xClx perovskite single crystals

Journal of

Materials Chemistry C View Article Online

PAPER

Published on 22 July 2017. Downloaded on 18/09/2017 11:16:14.

Cite this: J. Mater. Chem. C, 2017, 5, 8431

View Journal | View Issue

Extra long electron–hole diffusion lengths in CH3NH3PbI3xClx perovskite single crystals† Fengying Zhang,ab Bin Yang,bc Yajuan Li,bc Weiqiao Deng Rongxing He *a

*bc and

Long electron–hole diffusion lengths in organolead trihalide compounds play a key role in achieving the remarkable performance of perovskite photovoltaics. Diffusion lengths in solution-grown CH3NH3PbI3 single crystals have been found to be greater than 175 micrometer (mm). Herein, we report the diffusion lengths in CH3NH3PbI3xClx single crystals exceeding 380 mm under 1 Sun illumination, which is twice that Received 23rd June 2017, Accepted 21st July 2017

in CH3NH3PbI3 single crystals. Incorporation of chlorine is found to increase the density of trap-states and

DOI: 10.1039/c7tc02802d

charge transfer, respectively, are in a competing relation. As a result, the electron–hole diffusion lengths in

rsc.li/materials-c

This study provides a strategy for the design of perovskite optoelectronics.

reduce the valence band level; these two factors, which dominate the carrier recombination and the a CH3NH3PbI3xClx single crystal with an optimum Cl proportion (x = 0.005) reach the maximum values.

Introduction Organolead trihalide perovskite solar cells have reached over 22% power conversion efficiency (PCE).1,2 The remarkable PCE is closely related to the long carrier diffusion lengths in perovskite absorbers.3 CH3NH3PbI3xClx thin films have been developed to further improve the PCE, as it has longer carrier diffusion lengths compared with those in CH3NH3PbI3.3,4 The mechanism of enhanced optoelectronics is still a subject of debate.5–10 One point of view is that Cl affects the morphology of perovskite, in which Cl ions may enhance the formation of high-quality perovskite films by controlling the nucleation and growth of perovskite crystals without occupying the lattice.8,9 Another possibility is that Cl ions get incorporated into the crystal lattice and improve the crystallography of perovskite, and thus, they have a significant influence on the carrier recombination and transfer processes.10 All of these arguments are based on CH3NH3PbI3xClx thin films. However, a thin-film sample is not a good candidate for studying the intrinsic mechanism because it is difficult to separate the components, which are incorporated into the crystal lattice, from the improved morphology and crystallography thus obtained.10

A single crystal is a good candidate to study the intrinsic properties of materials such as the trap-states, carrier mobilities and carrier diffusion lengths, because they have free grain boundaries.11–17 Recently, CH3NH3PbI3(Cl) single crystals with low tap-state density have been synthesized.18 However, the function of Cl ions is still an open question, as they are present in trace amounts.18 Herein, we synthesized large-sized CH3NH3PbI3xClx (x = 0, 0.002, 0.005, 0.008) single crystals and quantitatively analyzed the Cl content by using ion chromatography method. We studied the effect of Cl on trap-state density, charge carrier lifetime and energy band structure in CH3NH3PbI3xClx single crystals with varying Cl content. It is found that these crystals exhibit lower trap density and longer carrier diffusion length at low Cl content, whereas a large amount of Cl is counterproductive. Combining the experimental results with density functional theory (DFT) calculation, we found that Cl incorporation can lower the valence band level and facilitate the charge injection. Consequently, CH3NH3PbI3xClx single crystals with optimum Cl content (x = 0.005) exhibit the best performance, and the electron–hole diffusion length can exceed 380 mm, which is twice that in CH3NH3PbI3 single crystals.

a

Key Laboratory of Luminescence and Real-Time Analytical Chemistry (Southwest University), Ministry of Education, College of Chemistry and Chemical Engineering, Southwest University, Chongqing 400715, China. E-mail: [email protected] b State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Science, Dalian 116023, China. E-mail: [email protected] c University of the Chinese Academy of Sciences, Beijing 10049, China † Electronic supplementary information (ESI) available: Synthesis and characterization of CH3NH3PbI3-xClx single crystals. See DOI: 10.1039/c7tc02802d

This journal is © The Royal Society of Chemistry 2017

Experimental Synthesis of CH3NH3I Methylammonium iodide (CH3NH3I) was synthesized by stirring a mixture of 40 mL of methylamine and 20 mL of HI in a flask that was placed in an ice bath at 0 1C for 2 h under argon atmosphere. CH3NH3I that was obtained as a solvent was carefully removed

J. Mater. Chem. C, 2017, 5, 8431--8435 | 8431

View Article Online

Paper

using a rotating evaporator at 60 1C. The white CH3NH3I powder was washed with isopropanol. The final product was collected by filtration and dried overnight in a vacuum oven at 70 1C.


Synthesis of single crystals and fabrication of PV devices To synthesize CH3NH3PbI3 single crystals, 1.59 g CH3NH3I and 4.60 g PbI2 were dissolved in 10 mL g-butyrolactone (GBL) at 60 1C. The resulting clear solution was heated to a temperature of 110 1C in an oil bath. A few seeds were formed initially. Further growth of the crystal was achieved by carefully removing the crystal and placing it in a fresh 1 M precursor solution. The large-sized CH3NH3PbI3 single crystals were obtained within 24 h. The mixed perovskite CH3NH3PbIxCl3x single crystals were synthesized using the same method, except for the addition of a mixture of different ratios of PbCl2. For example, for I : Cl = 15 : 1, 1.59 g CH3NH3I, 4.60 g PbI2 and 0.28 g PbCl2 were dissolved in 10 mL GBL. For the fabrication of PV devices from the single crystals, a 25 nm-thick Au anode was deposited by thermal evaporation on one surface of the single crystals. Metallic melted Ga was blade-coated onto glass substrates at 50 1C, and the single crystals were transferred onto the liquid Ga for contacting. Then, the substrates were cooled to below 20 1C to solidify the Ga. Measurement and characterization XPS measurements were carried out using Thermo ESCALAB 250Xi. The single crystals were fixed on the specimen disc with a special ultra-high vacuum conductive adhesive. The excitation source was a monochromatized Al Ka source. For ion chromatography measurements, the single crystals were peeled off piece by piece and then dissolved in deionized water and sulfuric acid (0.15 M). The perovskite single crystal was dissolved in an acidic aqueous medium. The obtained solutions were then injected into the Dionex ISC 100 ion analyzer. The concentration of all the anions was determined by matching their curves with the calibration curves obtained from standard solutions of known concentrations. Herein, standard solutions of KI and NaCl were used to obtain the concentrations of I and Cl, respectively. Powder X-ray diffraction was performed on the X’pert PRO diffractometer equipped with Cu Ka X-ray (l = 1.54186 Å) tubes. The single crystal powders were made by grinding each piece of the single crystal to fine powder in a mortar. Steady-state absorption spectra were recorded at room temperature on the JASCO V-550 UV-Vis absorption spectrometer with an integrating sphere attachment operating in the 190–900 nm region. PL spectra were recorded by laser scanning confocal microscopy (LSCM) of FV10000MPE from Olympus. The impedance spectroscopy (IS) spectra were recorded by Princeton Applied Research VersaSTAT 3, under one Sun illumination, and the simulative sunlight was produced by Abet Technologies Sun 2000. The equivalent circuit used for the curve fitting is provided in the ESI.† It was shown to fit the impedance spectra of the perovskite single crystal devices at different voltage bias and different light bias very well.12 Rinternal and Cgeometry are attributed to the bulk properties of the PV devices. Rrec and Cm are the recombination resistance

8432 | J. Mater. Chem. C, 2017, 5, 8431--8435

Journal of Materials Chemistry C

and the chemical capacitance, respectively, which are associated with the internal charge transfer dynamics in the PV devices. In addition, the recombination lifetime equals the reciprocal of the product of Rrec and Cm. Rseries is the series resistance consisting of the contact resistance of the PV device and electrode, as well as the PV device and LCR meter. The current as a function of the applied voltage was measured with Keithley 2400, using a rather simple geometry with two electrodes on opposite sides of the sample with the sample kept in the dark. Ohmic contacts were deposited on opposite sides of the sample by the consecutive thermal evaporation of Au. A nonlinear response was observed and analyzed according to the SCLC theory.12,13 A I–V curve of perovskite crystals exhibits different regions obtained from the log I versus log V plots. The regions are marked for ohmic (I p V n=1), TFL (I p V n43) and Child’s regime (I p V n=2). The trap density was obtained according to ntraps = 2ee0VTFL/qL2, where q is the elemental charge, L denotes the film thickness, VTFL represents the onset voltage of the trap-filled-limit (TFL) regime, and e and e0 are the crystal dielectric constant and vacuum permittivity, respectively; herein, we use e = 25.5. When operating in Child’s regime, the dark current of the PV device was well fitted by the Mott–Gurney Law: JD ¼

9ee0 mVb2 8L3

where Vb is the applied voltage. In addition, the surface potential measurement was performed using the Veeco MultiMode 3d AFM with SCM-PIT Pt/Ir-coated tips, 2.8 N m1 75 kHz, Pt/Ir reflective coating. This is a multifunction measuring instrument produced by BRUKER (formerly VEECO company). Density functional theory (DFT) calculation Both geometric optimizations and electronic structure calculations were performed on the basis of the Kohn–Sham formalism of the density functional theory, implemented in the Vienna ab initio package (VASP).19,20 The projector augmented wave method was applied to describe the interaction between an electron and an ion.21 It has been used successfully to predict the total-energy-related properties of various electron systems, such as crystal structures and molecular geometries. For the exchange–correlation energy functional, the generalized gradient approximation (GGA) was used in the Perdew–Burke–Ernzerhof (PBE) scheme, and the kinetic energy cutoff was taken to be 500 eV.22 Lattice constants and atomic positions were calculated until the total energies converged below 104 eV per atom for the forces acting on each atom that was less than 0.02 eV Å1. No symmetry constraint conditions were applied in the process of structure optimization. In our cases, we chose the tetragonalphase CH3NH3PbI3 as the research subject, which is the actual phase in the solar cell at room temperature. A Monkhorst–Pack k-point of 6 6 4 was chosen to guarantee the convergence. Though the spin–orbit coupling (SOC) is considered to be very important in the calculation of band structure, DFT-GGA calculations without SOC considerations can capture the semiquantitative behavior, yielding the correct trends for the band gap.


View Article Online


The optimized lattice constants of CH3NH3PbI3 are a = 9.07 Å, b = 9.00 Å and c = 12.91 Å, and the band gap of CH3NH3PbI3 without Cl-doping is 1.65 eV, both of which agree well with the experimental results.23


Results and discussion Four large-sized single crystals were synthesized using the inverse temperature method:24 CH3NH3PbI3 (hereafter referred to as I) and CH3NH3PbI3xClx with different I : Cl precursor ratios of 30 : 1, 15 : 1 and 10 : 1 were synthesized for exploring the effect of Cl on trap-state density, charge carrier lifetime and energy band structure in CH3NH3PbI3xClx single crystals with different Cl contents. Fig. 1a shows the tetragonal crystal structure of CH3NH3PbI311,12 or CH3NH3PbI3xClx with low Cl concentration.7,10 Cl was not visible in X-ray photoelectron spectroscopy (XPS, Fig. S1, ESI†) either due to its low content in the CH3NH3PbI3xClx (I : Cl = 30 : 1, 15 : 1 in precursor) single crystals, or because Cl was evaporated under the strong X-ray illumination.18,25 Ion chromatography was used to quantify the amount of Cl ions in single crystals (Fig. S2 and S3, ESI†).26 The content of Cl in CH3NH3PbI3xClx single crystals was proportional to the initial precursor (Fig. 1b). In single crystals, the resultant molar ratios of I to Cl were determined to be 1300 : 1 (x = 0.002), 600 : 1 (x = 0.005) and 375 : 1 (x = 0.008) (hereafter referred to as 1300 : 1, 600 : 1 and 375 : 1). The tetragonal phase of these samples was confirmed by X-ray diffraction (XRD) (Fig. 1c).11,12 The samples showed similar diffraction peaks at 14.21, 28.31 and 42.91 corresponding to the (110), (220) and (330) reflections, respectively. Diffraction peaks tended to move

Fig. 1 (a) Crystal structure of CH3NH3PbI3xClx. (b) The molar ratio in CH3NH3PbI3xClx single crystals determined by ion chromatography (inset: photo of the CH3NH3PbI3xClx single crystal). (c) XRD profiles corresponding to CH3NH3PbI3xClx single crystals. (d) Enlarged XRD profiles around 141. (e) Steady-state absorption and PL (lex = 532 nm) spectra of CH3NH3PbI3xClx single crystals.


Paper

to larger angles with the presence of Cl (Fig. 1d). This indicated that the lattice constants decreased because of Cl incorporation. The doping of Cl ions was further confirmed by steady-state absorption and photoluminescence (PL) spectra (Fig. 1e). For CH3NH3PbI3 single crystal, the calculated band gap is 1.49 eV (Fig. S4, ESI†), and the PL peak is located at 765 nm, which is consistent with the previous report.11,12 The absorption and PL spectrum were slightly blue-shifted in CH3NH3PbI3xClx crystals (Fig. 1e).6 To investigate the bulk trap density, dark current–voltage (I–V) traces were measured based on the space-charge-limited current (SCLC) method.11,12 Single crystals were sandwiched between two Au electrodes to form hole-only devices (insert of Fig. 2a and Fig. S5, ESI†). Three regions can be identified from the I–V trace (Fig. 2a): n = 1 is the ohmic region, n = 2 is the Child region, and in between is the trap-filled limited region. From ntraps = 2ee0VTFL/qL2, the CH3NH3PbI3 single crystal exhibits a trap-state density of (1.3 0.1) 1010 cm3. To prove that our results are reliable, we have listed the trap-state densities and carrier lifetimes (discussed below) of CH3NH3PbI3 single crystals from different measuring methods (Table S1, ESI†).11,12,18,24,27,28 Our results are consistent with the previous report. Furthermore, CH3NH3PbI3xClx single crystals exhibit trap densities of (1.1 0.3) 1010 cm3, (1.2 1.0) 1010 cm3 and (3.4 1.5) 1010 cm3 for 1300 : 1, 600 : 1 and 375 : 1, respectively, as shown in Fig. 2b. It is worth noting that CH3NH3PbI3xClx single crystals show lower trap-state density at low Cl proportion (1300 : 1, 600 : 1), which can be attributed to the improvement of crystalline quality.18 However, at a higher Cl proportion (375 : 1), the crystals exhibit a higher trap density, which can well explain the previously reported results in the solar cell: a low concentration of Cl is helpful to improve the PCE, whereas a high concentration Cl is detrimental to the device performance.10 In order to further investigate the carrier recombination and transfer dynamics, we fabricated these single crystals into PV

Fig. 2 (a) I–V traces and trap-state density. The regions are marked for ohmic (n = 1) and Child’s regime (n = 2). (b) Calculated trap-state densities (blue) and charge carrier mobility (red) for I, 1300 : 1, 600 : 1 and 375 : 1, respectively. (c) Schematic device structure of the perovskite single crystal PV devices. (d) IS measurements of the PV devices under one Sun illumination. (e) The lifetimes of the PV devices obtained from IS. (f) The carrier diffusion length of the PV devices.

J. Mater. Chem. C, 2017, 5, 8431--8435 | 8433

View Article Online


Paper

devices (see Fig. 2c and the PV device fabrication section) and measured their performance by impedance spectroscopy (IS). A widely equivalent circuit11,29–31 (Fig. S6, ESI†) was used to analyse the IS results and extract carrier lifetime in the whole device.13 The IS spectra are shown in Fig. 2d, which gives lifetimes of t B 127 12, 152 22, 774 168 and 489 221 ms for I, 1300 : 1, 600 : 1 and 375 : 1, respectively (Fig. 2e). These results were extracted from four nominally identical devices to reduce experimental error. Note that the carrier lifetime in the CH3NH3PbI3 single crystal was 127 12 ms, which is in agreement with the value of 95 8 ms reported in the literature (see Table S1, ESI†).11 Notably, the lifetime in 600 : 1 (774 168 ms) was approximately six times higher than that in I. We have also listed the carrier mobility of CH3NH3PbI3 through different methods (Table S1, ESI†).11,12,18,24,27,28 Using a reasonable mobility value of 70 cm2 V1 s1,18,24 the carrier diffusion length (LD) can be calculated using the formula LD = (kBT/emt)1/2, where kB is Boltzmann’s constant and T is the sample temperature. Combining the respective lifetimes and carrier mobility, LD values of 153 7, 230 8, 380 40 and 300 60 mm were obtained for I, 1300 : 1, 600 : 1 and 375 : 1, respectively, as shown in Fig. 2f. The carrier diffusion length in CH3NH3PbI3 in our measurement (153 7 mm) was in agreement with the diffusion length reported in the literature (175 25 mm).11 This indicates that our experimental approach is reliable. Remarkably, the bulk carrier diffusion length in 600 : 1 sample is more than twice that in I. As discussed earlier, incorporation of Cl into perovskite structures can have an influence on the trap-state density as well as the carrier recombination. Herein, the trap-state density is an intrinsic property of crystals, whereas the carrier lifetimes (in IS measurement) include both bulk recombination and interface charge transfer. The energy band structure at the interface is generally thought to affect the charge separation and transportation.10,32 To further study the charge transfer across the interfaces, the local surface potential in each sample was characterized by Kelvin-probe force microscopy (KPFM). The surface potential decreased with the increasing doping concentration of Cl in the crystals (see Fig. 3, S7, ESI†). Furthermore, the valence band maxima (VBM) can be estimated by photoelectron spectroscopy (see Fig. 3 and Fig. S8, ESI†). These results clearly indicated that Cl incorporation lowers the


Fig. 4 (a) Injection efficiency as a function of the energy gap DE. (b) Schematic illustrating the charge recombination within perovskite single crystals and charge transfer at interfaces.

surface potentials as well as VBM levels. This is in accordance with our DFT calculation. The first doped Cl atom induces changes in both the VBM and the conduction band minimum (CBM). However, the CBM did not change much when the second and third Cl atoms were doped into CH3NH3PbI3 (Fig. S9, ESI†). This is because the valence band (VB) is mainly composed of I atoms and the conduction band (CB) is mainly composed of Pb atoms,33 as observed from the local density of states (LDOS) of CH3NH3PbI3 and CH3NH3PbI3xClx around the Fermi level in Fig. S10 (ESI).† The band structure study demonstrates that the VBM decreased with Cl incorporation, which can affect the charge injection at the interface according to a two-band model.34 The charge injection efficiency as a function of the energy gap DE (the energy gap between the valence band of perovskite and the metal work function) can be expressed as Zinj E (1 + l exp(b DE))1, where l and b are constants; details are stated in ESI.† 34 As a result, the injection efficiency increases with the increasing energy gap, whereas the slope first increases and then decreases,34 as shown in Fig. 4a. Furthermore, the charge injection has shown a much faster behavior than the competing process of carrier recombination and has an influence on the carrier diffusion length.34 We illustrate the overall carrier recombination and charge transfer processes scheme as shown in Fig. 4b. For a low Cl concentration, CH3NH3PbI3xClx (1300 : 1, 600 : 1) single crystals show a lower trap-state density and a higher charge injection efficiency; thus, the CH3NH3PbI3xClx (1300 : 1, 600 : 1) single crystals show longer carrier lifetimes with respect to CH3NH3PbI3. However, at high Cl concentration (375 : 1), the crystal shows a higher trap-state density, whereas the injection efficiency may reach the situation value (Fig. 4a) and no longer be sufficient to compensate for the losses due to trap states. Thus, the carrier lifetime may decrease.

Conclusions

Fig. 3 KPFM characterization (red) and VBM value (blue) of perovskite single crystals.

8434 | J. Mater. Chem. C, 2017, 5, 8431--8435

In summary, Cl incorporation into CH3NH3PbI3 can simultaneously change the trap-state density and decrease the VBM. The charge recombination and transfer processes are affected as well. As a result, the carrier diffusion length shows Cl concentration dependence. CH3NH3PbI3xClx single crystals with optimum Cl content (x = 0.005) exhibit the best performance and the electron–hole diffusion length can exceed 380 mm, which is twice that in CH3NH3PbI3 single crystals.


View Article Online


Acknowledgements We would like to acknowledge S. Wang for the help of ion chromatography measurement. We are grateful to the National Natural Science Foundation of China (Grant No. 21525315, 91333116, and 21173169) for their financial supports.


Notes and references 1 C National Renewable Energy Laboratory, Best Research-Cell Efficiencies, www.nrel.gov/ncpv/images/efficiency_chart.jpg. 2 A. Kojima, K. Teshima, Y. Shirai and T. Miyasaka, J. Am. Chem. Soc., 2009, 131, 6050–6051. 3 S. D. Stranks, G. E. Eperon, G. Grancini, C. Menelaou, M. J. P. Alcocer, T. Leijtens, L. M. Herz, A. Petrozza and H. J. Snaith, Science, 2013, 342, 341–344. ¨tzel, 4 G. Xing, N. Mathews, S. Sun, S. S. Lim, Y. M. Lam, M. Gra S. Mhaisalkar and T. C. Sum, Science, 2013, 342, 344–347. 5 Q. Dong, Y. Yuan, Y. Shao, Y. Fang, Q. Wang and J. Huang, Energy Environ. Sci., 2015, 8, 2464–2470. 6 S. Colella, E. Mosconi, P. Fedeli, A. Listorti, F. Gazza, F. Orlandi, P. Ferro, T. Besagni, A. Rizzo and G. Calestani, Chem. Mater., 2013, 25, 4613–4618. 7 H. Yu, F. Wang, F. Xie, W. Li, J. Chen and N. Zhao, Adv. Funct. Mater., 2014, 24, 7102–7108. ¨tzel and 8 M. I. Dar, N. Arora, P. Gao, S. Ahmad, M. Gra M. K. Nazeeruddin, Nano Lett., 2014, 14, 6991–6996. 9 Y. Tidhar, E. Edri, H. Weissman, D. Zohar, G. Hodes, D. Cahen, B. Rybtchinski and S. Kirmayer, J. Am. Chem. Soc., 2014, 136, 13249–13256. 10 Q. Chen, H. Zhou, Y. Fang, A. Z. Stieg, T.-B. Song, H.-H. Wang, X. Xu, Y. Liu, S. Lu and J. You, Nat. Commun., 2015, 6, 7269. 11 Q. Dong, Y. Fang, Y. Shao, P. Mulligan, J. Qiu, L. Cao and J. Huang, Science, 2015, 347, 967–970. 12 D. Shi, V. Adinolfi, R. Comin, M. Yuan, E. Alarousu, A. Buin, Y. Chen, S. Hoogland, A. Rothenberger and K. Katsiev, Science, 2015, 347, 519–522. 13 G. Maculan, A. D. Sheikh, A. L. Abdelhady, M. I. Saidaminov, M. A. Haque, B. Murali, E. Alarousu, O. F. Mohammed, T. Wu and O. M. Bakr, J. Phys. Chem. Lett., 2015, 6, 3781–3786. 14 Y. Fang, Q. Dong, Y. Shao, Y. Yuan and J. Huang, Nat. Photonics, 2015, 9, 679–686.


Paper

15 M. I. Saidaminov, V. Adinolfi, R. Comin, A. L. Abdelhady, W. Peng, I. Dursun, M. Yuan, S. Hoogland, E. H. Sargent and O. M. Bakr, Nat. Commun., 2015, 6, 8724. 16 Y. Dang, Y. Zhou, X. Liu, D. Ju, S. Xia, H. Xia and X. Tao, Angew. Chem., Int. Ed., 2016, 55, 3447–3450. 17 J. Huang, Y. Shao and Q. Dong, J. Phys. Chem. Lett., 2015, 6, 3218–3227. 18 Z. Lian, Q. Yan, T. Gao, J. Ding, Q. Lv, C. Ning, Q. Li and J.-l. Sun, J. Am. Chem. Soc., 2016, 138, 9409–9412. 19 G. Kresse and J. Furthmuller, Phys. Rev. B: Condens. Matter Mater. Phys., 1996, 54, 11169–11186. 20 G. Kresse and J. Furthmuller, Comput. Mater. Sci., 1996, 6, 15–50. 21 P. E. Blochl, Phys. Rev. B: Condens. Matter Mater. Phys., 1994, 50, 17953–17979. 22 J. P. Perdew, K. Burke and M. Ernzerhof, Phys. Rev. Lett., 1996, 77, 3865–3868. 23 C. C. Stoumpos, C. D. Malliakas and M. G. Kanatzidis, Inorg. Chem., 2013, 52, 9019–9038. 24 M. I. Saidaminov, A. L. Abdelhady, B. Murali, E. Alarousu, V. M. Burlakov, W. Peng, I. Dursun, L. Wang, Y. He and G. Maculan, Nat. Commun., 2015, 6, 7586. 25 N. D. Pham, V. T. Tiong, P. Chen, L. Wang, G. J. Wilson, J. Bell and H. Wang, J. Mater. Chem. A, 2017, 5, 5195–5203. 26 L. Cojocaru, S. Uchida, A. K. Jena, T. Miyasaka, J. Nakazaki, T. Kubo and H. Segawa, Chem. Lett., 2015, 44, 1089–1091. 27 Y. Liu, Z. Yang, D. Cui, X. Ren, J. Sun, X. Liu, J. Zhang, Q. Wei, H. Fan and F. Yu, Adv. Mater., 2015, 27, 5176–5183. 28 Y. Liu, Y. Zhang, Z. Yang, D. Yang, X. Ren, L. Pang and S. F. Liu, Adv. Mater., 2016, 28, 9204–9209. 29 B. J. Leever, C. A. Bailey, T. J. Marks, M. C. Hersam and M. F. Durstock, Adv. Energy Mater., 2012, 2, 120–128. 30 I. Mora-Sero, G. A. Garcia-Belmonte, P. P. Boix, M. A. Vazquez and J. Bisquert, Energy Environ. Sci., 2009, 2, 678–686. 31 Z. Xiao, Q. Dong, C. Bi, Y. Shao, Y. Yuan and J. Huang, Adv. Mater., 2014, 26, 6503–6509. 32 Q. Chen, H. Zhou, T.-B. Song, S. Luo, Z. Hong, H.-S. Duan, L. Dou, Y. Liu and Y. Yang, Nano Lett., 2014, 14, 4158–4163. 33 G. Giorgi, J. Fujisawa, H. Segawa and K. Yamashita, J. Phys. Chem. Lett., 2013, 4, 4213–4216. 34 J. Cai, N. Satoh and L. Han, J. Phys. Chem. C, 2011, 115, 6033–6039.

J. Mater. Chem. C, 2017, 5, 8431--8435 | 8435