Hydrothermal Preparation and White-Light-Controlled Resistive Switching Behavior of BaWO4 Nanospheres

In this work, BaWO4 nanospheres were successfully prepared by hydrothermal process. The bipolar resistive switching behavior of Ag/BaWO4/FTO device is observed. Moreover, this resistive switching behavior can be modulated by white light. The device can maintain superior stability in the dark and under white-light illumination. This study is useful for developing the light-controlled nonvolatile memory devices.


Introduction
Reversible resistive-switching effect is a promising candidate for next-generation nonvolatile memories [1]. The resistive switching behavior, in which the reversible switching between a high-resistance state (HRS) and a low-resistance state (LRS) can be achieved by the applied voltage, is an attractive subject of scientific and technical research [2][3][4][5][6]. The resistive switching is classified into unipolar resistive switching and bipolar resistive switching [7]. The resistive switching memory cell usually has simple structure, in which an insulating oxide is sandwiched between two metal electrodes [8]. Therefore, the resistive switching device is suitable for wide application because of the simple preparation steps and relatively low cost.
In the past few years, a new control method (light controlled) has been involved in the resistive switching memory device. Ungureanu firstly reported the lightcontrolled resistive switching memory in Pd/Al 2 O 3 /SiO 2 device [9]. At the same time, Adachi and Park also added the light as extra control parameter in the switching memory device based on ZnO nanorods [10][11][12]. In addition, our group also found that light can act as a control method in some resistive switching systems [13][14][15]. The light-controlled resistive switching effect provides the potential for light-controlled nonvolatile memory device, which may be a promising developing trend of information science and storage technology. In addition, the white light, which is the most ordinary light source, is widely used. BaWO 4 is a wide gap semiconductor with Eg [4.9 eV and has a Scheelite structure [16,17]. BaWO 4 is an important material in the electro-optical industry owing to its emission of blue luminescence [18][19][20][21][22][23]. Therefore, BaWO 4 received more and more research interest [24].
Although there are many reports about various applications in BaWO 4 nanostructure in previous works, the resistive switching properties of BaWO 4 have not been reported yet. Herein we present the reversible bipolar resistive-switching effect in Ag/BaWO 4 /FTO device. Moreover, the resistive-switching effect can be controlled by white-light illumination.

Preparation of BaWO 4 Nanospheres
The BaWO 4 nanospheres were prepared by a hydrothermal process using cetyltrimethylammonium bromide (CTAB) as the surfactant. All the chemicals used in this work were of analytical grade and used directly without further purification. The distilled water was used as a solvent throughout the experiment. Firstly, Ba(NO 3 ) 2 (0.01 M) and Na 2 WO 4 Á2H 2 O (0.01 M) were dissolved in 40 ml distilled water under stirring continuously. Then 0.5 g cationic surfactant cetyltrimethylammonium bromide (CTAB) was added into above solution under strong stirring. After continuous stirring for 2 h, the solution was transferred to a 50-ml sealed Teflon-lined steel autoclave. Then, the sealed Teflon-lined steel autoclave was heated and kept at 200°C for 72 h. After the autoclave was cooled to room temperature, the powder obtained was washed with distilled water and ethanol and dried at 60°C for 12 h.

Preparation of Ag/BaWO 4 /FTO Device
Firstly, FTO substrates were cleaned by acetone, ethanol, and deionized water, and subsequently dried on the spin coater. Secondly, BaWO 4 films were prepared on FTO substrate by spin-coating method. The detail preparation process of BaWO 4 films is as follows: Firstly, we grinded the as-prepared BaWO 4 nanospheres powder for 2 h. Next, we dissolved the powder in toluene solution to prepare precursor gel. Then the precursor gel was spin-coated on the FTO substrate. The spin-coating process at 5,000 rpm for 10 s was used to prepare BaWO 4 films with thickness of about 2 lm. Then these samples were subsequently dried at 60°C in vacuum for overnight. The thickness of the BaWO 4 film was detected by the step profiler.

Characterizations
Crystal structure of BaWO 4 nanospheres was characterized by X-ray diffraction (XRD) with Cu Ka radiation at room temperature. Surface morphology of BaWO 4 nanospheres was characterized using scanning electron microscope (SEM). Microstructure, nanosphere size, selected area electron diffraction (SAED) pattern, and the energy-dispersive X-ray spectroscopy (EDX) spectra of the BaWO 4 nanospheres were observed by transmission electron microscopy (TEM) at an acceleration voltage of 200 kV. In the test of resistive switching characterizations, Ag is top electrode and FTO is bottom electrode, as shown in Fig. 1. Ag electrodes with area of *1 mm 2 and thickness of 200 nm were prepared by vacuum deposition. And the preparation process of Ag electrodes is as follows: Firstly, we covered a mask on surface of BaWO 4 /FTO. Secondly, we put it into the vacuum sputtering system to grow Ag electrodes. Finally, we chose the superior electrodes for characterization. Current-voltage (I-V) and resistance cycles curves were tested using the electrochemical workstation (CHI) at room temperature. In addition, we used an ordinary filament lamp as light source. The wavelength range of light is 400-760 nm. The crystalline structure of the BaWO 4 nanospheres was characterized by XRD. Figure 3a exhibits the XRD pattern of as-prepared BaWO 4 nanospheres. There are only the peaks of BaWO 4 , which reveals the purity of the BaWO 4 nanospheres. The XRD demonstrates the characteristic diffraction peaks of BaWO 4 . Moreover, the XRD profile matches very well with that in the reported work [25][26][27][28]. The result indicates that the BaWO 4 nanospheres have a   Fig. 3 a The XRD of as-prepared BaWO 4 nanospheres at room temperature. b The EDX spectrum of BaWO 4 nanospheres tetragonal scheelite unit cell (a = 5.62 ± 0.05 Å , c = 12.71 ± 0.07 Å ) according to the peak positions and their relative intensities, which is consistent with the reported value (JCPDS Cards 08-457). Therefore, the product contains only pure BaWO 4 , and the sharp peaks demonstrate good crystallinity of the BaWO 4 nanospheres. The composition of BaWO 4 nanospheres was further confirmed by elemental analysis carried out with energy-dispersive X-ray spectra (EDX). The EDX data in Fig. 3b confirm that the compositions of as-prepared product are only Ba, W, and O with an atomic ratio of 0.93:0.98:4, which is close to the stoichiometric ratio of BaWO 4 . Figure 4a displays the I-V characteristics curves of Ag/ BaWO 4 /FTO device in linear scale in the dark and under white-light illumination with power density of 30 mW cm -2 , we can see that I-V curves exhibit asymmetric behavior with significant hysteresis. The arrows in the figure denote the sweeping direction of voltage. Figure 4b presents a corresponding I-V curve of Ag/ BaWO 4 /FTO device in logarithmic scale. The arrows in the figure denote the sweeping direction of voltage. The Ag/ BaWO 4 /FTO device shows obvious resistive switching behavior in the dark. A sudden current increasing occurs at 3.0 V (V Set ), indicating a resistive switching from the highresistance state (HRS or 'OFF') to the low-resistance state (LRS or 'ON'), which was called the ''Set'' process. When the applied voltage sweeps from zero to negative voltage of about -3.5 V (V Reset ), the device can return to the HRS, which was called the ''Reset'' process. The resistances of HRS and LRS at negative bias are much larger than those at positive bias. During the successive ''Set'' and ''Reset'' cycles on the same device, the device shows the identical I-V curves. The V Reset and V Set are almost unchanged in subsequent cycles for the same device (not shown here). Moreover, the resistive switching behavior of Ag/BaWO 4 / FTO device is improved by white-light illumination. The I-V curve under white-light illumination is more symmetrical than that in the dark. And the resistive switching behavior at negative bias is more obvious than that in the dark. Furthermore, the resistance of LRS at negative bias is nearly as same as that at positive bias. In addition, the V Set (3.1 V) under white-light illumination is larger than that (3.0 V) in the dark.

Results and Discussion
In order to estimate the probable practicability of the white-light-controlled resistive switching behaviors of the Ag/BaWO 4  The mechanism for resistive switching in a metal/oxides/oxides structure has been extensively investigated [8,[29][30][31][32][33]. In our works, current-voltage curve of the Ag/ BaWO 4 /Ag structure is symmetrically linear without hysteresis (not shown here), indicating it is Ohmic contact between Ag and BaWO 4 . Therefore, the asymmetric behavior of I-V curve of Ag/BaWO 4 /FTO in the dark indicates that a Schottky barrier is formed at the interface of BaWO 4 /FTO. The bipolar resistive switching behavior of Ag/BaWO 4 /FTO should result from the trapped and detrapped charge in the Schottky-like depletion layer [26][27][28][29][30][31]. Moreover, the white light can generate a large number of charges, which can change the trapped state and detrapped state in the Schottky-like depletion layer [9][10][11][12]. Therefore, the white light can modulate the resistive switching behavior of Ag/BaWO 4 /FTO.

Conclusions
BaWO 4 nanospheres were prepared by hydrothermal process. The reversible bipolar resistive switching characteristics of Ag/BaWO 4 /FTO device were observed. In particularly, the resistance switching behavior can be controlled by white-light illumination. Therefore, the superior resistance switching characteristics of the Ag/ BaWO 4 /FTO device hold a promise for light-controlled nonvolatile memory applications.