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Optically Switchable Photocatalysis in Ultrathin Black Phosphorus Nanosheets Hui Wang, Shenlong Jiang, Wei Shao, Xiaodong Zhang, Shichuan Chen, Xianshun Sun, Qun Zhang, Yi Luo, and Yi Xie J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 16 Feb 2018 Downloaded from http://pubs.acs.org on February 16, 2018
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Optically Switchable Photocatalysis in Ultrathin Black Phosphorus Nanosheets ‡
‡
Hui Wang, Shenlong Jiang, Wei Shao, Xiaodong Zhang,* Shichuan Chen, Xianshun Sun, Qun Zhang,* Yi Luo, and Yi Xie* Hefei National Laboratory for Physical Science at the Microscale, iChEM, Synergetic Innovation Center of Quantum Information and Quantum Physics, University of Science and Technology of China, Hefei, Anhui, 230026, P. R. China ABSTRACT: Recently low‐dimensional materials hold great potential in the field of photocatalysis, whereas the concomitantly promoted many‐body effects have long been ignored. Such Coulomb interaction‐mediated effects would lead to some intriguing, nontrivial band structures, thus promising versatile photocatalytic performances and optimized strategies. Here, we demonstrate that ultrathin black phosphorus (BP) nanosheets exhibit an exotic, excitation‐energy‐ dependent, optical switching effect in photocatalytic reactive oxygen species (ROS) generation. It is, for the first time, observed that singlet oxygen (1O2) and hydroxyl radical (•OH) are the dominant ROS products under visible‐ and ultraviolet‐light excitations, respectively. Such an effect can be understood as a result of subband structure, where energy‐ transfer and charge‐transfer processes are feasible under excitations in the first and second subband systems, respectively. This work not only establishes an in‐depth understanding on the influence of many‐body effects on photocatalysis, but also paves the way for optimizing catalytic performances via controllable photoexcitation.
INTRODUCTION Photocatalysis based on low‐dimensional materials has drawn tremendous attention in recent years by virtue of the great promise in solving energy crisis and preventing environmental pollution.[1–6] Owing to the significantly reduced screening, robust many‐body effects mediated by Coulomb interactions would be expected in these low‐ dimensional materials, thus leading to distinct photocatalytic behaviors with respect to their bulk counterparts.[7–9] Notably, traditional viewpoints focusing on free charge carriers are somehow incomprehensive once the interactions between electrons and holes are taken into account. For instance, excitonic effects might dominate the photoexcitation processes of catalysts, and hence exciton‐based resonance energy transfer would be feasible as an alternative photocatalytic mechanism beyond carrier‐based charge transfer.[10–12] Aside from excitonic effects, band‐structure modifications (e.g., bandgap renormalization and subband structure) also tend to emerge in low‐dimensional materials with robust many‐body effects,[13,14] promising intriguing optical properties as well as optimized strategies based on controlled photoexcitation. As is well known, the subband structure, originating from giant electron– electron interactions and specific band dispersions, has mainly been observed in one‐dimensional materials, including single walled carbon nanotubes and silicon nanowires; such a modification leads to the formation of different subband systems, which endows the materials
with unique excitation‐energy‐dependent optical properties.[14–16] In term of photocatalysis, however, the topics pertaining to subband structure are far from being explored. In view of the distinct photoexcitation processes corresponding to different sets of subband systems, diverse photocatalytic behaviors might arise under different optical excitations. This conjecture inspires us to pursue novel, optically switchable photocatalysis in semiconductors with subband structure. Bearing this in mind, we concentrate on a newly emerging two‐dimensional material, black phosphorus (BP) nanosheet, which has recently attracted great interest due to its unique chemical and physical properties.[17–22] Benefiting from its high carrier mobility, high optical absorption, and novel electronic band structure,[23–25] BP‐based materials have been demonstrated, both theoretically and experimentally, to hold great potentials in a series of light‐driven energy conversion processes.[26,27] Notably, it has been theoretically predicted that BP nanosheets tend to exhibit robust many‐body effects and that its electronic structures are dispersive along one dimension, which may lead to significantly enhanced self‐energy corrections and excitonic effects.[28,29] Such a prediction may also hint the emergence of subband structure. The above theoretical hypotheses motivate us to interrogate the band structure of BP nanosheets and to exploit the possibility of optically switchable photocatalysis therein. Herein, we experimentally demonstrate, for the first time, that ultrathin BP nanosheets exhibit an exotic effect
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in photocatalytic reactive oxygen species (ROS) generation that strongly depends on the adopted excitation scenarios, that is, visible‐ and ultraviolet (UV)‐ light excitations lead to highly specific generation of singlet oxygen (1O2) and hydroxyl radical (•OH), respectively. Such an effect can only be understood in the framework of subband structure, as verified by a set of well‐designed experiments by means of photoluminescence (PL) and ultrafast transient absorption (TA) spectroscopy. Our findings are expected to open a new door to achieving high‐efficiency catalysis through controllable photoexcitation.
EXPERIMENTAL SECTION Synthesis of bulk black phosphorus (BP). Bulk BP was synthesized through a facile low‐pressure transport route according to the previous report based on the mineralization concept.[30] In detail, the mixture of red phosphorus (500 mg), tin (20 mg), and SnI4 (10 mg) were sealed in an evacuated Pyrex tube (~10‐3 Pa). The tube was then heated at 650oC for 5 h with a heating ramp rate of ~1.35oC min‐1, and then cooled to 500oC with a rate of ~0.33oC min‐1, followed by a natural cooling process. The obtained product was washed with hot toluene and acetone for several times to remove the residual mineralizers, and then dried under vacuum. Preparation of ultrathin BP nanosheets. Ultrathin BP nanosheets were prepared using a liquid exfoliation method. In detail, 50 mg of bulk black phosphorus was dispersed into 100 mL of distilled water, followed by argon bubbling for 10 min. After a sonication treatment in ice water for 8 h, the obtained brown suspension was treated with centrifugation at 3000 rpm for 10 min to remove the residual unexfoliated particles, and the supernatant with ultrathin BP nanosheets dispersion was collected for further use. Singlet oxygen (1O2) detection. Singlet oxygen sensor green (SOSG) was selected to detect the generated 1O2. Typically, the mixture solution of SOSG (5 μM) and BP nanosheet (20 μg mL–1) was allowed to be illuminated under different excitation scenarios. The 1O2 generation can be estimated by the PL intensity increment of the above mixture, where 490 and 525 nm were chosen as the excitation and emission wavelengths, respectively. A xenon lamp (PLS‐SXE300/300UV, Trusttech Co., Ltd., Beijing) with 420‐nm long‐wavelength pass filter and 420‐ nm short‐wavelength pass filter was employed as the visible‐ and ultraviolet‐light sources (100 mW cm–2), respectively. As for the wavelength‐dependent measurements, a xenon lamp with certain band‐pass filters (10 mW cm–2; full width at half‐maximum, ~10 nm) was employed as the light source. Photocatalytic degradation of methyl orange. For the degradation tests, the mixture solution of BP nanosheet (5 μg mL–1) and methyl orange (10 μg mL–1) was allowed to be stirred in the dark for 30 min to ensure the adsorption/desorption equilibrium before illumination.
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The degradation of methyl orange can be evaluated by the absorbance change of mixture solution monitored with an UV−vis spectrophotometer. The atmosphere‐dependent measurements were carried out with continuous gas bubbling. For the scavenger tests, certain amounts of scavengers (NaN3, 50 μg mL–1; mannitol, 300 μM) were added into the mixture solution before illumination. Hydroxyl radical (•OH) detection. Terephthalic acid was employed as the probe molecule for •OH detection. In detail, the mixture solution containing 5 μg mL–1 BP nanosheet, 10 mM NaOH, and 3 mM terephthalic acid was stirred in dark for 30 min to ensure the adsorption/desorption equilibrium. Within a specific time interval, a certain amount of the solution was removed and then centrifuged for fluorescence spectroscopy measurements. As for the wavelength‐dependent measurements, a xenon lamp with certain band‐pass filters (10 mW cm–2; full width at half‐maximum, ~10 nm) was employed as the light source. ESR‐trapping tests. As for the 1O2 trapping‐ESR tests, 50 μL of aqueous suspension of samples (4 g L–1) was mixed with 500 μL of 2,2,6,6‐tetramethylpiperidine (TEMP, 50 mM) solution. After being illuminated for 2 min, the mixture was characterized using a Bruker EMX plus model spectrometer operating at the X‐band frequency (9.4 GHz) at room temperature. As for the •OH trapping‐ESR tests, similar procedures were adopted except the use of 5,5‐dimethyl‐1‐pyrroline‐N‐oxide (DMPO) as the spin‐trapping agent. Characterizations. The X‐ray diffraction (XRD) patterns were recorded on a Philips X’Pert Pro Super diffractometer with Cu Kα radiation (λ = 1.54178 Å). The transmission electron microscopy (TEM) measurements were carried out on a JEM‐2100F field emission electron microscope with an acceleration voltage of 200 kV. The High‐angle annular dark‐field scanning transmission electron microscopy (HAADF–STEM) measurements were performed with a JEOL JEM‐ARM200F microscope with spherical aberration correction. The X‐ray photoelectron spectra (XPS) were collected on an ESCALAB MKII with Mg Kα (hν = 1253.6 eV) as the excitation source, using C 1s to 284.6 eV as reference. The electron paramagnetic resonance (EPR) measurements were carried out on a Bruker EMX plus model spectrometer operating at the X‐band frequency (9.4 GHz). The ultraviolet–visible (UV–vis) spectra were recorded on a Perkin Elmer Lambda 950 UV–vis–NIR spectrophotometer. The ultraviolet photoelectron spectroscopy (UPS, excitation energy, 170 eV) experiments were performed at the Photoemission Endstation at the BL10B beamline in the National Synchrotron Radiation Laboratory (NSRL) in Hefei, China. The steady‐state photoluminescence (PL) measurements were performed on an FLS920 fluorescence spectrometer (Edinburgh). The ultrafast transient absorption (TA) measurements were performed, under ambient conditions, on an ExciPro pump–probe spectrometer (CDP) coupled to an amplified femtosecond laser system (Coherent). The pump (~3
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μJ/pulse at the sample) were delivered by an optical parametric amplifier (TOPAS‐800‐fs) that was excited by a Ti:sapphire regenerative amplifier (Legend Elite‐1K‐HE; 800 nm, 35 fs, 3 mJ) seeded with a mode‐locked Ti:sapphire laser system (Micra 5). The white‐light continuum probe (500−700 nm) was generated by focusing a small portion of the 800‐nm beam onto a rotating CaF2 plate. The instrument response function was estimated to be 100 fs by a routine optical cross‐ correlation procedure. The sample cell containing the colloidal BP samples under investigation was mounted on a rapidly rotating stage (5000 rpm) to ensure that the photoexcited volume of the sample was kept fresh during the course of the TA measurements. The temporal and spectral profiles of the TA signal, i.e., the pump‐induced absorbance changes (in mOD; OD, optical density) were registered by a 1024‐pixel imaging spectrometer (CDP2022i).
RESULTS AND DISCUSSION The ultrathin BP nanosheets were prepared through a facile water‐exfoliation strategy, given the typically layered and relatively stable structure of pristine BP (Figure 1a, Figure S1 and Supporting Note 1), following similar procedures to our previous report.[31] The X‐ray diffraction (XRD) pattern of the reassembled BP nanosheets thin film is shown in Figure 1b, which is in accordance with JCPDS card No. 73‐1358, confirming the phase of the obtained sample. The exclusively observed diffraction peaks of {010} facets revealed the excellent orientation along the [010] direction of the as‐obtained sample. The crystal structure was further confirmed by Raman spectral analysis (Figure S2). A typical transmission electron microscopy (TEM) image (Figure 1c) revealed the ultrathin and freestanding features of the sample. The excellent dispersity of the sample in water was confirmed by Tyndall effect (Figure 1c, inset). The high‐angle annular dark‐field scanning transmission electron microscopy (HAADF–STEM) image and the corresponding selective area electron diffraction (SAED) pattern (Figure 1d) further indicated the high quality of the sample. The atomic force microscopy (AFM, Figure S3) characterization suggested an average thickness of approximately 2 nm of the as‐obtained nanosheets, leaving no more than four individual BP layers. The component and chemical state of the BP nanosheets were verified by X‐ray photoelectron spectroscopy (XPS, Figure S4) analyses, showing that the chemical composition was mainly phosphorus. The partial oxidation on the surface of the prepared BP nanosheets was also identified, given the sensitivity of BP to molecular water and oxygen. On the basis of the above characterizations and analyses, the ultrathin BP nanosheets have been successfully obtained. In a recent work of our group, we found that the BP nanosheets can result in efficient single oxygen (1O2) generation under visible‐light illumination (λ ≥ 420 nm), promising applications in photocatalysis and photodynamic therapy.[31] Herein, the illumination was extended to the ultraviolet (UV) region (λ
