All of the chemicals were of analytical purity and used as received without further purification. MnO₂ nanowires (NWs) were prepared by a simple hydrothermal method. Firstly, the 1.715 g KMnO₄ was dissolved in 25 mL of deionized water under magnetic stirring, and 2.5 mL concentrated HCl was added dropwise into the above solution. The mixture of the solution was stirred continuously for 1 h. Secondly, the mixed solution was transferred into a 50 mL Teflon-lined stainless steel autoclave and heated in an oven at 180˚C for 12 h. After the reaction, the autoclave was cooled down to room temperature in air, the sample was then collected by filtration and washed with ethanol and deionized water several times, and dried at 80˚C for 3 h. Finally, the dried sample was further calcined at 500˚C for 1 hour under N₂ environment, and MnO₂ NWs were obtained.

CuO_x/MnO₂ heterostructures were synthesized by a simple chemical both and calcination method. Firstly, the 0.174 g of the obtained MnO₂ NWs were dispersed in 50 mL deionized water followed by stirring for 10 min, then 0.72 g of glucose was dissolved in the above solution under the same conditions, 0.2 g of Cu(AC)₂・H₂O was added to the mixed solution of MnO₂ NWs and glucose while keeping stirring for 10 min, then the obtained mixed solution was heated in water-bath for 1 hours at 90˚C. Secondly, the product was collected and washed by filtration at room temperature, and dried at 50˚C overnight. Thirdly, the collected solid powders were put into a tubular furnace and heated to 500˚C at a rate of 5˚C・min⁻¹ and maintained for 180 min under N₂ protection. Finally, the CuO_x/MnO₂ heterostructures were obtained and named as the sample M-2. By adjusting the amount of Cu(AC)₂・H₂O, a series of CuO_x/MnO₂ heterostructures using 0.3 and 0.4 g of Cu(AC)₂・H₂O were obtained and denoted as M-3 and M-4, respectively. The synthesis of the CuO_x/MnO₂ heterostructures is shown in Figure 1.

X-Ray power diffraction (XRD) patterns were recorded on a Rigaku D/max-2550 PC X-ray power diffractometer employing Cu-K a radiation operating at 40 kV and 200 mA. The morphology of the samples was observed using a field emission scanning electron microscope (SEM, S-4800) at an acceleration voltage of 5

kV. The UV-vis diffuse reflectance spectra (DRS) of the sample were performed on a Perkin Elmer Lambda 35 spectrophotometer, using BaSO₄ as reference. The photoluminescence (PL) emission spectra of the samples were obtained using a fluorescence spectrophotometer (FS-5) at room temperature.

The photocatalytic activities of the as-prepared samples were evaluated using the degradation of methylene blue (MB) at room temperature under visible light irradiation. In photocatalytic experiment, 20 mg photocatalysts was suspended in 50 mL of MB solutions (5 mg・L⁻¹). Before lighting on, the suspension was stirred for 30 min in the dark to ensure absorption-desorption equilibrium between the photocatalyst and organic dye solution. After that, the suspension under magnetic stirring was placed approximately 20 cm below a xenon lamp (500 W, Model PLS-SXE300) with a cut-off filter that only emits visible light (λ > 400 nm). At a given time period, the 3 mL suspension was removed from the original solution and centrifuged at 8000 rpm for 3 min to remove the remnant photocatalyst, The absorbance of MB concentration were analyzed by a UV-vis spectrophotometer (UV-2550, Shimadzu) at wavelength 664 nm. After testing, the suspension was returned and the irradiation was resumed. Consequently, the degradation rate for MB could be calculated according to the change of the absorbance.

CuO_x/MnO₂ heterostructures with different amount of Cu(AC)₂・H₂O (0.2 g, 0.3 g and 0.4 g) were prepared via a simple chemical both and calcination method. MnO₂ NWs were produced by a reaction of KMnO₄ and concentrated HCl and then crystallized during the hydrothermal process. Meanwhile Cu²⁺ and glucose were deposited onto the surface of MnO₂ NWs. The conversion of CuO_x from Cu²⁺ was achieved by the subsequent calcination process under N₂ protection. The resulting catalysts are named as M-2, M-3 and M-4, respectively.

The phase of CuO_x/MnO₂ heterostructures was analyzed using the XRD measurements. Figure 2 presents the XRD patterns of the pure MnO₂ NWs, M-2, M-3 and M-4. In the XRD pattern of the pure MnO₂ NWs, all diffraction peaks at the 2θ values of 12.7˚, 18.0˚, 28.8˚, 37.5˚, 38.8˚, 41.9˚, 49.8˚, 56.1˚, 60.1˚, 65.3˚ and 69.3˚ correspond to the (110), (200), (310), (211), (330), (301), (411), (660), (521), (002) and (541) crystal faces of α-MnO₂ (JCPDS Card No: 44-0141), respectively. No impurity peaks are found within experimental error, indicating a high purity of α-MnO₂. In the XRD pattern of M-2, three characteristic peaks with 2θ values of 32.3˚, 35.8˚ and 57.5˚ are attributed to the (110), (11-1) and (202) crystal planes [27] of CuO (JCPDS Card No: 048-1548), and the peaks with 2θ values of 30.5˚, 37.5˚ and 62.9˚ could be indexed to the (110), (111) and (220) crystal planes of Cu₂O (JCPDS Card No: 78-2076), respectively. The Cu₂O (111)

peak is very close to the MnO₂ (211) peak and thus they are overlapped at around 37.5 in the pattern, and the other peaks can be indexed to MnO₂. Compared with the pure MnO₂ NWs, the intensity of MnO₂ diffraction peaks obviously decreases in the XRD pattern of M-2. Thus, according to the XRD results, the sample M-2 is consisted of three phases of CuO, Cu₂O and MnO₂, indicating that the information of CuO_x/MnO₂ heterostructures. In the XRD pattern of M-3, we can clearly see that there are four characteristic peaks with 2θ values of 32.3˚, 35.8˚, 38.7˚ and 57.5˚ corresponding to the (110), (111), (11-1) and (202) crystal planes of CuO, respectively, Three peaks with 2θ values of 30.5˚, 37.5˚ and 62.9˚ correspond to the (110), (111) and (220) crystal planes of Cu₂O, and one peak with 2θ value of 43.0˚ is assigned to the (111) crystal planes of Cu (JCPDS Card No: 04-0836) [28] , and the other peaks can be indexed to MnO₂. The intensity of MnO₂ diffraction peaks in the XRD pattern of M-3 was weaker than one in the XRD pattern of M-2. According to the XRD results, the sample M-3 is composed of four phases of CuO, Cu₂O, Cu and MnO₂, indicating that the information of CuO_x/MnO₂ heterostructures. In the XRD pattern of M-4, we can also clearly see there are two peaks at around 43.0˚ and 50.1˚ corresponding to the (111) and (200) crystal planes of Cu, and two peaks at around 35.8˚ and 61.2˚ are attributed to the (002) and (113) crystal planes of CuO, and one peak at 40.5˚ is consistent with the (200) crystal planes of Cu₂O. No peaks of MnO₂ are found due to the loading of CuO_x. So the sample M-4 is composed of four phases of CuO, Cu₂O, Cu and MnO₂. The above results verify the formation of CuO_x/MnO₂ heterostructures.

The morphology of MnO₂ NWs, M-2, M-3 and M-4 was observed in scanning electron microscopy (SEM) images, which are shown in Figure 3. Figure 3(a) and b are the SEM images of the pure MnO₂ NWs with different magnifications.

It can be clearly seen that the pure MnO₂ NWs with diameters of about 30 - 80 nm and lengths of 2 - 3 um have a relatively smooth surface, and MnO₂ NWs were uniform in diameter through the entire length and gathered in disorder. As can be seen from Figure 3(c) and Figure 3(d), a small and sparse nanoparticles with the sizes of 10 - 20 nm were deposited on the surface of MnO₂ NWs in the sample M-2 prepared using 0.2 g of Cu(AC)₂・H₂O. When increasing the amount of Cu(AC)₂・H₂O to 0.3 g in the sample M-3, as shown in Figure 3(e) and Figure 3(f), the sizes of nanoparticles on the surfaces of MnO₂ NWs increase to 20 - 40 nm. Using 0.4 g of Cu(AC)₂・H₂O in the sample M-4 (Figure 3(g) and Figure 3(h)), the bigger and more nanoparticles with diameters of 20 - 50 nm were deposited on the surfaces of MnO₂ NWs, and the surface of MnO₂ NWs is obviously rough. Additionally, it can not be observed the more density of nanoparticles grown on the surface of MnO₂ NWs with increasing the amount of Cu(AC)₂・H₂O to 0.5 g. These nanoparticles are considered to be the composite of CuO_x. So, a series of CuO_x/MnO₂ heterostructures of the sample M-2, M-3 and M-4 were successfully fabricated by changing the amount of Cu(AC)₂・H₂O.

We investigate the optical properties of the pure MnO₂ NWs and CuO_x/MnO₂ heterostructures using a combination of UV-vis and PL technologies. Figure 4(a) shows the UV-vis absorbance spectra of the pure MnO₂ NWs and CuO_x/MnO₂ heterostructures (M-2, M-3 and M-4). From the spectra, the characteristic absorption peak of the pure MnO₂ NWs is at 450 nm and the absorption band edge enlarges to almost the full visible region [29] . The sample M-2 and M-3 of

the CuO_x/MnO₂ heterostructures show two broad bands at 400 - 460 nm and 650 - 700 nm. With the increase of the amount of Cu(AC)₂・H₂O, the absorption intensity of the sample M-2 and M-3 was enhanced gradually, which further confirms that Cu²⁺ had been successfully anchored on the surface of the MnO₂ NWs. In comparison with the sample M-2 and M-3, the sample M-4 shows a broad and strong absorption in visible light range (400 - 700 nm), and the band gap energy (E_g) of the sample M-4 calculated on the basis of the corresponding absorption edge was 1.57 eV, showing a significant red-shift compared with the pure MnO₂ NWs. It suggested that the sample M-4 might display good photocatalytic activity under visible light.

The photoluminescence (PL) spectra were characterized the trapping, migration and separation of electron-hole pairs, and the lower PL intensity indicates the higher photocatalytic properties of the nanomaterials [30] . The PL spectrum of the samples is measured at room temperature using 300 nm as an excitation wavelength (Figure 4(b)). It is clear that two strong emission peaks of the pure MnO₂ NWs are at about 349 and 351 nm and it is similar to the emission peaks of M-3. Compared with the emission peaks of the pure MnO₂ NWs and M-3, the PL spectra of M-2 and M-4 consist of two weak emission peaks located at about 348 nm and 353 nm and two weaker emission peaks at 412 nm and 453 nm [31] [32] . The sample M-4 among all the samples showed the lowest PL emission intensity and indicated the enhancement of photocatalytic performance.

To evaluate the photocatalytic activity of the as-synthesized CuO_x/MnO₂ heterostructures, the degradation of MB (methylene blue, a standard organic dye) under visible light radiation is utilized as a probe reaction, and displays a characteristic absorption of MB at a wavelength of around 664 nm in absorption spectra. For comparison, the photocatalytic experiment using the pure MnO₂ NWs as the catalyst is also conducted under the same condition. One thing to note here is that before irradiation the mixed solution of the photocatalysts and MB is stirred for 30 min in the dark to establish the adsorption/desorption equilibrium on the photocatalyst surfaces. The corresponding photocatalytic properties have been demonstrated in Figure 5. Figure 5(a) shows the temporal evolution of the absorption spectra of MB over the sample M-4. The characteristic absorption peak of MB at wavelength of 664 nm decreases sharply as the reaction time increases and almost disappears after 140 min, confirming that MB can be efficiently degraded by the sample M-4. Figure 5(b) presents the degradation curves of MB over different catalysts. When using the pure MnO₂ NWs as the photocatalyst, only 47.2% of MB is degraded due to the fast recombination rate of charge carriers, resulting from the narrow band gap [18] . Interestingly, all the CuO_x/MnO₂ heterostructures (M-2, M-3 and M-4) display higher activities than the pure MnO₂ NWs, and the MB degradation efficiencies reach up to 69.3%, 89.2% and 96.1%, respectively. It is obvious that the photocatalytic activity of the pure MnO₂ NWs is improved due to different amount of CuO_x was loaded on the surface of MnO₂ NWs. This may be ascribed to a large specific surface area which can introduce more unsaturated coordination sites in the surface that exposed to MB molecules, that is more favorable for the photocatalytic degradation of MB molecules [33] .

In order to better understand the photocatalytic efficiency of the catalysis, the curve of ln(C₀/C) versus time of MB photodegradation process was also investigated. On the basis of the Langmuir-Hinshelwood model, the linear relationship of ln(C₀/C) versus time can be described as: −ln(C₀/C) = kt, where k represents the reaction rate constant of the pseudo-first-order. Figure 6 shows the effect of different samples on the kinetics of MB under visible light irradiation. The value of apparent rate constant k, which is equal to the corresponding slope of the simulation curve, is shown in the inset of Figure 6 in the form of a histogram graph. It can be seen that the k values for the pure MnO₂ NWs, M-2, M-3 and

M-4 are 0.005 min⁻¹, 0.015 min⁻¹, 0.008 min⁻¹ and 0.021 min⁻¹, respectively. These results show the introduction of CuO_x improve the visible light photocatalytic performance of the MnO₂ NWs and the sample M-4 exhibits the highest photodegraded efficiency.

The stability and reusability of photocatalysts is another important consideration for their practical applications. Therefore, MB recycling degradation experiments over the sample M-4 were also conducted. Figure 7 shows the degradation efficiencies of MB in three cycles, the first degradation rate of MB was 93.73% and 84.35% for the second time. After repeating the procedure 3 times, the degradation rate still remained at 84.07%, and there is a slight loss of photocatalytic activity, demonstration the good recyclability of M-4. This result indicates that the sample M-4 can still remain stable and efficient during organic dye degradation. Comparing these samples, the sample M-4 has better degradation

activity, which indicates CuO_x nanoparticles on the surface of MnO₂ nanowires played key roles in the enhanced photocatalytic performance. The enhancement may be due to the following reasons. Firstly, the higher photocatalysis efficiency of the heterostructure samples could be explained in terms of the enhancement of UV-vis absorbance spectra and the weaker PL emissions due to CuO_x nanoparticle depositions. Secondly, CuO_x nanoparticles on the surface of CuO_x/MnO₂ heterostructures have the increase of surface area, and inhibit the recombination of electron-hole pairs caused by electronic conduction of Cu nanoparticles [34] .