Homophase Junction for Promoting Spatial Charge Separation in

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Homophase Junction for Promoting Spatial Charge Separation in Photocatalytic Water Splitting Yu Bai, Yueer Zhou, Jing Zhang, Xuebing Chen, Yonghui Zhang, Jifa Liu, Jian Wang, Fangfang Wang, Changdong Chen, Chun Li, Rengui Li, and Can Li ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b05050 • Publication Date (Web): 04 Mar 2019 Downloaded from http://pubs.acs.org on March 4, 2019

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Homophase Junction for Promoting Spatial Charge Separation in Photocatalytic Water Splitting Yu Bai,[a]§ Yueer Zhou,[a]§ Jing Zhang,*[a] Xuebing Chen,[a] Yonghui Zhang,[a] Jifa Liu,[a] Jian Wang,[a] Fangfang Wang,[a] Changdong Chen,[a] Chun Li,[a] Rengui Li,*[b] and Can Li[b] [a]

School of Chemistry and Materials Science, Liaoning Shihua University, No.1 West Dandong

Road, Wanghua District, Fushun 113001,China [b]

State Key Laboratory of Catalysis, Dalian National Laboratory for Clean Energy, and The

Collaborative Innovation Center of Chemistry for Energy Materials (iChEM), Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Zhongshan Road 457, Dalian 116023, China

§These

authors contributed equally to this work

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Abstract: The widespread heterojunction or p-n junction strategies fabricated between different semiconductors are generally used to promote the spatial charge separation in photocatalysis and solar cells, which is originated from the principle that the junction composites possessing totally different crystalline and energy structures. A vagarious and supreme challenge remained is if a junction could be formed between the identical composites with the same semiconductors and the crystalline phases. Herein, taking model semiconductor TiO2 as prototype and proof-of-concept, a homophase junction was fabricated between the same crystalline phases of TiO2 with large and small nanoparticles. Photocatalytic H2 evolution and water splitting performances on three common TiO2 phases, brookite, anatase and rutile can be remarkably enhanced using such homophase junction strategy. The high photocatalytic activities are proposed to be attributed to the different surface band bending inducing the formation of built-in electric field at the interface of large and small particles, which facilitating the spatial charge separation and inhibiting the charge recombination. Our work provides a strategy for spatial charge separation in constructing highly efficient solar energy conversion systems, which is differentiated from the traditional junction strategies. Keywords: Spatial charge separation, Homophase junction, Photocatalysis, brookite TiO2, anatase TiO2, rutile TiO2


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INTRODUCTION The sunlight-driven photocatalytic artificial photosynthesis for hydrogen and chemicals production holds great promises in the field of solar energy conversion and utilization.1-9 However, the efficiencies of photocatalysts achieved so far are still low, mainly due to the low charge separation efficiency and the fast recombination of the charge carriers.10-15 Introducing built-in electric fields in the bulk or at the surface of photocatalysts by fabricating p-n junctions,16-20 heterojunctions,21-29 especially heterophase junctions30 have been becoming useful strategies to promote spatial charge separation in photocatalysis. The p-n heterojunction is a junction between two semiconductors, one doped with a donor (n-type) and one with an acceptor (p-type).31 In p-n heterojunction, the large Fermi-level difference in two semiconductors provides a very strong internal electrical field for efficient charge transfer and separation, increasing the quantum efficiency of the photocatalytic reactions. For example, Gong et al.32 described the synergetic enhancement of bulk charge separation and surface reaction kinetics by introducing discrete nanoisland p-type Co3O4 cocatalysts onto n-type BiVO4, forming a p-n Co3O4/BiVO4 heterojunction with an internal electric field to facilitate charge transport. Fabricating the type II heterojunctions is a common and effective strategy to improve the separation efficiency between photogenerated electron and hole. 31, 33-35 A built-in electrical field is formed due to the band bending at the interface of two semiconductors and could similarly enhance the separation of photogenerated electron-hole pairs. Thus, engineering the junction between semiconductors is essential for improving charge separation and transfer efficiency, and then photocatalytic activity. The heterophase junction (e.g., anatase/rutile TiO2 junction) is fabricated between two different polymorphs of a semiconductor with exactly the same composition, in which a built-in electrical field is formed due to the band bending at the junction layers, inducing photoexcited electrons and holes to migrate to the opposite sides, respectively.36,

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Compared with heterojunction, heterophase junction is more

advantages in photogenerated charges transfer between interface. Moreover, considering the structure or lattice matching, the preparation of heterophase junctions


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is more simple and controllable. Thus, the intensive research toward design of heterophase junction has been performed. Numerous heterophase junction photocatalysts,38-49 such as hexognal/cubic-CdS,38 /γ-Bi2O3,39 /-Ga2O3,40 2H/1TMoS2,41 hexognal/monoclinic-WO3,42 /-Bi4V2O11,43 BiOI/Bi5O7I,44 Sr2Ta2O7xNx/SrTaO2N,

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cubic/orthorhombic-NaNbO3,46 Ag2CO3/Ag2O,47 and so on have been

developed. For example, Yu et al.47 constructed Ag2CO3/Ag2O heterophase junction by phase transition and thus the silver based photocatalyst with high stability and high photocatalytic efficiency was obtained. The current research focus on the regulation of the type of the crystalline phase,48 ratio of two crystalline phases,49 morphology, and phase sequence50 in heterophase junction photocatalyst. Nevertheless, the heterophase junction can be roughly recognized as a specific kind of heterojunction formed between different semiconductors. A vagarious and supreme challenging question then comes out: is it possible for the spatial charge separation took place between the identical composites with the same semiconductors and the same crystalline phases, which determines the ultimate limit of the driven force for inducing the spatial charge separation in photocatalysis. However, there is still no any report focusing on this issue so far. Since the energy structures of semiconductors are significantly correlated with the particle size of a semiconductor material according to the quantum confinement effect,48,

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therefore, the semiconductors with varied particle sizes may possess

different band alignments. It suggests a possibility to rational construct a built-in electric field at the interface of the identical composites, e.g., the same semiconductors, the same crystalline phases and structures, but different particle sizes. It is meaningful to investigate whether such very slight difference in energy levels among different particle sizes can induce the spatial charge separation or not, which will be instructive to estimate the minimum energy differences that can be applied in constructing a builtin electric field for efficient charge separation in photocatalysis. Herein, we chose model semiconductor titanium dioxide (brookite, anatase and rutile) as prototype and proof-of-concept, and a homophase junction fabricated between the same crystalline phases of titanium dioxides was reported, which is differentiated from


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the traditional p-n junction or heterojunction strategies. The constructed homophase junction resulted in a dramatically enhanced activity in photocatalytic water splitting, and even more efficient than heterojunction strategy. We also demonstrated that the homophase junction is general and validated for spatial charge separation in all three common phases, brookite, anatase and rutile of TiO2 photocatalysts.

RESULTS AND DISCUSSION To fabricate the homophase junction, controllable TiO2 nanoparticles were decorated onto the surface of big TiO2 particles with the same crystalline phase as TiO2 nanoparticles in this study. Take brookite TiO2 as an example, brookite TiO2 (denoted as Be) was introduced in the experiment, and brookite nanoparticles were fabricated on large Be brookite surface (denoted as B/Be) via a hydrothermal process. From Figure 1a, all the identical XRD peaks of Be (25.3o, 25.6o, 30.8o, 48.0o, and 55.2o) corresponding to brookite phase. For B/Be, only the characteristic peaks of brookite and no other crystalline phases or impurities were presented, indicating that the as-prepared B/Be samples possess sole brookite phase. However, it should be noted that the hydrothermal treatment without adding Be only leads to the formation of anatase TiO2 (denoted as A) (Figure 1a). UV Raman spectroscopy (the laser line at 325 nm was selected as the excitation source), which is sensitive to differentiate the phase component of TiO2 surface,37, 52 was introduced to investigate the phase structure of B/Be samples (Figure 1b). In the UV Raman spectrum of Be sample, only typical brookite Raman bands (121, 150, 244, 284, 321, 363, 402, 487, 542, and 630 cm-1) were observed. For all the B/Be samples, all the Raman shifts are in good consistent with the brookite phase and no other species are formed even when the amount of TiO2 precursor is increased to more than 10 % in weight (Figure 1b). Both XRD and UV Raman results evidently verified that the newly formed species on the surface of Be brookite is also in brookite phase.


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TEM and HRTEM images of 5% B/Be revealed that the spacing of the lattice fringes of the large ellipsoidal particles are 0.290 and 0.351 nm corresponding to (121) and (120) planes of brookite, while the decorated nanoparticles with the lattice spacing of 0.237 and 0.241 nm are also ascribed to brookite TiO2 (Figure 1g and 1h). The results clearly verified that nanoparticles decorated on the surface of Be are also in brookite phase, possibly originated from the crystallographic-oriented epitaxial nucleation and growth on the surface of Be. Interestingly, the contact between Be and newly formed brookite nanoparticles are compact and smooth enough, exhibiting an atomic junction that formed at the interface between Be and brookite nanoparticles. As Be and the newly formed TiO2 are both in brookite phase, we called such new kind of junction as “homophase junction”. It is also found that the presence of Be play an significant role in the crystallization behavior for fabricating the homophase junction. The formation of “homophase junction” can also be found in the 3% B/Be sample (Figure S1). When pure anatase or rutile particles were added instead of Be, only anatase nanoparticles were produced on the surface of anatase or rutile TiO2 (Figure S2 and S3). Figure 2a shows the photocatalytic performance of B/Be photocatalysts possessing homophase junction in terms of hydrogen production via water splitting in the presence of methanol as sacrificial agent. It was interestingly found that only decorating a very small amount of brookite nanoparticles (less than 1%) on Be can result in an great improvement in the H2 evolution as compared with bare Be. With increasing the amount of brookite nanoparticles, more junction sites were formed between B and Be, inducing an improvement in the photocatalytic performance. The rate of H2 evolution reaches a maximum for 1% B/Be photocatalyst. However, an obvious decrease in the photocatalytic activity was observed when brookite nanoparticles was more than 5%, which could be attributed to an excess covering of B on the surface of Be. This suggests that the exposure ratio of B and Be influences the photogenerated charge separation and the following surface reactions. This phenomenon can also be seen in other junction systems, regardless of homojunction and heterojunction.53-56 After normalized the photocatalytic activity by surface area, which were calculated by dividing H2 evolution by surface area of


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corresponding B/Be sample, the activities of 1% B/Be photocatalysts are still about more than 3-fold of bare Be, clearly revealing the fact that homophase junction is indeed beneficial for the enhanced photocatalytic hydrogen production. The positive effect of homophase junction was also verified by performing photocatalytic hydrogen evolution from overall water splitting without sacrificial agent. As shown in Figure 2b for overall water splitting, a similar trend as Figure 2a can be evidently observed, although the optimized ratio of B/Be is not the same, which is possibly due to the different kinetics for hole-involving reactions e.g., water oxidation vs. methanol oxidation. Notably that many researchers have reported that brookite TiO2 can only generate H2 but no O2 during photocatalytic overall water splitting, which was further confirmed in our experiment.57 It should be pointed out that brookite is the least studied TiO2 photocatalyst due to its low photocatalytic activity58 while anatase TiO2 is generally recognized as the most active phase. However, we found that the brookite TiO2 with homophase junction exhibits a superior performance for photocatalytic hydrogen evolution in our experiment. To highlight the advantages of homophase junction, we also fabricated an anatase/brookite (A/Be) sample with traditional heterophase for comparison. It was found that A/Be sample exhibits the similar morphology with B/Be, among which uniform anatase nanoparticles with comparable sizes with brookite nanoparticles in B/Be (~10-15 nm) were deposited on the surface of Be (Figure S4). Although the A/Be displays a 2-fold phtocatalytic activity than bare Be, it is still much lower than B/Be with homophase junction (Figure 2c). The result implies that the TiO2 photocatalyst possessing brookite/brookite homophase junction is more active than that of anatase/brookite heterophase junction.


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Figure 2. (a) Photocatalytic H2 evolution from a 10 vol% methanol-water solution over B/Be homophase junction photocatalysts (0.5% platinum was photodeposited on the B/Be sample). Specific H2 evolution of B/Be samples were calculated by dividing H2 evolution by surface area of corresponding B/Be sample, (b) Photocatalytic H2 evolution via overall water splitting on different B/Be homophase junction photocatalysts (0.5% platinum was photodeposited on the B/Be sample), (c) the comparison of photocatalytic H2 evolution from a 10 vol% methanol-water solution between 1% A/Be and 1% B/Be (0.5% platinum was photodeposited on the A/Be or B/Be sample), (d) Surface photovoltage spectra of Be, B/Be and 1% A/Be samples, (e) Stability of photocatalytic H2 evolution via overall water splitting on 3% B/Be (0.5% platinum was photodeposited on the 3% B/Be sample). Notably, A/Be and B/Be samples have comparable specific surface area (28.0 and 27.2 m2/g) and particle sizes. Thus, the enhancement in photocatalytic activity is impossible due to the exposed surface of nanoparticles. As compared with A/Be, B/Be


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homophase junctions exhibits an atomic junction that formed at the interface between Be and brookite nanoparticles because B and Be are in the same crystalline phase, that is, B/Be have an advantage in charge separation and transfer than A/Be although particle size for the homojunction and heterojunction are almost the same. These results evidently manifest the roles that the well-constructed homophase junction must play an essential role in contributing the superior photocatalytic performances, making it even more efficient than traditional heterophase junctions or heterojunctions in some cases. To better understand the role of homophase junction for spatial charge separation, surface photovoltage (SPV) measurements were then conducted (Figure 2d). The signal of SPV is attributed to the change in surface potential barriers before and after light illumination.59-61 The intensity of SPV for bare Be is relative low. However, the SPV intensity was remarkably increased when only 0.5% of brookite nanoparticles were deposited. Furthermore, the SPV intensity can be even enhanced and reached a maximum when the amount of brookite nanoparticles is 1%. A decline of SPV intensity was observed when further increasing the amount of brookite nanoparticles. These SPV intensities almost follow the same trace as the photocatalytic activities shown in Figure 2a and 2b. Besides, A/Be heterophase junction exhibits a much lower SPV intensity than B/Be, agreeing with the difference in activities. The stronger SPV intensity means more photoexcited charge carriers are separated and transferred to the surface.62, 63 These results imply that the existence of homophase junction can indeed induce an efficient separation of photoexcited electron-hole pairs, and consequently results in a higher photocatalytic activity. In order to evaluate the stability of homophase junctions during photocatalysis process, the 3% B/Be sample was used as an example to test the H2 production over time in three repeated cycles, and the results were added as Figure 2e. As seen from Figure 2e and the XRD pattern of 3% B/Be before and after photocatalytic reaction (Figure S5), it is noteworthy that B/Be exhibits superior stability after more than 12 hr reaction.


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Figure 3. SEM images of (a) 5% B20/Be and (b) 5% B40/Be. (c), (d) TEM and HRTEM images of 5% B20/Be sample. (e) the photocatalytic H2 evolution from a 10 vol% methanol-water solution on B20/Be homophase junction (0.5% platinum was photodeposited on the B20/Be sample). Different sizes of brookite nanoparticles on the surface of Be were also precisely controlled for optimizing the homophase junction. The particle sizes of brookite were tuned via varying the acidity of solution. From Figure 3a and 3b, the average particle size of brookite nanoparticles on the Be was increased from ~20 to ~40 nm (labelled as B20/Be and B40/Be) with increasing the pH value from 4.2 to 7.8. HRTEM results further confirmed both the small nanoparticles and big particles are in the brookite phase with a d-spacing of 0.290 and 0.351 nm of brookite TiO2 (Figure 3c and 3d). To evaluate the advantage of B20/Be as a homophase junction photocatalyst with respect to Be, their photocatalytic activities for H2 evolution were also investigated, as shown in Figure 3e. As anticipated, B20/Be gives superior activities to Be, with the highest photocatalytic activity occurring for the 3% B20/Be sample, which is 2 times higher than that of pure Be. These results confirm that the homophase junction indeed benefits for the photocatalytic activities. SPV results demonstrated that B20/Be homophase junction is efficient in promoting spatial charge separation in photocatalysis (Figure S6). The positive role of homophase junction in promoting spatial charge separation and photocatalytic activity was also observed in B40/Be sample (Figure 4a and 4b). However, from the comparison of the


ACS Catalysis

activity on B/Be, B20/Be, and B40/Be with same loading amount of B, B20, and B40 nanoparticles (Figure 4a and 4b), it is worth pointing out that the photocatalytic activity of B20/Be was much lower than that of B/Be, and the activity declined further when the brookite particles were increased to about 40 nm. It is therefore believed that the photocatalytic performance of B/Be homophase junction photocatalyst is highly dependent on the particle size of B on the surface of Be. SPV characterization was conducted to evaluate the charge transport property of B/Be samples with different sizes of B, as shown in Figure 4c. It could be observed that the intensity of SPV was progressively increased from B40/Be, B20/Be, to B/Be demonstrating the gradually enhanced separate efficiency of created charges.

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crystalline phase. As expected, both the photocatalytic H2 evolution rate from a 10 vol% methanol-water solution and overall water splitting was obviously boosted after decorating different sizes (~ 5-25 nm) of anatase nanoparticles on the surface of Ae (Figure 5b and 5c), among which A5/Ae exhibits more than 3 (Figure 5b) and 7 times (Figure 5c) H2 evolution rate than Ae. 25

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ACS Catalysis

Figure 5. (a) SEM images of 1% A15/Ae, the photocatalytic H2 evolution from (b) a 10 vol% methanol-water solution and (c) overall water splitting of 1% A/Ae samples with different sizes of A, (d) SEM images of 1% R/Re, (e) Photocatalytic H2 production from a 10 vol% methanol-water solution on R/Re samples with different R loadings, and (f) Photocatalytic H2 and O2 evolution from overall water splitting on R/Re samples with different R loadings. In photocatalytic reaction, 0.5% platinum was photodeposited on the A/Be or R/Re samples.


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The results from electrochemical impedance spectra (Figure S9) clearly indicate that the separation and transfer of photogenerated electrons and holes is faster for A/Ae than Ae. Moreover, A5/Ae has an obvious advantage in charge separation and transfer among the A/Ae samples, which is in accordance with the photocatalytic performance. These observations indicate an accelerated dynamics process of photoinduced charge separation and transfer occurring on the interface between anatase nanoparticles (A5 or A15 or A25) and Ae, which is crucial to the photocatalytic H2 production activity of the homophase junction. Furthermore, as displayed in Figure 5b and 5c, with the increasing particle size of anatase A on the Ae, an obvious decrease in the photocatalytic activity of A/Ae samples was discovered, indicating that the particle size of A on the Ae is one of the key factors in the photocatalytic performance. With varying the particle size of A on the surface of Ae, the built-in electric field at the homophase interface are optimized, which benefits for the improvement of charge separation and the photocatalytic activities (Figure S9). After the in-situ generation of rutile nanoparticles on the surface of Re (45 nm diameter × 120 nm length ， Figure 5d), a significantly improved photocatalytic performance was observed, which is about 37 times higher than bare rutile Re (Figure 5e). It has been demonstrated that rutile can produce both H2 and O2 in photocatalytic overall water splitting while H2 can only be generated on anatase.57, 64 Interestingly, the homophase junction formed between R and Re was confirmed to be highly effective for H2 and O2 evolution from overall water splitting without methanol as sacrificial agent (Figure 5f). Moreover, the stoichiometric ratio of hydrogen and oxygen is about 2:1 on R/Re photocatalysts. The greatly enhancement of photocatalytic activities in R/Re were also verified by surface photovoltage characterizations (Figure S10), further elucidating the beneficial effect of the interface facilitated by the homophase junction between R and Re in improving charge separation and transfer in comparison to Re. It should be noted that brookite homophase (B/Be) shows more superior performance than anatase (A/Ae) and rutile (R/Re) phase, which is possibly due to the different crystallinity of pristine Ae, Re and Be samples as they were synthesized under different conditions.


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Figure 6. (a) A plausible mechanism for B/Be homophase junction. TEM images of 5% B/Be after (b) photo-oxidation deposition of MnOx or (c, d) photo-reduction deposition of Au nanoparticles (inset: HRTEM image of 5% B/Be after photo-reduction deposition of Au nanoparticles). These above results make us to draw a conclusion that the homophase junction can reliably induce the efficient spatial charge separation between large and small particles of brookite, anatase, and rutile TiO2. In order to better understand how the charge separation takes places between different particles of the same semiconductor, a plausible mechanism was then proposed (Figure 6). It is generally recognized that the surface band bending is varied from the bulk to nanoparticle of a semiconductor when changing the particle sizes.65 Mott-Schottky plots (Figure S11a) clearly showed that the prepared Be and 1% B/Be are both n-type semiconductor.66 An upward surface band bending is formed and the direction of the electric field increases the hole availability at the surface thus increasing a hole-induced surface reaction. Both Mott-schottky and valence band XPS results (Figure S11b) demonstrated that the valance band maximum of Be is slightly negative than B while conduction band minimum is more positive, indicating that the built-in electric field formed at


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homophase junction region will promote the photogenerated electrons to migrate from Be to B and photogenerated holes to transfer from B to Be (Figure 6a). Using the Poisson-Boltzmann equation, the potential distribution in a spherical particle with different diameters has been developed by Albery and Bartlett,67, 68 and the surface band bending is determined by the depletion length and the radius of the semiconductor nanoparticle. As shown in Figure 6a, the different surface band bending will induce the formation of the built-in electric field at the interface of large and small particles, which influences the rate of electron/hole transfer and recombination thus influencing the efficiency of the photocatalytic reaction. The built-in electric field formed at homophase junction region will promote the photogenerated electrons transfer from large TiO2 particles to small TiO2 nanoparticles, while the photogenerated holes migrate to the opposite direction. The proposed mechanism can be theoretically considered using the PoissonBoltzmann equation, the potential distribution in a spherical semiconductor particle with radius r0 has been developed by Albery and Bartlett.68 In an n-type semiconductor nanoparticle, the band bending (VBB), that is, potential difference, at distance r with reference to the center of the particle is: VBB(r0) = VBB(r0) =

eNdD2 2εrε0 eNdr2o 6εrε0

(r0 ≫ D)

(1)

(r0 < 3D)

(2)

where Nd is dopant concentration, D is the depletion length, εr and ε0 are the relative dielectric constant of the semiconductor and the vacuum permittivity, respectively, and r0 is the radius of the semiconductor nanoparticle. From Equation 1, VBB(r0)(r0 ≫ D) is approximately equal to a constant value, regardless the radius of the semiconductor particle, when the particle size is relatively large. However, three variables, particle size (r0), dopant concentration (Nd), and relative dielectric constant (εr), determine the magnitude of band bending in small particles. It has been commonly accepted for that the band bending will decrease with the size of the semiconductor particle. For TiO2, when the particle radius equals LD (for


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ACS Catalysis

TiO2, LD is ∼3.8-12 nm with εr ≈100, Nd ≈ 1024-1025 m-3),69, 70 the band bending from the particle center to surface is only ∼0.004 eV at room temperature. When the size of TiO2 increases, the band bending from the particle center to surface is 0.4 eV in bulk.71 Based on Equation 1, VBB(r0)(r0 ≫ D) is approximately equal to a constant value, when the particle size is larger than depletion length. Therefore, the band bending from the Be particle center to surface is approximated to be 0.4 eV, and band bending from the B particle center to surface is approximated to be 0.004 eV. There are two orders of magnitude difference in band bending between B and Be. However, the difference of the surface band bending between the Be and B (Ae and A, Re and R) become small with increasing the particle size of B (or A, or R). As a result, the driving force for spatial charge separation provided by the built-in electric field at homophase junction is consequently weakened. This is the main reason why the efficiency for the charge separation and the photocatalytic activity decrease with increasing the particle size of B in B/Be or A in A/Ae (Figure 4a, 4b, 5b). To verify the charge separation path at the homophase junction region, probing reactions using in-situ photo-reduction deposition of Au and photo-oxidation deposition of MnOx were examined in the experiment. As shown in Figure 6b, 6c and 6d, we can clearly observe that MnOx nanoparticles only appeared on large Be surface, while Au nanoparticles were deposited on small brookite nanoparticles. As the Femi level of Au is much lower than brookite and the Schottly junction at the interface will be formed, resulting in the photogenerated electrons are more preferred to accumulate on the Au clusters but not on brookite nanoparticles. The Au clusters will collect the photogenerated electrons of brookite nanoparticles and their surroundings. This is the reason why Au nanoparticles were only deposited on some of brookite particles. These results evidently demonstrated that the photogenerated electrons and holes are spatially separated to small brookite nanoparticles and large Be surface, respectively. These above results demonstrate the prominent advantage of the new homophase junction strategy to boost photocatalytic activity of photocatalyst.

CONCLUSIONS


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A homophase junction was fabricated between the same crystalline phases using TiO2-based photocatalysts as proof-of-concept in this study. The homophase junction which fabricated between large and small particles of TiO2 can remarkably improve the separation and transportation of photogenerated charges and consequently contributes to the great enhancement in photocatalytic overall water splitting. The enhanced photocatalytic activities are attributed to the different surface band bending inducing the formation of the built-in electric field at the interface of large and small particles, which influences the rate of electron/hole transfer and recombination thus influencing the efficiency of the photoreaction. The homephase junction strategies were also demonstrated to be general and validated in other systems, e.g., anatase and rutile phases. This work presents a strategy for spatial charge separation in photocatalysis and provides a new concept of homophase junction for constructing efficient solar energy conversion systems.

MATERIALS AND METHODS Chemicals Titanium tetrachloride (TiCl4), tetrabutyl titanate (TBOT: Ti[O(CH2)3CH3]4), triethanolamine (TEOA: N(CH2CH2OH)3), nitric acid (HNO3), methanol (CH3OH), chloroplatinic acid (H2PtCl6·6H2O), potassium ferricyanide (K3Fe(CN)6), potassium ferrocyanide (K4Fe(CN)6·3H2O), sodium sulphate (Na2SO4), manganese sulphate (MnSO4), chloroauric acid (HAuCl4·4H2O), sodium iodate (NaIO3), isopropanol ((CH3)2CHOH), and absolute ethyl alcohol (C2H5OH) were purchased from Sinopharm Chemical Reagent Co., Ltd (China). All the above chemicals were of analytical grade and directly used without further purification.

Synthesis of Materials The preparation procedure of ellipsoidal brookite TiO2 (denoted as Be), anatase TiO2 (denoted as Ae), rutile TiO2 (denoted as RS), and rod like rutile TiO2 (denoted as Re) can be seen in reference.52, 72-74


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The detailed procedure for B/Be is as following: a stock solution of Ti4+ was prepared by mixing a certain amount of TBOT with TEOA at a molar ratio of TBOT: TEOA=1:2 under dry air to avoid hydrolyzation of Ti4+ ions at room temperature. Then, the pH value (0.9 or 4.2 or 7.8) of the stock solution was adjusted by adding nitric acid, respectively, followed by diluting the volume of the stock solution with deionized water to 20 mL. The final solution was transferred into a serum bottle and aged at 60 °C for 24 h. Subsequently the certain amount of Be was added into the above suspension liquid under vigorous stirring at room temperature for 1 h. Then the mixed solution was transferred into a Teflonlined autoclave and aged at 140 oC for 72 h. The precipitate was collected, washed several times with deionized water, and dried at 60 oC overnight. The obtained products were denoted as n B/Be, where n is the loading amount (wt %) of B. In control synthesis experiments, pure anatase TiO2 was obtained in the absence of Be (pH=0.9). A/Ae or A/RS sample was obtained when Ae or RS not Be was added during the above preparation process of B/Be. A5/Ae, A15/Ae, and A25/Ae samples are prepared at pH=0.9, 4.2, and 7.8, respectively. A/RS sample is prepared at pH=0.9. A previously reported method was adopted to prepare A/Be or R/Re sample:75 the above pure anatase TiO2 sample was mixed with Be (or Re) at different weight ratios. The mixed oxide was then dispersed in 50 mL of isopropanol, followed by sonification and stirring for 1.5 h. After drying at 90 oC, the solid was sintered in air at 300 oC for 2 h to obtain A/Be sample (or the solid was calcined in air at 700 oC for 4 h to obtain R/Re sample).

Characterization The crystalline structures of the as-synthesized samples were characterized by X-ray diffraction, using Rigaku RotaflexRu-200 B diffractometer with Cu Kα radiation (λ=1.5418 Å) at a speed of 5 °/min. Raman spectra excited at 325 nm or 532 nm were acquired on a home-assembled UV Raman spectrograph (DL-3 UV Raman spectroscopy with operando system) with spectral resolution of 2 cm−1. The particle size and morphology of samples were obtained on a Quanta 200FEG scanning electron microscopy. HRTEM images were recorded on a FEI Tecnai F30 microscope with a


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point resolution of 0.20 nm operating at 300 kV. The Brunauer-Emmett-Teller (BET) surface area was determined by Quantachrome NOVA 4200e surface area and pore size analyzer. The X-ray photoelectron spectroscopy (XPS) investigations were carried out on a spectrometer (VG ESCALABMK-2) using Al Kα as the excitation source. The surface photovoltage measurements (SPV) were conducted by an assembled instrument including a photovoltaic cell where the photocatalyst powders are stuck in the middle of two ITO electrodes (d=1.2 mm, resistivity: 8-12 Ω•cm-2, Sigma-Aldrich, USA). A lock-in amplifier (SR830 DSP, Stanford research systems, USA) was applied to amplify the photovoltage signal of samples, and a 500 W xenon lamp (CHF-XM500W, Beijing Perfect Light Co., China) was used as the optical source coupling with a grating monochromator (Omni-λ3007, Zolix, China) providing monochromatic light. The electrochemical impedance spectra (EIS) of Ae and A/Ae samples were measured on an electrochemical workstation (Ivium Vertex One, Shanghai Chenhua, China) and carried out in a three-electrode system with a working electrode, Ag/AgCl (saturated KCl) as the reference electrode, Pt foil as the counter electrode. The electrolyte was 0.1 M Na2SO4 mixed with 0.01 M (1/1) K3Fe(CN)6/K4Fe(CN)6. A 300 W Xe lamp (Beijing Perfect Light Co., China) was used as the light source. The working electrode was prepared by dip-coating TiO2 slurry on an F-doped tin oxide (FTO) glass electrode (2 cm  0.5 cm) and dried at room temperature in the air. The EIS measurements were conducted at the open circuit potential with 10 mV amplitude from 100 kHz to 100 mHz. Mott-Schottky of Be or 1% B/Be was measured to get the semiconductor property. The 10 mg of Be or 1% B/Be powder was ultrasonically dispersed in 0.5 ml isopropanol with 50 μL 5 wt% Nafion solution, and 100 μL of the solution was dropped onto one piece of 1 cm2 FTO glass to form the working electrode. Then the working electrode was left dried at room temperature overnight. One piece of Pt and Ag/AgCl (sat. KCl) were applied as the counter and reference electrodes. The electrolyte was aqueous 0.1 M Na2SO4. The frequency and potential range of Mott-Schottky measurements were 1000 Hz and 1 ~ -1 V (vs. Ag/AgCl), respectively. The measurements were performed with Ivium Vertex One (Ivium Technologies, Dutch).


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Photocatalytic Reaction The photocatalytic H2 or O2 evolution from overall water splitting was investigated over Pt-loaded TiO2 samples. The photocatalytic reaction was carried out in a pyrex reaction cell connected to a closed gas circulation and evacuation system (LabsolarIIIAG, Beijing perfect light Co.). The light source was a 300 W Xe lamp (Beijing Perfect Light Co.) without filter. The TiO2 photocatalyst (0.05 g) was suspended in 100 mL water. Pt (0.5 wt%) was photodeposited on the TiO2 catalysts in situ from precursor H2PtCl6•6H2O under irradiation. The amount of evolved H2 or O2 was determined by an on-line gas chromatograph (GC7900 gas chromatography system, Shanghai Techcomp. LTD, Ar carrier). The activities of the B/Be (A/Be, A/Ae, R/Re) photocatalysts were also measured by monitoring hydrogen evolution from an aqueous solution containing 10 vol% of methanol sacrificial agent. The reaction conditions are similar to the above process, but the above TiO2 photocatalyst was suspended in mixed solution of H2O (90 mL) and CH3OH (10 mL).

Photodeposition of MnOx and Au In the photodeposition experiments, B/Be and a certain amount of MnSO4 (or HAuCl4·4H2O) were mixed in 100 ml water with NaIO3 (or methanol), and the suspension was then irradiated by a 300 W Xe lamp under stirring. After 3h photodeposition, the suspension was filtered, washed with deionized water for three times, and finally dried at 80 oC for overnight.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Details of XRD patterns of Ae, 5% A/Ae, RS, and 5% A/RS samples. XRD patterns of 3% B/Be sample before and after photocatalytic reaction. XRD patterns and Raman spectra of Ae, A5/Ae, A15/Ae, A25/Ae, Re, and 10% R/Re samples. SEM images of RS and 5% A/RS. SEM image with the size distribution and statistics of 5% A/Be photocatalyst. TEM and HRTEM images of 3% B/Be and 5% A/RS samples. SPV spectra of Re and 5% R/Re, Be, and B20/Be samples. EIS fitting data of Ae


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and 1% A/Ae samples. The equivalent circuit model. The relationship between R2 and H2 evolution on Ae, A5/Ae, A15/Ae, and A25/Ae samples. Mott-Schottky plots and XPS valence band spectra of Be and 1% B/Be samples. AUTHOR INFORMATION Corresponding Author *[email protected]; *[email protected]. ORCID

Jing Zhang: 0000-0002-9342-8473 Rengui Li: 0000-0002-8099-0934 Author Contributions Y. B. and Y. Z. made a significant contribution throughout the work. J. Z., R. L. conceived the original idea and supervised the project; X. C. supervised the H2 or O2 evolution performance of the samples and conducted SEM and XRD measurements. Y. Z. performed the H2 evolution from B/Be and A/Ae, J. L. performed the H2 and O2 evolution from R/Re. J. W. performed BET and SEM measurements. F. W. carried out Raman spectra measurements. C. C. carried out SPV characterization measurements. C. L. performed the EIS measurements. J. Z., R. L., and C. L. modified and finalized the manuscript. All authors have given approval to the manuscript. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported financially by National Natural Science Foundation of China (21573101), the Program for Liaoning Excellent Talents in University (LR2017011), the support plan for Distinguished Professor of Liaoning Province ([2015]153), Liaoning BaiQianWan Talents program ([2017]96), the fund of the State Key Laboratory of Catalysis in DICP (N-15-10), Liaoning Provincial Natural Science Foundation (20170540583 and 20180510002) and the talent scientific research fund of LSHU (2016XJJ-012). Rengui Li would like to thank the support from the Strategic Priority Research Program of Chinese Academy of Sciences (XDA21010207), Youth Innovation Promotion Association of Chinese Academy of Sciences and the R&D department of PetroChina.. REFERENCES (1) Jiang, C.R.; Moniz, S.J.A.; Wang, A.Q.; Zhang, T.; Tang, J.W. Photoelectrochemical Devices for Solar Water Splitting-Materials and Challenges. Chem. Soc. Rev. 2017, 46, 4645-4660.


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An efficient homophase junction was fabricated between the identical composite of semiconductors with different particle sizes, which can extremely enhance the spatial charge separation in photocatalysis for solar energy conversion. 66x44mm (300 x 300 DPI)


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Homophase Junction for Promoting Spatial Charge Separation in

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