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Crystallinity Engineering of Hematite Nanorods for High ‐ Efficiency Photoelectrochemical Water Splitting

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In particular, hematite (α‐Fe2O3) has been extensively investigated as a photo­anode for visible light‐driven PEC water splitting due to its availability, stability, and suitable bandgap of 1.9–2.2 eV. 1, 2, 3 Fabricating hematite nanostructures provides an effective way to enhance the PEC performance of hematite photoanodes since they can enhance charge separation, surface area, and light absorption. 4, 5, 6 High‐temperature activation (HTA) is a common method to eliminate the poor lattice mismatching between hematite and fluorine‐doped tin oxide (FTO) substrate. 7, 8, 9 However, such hematite nanostructures often suffer from severe morphology evolution under HTA, resulting in the loss of fine structures that are critical for high‐performance PEC. 7, 10, 11 High‐temperature annealing at ≈800 °C is a key step to activate hematite nanostructures for high‐performance PEC. 8, 10 For instance, annealing at 800 °C improved the photocurrent density of hematite nanorods from 0.035 to 1.24 mA cm at 1.23 V versus reversible hydrogen electrode (RHE), which represents one of the highest photocurrents reported for under hematite photoanodes under standard Air Mass (AM) 1.5 illumination. 8, 12, 13 Nevertheless, the PEC performance of hematite nanostructures after HTA is still far below the theoretical expectation, partially because of morphology evolution during high‐temperature treatment... Therefore, it is highly desirable to fabricate hematite nanostructures with minimal morphology evolution after annealing at 800 °C for further improving the PEC water splitting efficiency... Our rationale stemmed from the thermodynamics of activation and sintering... Previous studies showed that the energy barrier associated with hematite activation is greater than that for morphology evolution. 10 Hence, the temperature required for hematite activation is higher than the sintering temperature, resulting in significant morphology change... Figure 3a shows the (110) diffraction peak of HN is significantly enhanced to the strongest peak after annealing at 800 °C, which is attributed to the recrystallization of sintering... However, the (110) diffraction peak in AHN is slightly enhanced after annealing at 800 °C compared with HN, suggesting the absence of recrystallization or sinter... Morphology evolution during HTA seriously impedes the PEC performance using hematite nanostructures... Given that substrate significantly influences the nucleation and growth of hematite nanorods in PEC process,27, 28, 29 we employ ATO modification to change the surface of FTO substrate (Figure S3, Supporting Information) to tune the crystal structure of hematite nanorods... Of particular importance, nanorods on AHN possess enhanced crystallinity, which significantly increases the sinter temperature as compared to that on HN... Compared with the sintered nanorods on HN, the sintering‐resistant nanorods on AHN have smaller diameter size and larger surface area, which are beneficial for the PEC performance due to the charge separation and transfer at the electrode/electrolyte interface... Inside the depletion layer (Figure S4, Supporting Information), the core of hematite nanorod can be regarded as the diffusion region, where the electron–hole pairs are most likely lost via recombination due to the very short electron and hole diffusion length... Hence, photo‐excited electrons and holes are more effectively separated by electric field in the depletion layer than in the diffusion region. 1, 30 Because of the morphology evolution after HTA, the feature size of nanorods on HN is significantly larger than that of the sintering‐resistant hematite nanorods on AHN... Collectively, the efficiency of charge separation and transfer at the electrode/electrolyte interface of AHN is significantly higher than that of HN... In summary, we have developed a crystallinity engineering strategy to effectively retain the morphology of hematite nanorods under HTA.

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Structures of AHN and HN. a) The XRD patterns of AHN and HN before and after annealing at 800 °C. b) HRTEM images of AHN. c) HRTEM images of HN.
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advs201500005-fig-0003: Structures of AHN and HN. a) The XRD patterns of AHN and HN before and after annealing at 800 °C. b) HRTEM images of AHN. c) HRTEM images of HN.

Mentions: We further collected X‐ray diffraction (XRD) spectra to investigate the crystal structure of hematite nanorods on both AHN and HN. The XRD patterns (Figure3a) revealed that ATO modification tremendously intensified the (101) and (110) peaks of cassiterite phase and hematite nanorods with the intensified (012) and (014) faces grew on ATO–FTO. It indicates that ATO modification significantly influences the crystal structure of hematite nanorods. The lattice parameters calculated by MAUD software18, 19, 20 demonstrate that the lattice parameter of nanorods on AHN is closer to JCPDS card 33‐0664 (a = b = 5.0356, c = 13.7489) (Table S1, Supporting Information), suggesting the crystallinity of nanorods on AHN is higher than that on HN. Furthermore, the lattice energy was evaluated by Kapustinskii equation21 to quantitatively analyze the crystallinity. Given that the higher lattice energy indicates the higher crystallinity, the higher lattice energy of hematite nanorods on ATO–FTO confirms the crystallinity of AHN is higher than that of HN, as shown in Table S1, Supporting Information. Considering that higher lattice energy suggests higher temperature requirement for sintering, it further suggests that the sintering temperature of hematite nanorods on ATO–FTO is higher than that on FTO.


Crystallinity Engineering of Hematite Nanorods for High ‐ Efficiency Photoelectrochemical Water Splitting
Structures of AHN and HN. a) The XRD patterns of AHN and HN before and after annealing at 800 °C. b) HRTEM images of AHN. c) HRTEM images of HN.
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advs201500005-fig-0003: Structures of AHN and HN. a) The XRD patterns of AHN and HN before and after annealing at 800 °C. b) HRTEM images of AHN. c) HRTEM images of HN.
Mentions: We further collected X‐ray diffraction (XRD) spectra to investigate the crystal structure of hematite nanorods on both AHN and HN. The XRD patterns (Figure3a) revealed that ATO modification tremendously intensified the (101) and (110) peaks of cassiterite phase and hematite nanorods with the intensified (012) and (014) faces grew on ATO–FTO. It indicates that ATO modification significantly influences the crystal structure of hematite nanorods. The lattice parameters calculated by MAUD software18, 19, 20 demonstrate that the lattice parameter of nanorods on AHN is closer to JCPDS card 33‐0664 (a = b = 5.0356, c = 13.7489) (Table S1, Supporting Information), suggesting the crystallinity of nanorods on AHN is higher than that on HN. Furthermore, the lattice energy was evaluated by Kapustinskii equation21 to quantitatively analyze the crystallinity. Given that the higher lattice energy indicates the higher crystallinity, the higher lattice energy of hematite nanorods on ATO–FTO confirms the crystallinity of AHN is higher than that of HN, as shown in Table S1, Supporting Information. Considering that higher lattice energy suggests higher temperature requirement for sintering, it further suggests that the sintering temperature of hematite nanorods on ATO–FTO is higher than that on FTO.

View Article: PubMed Central - PubMed

AUTOMATICALLY GENERATED EXCERPT
Please rate it.

In particular, hematite (α‐Fe2O3) has been extensively investigated as a photo­anode for visible light‐driven PEC water splitting due to its availability, stability, and suitable bandgap of 1.9–2.2 eV. 1, 2, 3 Fabricating hematite nanostructures provides an effective way to enhance the PEC performance of hematite photoanodes since they can enhance charge separation, surface area, and light absorption. 4, 5, 6 High‐temperature activation (HTA) is a common method to eliminate the poor lattice mismatching between hematite and fluorine‐doped tin oxide (FTO) substrate. 7, 8, 9 However, such hematite nanostructures often suffer from severe morphology evolution under HTA, resulting in the loss of fine structures that are critical for high‐performance PEC. 7, 10, 11 High‐temperature annealing at ≈800 °C is a key step to activate hematite nanostructures for high‐performance PEC. 8, 10 For instance, annealing at 800 °C improved the photocurrent density of hematite nanorods from 0.035 to 1.24 mA cm at 1.23 V versus reversible hydrogen electrode (RHE), which represents one of the highest photocurrents reported for under hematite photoanodes under standard Air Mass (AM) 1.5 illumination. 8, 12, 13 Nevertheless, the PEC performance of hematite nanostructures after HTA is still far below the theoretical expectation, partially because of morphology evolution during high‐temperature treatment... Therefore, it is highly desirable to fabricate hematite nanostructures with minimal morphology evolution after annealing at 800 °C for further improving the PEC water splitting efficiency... Our rationale stemmed from the thermodynamics of activation and sintering... Previous studies showed that the energy barrier associated with hematite activation is greater than that for morphology evolution. 10 Hence, the temperature required for hematite activation is higher than the sintering temperature, resulting in significant morphology change... Figure 3a shows the (110) diffraction peak of HN is significantly enhanced to the strongest peak after annealing at 800 °C, which is attributed to the recrystallization of sintering... However, the (110) diffraction peak in AHN is slightly enhanced after annealing at 800 °C compared with HN, suggesting the absence of recrystallization or sinter... Morphology evolution during HTA seriously impedes the PEC performance using hematite nanostructures... Given that substrate significantly influences the nucleation and growth of hematite nanorods in PEC process,27, 28, 29 we employ ATO modification to change the surface of FTO substrate (Figure S3, Supporting Information) to tune the crystal structure of hematite nanorods... Of particular importance, nanorods on AHN possess enhanced crystallinity, which significantly increases the sinter temperature as compared to that on HN... Compared with the sintered nanorods on HN, the sintering‐resistant nanorods on AHN have smaller diameter size and larger surface area, which are beneficial for the PEC performance due to the charge separation and transfer at the electrode/electrolyte interface... Inside the depletion layer (Figure S4, Supporting Information), the core of hematite nanorod can be regarded as the diffusion region, where the electron–hole pairs are most likely lost via recombination due to the very short electron and hole diffusion length... Hence, photo‐excited electrons and holes are more effectively separated by electric field in the depletion layer than in the diffusion region. 1, 30 Because of the morphology evolution after HTA, the feature size of nanorods on HN is significantly larger than that of the sintering‐resistant hematite nanorods on AHN... Collectively, the efficiency of charge separation and transfer at the electrode/electrolyte interface of AHN is significantly higher than that of HN... In summary, we have developed a crystallinity engineering strategy to effectively retain the morphology of hematite nanorods under HTA.

No MeSH data available.