Surface-dependent generation of reactive oxygen species at pyrite–water interface

The generation of reactive oxygen species (ROS) at the pyrite–water interface is an important discovery for both early Earth’s and present environments in the past two decades. In these revealed reactions, pyrite can oxidize water to ROS and reduce O 2 and ROS to water. However, the two types of reactions are controversial. The underlying physical theory responsible for the reactions has yet to be elucidated. In this study, we established a surface structure-dependent model of oxidation and reduction potentials (ORP) for semiconductors. Surface atomic structure-dependent electronic structures were adopted to estimate the ORP of pyrite. We apply this model to gain insights into the generation of ROS at the pyrite–water interface. The results demonstrate that the surface structure of pyrite controls its ORP, and ROS production can only occur on certain pyrite facets. The {210} and {111} facets with certain defects (e.g., 210-2S ′ and 111-3S, respectively) could oxidize either H 2 O or OH − to O 2 , and ROS form during the oxidation of pyrite surfaces by O 2 . This suggests that surface effects play a crucial role in governing the ORP of semiconducting minerals.

Semiconducting minerals in engineering, as well as in nature, have caused multi-disciplinary interest in understanding their fundamental properties. 1 The oxidation and reduction potentials (ORP) are essential in semiconductors.Many of them are controls in chemical reactions, including dark redox and photocatalysis.Alternately, the ORP can be experimentally derived from the Pourbaix diagram, 2 but it is not easy to do the measurements.The ORP can be theoretically calculated based on the Gibbs energy of possible redox reactions of semiconductors. 3For instance, the possible oxidation and reduction of pyrite (FeS 2 ) under illumination can be described by the following reactions: where h + and e − are the photogenerated hole and photoelectron, respectively.The oxidation potential (OP) and the reduction potential (RP) are defined as where G is the Gibbs energy of formation and F is the Faraday constant.The evaluation of the ORP assumes that the reactions only occur under illumination ignoring the fact that (1) the reactions always occur at the semiconductor-water interface and (2) the estimated reactions may not be the real reactions occurring at interfaces.Such an estimation of the ORP cannot be employed to examine the dark redox reactions in natural sediments and deviate from real interface reactions.5][6] Therefore, surface effects should be considered in evaluating the ORP of semiconducting minerals.Regardless of illumination, the oxidation and reduction reactions of a semiconductor MmXn (M = metal; X = non-metal) can be When the Fermi energy level of a redox couple is higher than the OP of a semiconductor (e.g., ϕ 1 > OP), the semiconductor acts as an oxidant; when the Fermi level of a redox couple is between the OP and RP of a semiconductor (e.g., RP < ϕ 2 < OP), no redox reactions will occur; and when the Fermi level of a redox couple is lower than the RP of a semiconductor (e.g., ϕ 3 < RP), the semiconductor acts as a reductant.(b) OP and RP of semiconductors under illumination.Under illumination, the OP and RP are located at the VBM and CBM of a semiconductor, respectively.Either when the Fermi energy level of a redox couple is higher than the OP (e.g., ϕ 1 > OP) or when the Fermi energy level of a redox couple is lower than the RP (e.g., ϕ 3 < RP), the semiconductor under illumination acts the same role as under dark conditions.When the Fermi level of a redox couple is between the OP and RP of a semiconductor (e.g., OP < ϕ 2 < RP), the semiconductor can act as either an oxidant or a reductant, which is the fundamental of photocatalysis.
the semiconductor surfaces can spontaneously transfer to the CBM.Thus, the VBM and CBM of specific semiconductor surfaces are the energy thresholds of reduction and oxidation reactions on surfaces, i.e., the RP and the OP, respectively.Under appropriate illumination, electrons at VBM can be activated and can transition to CBM of the semiconductor surfaces; the opposite holes will form simultaneously.Then, the OP and the RP interchange to the VBM and the CBM levels, respectively [Fig.1(b)].This straightforward approach to directly align the ORP to surface band edges of semiconductors, instead of bulk band edges, can be applied to either a monocrystal with specific facets or powder semiconductors with fracture surfaces.For specific facets of a semiconductor, the ORP can be theoretically evaluated based on the facet structure.For broken particles of semiconductors without specific facets, usually for natural cases, the surfaces cannot be modeled by specific surface configurations.One should consider all the possible surfaces on the particles and then use the highest OP and the lowest RP as the real ORP of semiconductor powder.
1][12][13][14] Among the redox reactions on pyrite, the generation of reactive oxygen species (ROS, including H 2 O 2 and ⋅OH) involves both oxidation and reduction of pyrite. 15,16Under anaerobic conditions, more and more experimental evidence 12,17,18 has been observed since the first observation of ROS in the O 2 -free water suspension containing pyrite. 15In these experiments, water is oxidized by pyrite, but the mechanism is still unclear. 19,20Meanwhile, the formation of ROS during pyrite oxidation by dissolved molecular O 2 , an essentially natural process in environments, 21,22 has widely been confirmed.However, the Gibbs energy approach 3 cannot support these observations.The OP and RP of pyrite are estimated to be 0.518 and 0.260 V, respectively, vs standard hydrogen electrode (SHE).Based on these experimental observations, 12,14,15,17,18,21,22 one can get information that pyrite can oxidize water to ROS and reduce O 2 and ROS to water.The two sides of pyrite seem odd in a traditionally thermodynamic view.One material can only either oxidize or reduce another one. 20eanwhile, crystalline materials' facet and defect engineering suggest that various facets and defects host different reactivities of semiconductors. 4,23We hypothesize that the two-sided redox reactivities of pyrite may originate from its specific facets and surface defects.
To test this hypothesis, we designed several pyrite slabs to investigate the effect of facets and defects on the ORP of various pyrite surfaces under the density functional theory (DFT) framework.We chose the most stable pyrite {100}, {210}, and {111} facets as the initial models, 24 based on which defective configurations were designed (see the details in the Methods section of the supplementary material).We found that the ORP of pyrite is sensitive to its surface structures and the surface-dependent ORP can reasonably explain the two-sided redox reactivities of pyrite.Notably, we only consider the surface structure-dependent vacancies, while various other defects, such as impurity substitutions and dislocations, are present in actual pyrite crystals.
To estimate the ORP of pyrite surfaces, we designed both non-defective and defective configurations of pyrite surfaces.First, we investigated the stability of these pyrite surface models.The calculated surface energies of these models (see Figs. 1-3 of the supplementary material) show that the stability of defective pyrite surfaces is a function of environmental sulfur conditions-various defects formed during the growth of pyrite under various sulfur environments.Furthermore, all the considered defective models can be present on pyrite surfaces during the formation of broken surfaces under an external force.
We then calculated the electronic structure of pyrite surfaces.The obtained band structures of pyrite surfaces (see Figs. 4-6 of the supplementary material) could assess the surface ORP of pyrite.The evaluated results (Fig. 2) show that the standard OP and RP of pyrite surfaces are quite different from those of pyrite bulk.Thus, it is difficult to estimate the redox nature of the pyrite surface by using the bulk properties.All the pyrite {100} surfaces possess more negative OP and RP than the pyrite bulk [Fig.2(a)], suggesting that the {100} surfaces have a higher reduction ability than the bulk.For the pyrite {210} and {111} facets [Fig.2(b)], all the RPs are lower than that of pyrite bulk, while the OP can be either lower or higher than that of pyrite bulk.Thus, the {210} and {111} facets with certain defects (e.g., 210-2S ′ and 111-3S, respectively) could have a higher oxidation ability than the pyrite bulk.
Based on the observed standard OP and RP of pyrite surfaces, we can evaluate the possible redox reactions at the pyrite-water interface.The standard OP and RP of pyrite surfaces can be directly compared with those of aqueous redox couples. 25In such a scenario, the ORP of ⋅OH/OH − (2.02 V) and H 2 O 2 /H 2 O (1.776 V) are much higher than those of the pyrite surfaces presented in Fig. 2. Thus, the production of ⋅OH and H 2 O 2 from reactions between pyrite surface and H 2 O/OH − under anaerobic environments is thermodynamically unfavorable.Such an interpretation opposes the experimental fact that ROS can be detected at the pyrite-water interface under anaerobic environments. 12,14,15,17,18,21,22Alternatively, among the surfaces presented in Fig. 2, certain pyrite surfaces (e.g., 111-2S and 210-2S ′ ) possess a higher OP than the redox couples of O 2 /H 2 O (1.229 V) and O 2 /OH − (0.401 V), suggesting that these surfaces could oxidize either H 2 O or OH − to O 2 [(R5) and (R6)], The above-mentioned observations illustrate a separate redox mechanism (Fig. 3) at the pyrite-water interface through bulk conductions. 28The surface atomic configurations determine the surfaces' electronic structure, thereby constraining the redox ability of certain facets [Fig.3(a)].Because of the difference in the electronic structure of various facets, the charge will be separated on different facets through band bending. 29The consequent charge accumulation of different facets enables the separation of oxidation and reduction on different facets 4,30 [Fig.3(b)].Furthermore, the defective domains on specific facets can also share the separated redox model.Thus, oxidation and reduction can be separated into different defective zones with various redox abilities [Fig.3(c)].
The facet-specific ORP can be used to predict the redox reactivity of semiconducting minerals in aqueous environments.The ROS generation reactions at the pyrite-water interface were attributed to surface defective Fe sites 16 (R9).The hydroxylation on such Fe sites was the prerecorded reaction. 19  Although the reaction (R10) products include Fe 3+ , they will deplete at the pyrite-water interface because the ORP of Fe 3+ /Fe 2+ (0.771 V) is higher than that of the most pyrite surfaces presented in Fig. 2.This deduction is consistent with the previous experimental fact that aqueous Fe 3+ is a vital oxidant responsible for pyrite oxidation in environments.Fe 3+ has barely been detected when O 2 oxidizes pyrite in acid solutions. 31,32The surface Fe species present a higher oxidation state than the bulk. 33,34In such situations, no Fe 3+ could accumulate in the pyrite-water systems.Consequently, the depletion of Fe 3+ maintains a reduction state in ancient ocean 35 even though the ROS generation provides an oxidation driving force on early Earth. 15he surface structure-dependent ORP should be universal in semiconducting minerals.The standard OP and RP discussed above represent the ground state of semiconducting minerals.In this state, the OP is always lower than the RP of semiconducting minerals, meaning that semiconducting minerals cannot oxidize or reduce themselves.On the contrary, when a specific semiconducting mineral is excited, the OP and RP will exchange, i.e., OP > RP, thus enhancing both the oxidation and reduction abilities of semiconductor surfaces.However, this enhancement makes semiconducting minerals unstable.Such an excited state will deplete through self-redox (e.g., the combination of photo-induced holes and photoelectrons).In addition, the difference between the OP and RP in this study corresponds to the bandgap of specific surfaces of semiconductors (see Figs. 7 and 8 of the supplementary material).Therefore, the ORP can also evaluate the conductivity of semiconductor surfaces.
5][6] The ORP of semiconducting minerals are fundamental for such a design.Based on the surface structuredependent ORP, the stability of semiconductors in aqueous environments and the pursued catalysis reactivity can be tuned by designing various surface structures.The surface structure-dependent ORP and the following separated redox mechanism play a critical role in natural and synthetic systems.
See the supplementary material for methods and supplementary figures.

FIG. 1 .
FIG. 1.Schematic illustration of the redox potentials of semiconducting minerals.(a) Oxidation potentials (OP) and reduction potentials (RP) of semiconductors in dark reactions.The OP and RP are located at the conduction band minimum (CBM) and valence band maximum (VBM) of a semiconductor, respectively.When the Fermi energy level of a redox couple is higher than the OP of a semiconductor (e.g., ϕ 1 > OP), the semiconductor acts as an oxidant; when the Fermi level of a redox couple is between the OP and RP of a semiconductor (e.g., RP < ϕ 2 < OP), no redox reactions will occur; and when the Fermi level of a redox couple is lower than the RP of a semiconductor (e.g., ϕ 3 < RP), the semiconductor acts as a reductant.(b) OP and RP of semiconductors under illumination.Under illumination, the OP and RP are located at the VBM and CBM of a semiconductor, respectively.Either when the Fermi energy level of a redox couple is higher than the OP (e.g., ϕ 1 > OP) or when the Fermi energy level of a redox couple is lower than the RP (e.g., ϕ 3 < RP), the semiconductor under illumination acts the same role as under dark conditions.When the Fermi level of a redox couple is between the OP and RP of a semiconductor (e.g., OP < ϕ 2 < RP), the semiconductor can act as either an oxidant or a reductant, which is the fundamental of photocatalysis.

FIG. 2 .
FIG. 2. Standard oxidation potentials (OPs, red bars) and reduction potentials (RPs, blue bars) relative to SHE and vacuum level for pyrite surfaces.(a) The {100} surfaces with various vacancy numbers; (b) {210} and {111} with various atomic terminations.The cyan area represents the stability zone of water.The redox potentials of several common aqueous redox couples and the valence (blue dashed line) and conduction (red dashed line) band edges of pyrite bulk at pH = 0 are also plotted.

)
Most pyrite surfaces considered in this study possess lower OPs than the redox couples of O 2 /H 2 O (1.229 V) and O 2 /H 2 O 2 (0.695 V).Once O 2 forms during the interaction between pyrite and water, the produced O 2 then acts as an oxidant at the pyrite-water interface.Consequently, H 2 O 2 formed during the oxidation of pyrite surfaces by O 2 21 (R7).Then, the generated H 2 O 2 acts as oxidants and converts to ⋅OH through the Fenton reaction18,26,27 (R8),

FIG. 3 .
FIG. 3. Schematic illustrating the separated redox reactions on pyrite surfaces.(a) Redox ability of pyrite tuned by the surface electronic band structures of different facets; (b) accumulations of electrons or holes on different facets through the energy alignment of band bending; and (c) redox ability and electron accumulations on defective domains of specific facets.
20nsequently, ⋅OH became the dominant product, and the combination of two ⋅OH produced H 2 O 2 ,16However, the production of ⋅OH and H 2 O 2 from the reactions between pyrite surface and H 2 O/OH − under anaerobic environments is thermodynamically unfavorable.20Thefirst products of the ROS generation reactions should be O 2 rather than ⋅OH.Then, H 2 O 2 is produced during pyrite oxidation by O 2 .Subsequently, ⋅OH is generated by the Fenton reaction,