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Review

Sulfide-Based Photocatalysts Using Visible Light, with Special Focus on In2S3, SnS2 and ZnIn2S4

by
José C. Conesa
Instituto de Catálisis y Petroleoquímica, CSIC, 28049 Madrid, Spain
Catalysts 2022, 12(1), 40; https://doi.org/10.3390/catal12010040
Submission received: 4 July 2021 / Revised: 29 November 2021 / Accepted: 27 December 2021 / Published: 30 December 2021
(This article belongs to the Special Issue 10th Anniversary of Catalysts—Feature Papers in Photocatalysis)
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Abstract

:
Sulfides are frequently used as photocatalysts, since they absorb visible light better than many oxides. They have the disadvantage of being more easily photocorroded. This occurs mostly in oxidizing conditions; therefore, they are commonly used instead in reduction processes, such as CO2 reduction to fuels or H2 production. Here a summary will be presented of a number of sulfides used in several photocatalytic processes; where appropriate, some recent reviews will be presented of their behaviour. Results obtained in recent years by our group using some octahedral sulfides will be shown, showing how to determine their wavelength-dependent photocatalytic activities, checking their mechanisms in some cases, and verifying how they can be modified to extend their wavelength range of activity. It will be shown here as well how using photocatalytic or photoelectrochemical setups, by combining some enzymes with these sulfides, allows achieving the photo-splitting of water into H2 and O2, thus constituting a scheme of artificial photosynthesis.

1. Introduction

Photocatalysts are used for many purposes: energy-related applications, fine chemicals synthesis, environment protection, or detection of specific chemicals. Photocatalysis has been known for a long time. The first work on heterogeneous photocatalysis (to this author’s knowledge) was reported by Moore and Webster in 1913 [1]. The photoreduction to formaldehyde of CO2 was described there, using iron or uranium oxide colloids and utilizing visible light. Since there is currently an urgent need to revert the increase of CO2 in the atmosphere, this was certainly an important work. This paper appeared just after the work by G. Ciamician [2], which said in its last sentences, “So far, human civilization has made use almost exclusively of fossil solar energy. Would it not be advantageous to make better use of radiant energy?” Almost six decades later, an article was published by Fujishima and Honda [3] who proposed using photoelectrochemistry, a practical way to photodissociate H2O into O2 and H2.
Those authors used a single crystal of rutile TiO2 in their work. Given that its bandgap is 3.0 eV, it absorbs light in the near-UV range, thus it is unsuitable for converting much of the solar spectrum. Other oxides like SrTiO3, ZnO, or anatase TiO2, having bandgaps above 3.0 eV, have a similar limitation; nonetheless, for some fine chemical syntheses and especially for environment protection, anatase-type TiO2 remains unsurpassed as photocatalyst. Other materials have been tried in order to enlarge the amount of solar spectrum that can be used. Thus, anatase has been doped with cations or anions, and completely different oxides have been developed like BiVO4 (with Eg = 2.4 eV); this material, which is mentioned frequently as able to photogenerate O2, as well as the oxides mentioned before with bandgaps higher than 3.0 eV, are certainly resistant to photocorrosion. Additionally, (oxy)sulfides [4,5,6,7], (oxy)nitrides [8,9,10,11], selenides [12,13,14,15], several varieties of doped carbons [16,17,18,19], and a few more exotic materials were proposed in order to use a larger range of the solar spectrum. On the other hand, for energy applications, a co-catalyst is frequently required to facilitate the O2 and/or H2 evolution, the CO2 reduction or the conversion of substances derived from biomass.
Many such materials can be also used in photoelectrochemical (PEC) systems. For example, Fe2O3 in the hematite phase (see structure in [20]) has a small mobility of the photogenerated current carriers so that high recombination rates occur unless a very small thickness is used; it is, however, very actively studied for PEC uses thanks to its convenient bandgap (≈1.9 eV) and especially its abundance. Additionally, this material is resistant to photocorrosion. Examples of its use in photocatalysis can be found [21,22,23].
This review deals with the use of sulfide photocatalysts, which in many cases can use visible light (even infrared light in some cases). Many reviews have appeared dealing with photocatalysis using sulfides for protection of the environment [24] and transformation of organic molecules [25], as well as others dealing with more general photocatalysts (including sulfides), devoted to H2 generation [26,27] and CO2 reduction [28]; sulfides containing several cations have been also studied for photocatalysis or energy harvesting purposes [29,30]. Mixing sulfides with other phases has been utilized as well to better separate the photogenerated holes and electrons (leading, when both semiconductors absorb light, to the so-called Z-scheme) [31].
It is well known that sulfides, particularly in oxidizing conditions, can be photocorroded; efforts are thus made in order to avoid or at least minimize this process [32]. Therefore, sulfide photocatalysts are mainly used for photoreduction processes, as is the case of H2 generation or CO2 reduction. This might require using a sacrificial agent, e.g., sulphite or sulfide anions, which makes the process useful only for basic studies, except if the sacrificial agent is derived from biomass.

1.1. 2-Fold Coordinated Sulfides

One 2-fold coordinated sulfide that has been studied for photocatalysis is HgS (cinnabar). Its structure is given in [33]; its bandgap is 2.05 eV, as shown in [34]. Some other results in photocatalysis by HgS, combined with other phases, are found in [35,36].

1.2. Tri- and Tetrahedrally Coordinated Sulfides

There are some sulfides which contain cations in trigonal planar coordination. For example, Bi2S3, with both 4- and 3-fold coordinated Bi (due to a lone pair present in Bi; its structure is reported in [37]). Its bandgap is 1.3–1.7 eV and has been used for photocatalysis either alone [38,39] or in combination with other phases [40,41,42]. An additional example is CuS with covellite structure (structure reported in [43]), which also has Cu in trigonal and tetrahedral coordination. Its bandgap is 1.75 eV and has also been used alone [44,45] or combined with other materials, for photocatalysis applications [46,47,48]. It must be noted that, in spite of its formula, it contains mainly Cu+ [49], which means that disulfide ions exist in it.
Additionally, Ag2S (structure reported in [50]) and Cu2S (structure reported in [51]) contain, in their most stable phases, trigonally coordinated cations and have been used as well as photocatalysts. For example, Cu2S, which has a bandgap (indirect) of 1.2 eV [52], was utilized in degradation of herbicides combined with Bi2WO6 [53], in degradation of dyes in combination with TiO2 of P25 type [54] or with H2O2 [55]; combined with MoO3, it has been used for H2 generation, degradation of dyes, or reduction of Cr (VI) [56]. Ag2S, in turn, has an indirect bandgap of 1.0 eV [57]; it has been utilized as a photocatalyst in a number of composites for generation of H2 [58], degradation of dyes, or reduction of CO2 [59], as well as in disinfection [60]. Several Cu2−xS structures were used as well as photocatalysts [61].
A significant amount of sulfides contain only tetrahedrally coordinated cations. This is the case, for example, of ZnS (structure reported in [62]). It has a rather large bandgap (3.4 eV [63]), so that it can absorb only light in the UV range. It has been used, however, in photocatalysis for very different applications [64,65,66]. Note that this material can adopt different shapes, influencing its photocatalytic and photophysical properties [67].
The sulfide most studied for photocatalysis is probably CdS (structure reported in [68]), also tetrahedrally coordinated. Due to the high mobility of its photo-excited electrons and holes, and its significant ability to absorb light in the visible range (λ < 500 nm; its bandgap is 2.48 eV [63]), several reviews have studied its capabilities for generation of H2 [69], organic chemistry transformations [25] or degradation of dyes [24]. One problem is the toxicity of Cd; this sulfide is also prone to photocorrosion, particularly in oxidizing conditions [70]. Some attempts have been made to decrease this effect [71,72]. On the other hand, CdS easily undergoes (as in the case of CdSe) quantum confinement effects. Therefore, its particle size can be tailored by irradiating it with monochromatic light in oxidizing conditions; any photocorrosion will finish when the size of its particles is so small that the single-wavelength light can no longer be absorbed [73].
Another class of tetrahedrally coordinated sulfide photocatalysts that are studied include the chalcopyrite family. Thus, AgGaS2, CuGaS2, and CuInS2, all of them with chalcopyrite structure (see structure for AgGaS2 in [74]), have gaps of 2.6, 2.3, and 1.5 eV, respectively, and even their alloys have been used as photocatalysts. CuGaS2 [75,76], (Ag,Cu)GaS2 [77] or (Ag,Cu)(In,Ga)S2 [78] as well as doped AgGaS2 [79] are active for photogeneration of H2; mixing CuGaS2 with RGO-TiO2 has photocatalytic activity in reducing CO2 to CO [80]. Some systems of this kind can be used for the photocatalytic elimination of dyes [81], nitrate ions [82] or NO [83]; several years ago, a review dealt with the photocatalytic uses of CuInS2 [84]. Additionally, kesterites, which have structures similar to chalcopyrites (see [85]) and have a bandgap of 1.5 eV (like CuInS2), were used as well as photocatalysts [86].

1.3. Sulfides including 6-Fold Coordinated Cations

Materials that have been studied extensively as photocatalysts are WS2 and (especially) MoS2; there are several recent reviews on them [87,88,89]. They have layered structures with cations in prismatic coordination, held together by van der Waals forces (see their 2H structures in [90,91], respectively). They are also polymorphs [92,93]) and have indirect bandgaps of 1.35 eV and 1.23 eV, respectively [94]. These bandgaps can be increased by decreasing their particle sizes; in fact, isolated trilayers of MoS2 and WS2 have, according to photoluminescence data, direct bandgaps of respectively 1.89 and 2.03 eV [95]. This might position their conduction bands to levels more negative than the H2|H+ electrode potential [96], so that H2 photogeneration might be facilitated. MoS2. with small-to-moderate particle size. is much more efficient photocatalytically if its particle size falls below 4–5 nm [97,98]; this is certainly a quantum confinement effect. It must be noted, on the other hand, that there is another structure of MoS2, termed 1T, which has octahedral, not prismatic, coordination (see structure in [99]). It has metallic characteristics, so that it is very active in combining protons to achieve H2 evolution [100].
Other 6-fold coordinated sulfides have been studied for photocatalysis. This is the case of ZrS2 (see structure in [101]), which is also a layered structure held together by dispersion forces, but it has octahedral, not prismatic, coordination, at difference with 2H MoS2 or WS2 (i.e., it is similar to 1T MoS2). Its bandgap is ca. 1.7 eV [102]; however, as shown in [103] if it is made in 2–3-layer shape, it may attain a 2.0 eV bandgap making it ideal for photo-generation of H2 [103]. HfS2 has a similar structure [101] but has a smaller bandgap [104]. Its isolation from ZrS2 is difficult, however, and it has therefore rarely been used in photocatalysis [105].
Additionally, FeS2 with pyrite structure (including, in this case, only disulfide ions [106]) also has an octahedral coordination to S atoms; its bandgap is 0.95 eV and has been used sometimes for photocatalysis [107].
Finally, there is another octahedrally coordinated sulfide: PbS (its structure can be found in [108]). However, its bandgap is rather small (ca. 0.5 eV or less, as shown in [109]). Still, it has been considered for photocatalysis in combination with other phases [110,111].
This work will concentrate in the experience of our lab with three simple sulfides including octahedral coordination: In2S3, SnS2, and ZnIn2S4. They will be considered alone or with additions making them more photocatalytically active. It must be noted, therefore, that this review puts a special emphasis on the author’s own work.

2. Discussion and Comments on Previous Results of Our Group

2.1. In2S3

This material has a cation-defect spinel lattice (see its β structure in [112]; when the tetrahedral cations are disordered, the structure is named α) and a direct bandgap of 2.0–2.1 eV. It is frequently used as buffer layer in thin film photovoltaic cells. It has been used for H2 photoproduction in works beginning 15 years ago [113,114,115]. Its use for aqueous organics photodegradation is also relatively old [116,117,118]. Some 315 articles on the photocatalytic use of In2S3 (or its combination with other phases) have appeared until now, being combined frequently with TiO2 as well for dye or antibiotics degradation [119,120], H2 photogeneration, [121] or elimination of warfare agents [122]. Some recent reviews on the photocatalytic utilization of In2S3 have appeared [123,124].

2.1.1. Photocorrosion Resistance and Spectral Response of In2S3

We examined these aspects in a former work [125]. In2S3 was hydrothermally synthesized, and its specific surface (SBET ≈ 40 m2/g) was characterized; XRD revealed β-In2S3 with disordered cation vacancies, and diffuse reflectance spectroscopy confirmed the 2.1 eV bandgap. This sulfide was tested in the photocatalytic degradation of aqueous HCOOH, showing that In2S3 is more active (Figure 1a) and photocorrosion-resistant (Figure 1b) than CdS. Its spectral response was shown, using a series of λ-selecting filters, to agree with the bandgap (Figure 1c). The HCOOH degradation mechanism coincided with Equation (1) of [125].

2.1.2. Mechanism Research in the Degradation of the Dye Rhodamine B

The use of In2S3 for degrading this dye photocatalytically started more than 10 years ago [126]. This subject was undertaken by us recently [127], using the same hydrothermal method for making In2S3 and trying to better assess the mechanism of this process. The same nanocrystalline In2S3 was used as in the preceding section, using it now in degrading the rhodamine B dye. The evolution of the light absorption at λ = 554 nm of this dye in water solution (once the photocatalyst was filtered out), given in Figure 24.10 of [127], verified that higher wavelengths implied smaller activity in photocatalytic action.
The experiment revealed as well that the dye degradation involved more than two intermediate products, since no isosbestic point appeared in the absorption spectra of the solution. Besides, the evolution of the photodegradation depended on the presence of O2, as shown in Figure 2; with O2 the decay is much faster, while under N2 the component absorbing light at lowest wavelength takes much longer to be eliminated.
To understand this behavior, a principal component analysis (PCA) [128] of the absorption spectra of the dye was carried out. This allowed determining, first, that only three independent factors explained all the dye spectra. Besides the initial RhB dye, only another two components (by comparison with literature data) could be assigned: the same dye fully de-ethylated in just one N atom, or in both N atoms (leading to dye Rh110). The degradation steps sequence could be thus established:
Furthermore, with no O2 present, the last RhB degradation step in which the aromatic ring is broken takes much longer (see Figure 2). This implies that this step depends on the presence of O2H• or O2− radicals, formed by transfer to O2 of photogenerated electrons and subsequent protonation. The precedent steps involve thus the more aggressive OH• radicals (due to transfer of holes from In2S3). These OH• radicals might well survive much shorter time in solution; if the adsorption of the dye on In2S3 occurs mainly through the ethyl residues, once these disappeared the molecule fully de-ethylated, it may go mostly into solution, and there it may react only with the O2H• or O2− radicals, known to have longer lifetimes. The final part of that study involved decomposing with PCA also the dye absorption spectra found using the wavelength-selecting filters while bubbling the irradiated solution with O2; the results are shown in Figure 3.
Undertaking a semilogarithmic plot fitting of the initial RhB dye decays obtained in Figure 3 allowed determining the spectral response of that decay. It is shown in Figure 4, evidencing the agreement of this profile with the In2S3 absorption spectrum (not of the dye).

2.1.3. Two-Photon Processes Using V-Substituted In2S3

A proposal was made some years ago stating that by insertion of a narrow, delocalized band (partially filled) between the valence (VB) and conduction (CB) bands of a semiconductor could allow realizing electron transfers, using sub-bandgap photons from the VB to the CB in two steps, thus enhancing the theoretical photovoltaic efficiency beyond the Shockley–Queisser limit [129]. Several researchers (including this author) studied with DFT calculations how to achieve such structure (see [130] and references therein). This last work showed that by substituting with vanadium part of the In atoms in In2S3 could provide a proper structure to achieve this purpose.
Then, our laboratory carried out the preparation of this material, achieving it shortly after [131]. Here the VCl3 compound used reacted with protons generating much H2; a water- ethylene glycol (with 10% water) was therefore used, to decrease that reaction. The V4+ ions in the material, detected with EPR, were thanks to this strategy below 25%. We then tested it later in photocatalysis using the same aqueous HCOOH degradation reaction [132]. The results indicated (Figure 5a) that the HCOOH degradation spectral response was extended to longer wavelengths.
The most interesting result was provided by photoluminescence (PL) tests. These verified that while PL at ~600 nm (which corresponds roughly to the In2S3 gap) were excited in V-free In2S3 only by shorter wavelengths (as expected), the PL at that same wavelength could be excited in V-containing In2S3 also with wavelengths longer than the In2S3 bandgap. V-free In2S3 was unable to act in the same manner. (Figure 5b), evidencing an upconversion process requiring two photons. Furthermore, the range in which the PL was excited was the same as that in which photocatalysis took place (compare Figure 5a,c), proving that the process provoking this upconversion made possible as well the migration of holes and electrons to the surface, leading to chemical reactions. This was not due to a nonlinear process, as shown in Figure 5d, evidencing that PL does not depend on the degree of filling of the first transition, in agreement with both Figure 5e and the scCOHSEX + G0W0 result shown in Figure 7a of [132].
PL tests with terephtalic acid, which reacts with OH radicals to form the corresponding PL-active derivative (which has photoluminescence properties), showed as well (Figure 6) that the generation of these radicals (as shown in earlier work using also In2S3 [117]) occurs as well for longer wavelengths in the case of V-containing In2S3.

2.1.4. Photocatalytic Generation of H2 with an In2S3-Hydrogenase Combination

Hydrogenases are electroactive enzymes which contain dinuclear Ni-Fe or Fe-Fe complexes, bonded mainly to sulfur atoms, catalyzing efficiently the reaction
2 H+ + 2 e− ↔ H2
We thus published a work recently [133] in which a Ni-Fe hydrogenase, inserted in a hydrothermally prepared porous In2S3 structure (having a SBET area similar to that in [125]), was suspended in a sodium sulfite aqueous solution (used as sacrificial reagent) and irradiated then with white light. H2 was produced and detected with MS, as shown in Figure 7a), only when the In2S3 suspension at 37 °C and pH = 7 was irradiated in presence of the hydrogenase, behaving thus as co-catalyst. Comparing this production of H2 with that resulting when the hydrogenase, when no irradiation nor In2S3 were present, was contacted with a solution of reduced methylviologen (a very good substrate for generation of H2 with this enzyme), indicated a similar ability for generation of H2 in both cases (Figure 7b). This means that electrons photogenerated in In2S3 can be efficiently transferred to the enzyme, the latter being thus able to produce H2.

2.1.5. Photoelectrochemical Generation of O2 by an Electrode including Laccase and In2S3

Laccases are enzymes, which contain Cu-oxide clusters, the normal role of which is reducing O2 to water without stopping at the H2O2 intermediate product. Previous experience of another group in our institute [134] showed that the reverse reaction, i.e., direct evolution of O2 from water, could be carried out as well. We thus linked a laccase to an electrode and could verify how the same could be carried out irradiating an electrode which contained a visible light-responsive semiconductor (In2S3) so that an overpotential could be achieved which was lower than that needed for a nonirradiated electrode.
Thus, a recent publication by our group was made [135] using an electrode built by depositing hydrothermally prepared In2S3 (again with SBET area similar to that in [125]) on a FTO-covered glass, then linking covalently a laccase enzyme to the semiconductor. Several electrochemical measurements were carried out in phosphate-buffered solution (i.e., pH = 7.1) under Ar atmosphere using an Ag/AgCl reference electrode; a sensor of dissolved O2 allowed detecting this latter molecule.
Figure 8 gives a summary of the results. Part A shows cyclic voltammograms (CVs) of FTO/In2S3/laccase electrodes in the dark (a) and under illumination (b); inset shows the O2 sensor signal the same conditions (with delay because of the time that O2 needs to diffuse to the sensor); in the absence of In2S3 and/or laccase, the CV current and the signal from the O2 sensor were rather smaller or even negligeable. No relevant amount of H2O2 was found in the solution, evidencing the known ability of this laccase to catalyze the 4-electron process between O2 and H2O. Part B shows chronoamperograms recorded at 1 V vs. SHE upon irradiation of FTO/In2S3 (left) and FTO/In2S3/laccase electrodes (right); the O2 sensor signal is included in both cases. As it can be seen, a significant response of the O2 sensor appears only in the presence of the laccase enzyme. It could be also verified, after calibration of the O2 sensor signal, that the total current difference observed in both cases corresponded well with the quantity of O2 generated if a 4-electron process was assumed (it must be noted that the result observed without laccase corresponds to an electrode capacitance charging effect). On the other hand, in the absence of illumination FTO or FTO/laccase, electrodes require potentials higher that 1.5 V in order to generate some O2. The system implies, therefore, an overpotential decrease of at least 0.55 V because of the effect of the irradiated In2S3 semiconductor.
We can state that this was the first time ever in which combining an enzyme (used as co-catalyst) and a visible light-sensitive inorganic semiconductor showed ability to generate O2 upon illumination, as it occurs in natural photosynthesis.

2.2. A Semiconductor Related to In2S3: ZnIn2S4

This material (structure given in [136]; note that the c axis must be that of length 24.68 Å, as this is the one that gives a XRD diffraction compatible with that Figure 24.5 in [127]) has a layered structure, with a central layer of octahedrally coordinated In atoms having at one side a layer of In atoms tetrahedrally coordinated and at the other side a layer of Zn atoms tetrahedrally coordinated; the external atoms are always sulphur. The layers are held together by van der Waals forces; different stackings of them are possible [136,137,138]. For this material, bandgaps are in the 1.9–2.2 eV range, perhaps because of the several stacking possibilities. There are doubts as to whether this bandgap is direct or indirect, which may be again due to the different layer stackings possible [139,140]. The first work on photocatalysis using this material appeared less than 20 years ago [141]; since then, over 600 studies on its photocatalytic properties have appeared, either for dye degradation [142] or photogeneration of H2 [143]. Recent reviews of its photocatalytic properties have appeared [140,144].
We decided to undertake a study on its spectral response for photocatalysis; the results were reported in [127]. Its diffuse reflectance spectrum was measured; however, as previously stated, there are some doubts concerning its direct or indirect character. Thus, a DFT calculation using a hybrid functional was carried out, and the result (Figure 9A) shows that it has an indirect gap, but so close to the direct one that except for PL tests the bandgap can be considered direct in Tauc plots; thus a 2.6 eV bandgap was determined.
The photocatalytic spectral response of ZnIn2S4 was determined, like for In2S3, by means of the degradation rate of an HCOOH aqueous solution using a stirred suspension of ZnIn2S4, after verifying its crystallinity and SBET surface area (37.4 m2/g). The results, given in Figure 9B, show again that a good ability to absorb visible light makes this material interesting. However, its rate of photocorrosion is rather larger than that of In2S3 (Figure 9C), perhaps due to the presence of tetracoordinated Zn in one side of the layers (actually, the Zn fraction gone into solution is higher than that of In).

2.3. SnS2

This material (structure in [145]), which contains only octahedrally coordinated Sn, has as well a layered structure, in which each S-Sn-S trilayer is bonded to the next one by weak van der Waals forces, leading thus again to several stacking possibilities [146,147]. Its most stable phase has an indirect bandgap of 2.2 eV [148]; it can thus absorb a significant amount of visible light.
SnS2 has thus been studied for photocatalysis. The first publication of its photocatalytic properties appeared less than 15 years ago [149]; ca. 600 publications on these properties have appeared since then, related to H2 generation or dye degradation [150], but also publications on less common processes such as Cr(VI) photoreduction [151], removal of antibiotics [152], or reduction of CO2 to CH4 [153] or CO [154] have appeared. Works on photocatalysis using SnS2 done by our group are presented here.

2.3.1. SnS2 Spectral Response

In our group, we tested its spectral response [155] using once more the photodegradation of HCOOH dissolved in water using a stirred suspension. The material, synthesized with a hydrothermal method, achieved a rather good crystallinity, with a SBET area of 36 m2/g. Tauc plots derived from its diffuse reflectance spectrum provided a bandgap of 2.25 eV, thus agreeing well with literature.
It was verified that this semiconductor is clearly more active than CdS, as shown in Figure 10A. Using monochromatic light allowed verifying its spectral response as well; this is shown in Figure 10B together with the absorption spectrum of SnS2. One can see again that this material is active in photocatalysis in all the wavelength range in which it absorbs light. Even more interesting is its high resistance to photocorrosion, being much higher than that of CdS as evidenced in Figure 10C; this may be related to the higher cation charge and the octahedral coordination of SnS2, which may lead to a higher Madelung constant and consequently to a higher cohesion energy.

2.3.2. Two-Photon Processes Using V-Substituted SnS2

As in the case of V-substituted In2S3, DFT calculations indicated that V-substituted SnS2 could lead to two-photon processes. Therefore, the synthesis of such material was undertaken with success [156]. A summary of the obtained results is presented below.
EPR spectroscopy verified that over 90% of vanadium was in the V4+ state, as was assumed in the DFT calculations. The spectral response for the photocatalytic degradation of HCOOH dissolved in water using a suspension of V-free and V-substituted SnS2 is presented in Figure 11, showing again that this response is extended to longer wavelengths, as expected if V introduces an intermediate band in the gap. In this case, however, photoluminescence tests cannot prove an upconversion processes, because SnS2 is an indirect bandgap semiconductor; the recombination of photoproduced holes and electrons requires phonon participation, therefore the photoluminescence intensity at ambient temperature will be much smaller than for the In2S3 case.
It could be thus that, while DFT calculations predict that an in-gap band due to the inclusion of V would not overlap the conduction or valence bands [156], an overlap might exist after all, so that finally only a bandgap reduction might occur. On the other hand, the spectral response shows not much smaller photocatalytic activity at wavelengths lower than the intrinsic bandgap of V-free SnS2; this suggests that the much lower mobility expected for V-centered sites would not play a significant role, implying that a 2-photon process does occur in this V-containing SnS2.

2.3.3. Photoelectrochemical Generation of O2 by a SnS2 Electrode including a Laccase

That study was carried out by our group as well [157]. A FTO electrode was again used, covered this time by hydrothermally prepared SnS2 (with SBET area similar to that in [155]); as in our similar study involving In2S3, the same laccase enzyme was covalently linked to it. In this case the electrical contact was improved by including on top of the SnS2 nanoparticles ITO nanoparticles (ITOnp) at 1% level. Electrochemical tests were carried out, monitoring the dissolved O2 amounts with the same sensor. Ethanol, phosphate buffer at pH = 7.0, and acetate buffer at pH = 4.2 were the tested solvents.
The best results were achieved with the acetate buffer. Figure 12(Aa) shows that O2 appeared only when the electrode was illuminated; trace b shows the same electrode without the laccase enzyme bonded to it. In the absence of laccase, a higher photo current was detected; this might be due to a stronger SnS2 photocorrosion, due to its inability to transfer to the solution the photogenerated holes when the laccase co-catalyst was absent. With the laccase present one can expect that SnS2 will be more resistant to photocorrosion than In2S3. Besides, detecting O2 could be achieved in high yields with applied voltages as low as 0.4 V vs. SHE (see Figure 12B), implying a high decrease in the overpotential necessary to generate O2 under illumination; the faradaic efficiency could then reach levels as high as 75%, implying that with these smaller applied potentials the SnS2 photocorrosion is much decreased. This can be compared with another work showing the photoelectrochemical oxidation of water using as well SnS2, but now with a Pt co-catalyst to aid the same reaction [158].

2.4. In2S3 and SnS2 Band Alignment with O2 and H2 Standard Potentials

It now must be said whether the conduction bands of In2S3 and SnS2 lie above the H2 standard potential, so that H2 can be generated, and whether their valence bands lie below the O2 standard potential, so that O2 may be evolved. Here we have the help of [159], which leads to Figure 13:

3. Conclusions

This work has shown that very different sulfides can work as photocatalysts, in some cases combined with other phases, for a number of different reactions like dye degradation, reduction of water or CO2, herbicide removal, disinfection, or selective organic transformations. One main drawback is the possibility of photocorrosion; this is minimized if the photoreactions involved are reductive ones, or if the holes photogenerated in the system are kept in some oxide phase, e.g., in several Z-scheme combinations [31,46a),53,80]. Thus, the possibility of utilizing sulfide photocatalysts, which can absorb extended ranges of visible light (even near-infrared light in a few cases), is a very interesting alternative.

Funding

This work was made with help of the Programme FotoArt-CM of the Comunidad de Madrid (ref. S2018/NMT-4367).

Acknowledgments

Thanks are given to CSIC for the use of its parallel computer trueno in the hybrid DFT calculations.

Conflicts of Interest

The author declares no conflict of interest.

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Figure 1. Photocatalytic use of In2S3 for degrading aqueous HCOOH: (a) activity compared with CdS; (b) resistance to photocorrosion (from chemical analysis of S and metal ions gone into solution) compared with that of CdS; (c) spectral response of In2S3 in this process, evaluated through its first order rate constant, compared with the diffuse reflectance spectrum of the material (Adapted from Ref. [125]).
Figure 1. Photocatalytic use of In2S3 for degrading aqueous HCOOH: (a) activity compared with CdS; (b) resistance to photocorrosion (from chemical analysis of S and metal ions gone into solution) compared with that of CdS; (c) spectral response of In2S3 in this process, evaluated through its first order rate constant, compared with the diffuse reflectance spectrum of the material (Adapted from Ref. [125]).
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Figure 2. (A) Mechanism of the RhB degradation. Evolution of the absorption of light by the RhB dye (or by its intermediate degradation products) at different times under N2 (B) or O2 (C) flow (Adapted from Ref. [127]).
Figure 2. (A) Mechanism of the RhB degradation. Evolution of the absorption of light by the RhB dye (or by its intermediate degradation products) at different times under N2 (B) or O2 (C) flow (Adapted from Ref. [127]).
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Figure 3. PCA analysis results: evolution of the RhB dye and products RhEt2 and Rh110 irradiated in presence of In2S3 with light unfiltered or using band-pass filters of wavelength λ. Results scaled relative to the dye absorption at t = 0 (Adapted from ref. [127]).
Figure 3. PCA analysis results: evolution of the RhB dye and products RhEt2 and Rh110 irradiated in presence of In2S3 with light unfiltered or using band-pass filters of wavelength λ. Results scaled relative to the dye absorption at t = 0 (Adapted from ref. [127]).
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Figure 4. Spectral response of the initial RhB dye degradation (Adapted from Ref. [127]).
Figure 4. Spectral response of the initial RhB dye degradation (Adapted from Ref. [127]).
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Figure 5. (a) Spectral response of aqueous formic acid degradation of V-containing In2S3, compared with that of V-free In2S3 and with their respective absorption spectra. (b) PL tests using wavelengths above and below the bandgap energy; only V-containing In2S3 can excite PL at near 600 nm, while In2S3 cannot. (c) PL range in which emission at λ = 600 nm is excited only by V-containing In2S3. (d) Linear relationship between excitation intensity and resulting photoluminescence, which is in agreement with (e) (Adapted from Ref. [132]).
Figure 5. (a) Spectral response of aqueous formic acid degradation of V-containing In2S3, compared with that of V-free In2S3 and with their respective absorption spectra. (b) PL tests using wavelengths above and below the bandgap energy; only V-containing In2S3 can excite PL at near 600 nm, while In2S3 cannot. (c) PL range in which emission at λ = 600 nm is excited only by V-containing In2S3. (d) Linear relationship between excitation intensity and resulting photoluminescence, which is in agreement with (e) (Adapted from Ref. [132]).
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Figure 6. Increase in the fluorescence of hydroxy−terephtalic acid, detected at 425 nm, observed by irradiation at λ = 700 nm not in the case of (a), i.e., In2S3, while V−containing In2S3 (b) does show it (Adapted from Ref. [132]).
Figure 6. Increase in the fluorescence of hydroxy−terephtalic acid, detected at 425 nm, observed by irradiation at λ = 700 nm not in the case of (a), i.e., In2S3, while V−containing In2S3 (b) does show it (Adapted from Ref. [132]).
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Figure 7. (a) H2 production by light when both In2S3 and hydrogenase are present. (b) H2 production by irradiated In2S3-hydrogenase for several incubation times (striped columns) and % of H2 production by this same system, compared with the H2 formed after equal incubation times, in presence of MV+ and hydrogenase when no light nor In2S3 are present (black columns) (Adapted from Ref. [133]).
Figure 7. (a) H2 production by light when both In2S3 and hydrogenase are present. (b) H2 production by irradiated In2S3-hydrogenase for several incubation times (striped columns) and % of H2 production by this same system, compared with the H2 formed after equal incubation times, in presence of MV+ and hydrogenase when no light nor In2S3 are present (black columns) (Adapted from Ref. [133]).
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Figure 8. (A) Cyclic voltammograms of electrodes in dark (a) and under irradiation (b); the O2 sensor signal is shown in the inset in both cases. (B) Chronoamperograms of electrodes without (left) and with laccase (right) irradiated during ca. 2.3 min, the O2 sensor signal being also shown (Adapted from Ref. [135]).
Figure 8. (A) Cyclic voltammograms of electrodes in dark (a) and under irradiation (b); the O2 sensor signal is shown in the inset in both cases. (B) Chronoamperograms of electrodes without (left) and with laccase (right) irradiated during ca. 2.3 min, the O2 sensor signal being also shown (Adapted from Ref. [135]).
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Figure 9. (A) Results of a hybrid DFT calculation showing the indirect character of the ZnIn2S4 bandgap. (B) spectral response of ZnIn2S4 in photocatalytic degradation of aqueous HCOOH, compared with its diffuse reflectance absorption spectrum. (C) Time dependence of ZnIn2S4 photocorrosion during HCOOH photocatalysis: amount of each element gone into solution, as verified with chemical analysis (Adapted from Ref. [127]).
Figure 9. (A) Results of a hybrid DFT calculation showing the indirect character of the ZnIn2S4 bandgap. (B) spectral response of ZnIn2S4 in photocatalytic degradation of aqueous HCOOH, compared with its diffuse reflectance absorption spectrum. (C) Time dependence of ZnIn2S4 photocorrosion during HCOOH photocatalysis: amount of each element gone into solution, as verified with chemical analysis (Adapted from Ref. [127]).
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Figure 10. (A) Comparison of the aqueous HCOOH photodegradation activity between SnS2 and CdS; (B) SnS2 spectral response in the same process; (C) comparing the photocorrosion rate, evaluated from the amount of the sulfide components appearing in the solution, between CdS and SnS2 (Adapted from Ref. [155]).
Figure 10. (A) Comparison of the aqueous HCOOH photodegradation activity between SnS2 and CdS; (B) SnS2 spectral response in the same process; (C) comparing the photocorrosion rate, evaluated from the amount of the sulfide components appearing in the solution, between CdS and SnS2 (Adapted from Ref. [155]).
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Figure 11. Shift to longer wavelengths of the SnS2 photocatalytic response when part of the Sn cations are substituted by V cations (Reproduced from Ref. [156]).
Figure 11. Shift to longer wavelengths of the SnS2 photocatalytic response when part of the Sn cations are substituted by V cations (Reproduced from Ref. [156]).
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Figure 12. (A) Photoelectrochemical data obtained with FTO/SnS2/ITOnp electrodes, polarized at 1V vs. SHE: (a) with laccase enzyme; (b) without it. The inset presents the simultaneous O2 generation in this experiment. (B) O2 sensor signal for FTO/SnS2/laccase/ ITOnp electrodes when polarized at (a) 1 V, (b) 0.8 V, (c) 0.6 V, and (d) 0.4 V vs. SHE. Alternating dark/light periods were used in all cases (Adapted from Ref. [157]).
Figure 12. (A) Photoelectrochemical data obtained with FTO/SnS2/ITOnp electrodes, polarized at 1V vs. SHE: (a) with laccase enzyme; (b) without it. The inset presents the simultaneous O2 generation in this experiment. (B) O2 sensor signal for FTO/SnS2/laccase/ ITOnp electrodes when polarized at (a) 1 V, (b) 0.8 V, (c) 0.6 V, and (d) 0.4 V vs. SHE. Alternating dark/light periods were used in all cases (Adapted from Ref. [157]).
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Figure 13. Band alignments respect to the O2 and H2 standard potentials.
Figure 13. Band alignments respect to the O2 and H2 standard potentials.
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Conesa, J.C. Sulfide-Based Photocatalysts Using Visible Light, with Special Focus on In2S3, SnS2 and ZnIn2S4. Catalysts 2022, 12, 40. https://doi.org/10.3390/catal12010040

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Conesa JC. Sulfide-Based Photocatalysts Using Visible Light, with Special Focus on In2S3, SnS2 and ZnIn2S4. Catalysts. 2022; 12(1):40. https://doi.org/10.3390/catal12010040

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Conesa, José C. 2022. "Sulfide-Based Photocatalysts Using Visible Light, with Special Focus on In2S3, SnS2 and ZnIn2S4" Catalysts 12, no. 1: 40. https://doi.org/10.3390/catal12010040

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