Next Article in Journal
Interfacial Electric Fields and Chemical Bonds in Ti3C2O-Crafted AgI/MoS2 Direct Z-Scheme Heterojunction Synergistically Expedite Photocatalytic Performance
Previous Article in Journal
Recent Advances in Borylation and Suzuki-Type Cross-Coupling—One-Pot Miyaura-Type C–X and C–H Borylation–Suzuki Coupling Sequence
Previous Article in Special Issue
The Development of Ni-Al Aerogel-Based Catalysts via Supercritical CO2 Drying for Photocatalytic CO2 Methanation
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Catalysts
Volume 15
Issue 8
10.3390/catal15080739
catalysts-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

Visible Light Activation of Anatase TiO2 Achieved by beta-Carotene Sensitization on Earth’s Surface

by
Xiao Ge
1,2,
Hongrui Ding
1,2,*,
Tong Liu
1,2,
Yifei Du
1,2 and
Anhuai Lu
1,2,*
1
Beijing Key Laboratory of Mineral Environmental Function, School of Earth and Space Sciences, Peking University, Beijing 100871, China
2
The Key Laboratory of Orogenic Belts and Crustal Evolution, School of Earth and Space Sciences, Peking University, Beijing 100871, China
*
Authors to whom correspondence should be addressed.
Catalysts 2025, 15(8), 739; https://doi.org/10.3390/catal15080739
Submission received: 26 June 2025 / Revised: 24 July 2025 / Accepted: 29 July 2025 / Published: 1 August 2025
(This article belongs to the Special Issue Advancements in Photocatalysis for Environmental Applications)
Browse Figures
Versions Notes

Abstract

Photocatalytic redox processes significantly contribute to shaping Earth’s surface environment. Semiconductor minerals exhibiting favorable photocatalytic properties are ubiquitous on rock and soil surfaces. However, the sunlight-responsive characteristics and functions of TiO2, an excellent photocatalytic material, within natural systems remain incompletely understood, largely due to its wide bandgap limiting solar radiation absorption. This study analyzed surface coating samples, determining their elemental composition, distribution, and mineralogy. The analysis revealed enrichment of anatase TiO2 and β-carotene. Informed by these observations, laboratory simulations were designed to investigate the synergistic effect of β-carotene on the sunlight-responsive behavior of anatase. Results demonstrate that β-carotene-sensitized anatase exhibited a 64.4% to 66.1% increase in photocurrent compared to pure anatase. β-carotene sensitization significantly enhanced anatase’s electrochemical activity, promoting rapid electron transfer. Furthermore, it improved interfacial properties and acted as a photosensitizer, boosting photo-response characteristics. The sensitized anatase displayed a distinct absorption peak within the 425–550 nm range, with visible light absorption increasing by approximately 17.75%. This study elucidates the synergistic mechanism enhancing the sunlight response between anatase and β-carotene in natural systems and its broader environmental implications, providing new insights for research on photocatalytic redox processes within Earth’s critical zone.

Graphical Abstract

1. Introduction

Solar radiation constitutes the fundamental energy source sustaining life on Earth, with plant photosynthesis serving as the foundation for all biological activities—a concept that is well established. However, soils and rocks, ubiquitous on the Earth’s surface, are now recognized as also capable of utilizing solar energy. Research has revealed that natural semiconductor mineral particles, which exhibit the intrinsic property of long-term reception and transformation of solar energy, are commonly developed on the surfaces of terrestrial soils and rocks [1,2,3]. The surficial photocatalytic processes involving these minerals can play significant roles in Earth’s material cycles and environmental evolution [4]. This discovery has generated substantial interest in the fields related to solar energy utilization and catalytic effects by terrestrial semiconductor minerals.
Semiconductor minerals are widespread on the Earth’s surface, including iron oxides such as goethite and hematite [5,6]; manganese oxides such as birnessite and pyrolusite [1,7]; and titanium oxides such as rutile and anatase [8,9]. These minerals function as efficient semiconductor materials. Upon excitation by solar radiation, they undergo electron transitions, generating photogenerated electrons and holes. These charge carriers subsequently participate in photocatalytic redox reactions with substances in the environment [10,11], thereby exerting a crucial influence on shaping the surrounding environment. Photoelectrons generated by natural semiconductor minerals under solar excitation can promote the growth and metabolism of microorganisms [12]. Georgiou et al. [13] proposed that reactive oxygen species generated via the photocatalysis of substances like Fe2O3 and TiO2 enhance carbon cycling in arid regions. Within soil surfaces and the photic zone of seawater, the photocatalytic self-reduction of iron and manganese oxide minerals can release free Mn2+ and Fe2+ ions [14,15,16]. Iron oxides can then further catalyze the oxidation of Mn2+ [17,18], representing a key driver in the geochemical cycling of elements at the Earth’s surface. Collectively, these studies demonstrate that surficial coatings rich in natural semiconductor minerals possess substantial environmental significance.
However, the effective absorption and utilization of solar radiation by semiconductor minerals are constrained by their minimum excitation wavelength. Natural iron oxides and manganese oxides are recognized for their favorable visible-light responsiveness. Sherman determined the band gaps of a series of natural minerals using X-ray absorption and emission spectroscopy, finding that iron oxide minerals exhibit band gaps between 2.0 and 2.5 eV, while manganese oxide minerals have band gaps ranging from 1.0 to 1.8 eV [19]. This indicates that both possess good visible-light response capabilities. In contrast, titanium oxide minerals (TiO2) such as anatase (3.2 eV) and rutile (3.0 eV), despite their superior photocatalytic performance, are difficult to excite with visible light due to their excessively large band gaps [20]. In the materials field, techniques such as ion doping [21,22] and structural modification [20,23] are commonly employed to reduce the band gap of TiO2. However, these methods only enable utilization of the shorter-wavelength region within the visible spectrum [24,25]. In the 1990s, research discovered that semiconductor materials adsorbed with dyes can effectively absorb broad-spectrum visible light and transfer excited-state electrons to the semiconductor’s conduction band at the interface, thereby initiating subsequent photocatalytic reactions. The “dye-sensitized solar cell” system developed based on this principle further advanced research on TiO2 photocatalysis [26,27]. Natural photosynthetic pigments widely present in nature, such as chlorophyll [28], anthocyanins [29,30], and carotenoids [31], have also been demonstrated to function as natural sensitizers within such systems.
Notably, within the complex open system of surficial coatings, which are rich in minerals, organic matter, and microorganisms, natural pigments are highly likely to “sensitize” coexisting TiO2, thereby expanding its response range to the solar spectrum and enhancing its solar-light-responsive properties. Previous studies have detected β-carotene in various habitats, including Gobi rock varnish [32], karst mineral coatings [33], and soil [34]. Carotenoids, to which β-carotene belongs, represent a class of important photosynthetic pigments second only to chlorophyll in abundance. They are primarily synthesized by photosynthetic organisms, bacteria, and fungi, and are widely distributed in natural environments such as soil and seawater [35]. However, the coexistence of organic pigment molecules and semiconductor minerals in natural systems, along with their synergistic enhancement of solar-light response, has not been reported.
This study collected natural coating samples. The chemical composition and mineralogical characteristics of the samples were analyzed. Based on the detection results, a simulation experiment was designed to investigate the synergistic enhancement of solar-light response between anatase and β-carotene. The enhancement effect of β-carotene sensitization on the solar-light-responsive performance of anatase was quantitatively analyzed using photoelectrochemical testing techniques, and the corresponding intrinsic mechanisms were analyzed. Finally, this study proposes the mechanism of synergistic solar-light response enhancement by anatase and β-carotene in natural surficial systems, along with its potential environmental effects. This provides new insights for research related to electron transfer and energy conversion at interfaces of multi-sphere interactions on the Earth’s surface.

2. Results

2.1. Mineralogical Characterization of Natural Coating Samples

2.1.1. Chemical Composition

Field observations revealed that a black coating is commonly developed on exposed rock surfaces (Figure S1). Observing the sample cross-section under transmitted light mode in an optical microscope (Figure 1a), a distinct boundary between the coating and the underlying substrate rock is visible. The coating develops continuously over the substrate surface, with greater thickness of approximately 150 μm in depressions and thinner thickness of about 30 μm on protrusions. The morphological characteristics of the coating cross-section are clearer under SEM secondary electron imaging (Figure 1b), exhibiting a rough surface resulting from uneven components during polishing. Concurrently, the incorporated backscattered electron signal indicates the presence of higher-atomic-number metal components within the coating compared to the dark grey substrate rock. EDS (Figure 1c) was employed to obtain the average elemental composition within the boxed region in Figure 1b: Ti at 25.5 wt%, C at 9.4 wt%, Fe at 9.3 wt%, Si at 5.4 wt%, Al at 4.2 wt%, and Na at 4.1 wt%. The distribution of each element is shown in Figure 1d, where Ti exhibits significant enrichment within the coating. Elements Fe, Al, and Si are also present in the coating, which, based on previous research, may represent clay minerals formed through weathering and deposition. The enrichment of Ti within the coating is not a localized phenomenon but a general pattern. We analyzed the element enrichment factor (coating element content relative to substrate rock element content) for samples from five locations using ICP-OES (Figure 2). The enrichment level of Ti content consistently reached nearly 10-fold; specific values are provided in Table S1. This demonstrates a pervasive enrichment phenomenon of Ti within the coating.
The above test results indicate that Ti is generally enriched in coatings developed on the surfaces of exposed hilltops. However, the specific occurrence form of the Ti element requires further elucidation through structural characterization methods.

2.1.2. Mineral and Organic Composition

In situ micro-laser Raman spectroscopy was employed for multi-point analysis of the coating cross-section. Anatase, quartz, hematite, and albite were identified based on occurrence frequency (Figure 3). Raman peaks at approximately 145 cm−1 and 391 cm−1 correspond to the characteristic O-Ti-O bending vibration modes of anatase, while peaks near 512 cm−1 and 632 cm−1 correspond to Ti-O stretching vibration modes [36] (Figure 3a). Raman peaks at 208 cm−1 and 463 cm−1 represent the characteristic Si-O stretching vibration modes of quartz, and the peak near 123 cm−1 corresponds to the Si-O-Si bending vibration mode [37] (Figure 3b). Raman peaks at approximately 224 cm−1 and 615 cm−1 correspond to the characteristic Fe-O stretching vibration modes of hematite, while peaks near 244 cm−1, 291 cm−1, 410 cm−1, and 499 cm−1 correspond to Fe-O bending vibration modes [38] (Figure 3c). The main Raman peaks of albite are located at 475 cm−1 and 512 cm−1, resulting from the superimposition of Si-O-Si/Al bending and stretching vibration modes. Additionally, non-characteristic Si-O stretching vibration modes, such as the peak at 811 cm−1, as well as minor vibration modes below 460 cm−1 associated with crystallographic orientation, were detected [39] (Figure 3d). The Raman results indicate that the enriched Ti element in the coating primarily exists as anatase-type TiO2. Hematite, the main occurrence form of Fe, is one of the most common iron oxide minerals in surface oxidation environments. Quartz and albite are likely mechanically incorporated into the coating from minerals constituting the substrate rock.
Notably, signals of β-carotene (C40H56) were commonly detected on the cross-section of the coating sample (Figure 4 and Figure S2). Vibrational peaks at approximately 1001~1006 cm−1, 1154~1155 cm−1, and 1513~1524 cm−1 represent the δ(C=CH), v(C-C), and v(C=C) modes of β-carotene, respectively [40]. β-carotene is a tetraterpenoid compound with significant biological functions. Its core structure consists of two identical β-ionone rings connected by a long-conjugated polyene chain, forming a highly centrosymmetric molecule. The structural core features a carbon skeleton comprising 18 conjugated double bonds. These double bonds extend throughout the entire polyene chain and into both ring structures, creating an extensively delocalized π-electron system. This system results in a low energy requirement for electron transitions between molecular orbitals, corresponding to the visible light region. Specifically, it exhibits a very strong broad absorption band within the 400~500 nm range [41].
Characterization results of the composition and structure of natural samples indicate the widespread enrichment of anatase and β-carotene within coatings subjected to long-term solar radiation. Anatase is recognized as a high-performance photocatalytic semiconductor material, playing significant roles not only in industrial applications [20] but also within natural systems. It participates in photocatalytic redox reactions integral to terrestrial geochemical cycling processes [42]. However, previous research demonstrates that anatase possesses a limited capacity to absorb and utilize visible light due to its relatively wide band gap (3.2 eV), consequently constraining its ability to participate in photocatalytic redox reactions under solar irradiation. Notably, the β-carotene abundantly present within terrestrial coating systems is highly likely to function as a “natural photosensitizer” when combined with anatase, thereby enhancing its solar response characteristics. To test this hypothesis, experimental investigations into the synergistic enhancement of visible-light response by β-carotene and anatase, along with electrochemical studies, were conducted.

2.2. Mineralogical Characterization of Synthesized Anatase Electrode

The synthesized anatase electrode was examined using in situ Raman spectroscopy. The spectrum (Figure 5a) indicates well-crystallized anatase. Multiple-point testing confirmed the absence of signals from the FTO substrate within the anatase spectra, signifying nearly complete coverage of the electrode substrate surface by anatase crystals. This characteristic is further corroborated by the SEM micrograph (Figure 5b), which shows anatase nanoparticles stacked into layers, uniformly covering the FTO electrode surface. To determine the phase homogeneity of the synthesized anatase electrode, in situ powder X-ray diffraction (XRD) was performed directly on the electrode sample and on mineral powder scraped from the electrode surface (Figure 5c). The characteristic diffraction peaks in the XRD pattern at 25.3° (101), 48.07° (200), and 37.84° (004) confirm the anatase structure. Due to the finite penetration depth of X-rays, the XRD pattern of the anatase electrode contains characteristic diffraction peaks from both anatase and the FTO substrate.
UV-Vis DRS spectroscopy revealed the absorbance of the synthesized anatase electrode within the ultraviolet-visible range (Figure 6a). A sharp absorption edge is observed between 300 nm and 350 nm, with very low absorbance in the visible light region below 350 nm. This indicates that only ultraviolet light can induce electron transitions in the anatase. The corresponding Tauc plot (Figure 6b) yields a bandgap energy of approximately 3.04 eV, corresponding to a maximum excitation wavelength of about 408 nm.
Collectively, the synthesized anatase electrode exhibits a homogeneous phase, good crystallinity, and uniform coverage of the FTO electrode surface in the form of nanoparticles, resembling anatase particles found in natural coating samples. These characteristics meet the requirements for subsequent photoelectrochemical simulation experiments. Its bandgap is as high as 3.04 eV, rendering it nearly incapable of absorbing and utilizing visible light energy.
Raman spectroscopy (Figure 7a) confirms that β-carotene is successfully anchored on the sensitized anatase electrode. The spectrum simultaneously displays the characteristic Raman signatures of both β-carotene and anatase, evidencing intimate interfacial contact that mirrors the natural coating samples. FTIR-ATR data (Figure 7b) corroborate this conclusion. After the β-carotene/anatase composite (Ant-caro) is formed, new or enhanced absorption bands emerge at 966, 1350~1440, and 1670~1710 cm−1, positions that match the fingerprint vibrations of β-carotene and are absent from the pristine anatase spectrum [43]. These additional peaks, along with the attenuation of the 1619 cm−1 Ti-O stretching mode and the broad ~3400 cm−1 surface -OH stretching band, indicate that β-carotene perturbs the anatase lattice and engages the surface hydroxyl groups, most likely via hydrogen bonding [44]. The collective spectral changes therefore demonstrate robust adsorption of β-carotene on anatase, driven by a combination of hydrogen bonding, van der Waals forces, and electronic interactions.

2.3. Synergistic Photo-Response Properties of β-Carotene and Anatase

To determine whether the photosensitization of anatase by β-carotene enhances its photoelectrochemical response, photocurrent-time (i-t) curves were recorded for both pristine anatase electrodes and β-carotene-sensitized anatase electrodes over two consecutive 1000 s cycles (Figure 8). The current values exhibited a declining trend during the reaction and gradually stabilized, with steady-state current values summarized in Table 1. After stabilization at approximately 700 s, the dark current values of all three electrode groups were comparable, ranging from 0.05 to 0.06 mA. Upon illumination, the current of the pristine anatase electrode increased by only 0.021 mA compared to its dark current, indicating weak visible-light absorption and utilization. In contrast, the current of the β-carotene-sensitized anatase electrode reached 0.120 mA during the first cycle under illumination—twice its dark current value. The corresponding photocurrent (0.059 mA) increased by approximately 64.4% relative to that of the pristine anatase electrode. In the second i-t measurement cycle for the β-carotene-sensitized anatase electrode, the current remained stable compared to the first cycle, with a photocurrent of 0.062 mA, an increase of approximately 66.1% over the pristine anatase electrode. These results demonstrate that β-carotene adsorbed on anatase significantly enhances its visible-light response. The stability of dark currents across all three i-t tests and the consistent photocurrent of β-carotene-sensitized anatase electrodes over two cycles indicate that β-carotene acts as an effective photosensitizer to promote visible-light absorption and utilization by anatase.
Figure 9 presents the cyclic voltammetry (CV) results, which clearly reveal the differential modulation of the redox behavior of anatase (Ant) and β-carotene-sensitized anatase (Ant-caro) electrodes under illumination. The introduction of β-carotene markedly increases the oxidation and reduction peak currents and sharpens the peaks, indicating a greater number of active interfacial sites, accelerated electron-transfer kinetics, and a more reversible process. Notably, electrochemically active surface area (ECSA) measurements (Figures S3 and S4) show that the ECSA of Ant-Caro is actually ~2% lower than that of Ant, demonstrating that the current enhancement does not originate from surface area expansion. Instead, β-carotene dominates the performance gain through (i) its π-conjugated system forming an ordered monolayer on the TiO2 surface, providing a uniform and efficient electronic-coupling interface; (ii) acting as an additional electron-relay medium that opens new charge-transport pathways; and (iii) optimizing energy-level alignment to lower the interfacial charge-transfer barrier. These photosensitization and surface electronic-structure modulation effects fully offset the negative impact of the minor ECSA loss.
Electrochemical impedance spectroscopy (EIS, Figure 10) further corroborates the above mechanism. The Nyquist plots show that illumination reduces the semicircle diameter under any condition; when β-carotene is additionally loaded, the diameter shrinks further, indicating continuously decreasing charge-transfer resistance. The Bode plots exhibit only a single characteristic peak, confirming a sole charge-transfer process and justifying the use of a unified R(C(RW)) equivalent circuit (Figure S5, Table S2) for fitting. The fitted parameters reveal that, as the condition progresses from Ant-dark → Ant-light → Ant-caro-dark → Ant-caro-light, the charge-transfer resistance Rct decreases monotonically while the double-layer capacitance Cdl increases monotonically. The decrease in Rct arises from (i) photo-generated carriers under illumination, which raise charge density and compress the space-charge layer, and (ii) β-carotene-induced reorganization of the interfacial energy levels, shortening the electron-tunneling distance. The increase in Cdl is attributed to enhanced carrier concentration under illumination and modulation of the surface-state density by β-carotene. Through their synergistic action, the Ant-caro-light system achieves the lowest interfacial impedance and the fastest charge-transfer kinetics, confirming that β-carotene simultaneously functions as an “interfacial modifier” and a “natural photosensitizer.” Thus, illumination and β-carotene do not act as simple additives but generate a cooperative effect: illumination activates TiO2 and boosts carrier concentration, while β-carotene accelerates charge separation and transport by regulating the interfacial electronic structure, jointly yielding the smallest Rct and largest Cdl and thereby validating β-carotene’s efficacy as a natural sensitizer.
To further investigate the synergistic enhancement mechanism, ultraviolet-visible diffuse reflectance spectroscopy (DRS) was performed on β-carotene, the anatase electrode, and the β-carotene-sensitized anatase electrode (Figure 11). β-carotene exhibits a broad absorption peak between 400 nm and 600 nm, indicating its strong absorption of visible light and high propensity for excitation-induced electron transitions. The anatase electrode shows weak absorption in the visible region with almost no discernible absorption peaks. In contrast, the sensitized electrode displays an absorption peak within the 420–550 nm range, peaking at 450 nm. This peak aligns with the absorption range of β-carotene, suggesting that the broadening of the visible light absorption spectrum for the anatase electrode may be attributed to the surface-adsorbed β-carotene. Integration of the absorption area within the 400–700 nm range revealed that the visible light absorption of the sensitized anatase electrode increased by approximately 17.75% compared to the unsensitized electrode.

3. Discussion

This study provides the first direct evidence for the co-occurrence of anatase and β-carotene within natural rock-surface coatings. Using controlled experiments that precisely replicate field irradiance, pH, and moisture conditions, we demonstrate that this mineral–organic assemblage operates synergistically to enhance solar-energy conversion. Building on these results, we propose the concept of a “mineral-organic micro-battery” to explain how a sustained photoelectron flux at the Earth’s surface can drive elemental cycling within micro-environments. Specifically, β-carotene bound to anatase surfaces functions as a photosensitizer, extending the spectral response from the ultraviolet into the visible region by improving interfacial charge-transfer properties (Figure 12).
Solar radiation energy is concentrated in the visible spectrum, with only a minor portion in the UV range. Natural semiconducting minerals like anatase, possessing excellent photocatalytic properties within coatings, have relatively large band gaps that prevent efficient absorption and utilization of visible light, thereby limiting their environmental impact. We observed that β-carotene frequently coexists with anatase in complex surface coating systems. Crucially, β-carotene is known to exhibit strong absorption within the visible range of solar light. Our research demonstrates that β-carotene molecules adsorbed onto anatase are excited by sunlight from their ground state to an excited state. These excited β-carotene molecules then inject electrons into the conduction band of anatase. The conduction band position of TiO2 and the HOMO/LUMO energy levels of β-carotene ensure the feasibility of this process (Table S3) [45]. This process effectively broadens the utilization range of anatase for sunlight within the 420–550 nm wavelength band, increasing visible light absorption by approximately 17.75% and significantly enhancing its solar response characteristics.
Mineralogical analysis of collected natural coating samples aligns with morphologic features interpreted in previous studies [42]. These coatings predominantly contain poorly crystalline, fine-grained semiconducting minerals. Their mineral stacking bands host abundant micro-nano pores [42], facilitating close spatial contact between the minerals and environmental organic matter [46]. Furthermore, the coexistence of multiple pigments in the natural environment enables complementary absorption across the solar spectrum, leading to improved light absorption and conversion efficiency [47]. Protonation of the TiO2 surface is known to significantly enhance the photoelectric conversion efficiency of dye-sensitized solar cells [48]. Notably, weakly acidic rainfall in surface environments provides a suitable pH condition for the semiconducting minerals within this natural reaction pool. In the coating system, the Fe3+ ions present are adsorbed onto the anatase surface, thereby further enhancing its photocatalytic activity [49]. Consequently, micro-nano pores within surface coatings may function as “natural reaction pools” for the synergistic interaction between anatase and β-carotene, facilitating the continuous conversion of solar energy into mineral photoelectron energy.
The synergistic photo-response between β-carotene and anatase holds significant environmental implications. Its importance lies in the fact that this process regulates environmental redox catalysis not only through the inherent photocatalytic activity of anatase [50], but more crucially, by extending the spectral response range and stabilizing charge separation via synergy with pigments. This cooperative mechanism enhances interfacial electron transfer kinetics, particularly regulating the biogeochemical transformations of iron and manganese elements through wavelength-specific activation pathways [51]. The mineral-organic complex establishes an energy transduction channel between solar radiation and redox potential, generating bioavailable electron flows that reshape microbial metabolic priorities. This stable photoelectrochemical interface essentially constitutes a “bio-geo-battery system”. It sustains microbial communities by continuously delivering redox energy and exerts cascading effects on the kinetics of organic matter decomposition [52]. Collectively, our results expand current understanding of rock-surface photochemistry and provide a new theoretical framework for elucidating energy–matter coupling mechanisms across the Earth’s surface.

4. Materials and Methods

4.1. Natural Coating Samples Collection and Preparation

Natural coating samples were collected from the suburbs of Anqing, Anhui Province, in Eastern China (30°43′ N, 116°27′ E). This region experiences a subtropical monsoon semi-humid climate, with an average annual sunshine duration of approximately 1315 h and average annual precipitation ranging from 1250 to 1430 mm. The substrate rock in this area primarily consists of granite, medium- to coarse-grained monzonitic gneiss, medium- to fine-grained monzonitic gneiss, and biotite monzonitic gneiss. Multiple rock samples were collected at five equidistant points within the sampling area to ensure representative sampling. To preserve the integrity of the mineral coatings, parts of the samples were impregnated with polyaldehyde resin and cured at room temperature. They were then sectioned perpendicular to the mineral coating surface and polished to create optical thin sections approximately 50 μm thick. The morphological characteristics of the mineral coatings were observed under plane-polarized light using optical microscopy in transmission mode. These thin sections were also used for in-situ micro-Raman spectroscopy and scanning electron microscopy (SEM) analysis. Concurrently, powder samples were obtained by carefully scraping the surface mineral coatings from the remaining samples using a quartz rod. The powder was ground and sieved through a 200-mesh sieve for chemical composition analysis.

4.2. Characterization of Natural Coating Samples

Each thin section of mineral coating samples was analyzed using a Renishaw inVia Reflex Raman spectrometer (Confocal Raman Micro-spectrometer, Renishaw inVia Reflex, London, UK). Raman spectra were acquired using a laser excitation wavelength of 532 nm. To prevent sample damage from localized heating caused by high laser energy, the laser power was adjusted between 0.25 and 25 mW depending on the sensitivity of the specific sample under investigation. The acquisition time for each measurement was 10 s. Accumulation times ranged from 5 to 20, based on the spectrum quality and signal-to-noise ratio.
Representative coating sample thin sections were selected under an optical microscope for in situ observation and testing using a Field Emission Environmental Scanning Electron Microscope (FE-ESEM, Thermo Fisher Quattro S, Thermo Fisher Scientific Inc., Waltham, MA, USA) at the School of Physics, Peking University. Samples were sputter-coated with gold and placed in the microscope vacuum chamber. Secondary electron (SE) and backscattered electron (BSE) images were acquired. An energy-dispersive spectrometer (EDS) attached to the system was used to perform area scans across the substrate-coating transition zone to obtain elemental distribution maps. The operating voltage was 15.00 kV and the working distance was approximately 13.0 mm.
The elemental contents in the mineral coatings and bedrock were quantitatively analyzed using an Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES, Optima 8300, PerkinElmer, PerkinElmer Inc., Waltham, MA, USA) at the National Research Center for Geoanalysis, Chinese Academy of Geological Sciences. Prior to analysis, samples were dissolved using an open acid digestion method: 20 mg of sample powder was sequentially treated with 2 mL HCl, 3 mL HNO3, 5 mL HF, and 0.5 mL HClO4 (all Aristar grade). The mixture was heated on an electric hot plate at 120 °C and 150 °C for 1 h each, then raised to 180 °C until HClO4 fumes were completely expelled. The crucible walls were rinsed with 2 mL of 1:1 aqua regia, and the solution was cooled to room temperature before being diluted to 10.00 g with ultrapure water.

4.3. Preparation and Characterization of Anatase Electrodes

Anatase electrodes were prepared on fluorine-doped tin oxide (FTO) conductive glass substrates (effective area: 3.0 cm × 2.5 cm) using the sol-gel method. Before use, the FTO substrates were ultrasonically cleaned in acetone, anhydrous ethanol, and deionized water for 30 min each. The sol was prepared from Solution A (10 mL anhydrous ethanol, 1.26 mL deionized water, and 0.4 mL concentrated nitric acid, stirred thoroughly) and Solution B (6 g tetrabutyl titanate, 40 mL anhydrous ethanol, and 1 mL acetylacetone, stirred thoroughly). All reagents used were analytical grade. Solution A was added dropwise to Solution B, and the mixture was stirred vigorously to obtain a clear sol. The final sol was sealed and aged for 12 h. Electrodes were then fabricated using the dip-coating method and placed in a muffle furnace. Calcination was performed at 300 °C for 20 min and 500 °C for 30 min, with a heating rate of 5 °C/min.
The crystalline phase of the synthesized mineral was identified using an X’Pert Pro diffractometer (PANalytical, Almelo, The Netherlands) with Cu Kα radiation (λ = 1.5406 Å) at a tube voltage of 40 kV. Raman spectra of anatase were collected with a 532 nm laser. The laser intensity was 0.5 mW and the beam spot diameter was ~1 μm. The morphology of the anatase electrodes was observed by the SEM operated at 5 kV in secondary electron imaging mode.
FTIR-ATR spectra (400–4000 cm−1) were acquired on a Bruker Vertex 80 bench (Bruker, Karlsruhe, Germany), continuously purged with dry, CO2-free air and fitted with a Platinum single-bounce diamond ATR accessory. A Ge-on-KBr beam splitter directed light to a liquid-nitrogen-cooled, wide-band MCT detector. Each final spectrum represents the co-addition of 64 interferograms collected at 4 cm−1 spectral resolution.
UV-Visible diffuse reflectance spectra (DRS) were collected using a UV-VIS-NIR spectrophotometer (UV3600 Plus, Shimadzu, Tokyo, Japan) equipped with an integrating sphere, over the range of 300 to 700 nm. The slit width was 3.0 nm, and BaSO4 was used as a reflectance reference. The spectrum of the bare FTO substrate was subtracted as the baseline.

4.4. Solar Response and Electrochemical Experiments of Anatase and β-Carotene

β-Carotene (≥96%) was purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. (Shanghai, China). Some anatase electrodes (Ant) were immersed in a sealed 0.1 g/L β-carotene-acetone solution for 12 h in the dark, yielding β-carotene-sensitized anatase electrodes (Ant-caro). These were air-dried for subsequent testing and experiments. A Raman spectrometer was employed to investigate the adsorption of β-carotene on anatase, scanning over the range of 100–2000 cm−1. Electrochemical response experiments were performed using an electrochemical workstation (Autolab PGSTAT204, Metrohm AG, Herisau, Switzerland). A conventional three-electrode electrochemical cell was used for all measurements, with either the anatase electrode or the β-carotene-sensitized anatase electrode as the working electrode, a platinum wire as the counter electrode, and a saturated calomel electrode (SCE) as the reference electrode.
Dark/light current–time (I-t) curves were measured in an electrolyte consisting of 0.01 mol/L LiI and 0.001 mol/L I2 dissolved in 0.1 mol/L Na2SO4 solution. The measured pH of ≈6.3 is close to that of mildly acidic rainwater, ensuring relevance to field conditions. Alternating light and dark conditions were achieved using an external light-emitting diode (LED) with an emission wavelength range of 400 to 700 nm. The light illumination intensity, measured using an FGH-1 photosynthetic radiometer (Beijing Normal University Photoelectric Instrument Factory, Beijing, China), was 100 mW·cm−2 in back-illumination mode. The photocurrent density–time curves were recorded at a constant applied potential of −0.8 V (vs. SCE) while periodically blocking the light source.
Cyclic voltammetry (CV) curves of the anatase electrode and the β-carotene-sensitized anatase electrode were measured in the same electrolyte solution used for I-t measurements. The scan range was from −1.2 V to 0.8 V with a scan rate of 10 mV/s. The electrochemically active surface area (ECSA) of anatase TiO2 and β-carotene-sensitized TiO2 films was quantified by cyclic voltammetry (CV) in a three-electrode cell: film working electrode (geometric area 1 cm2), Pt foil counter electrode, Ag/AgCl reference electrode, and 0.1 M Na2SO4 electrolyte. After verifying the absence of redox processes between 0.1 and 0.3 V vs. Ag/AgCl, CVs were recorded at 0.02, 0.04, 0.06, 0.08, and 0.10 V s−1 (second cycle used) under both dark and AM 1.5 G (100 mW cm−2) illumination. The double-layer capacitance (Cdl) was obtained from the slope of Δj/2 at ~0.2 V vs. Ag/AgCl versus scan rate; relative ECSA changes were calculated with ECSA = Cdl/Cs.
Electrochemical impedance spectroscopy (EIS) was performed on both the experimental and control groups under both light and dark conditions in the same electrolyte solution used for CV measurements. The applied bias was set at −0.8 V (vs. SCE), and the frequency scan range was from 105 Hz to 0.1 Hz. The classical R(C(RW)) equivalent circuit model was employed to uniformly fit the data for all four experimental conditions. In this model, Rs represents the electrolyte solution resistance, Cdl denotes the double-layer capacitance at the TiO2 electrode/electrolyte interface, Rct is the charge transfer resistance, and Zw signifies the Warburg diffusion impedance.

5. Conclusions

Based on the inherent photo-response properties of natural semiconducting minerals within the Earth’s surface-developed coating system, this study proposes that β-carotene can synergize with anatase to broaden its sunlight response range and enhance its photocatalytic performance under solar irradiation, elucidating its significant implications for the natural environment. We systematically collected natural coating samples and conducted comprehensive mineralogical analyses. The results indicate enrichment of Ti relative to the substrate rock, with Ti occurring predominantly as anatase-type TiO2. Furthermore, Raman spectroscopy multi-point detection revealed the widespread presence of β-carotene within the coating. Photoelectrochemical simulation experiments demonstrated that the photocurrent of β-carotene-sensitized anatase increased by 64.4~66.1%. Both CV and EIS tests confirmed that β-carotene significantly enhances the electrochemical activity of anatase. DRS revealed an approximately 17.75% increase in visible light absorption by the sensitized anatase electrode. Collectively, these results demonstrate that β-carotene not only improves the interfacial properties of the anatase surface but also acts as a photosensitizer to enhance photo-response. This work provides novel insights into terrestrial electron transfer and energy transformation processes.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal15080739/s1: Figure S1: Outcrop photo of the sampling location for the natural coating sample; Figure S2: Raman spectrum of β-carotene in the natural coating sample; Table S1: Elemental contents of the coating and substrate rock in the natural samples; Figure S3: CV curves for (a) Ant-dark, (b) Ant-light, (c) Ant-caro-dark, and (d) Ant-caro-light in the region of 0.1~0.3 V vs. SHE with various scan rates; Figure S4: Current densities at various scan rates in the CV curves; Figure S5: Equivalent-circuit fitting analysis of electrochemical impedance spectroscopy: Bode plots and equivalent circuit diagrams for (a) Ant-dark, (b) Ant-light, (c) Ant-caro-dark, and (d) Ant-caro-light; Table S2: EIS fitting parameters; Table S3: The energy levels and band of anatase and β-carotene in Ph 7 (vs. SHE).

Author Contributions

Conceptualization, X.G. and H.D.; Methodology, H.D. and A.L.; Sampling, X.G.; Experimenting, X.G., Y.D. and T.L.; Formal Analysis, X.G.; Writing—Original Draft Preparation, X.G.; Visualization, T.L.; Project Administration, H.D. and A.L.; Funding Acquisition, A.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China, grant numbers 42192502, 92251301, 91851208, and 41820104003.

Data Availability Statement

The original contributions presented in the study are included in the article/Supplementary Material. Further inquiries can be directed to the corresponding author.

Acknowledgments

We thank Xiaobin Ge for his assistance in collecting natural coating samples.

Conflicts of Interest

The authors declare no conflicts of interest.

References

  1. Macholdt, D.S.; Jochum, K.P.; Pöhlker, C.; Arangio, A.; Förster, J.D.; Stoll, B.; Weis, U.; Weber, B.; Müller, M.; Kappl, M.; et al. Characterization and differentiation of rock varnish types from different environments by microanalytical techniques. Chem. Geol. 2017, 459, 91–118. [Google Scholar] [CrossRef]
  2. Huang, L.; Liu, F.; Tan, W.F.; Hu, H.Q.; Wang, M.K. Geochemical characteristics of selected elements in iron-manganese cutans and matrices of Alfisols in central China. J. Geochem. Explor. 2009, 103, 30–36. [Google Scholar] [CrossRef]
  3. Ju, Y.W.; Li, X.; Ju, L.T.; Feng, H.Y.; Tan, F.Q.; Cui, Y.S.; Yang, Y.; Wang, X.Q.; Cao, J.J.; Qiao, P.; et al. Nanoparticles in the Earth surface systems and their effects on the environment and resource. Gondwana Res. 2022, 110, 370–392. [Google Scholar] [CrossRef]
  4. Lu, A.H.; Li, Y.; Ding, H.R.; Xu, X.M.; Li, Y.Z.; Ren, G.P.; Liang, J.; Liu, Y.W.; Hong, H.; Chen, N.; et al. Photoelectric conversion on Earth’s surface via widespread Fe- and Mn-mineral coatings. Proc. Natl. Acad. Sci. USA 2019, 116, 9741–9746. [Google Scholar] [CrossRef]
  5. Schwertmann, U.; Taylor, R.M. Iron oxides. In Minerals in Soil Environments; The Soil Science Society of America: Madison, WI, USA, 1989; pp. 379–438. [Google Scholar]
  6. Waychunas, G.A.; Kim, C.S.; Banfield, J.F. Nanoparticulate iron oxide minerals in soils and sediments: Unique properties and contaminant scavenging mechanisms. J. Nanoparticle Res. 2005, 7, 409–433. [Google Scholar] [CrossRef]
  7. Post, J.E. Manganese oxide minerals: Crystal structures and economic and environmental significance. Proc. Natl. Acad. Sci. USA 1999, 96, 3447–3454. [Google Scholar] [CrossRef]
  8. Baioumy, H.M. Ti-bearing minerals in sedimentary kaolin deposits of Egypt. Appl. Clay Sci. 2014, 101, 345–353. [Google Scholar] [CrossRef]
  9. Imperial, A.; Pe-Piper, G.; Piper, D.J.W.; Clyburne, J. The use of titania polymorphs as indicators of mesodiagenesis during hydrocarbon charge. Mar. Pet. Geol. 2023, 149, 106075. [Google Scholar] [CrossRef]
  10. Doane, T.A. A survey of photogeochemistry. Geochem. Trans. 2017, 18, 1. [Google Scholar] [CrossRef]
  11. Xu, Y.; Schoonen, M.A.A. The absolute energy positions of conduction and valence bands of selected semiconducting minerals. Am. Mineral. 2000, 85, 543–556. [Google Scholar] [CrossRef]
  12. Lu, A.H.; Li, Y.; Jin, S.; Wang, X.; Wu, X.L.; Zeng, C.P.; Li, Y.; Ding, H.R.; Hao, R.X.; Lv, M.; et al. Growth of non-phototrophic microorganisms using solar energy through mineral photocatalysis. Nat. Commun. 2012, 3, 8. [Google Scholar] [CrossRef]
  13. Georgiou, C.D.; Sun, H.J.; McKay, C.P.; Grintzalis, K.; Papapostolou, I.; Zisimopoulos, D.; Panagiotidis, K.; Zhang, G.S.; Koutsopoulou, E.; Christidis, G.E.; et al. Evidence for photochemical production of reactive oxygen species in desert soils. Nat. Commun. 2015, 6, 7100. [Google Scholar] [CrossRef]
  14. Delgadillo-Hinojosa, F.; Segovia-Zavala, J.A.; Huerta-Díaz, M.A.; Atilano-Silva, H. Influence of geochemical and physical processes on the vertical distribution of manganese in Gulf of California waters. Deep Sea Res. Part I 2006, 53, 1301–1319. [Google Scholar] [CrossRef]
  15. Sunda, W.; Huntsman, S.; Harvey, G. Photoreduction of manganese oxides in seawater and its geochemical and biological implications. Nature 1983, 301, 234–236. [Google Scholar] [CrossRef]
  16. Wells, M.L.; Mayer, L.M.; Donard, O.F.X.; Sierra, M.M.D.; Ackelson, S.G. The photolysis of colloidal iron in the oceans. Nature 1991, 353, 248–250. [Google Scholar] [CrossRef]
  17. Wang, X.M.; Lan, S.; Zhu, M.Q.; Ginder-Vogel, M.; Yin, H.; Liu, F.; Tan, W.F.; Feng, X.H. The Presence of Ferrihydrite Promotes Abiotic Formation of Manganese (Oxyhydr)oxides. Soil Sci. Soc. Am. J. 2015, 79, 1297–1305. [Google Scholar] [CrossRef]
  18. Lan, S.; Wang, X.M.; Xiang, Q.J.; Yin, H.; Tan, W.F.; Qiu, G.H.; Liu, F.; Zhang, J.; Feng, X.H. Mechanisms of Mn(II) catalytic oxidation on ferrihydrite surfaces and the formation of manganese (oxyhydr)oxides. Geochim. Cosmochim. Acta 2017, 211, 79–96. [Google Scholar] [CrossRef]
  19. Sherman, D.M. Electronic structures of iron(III) and manganese(IV) (hydr)oxide minerals: Thermodynamics of photochemical reductive dissolution in aquatic environments. Geochim. Cosmochim. Acta 2005, 69, 3249–3255. [Google Scholar] [CrossRef]
  20. Chen, X.; Mao, S.S. Titanium dioxide nanomaterials: Synthesis, properties, modifications, and applications. Chem. Rev. 2007, 107, 2891–2959. [Google Scholar] [CrossRef] [PubMed]
  21. Piatkowska, A.; Janus, M.; Szymanski, K.; Mozia, S. C-,N- and S-Doped TiO2 Photocatalysts: A Review. Catalysts 2021, 11, 144. [Google Scholar] [CrossRef]
  22. Yu, W.L.; Zhang, J.F.; Peng, T.Y. New insight into the enhanced photocatalytic activity of N-, C- and S-doped ZnO photocatalysts. Appl. Catal. B Environ. 2016, 181, 220–227. [Google Scholar] [CrossRef]
  23. Low, J.X.; Cheng, B.; Yu, J.G. Surface modification and enhanced photocatalytic CO2 reduction performance of TiO2: A review. Appl. Surf. Sci. 2017, 392, 658–686. [Google Scholar] [CrossRef]
  24. Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. Visible-light photocatalysis in nitrogen-doped titanium oxides. Science 2001, 293, 269–271. [Google Scholar] [CrossRef]
  25. Kato, H.; Kudo, A. Visible-light-response and photocatalytic activities of TiO2 and SrTiO3 photocatalysts codoped with antimony and chromium. J. Phys. Chem. B 2002, 106, 5029–5034. [Google Scholar] [CrossRef]
  26. O’regan, B.; Grätzel, M. A low-cost, high-efficiency solar cell based on dye-sensitized colloidal TiO2 films. Nature 1991, 353, 737–740. [Google Scholar] [CrossRef]
  27. Ahmad, M.S.; Pandey, A.K.; Abd Rahima, N. Advancements in the development of TiO2 photoanodes and its fabrication methods for dye sensitized solar cell (DSSC) applications. A review. Renew. Sustain. Energy Rev. 2017, 77, 89–108. [Google Scholar] [CrossRef]
  28. Kay, A.; Gratzel, M. Artificial photosynthesis. 1. Photosensitization of TiO2 solar-cells with chlorophyll derivatives and related natural porphyrins. J. Phys. Chem. 1993, 97, 6272–6277. [Google Scholar] [CrossRef]
  29. Kohno, Y.; Haga, E.; Yoda, K.; Shibata, M.; Fukuhara, C.; Tomita, Y.; Maeda, Y.; Kobayashi, K. Adsorption behavior of natural anthocyanin dye on mesoporous silica. J. Phys. Chem. Solids 2014, 75, 48–51. [Google Scholar] [CrossRef]
  30. Polo, A.S.; Iha, N.Y.M. Blue sensitizers for solar cells: Natural dyes from Calafate and Jaboticaba. Sol. Energy Mater. Sol. Cells 2006, 90, 1936–1944. [Google Scholar] [CrossRef]
  31. Shanmugam, V.; Manoharan, S.; Anandan, S.; Murugan, R. Performance of dye-sensitized solar cells fabricated with extracts from fruits of ivy gourd and flowers of red frangipani as sensitizers. Spectrochim. Acta Part A Mol. Biomol. Spectrosc. 2013, 104, 35–40. [Google Scholar] [CrossRef]
  32. Malherbe, C.; Ingley, R.; Hutchinson, I.; Edwards, H.; Carr, A.S.; Harris, L.; Boom, A. Biogeological Analysis of Desert Varnish Using Portable Raman Spectrometers. Astrobiology 2015, 15, 442–452. [Google Scholar] [CrossRef]
  33. Xu, X.M.; Ding, H.R.; Li, Y.; Lu, A.H.; Li, Y.; Wang, C.Q. Mineralogical characteristics of Mn coatings from different weathering environments in China: Clues on their formation. Miner. Petrol. 2018, 112, 671–683. [Google Scholar] [CrossRef]
  34. Kumar, B.N.V.; Kampe, B.; Rösch, P.; Popp, J. Characterization of carotenoids in soil bacteria and investigation of their photodegradation by UVA radiation via resonance Raman spectroscopy. Analyst 2015, 140, 4584–4593. [Google Scholar] [CrossRef]
  35. Schroeder, W.A.; Johnson, E.A. Singlet oxygen and peroxyl radicals regulate carotenoid biosynthesis in phaffia rhodozyma. J. Biol. Chem. 1995, 270, 18374–18379. [Google Scholar] [CrossRef]
  36. Ohsaka, T.; Izumi, F.; Fujiki, Y. Raman-Spectrum of Anatase, TiO2. J. Raman Spectrosc. 1978, 7, 321–324. [Google Scholar] [CrossRef]
  37. Hemley, R.J. Pressure dependence of raman spectra of SiO2 polymorphs: A-quartz, coesite, and stishovite. In High-Pressure Research in Mineral Physics: A Volume in Honor of Syun-Iti Akimoto; American Geophysical Union: Washington, DC, USA, 1987; pp. 347–359. [Google Scholar]
  38. Legodi, M.A.; de Waal, D. The preparation of magnetite, goethite, hematite and maghemite of pigment quality from mill scale iron waste. Dye. Pigment. 2007, 74, 161–168. [Google Scholar] [CrossRef]
  39. Mernagh, T.P. Use of the laser raman microprobe for discrimination amongst feldspar minerals. J. Raman Spectrosc. 1991, 22, 453–457. [Google Scholar] [CrossRef]
  40. Inagaki, F.; Tasumi, M.; Miyazawa, T. Vibrational analysis of polyene chains—Assignments of resonance raman lines of poly(acetylene) and beta-carotene. J. Raman Spectrosc. 1975, 3, 335–343. [Google Scholar] [CrossRef]
  41. Saini, R.K.; Prasad, P.; Lokesh, V.; Shang, X.M.; Shin, J.; Keum, Y.S.; Lee, J.H. Carotenoids: Dietary sources, extraction, encapsulation, bioavailability, and health benefits-a review of recent advancements. Antioxidants 2022, 11, 795. [Google Scholar] [CrossRef] [PubMed]
  42. Xu, X.M.; Li, Y.; Li, Y.Z.; Lu, A.H.; Qiao, R.X.; Liu, K.H.; Ding, H.R.; Wang, C.Q. Characteristics of desert varnish from nanometer to micrometer scale: A photo-oxidation model on its formation. Chem. Geol. 2019, 522, 55–70. [Google Scholar] [CrossRef]
  43. Martin, D.; Amado, A.M.; Gonzálvez, A.G.; Marques, M.P.M.; de Carvalho, L.A.E.B.; Ureña, Á.G. FTIR Spectroscopy and DFT Calculations to Probe the Kinetics of β-Carotene Thermal Degradation. J. Phys. Chem. A 2019, 123, 5266–5273. [Google Scholar] [CrossRef]
  44. Mino, L.; Spoto, G.; Ferrari, A.M. CO2 Capture by TiO2 Anatase Surfaces: A Combined DFT and FTIR Study. J. Phys. Chem. C 2014, 118, 25016–25026. [Google Scholar] [CrossRef]
  45. Maddah, H.A. Investigation of charge transport mechanism at TiO2/MAPbI3/β-Carotene heterostructure in natural dye sensitized solar cells. Mater. Sci. Eng. B Adv. 2024, 302, 117197. [Google Scholar] [CrossRef]
  46. Dorn, R.I. Rock Varnish. Am. Sci. 1991, 79, 542–553. [Google Scholar] [CrossRef]
  47. Kumara, N.; Ekanayake, P.; Lim, A.; Liew, L.Y.C.; Iskandar, M.; Ming, L.C.; Senadeera, G.K.R. Layered co-sensitization for enhancement of conversion efficiency of natural dye sensitized solar cells. J. Alloys Compd. 2013, 581, 186–191. [Google Scholar] [CrossRef]
  48. Wang, Z.S.; Yamaguchi, T.; Sugihara, H.; Arakawa, H. Significant efficiency improvement of the black dye-sensitized solar cell through protonation of TiO2 films. Langmuir 2005, 21, 4272–4276. [Google Scholar] [CrossRef]
  49. Zhang, J.W.; Fu, D.F.; Gao, H.Y.; Deng, L. Mechanism of enhanced photocatalysis of TiO2 by Fe3+ in suspensions. Appl. Surf. Sci. 2011, 258, 1294–1299. [Google Scholar] [CrossRef]
  50. Zhang, J.; Zhou, P.; Liu, J.; Yu, J. New understanding of the difference of photocatalytic activity among anatase, rutile and brookite TiO2. Phys. Chem. Chem. Phys. 2014, 16, 20382–20386. [Google Scholar] [CrossRef] [PubMed]
  51. Wang, Y.; Suzuki, H.; Xie, J.; Tomita, O.; Martin, D.J.; Higashi, M.; Kong, D.; Abe, R.; Tang, J. Mimicking natural photosynthesis: Solar to renewable H2 fuel synthesis by Z-scheme water splitting systems. Chem. Rev. 2018, 118, 5201–5241. [Google Scholar] [CrossRef]
  52. Martin, J.H.; Fitzwater, S.E. Iron deficiency limits phytoplankton growth in the north-east Pacific subarctic. Nature 1988, 331, 341–343. [Google Scholar] [CrossRef]
Catalysts 15 00739 g001
Figure 1. The morphology and elemental distribution characteristics of the coating sample. (a) Optical micrograph of the coating cross-section; (b) scanning electron microscopy image; (c) elemental content analysis; and (d) distribution characteristics of individual elements.
Figure 1. The morphology and elemental distribution characteristics of the coating sample. (a) Optical micrograph of the coating cross-section; (b) scanning electron microscopy image; (c) elemental content analysis; and (d) distribution characteristics of individual elements.
Catalysts 15 00739 g001
Catalysts 15 00739 g002
Figure 2. The element enrichment factor of coating samples.
Figure 2. The element enrichment factor of coating samples.
Catalysts 15 00739 g002
Catalysts 15 00739 g003
Figure 3. Minerals detected by Raman spectroscopy in the coating samples. (a) Anatase; (b) quartz; (c) hematite; and (d) albite.
Figure 3. Minerals detected by Raman spectroscopy in the coating samples. (a) Anatase; (b) quartz; (c) hematite; and (d) albite.
Catalysts 15 00739 g003
Catalysts 15 00739 g004
Figure 4. β-carotene detected by Raman spectroscopy in the coating and its schematic chemical structure (inset).
Figure 4. β-carotene detected by Raman spectroscopy in the coating and its schematic chemical structure (inset).
Catalysts 15 00739 g004
Catalysts 15 00739 g005
Figure 5. Mineralogical characterization of the synthesized anatase electrode. (a) Raman spectra of the anatase electrode and FTO substrate (inset); (b) scanning electron microscopy image of the anatase electrode; and (c) XRD patterns of the anatase electrode, anatase powder, and FTO substrate.
Figure 5. Mineralogical characterization of the synthesized anatase electrode. (a) Raman spectra of the anatase electrode and FTO substrate (inset); (b) scanning electron microscopy image of the anatase electrode; and (c) XRD patterns of the anatase electrode, anatase powder, and FTO substrate.
Catalysts 15 00739 g005
Catalysts 15 00739 g006
Figure 6. (a) UV-Vis DRS spectrum and (b) corresponding Tauc plot of the anatase electrode.
Figure 6. (a) UV-Vis DRS spectrum and (b) corresponding Tauc plot of the anatase electrode.
Catalysts 15 00739 g006
Catalysts 15 00739 g007
Figure 7. (a) Raman spectrum of the β-carotene-sensitized anatase electrode; (b) FTIR-ATR spectra of anatase (green solid trace), β-carotene (orange dashed trace), and β-carotene-sensitized anatase (orange solid trace) electrodes.
Figure 7. (a) Raman spectrum of the β-carotene-sensitized anatase electrode; (b) FTIR-ATR spectra of anatase (green solid trace), β-carotene (orange dashed trace), and β-carotene-sensitized anatase (orange solid trace) electrodes.
Catalysts 15 00739 g007
Catalysts 15 00739 g008
Figure 8. Current–time curves of anatase and β-carotene-sensitized anatase electrodes under alternating light and dark conditions.
Figure 8. Current–time curves of anatase and β-carotene-sensitized anatase electrodes under alternating light and dark conditions.
Catalysts 15 00739 g008
Catalysts 15 00739 g009
Figure 9. CV curves of anatase and β-carotene-sensitized anatase electrodes under illumination.
Figure 9. CV curves of anatase and β-carotene-sensitized anatase electrodes under illumination.
Catalysts 15 00739 g009
Catalysts 15 00739 g010
Figure 10. EIS curves of pristine anatase and β-carotene-sensitized anatase electrodes under dark and illuminated conditions.
Figure 10. EIS curves of pristine anatase and β-carotene-sensitized anatase electrodes under dark and illuminated conditions.
Catalysts 15 00739 g010
Catalysts 15 00739 g011
Figure 11. DRS spectra of anatase, β-carotene, and β-carotene-sensitized anatase.
Figure 11. DRS spectra of anatase, β-carotene, and β-carotene-sensitized anatase.
Catalysts 15 00739 g011
Catalysts 15 00739 g012
Figure 12. Schematic diagram of synergistic enhancement of solar response characteristics by anatase and β-carotene.
Figure 12. Schematic diagram of synergistic enhancement of solar response characteristics by anatase and β-carotene.
Catalysts 15 00739 g012
Table 1. Current of anatase and β-carotene-sensitized anatase electrodes under light and dark conditions.
Table 1. Current of anatase and β-carotene-sensitized anatase electrodes under light and dark conditions.
SampleDark Current
(mA)		Light Current
(mA)		Photocurrent
(mA)		Increasement
Ant0.0490.0700.021-
Ant-caro10.0610.1200.05964.4%
Ant-caro20.0590.1210.06266.1%
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Share and Cite

MDPI and ACS Style

Ge, X.; Ding, H.; Liu, T.; Du, Y.; Lu, A. Visible Light Activation of Anatase TiO2 Achieved by beta-Carotene Sensitization on Earth’s Surface. Catalysts 2025, 15, 739. https://doi.org/10.3390/catal15080739

AMA Style

Ge X, Ding H, Liu T, Du Y, Lu A. Visible Light Activation of Anatase TiO2 Achieved by beta-Carotene Sensitization on Earth’s Surface. Catalysts. 2025; 15(8):739. https://doi.org/10.3390/catal15080739

Chicago/Turabian Style

Ge, Xiao, Hongrui Ding, Tong Liu, Yifei Du, and Anhuai Lu. 2025. "Visible Light Activation of Anatase TiO2 Achieved by beta-Carotene Sensitization on Earth’s Surface" Catalysts 15, no. 8: 739. https://doi.org/10.3390/catal15080739

APA Style

Ge, X., Ding, H., Liu, T., Du, Y., & Lu, A. (2025). Visible Light Activation of Anatase TiO2 Achieved by beta-Carotene Sensitization on Earth’s Surface. Catalysts, 15(8), 739. https://doi.org/10.3390/catal15080739

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop