Facet-dependent oxysulfidation of Cu₂O nanomaterials: implications for improving the efficacy of nanopesticides

Abstract

Copper (hydr)oxide nanomaterials are an important class of nanomaterials with various applications, including next-generation pesticides. The efficacy of these materials is largely affected by oxysulfidation, one of the most important transformation processes in the environment. Here, we show that the extent and route of oxysulfidation are facet-dependent for these materials. Specifically, oxysulfidation of Cu₂O_{100} and Cu₂O_{111}—two Cu₂O nanomaterials with predominantly exposed {100} and {111} facets—is fast and complete, with only a hollow shell left at the end of the experiment. In comparison, oxysulfidation of Cu₂O_{110}, a nanomaterial with {110} facet, is much less complete, in that, the end-product exhibits a yolk–shell structure with a large cuprite core. The varied degrees of oxysulfidation are attributable to the facet-dependent adsorption affinities of Cu₂O for both oxygen and sulfide ions, leading to the formation of different initial oxysulfidation products. Unlike the porous coatings of yarrowite on Cu₂O_{111} and a mixture of yarrowite and covellite on Cu₂O_{100}, the condensed layer of djurleite formed on Cu₂O_{110} passivates the material by sealing the surface of Cu₂O, hindering subsequent copper dissolution. Consequently, Cu ions released from Cu₂O_{100} and Cu₂O_{111} are 2.2 and 2.4 times higher than that from Cu₂O_{110}. These findings underline the important role of exposed facets in dictating the interfacial processes of soft metal-based nanomaterials, and have important implications for improving the efficiency of nanopesticides in redox dynamic rhizospheres to minimize the environmental impacts associated with the overuse of conventional pesticides.

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Environmental significance

With the increasing production and application of soft metal-based nanomaterials, their environmental release is inevitable. The fate, transport, and effects of these nanomaterials in the environment are largely dependent on their transformation processes, particularly, oxysulfidation in sulfur-rich environments such as wastewater treatment plants, sediments, and rhizosphere soils. The critical role of exposed facets in determining the route and propensity of oxysulfidation as demonstrated in this study underlines the importance in further understanding the structure–activity relationship governing the interfacial processes of soft metal-based nanomaterials. The findings also have important implications for the design of next-generation agricultural chemicals to minimize the environmental impacts associated with the overuse of conventional pesticides and fertilizers.

1. Introduction

Copper (hydr)oxide nanomaterials are an important class of nanomaterials that have shown great promise in a variety of applications, such as photocatalysis, gas sensing, and electrode materials for lithium-ion batteries, to mention a few.^1–5 The projected production of copper (hydr)oxide nanomaterials is 1900 metric tons by 2031.¹ Under such circumstances, environmental release of these nanomaterials during production, transportation, use and disposal is inevitable.^6–8 Understanding the environmental behavior and fate of these nanomaterials is crucial for the assessment of their potential risks and for the improvement of their environmental compatibility.^9–12 Oxysulfidation is one of the most important transformation processes of soft metal-based nanomaterials, and is induced by dissolved oxygen due to redox fluctuation as well as sulfide species that are ubiquitous as a result of microbial activities,^13,14 particularly in wastewater treatment plants,^15–17 sediments,¹⁸ and rhizosphere soils.¹⁹ Oxysulfidation results in (partial) transformation of the nanomaterials into metal sulfides, and consequently, can significantly alter their bioavailability and risks.^20–24 Note that understanding the oxysulfidation processes of copper (hydr)oxide also sheds light on nano-enabled agriculture, as these nanomaterials are considered next-generation pesticides for the prevention of root rot diseases of crops,²⁵ owing to their excellent antimicrobial activities.^26,27

Previous research has shown that the extent of oxysulfidation of copper (hydr)oxide nanomaterials is closely related to their physicochemical properties (e.g., size, shape and surface coating).^28–32 One structural property likely of particular importance is exposed facet – by dictating surface atomic arrangement of crystalline nanomaterials, exposed facets largely control the surface reactivities of nanocrystals, such as solubility and adsorption affinity.^33–38 For example, both experimental studies and theoretical calculations have shown that Cu₂O nanocrystals with the exposed {111} facet and those with the {100} facet exhibit different affinities for molecular O₂, leading to different extents of Cu²⁺ release and surface oxidation.^39–42 Since the primary oxysulfidation pathway of copper (hydr)oxides is oxidative dissolution–reprecipitation that involves surface reactions with both dissolved oxygen and sulfide species (H₂S(aq), HS⁻ and S²⁻),^22,43,44 we postulate that exposed facets can determine the kinetics and extent of oxysulfidation of copper (hydr)oxide nanomaterials, by regulating the adsorption affinities for oxygen and sulfide, and possibly, the specific reaction pathways.

In this study, we used three Cu₂O nanomaterials—with predominantly exposed {100}, {111}, and {110} facets, respectively—as model copper (hydr)oxide nanomaterials to understand how facets control oxysulfidation propensity. A range of microscopic and spectroscopic analyses, including scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS), were conducted to compare the physicochemical properties of each pristine Cu₂O nanomaterial and its oxysulfidized counterparts. Density function theory calculations were carried out to assess the affinities of different facets for the precursors of oxysulfidation reactions. The findings have important implications for improving the performance and durability of nanopesticides in redox dynamic rhizospheres to minimize environmental impacts associated with overuse of conventional pesticides. The conceptual model demonstrated herein likely can be extended to improve the efficacy of nanofertilizers to decrease the negative environmental effects due to the overuse of conventional fertilizers. Likely, the findings also inform the design of antimicrobial nanomaterials for sustained release of antimicrobial ions. Moreover, the significant effects of facet-dependent reaction intermediates on the specific oxysulfidation pathways highlight the critical importance to understand the time-dependent evolution of mineral formation and transformation.

2. Experimental section

2.1. Synthesis of Cu₂O nanomaterials with different exposed facets

Three types of Cu₂O nanomaterials with predominantly exposed {100}, {111}, and {110} facets (referred to as Cu₂O_{100}, Cu₂O_{111}, and Cu₂O_{110}, respectively) were prepared using previously reported methods.^35,45 For Cu₂O_{100} and Cu₂O_{111}, 50 mL of CuCl₂·2H₂O aqueous solution (0.01 M) was prepared with (for Cu₂O_{111}) or without (for Cu₂O_{100}) the addition of 4.44 g of PVP. Then, 5.0 mL of NaOH solution (2.0 M) was added slowly into the above light green solution. After stirring for 30 min, 5.0 mL of ascorbic acid solution (0.60 M) was added slowly to the above dark brown mixture. Next, the resulting turbid red suspension was constantly stirred for different times (5 h for Cu₂O_{100} and 3 h for Cu₂O_{111}) at 55 °C. The brick-red precipitate was centrifuged and washed three times with deionized (DI) water and absolute ethanol and finally dried in a vacuum drying oven at 40 °C for 24 h. For Cu₂O_{110}, 5 mL of oleic acid and 20 mL of absolute ethanol were added in succession into 40 mL of CuSO₄ aqueous solution (0.025 M) under vigorous stirring. Then, the mixture was heated to 100 °C, and 10 mL of NaOH solution (0.80 M) was added dropwise into the solution. After 5 min, 30 mL of D-(+)-glucose solution (0.63 M) was added to the resulting deep blue solution. After further stirring for 60 min, the resulting brick-red precipitate was centrifuged and washed with absolute ethanol and cyclohexane and dried in a vacuum drying oven at 40 °C for 24 h. Detailed information on the chemicals used is given in the ESI.†

2.2. Oxysulfidation of Cu₂O nanomaterials

Oxysulfidation experiments were carried out in 250 mL amber glass bottles at room temperature (25 ± 1 °C), using a literature method.^19,20 4-(2-Hydroxyethyl)-1-piperazinepropanesulfonic acid (HEPPS) was selected as the buffer for the oxysulfidation experiments because it would not induce metal complexation.⁴⁶ First, a 25 mM HEPPS buffer solution was prepared in DI water with 8.0 ± 0.2 mg L⁻¹ dissolved oxygen and adjusted to pH 8.0 ± 0.1 with NaOH solution. Next, 18 mg of Cu₂O nanomaterial was added and dispersed by bath sonication for 30 min. The resultant Cu₂O nanomaterial suspensions (with 0.5 mM Cu₂O) were amended with a Na₂S stock solution to yield an aqueous mixture with 1.2 mM Na₂S. At the reaction pH 8.0, HS⁻ is the predominant dissolved S(−II) species, accounting for 90.5% of the total dissolved sulfide (Fig. S1†). Each suspension was kept open to the atmosphere with no limitation of oxygen exchange. After incubation on a shaker at 150 rpm for 10 min or 1 day, the resulting suspensions were centrifuged at 13 000g for 15 min, and the pellets were washed three times with DI water to remove residual sulfide, collected, and allowed to dry in a vacuum freeze dryer. The oxidation of Cu₂O nanomaterials was also investigated to explore the different reactivities of the Cu₂O nanomaterials toward oxygen (see ESI† for details).

2.3. Characterization of pristine and oxysulfidized Cu₂O nanomaterials

The morphology of the Cu₂O nanomaterials before and after oxysulfidation was observed by SEM (JSM-7800F, JEOL Ltd.) and TEM (JEM-2800, JEOL Ltd.). The crystal phases were determined using MDI Jade 6 software based on the XRD patterns collected on an Ultima IV diffractometer (Rigaku) with Cu Kα radiation (λ = 1.5418 Å). The chemical compositions of the oxysulfidized Cu₂O nanomaterials were characterized by elemental mapping of the samples, collected with the JEM-2800 TEM system, and EDS with the JSM-7800F SEM system. The surface proportion of Cu(II) in the initial oxysulfidized Cu₂O nanomaterials was examined by XPS (K-Alpha, Thermo Fisher Scientific). The ζ potential of pristine Cu₂O nanomaterials in HEPPS buffer solution was measured on a Litesizer 500 particle size analyzer (Anton Paar GmbH) at room temperature (25 ± 1 °C).

2.4. Release of Cu ions from Cu₂O nanomaterials during oxysulfidation

Release of copper ions from Cu₂O nanomaterials during oxysulfidation was determined with the 2,9-dimethyl-1,10-phenanthroline (neocuproine) spectrophotometric method.^47,48 Briefly, 5 μL of concentrated sulfuric acid was added to 0.75 mL of reaction solution passing through a 0.22 μm membrane at selected time intervals. Then, 75 μL of 100 g L⁻¹ hydroxylamine hydrochloride solution or 75 μL of DI water was added to measure total Cu and Cu⁺, respectively. An additional 150 μL of 375 g L⁻¹ sodium citrate solution and 150 μL of sodium acetate buffer (pH 5.7) were added. After thorough mixing, 75 μL of 1.0 g L⁻¹ neocuproine reagent was spiked and the absorbance values at 457 nm were measured using a Cary 100 UV-vis spectrophotometer (Agilent Technologies). The Cu²⁺ concentration was obtained by subtracting the Cu⁺ concentration from the total Cu ion concentration. Each test group included three replicates, and statistical differences among the three groups were analyzed by one-way analysis of variance (ANOVA) with Dunnett's test; a p-value of less than 0.05 is considered statistically significant.

2.5. Density functional theory (DFT) calculations

To estimate the adsorption energies (E_ads) of O₂ and HS⁻ on different facets of Cu₂O nanomaterials (the optimized geometric structures are given in Fig. S2†), DFT calculations were performed using the Vienna Ab initio Simulation Package (VASP 5.3.5) code with the Perdew–Burke–Ernzerhof generalized gradient approximation and the projected augmented wave method. The cutoff energy for the plane-wave basis set was set to 400 eV. The Brillouin zone of the bulk and surface unit cell was sampled using Monkhorst–Pack (MP) grids for optimization of the Cu₂O structure. The {100}, {110}, and {111} surfaces were determined using 3 × 3 × 1, 2 × 2 × 1, and 3 × 3 × 1 MP grids. The convergence criteria for the electronic self-consistent iteration and force were set to 10⁻⁵ eV and 0.01 eV Å⁻¹, respectively. A vacuum layer of 12 Å was introduced to avoid interactions between periodic images. The adsorption energy was calculated as:

E_ads = E_total − E_surface − E_species (1)

where E_total is the total energy of the adsorbed species with the material surface, E_surface is the energy of the empty Cu₂O surface, and E_species is the energy of the species prior to adsorption.

3. Results and discussion

3.1. Characteristics of different faceted Cu₂O nanomaterials

The three as-prepared Cu₂O nanomaterials were similar in dimension, and exhibited well-defined exposed facets that differed among the three materials. The SEM images (Fig. 1a) show that the three nanomaterials had uniform cubic, octahedral, and rhombic dodecahedral morphologies, respectively. The side lengths of the three materials were 887 ± 135, 860 ± 97, and 509 ± 80 nm, respectively (Table S1†). The TEM images confirm the morphologies of the Cu₂O nanomaterials (Fig. 1b). The high-resolution TEM (HRTEM) images reveal the single-crystal structure of the three nanomaterials and further identify their predominantly exposed crystal facets (Fig. 1c). The HRTEM image of Cu₂O_{100} shows a lattice spacing of 0.21 nm, which can be indexed to the (200) plane of Cu₂O, indicating that the cubic material predominantly exposed {100} facets.⁴⁹ Cu₂O_{111} exhibited two sets of lattice fringes of 0.21 nm and 0.25 nm, corresponding to the (200) and (111) planes of Cu₂O, suggesting that the octahedral material was enclosed by eight {111} facets.⁵⁰ The two sets of lattice spacings observed for Cu₂O_{110} were 0.30 nm and 0.43 nm, corresponding to the (100) and (110) planes, showing that the rhombic dodecahedral material predominantly exposed {110} facets.⁵¹ The XRD patterns (Fig. 1d) of the three Cu₂O nanomaterials display sharp diffraction peaks that are characteristic of the cuprite phase (JCPDS No. 05-0667), with little impurities (e.g., CuO or Cu) detected, confirming that the samples were of good crystallinity and high purity.

Fig. 1 Characterization of the Cu₂O nanomaterials with different exposed facets. Scanning electron microscopy (SEM) (a), transmission electron microscopy (TEM) (b), and high-resolution TEM (HRTEM) (c) images, and X-ray diffraction (XRD) patterns (d), showing that the as-prepared Cu₂O nanomaterials have well-defined {100}, {111} and {110} facets with good crystallinity and high purity (I: Cu₂O_{100}; II: Cu₂O_{111}; III: Cu₂O_{110}).

3.2. Transformation products upon nano-Cu₂O oxysulfidation are facet-dependent

The oxysulfidation products of the three Cu₂O nanomaterials differed significantly, with two products taking the hollow–shell form and one exhibiting a yolk–shell structure. The morphological features of all three Cu₂O nanomaterials changed upon exposure to sulfide, in the presence of O₂. The surfaces of the materials became less smooth, and the shapes less well defined (Fig. 2a). Nonetheless, the changes were more pronounced for Cu₂O_{100} and Cu₂O_{111}, in that, nanoflakes were observed on the surfaces of these two materials and the appearance of the materials became nearly spherical (Fig. 2a). The changes of Cu₂O_{110} were more subtle, with the surfaces becoming slightly wrinkled while the rhombic dodecahedral morphology remaining largely intact (Fig. 2a). The TEM images of the transformation products corroborate these morphological changes (Fig. 2b). Remarkably, TEM images show that oxysulfidation of Cu₂O_{100} and Cu₂O_{111} led to the formation of hollow shells (Fig. 2b), consisting mainly of Cu and S, with only a trace amount of O, based on the elemental mapping results (Fig. 2c). In contrast, the transformation product of Cu₂O_{110} exhibited a yolk–shell structure, with the core consisting of Cu and O and the shell consisting of Cu, S, and a trace amount of O (Fig. 2b and c), indicating a much lower extent of transformation. The S/Cu ratios of the transformed Cu₂O_{100} and Cu₂O_{111}, calculated based on the EDS results, were close to the stoichiometric S/Cu ratio of covellite (i.e., 1), whereas that of Cu₂O_{110} was only 0.67 (Table S2†), consistent with the existence of the Cu₂O core. The formation of the hollow and yolk–shell structures suggested that the oxysulfidation process was in accordance with the Kirkendall effect.^45,52,53 That is, the reaction between Cu ions and HS⁻ ions generated a layer of Cu_xS_y on the surface of the Cu₂O nanomaterials. Due to the smaller effective diameters of Cu ions (1.54 Å for Cu⁺ and 1.46 Å for Cu²⁺) than that of HS⁻ (3.68 Å),^54,55 the outward diffusion of Cu ions from the Cu₂O core to the Cu_xS_y shell would prevail over the inward diffusion of HS⁻ ions, leading to the formation of the hollow and yolk–shell structures.

Fig. 2 Changes in the morphology and crystalline phases of Cu₂O nanomaterials during oxysulfidation are facet-dependent. SEM (a) and TEM (b) images, and elemental mapping (c) of oxysulfidized Cu₂O nanomaterials after 1 day of reaction, showing that the oxysulfidation products of Cu₂O_{100} and Cu₂O_{111} are in the forms of hollow shells, while that of Cu₂O_{110} exhibits a yolk–shell structure. XRD patterns (d) and HRTEM images (e) of oxysulfidized Cu₂O nanomaterials after 1 day of reaction, showing that the extent of sulfidation is more complete for Cu₂O_{100} and Cu₂O_{111}, while the crystalline phase of the Cu_xS_y shells of all three oxysulfidized Cu₂O nanomaterials is primarily covellite (I: oxysulfidized Cu₂O_{100}; II: oxysulfidized Cu₂O_{111}; III: oxysulfidized Cu₂O_{110}).

The crystalline phase compositions of the oxysulfidation products were also facet-dependent. Multiple new diffraction peaks were observed in the XRD patterns of all three Cu₂O nanomaterials upon oxysulfidation. For Cu₂O_{100} and Cu₂O_{111}, the diffraction peaks were identified as covellite (CuS; JCPDS No. 06-0464), with no visible characteristic diffraction peaks of Cu₂O left (Fig. 2d and Table S3†), indicating that the two materials were completely sulfidized. In comparison, the XRD pattern of Cu₂O_{110} shows that this material was only partially sulfidized, consisting of both covellite and cuprite (Fig. 2d and Table S3†). The HRTEM images of the three Cu₂O nanomaterials show the lattice fringes with spacings of 0.30, 0.28, 0.23, 0.27, and 0.32 nm, corresponding to the (102), (103), (105), (006), and (101) planes of covellite (Fig. 2e), showing that even though transformation of Cu₂O_{110} was incomplete, the shell of this material consisted of the same crystalline phase as Cu₂O_{100} and Cu₂O_{111}.

The extent of copper dissolution during the course of oxysulfidation also differed among the three Cu₂O nanomaterials. At any given time interval, the measured total concentration of copper ions, including both Cu²⁺ and Cu⁺, was significantly greater for Cu₂O_{100} and Cu₂O_{111} than that for Cu₂O_{110} (Fig. 3a; the copper dissolution patterns were similar between Cu₂O_{100} and Cu₂O_{111}). At the end of the 24 h reaction, the total amounts of copper ions released from Cu₂O_{100} and Cu₂O_{111} were 7.2 μM and 8.0 μM, respectively, 2.2 to 2.4 times higher than that from Cu₂O_{110} (3.3 μM) (Fig. 3a). The difference was mainly attributable to the different extents of Cu²⁺ release – Cu²⁺ released from Cu₂O_{100} and Cu₂O_{111} accounted for 79.2% and 68.8% of total copper ions, respectively, compared with 33.3% from Cu₂O_{110} (Fig. 3b). The measured concentrations of Cu⁺ were 1.5 μM for Cu₂O_{100}, 2.4 μM for Cu₂O_{111}, and 2.2 μM for Cu₂O_{110} (Fig. 3c). These observations are in line with the different morphological and compositional changes mentioned above, as Cu₂O_{110} appeared passivated during the oxysulfidation process (thus hindering subsequent copper dissolution), whereas the other two materials continued to release copper ions until the majority of Cu₂O was consumed. Thus, one explanation for the facet-dependent transformation of the Cu₂O nanomaterials is that a relatively condensed Cu_xS_y shell formed on the surface of Cu₂O_{110} during the initial stage of oxysulfidation, which impeded the outward diffusion of Cu ions (and possibly the inward diffusion of oxygen and HS⁻), whereas the nanoflakes on the surfaces of Cu₂O_{100} and Cu₂O_{111} left enough spaces for mass transfer. Previous studies on the formation of copper sulfides from Cu₂O also showed that the copper sulfide layer formed on the surface of Cu₂O nanomaterials may prevent the internal copper ions from reaching the external sulfur ions, and further reactions depend on how efficient the diffusion of copper and sulfur ions through the interface.^45,52,53

Fig. 3 Copper dissolution from Cu₂O nanomaterials during oxysulfidation is also facet-dependent. Copper dissolution during the 1 d oxysulfidation of Cu₂O, as total Cu ions (a), Cu²⁺ ions (b) and Cu⁺ ions (c), showing that greater amounts of Cu ions are released from Cu₂O_{100} and Cu₂O_{111}, mostly as Cu²⁺. The different letters indicate significant differences (p < 0.05) in the 24 h copper ion release according to the ANOVA followed by Dunnett's test.

3.3. Exposed facets determine the initial oxysulfidation products of Cu₂O nanomaterials by regulating adsorption affinities for O₂ and HS⁻

To characterize the evolution of the Cu_xS_y shells during the transformation process, we examined the initial oxysulfidation products of the Cu₂O nanomaterials with XRD. After a reaction time of 10 min, multiple diffraction peaks appeared on the XRD patterns of all three materials, indicating the formation of Cu_xS_y species, while the characteristic diffraction peaks of cuprite remained (Fig. 4a). Interestingly, the initial Cu_xS_y species formed on the three Cu₂O nanomaterials differed, with both yarrowite (Cu₉S₈; JCPDS No. 36-0379) and covellite (CuS; JCPDS No. 06-0464) observed on Cu₂O_{100}, yarrowite (Cu₉S₈; JCPDS No. 36-0379) on Cu₂O_{111}, and djurleite (Cu₃₁S₁₆; JCPDS No. 34-0660) on Cu₂O_{110} (Fig. 4a and Table S3†). Djurleite is composed of an ordered superlattice of copper in distorted hexagonal-close-packed sulfur layers (Fig. S3a†),⁵⁶ and the tiny tunnels between CuS₃ triangles and distorted CuS₄ tetrahedra in the crystal structure are not conducive to mass transport.^57,58 In comparison, covelline consists of an ordered structure of alternating CuS₃ triangles and CuS₄ tetrahedral layers connected by S–S bonds,⁵⁹ leaving large spaces between the two CuS₄–CuS₃–CuS₄ triple layers (Fig. S3b†). The layer spacing of covelline is 2.07 Å,⁶⁰ larger than the diameters of the Cu⁺ (1.54 Å) and Cu²⁺ (1.46 Å) ions, yet the pores of djurleite are much narrower. It has been reported that yarrowite has a similar structure to covelline.⁶¹ Thus, surface coverage by djurleite would inhibit the diffusion of Cu ions more significantly, compared with surfaces covered by yarrowite and covellite. The inward diffusion of O₂ would be hampered more by djurleite in a similar manner. The SEM images show that even at this very early stage of oxysulfidation, the morphological features differed substantially between Cu₂O_{110} and the other two nanomaterials (Fig. 4b).

Fig. 4 Exposed facets determine the initial oxysulfidation products of Cu₂O nanomaterials. XRD patterns (a) and SEM images (b) of the initial transformation products of Cu₂O nanomaterials after 10 min of oxysulfidation reaction, showing that the Cu_xS_y species formed on the three materials have different crystalline phases and morphologies (I: oxysulfidized Cu₂O_{100}; II: oxysulfidized Cu₂O_{111}; III: oxysulfidized Cu₂O_{110}).

It is also noteworthy that the initial Cu_xS_y species on Cu₂O_{110} were predominantly Cu(I), whereas those on Cu₂O_{100} and Cu₂O_{111} consisted mainly of Cu(II) (Table S3†). The XPS results of the initial oxysulfidation products were in general consistent with the XRD data, as indicated by the proportion of Cu(II) on the surface of the Cu₂O nanomaterials, calculated based on the deconvoluted Cu 2p XPS spectra (Fig. S4†). This indicates that oxidation dissolution of Cu₂O_{110} was hampered, likely because this specific facet had a lower affinity for O₂. Indeed, the E_ads values calculated based on DFT follow the order of Cu₂O_{100} > Cu₂O_{111} > Cu₂O_{110} (Fig. 5a–d). Thus, the weakest adsorption energy of O₂ on Cu₂O_{110} corroborates the overall lower valence of the Cu_xS_y species formed on Cu₂O_{110}. Moreover, the higher adsorption energy of Cu₂O_{100} than Cu₂O_{111} is also consistent with the different Cu_xS_y speciation between the two materials. The reactivity of the different faceted Cu₂O nanomaterials towards O₂ was further verified with a supplementary experiment, in which the suspensions of the three materials were treated with dissolved O₂ in the absence of sulfide. The XRD patterns (Fig. S5a†) show that tenorite (CuO; JCPDS No. 48-1548) was generated upon the oxidation of the materials, and the intensity ratio of the highest diffraction peak of tenorite (2θ at 38.7°) to the highest diffraction peak of Cu₂O (2θ at 36.4°) followed the order of Cu₂O_{100} > Cu₂O_{111} > Cu₂O_{110} (Fig. S5b†). Overall, these results collectively supported that the formation of djurleite, a lower-valence Cu_xS_y species, on Cu₂O_{110} was due to the low affinity of the {110} facet for dissolved O₂.

Fig. 5 Exposed facets of Cu₂O regulate adsorption affinities for O₂ and HS⁻. Side views of the optimized adsorption configurations of O₂ (a–c) and HS⁻ (e–g), and adsorption energy (E_ads) of O₂ (d) and HS⁻ (h) on the {100}, {111} and {110} facets of Cu₂O, showing that the adsorption affinities for both O₂ and HS⁻ are facet-dependent (a and e: Cu₂O_{100}; b and f: Cu₂O_{111}; c and g: Cu₂O_{110}).

The DFT results corroborate that adsorption affinity of sulfide to Cu₂O nanomaterials is facet-dependent. Clearly, this would also affect the speciation of Cu_xS_y on the surfaces of Cu₂O. The stable binding configuration of HS⁻ on the {111} and {100} facets is one S-atom binding to two Cu-atoms, while on the {110} facet one S-atom binds to one Cu atom (Fig. 5e–g). This difference in binding configuration results in a more negative adsorption energy of HS⁻ on the {111} and {100} facets than that on the {110} facet (Fig. 5h). Thus adsorption of HS⁻ to Cu₂O_{111} and Cu₂O_{100} is energetically more favorable than that to Cu₂O_{110}. Moreover, the {111} facet has a higher affinity for HS⁻ than the {100} facet, a reverse trend from their relative affinities for O₂. The different abilities of the two facets to bind HS⁻ and O₂ were likely the reason that different initial Cu_xS_y species formed on these two materials. This was consistent with the literature finding that the generation of different stoichiometric copper sulfides from Cu₂O may be modulated by controlling the concentrations of sulfur source and the O₂ content in the reaction system.³² The measured ζ potential values of the Cu₂O nanomaterials were consistent with the theoretical calculation results, in that, Cu₂O_{110} was more negatively charged than Cu₂O_{100} and Cu₂O_{111} (Fig. S6†), making the material less favorable for the binding of the anionic HS⁻. This is consistent with the higher degree of sulfidation of Cu₂O_{100} and Cu₂O_{111}. Overall, it is concluded that the facet-dependent affinities for both oxygen and sulfide would regulate the formation of the specific Cu_xS_y species in the very early stage of the oxysulfidation process, resulting in varied ion diffusion potentials and oxysulfidation propensities among different faceted Cu₂O nanomaterials.

4. Conclusions

In this study, we demonstrate that oxysulfidation of Cu₂O nanomaterials is facet-dependent, as Cu₂O nanomaterials with different exposed facets undergo different transformation routes, resulting in different extents of copper ion release. This is attributed to the facet-dependent adsorption affinities of Cu₂O for both oxygen and sulfide ions, which leads to the formation of different initial oxysulfidation products on Cu₂O, and the structural properties of these products control the subsequent copper dissolution and propensity of sulfidation. Such facet-dependent effects have important implications for the design of high-performance nanopesticides. Indeed, a common challenge for the application of nano-enabled agrichemicals is maintaining the performance of these nanomaterials in the dynamic environment of rhizosphere soils. The findings show that facet engineering can be exploited to ensure sustained release of active antimicrobial ingredients (i.e., copper ions), by mitigating the passivation of nanopesticides from surface precipitation of copper sulfides. Since the initial products formed on the two Cu₂O nanomaterials with predominantly exposed {100} and {111} facets are relatively loose in structure, these materials are less prone to passivation and better suitable as nanopesticides. The conceptual model demonstrated herein likely can be extended to improve the efficacy of nanofertilizers (e.g., apatite and hydroxyapatite nanoparticles), which are designed to achieve controlled release of nutrients to decrease the negative environmental effects due to the overuse of conventional fertilizers.

The findings of this study can also inform the design of antimicrobial nanomaterials such as Ag, ZnO and CuO nanoparticles, which are used as alternative antibiotics for preventing the development of antibiotic resistance.⁶² The performance of these nanomaterials relies on the sustained release of antimicrobial ions (e.g., Ag⁺), but is prone to passivation due to aging. Moreover, the significant effects of facet-dependent reaction intermediates on the specific oxysulfidation pathways highlight the critical importance to understand the time-dependent evolution of mineral formation and transformation. This is particularly important for the dynamic reactions occurring in relatively short time scales, in that, even subtle differences in the initial stage of the reactions can lead to markedly different environmental consequences. For such dynamic systems, attention should be paid to fully understand the mechanisms and kinetics of nano-scale intermediate formation.

Data availability

The data supporting the findings of this study are available within the article and/or its ESI.†

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This project was supported by the National Natural Science Foundation of China (22125603, 22020102004), Tianjin Municipal Science and Technology Bureau (21JCJQJC00060), the Anhui Provincial Natural Science Foundation (2108085QB55), Fundamental Research Funds for the Central Universities (63233056), and China–US Center for Environmental Remediation and Sustainable Development.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4en00545g

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