Morphology and Doping Engineering of Sn-Doped Hematite Nanowire

Subscriber access provided by University of Newcastle, Australia

Communication

Morphology and Doping Engineering of SnDoped Hematite Nanowire Photoanodes Mingyang Li, Yi Yang, Yichuan Ling, Weitao Qiu, Fuxin Wang, Tianyu Liu, Yu Song, Xiao-Xia Liu, Ping-Ping Fang, Yexiang Tong, and Yat Li Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b00184 • Publication Date (Web): 23 Mar 2017 Downloaded from http://pubs.acs.org on March 26, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Nano Letters is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 20

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Morphology and Doping Engineering of Sn-Doped Hematite Nanowire Photoanodes Mingyang Li,a,b Yi Yang,b Yichuan Ling,b Weitao Qiu,a Fuxin Wang,a Tianyu Liu,b Yu Song,b,c Xiaoxia Liu,c Pingping Fang,a Yexiang Tong,*,a and Yat Li*,b

a

KLGHEI of Environment and Energy Chemistry, MOE of the Key Laboratory of Bioinorganic

and Synthetic Chemistry, MOE of the Key Laboratory of Bioinorganic and Synthetic Chemistry, The Key Lab of Low-carbon Chemistry & Energy Conservation of Guangdong Province, School of Chemistry, Sun Yat-Sen University, Guangzhou 510275, People’s Republic of China b

Department of Chemistry and Biochemistry, University of California, Santa Cruz, California

95064, United States of America c

Department of Chemistry, Northeastern University, Shenyang 110819, People’s Republic of

China

ACS Paragon Plus Environment

Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract: High temperature activation has been commonly used to boost the photoelectrochemical (PEC) performance of hematite nanowires for water oxidation, by inducing Sn diffusion from fluorinedoped tin oxide (FTO) substrate into hematite. Yet, hematite nanowires thermally annealed at high temperature suffer from two major drawbacks that negatively affect their performance. First, the structural deformation reduces light absorption capability of nanowire. Second, this “passive” doping method leads to non-uniform distribution of Sn dopant in nanowire and limits the Sn doping concentration. Both factors impair the electrochemical properties of hematite nanowire. Here we demonstrate a silica encapsulation method that is able to simultaneously retain the hematite nanowire morphology even after high temperature calcination at 800 oC and improve the concentration and uniformity of dopant distribution along the nanowire growth axis. The capability of retaining NW morphology allows tuning the nanowire length for optimal light absorption. Uniform distribution of Sn doping enhances the donor density and charge transport of hematite nanowire. The morphology and doping engineered hematite nanowire photoanode decorated with cobalt oxide-based oxygen evolution reaction (OER) catalyst achieves an outstanding photocurrent density of 2.2 mA cm-2 at 0.23 V vs. Ag/AgCl. This work provides important insights on how morphology and doping uniformity of hematite photoanodes affect their PEC performance.

Keywords: Hematite; Photoanode; Morphology Engineering; Dopant Engineering; Water Oxidation.


Page 2 of 20

Page 3 of 20

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Earth abundant hematite (α-Fe2O3) is one of the most promising photoanode materials for photoelectrochemical (PEC) water oxidation due to its favorable bandgap (2.1 eV) and environmentally benign nature.1-7 Yet, the performance of the hematite photoanode for water oxidation has been severely restricted by its poor electrical conductivity, poor charge separation and transfer efficiency, and sluggish oxygen evolution reaction (OER) kinetics.8-12 A number of approaches such as element doping,13-16 morphology engineering17-19 and surface modification2022

have been explored to address these limitations. Among them, Sn-doped hematite photoanodes

have shown promising PEC performance and received extensive research efforts.23,24 In 2010, Sivula et al. reported a mesoporous hematite structure grown on fluorine-doped tin oxide (FTO) substrate with a pronounced photocurrent density.23 They observed the Sn diffusion from the FTO substrate into the hematite film when the sample was sintered at 800 °C. The Sn-doped hematite showed a 2-fold increment in optical absorption coefficient as a result of the structural distortion of hematite lattice. The unintentional Sn doping was believed to play an important role in the enhanced photoactivity of hematite. Ling et al. further revealed that intentional Sn doping could be achieved at relatively low temperature (650 °C) and it significantly increased the donor density of hematite, and consequently improved the efficiency of charge separation and photoactivity.24 However, both Sn doping approaches involve high temperature annealing processes. High-temperature thermal annealing poses two major drawbacks. First, hematite nanowires (NWs) shrink substantially after a high temperature annealing. Given that hematite is an indirect band-gap semiconductor,25,26 the reduced light absorption length induced by the structural deformation can significantly damage light absorption capability of the hematite NWs. Second, it is anticipated that the Sn-diffusion from the FTO substrate yields a non-uniform distribution of Sn doping along the hematite NW growth axis. The relatively few Sn doping at


Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

NW tips could impair the overall performance of a hematite NW photoanode. Here we demonstrate a silica encapsulation method that can simultaneously retain the morphology of hematite NWs during the high temperature annealing at 800 °C, and improve the doping concentration and distribution uniformity. This morphology and doping engineered hematite NW film shows a significant enhancement in PEC performance of water oxidation. Figure 1 illustrates the synthetic approach to prepare the morphology and doping engineered hematite NW film on a FTO substrate. Pristine Fe2O3 NW film was synthesized according to a previous reported method (Experimental Section, Supporting Information).27 Akaganeite (β-FeOOH) NWs were first grown through a hydrothermal method (Figure S1, Supporting Information). The sample was sintered in air under 550 °C for 30 minutes to be converted to hematite. Unintentionally Sn-doped hematite (denoted as Sn-Fe2O3) NWs were obtained by annealing the pristine Fe2O3 NWs at 800 °C for additional 20 minutes to allow a Sn diffusion from the FTO substrate.20,24 Scanning electron microscopy (SEM) images show that the pristine Fe2O3 NW film is composed of NWs with an average diameter of 60 nm and an average length of 700 nm (Figure S2, Supporting Information). The morphology of the hematite NWs changed considerably upon high temperature annealing at 800 °C. As shown in Figure 2a and 2c, the average diameter of NWs increases to 150 nm, while the length reduces considerably by almost half to 370 nm, which are consistent with the observations reported previously.17,28,29 Since hematite is an indirect bandgap semiconductor, the length reduction could significantly lower the light absorption capability of the hematite NW film. Additionally, previous studies have shown that the hole diffusion length of hematite is as short as 2-4 nm.12,30 The increase of NW diameter means the yields of photo-generated holes that can diffuse to hematite surface and participate in water oxidation reactions will be considerably reduced. Therefore the


Page 4 of 20

Page 5 of 20

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

morphological change upon high temperature annealing is believed to be detrimental to the PEC performance of hematite NWs. To retain the NW morphology, we encapsulated the pristine Fe2O3 NWs with a layer of silica shell (Figure 1). The silica shell is expected to be stable at 800 °C as silica has a very high melting point (~1700 °C). Silica shell was deposited onto pristine Fe2O3 NWs using a solution method reported previously.30 Meanwhile, to improve the distribution of Sn doping along the NW growth axis, we also intentionally doped hematite with Sn ions by adding tin (IV) chloride as a Sn precursor before the high temperature treatment. After annealing the sample in air at 800 oC, the silica shell was selectively etched by a fluorous mixture (2 M HF and 8 M NH4F). The obtained hematite sample is denoted as E-I-Sn-Fe2O3 (Figure 1). As shown in Figure 2b and 2d, the length and diameter of E-I-Sn-Fe2O3 NWs and pristine Fe2O3 NWs are comparable. It suggests that the silica encapsulation method is effective in preserving the morphology of the hematite NWs during high temperature annealing. They also have similar light absorption capability (Figure S3, Supporting Information). E-I-Sn-Fe2O3 NWs retained as single crystal hematite after the annealing and etching processes (inset in Figure 2d and Figure S4, Supporting Information). Infrared spectroscopy and energy-dispersive spectroscopy (EDS) data show that there is no detectable silica residual on the NWs (Figure S5, Supporting Information). More importantly, energy-dispersive spectroscopy (EDS) mapping (Figure 2e-g) results confirmed that the Sn signal is distributed uniformly over an individual E-ISn-Fe2O3 NW.


Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 1. A schematic illustration of the preparation of Sn-Fe2O3 NWs and E-I-Sn-Fe2O3 NWs.


Page 6 of 20

Page 7 of 20

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Figure 2. (a-b) SEM images of Sn-Fe2O3 NWs and E-I-Sn-Fe2O3 NWs. Scale bars are 500 nm. (c-d) TEM images of a Sn-Fe2O3 and E-I-Sn-Fe2O3 NW. Insets: SAED pattern of the Sn-Fe2O3 and E-I-Sn-Fe2O3 NW. Scale bars are 200 nm. (e-g) EDS mapping images of Fe-K, O-K, and SnK signals obtained from an E-I-Sn-Fe2O3 NW. Furthermore, X-ray photoelectron spectroscopy (XPS) measurements were carried out to gain insights into the amount and distribution of the Sn dopants in Sn-Fe2O3 and E-I-Sn-Fe2O3 NW films. As shown in Figure 3a, both samples exhibit peaks centered at 494.8 eV and 486.4 eV that are consistent with the signals reported for SnO2, suggesting the hematite NWs are substitutionally doped by Sn4+ ions.24 As expected, the intentionally doped sample (E-I-Sn-Fe2O3) has much higher amounts of Sn dopants than Sn-Fe2O3 NWs. To investigate the spatial


Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

distribution of Sn dopants, we collected secondary ion mass spectrometry (SIMS) depth profiles of Sn ions for both samples under identical etching rates of 0.36 nm s-1. SIMS depth profile technique collects element information from a large area of ~1000 µm in diameter, so its signal represents an average value of Sn doping in nanowire arrays within this probing area (Figure S6, Supporting Information). Figure 3b shows that Sn-Fe2O3 NW film exhibits a gradient of Sn doping from the NW tip (0.3% atomic percent of Sn) to bottom (0.8% atomic percent of Sn). The depth profile supports our hypothesis that the Sn diffusion from the FTO substrate resulting in a non-uniform distribution of Sn doping along the NWs growth axis. In contrast, E-I-Sn-Fe2O3 NW film has considerably higher Sn concentration than Sn-Fe2O3 NW film in the entire depth profile, consistent with the XPS data obtained from the film surface. The particularly high Sn concentration at the top (the first 100 nm) of E-I-Sn-Fe2O3 NWs is possibly due to the uneven deposition of the Sn precursor on the densely packed NW arrays. The rest part of the NWs (~600 nm thick) has a uniform Sn distribution with an average concentration of 2.5%. Significantly, the combination of intentional doping and silica encapsulation is an effective method to not only retain the NW morphology, but also increase the doped Sn concentration and ensure the uniform Sn distribution in the engineered hematite NW films.


Page 8 of 20

Page 9 of 20

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Figure 3. (a) XPS Sn 3d spectra of Sn-Fe2O3 and E-I-Sn-Fe2O3. Inset: magnified Sn 3d spectrum of Sn-Fe2O3. (b) SIMS depth profiles of Sn-Fe2O3 and E-I-Sn-Fe2O3. Error bars are evaluated based on parallel experiments. The PEC performance of Sn-Fe2O3 and E-I-Sn-Fe2O3 was measured in a three-electrode electrochemical cell using a platinum sheet as a counter electrode and an Ag/AgCl (saturated KCl) as a reference electrode. Figure 4a compares the linear sweep voltammograms (LSVs) collected in 1 M KOH electrolyte in dark and under front and back illuminations (100 mW cm-2, AM 1.5). Significantly, E-I-Sn-Fe2O3 exhibits an excellent photocurrent density of 1.36 mA cm-2 at 0.23 V vs. Ag/AgCl, which is about four times higher than that of Sn-Fe2O3. E-I-Sn-Fe2O3 also has a considerably higher incident photon-to-current conversion efficiency (IPCE) over the entire range of wavelengths between 300 and 600 nm. Both samples have negligible photoactivity beyond 600 nm as the bandgap of hematite is ~2.1 eV (Figure 4b). The enhanced photocurrent and IPCE of the E-I-Sn-Fe2O3 are expected to be attributed to reasons related to at least the four key processes: (i) generation of photo-induced charge carriers (light absorption), (ii) charge (electron-hole pair) separation, (iii) electron transport, and (iv) hole transfer. In the following sections, we will discuss the results obtained by a number of control experiments in order to elucidate these reasons, and more importantly, to understand the effect of morphology and Sn doping on these processes.


Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 20

Figure 4. (a) LSVs of Sn-Fe2O3 and E-I-Sn-Fe2O3 collected at 20 mV s-1 in a 1.0 M KOH aqueous electrolyte under one sun illumination (100 mW cm-2) and in dark. The solid and dashed lines represent the data collected under front (solid lines) and back (dashed lines) illuminations, respectively. (b) IPCE spectra of Sn-Fe2O3 and E-I-Sn-Fe2O3 collected at 0.23 V vs. Ag/AgCl. First, E-I-Sn-Fe2O3 is substantially longer than Sn-Fe2O3, so the enhanced photocurrent could be due to the increased light absorption path. The longer the NWs, the larger the amount of light can be absorbed and the more photo-induced electron-hole pairs can be generated. In this regards, the photocurrent is expected to increase with the length of the hematite NWs, on the assumption of charge transport is not a limiting factor. A control experiment indeed demonstrates that the photocurrent of E-I-Sn-Fe2O3 increases when the NW length increases from 380 nm to 690 nm (Figure S7, Supporting Information). There is no significant photocurrent improvement when the NW length is further increased to 1.2 µm, suggesting that most of the incident light is absorbed by the 690 nm thick film. The optimal hematite film thickness of 690 nm is consistent with the reported values.12,31,32 Due to the poor electrical conductivity of hematite, it is anticipated that charge transport would becoming a limiting factor with the increase of nanowire length, and in this case, front illumination will generates lower photocurrent than back


Page 11 of 20

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

illumination. Nevertheless, the photocurrents of E-I-Sn-Fe2O3 under the front and back illuminations are comparable when the NW length increases from 700 nm to 1.2 µm, which implies that the charge transport in E-I-Sn-Fe2O3 is excellent. Second, the photoactivity enhancement can be due to the improved charge separation and/or collection efficiency. A control sample (denoted as E-Sn-Fe2O3) was prepared under similar conditions as E-I-Sn-Fe2O3, except the addition of Sn precursor was skipped (Figure S8, Supporting Information). As shown in Figure S8, the E-Sn-Fe2O3 sample shows significantly lower photocurrent than the E-I-Sn-Fe2O3 ones, while they have similar NW length. This control experiment suggests that the Sn-doping plays an important role in facilitating the charge separation and transport. Furthermore, the comparison between front and back illuminations provides important information for understanding the mechanism of the charge transportation in hematite photoanodes. As shown in Figure 4a, the Sn-Fe2O3 sample has similar photocurrent densities under front and back illuminations when the applied bias is below 0.1 V vs. Ag/AgCl. Under this low bias, the photocurrent is mainly limited by the poor charge separation efficiency.33-35 When the applied bias increases, the Sn-Fe2O3 sample shows considerably higher photocurrent under back (vs. front) illumination. Considering the charge separation efficiency at high bias should no longer be a limiting factor, the photocurrent discrepancy between back vs. front illumination should be due to the poor charge transport as a result of the Sn-doping gradient along the NW growth axis. Apparently the photo-induced electrons generated at the NW tip are unable to reach the FTO substrate, which limits the photocurrent. One would expect that the ESn-Fe2O3 sample with longer NW length shows even more obvious photocurrent discrepancy than the Sn-Fe2O3 NWs (Figure S8, Supporting Information). Yet, E-I-Sn-Fe2O3 shows similar photocurrent profile under back and front illuminations under all applied bias, confirming that


Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 20

the photocurrent is not limited by charge transport. Likewise, the intentionally Sn doped hematite without silica encapsulation (denoted as I-Sn-Fe2O3) also shows similar photocurrent under back and front illuminations (Figure S9, Supporting Information). These consistent results suggest that the increased concentration and uniformity of Sn-doping are important for facilitating the charge transport in hematite NWs. Moreover, the uniform Sn-doping also substantially increases the donor density and enhances the electrical conductivity of hematite NWs. As shown in Figure S10 (Supporting Information), the donor density of E-I-Sn-Fe2O3 is calculated to be 6.5×1018 cm-3, which is an order of magnitude higher than that of Sn-Fe2O3 (2.3×1017 cm-3). The increased donor density can increase degree of band bending near hematite NW surface and improve charge separation efficiency.25,36 Furthermore, the electrical impedance spectra (EIS) measurements (Figure S11 and Table S1, Supporting Information) indicate that the charge transfer resistance (Rct) of E-I-Sn-Fe2O3 (572 Ω) is considerably lower than that of Sn-Fe2O3 (11000 Ω).37 This is due to the Sn dopants that reduce electron effective mass at conduction band minimum. The uniform Sn-doping thus can substantially enhance the electrical conductivity in E-I-Sn-Fe2O3.26,38 Table 1 summarizes the photocurrent density and structural properties of E-I-Sn-Fe2O3 and other control samples. Clearly, the results show that our morphology and doping engineering are critical for boosting the PEC performance of hematite NWs. Table 1. PEC performance of different hematite samples synthesized in this work

Hematite Samples Sn-Fe2O3

E-Sn-Fe2O3


I-Sn-Fe2O3

E-I-Sn-Fe2O3

Page 13 of 20

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Photocurrent

Front 0.34

0.03

0.57

1.36

0.45

0.23

0.54

1.38

Light absorption length

Insufficient

Sufficient

Insufficient

Sufficient

Charge transport

Inefficient

Inefficient

efficient

efficient

Donor density (cm-3)

2.3 × 1017

1.6 × 1017

3.1 × 1018

6.5 × 1018

Charge transfer resistance (Ω)

11000

65900

657

572

obtained at 0.23 V vs. Ag/AgCl

irradiation

Back

-2

(mA cm )

irradiation

Finally, to accelerate the hole transfer from hematite to electrolyte, we modified E-I-SnFe2O3 with a cobalt oxide-based OER catalyst (Experimental Section, Supporting Information).39,40 The sample is denoted as Co/E-I-Sn-Fe2O3. The addition of the OER catalyst shifts the onset potential of E-I-Sn-Fe2O3 by 300 mV and substantially increases the photocurrent density. As shown in Figure 5, the Co/E-I-Sn-Fe2O3 sample achieves a remarkable photocurrent density of 2.2 mA cm-2 at 0.23 V vs. Ag/AgCl, which is among the highest values reported for hematite photoanode.3,8,13,17,18,20 The photoanode also exhibits excellent photostability with ~4% decay after a ten hours measurement (Figure S12, Supporting Information).


Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 20

Figure 5. LSVs of E-I-Sn-Fe2O3 and Co/E-I-Sn-Fe2O3 collected in dark (dashed line) and under illumination (AM 1.5, 100 mW cm-2) (solid line); The arrows highlight the onset potentials. In summary, we have demonstrated a silica encapsulation strategy that can effectively retain the hematite NW morphology and improve uniformity of Sn doping distribution along the NW growth axis after high temperature activation. The capability of retaining NW morphology allows tuning the NW length for optimal light absorption. Uniform distribution of Sn doping enhances the donor density and charge transport of hematite NW that both factors are critical in enhancing its PEC performance for water oxidation. The morphology and doping engineered hematite NWs achieved excellent photocurrent density of ~1.36 mA cm-2 under front and back illuminations at 0.23 V vs. Ag/AgCl. Coating the hematite photoanode with a cobalt oxide-based OER catalyst further pushed the photocurrent density to a remarkable value of 2.2 mA cm-2 at 0.23 V vs. Ag/AgCl. This work provides fundamental insights on how morphology and doping uniformity of hematite photoanodes affect their PEC performance. In addition, we anticipate that our silica encapsulation strategy could be extended to improve the performance of hematite NWs for other applications including photocatalysis, batteries and supercapacitors.


Page 15 of 20

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

ASSOCIATED CONTENT Supporting Information Synthetic and analytical methods, SEM images, UV-visible, FT-IR, XRD and EDS spectra, as well as photoelectrochemical and electrochemical data. This material is available free of charge on the ACS Publications Website at DOI:

AUTHOR INFORMATION Corresponding Authors Y.T., E-mail: [email protected] Y. L., E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS Y.T. acknowledges the financial support of this work by Natural Science Foundation of China (21476271), NSFC-RGC (21461162003) and Science and Technology Planning Project of Guangdong Province, China (2014A030308012, 2014KTSCX004). M.L. and Y.S. thanks the China Scholarship Council for financial support. T.L. acknowledges the financial support from Chancellor’s Dissertation-year Fellowship awarded by University of California, Santa Cruz. We thank Dr. Tom Yuzvinsky for SEM image acquisition and the W. M. Keck Center for Nanoscale Optofluidics for use of the FEI Quanta 3D Dual-beam SEM instrument.

REFERENCES


Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(1)

Zandi, O.; Hamann, T. W. Nat. Chem. 2016, 8 (8), 778-783.

(2)

Shen, S.; Lindley, S. A.; Chen, X.; Zhang, J. Z. Energy Environ. Sci. 2016, 9 (9), 2744-

2775. (3)

Li, W.; He, D.; Sheehan, S. W.; He, Y.; Thorne, J. E.; Yao, X.; Brudvig, G. W.; Wang, D.

Energy Environ. Sci. 2016, 9 (5), 1794-1802. (4)

Wang, C. W.; Yang, S.; Fang, W. Q.; Liu, P.; Zhao, H.; Yang, H. G. Nano Lett. 2016, 16

(1), 427-433. (5)

Tang, J.; Li, J.; Zhang, Y.; Kong, B.; Yiliguma; Wang, Y.; Quan, Y.; Cheng, H.; Al-Enizi,

A. M.; Gong, X.; Zheng, G. Anal. Chem. 2015, 87 (13), 6703-6708. (6)

Kong, B.; Tang, J.; Selomulya, C.; Li, W.; Wei, J.; Fang, Y.; Wang, Y.; Zheng, G.; Zhao,

D. J. Am. Chem. Soc. 2014, 136 (19), 6822-6825. (7)

Xu, M.; Da, P.; Wu, H.; Zhao, D.; Zheng, G. Nano Lett. 2012, 12 (3), 1503-1508.

(8)

Kim, J. Y.; Youn, D. H.; Kang, K.; Lee, J. S. Angew. Chem. Int. Ed. 2016, 55 (36),

10854-10858. (9)

Tamirat, A. G.; Rick, J.; Dubale, A. A.; Su, W. N.; Hwang, B. J. Nanoscale Horiz. 2016,

1 (4), 243-267. (10)

Gurudayal; Sabba, D.; Kumar, M. H.; Wong, L. H.; Barber, J.; Gratzel, M.; Mathews, N.

Nano Lett. 2015, 15 (6), 3833-3839. (11)

Iandolo, B.; Wickman, B.; Zorić, I.; Hellman, A. J. Mater. Chem. A 2015, 3 (33), 16896-

16912. (12)

Sivula, K.; Le Formal, F.; Gratzel, M. ChemSusChem 2011, 4 (4), 432-449.

(13)

Zhang, Y.; Jiang, S.; Song, W.; Zhou, P.; Ji, H.; Ma, W.; Hao, W.; Chen, C.; Zhao, J.

Energy Environ. Sci. 2015, 8 (4), 1231-1236.


Page 16 of 20

Page 17 of 20

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

(14)

Qiu, Y.; Leung, S. F.; Zhang, Q.; Hua, B.; Lin, Q.; Wei, Z.; Tsui, K. H.; Zhang, Y.; Yang,

S.; Fan, Z. Nano Lett. 2014, 14 (4), 2123-2129. (15)

Ling, Y.; Wang, G.; Reddy, J.; Wang, C.; Zhang, J. Z.; Li, Y. Angew. Chem. Int. Ed.

2012, 124 (17), 4150-4155. (16)

Wang, G.; Ling, Y.; Wheeler, D. A.; George, K. E.; Horsley, K.; Heske, C.; Zhang, J. Z.;

Li, Y. Nano Lett. 2011, 11 (8), 3503-3509. (17)

Cho, I. S.; Han, H. S.; Logar, M.; Park, J.; Zheng, X. Adv. Energy Mater. 2016, 6 (4),

1501840. (18)

Ahn, H. J.; Yoon, K. Y.; Kwak, M. J.; Jang, J. H. Angew. Chem. Int. Ed. 2016, 55 (34),

9922-9926. (19)

Li, L.; Yu, Y.; Meng, F.; Tan, Y.; Hamers, R. J.; Jin, S. Nano Lett. 2012, 12 (2), 724-731.

(20)

Yang, Y.; Forster, M.; Ling, Y.; Wang, G.; Zhai, T.; Tong, Y.; Cowan, A. J.; Li, Y.

Angew. Chem. Int. Ed. 2016, 55 (10), 3403-3407. (21)

Jang, J. W.; Du, C.; Ye, Y.; Lin, Y.; Yao, X.; Thorne, J.; Liu, E.; McMahon, G.; Zhu, J.;

Javey, A.; Guo, J.; Wang, D. Nat. Commun. 2015, 6, 7447. (22)

Forster, M.; Potter, R. J.; Ling, Y.; Yang, Y.; Klug, D. R.; Li, Y.; Cowan, A. J. Chem. Sci.

2015, 6 (7), 4009-4016. (23)

Sivula, K.; Zboril, R.; Le Formal, F.; Robert, R.; Weidenkaff, A.; Tucek, J.; Frydrych, J.;

Gratzel, M. J. Am. Chem. Soc. 2010, 132 (21), 7436-7444. (24)

Ling, Y.; Wang, G.; Wheeler, D. A.; Zhang, J. Z.; Li, Y. Nano Lett. 2011, 11 (5), 2119-

2125. (25)

Wang, G.; Ling, Y.; Wang, H.; Xihong, L.; Li, Y. J. Photochem. Photobiol. C 2014, 19,

35-51.


Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(26)

Ling, Y.; Li, Y. Part. Part. Syst. Charact. 2014, 31 (11), 1113-1121.

(27)

Vayssieres, L.; Beermann, N.; Lindquist, S.-E.; Hagfeldt, A. Chem. Mater. 2001, 13 (2),

233-235. (28)

Wang, D.; Zhang, Y.; Peng, C.; Wang, J.; Huang, Q.; Su, S.; Wang, L.; Huang, W.; Fan,

C. Adv. Sci. 2015, 2 (4), 1500005. (29)

Annamalai, A.; Kannan, A. G.; Lee, S. Y.; Kim, D. W.; Choi, S. H.; Jang, J. S. J. Phys.

Chem. C 2015, 119 (34), 19996-20002. (30)

Brillet, J.; Gratzel, M.; Sivula, K. Nano Lett. 2010, 10 (10), 4155-4160.

(31)

Yang, J.; Bao, C.; Yu, T.; Hu, Y.; Luo, W.; Zhu, W.; Fu, G.; Li, Z.; Gao, H.; Li, F.; Zou,

Z. ACS Appl. Mater. Interfaces 2015, 7 (48), 26482-26490. (32)

Itoh, K.; Bockris, J. O. J. Electrochem. Soc. 1984, 131 (6), 1266-1271.

(33)

Cowan, A. J.; Durrant, J. R. Chem. Soc. Rev. 2013, 42 (6), 2281-2293.

(34)

Pendlebury, S. R.; Cowan, A. J.; Barroso, M.; Sivula, K.; Ye, J.; Grätzel, M.; Klug, D. R.;

Tang, J.; Durrant, J. R. Energy Environ. Sci. 2012, 5 (4), 6304-6312. (35)

Pendlebury, S. R.; Barroso, M.; Cowan, A. J.; Sivula, K.; Tang, J.; Gratzel, M.; Klug, D.;

Durrant, J. R. Chem. Commun. 2011, 47 (2), 716-718. (36)

Bak, T.; Nowotny, J.; Rekas, M.; Sorrell, C. C. Int. J. Hydrogen Energy 2002, 27 (10),

991-1022. (37)

Malara, F.; Minguzzi, A.; Marelli, M.; Morandi, S.; Psaro, R.; Dal Santo, V.; Naldoni, A.

ACS Catal. 2015, 5 (9), 5292-5300. (38)

Meng, X.; Qin, G.; Goddard, W. A.; Li, S.; Pan, H.; Wen, X.; Qin, Y.; Zuo, L. J. Phys.

Chem. C 2013, 117 (8), 3779-3784. (39)

Higashi, M.; Domen, K.; Abe, R. J. Am. Chem. Soc. 2012, 134 (16), 6968-6971.


Page 18 of 20

Page 19 of 20

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

(40)

Zhong, M.; Hisatomi, T.; Kuang, Y.; Zhao, J.; Liu, M.; Iwase, A.; Jia, Q.; Nishiyama, H.;

Minegishi, T.; Nakabayashi, M.; Shibata, N.; Niishiro, R.; Katayama, C.; Shibano, H.; Katayama, M.; Kudo, A.; Yamada, T.; Domen, K. J. Am. Chem. Soc. 2015, 137 (15), 5053-5060.


Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ToC GRAPHIC


Page 20 of 20

Morphology and Doping Engineering of Sn-Doped Hematite Nanowire

Recommend Documents