Spatial Separation of Charge Carriers in In2O3–x(OH)y Nanocrystal

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Spatial Separation of Charge Carriers in In2O3−x(OH)y Nanocrystal Superstructures for Enhanced Gas-Phase Photocatalytic Activity Le He,*,†,‡ Thomas E. Wood,‡ Bo Wu,§,∥ Yuchan Dong,‡ Laura B. Hoch,‡ Laura M. Reyes,‡ Di Wang,⊥ Christian Kübel,⊥ Chenxi Qian,‡ Jia Jia,‡ Kristine Liao,‡ Paul G. O’Brien,‡ Amit Sandhel,‡ Joel Y. Y. Loh,‡ Paul Szymanski,# Nazir P. Kherani,‡ Tze Chien Sum,∥ Charles A. Mims,‡ and Geoffrey A. Ozin*,‡ †

Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University, 199 Ren’ai Road, Suzhou, Jiangsu 215123, People’s Republic of China ‡ Materials Chemistry and Nanochemistry Research Group, Solar Fuels Cluster, Center for Inorganic and Polymeric Nanomaterials, Departments of Chemistry, Chemical Engineering and Applied Chemistry, and Electrical and Computing Engineering, University of Toronto, 80 St. George Street, Toronto, Ontario M5S 3H6, Canada § Singapore−Berkeley Research Initiative for Sustainable Energy (SinBeRISE), 1 Create Way, Singapore 138602 ∥ Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link, Singapore 637371 ⊥ Institute of Nanotechnology and Karlsruhe Nano Micro Facility, Karlsruhe Institute of Technology, Hermann-von-Helmholtz Platz 1, 76344 Eggenstein-Leopoldshafen, Germany # Laser Dynamics Laboratory, School of Chemistry and Biochemistry, Georgia Institute of Technology, 901 Atlantic Drive NW, Atlanta, Georgia 30332, United States S Supporting Information *

ABSTRACT: The development of strategies for increasing the lifetime of photoexcited charge carriers in nanostructured metal oxide semiconductors is important for enhancing their photocatalytic activity. Intensive efforts have been made in tailoring the properties of the nanostructured photocatalysts through different ways, mainly including band-structure engineering, doping, catalyst−support interaction, and loading cocatalysts. In liquid-phase photocatalytic dye degradation and water splitting, it was recently found that nanocrystal superstructure based semiconductors exhibited improved spatial separation of photoexcited charge carriers and enhanced photocatalytic performance. Nevertheless, it remains unknown whether this strategy is applicable in gas-phase photocatalysis. Using porous indium oxide nanorods in catalyzing the reverse water−gas shift reaction as a model system, we demonstrate here that assembling semiconductor nanocrystals into superstructures can also promote gas-phase photocatalytic processes. Transient absorption studies prove that the improved activity is a result of prolonged photoexcited charge carrier lifetimes due to the charge transfer within the nanocrystal network comprising the nanorods. Our study reveals that the spatial charge separation within the nanocrystal networks could also benefit gas-phase photocatalysis and sheds light on the design principles of efficient nanocrystal superstructure based photocatalysts. KEYWORDS: nanocrystal superstructures, photocatalysis, charge separation, gas phase, metal oxide semiconductor

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Nevertheless, the photocatalytic efficiency of metal oxide semiconductors is often limited by very short lifetimes of excited electron−hole pairs due to the fast recombination.11−13 Intensive efforts have been made in the rational engineering of nanostructured materials to increase the lifetime of photoexcited charge carriers.14−17 For example, Murray et al.

riving chemical reactions using sunlight is an important way of harnessing clean and renewable solar energy.1−4 Inorganic metal oxide based semiconductor nanostructures are favorable photocatalysts due to their large surface area, relatively low cost, and low toxicity, as well as long-term thermal, chemical, and photochemical stability.5−9 A semiconductor may facilitate the photocatalytic process when light with energy greater than the band gap excites valence band electrons into the conduction band, thereby generating electron−hole pairs which separate and migrate to surface-active sites to trigger redox reactions.10 © XXXX American Chemical Society

Received: April 7, 2016 Accepted: May 9, 2016

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ACS Nano demonstrated that the improvement of charge carrier separation by increasing the length of TiO2 nanorods could boost the efficiency of photocatalyzed processes.18 Recently, an alternative strategy for prolonging the lifetime of photoexcited charge carriers in semiconductors was reported based on the use of nanocrystal superstructures, which exhibited improved activity in liquid-phase photocatalytic dye degradation and water splitting.19−22 It was suggested that charge transport between neighboring nanocrystals in these superstructures enables an increased efficiency in the separation of photogenerated electron−hole pairs, which in turn prolongs their lifetime and improves their photocatalytic activity.20,21 However, the potential to increase the activity of photocatalytic nanocrystals toward gas-phase chemical reactions by assembling them into nanocrystal superstructures has not yet been investigated. Regarding gas-phase photocatalysis, we recently reported that indium oxide nanocrystals with surface defects in the form of oxygen vacancies and hydroxyl groups, denoted In2O3−x(OH)y, function as a single-component photocatalyst that activates the reverse water−gas shift (RWGS) reaction.23 The RWGS reaction occurs at frustrated Lewis pair (FLP) sites located on the In2O3−x(OH)y surface. These FLP sites comprise a Lewis acidic, coordinately unsaturated surface indium site proximal to an oxygen vacancy and a Lewis basic surface hydroxide site on In2O3−x(OH)y.24 The first step involves heterolytic dissociation of H2 to form a hydride-like H bonding to the In atom and proton-like H bonding to the hydroxide group. Following the heterolytic dissociative adsorption of H2 on the In2O3−x(OH)y surface, CO2 is then adsorbed and interacts with the proton and hydride at the active site InOH2+···InH−. Finally, the transfer of the proton and hydride to adsorbed CO2 subsequently leads to the production of CO and H2O with concomitant regeneration of the hydroxides and oxygen vacancies.24 Very recently, we uncovered that photoexcited charge carriers can lower the activation energy at these FLP sites, thereby accelerating the RWGS reaction rate.25 As a result of the deep understanding of the reaction mechanism, here we use In2O3−x(OH)y nanorods composed of smaller In2O3−x(OH)y nanocrystals as a model system to investigate the effects of nanocrystal superstructures for gas-phase photocatalysis. The charge transport between In2O3−x(OH)y nanocrystals in the nanorod superstructure is identified as the key factor responsible for extending the lifetimes of photoexcited carriers and is thus recognized as the origin of the observed improvement in the photocatalytic activity. This knowledge will enable a deeper understanding of the photocatalytic process and provide insight about general strategies for optimizing the efficiency of photocatalytic materials.

Scheme 1. Preparation of In2O3−x(OH)y Nanocrystal Nanorods

indicated by the roughness of the surface (Figure 1a, inset), which was also supported by the observed aggregation behavior of primary nanowires and thin nanorods (Figure S1). The bundle-like structures were further confirmed through TEM observation of a partially etched hydroxide nanorod sample using HCl as the etchant (Figure S2). A high-resolution (HR) TEM study was further performed to define details of the nanowire-bundle structure of the nanorods (Figure 1b). The diameter of the primary nanowire was around 10 nm. The lattice constant along the wire axis was 0.4 nm, in agreement with the lattice spacing of the (200) plane of the cubic In(OH)3 structure (d200 = 0.399 nm), implying that growth of the In(OH)3 nanowire proceeds along the [100] direction. The HRTEM study also revealed that the nanowires in each nanorod were well-aligned with an identical orientation, implying that the nanorod may form a quasi-single-crystal structure through the fusion of the nanowires by aging and ripening for a longer reaction time due to the absence of surfactants in the synthesis. The anisotropic growth along the [100] direction was further confirmed by the powder X-ray diffraction (PXRD) pattern of the sample, in which the (200) and (400) peaks of cubic In(OH)3 were particularly strong, which also implies that the nanorods are preferentially oriented such that their body length is aligned parallel to the substrate surface (Figure 1c). A detailed investigation of the phase conversion of the In(OH)3 nanorods into In2O3−x(OH)y was carried out using thermogravimetric differential scanning calorimetry (TG-DSC). Figure 1d depicts the TG-DSC plot of a typical nanorod sample synthesized with a reaction time of 6 h at 80 °C. A sharp weight loss was observed after the temperature reached ∼200 °C, accompanied by an endothermic peak at ∼240 °C, which corresponds to the conversion of In(OH)3 into In2O3−x(OH)y through dehydroxylation of the hydroxyl groups.26 The total weight loss was 19.62%, which is higher than the predicted weight loss of 16.28% from the complete conversion of In(OH)3 into In2O3, possibly due to the existence of physically and chemically adsorbed water molecules, as seen by the observation of a ∼6% weight loss before 200 °C. Another endotherm peak also exists at about 305 °C, which might be the result of an ordering transition of the In2O3−x(OH)y crystals. It is worth noting that the actual transition temperature could be lower as a relatively high heating rate of 5 °C/min was used in our TG-DSC study.27 Since no surfactant is involved in the synthesis of In(OH)3, its conversion into photoactive In2O3−x(OH)y devoid of residual carbon can be achieved through thermal posttreatment under mild conditions in air at a temperature as low as 250 °C, which benefits the formation of nanoporous structures during the dehydration process, resulting in highsurface-area carbon-free materials. The calcination at such low

RESULTS AND DISCUSSION Nanocrystalline In2O3−x(OH)y nanorods are synthesized through a two-step process. In(OH)3 nanorods are synthesized first and calcined to form the Bixbyite-structured indium oxide (Scheme 1). In(OH)3 nanorods were synthesized through a slow nucleation, growth, and crystallization method using InCl3 and urea as the precursors without any surfactant. Figure 1a depicts the transmission electron microscopy (TEM) image of a typical In(OH)3 sample with a reaction time of 6 h. The obtained products show a nanorod morphology with lengths ranging from a few hundred nanometers to greater than 1 μm. A close observation of the sample reveals that each nanorod may comprise parallel aligned bundles of primary nanowires, as B

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Figure 1. (a) TEM image, (b) HRTEM image, (c) XRD pattern, and (d) thermogravimetric differential scanning calorimetry curve of a typical In(OH)3 nanorod sample with a reaction time of 6 h. Inset in (a) shows the SEM image of the sample, wherein the scale bar is 500 nm.

154 m2/g, which is 20 m2/g higher than that of our previously reported randomly oriented In2O3−x(OH)y nanocrystal samples.23 The reaction time of the initial liquid-phase synthesis of the In(OH)3 precursors was varied to determine its effect on the structural parameters of the In2O3−x(OH)y nanorods and the corresponding photocatalytic performance. Specifically, a set of five different In2O3−x(OH)y nanorod samples were fabricated from a series of In(OH)3 precursors that were synthesized for a duration of 2, 3, 5, 8, and 12 h, and these samples are denoted as S1−S5, labeled with increasing reaction time. Further, all In2O3−x(OH)y nanorod samples (S1−S5) were similarly synthesized from their In(OH)3 precursors through an annealing treatment at 250 °C in air for 6 h. As depicted in the SEM images shown in Figure 3, the overall nanorod morphology was well preserved for all In2O3−x(OH)y samples. Moreover, the nanorod length increased with reaction time of the initial liquid-phase synthesis of the In(OH)3 precursors, while the average diameter varies little between nanorod samples S1−S5 (Figure 3, Table 1, and Figures S3 and S4). The PXRD patterns of all In2O3−x(OH)y nanorod samples were very similar and can be assigned solely to cubic In2O3 (Figure 3f). The grain size of samples S1−S5, calculated from the width of the strongest PXRD peak at 30.6°, increases slightly from 12 to 13 nm, possibly caused by a slow increase in the diameter of primary In(OH)3 nanowires during the initial synthesis. Furthermore, samples S1−S5 have similar surface areas and pore size distribution (Table 1 and Figures S5 and S6). The diffuse reflectance spectra are also very similar for all samples, except that the absorption edge moves slightly to longer wavelength from S1 to S5 (Figure 4a). On the other hand, Xray photoelectron spectroscopy (XPS) measurements reveal that the surface properties of the In2O3−x(OH)y nanorods

temperature may also favor the existence of more surface defects, which would function as the active sites for gas-phase CO2 reduction reactions.23 Accordingly, the aforementioned In(OH)3 nanorods were converted into In2O3−x(OH)y nanorods through the calcination at 250 °C for 3 h. The structure, morphology, and surface area of the obtained In2O3−x(OH)y sample were characterized using selected area electron diffraction (SAED), HRTEM, and N2 adsorption−desorption studies. As depicted in Figure 2a, the overall rod-like morphology was well maintained after calcination, but the structure became nanoporous. The related SAED pattern indicates a highly oriented polycrystalline structure, and the lattice constants match well with cubic indium oxide (Figure 2b). HRTEM images reveal that each In2O3−x(OH)y nanorod consists of nanocrystals with a size of ∼10 nm (Figure 2c,d). These nanocrystals have similar orientations inside each nanorod, evidenced by the short arcs in the SAED pattern, implying they form a quasi-mesocrystal superstructure, which creates nanopores with a diameter of a few nanometers between the constituent nanocrystals. Therefore, it is expected that the nanorods will exhibit a nanoporous superstructure. An isothermal N2 adsorption−desorption study was carried out to further investigate the surface area and porosity of the nanorods. Figure 2e shows a type IV adsorption isotherm, which is consistent with the existence of pore sizes in the mesopore range according to the IUPAC convention, which defines porosity length scales. The nanoporous structure was further defined by the relatively narrow pore size distribution between 2 and 6 nm with a maximum at 3.5 nm, as determined by analysis using nonlocal density functional theory (NLDFT) (Figure 2f). As a result of its small nanocrystal component size and the nanoporous structure, the In2O3−x(OH)y nanorod sample has a Brunauer−Emmett−Teller (BET) surface area of C

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Figure 2. (a) TEM image, (b) SAED pattern, (c,d) HRTEM images, (e) N2 isothermal adsorption−desorption plot, and (f) pore size distribution calculated by NLDFT of the In2O3−x(OH)y nanorod sample treated at 250 °C for 3 h.

RWGS reaction was then investigated. To avoid any falsepositive results caused by carbon contamination, carbon-13labeled carbon dioxide (13CO2) was used as an isotope tracer molecule to identify the products originating from CO2. After 20 h of reaction at 150 °C in the dark, negligible amounts of 13 CO product were detected for all samples. On the other hand, under similar conditions but with a simulated solar light source, 13 CO was detected as an unequivocal product of the CO2 reduction reaction, which confirms that the reaction with In2O3−x(OH)y nanorods is light-driven. It was found that the CO production rate increased as the length of the nanorods increased from S1 to S5, with an unexpected 4-fold increase in rate from the shortest to the longest nanorods (Figure 4c,d). As shown in the XPS results, less defects arising from surface hydroxyl groups and oxygen vacancies are present in the longer samples (Figure S7), which should have resulted in lower, rather than higher, photocatalytic activity due to fewer active

exhibit a more obvious dependence on the duration of the In(OH)3 precursor reaction. In addition to the main oxide peak at 530.4 eV, a shoulder peak also appears in the O 1s core level spectra for all samples, which corresponds to surface hydroxyl groups and oxygen vacancies (Figure 4b and Figure S7).23 In general, the number of surface hydroxyl groups and oxygen vacancies decrease for longer In2O3−x(OH)y nanorods from S1 to S5 (Table 1). With longer initial reaction time, it is likely that In(OH)3 precursors ripen and crystallize to a greater extent, and the subsequent transformation into In2O3−x(OH)y nanorods occurs more slowly with less defects present at the nanocrystal surfaces. As discussed above, our In2O3−x(OH)y nanorods exhibit nanoporous superstructures with 3D networks of nanocrystals populated with surface hydroxyl groups and oxygen vacancies, rendering them excellent candidates for studying the effect of nanocrystal superstructures in gas-phase photocatalysis. The activity of the above five samples in catalyzing the light-assisted D

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Figure 3. (a−e) SEM images of In2O3−x(OH)y samples obtained through the calcination of different In(OH)3 precursors at 250 °C for 6 h. The initial reaction time (ti) for the In(OH)3 precursors was 2 h for S1 (a), 3 h for S2 (b), 5 h for S3 (c), 8 h for S4 (d), and 12 h for S5 (e). Scale bar is 500 nm for all insets. (f) XRD patterns of samples S1−S5.

Table 1. Summary of Properties of Different In2O3−x(OH)y Samples sample

ti (h)a

length (nm)

Eg (eV)

D (nm)b

BET surface area (m2/g)

dp (nm)c

pore volume (cm3/g)

(OH + Ov)/oxided

S1 S2 S3 S4 S5

2 3 5 8 12

800 1140 1290 1810 1830

2.89 2.90 2.88 2.90 2.87

12 12 12.5 13 13

161 160 172 151 159

3.5 3.5 3.5 3.5 3.2

0.162 0.141 0.149 0.135 0.131

0.84 0.78 0.64 0.61 0.58

CO ratee 0.23 0.26 0.36 0.74 1.20

± ± ± ± ±

0.02 0.01 0.02 0.04 0.04

a The initial reaction time in the synthesis of In(OH)3. bGrain size calculated from the Scherrer equation. cPore diameter at the peak of the NLDFTcalculated pore diameter distribution curve. dThe peak area ratio of the sum of hydroxide and oxygen vacancy to the oxide from the O 1s core level. e CO production rate under the simulated sunlight illumination (intensity of 0.8 sun), and the unit is μmol·h−1·gcat−1. Each sample was tested for four runs, and the rates were reproducible between different runs.

for catalyzing photochemical reactions, the discussion herein focuses mainly on the transient absorption spectra at the nanosecond−microsecond time scale rather than the femtosecond−picosecond time scale. Figure 5a depicts the timeresolved absorption spectra of samples S1, S3, and S5 in the visible and near-infrared range immediately after being excited with a 325 nm femtosecond laser pulse at a fluence of ∼200 μJ cm−2. Significantly, all three samples become highly absorbing over the entire visible range within a sub-nanosecond time scale after laser excitation. This absorption is associated with subband-gap trapped holes, trapped electrons, and free conduction electrons.29,32 Our study shows that trapped holes exhibit an absorption maximum at 450 nm (Figure S8), which is similar to

sites, indicating the existence of other parameters contributing to the difference in the CO2 to CO conversion rates.24,25 A key factor influencing photocatalytic activity is the lifetime of photoexcited electron−hole pairs, which directly influences the probability of surface redox reactions.28,29 It is conceivable that charge transfer may occur between neighboring nanocrystals in the as-prepared In2O3−x(OH)y nanorods, resulting in prolonged lifetimes and thus improved photocatalytic activity.21,22 Transient absorption spectroscopy was employed to measure the lifetime of charge carriers, which may provide direct evidence of retarded charge recombination due to internanocrystal charge transfer in In2O3−x(OH)y nanorods.30,31 Since long-lived electron−hole pairs are primarily responsible E

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Figure 4. (a) Diffuse reflectance spectra, (b) O 1s core level XPS spectra, (c) CO production rates, and (d) surface area normalized rates of different In2O3−x(OH)y samples under simulated sunlight irradiation.

Figure 5. (a) Time-resolved absorption spectra (nanosecond to microsecond range) observed after the 325 nm laser pulse excitation of different In2O3−x(OH)y samples in N2. (b) Schematic illustration of the photoexcited electron and hole dynamics. Surface trapping states and interparticle charge transfer benefit the spatial separation of electron/hole pairs, which drives the redox reaction. (c) Normalized transient absorption traces observed at 750 nm for S1, S3, and S5. (d) Schematic illustration of the migration of a photoexcited electron between neighboring nanocrystals.

that of TiO2.29 It is believed the trapping states for the holes are associated with surface OH groups on the In2O3−x(OH)y nanorods. Indeed, upon laser excitation, the photoinduced absorption in the wavelength region from 400 to 500 nm increases with decreasing nanorod length, which is consistent with the existence of more OH groups in these samples from XPS results. Upon laser excitation, electrons exhibit photoinduced absorption over a broad spectral range, from 500 to

900 nm, due to the presence of various electronic trapping states with different potential energies created by bulk and surface defects, such as oxygen vacancies and coordinately unsaturated surface indium sites.33−36 Previous theoretical calculations have predicted that the energy position of the trapping states introduced by bulk oxygen vacancies is far from the conduction band, providing deep electron trapping states, while surface oxygen vacancies have energy positions closer to F

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ACS Nano the conduction band.33,34 Therefore, the photoinduced absorption at longer wavelength is most likely caused by the trapped electrons at surface states. The absorption by free electrons typically occurs at even longer wavelengths, which is believed to contribute to the tail observed in the transient absorption spectra in the near-infrared wavelength range.37 Accordingly, the decay of excited electron−hole pairs can be understood through different processes, as shown in Figure 5b. Upon illumination with solar light, electrons from the valence band are excited into the conduction band, leaving a hole in the valence band. The hole migrates into surface hydroxide trap states, while the electron in the conduction band may be trapped at an oxygen vacancy. It is important to note that the In2O3−x(OH)y nanorods are made up of many small nanocrystals in close contact with one another. This would give rise to a spatial separation of the excited charge carriers between neighboring nanocrystals. Figure 5c shows the decay transients of different In2O3−x(OH)y nanorod samples at 750 nm (surface-trapped electrons). The decay profiles can be wellfitted using a biexponential decay function, I(t ) = ∑i Ai exp( −t /τi), with time constants of (i) 0.35 and 2.89 μs for S1, (ii) 0.33 and 2.93 μs for S3, and (iii) 0.26 and 5.41 μs for S5. The amplitude-weighted average lifetimes of surface-trapped electrons were 1.78 μs for S1, 2.50 μs for S3, and 4.81 μs for S5. The lengthening of electron lifetimes with increased nanorod length could be attributed to (a) increased hole or electron surface traps and (b) improved internanocrystal charge transfer. From XPS results (Figure 4b and Figure S7), the density of OH groups gradually decreases while no obvious trend in the density of oxygen vacancies was observed from S1 to S5, which does not support mechanism (a). Hence, it is most possible that longer aging time of In(OH)3 precursors could result in a better contact between neighboring nanocrystals in In2O3−x(OH)y nanorods, which would facilitate efficient inter-nanocrystal charge transfer and improve the electron lifetime. It is also important to note that the presence of microsecond-long lifetimes in mesocrystalline TiO2 superstructures was suggested to result from the occurrence of an inter-nanocrystal charge transfer process through hopping.20,22 More importantly, the longer lifetimes of the surface-trapped electrons observed for the longer nanorod samples imply a more effective charge separation and transfer between neighboring nanocrystals, which may be responsible for the observed length-dependent photocatalytic activity of the In2O3−x(OH)y nanorods described above. On the other hand, the almost identical decay transients at 500 nm for the trapped holes (Figure S9) imply that the movement of holes is independent of the In2O3−x(OH)y nanorod length, indicating that the migration of holes between neighboring nanocrystals has a lower probability than the electron movement (Figure 5d). Recently, the presence of a 2D electron gas was discovered at the surfaces of defected In2O3, which may facilitate the transfer of electrons between nanocrystals and their trapping at surface defects in our case.33 While longer nanorod samples were observed to have more effective inter-nanocrystal charge transfer, further study is necessary to explain this trend. One plausible explanation takes into consideration the structural features of the In2O3−x(OH)y nanorods inherited from their parent In(OH)3 nanorods. Since each In(OH)3 nanorod has a mesocrystalline structure composed of a bunch of well-aligned primary nanowires, the

crystallization of nanowires is improved with longer reaction time. During the low-temperature conversion of In(OH)3 nanorods into In2O3−x(OH)y nanorods, confined nucleation and growth of nanocrystals occur mainly within each primary nanowire. Since longer nanorod samples are produced from In(OH)3 nanowires with a higher degree of crystallization, where better contact between neighboring nanocrystals might be achieved within the same nanorod, improved charge separation and transfer across the interfaces between nanocrystals would occur.

CONCLUSION In summary, we demonstrate enhanced gas-phase photocatalytic activity of nanocrystal superstructure based metal oxide semiconductors by using In2O3−x(OH)y nanorods as an model system for catalyzing the RWGS reaction (CO2 + H2 + hν → CO + H2O). It is found that interparticle charge transfer occurs within the nanocrystal superstructure such that the lifetime of photoexcited carriers is prolonged in In2O3−x(OH)y nanocrystal nanorods. The improvement in charge carrier lifetimes correlates well with the increase in conversion rate of the gas-phase, light-assisted RWGS reaction. The detailed synthesis, structural, and catalytic investigation presented in this paper shows that while the population of active sites diminishes with increasing length of the nanorods, the microsecond lifetimes of photoexcited electrons increase with increasing nanorod length as a result of enhanced inter-nanocrystal charge transport. It is the unique combination of nanocrystals assembled into a nanorod superstructure together with transport of photogenerated charge carriers from nanocrystalto-nanocrystal therein that ultimately controls the charge relaxation dynamics and surface chemistry responsible for the observed nanorod length dependence of the hydrogenation rate of carbon dioxide to carbon monoxide. Our study sheds light on the physicochemical design principles of nanocrystal superstructure based photocatalysts that will enable more efficient utilization of solar energy. METHODS Chemicals. Urea was obtained from Sigma-Aldrich. Anhydrous InCl3 was purchased from Alpha. All chemicals were used as received. Deionized water was used throughout the synthesis. Synthesis of In(OH)3 and In2O3−x(OH)y Nanorods. In a typical synthesis, 25 g of urea and 3 g of InCl3 were dissolved in 100 mL of deionized water. The aqueous solution was then heated at 80 °C in an oil bath under magnetic stirring for a controlled amount of time. After being cooled to room temperature, the white products were collected though centrifugation and washed with water to remove nonreacted residues. The sample was dried for 48 h at ambient temperature. The dried In(OH)3 precursors were then placed into an oven and treated at 250 °C in air for different periods of time to obtain the final In2O3−x(OH)y samples. For the study of the length effect, five initial reaction periods for In(OH)3 precursors were chosen as 2, 3, 5, 8, and 12 h, all of which were then treated at 250 °C for 6 h to obtain In2O3−x(OH)y samples with different lengths. For gas-phase photocatalytic tests, samples were prepared by drop-casting these In2O3−x(OH)y powders from an aqueous dispersion onto 1 in. by 1 in. binder-free borosilicate glass microfiber filters (Whatman, GF/F, 0.7 μm). Characterization. Sample morphology was determined using a Hitachi H-7000 TEM and a FEI Quanta FEG 250 environmental SEM/STEM. High-resolution TEM and electron diffraction studies were performed using an aberration (image) corrected FEI Titan 80300 operated at 300 kV and equipped with a Gatan US 1000 CCD camera. Sample weight loss during the calcination process was G

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determined by placing approximately 6 mg of indium hydroxide nanorod precursor in a TA Instruments SDT Q600 thermogravimetric analyzer/differential scanning calorimeter in an alumina pan under 100 mL/min flow of compressed air. The temperature was steadily increased from room temperature (21 °C) to 500 °C at a rate of 5 °C/ min. PXRD was performed on a Bruker D2-Phaser X-ray diffractometer, using Cu Kα radiation at 30 kV. Nitrogen adsorption isotherms were obtained at 77 K using a Quantachrome Autosorb-1-C. The surface area of each sample was determined using BET theory, and pore size distributions were determined with NLDFT. Diffuse reflectance of the samples was measured using a Lambda 1050 UV/ vis/NIR spectrometer from PerkinElmer and an integrating sphere with a diameter of 150 mm. XPS was performed using a PerkinElmer Phi 5500 ESCA spectrometer in an ultrahigh vacuum chamber with a base pressure of 1 × 10−9 Torr. The spectrometer uses an Al Kα X-ray source operating at 15 kV and 27 A. The samples used in XPS analyses were prepared by drop-casting aqueous dispersions onto p-doped Si(100) wafers in the case of the In2O3−x(OH)y samples. All data analyses were carried out using the Multipak fitting program, and the binding energies were referenced to the NIST-XPS database and the Handbook of X-ray Photoelectron Spectroscopy. Transient Absorption Measurements. Nanosecond−microsecond transient absorption measurements were performed using a commercially available integrated Helios and EOS setup (Ultrafast Systems LLC). The 325 nm pump pulse was generated from an optical parametric amplifier (OPerA Solo) that was pumped by a 1 kHz regenerative amplifier (Coherent Libra), which was seeded by a modelock Ti:sapphire oscillator (Coherent Vitesse, 80 MHz). The 800 nm beam from the regenerative amplifier has a pulse width of around 50 fs and an energy of 4 mJ per pulse. The pump beam was set to ∼200 μJ cm−2 per pulse. The probe beam for EOS was a white continuum generated from a photonic fiber using a Nd:YAG laser (center wavelength = 1064 nm). The probe beam was collected using a dual detector for UV−vis (CMOS sensor) and NIR (InGaAs diode array sensor). Gas-Phase Photocatalytic Measurements. These experiments were conducted in a custom-fabricated 1.5 mL stainless steel batch reactor with a fused silica view port sealed with Viton O-rings. The reactors were evacuated using an Alcatel dry pump prior to being purged with the reactant gases H2 (99.9995%) and CO2 (99.999%) at a flow rate of 6 mL/min and a stoichiometry of 1:1 (stoichiometric for RWGS reaction). During purging, the reactors were sealed once they had been heated to the desired temperature. The reactor temperatures were controlled by an OMEGA CN616 6-zone temperature controller combined with a thermocouple placed in contact with the sample. The pressure inside the reactor was monitored during the reaction using an Omega PX309 pressure transducer during the reaction. Reactors were irradiated with a 1000 W Hortilux Blue metal halide bulb for a period of 16 h. Product gases were analyzed with a flame ionization detector and thermal conductivity detector installed in a SRI-8610 gas chromatograph with a 3′ Mole Sieve 13a and 6′ Haysep D column. Isotope tracing experiments were performed using 13CO2 (99.9 atom %, Sigma-Aldrich). The reactors were evacuated prior to being injected with 13CO2 followed by H2. Isotope product gases were measured using an Agilent 7890A gas chromatographic mass spectrometer with a 60 m GS-carbonplot column fed to the mass spectrometer.

The authors declare no competing financial interest.

ACKNOWLEDGMENTS We acknowledge the support from the Collaborative Innovation Center of Suzhou Nano Science & Technology, the Priority Academic Program Development of Jiangsu Higher Education Institutions. G.A.O. is the Government of Canada Research Chair in Materials Chemistry and Nanochemistry. This work was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC), Connaught Innovation and Connaught Global Challenge Awards, University of Toronto, Ontario Ministry of Research and Innovation (MRI), and the Ontario Ministry of Economic Development and Innovation (MEDI). L.H. is deeply appreciative for support of his research by a NSERC Banting Post-Doctoral Fellowship. This work was also supported by start-up funding from Nanyang Technological University (M4080514), and the Ministry of Education AcRF Tier 2 Grants MOE2013-T2-1-081 and MOE2014-T2-1-044. B.W. and T.C.S. also acknowledge the financial support by the Singapore National Research Foundation through the Singapore−Berkeley Research Initiative for Sustainable Energy (SinBerRISE) CREATE Programme. We also greatly appreciate the helpful discussion with Prof. Frank Osterloh at the University of California, Davis. REFERENCES (1) Ozin, G. A. Throwing New Light on the Reduction of CO2. Adv. Mater. 2015, 27, 1957−1963. (2) Yan, T.; Long, J.; Shi, X.; Wang, D.; Li, Z.; Wang, X. Efficient Photocatalytic Degradation of Volatile Organic Compounds by Porous Indium Hydroxide Nanocrystals. Environ. Sci. Technol. 2010, 44, 1380−1385. (3) Gust, D.; Moore, T. A.; Moore, A. L. Solar Fuels via Artificial Photosynthesis. Acc. Chem. Res. 2009, 42, 1890−1898. (4) Lewis, N. S.; Nocera, D. G. Powering the Planet: Chemical Challenges in Solar Energy Utilization. Proc. Natl. Acad. Sci. U. S. A. 2006, 103, 15729−15735. (5) Tong, H.; Ouyang, S.; Bi, Y.; Umezawa, N.; Oshikiri, M.; Ye, J. Nano-Photocatalytic Materials: Possibilities and Challenges. Adv. Mater. 2012, 24, 229−251. (6) Chen, X.; Shen, S.; Guo, L.; Mao, S. S. Semiconductor-Based Photocatalytic Hydrogen Generation. Chem. Rev. 2010, 110, 6503− 6570. (7) Fujishima, A.; Rao, T. N.; Tryk, D. A. Titanium Dioxide Photocatalysis. J. Photochem. Photobiol., C 2000, 1, 1−21. (8) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Environmental Applications of Semiconductor Photocatalysis. Chem. Rev. 1995, 95, 69−96. (9) Fox, M. A.; Dulay, M. T. Heterogeneous Photocatalysis. Chem. Rev. 1993, 93, 341−357. (10) Tachikawa, T.; Fujitsuka, M.; Majima, T. Mechanistic Insight into the TiO2 Photocatalytic Reactions: Design of New Photocatalysts. J. Phys. Chem. C 2007, 111, 5259−5275. (11) Ronge, J.; Bosserez, T.; Martel, D.; Nervi, C.; Boarino, L.; Taulelle, F.; Decher, G.; Bordiga, S.; Martens, J. A. Monolithic Cells for Solar Fuels. Chem. Soc. Rev. 2014, 43, 7963−7981. (12) Liu, C.; Dasgupta, N. P.; Yang, P. Semiconductor Nanowires for Artificial Photosynthesis. Chem. Mater. 2014, 26, 415−422. (13) Habisreutinger, S. N.; Schmidt-Mende, L.; Stolarczyk, J. K. Photocatalytic Reduction of CO2 on TiO2 and Other Semiconductors. Angew. Chem., Int. Ed. 2013, 52, 7372−7408. (14) Wang, X.; Kafizas, A.; Li, X.; Moniz, S. J. A.; Reardon, P. J. T.; Tang, J.; Parkin, I. P.; Durrant, J. R. Transient Absorption Spectroscopy of Anatase and Rutile: The Impact of Morphology and

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DOI: 10.1021/acsnano.6b02346 ACS Nano XXXX, XXX, XXX−XXX