Research Article pubs.acs.org/acscatalysis
Bandgap- and Local Field-Dependent Photoactivity of Ag/Black Phosphorus Nanohybrids Wanying Lei,†,‡,§ Tingting Zhang,† Ping Liu,*,∥ José A. Rodriguez,∥ Gang Liu,*,† and Minghua Liu*,† †
CAS Key Laboratory of Standardization and Measurement for Nanotechnology, CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology, Beijing 100190, People’s Republic of China ‡ Academy for Advanced Interdisciplinary Studies, Peking University, Beijing 100871, People’s Republic of China § University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China ∥ Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973, United States S Supporting Information *
ABSTRACT: Black phosphorus (BP) is the most exciting post-graphene layered nanomaterial that serendipitously bridges the 2D materials gap between semimetallic graphene and large bandgap transition-metal dichalcogenides in terms of high charge-carrier mobility and tunable direct bandgap, yet research into BP-based solar to chemical energy conversion is still in its infancy. Herein, a novel hybrid photocatalyst with Ag nanoparticles supported on BP nanosheets is prepared using a chemical reduction approach. Spin-polarized density functional theory (DFT) calculations show that Ag nanoparticles are stabilized on BP by covalent bonds at the Ag/BP interface and Ag−Ag interactions. In the visible-light photocatalysis of rhodamine B by Ag/BP plasmonic nanohybrids, a significant rise in photoactivity compared with pristine BP nanosheets is observed either by decreasing BP layer thickness or increasing Ag particle size, with the greatest enhancement being up to ∼20-fold. By virtue of finite-difference time domain (FDTD) simulations and photocurrent measurements, we give insights into the enhanced photocatalytic performance of Ag/BP nanohybrids, including the effects of BP layer thickness and Ag particle size. In comparison with BP, Ag/BP nanohybrids present intense local field amplification at the perimeter of Ag NPs, which is increased by either decreasing the BP layer thickness from multiple to few layers or increasing the Ag particle size from 20 to 40 nm. Additionally, when the BP layer thickness is decreased from multiple to few layers, the bandgap becomes favorable to generate more strongly oxidative holes in the proximity of the Ag/BP interface to enhance photoactivity. Our findings illustrate a synergy between locally enhanced electric fields and BP bandgap, in which BP layer thickness and Ag particle size can be independently tuned to enhance photoactivity. This study may open a new avenue for further exploiting BP-based plasmonic nanostructures in photocatalysis, photodetectors, and photovoltaics. KEYWORDS: black phosphorus, Ag nanoparticles, photocatalysis, localized surface plasmon resonance, density functional theory
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surface to volume ratio.29 To date, the vast majority of photocatalytic studies on 2D nanomaterials are limited to graphene30 and transition-metal dichalcogenides (TMDs).31 Relatively few studies have been reported regarding other types of 2D materials.32,33 Since early 2014, great attention has been directed toward black phosphorus (BP), which is currently the most exciting post-graphene 2D layered crystal with substantial potential for exploring conceptually new materials and devices in electronics,34,35 optoelectronics,34 and sensing.36,37 As an allotrope of elemental phosphorus, orthorhombic BP is the most chemically stable.38 BP presents a honeycomb network with P atoms arranged hexagonally, which is slightly puckered to form armchair- and zigzag-shaped structure along the x and y axial directions, respectively.39,40 At room temperature bulk BP
INTRODUCTION Direct and efficient utilization of solar energy in photovoltaics,1,2 photodetectors,3,4 and photochemical reactions5−7 is an emerging topic. In this context, heterogeneous photocatalysis presents a promising prospective of implementing solar to chemical energy conversion and thereby enabling the sustainable development of energy and environment. Since Fujishima and Honda discovered photocatalytic water splitting on TiO2,8 the past decades have witnessed a significant rise in light-driven pollutant abatement,9−15 selective oxidation,16−18 CO2 reduction,6,19,20 and water splitting.14,21−28 In general, harvesting visible light using broad band responsive photocatalysts is a prerequisite for effective utilization of solar energy, taking into account a sizable fraction (∼44%) of visible light in the entire solar spectrum. Among the heterogeneous photocatalysts, two-dimensional (2D) nanomaterials are an important class of catalysts and catalyst supports due to their large © XXXX American Chemical Society
Received: September 2, 2016 Published: October 18, 2016 8009
DOI: 10.1021/acscatal.6b02520 ACS Catal. 2016, 6, 8009−8020
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Figure 1. Tapping mode AFM topographical images of (a) f-BP and (b) m-BP nanosheets. Inset: the height profiles of BP nanosheets. Representative low-magnification TEM images of (c) f-BP and (d) m-BP nanosheets. Atomically resolved HAADF-STEM images viewed along the (e) [001] and (f) [101] directions. Inset: the corresponding atomic structure models.
both components can coabsorb visible light via the formation of plasmons and excitons, respectively. On the other hand, metal− semiconductor nanohybrids could promote the spatial separation of photoexcited charge carriers. In general, metallic NPs show prominent localized surface plasmon resonance (LSPR) arising from the collective excitation of conduction electrons. Under resonance conditions, a maximum amount of incident energy in the visible regime can be absorbed. To date, LSPR has been extensively studied in photocatalysis,13,47,48 sensing,49 biotechnology,50 and solar cells.23 Despite the tremendous progress being made in heterogeneous photocatalysis, the relevant mechanisms are still far from being completely understood.17,51,52 In this work, a novel nanohybrid with Ag NPs supported on BP surface was prepared through a chemical reduction
exhibits respective electron and hole mobilities of 220 and 350 cm2 V−1 s−1.41 In addition, BP features a layer-dependent direct bandgap, ranging from ∼0.3 eV in the bulk to ∼1.5 eV in the monolayer.41,42 Indeed, BP serendipitously bridges the 2D materials gap between semimetallic graphene and large bandgap TMDs in terms of high carrier mobility and tunable direct bandgap; thereby it is indispensable in solar energy-based photocatalysis, photodetectors, and photovoltaics. Currently, research on BP is predominantly focused on its electrooptical43,44 and chemical properties;36,45 far less information is available regarding BP-based heterogeneous photocatalysis.39 In view of promotion of the overall performance of photocatalysts, metal−semiconductor nanohybrids combining a narrow-bandgap semiconductor and plasmonic metallic nanoparticles (NPs) are very attractive.17,46 On one hand, 8010
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Figure 2. Representative TEM images of (a) 3 wt % Ag/f-BP, (b) 5 wt % Ag/f-BP, (c) 3 wt % Ag/m-BP, and (d) 5 wt % Ag/m-BP. Insets: the aggregations of Ag NPs (upper left) and the size distributions of Ag NPs (lower left). (e) High-magnification ABF-STEM image in the proximity of the Ag particle perimeter. Corresponding line profiles display the intensity as a function of position (f) along a−a′ and (g) along b−b′ in (e).
layered crystals in water.53 Then a certain amount of AgNO3 solution was added dropwise into the as-obtained BP suspensions. Ag+ ions were subsequently reduced by aqueous NaBH4 to form Ag/f-BP and Ag/m-BP nanohybrids. The morphology, height profiles, and atomic structure of BP nanosheets were characterized by transmission electron microscopy (TEM), atomic force microscopy (AFM), and aberration-corrected scanning transmission electron microscopy (AC-STEM). Figure 1a shows a typical AFM image of fBP, and the corresponding height profiles are displayed in the inset. The thickness of f-BP nanosheets predominantly lies at ∼2 nm, which corresponds to approximately 4 atomic layers. Figure 1b displays that the thickness of m-BP is ∼10 nm, which is approximately equal to 20 layers. Figure 1c is a representative low-magnification TEM image of f-BP nanosheets that exhibit lateral sizes of hundreds of nanometers. In Figure 1d, m-BP
approach. Of particular interest are investigations of microscopic photocatalysis mechanisms including the effects of BP layer thickness and Ag particle size. We show that, under visible light illumination, a synergy between locally enhanced electric fields and BP bandgap governs the photocatalytic performance of Ag/BP nanohybrids that efficiently catalyze rhodamine B (RhB) in water, a model reaction of environmental purification. Our results represent an important step in demonstrating the potential of BP as a new 2D platform for engineering efficient solar energy conversion systems and significantly advancing our in-depth understanding of photocatalysis.
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RESULTS AND DISCUSSION First, few-layer BP (denoted as f-BP) and multiple-layer BP (denoted as m-BP) nanosheets were prepared by exfoliating BP 8011
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Figure 3. Micro-Raman spectra of (a) f-BP nanosheets and Ag/f-BP nanohybrids and (b) m-BP nanosheets and Ag/m-BP nanohybrids. Highresolution XPS spectra of the P 2p core level for (c) f-BP nanosheets and Ag/f-BP nanohybrids and (d) m-BP nanosheets and Ag/m-BP nanohybrids. High-resolution XPS spectra of the Ag 3d core level for (e) f-BP nanosheets and Ag/f-BP nanohybrids and (f) m-BP nanosheets and Ag/m-BP nanohybrids. UV−vis spectra of (g) f-BP nanosheets and Ag/f-BP nanohybrids and (h) m-BP nanosheets and Ag/m-BP nanohybrids.
nanosheets show lateral sizes larger than 10 μm. The representative atomic structure of BP nanosheets viewed along the [001] direction is resolved in Figure 1e using highangle annular dark-field (HAADF)-STEM. The bright spots represent the pairs of P columns with two adjacent P atoms overlapped along the [001] orientation. The interatomic distances are found to be 3.28 and 4.37 Å, which are consistent with the BP lattice parameters. Figure 1f shows a representative HAADF-STEM image viewed along the [101] direction, in
which the individual atomic P column is unambiguously distinguished. A typical electron energy loss spectroscopy (EELS) spectrum of BP nanosheets (Figure S1 in the Supporting Information) shows a peak located at ∼132 eV that is the near-edge fine structure of the P L2,3 edge. For Ag/ BP nanohybrids, the representative TEM images (Figure 2a−d) show that the nearly spherical Ag NPs are distributed across the BP surface. Furthermore, Ag particle aggregates were also frequently observed, as shown in the upper left insets of Figure 8012
DOI: 10.1021/acscatal.6b02520 ACS Catal. 2016, 6, 8009−8020
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ACS Catalysis 2a−d. Statistical TEM measurements show that the respective average sizes of Ag NPs are ca. 20 and 40 nm for 3 wt % Ag/BP and 5 wt % Ag/BP nanohybrids, respectively. Figure 2e shows a high-magnification annular bright-field (ABF)-STEM image in the proximity of the Ag particle perimeter. Both BP and Ag atomic structures are clearly resolved. The interatomic distances obtained by line profiles along a−a′ and b−b′ are 2.04 Å (Figure 2f) and 2.33 Å (Figure 2g), which are in accordance with Ag (100) and Ag (111) planes, respectively. The Raman spectra of f-BP and Ag/f-BP nanohybrids (Figure 3a) display three prominent peaks that are assigned to one outof-plane phonon mode (Ag1 at 361.5 cm−1) as well as two inplane phonon modes (B2g and Ag2 at 438.6 and 465.8 cm−1, respectively).42 In addition, the Raman peaks of f-BP are slightly red shifted in comparison with those of m-BP (Figure 3b), in agreement with previous reports.42,53−55 The peak at 521 cm−1 corresponds to the TO phonon mode of the silicon substrate. Upon Ag deposition, no significant changes in BP Raman spectra occur. The chemical compositions and oxidation states of BP nanosheets and Ag/BP nanohybrids were probed by X-ray photoelectron spectroscopy (XPS). The highresolution XPS spectra of the P 2p core level (Figure 3c,d) display 2p3/2 and 2p1/2 doublets located at 130.1 and 130.9 eV, respectively. Figure 3e,f exhibits the high-resolution Ag 3d corelevel spectra of Ag/BP nanohybrids. The Ag spectra are well fitted by a set of doublets located at 368.4 and 374.4 eV, which are ascribed to metallic silver (Ag0). UV−vis absorption spectra are shown in Figure 3g,h. Both f-BP and m-BP display a broad absorption over the entire visible regime from 300 to 800 nm. Furthermore, 3 wt % Ag/f-BP and 5 wt % Ag/f-BP nanohybrids exhibit prominent LSPR-derived absorption centered at ∼395 and ∼405 nm, respectively. Likewise, 3 wt % Ag/m-BP and 5 wt % Ag/m-BP show respective LSPR absorptions at ∼410 and ∼440 nm. The broadening of the LSPR peaks results from electron−electron and electron−phonon thermalization.56 Moreover, the intensity is also increased with increasing Ag content from 3 to 5 wt %. The numbers of photons absorbed in 3 wt % Ag/f-BP and 5 wt % Ag/f-BP nanohybrids are estimated to be respectively 8% and 12% greater compared with f-BP nanosheets, and the numbers of photons absorbed in 3 wt % Ag/m-BP and 5 wt % Ag/m-BP are increased by 13% and 15% in comparison with m-BP, respectively. The interaction between Ag and BP was studied using firstprinciples calculations. Herein, Ag extended adlayers (0.25−2 ML) on the slab models of the BP (010) tetralayer are employed to mimic realistic Ag NPs with average sizes of 20 and 40 nm (Figure 4a−h). The adsorption energies (Eads) and Ag−P bond lengths (dAg−P) as a function of Ag coverage is also calculated (Table S1 in the Supporting Information). The relative low adsorption energy compared to that of the gas phase confirms the favorable adsorption of Ag adlayers on the BP surface. With increasing Ag coverage, Eads decreases while dAg−P increases, indicating that the adsorption of Ag atoms on the BP surface attract more Ag atoms to form Ag adlayers. The deposition of Ag adlayers on the BP (010) surface is driven by the populated Ag−Ag interaction during this process, which is demonstrated by the decreased charges of Ag as shown by a Mulliken analysis (Table S1). The higher the Ag coverages, the more stable the supported Ag adlayers. The 2D slice of the projected electron localization function (ELF) perpendicular to the surface is plotted in Figure 4i−l. For 0.25 and 0.5 ML Ag (Figure 4i,j), there is some charge polarization at the Ag/BP interface. In the range of 1−2 ML Ag (Figure 4k,l), very little
Figure 4. Optimized structure of Ag adlayers on a BP (010) tetralayer as a function of Ag coverage. Top view of (a) 0.25 ML Ag, (b) 0.5 ML Ag, (c) 1 ML Ag, and (d) 2 ML Ag. Corresponding side views of (e) 0.25 ML Ag, (f) 0.5 ML Ag, (g) 1 ML Ag, and (h) 2 ML Ag. Color scheme: gray balls, Ag; purple balls, P. Calculated ELF for Ag adlayers supported on a P(010) tetralayer. The respective Ag coverages are (i) 0.25 ML, (j) 0.5 ML, (k) 1 ML, and (l) 2 ML, where the projected 2D slices perpendicular to the surface are displayed. The isosurface level is chosen as 0.04 e/a03, where a0 is the Bohr radius.
charge polarization is seen. The partial density of states (PDOS) was calculated and is shown in Figures S2 and S3 in the Supporting Information. Briefly, the metallic nature of Ag adlayers is predominant with a thickness greater than monolayer, which is in agreement with the aforementioned XPS experimental results. The deposition of Ag NPs onto the BP surface is energetically favorable. There is a transition of Ag−P bonds from ionic-like to covalent-like nature with increasing Ag coverage. With an increasing Ag coverage from 0.25 to 2 ML, one can see in Figure S2 that the PDOS of Ag becomes more and more delocalized and shifts toward the Fermi level to gain better overlapping with that of P; in the meantime the charge of Ag is decreased from partially positively charged Ag ion to neutral metallic Ag (Table S1). That is, at low Ag coverage the Ag−P bond is accompanied by charge transfer from Ag to P and localized electron distribution (Figure S2), which likely features an ionic nature; in contrast, at high Ag coverage the Ag−P bond is elongated (Table S1) due to the decreased charge transfer from Ag to P and more delocalized electron distribution (Figure S2), which likely indicates a covalent nature. Therefore, the stabilization of Ag 8013
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Figure 5. Photoactivity of (a) f-BP nanosheets and Ag/f-BP nanohybrids and (b) m-BP nanosheets and Ag/m-BP nanohybrids for RhB degradation under visible light illumination. Photocurrent responses of (c) f-BP nanosheets and Ag/f-BP nanohybrids and (d) m-BP nanosheets and Ag/m-BP nanohybrids. Control experiments in the presence of different scavengers over (e) f-BP, (f) Ag/f-BP nanohybrids, (g) m-BP, and (h) Ag/m-BP nanohybrids.
min−1). For Ag/m-BP nanohybrids, the reaction rate constant increases dramatically from 4.5 × 10−3 to 10.8 × 10−3 min−1 with increasing Ag particle size from 20 to 40 nm. In the case of the same Ag particle size, the photoactivity of Ag/BP nanohybrids increases with decreasing BP layer thickness. For instance, the reaction rate constant of 3 wt % Ag/f-BP displays nearly an order of magnitude greater than that of 3 wt % Ag/mBP. Notably, the photoactivity of 5 wt % Ag/f-BP is about 20fold greater than that of m-BP, the greatest enhancement of photoactivity observed in this work. The photoactivity of P25 was also investigated for comparison (Figure S4 in the Supporting Information). To gain insights into the BP layer thickness- and Ag particle size-dependent photoactivity, the photocurrent response was measured by means of on−off illumination under visible light (Figure 5c,d). The photocurrent density for f-BP is 0.04 μA cm−2, 2 times greater than that of mBP. For Ag/f-BP samples, the photocurrent density is increased dramatically from 0.18 to 0.75 μA cm−2 with increasing Ag particle size from 20 to 40 nm; these values are respectively 4-
NPs on BP is achieved by covalent bonds at the Ag/BP interface and Ag−Ag interactions. The photocatalytic activities of BP nanosheets and Ag/BP nanohybrids were evaluated toward RhB photodegradation in water under visible light illumination. To assess the photoactivity quantitatively, the apparent reaction rate constants of RhB degradation were calculated on the basis of the Langmuir− Hinshelwood kinetics, and the results are displayed in Figure 5a,b. First, the photoactivity of BP nanosheets is shown to be BP layer thickness-dependent; the reaction rate constant is increased from 2.9 × 10−3 to 5.1 × 10−3 min−1 with decreasing BP layer thickness from 10 to 2 nm. Further, Ag/BP nanohybrids significantly enhance the photoactivity with respect to the BP counterparts, with the activity dependent on the BP layer thickness and Ag content (i.e., Ag particle size). With the same BP layer thickness, the photoactivity increases with increasing Ag particle size. For example, the reaction rate constant of 5 wt % Ag/f-BP is 57.4 × 10−3 min−1, 1.3-fold larger than that of 3 wt % Ag/f-BP nanohybrids (k = 44.5 × 10−3 8014
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alternatively step by step in a sequence of time and space. In general, the size, shape, and local dielectric environment of plasmonic NPs strongly determine LSPR. To mimic the realistic shape and size of Ag NPs, half-sphere-shaped Ag monomers with two different sizes (20 and 40 nm for 3 wt % Ag/BP and 5 wt % Ag/BP, respectively) were selected for numerical simulations. Further, since the aggregates of Ag NPs are frequently observed by TEM imaging as mentioned above, we also simulated the electric fields of Ag dimers with an interparticle separation of 1 nm. Figure 7 displays the simulated spatial distribution of electric field enhancements in 20 and 40 nm diameter Ag monomers and associated dimers supported on either f-BP or m-BP nanosheets. First, the locally enhanced electric fields are spatially inhomogeneous, with the maximum field strength at the perimeter of Ag NPs. Second, the locally enhanced electric fields are Ag particle size-dependent. For Ag/ f-BP nanohybrids, a 1.8-fold enhancement of the maximum electric fields is seen as the size of Ag monomer increases from 20 to 40 nm. Notably, the intense local field amplification in the gap of dimers becomes much greater than that of the monomer counterparts and increases with increasing monomer size. In detail, the maximum local electric field of a Ag dimer comprising Ag monomers 40 nm in size is 2.1-fold larger than that of a Ag dimer comprising Ag monomers 20 nm in size, as shown in Figure 7c,d. Third, the locally enhanced fields are BP layer thickness-dependent. With the same Ag particle size, the intense local field amplication exhibited by Ag/f-BP is approximately 2−3 times larger than that of the Ag/m-BP counterparts. For instance, the maximum field enhamcement for a 40 nm Ag monomer on f-BP is 7.2, while it is 2.9 for the Ag conuterpart supported on m-BP. In a typical semiconductor-derived photocatalytic process, an incident photon with energy exceeding the bandgap of a given semiconductor is able to excite an electron from the VB to the conduction band (CB), forming a hole in the VB. The resulting energized electrons can be annihilated by recombination with hole counterparts or transferring to the adjacent electron acceptors. One efficient approach to suppress the e−−h+ recombination is to construct a heterostructure combining a semiconductor and plasmonic metal NPs. In this case, the scenario becomes complicated and the associated photocatalytic mechanisms have been elucidated to a lesser extent.58 On one hand, the electric dipole developed by LSPR can give rise to enhanced electric fields on metal NPs. The electric fields are often localized at hot spots whose field strengths are significantly maximized, along with the accumulation of enhanced electromagnetic energy. On the other hand, in addition to the excitons in the semiconductor support, LSPR on metal NPs can simultaneously generate plasmons. In general, there are two major decay channels for the excited plasmons. One is radiative decay with an extremely low fluorescence quantum yield (10−10 to 10−5).59 The other is nonradiative decay with the formation of an intraband electron−hole pair.60 The resulting electron−hole pair further undergoes two additional competing channels. One is relaxation through electron−electron scattering, and the other is electron transfer from plasmonic NPs into nearby acceptors such as the semiconductor support or adsorbates.61 Previous studies show that the electron transfer from plasmonic metal NPs to the semiconductor is an extremely inefficient route due to scattering effects etc.46,62,63 In the present study, we have shown how Ag/BP plasmonic nanohybrids enhance photoactivity by means of experiments
and 18-fold larger than that of f-BP nanosheets. Likewise, the photocurrent density for 5 wt % Ag/m-BP is about 0.06 μA cm−2, 1.5 and 2 times larger than those of 3 wt % Ag/m-BP and m-BP, respectively. Overall, the photocurrent density of 5 wt % Ag/f-BP is ∼25 times larger than that of m-BP, consistent with the above photodegradation results. It is generally accepted that a higher photocurrent density indicates a lower rate of charge carrier recombination. The above results prove that Ag/BP nanohybrids display more efficient charge separation in comparison with BP nanosheets, with the charge separation efficiency dependent on BP layer thickness and Ag particle size. In order to identify the major active species responsible for RhB degradation, control experiments with a number of scavengers were carried out. Figure 5e,f displays that all scavengers are able to decrease the RhB photodegradation efficiency to a certain degree. In detail, the RhB conversion over f-BP nanosheets is about 88.7% without scavengers in the photoreaction. In the presence of ammonium oxalate (AO), the RhB conversion is significantly decreased to 21.8%, indicating that h+ is the major active species. For Ag/f-BP nanohybrids, the conversion of RhB is 91.9% and decreases to 33.7% upon adding AO, as illustrated in Figure 5f, again suggesting that h+ is the dominant active species. In the case of m-BP, the photoactivity is significantly inhibited by adding AO and tert-butyl alcohol (TBA), showing that h+ and •OH are the major active species responsible for RhB degradation, as displayed in Figure 5g. Figure 5h illustrates that h+ is the dominant active species during photoreaction under Ag/m-BP nanohybrids. The energy band diagrams of Ag, f-BP, and m-BP nanohybrids are illustrated in Figure 6. As a p-
Figure 6. Energy band diagrams of Ag, f-BP, and m-BP.
type semiconductor, BP has a Fermi level of −3.9 eV,57 slightly higher than that of Ag NPs (−4.26 eV). When Ag NPs and BP are in contact, the energy bands of BP bend downward toward the interface to reach equilibrium. Under visible light illumination, the holes generated in the valence band (VB) could directly oxidize the adsorbed pollutant species. Moreover, the respective oxidation powers of holes for f-BP and m-BP is estimated to be +0.6 and −0.5 VNHE, indicating that the former possesses a lower VB maximum and thus generates more strongly oxidative holes. However, both of them are insufficient to oxidize either surface-adsorbed hydroxyl or hydroxyl ions to form •OH (E°ox > +1.6 or +2.72 VNHE). Thus, •OH involved in this photocatalytic system is probably generated by the photolysis of H2O2 molecules that are formed by two-electron reduction of O2. In order to further gain insights into the enhanced photocatalytic activity, the spatial electric field distribution induced by LSPR on the optically excited Ag NPs under 500 nm photon excitation was simulated using 3D finite-difference time domain (FDTD) method. In the FDTD method both space and time are divided into discrete segments. The space is segmented into Yell cells, in which electric fields are calculated 8015
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Figure 7. Simulated spatial distribution of electric field enhancements for Ag/BP nanohybrids under 500 nm photon excitation: Ag monomer with a size of (a) 20 nm and (b) 40 nm supported on f-BP; Ag dimer with an interparticle gap of 1 nm and individual Ag monomer size of (c) 20 nm and (d) 40 nm supported on f-BP; Ag monomer with a size of (e) 20 nm and (f) 40 nm supported on m-BP; Ag dimer with an interparticle gap of 1 nm and individual Ag monomer size of (g) 20 nm and (h) 40 nm supported on m-BP.
increase in absorption is observed for 3−5 wt % Ag/m-BP. Second, the intense local field amplification increases with increasing Ag particle size and increaes with decreasing BP layer
and numerical simulations. First, the photon absorption is increased by 8−12% for 3−5 wt % Ag/f-BP samples compared with f-BP. Further, in comparison with m-BP, a 13−15% 8016
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thickness. For the monomer-like Ag NPs supported on BP that are 20 and 40 nm in size, the maximum field enhancements induced by LSPR in the proximity of the Ag particle perimeter are increased by 1.3−1.8 folds, respectively. For Ag dimers with an interparticle gap of 1 nm, an approximately 2-fold increase of field enhancements within the gap is achieved. With the same Ag particle size, the maximum field enhancements by Ag/f-BP is increased by 2-fold in comparison with those of Ag/m-BP. The interaction of intense localized electric fields with adjacent BP promotes the formation rate of e−−h+ pairs in the BP nearsurface regime, in which the e−−h+ pairs with inherent short diffusion length can readily reach the BP surface and drive the photoreaction.47,64,65 The photoexcited electrons in BP relax by either injection into the Ag NPs or recombination with holes. The electrons on the Ag NPs can generate radicals with adsorbed O2 or transport back to BP. The resulting long-lived holes in the VB of BP are active toward RhB degradation. Third, the bandgap of BP is layer-dependent: 1.5 and 0.3 eV for f-BP and m-BP, respectively. As shown in Figure 6, f-BP possesses a lower VB maximum than m-BP, i.e., a favorable band-edge position, and thus generates more strongly oxidative holes for enhancing photoactivity.11,66 In a very recent study, Hersam and Chen et al. reported a layer-dependent ultrahigh sensing performance of phosphorene-based gas sensors, highlighting the crucial role of BP bandgap.36 Our mechanistic insights rationalize why RhB degradation by Ag/BP is significantly enhanced by 2−20 folds either by decreasing BP nanosheet layer thickness from 10 to 2 nm or increasing Ag particle size from 20 to 40 nm, and holes are dominating active species.
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EXPERIMENTAL SECTION
Preparation of Ag/BP Nanohybrids. m-BP nanosheets and f-BP nanosheets were obtained by the exfoliation of commercial BP (>99.998%, Nanjing XFNANO Materials TECH Co., Ltd., People’s Republic of China) that shows high crystallinity (Figure S5 in the Supporting Information).53 In detail, 80 mg of BP was dispersed into 60 mL of Milli-Q water (18 MΩ•cm, Millipore) that was bubbled with argon to remove the dissolved oxygen. The suspension was sonicated in an ice bath for about 4 h. m-BP nanosheets were subsequently collected from the above suspension by centrifugation (15000 rpm) and dried under vacuum. For f-BP nanosheets, the exfoliation time was extended up to 8 h, and then the suspension was centrifuged at 4000 rpm for 5 min to remove the unexfoliated nanocrystals and the supernatant was collected. The concentration of as-obtained f-BP is about 0.2 mg mL−1. Ag/BP nanohybrids were prepared via a reduction method. To prepare 3 wt % Ag/f-BP and 5 wt % Ag/f-BP nanohybrids, 48 and 80 μL of AgNO3 (99.9995%, metal basis, Alfa Aesar) solution was added to 40 mL of f-BP supernatant, respectively. Then 84 and 140 μL of NaBH4 (>97%, Alfa Aesar) solution (0.26 M) was injected into the above respective solutions and stirred for 1 h. For Ag/m-BP, 50 mg of BP nanosheets was dispersed into 20 mL of Milli-Q water in a 50 mL beaker. A certain amount of AgNO3 solution (300 and 500 μL for 3 wt % Ag/m-BP and 5 wt % Ag/m-BP, respectively) was added dropwise under ultrasonic agitation. Subsequently, 200 μL of NaBH4 solution with a concentration of 0.66 M. was rapidly injected into the above solution with vigorous stirring. The suspension was continually stirred for an additional 1 h. Then the as-prepared Ag/BP samples were isolated by centrifugation and repeatedly washed with Milli-Q water. Finally, the products were dried under vacuum for 3 h. The actual Ag weight was determined with an inductively coupled plasma atomic emission spectrometer (ICP-AES) in an Optima 4300 DV spectrometer (PerkinElmer). Characterization. Powder X-ray diffraction (XRD) patterns were conducted on a Shimadzu X-ray diffractometer (XRD-6000) with Cu Kα radiation (λ = 1.54178 Å, 50 kV, 300 mA). Raman spectra were collected using a Renishaw MicroRaman spectroscopy system (Renishaw in via plus) with 514 nm excitation at a power of 0.1 mW at room temperature. To prepare f-BP and Ag/f-BP samples for Raman experiments, a droplet of the respective supernatant was placed onto a silicon wafer and then dried under vacuum. XPS spectra were obtained on an ESCALab 250 electron spectrometer from Thermo Scientific Corp. Monochromatic Al Kα radiation (150 W) was used, and low-energy electrons were utilized for charge compensation. All binding energies (BE) were referenced to the adventitious C 1s line at 248.4 eV. Commercial software (Avantage) was used for curve fitting. The XPS spectra were modeled by Voigt peak profiles after subtracting a Shirley-type background. Further, the %Lorentzian−Gaussian for the spectra was fixed at 20%. UV−vis spectroscopy was performed using a Shimadzu UV-2600 spectrophotometer. TEM imaging was taken on a Tecnai G2 F20 U-TWIN microscopy with an acceleration voltage of 200 kV. ABF and HAADF-STEM images were acquired on a JEOL JEM ARM 200F instrument (Tokyo, Japan) equipped with a CEOS (Heidelberg, Germany) probe aberration corrector. EELS spectra were recorded using a Gatan (Pleasanton, CA, USA) Tridiem image filter with energy
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CONCLUSIONS A novel hybrid photocatalyst with Ag nanoparticles supported on BP nanosheets was successfully prepared via a chemical reduction approach. Spin-polarized DFT calculations reveal the formation of covalent bonds between Ag NPs and BP and Ag− Ag interactions, which are able to stabilize Ag NPs on BP. In the visible-light photocatalysis of rhodamine B by Ag/BP nanohybrids, a significant rise in photoactivity compared with BP is observed either by decreasing BP layer thickness or increasing Ag particle size, with the greatest enhancement up to 20-fold. FDTD simulations show that Ag/BP plasmonic nanohybrids enhance local electric fields at the perimeter of Ag nanoparticles. Further, the locally enhanced field amplification is shown to be dependent on BP layer thickness and Ag particle size: a few-layer sample is approximately 2−3 folds stronger than the multiple-layer counterpart for the same Ag particle size, and increasing the Ag particle size from 20 to 40 nm further enhances local fields by ∼2 folds. The numerical simulations are experimentally corroborated by photocurrent measurements. Additionally, when the BP layer thickness is decreased from 10 to 2 nm, the bandgap is increased from 0.3 to 1.5 eV and more strongly oxidative holes in the proximity of the Ag/BP interface are thus generated to enhance photoactivity. Our findings show a synergy between locally enhanced electric fields and BP bandgap, in which BP layer thickness and Ag particle size can be independently controlled to enhance photoactivity. The present study shows the potential of BP as a new 2D platform for engineering efficient solar energy conversion systems potentially useful in photocatalysis, photodetectors, and photovoltaics. 8017
DOI: 10.1021/acscatal.6b02520 ACS Catal. 2016, 6, 8009−8020
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calculations.71,72 The exchange-correlation energy and the potential were described by the Perdew−Burke−Ernzerhof functional (PBE).73 To model the BP tetralayer, four atomic layers along the [010] direction with AB stacking were considered, which are more stable than the AA and AC stacking counterparts.74 A p(2 × 1) slab was used on each layer, and a vacuum of 20 Å between the slabs was applied perpendicular to the surface. All atoms were allowed to relax. The calculations are sufficiently converged to allow the atomic structures to be optimized until the residual forces on the atoms are less than 0.005 eV Å−1. As for Ag/BP (010), the Ag coverage is defined with respect to the density of BP (010) surface atoms, with a monolayer (1 ML) equal to 1.3 × 1015 atoms/cm2. To model large supported Ag NPs, we considered 0.25, 0.5, 1, and 2 ML of Ag in a closepacked arrangement above the BP supercells. The respective coverages of Ag atoms in a unit cell (1 × 1) are 0.25, 0.5, 1, and 2 for 0.25, 0.5, 1, and 2 ML, respectively. The adsorption energy Eads is calculated using the equation
resolution of 0.5 eV. Tapping-mode AFM was performed with a Dimension 3100 instrument (Veeco). Evaluation of Photocatalytic Performance and Associated Active Species and Intermediate Products. The photoactivity of the as-prepared Ag/BP nanohybrids was evaluated toward aqueous RhB degradation at room temperature under ambient conditions. Commercial P25 (99.8% purity, Alfa Aesar) was tested as a reference. For m-BP and Ag/ m-BP samples, 50 mg of the photocatalyst was dispersed into a 50 mL RhB aqueous solution (1.6 × 10−5 M) in a 150 mL beaker. For f-BP and Ag/f-BP photocatalysts, 0.96 mL of RhB (1 × 10−4 M) was put into 30 mL of f-BP or Ag/f-BP supernatant (0.2 mg·mL−1), aiming to keep the same ratio of RhB concentration with photocatalyst mass, which is comparable with that of m-BP and Ag/m-BP. Subsequently, the solution was sonicated for 10 min and magnetically stirred in the dark for 40 min to ensure the complete adsorption− desorption equilibrium of RhB on the surface of photocatalysts. A 300 W xenon lamp with a 420 nm cutoff filter (λ >420 nm) was used. The light intensity in the center of the beaker was measured to be ∼300 mW cm−2 using a Newport optical power/energy meter (842-PE). During the illumination, the reaction temperature was maintained at room temperature. At certain time intervals, ∼3.0 mL of suspension was collected and centrifuged at 15000 rpm for 3 min to remove catalyst particulates. The photoactivity was monitored by measuring the solution absorbance at the maximum wavelength of 554 nm (RhB) using a Shimadzu UV-2600 spectrophotometer. The photocurrent measurements were carried on a CHI 660D electrochemical workstation using a standard three-electrode cell with a Pt-wire counter electrode, Hg/HgO reference electrode, and the working electrode in a electrolyte solution of 0.1 M Na2SO4 at zero bias voltage. The working electrode was prepared according to the following procedure. Suspensions of samples with a concentration of 0.2 mg mL−1 were sonicated for 20 min and then dropped on a 1 cm × 1 cm conductive substrate (glass coated with indium tin oxide) with micropipette. The working electrode was then dried under vacuum. The light source was a 350 W Xe lamp equipped with a 420 nm band filter, and the light intensity was about 90 mW cm−2. To detect the major active species during the photoreaction, AO (>99%, Acros), TBA (>99.5%, Alfa Aesar), and p-benzoquinone (PBQ, >99%, Acros) were introduced into RhB solution as respective scavengers for h+, •OH, and •O2−. The above control experiments were similar to photocatalytic experiments, except for the injection of scavengers prior to light illumination. The concentrations of AO, TBA, and PBQ were 10, 10, and 0.5 mM, respectively. DFT Calculations. The spin-polarized DFT calculations were performed using the CASTEP (Cambridge Serial Total Energy Package) suite of programs.67 The Kohn−Sham oneelectron equations were solved on a basis set of plane waves with an energy cutoff of 520 eV, and Vanderbilt-type ultrasoft pseudopotentials were used to describe the electron−ion interactions.68 Brillouin zone integration was approximated by a sum over special k points selected using the Monkhorst−Pack scheme.69 Enough k points (6 × 4 × 1) were chosen to ensure that there was no significant change in the calculated energies when a larger number of k points was used. The DFT-D approach to treat vdW interactions was employed by Grimme corrections to PBE.70 Our optimized lattice parameters for black P (a = 3.30 Å; b = 10.43 Å; c = 4.40 Å) are in good agreement with experimental values and previous DFT
Eads =
[En Ag/ surf − Esurf − nEAg ] (1)
n
where EnAg/surf, Esurf, and EAg represent the total energy of Ag supported on a P (010) surface, a bare P (010) surface, and a single gas-phase silver atom, respectively. n is the number of Ag atoms per unit cell. FDTD Simulations. The intensity and spatial distribution of electric fields were implemented by 3D FDTD method (solutions 8.0, Lumerical Solutions, Inc., Vancouver, Canada). The wavelength of incident light is 500 nm using the total field/ scattered field formalism and the energy of the source propagated along the z direction. Water was used as the background dielectric medium by specifying a constant background index of refraction of 1.33. The simulated area was 100 × 100 × 100 nm, and the mesh size was 0.5 nm. The refractive indices in the visible spectral regime reported by Zhang et al.75 were adapted to mimic the BP structure. The simulation geometry of BP was continuous in the x and y directions and the thicknesses in the z direction were 2 and 10 nm, respectively. Ag NPs were half-spheres with sizes of 20 and 40 nm, respectively. In the case of Ag dimers, the interparticle distance was 1 nm. All simulations were periodic in the x and y directions, and the perfectly matched layer (PML) was used in the z direction.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.6b02520. Calculated adsorption energy and Ag−P bond length for Ag/P(010), EELS spectrum of BP nanosheets, PDOS of Ag and P for Ag/P (010) tetralayer, PDOS of Ag adlayers before and after Ag deposition, and XRD patterns of bulk-like BP crystals (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail for P.L.:
[email protected]. *E-mail for G.L.:
[email protected]. *E-mail for M.L.:
[email protected]. Notes
The authors declare no competing financial interest. 8018
DOI: 10.1021/acscatal.6b02520 ACS Catal. 2016, 6, 8009−8020
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ACS Catalysis
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ACKNOWLEDGMENTS We gratefully acknowledge financial support from the National Natural Science Foundation of China (91027042, 21321063, 51272048). The first-principles calculations were performed at Brookhaven National Laboratory, which was founded by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract No. DE-SC-00112704. The first-principles calculations were performed using computational resources at the Center for Functional Nanomaterials, a user facility at Brookhaven National Laboratory, and at the National Energy Research Scientific Computing Center (NERSC), which is supported by the Office of Science of the U.S. DOE under Contract No. DE-AC02-05CH11231.
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