Review—Copper Oxide-Based Ternary and Quaternary Oxides: Where Solid-State Chemistry Meets Photoelectrochemistry

This review article addresses areas where solid-state chemistry concepts can contribute to the on-going search for a “magic bullet” inorganic semiconductor that can efﬁciently split water or reduce CO 2 . First, a methodology to visualize complex ternary oxide combinations is outlined using 31 examples based on copper in both + 1 and + 2 oxidation states. Then the synthetic aspects are reviewed followed by a discussion of the structural characteristics. The optoelectronic aspects are considered next, culminating the review with the state-of-the-art in the practical applicability of these materials in solar fuels photogeneration and environmental (e.g., azo dye) remediation. 2018. This paper is part of the JES Focus Issue on Processes at the Semiconductor-Solution Interface. This review article addresses the nexus between two seemingly disparate disciplines, namely solid-state chemistry (SSC) and photoelectrochemistry (PEC) using copper-based complex (ternary and quaternary) oxides as a discussion framework. The two disciplines, SSC and PEC, have more or less developed in a parallel fashion since their halcyon days in the mid-1970s. Both SSC and PEC shared an interest in similar materials (mainly oxides and chalcogenides) but for rather different applications. While SSC has focused mainly on magnetic or electronically and ionically conductive solid materials, and eventually evolving to superconducting (so-called high T c ) materials, PEC has dealt mainly with inorganic semiconductors . However, and interestingly enough, the two disciplines have coalesced in the area of energy conversion. The past couple decades have thus witnessed remarkable advances in the development of solid-state materials for conversion, photovoltaic solar cells, and solid-oxide fuel cells. Even high T c -superconductivity studies translated to niche applications in practical technologies. How- ever, in of a comparably early history of study, solar fuels (e.g., CO 2 N 2 not seen the light of day in terms of commercialization.

This review article addresses the nexus between two seemingly disparate disciplines, namely solid-state chemistry (SSC) and photoelectrochemistry (PEC) using copper-based complex (ternary and quaternary) oxides as a discussion framework. The two disciplines, SSC and PEC, have more or less developed in a parallel fashion since their halcyon days in the mid-1970s. Both SSC and PEC shared an interest in similar materials (mainly oxides and chalcogenides) but for rather different applications. While SSC has focused mainly on magnetic or electronically and ionically conductive solid materials, and eventually evolving to superconducting (so-called high T c ) materials, PEC has dealt mainly with inorganic semiconductors. However, and interestingly enough, the two disciplines have coalesced in the area of energy conversion. The past couple decades have thus witnessed remarkable advances in the development of solid-state materials for Li-ion batteries, thermoelectric conversion, photovoltaic solar cells, and solid-oxide fuel cells. Even high T c -superconductivity studies have translated to niche applications in practical technologies. However, in spite of a comparably early history of study, solar fuels (e.g., water splitting, CO 2 reduction, N 2 reduction) have not seen the light of day in terms of commercialization.
One may well argue that we lack a "magic bullet" inorganic semiconductor that can split water or reduce CO 2 in a stable, sustainable, and efficient manner because the connection between SSC and PEC has only occurred in fits and starts. With the entry of (a veritable melting pot of) scientists from a wide variety of technical backgrounds (e.g., colloid chemistry, ultrafast spectroscopy, solid-state physics, electrochemistry, etc.) into the PEC field, it was only natural that the emphasis on solid state chemistry has been somewhat diluted. For example, while the early history of PEC is liberally sprinkled with studies from solid-state research groups (see Refs. 1-4 as random examples), instances in the post 1990 s where SSC concepts have been rationally and deliberately applied, are sporadic, 5,6 as further discussed below.

Scope of Review and Literature Precedence
This review refocuses the key role that solid-state chemistry can play in the continuing search for an inorganic semiconductor (or a combination of semiconductors) for solar fuels photogeneration. First, means of visualizing complex oxide chemical compositions are introduced via suitable diagrams. Next, the synthetic aspects are considered followed by a review of the structural aspects of copper oxide-based ternary and quaternary oxides starting with the parent oxides, copper(I) oxide and copper(II) oxide (Cu 2 O and CuO). The optoelectronic attributes are discussed in turn, culminating in an examination of the extent to which these new materials have met the needs for applicability in the solar fuels and environmental remediation arenas.
We are not aware of a similar, overarching review on this topic although copper(I)-based p-type oxides were recently reviewed for photoelectrochemical and photovoltaic solar energy conversion applications. 6 Tantalum-and niobium-based ternary oxides were discussed in this particular article, as were four copper(I) delafossites and two Cu-V-O oxides (Cu 3 VO 4 and Cu 2 V 3 O 8 ). 6 Areas where the present article overlaps with this precedent review are identified later. The synthetic aspects of ternary metal oxide nanostructures have been reviewed 7 as were their applications in supercapacitors. 8 While not focusing per se on solar fuels generation, three recent articles [9][10][11] may be cited that share a similar objective to the present review: namely, targeted materials development for a specific application. Thus, we note a recent article that discusses how simple chemical concepts can be turned into viable strategies for thermoelectric materials development. 9 Another article 10 focuses on the accuracy of density functional theory in predicting the formation energetics of ternary oxides from their binary oxide counterparts. Finally, ab initio global structural prediction, and specifically the minima hopping method, was used to study 183 different compositions of the form, MXO 2 , where M = Cu, Ag, or Au (coinage metal), and X is an element in the Periodic Table. 11 The application target in this particular study was p-type transparent conducting oxide (TCO) development. 11 While the materials' application focus in the present discussion is mostly on solar water splitting, closely allied technology aspects related to photoelectrochemical CO 2 reduction and heterogeneous photocatalytic environmental remediation are also peripherally addressed, for the sake of completeness. It is worth noting that the photocatalysis application has seen the light of day, in terms of commercial realization, sooner than the other applications.
Tables Ia and Ib summarize the oxide candidates considered in the present review article; the two compilations relate to compounds based on copper in the +1 and +2 oxidation states respectively.

Visualizing Ternary and Quaternary Oxide Compositions and Their Relationship to the Binary Oxide Components
In this paper, A, B, and C represent metal cations in a ternary or quaternary oxide compound. Of course, for the present discussion, A = Cu in all the cases. At first glance, the various oxide compositions in Tables Ia and Ib appear to be complicated in terms of the combinations of formula subscripts, especially in the Cu-Nb-O, Cu-Ta-O, and Cu-V-O cases. However, a simple way to visualize these is to break up each ternary compound into a combination of the component binaries. Thus the two binaries are shown in columns 3 and 4 of Tables Ia and Ib and their combining ratios is shown in column 5 in each case. Thus the vast majority of the compounds considered are rather simple 1:1 molar combinations, although combinations such as 11:3 are observed in the Cu-V-O systems. Polymorphs of the same compound are also observed in many cases (designated in Tables Ia and Ib by α, β, etc.) and they will be further considered in the section below on structural aspects.
A final point to note is that the prescribed visualization of ternary compounds as combinations of corresponding binaries, is by no means, unique to oxides. This approach works equally well for nonoxides, e.g., chalcogenides, phosphides, arsenides etc. By extrapolation, quaternary compounds may be visualized as derived from the component binaries in the same manner. Thus, the quaternary compound, AgBiW 2 O 8 , studied by one of us recently, 95 is really a 1:1:4 combination of the three component oxides, namely, Ag 2 O, Bi 2 O 3 , and WO 3 . Certainly, this is not readily apparent from the above formula! Other Cu-based quaternary compounds are considered below.
Phase diagrams can also represent thermodynamically stable ternary and quaternary compounds based on their simpler components; this is depicted in Figure 2. The generic ternary phase diagram is shown in Figure 2a and a specific Cu-Bi-V-O system is considered in Figure 2b. Consider Figure 2a first. Each corner of the (equilateral) triangle represents 100% of one component (i.e., a binary compound). Points along each edge of the equilateral triangle represent a combination of two binaries on either end of the triangle. Any point on the edge must follow the lever rule; for example, the mid-point constitutes the 1:1 ternary compound, and AB 2 O x is represented as a point that is located 1/3 of the length toward the corner labeled "A-oxide" and 2/3 of the length toward the corner labeled "B-oxide." For example, a point in the middle of the triangle represents an oxide with the chemical formula, ABCO x , where one expects 33.3% each of the A-oxide, B-oxide and C-oxide. The convex-hull formalism, 10 commonly used for thermodynamic predictions, logically derives from similar ideas.
The Cu-V-O phase space is particularly rich and the four known compounds (Table I) Figure 2b, is the well-studied BiVO 4 . While not directly relevant for this review on Cu-based oxides, this particular compound is only shown in this diagram because it is a "champion" ternary oxide in terms of performance in solar water splitting. 96 The CuO-V 2 O 5 edge shows four stable ternary compounds that lie on the CuO-rich side of the edge, while the Bi 2 O 3 -CuO edge reveals one compound, CuBi 2 O 4 , with a 1:1 molar ratio of Bi 2 O 3 and CuO. The interior of the triangle in Figure 2 represents the quaternary phase space and one specific Cu-Bi-V-O compound (the 4:1:1 ratio of CuO-Bi 2 O 3 -V 2 O 5 ) is depicted in Figure 2b. Compositions beyond the quaternary case will necessitate the use of 3-D objects for visualization and their rarity puts them beyond the scope of the present discussion.
A final way to consider ternary compositions is to consider them as comprising of ionic fragments of the A cation (i.e., Cu + or Cu 2+ ) and the BO cluster. Indeed the compound name, in many cases, is derived by this underlying idea. To illustrate, CuBi 2 O 4 or "copper bismuthate" (Table Ib), may be considered to be composed of Cu 2+ and Bi 2 O 4 2− fragments. Similarly, entries #4 and #5 in Table Ib would read as "copper chromate" and "copper ferrate" respectively. There are other also more well-known such examples amongst non-Cu systems, e.g., bismuth titanates etc. Nonetheless, we favor the visualization based on component oxides instead, especially because the "ionic approach" is difficult to apply to quaternary cases and beyond.

Synthetic Aspects
As may be expected, a wide range of synthetic methodologies have been deployed for the preparation of ternary oxides based on Cu(I) and Cu(II). Figure 3 captures the distribution of a random sampling of these methods in the literature focusing only PEC or photocatalysis applications. Interestingly, ceramic preparation routes account for a significant number of instances in these studies. This trend is entirely in line with the theme of this review on the close relationship between solid-state chemistry and PEC. Worthy also of note is the fact that the "catch-all" (Others) category is the dominant category of methods in the Cu(II) case. This category includes variant techniques such as ion exchange, solution combustion synthesis, spin-coating, spraypyrolysis, or metal-organic decomposition.
Given that the set of materials attributes needed for a given technological application (e.g., solar water splitting) is rather well understood, 97 it would have been ideal if a given synthesis set of conditions could be tailored a priori to yield the material with precisely those attributes. Unfortunately, this is not the case given the present state-of-the-art. The targeted attributes may even be contradictory. For example, solar energy conversion applications dictate the use of very large active material area to compensate for the diluteness of the incoming energy source (i.e., sunlight), unless of course concentrator strategies are employed. On the other hand, when nanoparticles of the active material are used, larger crystallite sizes (approaching single crystal behavior!) must be sought to minimize the role of defects and optimize carrier collection.  Table (see  also Tables Ia and Ib). As in the crystal structures below, the elements are color-coded, blue being the A cation (Cu) while green is the B cation in the ternary oxide compound.  It is clear that in cases where an energy-harvesting material is to be synthesized, the energy investment for the synthesis itself, must be minimized such that the energy payback time is shortened. Solid-state (i.e., ceramic) synthesis routes are handicapped in this regard because they involve rather high temperatures. On the other hand, techniques such as solution combustion synthesis (SCS) 55,94 are attractive because the thermal energy required for the synthesis is contained within the heat of combustion of the fuel:oxidizer precursor mixture. Another important advantage of SCS is the versatility it offers for tuning the range of composition of the end product simply by varying the precursor ratio. Thus, an entire range of oxide compositions from neat CuO on the one side to the 1:1 ternary, CuBi 2 O 4 , and to Bi 2 O 3 on the other, could be generated using SCS. 55 Nanocomposites containing CuO and CuBi 2 O 4 nanoparticles in electronic contact or the Bi 2 O 3 /CuBi 2 O 4 material could also be synthesized. 55 Even more significantly, SCS has also afforded the scope for preparing doped oxides or solid solutions as elaborated later in this review.
Materials discovery in its current state of evolution has to rely on either tedious trial-and error methodologies or more efficiently, the combinatorial variant. In this regard, synthesis methods that are time-efficient (such as SCS) hold a significant advantage over prolonged techniques (spanning several hours) such as sol-gel or solidstate routes.

Structural Aspects
Oxides display a remarkable array of structural variations that could be visualized at different levels. At the outset, we can accommodate a particular structure within a mineral type such as perovskite, spinel, delafossite etc. Another approach derives from crystallographic examination and description of the solid-state arrangement in terms of a particular space group and unit cell dimensions. A localized ("inorganic chemist's") view considers the A-O or B-O clusters that are building blocks within the overall framework and the coordination environment around the Cu cations. All three descriptions are complementary and are adopted herein, see Tables IIa and IIb for the thirty-one compounds considered in this review.
The polyhedral representations are particularly illuminating in this regard, and we begin with the structure of the perovskite prototypea veritable "mother" of all structure types that are important in wide range of technological materials science applications. This is illustrated in Figure 4, with the Cu cation, the B cation, and the oxygens color coded the same way everywhere (c.f., Figure 1) for easy visualization. Rational and deliberate structure modifications, designed, to impart a certain characteristic (for example, a lowered optical bandgap), may be conveniently discussed with reference to this structural framework. Chemical alterations may be made either to the cationic sub-lattice or to the anionic framework ( Figure 4). Thus the B cation or the oxygens in the perovskite structure may be substituted with another cation or anion with comparable ionic radii respectively. Nitrogen (r N = 1.5 Å) readily substitutes for oxygen (r O = 1.4 Å) affording the corresponding oxynitrides, although note that the charges are not the same in the two cases. Rational and simultaneous chemical alterations of this sort, targeting both the sub-lattices, have been reported 98 for ABO 2 N type compounds, although such examples seem to be as yet sparse, for Cu-based oxides. Indeed, the so-called heteroanionic systems (e.g., oxynitrides, oxysulfides) 99.100 have garnered much interest in recent years, spurred by applications in solar fuels, thermoelectrics etc. However, these mixed anionic compounds are beyond the scope of the present review.
If two B cations are within ∼15% of the size of one another, a range of solid solutions may be derived with interesting gradation in structural (e.g., unit cell dimension) or optical (energy bandgap, E g ) attributes. This behavior is exemplified by the Cu-Fe-Cr-O system ( Figures 5 and 6); these new materials were prepared via SCS by simple compositional tuning of the precursor mixture. Note that both the lattice parameter (a, b with black and c with blue traces in Figure  5) and E g value (Figures 6) vary, albeit not in the same manner -the change in E g with the extent of substitution, x being much more drastic (c.f., Figures 5b and 6b). The linear change of the lattice parameter with composition confirmed that this ternary alloy followed Vegard's law. 101 It is worth noting that literature examples of solid solutions within the ternary compound space are not as common as within the binary compound space. Therefore, these new and interesting examples provide a guiding template for future studies on chemical compositionproperty relationships.
Consider now the structures of the two parents: Cu 2 O and CuO (Figures 7a and 7b respectively). The unit cells are also superimposed on the ball-and-stick representation in the two cases as dotted lines. While the Cu 2 O structure consists of a cubic unit cell, it is hexagonal for the CuO case. The local coordination geometry is also clearly seen to differ switching from linear in the former case to tetrahedral in the latter. Further structure details may be found for Cu 2 O in a preceding review article. 6 Table II lists all the crystallographic attributes for the two parent structures as well as the 31 derivatives considered in this study.
Delafossites ( Figure 8) are an important structural type for the Cu(I) case and several of these were also considered by previous authors; 6 therefore, their discussion is much abbreviated here. Interestingly, variant local coordination environments for the two cations are seen in the structural framework: linear for Cu(I) but octahedral for the B(III) cation. This structure type is observed for CuBO 2 (B = Rh, Fe, Al) ( Table IIa). By contrast, the two cations experience a similar pyramidal coordination environment in the wurtzite structure (not shown), typical of CuGaO 2 .
The structures of the other Cu(I) based oxides with Nb, Ta, or V as the B cation (see Table IIa) have been extensively discussed elsewhere by previous authors, 6 and therefore are not further considered here. Instead, we now turn to structures based on Cu(II): two common structure types are spinels and wolframite, shown in Figures 9a and 9b respectively. Copper-containing spinels with the general chemical formula, CuB 2 O 4 , have a tetragonal unit cell while Cu-containing wolframites (general formula: CuBO 4 ) possess a unit cell of much lower symmetry: either, monoclinic or triclinic, depending on the particular compound (Table IIb). Once again, the local coordination Table II. Structural details of copper(I)-and copper(II)-based oxides.

Cell parameters
No.
Compound Structure type   By contrast, V 5+ occurs as VO 6 or VO 4 clusters in the three cases.
Phase transitions have been studied in the Cu-V-O ternary system since the 1960s. 102,103 Polymorphs of both Cu 2 V 2 O 7 and Cu 3 V 2 O 8 are shown in Figures 11 and 12 respectively. These have been observed in recent SCS studies (as yet, unpublished) in our laboratories, and have been studied by thermal analysis and high-temperature XRD by previous authors. 102,103 Three polymorphs are known and their transitions (often reversible) occur at temperatures ranging from ∼ 500 • C to ∼700 • C. Other examples of polymorphism may be found in Table II. The wurtzite-derived β-CuGaO 2 structure has been studied using synchrotron X-ray radiation. 28 This structure irreversibly transforms to the delafossite α-phase at temperatures higher than ∼460 • C in an Ar atmosphere. 28 In general, there is much scope for similar detailed studies on phase transitions in ternary copper oxides. It is also worth noting that the impact of these polymorphs on the corresponding optoelectronic characteristics, has not been adequately explored.
Other interesting structural questions remain, at least on a semantic level. For example, one can envision a thought experiment wherein the two component binary oxide structures are completely dismantled and then reassembled into a ternary compound framework. What dictates the ultimate crystal structure and does that have a relationship to the initial binary compound structures?

Optoelectronic Aspects
Being semiconductors, an important optoelectronic property is the optical bandgap value, E g . Other details on the nature of the optical transition, whether direct or indirect, has a bearing on the strength of absorption of the excitation light by the semiconductor. Tables IIIa and IIIb assemble the available data on the 31 compounds. Methods for estimating E g have been reviewed elsewhere; 104 two of these are shown under the categories: "Absorption edge" and "Tauc plot" respectively. Again, for reasons outlined elsewhere, 104 values derived from the latter are considered to be more reliable. Nonetheless, in cases where both methods have been used, the two sets of results are in reasonable accord (see entry #2 in Table IIIb, for example). Copper tungstate and niobate are exceptions to this trend and considerable variability (as much as 1 eV or more!) exists in the reported values for reasons, again discussed elsewhere. 104 The E g values have been quoted to varying precision between 2 and 3 significant figures in Table III. Given the variability, however, only 2 significant figures may be justifiable. It is worth noting that, with rare exceptions, the vast majority of the E g  values also lie within the "sweet spot" of optimal overlap with the solar spectrum.
Many of the compounds listed in Table III feature both types of optical transitions; therefore two separate sets of values for E g have been listed. However, unlike in the notable case of BiVO 4 , few questions have been raised on the nature of the fundamental bandgap in these materials. While theory can contribute much to an understanding electronic band structures, we have avoided consideration of theoretically-derived E g values for the compilations in Table III. Accurate prediction of E g values remains an Achilles heel of methods such as DFT, although improvements may be anticipated in the future.
As in the preceding section, interesting questions remain on the electronic nature of the resultant ternary structures after they are assembled from the binary components. Unlike in the case of solid solutions of the two components (c.f., Figure 5b), the E g values for the ternaries are not bracketed by the values for the end (binary) members. Another way of looking at this is to appreciate that in many cases, the B oxide is not even a semiconductor. A case in point is entry #1 in Tables Ia and Ib above: Al 2 O 3 is an insulator. In other cases, e.g., entry #7 in Table Ib, the B oxide is an electronic conductor. In most of the cases, however, the B oxide indeed is a semiconductor.
Bonding or electronic band structure theory can contribute much to an understanding of how the ternary (or higher) compound band structures involve from the binary compound cases. For example, distortions in the cation or oxygen tetrahedra have been considered as a function of the r M(I) /r M(III) ratio in the β-NaFeO 2 structure (characteristic of wurtzite-derived β-CuGaO 2 ). 28 This distortion influences the energy splitting of d-orbitals in compounds containing the Cu(I) species, and in these cases, the Cu 3d orbitals strongly contribute to the top of the valence band. In general, the role of ions such as Cu + (or Ag + ) for that matter, as an effective electronic structure regulator, has been the topic of much recent interest in the PEC and photocatalysis communities. 105 This is because these ions affect the fundamental na-ture of the optical transition from the valence band to the conduction band. Strategies for "optoelectronic engineering" of the band structure to intensify the light absorption strength or even the nature of the optical transition (e.g., switch from indirect to direct), are only at a rudimentary level at present. Such efforts (e.g., doping with a rare earth element) have been attempted only with binary oxides, at least to our knowledge.
Carrier transport through the semiconductor bulk follows after the initial optical excitation of the inorganic semiconductor. The dynamics of this process exerts much influence on the overall efficiency of the energy conversion or catalysis process. In most instances (unless we are considering devices of the dye-sensitized variety), it is the minority carrier diffusion length (L D ) that is the crucial parameter. The larger the L D value is, the better is the material quality in terms of defects, traps etc. Thus for an n-type semiconductor, holes are the minority carriers while electrons are the minority carriers for p-type materials. Unfortunately, data on L D appear to be rather sparse of the compounds considered in this review.
One exception to this trend is the Cu(II) ternary oxide, CuBi 2 O 4 . Time-resolved microwave conductivity (TRMC) and surface photovoltage (SPV) measurements were performed to assess the optoelectronic quality of this semiconductor. 52 The TRMC transient decays showed the presence of two time constants leading to L D values of ∼10 nm and ∼52 nm respectively. 52 The coincidence of the SPV signal onset with the optical bandgap cut off wavelength was interpreted to signal the absence of optically active defects in the material. 52 The electrical transport mechanism has been studied and modeled as variable range small-polaron hopping at low temperatures (below 300 K) and phonon-assisted small polaron hopping at high temperatures. 106 In general, the need for more studies (both experimental and theoretical) aimed at understanding charge transport mechanism(s) in CuBi 2 O 4 , has been stressed in the literature. 52 The situation is even more pressing for the other 30 compounds in Table I.

Table III. Optical bandgaps for copper oxide-based ternary oxides. a) Copper (I)-based compounds. b) Copper(II)-based compounds.
Optical bandgap (eV) Indirect insights into the optoelectronic characteristics may be gleaned from photoelectrochemical and photocatalytic activity measurements; these are considered next.

Photoelectrochemical and Photocatalytic Aspects
At the outset, it is worth mentioning that comparisons of the efficacy of a given photoelectrode (or photocatalyst powder) material with other candidates in a given application, are immediately handicapped by the absence of an agreed-upon framework for reporting PEC data or a photocatalysis figure-of-merit. This stands in contrast to photovoltaic devices where such standards exist and cell efficiencies are certified. Nonetheless, with this caveat in mind, reported data on how the 31 ternary oxides fare in either water splitting, CO 2 photoreduction, or pollutant degradation can still be made.
An immediate assessment of the photoelectrochemical quality of the oxide material may be obtained either via a photovoltammetry scan or by measuring the photocurrent at a fixed bias potential in a suitable aqueous medium. 107 For an n-type material, the expectation is that the photocurrent arises from the (anodic) oxidation of solution species such as water (i.e., the oxygen evolution reaction (OER). For the ptype material in a de-oxygenated aqueous medium, the corresponding water-splitting reaction would be the hydrogen evolution reaction or HER. Such data on 13 Cu(I)-based ternary oxides were reported by previous authors (see Table I, Ref. 6); available data on the Cu(II) counterparts are contained in Table IV.
The above assumption of the source of the photocurrent rooted in OER or HER, hinges on the fact that the extent of photoelectro-chemical corrosion of the oxide semiconductor itself is negligible in the particular medium. This must be carefully verified by product (i.e., O 2 or H 2 ) collection and quantification, and comparison with the quantity expected from the charge passed (i.e., the current efficiency). In many of the cases in Table IV, this has not been done.
The other important variables in these data are the medium pH and the photon flux, both of which are specified in the compilation. Taken as a whole, CuBi 2 O 4 is easily seen to outperform the other compounds, and the photocurrent level is at least an order of magnitude higher in this case (entry #1, Table IV). Table V lists instances where the OER data have indeed been reported in studies on copper-based ternary oxides. The entries in this tabulation all pertain to Cu(II)-based compounds. The faradaic efficiency values indeed approach 100% except at lower bias potentials (e.g., entries # 1, 2 in Table V). Corresponding HER data are tabulated in Table VI, once again for Cu(II)-based ternary oxides. Unlike in Table V, these performance metrics pertain to powder suspensions of the oxide and not photoelectrodes. Therefore, it is more difficult to assess the efficacy in these cases, given that faradaic efficiency values, obviously, are not accessible.
Finally, CO 2 photoreduction data are summarized in Table VII; clearly, examples are rather sparce, especially for Cu(II)-based compounds. Both modes of operation, i.e., using photocathode (entry #1) or using powder suspensions (entries # 2,3) are considered in this tabulation on three ternary oxides based on Cu(II). A range of reduction products appear depending on the specific conditions used.  Corresponding reports on the use of Cu(I)-based ternaries for CO 2 photoreduction are contained in Refs. 5 and 26. A more general review on flow-based CO 2 photoreduction schemes, may be found in Ref. 108.
In general, the problems with the stability of Cu-based oxide semiconductors have been addressed by rapidly transferring the photogenerated electrons to a second phase, for example, carbon. Carbon has the advantage that the injected electrons are rapidly delocalized in the π-electron manifold. Further, many forms of nanocarbons are available. Two recent example studies on Cu 2 O, may be cited. 109,110 Corresponding examples for ternary oxides are as yet lacking, at least to our knowledge.
A second type of application of inorganic oxide semiconductors is in pollutant degradation. For this environmental remediation, n-type semiconductors are better suited in that highly oxidizing species such as holes or hydroxyl radicals can be generated at the semiconductormedium interface. However, the potential for simultaneously treating reducible pollutants (such as toxic metal ions) also exists with heterogeneous photocatalysis, and this feature is a bonus that is not available with advanced oxidation processes (AOPs) based on ozone, H 2 O 2 , and the like. Table VIII lists examples in the literature where Cu(II)-based ternary oxides have been deployed for such an application. All these examples concern the destruction of an azo dye such as methyl orange or methylene blue. While four of the reported cases show almost complete dye destruction (presumably to mineralized products), four other studies reveal only modest performance where amounts, even as low as only ∼42% of the initial dye was degraded. Even the simultaneous presence of H 2 O 2 , as a generator of reactive oxygen species (ROS) did not seem to have helped matters to a significant extent.
The combination of optoelectronic (i.e., bandgap) data and photoelectrochemical measurements (e.g., capacitance voltage or photoaction spectroscopy) allows us to map the band edge positions of the various Cu-containing ternary oxides on a common energy scale. This is shown in Figure 13; a few of these cases for the Cu(I) compounds were considered by previous authors in their review article. 6 The compilation in Figure 13 is more extensive; 24 compounds are compared here along with the two parent oxides, Cu 2 O and CuO.
The thermodynamic utility of diagrams such as those in Figure  13 resides with the fact that predictions may be made for a given compound's ability to oxidize (or reduce) a targeted solution species such as water, protons, or CO 2 . Since the band edges move with solution pH, it is judicious to do comparative assessments at a common pH, pH = 0 in this case.

Quaternary Oxides Based on Copper
These contain two other metal cations (B and C) besides copper in the oxide structure. One such compound, Cu 2 BiVO 6 , was already considered in Figure 2b earlier and consists of a 4:1:1 combination of Cu 2 O, Bi 2 O 3 and V 2 O 5 . Unlike the literature on ternary counterparts, instances where quaternary Cu-based oxides have been studied, are very sparse. This trend also contrasts with the rather rich literature on Ag-based compounds. Thus, four specific compounds appear to have been studied in the copper case: CuAlGaO 4 , 111 Cu 2 BiVO 6 (c.f., Figure 2), [112][113][114] and CuBiW 2 O 8. 115 The first compound is a 2:1:1 combination of CuO, Al 2 O 3 , and Ga 2 O 3 , while the third is a 1:1:4 combination of Cu 2 O, Bi 2 O 3 , and WO 3 . These have been prepared by a diverse range of methods including solid-state synthesis, 111 chemical solution, 113 and the use of a polymeric citrate complex precursor. 112 The compounds also have been evaluated for their propensity to evolve H 2 (from H 2 S), 111 O 2 114 or decompose a dye (e.g., rhodamine B) 112 under visible light irradiation. Clearly, studies on quaternary Cu-based oxides remain a fertile field of opportunity for further studies.

Concluding Remarks
Any research endeavor may be evaluated on the basis of two yard sticks: a) has it pushed the knowledge boundary forward in a transformative sense? And b) has it spawned a new technology? Recall also that the extension of the search for optimal inorganic semiconductors into the ternary (or higher) compound domain, was largely spurred by the perceived handicaps associated with the binary oxide counterparts. In the present instance, the lack of adequate photoelectrochemical stability of Cu 2 O was the major driver for the search for ternary copper-based semiconductors. Are the ternary compounds more stable in a photoelectrochemical sense? The jury is still out on this aspect although there are encouraging signs that insertion of the B cation (in to the copper oxide framework) can perturb the parent electronic band structure sufficiently to stabilize the compound against photocorrosion.
Returning to the two questions posed earlier, this review demonstrates that major strides have been made in the fundamental understanding of ternary oxide semiconductors. That being said, much more must be accomplished in selected areas, and identified in the preceding sections of this article. In a practical sense, more optimization has to be done in aspects related to materials quality etc., before performance levels (e.g., photocurrents in the 10-20 mA/cm 2 ) can be achieved in a PEC environment (c.f., Tables IV; the maximum observed to date is 2.5 mA/cm 2 , entry #1). Only then will the engineering community be motivated to tackle aspects related to photoreactor design, scale-up etc., and a viable technology emerge from all the science that would have been done up to that juncture.
Admittedly, this review has given short shrift to how theory can contribute to materials discovery, properties, and use. This reflects the authors' bias, more than anything else. Clearly, methods such as DFT (in spite of their well-recognized handicaps) and as briefly alluded to above, continue to provide much guidance to experimentalists in terms of understanding electronic band structures and their dependence on chemical composition. Although much more nascent, phase field models 116 can map morphological effects on materials properties on a mesoscale level. Such theoretical efforts could provide crucial support to the experimentalists in the continuing search and optimization of oxide semiconductors (such as the Cu-based compounds considered in this review) for solar fuels photogeneration and environmental remediation applications.