity become higher than ${\mathrm{TiO}}_2$ under visible light [22] where the activity of novel metals with oxides highly depending on the size of metal NPs. Based on this we have performed the electrical transport study of silver-nanocluster supported cerium oxide thin film based MOS devices under visible light and dark condition.

3.2 Application in optoelectronics

A schematic diagram of MOS structure of Ag-NCs/${\mathrm{CeO}}_2$ combination is depicted in Figure 6(a). The I-V curve of silver supported ceria thin film under dark and light condition is presented in Figure 6(b) under voltage sweep from 0 to 5 V and 5 to 0 V. The current density become increases under visible light environment as compare to the current under dark condition. Under the visible light illumination condition the Ag-NCs produces an electron-hole pair and after applying voltage the charge separation occur as a result the charge carrier density increasing more than that of dark condition. Therefore, the optical activity of silver supported ceria thin film under visible light appear i.e. this combination can be used as a photo catalysis.

Figure 6.

(a) Is the schematic diagram of MOS based structure, (b) is the I-V curve of Ag cluster supported cerium oxide thin film under dark and illumination of visible light.

3.3 Application as non-volatile memory devices

The novel metal nanoparticle embedded high -$\kappa$ dielectric based non-volatile memory (NVM) devices gives better performance as faster write and erase capacity, longer retention time and better endurance by smaller applied voltage [23]. Metal nanoparticle with transition metal oxide composite in the application of NVM devices was studied very few within 2009–2016 year range where researchers use mostly novel metals (Ag and Pt) with mainly two oxides such as ${\mathrm{HfO}}_2$ and ${\mathrm{Al}}_2{\mathrm{O}}_3$ [24, 25]. In these studies the effect of size of nanoparticles as well as the deposition conditions are the important factors for getting better storage devices. Inspired from this, we have used Ag-nano cluster supported cerium oxide combination based NVM devices. As it is good for us that ceria itself is known as higher oxygen reservoir by changing its oxidation states depending on the environmental circumstances. From other physical characteristics we have observed there is a change of oxidation state of ceria after annealing at a certain temperature in presence of silver and with heating up better redox reaction we have achieved.

Figures 7 and 8 are representing the C-V characteristics of Ag-NCs/${\mathrm{CeO}}_2$ composite of as deposited film and after annealing at ${250}^{°}\mathrm{C}$. It is clearly observed that as deposited Ag-cluster supported ceria thin film forming a hysteresis loop when the voltage sweeps from −5 to +2 V and + 2 to −5 V. After annealing at ${250}^{°}\mathrm{C}$, hysteresis loop area become negligible because of the charge store in silver clusters by electron or hole injections [26]. From this experimental result it is concluded that presence of silver within ceria thin film makes the device performance better than pristine ceria and after annealing the device performance decreases.

Figure 7.

High frequency C-V curve of as deposited Ag cluster supported cerium oxide thin film.

Figure 8.

High frequency C-V curve of Ag cluster supported cerium oxide thin film after heating at ${250}^{°}\mathrm{C}$.

4. Conclusions

We have studied the effect of interaction of silver metal in form of clusters of selected size with cerium oxide during heating and the corresponding change of electrical properties. Interaction of Ag-NCs with cerium oxide in form of thin film results the improvement of reduction temperature (at nearly ${100}^{°}\mathrm{C}$) and can change the oxidation state of ceria from 4+ to 3+. The composition of Ag-NCs/${\mathrm{CeO}}_2$ thin films involve in the SMSI effect at the interface of metal and supported cerium oxide as a result metal clusters encapsulated by cerium oxide layer after heat treatment as observed from HRTEM images. The oxidized form of deposited silver clusters with an reversible change of oxidation states can helps to improve the redox property of ceria by involving interaction via chemisorption process is verified from XPS spectra of Ag and Ce3d. This change in physical properties also helps to improve the electron charge transport of silver supported ceria and makes this combination applicable as photo catalysis in form of device under visible light illumination.

Acknowledgments

Authors wish to acknowledge Mr. Debraj Dey for helping in XPS and SEM measurement and Ms. Dimitra Das, Department of Physics, Jadavpur University, Kolkata for UV-Vis diffuse reflectance measurement. The first author wishes to acknowledge the University Grant Commission (UGC), Govt of India for financial help.

Conflict of interest

The authors declare no conflict of interest.

Abbreviations

DSC	differential scanning calorimetry
GIXRD	grazing incidence X-ray diffraction
XPS	X-ray photoelectron spectroscopy
NVM	non-volatile memory
SMSI	strong metal-support interaction
MOS	metal-oxide-semiconductor
UHV	ultra high vaccum
TEM	transmission electron microscopy
HRTEM	high resolution transmission electron microscopy
LSPR	localized surface plasmon resonance