Enhanced visible light photocatalytic activity of Fe₂O₃ modified TiO₂ prepared by atomic layer deposition

Abstract

In this work, commercial anatase TiO₂ powders were modified using ultrathin Fe₂O₃ layer by atomic layer deposition (ALD). The ultrathin Fe₂O₃ coating having small bandgap of 2.20 eV can increase the visible light absorption of TiO₂ supports, at the meantime, Fe₂O₃/TiO₂ heterojunction can effectively improve the lifetime of photogenerated electron–hole pairs. Results of ALD Fe₂O₃ modified TiO₂ catalyst, therefore, showed great visible light driven catalytic degradation of methyl orange compared to pristine TiO₂. A 400 cycles of ALD Fe₂O₃ (~ 2.6 nm) coated TiO₂ powders exhibit the highest degradation efficiency of 97.4% in 90 min, much higher than pristine TiO₂ powders of only 12.5%. Moreover, an ultrathin ALD Al₂O₃ (~ 2 nm) was able to improve the stability of Fe₂O₃-TiO₂ catalyst. These results demonstrate that ALD surface modification with ultrathin coating is an extremely powerful route for the applications in constructing efficient and stable photocatalysts.

Introduction

A rapid industrial development driven by unsustainable technology advances can cause plenty of industrial sewage, spreading chemical hazards into water resources. As a result, water pollution has emerged as one of the most serious environmental issues worldwide^1–4. Photocatalytic oxidation technology has shown great prospects in removing the toxic and harmful contaminants in aqueous environment^5–7. Semiconductors (e.g. TiO₂, ZnO, SnO₂) have been widely researched for organic pollutant degradation, however, the large band gap hinders their practical applications^8–12. For example, TiO₂ with band gap of 3.2 eV can only absorb the ultra-violet light, accounting for only 4–5% of entire solar spectrum¹³. Therefore, various visible light sensitive photocatalysts has also been widely explored, such as g-C₃N₄, BiVO₄, CdSe, Bi₂WO₆^14–19. On the other hand, TiO₂ is recognized as one of the excellent materials owning to its good inertness, eco-friendly, low cost, strong oxidizing power, and long-term stability against photo and chemical corrosion^9,13,20–22. Thus, plenty of works have been made to extend the absorption spectrum of TiO₂ to visible light so to make a full use of solar spectrum. Several different approaches can be employed, including doping^23–26 and coupling with small band gap semiconductors or metals^27–30.

Small band gap semiconductors not only increase the absorption of visible light but also inhibit photo-generated electrons-holes recombination when constructed as a semiconductor/semiconductor heterojunction structure, thus improving the photocatalytic performance dramatically³¹. Therefore, various TiO₂ based heterojunction photocatalysts have been proposed for visible light photocatalysis, including NiO/TiO₂^32,33, Ag₂O/TiO₂³⁴, CdTe/TiO₂³⁵, C₃N₄/TiO₂³⁶, Bi₂O₃/TiO₂³⁷, Cu₂O/TiO₂³⁸, Fe₂O₃/TiO₂³⁹, etc. For Fe₂O₃/TiO₂ heterojunction photocatalysts, a variety of composites have been investigated, such as Fe₂O₃ nanoparticles on TiO₂ nanotube⁴⁰, Fe₂O₃/TiO₂ nanoparticles⁴¹, TiO₂ coated cubic Fe₂O₃⁴², Fe₂O₃ nanosheet/TiO₂ hollow sphere³⁹, and Fe₂O₃ coated TiO₂⁴³. For instance, Lin et al. demonstrated that Fe₂O₃ coating can effectively enhance the visible light photocatalytic activity of TiO₂⁴³. Various fabrication methods were applied, including hydrothermal or solvothermal process and sol–gel, to prepare the heterojunction photocatalysts^44–46. Nevertheless, precise control of the interface between Fe₂O₃ and TiO₂ at atomic level by conventional methods remain challenges.

Atomic layer deposition (ALD) is a unique and promising thin film deposition technique based on self limited and saturated surface chemisorption reactions. It can deposit ultrathin, conformal, and uniform layers at sub-nanometer scale, which has attracted great attentions in surface engineering of nanostructures over the years^47–49. In catalysts design, ALD enables a conformal layer with precise thickness control and tunable film composition onto another nanostructures with high aspect ratio⁵⁰. The ALD coating can work as photo-active materials^51,52 or surface protection layer^53,54. Herein, we modified the commercial anatase TiO₂ powders with ultrathin Fe₂O₃ surface coating by ALD. The photocatalytic performance was investigated by visible light degradation of methyl orange (MO). The ultrathin Fe₂O₃ coating can enhance the absorption of TiO₂ supports for visible light. Fe₂O₃ modified TiO₂ powders show much better visible light photocatalytic degradation of MO than pristine TiO₂. A possible mechanism for improved photocatalytic performance is proposed. In addition, an ultrathin ALD Al₂O₃ (~ 2 nm) was used to promote the long-term durability of TiO₂@Fe₂O₃ catalyst.

Methods

ALD deposition on TiO₂ powders

Commercial TiO₂ powders with anatase phase (Nanjing Haitai nano materials Co.) were used as supports in this work. Ferrocene (Fe(Cp)₂, Suzhou Fornano Corporation Ltd., 99.99%) and ozone were adopted as Fe and oxygen precursors for ALD Fe₂O₃ deposition. Fe(Cp)₂ was vaporized at 85 °C. High purity nitrogen gas (N₂, 99.999%) was used as carrier gas at a total flow rate of 750 sccm and a pressure of 6 hPa in our ALD system (Picosun SUNALE™ R-150B). A particular container with porous mesh was used for ALD coating on powders, which has been reported elsewhere^24,55, as shown in Fig. 1. Herein, precursors can flow through the TiO₂ powders to achieve great conformality. X cycles of ALD Fe₂O₃ (X = 200, 400, 600, and 800) were coated on TiO₂ powder at 300 °C, the samples are marked as TiO₂@X-Fe₂O₃. One cycle of ALD Fe₂O₃ contains the following four steps, 5 s Fe(Cp)₂ injection, 20 s N₂ purge, 5 s O₃ injection, and 20 s N₂ purge. At the same system, 20 cycles of Al₂O₃ were deposited on TiO₂@400-Fe₂O₃ at 300 °C, where one ALD cycle of Al₂O₃ is consisted of 5 s trimethylaluminum dose, 20 s N₂ purging, 5 s H₂O dose, and 20 s N₂ purging.

Figure 1.

The schematic diagram of coating TiO₂ powders by ALD Fe₂O₃.

Materials characterizations

X-ray diffraction (XRD) using a Rigaku-D/MAX 2000 system was used for crystallinity and phase structure analysis. Scanning electron microscopy (SEM) images were taken using ZEISS Gemini SEM 500 instrument operated at 2 kV. The high-resolution transmission electron microscopy (HRTEM) was performed on a FEI Tecnai F20 S-Twin to observe the microstructures, where TiO₂ powders were loaded on the ultra-thin carbon coated copper grids. The surface chemical features and valence band spectra were explored by X-ray photoelectron spectroscopy (XPS) using Thermo Fisher K-Alpha. The adventitious carbon signal (C 1 s = 284.6 eV) was adopted to calibrate the binding energies. UV–visible absorption spectra were conducted on a UV–vis-NIR spectrophotometer (UV-3600, Shimadzu, Japan). Photoluminescence (PL) spectra were collected on a Horiba Jobin Yvon HR800 spectrometer.

Photocatalytic degradation

The photocatalytic performance of Fe₂O₃ coated TiO₂ catalysts was investigated by visible light degradation of MO. 100 ml MO solution (4 mg L⁻¹) with 100 mg photocatalysts were loaded into a glass reactor, which was magnetically stirred at 500 rpm. In order to establish the adsorption/desorption equilibrium between MO and catalysts before irradiation, the MO solutions with catalysts were magnetically stirred for 30 min in darkness. Then, the suspension was irradiated under a 300 W Xe lamp (MircoSolar300, Perfect Light). A 420 nm filter was adopted to cut off UV light. The lamp was placed at 15 cm above the suspension, whose average visible light intensity is around 80 mW cm⁻². Water cooling was applied throughout the experiment to maintain the temperature at 25 ± 1 °C. 3 mL solution was collected after each 15 min irradiation. The photocatalysts were removed by centrifugal separation. The residual MO concentration was determined using the absorption at 464 nm by UV–Vis–NIR spectrophotometer. The recycled usage experiment was performed for three times to explore the long-term stability of photocatalysts, e.g. after each photocatalytic experiment, the photocatalysts powders were gathered and rinsed by ethanol and water, then baked for 12 h at 100 °C. At last, a new MO solution was used to evaluate the photocatalytic activity of collected photocatalysts.

Photoelectrochemical measurements

Photoelectrochemical measurements were performed in a three-electrode electrochemical cell at room temperature using 1 M Na₂SO₄ as the electrolyte. The TiO₂ or TiO₂@400-Fe₂O₃ on FTO were used as the working electrode. A Pt wire and Ag/AgCl were used as the counter electrode and the reference electrode, respectively. Photocurrent densities were collected by an electrochemistry workstation (CHI660E, Shanghai) using a potentiostatic method at 0.50 V. Light was chopped on and off cyclically. A solar simulator (300 W Xe lamp, MircoSolar300, Perfect Light) with a 420 nm cut-off filter provides the visible-light irradiation.

Results

Figure 2 depicts the XRD patterns of pristine TiO₂ and Fe₂O₃ coated TiO₂ powders. All the samples show the similar characteristic diffraction peaks, in accord with anatase TiO₂ (JCPDS No. 21–1,272). This result indicates that ultrathin ALD Fe₂O₃ would not affect the crystal structure of TiO₂, consistent with our previous finding^24,55. In addition, signals related to Fe₂O₃ were absent.

Figure 2.

XRD patterns of pristine TiO₂ and Fe₂O₃ coated TiO₂.

SEM images of TiO₂ powders without and with 400 cycles ALD Fe₂O₃ deposition are shown in Fig. 3a,b. It can be seen that the pristine TiO₂ powder exhibits a disk-like morphology with a diameter of approximately 40 nm and a thickness of approximately 10 nm, with severe aggregation. After ALD Fe₂O₃ deposition, it was observed that Fe₂O₃ coated TiO₂ exhibited almost identical morphology, indicating that ultra-thin Fe₂O₃ coating did not have significant influence on particle size and morphology of TiO₂. HRTEM was further conducted to observe the microstructure of TiO₂@400-Fe₂O₃ and pristine TiO₂ powders as shown in Fig. 3c,d. While pristine TiO₂ shows a sharp well-ordered surface with good crystallinity (Fig. 3c), an amorphous layer of ~ 2.6 nm formed on the TiO₂ surface was observed which was contributed to ultra-thin Fe₂O₃ layer (400 cycles) formed by ALD deposition. Moreover, both samples exhibit the lattice spacing of 0.35 nm, corresponding to (101) planes of anatase TiO₂. Based on above XRD and TEM data, it was speculated that an ultra-thin amorphous Fe₂O₃ is coated on TiO₂ nanoparticles surface without significantly modifying the morphology of the catalyst supports.

Figure 3.

SEM images of (a) pristine TiO₂ and (b) TiO₂@400-Fe₂O₃. TEM images of (c) pristine TiO₂ and (d) TiO₂@400-Fe₂O₃.

To further determine the successful deposition of ALD Fe₂O₃, XPS was performed to characterize the surface chemical features of ALD Fe₂O₃ coated TiO₂ powders. Figure 4a shows the Ti 2p spectra of TiO₂@400-Fe₂O₃ and pristine TiO₂ powders. The spectra can be fitted into two peaks at 464.3 eV and 458.5 eV, which can be assigned to Ti 2p_1/2 and Ti 2p_3/2 peaks²⁸. In O 1 s spectra (Fig. 4b), both samples present the main peak at 529.9 eV related to Ti–O bonding from TiO₂. In addition, there is a peak at 531.7 eV can be ascribed to the surface -OH⁵⁶. Figure 4c exhibits the Fe 2p spectrum of TiO₂@400-Fe₂O₃, Fe 2p_1/2 and Fe 2p_3/2 peaks locate at 723.8 eV and 710.5 eV, in accord with Fe–O bonding value in Fe₂O₃⁵⁷. Based on XPS data, the Fe atom ratio (Fe/Fe + Ti) is determined to be 1.1% in TiO₂@400-Fe₂O₃. It is anticipated that an ultra-thin amorphous Fe₂O₃ is coated on TiO₂ nanoparticles surface successfully. Due to the low content of Fe₂O₃, Fe 2p spectrum shows a bad signal to noise ratio. Therefore, the Fe 2p spectrum of ALD Fe₂O₃ film is also presented for reference, as shown in Fig. 4d which can exhibit much better signal to noise ratio.

Figure 4.

XPS spectra of (a) Ti 2p and (b) O 1s for TiO₂ and TiO₂@400-Fe₂O₃, Fe 2p XPS spectra for (c) TiO₂@400-Fe₂O₃ and (d) Fe₂O₃ film.

The influence of ultra-thin Fe₂O₃ coating on absorption of TiO₂ powders in visible light was explored using UV–Vis diffuse reflectance spectra, as illustrated in Fig. 5a. The spectrum of pristine TiO₂ powders is also plotted for comparison. Pristine TiO₂ powders exhibit the absorption edge of around 371 nm without noticeable visible light absorption. However, noticeable absorption in the visible light region from 390 to 750 nm can be observed after ultra-thin Fe₂O₃ surface modification. The relationship of the absorption edge with the photon energy (hν) for the indirect bandgap semiconductor is shown in the following formula: (αhν)^1/2 = A(hν-E_g), where α and A are the absorption coefficient and absorption constant, respectively. Since the absorption coefficient α is determined by the scattering and reflectance spectra based on Kubelka–Munk theory, therefore, the bandgap values can be determined by the intercept of the tangent lines. As depicted from Fig. 5b, only one tangent line can be extrapolated for pristine TiO₂ powders showing a bandgap of 3.25 eV, while two bandgap values can be obtained from the plots for TiO₂@400-Fe₂O₃ powders, attributing to Fe₂O₃ coating with a bandgap value of ~ 2.20 eV and TiO₂ supports with a bandgap value of ~ 3.08 eV. It can be concluded that ultra-thin Fe₂O₃ coating results in a smaller bandgap which can increase the absorption of TiO₂ powder support in visible light.

Figure 5.

(a) UV–Vis diffuse reflectance spectra and (b) Tauc plot of TiO₂ with and without 400 cycles Fe₂O₃ coating.

The visible light photocatalytic activity of TiO₂ and Fe₂O₃ coated TiO₂ catalysts was compared by degrading MO. All samples exhibit low adsorption capacity of MO molecules. As reported in our previous work, MO is selected here for its stability under visible light irradiation in the absence of catalysts²⁴. Figure 6a shows the evolution of UV–vis absorption spectra of MO solution in the presence of pristine TiO₂ under visible light irradiation. It can been seen that the absorption peaks at 464 nm decreases slightly after 90 min, exhibiting very poor photocatalytic activity due to its large bandgap. In contrast, the peak intensity at 464 nm fades rapidly for TiO₂@400-Fe₂O₃ with the irradiation time extending, as shown in Fig. 6b. In addition, the orange MO solution turns into colorless after 90 min, as presented in the insert of Fig. 6b, indicating the degradation of MO. Figure 6c compares visible light photocatalytic activity of Fe₂O₃ coated TiO₂ catalysts. It can be found that a much-improved photocatalytic degradation efficiency of ~ 86.2% can be achieved with only 200 cycles of ALD Fe₂O₃ modification. And the TiO₂@400-Fe₂O₃ photocatalysts display the highest photocatalytic degradation efficiency of 97.4%. In comparison with reported Fe₂O₃-TiO₂ heterojunction catalysts for photodegradation of organic pollutants and antibiotics, ALD Fe₂O₃ coated TiO₂ (TiO₂@400-Fe₂O₃) in this work exhibit excellent removal efficiency for MO degradation, as summarized in Table 1. The photocatalytic degradation efficiency decreases to 95.8% and 90.4% for TiO₂@600-Fe₂O₃ and TiO₂@800-Fe₂O₃ samples, respectively, along with further increasing the thickness of ALD Fe₂O₃ coating. The reduced efficiency can be ascribed to the fact that more Fe₂O₃ coating would introduce more recombination sites for photoinduced electron–hole pairs⁴², diminishing the photocatalytic efficiency.

Figure 6.

UV–vis absorption spectra of MO exposed to different irradiation time in the presence of (a) pristine TiO₂ and (b) TiO₂@400-Fe₂O₃ catalysts under visible light irradiation. The inserts are the photos of MO solution before and after irradiation. (c) Visible light photocatalytic degradation curves of MO and (d) −In(C/C₀) vs. time curves by using TiO₂ with and without Fe₂O₃ coating as catalysts.

Table 1.

Comparison of photocatalytic activity for degradation of organic pollutants using TiO₂–Fe₂O₃ based catalysts.

Catalysts	Method	Power of Xe lamp (W)	Organic pollutants		Time (min)	D (%)	Ref
			type	C (mg L-1)
Fe2O3 decorated TiO2	calcination	350	MBa	3.2	80	64.5	58
Fe2O3-Doped TiO2	Sol–gel	500	MB	10	120	100	44
Fe2O3/TiO2 nanofibers	Electrospinning + calcination	800	RhBb	5	180	53.6	59
Fe2O3 decorated TiO2	hydrothermal	500	RhB	4.8	270	77.8	45
Fe2O3@SiO2@TiO2	vapor-thermal	300	RhB	4.8	60	100	46
Fe2O3 coated TiO2	solvothermal	300	TCc	50	90	100	39
Core–shell Fe2O3@TiO2	Precipitation	350	RhB	10	360	71.0	60
Core–shell TiO2@Fe2O3	hydrothermal	300	MO	10	16	96.6	43
Core–shell C@TiO2@Fe2O3	Impregnation	500	MB	20	240	80.8	61
Fe2O3/TiO2 composites	Impregnation	500	Orange II	20	240	53.4	62
Fe2O3 coated TiO2	ALD	300	MO	4	90	97.4	This work

^a MB is Methylene Blue, ^b RhB is Rhodamine B, ^c TC is Tetracycline.

The degradation data were also fitted by the pseudo-first-order kinetics. The rate constant k can be determined by In(C_t/C₀) = −kt at low initial pollutant concentration. Herein, C₀ is the initial MO concentration, while the C_t is the MO concentration after irradiation time of t. k is the first-order rate constant (min⁻¹). The −In(C_t/C₀) vs. t curves are plotted in Fig. 6d. It can be seen that −In(C_t/C₀) has a linear relationship with t, indicating the photocatalytic degradation of MO by Fe₂O₃ modified TiO₂ catalysts obeys the first-order kinetics. The first-order rate constant (k) is determined to be 3.8 × 10⁻² min⁻¹ for TiO₂@400-Fe₂O₃, which is much larger than pristine TiO₂ of 1.3 × 10^–3 min⁻¹. The result indicates that ALD Fe₂O₃ modification can effectively enhance the visible light photocatalytic performance of TiO₂ supports.

PL and photocurrent response measurements were conducted to explore the recombination rate of photo-generated electron–hole pairs. The lower the PL intensity, the higher separation efficiency of electron–hole pairs in the catalysts. Figure 7a shows the PL spectra of pristine TiO₂ and various cycles of ALD Fe₂O₃ coated TiO₂. It can be seen that all the Fe₂O₃ coated TiO₂ exhibit lower intensity than pristine TiO₂, indicating that the coupling of TiO₂ and Fe₂O₃ can effectively inhibit the recombination of the photo-generated electron–hole pairs. Moreover, it can be seen that the 400 cycles Fe₂O₃ coating results in the lowest intensity, thicker Fe₂O₃ coating would increase the recombination of the photo-generated electron–hole pairs. The PL data are consistent well with the results of photocatalytic degradation of MO. Figure 7b presents the photocurrent response curves of TiO₂ and TiO₂@400-Fe₂O₃, it can be seen that TiO₂@400-Fe₂O₃ exhibits a much higher photocurrent density than pristine TiO₂, indicating a more efficient separation of the photoexcited electron–hole pairs.

Figure 7.

(a) PL spectra and (b) photocurrent response curves of TiO₂ and Fe₂O₃ coated TiO₂.

The band alignment for Fe₂O₃ coated TiO₂ was determined by measuring the valence band offset ∆E_v (VBO) using XPS. Figure 8a shows VB spectra of TiO₂ and Fe₂O₃–TiO₂ determined by linear extrapolation method, respectively. The VB of TiO₂ and Fe₂O₃–TiO₂ are found to be 3.09 eV and 0.84 eV, respectively. The VB of Fe₂O₃ is higher than that of TiO₂, and ∆E_v at the interface of Fe₂O₃-TiO₂ is estimated to be 2.25 eV. The optical bandgaps of TiO₂ and Fe₂O₃ have been determined to be 3.25 and 2.20 eV, respectively, in Fig. 5. Therefore, the conduction band offset ∆E_c (CBO) at the interface of Fe₂O₃-TiO₂ is estimated to be 1.20 eV. Considering the band structure of TiO₂ vs. standard hydrogen electrode (NHE)^62–64, the energy band structure of Fe₂O₃ coated TiO₂ can be depicted in Fig. 8b. Under visible light, TiO₂ shows no photo-electronic response due to its large band gap. Only Fe₂O₃ can be excited, yielding photo-generated electron from its VB to CB. Due to the aligned equilibrium of Fermi level at the interface of TiO₂ and Fe₂O₃, as shown in Fig. 8b, the photogenerated electrons can transfer from CB of Fe₂O₃ to that of TiO₂ driven by the built-in electric field and the concentration gradient of electrons, while remaining the holes in VB of Fe₂O₃^60,62–64. Therefore, the separation efficiency of photoinduced electron–hole pairs can be improved, which has been demonstrated by PL and photocurrent response results in Fig. 7. There are a large number of literatures focusing on the photocatalytic activity of Fe₂O₃-TiO₂ composites catalyst^60,62–64. It is widely accepted that OH^. radicals can be formed via the reaction of water and photogenerated holes in VB of Fe₂O₃. And electrons in CB of TiO₂ can react with oxygen to form O₂^-. These radicals with high activities can degrade organic molecules into harmless substances.

Figure 8.

(a) Valence band spectra of TiO₂ and Fe₂O₃–TiO₂. (b) Schematic illustration of energy band structure of Fe₂O₃ coated TiO₂ and proposed charge transfer mechanism during visible light irradiation.

The stability of photocatalysts is one of the significant factors for practical applications, therefore, TiO₂@400-Fe₂O₃ and TiO₂@800-Fe₂O₃ catalysts were tested in recycling experiments of MO photodegradation. As shown in Fig. 9a, compared to the first usage, both TiO₂@400-Fe₂O₃ and TiO₂@800-Fe₂O₃ catalysts exhibit a declining degradation efficiency. The degradation efficiency retention compared to the first usage is only 59.6% and 66.7% for TiO₂@400-Fe₂O₃ and TiO₂@800-Fe₂O₃, respectively. It may be ascribed to the fact that stability of Fe₂O₃ during photocatalytic reactions is reduced due to photo-corrosion^42,65. ALD coatings have been widely used as surface protection layer to protect active materials from photo-corrosion^53,54. Therefore, an ultrathin Al₂O₃ (~ 2 nm) protective layer was deposited on catalysts surface by ALD in the same system to improve the stability of Fe₂O₃ coated TiO₂ catalysts. Figure 9b shows the photocatalytic degradation of MO using both TiO₂@400-Fe₂O₃ with Al₂O₃ passivation. The photocatalytic degradation efficiency decreases to 47.8% in 90 min, which is ascribed to the fact that Al₂O₃ coating with large band gap would also hinder the photogenerated carriers form migrating to the surface of electrode^62,66. But it is still much better than that of pristine TiO₂. More importantly, the stability of catalysts is persistent, as there is limited decline of degradation efficiency after three usage as shown in Fig. 8a, retaining 86.8% compared to the first usage. The results indicate that thin Al₂O₃ can act as a physical shell to protect Fe₂O₃ from photo-corrosion. This can be ascribed to that Al₂O₃ can prevent the direct contact between solution and Fe₂O₃.

Figure 9.

(a) Three cycles of MO degradation for TiO₂@400-Fe₂O₃, TiO₂@800-Fe₂O₃ and 20 cycles Al₂O₃ coated TiO₂@400-Fe₂O₃ in 90 min. (b) Comparison of MO degradation curves for pristine TiO₂, TiO₂@400-Fe₂O₃, and 20 cycles Al₂O₃ coated TiO₂@400-Fe₂O₃.

Conclusions

In summary, commercial anatase TiO₂ powders were modified using ultrathin ALD Fe₂O₃ surface coating. The ultrathin Fe₂O₃ layer with small bandgap of ~ 2.20 V can increase the absorption of TiO₂ supports in visible light. In addition, Fe₂O₃/TiO₂ heterojunction can suppress the photoinduced electron–hole pairs recombination. The above results indicate excellent visible light photocatalytic activity for the Fe₂O₃ modified TiO₂ powders. 400 cycles Fe₂O₃ (~ 2.6 nm) coated TiO₂ photocatalysts show excellent degradation efficiency of 97.4% in 90 min, far above the performance of pristine TiO₂ powders with only 12.5%. Moreover, an ultrathin Al₂O₃ (~ 2 nm) can improve the recycling usage performance of Fe₂O₃ coated TiO₂ catalyst effectively. As conclusions, ALD surface modification with ultrathin film is a promising route for improving the visible light activity and long-term stability of photocatalysts.

Author contributions

^#Y.Q.C. and T.Q.Z. contributed equally in this work. X.R.Z., T.Q.Z. and Y.Q.C. prepared samples and tested the photocatalytical performance. C.L. conducted the SEM test. C.L and Q.R. performed XPS characterization. J.B.F performed TEM test. Y.Q.C. drafted the manuscript. A.D.L. supervised the whole work. W.M.L. and A.D.L. revised the manuscript. All authors critically read and commented on the manuscript. All authors read and approved the final manuscript.

Competing interests

The authors declare no competing interests.