Tuning Thermal Catalytic Enhancement in Doped MnO2âAu Nano

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Tuning Thermal Catalytic Enhancement in Doped MnO2-Au Nano-heterojunctions Shuhuai Hu, Xiaoyun Liu, Chunrui Wang, Pedro HC Camargo, and Jiale Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b03879 • Publication Date (Web): 23 Apr 2019 Downloaded from http://pubs.acs.org on April 24, 2019

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ACS Applied Materials & Interfaces

Tuning Thermal Catalytic Enhancement in Doped MnO2-Au Nano-heterojunctions

Shuhuai Hu1,2, Xiaoyun Liu3, Chunrui Wang1,2, Pedro H. C. Camargo4,5, and Jiale Wang1,2*

1 College

2

of Science, Donghua University, Shanghai 201620, China

Shanghai Institute of Intelligent Electronics and Systems, Donghua University,

Shanghai 201620, China 3

Research Center for Analysis and Measurement, Donghua University, Shanghai

201620, China 4

Department of Chemistry, University of Helsinki, A.I. Virtasen aukio 1, FI-00014,

Helsinki, Finland 5

Departamento de Química Fundamental, Instituto de Química, Universidade de São

Paulo, Av. Prof. Lineu Prestes, 748, 05508-000, São Paulo-SP, Brazil

*Corresponding author: Email: [email protected]

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Abstract Sodium (Na) and potassium (K) doped δ-MnO2, which presented difference bandgaps, were synthesized by hydrothermal method. Then uniform Au nanoparticles (NPs) were deposited on MnO2 to form metal-semiconductor nano-heterojunctions (MnO2-Au). By comparing their temperature-dependent thermal

catalytic

p,p'-dimercaptoazobenzene

performances, conversion

the was

p-aminothiophenol used

as

to

proof-of-concept

transformations. MnO2-Au hybrid materials demonstrated better thermal catalytic performances relative to individual Au NPs. Meanwhile, K-doped MnO2-Au, with MnO2 support displaying narrower band gap, displayed superior catalytic activities relative to Na-doped MnO2-Au. To get the same catalytic performance by individual Au NPs, it can be ~50 K less by Na-doped MnO2-Au and ~100 K less by K-doped MnO2-Au. The enhancement is mainly attributed to the thermally-excited electrons in MnO2, which were transferred to Au NPs. The additional electrons in Au NPs increase the electron density and thus contribute to the improvement of thermal catalysis. Our findings show that the establishment of nano-heterojunction formed by metal NPs on a semiconductor support has a significant impact on thermal catalysis, where a narrower band gap can facilitate thermally-excited carriers and thus bring about better catalytic performances. Thus the results presented here shed light on the design of nano-heterojunctions catalyst to approach reactions with superior performance under moderate condition.

Key words: MnO2, Au nanoparticles, heterojunction, thermal catalysis, electron transfer

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Introduction

Traditional thermally activated catalytic reactions have been widely used in variety of important processes such as the removal of pollutants and the manufacture of chemicals.1-4 However, these processes usually require high temperatures and/or high pressures, which increase production costs and raise environmental concerns.5-6 Thus approaches to enable thermally activated transformations to take place under mild or milder reaction conditions are of paramount importance. It is well known that several noble metal nanoparticles (NPs) including gold (Au), palladium (Pd), and platinum (Pt) display outstanding catalytic properties towards a myriad of transformations, such as oxidation, reduction, and coupling.7-9 Haruta et. al. found that CO undergoes oxidation at room temperature by using dispersed Au NPs as catalyst. Gualteros et. al. reported the high thermal catalytic performance of Au NPs supported on TiO2, SiO2, or Al2O3.10 Bonarowska et. al. used Pt, Pd and Au as catalysts for low-temperature hydrodechlorination of tetrachloromethane.11 In nanocatalysis, in addition to the metal NPs, semiconductors can be used as supports in order to avoid NPs aggregation, facilitate catalyst recovery, provide additional catalytic sites due to metal-support interactions, and facilitate the interaction or activation of substrates or intermediates.12-16 However, to our knowledge, few work mentioned the metal-semiconductor heterojunction interactions towards thermal catalysis. Similar to photocatalysis, it has been established in thermal catalysis that semiconductors can contribute to prolong the lifetimes of thermally-excited carriers, and potentially boost the catalytic properties.17 Manganese dioxide (MnO2) is abundant in nature. It has five different phases (α, β, γ, λ, δ) and the different properties of each phase make it be enormously studied in the fields of catalysis.18-21 Particularly, δ-MnO2, with unique layered 3



crystalline structure, has aroused much investigation. By filling ions between MnO2 layers, its band gap can easily be tuned.22-24 In this context, doping of δ-MnO2 with sodium (Na) or potassium (K) enabled us to engineer the different band gaps. Surface-enhanced Raman spectroscopy (SERS) can conveniently obtain molecular structure information.25,26 Moreover, Au NPs are one of the most widely used SERS substrates due to the well-established synthesis process and large Raman signal amplification.27 Herein δ-MnO2 with different doping decorated with Au NPs was employed to investigate how a nano-heterojunction composed by divided metal NPs on a semiconductor support materials tuned its thermal catalytic performances. The p-aminothiophenol to p,p'-dimercaptoazobenzene (PATP-to-DMAB) conversions was applied as a proof-of-concept model reaction on MnO2-Au hybrid materials, which can be easily monitored by SERS.28,29 It is worthy to note that most of previous reports focused on employing SERS to explore photocatalysis,30,31 while its application in thermal catalysis was scarcely mentioned. In fact, photons are not the only excitation energy source, but thermal energy can also excite electrons from valance band (VB) to conduction band (CB) in a semiconductor. To avoid the influence of thermal effect from laser irradiation during Raman measurement, the PATP-to-DMAB conversions were measured at 78 K, where the samples were cooled down in liquid nitrogen after exposure to reaction temperature. In this paper, we report a tuning of thermal catalytic performances by using doped MnO2-Au hybrid materials. The formation of heterojunction promotes the thermally-generated electrons in MnO2 transferred to Au. The additional electrons in Au NPs increase the electron density and thus contribute to improved thermal catalytic activities. Meanwhile, our data indicate that K-doped MnO2-Au, which displayed narrower band gap, presented superior activities compared to Na-doped MnO2-Au and unsupported Au NPs. This enhancement in conversion was attributed to more thermally-excited electrons in MnO2 transferred to Au NPs and thus increased the PATP conversion percentages. 4


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Results and Discussion

We started by employing NaMnO4 or KMnO4 as precursor to synthesize the Naor K-doping δ-MnO2 materials, respectively. Figure S1a and b present the SEM images of Na- and K-doped MnO2, respectively. The MnO2 samples demonstrated a similar flower-like morphology, while the size of Na-doped MnO2 (~400 nm in diameter) was a little smaller than that of K-doped MnO2 (~450 nm in diameter). The thickness of petals/sheets were ~5.1 and ~5.6 nm for the materials obtained with Na+ and K+ ions doped, respectively. Figure S2 shows the XRD patterns of Na- (top) and K-doped (bottom) δ-MnO2. It can be clearly observed that they present very similar crystalline structures. The peaks at 12.3, 24.8, 36.6, 37.4 and 65.5° corresponding to the (003), (006), (101), (012) and (110) planes of δ-MnO2, and meanwhile no signal corresponding to other crystal phases was observed.32,33 Figure S3 present the TEM images (Figure S3a and b), as well as SAED patterns (Figure S3c and d) of Na- and K-doped MnO2, respectively. Rings of SAED in Figure S3c and d showed the same values of 1.42 and 2.45 Å. And these values were corresponded to δ-MnO2 (110) and (101) interplanar distance, respectively. Band gaps of Na- and K-doped δ-MnO2 were obtained by UV-VIS spectra as presented in Figure S4a and b. The band gaps are 0.87 and 0.75 eV for Na- and K-doped MnO2 sample, respectively. Previous reports have demonstrated ~2.23 eV bandgap for layered MnO2.34 The narrower band gaps here might attribute to the doping of Na+ or K+ ions between MnO2 layers,24 and this aspect will be discussed later in the present study. After the synthesis of the MnO2 materials, they were used as supports for the Au NPs deposition by the reduction of AuCl4-(aq).35 Figure 1a and b present typical SEM images of MnO2-Au materials. The Au NPs were uniform on the Na- and K-doped MnO2, respectively. Furthermore, before and after Au deposition, no changes in size and morphology for the MnO2 supports were detected. Figure 1c and 5



d show TEM images of Na- and K-doped MnO2, respectively. It is observed that the size of Au NPs deposited on MnO2 were relatively monodisperse corresponding to ~25 nm (Na-doped MnO2) and 24.5 nm (K-doped MnO2) in diameter (Figure S5). The high-resolution TEM (HRTEM) images depicted In Figure 1e and f display the lattice spacings of MnO2 (1.42 and 2.45 Å) coincident with the SAED results and these values were assigned to δ-MnO2 (110) and (101) interplanar distance, respectively. Furthermore, a lattice spacing of 2.04 Å which corresponded to Au (200) interplanar distance was detected in both Na- and K-doped MnO2 materials. Figure 2a shows the XPS survey spectra of MnO2. With ~10 mins exposure under UV irradiation, both of Na- (top) and K-doped (bottom) samples demonstrated a ~5 times minor intensity of surface C contamination relative to that of Mn 2p. The binding energy (BE) values of Mn 2p3/2, O 1s, Na 1s and K 2p3/2 were presented in Table 1 corresponding to the core-level spectra in Figure 2b-d. It is important to note that the intensity of the signals due to carbon contamination decreased under UV irradiation as the UV light is able to degrade several carbon-based contaminants.36 In Figure 2b, it was detected that the Mn 2p3/2 core-level spectra of both Naand K-doped MnO2 have two components. For Na-doped MnO2 (top trace), the low binding energy (LBE) main peak labelled I at 642.4 eV corresponds to bulk-coordinated Mn; the high binding energy (HBE) peak labelled II at 644.7 eV is attributed to MnO2 interacted with absorbed oxygen from air, as reported previously.37-42 For K-doped MnO2 (bottom trace), the BE of both peak I (642.3 eV) and peak II (644.6 eV) were 0.1 eV lower compared with the corresponding peaks of Na-doped MnO2. Also, peak I did not present significant shift relative to the undoped MnO2, while peak II was shifted to higher BE values for the doped MnO2 relative to undoped one.42 These variations of Mn BE indicate the occurrence of a charge transfer between MnO2 and doped Na+ or K+ ions, and the reason will be discussed later in detail in the present study.43,44 Figure 2c displays the O 1s core-level spectra for both Na- and K-doped MnO2. The O 1s spectra of both materials presented three components. For Na-doped 6


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MnO2 (top trace), the main peak (labeled I) at 529.9 eV corresponds to bulk-coordinated oxygen. Peaks II (BE 531.5 eV) is due to the surface component and peak III (BE 533.1 eV) is due to the chemically bound carbonate or hydroxyl groups on surface.39,45 For K-doped MnO2, a similar profile was detected. Nevertheless, the BE of peak I (529.8 eV), II (531.3 eV) and III (532.6 eV) shift 0.1, 0.2 and 0.5 eV to lower BE than those of Na-doped MnO2, respectively. The same trend in the shifts of O 1s core-level as that of Mn 2p3/2 further suggest the occurrence of charge transfer between MnO2 and doped Na+ or K+ ions. These values were close to the reported BE for undoped MnO2.42 Figure 2d presents the Na 1s (top) and K 2p3/2 (bottom) core-level spectra corresponding to Na- and K-doped MnO2, respectively. Both of Na 1s and K 2p3/2 spectra only presented one component. The BE of Na 1s of Na-doped MnO2 is 1070.9 eV, which is higher than that of NaF (~1070.6 eV) and NaOH (~1069.6 eV).46,47 Meanwhile, The BE of K 2p3/2 of K-doped MnO2 is 292.5 eV, which is higher than that of KF (~292.2 eV).48 These results indicate that the Na+ or K+ is chemically bound to MnO2, thus Na- or K-doping.49 Since only one component was detected in the Na or K core-level XPS spectra, this data suggests that all Na or K ions were filled between MnO2 layers. TPR experiments were also carried out to investigate the redox properties of MnO2 with and without Au NPs deposited on their surface. Figure S6 presents the reduction profiles for these samples. Each sample displayed 2 peaks. The low-temperature (LT) peak may be attributed to the reduction of MnO2 to Mn3O4, whereas the high-temperature (HT) peak is attributed to the consecutive reduction of Mn3O4 to MnO.50 For the MnO2-Au samples, these two reduction peaks shifted to lower temperatures relative to their pure MnO2 samples. This increased reducibility of the support in the presence of Au NPs indicates strong metal-support interactions.50 The K-doped MnO2 samples presented two reduction peaks with closer and lower temperatures compared to that of Na-doped MnO2 samples, which could be attributed to reinforced spillover effect caused by higher doping 7



concentration.51 As before mentioned, the band gaps of Na- and K-doped MnO2 are 0.87 and 0.75 eV, respectively. One Na or K atom can only donate one electron if it is used as the doping atom.52 Meanwhile, our previous work has proved that the band gap of δ-MnO2 is able to be varied by different quantity of doping.40 Furthermore, the FWHM values of XPS peak at 12.3° were 0.94 and 0.92° for Na- and K-doped MnO2, respectively. Calculated by using Scherrer formula,53 the grain sizes for Na- and K-doped MnO2 were 8.41 and 8.57 nm, respectively. Moreover, the average diameter and petals thickness of the Na and K-doped MnO2 nanoflowers are very similar. All the results confirmed that the MnO2 of different doping had similar crystal forms and morphologies. Thus the different band gaps between Na- and K-doped MnO2 come from a different quantity of doping. To determine the quantity of doping ions in the doped MnO2, the Na/Mn or K/Mn atomic radios are evaluated based on core-level XPS results, and the details have been described in Supporting Information.40 The Na/Mn atomic radio is 1:41, which is an order of magnitude less than that of K/Mn (1:5). The Na and K core-level XPS spectra have proved that Na+ or K+ exist as ions in the MnO2, where one electron transferred to MnO2 for each Na or K atom. Therefore, the amount of doping ions in Na-doped MnO2 is less than that in K-doped MnO2, which not only results in the detected difference in the bandgap values, but also contributes to the higher BE of Mn 2p3/2 and O 1s in Na-doped MnO2. The possible explanation might be that the size of Na+ ions is less than that of K+ ions, which makes the interaction between Na+ and the MnO2 layers weaker relative to K+.52 So for Na+ ions it’s easy to escape from the layers of MnO2. However, this interesting aspect is beyond the scope of the present study. Then we employed MnO2-Au materials displaying controlled band gaps as model materials, which were used to study how the metal-semiconductor nano-heterojunctions can influence the thermal catalytic properties. In this case, The p-aminothiophenol to p,p'-dimercaptoazobenzene (PATP-to-DMAB) conversions was applied as a proof-of-concept model reaction.54 This conversion can be monitored by 8


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SERS easily, as Raman bands of PATP and DMAB are at different positions.55 However, it has been shown that the thermal effect arising from the localized surface plasmon resonance (LSPR) decay caused by Raman laser may also contribute to the occurrence of LSPR mediated transformations.56-59 Figure S7 presents the SERS spectra of individual Au NPs with ~24.3 nm in diameter (shown in Figure S8) functionalized with PATP measured at different temperatures, employing 30 s as the exposure time under 633 nm irradiation. In this case, the samples were firstly heated to a certain temperature, kept at it for 10 mins, and then SERS was measured. The spectra have 2 sets of bands: (i) Raman peaks at 1081, 1489 and 1593 cm−1 corresponded to A1 modes of PATP; and (ii) Raman peaks at 1081, 1142, 1390, 1433 and 1575 cm−1 are attributed to Ag modes of DMAB.58,60,61 For convenience, all spectra have been normalized with respect to Raman peak at 1081 cm−1, and then the DMAB:(PATP+DMAB) intensity ratios is able to be directly observed from the SERS spectra. Figure S7a show the Au NPs SERS spectra functionalized with PATP recorded at 78 K. Even the laser powers increased from 0.037 to 0.780 mW, no bands of DMAB can be observed. As the temperature increased to 173 K (Figure S7b), peaks corresponding to DMAB were barely visible only at high laser power irradiation of 0.780 mW. As the temperature further increased to freezing point (273 K) (Figure S7c), room temperature (300 K) (Figure S7d) and 323 K (50 °C) (Figure S7e), the DMAB bands were obviously observed when the Au NPs were irradiated at high laser power of 0.330 and 0.780 mW. However, the DMAB bands were not observed under low laser power irradiation of 0.037 and 0.086 mW. Figure S7f shows the DMAB:(PATP+DMAB) intensity ratios obtained under different temperatures, employing 0.780 mW laser power. It is obvious that the DMAB:(PATP+DMAB) intensity ratios increased with temperature in which the Au NPs substrates were treated. Normally, the SERS is carried out at room temperature and thermal effect arising from the LSPR decay caused by Raman laser could also contribute to the occurrence of PATP-to-DMAB transformation.62 However, at low temperature 9



almost all the electrons in Au NPs are below the Fermi level,52 the thermal effect is inhibited as shown in Figure S9a. Here, in order to avoid the influence of thermal effect from laser irradiation during Raman measurement, the PATP-to-DMAB conversions were measured at 78 K, where the samples were cooled down by liquid nitrogen after exposure to reaction temperature. In this case, the samples were firstly heated to a certain temperature, kept at it for 10 mins, cooled down immediately to 78 K, and then SERS was measured. In order to have a better signal-to-noise ratio, the 0.780 mW laser power irradiation was used. Figure 3 show the PATP-to-DMAB conversion as a function of different thermal treatment temperature for the Au NPs and MnO2-Au materials employing the MnO2 supports displaying different band gaps. For the individual Au NPs sample (Figure 3a), at 300 K (27 oC, room temperature), there were no bands corresponding to DMAB. When the Au NPs sample was heated to 373 K (100 oC), the peaks assigned to DMAB were observed. If the temperature was elevated to 423 K (150 oC), the DMAB peaks were more obvious. Meanwhile, the peak at 1575 cm-1 started to appear. Finally all the bands corresponding to DMAB became more intense when the reaction temperature reached 473 K (200 oC). The results indicate that, at low or moderate temperatures for thermal treatment, very few electrons in Au NPs are upon Fermi level (Figure S9a),52 so the free electrons cannot overcome the energy barrier to activate O2 molecules (from air) and participate in the PATP oxidation. However, this trend increases with the rise in temperature, in which more electrons are excited to higher energy upon Fermi level (Figure S9b).52 Thus the DMAB:(PATP+DMAB) intensity ratios demonstrate a stable trend to increase with thermal treatment temperature (Figure 3d). It is noteworthy that we employed the intensity of 1433 cm-1 peak in the spectra to study the PATP-to-DMAB conversions and get insights into the PATP-to-DMAB conversion trends. For MnO2-Au hybrid materials functionalized with PATP, DMAB peak intensities showed a different variation with thermal treatment temperatures. Figure 3b presents the SERS spectra of MnO2-Au with Na-doped MnO2 as support. Even when 10


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the MnO2-Au sample was cooled downed to 78 K directly from room temperature (300 K), the Raman peaks at 1142 and 1433 cm-1 can be observed. And a shoulder at the left side of 1593 cm-1 Raman peak of DMAB can also be obviously observed. As previously reported by our group, in this system electrons can be excited from VB to CB under irradiation of visible light due to the narrow band gap of doped MnO2. The photo-generated electrons can transfer to the Au NPs and activate adsorbed O2 molecules, participating in PATP-to-DMAB conversions.40 When the Na-doped MnO2-Au sample was treated at 373 K, the intensity of the all DMAB peaks can be detected. If we consider the contribution from the DMAB formation due to the optical excitation of MnO2, the conversion percentage for the DMAB formation was almost the same relative to that of individual Au NPs (Figure S10). However, with the temperature continuous increasing to 423 K, bands corresponding to DMAB became much more intense. This indicates that the effect of thermal treatment temperature on Na-doped MnO2-Au begins from ~423 K. At this point, the conversion performance is better than that of individual Au NPs in the absence of the support. At higher temperature (473 K), higher conversion percentage was observed for Na-doped MnO2-Au as expected. After the whole thermal catalysis experiment, MnO2-Au samples kept the similar morphologies as before heating (Figure S11). At 0 K, all the electrons in MnO2 are under Fermi level, thus electrons are only at VB, while the CB is empty.52 The concentration of free electrons in semiconductor is proportional to exp(-Eg/2k0T), where Eg is the band gap value of semiconductor, k0 is the Boltzmann constant and T is the temperature.52,63 With increasing temperature, thermal energy excited electrons from VB to CB. These thermally-generated free electrons in MnO2 could transfer to Au NPs, participating in the activation of adsorbed O2 molecule and thus further contributing to the oxidation of PATP. Furthermore, the work functions of δ-MnO2 and Au are 6.8 and 5 eV, respectively.52,64 Thus as illustrated in Figure 4, the energy band of MnO2 bends towards smaller energy values.63 Due to this interfacial energy level structure, it 11



facilitate the charge-transfer of free electrons from MnO2 to Au. Since the concentration of free electrons in semiconductor is also related to the band gap value, a better thermal catalytic performance could be expected with a narrower band gap. In order to investigate this hypothesis, MnO2-Au with K-doped MnO2 as support was studied. Figure 3c presents the SERS spectra of MnO2-Au with K-doped MnO2 as support as a function of different thermal treatment temperatures. When the MnO2-Au sample was cooled downed to 78 K from 373 K, the DMAB peaks located at 1433 cm-1 demonstrated a higher intensity than that of MnO2-Au with Na-doped MnO2 as support. When the sample was heated to 423 K, the thermal catalytic performance was comparable to that of MnO2-Au with Na-doped MnO2 as support reacted at 473 K. With the temperature continuously increasing to 473 K, the catalytic performance became much more obvious. The comparison between the conversion percentages for all materials can be better visualized on Figure 3d. It has been pointed out that MnO2 may also act as an active catalyst in thermal or light-induced thermal driven reactions,65,66 In this context, we also probed the PATP-to-DMAB conversions promoted by MnO2 eliminating the effect of Au NPs. To this end, Au covered by ultrathin SiO2 (Au@SiO2) core-shell NPs were synthetized67,68 and then were deposited on MnO2 nanoflowers (MnO2-Au@SiO2). The SEM image (Figure S12a) of Au@SiO2 NPs indicates that they were relatively monodisperse and their size corresponded to ~27.5 nm in diameter (Figure S12b), which is a little larger relative to the initial Au NPs as a result of the coverage by ultrathin SiO2 shell. Moreover, Figure S12c and d suggest that the Au@SiO2 NPs were uniformly deposited on Na- and K-doped MnO2, respectively. In this system, it is expected that the heterojunction between Au and MnO2 is broken while still allowing for the detection of PATP and DMAB molecules by shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS). For the individual Au@SiO2 NPs sample (Figure S13a), there were nearly no bands corresponding to DMAB when the sample was heated from 300 K to 473 K, 12


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confirming the coverage of ultrathin silica shell on Au NPs which inhibited the oxidation of PATP. Similar to MnO2-Au samples, even when the MnO2-Au@SiO2 samples were cooled downed to 78 K directly from room temperature (300 K), the Raman peaks at 1142 and 1433 cm-1 can also be observed (Figure S13b and c), though their intensities were much lower relative to the MnO2-Au samples (Figure S13d). As mentioned previously, electrons can be excited from VB to CB under irradiation of visible light due to the narrow band gap of doped MnO2. The photo-generated electrons can transfer to adsorbed O2 molecules, participating in PATP-to-DMAB conversions.40 If we consider the contribution of DMAB formation due to the optical excitation of MnO2 as that in the MnO2-Au samples, the MnO2 contribution to the PATP-to-DMAB conversion was less than 8 % (Figure S14), which is much lower than that of individual Au NPs or MnO2-Au samples.

Conclusions

Na-doped and K-doped δ-MnO2 nanoflowers, uniformly decorated with Au NPs, were successfully synthesized by hydrothermal method. They were employed as model

systems

to

investigate

the

thermal

catalytic

enhancement

in

metal-semiconductor nano-heterojunctions caused by thermally-excited electrons. MnO2-Au hybrid materials demonstrate better thermal catalytic activities relative to individual Au NPs. Furthermore, the variation of MnO2 band gap can be mediated as a function of different doping with K+ or Na+ ions. MnO2-Au with K-doped MnO2 as support, presenting a narrower band gap, shows a superior thermal catalytic performance relative to that with Na-doped MnO2 as support. In this case, thermally-generated electrons in MnO2 can be transferred to Au NPs, which increased free electron density in Au NPs, further contributing to activate adsorbed O2 molecules and thus the oxidation of PATP. For narrower bandgap, more free electrons can be thermally-excited in semiconductor and then transfer to Au NPs, resulting in a better catalytic performance. Our results exhibited here may provide a 13



new insight to design metal-semiconductor nano-heterojunctions for applications in thermal catalysis, which can significantly decrease its reaction temperature.

Acknowledgements This work was supported by the National Natural Science Foundation of China (21703031, 61376017 and 51503033), and Shanghai Talent Development Funding. J.W. thanks the funds from Donghua University for Distinguished Research Fellow. P.H.C.C thanks CNPq for the research fellowship and FAPESP for financial support (15/26308-7).

Associated Content Supporting Information Experimental details and supplementary figures. This material is available free of charge via the Internet at http://pubs.acs.org.

Notes The authors declare no competing financial interest.

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Figure 1. SEM (a and b), TEM (c and d), and HRTEM (e and f) images of MnO2-Au with Na- and K-doped MnO2 as support.

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Figure 2. (a) Survey XPS scan of Na-doped MnO2 (top trace) and K-doped MnO2 (bottom trace). Core-level spectra for Na-doped MnO2 (top trace) and K-doped MnO2 (bottom trace): (b) Mn 2p3/2, (c) O 1s, (d) Na 1s and K 2p3/2 .

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Figure 3. SERS spectra obtained at 78 K after thermally treated at different temperatures for (a) Au NPs, (b) MnO2-Au with Na-doped MnO2 as support, and (c) MnO2-Au with K-doped MnO2 as support. The samples had been functionalized with PATP employing 30 s as the exposure time and 633 nm as the excitation wavelength. All spectra were normalized with respect to the band at 1081 cm-1. (d) 1433:1081 cm-1 DMAB:(PATP+DMAB) intensity ratios obtained from (a), (b) and (c). The laser power corresponded to 0.780 mW.

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Figure 4. Schematic for the energy-level and charge transfer pathways for the MnO2-Au thermally treated at low (a) and high (b) temperatures.

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Na-MnO2

K-MnO2

MnO2[42]

Peak position

Peak position

Peak position

(eV)

(eV)

(eV)

MnI

642.4

642.3

642.2

MnII

644.7

644.6

642.7

OI

529.9

529.8

529.7

OII

531.5

531.3

531.2

OIII

533.1

532.6

533.0

Component

K Na

292.5 1070.9

Table 1. Binding energy of the Mn 2p3/2, O 1s, Na 1s and K 2p3/2 core-level components of Na- and K-doped MnO2, respectively. The Binding energy of undoped MnO2 from literature were also presented for comparison.42

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Table of Contents Graphic:

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Tuning Thermal Catalytic Enhancement in Doped MnO2âAu Nano

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