TiO2 NRs based Self-powered UV Photodetector: Heterojunction with

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Surfaces, Interfaces, and Applications

TiO2 NRs based Self-powered UV Photodetector: Heterojunction with NiO Nanoflakes and Enhanced UV Photoresponse Yanyan Gao, Jianping Xu, Shaobo Shi, Hong Dong, Yahui Cheng, Chengtai Wei, Xiaosong Zhang, Shougen Yin, and Lan Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b18815 • Publication Date (Web): 20 Mar 2018 Downloaded from http://pubs.acs.org on March 21, 2018

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ACS Applied Materials & Interfaces

TiO2 NRs based Self-powered UV Photodetector: Heterojunction with NiO Nanoflakes and Enhanced UV Photoresponse Yanyan Gao†, Jianping Xu*†, Shaobo Shi‡, Hong Dong§, Yahui Cheng§, Chengtai Wei†, Xiaosong Zhang†, Shougen Yin†, and Lan Li*† †

School of Materials Science and Engineering, Institute of Material Physics, Key

Laboratory of Display Materials and Photoelectric Devices, Ministry of Education, and Tianjin Key Laboratory for Photoelectric Materials and Devices, and National Demonstration Center for Experimental Function Materials Education, Tianjin University of Technology, Tianjin 300384, China ‡

School of Science, Tianjin University of Technology and Education, Tianjin 300222,

China §

Department of Electronics and Tianjin Key Laboratory of Photo-Electronic Thin

Film Device and Technology, Nankai University, Tianjin 300350, China

ABSTRACT: The self-powered ultraviolet photodetectors (UV PDs) have attracted increasing attention due to their potential applications without consuming any external power. It is important to obtain the high performance self-powered UV PDs by a simple method for the practical application. Herein, TiO2 nanorod arrays (NRs) were synthesized by hydrothermal method, which were integrated with p-type NiO nanoflakes to realize a high performance pn heterojunction for the efficient UV photodetection. TiOx thin film can improve the morphological and carrier transport properties of TiO2 NRs, and decrease the surface and defect states, resulting in the enhanced photocurrent of the devices. NiO/TiO2 nanostructural heterojunctions show excellent rectifying characteristics (rectification ratio of 2.52×104 and 1.45×105 for NiO/TiO2 NRs and NiO/TiO2 NRs/TiOx, respectively) with a very low reverse saturation current. The PDs based on the heterojunctions exhibit good spectral selectivity, high photoresponsivity and fast response and recovery speeds without external applied bias under the weak light radiation. The devices demonstrate good stability and repeatability under UV light radiation. The self-powered performance 1

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could be attributed to the proper built-in electric field of the heterojunction. TiO2 NRs and NiO nanoflakes construct the well aligned energy-band structure. The enhanced responsivity and detectivity for the devices with TiOx thin films is related to the increased interfacial charge separation efficiency, reduced carrier recombination and relatively good electrons transport of TiO2 NRs. KEYWORDS: TiO2 nanorod arrays, NiO nanoflakes, surface and defect states, heterojunctions, self-powered photodetector

2


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1. INTRODUCTION High-performance ultraviolet (UV) photodetectors (PDs) with high responsivity and fast response speed are highly desired for applications in many fields, such as environmental monitoring, flame sensing, optical communications and remote control.1-3 In recent decades, various wide band-gap semiconductors including ZnO, GaN, NiO and TiO2 have been utilized in the fabrication of UV PDs because of their excellent physical and chemical properties.4-7 Among them, TiO2 nanostructures such as one-dimensional nanotubes, nanowires and nanorods possess advantageous features for the fabricating highly sensitive UV PDs due to the carrier-oriented transfer and high UV absorption coefficient. However, the UV PDs based on TiO2 nanostructures exhibit a large dark current, a low sensitivity (Ilight/Idark ratio) and low photoresponse speed for the self-recombination of surface and defect states. It is necessary to efficiently improve the separation and inhibit the recombination of photogenerated carriers through special methods, such as the device structure design and the improvement of the crystalline quality of TiO2. The heterojunction structures based on nanostructural TiO2 integrated with other semiconductors such as SnO2, NiO, CdS and MoS2 have been performed.8-11 It has been proved that the build-in electric field of the Schottky junction, pn homojunction and heterojunction provides more efficient charge separation and the separated electrons and holes collection at cathode and anode, respectively, which can lead to the increased number of carriers and prolong the carrier lifetime.12 In addition, the PDs based on the junction structure demonstrate self-powered photoresponse properties based on the photovoltaic behavior characteristics. The self-powered PDs demonstrate high response speed due to the decreased local perturbations of the depletion regions without any power supply, which can be operate wirelessly and independently for saving energy and reducing device size. Among those TiO2-based pn heterojunctions, NiO/TiO2 heterojunctions have attracted considerable interest and have been applied to many fields, such as photocatalysis, optical detection and so on, because NiO possesses strong UV absorption and high hole mobility.13,8 TiO2 and NiO are intrinsically n- and p-type 3



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semiconductors, respectively, and their type II energy band alignments make them promising candidates for the self-powered heterojunction PDs without a external applied bias. The nanostructural NiO/TiO2 heterojunction can be prepared by relatively simple, inexpensive and convenient non-vacuum chemical synthesis method. For examples, TiO2 nanowires decorated with NiO were prepared using a two-step method involving hydrothermal and solvothermal processes.14 Mesoporous NiO/TiO2 nanofibers with excellent catalytic activity were fabricated by an electrospinning and calcination method.15 Nanostructural heterojunction can provide more interface area for the separation of the photogenerated electron-hole pairs and the good electron conductivity remaining in the TiO2 vertical nanorods. Previous studies have indicated that NiO can be considered as one of the most effective hole transfer layer to enhance UV photoresponse speed of the UV PDs based on TiO2 nanorods.8 Nevertheless, the inherent poor conductivity of metal oxide semiconductors inhibits the efficient transfer of excitons, leaving room for further improvement of the UV photoresponse properties of NiO/TiO2 heterojunctions. It is nessesary to effectively control the diameter, length, orientation, density and crystallization of TiO2 nanorods and decrease the trap states of the carriers. In this work, TiO2 nanorod arrays (NRs) were fabricated by hydrothermal synthesis and then covered by NiO nanoflakes using chemical bath deposition (CBD) method. The NiO/TiO2 heterojunctions demonstrate good rectifying characteristics and distinct photovoltaic effect. The additional TiOx thin film on the FTO substrates improve the orientation, density and crystal crystallization, and reduce the surface defect states of TiO2 NRs, which leads to the increased area of heterojunction and the enhanced built-in electric field. The UV PDs based on NiO/TiO2 NRs demonstrate an excellent self-powered photodetection performance under low UV illumination. 2. EXPERIMENTAL SECTION 2.1 Synthesis of TiO2 NRs on FTO and TiOx thin film substrates. Firstly, FTO substrates were sequentially ultrasonically cleaned in deionized water, acetone, isopropyl alcohol and ethanol solutions for 10 min. The TiOx thin films (The layer is thin and the crystal phase was not recognized by XRD, so defined TiOx thin film) 4


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with ~ 60 nm thickness were sputtered on the cleaned FTO substrates by rf-sputter and then thermally annealed in air at 450 °C for 30 min. TiO2 NRs were prepared on FTO and TiOx thin films by a typical hydrothermal method.16 The mixed solution containing 15 mL of deionized water and 15 mL of concentrated hydrochloric acid (HCl, mass fraction 36.5-38%) was stirred for 10 min, then adding 0.5 mL of tetrabutyl titanate (Ti(OC4H9)4) into the solution. After stirred for 30 min, 30 mL precursor solution was transferred into a Teflon-lined stainless steel autoclave with 100 mL volume. The FTO and TiOx thin film substrates facing down were placed in the autoclave. The sealed autoclave was kept at 180 °C for 3 h in a drying oven and then naturally cooled to room temperature. The TiO2 NRs were rinsed with deionized water and alcohol for several times and dried in ambient air. Finally, the samples were thermally annealed in air at 450 °C for 2 h to crystallize. TiO2 NRs grown on TiOx thin film were denoted TiO2 NRs/TiOx. 2.2 Preparation of NiO/TiO2 NRs heterojunction. The NiO nanoflakes were prepared using by CBD method.17 0.08 mol of nickel sulfate (NiSO4) and 0.015 mol of potassium persulfate (K2S2O4) were added into 180 mL of deionized water and stirred at ambient condition for 20 min. Then, 20 mL of aqueous ammonia (NH3·H2O) was poured into the solution. The as-obtained TiO2 NRs were used as the substrates and the back sides were masked with polytetrafluoroethylene tape to prevent the deposition on the glass side. The substrates were placed vertically against on the Pyrex beaker containing the precursor solution and kept for 8 min to deposit the precursor films. The films were taken out, removed the tape, and then extensively rinsed with deionized water, and dried in air. After annealed at 350 °C in air for 2 h, the NiO/TiO2 NRs heterojunctions were obtained. For comparison, a pure NiO nanoflakes film on FTO substrate was also prepared with the same process. Au metal (0.0625 cm2) was sputtered on the surface of NiO nanoflakes as the electrode and the PDs based on NiO/TiO2 NRs heterojunctions were obtained. 2.3 Characterization Methods. Field-emission scanning electron microscopy (SEM, Hitachi S-8010) characterized the surface morphologies of the samples. X-ray diffraction (XRD, Rigaku, 2500 V/PC) patterns and Raman spectra (MICRO-Raman, 5



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LabRAM HR Evolution, France) investigated the crystal structure. X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB-250) were used to analyze the chemical state of the element and valence state of the samples. The Fermi level of each single material was obtained by a Kelvin probe. The wavelength-dependent absorption and diffuse reflectance spectra were monitored by a UV-vis spectrophotometer (Hitachi V4100) with an integrated sphere. Photoluminescence (PL) spectra at room temperature were performed on a FluoroLog 3 spectrometer (Horiba Jobin Yvon). Mott-Schottky (M-S) plots were obtained using a threeelectrode electrochemical workstation (CHI660D, Shanghai, China). The current versus voltage (I-V) curves under dark and light illumination and the temporal photoresponse of the formed heterojunctions were measured using a Keithely 2400 sourcemeter. A monochromatic light was provided by the PL-365 nm LED and spectrometer. 3. RESULTS AND DISCUSSION Figure 1a-d illustrate the surface and cross-sectional SEM images of TiO2 NRs grown on FTO and TiOx thin film substrates. The both substrates are uniformly covered with ordered TiO2 NRs. The TiO2 NRs have cubic-columnar shape with square top facets consisting of many small grids. The length and diameter of TiO2 NRs grown on the both substrates have no significantly difference for the same growth conditions such as the same precursor solution, growth temperature and time. The average length and diameter of TiO2 NRs is estimated to be ~1.5 µm and 100-150 nm, respectively. However, TiO2 NRs have a significantly stronger preferred orientation and more densely arranged on TiOx thin film substrates than that grown on FTO substrates. The flat thin film provide more uniform nucleation points for the growth of TiO2 NRs and facilitate the growth of nanorods perpendicular to the substrates.18 Furthermore, much more small grids on square top facets of the TiO2 NRs on TiOx thin film substrates can provide the nucleation sites for the growth of NiO nanoflakes on the surface of TiO2 NRs. Figure 1e-h shows the surface and cross-sectional SEM images of the NiO nanoflakes grown on the TiO2 NRs by the CBD method. SEM images revealed NiO nanoflakes cover the surface and lateral 6


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sides of TiO2 NRs grown on both substrates. Since TiO2 NRs on FTO substrates are relatively sparse and have large voids between the nanorods, the lateral side of TiO2 NRs covering some NiO nanoflakes can be observed in the top-view SEM images. The NiO nanoflakes are cross-linked to each other on the surface and attached on the lateral side of the dense TiO2 NRs grown on TiOx substrate, indicating the contact area between NiO nanoflakes and TiO2 NRs on TiOx substrate is larger than that of FTO substrate. Enhanced area of the junction between NiO nanoflakes and TiO2 NRs is beneficial to separate the potogenerated carriers at the interface and decrease the recombination probability, thus improve the photoelectric properties of NiO/TiO2 NRs heterojunctions. In order to study the crystal structure of TiO2 NRs and NiO/TiO2 NRs, XRD and Raman spectra were measured. As shown in Figure 2a, for TiO2 NRs grown on both substrates, except the FTO substrate, the diffraction peaks at 36.1°, 41.4°, 62.7°, 69.0° and 69.8° are well consistent with rutile phase TiO2, which can be attributed to (101), (111), (002), (301) and (112) planes of characteristic XRD peaks according to PDF card No. 65-0192.16 Among them, the (101) and (002) diffraction peaks are relatively strong and other diffraction peaks are weak for TiO2 NRs grown on both substrates. The relative intensities of (101) and (002) have a significantly difference for TiO2 NRs grown on both substrates. For TiO2 NRs grown on TiOx thin film substrate, the intensity of (002) peak is stronger and (101) peak is weak compared to that of FTO substrates, indicating the TiOx thin film thin film is benefit to the preferred orientation along the [001] direction, in agreement with the SEM results.19 Ordered single-crystalline nanorods can offer a direct pathway for the efficient photogenerated charge transport.20 After NiO nanoflakes deposition, the XRD diffraction peaks of TiO2 NRs become weak. The XRD diffraction peaks of the NiO nanoflakes can not be identified due to the thin and porous structure. To obtain XRD diffraction peaks of NiO nanoflakes, NiO nanoflakes were deposited on glass substrate on the same growth conditions and the typical (111), (200), and (220) peaks at 37.1o, 43.1o and 62.3o appear for the cubic NiO phase (PDF # 65-2901) (Figure S1 in Supporting Information). The rutile phase structure of TiO2 NRs can also be confirmed by Raman 7



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spectra (see Figure 2b). The Raman peaks of all samples appear at 443, 609 and 241 cm-1, which are attributed to Eg, A1g and multi-photon Raman vibration modes of rutile TiO2.21 The surface states of the semiconductor nanostructures usually have a significant effect on the photoelectric properties. The chemical composition and element valence states of the surface of TiO2 NRs and NiO nanoflakes were analyzed by XPS spectra, as shown in Figure 3. All the XPS spectra were calibrated by C 1s (284.8 eV). The survey scan spectra of TiO2 NRs grown on both substrates and NiO nanoflakes in Figure 3a revealed that all the peaks can be only ascribed to Ti, O and C elements for TiO2 NRs and Ni, O and C elements for NiO nanoflakes, indicating that no other impurities exist in TiO2 NRs and NiO nanoflakes. The C element could be attributed to the contaminant carbon. Similar Ti 2p XPS spectra of TiO2 NRs grown on both substrates are shown in Figure 3b. Two major peaks at 458.8 and 464.6 eV correspond to the Ti 2p3/2 and Ti 2p1/2 energy levels, respectively. The Ti element is mainly in the form of Ti4+ state in TiO2 NRs.22 Figure 3c shows the O 1s XPS spectra of TiO2 NRs grown on both substrates. The O 1s XPS spectra have obviously an asymmetric shape and consist of two peaks (Figure S2b and d). The dominant one (O1) located at 530.2 eV can match the O lattice of TiO2 and the higher binding energy peak (O2) at 532.4 eV can be ascribed to the absorbed oxygen-based species such as H2O or O2- groups on TiO2 NRs surface.23 The atomic percentages of O1 and O2 are calculated (Table S1 in the Supporting Information). The low atomic percentage of O2 for TiO2 NRs grown on TiOx thin film may be related to the decreased surface states and defect states.23 TiO2 NRs grown on TiOx thin film has better crystal quality. The Ni 2p XPS spectrum shown in Figure 3d for NiO nanoflakes grown on FTO revealed that Ni 2p XPS spectrum consists of multiple peaks and could be deconvoluted into five peaks (see Figure S2e in Supporting Information). The binding energies of 853.6, 855.4 and 860.6 eV correspond to the Ni 2p3/2 peaks, and the binding energies of 872.5 and 879.0 eV to the Ni 2p1/2 peaks, among which, the higher binding energies of 860.6 and 879.0 eV are ascribed to the satellite peaks of Ni 2p3/2 and Ni 2p1/2 of Ni2+ ions, respectively.24 The peak located at 853.6 eV is attributed to NiO6 in octahedral 8


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symmetry and the peak at 855.4 eV to NiO5 or Ni2+ in cone symmetry.25 The O 1s spectrum of NiO nanoflakes (Figure 3e) shows two peaks at 529.3 and 532.0 eV are assigned to lattice oxygen and surface adsorbed gas molecules in air molecular, respectively. The strong O 1s peak at 532.0 eV of NiO corresponds to the porous structure of NiO nanoflakes with the larger surface area. XPS is a powerful technique to characterize the valence band maximum (VBM) of a semiconductor. In order to determine VBM of TiO2 NRs and NiO nanoflakes, the XPS valence band spectra of the samples were characterized. The intersection between extrapolating the low binding energy edge of the valence band spectra and the background can determine the relative energy between the VBM and Fermi level (EF). As shown in Figure 3f, the EF position relative to the valence-band edge was estimated to be 2.77 and 2.89 eV for TiO2 NRs grown on FTO and TiOx thin film, and 0.20 eV for NiO nanoflakes, which indicated n-type semiconductor of TiO2 NRs grown on both substrates and p-type semiconductor of NiO nanoflakes. The EF relative to the vacuum level for TiO2 NRs and NiO nanoflakes can be obtained by a Kelvin probe force microscopy. The calculated work function (defined as the energy difference between EF and vacuum energy level) values were 4.95 and 4.82 eV for TiO2 NRs grown on FTO and TiOx thin film, and 4.97 eV for NiO nanoflakes. The excited electrons in TiO2 NRs grown on TiOx thin film have higher activity. The light absorption properties of TiO2 NRs, NiO nanoflakes and NiO/TiO2 NRs heterojunctions are investigated, as shown in Figure 4a-b. Figure 4a shows the absorption spectra of the TiO2 NRs grown on the both substrates, NiO nanoflakes and TiO2/NiO heterojunctions in 300-1000 nm wavelength range under transmissive mode. The absorption spectra of NiO nanoflakes exhibit a strong absorption in the UV region with an absorption edge at about 350 nm and weak absorption in the visible region, indicating NiO nanoflakes with a wide band gap. For TiO2 NRs grown on FTO and TiOx thin film, the absorption spectra show a strong UV absorption with the absorption edges at about 410 nm and the absolute absorption values are almost identical. After deposition of NiO nanoflakes, the absorption spectra of TiO2 NRs have no significantly change, which can be due to the small amount of NiO 9



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nanoflakes and their short-wavelength UV absorption. Moreover, the absorption in the visible region (λ > 410 nm) was observed for TiO2 NRs grown on both substrates, which is generally related to the visible light absorption by the surface defects of TiO2 NRs and the FTO substrate.19 A portion of the photons in TiO2 NRs coupled into the glass substrate can be internally reflected and exit without measured by the spectrophotometer, resulting in the light loss through the side of glass substrate. The absorption intensity of the TiO2 NRs grown on TiOx thin film substrates has slightly lower than that of the TiO2 NRs grown on FTO substrates, which can be less scattering of dense TiO2 NRs and low absorption of FTO substrates for TiOx thin film substrates.26 In order to eliminate the effect of light scattering on the calculated optical band gap energy of TiO2 NRs and NiO nanoflakes, the UV-visible diffuse reflectance spectra were measured (Figure S3) and the absorption spectra can be obtained according to the Kubelka-Munk function (Equation 1), as shown in Figure 4b. The optical band gap energy of TiO2 NRs and NiO nanoflakes are calculated from Tauc's relationship (Equation 2).

F ( R) =

(1 − R) 2 2R

(αhν )1/ n = A(hν − E g )

(1) (2)

where A, h and ν are a constant, the Planck constant and the frequency, respectively. α (α ∝ F(R)) is the absorption coefficient at a certain wavelength λ, n is 2 for indirect transition and 1/2 for direct transition. TiO2 and NiO are indirect semiconductors and n is 2. The calculated absorption spectra have strong UV absorption and weak visible absorption, indicating that TiO2 NRs and NiO nanoflakes are suitable for the active materials of UV PDs. In the inset of Figure 4b, the values of the optical band gap energy of TiO2 NRs and NiO nanoflakes are 3.00 and 3.55 eV determined from a plot of (F(R)hν)1/2 (hν=1240/λ) as a function of the energy of the incident radiation and intercept of the extrapolation of absorption edge.12 There is no difference in the optical band gap energy for TiO2 NRs grown on FTO and TiOx thin film although there are differences in morphology. 10


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Figure 4c shows the PL spectra of TiO2 NRs grown on both substrates and NiO/TiO2 NRs heterojunctions under excitation of 320 nm wavelength. All samples exhibit similar PL spectra with the emission bands dominated by two peaks at around 415 and 440 nm, which can be considered to be related to TiO2 NRs. The higher emission band at 415 nm corresponds to the band gap transition of TiO2 from the edge to the center of the Brillouin zone, namely, X1a/X1b→Γ1b.27 The process is phonon-assisted indirect transition. The emission band at about 440 nm can be ascribed to self-trapped excitons localized on TiO6 octahedra.27-29 As revealed in PL spectra, there is a tail at the longer wavelength besides two emission bands in blue region. The emission band at 500-550 nm was generally observed in TiO2 nanostructures, which is attributed to oxygen vacancies or surface defects.30 It is difficult to observe the PL spectra at room temperature for the conventional TiO2 crystalline due to the translation-symmetry and the electron transition following the rules of the vertical transition in K space. The probability of radiation transition is low and the phonons participate in the process, resulting in the low luminous efficiency for TiO2. For TiO2 nanostructures, the translation symmetry is damaged in local lattice and the selection rule of electron transition is break, resulting in the increased probability of radiation transition. PL spectra of TiO2 NRs grown on TiOx thin film is weak than that of TiO2 NRs grown on FTO. While the absorption spectra in UV region revealed that no significantly difference between the TiO2 NRs grown on both substrates. Therefore, the reduced intensity of PL spectra may be related to the high crystal quality of TiO2 NRs grown on TiOx thin film, which is consistent with the observation from XPS results. After NiO nanoflakes deposition, the intensity of PL spectra significantly decreases for TiO2 NRs grown on both substrates. Similar results were

observed

in

NiO/ZnO

NRs

and

NiO

nanoflakes/A-TiO2

NRs

heterostructures.31,32 There are two possible reasons, one involving the reduced number of photoexcited electrons of TiO2 NRs for NiO nanoflakes covering, and one involving the interface charge transfer between TiO2 NRs and NiO nanoflakes for type II heterojunction formation. The photo-excited holes in the valence band of TiO2 can transfer to NiO and reduce the recombination probability of the photo-excited 11



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electron-hole pairs, which presumably causes a significant decrease in the PL spectra intensity. As revealed in the absorption spectra shown in Figure 4a, the absorption in UV region for TiO2 NRs after NiO nanoflakes deposition is not increased, which can exclude the first possible reason. The flat band potentials, carrier densities and depletion width of TiO2 NRs grown on both substrates and NiO nanoflakes were determined from Mott-Schottky plots (1/C2 versus V). Figure 5a and b show a positive slope for TiO2 NRs and negative slop for NiO nanoflakes in the linear region of Mott-Schottky plots, which implies that TiO2 NRs act as a n-type semiconductor and NiO nanoflakes as a p-type semiconductor.33,34 The flat band potential obtained by extrapolating to the potential axis intercept of this curves is -0.28 V, -0.45 V and -0.18 V (versus Ag/AgCl) for TiO2 NRs grown on FTO and TiOx thin film, and NiO nanoflakes, which is consistent with the Fermi level (EF) position obtained by a Kelvin probe force microscopy (see Table S2). The more negative flat potential of TiO2 NRs indicates that the heterojunction based on TiO2 NRs grown TiOx thin film and NiO nanoflakes has a large built-in electric field. The donor concentration (ND) of TiO2 NRs and the acceptor concentration (NA) of NiO nanoflakes can be calculated from the slopes of Mott-Schottky plots using the equation ( 3 ) and ( 4 ):

(

)

  d 1 / C 2  2  N D =   2   eε r ε 0 A   dV 

(

−1

)

  d 1 / C 2  2  N A = −  2   eε r ε 0 A   dV 

(3)

−1

(4)

where e is electronic charge (1.602×10-19 C), εr is the relative dielectric constant of sample (εrTiO2 =55, εrNiO=12), ε0 is vacuum dielectric constant (ε0=8.854×10-12 F/m), A is the effective working area (A=1 cm2), C is capacitance and V is the applied potential at the electrode.35-37 The calculated donor concentration is 1.71×1019 and 7.47×1019 cm-3 for TiO2 NRs grown on FTO and TiOx thin film, and the acceptor concentration is 2.90×1021 cm-3 for NiO nanoflakes. The donor concentration of TiO2 12


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NRs grown on TiOx thin film is slightly higher than that of TiO2 NRs grown on FTO, which induces the Fermi level is closer to the conduction band position for the TiO2 NRs grown on TiOx thin film. The increased donor concentration improves the charge transport in TiO2 NRs. The upward shift of the Fermi level facilitates the charge separation at NiO/TiO2 NRs interface. The XPS results revealed the presence of many surface states and defect states on the TiO2 NRs, which can induce the formation of surface depletion region. The degree of band bending of TiO2 can effect the charge separation and transfer of the NiO/TiO2 NRs heterojunctions. The depletion width (W) in the TiO2 NRs and liquid interface can be determined from the Mott-Schottky plots using the equation ( 5 ):

 2ε ε (V − VFB )  W = r 0  eN D  

1/ 2

(5)

where VFB is flat band potential, ND is donor concentration of TiO2 NRs. The depletion width is 19.58 nm and 10.08 nm for TiO2 NRs grown on FTO and TiOx thin film as the the applied potential is chosen to 0.8 V. It is clear that the TiO2 NRs grown on the TiOx thin film have a higher carrier concentration and a thinner space depletion width. TiO2 NRs with the less amount of the surface states on the surface possess the thinner depletion width.38 Figure 6a illustrates the schematic diagram of NiO/TiO2 NRs heterojunction based PDs. The current−voltage (I−V) curves of NiO/TiO2 NRs heterojunction devices under dark and 365 nm UV irradiation (1.5 mW/cm2) from FTO side are shown in Figure 6b. The devices have a good rectifying characteristic with a small leakage current in the dark. The linear I-V curve of Au/NiO reveals good ohmic contacts between Au and NiO, indicating the formation of a p-n junction interface between NiO nanoflakes and TiO2 NRs (see Figure S4 in the Supporting Information). The rectification ratios are 2.52×104 and 1.45×105 at bias voltages of ±3 V for NiO/TiO2 NRs and NiO/TiO2 NRs/TiOx heterojunction devices, respectively. Clearly, NiO/TiO2 NRs/TiOx heterojunction devices show better rectification characteristics and non-linearity. The both devices exhibit an increased current under the UV light illumination and high sensitivity in detecting UV light. As shown in the enlarged 13



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curves around zero bias (inset of Figure 6b), all the devices exhibit a small, but measurable photovoltaic effect, indicating that no external voltage, self-powered UV light detection can be realized. The reverse bias currents of the both heterojunction devices have an obviously increase on UV illumination. The transient photoresponse of the devices were measured under on/off switching radiation of 365 nm UV light (1.5 mW/cm2) with an on/off internal of 60 s at zero bias and a small reversed bias of -1 V. Figure 6c and d shows good stability and repeatability of the photoresponse of the devices for four repeat cycles in the absence of external bias voltage and under applied bias of -1 V. Figure 6c shows the photocurrent of the devices rises sharply to saturation and reduces rapidly to the background current at the moment of switching the UV light on and off without applied bias. The enlarged rising and decaying edges of the photocurrent response (Figure S5a-b) revealed that the rise time and decay time is lower than 0.1 s for the both devices (The typical collection time is around 0.1 s per point in the current measurement), indicating the devices have a fast UV photoresponse characteristics under zero bias. Under a reversed bias of -1 V, the photocurrent of the devices increases but does not reach saturation after turning on the UV light and reduces sharply after switching the UV light off. The rise/decay times (defined as the time required for the photocurrent to increase from 10% to 90% and drop from 90% to 10%) are 38.5/4.2s and 28.9/2.0s for the devices based on NiO/TiO2 NRs heterojunction and NiO/TiO2 NRs/TiOx heterojunction under the bias of -1 V (Figure S4c-d). The photocurrent can not reach the saturation under applied bias of -1 V, which indicates that the generation rate is larger than the recombination rate of photogenerated electron-hole pairs during the short-time light illumination.33 The photocurrent under the reversed bias is obviously higher than that at zero bias. The applied reversed bias can broaden the depletion region and increase the built-in electric field, leading to separate efficiently more photogenerated carriers and produce a larger concentration of free electrons. As revealed from XPS and PL spectra, surface defects states exist in TiO2 NRs, in generally, which can capture the photogenerated electrons. The trapped photogenerated electrons can release under the applied bias, resulting in the enhanced photocurrent. In fact, the carrier trapping/release process is 14


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extremely slow, resulting in the slow photoresponse to the UV illumination at applied bias of -1 V. However, the photogenerated carriers trapped by the surface defect states do not contribute to the photocurrent at zero bias, resulting in a fast photocurrent response behavior.39 The light sensitivity (Ilight/Idark) are 410 and 635 for the devices based on NiO/TiO2 NRs and NiO/TiO2 NRs/TiOx under -1 V bias, respectively. The dark currents of the both devices are comparative and have little variation for the repeated cycles. The low dark current results in a high light sensitivity. Between the both heterojunction devices, the device with TiOx thin film exhibits a larger photocurrent and faster photoresponse, demonstrating very more excellent UV light sensitivity. Figure 6e and f show schematic energy band diagrams of the p-type NiO/n-type TiO2 heterojunction at zero bias and reversed bias of -1 V on the basis of the above experimental results. NiO and TiO2 can form the heterojunction with type II energy band alignment (Figure S6). The Fermi energy equilibrates across the TiO2 and NiO, resulting in the formation of a depletion region at NiO/TiO2 NRs interface due to the diffusion and drift of electrons and holes under the thermal balance condition. The built-in electric field in the interface depletion region can act as an electromotive force to drive the electrons to n-type TiO2 NRs as well as holes to p-type NiO nanoflakes. As the incident photons with energies exceeding or equal to the band gaps of NiO and TiO2, the electron-hole pairs are generated in the NiO/TiO2 NRs heterojunctions. The built-in electric field facilitates the electron-hole pair separation before recombination, resulting in the generation of a photovoltaic current in the external circuit. The result is faster transport of the charge carriers and a decreased probability of charge recombination. Under the 365 nm (3.40 eV) UV light irradiation from FTO side, conduction band electrons and valence band holes are generated in TiO2 NRs for the photon energy is greater than TiO2 NRs band gap (3.00 eV) and lower than NiO band gap (3.55 eV). The photogenerated holes are transferred from TiO2 to NiO and collected by Au electrode with simultaneous electrons collected by FTO, suggesting that the photogenerated electrons and holes can be efficiently separated under the built-in electric field and produce the photocurrent. The device with TiOx thin film shows the enhanced photocurrent. The possible reasons are: (1) 15



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relative growth orientation and density distribution of TiO2 NRs grown on TiOx thin film provide the enlarged heterojunction area, (2) larger built-in electric field from the high Fermi level of TiO2 NRs grown on TiOx thin film enhances the photogenerated electron-hole pairs separation and separated charges transform, and (3) the decreased surface defect states in TiO2 NRs grown on TiOx thin film reduces the probability of the carrier recombination and promotes the electrons to n-type TiO2 NRs as well as holes to p-type NiO nanoflakes. The photocurrent response of the devices based on NiO/TiO2 NRs heterojunctions at a zero-bias with various incident UV intensity (5-25 mW/cm2) are shown in Figure S7 in supporting information. The photocurrent increases steadily as the incident UV intensity increases from 5 to 25 mW/cm2 for both devices. The nonlinear dependence of photocurrent density (J) on the incident light intensity (P) can be expressed by a power law: J=APθ, where A is a constant and θ is a parameter related to the trapping or recombination processes of the photocarriers in the devices. The stronger the intensity of the incident light, the greater the current density. The more electron-hole pairs are generated and separated into free carriers and contribute to the photocurrent with the enhanced photon flux. The measure value θ are 0.69 and 0.50 for the NiO/TiO2 NRs and NiO/TiO2 NRs/TiOx based devices (see Figure 7a) by fitting the curves, which is related to the complicated process of generation, trapping and recombination of the photo-carriers within the heterojunctions.40,41 The responsivity (R) and detectivity (D*) are the important parameters for the PDs and can be expressed by using the following equations:

R=

D* =

I light − I dark

AP A1/ 2 R (2eI dark )1/ 2

(6)

(7)

where Ilight and Idark are the photocurrent and dark current, A is the working area (0.0625 cm2), P is the light power density (1.5 mW/cm2), and e is the electron charge, respectively. As shown in Figure 7a and b, with the increasing illumination intensity, the responsivity and detectivity decrease for both devices at zero bias. The 16


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responsivity and detectivity are large at low incident light intensity, i.e. the device exhibit higher photosensitivity to weaker UV light. Similar results have been reported in ZnO/NiO device.42 This phenomenon can be related to the increased recombination rate and decreased average life time of carriers due to the surface states and defect states on the TiO2 NRs and NiO nanoflakes. The larger photocurrent for the devices based on NiO/TiO2 NRs/TiOx results in the higher responsivity and detectivity. The wavelength selectivity is an important performance index of PDs.43,44 The wavelength selectivity of the devices based on NiO/TiO2 NRs heterojunctions were measured in the light wavelength ranging from 280 to 650 nm under the weak light radiation intensity (0.5 mW/cm2) at zero bias and -1V applied bias, as shown in Figure 7c and d. The both devices exhibit excellent wavelength selectivity and high UV responsivity under the weak light radiation. A strong and narrow peak located at 380 nm with a shoulder peak at 350 nm were demonstrated for the devices, which correspond to the strong band-gap absorption of TiO2 NRs and NiO nanoflakes. The results reveal that the NiO/TiO2 NRs heterojunctions can be a promising candidate for high spectrum selectivity UV PDs applications. The maximum values of responsivity at 380 nm are 1.34 and 5.66 mA/W for the devices based on NiO/TiO2 NRs and NiO/TiO2 NRs/TiOx without external applied bias under the weak light radiation. Compared with the recent reports (Table S3), the UV responsivity and detectivity are relatively large for both self-powered devices. Under the -1V reversed bias, the responsivity of the devices is remarkably enhanced due to the increased photocurrent. The carrier drift velocity increases under the external bias, resulting in the increased photocurrent. In addition, the difference of UV responsivity between the both devices is large under zero bias and small under -1V applied bias. The photogenerated excitons in NiO nanoflakes are more easy separated and the separated electrons are transferred to TiO2 NRs under the external electric field driving. On the other hand, the release of the surface trapped charges on TiO2 NRs also contribute to the enhanced photocurrent in the applied electric field. 4. CONCLUSIONS

17



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In this work, the devices with NiO nanoflakes/TiO2 NRs heterojunctions were fabricated and the photoelectric response characteristics were studied. The devices with TiO2 NRs grown on FTO and TiOx thin film exhibit good spectral selectivity, high photoresponsivity and a fast response speed (

TiO2 NRs based Self-powered UV Photodetector: Heterojunction with

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