Oriented Built-in Electric Field Introduced by Surface Gradient

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Oriented Built-in Electric Field Introduced by Surface Gradient Diffusion Doping for Enhanced Photocatalytic H Evolution in CdS Nanorods 2

Hengming Huang, Baoying Dai, Wei Wang, Chunhua Lu, Jiahui Kou, Yaru Ni, Lianzhou Wang, and Zhongzi Xu Nano Lett., Just Accepted Manuscript • Publication Date (Web): 25 May 2017 Downloaded from http://pubs.acs.org on May 25, 2017

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Oriented Built-in Electric Field Introduced by Surface Gradient Diffusion Doping for Enhanced Photocatalytic H2 Evolution in CdS Nanorods Hengming Huang,†,‡,§,# Baoying Dai,†,‡,§,# Wei Wang,†,⊥ Chunhua Lu,*,†,‡,§ Jiahui Kou,*,†,‡,§ Yaru Ni,†,‡,§ Lianzhou Wang,# Zhongzi Xu†,‡,§ *E-mail: [email protected]. Tel.: +86 25 83587252. Fax: +86 25 83587220 *E-mail: [email protected]. Tel.: +86 13913004282 †

State Key Laboratory of Materials-Oriented Chemical Engineering, College of Materials

Science and Engineering, ‡Jiangsu Collaborative Innovation Center for Advanced Inorganic Function Composites, §Jiangsu National Synergetic Innovation Center for Advanced Materials(SICAM), Nanjing Tech University, Nanjing 210009, China ⊥

School of Physics and Optoelectronic Engineering, Nanjing University of Information Science

& Technology, Nanjing 210044, China. #

Nanomaterials Center, School of Chemical Engineering and Australia Institute for

Bioengineering and Nanotechnology, University of Queensland, Brisbane, Queensland 4072, Australia

ACS Paragon Plus Environment

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ABSTRACT. Element doping has been extensively attempted to develop visible-light-driven photocatalysts, which introduces impurity levels and enhances light absorption. However, the dopants can also become recombination centers for photogenerated electrons and holes. To address the recombination challenge, we report a gradient phosphorus-doped CdS (CdS-P) homojunction nanostructure, creating an oriented built-in electric-field for efficient extraction of carriers from inside to surface of the photocatalyst. The apparent quantum efficiency (AQY) based on the cocatalyst-free photocatalyst is up to 8.2% at 420 nm while the H2 evolution rate boosts to 194.3 µmol∙h-1∙mg-1, which is 58.3 times higher than that of pristine CdS. This concept of oriented built-in electric field introduced by surface gradient diffusion doping should provide a new approach to design other types of semiconductor photocatalysts for efficient solar-tochemical conversion.

KEYWORDS. Hydrogen evolution, CdS, homojunction, gradient doping, built-in electric-field

To ease the ever-increasing global energy crises, photocatalytic conversion of solar energy to fuels is attracting much attention from research community.1,2 Since photocatalytic water splitting on TiO2 electrodes was reported,3 H2 production from water has been considered to be a promising route for efficient conversion of solar energy to chemical energy. However, low solarto-hydrogen efficiency (STH) of UV-light-responsive semiconductors gives vitality to research focused on the development of visible-light-responsive semiconductors with high theoretical STH.4


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Metal sulfide has been proved to be a potential candidate for visible-light-driven photocatalysts due to their low cost and excellent light responses.5-7 In order to further improve the performance, cation doping in metal sulfide is employed to enhance visible light absorption. It has been reported that metal dopants will introduce impurity levels below the conduction band (CB) via the d-orbitals hybridization of Cd, Zn and the metallic impurities.8 Nevertheless, cationic defects also play the role of trapping sites for photo-induced electrons, hindering the reduction of protons. For this reason, metal elements doped CdS or ZnxCd1-xS barely bring a noteworthy improvement for H2 production.9,10 To improve the photocatalytic activity, surface Ba-doped Cd0.8Zn0.2S photocatalyst was intentionally designed to create electron trapping sites centered on the surface of the catalyst obtaining a 2-fold enhancement for H2 evolution.11 By comparison, almost no work focused on the non-metal doped CdS for H2 evolution though non-metal doping significantly affects the electrical properties of CdS.12,13 Meanwhile, metal phosphides were reported to be one of the best catalysts for the electrochemical H2 generation. The moderate free energy of H2 adsorption and corrosion resistance can be introduced via doping P atoms into the lattices of the transition metal.14 Moreover, a well-designed multilayer coreshell structure with specific “quantum well” potential structure was demonstrated to be a bright and stable fluorescent material.15 Band offsets between the core and shell regions are sufficiently high so that carriers are confined into the core region and kept separated from the surface.15-16 Inspired by this ingenious design, taking the opposite approach, we present gradient P-doped CdS nanorods (NRs) as cocatalyst-free homojunction photocatalysts with remarkably high activity for H2 evolution. Our detailed study revealed a unique gradient P-doping structure (an “anti-quantum well” band structure) in the material which generates a built-in oriented electric field to facilitate the charge extraction and subsequent photocatalytic reactions.


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Figure 1. (a) XRD patterns and (b) Diffuse reflection spectra of CdS, CdS-P1 and CdS-P5. TEM images of (c) CdS and (d) CdS-P1. HRTEM images of (e) CdS and (f) CdS-P1. (g) Atomicresolution aberration-corrected HAADF-STEM image the corresponding model of the CdS-P1. (h) EDX-STEM elemental mapping images of CdS-P1 NRs.

The CdS NRs were synthesized according to a reported procedure.17 Phosphine (PH3) gas was chosen to be the source of dopants which are rapidly decomposed from sodium hypophosphite


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(NaH2PO2·H2O) when heated over 200℃. By varying the mass ratio of NaH2PO2·H2O to CdS from 0.5 to 1, 3 and 5, four types of CdS-P homojunction (named as CdS-P0.5 to CdS-P5) with various compositions were obtained. Figure 1a shows the XRD patterns of catalysts indexed to the P63mc space group of wurtzite CdS. The inset in Figure 1b displays the enlarged peaks corresponding to (100), (002) and (101) planes of CdS. With the increase of P amount, the three peaks obviously shift to a small angle, owing to the expansion of the lattice. The detailed lattice parameters calculated from XRD data are showed in Table S1. UV−visible diffuse absorption spectra of all samples present optical absorption in the visible region (Figure 1b). The color of catalyst solutions changes from bright yellow to earthy yellow with the increase of dopant (Figure 1b inset). It indicates the enhanced visible light absorption for P-doped CdS NRs. As displayed in Figure S1a~b, the absorption trailing below 2.4 eV indicates that the increase of doping defects from CdS-P1 to CdS-P5. According to the previous research, the slightly redshifted absorption has been interpreted as evidence of band trailing effects related to the disorder induced by impurity insertion into the CdS lattice.18 SEM (Figure S1c~d) and TEM (Figure 1c~f) images display the catalysts with the average diameter of ~60 nm and the length of 1~2 µm. After the P elements doping, the typical lattice distances measured for the CdS-P1 are still ~0.338 nm, which correspond well with lattice distances of (002) crystal face. Elemental mapping and a subtle linear scan atomic composition of CdS-P1 (Figure 1h) displays that Cd element concentrates at a narrow region while S and P element uniformly distribute on the surface of CdS-P1, suggesting the non-metal termination feature. Some parts (red loop in Figure 1f) on surface of CdS-P1 even become amorphous while most areas match well with the lattice patterns with wurtzite CdS (Figure 1g). This may be caused by the P species anchored on the


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surface. However, the P dopants have no evident influence on the crystal structure of CdS and introduce no heterogenous species.

Figure 2. (a) The H2 evolution rates on CdS-P0.5~5 at room temperature under visible light (λ > 420 nm). (b) Comparison of H2 evolution rates between pure CdS, 0.5%Pt/CdS and P-doped CdS NRs. (c) The time courses of H2 evolution and AQY on CdS-P1 under monochromatic 420 nm light irradiation. (d) Cycling runs of CdS-P1 for photocatalytic H2 evolution. After every 3 hours, the produced H2 is evacuated. (e) Possible carriers transfer in P-doped CdS NRs leading to efficient photocatalytic H2 evolution.

Photocatalytic H2 evolution was performed at room temperature under visible light (λ > 420 nm) illumination. Figure 2a displays the H2 evolution rates of P-doped CdS NRs with different P concentrations. The average H2 evolution rates of pure CdS and 0.5%Pt/CdS were compared in Figure 2b. Only a little amount of H2 (3.33 µmol∙mg-1∙h-1) is obtained when pure CdS alone is used as the photocatalyst, revealing that pristine CdS is not active for H2 evolution. Lack of


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active sites and high recombination rate of carriers are responsible for the low acitivity.19,20 When coupled with 0.5wt % Pt, the compound achieves a considerable enhancement of 6 times. Though Pt is an acknowledged efficient cocatalyst, Pt-CdS photocatalyst here displays much lower activity than P-doped CdS for H2 evolution. The sample CdS-P1 boosts to a rate of 194.3 µmol∙h-1∙mg-1, which is 58.3 and 9.7 times of pure CdS and 0.5%Pt/CdS, respectively. Surprisingly, the H2 bubbles generated from the solution can be observed by the naked eyes (Figure S1e and Video). Nevertheless, CdS-P5 get a relative low H2 evolution rate, suggesting the moderate doping is imperative for achieving remarkable activity. AQY for CdS-P1 at discrete wavelengths was determined in Figure S2. The highest AQY is obtained at 420 nm, under which the H2 evolution rate is 35.8 µmol∙h-1∙mg-1 with AQY > 7.90% after 2 hours (Figure 2c). Figure 2d shows the durability of CdS-P1 for H2 evolution. After six runs, no significant decrease for H2 evolution can be observed during the long time irradiation under visible light, revealing the good stability and photo-corrosion resistance. We surmise the possible transfer ways of carriers in Pdoped CdS NRs. With the P dopants abundant on the surface of catalysts, a large number of defects could play the role of trapping sites which promote efficient charge transfer from inside to surface. The extraction of carriers may lead to the enhanced photocatalytic H2 evolution.


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Figure 3. (a) XPS spectra of P 2p for CdS-P1 by Ar+ beam etching for 0, 3, 6 and 15 nm. (b) Elements depth distribution recorded by Ar+ beam etching. (c) VB spectra of CdS-P1 by Ar+ beam etching for 0, 3 and 6 nm. (d) Mott-Schottky plots of CdS and CdS-P1 film in the dark at frequencies of 250 (circle) and 500 Hz (triangle) with a three-electrode system.

To gain insight into the intrinsic reason for the excellent performance of P-doped CdS NRs, the electronic band structure of catalysts was investigated in details. Based on above structural analysis, the DFT calculation was carried out to predict properties of catalysts (Details are provided in the Supporting Information Figure S3~S6, Table S2~S3). Calculation results indicate the lengths of Cd-S bonds in P-doped models are slightly larger than those in undoped models (Figure S3), leading to the expansion of the crystal lattice. P-doped models present a narrower


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band-gap due to the upshift of VB by the hybridization of S 3p and P 3p states (Figure S4~S5), corresponding to the absorption trailing after 520 nm in Figure 1b. It indicates that P element was successfully doped into CdS, changing the band structure and light absorption. Further investigation of the electron density difference (Figure S6) showed that more electron transfers occur between doped P and Cd due to the more positive electronegativity of P than S, suggesting the strong covalent interactions between electron cloud of P and Cd atoms. The XPS spectra (Figure S7) depict two peaks at 404.9 and 411.6 eV for Cd 3d5/2 Cd 3d3/2, together with 161.2 and 162.4 eV for S 2p3/2 and S 2p1/2, which are the typical character of pure CdS. The spectrum of P 2p shows a single peak at 133.5 eV for CdS-P1, probably in corresponding to P element on the surface, which is consistent with the previous work.13 However, for CdS-P5, the spectrum of P 2p presents three peaks at 133.5, 130.6 and 129.8 eV, respectively (Figure S8). The last two peaks are agreed with the previous study on Cd3P2,21 which might result in the decrease of H2 evolution rate of CdS-P5. To further confirm the distribution of P dopants in the near surface of CdS-P1 NRs, XPS spectra in depth were performed by Ar+ beam etching (Figure 3b and Table S4). Three different independent areas of CdS-P1 were detected by Ar+ beam etching (Figure S9). As the detection depth increases, the signal of P 2p sharply decreases, which is the mirror image of S 2p. In contrast, the concentration change of Cd 3d is relatively more stable than anions, suggesting the substitution of P in place of S. Importantly, the gradient distribution of nonmetal elements provides the basis for constructing a built-in electric field. The VB XPS pattern of CdS-P1 (Figure 3c) before etching shows the onset at 1.41 eV. After Ar+ beam etching, the valence band maximum (VBM) shifts to a higher region at 1.52 and 1.58 eV for 3 and 6 nm etching, respectively. It indicates that the VBM of


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surface layer in P-doped CdS is cathodic-shifted by about 0.11 and 0.17 eV compared to inlayer (approximate undoped CdS). That is, the potential distribution of VB can be identified. The flatband potential (Vfb) and carrier density at photocatalyst/electrolyte interface can be quantified by the Mott-Schottky (M-S) equation (Table S5).22 Vfb value is the difference between Fermi level (Ef) and water-reduction potential. M-S plots collected from pure CdS and CdS-P1 film samples are presented in Figure 3d. The Vfb value of pure CdS is found to be -0.85 V, indicating an n-type semiconductor. Meanwhile, Vfb value of CdS-P1 is found to be -0.94 V, proving the formation of n-n+ homojunction between inlayer and surface of CdS-P1. Furthermore, after exposed to light, related Ef of CdS-P1 equilibrates and additional band bending between inlayer and surface generates. As a consequence, a built-in oriented electric field is established for extraction of carriers. In addition, the electron density of CdS-P1 is calculated to be 3.74 × 1020 and 2.85 × 1020 cm-3 at the frequency of 250 and 500 Hz, respectively, nearly 2 times more than pure CdS. Due to donor doping of P atoms, the increase in carrier density is expected.12 As is known to all, the polarization curves can be employed to study the charge transfer processes between surface of catalysts and electrolyte solution. Photoelectrochemical behavior of CdS, Pt/CdS, PdS/CdS, and Pt– PdS/CdS has been studied by Li’s group in detail.23,24 The cocatalysts, Pt and PdS, make a critical difference on the cathodic current density. Before talking over how the built-in electric field affects the activity of photocatalytic H2 evolution, we need to quantify the contribution of catalytic activity change after P-doping. So, the polarization curves are displayed in Figure S10a to compare the catalytic activity of CdS, CdS-P1, CdS-P5 and CdS-Pt (or 0.5%Pt/CdS). The obvious increase of the cathodic current of CdS-Pt is an indication that the cocatalyst Pt on CdS can catalyze proton reduction. However, it shows a similar change on the cathodic current with the polarization


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process, which is much lower than CdS-Pt. Conclusively, the (electro)catalytic activity did not change much after P-doping before the generation of band alignment. However, photocurrent intensity (Figure S10b) significantly increased for CdS-P1 and CdS-P5 compared with pristine CdS when exposed to light irradiation. This means the photo-induced band alignment plays an important role in the enhancement of photocatalytic activity which will be discussed in the following.

Figure 4. (a) Photoluminescence spectra of CdS and CdS-P1 at an excitation wavelength of 405 nm. (b) Energy Fluorescence emission decay spectra of CdS and CdS-P1 monitored at 500 nm (inset: 700 nm) by time-correlated single photon counting. The powder samples in a quartz cell were excited by 405 nm laser in air at room temperature.

The recombination of electron–hole pairs in semiconductors leads to photoluminescence (PL), the intensity of which reveals the recombination rate of carriers. Room temperature PL spectra in the solid-state are displayed in Figure 4a. The CdS and CdS-P1 samples consist of a sharp emission band at 505 nm and a broad emission band centered at 680~700 nm, attributed to the


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band-gap emission and the radiative transition resulting from defect states, severally. After P implantation, the band-gap emissions decrease, demonstrating a low recombination rate of carriers. In contrast, defect states emission become stronger and broader. It indicates that the defect states simultaneously become more abundant. Based on the above analysis of the band structure, photon-induced carriers can be extracted out to the surface, profiting from the increase of P-related defects located on the surface. There is a similar change for CdS-P5 sample in Figure S11a and a detailed discussion can be found in Supporting Information. In consequence, the recombination of electron-hole pairs occurs on the surface where defects are abundant. When the photocatalyst is used for H2 evolution in solution, the electrons on the surface can easily transfer to protons or water molecules. This deduction is proved by the solution PL spectra in Figure S11b. When CdS-P1 powders are dispersed in water, the relative intensity of the peak at 600~850 nm decreases compared with solid-state PL. Furthermore, when the powders are dispersed in the solution containing 0.1 M Na2S/Na2SO3 as hole scavenger, the peak at 600~850 nm almost disappears. When further adding electron capture agent, methyl-viologen (MV2+), into the above solution, the peak completely disappears. So, the broad PL peak at 600~850 nm is confirmed to be related to the surface defect states and can be quenched by sacrificial agents in water.25,26 To further verify the lifetime of photo-induced charge carriers, the time-resolved PL spectra were obtained (Figure 4b and Figure S11c). The decays at 505 nm and 700 nm stay the orderliness of CdS-P1 ≈ CdS-P5 > CdS and CdS-P1 > CdS-P5 > CdS, respectively. It demonstrates that the carriers trapped at surface defect states can stay excited for a longer time. As we know, in single photocatalyst system without cocatalysts, if the lifetime of photogenerated charge carriers is longer, the carriers will more likely transfer to electron acceptor for the redox reaction.27


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Figure 5. (a) The band structure evolution for the formation of homojunction. (b) Density of state of the simulated P-doped CdS system. (c) Schematic energy level and exciton transfer pathways in CdS-P homojunction with a gradient distribution of P element from inner to surface of the catalyst.

Based on the above results, the schematic energy levels and exciton transfer pathways in CdSP homojunction are illustrated in Figure 5. After P doping, the VBM of P-doped parts rises due to the orbital hybridization of P 2p and S 2p. Meanwhile, the donor doping with P raises the Ef. After the exposure to light, Ef equilibrates and band bends owing to the generation and migration of carriers, forming a n-n+ homojunction with unique band structure. As a consequence, the


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photoexcited carriers will be extracted by the built-in oriented electric field to the surface defect sites, where the water molecules can be reduced to H2 by photo-induced electrons (Cd-sites) while the sacrificial agents are oxidized by photo-induced holes (P-sites). In addition, the photocorrosion is suppressed because the S sites are “hidden” inside. As is known to all, in traditional “type Ⅱ” heterojunction, the conduction band of one semiconductor is lower than the other. However, the valence band of one semiconductor has higher value than the other. As a result, excited electrons can move from one semiconductor 2 to 1, although generated holes migrate vice versa. This type of heterojunction was considered to be the most efficient one and had been widely studied.28 However, it is inapplicable in this case, the “core-shell”-like homojunction. If the holes were spatially confined in the “core” (undoped CdS), photocorrosion will be unavoidable. The remove of holes will become “rate-determining step” ultimately.29 On the contrary, the confinement of electrons in the “core” will restrain the proton reduction reaction. So, it is beneficial for both proton reduction and hole elimination if the energy band align like what we have described and demonstrated above. In summary, a diffusion doping method for improving the photocatalytic activity of metal sulfide semiconductors via gradient P-doping was demonstrated. Our technique avoids the random distribution of defects resulted from the simple and inefficient elements doping. The combined study of experiments and DFT calculations clarifies that a smooth built-in oriented electric field, which is based on an “anti-quantum well” band structure, is established all for extraction of charge carriers. The activity for photocatalytic H2 evolution achieves enormous enhancement profiting from the enrichment of surface defects. The H2 evolution rate is among the highest reported to date based on CdS without any cocatalyst or heterojunction. We hope this


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concept of oriented built-in electric field could shed light on the rational design of other semiconductors used in solar-to-chemical conversion.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. Experimental procedures and additional data (PDF) Video for H2 bubbles generated from photocatalytic solution (AVI)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT Natural Science Foundation of Jiangsu Province (No. BK20141459), Project on the Integration of Industry, Education and Research of Jiangsu Province (No. BY2015005-16, BK20150919), Key University Science Research Project of Jiangsu Province (No. 15KJB430022), Qing Lan


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Project, Six Talent Peaks Project in Jiangsu Province (No. XCL-029) and Priority Academic Program Development of the Jiangsu Higher Education Institutions (PAPD) is gratefully acknowledged.

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(24) Yang, J.; Yan, H.; Wang, X.; Wen, F.; Wang, Z.; Fan, D.; Shi, J.; Li, C. J. Catal. 2012, 290, 151. (25) Halivni, S.; Sitt, A.; Hadar, I.; Banin, U. ACS Nano 2012, 6, 2758. (26) Liu, I. S.; Lo, H. H.; Chien, C. T.; Lin, Y. Y.; Chen, C. W.; Chen, Y. F.; Su, W. F.; Liou, S. C. J. Mater. Chem. 2008, 18, 675. (27) Wang, X.; Liu, G.; Chen, Z.; Li, F.; Wang, L.; Lu, G. Q.; Cheng, H. Chem. Commun. 2009, 23, 3452. (28) Gholipour, M. R.; Dinh, C. T.; Béland, F.; Do, T. O. Nanoscale 2015, 7, 8187. (29) Wu, K.; Chen, Z.; Lv, H.; Zhu, H.; Hill, C. L.; Lian, T. J. Am. Chem. Soc. 2014, 136, 7708.


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Nano Letters

For TOC only

Gradient P-doped CdS nanorods as cocatalyst-free homojunction photocatalysts show remarkably high activity for H2 evolution. A unique gradient P-doping structure in the material generates a built-in oriented electric field to facilitate the charge separation and subsequent photocatalytic reactions.


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Oriented Built-in Electric Field Introduced by Surface Gradient

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