AgCl Nanostructures - ACS Publications

Dec 9, 2014 - Spherical and Sheetlike Ag/AgCl Nanostructures: Interesting. Photocatalysts with Unusual Facet-Dependent yet Substrate-. Sensitive React...
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Spherical and Sheetlike Ag/AgCl Nanostructures: Interesting Photocatalysts with Unusual Facet-Dependent yet SubstrateSensitive Reactivity Yunfan Shen,†,‡ Penglei Chen,*,†,‡ Dan Xiao,†,‡ Chuncheng Chen,† Mingshan Zhu,† Tiesheng Li,‡ Wangong Ma,† and Minghua Liu† †

Beijing National Laboratory for Molecular Science, CAS Key Laboratory of Colloid, Interface, Chemical Thermodynamics, Institute of Chemistry, Chinese Academy of Sciences, No. 2 Zhongguancun Beiyijie, Beijing 100190, People’s Republic of China ‡ College of Chemistry and Molecular Engineering, Zhengzhou University, 100 Science Road, Zhengzhou, Henan 450001 S Supporting Information *

ABSTRACT: We herein report that spherical and sheetlike Ag/AgCl nanostructures could be controllably synthesized by means of chemical reactions between AgNO3 and cetyltrimethylammonium chloride (CTAC) surfactant. In this synthesis system, AgNO3 works as the silver source, while CTAC serves not only as the chlorine source but also as the directing reagent for a controllable nanofabrication. We show that compared to the spherical Ag/AgCl nanostructures, the sheetlike counterparts, wherein the AgCl nanospecies are predominantly enriched with {111} facets, could exhibit superior catalytic performances toward the photodegradation of methyl orange. Interestingly, we further demonstrate that when 4-chlorophenol or phenol is used as the substrate, the sheetlike Ag/AgCl nanostructures exhibit inferior catalytic reactivity, whereas the spherical counterparts display superior catalytic performances comparatively. Our results disclose new insights on the facet-dependent catalytic performances with regard to a facet-selective but substrate-sensitive photoinduced electron−hole separation.

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INTRODUCTION In recent years, nanostructured architectures with a controlled yet well-defined morphology have drawn broad attention from a wide variety of scientific communities.1−6 This stems from their intriguing morphology−sensitive physicochemical properties, which make them of fundamental interest in numerous frontier areas of paramount importance, including energy harvesting/conversion/storage, sensors, optoelectronics, and catalysis, etc.1−6 Among these issues of general concern, heterogeneous nanocatalysts of a unique architecture, which are predominately enriched with a uniform crystal facet, have gained particular attention.1−4 On one hand, by taking advantage of their well-defined crystal facets, the intrinsic correlation between surface atomic arrangement and catalytic performance, which is one of the most important issues in nanocatalysis, could be disclosed. This enables them to be the ideal scientific platform for heterogeneous catalysis research. On the other hand, by virtue of the derived scientific principles, promising catalysts of superior catalytic performances could be selected.1−4 So far, numerous state-of-the-art nanocatalysts have been developed with regard to morphology-dependent catalytic performances.1−4 The morphology-sensitive catalytic reactivity could generally be understood in terms of the different reactivity induced by specific crystal facets selectively exposed by an anisotropically shaped nanoarchitecture.1−4 However, in © XXXX American Chemical Society

most of the cases, the conventional investigations focus on the catalytic behaviors of catalysts of different facets toward the same model reactions. Although the relationship between surface/electronic structure and catalytic reactivity could be clarified to some extent, and advanced nanocatalysts with superior performances could thus be identified, it is evident that our current understanding on the correlation between facet and catalytic performance is still insufficient. For instance, a crucial question in this aspect is that, besides the probed model reactions, does the high-reactive facet conventionally identified via facet-dependent catalytic behaviors still display superior catalytic performances toward other model reactions? If not, what might be the underlying causes for such facet-dependent but substrate-sensitive behaviors? Undoubtedly, an investigation on a substrate-sensitive catalytic behavior might shed some insights on a deep understanding of the underlying facetdependent catalytic reactivity. More importantly, it might provide researchers with varied opportunities for the investigation of nanocatalysts of desired yet tunable selectivity and reactivity, which is an issue of significant importance in nanocatalysts. In this contribution, we report that in terms of chemical reactions between AgNO3 and cetyltrimethylammonium Received: April 7, 2014

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As known, CTAC surfactant is one of the most frequently addressed directing reagents for controllable nanofabrication, where the shape/morphology of the produced nanostructures could be easily tuned by the concentration of the surfactants.8−10 For the surfactant-assisted controllable nanofabrication, it is widely known that the morphology/shape of the produced nanostructures could be easily controlled by the concentration of the employed surfactant.8−10 Generally, welldefined anisotropically shaped nanoarchitectures are inclined to be produced when a high-concentration surfactant solution is used and vice versa. Together with this basic information, our present results indicate that the CTAC surfactant in our synthesis system works not only as the chlorine source for the production of AgCl species but also as a directing reagent for a controllable nanofabrication. The component of our nanostructures was investigated by means of energy-dispersive X-ray spectroscopy (EDX) analysis. As shown in Supporting Information Figure S1a, in the case of the spherical nanostructures, except for Ag and Cl species, no other elements could be detected. A semiquantitative analysis indicates that the atomic ratio between Ag and Cl elements is approximately 1.04:1. This value is higher than the theoretical stoichiometric atomic ratio between Ag and Cl species in AgCl, which should be 1:1. These results indicate an ambient light induced generation of metallic Ag0 species in the formulated nanostructures, resulting in the formation of Ag/AgCl nanospecies.7,8,11 This could be further confirmed by the X-ray photoelectron spectroscopy (XPS) investigations. As shown in Figure 2a, for the spherical nanostructures, the Cl species

chloride (CTAC) surfactant, Ag/AgCl nanoarchitectures of spherical and sheetlike morphologies could be controllably fabricated by means of tuning the concentration of CTAC aqueous solution. Spherical nanospecies are produced when a low-concentration CTAC aqueous solution is used, while sheetlike nanostructures are generated when a high-concentration CTAC aqueous solution is employed. Importantly, we show that compared to the spherical Ag/AgCl nanostructures, the sheetlike counterparts, wherein the AgCl nanospecies are predominantly enriched with {111} facets, could display distinctly boosted catalytic performances toward the photodegradation of methyl orange (MO) pollutants under visible light or UV light irradiation. Our results indicate that a more effective photoinduced electron−hole separation, which is promoted by the unique structure and electronic features of AgCl {111} facets, plays a crucial role for their intrinsic superior catalytic reactivity. More fascinatingly, we further demonstrate that when our catalysts are used to catalyze the photodegradation of 4-chlorophenol (4-CP) or phenol, the sheetlike Ag/AgCl nanostructures display inferior catalytic reactivity, while the spherical counterparts exhibit superior catalytic performances comparatively. Beyond the high-reactive catalytic facets conventionally validated by facet-dependent catalytic investigations, our new results might shed deep insights into the facet-dependent catalytic performances with regard to a facet-selective but substrate-sensitive photoinduced electron−hole separation, which might be further extended to other nanocatalysts. This might provide researchers with important clues for the design of nanocatalysts of well-defined facets and desirable yet tunable catalytic selectivity and reactivity, which are potentially applicable in separation, selective transformation, and purification of targeted organics, etc.

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RESULTS AND DISCUSSION Experimentally, our Ag/AgCl nanostructures were synthesized by means of adding an aqueous solution of AgNO3 into an aqueous solution of CTAC of different concentrations under ambient conditions. In this fabrication system, AgNO3 worked as the silver source, whereas CTAC worked as the chlorine source.7,8 The obtained nanomaterials were washed thoroughly with Mill-Q water by means of repeated centrifugation and resuspension, after which they were subjected to various characterizations and catalytic uses. As shown in Figure 1a,

Figure 2. XPS spectra of Cl 2p (a and b) and Ag 3d (a′ and b′) of our nanospheres (a and a′) and nanosheets (b and b′).

display a binding energies of Cl 2p3 and Cl 2p1 at about 197.7 and 199.3 eV, respectively. Besides, two bands at ca. 367.5 and 373.5 eV, which could be ascribed to Ag 3d5/2 and Ag 3d3/2 binding energies, respectively, could also be observed (Figure 2a′). These two bands could be further deconvoluted into two sets of peaks at 367.5, 368.4 eV, and 373.5, 374.4 eV, respectively. Those at 367.5 and 373.5 eV could be assigned to the Ag+ species of AgCl, while those at 368.4 and 374.4 eV could be attributed to the metallic Ag0 species.7,8,11,12 The semiquantitatively calculated mole ratio between Ag0 and Ag+ is

Figure 1. Typical SEM images of our nanospheres (a) and nanosheets (b).

nanospheres with a size of ca. 140−240 nm were produced when a low-concentration aqueous solution of CTAC was used. On the other hand, well-defined nanosheets (Figure 1b) with a thickness of ca. 55−120 nm and a lateral dimension of ca. 240− 590 nm were produced, when the concentration of the CTAC aqueous solution was 6 times increased. B

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approximately 1:28. These results confirm the existence of Ag0 species in the obtained spherical nanostructures, further indicating the formation of Ag/AgCl. As shown in Supporting Information′ Figure S1b and Figure 2b,b′, nearly similar EDX and XPS results are obtained in the case of the sheetlike nanoarchitectures, indicating that our nanosphere and nanosheet basically have a similar component. The UV−visible diffuse reflectance spectra of our nanostructures were measured. As shown in Figure 3a, in addition to the

These results indicate that there exist some differences between the AgCl species of our spherical and sheetlike nanostructures. Our Ag/AgCl nanostructures were fabricated under ambient conditions without special protections. In order to disclose how the Ag0 species were generated, we carried out the synthesis work in a strictly darkened room. As shown in Supporting Information Figure S3a, the obtained products in these cases display almost no plasmon resonance absorptions in the visible region, suggesting that the generation of the Ag0 species could be nearly forbidden under this circumstance. This deduction could be further confirmed by the powder X-ray diffraction (PXRD) pattern of the thus-synthesized products shown in Supporting Information Figure S3b, wherein only diffraction peaks attributed to AgCl could be observed, while those ascribed to Ag0 species could not be discerned. These observations indicate that the generation of Ag0 species in our samples resulted from the ambient light. Similar results have been observed in our previous works.7,8,11 The susceptibility of our Ag/AgCl nanostructures toward the high-energy electron beam makes it difficult to investigate the internal structure of their AgCl species via TEM.19 We then turned to the PXRD investigations, as shown in Figure 3b. In the case of the nanosheets, their PXRD exhibits diffraction peaks (2θ) at ca. 27.7°, 32.2°, 46.2°, 54.8°, and 57.4°, which could be indexed to the {111}, {200}, {220}, {311}, and {222} facets of the typical cubic phase AgCl (JCPDS file no. 311238), respectively.8 At the same time, a weak diffraction peak at ca. 38.1°, which could be indexed to the {111} facet of the cubic phase metallic Ag (JCPDS file no. 65-2871), could also be observed.8 Together with the SPR absorptions observed in the visible region (Figure 3a), and the results of EDX (Supporting Information Figure S1) and XPS (Figure 2) investigations, these present experimental facts solidly indicate the formation of Ag/AgCl species. Notably, in the PXRD pattern of the AgCl species of our nanosheets, the intensity ratio of the {111} and {222} peaks to the {200} peak, estimated using {200} as a reference (that is, 100), is ca. 468.3 and 125.9, respectively. These values are substantially larger than those reported for the standard JCPDS file of AgCl, which are generally ca. 51.3 and 17.4, respectively. The observation of the exceptionally prominent {111} and {222} peaks indicates that the AgCl of our nanosheets are enriched with {111} facets. In contrast, the PXRD curve of our nanospheres manifests itself as a pattern nearly similar to that of the standard JCPDS file of AgCl except that the intensity ratios of the {111}, {220}, {311}, and {222} peaks to the {200} peak (using {200} as a reference), are ca. 44.8, 51.5, 14.2, and 15.1, respectively. These values are smaller than those of the corresponding data of the standard JCPDS file of AgCl, which are 51.3, 59.4, 18.2, and 17.4, respectively. These results indicate that the AgCl species of our nanospheres are relatively enriched with {100} facets, although they are enclosed by mixed crystal planes. The success in controlling the exposed facets motivates us to investigate their facet-dependent catalytic performances in terms of photodegradation of MO and 4-CP pollutants, which are frequently addressed model reactions for photocatalytic reactivity evaluation.7,8,13,19,20 The visible-light-driven (λ > 420 nm) photodegradation of MO was explored first, where the progress of the reaction was monitored by measuring the real-time UV−vis absorption of MO at ca. 463 nm, as representatively shown in Supporting Information Figure S4. The rate constant of the photocatalytic performances, in terms

Figure 3. Panel a: UV−visible diffuse reflectance spectra of our nanospheres (black) and nanosheets (red). Panel b: XRD patterns of the standard JCPDS file of AgCl (black) and of our nanospheres (red) and nanosheets (blue). The diffraction peaks ascribed to AgCl and Ag0 species are marked with ▼ and ◆, respectively.

absorptions in the ultraviolet region, which are aroused by AgCl species, both samples display broad and strong absorptions in the visible region. Commonly, plain AgCl species could only display evident absorptions in the ultraviolet region but negligible absorptions in the visible region.7,8 Together with the above-mentioned results of EDX and XPS investigations, these observations further imply the existence of metallic Ag0 species in our nanostructures, which can provoke surface plasmon resonance (SPR) absorptions in the visible region.13 Compared to the SPR peak maximum of our nanospheres (ca. 500 nm), that of nanosheets displays an evident bathochromic shift (ca. 550 nm). It is known that the plasmonic absorption of nanostructured Ag species depends strongly on their size and shape and so on, wherein broad SPR peaks could generally be observed from the system composed of Ag nanoarchitectures of various shapes or sizes.13−17 A similar broad SPR peak has been reported by other researchers during their investigation on the UV−visible spectra of the Ag/AgCl species.12,18 We have attempted to investigate the size/shape of the Ag 0 species of our nanostructures via transmission electron microscopy (TEM). Unfortunately, it is found that the samples suffered a fast decomposition soon after the focusing. This makes it difficult to identify the shape/size of the Ag0 species via TEM. Nevertheless, we suggest that the broad SPR peaks observed from our samples might be, to a great extent, owing to the existence of Ag0 species of various sizes and shapes.13−17 The experimental facts observed from the UV−visible diffuse reflectance spectra of our samples indicate that there might be some differences in the Ag0 species of our nanospheres and nanosheets either in size or shape.13−17 At the same time, it is noted that, in contrast to the absorption edge of the nanospheres (ca. 396 nm), that of the sheetlike nanostructures bathochromically shifts to a longer wavelength (ca. 412 nm). As shown in Supporting Information Figure S2, the bandgap of the nanospheres is estimated to be ca. 3.0 eV, which is larger than the ca. 2.9 eV of the nanosheets. C

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550 ± 15 nm (around the SPR peak maximum of nanosheets). As shown in Figure 4b,b′ and Supporting Information Figures S5b, S6, and S7 and Table S1, similar to the results of visiblelight-driven catalytic reactions, compared to the nanospheres, our nanosheets in these cases also display superior catalytic performances toward MO photodegradation. A correlation of the rate constants with the extinction coefficients was conducted in terms of calibrating the rate constants using extinction coefficients of the catalysts at various wavelengths. As summarized in Supporting Information Table S1, it is found that a similar comparison tendency as that before the correlation could be obtained. These results indicate that compared to the Ag/AgCl nanospheres, the sheetlike counterparts are superior catalysts toward MO photodegradation. To elucidate these results, the electronic structure of the {111} and {100} facets of AgCl, which are enriched in our nanosheets and nanospheres, respectively, were investigated via density functional theory (DFT) method. In agreement with the recent results,22 the bandgap of the {111} facet is estimated to be much narrower than that of the {100} facet, as shown in Figure 5. This verifies that the different facets of AgCl could endow our nanospheres and nanosheets with distinct absorption edges and bandgaps, as shown in Figure 3a and Supporting Information Figure S2. One the other hand, our calculation result indicates the AgCl {111} facet might manifest itself in terms of two possible surface structures, which are entirely exposed with Cl− (AgCl {111}−Cl) or Ag+ (AgCl {111}−Ag) in the outer layers, as shown in Figure 6a,b,

of the correlation between ln(C/C0) and the reaction time (t), was deduced by a kinetic linear simulation of the photocatalytic performance curves.21 As plotted in Supporting Information Figure S5, there is a linear correlation between ln(C/C0) and the reaction time for the catalytic reactions. This indicates that the decomposition reaction of MO over our Ag/AgCl-based catalysts follows the first-order kinetics. As shown in Figure 4a, when our nanospheres are used as photocatalyst, ca. 44.6% MO molecules are decomposed after

Figure 4. Photocatalytic performances (a and b) and the corresponding reaction rate constants (a′ and b′) for the photodegradation of MO over our nanospheres (black) and nanosheets (red) under visible (a and a′, λ > 420) and UV (b and b′, λ = 365 nm) light irradiations.

the visible-light irradiation proceeds for 30 min. The rate constant of the photocatalytic reaction in this case is estimated to be ca. 1.8 × 10−2 min−1, as shown in Figure 4a′ and Supporting Information Figure S5a. In contrast, when our nanosheets are employed as catalyst, ca. 95.9% MO molecules are photodecomposed immediately under similar experimental conditions (Figure 4a). In this case, the rate constant is evaluated to be ca. 1.1 × 10−1 min−1 (Figure 4a′ and Supporting Information Figure S5a). This value is substantially larger (specifically speaking, 6.1 times) than that of the nanosphere-catalyzed system. Experimentally, our photocatalytic reactions were also performed under UV light (λ = 365 nm) irradiation or in combination with a bandpass filter of 500 ± 15 nm (around the SPR peak maximum of nanospheres) and

Figure 6. Relaxed geometries of the {111}−Cl (a), {111}−Ag (b), and {100} (c) surfaces of AgCl optimized via DFT calculations.

respectively. A total energy calculation suggests that the AgCl {111}−Ag surface, which is enclosed by Ag+, is more stable than the AgCl {111}−Cl surface, which is enclosed by Cl−. This indicates that our AgCl {111} facets are most possibly exposed with more Ag+, as reported for the {111} facets of AgBr.23,24 It can be seen from Figure 6a,b that the AgCl {111} facet displays a layered structure, which is characterized by Ag+ slabs interleaved with Cl− slabs. These unique surface structure and electronic features of the AgCl {111} facets would induce the

Figure 5. Total density of states (TDOS) and projected DOS (PDOS) of AgCl {100} (a), AgCl {111}−Cl (b), and AgCl {111}−Ag (c). D

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presence of surface-perpendicular internal static electric fields directing from the outer to the inner of the AgCl species.25 As suggested by a projection of the total density of states (DOS) shown in Figure 5, the valence band (VB) top of AgCl is composed of hybrid orbitals of Cl and Ag, while the conduction band (CB) bottom mainly consists of Ag orbital. Consquently, the existence of such electric fields would promot a more effective photoinduced electron−hole separation, thus endowing our nanosheets with intrinsic superior catalytic reactivity. In contrast, the calculation result also indicates that the slabs of the AgCl {100} facet are structurally composed of both Ag+ and Cl−, as shown in Figure 6c. In this case, owing to the coexistance of Ag+ and Cl− in each slab, there exists no surfaceperpendicular internal static electric fields in the AgCl {100} species. Consquently, the promoted photoinduced electron− hole separation occurring in the case of AgCl {111} could not ocurr in AgCl {100}. Accordingly, compared to the Ag {111} facet, Ag {100} exhibts relatively less effective photoinduced electron−hole separation. This endows our nanospheres with inferior catalytic reactivity comparatively. To experimentally verify the preceding theoretical results, the decay kinetics of the CB photoelectrons were investigated by means of time-resolved microwave photodielectric spectroscopy.26−30 As shown in Figure 7, the photoelectron decay

estimated by a kinetic linear simulation of the photocatalytic activity curves with regard to the correlation between ln(C/C0) and the reaction time (t). As plotted in Supporting Information Figure S9, a linear correlation between ln(C/C0) and the reaction time (t) could be observed, suggesting that the photocatalytic degradation of 4-CP molecules over our catalysts also follows the first-order kinetics. As shown in Figure 8a,

Figure 8. Photocatalytic performances (a and b) and the corresponding reaction rate constants (a′ and b′) for the photodegradation of 4-CP over our nanospheres (black) and nanosheets (red) under visible (a and a′, λ > 420) and UV (b and b′, λ = 365 nm) light irradiations.

Figure 7. Kinetic decay traces of the photoelectrons of our nanospheres (a) and nanosheets (b) adsorbed without (bottom panels) and with (top panels) 4-CP. The red lines show the best fit to the measured traces.

when our nanospheres are used as photocatalyst, ca. 63.2% 4CP molecules are eliminated after the visible-light irradiation proceeds for 90 min, wherein a rate constant of ca. 1.1 × 10−2 min−1 (Figure 8a′ and Supporting Information Figure S9a) is obtained. Unexpectedly, when our nanosheets are used as photocatalyst, only ca. 10.3% 4-CP molecules are decomposed under similar experiemental condictions (Figure 8a). In this case, a rate constant as low as ca. 5.9 × 10−4 min−1, which is substantially smaller (specifically speaking, 18.6 times) than that obtained over our nanospheres, is obtained (Figure 8a′ and Supporting Information Figure S9a). Moreover, when these photocatalytic reations were performed under UV (λ = 365 nm) light irradiation or in combination with a bandpass filter of 500 ± 15 or 550 ± 15 nm, similar comparison results are observed, as shown in Figure 8b,b′ and Supporting Information Figures S9b, S10, and S11 and Table S2. A correlation of the rate constants with the extinction coefficients was also conducted in terms of calibrating the rate constants using extinction coefficients of the catalysts at various wavelengths. As summarized in Supporting Information Table S2, it is found that a comparison tendency similar to that before the correlation could be obtained. These surprising results indicate that our nanosheets of an intrinsic superior catalytic performance toward the photodegradation of MO pollutant become an inferior catalyst toward the photodecomposition of 4-CP molecules. This suggests a facet-dependent but substratesensitive photocatalytic reactivity.

characteristics of our nanospheres and nanosheets both display an exponential attenuation. The lifetime of the photoelectrons (τ = −dt/d[ln(Vt/V0)], where Vt represents the photoelectron signal intensity at a time t and V0 represents the maximum of the photoelectron signal intensity), estimated from a semilogarithmic fitting of the photoelectron decay curves,26−28 is ca. 56.9 ns for our nanospheres (Figure 7a, bottom panel). On the other hand, in the case of the nanosheets, a lifetime of ca. 124.2 ns (Figure 7b, bottom panel), which is distinctly larger than that of nanospheres, is obtained. These experimental results are in good agreement with the theoretical results, confirming that compared to those of AgCl {100} facets, the photoelectrons within the CB of AgCl {111} facets are more stable. This suppresses the electron−hole recombination and facilitates a more effective photoinduced electron−hole separation, and accordingly endows our nanosheets with intrinsic superior catalytic reactivity. To further verify the intrinsic superior photocatalytic activity of our nanosheets, their catalytic performances toward 4-CP photodegradation were also investigated. Here, the progress of the photodegradation was investigated by measuring the realtime UV−vis absorption of 4-CP at ca. 280 nm, as representatively shown in Supporting Information Figure S8. The rate constants of the photocatalytic performances were also E

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compared to the nanospheres, the sheetlike counterparts could display boosted catalytic reactivity toward MO photodegradation. Note that owing to the large delocalization system of the MO molecules, besides the photoexcitation of the catalyst itself, the dye-sensitization mechanism could play an important role for their decolorization. However, no matter where the photogenerated electrons in the CB bond come from, the long lifetime of the photogenerated electrons in the CB bond of AgCl {111} is responsible for the higher catalytic reactivty of the nanosheets. It has been recently reported that driven by the synergistic effect of cation−π and cation−hydroxyl interactions, Ag+ could form complexes with phenol derivatives,35−37 wherein there occurs electron transfer from phenol to Ag+.35 Consequently, the photoinduced electron transfer from VB to CB in the case of the nanosheets could be hampered to some extent when 4CP is used as substrate, since the unique surface structure of the Ag {111}, which is almost entirely enclosed by Ag+, facilitates the formation of a Ag+−phenol complex. This favors a less efficient photoinduced electron−hole separation and promotes electron−hole recombination, resulting in a shortened lifetime of CB photoelectrons (Figure 7b, top panel), and thus making our nanosheets an inferior catalyst toward the photodegradation of 4-CP. In contrast, for nanospheres, this 4-CP accelerated electron−hole recombination effect could be avoided to a great extent, because their AgCl {100} facets are composed of both Ag+ and Cl−, which dilutes the density of Ag+ and thus disfavors the formation of a Ag+−phenol complex, rendering our nanospheres a comparatively superior catalyst toward 4-CP photodegradation. This could be supported by the fact that the lifetime of the photoelectrons of the nanospheres adsorbed with 4-CP molecules (ca. 74.0 ns) is evidently larger than that of the sheetlike counterparts (ca. 54.1 ns), as shown in Figure 7. Experimentally, we further verify this by means of the photocatalytic degradation of phenol (Supporting Information Figures S12−S16 and Table S3), wherein compared to the nanospheres, our nanosheets enriched with the so-called intrinsic high-reactive {111} facet display relatively inferior catalytic performances.

However, as experimentally and theoretically clarified in the above paragraphs, the unique layered structure of the AgCl {111} facets and their entirely exposed Ag+ could induce a facet-perpendicular internal static electric fields, which directs from the outer to the inner of the AgCl {111} planes. The presence of such static electric fields could stabilize the CB photoelectrons, rendering our nanosheets an intrinsic superior photocatalyst. On the other hand, this could not ocurr in the case of the spherical nanostructures, which is due to the coexitance of the Ag+ and Cl− species in each slab of the Ag {100} facet, making the nanospheres have relatively inferior catalytic reactivity. The preceding unexpected results indicate that the decay kinetics of the CB photoelectrons might be altered when 4-CP is used as the substrate. To verify this, the time-resolved microwave photodielectric spectra of our nanospheres and nanosheets adsorbed with 4-CP were investigated, as shown in Figure 7. For nanospheres (Figure 7a, top panel), a lifetime of ca. 74.0 ns, which is larger than that of the corresponding value of the bare nanospheres measured before the 4-CP adsorption (ca. 56.9 ns), is obtained. This is in good accordance with the results of photocatalytic experiments, suggesting the occurrence of 4-CP photodegradation in the nanosphere system. Interestingly, for our nanosheets (Figure 7b, top panel), a lifetime of ca. 54.1 ns, which is substantially smaller than the corresponding value of the bare nanosheets measured before the 4-CP adsorption (124.2 ns), is obtained. The shortened lifetime in this case indicates that when our nanosheets are used to catalyze 4-CP photodegradation, the intrinsic effective photoinduced electron−hole separation occurred in AgCl {111} is suppressed greatly. This promotes the electron−hole recombination and thus leads to an inferior catalytic performance. For the catalytic mechanism of the Ag/AgCl-based photocatalysts, it is widely accepted that the photogenerated electrons and holes would be trapped by oxygen and would combine with Cl−, which would lead to the formation of reactive oxygen species and Cl0, respectively.18,31−34 The asgenerated reactive oxygen species and Cl0 would work as reactive species for the degradation of organic pollutants, where the catalytic reactivity depends, to a great extent, on the separation of the photoinduced holes and electrons. As indicatecd by our calculation results, the AgCl {111} facets have a layered structure, which is characterized by interleaved Ag+ and Cl− slabs with the former exposed in the outer layer (Figure 6b). This could induce surface-perpendicular internal static electric fields in Ag {111}, which direct from the outer to the inner of the structure. As verified by a projection of the DOS shown in Figure 5, the VB top of AgCl is composed of hybrid orbitals of Cl and Ag, while the CB bottom mainly consists of Ag orbital. Thus, the photoinduced electrons in CB could be stabilized by these internal static electric fields, promoting an efficient separation of the photoinduced holes and electrons. On the other hand, as illustrated in Figure 6c, both Ag+ and Cl− coexist in each slab of Ag {100}. This leads to the absence of such internal static electric fields in Ag {100}. As a result, the as-facilitated photoinduced electron−hole separation in the nanosheets could not happen in the nanospheres. These proposals could be experimentally verified by the time-resolved microwave photodielectric spectroscopy shown in Figure 7, wherein it is shown that the lifetime of the photoelectrons of the nanospheres (ca. 56.9 ns) is evidently smaller than that of the nanosheets (ca. 124.2 ns). The result of these issues is that

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CONCLUSION In conclusion, using spherical and sheetlike Ag/AgCl nanoarchitectures as examples, our investigation indicates that the intrinsic highly catalytically reactive facets, which are traditionally identified via a facet-dependent catalytic performance toward specific model reactions, might not be necessarily superior catalysts toward some other model reactions. The electronic and surface structures of a certain facet could endow it with intrinsic superior catalytic reactivity, while they could instead endow them with inferior catalytic performances in some other cases, which depend not only on the crystal facets but also on the investigated substrates. These insights in terms of a facet-dependent but substrate-sensitive photoinduced electron−hole separation might be further extended toward other nanocatalysts. This likely provides a varied avenue for the design of high-quality nanocatalysts of desirable yet tunable catalytic selectivity and reactivity, which is of crucial importance for highly efficient organics separation, fine synthesis, selective/ target chemical transformation, and purification, etc.

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EXPERIMENTAL SECTION

Chemicals and Materials. Silver nitrate (AgNO3, Alfa Aesar, >99%), cetyltrimethylammonium chloride (CTAC, Alfa Aesar, 96%), F

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methyl orange (MO, Alfa Aesar, >98%), 4-chlorophenol (4-CP, Alfa Aesar, 99%), and phenol (Beijing Chemical Works, A.R. > 98%) were used as received without further purification or treatment. Milli-Q water (18 MΩ cm) was used as solvent in all cases. Fabrication of Ag/AgCl Nanoarchitectures. To synthesize Ag/ AgCl nanospheres, a 600 μL aqueous solution of AgNO3 (5 × 10−2 mol L−1) was added dropwise into a 10 mL aqueous solution of CTAC (2 × 10−2 mol L−1) within ca. 6 min at room temperature under vigorous magnetic stirring. Herein, AgNO3 served as the silver source, while CTAC worked as the chlorine source and also as the surfactant template for a controllable synthesis.8 The stirring was maintained for another 20 min under ambient conditions, after which a milky dispersion was obtained. Then, the as-produced dispersion was treated by centrifugation (10000 rpm, 10 min), and the obtained solids were collected and washed thoroughly with ultrapure Milli-Q water by repeating centrifugations. In the case of the sheetlike Ag/AgCl nanostructures, almost similar operations were carried out, except that a concentrated aqueous solution of CTAC (1.2 ×10−1 mol L−1) was employed. The synthesized nanstructures were subsequently subjected to vairous characterizations or uses. Our Ag/AgCl nanostructures were fabricated in our laboratory under ambient conditions on a sunny day or a cloudy day with or without special protection, and the storage of the as-fabricated products was also performed under similar conditions, where it was not so easy to artificially control the intensity of the ambient light. Under the above-mentioned ambient conditions, we have measured the intensity of the ambient light during our nanofabrication. The results showed that the intensity of the ambient light was about 0.05− 0.75 mW cm−2. To characterize the samples using various apparatuses, it generally needed to wait for 5 h to ∼7 days to gain access to the related apparatus. The specific waiting time depended essentially on whether the instruments were available or not. Thus, the irradiation time of our samples by the ambient light roughly ranged from ca. 5 h to 7 days, where the intensity of the ambient light ranged approximately from 0.05 to 0.75 mW cm−2. We found that nearly similar results could be obtained from the samples formulated under different intensities of the ambient light (0.05−0.75 mW cm−2) and with different irradiation times (from ca. 5 h to 7 days). This was similar to our previous investigations on the Ag/AgCl-based photocatalysts.11 Together with the results presented in Results and Discussion, we suggest that the generation of Ag0 in our samples was attributed to the ambient light, while the intensity of the ambient light and the irradiation time under our experimental conditions might affect the nature of our sample negligibly, although we could not exclude their contribution absolutely. Photocatalytic Performances. For photodegradation of MO pollutants, photocatalysts typically involving 2.5 mg of Ag/AgCl nanospecies were dispersed in a 4 mL aqueous solution of MO (40 mg L−1), wherein a quartz cuvette was used as the reactor. In the cases of the photodegradation of 4-CP and phenol pollutants, almost similar experimental conditions were employed except that 5 mg amounts of Ag/AgCl photocatalysts were invloved. A 500 W xenon arc lamp installed in a laboratory lamp housing system (CHF-XM35-500 W, Beijing Trusttech Co. Ltd., China) was used as the light source. In the case of the visible-light-driven photocatalytic performances, the light passed through a 10 cm water filter and a UV cutoff filter (> 420 nm) before entering the reactor. The photocatalytic performances of our catalysts were also evaluated in combination with a bandpass filter of 500 ± 15 or 550 ± 15 nm, wherein the light generated by the 500 W xenon arc lamp passed the assigned bandpass filter before entering the reactor. On the other hand, the photocatalytic experiments were also carried out under UV light (λ = 365 nm) irradiation. In this case, the catalytic system was irradiated by a UV−light emitting diode (UV−LED; UVEC-4II, Beijing Trusttech) light source of a wavelength of 365 nm. Experimentally, the reaction system was kept for 30 min in a dark room to achieve an equilibrium adsorption state before light irradiation. Actually, the stirring time that was performed in the dark had been extended from 30 min to 24 h, wherein we found that the equilibrium adsorption state could be achieved within 30 min. During

the photocatalytic performances, an aliquot of the dispersion (0.3 mL) was taken out from the reaction system for real-time sampling. The progress of the photodegradation of MO pollutants over our photocatalysts was investigated by measuring the real-time UV−vis absorption of MO at ca. 463 nm. In the case of 4-CP and phenol, the progress of the degradation was investigated by measuring the realtime UV−vis absorption of 4-CP and phenol at ca. 280 and 270 nm, respectively. For the evaluation of the photocatalytic activities, C is the concentration of the respective organic pollutant at a real-time t, and C0 is that immediately before it was kept in the dark. The rate constant of the photocatalytic performances, in terms of the correlation between ln(C/C0) and the reaction time (t), was deduced by a kinetic linear simulation of the curves of the photocatalytic performances. Theoretical Calculations. The electronic structures of AgCl {111} and AgCl {100} were investigated via the DFT + U approach.24,38 The exchange-correlation energy functional was represented by the local-density approximation (LDA).24 An Up of 7.0 eV and an Ud of 7.2 eV were adopted. At these U values, the bandgap of AgCl was estimated to be 3.2 eV, which was consistent with the experimental value.18 The calculations were performed using the projector-augmented wave pseudopotentials as implemented in the CASTEP code.24,39 The valence configurations of the pseudopotentials were 4d105s1 for Ag and 3s23p5 for Cl. The stoichiometric 1 × 1 slab models using the rock salt (RS) cubic lattice parameters (5.41 Å), as adopted by Dai and Huang,22,40 were used to construct the surface models consisting of eight atomic layers and a total of 16 atoms for {100} and {111} surfaces with a 20 Å thickness of the vacuum layer. The energy cutoff for a plane wave basis set was 270 eV, and a Monkhorst−Pack k-mesh of 3 × 3 × 1 was used for all of the slabs. The atomic positions were fully relaxed to a force convergence of 0.03 eV/Å. Measurements of Time-Resolved Microwave Photodielectric Spectroscopy. To prepare the samples for these measurements, our original nanospheres and nanosheets (5 mg) were dispersed in a 4 mL aqueous solution of 4-CP (40 mg L−1). The dispersions were stirred for 30 min in a dark room to achieve an equilibrium adsorption state. Subsequently, the nanostructures absorbed with 4-CP molecules were obtained by centrifugation (10000 rpm, 10 min). Then, our Ag/ AgCl-based nanoarchitectures adsorbed with or without 4-CP were drop-cast onto a slice of transparent polyethylene naphthalate (PEN) film (3 × 15 mm) of a thickness of ca. 1 mm. The samples were dried in a vacuum oven at room temperature overnight, after which the asobtained specimens were subjected to the measurement of the timeresolved microwave photodielectric spectroscopy. The principle of the time-resolved microwave photodielectric spectroscopy has been described in pervious publications.28,30,41 Our measurements were performed using the transient microwave photodielectric spectrum detection system established by Fu, Li, and co-workers of the College of Physics Science and Technology, Hebei University.26−28 Experimentally, the above-prepared film samples were inserted into the middle of the microwave cavity, wherein a 35 GHz Gunn oscillator with a power of 143 mW, a quality coefficient of 186, a bandwidth of 1 GHz, and a time resolution of 1−2 ns was used as the microwave source. The samples were exposed to a YAG pulse laser (Quantel-YG901C, France; 35 ps, 355 nm) of 3 mJ. The signals were amplified by a high-magnification amplifier (SR440) and recorded with a digital storage oscilloscope (Tek. TDS3052) and sent to a computer for analysis. The lifetime of the photoelectrons (τ = −dt/d[ln(Vt/V0)], wherein Vt represents the photoelectron signal intensity at a time t and V0 represents the maximum of the photoelectron signal intensity), is estimated from a semilogarithmic fitting of the photoelectron decay curves.26−28 Apparatus and Measurements. The scanning electron microscopy (SEM) measurements were performed by using a Hitachi S-4800 system, wherein an accelerating voltage of 10 kV was employed. The energy-dispersive X-ray spectroscopy (EDX) was measured with a Horiba EMAX X-act energy-dispersive spectroscope that was attached to the Hitachi S-4800 system. In this case, an accelerating voltage of 15 kV was employed. X-ray photoelectron spectroscopy (XPS) was measured on an ESCALab220i-XL electron spectrometer from VG G

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Scientific using 300 W Al Kα radiation. The binding energies were referenced to the C 1s line at 284.8 eV from adventitious carbon. X-ray diffraction (XRD) measurements were performed on a PANalytical X’Pert PRO instrument with Cu Kα radiation. The UV−visible diffuse reflectance spectra (DRS) of the samples were obtained on an UV−vis spectrophotometer (JASCO UV-3900) using BaSO4 as the reference. The photodegradation of the organic pollutants was monitored by using a JASCO UV-3900 spectrophotometer. All of the involved measurements were conducted at room temperature.

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ASSOCIATED CONTENT

S Supporting Information *

Figures showing EDX elemental analyses of our Ag/AgCl nanostructures, the typical real-time absorption spectra of the organic pollutants during the photodegradation process, the kinetic linear simulation curves of the photocatalytic performances, and some of the photocatalytic results and tables listing summaries of the rate constants of photodegradation reactions and the corresponding calibrated rate constants. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the financial support from the National Natural Science Foundation of China (Grants 21372225, 20873159, 21321063, and 91027042), the National Key Basic Research Project of China (Grants 2011CB932301 and 2013CB834504), and the Chinese Academy of Sciences (Grants XDA09030200 and 1731300500015). Dr. Penglei Chen thanks Zhengzhou University for the “Talent Project of Distinguished Professors”.

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