Understanding the Nature of Ammonia Treatment to Synthesize Oxygen Vacancy-Enriched Transition Metal Oxides

Article

Understanding the Nature of Ammonia Treatment to Synthesize Oxygen VacancyEnriched Transition Metal Oxides Dali Liu, Changhong Wang, Yifu Yu, Bo-Hang Zhao, Weichao Wang, Yonghua Du, Bin Zhang [email protected] (W.W.) [email protected] (B.Z.)

HIGHLIGHTS Combining experimental and theoretical studies to reveal OV formation mechanism A general low-temperature ammonia-assisted reduction strategy to create OVs in TMOs The sample is highly active for photothermal CO2 conversion without sacrificial agents

This work raises a universal low-temperature ammonia-assisted reduction strategy to synthesize OV-enriched TMOs without N-doping. The H and N atoms can extract O atoms in TMOs to form H2O and N2O at low temperature, and thus lead to OV-rich TMO including OV-enriched blue WO3 x porous nanorods (OBWPN). Compared with the corresponding yellow WO3 porous nanorods and N-doped WO3 x porous nanorods, OBWPN exhibits greatly enhanced photothermal CO2 conversion performance.

Liu et al., Chem 5, 1–14 February 14, 2019 ª 2018 Elsevier Inc. https://doi.org/10.1016/j.chempr.2018.11.001

Please cite this article in press as: Liu et al., Understanding the Nature of Ammonia Treatment to Synthesize Oxygen Vacancy-Enriched Transition Metal Oxides, Chem (2018), https://doi.org/10.1016/j.chempr.2018.11.001

Article

Understanding the Nature of Ammonia Treatment to Synthesize Oxygen Vacancy-Enriched Transition Metal Oxides Dali Liu,1,4,6 Changhong Wang,2,3,6 Yifu Yu,1 Bo-Hang Zhao,1 Weichao Wang,3,4,* Yonghua Du,5 and Bin Zhang1,4,7,*

SUMMARY

The Bigger Picture

Oxygen vacancies (OVs) have emerged as an important strategy to modulate the electronic structures, conductivity, and catalytic performance of transition metal oxides (TMOs). A few studies reported that OVs could be formed in N-doped TMOs during ammonia treatment. However, the OV-enriched TMOs without N-doping obtained through ammonia treatment are still unreported and their mechanism is unclear. Herein, we adopt experimental and theoretical investigations to demonstrate the mechanism of ammonia treatment. Based on this mechanism, we develop a facile method to synthesize OV-enriched blue WO3 x porous nanorods (OBWPN) without N-doping. OBWPN exhibit promising performance for photothermal reduction of CO2-H2O to CH4 without any external cocatalysts or sacrificial agents. In addition, the low-temperature ammonia-assisted reduction treatment is a universal strategy to generate OVs in other TMOs with enhanced performance of photocatalytic hydrogen generation. This work is significant for understanding the nature of ammonia treatment and promoting the wide application of OV-enriched TMOs.

Ammonia, as an abundant and cheap resource, is often adopted in the nitridation process. In addition, a few studies reported that oxygen vacancies (OVs) could be formed in N-doping transition metal oxides (TMOs) during ammonia treatment. However, the mechanism still lacks fundamental understanding. Thus, it is of interest to study the nature of ammonia treatment. Herein, we reveal that the H and N atoms can extract O atoms in WO3 to form H2O, N2, N2O, and NO in ammonia treatment. Meanwhile, WO3 are converted sequentially into WO3 x and WN. Based on this mechanism, we present a universal low-temperature ammonia-assisted reduction strategy to synthesize OVenriched TMOs without Ndoping. Interestingly, the asprepared OV-enriched blue WO3 x porous nanorods (OBWPN) and Nb2O5 x show enhanced photocatalytic performance owing to the existence of OVs. This work provides a fundamental mechanism study of ammonia treatment and opens a new perspective on the design of OVenriched TMOs with enhanced performance.

INTRODUCTION Intriguing applications of transition metal oxide (TMO) nanostructures in catalysis, electronics, and photonics are driving the exploration of synthetic approaches to control and manipulate their chemical composition, morphology, and structure.1–7 In this regard, oxygen vacancies (OVs) have emerged as an important tool to modulate the electronic structure, conductivity, band gap, and catalytic performance of TMOs.8–11 For instance, Chen’s group reported the preparation of black TiO2 x with narrowed band gap12 and Xie’s group utilized OVs in ultrathin nanosheets to greatly improve catalytic properties of TMOs toward the oxygen evolution reaction (OER), CO oxidation, and electrochemical CO2 reduction.13–15 Some general methods, such as thermal treatment by H2,16 electrochemical etching,17 metal powder reduction,18 sodium borohydride reduction,19 and calcination of inorganic/ organic hybrids,20 have been reported to generate OVs in TMOs. Ammonia, as an abundant and cheap resource, is often adopted in the N-doping and nitridation process.21–24 Interestingly, some studies reported that OVs could be formed in N-doped TMOs during ammonia treatment.25–27 However, OV-enriched TMOs without N-doping obtained through ammonia treatment is still unreported. In addition, the mechanism of OV formation in the process of ammonia treatment is unclear, which severely hinders the wider application of ammonia treatment for producing OVs in various kinds of TMOs. Thus, it is of interest to understand the nature of ammonia treatment to open up a new perspective on OV generation in TMOs.

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The conversion of CO2 to value-added chemicals offers a sustainable method for carbon cycling and is of significance in solving environmental problems and energy crises.28–33 Photocatalytic CO2 reduction as a significant method has been investigated thoroughly.34–39 However, the chemical inertia of CO2 limits the efficiency of solely photocatalytic CO2 conversion.40,41 Recently, photothermal CO2 reduction has drawn extensive research interest because of its high efficiency and selectivity.42–44 However, H2 (as hydrogen source), noble-metal cocatalysts, and sacrificial agents are often necessary in these conversion reactions.45 In addition, high temperature and pressure (e.g., 473 K and 6 bar) are essential in photothermal coupling catalytic reactions.46,47 Therefore, a simple and low-cost photothermal catalyst for efficient CO2 conversion, especially water-based CO2 conversion under facile conditions, is highly desirable but challenging. Herein, we experimentally and theoretically revealed the generation mechanism of OVs in the process of ammonia treatment. The H and N atoms in ammonia were able to extract O atoms in TMOs to form H2O, N2, N2O, and NO. As a result, TMOs are converted into OV-enriched TMOs and metal nitrides sequentially with the increase in calcination temperature. Based on this mechanism, we developed a low-temperature ammonia-assisted reduction strategy to synthesize OV-enriched blue WO3 x porous nanorods (OBWPN) without N-doping via calcination of yellow WO3 porous nanorods (YWPN) in ammonia atmosphere (see Supplemental Information for details). This method not only can avoid danger caused by imperceptible gas leakage (e.g., H2 and CO); compared with YWPN and N-doped WO3 x porous nanorods (NWPN), the as-prepared OBWPN shows excellent activity and selectivity for photothermal reduction of CO2 to CH4 (45.7 G 1.3 mmol g 1 h 1) in the presence of water without any external cocatalysts or sacrificial agents. In addition, the low-temperature ammonia-assisted reduction strategy not only avoids danger caused by imperceptible leakage of H2 and CO but also is ubiquitous in generating OVs in other TMOs, including MnO2, Nb2O5, and MoO3. This work is significant for understanding the nature of ammonia treatment and promoting wide application of a lowtemperature ammonia-assisted reduction strategy for producing OVs in various kinds of TMOs.

RESULTS AND DISCUSSION Mechanism of Ammonia Treatment To investigate the mechanism of ammonia treatment, we performed X-ray diffraction (XRD), on-line gas mass spectrometry, and theoretical calculations based on density functional theory (DFT) (Figures S1 and S2). Figure 1A shows the XRD patterns of WO3 calcinated in ammonia atmosphere at different temperatures. The XRD pattern of the product treated at 300 C is consistent with the standard pattern of WO3 (JCPDS no. 20-1324). When the calcination temperature increases to 450 C, WO3 is partially reduced to W10O29 (JCPDS no. 05-0386) and a small quantity of WN (JCPDS no. 65-2898) appears.48 Finally, WO3 is totally transformed into WN at 700 C.49 DFT calculations are adopted to calculate the relative reaction energy of adsorbed ammonia molecules on W atmospheres of the WO3 surface. First, we calculate the stability of different planes of WO3. As shown in Table S1, the (001) plane is more stable than (010) and (100). Thus, our subsequent simulations are all based on the (001) surface (Figure S1). As demonstrated in Figure S2, no difference is observed for the band structure of WO3 slabs containing 4–6 W-O layers, and the pseudo surface does not introduce any gap states. Thus, the four-layer WO3 slab model is adopted in the following calculations. The nitridation of WO3 goes through multiple processes as presented in Figure 1C. Firstly, NH3 reacts

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1Tianjin

Key Laboratory of Molecular Optoelectronic Sciences, Department of Chemistry, School of Science, Tianjin University, Tianjin 300354, China

2School

of Science, Tianjin University of Technology, Tianjin 300384, China

3Department

of Electronics, Nankai University, Tianjin 300071, China

4Collaborative

Innovation Center of Chemical Science and Engineering, Tianjin 300072, China

5Institute

of Chemical and Engineering Sciences, A*STAR (Agency for Science, Technology and Research), Jurong Island, Singapore 627833, Singapore

6These 7Lead

authors contributed equally

Contact

*Correspondence: [email protected] (W.W.), [email protected] (B.Z.) https://doi.org/10.1016/j.chempr.2018.11.001

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Figure 1. Mechanism of Investigation of Ammonia Treatment (A) XRD patterns of products treated in ammonia atmosphere at different temperatures. (B) Calculated relative reaction energy of N-doping at different locations on (001) plane of WO 3 . (C) Calculated reaction energy diagram of ammonia treatment over WO 3 (001) plane. (D) Mass spectrometer signals of N 2 , H2 , and H 2 O produced by temperature-programmed reaction of WO 3 with ammonia. (E) Mass spectrometer signals of N2 O, and NO produced by temperatureprogrammed reaction of WO3 with ammonia.

with the surface O* to form NH2* and OH* with a total energy change of 0.46 eV, indicating that this reaction would take place spontaneously. During this reaction, surface W atoms form a chemical bond with N in NH3 molecule with an adsorption energy of 1.2 eV, which is crucial for the subsequent reduction reactions. NH2* is then dehydrogenated to NH* by another surface O* with an energy increase of 0.96 eV. Further dehydrogenation of NH* to N* is energetically unfavorable because of the uphill reaction energy as large as 1.52 eV. Instead, NH* cooperates with the as-generated OH* to form HNOH* with DE = 0.765 eV. Meanwhile, OH* can be easily transformed to H2O. These processes are consistent with the results of online gas mass spectrometry (Figure 1D), in which only H2O is detected under low temperature (350 C, blue line in Figure 1D). NH3 removes some of the surface O* to form N-containing species, which are adsorbed on the unsaturated surface W site. With the increase in temperature, endothermic reactions with uphill energy larger than 1 eV appear and more ammonia is consumed. HNOH* reacts with O*

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to form NO* and H2O with 1.602 eV energy increase. NO* is then transformed into different N species by different reaction paths. As shown in Figure 1C, NO* reacts with as-obtained HNOH* and NH2* to form N2O* and N2, respectively, with 0.002 and 0.659 eV energy decrease, then N2O is released from the catalyst surface. By contrast, NO* could also react with O* to turn into NO2* (DE = 1.253 eV), which will further react with NH2* to form N2O* and H2O with energy increase of 0.414 eV. N2O* can then be released from the catalyst surface. Finally, NO* could also directly desorb from the catalyst surface with 1.403 eV energy increase. These results were verified experimentally by on-line gas mass spectrometry as shown in Figure 1E. N2O is detected at 320 C (Figure 1E, green line), which is prior to NO (Figure 1E, pink line), consistent with the theoretical calculation. However, no NO2 is observed because much higher energy is needed for NO2 release than the transformation from NO2* to N2O*. N2 is detected at about 500 C (Figure 1D, black line) and H2 is present at 570 C (Figure 1D, red line) because of NH3 decomposition. The nitrogen doping is accompanied by the reduction reactions under high temperature. Two H atoms of NH2* take one lattice O* away, and N* is introduced into different locations of WO3 lattice with increasing energy (Figure 1B). Specifically, N* prefers to occupy the vertex of WO6 octahedron (-O-(2)). Thermal diffusion of N* from surface to bulk will transform WO3 to WON or WN. Based on these results, we could introduce OVs into YWPN via a low-temperature ammonia-assisted reduction strategy. Synthesis and Characterization of YWPN and OBWPN YWPN were synthesized according to our previous work.20 The photograph (inset in Figure 2A) and scanning electron microscopy (SEM) images (Figure 2A) reveal that YWPN with an average length of around 2 mm were successfully prepared. The transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images (Figure 2B and its inset) indicate the porous structure of obtained YWPN. After calcination in ammonia gas flow at 350 C for 1 hr, YWPN are transformed into OBWPN. Note that high calcination temperature (>450 C) can lead to the formation of N-doped WO3 x porous nanorods (NWPN, Figure S3).27,50 The photograph (inset in Figure 2C) shows the blue color of the products. The SEM (Figure 2C) and TEM (Figure 2D) images demonstrate that the as-prepared OBWPN preserve porous nanorod morphology. In the HRTEM image (Figure 2E), the lattice fringe of 0.375 nm can be clearly observed, which is indexed to the (020) plane of WO3. Meanwhile, the existence of amorphous areas (marked with yellow circles) in OBWPN further confirms low crystallinity, which results from generation of OVs.46 The energy-dispersive X-ray spectroscopy (EDS) analysis (Figure S4) illustrates that OBWPN are composed of W and O. The high-angle annular dark-field image and the associated scanning TEM EDS (STEM-EDS) element mapping images (Figure S5) further reveal uniform distribution of W and O in OBWPN. Figure 2F shows XRD patterns of WO3 before and after the ammonia calcination at 350 C. OBWPN display an XRD pattern similar to that of YWPN, but weaker peak intensities. This phenomenon may be associated with lower crystallinity of OBWPN caused by generation of OVs. In addition, compared with the (211) diffraction peak of YWPN, the peak position of OBWPN slightly shifts to a lower angle (Figure 2F), implying that the ammonia-assisted reduction method can induce the expansion of lattice resulting from presence of OVs in OBWPN.51 To further confirm the existence of OVs in OBWPN, we carried out X-ray photoelectron spectroscopy (XPS) to analyze the valence of W and O before and after ammonia treatment. The survey scan of YWPN (Figure S6) indicates the presence of W, O, and C from the reference and the absence of other impurities. Besides two characteristic peaks centered at 35.8 (W2, W 4f5/2) and 37.9 eV (W1, W 4f7/2) of W6+ in WO3,52

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Figure 2. Characterization of YWPN and OBWPN (A and B) SEM image (inset: photograph) (A) and HRTEM image (inset: TEM image) (B) of YWPN. (C–E) SEM (inset: photograph) (C), TEM (inset: low-magnification TEM) (D), and HRTEM (E) images of OBWPN. (F) XRD patterns of YWPN (black) and OBWPN (red).

OBWPN owns two new W3 and W4 peaks centered at 37.0 and 34.6 eV (Figure 3A), which are attributed to characteristic W 4f7/2 and W 4f5/2 of W5+.38 The low valence of W suggests the existence of OVs in OBWPN. As shown in Figure 3B for O 1s XPS spectra, two typical peaks at O1 (530.3 eV) and O3 (532.6 eV) can be clearly seen for both YWPN and OBWPN. The O1 and O3 peaks can be ascribed to oxygen atoms bonded to metal and hydroxyl species of the surface-adsorbed water molecules. However, for OBWPN there is an O2 (531.2 eV) peak corresponding to oxygen defect sites with low oxygen coordination,53 confirming that OVs are introduced into OBWPN by the facile ammonia-assisted reduction strategy. Meanwhile, a survey scan and N 1s XPS spectra (Figure S7) indicate that there is no N-doping in OBWPN or the content of N-doping is below the limit of XPS detection. To further confirm the existence of OVs in OBWPN, we collected the X-ray absorption fine-structure spectroscopy (XAFS) at W L3-edge (Figures 3C and 3D) and photoluminescence (PL) spectra (Figure 3E) of samples. Figure 3C shows the X-ray absorption near-edge structure (XANES) of YWPN and OBWPN. The peak for OBWPN

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Figure 3. XPS, XAFS, PL, and UV-Vis-NIR Diffuse Reflectance Characterization (A and B) W 4f (A) and O 1s (B) XPS spectra of YWPN and OBWPN. (C) Normalized W L 3 -edge XANES spectra of YWPN and OBWPN. (D) Fourier transform (FT) of k-weighted W EXAFS spectra of YWPN and OBWPN. (E) Photoluminescence spectra of YWPN and OBWPN. (F) UV-vis-NIR absorption spectra of YWPN and OBWPN.

exhibits an obvious decrease in comparison with that of YWPN, indicating the decrease of W valence in OBWPN caused by OVs, which is in line with the XPS results (Figure 3A). Figure 3D demonstrates the Fourier transform (FT) of k-weighted W EXAFS spectra for YWPN and OBWPN. The peak at 1.33 A˚ represents the W-O bond. The W-O peak magnitude of OBWPN is lower than that of YWPN, indicating a drop of W-O coordination number in OBWPN originating from OVs. Figure 3E shows the PL spectra of YWPN and OBWPN. Compared with YWPN, the weak luminescence peak of OBWPN at 440 nm corresponds to a lower recombination rate of the electrons and holes under light irradiation, caused by OVs.54 These results strongly support that low-temperature ammonia-assisted reduction treatment can generate OVs into WO3. To study the influence of OVs on its light absorption, we measured the ultraviolet-visible near-infrared (UV-vis-NIR) absorption spectra. As shown in Figure 3F, compared with YWPN, OBWPN have stronger and wider absorption ability for UV-vis-NIR light. This greatly enhanced absorption capability

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Figure 4. Photothermal CO2 Conversion Measurement (A) Performance comparison of YWPN, OBWPN, and NWPN under simulative sunlight at 150  C for 5 hr. (B) OBWPN photocatalyst production of H 2 and CH 4 as a function of reaction time. (C) Stability test of OBWPN photocatalyst (5 hr for one cycle). (D) Mass spectra of GC-MS analysis in 13 CO2 and D 2 O (inset) isotope experiment.

is mainly associated with an OV-induced localized surface plasmon resonance (LSPR) effect.55,56 Performance of Photothermal CO2 Conversion The OVs have attracted wide attention because of their promising applications in various fields including oxygen evolution/reduction reactions, lithium batteries, solid fuel cells, nanodevices, photocatalysis, and organic catalysis.57–63 As a proof-of-concept application, we demonstrated the photothermal CO2 conversion performance of OBWPN (Figure 4). The photothermal CO2 reduction is performed in a sealed reaction chamber with a quartz window under illumination of a 300-W Xe lamp with an AM 1.5G light filter in thermal condition. Figure 4A shows the CO2 reduction performance of OBWPN, YWPN, and NWPN in the presence of H2O under simulative sunlight irradiation. The YWPN exhibits poor activity with little hydrogen (27.7 G 0.6 mmol g 1) and negligible CH4 after 5 hr of irradiation. Compared with YWPN, the as-prepared OBWPN shows excellent activity. The yield of H2 and CH4 after 5 hr of irradiation reaches 59.7 G 2.2 and 228.5 G 6.1 mmol g 1, respectively. In addition, methane is the sole hydrocarbon, revealing high selectivity of OBWPN for CO2 conversion. However, the NWPN displays a low yield of CH4 (62.8 G 3.3 mmol g 1) because of the decrease in OV content in the process of N-doping. Furthermore, the yield of H2 over NWPN (63.0 G 2.7 mmol g 1) is similar to that with OBWPN. Figure 4B shows the formation rate of CH4 and H2 on OBWPN, the average being 45.7 G 1.3 and 11.9 G 0.5 mmol g 1 h 1, respectively. These results demonstrate that OBWPN shows much higher activity of photothermal CO2 conversion than YWPN because of the existence of OVs. This high conversion rate indicates the promising potential of OBWPN as a photothermal catalyst for CO2 conversion. To examine its stability, we used OBWPN repeatedly for 25 hr

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and measured the yield of CH4 and H2 every 5 hr. As shown in Figure 4C, no obvious deactivation was observed. Moreover, SEM, XRD, TEM, and XPS results (Figure S8) of OBWPN after long-term photocatalytic testing further confirmed its high stability. To explore the roles of light and heat in the CO2 conversion process, we carried out a series of control experiments (Table S2). No products could be detected under darkness, indicating the photocatalytic nature of our conversion reaction. When H2O is replaced by H2 under otherwise identical conditions, there is also no fuel product under darkness, suggesting that the reaction is not triggered by heat. However, after irradiation of the CO2-H2 system, the CO2 conversion activity into CH4 and CO is similar to those of CO2-H2O. Thus, our CO2-H2O conversion reaction over OBWPN is driven by the photocatalysis and accelerated by the heat. When CO2 is only replaced by argon (Ar) with other conditions remaining unchanged, the only detected product is H2 with the absence of CH4, implying that the carbon source of evolved CH4 is from CO2 molecules. To directly demonstrate the carbon and hydrogen source of evolved CH4, we performed 13CO2 and D2O isotopic tracer experiments (Figure 4D). The results of mass spectrometry show that 13CH4 (m/z = 17) is detected once CO2 is replaced by 13CO2, indicating that the carbon atom of produced CH4 stems from CO2 feedstock. In addition, the 12CD4 (m/z = 20) is found when H2O is replaced by D2O, suggesting that the H atom of evolved CH4 derives from H2O. The CH4 (m/z = 16) is still present owing to the adsorbed CO2 and H2O on OBWPN in the isotope experiment. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was conducted to identify the surface intermediates and explore the roles of OVs for CO2 activation and conversion.64 As shown in Figure S9A, the introduction of CO2/H2O into YWPN induces the appearance of bidentate carbonate (b-CO32 ) at 1,640 and 1,368 cm 1, weak monodentate carbonate (m-CO32 ) at 1,548 and 1,508 cm 1, and negligible CO2 at 1,685 cm 1. In addition, the peak intensities of CO2 , b-CO32 , and m-CO32 almost remain unchanged after UV-vis irradiation for 60 and 120 min over YWPN. This result demonstrates the inefficient activation and conversion of the adsorbed CO2 species, which is coincident with the poor activity of YWPN. When CO2/H2O was introduced onto OBWPN, besides the obvious peaks of CO2 (1,688 cm 1), b-CO32 (1,642 and 1,388 cm 1), and m-CO32 (1,530, 1,465, and 1,377 cm 1), a new bicarbonate (HCO3 ) at 1,425 cm 1 is also observed, as shown in Figure S9B. The obvious peaks of CO2 and HCO3 reveal the presence of rich metal ionic sites and OH groups,65 indicating that OVs in OBWPN can enhance the activation of CO2 compared with YWPN. After irradiating OBWPN by UV-vis light for 120 min, all the peaks experience a dramatic decrease, confirming that OVs can facilitate the conversion of the surface intermediates. The in situ DRIFTS analysis demonstrates that OVs not only induce the appearance of a new intermediate (HCO3 ) but also facilitate the activation and conversion of the adsorbed CO2. These results suggest that the performance of WO3 for photothermal CO2 conversion can be greatly improved by the generation of OVs via the ammonia-assisted reduction strategy. Universality of the Ammonia-Assisted Reduction Strategy The low-temperature ammonia-assisted reduction strategy is found to be highly generalized for producing OVs in other TMOs. For instance, when MnO2 nanorods are treated at 200 C by ammonia, the morphology (Figure S10) and XRD patterns of the product (Figure 5A) almost show no change before (black) and after (red) treatment. The obvious O2 (531.2 eV) peak (Figure 5B) demonstrates the existence of OVs.15,54 The survey scan (inset in Figure 5B), N 1s XPS spectra (Figure S11), EDS analysis (Figure S12), and STEM-EDS elemental mapping images (Figure S13) indicate that there is no N-doping or the content of N-doping is below the limit of detection. Thus, MnO2 can be converted to the corresponding OV-rich counterparts

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Figure 5. Characterization of MnO2 x, Nb2O5 x, and MoO3 x (A) XRD patterns of MnO 2 (black) and the as-converted products treated with ammonia at 200  C (red) and 300  C (blue). (B) XPS spectra for the O 1s and survey scan (inset) of MnO 2 x . (C) Color change of MoO 3 and Nb 2 O5 before and after ammonia thermal treatment. (D) Photocatalytic water splitting for H 2 generation over Nb 2 O5 and Nb 2 O 5 x .

(MnO2 x) by the low-temperature ammonia-assisted reduction strategy. MnO2 x can be further reduced to MnO (Figure 5A, blue) by ammonia treatment at 300 C, further confirming the reducibility of ammonia in our method. Such an OV-introduction strategy is highly feasible for Nb2O5 (Figures S14–S18) and MoO3 (Figures S19– S23). Figure 5C shows the obvious color changes of MoO3 and Nb2O5 before and after ammonia treatment. To demonstrate the other promising applications of OV-enriched TMOs, we also investigated the photocatalytic hydrogen generation activities of Nb2O5 and Nb2O5 x (Figure 5D). Under the irradiation of simulated solar light, the Nb2O5 x steadily produces hydrogen at a rate of 41.6 mmol h 1 g 1, which is 2-fold higher than that of Nb2O5 (21.4 mmol h 1 g 1). These additional results indicate that our proposed low-temperature ammonia-assisted reduction strategy is a facile and universal method to generate OVs into TMOs with improved photocatalytic performance. Conclusion In summary, we demonstrate the mechanism of ammonia treatment through combining theoretical calculation with experimental investigation. H and N atoms in ammonia were able to extract O atoms in WO3 to form H2O, N2, N2O, and NO. As a result, TMOs were converted to OV-enriched TMOs and metal nitrides with the sequential increase in calcination temperature. Based on this mechanism, we developed a facile method to synthesize OV-enriched WO3 x OBWPN by a lowtemperature ammonia-assisted reduction strategy. The OBWPN show enhanced UV-vis-NIR photoabsorption capability induced by an LSPR effect. By selecting the performance of photothermal CO2 conversion with H2O as the proof-of-concept application, OBWPN can efficiently generate CH4 (45.7 G 1.3 mmol g 1 h 1) in the

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absence of noble-metal cocatalysts and sacrificial agents under solar light irradiation, 45.7- and 3.6-fold higher than that of primal YWPN and NWPN. In addition, the low-temperature ammonia-assisted reduction treatment is a universal strategy able to generate OVs in other TMOs including MnO2, Nb2O5, and MoO3. This work is significant for understanding the nature of ammonia treatment and promoting the wider application of a low-temperature ammonia-assisted reduction strategy to produce OVs in various kinds of TMOs.

EXPERIMENTAL PROCEDURES Preparation of Yellow WO3 Porous Nanorods WO3 porous nanorod was synthesized according to the reported literature. Commercial WO3 (0.4 g) was dispersed in 12 mL of ethylenediamine (EDA) under vigorous stirring for 30 min in a 15-mL Teflon-lined stainless-steel autoclave. The system was then treated at 180 C for 8 hr and allowed to cool naturally. The white product formed at the end of the reaction was washed by deionized water and absolute ethanol to remove the absorbed EDA molecules and dried in a vacuum oven. The white product was put into a quartz tube of diameter 6 mm. The sample was heated to 700 C at a rate of 1 C/min and kept at 700 C for 5 hr in air. It was then allowed to cool naturally. The yellow product formed at the end of the process was YWPN. Preparation of OV-Enriched Blue WO3 x Porous Nanorods To fabricate the OBWPN, we put the as-prepared YWPN into a quartz tube. The tube was purged under ammonia (NH3) gas for 30 min. The sample was then heated to 350 C at a rate of 2 C/min and kept at 350 C for 1 hr. The whole procedure was conducted under constant NH3 gas flow (30 mL/min). Finally, the product was treated at 500 C in argon atmosphere for 1 hr to remove the adsorbed ammonia on the surface. Preparation of N-Doped WO3 x Porous Nanorods To fabricate the NWPN, we put the as-prepared YWPN into a quartz tube. The tube was purged under NH3 gas for 30 min. The sample was then heated to 450 C at a rate of 2 C/min and kept at 450 C for 1 hr. The whole procedure was conducted under constant NH3 gas flow (30 mL/min). Finally, the product was treated at 500 C in argon atmosphere for 1 hr to remove the adsorbed ammonia on the surface. Preparation of MnO2, MoO3, and Nb2O5 The synthesis of MnO2 nanorods is based on a modified hydrothermal method.66 In brief, 0.50 g of KMnO4 and 0.21 g of MnSO4$H2O were mixed in distilled water (32 mL) and magnetically stirred for about 10 min to form a homogeneous mixture. The mixture was then transferred into a Teflon-lined stainless-steel autoclave (40 mL) and heated at 160 C for 12 hr. The product was collected, washed, and dried at 80 C. The synthesis of MoO3 nanorods is based on a modified hydrothermal method.67 In a typical synthesis, 1.67 g of ammonium heptamolybdate tetrahydrate (AHM, (NH4)6Mo7O24$4H2O) was dissolved in a mixed solution of 65% HNO3 and deionized H2O with a volume ratio of 1:5. After being fully dissolved, the reaction solution was transferred into a Teflon-lined stainless-steel autoclave (50 mL capacity) and heated at 160 C in a preheated electric oven for 20 hr. After cooling, the light-gray product was harvested by centrifugation and washed thoroughly with ultrapure water before drying at 60 C overnight.

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The Nb2O5 nanobelts were synthesized by sequential hydrothermal treatment, ion exchange, and annealing in air.68 In brief, 0.26 g of niobium (Nb) powders (99.5%) was dissolved in 40 mL of 10 M NaOH under stirring for 10 min and transferred to a 60-mL Teflon-lined stainless-steel autoclave. After heating at 130 C for 18 hr in an oven, the products were filtered and rinsed with deionized water several times to obtain the sodium niobate nanobelts. The niobic acid nanobelts were obtained after ion exchange in diluted HCl. The protonated products were rinsed with deionized water to obtain a neutral pH and then dried at 80 C overnight. The solid Nb2O5 nanobelts were then obtained after annealing at 700 C for 3 hr in air. Preparation of MnO2 x, MoO3 x, and Nb2O5 x The synthesis of MnO2 x, MoO3 x and Nb2O5 x is similar to that with OBWPN. To obtain the MnO2 x, we heated the prepared MnO2 to 200 C at a rate of 2 C/min and kept it at 200 C for 1 hr under constant NH3 gas flow; to obtain the MoO3 x, we heated the prepared MoO3 to 200 C at a rate of 2 C/min and kept it at 200 C for 1 hr under constant NH3 gas flow; to obtain the Nb2O5 x, we heated the prepared Nb2O5 to 500 C at a rate of 2 C/min and kept it at 500 C for 1 hr under constant NH3 gas flow. Finally, the product was treated in argon atmosphere for 1 hr to remove the adsorbed ammonia on the surface. Materials Characterization Powder XRD was performed on a Bruker D8 Focus Diffraction System using a Cu Ka source (l = 0.154178 nm). SEM and EDS analyses were conducted with a Hitachi S-4800 scanning electron microscope equipped with the Thermo Scientific energy-dispersion X-ray fluorescence analyzer. TEM, HRTEM, and elemental distribution mapping images were obtained with a JEOL-2100F system equipped with an EDAX Genesis XM2. XPS measurements were conducted with a PHI-1600 X-ray photoelectron spectrometer equipped with Al Ka radiation. The XAFS measurements were undertaken at Beamline 1W1B of Beijing Synchrotron Radiation Facility. PL measurements were carried out on a Horiba Jobin Yvon fluorescence photometer. UV-vis-NIR diffuse reflectance spectra (UV-vis-NIR DRS) were recorded on a Lambda 750 UV-vis-NIR spectrometer (PerkinElmer) equipped with an integrating sphere. The UV-vis-NIR DRS spectra of solid samples were collected in 200– 1,300 nm against BaSO4 reflectance standard. To investigate the reduction and nitridation mechanism of WO3, we monitored the component of off-gas produced in the nitridation process by on-line gas mass spectrometry. WO3 (0.2 g) was put into a quartz tube. The sample was pretreated at 150 C under ammonia (NH3) gas for 30 min. The sample was directly heated to 700 C at a rate of 2 C/min. Meanwhile, the off-gas was introduced into a gas mass spectrometer. In situ DRIFTS analysis is carried out in two sequential steps in a continuous-flow mode. First, CO2 adsorption on the sample surface was studied by introducing a CO2/H2O mixture to the IR cell for 60 min in the dark when the intensities of adsorption peaks reached saturation. Next, the UV-vis light was turned on for 60 and 120 min to investigate the photocatalytic conversion of reaction intermediates. An QP2010Ultra gas chromatographymass spectrometer was employed to analyze the produced CH4 from the 13CO2 and D2O isotopic experiments. Photothermal CO2 Conversion Measurement Photothermal CO2 conversion with H2O was conducted in a Teflon-lined stainlesssteel reaction chamber with a quartz window at the top for light irradiation. The volume of the chamber was 90 mL. The sample (50 mg) was spread in the reaction chamber. Prior to the photoreaction, the chamber was evacuated by a mechanical pump. CO2 (Air Liquide [Tianjin], 99.999%) bubbled through water was then added

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to the chamber to achieve atmospheric pressure. The products formed under irradiation (300-W Xe lamp with AM 1.5G light filter, Beijing Perfectlight PLS-SXE-300UV) in thermal condition were determined at regular intervals from the chamber. The products were analyzed by barrier discharge ionization detection-gas chromatography using a Shimadzu GC-2010 chromatograph, equipped with an active-carbonpacked column (carrier gas: He). Photocatalytic H2 Generation Measurements Photocatalytic H2 evolution from water reduction was carried out in a Pyrex reaction cell (irradiation area 12.56 cm 2) connected to a closed gas circulation and evacuation system. Typically, 50 mg of Nb2O5 or Nb2O5 x loaded with 0.5 wt % Pt were dispersed into 50 mL of mixed solution comprising EtOH (99.7%) and H2O (v/v = 9:1). The solution was then degassed for 30 min, followed by irradiation with a 300-W Xe lamp. The temperature was maintained at room temperature by a flow of cooling water during the photocatalytic reaction. The composition of products mixture was analyzed with an on-line Agilent 7890A gas chromatograph equipped with a thermal conductivity detector. The Xe lamp and the gas chromatograph were regularly calibrated to ensure reproducibility.

SUPPLEMENTAL INFORMATION Supplemental Information includes Supplemental Experimental Procedures, 23 figures, and 2 tables and can be found with this article online at https://doi.org/10. 1016/j.chempr.2018.11.001.

ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (no. 21871206 and no. 21422104) and the Natural Science Foundation of Tianjin City (no. 17JCJQJC44700). We would like to thank Dr. Lequan Liu for gas chromatography-mass spectrometry analysis.

AUTHOR CONTRIBUTIONS B.Z. conceived and directed the research. D.L. carried out the experiments, most characterizations, and measurements. C.W. and W.W. contributed to the DFT calculations. Y.D. contributed to the discussion on XAFS. All authors analyzed the data and discussed the results. D.L. and C.W. wrote the paper. Y.Y., B.-H.Z., W.W., and B.Z. revised the manuscript.

DECLARATION OF INTERESTS The authors declare no competing interests. Received: July 13, 2018 Revised: August 23, 2018 Accepted: November 2, 2018 Published: November 29, 2018

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