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Abstract
Material with low reflectance in the ultra-broadband range of UV-Vis-IR is strongly desired for advanced optical instruments. Herein, in situ one-step hydrothermal methods were first introduced to prepare nanostructured spinel films on a low-priced Fe-Cr alloy substrate. The films show a superior low-reflectance feature of 4.71% in 300–2500 nm and 6.16% in 2500–13000nm, which attribute to its uniform nano-micron pores distributed on the surface. Besides, they can tolerate 180 °C due to strong film adhesion stemming from in situ growth. The preparation process is convenient, low-cost, non-poisonous, pollution-free, and suitable for quantity production. Moreover, it unnecessarily requires any template, catalyst, or expensive apparatuses. The resultant spinel films are a promising candidate for developing optical instruments with higher accuracy in the ultra-broadband range.
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1. Introduction
Low-reflectance materials are the core component of optical instruments. Especially, it is a challenge and urgent need to prepare low-reflectance materials in ultra-broadband wavelength, due to that such materials can help optical instruments achieve high accuracy in ultra-broadband wavelength by reducing stray light and reflected light of the inner cavity in UV-Vis-IR wavelength [1,2].
Recently, many nano-micro films were designed to obtain ultra-broadband low-reflectance materials by spraying [3], etching [4], chemical vapor deposition (CVD) [5], solution process [6] and so on. Such as, passivation black silicon surfaces by CVD exhibited less than 4% reflection in 430–1020 nm [5]. The ZnO nanorods by solution process exhibited average reflectance of ∼ 9.5% in 400–1000 nm [6]. However, the low-reflection wavelength region of above materials cannot cover UV-Vis-IR. In order to address these challenges, our research group has also carried out several exploration. For example, CuO nanowire arrays were prepared by air thermal oxidation, acquiring ultra-low reflectance namely 0.078%, but only in 200-700 nm wavelength [7].
In this work, We designed a microstructure film with nano-micron particles stacking and multi-stage nano-micron pores to achieve light-trap effect in ultra-broadband region. The above structural films were prepared in situ by one-step hydrothermal method on low-priced Fe-Cr alloy substrate firstly. The films successfully achieve low-reflectance of 4.71% in 300–2500 nm and 6.16% in 2500–13000 nm due to nano-micron pores and nano effect. Besides, they can tolerate 180 °C due to strong film adhesion stemming from in situ growth. The films provide convenient, low-cost and operable method for the preparation of ultra-broadband low-reflectance materials. The films are promising to helping optical instruments enhance accuracy in UV-Vis-IR [7,8].
2. Experimental
The Fe-Cr alloy substrate were placed into Teflon-lined autoclaves along with KOH solution. Then the autoclaves were subjected to oven for 48 h at 150°C, 200 °C or 250°C, respectively. Finally, the low-reflectance spinel films could be obtained after ultrasound, washing and drying treatment.
The XRD was operated by a Rigaku D/Max 2550 diffractometer with a monochromator using CuKα radiation (λ = 1.5418 Å) to identify the crystal structures of spinel films. The EDS and morphologies of the spinel films were detected using SEM on a Helios NanoLab 600 I from FEI Company. Reflectance in 300 - 2500 nm wavelength was measured by a Perkin-Elmer Lambda 950 UV / vis / NIR double beam spectrophotometer equipped with an integrating sphere (150 mm). Reflectance in 2500 – 13000nm wavelength were measured by Bruker sensor II Fourier infrared spectrometer of Brooke company. The XPS was measured by X-ray photoelectron spectrometer of American thermoelectric company.
3. Results and discussion
The SEM, XRD, EDS and XPS were used to comprehensively study the surface morphology, crystal structure, composition and metal valence of the spinel films. In addition, the reflection curves of the spinel films in the range of 300-2500nm and 2500-130000 nm were also characterized to study the low-reflectance performance.
3.1 SEM analysis
In this experiment, samples a - l were prepared by adjusting the reaction temperature, time and alkali concentration, respectively.
For the SEM (Fig. 1–3), from sample a to sample l, with the increase of the KOH solution concentration, the average size of spinel crystal particles decreases from 2.51 µm to 0.61 µm, the crystal particles on film surface stack looser and the number of the nano-micro pores becomes more.
Fig. 1. The SEM of the spinel film surfaces: sample a (150°C, 2d, 5 mol/L), sample b (150°C, 2d, 7.5 mol/L), sample c (150°C, 2d, 10 mol/L), sample d (150°C, 2d, 12.5 mol/L), respectively. Scale bar equals 1 µm.
Fig. 2. The SEM of the spinel film surfaces: sample e (200°C, 2d, 5 mol/L), sample f (200°C, 2d, 7.5 mol/L), sample g (200°C, 2d, 10 mol/L), sample h (200°C, 2d, 12.5 mol/L), respectively. Scale bar equals 1 µm.
Fig. 3. The SEM of the spinel film surfaces: sample i (250°C, 2d, 5 mol/L), sample j (250°C, 2d, 7.5 mol/L), sample k (250°C, 2d, 10 mol/L), sample l (250°C, 2d, 12.5 mol/L), respectively. Scale bar equals 1 µm.
From sample a to sample d prepared at 150 °C (Fig. 1), the spinel grains on the film surface are closely stacked, and the average particle size of the crystals varies from 2.51 µm to 0.88 µm.
From sample e to sample h prepared at 200 °C (Fig. 2), the average spinel grain size on the film surface vary from 2.07 µm to 0.82 µ m. And the stacking of grains is becoming looser, and the number of nanomicro voids is gradually increasing.
From sample i to sample l prepared at 250 °C (Fig. 3), the average spinel grain size on the film surface vary from 1.14 µm to 0.61 µm. The nano-micro pores on the film surface are intensive obviously, gradually forming a three-dimensional, multi-scale arrangement, being conducive to the light absorption [7,9].
3.2 XRD analysis
For the XRD of the spinel film sample a - l (Fig. 4–6), 64.6° corresponds to the characteristic peak of stainless-steel substrate. 29.9°, 35.2°, 36.8°, 42.8°, 53.2°, 56.7° and 62.3° respectively correspond to (220) (311) (222) (400) (422) (511) (440) characteristic peaks of cubic spinel phase (JCPDS: 653107) [10]. In hydrothermal reaction, from 150 °C to 250°C, with the increase of alkali concentration, the characteristic peak intensity of stainless-steel substrate gradually decreases, and the characteristic peak intensity of spinel gradually increases, showing good crystallinity of spinel particles.
Fig. 4. The XRD of the alloy substrate and the spinel film samples: sample a (red) (150°C, 2d, 5 mol/L), sample b (blue) (150°C, 2d, 7.5 mol/L), sample c (pink) (150°C, 2d, 10 mol/L), sample d (green) (150°C, 2d, 12.5 mol/L).
Fig. 5. The XRD of the alloy substrate and the spinel film samples: sample e (red) (200°C, 2d, 5 mol/L), sample f (blue) (200°C, 2d, 7.5 mol/L), sample g (pink) (200°C, 2d, 10 mol/L), sample h (green) (200°C, 2d, 12.5 mol/L).
Fig. 6. The XRD of the alloy substrate and the spinel film samples: sample i (red) (250°C, 2d, 5 mol/L), sample j (blue) (250°C, 2d, 7.5 mol/L), sample k (pink) (250°C, 2d, 10 mol/L), sample l (green) (250°C, 2d, 12.5 mol/L).
3.3 EDS analysis
The EDS results prove that the alloy substrate is composed of Fe and Cr elements, whose proportion is 86.92% and 13.08%, respectively. The atomic average ratio of O, Cr and Fe of the spinel film is 58.02%, 4.15%, and 37.83% (Fig. 7), respectively. It is basically consistent with the results of ICP.
Fig. 7. The EDS of the alloy substrate (a) and the spinel film samples (b). The EDS of spinel film samples obtained at 150°C and 250°C are similar to that obtained at 200°C.
3.4 XPS analysis
From the XPS of 2p1/2 and 2P3/2 spin orbit double peaks, it is certain that Cr remains + 3, Fe owning mixed valence states of + 2 and + 3 [10] (Fig. 8(a), 8(c)). After 180°C heat treatment for 1 day, the valence states of Cr and Fe on spinel film remain unchanged (Fig. 8(b), 8(d)). The presence of mixed valent transition metal is conducive to the light absorption of spinel films.
Fig. 8. The XPS of the spinel film samples. (a) and (c) are the XPS of spinel films before 180°C heat treatment; (b) and (d) are the XPS of spinel films after 180°C heat treatment. The XPS of spinel film samples obtained at 150°C and 250°C are similar to that obtained at 200°C.
3.5 Reflectance analysis in 300-2500 nm
The arithmetic average reflectance of sample a - sample l in 300-2500 nm are 12.85% - 4.71% (Table 1, Fig. 9– Fig. 12), respectively. With the increase of reaction temperature and KOH concentration, the low-reflectance performance of spinel films increases gradually. Among all samples, sample l obtains the best low-reflectance, i.e. 4.71% in 300-2500 nm, due to that the spinel film surface of sample l is rich in micro-nano particles and multilevel micro-nano pores [7–11].
Fig. 9. The reflectance of the spinel films in NUV-vis-NIR (300 - 2500 nm): sample a (150°C, 2d, 5 mol/L), sample b (150°C, 2d, 7.5 mol/L), sample c (150°C, 2d, 10 mol/L), sample d (150°C, 2d, 12.5 mol/L), respectively.
Fig. 10. The reflectance of the spinel films: sample e (200°C, 2d, 5 mol/L), sample f (200°C, 2d, 7.5 mol/L), sample g (200°C, 2d, 10 mol/L), sample h (200°C, 2d, 12.5 mol/L), respectively.
Fig. 11. The reflectance of the spinel films: sample i (250°C, 2d, 5 mol/L), sample j (250°C, 2d, 7.5 mol/L), sample k (250°C, 2d, 10 mol/L), sample l (250°C, 2d, 12.5 mol/L), respectively.
Fig. 12. The reflectance measured in 300 – 2500 nm of the spinel films and heated spinel films of sample a - l, respectively.
Table 1. The average reflectance in 300 - 2500 nm wavelength of the spinel film samples prepared at different conditions.
After heat treatment in air at 180 °C for 1 d, the average arithmetic reflectance of sample a - sample l in 300-2500 nm were 13.34% - 4.91%, respectively. Compared with that before heat treatment, the heat-treated sample a - l have little loss in low-reflectance in 300-2500 nm, not more than 1%, maintaining excellent thermal stability (Fig. 12, Figure S1-S3 and Table 1).
3.6 Reflectance analysis in 2500-13000 nm
The arithmetic average reflectance of sample a - sample l in 2500–13000nm, are 23.04% - 6.16% (Table 2, Fig. 13–Fig. 16), respectively. With the increase of reaction temperature and KOH concentration, the low-reflectance performance of spinel films increases gradually.
Fig. 13. The reflectance of the spinel films in mid-IR (2500-13000nm): sample a (150°C, 2d, 5mol/L), sample b (150°C, 2d, 7.5mol/L), sample c (150°C, 2d, 10mol/L), sample d (150°C, 2d, 12.5mol/L), respectively.
Fig. 14. The reflectance of the spinel films in mid-IR (2500-13000 nm): sample e (200°C, 2d, 5 mol/L), sample f (200°C, 2d, 7.5 mol/L), sample g (200°C, 2d, 10 mol/L), sample h (200°C, 2d, 12.5 mol/L), respectively.
Fig. 15. The reflectance of the spinel films in mid-IR (2500-13000 nm): sample i (250°C, 2d, 5 mol/L), sample j (250°C, 2d, 7.5 mol/L), sample k (250°C, 2d, 10 mol/L), sample l (250°C, 2d, 12.5 mol/L), respectively.
Fig. 16. The reflectance measured in 2500-13000 nm of the spinel films and heated spinel films of sample a - l, respectively.
Table 2. The average reflectance in 2500 - 13000 nm wavelength of the spinel film samples prepared at different conditions.
After heat treatment in air at 180 °C for 1 d, the average arithmetic reflectance of sample a - sample l in 2500 – 13000nm were 21.37% - 6.70%, respectively. Compared with that before heat treatment, the heat-treated sample a - l have little loss in low-reflectance, not more than 2%, maintaining excellent thermal stability (Fig. 16, Fig. S4-S6 and Table 2).
Compared with the reflectance of alloy substrate, namely 64.2% in 300-2500 nm and 87.03% in 2500 – 13000nm, it is found that the spinel film samples prepared by in situ hydrothermal method successfully achieve significant improvement of ultra-broadband low-reflectance performance (Figure S7-S8). Among all samples, sample l obtains the best low-reflectance, i.e. 4.71% in 300-2500 nm and 6.16% in 2500 – 13000nm, due to that the spinel film surface of sample l is rich in micro-nano particles and multilevel micro-nano pores [7–11]. More importantly, due to in situ growth, the spinel films have excellent thermal stability and outstanding low-reflection performance in ultra-wide wavelength after heat treatment. The spinel film will greatly help to improve the precision of spectrophotometer, infrared spectrometer, telescope, camera and other optical instruments, by reducing the reflected light and stray light of the inner cavity in UV-Vis-IR wavelength.
3.7 Ultra-broadband low-reflectance theoretical analysis
Herein, the ultra-broadband low-reflectance principle of the spinel films is analyzed as follows: For the spinel film which is composed of multiscale nano-micro particles and pores, the multilevel nano-micro pores on spinel film can produce ultra-broadband light-trap effect for the incident light in UV-Vis or IR wavelength [7–16]. As the Fe-Cr-O spinel particle is an excellent light absorber, therefore every light reflection is accompanied by light absorption, eventually multiple reflections produce high absorption (Fig. 17(a)). In addition, the nano particles on the spinel film surface have many grain boundaries, dangling bonds, interfacial polarization and interfacial atomic surface area, making light effectively converse into heat [15–18]. However, for the spinel film whose surface consists of large crystal blocks (Fig. 17(b)), when the incident light arrives at the interface of the film and air, it will be strongly reflected according to the refractive index mutation from the air to the spinel crystal, which is averse to low reflectance.
Fig. 17. Schematic diagrams illustrating the ultra-broadband low-reflectance of the spinel films: (a). For the spinel film which is composed of multiscale nano-micro spinel particles and pores. (b). For the spinel film whose surface consist of densely packed crystal blocks.
4. Summary
In conclusion, in situ one-step hydrothermal method was firstly introduced to prepare nano-micro structured spinel films on low-priced Fe-Cr alloy substrate. The preparation process is convenient, low-cost, non-poisonous, pollution-free and suitable for quantity production. What's more, it unnecessarily requires any template, catalyst, or expensive apparatuses. The spinel films successfully achieve low-reflectance of 4.71% in 300 - 2500 nm and 6.16% in 2500 – 13000nm due to multiscale nano-micron pores. Besides, they can tolerate 180 °C due to strong film adhesion stemming from in situ growth. The spinel films are promising to helping optical instruments enhance accuracy in UV-Vis-IR. It provides a new strategy for the preparation of low-priced low-reflection films in ultra-broadband region.
Funding
the Project on Experimental Technique of Jilin University (409020720202); the Scientific Research Project in the Education Department of Jilin Province (JJKH20211044KJ); National Natural Science Foundation of China (21801090).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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