Efficient Photocatalysts Made by Uniform Decoration of Cu2O Nanoparticles on Si Nanowire Arrays with Low Visible Reflectivity
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Highly uniformed decorations of Cu2O nanoparticles on the sidewalls of silicon nanowires (SiNWs) with high aspect ratio were prepared through a two-step electroless deposition at room temperature. Morphology evolutions and photocatalytic performance of SiNWs decorated with aggregated and dispersed Cu2O nanoparticles were unveiled, and the correlated photodegradation kinetics was identified. In comparison with the conventional direct loadings where the aggregated Cu2O/SiNW structures were created, the uniform incorporation of Cu2O with SiNWs exhibited more than three and nine times of improved photodegradation efficiency than the aggregated-Cu2O/SiNWs and sole SiNWs, respectively.
KeywordsPhotocatalysts Silicon nanowires Electroless deposition Copper oxide
Field emission scanning electron microscopy
Transmission electron microscopy
Cu2O nanostructures, with direct band gap energy of 2.0–2.2 eV, have been emerged as the efficient photocatalytic materials that could decompose the organic pollutants directly through the activation of visible light [1, 2, 3, 4]. The benefited effects also correlated with their environmental acceptability, low toxicity, and sustainable availability, which could be potential for many practical applications including hydrogen production, solar cells, and chemical sensing [5, 6, 7]. Nevertheless, these features were limited by the usage instability and unrepeatability due to the intended aggregation or significant morphological changes of nanostructures upon operation within an aqueous environment. In this regard, the supporters that enabled to disperse the photoactive Cu2O nanostructures in a sable manner were considered to be highly desirable, which could maintain the long-term capability of photodegradation process of organic pollutants and represented the reproducible, efficient, and reliable platform for the practical requirement of photocatalytic operation.
Silicon nanowires (SiNWs), with good surface hydrophilicity, mechanical robustness, and chemical stability could be the potential supporting materials for strengthening the photocatalytic activity of Cu2O nanostructures, rendering a promising design of visible-responsive photocatalysts [8, 9, 10, 11, 12]. Moreover, SiNWs further possessed highly visible-absorption capability due to confining the incident lights through the effects of multiple scattering . However, the difficulties of processing lied in the deposition non-uniformity of Cu2O nanoparticles on the sidewalls of SiNWs using the inexpensive solution-based processing because of the nature of high aspect ratio in SiNWs. Therefore, in this study, such limitations were overcome by adapting the two-step electroless deposition in order to achieving uniform incorporation of Cu-oxide nanoparticles with the SiNW arrays. In addition, the controlled formation of heterostructural n-type Cu2O/p-type Si could be particularly potential to act as efficient photocatalysts  because the photo-excited electrons and holes might be separated to initiate photodegradation reaction of dye removal prior to rapid carrier recombination [14, 15]. Based on such designs, investigations of surface morphologies, chemical compositions, and crystallographic analysis were performed to characterize the synthesized Cu2O/Si nanostructures.
Next, light-reflection and photoluminescent properties of Cu2O/SiNW arrays were measured to identify the light properties and examine the influences of adding Cu2O nanoparticles on the decreased recombination of photogenerated carriers. In addition, photocurrent measurements were further performed to clarify the carrier separation of Cu2O/SiNW heterostructures under light illuminations. Finally, the detailed photocatalytic evaluations were performed, which clarified the efficient photocatalytic reactivity of degrading organic dyes and explained the involving photodegradation mechanism of such nanostructured photocatalysts.
The utilized Si substrate was p-type Si (100) made with Czochralski process. Silver nitrate (99.85%, Acros Organics, Geel, Belgium), hydrofluoric acid (48%, Fisher Scientific UK, Loughborough, UK), and nitric acid (65%, AppliChem PanReac, Germany) were used for the fabrication of Si nanowire arrays. CuSO4 (98+%, Acros Organic, Geel, Belgium) and hydrofluoric acid were used in the synthesis of Cu2O nanoparticles. Methylene blue (pure, Acros Organics, Geel, Belgium) was utilized for photodegradation test.
Fabrication of Si Nanowire Arrays
SiNW arrays were prepared by dipping the as-cleaned p-type Si (100) substrates in the mixed solutions (20 ml) containing 0.02 M of AgNO3 and 4.8 M of HF under a gentle magnetic stir at room temperature. Subsequently, the samples were rinsed with deionized (DI) water and then immersed in the concentrated nitric acid (63%) for 15 min in order to completely remove the residual Ag particles. Finally, the as-prepared SiNWs were rinsed with DI water and preserved in the vacuum chamber.
Synthesis of Cu2O Nanoparticles
Electroless deposition of Cu was made by two distinct methods. In method 1, the as-etched SiNWs were directly immersed in the aqueous solutions (20 ml) with 0.047 g (0.015 M) of CuSO4 powders and 4.5 M of HF for 3 min. This facilitated the reduction of Cu2+ ions preferentially at the primarily created Cu aggregates and therefore limited the well incorporation of Cu2O nanoparticles on SiNW arrays. The as-prepared samples were described as aggregated-Cu2O/SiNWs (A-Cu2O/SiNWs). In method 2, the as-prepared Si nanowires was dipped in 0.015 M of CuSO4 solutions (20 ml) for 15 min and followed by gently introducing HF (4.5 M) for initiating the reduction of Cu2+ ions. Through the separation of introducing Cu2+ ions and HF etchants in the sequential processes, each Cu2+/Si interface were activated for the electroless reduction of Cu2+ ions, which essentially led to the formation of well-dispersed Cu nanoparticles directly on SiNW arrays. The formed nanostructures prepared with method 2 were described as dispersed-Cu2O/SiNWs (D-Cu2O/SiNWs). After conducting the electroless deposition, all the fabricated samples were rinsed in DI water and then subjected to the oven at 90 °C for thermal oxidation (30 min).
Morphology and chemical compositions of as-prepared photocatalysts were characterized with field emission scanning electron microscopy (FESEM; Hitachi JSM-6390) and energy-dispersive X-ray (EDX) spectrometer (Oxford INCA 350), respectively. Prior to SEM investigation, the samples were deposited with a thin layer of Au to improve imaging resolution. Transmission electron microscope (TEM; JEM-2100F) was further performed to characterize the surface morphologies of samples. The samples were carefully scratched from substrates and dispersed in ethanol using an ultrasonicator, and then the dispersed solutions were dipped on the TEM grids. Crystallographic characterizations were performed with Rigaku Multiflex X-ray diffractometer using the Cu-K radiation. Light reflection spectra were measured with a UV-Vis-NIR spectrophotometer (Varian, Cary 5000, Australia). Photocatalytic experiments of various hybrid photocatalysts were conducted with a PanChum multilamp photoreactor (PR-2000) under the illuminations of light source with center wavelength of 580 nm. In each test, 0.2 mM of methylene blue (MB) was utilized as the tested targets. Prior to the light irradiations, the samples were placed in the dark for 40 min in order to establish the adsorption equilibrium, as presented in the Additional File 1. In photodegradation tests, at each time interval, 0.1 ml of suspension was withdrawn and then diluted with 5 ml distilled water. The concentrations of MB dyes were evaluated with UV/visible spectrophotometer (Shimadzu UV-2401 PC).
Results and Discussion
Thus, it resulted in the Cu aggregations on the top surfaces of SiNWs. The succeeding reduction of Cu2+ ions was preferentially occurred nearby these primarily formed Cu aggregates, where the effective transfer of holes from newly arrived Cu2+ ions into Si was facilitated, as presented in Fig. 1a. Meanwhile, the diffusion of Cu2+/HF reactants turned to hardly reach the bottom sides of SiNWs owing to the involved steric hindrance of the existed Cu nanoparticles. These combined effects might limit the possible formation of well decoration of Cu2O nanoparticles on SiNW arrays after experiencing the heat treatment, which therefore degraded their photocatalytic activity.
Figure 1b presented the possible electroless pathway that significantly reduced the aggregated formation of Cu nanoparticles which were locally confined on the nanowire tips. This was accomplished by separating the introductions of Cu2+ ions and HF etchants in the sequential processes. Accordingly, the incorporation of Cu2+ ions with SiNWs facilitated the uniform reduction of Cu2+ ions initiated by adding HF solutions, where each Cu2+/Si interface could be activated for the electroless deposition. Evidently, the distinct morphologies of Cu2O nanoparticles formed on SiNW arrays were presented in Fig. 1c, d, respectively. In comparison with the aggregated Cu2O nanoparticles dominantly at the nanowire tips (Fig. 1c), the incorporations of Cu2O nanoparticles with SiNW arrays prepared with two-step electroless deposition appeared to be highly uniformed throughout the sidewalls of nanowires, as shown in Fig. 1d. These features could be clearly observed from the high-magnification SEM image, as presented in the Additional File 1.
Nevertheless, the aggregated Cu2O/SiNW arrays possessed the high reflectivity covering the whole measured spectral regions (average reflectivity = 7.7%) due to the strong reflection of incoming lights directly from the Cu2O aggregates on SiNW tops, which could significantly degrade the effective lights/photocatalysts interaction. In addition, photoluminescent (PL) analysis was performed. It was found that all the samples demonstrated the PL peak centered at 522 nm, whereas the corresponding PL intensities of both D-Cu2O/SiNWs and A-Cu2O/SiNWs were much lower than that of sole SiNWs, as shown in Fig. 2d. These features implied that the recombination of photogenerated carriers was greatly reduced due to the introduction of Cu2O/SiNW heterostructures. On the other hand, compared with D-Cu2O/SiNWs, the slightly lower PL intensity was observed from A-Cu2O/SiNWs, which could be attributed to the strong reflection of incoming lights occurred at top Cu2O aggregates, thus decreasing the possibility of light emission. The possible interaction pathways between incoming lights and samples could be found in the Additional File 1. Moreover, the measurements of photocurrents were performed to clarify the carrier separation of Cu2O/SiNW heterostructures under light illuminations, as presented in Fig. 2e. It was found that the photocurrent enhancement, evaluated by Iphotocurrent − Idark current, was 0.216 mA (sole SiNWs), 0.527 mA (A-Cu2O/SiNWs), and 0.823 mA (D-Cu2O/SiNWs) at bias of 4 V. These results clearly supported our findings from light reflectivity and PL measurements, where the effective separation of photoexcited carriers occurred at D-Cu2O/SiNWs that contributed to the improved photocurrent enhancement.
To further unveil the reaction kinetics of dye degradation and evaluate the involving reaction constant, three possible kinetic models, including first-order kinetic model, second-order kinetic model, and Langmuir–Hinshelwood kinetic model were examined, as given below,
in which k1, k2, and k3 are the first-order, second-order, and Langmuir–Hinshelwood kinetic rate constants, respectively. Concentrations of dyes are denoted as C0 at reaction time = 0 and Ct at reaction time = t. In addition, KL represents the constant of Langmuir absorption equilibrium. The results were summarized in Fig. 4c, where the corresponding correlation coefficients (R2) were 0.82 in first-order kinetic model, 0.96 in second-order kinetic model, and 0.71 in Langmuir–Hinshelwood kinetic model.
These findings clearly verified that the photodegradation kinetics of MB dyes using Cu2O-decorated SiNW arrays corresponded to second-order kinetic model. Thus, the explicit reaction constant could be evaluated, indicating that the most pronounced value appearing in the D-Cu2O/SiNW arrays, as demonstrated in Fig. 4d. These effects explained the significant influences on the spatial incorporation of Cu2O with SiNW hosts, which could act as a decisive rule on dye removal. Based on the detailed investigations, the photodegradation of MB dyes in the presence of Cu2O/SiNW arrays could be elucidated, as illustrated in Fig. 4e. The photoexcited electrons and holes from highly light-absorptive SiNWs could be efficiently separated, where the photogenerated electrons were energetically transferred to the conduction band of Cu2O nanoparticles, thus initiating the photodegradation of MB dyes. Therefore, the uniform Cu2O dispersions facilitated such carrier separation that enabled to effectively reduce the electron-hole recombination. Moreover, these dispersed features also allowed the high-penetration depth of incoming lights irradiated on nanowire structures, thus being favorable for the efficiency improvement in photocatalytic process. Finally, we have performed the repeated photodegradation experiments, termed as first run, second run and third run, and the corresponding XRD measurements along with corresponding SEM images after each photodegradation test were presented in the Additional File 1: Figure S5 and S6, respectively. The above results further evidenced that the photocatalysts could stably and reliably possess the photodegradation of MB dyes.
In conclusion, we demonstrated an efficient visible-driven photocatalysts with a facile, inexpensive and reliable two-step electroless deposition for large-area production. These hybrid Cu2O/SiNW arrays with uniform Cu2O decorations were feasible for reducing the recombination of photogenerated carriers under visible-light irradiations. The investigations of photodegradation kinetics, along with facile synthetic process might benefit the development of high-performance and miniaturized photoactive substrates for diverse applications, including water treatment, water splitting, and other function devices.
This study was financially supported by the Ministry of Science and Technology of Taiwan (MOST 106-2221-E-006-240-) and (MOST 107-2221-E-006-013-MY3), and Hierarchical Green-Energy Materials (Hi-GEM) Research Center, from The Featured Areas Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education (MOE) in Taiwan. The authors greatly thank Center for Micro/Nano Science and Technology, National Cheng Kung University with the facilities provided for conducting material characterizations.
Availability of Data and Materials
The datasets supporting the conclusions of this article are included within the article.
CYC proposed the idea and was involved in the design of the experiments and wrote the main manuscript text. CHT and PHH performed the experimental synthesis and material characterizations, evaluated the photocatalytic performance, and studied the underlying mechanism. All authors read and approved the final manuscript.
The authors declare that they have no competing interests
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