Ag2O and Ag-Doped CeO2

In examining the nucleation effect and morphology of the resulting ALD of Ag2O, several AFM images were taken with variable ALD cycles (Figure 6). The average size of Ag2O nanoparticles after 25 ALD cycles was around 22 nm with film roughness of 1.5 nm. After 100 ALD cycles, large nanoparticles with size of around 40 nm were present with a high nanoparticle density (Figure 6a) and overall surface coverage. The AFM image of 500 cycles of Ag2O film is shown in Figure 6b, which demonstrates that the surface is now fully covered with silver, having grain sizes between 40 and 46 nm. As was shown in Figure 3, the nucleation region for Ag2O ALD occurs over 100 cycles; Ag2O films can nucleate and grow by the Volmer–Weber (VW) growth mechanism, where the deposited atoms form islands or clusters and three dimensional aggregates on the substrate. Growth of these clusters, along with coarsening, can be a cause of rough thin films on the substrate surface [30].

**Figure 6.** The atomic force microscopy (AFM) non-contact mode images of Ag2O deposited with: (**a**) 25 cycles (*R*a 0.7 nm); (**b**) 100 cycles (*R*a 1.5 nm); (**c**) 250 cycles (*R*a 1.6 nm); and (**d**) 500 cycles (*R*a 1.7 nm). Axis scales are in nm.

All the Ag2O thin films deposited on Si had a visual matt finish, which is a sign of rough microstructure. SEM studies supported the AFM results (Figure 7). The 250 and 500 ALD cycles films were confirmed to consist of particles with widely different sizes as a result of coalescence and secondary nucleation on existing particles.

**Figure 7.** Scanning electron microscope (SEM) images of Ag2O thin films deposited with different number of cycles at 200 ◦C resulting in different thicknesses (5.8, 8.8, and 13.8 nm).

The AFM and SEM analyses on Figures 8 and 9 show that the surface morphology of Ag doped CeO2 films changes in accordance with silver concentration in the film. It can be seen that the reduction of Ag concentration dramatically decreases crystal and cluster sizes in doped films. Figures 8a and 9a suggest that higher concentration of Ag (CeO2:Ag 10:1) inhibits CeO2 growth so that the Ag nuclei are not covered with CeO2 and so the next Ag cycle nucleates more easily on top of the Ag and can therefore form bigger crystals of about 50 nm in size. Such large nanoparticles were noticed on pure Ag2O and described above. With lower doping concentration of Ag in CeO2 thin films, no crystals larger than 25 nm were noticed.

**Figure 8.** The AFM non-contact mode images of Ag doped CeO2 in different CeO2:Ag ratios deposited at 200 ◦C: (**a**) 10:1; (**b**) 20:1; (**c**) 30:1; and (**d**) CeO2. The scales are in nm.

**Figure 9.** SEM of Ag doped CeO2 in different CeO2:Ag ratios (**a**) 10:1, (**b**) 20:1, and (**c**) 30:1 deposited at 200 °C.

Figure 10a shows XRD spectra of Ag2O, CeO2, and Ce:Ag 10:1, 20:1, and 30:1 Ag doped CeO2 films in the 2θ range of 20◦–90◦. The Ag2O films showed strong X-ray diffraction peaks at 2θ = 32.7◦ and 38.3◦ related to the (111) and to (200) cubic planes of Ag2O, respectively (ICCD Card No: 00-41-1104). They revealed that the films deposited at 200 ◦C contained only Ag2O crystallites. The diffraction peaks at 2θ angles of 28.6◦, 33.6◦, 47.6◦, and 56.3◦ can be identified for all the other samples and attributed to (111), (200), (220), and (311) planes of cubic cerium oxide, respectively (COD database, card No 9009008). The XRD patterns of CeO2, Ag2O, and Ag doped CeO2 in the 2θ range 24–40◦ were expanded (Figure 10b) to analyse the position of CeO2 and Ag2O reflections in the X-ray spectra in more detail. Figure 10b shows that the cerium oxide peak intensity and the shape of (200) plane reflection in the Ag doped CeO2 samples decreased and broadened, respectively, with increasing Ag doping, compared to the pure CeO2 catalyst.

**Figure 10.** (**a**) X-ray diffraction (XRD) patterns of Ag2O, CeO2 and Ag doped CeO2 with different Ce:Ag ratios 10:1, 20:1, and 30:1 deposited at 200 ◦C. (**b**) XRD patterns of slow scans of CeO2, Ag2O and Ag doped CeO2 with different Ce:Ag ratios 10:1, 20:1, 30:1 in the 2θ range of 24–40◦.

The mean grain size, assuming spherical grains, of CeO2 can be determined from the full width at half maximum (FWHM) of the (111) XRD peak, through Scherrer's equation. The grain sizes calculated from the (111) plane reflection of CeO2 are indicated in Table 1. XRD analyses confirm the decrease of the CeO2 crystallite size of (111) plane as the Ag amount increases. This behaviour is related to the occurrence of lattice defects due to the presence of the dopant, which leads to deformations in the crystalline structure and smaller crystallites. The XRD data do not show any peaks related to Ag species for CeO2:Ag 20:1 and 30:1 catalysts, which is, most probably, demonstrative of a high distribution of the dopants in the CeO2 samples. The CeO2 (200) and Ag2O (111) appear at very similar positions. However, for higher amount of Ag doping Ce:Ag 10:1, a small peak from the Ag2O (200) plane reflection can be observed (Figure 10b). In the CeO2 lattice, the radius of Ce4+ ion is 0.97 Å. However, the ionic radius of Ag+ ions is 1.28 Å [31]. As such, substitution or replacement of Ag+ ions

for Ce4+ ions in the CeO2 lattice requires high energy [32] and from the XRD there is no evidence of significant substitutional doping during ALD of Ag doped CeO2 thin films from a shift in the position of the CeO2 peaks to smaller angles. Ag2O forms as metal oxide or alloy clusters in CeO2:Ag 10:1 catalyst and inhibits CeO2 crystal formation.


**Table 1.** Grain size of CeO2 (111) plane reflection based on Scherrer's equation.

XPS was employed to analyse the chemical state of the as-deposited Ag2O thin films and the Ag doping in CeO2 films, which were controlled by varying the Ce:Ag supercycle binary process pulse ratio. The information on silver and cerium oxidation states was obtained from the high resolution Ag 3*d* and Ce 3*d* spectra after Ar<sup>+</sup> bombardment to exclude surface contaminations.

Using the values of surface atomic composition from Table 2, an estimation of the O/Ce and O/Ag atomic ratio can be obtained. The ratio O/Ce for the cerium oxide deposited at 200 ◦C is around two, which indicates that the pure CeO2 is stoichiometric. The carbon impurity level is around 21 at.%, which arise from the Ce(thd)4. The Ag to O ratio in the Ag2O films was estimated to be close to 2:1, which indicates that the film primarily consists of Ag2O with 14.2% of carbon, 0.5% of F, and 3% of N as the main impurities in that film. With regard to the Ag/Ce surface atomic ratio, an important enhancement with Ag loading is observed, indicative of an increase in the number of Ag surface atoms. We found from survey spectra that by changing the Ce:Ag ratio from 30:1 to 10:1 the amount of Ag increases from ~2 at.% to ~9.7 at.%., as measured by XPS (Table 2).


**Table 2.** Surface elemental composition of Ag doped CeO2, Ag2O, and CeO2 thin films.

High resolution spectra of the Ag 3*d* peaks of Ag2O and Ag doped CeO2 thin films with the nominal ratio Ce:Ag from 30:1 to 10:1 give us indications of the chemical state of Ag atoms (Figure 11). The pure Ag2O films showed only one peak at 368.2 eV. The binding energy which has been observed for pure Ag2O thin film is 367.2 eV, which consists of the dominant oxidation state Ag+ [33]. The spectrum here shows a shift of ~1 eV in the peak position compared to the previously found results which may be due to sample charging. The spectrum showed core level binding energies at about 368.2 ± 0.1 eV and 374.2 ± 0.1 eV related to the Ag 3*d*5/2 and Ag 3*d*3/2 respectively with spin orbit separation of 6 eV [34]. Each Ag 3*d* level in Ag doped CeO2 films can be deconvoluted into three peaks, with corresponding binding energies 368.2, 369.2, and 367.2 eV, which are consistent with those of Ag+, Ag0, and Ag2+ (Table 3), allowing for the shift due to sample charging [35–37]. The estimated percentages of the three peaks, shown in Table 3, indicates that with increasing Ag doping concentration in CeO2 films from 30:1 to 10:1, the Ag<sup>+</sup> oxidation state also grows from 38.4% to 85%, respectively. At the same time the Ag0 oxidation state decreases from 59% to 9.7% for CeO2 doped Ag films deposited with the ratio Ce:Ag from 30:1 to 10:1, respectively.

**Figure 11.** High resolution Ag 3*d* X-ray photoelectron spectroscopy (XPS) spectra of (**a**) pure Ag2O and Ag doped CeO2 in different CeO2:Ag ratio (**b**) 10:1, (**c**) 20:1, and (**d**) 30:1.

**Table 3.** Binding energies and integrated peak areas of Ag 3*d* spin-orbit doublets in Ag2O, Ag doped CeO2 and CeO2 thin films.


Figure 11 and Table 3 show that at low doping concentration, Ag species mainly exist as Ag0, while as doping concentration increases, Ag<sup>+</sup> species increase remarkably. It is likely that at low concentrations of Ag2O doping, some of the silver oxide is reduced by the CeO2; a similar effect has been reported on Ag2O-doped TiO2 [38]. For higher Ag dopant concentration, most of the silver present in the catalysts remains as cations and probably interacts with CeO2 through the Ag–O bonds. Based on previous studies involving silver oxides [39–42], it can be concluded that some electrons may transfer from CeO2 to the Ag dopant and there is strong interaction between the Ag species and the CeO2 catalyst.

It is interesting to note that the concentration of Ce4+ decreased from 82% to 77% and the concentration of Ce3+ increased from 18% to 23% with increasing Ce:Ag doping from 30:1 to 10:1, respectively, further suggesting the existence of the interaction between Ag and CeO2. This is probably because the Ag<sup>+</sup> ions in Ag doped CeO2 can partially substitute Ce4+ in the CeO2 matrix in the form of Ce1−*x*Ag*x*O2−δ. As was shown earlier from the XRD spectra, there is no evidence of substitutional doping. However, the increasing Ag content also produced smaller crystallites so it may be that the

increasing Ce3+ arises as a consequence of interaction between the CeO2 and the Ag in the disordered regions at the grain boundaries. In summary, Ag atoms deposited on a stoichiometric CeO2 surface tend to result in reduction of the Ce ions, which leads to the stabilization of the Ag in the +1 oxidation state. These results are in good agreement with the literature reports [43–45].
