*3.1. Structural, Compositional, and Morphological Characterizations*

Figure 1a shows XRD patterns of CCO (black curve), 5% Mg:CCO (red curve), and 10% Mg:CCO (blue curve) powders, respectively. XRD peaks are indexed as a mixture of two CCO polytypes, rhombohedral R3m (3R-CCO, pink sticks), and hexagonal P63/mmc (2H-CCO, purple sticks), for all three compounds. No impurity phases are detected. For the (110) reflection at ~62.0◦, Mg doping results in a ~0.05◦ peak shift to the lower angle, indicating a larger lattice spacing. The (004) reflection at 31.4◦ exhibits broadening with Mg doping, indicating decreased crystal size along the *c* axis (Figure 1a and Table 1). Rietveld refinement was carried out in order to quantitatively determine the polytype composition, crystal size, and lattice parameters for each XRD pattern. Figure 1b shows the experimental (blue solid circle), calculated (red curve), and difference between experimental and calculated (grey curve) patterns for CCO, 5% Mg:CCO, and 10% Mg:CCO. Table 1 shows the results extracted from the Rietveld refinement. The polytype compositions for all three compounds are found to be ~60 ± 3% for 3R and ~40 ± 3% for 2H. The crystal sizes decrease monotonically from CCO to 10% Mg:CCO, from 7.8 to 4.5 nm for the 2H polytype calculated from the (004) reflection and from 9.6 to 8.7 nm for the 3R polytype calculated from the (110) reflection. However, the crystal size increases monotonically from 10.2 nm for CCO to 13.1 nm for 10% Mg:CCO in the (110) reflection for the 2H polytype. The tradeoff of the size changes for both polytypes leads to similar widths of the

(110) reflection independent of Mg doping. Since delafossite nanocrystals often exhibit anisotropic morphology [25,27], it is reasonable that size changes differ for the (004) and (110) reflections. Similar orientation dependent size variation in Mg:CCO was reported by Bywalez et al. [35]. They attributed the decrease of crystal sizes along the *c* axis to Mg2<sup>+</sup> obstructing growth of the delafossite crystal structure and stabilizing the spinel phase. However, there is no indication that this phase exists in our samples.

**Figure 1.** (**a**) X-ray diffraction (XRD) patterns of CuCrO2 (CCO) (black curve), 5% Mg:CCO (red curve), and 10% Mg:CCO (blue curve) powders. Two CCO polytypes, 3R-CCO (Powder diffraction files (PDF) #39-0247, pink sticks) and 2H-CCO (PDF#89-6743, purple sticks) are detected in all three XRD patterns. Prominent reflections are indexed between 30◦ to 40◦, and ~62.0◦. (**b**) Rietveld refinement of XRD patterns. The experimental (blue solid circle), calculated (red curve) and, difference (grey curve) patterns are shown. Structural files for the refinement are 3R-CCO (Crystallography Open Database (COD) No. 8104066) and 2H-CCO (COD No. 8104065).


<sup>1</sup> *Rwp* is the weighted profile R-factor and the squared *Rwp* is equal to the weighted sum of squared difference between the experimental and calculated intensity values over the weighted sum of squared experimental intensity values [44]. <sup>2</sup> *Rexp* is the expected R-factor and the squared *Rexp* is equal to the number of data points over the weighted sum of squared experimental intensity values [44]. <sup>3</sup> *Rp* is the profile R-factor and is equal to the weighted sum of difference between the experimental and calculated intensity values over the weighted sum of experimental intensity values [45]. 4 and 5 *a* and *c* are the in-plane and out-of-lane lattice constants in the unit cell.

The lattice parameters for 3R-CCO and 2H–CCO polytypes were similar for CCO and 5% Mg:CCO. However, for 10% Mg:CCO, a larger *a* lattice parameter in both phases (3.00 Å versus 2.99 Å) and an increase in the *c* lattice parameter in the 2H–CCO phase (11.46 Å versus 11.43 Å) were observed. This lattice expansion was consistent with Mg substituting on the Cr site, rather than the Cu site [36], because the ionic radius of Mg2<sup>+</sup> (0.72 Å) is larger than that of Cr3<sup>+</sup> (0.62 Å) and smaller than that of

Cu<sup>+</sup> (0.77 Å). This result is consistent with the bond length increase between Cr and O sites after Mg doping predicted from theoretical calculations [33].

In order to measure Mg concentration in CCO, EDX was performed on 5% Mg:CCO and 10% Mg:CCO. The insets of Figure 2a,b show that Mg is present and distributed uniformly in the Mg:CCO films. Mg/(Mg+Cr) represents the Mg concentration in the Mg:CCO films, which is calculated by atomic number effects (Z), absorption (A), and fluorescence (F) method from EDX spectra in Figure 2 [46]. Table 2 shows that the averaged Mg concentration in 5% Mg:CCO and 10% Mg:CCO measured from EDX is 4.0% and 9.8%, respectively. A possible Mg doping process is proposed similarly to the CCO formation mechanism as described by Miclau et al. [47]. During the hydrothermal synthesis of Mg:CCO nanoparticles, Cu1<sup>+</sup>, Cr3<sup>+</sup>, and Mg2<sup>+</sup> ions can form Cu(OH) − <sup>2</sup> , Cr(OH) − <sup>4</sup> , and Mg(OH)2, respectively, at alkaline pH environment according to equations (1-3) below. Mg:CCO nanoparticles can then be formed from the metal hydroxides according to Equation (4) [48–50].The formation process of Mg:CCO nanoparticles are given in the following equations:

$$\text{Cu}^{1+} + 2\text{H}\_2\text{O} \rightarrow \text{Cu(OH)}\_2^- + 2\text{H}^+ \tag{1}$$

$$\text{Cr}^{3+} + 4\text{H}\_2\text{O} \rightarrow \text{Cr(OH)}\_4^- + 4\text{H}^+ \tag{2}$$

$$\text{Mg}^{2+} + 2\text{OH}^- \rightarrow \text{Mg(OH)}\_2 \tag{3}$$

$$\text{Cu(OH)}\_{2}^{-} + (1-\text{x})\cdot\text{Cr(OH)}\_{4}^{-} + \text{x-Mg(OH)}\_{2} + 2\text{H}^{+} \rightarrow \text{CuCr}\_{1-x}\text{Mg}\_{x}\text{O}\_{2} + 4\text{H}\_{2}\text{O} - 2\text{x} \cdot \text{(OH)}^{-} \tag{4}$$

**Figure 2.** Energy-dispersive X-ray spectroscopy (EDX) spectra for (**a**) 5% Mg:CCO and (**b**) 10% Mg:CCO. Mappings of Mg element (inset) show uniform distribution.

**Table 2.** Measured Mg doping concentration of CCO, 5% Mg:CCO, and 10% Mg:CCO films and average transmission electron microscopy (TEM) nanoparticle sizes of CCO, 5% Mg:CCO, and 10% Mg:CCO nanoparticles.


<sup>1</sup> Mg concentration is averaged over five EDX measurements. <sup>2</sup> Nanoparticle size is calculated from TEM images and mean size for each sample is averaged over 50 individual nanoparticles (Figure 3).

TEM images of CCO (Figure 3a), 5% Mg:CCO (Figure 3b), and 10% Mg:CCO (Figure 3c) show the nanoparticles exist in individual or small clusters as well as large agglomerates. We only use individual or double nanoparticles (indicated by white circles) to determine particle sizes. The average nanoparticle size for CCO, 5% Mg:CCO, and 10% Mg:CCO is 10.3 ± 2.1, 8.2 ± 2.1, and 9.8 ± 3.0 nm, respectively. The size trend according to TEM results differs slightly from that of Rietveld-refined XRD data. One difference is that the particle size determined from XRD is analyzed for specific reflection

and polytype (Table 1), while TEM images are two-dimensional projections of nanoparticles with random orientation. To examine the TEM size results in details, Figure 4a shows box plots of TEM particle sizes for CCO (black), 5% Mg:CCO (red), and 10% Mg:CCO (blue) nanoparticles. The ranges of CCO and 5% Mg:CCO nanoparticle sizes are smaller than that of 10% Mg:CCO nanoparticles. It is clear that 90% of the CCO nanoparticles are larger than 8 nm. In contrast, significant fractions of both types of Mg:CCO nanoparticles are between 6 and 8 nm. Thus, the statistics of TEM results show that Mg:CCO samples have greater numbers of smaller-sized particles, although 10% Mg doping appears to broaden the distribution. Considering the particle size results from XRD and TEM, overall Mg doping decreases CCO nanoparticle sizes because the XRD results show the size decreases along the *c* axis and TEM results show greater numbers of smaller sized Mg:CCO particles.

**Figure 3.** TEM images of (**a**) CCO, (**b**) 5% Mg:CCO, and (**c**) 10% Mg:CCO nanoparticles. Individual and double nanoparticles, as shown inside white circles in (**a**)–(**c**) are used to calculate size distributions shown in Table 2 and Figure 4a.

(**a**) (**b**) (**c**)

**Figure 4.** Box plots of (**a**) TEM-determined particle sizes from Figure 3 and (**b**) dynamic light scattering (DLS)-determined sizes for CCO (black color), 5% Mg:CCO (red color), and 10% Mg:CCO (blue color) nanoparticles. The percentiles are set to 90% whisker top, 75% box top, 25% box bottom, and 10% whisker bottom for each data set. In (**a**), ~50 individual nanoparticles are used in each data set. In (**b**), ~12 batches of DLS measurements are used in each data set.

Figure 4b shows box plots of DLS sizes for CCO (black), 5% Mg:CCO (red), and 10% Mg:CCO (blue) nanoparticles dispersed in 2-MOE. For all nanoparticles, the 25–75% distributions are between 20 to 30 nm, indicating nanoparticles disperse well in 2-MOE. There are no significant differences among the three doping concentrations due to possibly similar hydrodynamic layer thickness. This is expected because the hydrodynamic layer is determined by solution ionic strength and hydrodynamic size and is often larger than dry particle size [51,52]. Since the Mg concentration in these nanoparticles is low, it is not surprising that hydrodynamic sizes for all samples are similar.

The HR TEM image of 5% Mg:CCO nanoparticles shows clear lattice fringes (Figure 5). The lattice spacing of 2.47 Å corresponds to the (012) reflection for 3R–CCO polytype. The lattice spacing of 2.33 Å corresponds to the (102) reflection for 2H–CCO polytype. No other lattice spacings corresponding to impurity phases are detected, consistent with XRD results.

**Figure 5.** High resolution TEM (HR TEM) image of 5% Mg:CCO nanoparticle. Lattice spacings corresponding to the (012) reflection in 3R-CCO and the (102) reflection in 2H-CCO polytypes are indicated.

XPS studies were carried out in order to confirm the oxidation states of Cu, Cr, and Mg in the Mg:CCO powders. XPS data was analyzed using PHI Multipak software and peak fitting was done using a Gaussian–Lorentzian profile after a Shirley type background subtraction [53]. The binding energy was shifted using the valence band edge. The measured (cross symbol) and fitted (solid curve) XPS spectra of Cu 2p3/2, Cr 2p3/2, Mg 1s, and O 1s core levels for 5% Mg:CCO (red color) and 10% Mg:CCO (blue color) nanoparticles are shown in Figure 6. Deconvolution of the Cu 2p3/<sup>2</sup> spectrum for both 5% and 10% Mg:CCO (Figure 6a) results in two peaks at 934.6 eV and 932.3 eV corresponding to binding energies of Cu(OH)2 and Cu1+, respectively, consistent with the literature [54,55]. The Cr 2p3/<sup>2</sup> spectrum (Figure 6b) can be fitted to two peaks at 577.3 eV and 576.5 eV corresponding to binding energies of Cr3<sup>+</sup> as hydroxide and Cr3<sup>+</sup> as oxide, respectively [37,56]. These are similar to the binding energy peak positions of Cu1<sup>+</sup> and Cr3<sup>+</sup> oxide of undoped CCO nanoparticles reported in the literature [24]. Figure 6c shows the Mg 1s spectra, wherein the peak is located at binding energy of 1303.1 eV, corresponding to the Mg2<sup>+</sup> oxidation state [39]. The O 1s spectrum (Figure 6d) shows peaks corresponding to lattice oxygen (OI) at 529.9 eV and hydroxyl groups (OII) at 531.5 eV for both 5% Mg:CCO and 10% Mg:CCO nanoparticles. A small-intensity peak at 533 eV corresponding to adsorbed water for 5% Mg:CCO nanoparticles is observed [54,57].

**Figure 6.** X-ray photoelectron spectroscopy (XPS) spectra of (**a**) Cu 2p3/2, (**b**) Cr 2p3/2, (**c**) Mg 1s, and (**d**) O 1s orbitals for 5% Mg:CCO (red) and 10% Mg:CCO (blue) nanoparticles. The measured XPS spectra are represented by cross symbols. The fitted XPS spectra are represented by solid curves. The black lines show the binding energies for Cu(OH)2, Cu<sup>1</sup>+, Cr3<sup>+</sup>I, Cr<sup>3</sup><sup>+</sup>II, Mg<sup>2</sup><sup>+</sup>, OI, OII, and OIII. Cr<sup>3</sup>+<sup>I</sup> represents Cr3<sup>+</sup> as oxide, Cr3+II is Cr3<sup>+</sup> as hydroxide; OI represents the lattice oxygen, OII is hydroxyl species, and OIII is adsorbed water.
