# Electron Spin Dynamics of Single and Paired Methyl Radicals at the ZnO Nanoparticle Surface

Adrien Savoyant, D. Marin, Z. Jradi, O. Margeat, S. Bertaina

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Adrien Savoyant, D. Marin, Z. Jradi, O. Margeat, S. Bertaina. Electron Spin Dynamics of Single and Paired Methyl Radicals at the ZnO Nanoparticle Surface. Journal of Physical Chemistry C, 2025, 129 (21), pp.9778 - 9785. 10.1021/acs.jpcc.5c01638. http://dx.doi.org/10.1021/acs.jpcc.5c01638

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# Electron Spin Dynamics of Single and Paired Methyl Radicals at the ZnO Nanoparticle Surface

A. Savoyant,* D. Marin, Z. Jradi, O. Margeat, and S. Bertaina

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Cite This: https://doi.org/10.1021/acs.jpcc.5c01638

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Supporting Information

**ABSTRACT:** The photoinduced paramagnetic behavior exhibited by ZnO nanoparticles (NPs) grown by hydrothermal methods is investigated by means of continuous and pulse electron paramagnetic resonance (EPR) as a function of temperature and microwave power. Aside of the usual light-induced signals of methyl radicals (•CH₃) and core defects (CDs), some other intermediate lines are often detected, which are here demonstrated to originate from radical pairs (•CH₃)–(•CH₃) coupled by an almost isotropic Heisenberg exchange interaction. Continuous and pulsed EPR experiments show that methyl radicals undergo irreversible chemical reactions above 40–50 K and reversible rotation hindrance below 30–40 K. On the contrary, radical pairs do not disappear up to 110 K, with no decay after illumination is removed. These surface methyl radical pairs are the most chemically and thermally stable photogenerated species. The paired spins have longer spin–lattice and spin–spin relaxation times, with a phase memory time of about 4.3 µs at 40 K.

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## INTRODUCTION

Zinc oxide (ZnO) stands out as a highly investigated semiconductor with significant applications in optoelectronics, energy harvesting, and spintronics technologies due to its remarkably high electron mobility, large direct band gap energy, direct and inverse piezoelectric properties, and stable excitons at room temperature.¹⁻³ It holds significant advantages for electronic devices due to its thermal and chemical stability, photoelectric responsiveness, nontoxicity, and low-cost production. In addition to these attributes, ZnO synthesized through the sol–gel method positions this material as a promising candidate for the development of nano-technologies.⁴,⁵ This particular method of synthesis imparts unique properties to the material, allowing for the production of nanoparticles (NPs) with small diameters, averaging around 10–15 nm. Furthermore, it has been demonstrated that these NPs interact strongly with visible light, generating charge carriers to induce photocatalyst⁶⁻⁸ or surface magnetic states.⁹⁻¹¹ This characteristic holds potential for the advance-ment of novel light-based devices, such as spin-polarized solar cells and spin light-emitting diodes (LEDs).¹²,¹³

In nanostructured ZnO systems, the photoinduced effects are mainly influenced by the surface-to-volume ratio, the presence of specific defects, and various materials present at the NP surface. Charge transfer from the NP bulk to the surface first implies a certain point-defect photoionization, releasing a charge that can then reach the surface to initiate a surface chemical reaction.¹⁴ Previous studies have demon-strated that ZnO NPs synthesized via the sol–gel method

present a stabilizing shell mainly composed of acetic acid (CH₃COOH) and/or acetates (CH₃COO⁻),¹⁵ depending on whether the proton has been released or not. The negatively charged acetate molecules are thereby natural candidates for the passivation of the ZnO NPs’ polar surfaces, together with their released protons. More precisely, it has been shown that below 120 K and upon subband-gap laser excitation (<3.4 eV), an initially neutral bulk acceptor releases a hole, which in turn reaches the surface to dissociate an adsorbed acetate molecule to form a quasi-stable surface methyl radical.¹¹,¹⁶ To study this kind of mechanism, continuous wave (CW) Electron Para-magnetic Resonance (EPR) is a tool of choice, allowing for the detection of the unpaired spin borne by the ionized acceptor together with that of the methyl radical and other para-magnetic species. Complementarily, pulse wave (PW) EPR brings crucial information on the electron spins’ dynamics, their concentration, and their coupling with environment.

The above-mentioned responsivity of the ZnO NPs system to visible light excitation renders it possible to monitor it from a totally diamagnetic state to a quasi-stable paramagnetic state, with contributions from the inner bulk part of the NP and from

Received: March 11, 2025

Revised: May 5, 2025

Accepted: May 5, 2025
the organic stabilizing shell as well. The adjective quasi-stable is used because, above 40–50 K, some part of the paramagnetic signal disappears when removing the illumination, while some others remain constant. Below 40 K, no loss of magnetization is observed after the illumination removal, with the system indefinitely remaining in its paramagnetic state. These features make the ZnO NP system, coupled with light, of particular interest for spin-based devices,¹⁷ nanoscale magnetic memory elements, and quantum computing.¹⁸

In this work, we aim to study the various contributions to the paramagnetic response of the excited ZnO NPs system as a function of temperature. First, it is shown that alongside the contributions of the core-defect ionized acceptor and the surface methyl radical, another weaker signal exists, which is attributed to the methyl radical pair with almost isotropic Heisenberg coupling. Contrary to the methyl radical signal, the spin pair signal remains totally stable after illumination removal up to 110 K. The PW-EPR experiments for temperatures in the range of 18–65 K give valuable information into the energy exchange processes of the spins with their environment, namely, direct phonon for the core-defect and Orbach mechanism for single and paired surface radicals. The study of the transverse (spin-spin) relaxation allows for the phase memory time measurement of each spin, which is a few microseconds for the radical species. All the results point to demonstrate that the newly identified photogenerated radical pair spin system is the most interesting in terms of chemical/thermal stability and spin dynamics. Understanding these aspects is crucial for advancing the application of NPs in various fields, such as quantum information technologies and spintronics, where the manipulation and control of spin states at manageable temperatures are pivotal for practical implementation.

## EXPERIMENTAL SECTION

ZnO NPs were synthesized using the sol-gel method,⁴ following the procedure described by Pacholski et al.¹⁹ The procedure consisted of two steps. Initially, 2.0694 g of zinc acetate [Zn(Ac)₂] (Aldrich Chemicals, 99.99%) was solubilized in 105 mL of methanol and 0.625 mL of distilled water. A second solution with 1.1236 g of potassium hydroxide (KOH, Aldrich Chemicals, 99.99%) and 57.5 mL of methanol was added dropwise at 60°C under magnetic stirring. After 3 h 15 min, the final solution was removed from the thermal bath and subjected to centrifugation for 12 min at 7800 rpm. The precipitate was washed with the solvent and centrifuged once more. The white solid was dried in an oven at 100°C. Subsequently, the recovered powder was macerated using a macerator to disaggregate agglomerations. The morphological analysis conducted using transmission and scanning electron microscopy (TEM and SEM), revealed that the NPs possess an average diameter of 15 nm and exhibit an almost spherical shape with hexagonal facets. The NPs exhibit a high degree of crystallinity and feature a hexagonal Wurtzite structure.

The continuous wave (CW) EPR experiments were performed on a Bruker EMX spectrometer operating at the X-band (9.39 GHz), equipped with a standard TE₁₀₂ cavity, allowing for external optical excitation. The power of the microwave (MW) was attenuated following the relation a = 10 log(P₀/P), where P₀ is the maximal MW power (P₀ = 200 mW). Temperature control was made by two different cooling systems: for temperatures higher than 80 K, a liquid-nitrogen flux and an intelligent temperature controller (Oxford, ITC

503) were employed. Lower temperatures (down to 8 K) were reached by using a Bruker Cryocooler.

X-band (9.73 GHz) and Q-band (33.73 GHz) pulse wave (PW) EPR experiments were carried out on a Bruker Elexsys E580 spectrometer equipped with a standard dielectric ring resonator (ER4118X-MD5) and an optical helium flow cryostat (Oxford CF9350), allowing for external optical excitation. The MW pulses were amplified with a 1 kW TWT amplifier. A typical π/2 pulse was 16 ns for the 1/2-spin system. Low temperatures were reached using a Bruker Cryocooler.

The optical excitation performed in CW and PW EPR experiments was conducted *in situ* using an OBIS LX Coherent laser with a wavelength of 405 nm set at 30 mW. In each case, the illumination was performed at the lowest possible temperature: 8 K for CW experiments and 18 K for PW experiments.

## RESULTS

**Identification of Radicals Pairs.** Before laser illumination (dark conditions), the NP powder does not exhibit any noticeable magnetic activity above the noise level. Similarly, for excitation wavelengths longer than 500 nm, no magnetic response is observed. At a threshold of about 450 nm, an EPR signal is detected, with almost maximal intensity in the violet region (~400 nm).¹⁰ The corresponding energy (3 eV) is that of an intraband gap transition, involving point-defect ionization or (equivalently) bound-exciton generation. The resulting EPR spectrum consists of two main groups of lines originating from a surface methyl radical (•CH₃)¹¹ and a core defect (CD), the latter being attributed to an intrinsic ionized acceptor within the core ZnO matrix.²⁰⁻²⁵ The underlying generation mechanism has been stated to be a photo-Kolbe reaction, initiated by the release of a photogenerated hole from the CD and its migration toward the surface to dissociate an adsorbed acetate molecule (CH₃COO⁻).¹⁶ The characteristic signal of •CH₃ is shown in Figure 1a,c, consisting of four lines (L₁ to L₄) spaced by 2.3 mT, with a gravity center at g = 2.003 and an intensity ratio of 1:3:3:1 at 85 K. At a higher field (g = 1.96), the CD signal is an asymmetric single broad line, not shown in Figure 1a,–c for clarity. The whole spectra in X- and Q-band are shown in Figure S1.

Inserted between the methyl's lines, three lines (L₅ to L₇) are also identified with weaker intensity (Figure 1a–c). These lines are always present together with those of methyl and cannot be observed separately. This indicates a causal relationship between their respective originating systems. Moreover, these two groups of lines share the same gravity center (g = 2.003), pointing to the common nature of the two systems. The identity of the two gravity centers is confirmed by a Q-band EPR experiment (Figure 1b), in which the distance between the two groups of lines (1.3 mT) remains exactly the same as in the X-band (Figure 1a).

Searching for a clear identification of these three lines, the following additional facts have been established: (i) 2-pulse ESEEM experiments indicate the presence of vicinal protons close to the electron spin (see Supporting Information), (ii) disappearing upon annealing, together with methyl radicals, these three lines are assumed to also arise from organic surface material, (iii) these lines display different (slower) electron spin dynamics in both the spin-lattice and spin-spin relaxation times, and (iv) by heating the NPs up to 110 K
<figure><img src="image_8.png" /><figcaption>**Figure 1.** EPR spectra of ZnO NPs centered around the ⋆CH₃ signal. (a) X-band CW spectrum, (b) derivative of the Q-band PW field-sweep spectrum, (c) X-band CW spectra before (0 min) and after (480 min) illumination extinction. (d) EPR intensities recorded every 10 min for each system.</figcaption></figure>

under constant illumination, the L<sub>5</sub>–L<sub>7</sub> intensities decrease much more slowly than L<sub>1</sub>–L<sub>4</sub>.

The consideration of all these experimental facts about the L<sub>5</sub>–L<sub>7</sub> lines fundamentally points toward two possibilities: (1) either a ⋆CH<sub>2</sub> radical fragment (giving three lines), or (2) a (⋆CH<sub>3</sub>)–(⋆CH<sub>3</sub>) methyl radical pair (giving intercalated lines). Possibility (1) has been considered by Maurel and co-workers in the context of a surface methyl radical at the TiO<sub>2</sub> NPs surface,<sup>26</sup> where three similar intercalated lines (between those of methyl) have been attributed to a carboxymethyl radical (⋆CH<sub>2</sub>COOH)<sup>27</sup> resulting from the recombination of a photogenerated methyl radical with an acetic acid molecule.<sup>28,29</sup> However, this possibility for explaining the L<sub>5</sub>–L<sub>7</sub> lines must be rejected here for the two following reasons: first, an intensity ratio of 1:2:1 is expected for a ⋆CH<sub>2</sub> fragment (not observed here), and second, the difference in g-factors between methyl (2.0025) and carboxymethyl (2.0032)<sup>28</sup> would imply a shift of 0.3 mT between the two spectra when passing from Q-band to X-band (not observed here, Figure 1b).

The radical pair hypothesis thus appears to be the most probable one. To investigate this, an experiment is performed where the temperature is maintained constant at 85 K and the EPR signal was recorded over time after illumination removal. The spectra were recorded just after the laser's extinction and then each 10 min for a total duration of 480 min. The first and last EPR spectra are shown in Figure 1c which shows that while the methyl's lines has experienced a drastic decay, the three-lines group remains unchanged, pointing to a noticeable stability of the latter. The detail of the intensities evolution is shown in Figure 1d for the three systems under study, including the CD. The different decay rates of the methyl and the unknown system allows to obtain different proportions of both systems, and thus to test the radical pair hypothesis in different situations.

In order to model such a ⋆CH₃–⋆CH₃ radical pair, the simplest coupling scheme is considered, that is, an isotropic Heisenberg-like interaction between the two unpaired electrons. Superhyperfine interactions between electrons and protons of different radicals, as well as dipolar interactions, are not considered. The simulations of the two experimental spectra of Figure 1c are made with the EasySpin MATLAB toolbox<sup>30</sup> and are shown in Figure 2 for a coupling constant of

<figure><img src="image_9.png" /><figcaption>**Figure 2.** Simulated and experimental (same data as Figure 1c) EPR spectra before (a) and 480 after illumination removal (b).</figcaption></figure>

350 MHz, displaying in each case the total simulation and the separate contributions of both single and paired radicals. The L<sub>5</sub>, L<sub>6</sub>, and L<sub>7</sub> line positions are reasonably reproduced, together with the satellite lines around each of the methyl lines (L<sub>1</sub> to L<sub>4</sub>), which are commonly observed but never explained. The main discrepancy is the relative intensity of the central line L<sub>6</sub>, which is notably overestimated in the simulations. However, involving only the *J* coupling value adjustment and the two contribution weightings, the resulting simulations in both situations (before and after decay) are reasonably convincing, considering that significant improvement could be obtained by introducing anisotropic coupling, super-hyperfine interaction, dipolar interaction, and other refinements. Such a more realistic but more complex radical-pair model would introduce too many adjustable parameters and has not been considered here. More details on the spectra simulation are given in Section SII.

In particular, the transition from single radical to radical pair simulated spectra with increasing *J* coupling value is given in Figure S2, which shows that above 300 MHz, the radical pair spectrum does not evolve significantly, and the intermediate line positions are fixed, intercalated between the methyl's lines. Thus, the *J* = 350 MHz value has to be seen as a lower limit for the exchange coupling. Alternatively, considering a dipolar coupling between the electron spins results in possible intermediate lines, but with positions and line widths highly dependent on the interaction intensity, i.e., on the distance between radicals (Figure S3). This distance has no reason to be exactly the one giving intercalated lines and, in fact, is certainly distributed. It must be concluded that the main coupling between radicals is mostly an exchange one, although some weak dipolar contributions are certainly present to disturb the spectra. It is also important to note that the apparent three-line group is, in fact, a seven-line group, with four of them superimposed on the methyl's lines. This may explain why the decay of the L<sub>2</sub> line after illumination removal,
as depicted in Figure 1d, does not go to zero. Finally, the radical pair hypothesis for these additional lines is quite consistent, with a single/pair ratio equal to 2.1 before and 0.7 after decay. The pair number apparently remains constant; this corresponds to a dispartition of 70% of the methyl radicals.

**Spin Dynamics: Continuous Regime.** In order to investigate the spin dynamics and concentration of the methyl and CD species, saturation curves in the CW regime are measured, enabling the qualitative characterization of spin relaxation mechanisms at different temperatures. In CW EPR, the response of the system for different MW powers leads to saturation curves, plotted as the EPR intensity vs the square root of the MW power, so that the saturation regime starts with the departure from linear behavior. At very high MW power, the EPR intensity finally falls to zero, as the excited spins do not have time to relax before being excited again. The saturation curves were analyzed for both the CD and the methyl radical L₂ line. Relevant data could not be obtained for paired radicals due to line overlap and too weak intensity above 15 dB attenuation. Experimental data for methyl and CD are displayed in Figure 3, showing typical saturation

<figure><img src="image_10.png" /><figcaption>**Figure 3.** Saturation curves for methyl radicals and CD are shown with the corresponding attenuation levels indicated in the figure. The curves were normalized by using the corresponding A value for each temperature.</figcaption></figure>

behavior. To extract qualitative information, the experimental points are simply fitted with a classical isotropic 1/2 spin Bloch model, in which the EPR intensity of a homogeneous line is described by the function:

$$I(p) = \frac{A \cdot p}{1 + sp^2} \qquad (1)$$

where $p = \sqrt{P/P_0}$ is the square root of the normalized MW power. For a purely homogeneous line, parameters A and s are related to the phenomenological relaxation times $T_1$ (longitudinal) and $T_2$ (transverse), by $A \propto N \cdot T_2/T$, and $s \propto T_1 \cdot T_2$ ($T$ the temperature, $N$ the number of spins). For the present case of inhomogeneous lines, the saturation parameter s is only effective and simply describes the departure from the linear regime.

Fits of the experimental points by (eq 1) are shown in Figure 3, with each fit and data normalized by the corresponding A parameters in order to unify the linear low-power behavior (with unity slope). At higher power, low accuracy of the fits is expected because of inhomogeneities and the Bloch-model inadequacy, especially at low temperatures. As a consequence, the A parameter deduced from the fits has low uncertainty, while the saturation s parameters have relatively high uncertainty and must be seen as purely qualitative.

First, the dependence of the A parameters on temperature is analyzed. For a usual EPR signal with a constant number of spins N and constant line width, the A parameter is expected to follow Curie law ($A \propto 1/T$). For that reason, 1/A is plotted as a function of temperature in Figure 4a, so that a linear behavior

<figure><img src="image_11.png" /><figcaption>**Figure 4.** Analysis of parameters A (a) and s (b) for the CD and methyl radicals. The CD exhibits standard Curie behavior with an AF contribution of 15 K. In contrast, the methyl radicals display nonstandard behavior, with significant changes in both parameters occurring around 40 K, highlighted by the dotted vertical line.</figcaption></figure>

is expected if N is constant. For the CD signal, 1/A is actually linear, indicating a constant number of CD spins. However, the fit line does not pass through 0 at zero temperature. This indicates a slightly modified Curie's law expressed by $A \propto N/(T-T_a)$, with $T_a = -15$ K, corresponding to a weak antiferromagnetic coupling between the CD spins.

For the methyl radical, no linear behavior is observed except around 40 K. Above this temperature, the line width is almost constant ($\Delta B \approx 3$ G), and the departure from the Curie law must corresponds to a decrease in the number of the spins. When cooling down the sample, the former intensity is not recovered, indicating an irreversible disparition of the methyl radical above 40 K. Below 40 K, the line width undergoes a substantial increase (Figure 1a,c) which may explain the observed reduction of the A parameter by a saturation effect rather than by a change in the number of spins. This last point is supported by the observed reversible change (with temperature) of the EPR intensity below 40 K. In the range of 30–50 K, the A parameter follows the Curie law.

Second, the s saturation parameters are analyzed. For usual EPR lines, with a constant spin number and environment, the s parameter continuously decreases toward zero with increasing
temperature. Figure 4b shows that this is actually the case for the CD line. The temperature increase tends to desaturate a saturated transition by speeding up the longitudinal (spin-lattice) relaxation. On the contrary, the s parameter for the methyl radical displays quite different and unusual behavior above 40 K, with an increasing saturation parameter. This can be understood by the reduction of methyl radical concentration above—deduced from the 1/A curve—which implies an augmentation of the effective spin-spin relaxation time $T_2$ and, thus, an increasing saturation. These CW results tend to qualitatively demonstrate that, while the CD spins behave normally with a constant concentration over the whole temperature range, the methyl's spins disappear above 40 K, undergoing an augmentation in their spin-spin relaxation time.

**Spin Dynamics: Pulse Regime.** Quantitative information on the relaxation times was obtained from PW EPR experiments probing the spin dynamics.30–32 For obtaining the spin-lattice relaxation time $T_1$, the inversion recovery sequence was applied to the prominent line of each system: CD, $L_2$ (single radical), and $L_6$ (paired radicals).35 The inversion recovery curves were fitted by using a simple monoexponential model. An example of such fits is given in the Supporting Information, together with all obtained fitting parameters. The $T_1(T)$ values for each temperature are presented in Figure 5. Analysis of the $T_1(T)$ behavior for

<figure><img src="image_12.png" /><figcaption>Figure 5. (a) $T_1$ in linear scale for the three systems with fits by eq 2 for CD and by eq 3 for $L_2$ and $L_6$; (b) same data for $1/T_1$ in logarithmic scales with fits by eq 2.</figcaption></figure>

each system gives insight into the mechanisms involved of the spin-lattice relaxation. In most of the solid-state cases, the temperature dependence of the longitudinal relaxation time $T_1$ has the form:

$$T_1(T) \propto \frac{1}{T^n} \qquad (2)$$

where n exponent indicates the kind of mechanism. Specifically, n = 1 indicates a direct relaxation process (1 phonon is emitted/absorbed), while n = 5, 7, and 9 correspond to Raman processes in solids (1 phonon is absorbed, 1 phonon is emitted).34,35 To determine the n exponent, $1/T_1(T)$ is plotted on logarithmic scales (Figure 5b). First, it is clear that the CD spin relaxes through a direct process (n = 1) up to 35 K, where the slight slope's change indicates the progressive

involvement of the Raman process (n > 1). This is consistent with a CD being a crystalline point defect within the wurtzite structure, exchanging energy with the lattice's phonons: single phonon absorption/emission at low temperature (direct process) and progressive phonon scattering at higher temperatures (Raman process).

For the single radical line $L_2$, a linear behavior is also observed above 35 K (n = 0.55), whereas below this temperature, a progressive slope inversion is observed. Such a temperature dependence is quite unusual and counterintuitive because the inverse spin-lattice relaxation time $1/T_1$ is only supposed to decrease with decreasing temperature (fewer and fewer phonons are available). This indicates a substantial change in the spin-lattice coupling, which is to be correlated to the change in the 1/A and s parameters occurring around 40 K (Figure 4) in the CW regime (section Spin Dynamics: Continuous Regime). Moreover, the linear behavior above 35 K is characterized by n < 1, which cannot correspond to an energy exchange with lattice phonons, either by direct or Raman processes. Other possibility is the Orbach mechanism, for which the spins do not relax energy with phonons but with a certain accessible level at an energy $\Delta$ from the electron spin energy.36 The temperature dependence of $T_1$ thus has the form:

$$T_1(T) \propto e^{\Delta/kT} \qquad (3)$$

Such a function is used to fit $T_1(T)$ for the methyl radical in the range 35–65 K, leading to a value $\Delta/k = 26$ K (Figure 5a). Discussion on the behaviors below 35 K is left to the section Discussion. Regarding the radical pair spin-lattice relaxation time, an exponent n = 0.38 is obtained, without any slope change in the whole temperature range (Figure 5b). As for the methyl radical, this indicates an Orbach mechanism with $\Delta/k = 10$ K (Figure 5a). These results are consistent with a surface location of both single and paired radicals, where no bulk phonons are available to relax energy. Rather, an internal excited state of energy $\Delta$ may allow for this relaxation. The maximal observed $T_1$ time over the whole temperature range is that of the radical pair, reaching a maximum value of 130 µs at 18 K.

The spin-spin (transverse) relaxation time measurement and analysis are not as straightforward as those for $T_1$. First, for inhomogeneous lines, the effective $T_2$ time is primarily affected by the in-plane defocusing, inversely proportional to the CW line width. For example, the $L_2$ line width of 3 G corresponds to an effective transverse time $T_2 \sim 120$ ns, below the pulse spectrometer dead time (140 ns). The rapidity of this defocusing masks the decoherence time relative to a single spin packet. However, this decoherence time (or phase-memory time) $T_2^m$ can be investigated using the basic 2-pulse echo decay sequence, canceling the in-plane defocusing by a refocusing $\pi$ pulse. Examples of the experimental $M_\gamma(\tau)$ decay curves and their fits with monoexponential curves for the three systems are given in Section SIV. Such fits are not perfect but allow for obtaining the trend for the decoherence time $T_2^m$ as a function of temperature, depicted in Figure 6. The obtained $T_2^m$ for each temperature must be seen as semiquantitative because they depend on the peculiar pulse sequence used (here $\pi/2-\tau-\pi$) and also on side effects such as instantaneous and spectral diffusion.37 Incidentally, a proton modulation is observed for the radical pair line, with almost the same modulation depth as that of the methyl radical.
<figure><img src="image_13.png" /><figcaption>**Figure 6.** $T_2^m$ times for the three systems: core defect (CD), methyl radical ($L_2$), and methyl biradical ($L_6$), as a function of temperature.</figcaption></figure>

For the CD spin, a constant and very short $T_2^m$ (170 ns) is observed, just above the spectrometer dead time (140 ns), consistent with a constant number of spins with relatively high concentration, as previously deduced from CW measurements (Sec.).

On the contrary, for methyl line $L_2$, $T_2^m$ is almost constant below 30 K and increases linearly above this temperature. This is consistent with a diminution in the methyl radical number above ~35 K, which implies a reduction of the spin-spin relaxation mechanism ($T_2^m$ time increase), consistent with the study of the 1/A and s parameters (section Spin Dynamics: Continuous Regime). This finding supports the hypothesis that, starting from this temperature, methyl radicals become subject to irreversible chemical reactions, reducing their absolute number and consequently affecting their relaxation time $T_2^m$. In a more diluted spin environment, the spins interact less, allowing coherence to be maintained for a longer time.

For the radical pair line $L_6$, only a slight linear increase is observed within the available temperature range, with no particular change around 35 K. For this line, the $T_2^m$ time can be considered almost constant, slightly influenced by the methyl radical spins.

## DISCUSSION

The ZnO NPs system studied in this work displays interesting interactions between an inorganic crystalline bulk core and an organic amorphous shell. In many cases, the organic shell resulting from the hydrothermal growth method is removed by annealing the dried powder in order to obtain ZnO NPs as pure as possible. However, an advantage can be taken from this organic shell via a magnetic functionalization of the surface, monitored by light-induced charge transfer from the NP core. More precisely, under 405 nm light irradiation, a neutral intrinsic acceptor defect is negatively ionized, thereby releasing a hole which subsequently reaches the surface to dissociate a surface-adsorbed acetate molecule, thus generating a methyl radical and carbon dioxide. This optical excitation makes the system to pass from a diamagnetic state to a paramagnetic state with two main contributions: an inner spin borne by the ionized CD, and a surface spin borne by the methyl radical $^3\text{CH}_3$. Additionally, some radical spin pairs are detected, showing much more thermal stability, less reactivity, and longer coherence time.

First, CW EPR results show that the photogenerated CD spin has a classical paramagnetic behavior, with a constant number of spins characterized by an A parameter following Curie law (Figure 4a) and a decreasing saturation s parameter with temperature (Figure 4b). Second, the PW EPR results indicate a clear direct process of energy exchange with lattice phonons below 35 K, with a progressive contribution of the Raman process as the temperature increases (Figure 5b). Finally, the observed constant phase memory time over the whole temperature range (Figure 6) is consistent with a constant number of CD spins. It is also to be noted that the slight antiferromagnetic interaction ($T_a = -15$ K) between these CD spins, deduced from the modified Curie law (Figure 4a), suggests a relatively high concentration of these defects, which is fully compatible with the very short measured $T_2^m$. All these results confirm that the CD is a crystalline bulk defect, stable with temperature and with relatively long spin-lattice dynamics and short spin-spin dynamics.

Regarding the surface methyl radical spin, the situation is not usual. The combined CW and PW EPR results show that a kind of transition happened at around 35–40 K, pointing to a regime change. First, a departure from the Curie law is observed below and above this transition temperature, with a decreasing A parameter in both cases (Figure 4a). Above the transition, the situation is clear because the decrease of the A parameter, together with the increase of the saturation s parameter and the phase memory time $T_2^m$, can be simply interpreted as a reduction in the number of methyl radical spins. The lower the methyl radical concentration, the longer the spin-spin transverse relaxation time, corresponding to a longer coherence time. Staying below 110 K, this disappearance of methyl radicals is irreversible because, upon cooling again the system, the initial EPR intensity is not recovered. The irreversibility of this disparition, together with the fact that the CD spin number does not change meanwhile, suggests that the methyl radicals undergo a surface chemical reaction to form some diamagnetic species. Note that this intermediate species must finally recombine to reform the initial acetate surface molecule because, by heating the system up to 300 K and cooling it again below 110 K, the same experiment can be performed again to give the same results. This implies that the ionized acceptor (the CD) must come back to its neutral form during this room-temperature heating process. As a consequence, the whole system (core + shell) is closed in the sense that all of the different surface species remain on the NP surface.

Now the situation below the transition temperature is examined. While the A parameter slightly decreases (Figure 4a), the phase memory time $T_2^m$ remains almost constant between 30 and 22 K and finally slightly increases at 18 K (Figure 6). These changes are reversible between 10 and 40 K. This may suggest that the number of methyl radicals remains constant below the transition, but their features undergo a reversible qualitative change. This change in the methyl radical features is confirmed by the examination of the spin-lattice relaxation time $T_1$. The analysis of $T_1(T)$ performed in the previous section has brought to light the existence of an Orbach process for the spin-lattice energy exchange of the methyl radical above the transition temperature. In such a process, no lattice phonons are involved to relax energy but rather a certain excited level of the system, consistent with a surface location of the methyl radical. For a free quantum
rotor, the rotational energy $E_R$ is quantified by a quantum number *m*, $E_R = m^2\hbar^2/2I$, where *I* is the moment of inertia.³⁸ For the methyl radical, $\hbar^2/2I = 7.5$ K³⁹ (in temperature units) and the observed EPR lines ratio 1:3:3:1 corresponds to the occupation of the two lowest rotational levels *m* = 0 and *m* = 1 with respective total nuclear spin *I* = 3/2 and *I* = 1/2 (see ref.¹¹ The next rotational level is that of *m* = 2 (*I* = 1/2) at a distance of 30 and 22.5 K from the lowest and first excited states, respectively. The obtained value for the Orbach excited level $\Delta/k = 26$ K thus suggests the involvement of this second excited rotational level in the longitudinal relaxation of the methyl radical. Below the transition, the $T_1(T)$ behavior is quite unusual, with a constant value between 30 and 22 K and a weaker value at 18 K. This unusual behavior of $T_1(T)$ is completely correlated to that of the phase memory time $T_2^m(T)$ between 18 and 30 K, as mentioned above. This qualitative change in the methyl radical features has previously been suggested to be linked to the methyl radical rotationhindering at low temperature.

One novel finding of this study is the attribution of the usually observed intermediate lines $L_5-L_7$ to some methyl radical pairs, which display interesting features in terms of thermal and chemical stability, as well as in their electron spin dynamics. First, CW EPR experiments show that even at *T* = 85 K, and after illumination removal, these methyl radical pairs $^{\bullet}$CH₃–$^{\bullet}$CH₃ remain constant in number with no chemical reaction up to 110 K. It is likely that these pairs are formed by methyl radicals generated very close to each other at the NP surface, whose unpaired spins are coupled by a Heisenberg-like interaction. This system may be seen as the excited state (*S* = 1) of an ethane molecule ($C_2H_6$) going back to its ground state (*S* = 0) above 110 K. These paired species have much more interesting dynamics than their methyl radical counterparts, both in the longitudinal relaxation time ($T_1$ = 130 µs vs 88 µs at 18 K) and in the transverse coherence time ($T_2^m$ = 4.2 µs vs 3.3 µs at *T* = 18 K). The longitudinal relaxation is shown to operate via an Orbach process, with an excited state separated from the ground state by $\Delta/k$ = 10 K, a value of the same order as that for the methyl radical (26 K), also pointing to a surface location of this species. Contrary to the methyl radical dynamics, no regime transition is observed over the studied temperature range. This may be understood by assuming that the radical pairs naturally have no rotational freedom and that the spin coupling prevents the chemical reaction occurring for a single radical. By making heating/cooling cycles after illumination removal at temperatures between 40 and 110 K, it is possible to suppress almost all of the surface methyl radicals and to magnetically functionalize the NPs' surface with only stable radical pairs. This opens the way for encoding and processing information into the spin multiplicity of a single nanoscale object, activated by visible light irradiation.

## CONCLUSIONS

ZnO NPs synthesized via the sol–gel method exhibit a complex system where the inorganic properties of the NPs are coupled with those of the organic shell formed around them during growth. Upon subband gap photoexcitation, the absorbed photons promote defects in the crystalline lattice to an excited state, releasing a hole able to reach the surface, where it can dissociate an acetate molecule into a methyl radical and carbon dioxide. Incidentally, some radical pairs are formed by close-enough photogenerated methyl radicals

interacting via an exchange-like coupling. The photogenerated surface single and paired methyl radicals are detected by CW and PW EPR experiments, which give insight into their respective dynamics. As a result, methyl radical pairs appear to be remarkably stable, both thermally and chemically, with absolutely no decay after illumination removal up to 110 K. The longest phase-memory time for the paired radical species is reached at *T* = 40 K, with a value of 4.3 µs, allowing for spin processing and manipulation at MHz frequency.

## ASSOCIATED CONTENT

### Supporting Information

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.jpcc.5c01638.

Full EPR spectra; spectra simulation; $T_1$ measurements and fits; $T_2$ measurements and fits (PDF)

## AUTHOR INFORMATION

### Corresponding Author

A. Savoyant – Aix-Marseille Univ, CNRS, Université de Toulon, IM2NP (UMR 7334), Marseille F-13397, France;

● orcid.org/0000-0003-0626-3082;

Email: adrien.savoyant@im2np.fr

### Authors

D. Marin – Aix-Marseille Univ, CNRS, Université de Toulon, IM2NP (UMR 7334), Marseille F-13397, France

Z. Jradi – Aix-Marseille Univ, CNRS, Université de Toulon, IM2NP (UMR 7334), Marseille F-13397, France

O. Margeat – Aix-Marseille Univ, CNRS, CINaM, Marseille 13288, France; ● orcid.org/0000-0003-3716-2399

S. Bertaina – Aix-Marseille Univ, CNRS, Université de Toulon, IM2NP (UMR 7334), Marseille F-13397, France;

● orcid.org/0000-0002-6466-8830

Complete contact information is available at: https://pubs.acs.org/10.1021/acs.jpcc.5c01638

### Notes

The authors declare no competing financial interest.

## ACKNOWLEDGMENTS

This work is supported by the Agence Nationale de la Recherche (ANR project "SPIMAN", ANR-21-CE09-0027-01). Financial support from the IR INFRANALYTICS FR2054 for conducting the research is gratefully acknowledged.

## REFERENCES

(1) Özgür, Ü.; Alivov, Y. I.; Liu, C.; Teke, A.; Reshchikov, M. A.; Dogan, S.; Avrutin, V.; Cho, S.-J.; Morkoç, H. A comprehensive review of ZnO materials and devices. *J. Appl. Phys.* **2005**, *98*, 041301.

(2) Sharma, D. K.; Shulka, S.; Sharma, K. K.; Kumar, V. A review on ZnO: Fundamental properties and applications. *Mater. Today Proc.* **2022**, *49*, 3028–3035.

(3) Ayoub, I.; Kumar, V.; Abolhassani, R.; Sehgal, R.; Sharma, V.; Sehgal, R.; Swart, H. C.; Mishra, Y. K. Advances in ZnO: Manipulation of defects for enhancing their technological potentials. *Nanotechnol. Rev.* **2022**, *11* (1), 575–619.

(4) Hasnidawani, J. N.; Azlina, H. N.; Norita, H.; Bonnia, N. N.; Ratim, S.; Ali, E. S. Synthesis of ZnO Nanostructures Using Sol-Gel Method. *Proc. Chem.* **2016**, *19*, 211.

(5) Alnoor, H.; Savoyant, A.; Liu, X.; Pozina, G.; Willander, M.; Nur, O. An effective low-temperature solution synthesis of Co-doped [0001]-oriented ZnO nanorods. *J. Appl. Phys.* **2017**, *121*, 215102.
(6) Bernardini, S.; Benchekroun, M. H.; Fiorido, T.; Aguir, K.; Bendahan, M.; Dkhil, S. B.; Gaceur, M.; Ackermann, J.; Margeat, O.; Videlot-Ackermann, C. Ozone Sensors Working at Room Temperature Using Zinc Oxide Nanocrystals Annealed at Low Temperature. In *Proceedings of Eurosensors 2017*; MDPI, 2017, Vol. 1, pp. 423

(7) Abou Zeid, S.; Leprince-Wang, Y. Advancements in ZnO-Based Photocatalysts for Water Treatment: A Comprehensive Review. *Crystals* **2024**, *14*, 611.

(8) Singh, K.; Nancy; Bhattu, M.; Singh, G.; Mubarak, N. M.; Singh, J. Light-absorption-driven photocatalysis and antimicrobial potential of PVP-capped zinc oxide nanoparticles. *Sci. Rep.* **2023**, *13*, 13886.

(9) Savoyant, A.; Rollo, M.; Texier, M.; Adam, R. E.; Bernardini, S.; Pilone, O.; Margeat, O.; Nur, O.; Willander, M.; Bertaina, S. Light-induced high-spin state in ZnO nanoparticles. *Nanotechnology* **2020**, *31*, 095707.

(10) Savoyant, A.; Sebastiao, C.; Margeat, O. Long-Standing Optically Excited Magnetic States in ZnO Nanoparticles. *Phys. Status Solidi* RRL **2020**, *14* (14), 2000176.

(11) Marin, D.; Gerbaud, G.; Margeat, O.; Ziarelli, F.; Ferrer, F.; Ouari, O.; Campos, A.; Bertaina, S.; Savoyant, A. Magnetic functionalization of ZnO nanoparticles surfaces via optically generated methyl radicals. *J. Chem. Phys.* **2023**, *158*, 184704.

(12) Kikkawa, J.; Awschalom, D. Lateral drag of spin coherence in gallium arsenide. *Nature* **1999**, *397*, 139–141.

(13) žutić, I.; Fabian, J.; Sarma, S. D. Spin injection through the depletion layer: A theory of spin-polarized p-n junctions and solar cells. *Phys. Rev. B* **2001**, *64*, 121201.

(14) Giamello, E. Reactive intermediates formed upon electron transfer from the surface of oxide catalysts to adsorbed molecules. *Catal. Today* **1998**, *41*, 239–249.

(15) Yang, R. D.; Tripathy, S.; Li, Y.; Sue, H. J. Photoluminescence and micro-Raman scattering in ZnO nanoparticles: The influence of acetate adsorption. *Chem. Phys. Lett.* **2005**, *411*, 150.

(16) Jradi, Z.; Marin, D.; Campos, A.; O'Byrne, M.; Margeat, O.; Bertaina, S.; Savoyant, A. Influence of Cobalt Doping on the Photogenerated Holes in ZnO Nanoparticles. *Phys. Status Solidi RRL* **2024**, *18*, 2300341.

(17) Awschalom, D. D.; Bassett, L. C.; Dzurak, A. S.; Hu, E. L.; Petta, J. R. Quantum Spintronics: Engineering and Manipulating Atom-Like Spins in Semiconductors. *Science* **2013**, *339* (6124), 1174–1179. 8

(18) Weber, J. R.; Koehl, W. F.; Varley, J. B.; Janotti, A.; Buckley, B. B.; Van de Walle, C. G.; Awschalom, D. D. Quantum computing with defects. *Proc. Natl. Acad. Sci. U.S.A.* **2010**, *107* (19), 8513–8518.

(19) (a) Pacholski, C.; Kornowski, A.; Weller, H. Self-Assembly of ZnO: From Nanodots to Nanorods. *Angew. Chem., Int. Ed.* **2002**, *41* (7), 1188–1191. (b) Kaftelen, H.; Ocakoglu, K.; Thomann, R.; Tu, S.; Weber, S.; Erdem, E. EPR and photoluminescence spectroscopy studies on the defect structure of ZnO nanocrystals. *Phys. Rev. B* **2012**, *86*, 014113.

(20) Parashar, S. K. S.; Murty, B. S.; Repp, S.; Weber, S.; Erdem, E. Investigation of intrinsic defects in core-shell structured ZnO nanocrystals. *J. Appl. Phys.* **2012**, *111* (11), 113712.

(21) Erdem, E. Microwave power, temperature, atmospheric and light dependence of intrinsic defects in ZnO nanoparticles: A study of electron paramagnetic resonance (EPR) spectroscopy. *J. Alloys Compd.* **2014**, *605*, 34.

(22) Repp, S.; Weber, S.; Erdem, E. Defect Evolution of Nonstoichiometric ZnO Quantum Dots. *J. Phys. Chem. C* **2016**, *120* (43), 25124.

(23) Savoyant, A.; Alnoor, H.; Bertaina, S.; Nur, O.; Willander, M. EPR investigation of pure and Co-doped ZnO oriented nanocrystals. *Nanotechnology* **2017**, *28*, 035705.

(24) Savoyant, A.; Alnoor, H.; Pilone, O.; Nur, O.; Willander, M. Core-defect reduction in ZnO nanorods by cobalt incorporation. *Nanotechnology* **2017**, *28*, 285705.

(25) Nadupalli, S.; Repp, S.; Weber, S.; Erdem, E. About defect phenomena in ZnO nanocrystals. *Nanoscale* **2021**, *13*, 9160–9171.

(26) Bouhadoun, S.; Guillard, C.; Sorgues, S.; Hérissan, A.; Colbeau-Justin, C.; Dapozze, F.; Habert, A.; Maurel, V.; Herlin-Boime, N. Laser synthesized TiO2-based nanoparticles and their efficiency in the photocatalytic degradation of linear carboxylic acids. *Sci. Technol. Adv. Mater.* **2017**, *18*, 805–815.

(27) Shkrob, I. A.; Chemerisov, S. D.; Marin, T. W. Photocatalytic Decomposition of Carboxylated Molecules on Light-Exposed Martian Regolith and Its Relation to Methane Production on Mars. *Astrobiology* **2010**, *10* (4), 425–436.

(28) Kaise, M.; Nagai, H.; Tokuhashi, K.; Kondo, S.; Nimura, S.; Kikuchi, O. Electron Spin Resonance Studies of Photocatalytic Interface Reactions of Suspended M/TiO2 (M = Pt, Pd, Ir, Rh, Os, or Ru) with Alcohol and Acetic Acid in Aqueous Media. *Langmuir* **1994**, *10*, 1345–1347.

(29) Nosaka, Y.; Koenuma, K.; Ushida, K.; Kira, A. Reaction Mechanism of the Decomposition of Acetic Acid on Illuminated TiO2 Powder Studied by Means of in Situ Electron Spin Resonance Measurements. *Langmuir* **1996**, *12*, 736–738.

(30) Stoll, S.; Schweiger, A. EasySpin, a comprehensive software package for spectral simulation and analysis in EPR. *J. Magn. Reson.* **2006**, *178* (1), 42–55.

(31) Hahn, E. L. Spin Echoes. *Phys. Rev.* **1950**, *80*, 580.

(32) Schweiger, A.; Jeschke, G. Principles of pulse electron paramagnetic resonance; Oxford university press, pp. 2001. DOI:.

(33) Hahn, E. L. An Accurate Nuclear Magnetic Resonance Method for Measuring Spin-Lattice Relaxation Times. *Phys. Rev.* **1949**, *76*, 145.

(34) *Multiple electron resonance spectroscopy, Plenum*; Dorio, M. M.; Freed, J. H., Eds.; Springer: New York, 1979. .

(35) Gu, L.; Wu, R. Origin of the anomalously low Raman exponents in single molecule magnets. *Phys. Rev. B* **2021**, *103*, 014011.

(36) Culvahouse, J. W.; Richards, P. M. T1 and T2 for Orbach Relaxation Processes. *Phys. Rev.* **1969**, *178*, 485.

(37) Boscaino, R.; Gelardi, F. M. Instantaneous diffusion in spin-echo dynamics: A frequency-domain experimental investigation. *Phys. Rev. B* **1992**, *46*, 14550.

(38) Freed, J. H. Quantum Effects of Methyl-Group Rotations in Magnetic Resonance: ESR Splittings and Linewidths. *J. Chem. Phys.* **1965**, *43*, 1710.

(39) Somoza, M. M.; Friedrich, J. The physics of rotational tunneling: hole-burning spectroscopy of methyl groups. *Low Temp. Phys.* **2006**, *32*, 1020–1027.