Enhanced Charge Injection and Collection of Niobium-Doped TiO2

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Enhanced Charge Injection and Collection of Nb-Doped TiO2/ Gradient W-doped BiVO4 Nanowires for Efficient Solar Water Splitting Zhangliu Tian, Feng Shao, Wei Zhao, Peng Qin, Jianqiao He, and Fuqiang Huang ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.7b00322 • Publication Date (Web): 13 Feb 2018 Downloaded from http://pubs.acs.org on February 14, 2018

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ACS Applied Energy Materials

Enhanced Charge Injection and Collection of NbDoped TiO2/Gradient W-doped BiVO4 Nanowires for Efficient Solar Water Splitting Zhangliu Tian,1,2 Feng Shao,1 Wei Zhao,1 Peng Qin,1 Jianqiao He,1,2 and Fuqiang Huang1* 1

State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai

Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, P.R. China; 2

University of Chinese Academy of Sciences, 19 Yuquan Road, Beijing 100049, P.R. China.

KEYWORDS: Nb-doped TiO2, gradient W-doped BiVO4, photoanode, charge injection efficiency, charge collection efficiency

ABSTRACT: Extensively investigated BiVO4 photoanode for solar water splitting suffers from low product of light absorption and charge separation efficiency (ηabs × ηsep) due to the lack of high surface area supporting materials as a charge collector. Such a host|guest heterostructure is not only effective but also attractive but it is too complicated to understand the original process of ηsep. Here, a host-guest heterostructure of Nb-doped TiO2 nanowires supporting BiVO4 nanoparticles is fabricated to investigate its visible-light charge injection efficiency (ηinj) and charge collection efficiency (ηcol). With the aid of gradient W doping in BiVO4 guest, the Nbdoped TiO2|gradient W-doped BiVO4 (N:T|g-W:B) produces ηinj=82% and ηcol=95% to yield a very high value for ηabs × ηsep of 55.3% at 0.6 VRHE, which is one of the highest values among

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these nanostructure-host|BiVO4-geust photoanodes. By further coated with Co-pi, the photoanode simultaneously achieves high value of ηtrans for efficient solar water splitting.

The solar-driven photoelectrochemical (PEC) water splitting is one of the promising means to harness energy from sunlight via production of hydrogen.1-3 BiVO4 is a well-studied photoanode in PEC devices due to its relatively small band gap of ~2.5 eV and favorable conduction band (CB) edge position (∼0 V versus RHE).4-6 However, the practical water oxidation photocurrent (JH2O) of BiVO4 is far below its theoretical value, mainly due to its low product of light absorption and charge separation efficiency (ηabs × ηsep). Constructing host|guest structure, where high surface area materials (host) serve as charge conductor to support BiVO4 (guest), is a facile strategy to improve ηabs × ηsep. The ηabs can be increased due to the enhanced light absorption paths by the hierarchical structure. However, the improvement of ηsep is a critical issue because charge separation in the host|guest structure is more complex and lacking of fundamental studies. If the illumination only in the guest absorption region, the processes of ηsep for the host|guest photoanodes can be simplified, which should include the charge injection determined by the interaction between the host and guest and charge collection determined by the charge transport in the host and collected on the counter electrodes. Therefore, it is reasonable to express ηsep by ηsep = ηinj × ηcol, where ηinj and ηcol represent charge injection efficiency and charge collection efficiency, respectively, which may provide us a deeper insight into ηsep. In consideration of high electron mobility in their bulk single-crystal phases and energy band structure matching, SnO2 and WO3 are charming as the supporting materials. WO3 has a low CB edge (∼0.4 VRHE)7, suggesting the ηinj should be high. Therefore, a lot of work focused on constructing WO3|BiVO4 host|guest heterostructure,8-12 and a record ηabs × ηsep at 1.23 VRHE was


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achieved by 2014.9 Nevertheless, the low CB edge lowered the energy of injected electrons then limiting the electrons collection in low potential region, which was reflected by its high onset potential of photocurrent (Eonset) (~ 0.5 VRHE) for hole scavenger oxidation and low ηabs × ηsep (~33.3%) at 0.60 VRHE. While performance in low potential region is the key to obtain high efficiency for a series-connected tandem system, which is operated at a potential between the EonsetS of the photoanode and photocathode, at the same time the Eonsets of the photocathodes are always in the range of 0.5-0.8 VRHE.13-22 While using SnO2 to fabricate Sb:SnO2|BiVO4 NW photoanode, one of the highest of ηabs × ηsep (~51%) at 0.6 VRHE and lower Eonset (~ 0.3 VRHE) for hole scavenger oxidation were achieved.23 They attributed the higher ηabs × ηsep than that of WO3|BiVO4 at low potential to the higher conductivity of SnO2, while ignoring the favorable factor of higher CB edge. Compared to WO3 and SnO2, rutile TiO2 has more negative CB edge2425

but lower than that of BiVO4,26-28 which makes it possible for achieving higher ηcol in low

potential region. If only from the point of view of the CB edge position, the ηinj in SnO2 from BiVO4 could be more efficient than in their TiO2 counterparts. However, it is not sufficient to come to this conclusion by referring to DSSC, in which the ηinj in SnO2 is much lower than that in TiO2,29-31 accounting for the main reasons for the limiting performance. Therefore, it is very likely to achieve higher ηcol and ηinj simultaneously in low potential region by using TiO2 to support BiVO4 with high ηabs, namely higher ηabs × ηsep than that of using SnO2, in spite of that the electron mobility of TiO2 is much less than that of SnO2.31-32 In our previous work, we demonstrated high surface area and high-crystallized NWs of Nbdoped rutile TiO2, in which Nb acts as substitutional electron donor, and its efficient electron transport.33 Here, we directly use the Nb-doped NWs with doping content of 0.5 mol% in Nb/Ti ratio (N:T) as the scaffold for BiVO4. Through gradient W doping in the BiVO4 guest, one of


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highest ηabs × ηsep at 0.6 VRHE (55.3%) among nanostructure-host|BiVO4-geust photoanodes was achieved. To further find out the reasons for the high ηabs × ηsep of N:T|g-W:B photoanode, the ηinj and ηcol were investigated under the condition of visible light illumination. The results show that the distributed n+-n homojunction in BiVO4 guest created by the gradient W doping greatly enhances the ηsep of the photoanode at low potential by achieving simultaneous enhancements in charge collection and charge injection compared to the photoanode with pure BiVO4 guest. RESULTS AND DISCUSSIONS


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Figure 1 (a) Structure schematic and energy band diagram of the N:T|g-W:B host|guest NWs on FTO substrate. Scanning electron microscopy (SEM) images of N:T NWs (b) and N:T|g-W:B host|guest NWs (c, d). (e)Scanning transmission electron microscopy (STEM) image and linear energy dispersive X-ray spectroscopy (EDX) mapping of 1 at% W-doped BiVO4 coated on N:T NWs. Figure 1a shows the architecture and energy band diagram of the heterostructure, in which N:T NWs and gradient W doped BiVO4 (g-W:B) particles serve as host and guest, respectively. In such a heterostructure, the N:T NWs account for ultraviolet absorption and electron transport, while coated g-W:B particles are capable of visible light harvesting. The N:T NWs were prepared by hydrothermal growth according to our previous work33 and the coated g-W:B particles were deposited by spray pyrolysis with pulsed deposition mode. By changing BiVO4


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precursors at each step, BiVO4 coated on N:T NWs (N:T|B), 1 at% W-doped BiVO4 coated on N:T NWs (N:T|W:B) and gradient W-doped BiVO4 with doping range from 1 to 0 at% coated N:T NWs (N:T|g-W:B) could be obtained. Due to that the NW is thick enough, the BiVO4 particles are mainly deposited on the top of the NWs and excess BiVO4 particles are highly dispersed coated on the side surface of the NWs. As shown in Figure 1b, N:T NWs have the diameters of ~300 nm. The agminated particles on the top of NW with the thickness of about 100 nm (Figure 1d and Figure S1) allow high light absorption efficiency and the distributed n+-n homojunction created by the gradient W concentration in BiVO434 allows high charge separation efficiency. As the energy band diagram illustrated, at the function of the Type II Band Alignment and n+-n homojunction, the photo-induced electrons and holes are efficiently transferred through TiO2 and BiVO4 to the out circuit and BiVO4/electrolyte interface, respectively. The latter is used for water oxidization. The deposited particles are indexed to monoclinic scheelite BiVO4 phase (PDF 14-0688) evidenced by the XRD pattern (Figure S2). There is no obvious migration of the diffraction peaks due to the low doping concentrations of W atoms, which has been proved to be stabilizing the tetragonal scheelite structure.35-37. Besides, the XRD patterns of N:T NWs and g-W:B in the N:T|g-W:B host|guest structure remain unchanged compared to those of N:T NWs and g-W:B (Figure S2), indicating that inter-doping or inter-reaction were unlikely to happen between the two phase during the spray pyrolysis and annealing process. In addition, the STEM-EDX (Figure 1e and Figure S3) clearly shows that there are strong Ti signals observed on NWs and strong Bi and V signals observed on the particles. Though the W signals can hardly be observed by comparison with the strong Bi and V signals, to further enlarge the EDX mapping (left down inset of Figure 2b and Figure S3a), weak W signals are observable, evidencing the


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incorporation of W into BiVO4 particles. As shown in Figure S3b, the spacing distance of the NW is 0.326 nm, in good consistent the d-spacing of rutile TiO2 in (110) plane, and the spacing distance of the particle is 0.475 nm, which is corresponding to the (110) plane of monoclinic BiVO4.

Figure 2 Band alignment between N:T and W:B. (a) Ultraviolet photoelectron spectroscopy (UPS) of N:T and W:B. (b) Band structure diagram of N:T and W:B based on the results of UPS. The designed host|guest photoanodes rely on a type II band alignment so that the electrons can transfer from the guest to the host. To determine the band structure of TiO2 and BiVO4, ultraviolet photoelectron spectroscopy (UPS) spectra were recorded as shown in Figure 2a. Considering that the heterostructure interface of N:T|g-W:B NW is comprised of N:T NWs and W:B particles, the N:T and W:B were chosen to conduct the UPS spectra. Work function of materials can be obtained by calculating the difference value between the low kinetic energy cutoff (Ecutoff) of the UPS spectrum and the photon energy of He I (21.21 eV). The Ecutoff of N:T and W:B (Figure 2a) are 16.60 and 16.70 eV, respectively, corresponding to work function of 4.61 and 4.51 eV. As -4.44 eV vs vacuum level (vac) equals 0 VRHE, the Fermi levels (EF) of N:T and W:B are 0.17 and 0.07 VRHE, and valence band levels (EV) are 3.16 VRHE and 2.55 VRHE,


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respectively. The CB levels (EC) were calculated by subtracting the energy band gap (Eg), which are 3.04 eV and 2.51 eV for N:T (Figure S4) and W:B (Figure S5). Through this analysis, it yielded EC of 0.12 VRHE for N:T and 0.04 VRHE for W:B (Figure 2b), declaring a favorable type II band alignment existing the host|guest NWs.

Figure 3 Optical and PEC performance of loading BiVO4 particles with spraying 21 cycles on N:T NWs, and the corresponding BiVO4 with the same mass deposited directly on FTO substrates. (a) Light harvesting efficiency (LHE) and integrated ηabs for BiVO4 with different doping method coating on NWs and FTO substrates. (b) Linear sweep voltammograms for sulfite oxidation (Jsulfite) measured in 0.5 M potassium phosphate electrolyte buffered to pH 7 with 1 M


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Na2SO3 under the illumination and in the dark. (c) The calculated ηabs × ηsep from (b). (d) The calculated ηsep from the ηabs × ηsep by dividing by ηabs. After optimization of the g-W:B amount deposited on N:T NWs, maximum of Jsulfite can be obtained, which realized by spraying 21 cycles as shown in Figure S8 and Figure S9. To further understand the mechanism of the optimal ηabs × ηsep for N:T|g-W:B with spraying 21 cycles, the effect of W doping BiVO4 was investigated. As shown in Figure 3a, BiVO4, W:B and g-W:B give almost similar ηabs of around 69.5%, and the ηabs of N:T|B, N:T|W:B and N:T|g-W:B also show similar values of around 73.1%. The 3.6% increase in ηabs is mainly attributed to the enhanced light absorption areas in the dispersed BiVO4 coated on the side surface of NWs. As shown in Figure 3b, the Jsulfites of N:T|B, N:T|W:B and N:T|g-W:B are 3.57, 3.76 and 4.42 mA cm-2 at 0.6 VRHE (4.61, 4.96 and 5.24 mA cm-2 at 1.23 VRHE), corresponding to ηabs × ηsep (Figure 3c) of 44.6, 47.0 and 55.3% at 0.6 VRHE (57.6, 62.0 and 65.5% at 1.23 VRHE). Thus, the obtained ηsep (Figure 3d) are 61.1, 64.3 and 75.5% at 0.6 VRHE (78.9, 84.8 and 89.5% at 1.23 VRHE). Remarkably, the gradient doping in BiVO4 can significantly enhance the ηsep. Though the agminated BiVO4 particles by spraying 21 cycles on the top of NWs can absorb light sufficiently, the 100 nm thickness is an obstacle to efficient carrier separation for BiVO4 due to its slow photo-induced carriers transport. The incorporation of W into BiVO4 obviously enhances ηsep, ascribed to increase the conductivity of BiVO4. However, because of the low intrinsic mobility of BiVO4, W doping as donor-type dopant to substitute V5+ site is not sufficient by itself, thus the ηsep just can be increased to a limited value. By introducing a gradient W into the agminated BiVO4 particles, a series of n+-n homojunctions is formed.34 Therefore, under the function of the built-in electric field caused by the distributed n+-n homojunctions and the type II band


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alignment, photo-induced carriers can separate and transport efficiently, thus further enhancing ηsep. Table 1 Comparison of various NW-host|BiVO4-guest photoanodes for PEC water splitting

Photoanodes

at 1.23VRHE

at 0.6 VRHE

ηabs (%)

ηsep (%)

ηabs × ηsep (%)

ηsep (%)

ηabs × ηsep (%)

WO3|W:BiVO4

69.0

~ 48.3

~ 33.3

77.0

53.1

9

Sb:SnO2|BiVO4

72.0

71.1

51.2

92.4

66.5

23

Ta:TiO2|BiVO4

-

-

~ 22

-

~ 33

27

N:T|g-W:B

73.2

75.5

55.3

89.5

65.5

This work

ref

By comparison these recently reported highest values of ηabs × ηsep for NW-host|BiVO4-guest photoanodes as listed in Table 1, we find that the excellent ηabs × ηsep of N:T|g-W:B NW photoanode is one of the highest efficiencies achieved to date at 0.6 VRHE. To date, the reported highest ηabs × ηsep of metallic oxide|BiVO4 photoanode was achieved by Zhou et al.23 They attributed the higher ηsep of Sb:SnO2|BiVO4 than that of WO3|BiVO49 to the higher conductivity in Sb doped SnO2 (∼33 S/cm) than WO3 (∼1 S/cm). Nevertheless, ηsep based on metallic oxide|BiVO4 photoanode is still a complex process, and the conductivity of metallic oxide host is only one part of the factors that influence ηsep. Although the authors pointed out that heavily W doping (up to 7%) in BiVO4 of WO3|BiVO4 photoanode would bring about detrimental recombination exceeding the beneficial effect of increase electron concentration, they did not give powerful evidence that access the efficiency of charge separation between the metallic oxide and BiVO4 heterojunction. Therefore, some measures need to be taken to further investigate the electronic dynamics process to have a deep insight into ηsep instead of deduction by intuition.


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Figure 4 (a) Electron lifetime (τn) and transport time (τt) of three photoanodes measured under illumination with a narrow-band blue LED light (448nm) via IMVS/IMPS in 0.5 M potassium phosphate electrolyte buffered to pH 7 with 1 M Na2SO3. (b) The incident-photon-to current conversion efficiency (IPCE) spectra measured the same electrolyte. Intensity-modulated photovoltage/photocurrent spectroscopy (IMVS/IMPS) are effective tools to determine the mean transit time of photoinjected electron (τt) and electron lifetime (τn).38-39 As shown in Figure 4a, both W doping and gradient W doping in BiVO4 guest just lead to a small change for τt. Meanwhile, the significantly increased τn for N:T|g-W:B photoanodes can be clearly seen, accounting for the improvement of ηsep. Note that 448 nm light is only in BiVO4 absorption region, the increased τn is mainly benefitted from the gradient doping BiVO4 or the physical interaction between the interface of the doping BiVO4 and N:T. To quantitatively analyze these factors, the processes of ηsep should be quantified. Now that the light is only absorbed by BiVO4, the progresses referred to ηsep can be simplified into two processes as follows. First, the photo-induced electrons in BiVO4 inject into N:T host, which can be quantitatively analyzed by ηinj. Second, the injected electrons transport in N:T host and are collected by the counter electrode, which can be quantitatively analyzed by ηcol. Therefore, under


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this condition, the ηsep can be expressed as: ηsep = ηinj × ηcol. ηcol can be obtained via the relation ηcol = 1 – τt/τn.40 On account of that the obtained τt and τn are measured at 448 nm wavelength, the calculated ηcol are corresponding to the efficiencies of electrons collection under 448 nm wavelength light illumination. Thus, in order to calculate ηinj, the ηsep under 448 nm wavelength light illumination should be known. The incident photon-to-current efficiency (IPCE), taking into consideration the efficiencies for generation of photo-excited charges (LHE), ηsep and ηtrans at each wavelength, is a measure of the ratio of the photocurrent versus the rate of incident photons as a function of wavelength. Therefore, the IPCE is itself a measure of the LHE × ηsep × ηtrans product at each wavelength. It is important to note that the IPCE were measured under the condition of exiting 1 M Na2SO3 as a hole scavenger (ηtrans ≈ 100%), thus the IPCE for sulfite oxidation (IPCEsulfite) can be simplified as: IPCEsulfite = LHE × ηsep. Combined with the equation of ηsep = ηinj × ηcol, the IPCEsulfite is further expressed as: IPCEsulfite = LHE × ηinj × ηcol. The values determined for IPCEsulfite, LHE, ηcol and ηinj at the wavelength of 448 nm are summarized in Table S1.


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Figure 5 Three dimensional histogram of ηinj and ηcol determined for N:T|B N:T|W:B and N:T|gW:B photoanodes. In order to intuitively observe the influence of W doping and gradient W doping on the ηcol and ηinj, the values of ηcol and ηinj both at 0.6 VRHE and1.23 VRHE for the three photoanodes are plotted as three dimensional histogram as shown in Figure 5. W doping in BiVO4 has been reported as electron donor to substitute V5+, which is an effective method to break through the bottleneck of slow electron transport in BiVO4, consequently enhancing ηsep.35, 41-43 Although W doping can decrease the carrier lifetime of BiVO4 guest,44 the enhanced electron transport drives more electrons to the interface of N:T and W:B and these electrons can be injected into N:T host under the function of the Type II Band Alignment. As a result, the τn of N:T|W:B is probably unchanged and even slightly increased compared to that of N:T|B, which is depending on the competition result of the decreased carrier lifetime and the increased electron transport. Compared to the unchanged τn, the decreased τt both at 0.6 VRHE and 1.23 VRHE is the main reason for the enhanced ηcol of N:T|W:B. Besides, the ηinj of N:T|W:B is also increased, due to the improved electron transport of BiVO4 which drives more electrons migrate to the interface of N:T and W:B under the applied potential driving. The distributed n+-n homojunction in N:T|gW:B photoanode can greatly enhance the photoinduced charge separation in BiVO4 guest,34 corresponding to the greatly enhanced τn, thus the ηcol of N:T|g-W:B is further increased compare to that of N:T|W:B. In addition to the noteworthily increased ηcol of N:T|g-W:B, its ηinj is further improved to a relative high level, the reasons of which can be attributed to the greatly enhanced electron lifetimes in BiVO4 guest and more energy difference between the guest and the host provided by the continuously upward CB. As shown in Figure 5, the ηinj is the main limiting factors for ηsep of N:T|B and it is more susceptible to applied potential than ηcol for all


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photoanodes. Though the ηinj of N:T|g-W:B is greatly enhanced at low applied potential, it is still the main limiting factors for ηsep instead of ηcol. The electron mobility (µ) in single-crystalline rutile TiO2 has been reported in the range of 0.1–1 cm2 V−1 s−1 and the conductivity of N:T (σ) can be determined by the equation of σ = qµn, where q is the elementary charge and n is the density of free electrons, which is 7.99 × 1017 cm-3 obtained from our previous work.33 As a result, the σ of N:T is not exceeding the limit of 0.13 S cm-1, which is much less than that of WO3 (~1 S cm-1) and Sb:SnO2 (~33 S cm-1). However, the ηsep of N:T|g-W:B (75.5%) is higher than that of WO3 (48.3%) and Sb:SnO2 (71.1%) at 0.6 VRHE, indicating that the σ is not the limiting factor for ηsep at low applied potential. In order to further figure out the reasons of differences of ηsep for these photoanodes, the processes of ηsep mentioned above should be reconsidered. The CB edge of SnO2 is higher than that of WO3, but lower than that of TiO2 (Figure S10), 24-25, 45 suggesting that the electron injection in SnO2 could be more effective than in TiO2. However, both the CB edges of SnO2 and TiO2 are lower than the position of reversible hydrogen electrode, consequently, it need extra energy to increase the injected electrons energy to produce hydrogen in spite of existing overpotential, corresponding to the electrons collection on the counter electrode. In fact, the Eonset for Jsulfite can reflect the influence of CB edge on the hydrogen production, due to that the oxidation of Na2SO3 is very fast and the Eonset for Jsulfite is main determined by the reduction reaction on the counter electrode. The Eonset of N:T|g-W:B is 0.22 VRHE, which is more negative than that of Sb:SnO2|BiVO4(~0.30 VRHE), in consistent with the lower CB edge of SnO2. Based on this analysis, Sb:SnO2|BiVO4 photoanode needs more extra energy to break through the limit of electrons collection on the counter electrode than that of N:T|g-W:B. Therefore, at low applied potential meaning without enough extra energy, the limiting ηcol of Sb:SnO2|BiVO4 accounts for the low ηsep. The same


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condition for WO3|W:BiVO4, the lower CB edge of WO3 than that of SnO2 is the main reason for lower ηsep instead of electron conductivity at low applied potential. As the applied potential increases to high value meaning with enough extra energy, the ηcol of Sb:SnO2|BiVO4 is not the limiting factor for ηsep, thus both the ηcol and ηinj could be the reasons for the higher ηsep than that of N:T|g-W:B.

Figure 6 Performance of TiO2-BiVO4 NW heterostructure photoanodes with and without Co-Pi catalyst for PEC water oxidation, measured using a 3-electrode configuration in 0.5 M potassium phosphate electrolyte buffered to pH 7. (a) Photocurrent for water oxidation (JH2O). (b) Surface charge transfer efficiency (ηtrans). As shown in Figure 6a, without hole scavenger, the surface catalytic efficiency for water oxidation is obviously dull, reflected by the decreased photocurrents and ∼80 mV anodic shift in the Eonset (the three photoanodes showing almost the same Eonset of ∼0.30 VRHE). Impressively, the ηtrans of N:T|g-W:B, N:T|W;B and N:T|B show tiny differences (Figure 6b), demonstrating that gradient W doping and W doping in BiVO4 guest hardly affect the surface catalytic of TiO2BiVO4 heterostructure photoanodes. Therefore, the enhanced PEC performance of N:T|g-W:B


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and N:T|W;B photoanodes is mainly coming from the improved ηsep. The ηtrans of the three photoanodes show low values of ∼30% at 0.6 VRHE and ∼64% at 1.23 VRHE. To break through this bottleneck, the photoassisted electrodepostion method was used to deposit cobalt based catalyst containing phosphates (Co-Pi) on TiO2-BiVO4 NW photoanodes.43 As oxygen evolution catalyst, Co-Pi has been shown to have excellent activity to improve surface catalytic efficiency of the photoanodes.27,

43, 46-47

After the deposition of Co-Pi catalysts on TiO2-BiVO4 NW

photoanodes, all the JH2Os are greatly enhanced. Similar to the situation of without Co-Pi catalysts, the three photoanodes with Co-Pi catalysts also show almost same ηtrans. The ηtrans of three photoanodes with Co-Pi catalysts are enhanced to high values of ∼58% at 0.6 VRHE and ∼83% at 1.23 VRHE. Except the enhanced ηtrans, the EonsetS show ∼70 mV cathodic shift, showing that Co-Pi catalysts can improve water oxidation kinetics and decrease surface recombination. The equal loading of BiVO4, W doping BiVO4 and gradient W doping BiVO4 on FTO also show increased JH2Os and cathodic shift in EonsetS after being deposited Co-Pi catalysts (Figure S11). Furthermore, the applied potential photon-to-current efficiency (ABPE) was shown in Figure S12. The N:T|W:B photoanode exhibits a maximum ABPE of 0.812% at 0.792 VRHE, while both the N:T|W:B and N:T|g-W:B photoanodes achieve higher ABPE of 0.872% at 0.783 VRHE and 0.979% at 0.751 VRHE, respectively. The highest ABPE and lowest applied potential of N:T|gW:B photoanode indicates that gradient W doping not only improve the photoconversion efficiency of the photoanode, but also reduce the current saturation potential. By deposited Co-Pi catalysts on these photoanodes, the ABPEs were further increased, which show 1.349% at 0.694 VRHE for N:T|B|Co-Pi, 1.476% at 0.685 VRHE for N:T|W:B|Co-Pi and 1.697% at 0.672 VRHE for N:T|g-W:B|Co-Pi, respectively. The almost twice ABPE value of N:T|g-W:B|Co-Pi and 79 mV reduction in current saturation potential compare to those of N:T|g-W:B demonstrate that Co-Pi


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catalysts largely eliminate a major performance limitation of the photoanodes. Finally, the longterm stability of N:T|g-W:B|Co-Pi photoanode was examined at 1.23 VRHE for water oxidation as shown in Figure S13. The sample has not shown noticeable JH2O decreases after 4 hours test, proving its long-term stability. Finally, the amounts of H2 and O2 evolution produced by N:T|gW:B|Co-Pi photoanode were quantified as shown in Figure S14. The Faradaic efficiency for O2 evolution at 1.23 VRHE was 97 %, indicating that the photocurrent of the photoanode substantially comes from water splitting. CONCLUSIONS In summary, we have demonstrated the use of Nb doping TiO2 NWs as scaffold for gradient W doping BiVO4, to provide simultaneously high charge separation efficiency and light absorption efficiency, thereby obtaining ηsep of 75.5% at 0.6 VRHE and ηabs of 73.2%, for a combined ηabs × ηsep of 55.3%, which is one of highest value at 0.6 VRHE by comparison the recently reported highest ηabs × ηsep of 51.2% at 0.6 VRHE for Sb:SnO2|BiVO4 photoanode. To further investigate the reason for high ηsep, we divide ηsep into ηinj and ηcol by analyzing the process of ηsep. Through gradient W doping in BiVO4 guest, the ηinj and ηcol of N:T|g-W:B photoanode are increased to 82.91% and 95.57% at 0.6 VRHE compared to the ηinj of 73.94% and ηcol of 88.02% for N:T|B photoanode. Though the conductivity of Sb:SnO2 is over 2 orders of magnitude higher than that of N:T, the poor ηcol of Sb:SnO2|BiVO4 in low applied potential region, limited by the low CB edge, should be accounting for the low ηsep compared to that of N:T|g-W:B. Moving forward, the ηsep at low applied potential can be further enhanced by improving the ηinj through some physical or chemical methods, thus further increasing ηabs × ηsep. Limited by the slow kinetics for water oxidation, the ηtrans of N:T|g-W:B is low, which is largely improved by deposited Co-Pi catalyst. ASSOCIATED CONTENT


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Supporting Information. Experiment section, additional SEM, XRD and STEM, spectral irradiance of Xe lamp solar simulator and standard AM 1.5G solar spectrum, UV/vis absorption, Tauc plots, Mott−Schottky plots, photocurrents and LHE of different loadings of g-W:B partilces on N:T NWs, schematic diagram illustrating the energy levels of conduction band and valence band of rutile WO3 and SnO2, table of ηinj and ηcol, photocurrents and ABPE for different samples. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT Financial support from the National Key Research and Development Program (Grant No. 2016YFB0901600), NSF of China (Grants 61376056, 51672301 and 51672295), Science and Technology Commission of Shanghai (Grants 14520722000, 16ZR1440500 and 16JC1401700), and Shanghai Science and Technology Development Funds (Grant 16QA1404200) is acknowledged. REFERENCES 1. Fujishima, A.; Honda, K., Electrochemical photolysis of water at a semiconductor electrode. Nature 1972, (238), 37-8. 2. Bensaid, S.; Centi, G.; Garrone, E.; Perathoner, S.; Saracco, G., Towards Artificial Leaves for Solar Hydrogen and Fuels from Carbon Dioxide. Chemsuschem 2012, 5 (3), 500-521. 3. Prevot, M. S.; Sivula, K., Photoelectrochemical Tandem Cells for Solar Water Splitting. Journal of Physical Chemistry C 2013, 117 (35), 17879-17893.


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Table of Contents graphic

Host-guest heterostructure photoanode is fabricated by using Nb-doped TiO2 nanowires to support BiVO4 nanoparticles. The poor ηinj at low potential is improved through gradient W doping in BiVO4 guest, simultaneously leading to enhancement of ηcol. As a result, one of the highest ηabs × ηsep efficiencies (55.3%) at 0.6 VRHE for nanostructure-host|BVO4-geust photoanodes is achieved.


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Enhanced Charge Injection and Collection of Niobium-Doped TiO2

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