Zr-Doped β-In2S3 Ultrathin Nanoflakes as Photoanodes: Enhanced

Apr 11, 2016 - In this study, we investigated metal ion doping application to prepare 2, 5, and 8 mol % Zr-doped β-In2S3 two-dimensional nanoflakes; ...
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Research Article pubs.acs.org/journal/ascecg

Zr-Doped β‑In2S3 Ultrathin Nanoflakes as Photoanodes: Enhanced Visible-Light-Driven Photoelectrochemical Water Splitting Ligang Wang, Lu Xia, Yanjie Wu, and Yang Tian* Department of Chemistry, Beijing Key Laboratory for Optical Materials and Photonic Devices, Capital Normal University, Haidian District, Beijing 100048, China S Supporting Information *

ABSTRACT: Photoelectrochemical (PEC) water splitting via semiconductor is a promising approach to the scalable generation of renewable H2 fuels. Several characteristics are crucial for efficient water splitting in PEC cell systems, including low onset potential for the photoanode, high photocurrent, and long-term stability. In this study, we investigated metal ion doping application to prepare 2, 5, and 8 mol % Zr-doped β-In2S3 two-dimensional nanoflakes; we then used the material to create improved photoelectrodes for PEC water splitting. That Zr4+ doping in the crystal lattice of β-In2S3 led to red-shift absorption of the 40 nm wavelength, which benefits visible-light utilization. Three nanoflake samples were tested for use as PEC water splitting electrodes and compared to pure β-In2S3 nanoflakes. We found that the photocurrent density of 2 mol % Zr-doped β-In2S3 nanoflakes was nearly 10 times higher than that of pure β-In2S3 nanoflakes at 1.2 V versus a reversible hydrogen electrode (RHE). In addition, the anodic photocurrent onset had a 0.15 V negative shift compared to pure β-In2S3 nanoflakes. The strategy we propose here can likely be used to develop other n-type semiconducting photoanodes for use in low-cost, solar-fuel-generation devices. KEYWORDS: Semiconductor, Nanoflakes, Photoelectrochemical, Zr4+ doping, In2S3

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INTRODUCTION In response to continual, dramatic increase in global energy demand (and the environmental concerns it inevitably raises,) developing clean and renewable fuels has become an imperative, and urgent, endeavor.1−6 Hydrogen fuel generated by photoelectrochemical (PEC) water splitting is considered to be one of the most attractive chemical methods for renewable energy storage.7−12 When coupled with solar energy capture and conversion, PEC water splitting can facilitate the direct utilization of unlimited solar energy with minimal carbon footprint.9,10 The development of efficient photoelectrocatalysts that can effectively enhance slow-moving kinetic processes is central to ongoing research in this area, as they can be designed to allow for both water reduction and oxidation at low overpotentials. Pt13 and noble metal oxides (e.g., IrO2, RuO2)14 are considered quite favorable catalysts for both hydrogen evolution reactions (HER) and oxygen evolution reactions (OER), and can be processed highly efficiently in a large range of solution pHs; unfortunately, any large-scale application of these catalysts is limited by general lack of supply, elemental scarcity, and the high cost of the materials. Alongside the recent population of graphene,15,16 twodimensional (2D) nanostructures such as nanosheets, nanoplates, and nanoflakes have garnered considerable amounts of attention for their promising application in optoelectronic devices, light-emitting diodes, and solar energy conservation.17−25 Because of competitive advantages in terms of © 2016 American Chemical Society

electrochemical, adsorption, and catalytic performance characteristics (including large specific surface and confined thickness on the atomic scale), these structures allow effective contact between the electrode materials and the electrolyte. Research has continued to thrive as researchers worldwide have continued to develop advanced nanomaterials using structural design and functionalization, especially in pursuit of innovative approaches to PEC water splitting. As an important semiconductor materials, indium sulfide (In2S3) is often studied for uses in phothocatalysis, solar cells, and photovoltaics.26,27 In2S3 is a material well-suited to the fundamental research of quantum confinement effects for its large exciton Bohr diameter (33.8 nm).28 In2S3 is known to possess three different crystal structures: defect cubic structure (α-In2S3), defect spinel (β-In2S3), and layered structure (γIn2S3). Among them, β-In2S3 is an n-type semiconductor with particularly favorable band gap (2.0−2.3 eV) for photocatalysis with visible-light absorption, a relatively negative conduction band edge, and moderate charge transport properties.29 Several previous research groups have reported the use of β-In2S3 nanostructures for PEC water splitting to generate H2 or O2.30−33 We also have prepared 2D β-In2S3 nanobelts and nanoflakes and investigated their PEC properties in a previous Received: January 15, 2016 Revised: March 11, 2016 Published: April 11, 2016 2606

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Figure 1. XRD patterns (part I) and precise-scanning patterns for (440) planes (part II) of the prepared nanoflakes. Curve (a) pure β-In2S3; (b) 2 mol %, (c) 5 mol %, and (d) 8 mol % Zr4+-doped nanoflakes.

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study.34 The efficiency of PEC water oxidation on pure β-In2S3 semiconductor nanocrystals, however, is still far from satisfactory. Various attempts have been made to increase PEC efficiency for water splitting purposes. Compositional modification via introducing foreign ions is a popular strategy for adjusting semiconductor properties; certain such modifications have been shown to enhance photoelectrolysis/catalysis considerably. For example, doping hematite films with Sn improves PEC water oxidation.26 Na+ or Ba2+ can be doped in Ta3N5 electrode to enhance PEC water splitting,35,36 and Zr4+ doped in Fe2O3 nanocrystals for PEC water splitting.37−39 In a very recent study, Mg−Zr was consubstituted in Ta3N5 for lower-onsetpotential photoanode in PEC water splitting.40 Owing to the difference in valence between the foreign and parent cations, energy levels, electron density, trap states, and surface defects can be tailored to maintain necessary charge balance by foreign metal ion doping. Foreign metal ion doping can also accelerate the separation and transfer of charge and decrease interfacial recombination to enhance effectively PEC efficiency. These valuable contributions to the literature inspired us to test the ability of Zr to function as a dopant for enhancing the PEC efficiency of β-In2S3. Notably, Zr4+ (72 pm), which has an ionic radius comparable to that of In3+ (81 pm), is a suitable foreign ion for forming β-In2S3 variants. In this study, we investigated the introduction of foreign Zr4+ ions into β-In2S3 2D ultrathin nanoflakes via a facile solvothermal method at relatively low temperature. Compared to our previous synthesis of β-In2S3 nanoflakes in cyclohexane with foreign metal ions assistance (Al3+),34 the present synthesis of both pure and Zr-doped β-In2S3 nanoflakes in octanol is straightforward and does not require Al3+. We also found a red shift of 0.22 eV in the UV−visible spectrum and improved PEC activity in the visible edge extending from 450 to 490 nm after Zr4+ doping. We also conducted a comparison experiment between pure β-In2S3 and Zr-doped β-In2S3 nanoflakes to clarify the role of Zr4+ ions in the enhancement of β-In2S3 PEC activity. To the best of our knowledge, there are rarely reports of the effects of Zr4+ doping in β-In2S3 used as photoanodes for PEC water oxide applications under visiblelight irradiation. Interestingly, the onset photocurrent potential was lower after the β-In2S3 was doped with Zr; further, the βIn2S3 supplied the optimal doping amount (2 mol % Zr4+) generated far superior photocurrent density compared to pure β-In2S3, though the 5 and 8 mol % Zr-doped β-In2S3 samples also performed well.

EXPERIMENTAL SECTION

Chemical Agents. In(NO3)3·4H2O (99%, AR), n-octylamine (99%, AR), and thioacetamide (AR) were provided by J&K Ltd. Zirconium tetrachloride, octanol, and cyclohexane were of analytical grade (Tianjin institute of fine chemicals). All chemicals were used after received with no further purification. Synthesis of Pure β-In2S3 Nanoflakes. In a typical route to synthesize pure β-In2S3 nanoflakes, 0.2 mmol In(NO3)3·4H2O (0.0764g,), octanol (10.0 mL), and octylamine (5.0 mL) were poured into a 20 mL Teflon-lined stainless steel autoclave at 25 °C with stirring for 10 min. Moreover, 0.3 mmol thioacetamide was poured into the above reaction solution with stirring for 6 min again. After the autoclave was sealed and heated at 220 °C for 3 h in an oven, it was cooled to 25 °C naturally. Yellow powders were obtained after being fully washed by absolute ethanol and cyclohexane, and then were dried for 12 h in an oven. Finally, the prepared powder was dispersed in 6 mL cyclohexane and stored for next uses. Synthesis of 2 mol % Zr-Doped β-In2S3 Nanoflakes. The ultrathin 2 mol % Zr-doped β-In2S3 nanoflakes were synthesized through the same way except for adding 0.0009 g of zirconium tetrachloride into the precursor solution. Synthesis of 5 mol % Zr-Doped β-In2S3 Nanoflakes. The ultrathin 5 mol % Zr-doped β-In2S3 nanoflakes were prepared via a similar method except for adding Zr4+ (zirconium tetrachloride, 0.00239 g) to the precursor reaction solution. Synthesis of 8 mol % Zr-Doped β-In2S3 Nanoflakes. The ultrathin 8 mol % Zr-doped β-In2S3 nanoflakes were prepared through a same way except for adding zirconium tetrachloride (0.0038 g) to the precursor reaction solution. Characterization. Powder X-ray diffraction (XRD) was performed on a Rigaku D/Max 2200 PC diffractometer (λ = 0.154 18 nm) with graphite monochromator at 5.0°/min. An accurate scanning was collected between 46° and 51° at 0.2°/min. Crystalline-cell parameters were calculated from JADE 5 program. Transmission electron microscopy (JEM100-CXII) was used to characterize the size and morphology of samples with equipped selected area electron diffraction (SAED). The high-resolution TEM (GEOL-2010) was employed to observe the crystalline structure and lattice fringes. The components and surface of nanocrystals were characterized by X-ray photoelectron spectroscopy (XPS) analyses based on the C−C standard peak at 284.6 eV. Energy dispersive X-ray spectroscopy of Oxford was used to analyzed the chemical composition of products. The elements of the products were analyzed via high-angle annular dark field-scanning TEM (HAADF-STEM) imaging with EDX mapping. The UV−visible absorption spectra were achieved on the spectrometer of Shimadzu (UV2550). Fabrication of Photoelectrochemical Device. Glass substrates with fluorine-doped tin oxide (FTO) (1 cm × 2 cm) were carefully cleaned several times with acetone (for 3 times) and deionized water (for 3 times), followed by pure nitrogen blowing. The work electrode was prepared by spin-coating pure β-In2S3 and Zr-doped β-In2S3 nanoflakes that was dispersed in 1 mL of cyclohexane (0.5 mg·mL−1) 2607

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ACS Sustainable Chemistry & Engineering onto the FTO-glass substrate rotated at 3200 rpm. During the spincoating, the product solutions were dropped onto the FTO glass for 4−5 times, respectively. Finally, the obtained product was annealed under nitrogen atmosphere at 60 °C for 1 h before it was used for the water splitting experiment. Photoelectrochemical Water Splitting Test. The PEC characterization was performed using a three-electrode system on an electrochemical workstation (CH Instruments 660D) at ∼28 °C. A 350 W xenon lamp provided visible-light illumination with an optical filter (400−700 nm wavelength). The working electrode with pure βIn2S3 or Zr-doped β-In2S3 nanocrystals exposed the area of 1.0 cm2. The electrode of Ag/AgCl (1 mol L−1 KCl) was used as reference, Pt mesh (0.8 mm2 surface area) as cathode electrode, and 0.5 M NaOH as electrolyte solution (pH = 13.7). Potentials are showed versus RHE, which means V versus Ag/AgCl (volt) + [0.059 (volt) × pH] + 0.244 (volt). The light-source power was measured by using a laser power meter (LP-3A). IPCE was measured at 1.2 V versus RHE with a monochromator in the range of 400−800 nm (71MS1021, Saifan). The IPCE is expressed as the following equation: IPCE = hcI /λJlight It includes the Planck’s constant (h), the measured photocurrent density at a specific wavelength (I), the speed of light (c), the incident light wavelength (λ) and the recorded irradiance intensity at a specific wavelength (Jlight). Mott−Schottky (MS) measurements were conducted by electrochemical-impedance/potential at a fixed frequency (1 kHz). All the electrolytes were purged with N2 before measurements.

Figure 2. (a) TEM image; (b) SAED pattern; (c) HRTEM image; (d) HAADF-STEM image; element-mappings of In (e), Zr (f), and S (g) of the prepared 2 mol % Zr-doped β-In2S3 nanoflakes.

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RESULTS AND DISCUSSION Figure 1I shows the powder X-ray diffraction (XRD) patterns of pure β-In2S3 and Zr-doped β-In2S3 nanoflakes. Curve a shows where the diffraction peaks of pure β-In2S3 nanoflakes all were well indexed as a cubic phase structure (JCPDS No. 320456), and no impurities were detected. Curves b−d show the XRD patterns of 2, 5, and 8 mol % Zr-doped β-In2S3 nanoflakes, respectively, which were in agreement with those of β-In2S3 with the Fd3m ̅ space group (Figure 1I). The unit cell parameters of pure β-In2S3 were calculated as a = b = c = 10.73 Å, in good agreement with the parameters in the JCPDS card (a = b = c = 10.73 Å). The lower intensity and broader peak width in the XRD patterns was likely accounted for the small size effect and weak crystalline nature of the ultrathin nanoflakes compared to the traditional nanoflakes. Figure 1II shows the precise XRD scan patterns of the samples’ (440) peak degrees. Using Jade software, we determined that the unit cell parameters of 2, 5, and 8 mol % Zr-doped β-In2S3 were a = 10.72 Å, a = 10.71 Å, and a = 10.70 Å, respectively. The peaks shifted rightward as Zr4+ concentration increased due to crystal lattice distortion of the β-In2S3 after Zr doping. The radius of In3+ (0.81 Å) was larger than that of Zr4+ (0.79 Å), so the cell volume of β-In2S3:Zr decreased as Zr4+ replaced In3+ in the cell, causing the XRD peaks to shift to higher degrees relative to pure β-In2S3. These results also indicate that the Zr4+ was indeed doped into the crystalline lattices of β-In2S3 nanoflakes successfully. The prepared Zr-doped β-In2S3 nanoflakes were observed by transmission electron microscopy (TEM). The TEM image (Figure 2a) of the Zr-doped β-In2S3 nanoflakes (2 mol %) shows that the obtained nanocrystals were in flake-like shape with wrinkling and rolling. The standing edges of the flakes in the TEM and high-resolution TEM (HRTEM) images show that flake was in ∼3 nm thickness. The selected area electron diffraction (SAED) pattern indicated the cubic structure of the prepared nanocrystals. The intrinsic crystallography of the 2

mol % Zr-doped β-In2S3 nanoflakes was characterized by HRTEM, where the image (Figure 2c) clearly revealed two suites of planes, where the distance is ∼0.30 nm and the crossing angle was in approximately 70 degree, corresponding to {222} planes in β-In2S3. Thus, they have ⟨110⟩ zone axis, demonstrating that the nanoflakes were exposed by {110} planes. TEM images of the pure β-In2S3, 5, and 8 mol % Zrdoped β-In2S3 nanoflakes (see Supporting Information, Figure S1) revealed virtually no change in morphology after the introduction of Zr ions to the precursor solution. The prepared 2 mol % Zr-doped β-In2S3 nanoflakes were then further characterized via high-angle annular dark-fieldscanning transmission electron microscopy (HAADF-STEM) imaging in conjunction with energy-dispersive spectroscopy (EDS) mapping, as shown in Figure 2d. It shows the correct elemental signals and stoichiometry within the error, and the elements of In, Zr, and S were distributed uniformly along the Zr-doped β-In2S3 nanoflakes as shown in Figure 2e−g; the EDS elemental mapping similarly demonstrated that the Zr elements were homogeneously distributed within any single nanoflake, indicating that the foreign ions (Zr4+) were incorporated into the β-In2S3 crystal structure successfully. These results were consistent with those obtained through XRD analysis. The obtained pure β-In2S3 and Zr-doped β-In2S3 products were examined by X-ray photoelectron spectrometry (XPS) analysis (Figure 3). The survey spectra (Figure 3a) showed In, Zr, and S elements with O and C elements, induced by the adsorbed organic molecules. The molar ratio of S to In was determined to be approximately 1:0.7, which was in accordance with EDS results. Figure 3b shows the high-resolution XPS spectrum of the In 3d peak; two bands around 452.1 and 444.5 eV are attributed to the 3d3/2 and 3d5/2 of the In (III) in In2S3.41,42 Figure 3c shows the high-resolution XPS spectra of Zr (3d peaks) for all three Zr-doped β-In2S3 nanoflakes, where the Zr 3d5/2 and Zr 3d3/2 peaks for the 2 mol % Zr-doped βIn2S3 sample were unexpectedly located at a higher binding 2608

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Figure 3. XPS patterns for survey (a); In 3d (b); and Zr 3d (c) of four synthesized samples: pure β-In2S3 nanoflakes (black lines); 2 mol % (red lines); 5 mol % (blue lines); and 8 mol % Zr-doped β-In2S3 (pink lines).

Figure 4. UV−visible absorption spectra of pure, 2, 5, and 8 mol % Zr-doped β-In2S3 nanoflakes (a). Plots of (ahv)2 vs hv for the pure β-In2S3 and 2 mol % Zr-doped β-In2S3 nanoflakes. Two intersecting lines denote Eg values of the two samples (b).

the photocurrent density−voltage (J−V) curve under 350 W Xe lamp irradiation. To complete the PEC characterizations, standard Ag/AgCl electrode and Pt mesh (0.8 mm2 surface area) were applied as reference and counter electrodes, respectively, in 0.5 M NaOH solution. Figure 5a depicts the PEC working mechanism and shows the electron (e−)/hole (h+) separation and transfer processes, and the electrode reactions we observed in the PEC test for n-type semiconductor. All samples revealed an rare dark-current in the potential between 0 and 1.3 V versus RHE. Figure 5b shows the J−V curves of all the prepared photoanodes scanned at a power density of 60 mW cm−2. The photocurrent density of pure β-In2S3 nanocrystals had an onset potential of ca. 1.2 VRHE that reached 0.04 mA cm−2 at 1.2 VRHE and 0.1 mAcm−2 at 1.3 VRHE. The photocurrent densities of the other Zr-doped β-In2S3 samples varied, as shown in Figure 5b. The J−V curves of the 2, 5, and 8 mol % Zr-doped β-In2S3 were 0.38, 0.1, and 0.06 mA cm−2 at 1.2 VRHE, respectively, and the photocurrent densities at 1.3 VRHE were 1.1, 0.55, and 0.3 mA cm−2, respectively. On the basis of these observations, we concluded that the photocurrent density of 2 mol % Zr-doped β-In2S3 nanoflakes was nearly 10 times higher than that of pure β-In2S3 nanoflakes at 1.2 VRHE. We also found that the 2 mol % Zr-doped β-In2S3 nanoflakes had a lower onset potential (negative shift) compared to pure β-In2S3. Taken together, these results demonstrate that the 2 mol % Zrdoped β-In2S3 nanoflakes generated the greatest PEC activity for water splitting out of all the samples we tested. We further listed the reported In2S3-based electrodes for water splitting compared with our Zr-doped In2S3 in Table 1, including both as anode and cathode materials. It shows that, among all the anode cases, the onset of our Zr-doped In2S3 nanoflakes is very near that of reported Co-doped In2S3, but lower than the pure In2S3 nanobelts; the photocurrent density of our Zr-doped In2S3 is much higher than that of the pure In2S3 nanobelts at

energy, possibly due to the presence of a few oxygen molecules and the formation of In2ZrxS3O2x oxysulfide having maintained the charge balance. With increased amounts of doped Zr4+ to 5 mol % and 8 mol %, the Zr 3d5/2 and Zr 3d3/2 peaks shifted slightly left−in the 8 mol % Zr-doped sample, specifically, the Zr 3d5/2 and 3d3/2 peaks were located at 181.7 and 184.1 eV, respectively, consistent with those reported in ZrS2,43 which indicated that the Zr4+ partially substituted the In3+ to form Zr−S bonds (In2−xZrxS3+0.5x) as doping level increased. β-In2S3, as mentioned above, is an n-type semiconductor (band gap of 2.0−2.3 eV) that is in stable form at room temperature. The band gap in the visible part of the spectrum allows the use of β-In2S3 to fabricate materials that may be very well-suited to photocatalytic and photoelectric applications.44 With this in mind, we characterized the four β-In2S3 samples prepared in ethanol according to their UV−visible absorption spectra (Figure 4). As shown in Figure 4a, the pure β-In2S3 showed obvious absorption at 450 nm, matching well with the characteristic UV−visible absorption of the reported 2D βIn2S3.27,45 We also found that the absorption bands shifted red as Zr4+ doping amount increased. We postulate that the red shift is attributable to the fact that the band gap of the obtained Zr-doped β-In2S3 nanoflakes had a relatively smaller value compared to the pure β-In2S3 nanoflakes. By exploiting the spectral deformation into (ahv)2 versus photon energy (hν/eV) plotted at the onset region, the band gap energy (Eg) of the pure β-In2S3 and β-In2S3 film under the 2 mol % Zr-doped stacked modifier was estimated to be 2.05 and 2.02 eV (Figure 4b), respectively. These results altogether confirmed that the red shift in Zr-doped β-In2S3 nanoflakes increased the number of photogenerated electrons and holes available for photocatalytic reaction, which then enhanced the photocatalytic activity of Zr-doped β-In2S3. The as-prepared series of products were evaluated for their ability to generate O2 as a photoathode material by measuring 2609

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Figure 5. PEC water splitting working mechanism with the pure β-In2S3 and Zr-doped β-In2S3 as photoanodes (a). J−V curves of pure β-In2S3 and Zr-doped β-In2S3 photoanodes under chopped 350 W Xe lamp irradiation (b). On−off switching of 2 mol % Zr-doped β-In2S3 nanoflake based device at 1.2 VRHE (c). Current density versus time (J−t) curves of β-In2S3 and Zr-doped β-In2S3 photoanodes tested at 1.2 VRHE (d). OER activities of 2 mol % Zr-doped β-In2S3 nanocrystals at different loadings: 0.5, 1.0, and 2.0 mg cm−2 (e). Specific current densities at 1.0, 1.1, and 1.2 VRHE of the 2 mol % Zr-doped β-In2S3 nanocrystals at different loading masses: 0.5, 1.0, and 2.0 mg cm−2 (f).

Table 1. PEC Water Splitting Performances of the Reported In2S3-Based Photoelectrodes materials

morphology

electrode type

electrolyte

onset potential (VRHE)

J (mA cm−2)

ref.

Zr-doped In2S3 In2S3-graphene MoS2-In2S3 Co-doped In2S3 pure In2S3 Pt/In2S3/CdS/Cu2ZnSnS4

flakes composite plates slab belts thin Film

anode cathode cathode anode anode cathode

NaOH Na2SO4 Na2S/Na2SO3 KOH NaOH Na2HPO4/ NaH2PO4

1.05

1.1 (1.3 VRHE) 0.003 (1.0 VRHE) 0.001 (0.5 VRHE) 1.17 (1.5 VRHE) 0.010 (1.3 VRHE) −4.5 (0.62 VRHE)

this work 31 32 33 34 30

1.0 1.17 0.62

as fast as about 0.51 s.46,47 So, the above results also show that the time responses of the 2 mol % Zr-doped β-In 2 S 3 nanocrystal-based devices were highly stable and reproducible. Further, no degradation was found after 2 min of on−off switching cycles, confirming the stability and rapid response speed of our device. Details regarding the PEC of the samples showing catalytic activity, pure β-In2S3 and Zr-doped β-In2S3 nanocrystals, were also investigated carefully. As mentioned above, chemical stability is also a necessary feature for photoanode materials. The J−t curves in Figure 5d were obtained by illuminating the photoanodes at the bias of 1.2 VRHE for 240 min. The results demonstrated that the five β-In2S3 products displayed good

the bias of 1.3 VRHE, and even is very close to that of the Codoped In2S3 at 1.5 VRHE. All of the these indicate the outstanding photocatalysis performances of our Zr-doped (2 mol %) In2S3 sample. Figure 5c shows the photocurrent density to time (J−t) cycle of 2 mol % Zr-doped β-In2S3 nanoflakes tested with light irradiation on/off at 60 mW cm−2. The switching in these on/ off states was very reversible and quick, similar to a good photosensitive switch. As shown in Figure 5c, the obtained 2 mol % Zr-doped β-In2S3 nanocrystals easily shifted from on (“high”-current state) to off (“low”-current state). Furthermore, the photoresponse time, which is crucial for detection performances, is estimated to be 0.45 s, and the decay time is 2610

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Figure 6. Photocurrent density curves of 2 mol % Zr-doped β-In2S3 nanocrystals displaying photocurrent dependence on illumination power (a). Measured J−t curves, corresponding with functions I = 30.05P0.82 (b). IPCE spectra of pure β-In2S3 and 2 mol % Zr-doped β-In2S3 as photoanodes in 0.5 M NaOH electrolyte at 1.2 VRHE (c). Plots of [hv ln(1 − IPCE)]2 versus hv for pure β-In2S3 and 2 mol % Zr-doped β-In2S3 nanocrystal film, where two intersecting lines denote Eg value (d).

Figure 7. Mott−Schottky plots (a) and band structure diagram (b) of pure β-In2S3 and 2 mol % Zr doped β-In2S3 nanoflakes.

the Xe lamp light power from 0 to 60 mW cm−2, the photocurrent densities gradually increased and the photocurrent density of the 2 mol % Zr-doped β-In2S3 nanocrystals increased as light power was amplified due to increased absorption of photons (Figure 6a). In general, photocurrent is related to the power function of the light intensity: I ∼ PC2, where C2 corresponds to the photocapture coefficient that determines the photocurrent to light intensity.48−50 The relationship between the photocurrent and incident optical power densities reflected a power dependence of ∼0.82 (I ≈ P0.82), which is close to 1, indicating superb photocapture efficiency by the obtained 2 mol % Zr-doped β-In 2 S 3 nanocrystals (Figure 6b). These findings further support the potential of the prepared 2 mol % Zr-doped β-In 2 S 3 nanocrystals for use in photoanode of PEC water splitting. The incident photon-to-current conversion efficiency (IPCE) of the obtained pure β-In2S3 and 2 mol % Zr-doped β-In2S3 photoanodes were also measured under irradiation by monochromatic light of various wavelengths at 1.2 VRHE, as shown in Figure 6c. The spectrum of pure β-In2S3 showed

stability in PEC tests for water splitting, suggesting that this highly stable material is rather ideal for real-world clean energy production applications. We further examined the advantages of the 2 mol % Zrdoped β-In2S3 nanoflakes by comparing their mass loading and photocurrent densities in terms of OER performance (Figure 5e,f). At three selected potentials, 1.0, 1.1, and 1.2 V versus RHE, the OER current densities increased almost linearly with different nanocrystal loading amounts (0.5, 1.0, and 2.0 mg cm−2,) illustrating that mass loading, to some extent, impacted the photocurrent density of the 2 mol % Zr-doped β-In2S3 nanoflakes, and further that there was a practically linear relationship between photocurrent density and mass loading at specific potential. In other words, these results confirmed that the as-synthesized electrode materials caused the observed change in photocurrent. Taken together, these results demonstrate the excellent potential of 2 mol % Zr-doped βIn2S3 nanocrystals as effective and stable OER photocatalysts. Power-dependent photocurrent response was used to confirm fully the origin of the photocurrent. By increasing 2611

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doped β-In2S3 nanoflakes. Notably, the successful incorporation of 2 mol % Zr4+ into the β-In2S3 helped enhance the PEC activity for water oxidation through two main features: (1) improving the photocurrent density, and (2) lowering the onset potential. The proposed partial foreign metal ion doping strategy we demonstrated in this study may be quite applicable to a wide range of semiconductors, and may enable the fabrication of much more efficient photoelectrodes in the future.

IPCE values of 0−0.5% at wavelengths ranging from ca. 400 to 800 nm; the spectrum of 2 mol % Zr-doped β-In2S3 showed IPCE values of 0−2.5% at wavelengths ranging from ca. 400 to 800 nm; and the IPCE value declined steeply at wavelengths longer than 650 nm. We were intrigued to find that the prepared 2 mol % Zr-doped β-In2S3 showed the highest IPCE at 400 nm−dramatically higher than that of pure β-In2S3 (0.5%) in Figure 6c. We assumed that the IPCE performance of 2 mol % Zr-doped β-In2S3 had improved according to the adjustments made to energy levels, trap states, and electron density after incorporating Zr into the β-In 2 S 3 . By exploiting the deformation of the spectra into [hν ln(1 − IPCE)]2 versus photon energy (hν/eV) plotted at the onset region, the band gap energy (Eg) of the pure β-In2S3 and β-In2S3 films under the 2 mol % Zr-doped stacked modifier were estimated to be 2.04 and 2.02 eV (Figure 6d), respectively; these results are consistent with the UV−visible results, confirming the considerable potential of the 2 mol % Zr-doped β-In2S3 nanoflakes for use in solar water splitting/photoswitching applications and photoelectronic device. To probe the detail structure of band gap, we measured electrochemical impedance spectra on the pure β-In2S3 and 2 mol % Zr doped β-In2S3 nanoflakes in the dark. Figure 7a is Mott−Schottky plot, which shows the interface capacitances between the active material and electrolyte depended on equivalent circuit51 This Mott−Schottky plot expresses eq 1: ⎛ 2 ⎞⎛ k T⎞ 1 =⎜ ⎟⎜V − Efb − B ⎟ 2 e ⎠e C ⎝ eεε0Nd ⎠⎝

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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/acssuschemeng.6b00090. TEM and SEM images, EDX spectra, UV−vis absorption spectra, electrochemical impedance spectra, and cyclic voltammetry curve (PDF).

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AUTHOR INFORMATION

Corresponding Author

*Y. Tian. Email: [email protected]. Tel: +86-10-68903033. Fax: +86-10-68903040. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the support of National Natural Science Foundation of China (51402201), Beijing Youth Excellent Talent Program (CIT&TCD201404162); Beijing Local College Innovation Team Improve Plan (IDHT20140512); Scientific Research Base Development Program of the Beijing Municipal Commission of Education.

(1)

where C is semiconductor capacitance, N d is carrier concentration, Efb is flat-band potential, e is fundamental electric charge, ε is relative permittivity (β-In2S3 value is 13), εo is permittivity of vacuum, T is temperature, and kB is Boltzmann constant.40,51−53 From the Mott−Schottky plots in Figure 7a, the positive slope of the linear part indicates the nature of n-type semiconductor of the materials. At the same time, the flat-band potential can be obtained from the abscissa intercept from the linear part of Mott−Schottky plots. The values of the flat-band potential for pure β-In2S3 and 2 mol % Zr doped β-In2S3 nanoflakes are −0.52 and −0.55 V from Figure 7a, respectively. Further considering the above band gap from Figure 6d, the edges of the valence band and conduction band of pure β-In2S3 and 2 mol % Zr doped β-In2S3 nanoflakes are shown in Figure 7b with the reduction and oxidation potentials of H2O. It shows an observed negative shift in energy levels after the Zr-doping, which accounted for the lower onset potential for the 2 mol % Zr-doped β-In2S3 used in PEC water splitting.

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REFERENCES

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CONCLUSIONS In this study, we explored the controlled synthesis of ultrathin 2D β-In2S3 nanoflakes via a simple solvothermal method with foreign Zr4+ ion doping at relatively low temperature. The results of XRD, XPS, and PEC tests indicated that Zr4+ ions were very successfully incorporated into the crystal lattice of the β-In2S3, and that the doped percentage within the crystal lattices was the determining factor for enhanced PEC activity. UV−visibleible spectra revealed that Zr-doped β-In2S3 nanoflakes, with a red shift of 0.22 eV, exhibited enhanced PEC activity for water splitting. We also found that the synthesized ultrathin 2 mol % Zr-doped β-In2S3 nanoflakes possessed better PEC water-splitting activity than pure β-In2S3, 5, or 8 mol % Zr2612

DOI: 10.1021/acssuschemeng.6b00090 ACS Sustainable Chem. Eng. 2016, 4, 2606−2614

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DOI: 10.1021/acssuschemeng.6b00090 ACS Sustainable Chem. Eng. 2016, 4, 2606−2614