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C: Energy Conversion and Storage; Energy and Charge Transport

Insight into the Transfer Mechanisms of Photogenerated Carriers for WO/TiO Heterojunction Photocatalysts: Is It the Transfer of Band-Band or Z-Scheme? Why? 3

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Sugang Meng, Wenting Sun, Sujuan Zhang, Xiuzhen Zheng, Xianliang Fu, and Shifu Chen J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b07524 • Publication Date (Web): 22 Oct 2018 Downloaded from http://pubs.acs.org on October 25, 2018

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The Journal of Physical Chemistry

Insight into the Transfer Mechanism of Photogenerated Carriers for WO3/TiO2 Heterojunction Photocatalysts: Is It the Transfer of Band-Band or Z-scheme? Why? Sugang Meng, Wenting Sun, Sujuan Zhang, Xiuzhen Zheng, Xianliang Fu, Shifu Chen *

Department of Chemistry, Huaibei Normal University, Anhui Huaibei, 235000, People’s Republic of China.

*Corresponding

author,

Tel:

+86-561-3806611,

Fax:

[email protected], [email protected]

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Abstract: The transfer mechanism of photoexcited charge carriers is alway a hot topic in photocatalysis research field. Coupling a photocatalytst with other photocatalyst is one of the most widely used strategies to realize effective transfer of the photogenerated carriers. In the paper, a series of WO3/TiO2 composites with different weight ratios were prepared. And the WO3/TiO2 composites were characterized in detail. The result showed that no matter whether the primary part of WO3/TiO2 composites is TiO2 or WO3, the photocatalytic activities of WO3/TiO2 are much higher than that of pure TiO2 or WO3. The reason may be ascribed to the generation of a relative p-n heterojunction between WO3 and TiO2. Under the effect of the built-in electric field, the transfer directions of the photogenerated charge carriers in the heterojunctions are opposite to the migration directions of the photogenerated charge carriers in the conduction band (CB) and valence band (VB) of WO3 and TiO2. Thus, the transfer of the photogenerated charge carriers adopts a Z-scheme system in the WO3/TiO2 heterojunctions. The accumulated photogenerated electrons in the CB of TiO2 with more negative potential can reduce O2 to superoxide radical (•O2-), and the photogenerated holes in the VB of WO3 with more positive potential may oxidate H2O (or OH-) into hydroxyl radical (•OH). The photocatalytic activities of the WO3/TiO2 heterojunctions are significantly promoted. The transfer mechanisms and natural law for the WO3/TiO2 heterojunction photocatalysts were proved by physical and chemical methods. This work not only reveals the transfer mechanisms of photogenerated carriers and internal natural behavior of heterojunction photocatalysts, but also guides the design and constructing of composite photocatalysts, and thus has theoretical and practical significance.

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1. Introduction Photocatalysis as a promising technology has been researched extensively because of its possible applications in environmental remediation,1-8 selective transformation of organics,9-10 hydrogen production from water,11-12 CO2 reduction,13-14 etc. In order to seek excellent photocatalysts, scientists have carried out a wide range of screening work for different semiconductor materials in the past decades, among which the single semiconductor materials, such as metal oxides,15-16 nitrides,17-18 sulfides,19-20 etc. have been investigated particularly. However, the results showed that the photocatalytic activity of the single component could not meet people's expectations. One primary reason is attributed to the rapid recombination of the photogenerated charge carriers in the interior of the single semiconductor materials. The other reason is that the charge carriers are long-lived but end up being deeply trapped within the material.21-23 In order to solve the problem, the scholars have focused on the preparation of multicomponent photocatalysts.24-25 They adopted noble metal deposition,26-27 ion doping of metals or nonmetals,28-29 coupled semiconductors,30-31 etc. to modify the single component photocatalysts. The results showed that, compared with the single component photocatalysts, the composite photocatalysts exhibit higher photocatalytic activity. Among them, the heterojunction photocatalysts formed by different semiconduction materials with band gap matching are always a reasearch hotspot.32-35 For a band-band transfer heterojunction photocatalyst, because of the difference of CB and VB positions for the different semiconductors, the photoexcited electrons in the CB with more negative potential will be transfered into the CB with low negative potential, and the photogenerated holes in the VB with more positive potential will be transfered into the VB with low positive potential. Therefore, the photogenerated electron-hole pairs in different semiconductions are separated effectively, and the quantum efficiency is increased.36-41 However, the redox abilities of the transferred charge carriers are lower than that of the original photoinduced charge carriers due to different positions of CB and VB.42 In order to avoid the above drawbacks, the Z-scheme of heterojunction photocatalysts has 3



become the hotspot of photocatalysis due to its strong redox potential and attractive performance than single photocatalyst.43-48 For instance, g-C3N4/WO3,45 ZnO/CdS Z-scheme photocatalysts can prolong the lifetime of the photoexcited carriers and increase photocatalytic activity.47 However, it can be seen from the reported results that, for the same heterojunction photocatalysts, because the difference of preparation and experimental conditions, the reported transfer mechanisms of the photogenerated charge carriers are different, such as TiO2/C3N449-50 and TiO2/CdS.51-52 At the same time, from the previous reports, the transfer mechanisms of the photogenerated electron-hole pairs in the heterojunction photocatalysts are judged by comparing the photocatalytic activity and the active species generated in the reaction system to determine whether it is a band-band transfer or a Z-scheme transfer. Therefore, how to accurately determine the transfer mechanisms of the photoexcited electron-hole pairs in the heterojunction photocatalysts needs further study, and the underlying reasons need to be further explored. It is known that TiO2 is an excellent photocatalyst because of its high redox ability, corrosion resistance, non-toxicity, durability and low cost.53 Tungsten oxide (WO3) is considered to be the promising materials due to their special photocatalytic and electrochromic properties. The CB and VB positions of TiO2 are -0.30 and 2.90 eV54 and the CB and VB positions of WO3 are 0.80 eV and 3.40 eV,45 respectively. When WO3 is combined with TiO2, WO3/TiO2 heterojunction photocatalyst can be formed between WO3 and TiO2. The reported results showed that compared with the single WO3 or TiO2 photocatalyst, the WO3/TiO2 heterojunction photocatalyst exhibits excellent photocatalytic activity.1,53,55 The reason for the improved activity was attributed to the band-band transfer of the photogenerated charge carriers. However, the transfer mechanisms of WO3/TiO2 heterojunction photocatalyst have not been investigated extensively. Especially, the underlying reasons have not been promulgated. In this paper, a series of WO3/TiO2 composites with different weight ratios were prepared by an in situ calcination method. The activity was evaluated by 4


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photocatalytic degradation of coloring pollutants (methyl orange (MO) and rhodamine B (RhB)) and endocrine disrupting chemicals (bisphenol A (BPA)). The migration processes of the photogenerated charge carriers were studied comprehensively by the photoelectrochemical technique (PEC), the electron paramagnetic resonance (EPR), the time-resolved photoluminescence (TR-PL), and the effect of reactive species in the photocatalytic reactions. The transfer mechanisms of Z-scheme for the WO3/TiO2 heterojunction photocatalysts were proposed and verified. The reason for the Z-scheme mechanism was promulgated. Some interesting results were obtained. It is proposed that the internal electric field of the relative p-n junction formed by WO3 and TiO2 photocatalysts is the inner impetus for the Z-scheme migration in the heterojunctions. We hoped that this work could give inspiration to understand the transfer mechanism of the photoexcited charge carriers and open a pathway for constructing photocatalysts with excellent photocatalytic performance. 2. Experimental The WO3/TiO2 composite photocatalysts were fabricated as follows: commercial TiO2 (anatase) and (NH4)10W12O41•xH2O with a certain ratio of WO3/TiO2 were mixed into a zirconia tank. Zirconia balls with two different diameter sizes and absolute ethyl alcohol (1 mL) were then added into the tank. The mixture was milled at 200 rpm for 60 min and dried at 60 ◦C for 120 min. And then, this mixture was successive heated at 500 ◦C and 520 ◦C for 2 h, respectively. In this way, the WO3/TiO2 composite photocatalyst

was

obtained.

By

changing

the

weight

ratio

of

TiO2/(NH4)10W12O41•xH2O, the WO3/TiO2 composite photocatalysts with different weight ratios of WO3 (99%, 97%, 95%, 90%, 70%, 50%, 30%, 15%, 10%, 5%, 3% and 1%) were prepared. Pure WO3 and TiO2 samples were also prepared by the same method. Materials, Characterization and Photoreaction apparatus and procedure were presented in the supporting information.

3. Results and discussion 3.1. Catalyst characterization The X-ray powder diffraction patterns of the WO3/TiO2 composites with different 5



amounts of WO3 are shown in Figure 1. The XRD patterns of pure TiO2 and WO3 are also given for comparison. It can be seen that the TiO2 powder is in good agreement with standard tetragonal phase of TiO2 (anatase phase) indexed to be the data in the JCPDS cards (21-1272),54 while the diffraction peaks of WO3 are identical to the reported data of a orthorhombic phase in the JCPDS (20-1324).45 It is clear that when the weight ratio of WO3 is lower than 3%, no distinct peaks of WO3 are found on the WO3/TiO2 samples surface. However, when the weight ratio of WO3 is higher than 3%, the peaks of WO3 located at 23.7 and 24.1° can be observed. It is clear that with the increase of the amount of WO3, the characteristic peaks of TiO2 located at 25.3° are weakened gradually. When the amount of WO3 is higher than 97%, the characteristic peaks of TiO2 disappear because of the low amount or high disperse of TiO2 on the samples surface. Meanwhile, with the increase of the amount of WO 3, the diffraction peaks of WO3 increase remarkably. It is notable that no other new diffraction peaks are detected in WO3/TiO2 composites, indicating that TiO2 and WO3 keep pure phase and no impurities formed in the preparation process of the WO3/TiO2 heterojunction photocatalysts.

Figure 1. XRD patterns of TiO2, WO3 (wt.%)/TiO2 photocatalysts and WO3. 6


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The optical properties of TiO2, WO3 and WO3/TiO2 photocatalysts were shown in Figure 2. The absorption curves of the WO3/TiO2 photocatalysts are between the TiO2 and WO3 (Figure 2a). The absorption edges of the pure WO3 and TiO2 samples are at about 480 nm and 388 nm, respectively. The band gap energies of the photocatalysts were calculated by the reported equation: ɑhʋ = A ( hʋ – Eg )n/2, where ɑ, h, ʋ, A, and Eg are absorption coefficient, Planck constant, light frequency, proportionality and band gap energy, respectively;56 The values of n for anatase TiO2 and orthorhombic WO3 are 4. The band gap energies of TiO2 and WO3 are about 3.21 and 2.58 eV, respectively (Figure 2b), which is in line with the reported data.45, 54 Valence-band XPS (VB-XPS) was further adopted to analyze their VB potentials (Figure S1). The VB potentials of WO3 and TiO2 are 3.36 and 2.87 eV, respectively. Therefore, The CB potentials of WO3 and TiO2 are 0.78 and -0.34 eV, respectively. The above experimental results are close to the calculated results (the CB/VB potentials of WO3 and TiO2 are calculated to be 0.81/3.39 and -0.305/2.905, respectively. The detail was in the supporting information). Moreover, the results are in good agreement with the previous reports.45, 54

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Figure 2. a. UV-vis absorption of WO3, TiO2 and WO3/TiO2 composite photocatalysts. b. Band gap energies (Eg) of pure TiO2 and WO3. In order to observe the morphology and interface state between TiO2 and WO3, the TEM and HRTEM images of 30% WO3/TiO2 and 90% WO3/TiO2 are shown in Figure 3. As comparison, the TEM and HRTEM images of the pure TiO2 and WO3 are also presented. It is clear that the pure TiO2 appearance is composed of regular nanoparticles with a mean size of about 60 nm,57 and WO3 appearance is composed of irregular agglomerate grains with a particle size of ~100 nm (Figure 3a and 3c). From Figure 3b and 3d, it can be seen that TiO2 exhibits an interlayer distance of 0.24 nm, corresponding to the (004) crystal face of TiO2, and WO3 displays a lattice distance of 0.31 nm, originating from the (111) crystal face of WO3. The HRTEM image of 30% WO3/TiO2 is shown in Figure 3f. It is clear that there exist two set of fringes with the interlayer distances of 0.24 and 0.31 nm, corresponding to the (004) plane of TiO2 and the (111) plane of WO3, respectively. Two different lattice fringes are also distinctive displayed on 90% WO3/TiO2 (Figure 3i). Meanwhile, a distinguished amorphous region (the orange dotted area) between WO3 and TiO2 nanoparticles can be observed in both Figure 3f and 3l. These interfacial transition zones reveal that the nanojunction is indeed formed in WO3/TiO2 heterojunction. Thus, it is indicated that the 8


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heterojunction structure is produced between WO3 and TiO2 nanoparticles. The distinct interface may be more favourable for the separation and transfer of photogenerated carriers. The EDS (energy-dispersive X-ray spectrometry) mapping of the 30% WO3/TiO2 and 90% WO3/TiO2 are presented in Figure 3g-j and 3m-p. It is clear that the WO3/TiO2 composite photocatalyst is composed of Ti, W, and O elements. Moreover, the distribution of the constituted elements (Ti, W, and O) in the WO3/TiO2 composite photocatalyst is well-proportioned. These results are in accordance with the results of SEM and SEM-EDS (Figure S2).

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Figure 3. TEM and HRTEM images of (a, b) TiO2, (c, d) WO3, (e, f) 30% WO3/TiO2 10


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and (k, l) 90% WO3/TiO2 samples. (g - j) EDS mapping of 30% WO3/TiO2 sample. (m - p) EDS mapping of 95% WO3/TiO2 sample. XPS analysis has demonstrated the surface chemical composition and element valence states of TiO2, 30% WO3/TiO2, 90% WO3/TiO2 and WO3 (Figure S3). It is clear that the valence states and chemical compositions shown in the XPS data are in accordance with those in the as-prepared samples. The result is in line with the previous reports.58-61 Moreover, The BET (Brunauer-Emmett-Teller) surface areas of TiO2, 10% WO3/TiO2, 30% WO3/TiO2, 90% WO3/TiO2, 95% WO3/TiO2 and WO3 samples are about 10.57, 8.05, 6.97, 5.75, 5.34 and 5.09 m2 g-1, respectively (Figure S4). Meanwhile, mesoporous structure demonstrated by type IV isotherm with a typical H3 hysteresis loop characteristic is resulted from the aggregation of nanoparticles.62 It is clear that the BET surface area can not be the primary reason accounting for the photocatalytic performance of the samples. 3.2. Photocatalytic activities The activities of the as-prepared photocatalysts were studied by photocatalytic degradation of RhB, MO and BPA. The self-degradation activities of RhB, MO and BPA without the as-prepared photocatalysts under UV-light irradiation for ten minutes could be negligible. Moreover, the dark absorption of the as-prepared photocatalysts could also be negligible. It is clear from Figure 4a and Figure 4b that the photocatalytic degradation efficiencies of TiO2 for MO and RhB are 26 and 25%, respectively, and the photocatalytic degradation efficiencies of pure WO3 for MO and RhB are 5.5% and 6%, respectively. When the main part of WO3/TiO2 composites is TiO2 (WO3/TiO2), the photocatalytic degradation efficiency increases with the increase in the amount of WO3. When the loaded amount is 30%, the 30% WO3/TiO2 composite shows the highest photocatalytic degradation efficiency, and the degradation efficiencies for MO and RhB are 37 and 40%, respectively. However, the photocatalytic degradation efficiency of the WO3/TiO2 composite is decreased gradually, when the loaded amount of WO3 is higher than 30%. Furthermore, when WO3 is the main part of WO3/TiO2 heterojunctions, compared with the pure WO3, the 11



photocatalytic activity of the WO3/TiO2 photocatalysts also increases greatly with the the increase of the TiO2. When the amount of TiO2 is 70% (30% WO3/TiO2), the photocatalyst exhibits the best photocatalytic activity. In order to further prove the result of the photocatalytic performance, the BPA degradation was carried out (Figure 4c). The result is in accordance with the MO and RhB. The photocatalytic degradation efficiency of 30% WO3/TiO2 sample for BPA is about 37%. The apparent quantum efficiencies of 30% WO3/TiO2 composite for RhB, MO and BPA degradation are estimated to be about 0.406%, 0.308% and 0.591%, respectively. Generally, the process of photocatalytic degradation of organic pollutant follows first order kinetics, the kinetic constants (kapp) can be calculated according to first order kinetics equation.63 The kapp values of WO3/TiO2 photocatalysts change prominently and the maximal kapp values are obtained over 30% WO3/TiO2 composite for RhB (0.034 min-1, SD = 1.2×10-3 min-1), MO (0.023 min-1, SD = 8.1×10-4 min-1) and BPA (0.047 min-1, SD = 6.5×10-4 min-1) degradation under UV-light illumination (Figure S5). From the above results, it can be found that the amount of TiO2 and WO3 both play an important role in the photocatalytic reactivity of the WO3/TiO2 composites. No matter whether the main part of WO3/TiO2 heterojunctions is TiO2 or WO3, the photocatalytic activities of WO3/TiO2 are much higher than that of pure TiO2 or WO3. This phenomenon may be caused by the same mechanisms of the photogenerated charger carriers for WO3/TiO2 composites with different components. The stability of the photocatalyst was further confirmed by the XRD and TEM results of the used sample (measured after photocatalytic degradation of BPA). It can be found in Figure S6 that the XRD pattern of the used WO3/TiO2 composite is identical to that of the fresh one. Additionally, compared to the fresh sample, the morphology and microstructure of the used sample exhibit no obvious change. Therefore, it is clear that the photocatalyst has a good stability.

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Figure 4. The degradation efficiencies of different photocatalysts for (a) MO, (b) RhB and (c) BPA under UV-light irradiation for 20 min, 15 min, 10 min, respectively. 3.3. Photocatalytic mechanism 3.3.1. Detection of the reactive species (•O2- and •OH) Based on the above analysis, the CB and VB potentials of TiO2 are about -0.34 eV and 2.87 eV, respectively. The CB and VB potentials of WO3 are about 0.78 eV and 3.36 eV, respectively. The standard redox potentials of Eɵ( O2/•O2-) and Eɵ(OH-/•OH) are about -0.05 eV and 2.4 eV (vs NHE), respectively. Therefore, the photogenerated holes (h+) in the VB of TiO2 and WO3 both can oxidize OH- to give hydroxyl radical (•OH). The photogenerated electrons (e-) in the CB of TiO2 can reduce O2 to produce superoxide radical (•O2-),64 while the photogenerated electrons in the CB of WO3 hard to reduce O2 to give •O2-. EPR experiments are performed to prove the existence of the •OH and •O2- active species (Figure 5). It can be seen from Figure 5a that the six characteristic peaks of the DMPO-•O2- adducts are presented obviously over pure TiO2 and WO3/TiO2 composites under UV- light irradiation. However, there are no formation of •O2- active species for WO3 and blank condition. It indicates that •O2active species are formed on TiO2 and WO3/TiO2 heterojunctions under light irradiation. At the same time, the peaks intensity of the DMPO-•O2- can be observed 14


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in the following order: 30% WO3/TiO2 > 10% WO3/TiO2 > TiO2. It can be seen that the peaks intensity of the DMPO-•O2- is increased with increasing the amount of WO3, when the primary part of WO3/TiO2 composites is TiO2 (WO3/TiO2). Moreover, the peaks intensity of the DMPO-•O2- is also increased with increasing the amount of TiO2, when WO3 is the main part of the composites (TiO2/WO3). The DMPO-•O2peaks of 90% WO3/TiO2 are higher than that of 95% WO3/TiO2, indicating that the amount of •O2- generated for 90% WO3/TiO2 is higher than that of 95% WO3/TiO2. The results are in line with the photocatalytic performance of the samples. Based on the results, it is demonstrated that the photogenerated e - in the CB of TiO2 are not transferred into the CB of WO3. If not, the signals of the DMPO-•O2- can not be detected because the potential of the photogenerated e- in the CB of WO3 is more positive than Eɵ( O2/•O2-). It can be found from Figure 5b that four characteristic peaks of DMPO-•OH are presented obviously over pure TiO2, WO3/TiO2 composites and pure WO3. There are no signals of DMPO-•OH adducts in blank control group. It is demonstrated that •OH is generated on the samples surface after illumination. Additionally, it is clear that the peak intensities of the DMPO-•OH for 15% WO3/TiO2 and 30% WO3/TiO2 composites are higher than that of pure TiO2. It is concluded that when the primary part of WO3/TiO2 composites is TiO2 (WO3/TiO2), the WO3/TiO2 samples are more favorable for the formation of •OH compared with pure TiO2. When WO3 is the main part of the heterojunction photocatalysts (TiO2/WO3), the intensity of the DMPO-•OH signal increases with increasing the amount of TiO2. The characteristic peaks of the DMPO-•OH adduct of WO3/TiO2 samples are stronger than the pure WO3. It is clear that the DMPO-•OH peak intensity of 90% WO3/TiO2 is greater than that of 95% WO3/TiO2 sample, and the intensity of 95% WO3/TiO2 sample is stronger than that of WO3. The result is in accordance with the result of photocatalytic performance. According to the results of EPR, it is suggested that the WO3/TiO2 samples are more favorable for the formation of •O2- and •OH than the pure TiO2 and WO3. The reason may be attributed to the fact that the transfer mechanisms of the photogenerated e - and 15



h+ adopt a Z-scheme in the WO3/TiO2 photocatalysts. That is, the photogenerated h+ in the VB of TiO2 and photogenerated e- in the CB of WO3 are recombined.

Figure 5. EPR signals of (a) DMPO-•O2- adduct and (b) DMPO-•OH adduct over different samples under UV-light irradiation for 60 s in methanol and aqueous dispersion, respectively. In order to further confirm the generation of •O2- and •OH on pure TiO2, WO3/TiO2 composites and pure WO3, NBT (Nitroblue tetrazolium)65 and TA-PL (Terephthalic acid-photoluminescence)45,66 tests were performed for detecting •O2- and •OH (Figure S7 and Figure S8), respectively. It is clear that the results of •O2- and •OH radicals are both in accordance with the results of EPR and photocatalytic activities of the samples. 16


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So no matter whether the main part of WO3/TiO2 heterojunction is TiO2 or WO3, the amount of •O2- and •OH radicals of WO3/TiO2 composites are more than that of the pure WO3 and pure TiO2. 3.3.2. Behavior of photogenerated charge carriers The

behavior

(separation-recombination,

transportation

and

lifetime)

of

photoexcited e- - h+ is the most crucial factor on the activity of a photocatalyst and could be characterized by photoelectrochemical experiment, photoluminescence (PL) emission spectroscopy and time-resolved photoluminescence decay techniques. 67-75 As shown in Figure 6a, when the primary part of WO3/TiO2 heterojunction photocatalyst is TiO2 (WO3/TiO2), the WO3/TiO2 composites show much higher photocurrent response compared with pure TiO2, and the photocurrent response of composites increases with the increase of WO3. When the main part of the WO3/TiO2 composite is WO3 (TiO2/WO3), the photocurrent response of the TiO2/WO3 composites increases with the increase of TiO2 (Figure 6b). It also can be seen that the highest photocurrent density of all the samples is the 30% WO3/TiO2. It indicates that no matter whether the primary part of WO3/TiO2 heterojunction is TiO2 or WO3, the WO3/TiO2 (or TiO2/WO3) photocatalysts demonstrate excellent separation efficiency for the photogenerated charges. It can be found that the photocurrent changes are in line with their photocatalytic activities. Electrochemical impedance spectroscopy (EIS) was used to investigate the migration of charge carriers in the electrode-electrolyte interface.69,70 As shown in Figure 7, just one arc can be found at high frequency in the EIS spectra of all the samples, implying that just surface charge-transfer happened on the as-prepared photocatalysts in the photocatalytic reaction. Compared to TiO2 (Figure 7a), smaller arc radius of WO3/TiO2 composites means that the separation efficiency of the photogenerated e- - h+ and the charge transfer over WO3/TiO2 composites are superior to those over the pure TiO2. Consequently, when the primary part of the WO3/TiO2 composites is TiO2 (WO3/TiO2), the photoelectrochemical results are in agreement with that of photocatalytic activity test. However, as shown in Figure 7b, an 17



interesting result was found. When WO3 is the main part of the WO3-TiO2 composites (TiO2/WO3), the arc radius of pure WO3 is the smallest among these composites (TiO2/WO3). The arc radius of the TiO2/WO3 composites gradually increases with the increase of TiO2, and the arc radiuses of TiO2/WO3 composites are bigger than the pure WO3. Superficially, the result indicates the WO3 owns a more effective separation of photogenerated e- - h+ pairs than the TiO2/WO3 composites.

Figure 6. (a and b) Photocurrent response of the pure TiO2, WO3-TiO2 composites and pure WO3 without bias versus Ag/AgCl electrode (the init potential is 0 V) under 18


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UV-light irradiation.

Figure 7. EIS Nyquist plots of the pure TiO2, WO3-TiO2 composites and pure WO3.

To further evaluate the separation capability, trapping and migration rate of the photogenerated charge carriers for the as-prepared samples, PL emission spectroscopy was carried out (Figure S9). Generally, a higher PL intensity means a higher recombination rate of the photogenerated charge carriers.23,45,72 The results are consistent with the fascinating results of the EIS. Specifically, when TiO2 is the main part of the WO3/TiO2 composite, the recombination of the photoinduced charge 19



carriers in the WO3/TiO2 heterojunctions is suppressed (Figure S9a), which would accelerate the interfacial charge transfer and increase their lifetime, so the photocatalytic activity could be improved significantly. However, when WO3 is the main part of the WO3/TiO2 composite (TiO2/WO3), the PL intensity of the TiO2/WO3 composite is much higher than that of pure WO3. In addition, the PL intensity increases with the increase of TiO2, indicating that the WO3 owns a more effective separation of photogenerated electron-hole pairs than the TiO2/WO3 heterojunctions. Based on the experimental results of EIS and PL, we think the reason for the interesting results of EIS and PL may be attributed to the Z-scheme transfer mechanisms of the photogenerated e- and h+. It is proposed that when TiO2 is the primary part of the WO3/TiO2 composites, the arc radiuses of the WO3/TiO2 composites reduce with the increase of WO3, suggesting that the composites exhibite excellent transfer and separation efficiency of the photogenerated e- and h+. Namely, the separation superiority for the photogenerated e- in the CB of TiO2 and the photogenerated h+ in the VB of WO3 is greater than the combination trend for photogenerated e- in the CB of WO3 and photogenerated h+ in the VB of TiO2, resulting in long lifetime of photogenerated e - and h+. Therefore, the arc radiuses and solid fluorescence intensity of the WO3/TiO2 composite show a decrease trend with the increase of WO3. However, when the main part of the WO3-TiO2 composites is WO3, the arc radiuses of the TiO2/WO3 samples graudally increase with the increase of TiO2, indicading that the separation and transfer of photogenerated e - - h+ is poor. That is, the combination advantage of the photogenerated e- in the CB of WO3 and the photogenerated h+ in the VB of TiO2 is greater than separation superiority of the photogenerated e- in the CB of TiO2 and photogenerated h+ in the the VB of WO3, resulting in a relatively shorter lifetime of photogenerated e - and h+. Therefore, the arc radiuses and solid fluorescence intensity of composite TiO2/WO3 exhibit an increase trend. In order to prove the above assumptions, the time-resolved photoluminescence decay techniques were used to investigate the separation efficiency of photogenerated 20


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carriers.73-75 As shown in Figure S10, the calculated lifetime of the 30% WO3/TiO2 heterojunction is 18.56 ns (SD = 0.41 ns), which is longer than that of TiO2 (16.22 ns, SD = 0.65 ns). That is, the lifetime of photogenerated electrons and holes for 30% WO3/TiO2 heterojunction is higher than that of the pure TiO2. The calculated lifetime for the 90% WO3/TiO2 heterojunction (12.32 ns, SD = 0.37 ns) is lower than the monomer WO3 (17.36 ns, SD = 0.68 ns). Namely, the lifetime of photogenerated charge carriers for the 90% WO3/TiO2 is shorter than that of pure WO3. In other words, when the primary part of WO3/TiO2 composite photocatalyst is TiO2 (WO3/TiO2), the photoexcited charge carriers of the samples exhibite a longer lifetime. However, when WO3 is the main part of the heterojuction photocatalysts (TiO2/WO3), the photogenerated charge carriers show a relatively shorter lifetime. The result proves the above assumption. 3.3.3. Conductivity of the heterojunctions Theoretically, the build-in internal electric field would also be generated on the interface of two same type semiconductors (n-n and p-p) due to the differences in the number of carriers between two semiconductors, which is similar to p-n junction.76,77 The carriers will diffuse from the semiconductor with large amount of carriers into the other semiconductor with small amount of carriers. Thus, the build-in internal electric field is formed on the interface of two same type semiconductors. To better illustrate the point, the carrier density (ND) over the samples has been determined from Motto-Schottky (M-S) curves according to the following equation: N D = (2/eεε0) [dUFL/d(1/C2)],78,79 where e = 1.6 × 10-19 C, ε0 = 8.86 × 10-12 F m-1,25 ε is dielectric constant, and C is the capacitance. Dielectric constant of anatase TiO2 and WO3 are about 48 and 1000, respectively.78-80 The linear slopes of the M-S plots of the TiO2 and WO3 are positive (Figure 8 and Figure S11), suggesting that both TiO2 and WO3 belong to n-type semiconductors.81-83 The slopes of TiO2 and WO3 are 6.39E9 and 3.34E9, respectively. Therefore, the ND values of TiO2 and WO3 are about 4.6 × 1018 and 4.2 × 1017, respectively. The majority carriers (electrons) in TiO2 will diffuse to WO3, and the build-in internal electric field with direction of TiO2 → WO3 will be 21



formed like n → p in p-n junction. In the heterjunction of WO3/TiO2, WO3 and TiO2 are analogous to p-type semiconductor and n-type semiconductor in p-n junction, respectively. Therefore, we suggest that this homo-type heterojunction may be called as a relative p-n junction. The WO3 is predominant in the form of p-type, and TiO2 is partial to n-type. The slopes of the WO3/TiO2 samples are all between TiO2 and WO3 (Figure 8), indicating that the conductivities of WO3 and TiO2 samples in the heterojunction photocatalysts do not change.

Figure 8. The linear slopes of the M-S plots for the samples.

3.3.4. Proposed mechanisms of the WO3/TiO2 heterojunctions Based on the above results and discussion, the transfer mechanisms of photogenerated e- and h+ over the WO3/TiO2 heterojunction photocatalysts was proposed and represented in Figure 9. As shown in Figure 9a, the CB of TiO2 (-0.34 eV) is more negative than that of the WO3 (0.78 eV) and the VB of WO3 (3.36 eV) is more positive than that of the TiO2 (2.87 eV). In theory,7 the photogenerated e- in the CB of TiO2 will migrate into the CB of WO3, and the photogenerated h+ in the VB of WO3 will migrate into the VB of TiO2 (Figure 9b). Under the circumstances, the accumulated e- in the CB of WO3 can not reduce O2 to •O2-, because the potential of the e- in the CB of WO3 is more positive than standard redox potential of Eɵ( O2/•O2-) 22


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(-0.05 eV vs. NHE). Thus, if the charge carriers transfer in accordance with this traditional band-band transfer, it is not favorable for the production of the •O2-. However, according to the above experimental results, it is known that the WO3/TiO2 heterojunctions are more favorable for the generation of •O2- radicals. Therefore, the transfer of photogenerated charge carriers for the WO3/TiO2 heterojunctions should not adopt band-band mechanism, but Z-scheme mechanism. The reason is that when WO3 and TiO2 are combined, a homo-type heterojunction (n-n) is formed. Under the effect of the internal electric field produced between WO3 and TiO2 (TiO2 → WO3), the photogenerated e- will migrate to the TiO2 and the photogenerated h+ will move to the WO3. Therefore, the migration directions of the photogenerated e- and h+ in the CB and VB of WO3 and TiO2 are opposite to the transfer directions of e- and h+ in the homo-type heterojunction. As shown in Figure 9c, the fast combination is achieved between the photogenerated e- in the CB of WO3 and photogenerated h+ in the VB of TiO2 (Conversely, if the migration directions of the photogenerated charge carriers in the CB and VB of the two contacted semiconductors are in line with the direction of built-in electric field in this formed heterojunction, we think the band-band transfer of the photogenerated charge carriers would be occurred.). Meanwhile, the accumulated photogenerated e- in the CB of TiO2 with more negative potential can reduce O2 to •O2-. And the photogenerated h+ in the VB of WO3 with more positive potential may oxidate H2O (OH-) into •OH radicals. The photocatalytic activities of the WO3/TiO2 heterojunctions are significantly promoted. Namely, no matter whether the primary part of WO3/TiO2 composites is TiO2 or WO3, the transfer mechanisms of photogenerated e- and h+ for the WO3/TiO2 heterojunction photocatalysts all adopt a type of Z-scheme under our experimental conditions.

23



Figure 9. a. Scheme illustrating band positions of WO3 and TiO2 together with OH-/•OH and O2/•O2- potentials. b. Illustration of conventional WO3/TiO2 heterojunction, which do not form. c. Schematic of the charge carriers transfer and separation in a Z-scheme system of WO3/TiO2 heterojunctions.

4. Conclusions The heterojunction photocatalysts WO3/TiO2 were fabricated via in situ calcination methods. No matter whether the primary part of WO3/TiO2 composites is TiO2 or WO3, the photocatalytic activities of the WO3/TiO2 are much higher than that of pure TiO2 or WO3. When WO3 and TiO2 are combined, the built-in electric field will be produced due to the difference of conductivity (carrier density) for WO3 and TiO2. Because the migration directions of the photogenerated charge carriers in the CB and VB of WO3 and TiO2 are opposite to the direction of the built-in electric field in the formed heterojunction, the transfer mechanisms of photogenerated charge carriers for the WO3/TiO2 heterojunction photocatalysts only adopts a Z-scheme system. In this photocatalytic system, the photogenerated electrons in the CB of WO3 and holes in the VB of TiO2 are quickly combined; the accumulated electrons in the CB of TiO2 display high reducibility to reduce O2 to •O2-, and the holes in the VB of WO3 exhibit high oxidizability to oxidize H2O (or OH-) to •OH. The built-in electric field is the inner impetus for the Z-scheme migration in the heterojunctions photocatalysts.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI ： XXXXXX. Experimental details (Materials, Characterization, Photoreaction apparatus and procedure); VB-XPS, SEM, XPS and Nitrogen adsorption-desorption isotherms; the kapp values of different photocatalysts for MO, RhB and BPA degradation; XRD patterns of fresh and used 30% WO 3/TiO2 and TEM 24


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image of the used 30% WO3/TiO2; •O2- and •OH detected by NBT and TA-PL methods; PL spectra, carrier lifetime and Mott-Schottky plots of the samples. AUTHOR INFORMATION

Corresponding Authors *E-mail: [email protected], [email protected]. Notes The authors declare no competing financial interest.

Acknowledgement This work was supported by the Natural Science Foundation of China (NSFC, grant Nos. 51472005 and 51772118), and the Natural Science Foundation of Anhui Province (grant No. 1608085QB37), and the Natural Science Foundation of Educational Committee of Anhui Province (grant No. KJ2018A0387).

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Insight into the Transfer Mechanism of ... - ACS Publications

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