The Fine Characterization and Potential Photocatalytic Effect of Semiconducting Metal Minerals in Danxia Landforms

Article The Fine Characterization and Potential Photocatalytic Effect of Semiconducting Metal Minerals in Danxia Landforms

Yuxiong Xiao †, Yanzhang Li †, Hongrui Ding, Yan Li and Anhuai Lu *

The Key Laboratory of Orogenic Belts and Crustal Evolution, Beijing Key Laboratory of Mineral Environmental Function, School of Earth and Space Sciences, Peking University, Beijing 100871, China; [email protected] (Y.X.); [email protected] (Y.L.); [email protected] (H.D.); [email protected] (Y.L.) * Correspondence: [email protected]; Tel./Fax: +86-10-6275-3555 † These authors contributed equally to this work.

Received: 19 October 2018; Accepted: 26 November 2018; Published: 29 November 2018

Abstract: The Danxia landform is representative of the Cretaceous continental red sediment. The careful identification and potential environmental effects of minerals in Danxia red beds have yet to be clearly reported. In this work, reddish sandstone samples were collected from Lang Mountain Danxia landform in Xinning, Hunan province, China, and their mineral phases, element distribution, microstructure, and the spatial relationship of different minerals were investigated using polarizing optical microscope, environmental scanning electron microscopy, energy-dispersive X-ray analysis, electron probe microanalysis, micro-Raman spectra, micro- X-ray diffraction, X-ray fluorescence spectroscopy, and high-resolution transmission electron microscopy. The results revealed that iron oxide (mainly hematite) and titanium oxide (mainly anatase) were the dominant minerals in Danxia red layers. Microcrystalline hematite was suggested as being the coloring mineral. Anatase, reported here for the first time in Danxia red beds, constituted the content of titanium in the red layer (0.17–0.57%) and was present in a significantly higher amount than the adjacent limestone formation (0.13%). Over 95% of Fe/Ti oxides served as a cementation agent along the framework of coarse-grain minerals (quartz and feldspar). The hematite and anatase were visible-light-responsive semiconductors, with a band gap of 2.01 eV and 3.05 eV, respectively. Photoelectrochemical experiments were performed on synthetic hematite, anatase, and their coupled material. The inactive hematite displayed an enhanced 23-fold photocurrent at 0.8 V (vs. Ag/AgCl) when coupled with anatase. Furthermore, in a photodegradation experiment using methyl orange dye under simulated sunlight, the coupled material showed decolorizing efficiency 2.4 times that of hematite. The anatase, therefore, prominently improved the photocatalytic activities of hematite. It is proposed that these semiconducting minerals in red beds produce oxygen reactive species and have significant environmental effects, which is of great importance.

Keywords: Danxia landform; hematite; anatase; photocatalysis; environmental effect

1. Introduction The Danxia landform, covering approximately 8.6% of the total land mass of China, is a unique type of petrographic geomorphology [1–4]. With vast red-colored sandstones and conglomerates, which were mainly deposited in the Cretaceous period (145–65 million years ago), these spectacular red cliffs and erosional landforms joined the World Heritage List in 2010 due to their classic geomorphological and biogeographic features [5]. The uniqueness of the Danxia landform is attributed not only to its magniﬁcent reddish geomorphology but also to its great value in scientiﬁc research as an

Minerals 2018, 8, 554; doi:10.3390/min8120554 www.mdpi.com/journal/minerals Minerals 2018, 8, 554 2 of 16 important part of the critical zone on Earth’s surface [4,6,7]. These red stratified sediments, or red beds, could be used to deduce depositional environments, such as paleomagnetism [8], atmospheric oxygen content [9], paleoceanography and paleoclimate [10], etc. Furthermore, as a significant component of the surface ecosystem and environment on Earth, red beds maintain interactions among minerals, microorganism, water, light, organics, and other substances [7,8,11,12]. Previous researches on the Danxia landform have mainly focused on its developing mechanism, erosion rate, topography, environmental ecology, and petrostratigraphy using macroscopical, traditional, and qualitative methods, such as field study, remote sensing observation, taxonomy, and description [2–4,6,7]. Aside from these, the Danxia red beds cover such a large continental area that their environment effects are of great interest. For conducting intensive studies, minerals are powerful probes to further investigate about Danxia red beds. For example, through SEM microanalysis of the biotite occurring in the red sandstone rock of the northern Scotland Devonian, the mechanical disintegration effect of the alluvial plain during the process of redeposition was thought to turn biotite into hematite, with the latter reddening the rock [11]. A high-resolution transmission electron microscope was used on hematite collected from red beds in the Vispi Quarry section of central Italy, and the results suggested that the hematite with nanograins were authigenic in oxic or suboxic conditions [12]. Batches of adsorption experiments of Cu, Pb, Zn, Co, Ni, and Ag on goethite and hematite were conducted to clarify the metallogenic mechanism and the environmental effect of red beds [13]. Furthermore, the chemical solubility of hematite and goethite was studied via experiments and simulated calculations, which indicated the phase transformation from goethite to hematite during the diagenesis of red beds [14]. Microtechniques and simulated experiments applied to minerals can thus help shed light on the red beds as well as their effects on the surrounding environment in microscale and nanoscale. However, they have seldom been reported in the research of the Danxia landform so far. As with other natural red beds worldwide, oxidized iron (Fe3+) in the Danxia regions are thought to be the dominant coloring element. Furthermore, hematite (Fe2O3) and goethite (α-FeOOH) have been suggested as the main hosted minerals of Fe3+ in those red beds, which dye the rock a reddish color [13–15]. In particular, hematite has been thought to be the most common and stable mineral in red beds and is regarded as the terminally oxidation state of Fe-bearing minerals, e.g., green rust, ferrihydrite, goethite, lepidocrocite, and magnetite [16]. Furthermore, hematite is one of the most abundant minerals in soil, especially fertile red soil exposed in southern China and some regions of South America and the tropics [17]. One of the environmental effects of hematite, as well as other widely distributed minerals such as goethite, anatase, and rutile, derives from their brilliant semiconducting and photon-responsive properties by which these minerals can convert solar energy into chemical energy and nourish microorganisms [18,19]. Specifically, when the incident light energy is greater than the threshold value (i.e., band gap) between the valence band and the conduction band of these minerals, electron-hole pairs can be generated, and some redox reactions are triggered with the release of energy [20,21]. Such vital redox reactions, the so-call photocatalytic effect, which occurs among light, minerals, water, organics, and microbes, could play a significant role in the geochemical cycle of elements, materials, and energy in Earth’s critical zone [22,23]. Danxia red beds develop exposed cliff and outcrops under continuous solar light, which merits further investigation to find out whether the inside minerals could exert their photon-absorption property in sunlight and catalyze some reactions. In this work, a series of measurements, such as X-ray fluorescent (XRF), electron microprobe Analysis (EMPA), polarizing optical microscope (POM), environmental scanning electron microscopy (ESEM), energy-dispersive X-ray analysis (EDX), micro-Raman spectra, micro- X-ray diffraction (XRD), and high-resolution transmission electron microscopy (HRTEM), were used to characterize the composition of elements and the distribution and morphology of the mineral phases in the red bed samples collected from Lang Mountain in southern China. To shed light on the environmental effect of the semiconducting minerals found in red beds, some photoelectrochemical experiments were Minerals 2018, 8, 554 3 of 16 conducted on synthetic metal oxides. Subsequently, the potential impact of vast Danxia landforms on theMinerals special 2018 ecology, 8, x FOR andPEER environment REVIEW of southern China may be clarified from a microscopic perspective.3 of 16

2. Materials and Methods

2.1.2.1. Sample Preparation The collection of sandstone samples took place at at Lang Lang Mountain,Mountain, Hunan province, China (Figure(Figure1 1).). Seven Seven sampling sampling sites sites were were chosen chosen around around the the Lang Lang Mountain; Mountain these; these locations location ares are shown shown in Figurein Figure S1 S1 and and Table Table S1 S1 (in (in Supplementary Supplementary Materials). material).The The looseloose andand fragilefragile sandstonesandstone samplessamples were separated into two parts. One was glued with resin and then air air-dried,-dried, followed by cuttingcutting along the vertical directiondirection andand burnishing burnishing into into 30- 30µ-m-thickμm-thick thin thin sections sections for for the the in situin situ observation observation of POM of POM and SEMand SEM and theand measurementsthe measurements of EPMA, of EPMA, XRD, XRD, and Ramanand Raman spectrum. spectrum The. otherThe other part part was groundwas ground into ﬁneinto powderfine powder (particles (particle lesss thanless than 36 µ m)36 forμm) the for XRF the andXRF TEMand TEM measurements. measurements.

Figure 1.1. ((aa)) DistributionDistribution of of Danxia Danxia landforms landforms in in China, China, modiﬁed modified from from Peng Peng et al. et [4 al.]. (b[4]) The. (b) fullThe view full ofview Lang of Lang Mountain Mountain with awith typical a typical Danxia Danxia landform. landform. (c) The (c) reddish The reddish sandstone sandstone in Lang in Lang Mountain; Mountain inset:; mineralinset: mineral particles particles (in microscale) (in microscale separated) separated fromthe from sandstone. the sandstone.

According to the analytic results ofof the field field samples, a series of simulation experiments focusing on the photoelectrochemical photoelectrochemical a activitiesctivities of of detected detected semiconductor semiconductor minerals minerals were were conducted conducted in the in lab. the lab.Fe2O Fe3 (hematite2O3 (hematite phase) phase) and and TiO TiO2 (anatase2 (anatase phase) phase) were were deposited deposited onto onto fluorine fluorine-doped-doped tin tin oxide conductive glasses (abbreviated as FTO, South China Xiang Xiang Science Science & & Technology Technology Company Company Limited, Limited, Shenzhen,Shenzhen, China) and manufactured into electrodes electrodes,, while the FTO glass was wash washeded with acetone, ethanol,ethanol, and deionized water (in that order) inin advance.advance. The preparation of the TiOTiO22 electrode was realized with the sol-gelsol-gel method. The mixture containing 10 10 mL ethano ethanol,l, 1.26 1.26 mL mL deionized deionized water water,, and 11 mLmL concentratedconcentrated nitric nitric acid acid was was added added into into the the other other mixture mixture with with 6 g 6 tetrabutyl g tetrabutyl titanate, titanate, 40 mL 40 ethanol,mL ethanol and, and 1 mL 1 acetylacetone.mL acetylacetone. After After one day,one day, the precursor the precursor in yellow in yellow was extractedwas extracted and spreadand spread onto ◦ theonto FTO the glassFTO glass evenly, evenly, followed followed by calcination by calcination in a furnace in a furnace in air in at air 400 at 400C for °C 24 for h 24 with h with a heating a heating rate ◦ ofrate 10 ofC/min. 10 °C/min The. The deposition deposition of Fe of2O Fe3 used2O3 used the hydrothermalthe hydrothermal method. method A Teflon-lined. A Teflon-lined stainless stainless steel autoclavesteel autoclave containing containing 0.15 0.15 mol/L mol/L FeCl FeCl3, 13 mol/L, 1 mol/L NaNO NaNO3,3 and, and a a piece piece of of FTO FTO glassglass waswas sealedsealed and ◦ ◦ then heated at 94 °CC for 4 h with a heating rate ofof 1515 °C/minC/min.. After After natural natural cooling, cooling, the FTO glass was ◦ taken out and washed with deionized water, andand itit waswas finallyfinally calcinedcalcined inin thethe furnacefurnace inin airair atat 550 °CC ◦ for 2 h with a heating rate ofof 1010 °C/minC/min.. The The Fe Fe22OO3 3electrodeelectrode was was well well prepared prepared when when it it cooled cooled down. down. A coupled FeFe22O3––TiOTiO2 materialmaterial was was synthesized synthesized according according to to the the abovementioned abovementioned methods in which Fe2O33 waswas covered covered onto onto the the as as-prepared-prepared TiO TiO22 directlydirectly..

2.2. Characteristic Methods The polished thin sections of rock samples were first observed by a polarizing optical microscope (LV100POL, Nikon, Tokyo, Japan). In particular, metallic minerals were relucent under reflected Minerals 2018, 8, 554 4 of 16

2.2. Characteristic Methods The polished thin sections of rock samples were first observed by a polarizing optical microscope (LV100POL, Nikon, Tokyo, Japan). In particular, metallic minerals were relucent under reflected light, whose positions were located accurately for follow-up tests. EMPA was carried out on an electron microprobe analyzer (EPMA-1720, Shimadzu, Kyoto, Japan) to measure the composition and concentration of elements. When the selected regions were fixed (backscattered mode), mapping patterns were obtained under 15 kV operating voltage and 10 nA operating current, with a spot size of 1 µm. Rock thin sections and prepared electrodes were sprayed with a film of thin gold due to its poor conductivity before being observed by a SEM (Quanta 650FEG, FEI, Hillsboro, OR, USA) equipped with an EDX detector. The compositions of major elements in powder samples were investigated using an XRF spectrometer (Advant’XP+, Thermo ARL, Écublens, Switzerland) on the basis of the weight fractions of the equivalent oxides. Prior to this, the powder was made into glass disks by melting at 1100 ◦C for 4 min in lithium tetraborate and lithium metaborate. The loss on ignition was the calculated weight difference before and after calcination at 980 ◦C for 20 min. The phases of metallic minerals found in the polarizing optical microscope on the rock thin section were in situ identified by micro-XRD (Rapid IIR, Rigaku, Tokyo, Japan). The data were collected at 40 kV operating voltage and 250 nA operating current with 1◦/s step size. Micro-Raman spectra were recorded by a Renishaw inVia Reflex Raman microscope equipped with a 532 nm laser. The beam was through a 65 µm slit and a 50× objective with 50 mW power. Each spectrum was captured at 30 s and averaged by 10 successive scans with ±1 cm−1 resolution. Before TEM measurement, the solution containing powder samples was dropped onto a holey carbon film supported by a Cu grid. After air-drying, the prepared sample was loaded into the holder of TEM (TECNAIF 20, FEI, Hillsboro, OR, USA) equipped with an EDX detector at 200 kV. Digital micrograph version 3.6.5 (Gantan Ltd., Pleasanton, CA, USA) was applied for the image processing. The ultraviolet-visible diffuse reflection spectrum (UV-vis DRS) of synthetic samples in 300–1000 nm was detected on a spectrometer (UV-3600 Plus, Shimadzu, Japan) equipped with a diffuse integrating sphere attachment. BaSO4 was used as a reference. The slit width of the incident light was 2.00 nm.

2.3. Photoelectrochemical and Photocatalytic Experiments Photoelectrochemical measurements were carried out in a quartz glass reactor with a three-electrode configuration using CHI 660C electrochemical apparatus (Chenhua, Shanghai, China). The oxide electrode covering on the FTO glass was set as working electrode, a platinum sheet (1 × 1 cm) was used as the counter electrode, and the reference electrode was Ag/AgCl (3 mol/L KCl). The electrolyte composition was 0.5 mol/L Na2SO4 and 0.5 mol/L ethanol. An externally simulated solar light came from a 500 W high-voltage light-emitting diode (LED) lamp (λ > 400 nm, 130 mW/cm2), straight irradiating the backface of the working electrode (the side without oxide covering). The linear sweep voltammetry (LSV) method was used from 0 V to 1.6 V (vs. Ag/AgCl) to separately record the current under light-on and light-off conditions. A potential of 0.8 V (vs. Ag/AgCl) was applied in the photocurrent response measurement. The on and off conditions of light were controlled by the shielding separator in the same time period (60 s). The photocatalytic experiment was performed to the degradation of methyl orange (MO) dye. The oxide electrode was placed in the quartz glass reactor, which contained 90 mL MO solution with 3.2 mg/L initial concentration and pH value of 4.4. The light source configuration was the same as the above photoelectrochemical measurements. After a certain period of irradiation (1 h), 1.0 mL suspension was extracted to measure the adsorption spectra of MO at 476 nm by UV-vis spectrophotometer (Thermo Fisher Scientific, Waltham, MA, USA). All the reagents used for the experiments (such as acetone, ethanol, nitric acid, tetrabutyl titanate, acetylacetone, methyl orange, FeCl3, NaNO3, and Na2SO4) were analytically pure (>99.99%, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China). The water was distilled deionized water (18 MΩ·cm). Minerals 2018, 8, 554 5 of 16 Minerals 2018, 8, x FOR PEER REVIEW 5 of 16

3. Results and Discussion

3.1.3.1. The Mineralogical Identification Identification of Fe OxidesOxides and Ti OxidesOxides Some mineral particlesparticles with with microscale microscale separated separated from from the the sandstone sandstone in in Lang Lang Mountain Mountain (Figure (Figure1c) showed1c) showed that that the the reddish reddish substance substance was was just likejust alike coated a coated material, material while, while the interior the interior particle particle with with light colorlight color hadmore had more light light penetration. penetration Under. Under the microscope the microscope with with reflected reflected light light (Figure (Fig2a,b),ure 2 somea,b), some dim grainsdim grain withs with hundreds hundreds of micrometers of micromete werers encircledwere encircled by numerous by numerous particles, particles which, which presented presented a bright a whitebright colorwhite andcolor dozens and dozens of micrometers of micromete orrs smaller. or smaller. The The central central particles particles were were furtherfurther identifiedidentified as silicate minerals in transmitted light (Figure(Figure2 2c,c,d).d). ThisThis indicatedindicated thatthat somesome metallic metallic minerals, minerals, actingacting as cement, were almost uniformly distributed around some nonmetallic minerals. Elementary composition results results determined determined by by XRF XRF (Table (Table S2) showed S2) showed that the that red the beds, red compared beds, compared to local siltstone to local insiltstone grey, were in grey, enriched were in enriched Fe and Ti byin 2.6Fe and 2.2 Ti times by 2.6 on and average, 2.2 times respectively. on average The, abundance respectively. of Fe The in redabundance beds has of always Fe in red been beds reported has always to be high,been butreported the enrichment to be high of, but Ti hasthe enrichment never been reportedof Ti has before.never Moreover,been reported EPMA before. was usedMoreover, to obtain EPMA elementary was used mapping to obtain of Fe elementary and Ti on amapping silicate particle, of Fe and which Ti on was a coatedsilicate byparticle a thin, (severalwhich was microns) coated ant by densea thin metallic (several shell microns) (Figure ant3a,b). dense Notably, metallic the shell distribution (Figure of3a, Feb). andNotably, Ti fell the exactly distribution into the of region Fe and containing Ti fell exactly metallic into minerals, the region with containing some regions metallic enriched minerals with, with both Fesome and regions Ti, as shown enriched in Figurewith both3c,d. Fe and Ti, as shown in Figure 3c,d.

Figure 2.2. (a,b) RockRock thin section in opticaloptical microscope with reﬂectedreflected light.light. (c) PlanePlane polarizedpolarized light and ( (dd)) perpendicular perpendicular polarized polarized light light.. Panel Panelss (b– (db)– dshow) show the thesame same mineral mineral particle. particle. “Kfs” “Kfs” refers refers to K- tofeldspar. K-feldspar. Minerals 2018, 8, 554 6 of 16 Minerals 2018, 8, x FOR PEER REVIEW 6 of 16

FigureFigure 3.3.In In thethe rockrock thinthin section,section, aa silicatesilicate particleparticle surrounded by by metallic metallic shell shell structure structure enriched enriched withwith Fe Fe and and Ti.Ti. ImagesImages fromfrom ((aa)) opticaloptical microscope,microscope, ( b) backscattered mode mode of of electron electron microprobe microprobe analyzer,analyzer, ( c(c)) Fe Fe elementary elementary distribution distribution map,map, andand ((dd)) TiTi elementaryelementary distributiondistribution map.

FromFrom aa moremore microscopicmicroscopic perspectiveperspective using SEM, some crystal granules granules that that were were enriched enriched in in FeFe werewere foundfound toto displaydisplay rhombohedralrhombohedral form,form, as shown in FigureFigure 44a,b.a,b. According to the element compositioncomposition and and crystal crystal morphology, morphology, these these particles particles distributed distribut amonged among silicate silicate minerals minerals were probably were hematiteprobably (α hematite-Fe2O3). Another(α-Fe2O3). mineral Another aggregation mineral aggregation consisted of consisted several tetragonal of several crystals tetragonal and contained crystals considerableand contained Ti considerable (Figure4c), which Ti (Fig couldure 4c be), which assigned could as be Ti oxides.assigned According as Ti oxides. to theirAccording crystal to shape, their theycrystal were shape, probably they were anatase probably or rutile, anatase whose or chemical rutile, whose formula chemical and crystallographic formula and crystallographic system are the same.system The are silicate the same. minerals, The silicate by contrast, minerals, were by mainly contrast, made were of abundant mainly made Si, Al, of Mg, abundant and Na Si, (Figure Al, Mg4d)., and Micro-XRD,Na (Figure 4d). micro-Raman spectra and HRTEM were used to precisely identify the metallic oxide mineralsMicro in-XRD, this study. micro-Raman The XRD spectra pattern and HRTEM of one rock were thin used section to precisely sample identify is shown the metallic in Figure oxide5a. Thoughminerals the in this spot study. size (~100 The XRDµm) pattern of X-ray of one in XRD rock measurementthin section sample was largeris shown than in Figure the metallic 5a. Though oxide particles,the spot size the signal(~100 μ fromm) of Fe X oxide-ray in minerals XRD measurement was collected was owing larger to than its relatively the metallic high oxide concentration particles, (comparedthe signal from with Fe Ti oxide oxide). minerals Quartz was was collected identified owing as the to its main relatively component high concentration with the three (compared strongest peakswith Ti at oxide). 20.8◦, 26.6 Quartz◦, and was 50.1 identified◦, corresponding as the main to component (100), (011), with and the (112) three planes strongest (JCPDS peaks No. at 89-8935), 20.8°, 26.6°, and 50.1°, corresponding to (100), (011), and (112) planes (JCPDS No. 89-8935), respectively. respectively. The secondary mineral phase was designated to hematite (α-Fe2O3), whose three strongest peaksThe secondary at 33.1◦, 35.6 mineral◦ and 54.1phase◦ were was assigneddesignated to (104),to hematite (110), and (α-Fe (116)2O3) lattice, whose planes three (JCPDS strongest No. peaks 89-0599), at respectively.33.1°, 35.6° andFigure 54.15b° illustrates were assigned the Raman to (104), spectra (110) of, and hematite (116) from lattice different planes (JCPDSsampling N siteso. 89- as0599) well, asrespectively the standard. Figure pattern 5b illustrates from the RRUFFthe Raman database. spectra The of hematite hematite from in our different case contained sampling allsites the as above well Ramanas the standard modes, whosepattern activefrom the Raman RRUFF modes database. of trigonal The hematite hematite in our include case contained seven optical all the vibration above Raman modes, whose active Raman modes of− trigonal1 hematite include seven− 1 optical vibration modes (2A1g + 5Eg), located at about 225, 498 cm and 247, 293, 299, 412, 613 cm , respectively [24]. modes (2A1g + 5Eg), located at about 225, 498 cm−1 and 247, 293, 299, 412, 613 cm−1, respectively [24]. Furthermore, high-resolution TEM image (Figure5c,d) showed the interplanar distances of (012) and Furthermore, high-resolution TEM image (Figure 5c,d) showed the interplanar distances of (012) and (01−2) planes of α-Fe2O3. The fast Fourier transform pattern (Figure5d) also supported this conclusion (01−2) planes of α-Fe2O3. 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 with the [400] zone axis and angle of 86°, which was identical to the theoretical value. Given the above, the occurrence of α-Fe2O3 was confirmed in samples collected from the red beds of Danxia, above, the occurrence of α-Fe2O3 was confirmed in samples collected from the red beds of Danxia, as as reported in previous studies [13,15]. Surprisingly, the existence of TiO2 was found by micro-Raman reported in previous studies [13,15]. Surprisingly, the existence of TiO2 was found by micro-Raman as the mineral phase of anatase in samples collected from different sedimentary layers of all sampling as the mineral phase of anatase in samples collected from different sedimentary layers of all sampling Minerals 2018, 8, 554 7 of 16 Minerals 2018, 8, x FOR PEER REVIEW 7 of 16 sites (Figure 5 5e).e). TheThe opticaloptical phononsphonons atat Brillouin zonezone ofof anatase conform toto the following irreducible −1 −1 representation: ΓΓ == AA1g1g + A A1u1u ++ 2B 2B1g1g + +B B2u2u + +3E 3Eg +g +2E 2Eu, uwhere, where A A1g 1g(513(513 cm cm−1), B),1g B (399,1g (399, 519 519 cm cm−1) and) and Eg −1 (144,Eg (144, 197, 197, 639 639 cm cm−1) are) areRaman Raman-active-active [25]. [25 The]. The above above modes modes were were all all included included in in our our measurement, measurement, providing solid evidence for the universality of anatase anatase-phase-phase TiO 2 inin the the red red beds beds of of Danxia. Danxia. To To the best of of our our knowledge knowledge,, this this is is the the first first time time the the anatase anatase-phase-phase TiO TiO2 in2 inDanxia Danxia has has been been discovered, discovered, as itas is it a is vital a vital but buteasily easily overlooked overlooked mineral. mineral. Notably, Notably, it was it hard was hardto find to rutile find rutileor brookite or brookite—the—the other twoother titanium two titanium oxides oxides in the insamples the samples—while—while some someother otherFe oxid Fees, oxides, such suchas goethite, as goethite, maghemite maghemite,, and ilmenite,and ilmenite, were were all nanominerals all nanominerals and andtherefore therefore identified identified by HRTEM by HRTEM (Figure (Figures S2–S4). S2–S4).

Figure 4 4.. BackscatteredBackscattered electronelectron images BSE images in SEM in of SEM (a) silicateof (a) silicate and Fe and oxide Fe particles, oxide particles, (b) hematite (b) hematite particles particles(inset: EDX (inset: pattern), EDX pattern (c) silicate), (c and) silicate Ti oxide and particlesTi oxide (inset:particles EDX (inset: pattern), EDX andpattern), (d) EDX and pattern (d) EDX of patternsilicate mineral.of silicate mineral.

According to the EPMA mapping results (Figure 3 3),), thethe elementselements FeFe andand TiTi couldcould coexistcoexist inin somesome regions, which implimplyy that anatase and hematite might be close in in spatial spatial scale. scale. In some samples ccollectedollected from from different different sites, sites, it was it was found found that that anatase anatase and andhematite hematite could could be very be close very in close the spatial in the distributionspatial distribution (Figure (Figure6). The6 micro). The-Raman micro-Raman focused focused on the area on the in areamicroscale in microscale (several (several microns), microns), whose peakswhose could peaks be could assigned be assigned to anatase to anataseand hematite and hematite as well as quartz well as (Figure quartz (Figure6a). Furthermore,6a). Furthermore, EDX in TEMEDX inconfirmed TEM conﬁrmed that two that small two individual small individual nanoparticles nanoparticles were enriched were enriched in Fe and in Fe Ti and, respectively, Ti, respectively, and theirand their particle particle sizes sizes were were just just below below 150 150 nm nm (Figures (Figure6 b6b and and Figure S5). Specifically, S5). Speciﬁcally, the theclear clear resolved resolved d- spacingsd-spacings of of0.270 0.270 and and 0.252 0.252 nm nm in in the the region region denoted denoted as as “c” “c” corresponded corresponded to to the the lattice lattice fringes of (104) and (110) facets in α-Fe-Fe2O33 (Figure(Figure 66c),c), andand twotwo planesplanes highlightedhighlighted spotsspots inin thethe correspondingcorresponding simulated fast fast Fourier Fourier transform transform pattern pattern (inset (inset of ofFigure Figure 6c)6 c)that that were were well well indexed indexed to the to the[−441] [ − zone441] axis.zone The axis. high The-resolution high-resolution TEM image TEM and image selected and selected area electron area electron diffraction diffraction pattern indicated pattern indicated that the particlethat the marked particle with marked a green with color a green in Figure color 6b in Figurewas anatase,6b was whose anatase, interplanar whose interplanar spacings were spacings 0.168 nmwere and 0.168 0.482 nm nm and, corresponding 0.482 nm, corresponding to (21−1) and to (002) (21− 1)facets and with (002) the facets [−240] with zone the axis [−240], respectively zone axis, (Figurerespectively 6d). (Figure6d).

Minerals 2018, 8, 554 8 of 16 Minerals 2018, 8, x FOR PEER REVIEW 8 of 16

Figure 5 5.. ((aa)) XRD XRD pattern pattern of of the the selected selected sample sample as as well well as as the the standard quartz and hematite as reference.reference. ( b) The RamanRaman spectraspectra ofof hematitehematite collected collected from from all all sampling sampling sites. sites (.c ()c A) A hematite hematite particle particle in inTEM TEM image. image. (d) The (d) The high-resolution high-resolution TEM TEMimage image and simulated and simulated fast Fourier fast Fouriertransform transform pattern (inset) pattern of (inset)the hematite of the hematite particle. (particle.e) The Raman (e) The spectra Raman of spectra anatase of collectedanatase collected from all samplingfrom all sampling sites. The sites location. The locationand full and names full of names these samplingof these sampling sites are sites shown are in shown Figure in S1 Figure and Table S1 and S1, Table respectively. S1, respectively. Minerals 2018, 8, 554 9 of 16 Minerals 2018, 8, x FOR PEER REVIEW 9 of 16

Figure 6.6. (a) Raman spectra of samples in several different sampling sites sites.. The The dash dash lines lines in in red, red, blue blue,, and greengreen markmark thethe Raman Raman modes modes of of anatase, anatase, hematite, hematite and, and quartz, quartz, respectively. respectively. The The location location and and full namesfull names of these of these sampling sampling sites are sites shown are inshown Figure in S1 Figure and Table S1 and S1, respectively. Table S1, respectively (b) EDX elementary. (b) EDX mappingelementary of mapping Si, Fe, and of Ti Si, using Fe, and TEM. Ti (usingc) High-resolution TEM. (c) High TEM-resolution image capturedTEM image from captured the Fe-enriched from the regionFe-enriched in (b). region Inset: simulatedin (b). Inset: fast simulated Fourier transform fast Fourier pattern transform and its identiﬁcation.pattern and its (d identification.) High-resolution (d) TEMHigh- imageresolution captured TEM from image the captured Ti-enriched from region the Ti in-enriched (b). Inset: region selected in ( areab). Inset: electron selected diffraction area electron pattern anddiffraction its identiﬁcation. pattern and its identification.

3.2.3.2. Simulated Photo Photoelectrochemicalelectrochemical Experiments of Fe Oxides and Ti OxidesOxides

Anatase-phaseAnatase-phase TiO TiO22 andand hematite hematite-phase-phase Fe Fe2O23 Oare3 arewell well-known-known semiconducting semiconducting materials, materials, thus thusthey c theyan harvest can harvest and convert and convert solar light solar into light electric into electric energy energy and chemical and chemical energy. energy. In terms In of terms this ofstudy this, t study,he first the discovery ﬁrst discovery of anatase of anataseand its concomitance and its concomitance with hematite with hematite in Danxia in red Danxia beds redbegs beds the begsquestion the questionof whether of whetherthese widespread these widespread minerals minerals in sun exposure in sun exposure have some have potential some potential effects on effects the onlocal the environment. local environment. To investigate To investigate this, some this, synthetic some synthetic Fe2O3, TiO Fe2O2. 3and, TiO coupled2. and coupled Fe2O3–TiO Fe22 Osamples3–TiO2 sampleswere used were to measure used to measure their photoelectrochemical their photoelectrochemical properties properties and photocatalytic and photocatalytic performance. performance. XRD XRD(Figure (Figure 7a) and7a) micro and micro-Raman-Raman spectra spectra methods methods (Figure (Figure 7b) confirmed7b) conﬁrmed that pure that purehematite hematite-phase-phase Fe2O3 Feand2O anatase3 and anatase-phase-phase TiO2 electrodes TiO2 electrodes were successfu were successfullylly synthesized. synthesized. In particular, In particular, the coupled the coupled Fe2O3– FeTiO2O2 sample3–TiO2 containsampleed contained all the spectroscopic all the spectroscopic features of featureseach component. of each component.The diffraction The peaks diffraction of TiO2 peakswere wider of TiO 2 thanwere Fe wider2O3, implying than Fe2 O the3, implying smaller crystalline the smaller size crystalline of the former. size of the Based former. on Scherrer’s Based on Scherrer’sequation, the equation, crystalline the crystallinesize of TiO size2 was of TiOestimated2 was estimated as approximate as approximatelyly 20 nm, according 20 nm, according to the full to thewidth full at width half maximum at half maximum (FWHM) (FWHM) value of valuepeak assigned of peak assigned as (011) plane as (011). Under plane. the Under scanning the scanning electron microscope, the as-prepared Fe2O3 exhibited as rod-like, which was at most 1 μm in length and hundreds of nanometers in diameter (Figure 7c); the plate-like TiO2 with a smooth surface showed Minerals 2018, 8, 554 10 of 16

electron microscope, the as-prepared Fe2O3 exhibited as rod-like, which was at most 1 µm in length Minerals 2018, 8, x FOR PEER REVIEW 10 of 16 and hundreds of nanometers in diameter (Figure7c); the plate-like TiO 2 with a smooth surface showed crisscross fissuring, probably due to the considerable contraction when cooling (Figure7d). crisscross fissuring, probably due to the considerable contraction when cooling (Figure 7d). While While depositing Fe2O3 on the as-prepared TiO2, rod-like Fe2O3 particles assembled on the surface depositing Fe2O3 on the as-prepared TiO2, rod-like Fe2O3 particles assembled on the surface of TiO2 of TiO2 and filled in the fissuring between the plate-like TiO2 (Figure7e). The coupled Fe 2O3–TiO2 and filled in the fissuring between the plate-like TiO2 (Figure 7e). The coupled Fe2O3–TiO2 sample in sample in this case was just like those natural samples found in the Danxia red beds; the close contact this case was just like those natural samples found in the Danxia red beds; the close contact between between the two components made their interaction possible. EDX elementary mappings for the the two components made their interaction possible. EDX elementary mappings for the synthesized synthesized Fe2O3–TiO2 sample are shown in Figure S6. The distribution of Fe was found to overlap Fe2O3–TiO2 sample are shown in Figure S6. The distribution of Fe was found to overlap with that of with that of Ti, inferring the close contact of hematite and anatase. Ti, inferring the close contact of hematite and anatase.

Figure 7. The spectroscopic identification of Fe2O3, TiO2, and coupled Fe2O3–TiO2 using (a) XRD and Figure 7. The spectroscopic identiﬁcation of Fe2O3, TiO2, and coupled Fe2O3–TiO2 using (a) XRD and (b) Raman spectrum. Secondary electron images of (c) Fe2O3, (d) TiO2, and (e) coupled Fe2O3–TiO2 in (b) Raman spectrum. Secondary electron images of (c) Fe2O3,(d) TiO2, and (e) coupled Fe2O3–TiO2 SEM.in SEM.

TheThe photoresponsive photoresponsive properties properties and optical band gap values ( (Egg)) of of the the as as-prepared-prepared oxides oxides were were determineddetermined by by UV UV-vis-vis DRS DRS and T Tauc’sauc’s plot [[2266].]. The The direct transition of photogenerated carriers carriers betweenbetween the the valence valence band band and and conduction conduction band band is is described by the Tauc model: (Aℎ) = 2 ℎ − (1) (Ahv) = K hv − Eg (1)

Aℎ = 0, = ℎ (2) 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 (Ahv)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 TiO2 was UV-light active and its band Minerals 2018, 8, 554 11 of 16

Ahv = 0, Eg = hv (2) 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 (Ahv)2 versus hv; therefore, it is Minerals 2018, 8, x FOR PEER REVIEW 11 of 16 used to determine the direct optical Eg through linear extrapolation. The results, as well as actual gapphotos was of estimated synthetic as electrodes, 3.05 eV (Figure are shown 8a,b) in, resulting Figure8. Thein its pure off- TiOwhite2 was color UV-light due to limited active and absorption its band ingap solar was light estimated (Figure as 8d) 3.05. eVIn contrast, (Figure8a,b), Fe2O resulting3 exhibited in itsa r off-whiteeddish color color and due strong to limited absorption absorption below in 600solar nm light (Figure (Figure 8a8,d).d), Inand contrast, its band Fe gap2O3 exhibitedvalue was a reddishapproximate colorly and 2.01 strong eV (Figure absorption 8c). belowThe coupled 600 nm Fe(Figure2O3–TiO8a,d),2 sample, and its interestingly, band gap value presented was approximately as orange and 2.01 eVan (Figurealmost 8similarc). The absorption coupled Fe feature2O3–TiO as2 Fesample,2O3 (Figure interestingly, 8d). This presented indicate asd orangethat the and light an almostabsorption similar property absorption of Fe feature2O3–TiO as2 Fe coupled2O3 (Figure sample8d). mustThis indicatedhave been that mainly the light determined absorption by property Fe2O3; ofin Feother2O3 –TiOwords,2 coupled the incorporation sample must of have moderate been mainly TiO2 woulddetermined not considerably by Fe2O3; in change other words, the photoresponsive the incorporation property of moderate of Fe2O TiO3. The2 would Eg of notsemiconducting considerably mineralschange the determines photoresponsive their longest property absorbed of Fe2O 3 wavelength. The Eg of semiconducting of light, thus indicating minerals determines the utilization their efficiencylongest absorbed of sunlight. wavelength The smaller of light, the E thusg of indicatingthe semiconducting the utilization minerals, efﬁciency the more of sunlight. flux of light The smallerwould bethe absorbedEg of the semiconductingby them, which minerals,helps improve the more the ﬂuxproduction of light wouldof oxidizing be absorbed photogenerated by them, which holes helps and reductiveimprove thephotogenerated production of electrons. oxidizing As photogenerated such, naturally holes occurring and reductive hematite photogenerated and anatase can electrons. evoke redoxAs such, reactions naturally under occurring solar light. hematite and anatase can evoke redox reactions under solar light.

Figure 8. 8. (a()a UV) UV-vis-vis diffuse diffuse reflection reﬂection spectrum spectrum (DRS (DRS)) of Fe of2O Fe3, TiO2O32,, and TiO 2coupled, and coupled Fe2O3–TiO Fe22O. Optical3–TiO2 . bandOptical gap band estimated gap estimated from Tauc’s from plot Tauc’ss of ( plotsb) TiO of2 (andb) TiO (c) 2Feand2O3. ( c()d Fe) Actual2O3.( dphotos) Actual of photosthree synthetic of three electrodes.synthetic electrodes.

The a abilitybility to to convert convert light light into into electricity electricity w wasas verified veriﬁed by by photoelectrochemical photoelectrochemical measurements. measurements. The c current-voltageurrent-voltage characteristics characteristics are are shown shown in in Figure Figure 9aa.. C Comparedompared with with the the smaller smaller yield yield of currentof current in inthe the dark, dark, strong strong dependences dependences of of the the photocurrent photocurrent density density on on light light were were observed, observed, indicatingindicating that that light light (simulated (simulated sunlight) sunlight) indeed indeed facilitated facilitated the the formatio formationn of of extra electrons from from semiconducting hematite hematite and anataseanatase.. In In addition, addition, the the three three oxide oxide m materialsaterials showed different performanceperformances.s. The The h hematite-phaseematite-phase Fe Fe22OO3,3 ,the the foremost foremost concern concern in in this this work work due due to to its dominant concentration, showed the worst photoelectric conversion capacity compared with its two counterparts. Notably, when Fe2O3 was coupled with TiO2, a remarkable enhancement of photocurrent density was observed, which must be ascribed to the assistance of anatase-phase TiO2 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 Minerals 2018, 8, 554 12 of 16 concentration, showed the worst photoelectric conversion capacity compared with its two counterparts. Notably, when Fe2O3 was coupled with TiO2, a remarkable enhancement of photocurrent density was observed, which must be ascribed to the assistance of anatase-phase TiO2 with brilliant photoelectric conversion capacity. Furthermore, when the bias was ﬁxed at 0.8 V (vs. Ag/AgCl), the current

Mineralsdensity 2018 was, 8,obtained x FOR PEER under REVIEW alternant dark and simulated sunlight, as shown in Figure9b. The12 dark of 16 currents in all cases were negligible, while enhanced, sensitive, and stable current signals were currentcaptured signals when were photons captured were incident.when photons In particular, were incident coupled. In Feparticular,2O3–TiO 2coupledproduced Fe2O a3 stable–TiO2 producedphotocurrent a stable density photocurrent of about 42 densityµA/cm of2 aboutunder 42 this μA/cm condition,2 under preponderating this condition, 23preponderating times over pure 23 timesFe2O3 over. The pure highest Fe2O photocurrent3. The highest density photocurrent was assigned density towas pure assigned TiO2 (inset), to pure whose TiO2 (inset), photocurrent whose photocurrentdensity reached density almost reach 300edµ A/cmalmost2 .300 With μA/cm its narrower2. With its absorption narrower absorption range of solar range light, of solar the betterlight, thephotoelectrochemical better photoelectrochemical performance performance of TiO2 than of TiO Fe2O2 3thancould Fe2 likelyO3 could be attributedlikely be attributed to its nanoscale to its nanoscaleparticles, which particles, decreased which migration decreased distance migration and distance the probability and the of recombinationprobability of of recombination photogenerated of photogeneratedcarriers [27]. Based carriers on these [27]. results,Based on it these is suggested results, thatit is suggested even with that a low even concentration with a low concentration of TiO2 in red ofbeds, TiO their2 in red extraordinary beds, theirphotoelectric extraordinary conversion photoelectric capacity conversion should makecapacity a difference. should make More a importantly, difference. Moreonce TiOimportantly,2 is involved, once theTiO massive2 is involved but, inferior the massive Fe2O but3 in inferior red beds Fe2 couldO3 in red signiﬁcantly beds could improve significantly their improvephotoelectric their conversionphotoelectric performance. conversion performance.

FigureFigure 9. 9. PhotoelectrochemicalPhotoelectrochemical measurements measurements ofof the the three three oxide oxide electrodes electrodes in 0.5 in 0.5mol/L mol/L Na2SO Na42 andSO4 ethanol.and ethanol. (a) Current (a) Current density density vs. bias vs. curves bias curves and ( andb) current (b) current density density vs. time vs. timecurves curves at 0.8 at V 0.8 (vs. V

Ag/AgCl)(vs. Ag/AgCl).. Inset: Inset: TiO2 case. TiO2 case.

MethylMethyl orange orange (MO) (MO) dye dye is is a model compound to measure the photocatalytic activity of photocatalyst, which which was was used used in in this this case case to compare to compare the thephotocatalysis photocatalysis-related-related redox redox reaction reaction rate ofrate the of three the three as-prepared as-prepared oxides oxides.. The results The results are presented are presented in Figure in Figure 10. In 10 simulated. In simulated solar solarlight, light, MO moleculesMO molecules were wereunder under slight slightphotolysis photolysis with a with19% remo a 19%val removal rate after rate 12 afterh irradiation 12 h irradiation time. When time. a photocatalystWhen a photocatalyst was added,was as added, expected, as expected, the photodegradation the photodegradation efficiencies efficiencies were enhanced were enhanced at different at levels.different The levels. change The of abso changerption of absorption spectra of MO spectra in the of presence MO in the of presencecoupled Fe of2O coupled3–TiO2 is Fe illustrated2O3–TiO2 inis Figureillustrated S7 (the in Figurespectra S7 in (theother spectra photocatalyst in other cases photocatalyst were similar cases and were are similarnot shown) and. areThe not absorption shown). peakThe absorption at λ = 476 peaknm declined at λ = 476 gradually nm declined and graduallypresented and a slight presented blue ashift slight with blue increasing shift with irradiation increasing time,irradiation suggest time,ing suggestingthe progressive the progressive decolorization decolorization of the MO during of the MO the duringreaction. the Specifically, reaction. Specifically, only 27% ofonly MO 27% molecules of MO molecules were removed were removed in the case in the of case Fe2O of3, Fewhile2O3, whilea 48% a removal 48% removal rate was rate was reach reacheded for coupledfor coupled Fe2O Fe3–2OTiO3–TiO2 after2 after 12 h. 12 The h. The pure pure TiO TiO2, surprisingly2, surprisingly,, degraded degraded MO MO by by 75% 75% within within the the same same timetime (inset(inset in in Figure Figure 10 a).10a) The. The photocatalytic photocatalytic performance performance of the ofthree the materials three materials above were above extremely were extremelysimilar to theirsimilar photoelectrochemical to their photoelectrochemical measurements measurements (Figure9), indicating (Figure 9), that indicating passivated that Fe passivated2O3 could Febecome2O3 could active become when active coupled when with coupled superior with TiO superior2. Notably, TiO in2. Notably, Figure 10 ina, Figurethe drastic 10a, decrease the drastic in decreaseMO concentration in MO concentration in the first hourin the could first hour most could likely most be attributed likely be to attributed the strong to physical the strong adsorption, physical adsorption,followed by follow the slowed by photocatalytic the slow photocatalytic degradation degradation for the rest offor the the reaction rest of time.the reaction Thus, thetime. removal Thus, tratehe removal by 20% ofrate TiO by2 in20% the of first TiO hour2 in the might first be hour due tomight its nanoscale be due to particle its nanoscale size, huge particle specific size, surface huge specificarea, and surface strong area absorption, and strong capability. absorption Figure capability. 10b demonstrates Figure 10b the demonstrates photocatalytic the degradationphotocatalytic of 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 Fe2O3, coupled Fe2O3–TiO2, and TiO2, respectively. Based on this, the coupled Fe2O3–TiO2 exhibited a removal efficiency of MO 2.42 times that of inactive Fe2O3, which highlighted the assistance effect of TiO2. Minerals 2018, 8, 554 13 of 16

MO following the pseudo-ﬁrst-order law, with an apparent rate constant of k = 0.019 h−1, 0.046 h−1, −1 and 0.103 h for Fe2O3, coupled Fe2O3–TiO2, and TiO2, respectively. Based on this, the coupled Fe2O3–TiO2 exhibited a removal efﬁciency of MO 2.42 times that of inactive Fe2O3, which highlighted Mineralsthe assistance 2018, 8, x effectFOR PEER of TiO REVIEW2. 13 of 16

FigureFigure 10. 10. (a)( aC)hange Change in relative in relative MO MOconcentration concentration under under irradiation irradiation for Fe for2O3 Feand2O coupled3 and coupled Fe2O3– TiOFe2O2 as3–TiO well2 asas the well situation as the situationwithout photocatalyst. without photocatalyst. Inset: TiO2 Inset: case. ( TiOb) The2 case. variation (b) The of ln variation (Ct/C0) of of MOln (C againstt/C0) ofirradiation MO against time irradiation and linear timefitting and for linear Fe2O3 fittingand coupled for Fe 2FeO32Oand3–TiO coupled2. The linear Fe2O 3fitting–TiO2 . wasThe not linear involve fittingd wasin the not first involved data point in the to firstexclude data the point factor to excludeof physical the absorption. factor of physical Inset: TiO absorption.2 case.

Inset: TiO2 case. 3.3. The Potential Environmental Effect of Fe Oxide and Ti Oxide Minerals on the Danxia Landform 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 whichThe countless Danxia landform minerals in assemble southern and China lithify is a into unique rock environment,. Semiconducting represented minerals as, represented erosional cliffs as hematitein which and countless anatase minerals, once formed assemble and and exposed lithify in into sunlight rock. should Semiconducting be excited minerals, and generate represented reductive as electronshematite and oxidizing anatase, once holes. formed The photogenerated and exposed in sunlightelectrons should from natural be excited minerals and generate have been reductive found toelectrons synthesize and oxidizing prebiotic holes. organics The photogenerated and facilitate electronsthe growth from of natural microorganism minerals haves in been soil found[28,29] to. Furthermoresynthesize prebiotic, the photogenerated organics and facilitateholes from the natural growth minerals of microorganisms are able to in decompose soil [28,29]. organics Furthermore, and killthe bacteria photogenerated in wastewater holes from[30,31] natural. In this minerals work, anatase are able was to decompose found for the organics first time and as kill a mineral bacteria in in thewastewater red beds [30of ,Danxia.31]. In this In addition work, anatase, with wasthe assistance found for theof anatase, first time the as major a mineral species in the hematite red beds was of foundDanxia. to Inexhibit addition, significant with the photoelectrical assistance of performance anatase, the and major high speciesly efficient hematite photocatalytic was found activities. to exhibit Accordingsignificant photoelectrical to our simulated performance experiment, and highly these efficient two photocatalyticminerals could activities. utilize According solar light to our to photocatalyticsimulated experiment,ally decompose these two and minerals destroy could organics. utilize In solar fact, lightseveral to photocatalyticallystudies have reported decompose that there and aredestroy different organics. levels In of fact, desertification several studies in the have red reported beds of Danxia that there, even are differentthough this levels landform of desertification is located inin the the humid red beds region of Danxia, of southern even thoughChina [32 this,33 landform]. Many factors is located are thought in the humid to be responsible region of southern for the continuousChina [32,33 desertification]. Many factors of are Danxia, thought such to be as responsible lithological for features, the continuous natural desertification impacts, and of human Danxia, activities,such as lithological resulting in features, sparser natural vegetation impacts, and anddecreased human microbial activities, community resulting in year sparser on year. vegetation Based and on thisdecreased study, another microbial mechanism community is proposed year on year. with Based the photocatalytic on this study, effects another of hematite mechanism and is anatase proposed on anwith increasing the photocatalytic number of effectsbarren ofred hematite beds. A andsimilar anatase mechanism on an increasing has been put number forward of barren in abiotic red Mars beds. andA similar oligotrophic mechanism desert has, where been titanic put forward oxides in(anatase abiotic and Mars rutile) and oligotrophicand iron oxide desert, (mainly where hematite) titanic areoxides assumed (anatase to produce and rutile) considerable and iron oxidereactive (mainly oxygen hematite) species (ROSs) are assumed with extreme to produce solar considerable irradiation, andreactive these oxygen strong species oxidizing (ROSs) agents with extreme have the solar ability irradiation, to decompos and thesee organics strong oxidizingand exacerbate agents desertificationhave the ability [34,35 to decompose]. On a positive organics note, and the exacerbate photocatalytic desertification effects of[34 natural,35]. On semiconducting a positive note, mineralsthe photocatalytic are suggested effects as ofpossibly natural stimulat semiconductinging metabolism minerals and are the suggested growth of asmicroorganism possibly stimulatings in the soilmetabolism [29,36], thus and thepromoting growth ofinternal microorganisms biodiversity in and the soilsoil [ 29fertility,36], thus. Hematite promoting and internalanatase biodiversityin this case couldand soil also fertility. exert a Hematitesimilar effect and on anatase the Danxia in this landform. case could In also summary, exert a semiconducting similar effect on minerals the Danxia in thelandform. red beds In summary, of Danxia semiconducting might simultaneously minerals inplay the reconstructive red beds of Danxia and might constructive simultaneously roles in playthe environment.reconstructive Though and constructive such effect roless inof the every environment. mineral particle Though remainsuch effects minimal, of every the mineral considerable particle distributionremain minimal, of all thesemiconducting considerable minerals distribution and of the all long semiconducting history of solar minerals irradiation and thecould long lead history 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.

4. Conclusions Minerals 2018, 8, 554 14 of 16 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.

4. Conclusions The Danxia landform, which developed mainly from terrestrial coarse clastic rocks, constitutes a special red bed geomorphology in China. However, the fine identification of its environmentally functional minerals has received little attention. In this study, the Lang Mountain Danxia landform served as a case study to investigate the phase, distribution, as well as the microstructural and spatial relationship, of these minerals. Hematite and anatase were verified as being the main metallic minerals. The first reported anatase found in the Danxia landform could keep a close contact with hematite, whose configuration was shown to be beneficial to their interaction. The brilliant semiconducting and photoelectric properties of hematite, anatase, and their coupled product were examined on synthetic materials. When coupled with anatase, the inactive hematite demonstrated enhanced photoelectric conversion capacity and photocatalytic efficiency of degrading methyl orange in sunlight. Therefore, these natural semiconducting minerals could play a potential role in the destruction of the organic network in vast red beds, most likely resulting in the desertification of the Danxia landform. Other positive effects of these minerals on microorganisms are also proposed. This study reconsiders the environmental functions of minerals in red beds and offers a new perspective to understand the interaction among light, mineral, and organic materials in a long-term geohistory.

Supplementary Materials: The following are available online at http://www.mdpi.com/2075-163X/8/12/554/ s1. Figure S1: The distribution map of sampling sites in Lang mountain geopark, Figure S2: The TEM images of some mineral nanoparticles (a) and the identification of goethite through its interplanar spacing of (010) (b), Figure S3: The TEM images of a well-crystalline maghemite particle (a) and its lattice fringes (b). Inset: selected area electron diffraction pattern and its identification, Figure S4: The TEM images of ilmenite (a) and its lattice fringes (b). Inset: simulated fast Fourier transform pattern and its identification, Figure S5: The change of absorption spectra of MO with irradiated time in the presence of coupled Fe2O3-TiO2, Table S1: The location of sampling sites, Table S2: The concentration of major elements of sandstone samples through XRF method (wt %). Author Contributions: Conceptualization, A.L. and Y.X.; Methodology, Y.X. and Y.L. (Yanzhang Li); Software, H.D.; Validation, A.L. and Y.X.; Formal Analysis, Y.L. (Yanzhang Li); Investigation, Y.X. and Y.L.; Writing–Original Draft Preparation, Y.X.; Writing-Review & Editing, Y.L. (Yanzhang Li); Visualization, Y.L. (Yan Li); Supervision, H.D.; Project Administration, A.L. Funding: This research was supported by the National Basic Research Program of China (Program No. 2014CB846001) and the National Natural Science Foundation of China (Grant No. 41230103, 41272003 & 41522201). Conflicts of Interest: The authors declare no conflict of interest.

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