3.1. The Mineralogical Identification of Fe Oxides and Ti Oxides
Some mineral particles with microscale separated from the sandstone in Lang Mountain (
Figure 1c) showed that the reddish substance was just like a coated material, while the interior particle with light color had more light penetration. Under the microscope with reflected light (
Figure 2a,b), some dim grains with hundreds of micrometers were encircled by numerous particles, which presented a bright white color and dozens of micrometers or smaller. The central particles were further identified as silicate minerals in transmitted light (
Figure 2c,d). This indicated that some metallic minerals, acting as cement, were almost uniformly distributed around some nonmetallic minerals. Elementary composition results determined by XRF (
Table S2) showed that the red beds, compared to local siltstone in grey, were enriched in Fe and Ti by 2.6 and 2.2 times on average, respectively. The abundance of Fe in red beds has always been reported to be high, but the enrichment of Ti has never been reported before. Moreover, EPMA was used to obtain elementary mapping of Fe and Ti on a silicate particle, which was coated by a thin (several microns) ant dense metallic shell (
Figure 3a,b). Notably, the distribution of Fe and Ti fell exactly into the region containing metallic minerals, with some regions enriched with both Fe and Ti, as shown in
Figure 3c,d.
From a more microscopic perspective using SEM, some crystal granules that were enriched in Fe were found to display rhombohedral form, as shown in
Figure 4a,b. According to the element composition and crystal morphology, these particles distributed among silicate minerals were probably hematite (α-Fe
2O
3). Another mineral aggregation consisted of several tetragonal crystals and contained considerable Ti (
Figure 4c), which could be assigned as Ti oxides. According to their crystal shape, they were probably anatase or rutile, whose chemical formula and crystallographic system are the same. The silicate minerals, by contrast, were mainly made of abundant Si, Al, Mg, and Na (
Figure 4d).
Micro-XRD, micro-Raman spectra and HRTEM were used to precisely identify the metallic oxide minerals in this study. The XRD pattern of one rock thin section sample is shown in
Figure 5a. Though the spot size (~100 μm) of X-ray in XRD measurement was larger than the metallic oxide particles, the signal from Fe oxide minerals was collected owing to its relatively high concentration (compared with Ti oxide). Quartz was identified as the main component with the three strongest peaks at 20.8°, 26.6°, and 50.1°, corresponding to (100), (011), and (112) planes (JCPDS No. 89-8935), respectively. The secondary mineral phase was designated to hematite (α-Fe
2O
3), whose three strongest peaks at 33.1°, 35.6° and 54.1° were assigned to (104), (110), and (116) lattice planes (JCPDS No. 89-0599), respectively.
Figure 5b illustrates the Raman spectra of hematite from different sampling sites as well as the standard pattern from the RRUFF database. The hematite in our case contained all the above Raman modes, whose active Raman modes of trigonal hematite include seven optical vibration modes (2A
1g + 5E
g), located at about 225, 498 cm
−1 and 247, 293, 299, 412, 613 cm
−1, respectively [
24]. Furthermore, high-resolution TEM image (
Figure 5c,d) showed the interplanar distances of (012) and (01−2) planes of α-Fe
2O
3. The fast Fourier transform pattern (
Figure 5d) also supported this conclusion with the [400] zone axis and angle of 86°, which was identical to the theoretical value. Given the above, the occurrence of α-Fe
2O
3 was confirmed in samples collected from the red beds of Danxia, as reported in previous studies [
13,
15]. Surprisingly, the existence of TiO
2 was found by micro-Raman as the mineral phase of anatase in samples collected from different sedimentary layers of all sampling sites (
Figure 5e). The optical phonons at Brillouin zone of anatase conform to the following irreducible representation: Γ = A
1g + A
1u + 2B
1g + B
2u + 3E
g + 2E
u, where A
1g (513 cm
−1), B
1g (399, 519 cm
−1) and E
g (144, 197, 639 cm
−1) are Raman-active [
25]. The above modes were all included in our measurement, providing solid evidence for the universality of anatase-phase TiO
2 in the red beds of Danxia. To the best of our knowledge, this is the first time the anatase-phase TiO
2 in Danxia has been discovered, as it is a vital but easily overlooked mineral. Notably, it was hard to find rutile or brookite—the other two titanium oxides in the samples—while some other Fe oxides, such as goethite, maghemite, and ilmenite, were all nanominerals and therefore identified by HRTEM (
Figures S2–S4).
According to the EPMA mapping results (
Figure 3), the elements Fe and Ti could coexist in some regions, which imply that anatase and hematite might be close in spatial scale. In some samples collected from different sites, it was found that anatase and hematite could be very close in the spatial distribution (
Figure 6). The micro-Raman focused on the area in microscale (several microns), whose peaks could be assigned to anatase and hematite as well as quartz (
Figure 6a). Furthermore, EDX in TEM confirmed that two small individual nanoparticles were enriched in Fe and Ti, respectively, and their particle sizes were just below 150 nm (
Figure 6b and
Figure S5). Specifically, the clear resolved d-spacings of 0.270 and 0.252 nm in the region denoted as “c” corresponded to the lattice fringes of (104) and (110) facets in α-Fe
2O
3 (
Figure 6c), and two planes highlighted spots in the corresponding simulated fast Fourier transform pattern (inset of
Figure 6c) that were well indexed to the [−441] zone axis. The high-resolution TEM image and selected area electron diffraction pattern indicated that the particle marked with a green color in
Figure 6b was anatase, whose interplanar spacings were 0.168 nm and 0.482 nm, corresponding to (21−1) and (002) facets with the [−240] zone axis, respectively (
Figure 6d).
3.2. Simulated Photoelectrochemical Experiments of Fe Oxides and Ti Oxides
Anatase-phase TiO
2 and hematite-phase Fe
2O
3 are well-known semiconducting materials, thus they can harvest and convert solar light into electric energy and chemical energy. In terms of this study, the first discovery of anatase and its concomitance with hematite in Danxia red beds begs the question of whether these widespread minerals in sun exposure have some potential effects on the local environment. To investigate this, some synthetic Fe
2O
3, TiO
2. and coupled Fe
2O
3–TiO
2 samples were used to measure their photoelectrochemical properties and photocatalytic performance. XRD (
Figure 7a) and micro-Raman spectra methods (
Figure 7b) confirmed that pure hematite-phase Fe
2O
3 and anatase-phase TiO
2 electrodes were successfully synthesized. In particular, the coupled Fe
2O
3–TiO
2 sample contained all the spectroscopic features of each component. The diffraction peaks of TiO
2 were wider than Fe
2O
3, implying the smaller crystalline size of the former. Based on Scherrer’s equation, the crystalline size of TiO
2 was estimated as approximately 20 nm, according to the full width at half maximum (FWHM) value of peak assigned as (011) plane. Under the scanning electron microscope, the as-prepared Fe
2O
3 exhibited as rod-like, which was at most 1 μm in length and hundreds of nanometers in diameter (
Figure 7c); the plate-like TiO
2 with a smooth surface showed crisscross fissuring, probably due to the considerable contraction when cooling (
Figure 7d). While depositing Fe
2O
3 on the as-prepared TiO
2, rod-like Fe
2O
3 particles assembled on the surface of TiO
2 and filled in the fissuring between the plate-like TiO
2 (
Figure 7e). The coupled Fe
2O
3–TiO
2 sample in this case was just like those natural samples found in the Danxia red beds; the close contact between the two components made their interaction possible. EDX elementary mappings for the synthesized Fe
2O
3–TiO
2 sample are shown in
Figure S6. The distribution of Fe was found to overlap with that of Ti, inferring the close contact of hematite and anatase.
The photoresponsive properties and optical band gap values (
Eg) of the as-prepared oxides were determined by UV-vis DRS and Tauc’s plot [
26]. The direct transition of photogenerated carriers between the valence band and conduction band is described by the Tauc model:
where
K is a proportionality constant,
hv is the incident photon energy, and A is the absorbance measured by UV-vis DRS. The Tauc plot is presented as the change of (A
hv)
2 versus
hv; therefore, it is used to determine the direct optical
Eg through linear extrapolation. The results, as well as actual photos of synthetic electrodes, are shown in
Figure 8. The pure TiO
2 was UV-light active and its band gap was estimated as 3.05 eV (
Figure 8a,b), resulting in its off-white color due to limited absorption in solar light (
Figure 8d). In contrast, Fe
2O
3 exhibited a reddish color and strong absorption below 600 nm (
Figure 8a,d), and its band gap value was approximately 2.01 eV (
Figure 8c). The coupled Fe
2O
3–TiO
2 sample, interestingly, presented as orange and an almost similar absorption feature as Fe
2O
3 (
Figure 8d). This indicated that the light absorption property of Fe
2O
3–TiO
2 coupled sample must have been mainly determined by Fe
2O
3; in other words, the incorporation of moderate TiO
2 would not considerably change the photoresponsive property of Fe
2O
3. The
Eg of semiconducting minerals determines their longest absorbed wavelength of light, thus indicating the utilization efficiency of sunlight. The smaller the
Eg of the semiconducting minerals, the more flux of light would be absorbed by them, which helps improve the production of oxidizing photogenerated holes and reductive photogenerated electrons. As such, naturally occurring hematite and anatase can evoke redox reactions under solar light.
The ability to convert light into electricity was verified by photoelectrochemical measurements. The current-voltage characteristics are shown in
Figure 9a. Compared with the smaller yield of current in the dark, strong dependences of the photocurrent density on light were observed, indicating that light (simulated sunlight) indeed facilitated the formation of extra electrons from semiconducting hematite and anatase. In addition, the three oxide materials showed different performances. The hematite-phase Fe
2O
3, the foremost concern in this work due to its dominant concentration, showed the worst photoelectric conversion capacity compared with its two counterparts. Notably, when Fe
2O
3 was coupled with TiO
2, a remarkable enhancement of photocurrent density was observed, which must be ascribed to the assistance of anatase-phase TiO
2 with brilliant photoelectric conversion capacity. Furthermore, when the bias was fixed at 0.8 V (vs. Ag/AgCl), the current density was obtained under alternant dark and simulated sunlight, as shown in
Figure 9b. The dark currents in all cases were negligible, while enhanced, sensitive, and stable current signals were captured when photons were incident. In particular, coupled Fe
2O
3–TiO
2 produced a stable photocurrent density of about 42 μA/cm
2 under this condition, preponderating 23 times over pure Fe
2O
3. The highest photocurrent density was assigned to pure TiO
2 (inset), whose photocurrent density reached almost 300 μA/cm
2. With its narrower absorption range of solar light, the better photoelectrochemical performance of TiO
2 than Fe
2O
3 could likely be attributed to its nanoscale particles, which decreased migration distance and the probability of recombination of photogenerated carriers [
27]. Based on these results, it is suggested that even with a low concentration of TiO
2 in red beds, their extraordinary photoelectric conversion capacity should make a difference. More importantly, once TiO
2 is involved, the massive but inferior Fe
2O
3 in red beds could significantly improve their photoelectric conversion performance.
Methyl orange (MO) dye is a model compound to measure the photocatalytic activity of photocatalyst, which was used in this case to compare the photocatalysis-related redox reaction rate of the three as-prepared oxides. The results are presented in
Figure 10. In simulated solar light, MO molecules were under slight photolysis with a 19% removal rate after 12 h irradiation time. When a photocatalyst was added, as expected, the photodegradation efficiencies were enhanced at different levels. The change of absorption spectra of MO in the presence of coupled Fe
2O
3–TiO
2 is illustrated in
Figure S7 (the spectra in other photocatalyst cases were similar and are not shown). The absorption peak at λ = 476 nm declined gradually and presented a slight blue shift with increasing irradiation time, suggesting the progressive decolorization of the MO during the reaction. Specifically, only 27% of MO molecules were removed in the case of Fe
2O
3, while a 48% removal rate was reached for coupled Fe
2O
3–TiO
2 after 12 h. The pure TiO
2, surprisingly, degraded MO by 75% within the same time (inset in
Figure 10a). The photocatalytic performance of the three materials above were extremely similar to their photoelectrochemical measurements (
Figure 9), indicating that passivated Fe
2O
3 could become active when coupled with superior TiO
2. Notably, in
Figure 10a, the drastic decrease in MO concentration in the first hour could most likely be attributed to the strong physical adsorption, followed by the slow photocatalytic degradation for the rest of the reaction time. Thus, the removal rate by 20% of TiO
2 in the first hour might be due to its nanoscale particle size, huge specific surface area, and strong absorption capability.
Figure 10b demonstrates the photocatalytic degradation of MO following the pseudo-first-order law, with an apparent rate constant of k = 0.019 h
−1, 0.046 h
−1, and 0.103 h
−1 for Fe
2O
3, coupled Fe
2O
3–TiO
2, and TiO
2, respectively. Based on this, the coupled Fe
2O
3–TiO
2 exhibited a removal efficiency of MO 2.42 times that of inactive Fe
2O
3, which highlighted the assistance effect of TiO
2.
3.3. The Potential Environmental Effect of Fe Oxide and Ti Oxide Minerals on the Danxia Landform
The Danxia landform in southern China is a unique environment, represented as erosional cliffs in which countless minerals assemble and lithify into rock. Semiconducting minerals, represented as hematite and anatase, once formed and exposed in sunlight should be excited and generate reductive electrons and oxidizing holes. The photogenerated electrons from natural minerals have been found to synthesize prebiotic organics and facilitate the growth of microorganisms in soil [
28,
29]. Furthermore, the photogenerated holes from natural minerals are able to decompose organics and kill bacteria in wastewater [
30,
31]. In this work, anatase was found for the first time as a mineral in the red beds of Danxia. In addition, with the assistance of anatase, the major species hematite was found to exhibit significant photoelectrical performance and highly efficient photocatalytic activities. According to our simulated experiment, these two minerals could utilize solar light to photocatalytically decompose and destroy organics. In fact, several studies have reported that there are different levels of desertification in the red beds of Danxia, even though this landform is located in the humid region of southern China [
32,
33]. Many factors are thought to be responsible for the continuous desertification of Danxia, such as lithological features, natural impacts, and human activities, resulting in sparser vegetation and decreased microbial community year on year. Based on this study, another mechanism is proposed with the photocatalytic effects of hematite and anatase on an increasing number of barren red beds. A similar mechanism has been put forward in abiotic Mars and oligotrophic desert, where titanic oxides (anatase and rutile) and iron oxide (mainly hematite) are assumed to produce considerable reactive oxygen species (ROSs) with extreme solar irradiation, and these strong oxidizing agents have the ability to decompose organics and exacerbate desertification [
34,
35]. On a positive note, the photocatalytic effects of natural semiconducting minerals are suggested as possibly stimulating metabolism and the growth of microorganisms in the soil [
29,
36], thus promoting internal biodiversity and soil fertility. Hematite and anatase in this case could also exert a similar effect on the Danxia landform. In summary, semiconducting minerals in the red beds of Danxia might simultaneously play reconstructive and constructive roles in the environment. Though such effects of every mineral particle remain minimal, the considerable distribution of all semiconducting minerals and the long history of solar irradiation could lead to a great accumulation and make a significant difference. This study provides a completely new perspective to understand the minerals in the Danxia landform. Moreover, it should be noted that semiconducting minerals, such as hematite and anatase, are widespread on the surface of Earth, thus the photocatalytic performance and the environmental effects of semiconducting minerals in the critical zone of Earth should be highly regarded.