Theoretical Investigation of Ru-Doped Wurtzite Zno: Insights into Electronic Structure and Photocatalytic Potential

Abstract

Zinc oxide (ZnO), a wide-bandgap semiconductor, has garnered significant interest for photocatalytic applications due to its excellent chemical stability, non-toxicity, and strong oxidative capability. In this study, density functional theory (DFT) calculations were employed to explore the impact of ruthenium (Ru) doping on the structural, electronic, and magnetic properties of wurtzite ZnO. The introduction of Ru leads to bandgap narrowing and the emergence of impurity states, thereby enhancing visible light absorption. Charge density analysis reveals enhanced electron delocalization, while the projected density of states (PDOS) indicates strong hybridization between the Ru 4d orbitals and the ZnO electronic states. The density of states at the Fermi level, N(EF), exhibits a notable dependence on doping concentration and magnetic configuration. For non-magnetic states, N(EF) reaches 11 states/eV and 9.5 states/eV at 12.5% and 25% Ru concentrations, respectively. In ferromagnetic configurations, these values decrease to 0.65 states/eV and 1.955 states/eV, while antiferromagnetic states yield 4.945 states/eV and 0.65 states/eV. These variations highlight Ru’s crucial role in regulating electronic density, thereby affecting electrical conductivity, magnetic properties, and photocatalytic efficiency. The results offer theoretical guidance for designing high-performance Ru-doped ZnO photocatalysts.

1. Introduction

Photocatalysis has emerged as a sustainable and eco-friendly approach for environmental remediation and renewable energy conversion, with semiconductor materials playing a pivotal role in harnessing solar energy for driving chemical transformations [1]. Among various semiconductors, zinc oxide (ZnO) has attracted considerable attention due to its notable photocatalytic characteristics, including high charge carrier mobility, strong redox potential, non-toxicity, and environmental compatibility [2]. Despite these advantages, the practical deployment of ZnO is hindered by two major limitations: its wide bandgap of 3.37 eV, which confines light absorption primarily to the ultraviolet (UV) region, and the rapid recombination of photo-generated electron-hole pairs, which diminishes photocatalytic efficiency [3]. To address these drawbacks, several modification strategies have been explored, including nanostructuring, heterojunction engineering, and elemental doping [4]. Among these, doping ZnO with transition metals (TMs) has proven especially effective in tuning its electronic structure, enhancing visible light absorption, and promoting charge carrier separation. Ruthenium (Ru), a 4d transition metal, stands out as a promising dopant due to its excellent catalytic activity, favorable electronic configuration, and ability to introduce mid-gap states [5]. Ru doping has been reported to significantly improve the photocatalytic performance of ZnO by extending optical absorption into the visible region and mitigating charge recombination [6]. However, a detailed theoretical understanding of the effects of Ru doping on the structural, electronic, magnetic, and optical properties of ZnO remains incomplete [7,8]. ZnO-based materials have long been utilized in diverse technological applications, including gas sensors, solar cells, detectors, flat-panel displays, liquid crystal displays (LCDs), and UV semiconductor lasers [9,10]. Apart from being non-toxic, ZnO exhibits intriguing multifunctional properties such as piezoelectricity, ferroelectricity, and ferromagnetism [11]. In the field of nanotechnology, ZnO nanoparticles are of particular interest due to their size and shape-dependent antibacterial activity [12]. The exceptional optoelectronic properties of ZnO, as a representative II–VI compound semiconductor, have inspired extensive research for its industrial applicability [13]. ZnO nanostructures are typically classified by their dimensionality: one-dimensional (1D) forms such as nanorods, nanowires, belts, and combs [14,15], two-dimensional (2D) nanosheets and nanoplates [16], and three-dimensional (3D) architectures like flower-like, snowflake, and urchin-like morphologies [17].

Hexagonal wurtzite ZnO is an n-type semiconductor with a direct bandgap and excellent photoelectric properties [18], along with high chemical stability [19], low static dielectric constant, and strong optoelectronic response [20]. Doping with various elements such as Al, Mg, and Fe has been shown to enhance its n-type conductivity and tailor its bandgap [21,22]. More generally, metal ion doping can significantly increase electrical conductivity while maintaining transparency in the visible and near-UV regions [23]. Ruthenium substitution has also been demonstrated to enhance the electronic and magnetic behavior of complex oxide systems, such as cation-deficient manganites and thin-film TiO₂ structures, as well as to boost superconductivity in Nb₂Pd_1−xRu_xS₅ [24,25,26,27]. Furthermore, numerous dopants—including F, B, Al, Ga, In, and Sn—have been employed to fabricate conducting ZnO films [28]. Rare-earth and noble-metal doping (Eu, Pd) has also been investigated for tailoring the magnetic and electronic properties of ZnO [29,30,31]. In recent years, such modifications have expanded the scope of ZnO for use in optoelectronics and photocatalysis. Nevertheless, further insight into the role of Ru doping, especially from a first-principles perspective, is essential for the rational design of ZnO-based photocatalysts with enhanced efficiency and broader functionality [32,33]. Our results show that Ru doping significantly modifies the band gap and d-state distribution in ZnO. Similar effects were observed by Liu et al. [34] for Fe-doped ZnO, where transition-metal d–O 2p hybridization played a key role in tailoring electronic and magnetic properties. This highlights the versatility of transition-metal doping for engineering ZnO-based photocatalysts and spintronic materials. In addition to these strategies, another crucial and widely adopted method is the introduction of intrinsic surface defects, especially oxygen vacancies. Oxygen vacancies can act as active sites for catalytic reactions and are known to generate localized states within the bandgap, which improve visible light harvesting and promote charge separation. Recent studies have emphasized the key role of such defects in modulating the electronic structure and improving photocatalytic efficiency [35]. For instance, defect engineering through controlled creation of oxygen vacancies has been shown to significantly enhance ZnO’s activity under solar irradiation [36]. Therefore, understanding and integrating both dopant effects and defect-induced modifications is critical for tailoring the material properties for energy and environmental applications. Motivated by these insights, the present work investigates the impact of Ru doping on the electronic and photocatalytic properties of wurtzite ZnO using density functional theory (DFT). The study focuses on how substitutional Ru atoms influence the band structure, magnetic behavior, and electronic density of states at various doping concentrations, providing a theoretical framework to assess their potential in visible-light-driven photocatalysis. The system under study comprises Zinc (Zn: [Ar]3d¹⁰4s²2), Oxygen (O:[He]2s²2p⁴), and Ruthenium (Ru: [Kr]4d⁷5s¹. Except for oxygen, both Zn and Ry possess d-orbital electrons in their electronic configurations [37]. Upon doping of Ru at Zn sites, the calculations reveal a significant narrowing of the bandgap along with the emergence of localized impurity states near the Fermi level. This electronic restructuring is consistent with previous theoretical studies on transition metal (TM)-doped ZnO systems, where similar effects have been reported for dopants such as Co, Fe, and Ni [38]. Transition metal (TM) doping is widely recognized for its ability to introduce mid-gap states that extend light absorption into the visible region an essential requirement for effective photocatalytic applications [39]. Although direct experimental studies on Ru-doped ZnO remain scarce, related research on Ru-doped TiO₂ and RuO₂-based photocatalysts has shown enhanced photocatalytic performance under visible light irradiation [40,41,42]. These improvements have been primarily attributed to the formation of intermediate energy levels within the bandgap and improved charge carrier separation, both of which are consistent with our theoretical predictions. These parallels suggest that incorporating Ru into the ZnO lattice can effectively modulate its electronic structure to favor enhanced photocatalytic activity, thus motivating further experimental validation. In this study, we conduct a comprehensive theoretical investigation of Ru-doped wurtzite ZnO using first-principles density functional theory (DFT) calculations. Our focus is on understanding how Ru incorporation at varying doping concentrations (12.5%, 25%, and 50%) influences the structural, electronic, and magnetic properties of ZnO. Detailed analyses of the band structure, density of states (DOS), and charge density distribution are carried out to evaluate the photocatalytic potential of Ru-doped ZnO under visible light. To the best of our knowledge, this work represents one of the first in-depth theoretical studies specifically targeting the role of Ru doping in ZnO for photocatalytic applications. The insights gained here offer predictive guidance for the rational design and experimental development of efficient Ru-based ZnO photocatalysts. The La-doped ZnO photocatalyst demonstrated enhanced visible-light-driven photocatalytic degradation of paracetamol compared to pure ZnO. Lanthanum doping improved light absorption and suppressed electron-hole recombination, resulting in higher degradation efficiency and catalyst stability over multiple cycles [43].

2. Computational Methods

First-principles calculations based on density functional theory (DFT) were carried out using the Quantum ESPRESSO simulation package, specifically employing the PWscf Plane-Wave Self-Consistent Field (PWscf) code. We investigated the electronic properties of Ru-doped ZnO (Zn_1–xRu_xO) for doping concentrations of x=0.125,0.25, and 0.50. The electronic band structure, partial density of states (PDOS), and total density of states (DOS) were computed to evaluate the effects of Ru substitution on the electronic characteristics of ZnO. The Brillouin zone was sampled using an 8 × 8 × 10 Monkhorst-Pack k-point mesh. Ultrasoft pseudopotentials (USPP) were used in conjunction with the generalized gradient approximation (GGA) utilizing the Perdew–Burke–Ernzerhof revised for solids (PBEsol) exchange-correlation functional. The hexagonal lattice parameters were set to a = 3.2490 Å and c = 5.20899 Å, as reported in previous studies [44]. Atomic positions were initialized with Zn at (0.1666667, 0.3333333, 0.0000000) and O at (0.1666667, 0.3333333, 0.1900000), obtained through structural visualization using the VESTA software. The electronic band structures were calculated along high-symmetry directions in the conventional hexagonal Brillouin zone: Γ–M–K–Γ–A–L–H–A–L–M–K–H [45]. For accurate DOS calculations, the tetrahedron method was employed. Supercells were constructed to model different Ru doping concentrations using the Phonopy package, with a 2 × 2 × 1 supercell for 12.5% doping and a 2 × 1 × 1 supercell for 25%. Band structures were plotted for each case to compare the evolution of electronic states within the first Brillouin zone. Both non-magnetic and spin-polarized (ferromagnetic and antiferromagnetic) calculations were performed to evaluate the influence of Ru incorporation on the electronic and magnetic properties of ZnO.

3. Results and Discussion

In this section, we present and analyze the structural, electronic, and magnetic properties of Ru-doped wurtzite ZnO to evaluate its potential for enhanced photocatalytic activity. First-principles calculations based on density functional theory (DFT) were employed to explore the influence of ruthenium (Ru) substitution at the Zn site on the host ZnO lattice. The motivation for this study arises from the inherent limitations of pristine ZnO in photocatalytic applications, primarily due to its wide bandgap and limited visible light absorption. Transition metal doping has been widely proposed as an effective strategy to overcome these limitations by inducing mid-gap states and narrowing the bandgap [46]. Among various dopants, Ru has attracted interest due to its favorable electronic configuration and ability to tailor the photocatalytic properties of oxide semiconductors [47]. Figure 1 shows the optimized crystal structures of Ru-doped wurtzite ZnO at three different doping concentrations: 50%, 25%, and 12.5%. The wurtzite framework remains largely intact, with only slight distortions observed around the dopant site due to local lattice relaxation. As the Ru concentration increases to 25%, two Zn atoms are substituted by Ru atoms, introducing more noticeable distortions in the lattice. Although the hexagonal wurtzite symmetry is preserved, localized strain becomes more evident, especially around the Ru-O coordination environment. At the highest doping level of 50%, half of the Zn atoms are replaced with Ru, resulting in significant structural distortion. The proximity of multiple Ru atoms leads to strong Ru–O interactions and possible Ru–Ru coupling, which destabilizes the original wurtzite structure [48]. These changes are indicative of increasing lattice strain and potential phase instability at high doping levels, which are critical factors influencing the material’s photocatalytic behavior. At 12.5% Ru doping, we observe the formation of localized impurity states near the conduction band minimum, indicating an initial narrowing of the bandgap and enhancement of visible light absorption. For 25% doping, stronger Hybridization between Ru 4d and O 2p orbitals leads to notable changes in both the valence and conduction bands, introducing mid-gap states that facilitate better charge separation and mobility features that are beneficial for photocatalysis. At the highest concentration of 50%, the system approaches metallic behavior, as evidenced by the increased density of states at the Fermi level, suggesting a trade-off between doping level and photocatalytic efficiency due to possible recombination losses.

The electronic structure of Ru-doped wurtzite ZnO at varying concentrations (12.5%, 25%, and 50%) is explored through the calculated band structures and projected density of states (PDOS), as illustrated in Figure 2. The band structure diagrams (Figure 2a–c) and the corresponding PDOS plots (Figure 2d–f) reveal the impact of Ru substitution on the host ZnO electronic states. In the case of 12.5% Ru doping (Figure 2a,d), the conduction band minimum (CBM) shifts downward, and new impurity states emerge near the Fermi level, indicating partial metallization. These states are primarily derived from the Ru 4d orbitals hybridized with the Zn 3d and O 2p states [49]. The overall bandgap is reduced compared to pure ZnO, suggesting that Ru doping effectively narrows the bandgap and modifies the electronic environment to promote potential semiconducting-to-metallic transitions. With an increased doping concentration of 25% (Figure 2b,e), the electronic structure undergoes more profound changes. The band structure indicates a further reduction of the bandgap, with Ru-induced states becoming more dominant around the Fermi level. This indicates enhanced carrier concentration and increased electronic conductivity. In the PDOS (Figure 2e), a clear broadening and shift of the Ru 4d states can be seen, overlapping with the conduction and valence band edges. Such hybridization points to a stronger perturbation of the ZnO matrix by Ru dopants. The spin polarization seen in the PDOS suggests a possibility of magnetic ordering emerging due to exchange interactions among Ru sites, which becomes more likely as the distance between dopant atoms decreases with increasing concentration [50]. At 50% Ru doping (Figure 2c,f), the system shows metallic characteristics with a significant density of states crossing the Fermi level. The band structure no longer exhibits a clear gap, confirming a full semiconductor-to-metal transition. The Ru 4d states dominate the vicinity of the Fermi level, strongly overlapping with the Zn and O states, thereby indicating complete delocalization of charge carriers. The PDOS plot in Figure 2f shows substantial contributions from Ru 4d orbitals in both spin channels, further confirming the metallic behavior. A high concentration of Ru results in a band-filling effect and may introduce disorder, potentially causing electron localization or magnetic fluctuations depending on the synthesis conditions and temperature [51,52,53]. The first-principles study revealed that Co doping in ZnO induces ferromagnetism primarily through strong hybridization between Co 3d and O 2p orbitals, significantly modifying the electronic structure and magnetic properties of ZnO [54].

Table 1. Fermi energy (eV), total energy (Ry), lattice parameter ($\mathring{A}$), and volume (${\mathring{A}}^3$) of ${\mathrm{Zn}}_{1-x}$${\mathrm{Ru}}_x$O.

Composition	Fermi Energy	Total Energy of FM	Lattice Parameter	Volume
0.125	9.695	−1489.211	12.431	1332.843
0.25	10.593	−767.574	12.431	666.421
0.5	13.250	−406.796	6.095	316.713

influence of Ru doping on the structural and electronic properties of ZnO was systematically examined for ${\mathrm{Zn}}_{1-x}$${\mathrm{Ru}}_x$O compositions with Ru concentrations of 12.5%, 25%, and 50%. A clear upward trend in Fermi energy is observed with increasing Ru content, rising from approximately 9.70 eV at 12.5% to 13.25 eV at 50%, marking an increase of around 36%. This indicates a progressive shift of the Fermi level into higher energy states, likely due to the introduction of Ru 4d orbitals, which enhances the metallic nature of the system. Concurrently, the total energy of the ferromagnetic (FM) phase becomes progressively less negative, changing from −1489.21 Ry at 12.5% Ru to −406.80 Ry at 50%, reflecting a notable decrease in thermodynamic stability. This reduction may be attributed to local lattice distortions and electronic interactions introduced by the substitution of Zn with Ru ions of different sizes and electronic configurations. Structurally, the lattice parameter remains unchanged at 12.431 Å for both 12.5% and 25% Ru doping but shows a sharp contraction to 6.095 Å at 50%, indicating a decrease of nearly 51%. This pronounced shrinkage suggests a possible structural phase and increased lattice compaction due to high Ru incorporation. Overall, these results demonstrate that Ru doping substantially alters both the electronic structure and the lattice characteristics of ZnO, promoting a transition toward metallicity while inducing significant structural changes at higher doping levels. These reductions are due to the smaller size of the unit cell used for this higher doping level but also suggest increased lattice distortion or possible structural phase transition as more Zn atoms are replaced with Ru [55].

The overlap between Ru 4d and ZnO host orbitals leads to a broadened electronic structure, diminishing spin polarization due to the suppression of localized magnetic moments and reduced exchange splitting [56]. This progressive transition from spin-polarized semiconducting to half-metallic and ultimately metallic behavior with increasing Ru doping showcases the tunability of the electronic and magnetic properties of wurtzite ZnO through transition metal substitution. The emergence of half-metallicity at 25% Ru doping is particularly promising for spintronic applications, where control over spin channels is vital. However, excessive Ru incorporation (50%) may result in the loss of spin selectivity due to strong band overlap and metallic screening. Therefore, an optimal doping range exists where the material exhibits the desired combination of magnetic ordering and electronic conductivity suitable for spin-injection or magnetoresistive devices.

Figure 3 states that the spin-up (I) and spin-down (II) channels are plotted separately, allowing for a clear observation of spin asymmetry and magnetic exchange effects induced by Ru substitution. The evolution of the band structure with increasing Ru content reflects the profound impact of Ru 4d states on the host ZnO electronic configuration and spin polarization behavior. At 12.5% Ru doping, the band structure reveals a semiconducting nature for the spin-up channel, with a small bandgap between the valence band maximum (VBM) and conduction band minimum (CBM). In contrast, the spin-down channel displays mid-gap states that appear close to the Fermi level, significantly narrowing the bandgap or even making it nearly metallic. This asymmetry suggests spin polarization and the possible emergence of half-metallicity, where one spin channel remains semiconducting while the other becomes conducting. The PDOS analysis (Figure 3d) confirms this observation: Ru 4d states hybridize with O 2p states and contribute strongly near the Fermi level, particularly in the spin-down channel. This hybridization introduces localized magnetic moments and exchange splitting, essential for spintronic functionality. With increasing Ru concentration to 25%, more pronounced modifications are observed. The spin-up bands shift upward in energy, and the bandgap becomes narrower, while the spin-down channel exhibits fully metallic behavior with states crossing the Fermi level. This indicates a transition toward robust half-metallicity. The PDOS (Figure 3e) shows an enhanced contribution of Ru 4d states at the Fermi level, especially for spin-down states, suggesting increased itinerancy of Ru-induced carriers and stronger exchange interactions. The presence of delocalized Ru states reduces the bandgap and increases spin asymmetry, which could promote spin-polarized current in practical devices. At the highest doping level (50% Ru), the system undergoes a complete metallization in both spin channels. As seen in the band structure (Figure 3c), several bands cross the Fermi level for both spin-up and spin-down states, indicating the loss of semiconducting behavior and transition into a metallic phase. The PDOS (Figure 3f) supports this observation with a high density of Ru 4d and Zn 3d states at the Fermi level for both spin directions.

Figure 4 presents the spin-resolved projected density of states (PDOS) for ferromagnetic (FM) Figure 4a–c and antiferromagnetic (AFM) Figure 4d–f configurations of Ru-doped wurtzite ZnO at doping concentrations of 12.5%, 25%, and 50%. These plots highlight the role of Ru 4d states and their interaction with Zn 3d and O 2p orbitals in influencing the magnetic and electronic properties of the system. At 12.5% Ru doping, the FM PDOS (Figure 4a) shows significant spin asymmetry, particularly around the Fermi level, indicating spin polarization due to exchange splitting of Ru 4d states. The AFM counterpart (Figure 4d), however, displays nearly symmetric spin channels, consistent with antiparallel spin alignment and reduced net magnetization. The sharp peaks near the Fermi level suggest the localized nature of Ru states, which dominate the magnetic interaction at this concentration. For 25% doping, the FM PDOS (Figure 4b) exhibits enhanced hybridization between Ru 4d and O 2p states, leading to broader features near the Fermi level and stronger spin polarization. The AFM configuration in Figure 4e again shows a more symmetric profile but with some residual imbalance, suggesting competing magnetic interactions and partial moment compensation. The system remains magnetic, with the FM configuration likely energetically favorable. At 50% Ru doping, both FM Figure 4c and AFM Figure 4f PDOS profiles become increasingly metallic with broadened Ru-derived states crossing the Fermi level in both spin channels. The FM configuration still shows a slight spin asymmetry, but the exchange splitting is reduced, indicating weakening ferromagnetic order at high Ru concentrations due to band broadening and delocalization of Ru 4d electrons [57]. These observations suggest that Ru doping not only tunes the magnetic configuration of ZnO but also drives a transition from localized magnetic moments (low doping) to itinerant magnetism and eventual metallicity (high doping). The FM configuration is generally more spin-polarized at low and intermediate doping, making it favorable for spintronic applications, while AFM ordering becomes increasingly competitive at higher doping levels.

Table 2. Fermi level (EF) and total energy (TE) of FM and AFM ${\mathrm{Zn}}_{1-x}$${\mathrm{Ru}}_x$O.

Composition	EF of FM (eV)	TE of FM (Ry)	EF of AFM (eV)	TE of AFM (Ry)	$\mathbf{\Delta }$ E
0.125	9.627	−297.627	9.803	−2977.641	0.014
0.25	10.888	−1534.436	9.889	−1534.564	0.127
0.5	12.944	−812.799	12.703	−812.812	0.013

provides a detailed comparison of the Fermi level (EF) and total energy (TE) for both the ferromagnetic (FM) and antiferromagnetic (AFM) configurations of Ru-doped ZnO at different Ru doping concentrations: 12.5%, 25%, and 50%. For 12.5% Ru doping, the Fermi level for the FM configuration is 9.6266 eV, while for the AFM configuration, it shifts to 9.8890 eV. The total energy for the FM configuration is −2977.6275 Ry, while the energy for the AFM configuration is slightly more negative at −2977.641 Ry, implying that the AFM state is marginally more stable than the FM state by an energy difference of $\Delta$E = 0.0143 Ry. This small energy difference indicates that both FM and AFM configurations are nearly degenerate at this low doping concentration, suggesting that the material can exist in either magnetic state with similar stability. For 25% Ru doping, the Fermi level for the FM configuration increases to 10.8881 eV, while the AFM Fermi level drops to 9.8025 eV. The total energy for the FM configuration is −1534.4359 Ry, whereas the energy for the AFM configuration is −1534.5635 Ry. The energy difference ($\Delta$E = 0.127565 Ry) between the FM and AFM states is significantly larger than at 12.5% doping, indicating that the FM state is now more stable than the AFM state, favoring ferromagnetism at this concentration [58]. For 50% Ru doping, the Fermi level for the FM configuration rises further to 12.9443 eV, and for the AFM configuration, it is 12.7034 eV. The total energy for the FM configuration is −812.7992 Ry, and for the AFM configuration, it is −812.8123 Ry, with a very small energy difference ($\Delta$E = 0.0131 Ry). At this high doping concentration, the energy difference between the FM and AFM configurations is again small, suggesting that both states are nearly degenerate. However, the very slight preference for the AFM configuration at this doping level implies that the system may exhibit more complex magnetic behavior as the Ru concentration reaches higher levels.

Table 3. Total magnetization and absolute magnetization for FM and AFM of 12.5%, 25%, and 50% ruthenium-doped wurtizte ZnO.

	Ferromagnetic (FM)		Antiferromagnetic (AFM)
Composition	Total Mag.	Absolute Mag.	Total Mag.	Absolute Mag.
0.125	3.85	4.38	0.0000	5.31
0.25	5.51	5.74	0.0000	7.34
0.5	8.44	8.53	0.0000	7.47

presents the total magnetization and absolute magnetization for both ferromagnetic (FM) and antiferromagnetic (AFM) configurations of Ru-doped wurtzite ZnO at various doping concentrations: 12.5%, 25%, and 50%. These values provide insights into the magnetic properties of the material at different doping levels and magnetic ordering. In the FM configuration, the total magnetization is the sum of the magnetic moments of all atoms in the system, while the absolute magnetization refers to the magnitude of the total magnetization, irrespective of the direction of the magnetic moments. At 12.5% Ru doping, the total magnetization is 5.51 $\mu$_B, and the absolute magnetization is 5.74 $\mu$_B. The slight difference between total and absolute magnetization suggests a small degree of magnetic moment cancellation, indicating a weaker ferromagnetic ordering. The material exhibits noticeable ferromagnetism, but the moments are not perfectly aligned, which is consistent with a moderate doping level where spin polarization is weaker. At 25% Ru doping, the total magnetization drops to 3.85 $\mu$_B, but the absolute magnetization increases to 4.38 $\mu$_B. The decrease in total magnetization suggests a reduction in the alignment of magnetic moments, possibly due to the higher concentration of Ru, which might introduce competing magnetic interactions. However, the increase in absolute magnetization indicates that the material still retains significant spin polarization, though the magnetic ordering might be less stable than at lower doping levels. At 50% Ru doping, the total magnetization increases significantly to 8.44 $\mu$_B, and the absolute magnetization is 8.53 $\mu$_B. This shows that, at high doping levels, the magnetic moments align more strongly in the FM configuration, resulting in a higher total magnetization.The total and absolute magnetization values are nearly identical, indicating a robust ferromagnetic order with negligible moment cancellation within the system. This finding supports previous observations that increasing Ru doping enhances ferromagnetism in ZnO. In contrast, the antiferromagnetic (AFM) configuration exhibits a total magnetization close to zero ${\mu }_{\mathrm{B}}$ across all doping levels, consistent with the expected cancellation of opposing magnetic moments in AFM materials. However, a small residual magnetization persists in the AFM state, arising from the partial alignment of individual magnetic moments.

Recent advances in magnetic field-assisted photocatalysis have demonstrated that applying external magnetic fields can significantly enhance photocatalytic efficiency by promoting charge carrier separation, modulating spin states, and influencing reaction kinetics. Li et al. [59] comprehensively reviewed these effects, highlighting the potential of magnetic fields to improve photocatalytic performance in environmental remediation and energy conversion applications. At 25% Ru doping, the absolute magnetization is 7.34 $\mu$_B, which is higher than that at 12.5% doping, suggesting that the AFM configuration becomes more stable or more defined with increased Ru concentration. At 50% Ru doping, the absolute magnetization is 7.4 ${\mu }_B$. The absolute magnetization values are reported as well: At 12.5% Ru doping, the absolute magnetization is 5.31 ${\mu }_B$, indicating that while the material shows no net 7 ${\mu }_B$, which is slightly higher than at 25%, further suggesting that the AFM configuration remains somewhat stable at higher doping levels, with stronger magnetic interactions among the dopant atoms.

4. Conclusions

A theoretical investigation of Ru-doped wurtzite ZnO was carried out using first-principles DFT calculations. A structural analysis confirmed minor lattice distortions upon Ru incorporation without disrupting the wurtzite framework. Band structure calculations revealed significant bandgap narrowing and impurity states introduced by Ru 4d orbitals near the Fermi level. Density of states (DOS) and partial DOS (PDOS) analyses showed strong hybridization between Ru 4d and O 2p orbitals, altering the electronic characteristics of ZnO. Ru doping transformed ZnO from a semiconductor to a metallic system across all non-magnetic (NM) and most magnetic states. In NM configurations, N(EF) values of 11, 9.5, and 8 states/eV were recorded for 12.5%, 25%, and 50% Ru doping, respectively. In FM configurations, N(EF) decreased to 0.824 and 2.2 states/eV for 12.5% and 25% doping. Notably, at 25% Ru doping in the FM state, a bandgap opened in the down-spin, indicating a possible half-metallic behavior. AFM configurations yielded N(EF) values of 4.9 and 0.48 states/eV at corresponding doping levels.

5. Recommendation

Further experimental synthesis and photocatalytic performance testing of Ru-doped ZnO at 12.5–25% concentrations are strongly recommended to validate the theoretical predictions and optimize the material for visible light photocatalytic applications.